Recent advances in dielectric and two-dimensional material-based surface plasmon resonance biosensors for ultrasensitive refractive index sensing and diverse applications

1 Fundamentals of SPRBiosensing

1.1 Physical principles of surface plasmon resonance

SPR(SPR) is a label-free optical sensing method based on the resonant excitation of collective oscillations of free electrons at a metal–dielectric interface with p-polarized light (Helmerhorst et al., 2012; Puiu and Bala, 2016; Zanchetta et al., 2017). Due to its real-time operation mode and no need of fluorescent or radioactive labeling, SPR has emerged as one of the most popular sensing technologies for biological, chemical and environmental analysis because it is able to detect very small RI (RI) changes. A typical SPR sensor design based on prism utilizes Kretschmann configuration and consists of a thin layer of noble metal, mostly gold (Au) or silver (Ag) coated onto the base of a high RI prism (Naik et al., 2013; Dreaden et al., 2012; Di Pietro et al., 2016). Where the incident light in plane component coincides with surface plasmon polariton (SPP) propagation constant, results in resonance which creates a sharp minimum of reflected light intensity (Ghosh and Pal, 2007; Pettinger, 1986). Equations 1, 2 express the resonance and prism-coupling conditions (Boddu et al., 2025).where ε m and ε d represent the dielectric constants of the metal and sensing medium, respectively.where n p is the prism RI and θ is the incidence angle. The resonance angle is highly sensitive to changes in the RI of the surrounding medium. Therefore, biomolecular binding events that occur at the surface of the metal change its local optical environment and result in shifts in the resonance angle or wavelength that can be quantified. The evanescent field produced by SPR excitation usually extends approximately 200 nm–300 nm into the dielectric medium, and SPR is a particularly well-suited technique for monitoring surface-confined interactions, such as antigen–antibody binding, DNA hybridization, and protein adsorption. Despite these strengths, traditional metal-film SPR sensors suffer from inherent optical losses, which lead to the broadening of the resonance curve and decreased detection accuracy for low-concentration analytes. These limitations have motivated the development of advanced SPR platforms incorporating dielectric materials and two-dimensional nanomaterials, which are discussed in the following sections. DNA hybridization and protein adsorption. An illustration of the principle of operation for a general prism-based dielectric–2D material-assisted SPR biosensor is shown in Figure 1. The sensing is based on prism coupling to generate the surface plasmons at the metal–dielectric interface, while confinement of electromagnetic fields and biomolecular interactions can be further enhanced by dielectric as well as 2D materials.

k S P P = k 0 ε m ε d ε m + ε d (1)

k x = 2 π λ n p ⁡ sin ⁡ θ (2)

1.2 Performance parameters of SPR sensors

Several quantitative metrics are typically used to evaluate the performance of SPR biosensors including sensitivity, detection accuracy (DA), full width at half maximum (FWHM), figure of merit (FOM), signal-to-noise ratio (SNR) and limit of detection (LOD). All these parameters together define the ability of an SPR platform that can detect small RI variances with high precision and reliability.

1.2.1 Sensitivity

Sensitivity is one of the critical performance characteristics of an SPR sensor, and indicates the change in resonance angle or resonance wavelength due to a unit variation of RI associated with the shifted medium. The angular and wavelength sensitivities are computed with Equations 3, 4, respectively (Efimov et al., 2024; Shalabney and Abdulhalim, 2011). Conventional gold-based SPR sensors are generally characterised by the sensitivity about in the range of 100°–300°/RIU, however multilayer and nanomaterial-assisted SPR configurations have been shown to provide the sensitivities beyond 500°/RIU.where Δ θ is the shift in resonance angle corresponding to RI variation Δ n.

S = Δ θ Δ n (3)

S λ = Δ λ Δ n (4)

1.2.2 Detection accuracy and FWHM

The narrower the SPR resonance curve, the higher the detection accuracy, which is related to FWHM. A lower FWHM means a sharper resonance dip, allowing the resonance angle to be determined more precisely and providing narrower measurement windows. Resolving to improve resonance sharpness is a complementary approach as enhancing sensitivity in pursuit reliable sensing performance. The detection accuracy was calculation b using the Equation 5.

D A = 1 F W H M (5)

1.2.3 Figure of merit (FOM)

The FOM can be calculated using Equation 6 provides a comprehensive assessment of the sensor performance by simultaneously considering the sensitivity and resonance linewidth. Therefore, a high FOM indicates that the sensor exhibits both a large resonance shift and a narrow resonance curve, making it a more reliable indicator than sensitivity alone when comparing different SPR-sensor architectures.

F O M = S F W H M (6)

1.2.4 Limit of detection (LOD)

The LOD, given by Equation 7, is the minimum index variation that can be detected by the sensor with high certainty (Armbruster and Pry, 2008; Saadati et al., 2013). Recent multilayer SPR biosensors with novel functional materials have been fabricated, yielding LOD values close to 10^–6 to 10^–7 RIU, thus making them promising candidates for trace-level biochemical sensing.

L O D = δ min S (7)

1.2.5 Signal-to-noise ratio (SNR)

The signal-to-noise ratio (SNR) represents the capability of an SPR sensor to separately identify valuable sensing signals from noise, which is critical for analyzing biological samples with complicated compositions. A higher SNR allows the measurement to be more reliable and repeatable because it reduces the influence of optical scattering and external disturbances. Exciting advances have been reported in SNR by using multilayer and nanomaterials-assisted SPR sensors that leverage their strong resonance characteristics and lower optical losses.

1.3 Motivation for ultrasensitive SPR biosensor development

The increasing demands for highly sensitive, rapid and label-free detection techniques in biomedical diagnostics, environmental monitoring, food safety and chemical sensing have accelerated the development of SPR biosensing technology. Target analytes are often present at very low level in real world applications and lead to negligible RI changes, which are not practicable with traditional sensing platform. Thus, in recent years the enhancement of SPR sensors sensitivity and reliability have been considered a considerable research field. Conventional metal-based SPR sensors enable real-time as well as label-free detection, but their performance is hampered by broad resonance curves, inherent optical loss, and moderate sensitivity. Among numerous biomedical applications, the detection of trace biomarkers in biological fluids continues to play a crucial role in early-stage diagnosis of ailments. Examples of highly sensitive materials used for presymptomatic detection include metabolic biomarkers, such as urea, creatinine, and albumin in urine, which typically evoke a RI change at an order of 10^–6 RIU; hence, ultra-sensitive sensing platforms with low detection limits and narrow resonance linewidths are essential. Pathogenic microorganism detection and environmental pollutant and chemical analysis are some examples where trace-level analyte or measurement is essential for very precise data (Wang A. et al., 2022; Fu et al., 2023). To address these limitations, substantial efforts have been directed toward engineering multilayer SPR platforms through the incorporation of functional materials and optimized device architectures. These advances have facilitated strongly enhanced electromagnetic field confinement, ultimately improving sensing performance and detection capability for the fabrication of next-generation ultrasensitive SPR biosensors. The material systems, improvement methods, and architectural techniques that enable these improvements are detailed in the following sections.

2 Dielectric and two-dimensional materials for SPR enhancement

2.1 Role of dielectric layers in SPR sensors

However, conventional metal-film SPR biosensors are fundamentally limited by the intrinsic optical losses found in noble metals, which leads to broadening of resonance curves and poorer sensitivity. To overcome these limitations, thick high-RI dielectric materials have been widely used in multilayer SPR structures to further enhance light–matter interaction at the sensing interface via better confinement of the electromagnetic field (Jiang et al., 2017). Consequently, dielectric-assisted SPR platforms offer an effective means of improving the sensitivity and performance of label-free biosensors (Gupta, 2011). The dielectric layers are either placed between the prism and metal film or placed between the metal layer and sensing medium depending on the sensor layout.

The insertion of particles changes the effective propagation constant of the SPP, thereby improving momentum matching between incident photons and surface plasmons. The resonance condition is represented by Equation 8, in which the effective permittivity of multilayer structures determines the propagating properties of the plasmonic mode.

k s p = 2 π λ ε e f f ε d ε e f f + ε d (8)

Many dielectric materials have been studied for the enhancement of SPR detection, such as titanium dioxide (TiO₂), zinc oxide (ZnO), silicon nitride (Si₃N₄), and barium titanate (BaTiO₃). These materials have high refractive indices combined with low optical absorption, so they can localize the evanescent electromagnetic field near the sensor surface. This in turn greatly increases the interaction between the optical field and target analytes, resulting in better resonance characteristics and larger sensing sensitivity. Besides enhancing the field localization in the nano-tip region, dielectric coatings lower plasmon damping and narrow down the FWHM of the resonance curve more effectively than gold coatings, which leads to an improvement in the detection capability and FoM. In addition, the dielectric layer thickness serves as an extra design parameter in this sensor, which enables the tailoring of the resonance condition for different sensing scenarios (Dutta et al., 2023; Chang et al., 2021). Due to these benefits, dielectric engineering has been one of the most widely deployed strategies for achieving high-performance SPR biosensors. Figure 2 gives an overview of the main dielectric and 2D materials used in current SPR biosensors and summarizes their individual roles for improving the performances of biosensor.

2.2 Optical properties of two-dimensional materials

One promising strategy for improving SPR biosensor performance is to integrate 2D materials. The inherent atomic thickness, high specific surface area, outstanding optical performance, and tunable electrical properties of 2D materials encourage more effective light-matter interactions with sufficient active sites for immobilizing biorecognition molecules. These specific properties allow for substantial improvements in the sensitivity of sensing, precision in detection, and surface functionalization when using traditional metal-film SPR platforms (Zheng and Wang, 2015; McCann et al., 2025; Ma et al., 2018). Graphene is the most studied out of all 2D materials due to its high carrier mobility, exceptional chemical stability and efficient π–π interactions which assist in biomolecule adsorption. Transition metal dichalcogenides (TMDs) semiconductor materials such as MoS₂ and WS₂ have also attracted significant interest as photodetector platforms due to their large light absorption coefficient and high exciton binding energy, resulting in a favourable dielectric environment for strong confinement of the electromagnetic field near the sensor interface. Moreover, black phosphorus (BP) exhibits notable in-plane carrier mobility and anisotropic optical properties; MXenes have been designed as high electrical conductivity 2D materials possessing abundant surface functional groups to capture biomolecules (Xi et al., 2008; Fang, 2014). In addition to acting solely as plasmonic enhancement layers, 2D materials serve as effective functional interfaces for immobilizing recognition molecules, thereby improving both the sensitivity and selectivity of the sensor. Consequently, hybrid SPR biosensors incorporating 2D materials have demonstrated superior sensing performance compared to conventional metal-based configurations across a wide range of biochemical sensing applications. Detailed comparisons of different 2D material platforms are presented in the following sections.

2.3 Dielectric–2D material interaction for sensitivity improvement

2.3.1 Graphene-based SPR biosensors

Among all 2D materials, graphene is one of the most widely studied SPR biosensing two-dimensional material owing to its excellent electrical conductivity, high carrier mobility, large specific surface area, and good chemical stability. Such characteristics facilitate the adsorption of biomolecules via π–π interactions with resonance energy transfer from nanoparticles to the layering or enhanced interaction of the evanescent electromagnetic field and the sensing medium (Suvarnaphaet and Pechprasarn, 2017; Bai et al., 2020). Consequently, graphene has become a widely used function layer for the enhancement of SPR biosensor sensitivity as well as reliability. Graphene is known to change the local dielectric environment and improve electromagnetic field confinement at a metal–analyte interface when used in multilayer SPR structures. The Fano resonance angle shift caused by tiny changes in the RI is amplified by this better field localization, which improves the sensitivity of sensor. As well as being a biomaterial, graphene offers a biocompatible platform for the immobilization of antibodies, DNA, enzymes or any other recognition element potentially increasing the selectivity and stability of biosensing systems (Szunerits and Boukherroub, 2018; Karki et al., 2022).

The performance improvement owing to graphene is characterized by the number of graphene layers, the plasmonic metal below graphene, and multilayer architecture. Graphene is also known as a tunable absorber, and when multiple layers are dispersed to form the sensing membrane, although it can effectively improve sensor performance, an excessive number of layers will lead to great absorption of light and widening optical resonance caused by a large E-field distribution range, which leads to limited detection accuracy (Verma et al., 2015). Hence, detailed tuning of the graphene layer is needed to find a suitable trade-off between sensitivity and resonance quality. Owing to these benefits, graphene-based SPR biosensors have been explored for the detection of disease biomarkers, proteins, nucleic acids, environmental pollutants, and chemical analytes (Bai et al., 2020). Although graphene has outstanding electrical and surface functionalization properties, further improvements can be realized by building composites of 2D nanomaterials such as dielectric materials with graphene, which will be described in the following sections.

2.3.2 MoS₂-based SPR biosensors

Molybdenum disulfide (MoS₂) has emerged as a promising TMD for SPR biosensing owing to its unique optical and electronic properties. In contrast to graphene, monolayer molybdenum disulfide (MoS₂) has a direct bandgap, leading to stronger light absorption and more significant interaction with the evanescent electromagnetic field. These properties endow MoS₂ not only with efficient electron transport but also ultra-high light-matter interactions, making it an excellent candidate to enhance plasmonic sensing performance (Bahmani et al., 2026). The mechanical effect of MoS₂ in a multilayer SPR structure is to provide more effective confinement of the electromagnetic field at the metal–dielectric interface, resulting in larger resonance angle shifts for small variations in the RI. Moreover, the optical absorption and dielectric properties of the composite increase the coupling between the incident light and surface plasmons, reducing losses due to scattering and improving the sensing sensitivity with effective resonance quality (Dey et al., 2026).

MoS₂ also offers an outstanding active site with a high surface-to-volume ratio for the immobilization of biomolecules (antibodies, proteins, enzymes, and nucleic acids). The addition of these functionalization steps increases the efficiency with which analytes can be bound to the sensor and therefore results in improved sensing stability overall. In addition, the hybridization of MoS₂ with a dielectric layer or other nanomaterials can enhance plasmonic field localization to achieve high-sensitivity and high-efficiency SPR biosensors (Tene et al., 2025a). On the one hand, although these merits make MoS₂ a promising plasmonic material, the sensitivity of MoS₂-based SPR platforms is basically determined by the deposited layer arrays as well as the whole device configuration. If the layer thickness is excessive, it will subsequently lead to increased optical losses and an increase in the broadening of the resonance dip, which reduces the detection accuracy. This underscores the necessity of tuning the MoS₂ thickness to reach a suitable compromise between sensitivity and resonance quality. These favorable properties render MoS₂-based SPR biosensors potential candidates for biomedical diagnostics, environmental monitoring, and chemical sensing (Tene et al., 2025b).

2.3.3 WS₂-based SPR biosensors

Recently, tungsten disulfide (WS₂), a more significant transition metal dichalcogenide (TMD), has captivated considerable research for SPR biosensing owing to its distinctive optical properties and high effective mass, RI, and intense excitonic absorption. Analogous to MoS₂, the metallic properties of WS₂ can also strengthen the hybridization between the optical cavity mode and sensing medium, which further helps in enhancing plasmonic coupling simultaneously supporting high-order resonance features (Wekalao et al., 2025). The addition of WS₂ to multilayer SPR platforms improves the confinement of the electromagnetic field at the sensing interface and extends the effective interaction length between the surface plasmon and target analytes. This improved confinement leads to more pronounced resonating angle shifts, higher sensing sensitivity, and less signal loss. Moreover, the atomically thin feature of WS₂ provides a large surface area for biomolecular immobilization without causing appreciable increases in sensor thickness (Haque et al., 2025).

Compared with the emerging 2D materials, WS₂ also displays better environmental tolerance, which is essential for biosensing applications because of the practical operating conditions. Furthermore, stable energy levels in wide band gap semiconductors allow strong enhancement of light absorption without indirect transitions. However, similar to other TMD-based SPR sensors, the sensing performance is largely determined by the thickness of the WS₂ layer, plasmonic metal, and multilayer structure (Mao et al., 2022). Accordingly, fine optimization of the structure is required to achieve high sensitivity with a narrowly resonating response. Owing to its ideal optical characteristics and high compatibility with multilayer plasmonic devices, WS₂ has emerged as an excellent candidate for future SPR biosensors that are critical in biomedical diagnostics, environmental assessments, and chemical sensing. Moreover, WS₂ is an attractive option as it is usually complemented in combination with dielectric spacer layers and rare metals to advance sensing performance (Xu et al., 2019).

2.3.4 Black phosphorus (BP)-based SPR biosensors

Black phosphorus (BP) has emerged as a promising 2D material for SPR biosensing owing to its unique optical and electronic anisotropic properties. Unlike graphene and most transition metal dichalcogenides, BP displays directional optical responses, which allow for orientation-sensitive interaction of polarized light with the sensing interface (Kumar et al., 2024). Owing to these characteristics, BP is one of the most promising materials for enhancing the sensitivity and accuracy of plasmonic biosensors. BP increases the localization of electromagnetic fields at the metal-analyte interface in multilayer SPR structures, thereby improving the resonance performance and facilitating the attainment of high sensitivity to small RI changes. Furthermore, high surface-to-volume ratio provides abundant active sites for the immobilization of biomolecular recognition elements and thus enables efficient analyte binding and improves biosensing performances (Wu et al., 2017).

While these advantages exist, the environmental stability of BP-based SPR biosensors severely restricts their potential in practice as detection systems. Black phosphorus is an intrinsically unstable material under ambient conditions, and exposure to moisture and oxygen can significantly deteriorate its optical properties and render poor long-term stability as a sensor. To overcome this restriction, some protective encapsulation approaches combined with a dielectric passivation layer have been suggested to maintain material stability while preserving plasmonic enhancement. Significantly, by harnessing the unique optical anisotropy and high sensing precision of these two-dimensional materials, black phosphorus shows great potential for the development of ultra-sensitive SPR biosensors. Subsequent progress in material stabilization and device engineering is expected to extend its applicability to next-generation plasmonic sensing platforms (Srivastava et al., 2020).

2.3.5 MXene-based SPR biosensors

MXenes are an exciting new class of two-dimensional transition metal carbides and nitrides that have gained significant attention in the context of plasmonic biosensing. The aforementioned properties of metallic electrical conductivity, large specific surface area, and surface chemistry make it a suitable candidate for multilayer integration of SPR sensor architectures (Wu et al., 2019). Unlike most conventional 2D materials, MXenes have hydrophilic surface functional groups that help in the adsorption of biological molecules and promote effective surface functionalization. The incorporation of MXenes in sensing platforms can enhance the interaction between the evanescent electromagnetic field and target analytes because they provide an active interface that allows for higher biomolecular adsorption on the surface during detection compared to planar surfaces, including metals (Wu et al., 2019; Kumar et al., 2021). Moreover, their excellent electrical conductivity promotes effective charge transfer at the sensing interface, resulting in an increase of plasmonic coupling and resonance characteristics. Therefore, MXene-assisted SPR biosensors are promising candidates for achieving high sensitivity and better detection performance in various biological assays.

In addition to their good optical and electrical properties, the surface chemistry of MXenes facilitates simple functionalization with antibodies, enzymes, nucleic acids, and other recognition molecules. This varied functionality broadens the potential application of MXenes in biosensing toward different biomedical, environmental, and chemical analytes (Srivastava et al., 2020). However, structural optimization remains the key factor in sensing performance, which can be affected by multilayer design, material compositions, and fabrication conditions. With these advantages, MXenes are increasingly being developed as substitutes for traditional plasmonic enhancement materials and have shown potential in next-generation SPR biosensors. This combination with dielectric spacer layers and noble metal films is predicted to further enhance sensing functionality and extend the areas of practical biosensing applications (Srivastava et al., 2020).

2.4 Dielectric–2dD material hybrid structures for SPR enhancement

Hybrid SPR architectures integrating dielectric layers with 2D materials have emerged as one of the most effective approaches for overcoming the limitations of conventional metal-based plasmonic sensors. In these multilayer configurations, dielectric materials primarily enhance electromagnetic field confinement and reduce plasmon damping, whereas 2D materials improve biomolecular adsorption, surface functionalization, and charge transfer at the sensing interface (Kravets et al., 2022). The synergistic interaction between these material classes results in substantially improved sensing performance compared with individual dielectric- or 2D-material-assisted SPR platforms. The improved performance of hybrid structures derives from the synergistic effects of constituent layers. Dielectric spacers with a high RI redistribute the evanescent electromagnetic field to a region close to the sensing interface, enhancing the interaction between surface plasmons and target analytes (Rahman et al., 2020). At the same time, atomically thin 2D materials lead to plenty of active sites for immobilization of biomolecules and rapid carrier interactions at the metal surface. Collectively, these effects enable significantly wider resonance angle shifts, sharper resonance linewidths, and better figures of merit than conventional SPR sensors (Rahman et al., 2021).

Several hybrid material combinations have been investigated, including dielectric–graphene, dielectric-MoS₂, dielectric–WS₂, and dielectric–BP platforms. The selection of a particular material combination depends on the sensing application and the desired balance between sensitivity, detection accuracy, and long-term stability. Rather than relying on a single enhancement mechanism, these hybrid platforms simultaneously optimize optical confinement, plasmonic coupling, and biomolecular recognition, making them particularly attractive for ultrasensitive RI sensing and label-free biosensing applications. Recent studies have demonstrated that appropriately designed dielectric–2D material hybrid structures can achieve substantially higher sensing performance than conventional metal-film SPR sensors (Rahman et al., 2021). However, the overall performance remains strongly dependent on factors such as layer thickness, material sequence, and fabrication quality. Consequently, systematic optimization of multilayer architectures is essential for achieving high sensitivity, narrow resonance curves, and reliable sensing performance (Sriram et al., 2020). For the convenience of a systematic comparison between the materials used in SPR biosensors, Table 1 summarizes primary properties of the most studied dielectric and 2D materials. Different materials play a unique role in influencing the confinement of electromagnetic field, plasmonic coupling, biomolecule immobilization, chemical stability and sensing efficiency. Therefore, choosing the right material should consider all factors of the sensing application implemented in terms of sensitivity, fabrication complexity, detection accuracy stability. Table 1 demonstrates that, across a single sensing application, there does not exist one material that achieves the optimum optical, electrical, chemical and mechanical properties for all of the desired sensitivities. High-RI dielectric materials mainly increase the electromagnetic field localisation and decrease plasmon damping, whereas 2D materials can improve the binding of biomolecules and surface functionalization. Consequently, recent studies have been centred on hybrid multilayer architectures that leverage the complementary characteristics of both material families in order to deliver superior sensing capabilities. This design approach has evolved into an important guideline for SPR biosensors, including biomedical, environmental, and chemical sensors for future applications.

TABLE 1. Comparison of dielectric and 2D materials employed in SPR biosensors.

3 SPR sensor configurations and multilayer design strategies

3.1 Prism-based SPR excitation techniques

However, prism-based excitation remains the most popular method for producing SPRin label-free optical biosensors because of its simple optical configuration, high coupling efficiency, and compatibility with the multilayer plasmonic structures. Here, the prism allows for additional momentum to fulfill the phase-matching condition between the incident photons and surface plasmon polaritons (SPPs), facilitating the efficient excitation of surface plasmons at the metal–dielectric interface. Prism-coupled SPR platforms have advanced over the last 10 years from traditional single-metal structures to complex multilayer configurations that include dielectric coatings and further layers of two-dimensional (2D) materials to improve sensing performance. Different prism coupling configurations have been introduced to enhance plasmon excitation in distinct sensing situations. The Kretschmann configuration is the most widely used because of its simple fabrication, high stability, and compatibility with liquid-phase biosensing applications. In addition, unconventional methods, such as the Otto configuration, long-range SPR(LRSPR), and phase-sensitive SPR techniques, have been explored to obtain better resonant properties, increased accuracy in detection, and lower limits of detection. The choice of excitation geometry depends on the application, ease of fabrication, and compromise between sensitivity and practical or implementation concerns. The following subsections summarize the operating principles, advantages, limitations, and common applications of the core prism-based SPR excitation methods widely published in the literature. Prism coupling is the most commonly used SPR biosensor excitation mechanism due to its high sensitivity, simple optical alignment and multilayer compatibility. To date, a myriad of optical excitation configurations using prisms have been explored to enhance plasmon coupling and localized electromagnetic field confinement, therefore improving sensor performance. As illustrated in Figure 3, Among these major configurations, Kretschmann configuration, Otto configuration, LRSPR, and hybrid dielectric-multilayer architectures have individual advantages for different types of biosensing applications.

3.1.1 Kretschmann configuration

The Kretschmann configuration is still the most popular excitation geometry where efficient coupling of incident light to surface plasmons can be achieved. Owing to its simple structure, ease of fabrication, and excellent sensor performance compared with more complex structures, the Kretschmann configuration is the most common excitation geometry for prism-based SPR biosensors (Shukla et al., 2022). Here, a thin noble metal film is deposited directly at the end of a high-RI prism. Coupling through the prism at an angle greater than the critical angle of incidence produces an evanescent electromagnetic field around the metal-dielectric interface under p-polarized light irradiation. The resonance between the incident light in the plane momentum and the surface plasmon polaritons resulted in a minimum reflected light intensity. The resonance condition can be expressed as:Where, k s p = surface plasmon wave vector, k 0 = free-space wave vector, n p = RI of prism, θ r e s = resonance angle. The resonance condition is altered by changes in the RI of the sensing medium, generating shifts in the measurable angle or wavelength (Zanchetta et al., 2017). These changes are correlated with the molecular interactions at the sensing surface; therefore, this configuration supports high sensitivity and good reproducibility in real-time and label-free detection. These properties make it the preferred platform for several chemical and biological sensing applications. The performance of the Kretschmann configuration has recently been improved by adding high-RI dielectric spacer layers and two-dimensional materials (Pandey et al., 2021). In conclusion, dielectric coatings enable better prompt dispersion confinement and reduced plasmon damping, whereas atomic-layer-thin materials, such as graphene and several covalent compounds (transition metal dichalcogenides), improve biomolecule adsorption and plasmons. Thus, modern Kretschmann-based multilayer configurations perform significantly better in terms of sensitivity, narrower resonance curves, and surface-enhanced figures of merit than the original single-metal configuration. Owing to its suitability for multilayer structures, ease of optical alignment, and reliable sensing performance, the Kretschmann configuration has served as the basis for most experimentally proven SPR biosensors and remains the excitation scheme of choice for most biomedical, environmental, and chemical sensing applications (Gwon and Lee, 2010; Heckmann et al., 2018).

k s p = k 0 n p ⁡ sin ⁡ θ r e s

3.1.2 Otto configuration

Otto configuration, one of the earliest prism-coupled SPR excitation geometries used solely to consider theoretical statistical mechanics studies and highly specialized types of sensors. Unlike the Kretschmann configuration, where a very thin metal film is placed directly on top of the prism surface. Instead, it maintains a thin dielectric gap between the prism and metal layer (Akowuah et al., 2009). The p-polarized light propagating through the prism undergoes total internal reflection as it is incident on the metal–dielectric interface, and near this interface, an evanescent field is generated, which tunnels through the gap, offering a virtual coupling of surface plasmon polaritons into the system. The resonance condition in the Otto configuration is highly sensitive to the dielectric gap thickness. This gap must be appropriately controlled to allow for effective plasmon excitation and to minimize optical losses from any direct prism–metal contact. As a result, Otto-based SPR systems exhibit higher resonance curves and better detection sensitivity in carefully controlled experiments. These features make the configuration ideally suited for both studies of basic plasmonic physics and applications in which spectral resolution needs to be improved (Lee et al., 2021).

However, despite these advantages, the practical utility of Otto configuration is limited as maintaining a uniform nanometre-scale air gap is technically challenging in liquid-phase biosensing environments. Subtle changes in gap thickness can lead to dramatically reduced coupling efficiency and measurement stability, leading to the limited applicability of coupling-based methods on routine sensing platforms (Fontana et al., 2015). This makes Otto-based SPR systems much less popular than Kretschmann-based configurations for biosensors in experimental use. Although the practical application is limited, the Otto configuration remains a theoretical point of reference taking part in providing insight into surface plasmon excitation mechanisms well applicable to advanced SPR sensing platforms (Li et al., 2017). Although the Otto configuration offers significant theoretical advantages, practical limitations have prompted researchers to develop alternative excitation schemes to attain sharper resonance control: Long-Range SPR(LRSPR) enables a broader range of simpler pumping methods, yet still without necessitating an accurate air gap (Maegawa et al., 2016).

3.1.3 Long-range SPR (LRSPR) coupling

Long-range SPR(LRSPR) is an advanced SPR excitation technique that was developed to eliminate the problems associated with the propagation losses of standard surface plasmons. Ultra-thin metal films are embedded between two dielectric media with refractive indices that are almost identical and of low magnitude, as in the LRSPR configuration (Yang et al., 2025). The symmetric dielectric environment diminishes energy losses in the metal layer and makes it possible to excite long-range surface plasmon modes with dramatically longer propagation lengths. The low propagation loss in LRSPR results in a much narrower resonance curve and higher sensitivity compared to traditional prism-coupled SPR systems. Because the resonance dip is sharper, even very small values of RI can be detected with higher precision. This property makes LRSPR an excellent candidate for trace biomolecule detection because the changes in optical properties are at very minimal levels (Bakar et al., 2025; Thi Tran, 2021).

More recently, dielectric spacer layers and two-dimensional materials (e.g., MoS₂ and WS₂) have further improved the performance of LRSPR. These unique hybrid structures allow confinement of electromagnetic fields and increase light–matter interactions, which overall lead to increased sensitivity, reduced detection limits, and enhanced figures of merit. These multilayer LRSPR platforms have gained significant interest in biomedical diagnostics, environmental monitoring, and other ultrasensitive label-free sensing applications (Uniyal et al., 2023; Cheng et al., 2021). Although LRSPR also shows a very good sensing performance, it requires strict control of the metal film thickness and dielectric symmetry during preparation. Even small discrepancies in structural parameters can significantly affect the resonance characteristics, leading to an undesirable drop in sensing performance. Thus, although LRSPR provides more advanced analytical capabilities than traditional Kretschmann-based SPR sensors, its fabrication is usually more complicated. In summary, LRSPR is a significant breakthrough for prism-based plasmonic sensing, with improved detection sensitivity and lower initiation limits, as an attractive platform to develop next-generation biosensors for detecting low-concentration target analytes (Pal and Maurya, 2023).

3.1.4 Phase-sensitive and imaging SPR

Phase-sensitive and imaging SPR strategies are advanced interrogation methods devised to enhance the sensitivity, resolution, and multiplexing capacity of prism-based SPR biosensors. In contrast to angular or wavelength interrogation techniques, which rely on shifts of a resonance position, these methods leverage additional optical information beyond defining a certain degree of sensitivity to detect minute RI variations with higher accuracy (Su et al., 2026). They have received great interest in the applications toward ultrasensitive and real-time detection of biomolecules. Phase-sensitive SPR detects the ultra-fast phase variation of the reflected light close to resonance. This technique offers much better detection resolution because the phase changes more steeply than the reflected intensity around resonance, and it also permits the measurement of very small RI changes. The sensitivity of phase-sensitive SPR is highly advantageous for applications requiring high precision detection of biomolecules, early disease diagnosis, and other use cases where such minute optical perturbations need to be resolved (Tayef et al., 2025).

Imaging SPR(iSPR): iSPR is an extension of conventional SPR that enables the simultaneous observation of several sensing regions using imaging detectors such as CCDs or CMOS camera. Imaging SPR thus overcomes the limitation of measuring only one sensing spot by generating spatially resolved reflectivity maps and providing a robust means for parallel analysis of multiple analytes on a single sensor platform (Sterzik et al., 2025). Such multiplexing capacity results in a tremendous increase of analytical throughput, which is especially important for clinical diagnostics, biomarker screening, drug discovery and high-throughput biosensing applications. The relative complexity of phase-sensitive and imaging SPR systems is mainly due to the inherent requirements of more elaborate optical instrumentation, rigor in alignment owing to its sensitivity, and signal processing over traditional angular interrogation systems (Koutník et al., 2025; Zhang et al., 2025). These requirements increase both the system complexity and overall cost, currently limiting their deployment on a wide scale using portable sensing platforms. However, ongoing innovations in optical elements, imaging methods, and computational data processing should enhance their feasibility and hasten their incorporation into emerging SPR sensor platforms. For clarity, Table 2 summarizes the key characteristics of the major prism-based SPR excitation techniques discussed in this section. Each configuration offers distinct advantages and limitations, and the appropriate choice depends on the required sensing performance, fabrication complexity, and intended application.

TABLE 2. Comparison of prism-based SPR excitation techniques.

3.2 Multilayer SPR sensor architectures

High-performance RI sensing has been demonstrated using conventional SPR biosensors, where the metal layer places between the substrate/prism and dielectric layer. However, their performance is associated with the limited confinement of electromagnetic field lines, wide resonance curves, and moderate sensitivity, particularly considering that some analytes can lead to very weak RI changes. In light of these limitations, multilayer SPR architectures have recently provided an important approach for controlling plasmonic behavior and enhancing overall performance in sensing (Tao et al., 2020; Jha and Sharma, 2009). This approach integrates plasmonic metals with dielectric materials or 2D nanomaterials, or both, to maximize light–matter interactions at the sensing interface using multilayer SPR architectures. Each functional layer is responsible for a particular task in the sensor architecture. Metallic films enable the excitation of surface plasmons, whereas dielectric spacer layers promote better electromagnetic field confinement and reduce optical losses; 2D materials offer high surface areas owing to their unique structures and abundant active sites for biomolecule immobilization (Chew et al., 2017; Qiulian et al., 2023). The interaction between these layers synergistically leads to sharper resonance characteristics, improved sensitivity, lower detection limits, and overall better figures of merit than single-layer SPR platforms.

Multilayer SPR sensors are important because the performance of multilayers can be affected by the selection, arrangement, thickness, and optical properties of each component. Consequently, while high plasmon coupling efficiency, low level of damping losses, and measurement stability are all prerequisites for SPELT devices, careful design of their structure is required. Consequently, different multilayer configurations, such as metal-dielectric bilayers, metal-2D material hybrids, dielectric-metal-2D heterostructures, and nanostructured plasmonic surfaces, have been suggested with certain advantages for specific applications. In the following subsections, we examine the major multilayer SPR architectures in the literature in terms of their working principle, structural merits, and role in sensitivity enhancement, detection accuracy, and overall biosensor performance improvement (Gholipour et al., 2023).

3.2.1 Metal-dielectric bilayer structures

One of the earliest and most widely applied strategies to improve the performance of mainstream SPR biosensors involves metal–dielectric bilayer structures. In such architectures, a thin dielectric layer is added between the plasmonic metal film and either the sensing medium or prism (based on sensor design). A dielectric layer alters the dispersion of the evanescent electromagnetic field, thereby providing better plasmon confinement at lower optical damping losses (Ivanchenko and Jing, 2023; Gholipour et al., 2018). Consequently, bilayer structures generally exhibit better resonance properties than single-metal SPR sensors. Dielectric materials are an important consideration when designing optimal sensor performance. Commonly used high-RI materials, such as titanium dioxide (TiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), and silicon (Si), have excellent optical transparency, low absorption loss, and provide electromagnetic field localization near the sensor interface (Priyadarshini et al., 2022). These dielectric layers help detect larger resonance shifts and achieve better RI sensitivity by increasing the interaction between surface plasmons and target analytes.

In addition to sensitivity enhancements, dielectric spacer layers establish the structural stability of multilayer SPR platforms. They lead to less direct exposure of the metal surface to the local environment, thereby reducing oxidation, mitigating plasmon damping, and potentially enhancing operational stability over long timescales. Moreover, dielectric layers also serve as a suitable platform to accommodate functional nanomaterials and biomolecular recognition elements (REs), making them indispensable contributors to modern hybrid SPR biosensors (Chung et al., 2022). Adding a dielectric layer to metal surfaces can significantly enhance sensing performance; however, the proper thickness of the dielectric layer must be optimized. Too-thin layers only minimally enhance above the theoretical limit, whereas too-thick dielectric coats decrease the evanescent field penetration depth, reduce plasmon coupling, widen the resonance curves, and reduce diagnostic capacity. Hence, careful optimization of the dielectric material and thickness is crucial to achieve high-performance SPR sensors (Hancock, 2021; Naeem et al., 2021).

3.2.2 Metal-2d material hybrid structures

Metal-2D material hybrid structures are considered to be one of the most significant strategies for improving the performance of SPR biosensors. These designs make use of atomically thin two-dimensional materials incorporated with conventional plasmonic metals, allowing for simultaneous enhancement of optical confinement with surface biochemical interactions. Hybrid structures take advantage of the synergistic aspects of both components (in contrast to what happens in bare metal films, which only support plasmon excitation) and provide higher sensing performance. The integration of 2D materials alters the electromagnetic field distribution at the metal–dielectric interface and enhances the interactions between the evanescent field and target analytes. At the same time, the large specific surface area and rich active binding sites of these materials promote effective immobilization of biomolecular recognition elements, which can enhance analyte adsorption and sensor response. This results in wider resonance shifts, narrower resonance curves, and higher figures of merit of the general metal–2D material hybrid localized surface plasmon resonances compared to conventional metal-film SPR sensors.

To date, various 2D materials have been integrated into hybrid SPR platforms, including graphene, transition metal dichalcogenides (TMDCs) such as MoS₂ and WS₂, black phosphorus, and MXenes. Although each material has its specific optical and physicochemical properties, when applied within hybrid architectures, they mostly serve as interfaces that provide strong plasmon–analyte interactions. Thus, the choice of suitable 2D materials depends on the sensing application, operating environment, and trade-off between sensitivity, selectivity, and stability. Recent reports on the integration of plasmonic metals with 2D materials have demonstrated enhanced analytical performances for low-concentration biomarkers and environmental contaminants, further demonstrating their potential applications in SPR biosensors (Prabakaran et al., 2022). However, this improvement of sensing relies heavily on the number of 2D layers, optical absorption, interfacial coupling, and fabrication quality. However, a high substrate thickness will induce some optical losses and widen the resonance curve, thus increasing the requirements for structural optimization in sensor design. In summary, metal–2D material hybrid systems represent an effective means to improve conventional SPR sensors by combining high optical activity with more efficient biomolecular interactions. Their remarkable optical characteristics, along with tunable structures, have made them an attractive candidate for next-generation label-free biosensors for biomedical detection, environmental monitoring, and chemical sensing (Wang et al., 2021).

3.2.3 Dielectric-metal-2D multilayer heterostructures

Dielectric-metal-2D multilayer heterostructures are the most advanced configuration of prism-based SPR sensors that incorporate dielectric spacer layers, plasmonic metals, and atomically thin two-dimensional materials into a single sensing platform. These heterostructures overcome the limitations of traditional single-metal SPR sensors by combining materials with complementary functionalities, providing major improvements in sensitivity, detection accuracy, and sensing performance (Priyadarshini et al., 2022). A standard multilayer heterostructure includes a fugacious high-RI prism, ultra-thin plasmonic metal film, one and/or numerous dielectric spacer layers, ultrathin two-dimensional (2D) material coating, and analyte media. In this architecture, the metal layer facilitates surface plasmon excitation; the dielectric layer contributes to greater field confinement with lower damping losses; and the 2D material helps with efficient biomolecular adsorption and enhances light–matter interactions (Lei and Guo, 2022). The combined effect of these functional layers gives rise to enhanced evanescent fields and larger resonance shifts than typical bilayer structures. The performance improvement of multilayer heterostructures is very sensitive to the structural design of the sensor. The resonance characteristics are dictated by parameters such as the thickness of the metal and dielectric layers, number of 2D material layers, and optical properties of each constituent material. The optimization of these parameters leads to correspondingly sharper resonance dips, higher quality factor (QF), lower detection limits, and better figures of merit, making these architectures very attractive for detecting extremely low RI changes (Li et al., 2023).

Recently, multilayer heterostructures, such as those made of graphene or carbon nanotubes (CNT), have been shown to be highly suitable for the label-free detection of ultra-sensitive low-concentration biomarkers and environmental pollutants or other trace analytes. Moreover, 2D NTs provide considerable freedom in adjusting the composition of sensors to achieve different functionalities by combining various dielectric materials and/or types of 2D nanomaterials under a modular design according to specific applications (Zeng et al., 2021; Chu et al., 2019). These results inspired us to shift from plasmonic materials with high liability towards dielectric–metal–2D heterostructures, which have emerged as a favorable platform for next-generation SPR biosensing design that requires both enhanced analytical sensitivity and operational stability. However, although these benefits are tremendous, multilayer heterostructure processing is presently very challenging and demands nanometer-scale layer thickness uniformity of interface quality to ensure reproducible sensor performance (Tame et al., 2013). Advances in thin-film deposition techniques and nanofabrication technologies are expected to improve manufacturing precision and facilitate the large-scale implementation of these high-performance SPR platforms.

3.2.4 Nanostructured plasmonic surfaces

A novel approach to enhance the performance of conventional prism-based SPR biosensors is to use nanostructured plasmonic surfaces. In contrast, polymer-encapsulated nanoparticle-based sensors incorporate nanoscale features that modify the localized electromagnetic field at the sensing interface] than planar metal films when placed under a bimetallic (Choi and Kim, 2018). Such a result leads to an enhanced plasmonic interaction, which thus ensures more confinement of light and higher sensitivity to small RI changes. Nanostructures are introduced in a way that support localized SPR(LSPR), where collective oscillation of electrons is committed to nanoscale metallic features. These localized plasmon modes couple with surface plasmon polaritons in conventional propagating SPR platforms, leading to enhanced electromagnetic field localization close to the sensing surface. Moreover, this plasmonic hybrid behavior enhances the interaction of the evanescent field with target analytes, resulting in larger resonance responses and improved sensing performance (Tkach et al., 2011).

Nanostructured plasmonic surfaces are particularly suitable for the detection of low-molecular-weight analytes, for which RI variations upon molecular binding are tiny. The amplification of the local electromagnetic field produced around nanostructures also increases the likelihood of finding these weak interactions, thereby boosting analytical sensitivity and lowering the practical detection limits. Consequently, hybrid SPR–LSPR platforms have been rapidly developed for applications in biomedicine, environmental monitoring, and chemical sensing, which require highly sensitive label-free detection. Although promising for sensing, the fabrication of nanostructured plasmonic surfaces requires strict control over feature size, geometry, and spatial arrangement because these physical parameters can significantly modify the resonance behavior and sensor reproducibility (Wood et al., 2014). Recently, improvements in nanofabrication methods have allowed for the fabrication of more complicated plasmonic structures, and thus for the integration of enhanced nanostructured surfaces with multilayer dielectric–metal–2D architectures that can yield much higher-performing sensors. This section describes how the development from traditional metal films through multilayer heterostructures to nanostructured plasmonic surfaces has positively influenced the analytical performance of SPR biosensors (Tanaka et al., 2013; Ahn et al., 2018). While sophisticated designs achieve better electromagnetic confinement and sensing performance, the suitability of a particular structure still relies heavily on revealing the optimal structural parameter choices, including material selection, layer thickness, and nanostructure geometry. To this end, systematic structural optimization has become a critical step in the design of high-performance SPR biosensors.

3.3 Structural optimization for improved sensing performance

The performance of SPR biosensors depends not only on the nature of the sensing materials but also on different structural parameters. Even if the materials are the same, different layer thicknesses, material order, operating wavelengths, and sensor geometries can all dramatically affect plasmon excitation, resonance properties and sensing performance. This has led to structural optimization being an essential component of SPR sensor designs, ensuring that researchers are able to maximize the sensitivity while maintaining detection accuracy, narrow resonance linewidths and excellent operational stability (Lahav et al., 2009). The optimization process is performed to maximize the coupling of the incident optical wave and surface plasmons, simultaneously reducing optical losses in the multilayer structure. Optimization improves electromagnetic field confinement at the sensing interface, produces a stronger interaction between the evanescent field and target analytes, with sharper resonance dips that allow precise RI shift measurements. Owing to the strong inter-dependence of structural parameters, one characteristic improvement commonly affects multiple other sensing metrics simultaneously. Hence, sensor optimization is a multi-objective operation that must accommodate different performance metrics rather than only maximizing a single parameter (Park et al., 2010; Hossain and Talukder, 2021).

Although multilayer SPR biosensors are mature sensor types, recent advances have shown that optimizing multiple parameters, such as the spacer layer thickness, metal film thickness, and number of 2D material layers, overall structure configuration, greatly improves the analytical performance. Beyond using conventional parameter optimization methods, computational algorithms and artificial intelligence techniques are coupled with these settings to attain an optimal multilayer configuration that reduces the cost of design. These advances have driven the development of high-performance SPR-based biosensors for biomedical diagnostics, environmental monitoring, and chemical sensing. The main structural parameters that affect sensor performance are metal thickness, dielectric layer thickness, number of 2D material layers, and optimization algorithms, which are covered in the next subsections along with the sensitivity–stability trade-off and optimization strategy for biological samples. These factors lay the framework for robust and ultra-sensitive multilayer SPR biosensor design. The sensor performance of SPR biosensors relies not only on material choice, but also requires the careful optimization of various structural, optical and sample-related parameters. The characteristics that can coalesce to influence plasmon excitation, electromagnetic field corporeality and resonance behaviour include metal film thickness, dielectric layer thickness, 2d material layers number, operating wavelength/prism type and analyte RI (Tene et al., 2025a). As summarized in Figure 4, and the systematic tuning of these parameters represents an important route to achieving high sensitivity and reproducible operation of biosensing devices.

3.3.1 Metal thickness optimization

The performance of prism-based SPR biosensors is largely governed by the thickness of the plasmonic metal film. Because surface plasmons require excitation at the metal-dielectric interface, the metal layer must be appropriately thick for effective coupling between the incident optical wave and surface plasmon polaritons (Fontana, 2006). Nanometre-scale variations in the metal thickness change the resonance angle, resonance linewidth, and electromagnetic field distribution, which fundamentally alter the overall sensing performance. Maximum plasmon excitation efficiency is obtained for an appropriate thickness; thus, a thin film of plasmonic metal, such as gold and silver, is required. At very thin thicknesses, incomplete formation of the plasmon and higher optical transmittance produce a weakening effect on this type of resonance signal. In contrast, very thick metal films extract a higher proportion of the incoming optical energy, which decreases the distance that an evanescent field extends into the sensing medium, thus reducing plasmon coupling between adjacent metal structures. Both lead to wider resonance lines, reduced sensitivity, and lower detection precision (El Biyari, 2026).

The optimum metal thickness depends on the plasmonic material, operating wavelength, prism geometry, and multilayer structure. Gold films normally need to be a little thicker owing to their higher chemical stability, whereas silver films often display stronger plasmon resonance at a relatively lower thickness because of their lower optical losses. Thus, selecting the correct metal thickness is a trade-off between sensitivity, resonance Q-factor, and long-term performance stability. Optimally designed metal films have been previously shown to produce narrower FWHM, higher Q-factors, and FoM, and stronger RI sensitivity. Accordingly, prior to optimizing any dielectric and two-dimensional material layers in multi-layered SPR sensors, metal thickness optimizations are typically performed at the initial stage of sensor design. This sequential optimization approach guarantees effective plasmon excitation and establishes a solid basis for additional improvement through sophisticated multilayer architectures.

The ranges of optimal thickness presented in Table 3 are typical findings for classical prism-based SPR biosensors. This optimal value is highly dependent on the field of application, which operates in a wide spectral range and must be calculated numerically for any specific combination of wavelength, prism material, multilayer configuration and sensing application. Moreover, Figure 5 shows workflow of a hybrid dielectric–2D material-assisted SPR biosensor for urinary biomarker detection and clinical diagnosis. In addition to it, Table 4 reports the influence of structural optimization on sensitivity and stability in SPR biosensors.

TABLE 3. Typical optimized thickness ranges of commonly used plasmonic metals in prism-based SPR biosensors.

TABLE 4. Influence of structural optimization on sensitivity and stability in SPR biosensors.

3.3.2 Dielectric layer thickness

The dielectric spacer layer thickness is another vital parameter that affects the behavior of multilayer SPR biosensors. The introduction of a dielectric layer generally aims to restrict the electromagnetic field lines and minimize plasmon damping, as well as enhance the coupling between the evanescent field and analyte. However, these benefits can only be fully exploited when coupled with the correct optimization of the dielectric layer. The dielectric spacer changes the propagation characteristics of the surface plasmon waves by adjusting the depth at which the evanescent field penetrates the sensing medium. The role of the thin dielectric layer in field confinement is limited because the thickness is less than a few hundred nanometers, and it only leads to some improvement compared to conventional metal-film SPR sensors. Conversely, very thick dielectric layers remove the analyte from the plasmonic metal surface, leading to a weaker coupling of plasmons that broadens the resonance curve (Hatamie et al., 2020). Therefore, dielectric thickness that is insufficient or excessive has a negative effect on sensor sensitivity and detection accuracy.

In the case of majority prism-based multilayer SPR biosensors, nanometer-scale dielectric layers yield an optimal compromise between field enhancement and optical coupling efficiency. The optimal thickness depends on the RI of the dielectric, applied wavelength, layer film thickness, or multilayer design. Thus, the optimization of the dielectric layer is generally carried out with a metal layer to ensure the highest performance. Recent multilayer SPR designs using high-RI dielectric materials have shown that a properly optimized spacer layer leads to sharper resonance dips, an increased QF, an improved FOM, and enhanced RI sensitivity. Therefore, optimizing the thickness of the dielectric material is an essential step in the design of high-performance SPR biosensors for biomedical diagnostics, environmental monitoring, and other relevant applications of label-free sensing (He et al., 2024).

3.3.3 Number of 2D layers

Schematic of a multilayer SPR biosensor imaged by visible light to construct a multilayer SPR sensor based on 2D materials, various layers of them are transferred onto the plasmonic surface. If the atomically thin materials directly interact with the evanescent electromagnetic field and target analytes, tuning its number of layers becomes a widely used strategy to maximize sensing performance (Manoharan et al., 2024). The improvements derived from 2D materials result from both their capability to reinforce the coupling between plasmon and analyte, as well as their large available satisfactory sites for biomolecule adhesion. One small caution, though this improvement does not keep increasing as you add more layers. For cases with a single-layer material, sensitivity enhancement is limited because the electromagnetic field interacts only weakly with the functional materials. Generally, increasing the number of layers enables more biomolecules to be adsorbed on this highly confined electromagnetic field, giving larger resonance shifts and potentially higher sensitivity. However, dense stacking of 2D materials also adds more optical absorption and scattering losses, which can increase the width of the resonance dip to mitigate plasmon coupling efficiencies. Hence, after a certain number of layers, the improvement of the sensor functions will stop by adding more material and in some cases lead to degradation (Raghunathan et al., 2024).

The ideal number of layers of the 2D material will be determined by three factors: the optical properties of the chosen thin layer, the operational wavelength and configuration (i.e., the entire multilayer). A few recent studies have shown that a low number of well-controlled layers usually balances improved analyte adsorption with optimal optical losses. Hence, it is common to perform 2D material thickness optimization alon g with the metal and dielectric layer optimizations when designing advanced multilayer SPR biosensors. Multilayer SPR biosensors commonly achieve optimal performance with approximately three to five 2D material layers, providing enhanced analyte adsorption while avoiding excessive optical absorption losses. The exact optimum value depends on the specific 2D material and sensor configuration (Mirzaei et al., 2023).

3.3.4 Optimization algorithms

Computational optimization is an integral part of the design of sensors because of the rising complexity of multilayer SPR biosensor architectures. In modern SPR structures, there exist multiple design variables, such as the thickness of the metal layer and dielectric spacers, the number of 2D material layers and their multilayer sequence, and optical design parameters, such as the operating wavelength [633 nm]. Owing to their strong interdependencies, conventional trial-and-error simulations are computationally expensive and time-consuming. As a result, the use of optimization algorithms has increased to facilitate sensor design by optimizing the overall sensing performance. Specifically, particle swarm optimization (PSO) and genetic algorithms (GA), two of the most widely used computational approaches reported in the literature for optimal structural parameter localization, allow for the efficient exploration of a relatively large design space (Khodabakhshian et al., 2026; Chen et al., 2017). These algorithms assess numerous candidate configurations over many iterations and progress towards provisions that optimize desirable performance indicators (such as sensitivity, QF and FOM). Because multilayer SPR biosensors comprise many tuning parameters, global optimization methods are especially proficient for this purpose (El Biyari, 2026).

AI- and neural network-based techniques for the optimization of SPR sensors have also been recently reported. These models aid in the rapid prediction of favorable sensor layouts while minimizing computational resources during the design process by learning the relationship between the structural parameters and sensor performance as a function of the simulation data (Singh et al., 2026). Feedforward neural networks for direct volume optimization of modulus-shaped components Optimization modelling. Based on the evidence of traditional numerical optimization techniques, such data-driven approaches are increasingly being investigated as a supplementary approach, ultimately used together with typical numerical strategies. In conclusion, optimization algorithms can greatly facilitate the efficient development of SPR biosensors by reducing the computational cost and design time and allowing for the simultaneous optimization of multiple structural parameters (Chang et al., 2025). With the development of multilayer sensor architectures for higher optimization sophistication, computational optimization will be increasingly important for the design of next-generation high-performance SPR biosensors.

3.3.5 Sensitivity-stability trade-off

In developing high-performance SPR biosensors, this trade-off should be considered very carefully between sensing sensitivity and operational stability. While achieving the highest possible sensitivity typically serves as a design goal for sensors, extreme sensitivity does not always yield properly functioning systems with better overall performance. In addition, real-time biosensors should also be characterized with stable resonance properties, reproducible measurements, mechanical robustness, and long-term reliability under various operating conditions (Choi and Kim, 2018; Anasori et al., 2023). Thus, finding a good trade-off between sensitivity at the detection limit and stability is critical for the implementation of real-world sensors. Some structural adjustments that have been made to improve sensitivity, such as thinning the metal thickness, adding more dielectric layers, or increasing the amount of 2D material, might also exacerbate a sensor’s response to fabrication deviations or environmental variations or optical loss. These alterations can enhance the coupling of light energy into the evanescent field and subsequently improve the interaction with target analytes, but they can also lead to an increasing resonance dip width, decreasing repeatability in measurement, or reduced long-term stability of the sensing structure. Overall, this can lead to excessive optimization towards sensitivity alone, which can be detrimental to the performance and reliability of the biosensor (Srivastava et al., 2020).

Therefore, for practical sensor development, optimization should consider multiple performance metrics, such as sensitivity, FWHM, QF, FoM, detection accuracy, and operational stability. A holistic optimization approach guarantees that gains in one performance metric are not at the cost of other important factors. This type of multi-objective optimization is especially crucial for biosensors applied to clinical diagnostics, environmental monitoring, and continuous sensing applications, where retaining measurement consistency (accuracy) and long-term performance is as critical as achieving high sensitivity. Massively multiplexed SPR biosensors are crucial tools for applications in disease detection, biomarker discovery, and drug screening (Singh et al., 2020). Therefore, next multilayers sensor designs must more often collapse integrated optimized strategies introducing sensitivity via robustness, fabrication possibilities, and stability during device operation. Table 5 summarizes the impact of various structural optimization strategies on both the sensing sensitivity and operational stability. Each optimization approach provides distinct benefits while imposing respective limitations, suggesting a balanced design strategy rather than the maximization of a single performance parameter.

TABLE 5. A Comparison of Representative SPR Biosensors with existing works for Urea Detection.

3.3.6 Optimization for biological fluids

The design of SPR biosensors for biological fluids is more challenging than that of conventional RI sensing. Such body fluids as blood, serum, plasma, saliva, urine, and other physiological fluids consist of complicated mixtures of proteins (from albumin to antibodies), lipids, salts, and various biomolecules that can affect plasmon resonance and interfere with the detection of target analytes (Balouch et al., 2025). Therefore, sensor structures engineered in the lab may need to be optimized for effective performance under real biological conditions. The RI and composition of biological fluids are sample type, analyte concentration, temperature, and physiological condition-dependent characteristics. These changes might generate broader resonance shifts that would be non-specific or increase the variability in measurements of the same sample if not paired with optimization processes. Thus, multilayer SPR biosensors must be developed and designed as per their high sensitivity responses against background constituents as well as surrounding environmental changes. In biological sensing, this may require an appropriate selection of the metal and dielectric layer thicknesses, optimization of the number of functional nanomaterial layers, and suitable surface functionalization to enhance analyte selectivity. Moreover, the narrow FWHM, QF, and enhanced FoM resulting from the resonance characteristics lead to accurate discrimination of small sensor signals attributed to RI variations caused by target biomolecules. Therefore, an optimization scenario for biological tissues must simultaneously consider both optical performance and practical sensing scenarios (Long et al., 2022).

Multilayer prism-coupled SPR biosensors represent a promising platform for these applications, and recent advances have shown that integrating optimally designed structural parameters with an appropriate surface modification strategy can greatly enhance the analytical performance of complex biological samples. This integrated optimization improves sensor reliability, reproducibility, and detection sensitivity, facilitating label-free biosensing for clinical diagnostics, point-of-care testing, and biomedical research. Therefore, for practical sensor development, optimization should consider multiple performance metrics, such as sensitivity, FWHM, QF, FoM, detection accuracy, and operational stability. A holistic optimization approach guarantees that gains in one performance metric are not at the cost of other significant factors. This type of multi-objective optimization is especially crucial for biosensors applied to clinical diagnostics, environmental monitoring, and continuous sensing applications, where retaining measurement consistency (accuracy) and long-term performance is as critical as achieving high sensitivity (Chen et al., 2026). Massively multiplexed SPR biosensors are crucial for disease detection, biomarker discovery, and drug screening. Therefore, future multilayer sensor designs must incorporate optimized strategies that introduce sensitivity via robustness, fabrication possibilities, and stability during device operation. This section introduces general structural optimization strategies of multilayer SPR biosensors for achieving high-performance. Nevertheless, the real performance of these optimized architectures is dependent on the selection of functional materials used in the sensing framework. In this sense, the following chapter provides an overview of sophisticated material platforms that have been explored to further increase the sensitivity, selectivity and performance deformable SPR biosensors.

4 Theoretical modeling and performance evaluation methods

SPR biosensor design is heavily dependent on theoretical modeling and numerical analysis to optimize sensing performance without prior experimental fabrication. Analytical models allow the analysis of the interaction of electromagnetic waves with multilayer plasmonic structures and optimization of material combinations and sensing characteristics based on different working conditions. These models can predict resonance behavior reasonably accurately, and reduce experimental cost and development time substantially while aiding the design of high-performance biosensors (Srivastava et al., 2016; Tene et al., 2026). The Transfer Matrix Method (TMM) is still the most commonly used analytical method for multilayer SPR biosensors due to its lower computation cost and physical nature of electromagnetic wave propagation through layered media. Apart from predicting reflectance characteristics, theoretical modeling allows for systematic optimization of various structural parameters, including metal thickness, dimensions of dielectric spacer layers, number of 2D material layers, operating wavelength and interrogation conditions. Therefore, numerical analysis is the key to engineering dielectric and 2D material-based SPR biosensors with high sensitivity and detection precision (Panyaboon et al., 2026). The predicted theoretical performance was measured using quantitative metrics, including sensitivity, detection accuracy, FWHM, FoM, and LoD. These provide objective criteria for the comparison of various multilayer sensor architectures and a means to identify designs that yield higher analytical performance in biomedical and environmental sensing. This section provides a brief review of the theoretical framework adopted for dielectric–2D material SPR biosensors. This study covers the TMM, different interrogation techniques, reflectivity features, and important performance parameters for multilayer SPR sensor design optimization in terms of ultra-sensitive RI detection and diagnostics use in urine.

4.1 Multilayer SPR analysis by transfer matrix method

The Transfer Matrix Method (TMM) is the most commonly used analytical method for modeling multilayer prism configurations based on SPR biosensors because of its efficient and accurate solution of electromagnetic wave propagation through planar stratified media. TMM is based on Maxwell’s equations, electromagnetic boundary conditions, and allows the precise calculation of optical reflectance of a certain layer structure by considering multiple reflections and interference at each interface in the multilayer stack. TMM has been the conventional modeling method for multilayer SPR biosensor design and optimization because of its computational speed, numerical precision, and analytical correctness. In a common prism-coupled SPR setting, a p-polarized monochromatic light beam is incident on the multilayer structure, which contains a prism, plasmonic metal film, dielectric spacer layers, and two-dimensional (2D) materials as well as target analyte are placed on top. When light passes through these subsequent layers, every interlayer interface causes phases to accumulate and reflections to occur. The TMM processes the optical response of every layer into a single global transfer matrix, which allows one to directly predict both the resonance condition and resultant reflectance spectrum (Rishi et al., 2026; Uniyal et al., 2025; Kvist et al., 2026).

The characteristic matrix of the k ^th layer for an N-layer multilayer structure is expressed as

M k = cos ⁡ δ k i q k sin ⁡ δ k i q k ⁡ sin ⁡ δ k cos ⁡ δ k

Here:

• n k = RI of the kth layer
• d k = thickness of the kth layer
• λ = wavelength of the incident light
• θ k = propagation angle within the kth layer
• q k = optical admittance for TM (p-polarized) waves

Were,

δ k = 2 π λ n k d k ⁡ cos ⁡ θ k

The characteristic matrices of all individual layers are subsequently multiplied to obtain the overall transfer matrix of the multilayer structure.

M = ∏ k = 1 N M k

Using the global transfer matrix, the reflection coefficient at the prism–multilayer interface is determined as

r p = M 11 + M 12 q s q 0 − M 21 + M 22 q s M 11 + M 12 q s q 0 + M 21 + M 22 q s

The reflectance of the SPR sensor is then calculated using

R p = ∣ r p ∣ 2

This resonance condition is at the incident angle where the reflected optical intensity drops to a minimum, owing to the maximal transfer of optical energy from the incident light to the surface plasmon polaritons propagating along the metal–dielectric interface. Small RI variations induced by biomolecular interactions lead to measurable shifts of the reflectance minimum in angular interrogation SPR systems; this resonance angle is considered to be the main sensing parameter. TMM is especially effective in dielectric–2D material hybrid SPR biosensors because each ultrathin functional layer can be introduced into the multilayer model based on its effective complex RI or optical conductivity. The reviewed literature typically represents graphene via the Kubo conductivity model, whereas transition metal dichalcogenides (TMDCs) such as MoS₂ and WS₂ are modeled as ultrathin dielectric films possessing complex permittivity. The flexibility in modeling capabilities allows for an accurate analysis of a wide variety of multilayer architectures without substantially increasing the complexity of computation (Basumatary et al., 2026). The Transfer Matrix Method can be used to systematically optimize key structural parameters, including

• ➢ Metal film thickness
• ➢ Dielectric spacer thickness
• ➢ Number of two-dimensional material layers
• ➢ Operating wavelength

These parameters directly affect the resonance angle, resonance linewidth, electromagnetic field confinement, and ultimately the sensing performance. As a result, TMM not only acts as a reflectance calculation method but also effectively works for multilayer SPR biosensor design optimization. For planar multilayer structures, the transfer matrix method provides relatively accurate analytical solutions with much lower computational costs than full-wave numerical methods, which are more computationally intensive. As a result, it continues to be the theoretical framework of choice for initial sensor design, parameter tuning, and performance prediction before experimental fabrication and testing (de Oliveira et al., 2026).

4.2 Interrogation techniques and reflectivity characteristic

SPR biosensors operate based on the detection of changes in the resonance condition from variations in the RI of the sensing medium. The binding of biomolecules to the functionalized sensor surface changes the local RI, which affects the propagation characteristics of the surface plasmon polaritons. These changes manifest as quantifiable deviations in the reflectance spectrum and can be detected using diverse interrogation methods. The interrogation method directly affects the sensitivity, detection resolution, instrumentation complexity, and ease of use of an SPR biosensor for certain applications (Islam et al., 2024; Mustaffa et al., 2023; Hma Salah et al., 2024).

4.2.1 Angular interrogation

Angular interrogation is the most commonly used approach in prism-based SPR biosensors. The key step in this approach is keeping the incident wavelength constant, varying the angle of incidence, achieving resonance condition. Its resonance angle changes in response to a change of the analyte RI, therefore allowing for quantitative analysis of biomolecular interaction (Chen et al., 2026).

Δ θ r e s ∝ Δ n a

Angular interrogation, known for its large angular sensitivity and high measurement accuracy, is widely used in laboratory-based SPR systems for biochemical sensing and RI measurement. It is especially useful for assessing multilayer sensor architectures in which it is important to accurately ascertain the shifts in resonance angles (Mustaffa et al., 2023).

4.2.2 Wavelength interrogation

In wavelength interrogation, the angle of incidence is kept constant while the wavelength of light incident on the sensor surface varies. The light that achieves SPRis at a defined wavelength which satisfies the phase-matching condition, further changes in analyte RI result in observable shifts of this resonance wavelength (Abu Bakar et al., 2025; Yi et al., 2003).

k s p = 2 π λ r e s n p ⁡ sin ⁡ θ

This interrogation technique is well suited for compact optical systems and fiber-optic SPR biosensors, where varying the incident angle is impractical. Wavelength interrogations are also attractive for the use of broadband light sources and spectrometers in portable sensing platforms.

4.2.3 Phase interrogation

Phase interrogation is a measurement of the rapid phase variance of reflected light, which occurs close to the SPR resonance condition. Rather than directly monitoring the resonance angle or wavelength, this technique measures variations in the optical phase which are highly sensitive towards small changes in the RI (Chen et al., 2026).

ϕ = arg r p

Due to the sharp phase change in resonance, phase interrogation allows for ultra-high-resolution detection all the way down to very low detection limits. Thus, it is perfect for the detection of trace biomolecules and ultra-small RI changes (Su et al., 2026).

4.2.4 Intensity interrogation

The simplest measurement method based on SPR is intensity interrogation. In this method analyte-induced variations in RI appear as changes in reflected optical intensity from a sample when the intensity is measured at a fixed angle or wavelength. Although intensity interrogation usually has a lower sensitivity compared to angular or phase interrogation schemes, it has much simpler optical configuration, lower instrumentation cost and implementable for portable, miniaturized and point of care diagnostic systems (Islam et al., 2024).

5 SPR-based biosensing applications

5.1 Optical properties of biological specimens

SPR biosensors can be used for the label-free detection of biological fluids because of their specific optical properties. The RI, which is determined by the concentration and composition of dissolved biomolecules in these fluids, can vary significantly between healthy and diseased patients, making it an important optical parameter for disease detection. These devices play a major role in highly sensitive biosensors, as the SPR sensing mechanism relies on detecting very small RI changes at the metal–dielectric interface; thus, a good knowledge of the optical characteristics of all biological specimens that need to be analyzed is required (Liu et al., 2026). Among all biological fluids examined in clinical diagnostics, urine has gained significant attention because its collection is non-invasive, accessible in adequate volumes, and has relatively high concentrations of several biochemical constituents relevant to human health. The RI of urine is usually in the range reported in previous studies, and it also changes depending on the concentrations of proteins, urea, creatinine, glucose, and other metabolites in urine. Small variations in the concentrations of these markers can generate detectable RI increments from optimized multilayer SPR biosensors (Jin et al., 2025).

Multiple molecules are utilized to form optical responses depending on the concentration of biomolecules, molecular size, and temperature in bulk-level samples. This is mainly due to the higher polarizability and molecular weight of dissolved proteins compared with molecules such as urea or salts, which produce relatively smaller optical changes. Consequently, SPR biosensors used in biological applications require high sensitivity (3–10 nm/RIU), small resonance linewidths (<5.0 nm), and detection accuracy to detect such subtle values of RI changes accurately. The dielectric layers in conjunction with the 2D nanomaterials further improve the interaction of the evanescent electromagnetic field around the sensing surface and biomolecules located close to it. Materials such as graphene or MoS₂ offer large surface areas and conducive surface chemistry for biomolecule immobilization, allowing for greater adsorption of analytes, yielding a higher local effective RI change to which the propagating surface plasmons are sensitive. These hybrid dielectric–2D material SPR biosensors are particularly interesting for the sensitive detection of urinary biomarkers related to kidney function and disorders.

5.2 SPR biosensors for diverse applications

Urea is the main end product of protein metabolism and is produced from ammonia in the urea cycle that occurs in hepatocytes and excreted by the kidneys. The ability to concentrate in urine is highly informative of renal function, hydration status, and metabolic disorders. The kidneys fail and urea accumulates in the circulation due to renal impairment; however, elevated blood urea levels can result from hepatitis, liver dysfunction, or other pathological conditions. Indeed, the correct and fast measurement of urinary urea has emerged as an important goal in clinical diagnostics and point-of-care healthcare systems (Cheng et al., 2025). Traditional methods for urea determination, such as enzymatic assays, spectrophotometry, and chromatographic techniques, typically offer accurate quantification but require time-consuming sample processing steps, expensive instrumentation, and skilled personnel to conduct the analysis. These limitations have driven the need for label-free optical biosensors with fast, high sensitivity, and real-time readout capabilities while minimizing sample preparation. SPR biosensors are especially promising because these technologies can rapidly measure both the magnitude and direction of RI variations resulting from urea concentration changes without fluorescent or radioactive labelling (Gong et al., 2025). Recent studies have indicated that multilayer SPR biosensors using dielectric materials and two-dimensional nanomaterials outperform simple single-metal SPR configurations in terms of sensitivity and QF. Table 5 represents the comparison of representative SPR biosensors reported for different applications along with urea detection, highlighting sensor configuration, analytical performance, and key contributions toward kidney disease diagnostics.

Dielectric spacer layers can restrict electromagnetic fields, whereas graphene, MoS₂, black phosphorus, and other 2D materials with high surface areas and appropriate chemical compositions favour analyte adsorption. The combined effect of the two transformations leads to particularly large resonance angle shifts, narrower resonance curves, and better detection performance in all the explored tested concentrations for urea sensing. Optimizing structural parameters such as metal film thickness, dielectric layer thickness, number of 2D material layers, and wavelength has also been shown to have a significant influence on sensor performance. This approach dispenses with the use of a single material, and investigations increasingly use hybrid multilayer architectures that leverage the complementary optical and physicochemical properties of several functional layers. These approaches have led to consistent enhancements in sensitivity, FOM, and overall reliability of urinary urea sensing. Despite such advancements, there are fundamental challenges that must be resolved before the clinical translation of SPR-derived urea sensors becomes a reality. Real urine specimens contain many dissolved biomolecules that can either account for the non-specific changes in RI or interfere with selective analyte recognition. Consequently, further studies must focus on the optimization of surface functionalization and sensor selectivity, as well as testing with clinical samples and compact, portable SPR platforms suitable for point-of-care diagnostics.

Albumin is one of the most abundant plasma proteins and is an important biomarker for assessing kidney function. Urinary albumin is only detected in trace amounts in urine under normal physiological conditions, whereas high levels (albuminuria) are well established as early signs of diabetic nephropathy, chronic kidney disease, and other renal diseases. Therefore, careful and reliable measurement of urinary albumin is highly significant for early recognition, patient monitoring, and clinical treatment (Vashist et al., 2012; Yue et al., 2025). Standard methods for albumin determination, such as immunoassays and laboratory biochemical analyses, yield reliable quantitative results but typically require specialized equipment, trained personnel, and relatively long sample processing times. In contrast, label-free SPR biosensors have several notable advantages, including the real-time sensing of biomolecular interactions without the need for fluorescent or enzymatic labels, making them attractive for rapid clinical diagnostics and point-of-care applications. The detection of albumin using SPR biosensors relies on the specific interaction between the immobilized recognition molecule and albumin in the sample, leading to a localized change in the RI at the sensing interface. A Comparison of Representative SPR Biosensors with existing works for different applications along with Albumin Detection was tabulated in Table 6, respectively.

TABLE 6. A Comparison of Representative SPR Biosensors with existing works for Albumin Detection.

Albumin is a moderately high molecular weight molecule (∼66 kDa), and its binding induces significant local RI changes that cause readily detectable shifts in the SPR resonance condition, even at low concentrations. This characterizes SPR as being especially relevant for the discovery of microalbuminuria beforehand. It has long been proven that by adding dielectric layers in combination with two-dimensional nanomaterials, albumin sensing can be further improved. Graphene has rich surface functionalization sites and excellent antibody immobilization, while MoS₂ increases the wettability and affectivity of biomolecules on surfaces, which is essential for efficient capture of analytes. Such composite materials invigorate the coupling effect of the evanescent field with target biomolecules, which in turn improves the sensitivity and offers more selective resonance curves and higher reliability of sensing (Karakoç et al., 2025). The state-of-the-art literature continuously shows that hybrid dielectric–2D material SPR architectures outperform traditional metal-film sensors in detecting low levels of albumin associated with early-stage kidney disease. However, clinical implementation requires bolstering by improving surface functionalization, long-term stability, selectivity towards complex urine samples, and validation against large-scale clinical datasets. The studies reviewed in this chapter proved that the dielectric and 2D material-enhanced SPR biosensor technology play an important role in the detection of clinically relevant urinary biomarkers, such as urea and albumin. Due to their high sensitivity, label-free operation and real-time detection capabilities, they are promising candidates for early diagnosis of kidney disease. Nonetheless, despite these advances, there are persistent practical issues surrounding material stability, sensor fabrication, biofouling and large-scale clinical validation. The next chapter talks about these challenges desperately.

6 Challenges, future perspectives, and conclusion

Despite impressive strides in dielectric and 2D material-assisted SPR biosensors, many scientific, engineering, and translational challenges remain to be resolved before widespread translation of these technologies into clinical diagnostics is possible. Although most studies show major improvements in sensitivity, detection accuracy, and figure-of-merit under controlled environments, a sensor also needs to be used reliably in complex biological matrices with high stability, reproducibility, cost-effectiveness, and ability to function unattended instead of just being highly sensitive. This chapter reviews key challenges related to material stability, fabrication, biofouling, and large-scale clinical translation intimately associated with SPR biosensors alongside emerging avenues of research capable of advancing the next-generation of such sensing platforms.

6.1 Practical implementation challenges and material stability

While multilayer SPR biosensors have shown exceptional analytical performance in both simulation studies and proof-of-concept experiments, translating these devices into commercial- and clinical-based applications remains challenging. Advancements in material science for the inclusion of 2D nanomaterials and more advanced ultra-thin dielectric materials celebrate different quality control issues pertaining to stability, fabrication ability, long-term reliability, and sensor reproducibility. It is thus crucial to deal with these practical issues for ensuring effective operation in real world conditions.

6.1.1 Metal film stability

The long-term stability of the plasmonic metal layer, which forms the core sensing element of prism-based SPR biosensors, is essential for ensuring the reliability of the sensor performance. Au has been the most common plasmonic material because of its favorable chemical stability, corrosion resistance, and biocompatibility. Nevertheless, its comparatively high optical losses tend to result in broader resonance curves compared to Ag, which can degrade the resolution of sensing and detection accuracy. Due to its low optical absorption and sharp resonance, Ag has better plasmonic performance with higher sensitivity and narrower linewidth than the other metals. However, during the operation of the sensor for a longer time in a liquid or biological environment, Ag undergoes oxidation, which leads to its surface degradation. This restriction of silver has driven scientists to create bimetallic structures with compositions aiming at interplaying the chemical stability of Au and optical characteristics with those intrinsic in Ag. However, long-term functional maintenance when continuously exposed to a plethora of biological samples remains an open area of research.

6.1.2 Stability of two-dimensional materials

2D materials have advanced SPR biosensor performance through sophisticated electromagnetic field confinement and biomolecule adsorption. However, environmental stability is material-dependent. Graphene possesses excellent chemical stability and can retain its structural integrity under a wide range of operating conditions; therefore, it is ideal for long-term biosensing applications. In contrast, materials such as black phosphorus oxidize very quickly upon contact with oxygen and moisture, severely restricting their usability without surface protection. Compared to black phosphorus, transition metal dichalcogenides, including MoS₂ and WS₂, have better stability in air; however, they can still suffer from oxidation, contamination during processing, and structural defects due to prolonged operation. As a result, encapsulation strategies and dielectric passivation layers have been developed to enhance environmental stability while maintaining the optical properties necessary for efficient plasmon excitation.

6.1.3 Biofouling and surface functionalization

Biofouling, which is the non-specific adsorption of proteins, salts, and other biomolecules on the sensor surface, is among the most persistent problems and challenges in clinical SPR sensing. Unwanted adsorption can change the inherent resonance condition, decrease sensor repeatability, and negatively affect the measurement accuracy, especially during continuous analyses of biological fluids. Thus, surface functionalization approaches are key to reducing these effects while enhancing the selective recognition of target analytes. Polymer-based coatings (e.g., PEG and zwitterionic materials) have been extensively studied because they accurately shield non-specific adsorption while maintaining biomolecular recognition. Nonetheless, in the case of too high coating thickness, it may reduce the interaction between the evanescent field and analyte, emphasizing that additional analytical performance should be executed without compromising sensing abilities.

6.1.4 Fabrication and cost issues

Multilayer SPR biosensors require precise nanometer-scale control in terms of the thickness and uniformity of each functional layer. For example, small differences in layer thickness can have very large effects on the rate of plasmon coupling and resonance linewidth, while affecting sensing performance. Consequently, high-accuracy deposition methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD) are frequently utilized to fabricate high-quality multilayers. Although these methods provide good control over the quality and reproducibility of the film, they also increase fabrication complexity and cost. These obstacles still present challenges for the bulk adoption of SPR biosensors. Consequently, developing scalable and cost-effective specifications that can manufacture multilayer sensor platforms in customizable/transferable reproducible vectors could be a future milestone for practical routine clinical or point-of-care applications.

6.2 Emerging trends and future research directions

The development of plasmonic materials, computational methods, and miniaturized sensing platforms has broadened the applications of SPR biosensors beyond standard laboratory use. Emerging research has therefore shifted attention to the amalgamation of sophisticated materials, machine learning through artificial intelligence, miniaturized sensing platforms, and multiplexed detection strategies (decreasing time of analysis) for improved analytical performance, with a particular emphasis on clinical translation. Major advances are anticipated to additionally develop the touchiness, selectivity, size scalability, and pragmatic applicability of next-generation dielectric–2D material-based SPR biosensors.

6.2.1 Integration with artificial intelligence

The use of artificial intelligence (AI) and machine learning techniques is growing into one of the effective tools for design and optimization of SPR biosensors. Machine learning algorithms circumvent the need for iterative numerical simulations by efficiently optimizing multilayer configurations to predict resonance features and determining optimal combinations of material properties and structural parameters. In addition, supplementary AI-augmented analysis of SPR reflectance curves enables enhanced signal processing, noise attenuation and expedited interpretation of biosensing data, which in turn bolster more dependable clinical decision-making.

6.2.2 Fiber-compact & portable SPR systems

Advances in compact fiber-optic SPR platforms are a decisive step toward the development of point-of-care diagnostic tools. Compared to the conventional prism-based systems, fiber-optic SPR sensors have advantages regarding reduced size, simplified optical alignment and better portability at high sensing performance. Microfluidic systems could be especially compatible with these devices, permitting handling of minimal sample volumes, automated sample processing and multiplexed biomarker detection. Due to these advantages, portable SPR platforms show a very promising potential for rapid clinical screening and real-time healthcare monitoring.

6.2.3 Phase-sensitive and imaging SPR

These advanced interrogation techniques are further enhancing the analytical performance of SPR biosensors. By measuring the rapid phase variations around the resonance condition, Phase-sensitive SPR remarkably improves detection resolution for ultralow RI changes. Imaging SPR also takes this a step further, not only because it can observe multiple sensing regions at once but also because it allows multiplexed detection of biomarkers in one measurement. This makes these approaches especially attractive for high-throughput clinical diagnostics and personalized healthcare application.

6.2.4 Emerging materials beyond graphene

Despite graphene being among the most studied 2D materials for SPR biosensing, other materials attracting considerable interest due to their distinct optical and electronic features are quickly emerging on new horizons. Emerging materials like MXenes, topological insulators, and perovskite-based nanostructures show strong light–matter interaction, tunable optical properties as well as surface chemistry that enable biomolecular adsorption. And among them, plasmonic materials have shown particularly great promise for improving electromagnetic field confinement and the attenuation length; that is, a much higher sensitivity and wider range of operational capabilities in potential future biosensors.

6.2.5 Toward clinical translation

High-performance analyzers cannot guarantee successful clinical translation despite significant advances in sensor design and material engineering. Future SPR biosensors must achieve repeatable, long-term stable, rapid, inexpensive measurements with low limits of detection while maintaining reliable function in complex biological samples. Furthermore, large clinical validation in patients with standardized testing protocol and involved regulatory created an approval process are required for these technologies to become the standard part of routine clinical diagnostics. Meeting these needs for practicality will be the key step towards the clinical application of dielectric–2D material-assisted SPR biosensors. Table 7 shows the critical comparison of major challenges, mitigation strategies, and remaining limitations in SPR biosensors for practical and clinical applications.

TABLE 7. Critical comparison of major challenges, mitigation strategies, and remaining limitations in SPR biosensors for practical and clinical applications.

7 Conclusion and future challenges

In this review, we critically discussed recent developments of the dielectric- and two-dimensional (2D)-material-assisted surface plasmon resonance (SPR) biosensors covering material engineering, multilayer structures, sensing mechanism, performance metrics, theoretical modelling, and various biomedical applications as well as analytical applications. The reviewed studies provide valuable evidence on dielectric and 2D materials for modifying electromagnetic field confinement, plasmonic interactions and optical losses in traditional metal-film SPR configurations, which result in better refractive-index sensitivity, figure of merit (FOM), and detection abilities than conventional configurations. But the amount of improvement will be different depending on the layers, the conditions under which interrogation is performed and the material properties, and the structural configuration. Hence, it is important to understand that reported sensitivity and FOM figures should only be understood in the context of the applied sensing configuration and are not, in themselves, a good measure of the superior selection of the sensor.

For this review, another significant difference between theoretical predictions and experimental observations of performance is pointed out. A significant percentage of high-performance platforms described in the literature rely on numerical simulation, where usually ideal properties of the materials, thickness of the layers and conditions at the interfaces are assumed. When applied in practical devices fabrication tolerances, roughness of the fabricated surfaces, degradation of the materials and Fines due to optical losses, effects of varying environmental conditions and measuring noise can lead to limitations on the actual performance achieved. Thus, additional experimental testing is needed to demonstrate the impact of theoretically ideal multi-layer designs. A number of hindrances remain to be overcome to translate these platforms into viable sensing platforms. These encompass stability of material and interface over time, agreement between different types of sensors, bio fouling and non-specific adsorption in complex biological matrices, fabrication difficulties, and significant lack of standardised benchmarking protocols. Furthermore, the sensitivity alone or the low theoretical detection limit is not enough to prove the clinical applicability. The following methods are required to establish clinical relevance: real-sample analysis, matrix effects, regeneration capability, large-cohort validation, comparison with good diagnostic methods and standardization. The development of improved sensor architectures should therefore not be aimed just at improving sensor sensitivity, and should rather be those sensor architectures which have been tested and proven and which can easily be reproduced in the future. Stable and scalable material deposition, quantitative benchmarking, the engineering of non-fouling surfaces and the systematic evaluation on real biological samples will be of paramount importance. The integration of SPR platforms with microfluidics, compact fiber-optic platforms, portable detection instrumentation, and multiplexed detection may further contribute to deployment. AI and computational approaches can help narrow down material combinations and key parameters but the predictions must be verified experimentally. There is also a need for systematic investigation of emerging materials like MXenes, black phosphorus, h-BN and other layer materials, including their stability under processing conditions and manufacturability.

In summary, the use of dielectric and 2D-materials to enhance the SPR biosensor functionality is a promising approach for optical sensing with high sensitivity and without the need of molecules labelled with dyes. Their future impact will be determined by their sensitivity, selectivity, stability, reproducibility, manufacturability, cost and clinical validation in the right balance as well, in addition to meeting high and absolute theoretical values for these parameters. A focus on these needs will be critical for bringing laboratory scale SPR studies into good, portable and clinically useful sensing applications.

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Footnotes

Edited by: Sreeramulu Dr. Valligatla, Q.ANT GmbH, Germany

Reviewed by: Semra Akgönüllü, Hacettepe University, Türkiye

Xin Ding, Guangdong Polytechnic, China