Structural, biomedical and metal-ion sensing properties of the eco-friendly synthesized CrVO 4 nanoparticles using Saussurea obvallata flower extract

1 Introduction

The growing demand for environmentally responsible technologies has significantly influenced the direction of nanomaterials research (Jadoun et al., 2021; Muzammal et al., 2025a; Muzammal et al., 2025b). As nanotechnology continues to expand into biomedical, environmental, and sensing applications, the need for safer and more sustainable synthetic approaches has become increasingly evident (Smith et al., 2013; Giraldo et al., 2019; Huang et al., 2024; Patel et al., 2026; Patel et al., 2025; Sanaullah et al., 2025). High temperatures, hazardous reducing agents, and energy-intensive processes are frequently utilized in conventional methods for the creation of metal oxide nanoparticles, which may limit their environmental acceptability and biomedical compatibility (Roy et al., 2019; Radulescu et al., 2023; Singh et al., 2018; Selvaraj et al., 2025; Bhardwaj et al., 2024). In response to these challenges, green synthesis strategies based on plant extracts have emerged as attractive alternatives (Rameshkumar et al., 2023; Gokul et al., 2026; Cuong et al., 2022). Biomolecules derived from plants can cap nanoparticle surfaces, stabilize developing nuclei, and simultaneously reduce metal ions, thereby enabling regulated synthesis under moderate, environmentally acceptable conditions (Iravani, 2011; Singh et al., 2016; Ahmed et al., 2016). This approach not only reduces chemical hazards but can also introduce biologically relevant surface functionalities that enhance the performance of the resulting nanomaterials (Patil et al., 2025; Muzammal et al., 2025c).

Among various functional nanomaterials, mixed-metal oxides have attracted attention due to their cooperative electronic interactions and tunable redox properties (Wu et al., 2025; Cheng et al., 2026; Duglet et al., 2025). One such ternary oxide semiconductor is CrVO₄, which exhibits favorable optical absorption and effective charge-transfer properties due to the interaction between the oxidation states of chromium and vanadium (Botella et al., 2020). These properties have led to investigations of CrVO₄ in photocatalytic and electrochemical systems (Bera et al., 2018). However, environmentally benign plant-mediated fabrication techniques remain largely unexplored, and most published syntheses rely on traditional chemical or thermal processes. Moreover, the biological and sensory capabilities of CrVO₄ NPs have not been well investigated, particularly when synthesized using environmentally friendly methods. Developing a sustainable method for CrVO₄ NP synthesis while broadening its functional applications, therefore, represents a meaningful scientific advancement.

In this regard, S. obvallata (Brahma Kamal) offers a promising, biologically rich platform for nanoparticle synthesis. Traditionally, this high-altitude Himalayan medicinal herb has been utilized to treat infections, wounds, respiratory disorders, and inflammatory illnesses (Semwal et al., 2020). Numerous secondary metabolites have been identified in S. obvallata through phytochemical research, including flavonoids such as quercetin and kaempferol derivatives, phenolic acids, tannins, terpenoids, sesquiterpene lactones, alkaloids, glycosides, and steroidal components (Semwal et al., 2020; Mukhopadhyay et al., 2024). These compounds possess functional groups, such as hydroxyl, carbonyl, and carboxyl moieties, that can donate electrons and coordinate metal ions. During nanoparticle formation, such biomolecules can facilitate the reduction of chromium and vanadium precursors, stabilise the resulting nanostructures, and limit aggregation. The plant extract is known to exhibit cytotoxic and antioxidant properties, suggesting that phytochemical residues on nanoparticle surfaces could enhance biological interactions (Semwal and Painuli, 2019). Therefore, using Saussurea obvallata flower extract provides an environmentally friendly synthesis method and the potential for increased bioactivity through phytochemical functionalization.

The relevance of this strategy extends to important biomedical challenges. Cancer continues to be a major cause of mortality worldwide; attempts are being made to find substances that can target cancerous cells specifically while posing less of a risk to the body (Sung et al., 2021). Metal oxide NPs are known to exert anticancer effects through mechanisms such as the production of reactive oxygen species, the induction of oxidative stress, and the activation of apoptosis (Akhtar et al., 2012; Shi et al., 2017). At the same time, drug-resistant forms of Mycobacterium tuberculosis, which causes tuberculosis (TB), continue to pose a severe threat to world health (Dastidar et al., 2025). Nanostructured metal oxides have demonstrated antimicrobial and antitubercular potential by disrupting cellular membranes, interfering with metabolic pathways, and promoting oxidative damage in bacterial cells. Vanadate-based materials exhibit inherent redox behavior and semiconducting properties, making them suitable for analytical sensing and therapeutic applications (Dubey et al., 2025; Ghotekar et al., 2023; Bhardwaj et al., 2026). CrVO₄ NPs, with their high surface area and favourable electron-transfer properties, may enable the sensitive detection of environmentally relevant metal ions, supporting applications in environmental monitoring and public health protection (Anwar et al., 2023).

In light of these considerations, the present study explores a plant-mediated approach to synthesizing CrVO₄ NPs, using S. obvallata flower extract as a natural reducing and stabilizing agent. To the best of our knowledge, the environmentally benign fabrication of CrVO₄ via a phytochemical route has not been systematically investigated, particularly in conjunction with combined anticancer, antitubercular, and metal ion-sensing evaluations. To fully comprehend the structural, topological, and optical characteristics of the synthesized NPs, their biological and sensing capabilities were evaluated. By combining green synthesis with multifunctional applications, this work aims to expand the scope of CrVO₄ nanomaterials and to contribute to the development of sustainable nanotechnology solutions for environmental and biomedical challenges.

2 Experimental

2.1 Materials

Chromium(III) nitrate nonahydrate [Cr(NO₃)₃·9H₂O] and ammonium metavanadate (NH₄VO₃) of analytical grade were received from SRL Chem, India, and were utilized without further purification. Fresh blossoms of S. obvallata (Brahmakamal) were procured locally. All aqueous solutions were prepared using double-distilled water to avoid extraneous ionic interference.

2.2 Preparation of flower extract

Fresh S. obvallata flowers were rinsed thoroughly under running tap water, then with distilled water (dH₂O), to remove adhering impurities. After being cleansed, the petals were coarsely chopped and shade-dried to retain thermolabile phytoconstituents. Approximately 5 g of the processed floral material was gently heated in 50 mL of dH₂O at 60 °C–70 °C for 20 min. Whatman filter paper (No. 1) was used to filter the mixture once it had cooled to room temperature. The clear filtrate was preserved at 4 °C and employed within 48 h as a bioreducing and stabilizing medium.

2.3 Green synthesis of CrVO₄ NPs

An equimolar amount of Cr(NO₃)₃·9H₂O and NH₄VO₃ was taken in a beaker to make a homogeneous solution. Subsequently, the solution was placed on a magnetic stirrer, followed by dropwise addition of 10 mL of the freshly prepared S. obvallata extract. The mixture was agitated for a further half hour at room temperature after the addition was finished. A perceptible change in color from pale green to deep brown signified the onset of nucleation and nanoparticle formation, attributable to phytochemical-mediated reduction and complexation. The mixture was then dried in a hot air oven. To enhance crystallinity and eliminate residual organic moieties, the dried powder was calcined at 500 °C (2 h) in a furnace, yielding phase-pure CrVO₄ NPs. The bio-inspired synthetic pathway for producing CrVO₄ NPs is schematically depicted in Figure 1.

2.4 Characterizations

The crystalline phase and structural assessment of the prepared NPs were confirmed employing X-ray diffraction analysis (PANalytical X'Pert PRO). Using Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX) (MIRA-3 TESCAN), and High-Resolution Transmission Electron Microscopy (HRTEM) (JEM-2100) methods, morphological characteristics and particle size distribution were investigated. Fourier Transform Infrared Spectroscopy (FTIR) (Jasco 6300) was performed to identify characteristic metal–oxygen vibrations. Ultraviolet–Visible Spectroscopy (UV) (Agilent Cary 5000) was utilized to study optical characteristics and estimate the band gap.

2.5 In vitro anticancer activity

The cytotoxic potential of the biosynthesized CrVO₄ NPs was assessed against human breast adenocarcinoma (MDA-MB-231) cells (cells were procured from National Centre For Cell Science (NCCS), Pune) employing the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay (Hattiholi et al., 2022). Cells were cultured in 96-well plates and exposed to graded concentrations of NPs (100, 50, 25, 12.5, 6.25, and 3.12 μg mL^-1) for 48 h under standard incubation conditions, according to the standard protocol followed by the testing laboratory. The concentration range (100, 50, 25, 12.5, 6.25, and 3.12 μg mL^-1) was selected according to the standard protocol followed by the testing laboratory, which was adapted from (Hattiholi et al., 2022). A two-fold serial dilution was used to evaluate dose-dependent cytotoxicity and reliably determine the IC₅₀. After treatment, the wells were gently washed twice with phosphate-buffered saline (PBS). Then, MTT reagent was added, and the wells were incubated for 4 h to allow metabolically active cells to generate insoluble formazan crystals. Dimethyl sulfoxide (DMSO) was used to dissolve the crystals, and absorbance was measured at 570 nm using a microplate reader. Optical density (OD) measures a sample’s light absorbance at a specific wavelength. Equation 1 was used to calculate cell viability.

% Viability = OD of treated sample OD of control × 100 (1)

The dose-response curve was used to calculate the IC₅₀, the concentration required to reduce cellular viability to 50%. To guarantee reproducibility, every experiment was conducted in triplicate.

2.6 Antitubercular activity

The antimycobacterial efficacy of CrVO₄ NPs was evaluated against the standard strain Mycobacterium tuberculosis H37Rv employing the microplate Alamar Blue assay (MABA). A standardized bacterial solution was used to inoculate serial dilutions of nanoparticles prepared in Middlebrook 7H9 broth. For 5 days, the microplates were incubated at 37 °C. Alamar Blue was used as a redox indicator to assess bacterial vitality after incubation. The minimum inhibitory concentration (MIC) was defined as the lowest nanoparticle concentration required to prevent the characteristic colour transition from blue to pink, thereby indicating effective inhibition of bacterial growth. As comparison reference controls, common antitubercular medications, including rifampicin, isoniazid, pyrazinamide, ethambutol, and streptomycin, were utilized (Lourenco et al., 2007).

3 Results and discussion

3.1 XRD analysis

The crystallographic elucidation and phase purity of the greenly prepared CrVO₄ NPs were investigated by X-ray diffraction (Figure 2). The diffraction pattern showed sharp, intense Bragg reflections, confirming the material’s crystalline nature. The major diffraction peaks observed at 2θ values of 12.59°, 13.89°, 17.38°, 19.26°, 21.65°, 24.54°, 27.93°, 29.70°, 32.00°, 33.50°, 34.89°, 35.77°, 36.68°, 38.66°, 41.66°, 42.36°, 45.54°, 50.02°, 53.70°, 54.89°, 56.28°, 60.67°, 64.25°, 67.73°, 72.01°, and 78.39° were indexed to their respective crystallographic planes of orthorhombic CrVO₄. The standard data shown in JCPDS card No. 83–0761 and the measured diffraction peaks correlate well (Liu et al., 2024). The excellent match between the experimental and conventional peak positions confirms the effective production of single-phase CrVO₄. Interestingly, no further peaks were observed, suggesting that the NPs produced were highly pure-phase. The mean crystallite size was computed employing Scherrer’s equation (Equation 2). In this context, D represents the average crystallite size, K denotes the shape factor (typically 0.9), λ refers to the wavelength of the X-ray radiation (1.5406 Å for Cu Kα radiation), β indicates the full width at half maximum (FWHM) of the diffraction peak in radians after correction for instrumental broadening, and θ corresponds to the Bragg diffraction angle (Holzwarth and Gibson, 2011).

D = k λ β ⁡ cos ⁡ θ (2)

The calculated mean diameter was 21.16 nm, confirming the nanocrystalline nature of the material. Overall, the XRD results validate the successful green synthesis of phase-pure, structurally stable CrVO₄ NPs with nanoscale dimensions.

3.2 FTIR analysis

The FTIR spectrum (Figure 3) of the synthesized CrVO₄ NPs clearly supports the formation of the metal vanadate framework along with minor surface-bound species. The O–H stretching vibrations originating from surface-adsorbed moisture or hydroxyl groups, which are frequently found in metal oxide nanomaterials, are responsible for the broad absorption band seen at about 3,435 cm^‒1. The weak band at approximately 1,643 cm^‒1 corresponds to the bending vibration of H–O–H of physically adsorbed water molecules (Zhang and Zhang, 2009). The presence of vanadate groups is confirmed by a strong and definite absorption band at around 964 cm^‒1, which is characteristic of the terminal V=O stretching vibration of the VO₄ ^3‒ tetrahedral unit. The band located near 838 cm^‒1 is assigned to the asymmetric stretching vibration of V–O–V linkages within the CrVO₄ lattice (He et al., 2020). Additionally, the absorption peaks at approximately 668 cm^‒1 and 549 cm^‒1 are attributed to Cr–O stretching vibrations, indicating the formation of the CrVO₄ phase and the incorporation of chromium into the vanadate framework. The observed vibrational features are consistent with previously described reports on chromium vanadate materials (He et al., 2020).

3.3 UV-vis analysis

The UV–Vis absorption spectrum of the as-prepared CrVO₄ NPs exhibits strong absorption in the ultraviolet region (285 nm), which is attributed to ligand-to-metal charge transfer transitions from O^2‒ (2p) to V⁵⁺ (3d) orbitals within the VO₄ ^3‒ tetrahedral units as depicted in Figure 4a. The inclusion of chromium into the lattice is confirmed by a broad absorption extending into the visible range (≈350–750 nm), which is attributed to metal–oxygen charge-transfer interactions and the d–d electronic transitions of Cr³⁺ ions (Elhamdi et al., 2024). The optical band gap energy, estimated from a Tauc plot, was 3.44 eV (Figure 4b), indicating the semiconducting nature of CrVO₄. This result is in good agreement with previously reported band gap values for CrVO₄ in the visible region (Botella et al., 2020). Its semiconducting properties are further supported by the progressive decrease in absorbance at longer wavelengths, underscoring its potential for optoelectronic and photocatalytic applications.

3.4 FESEM and EDX analysis

Field-emission scanning electron microscopy (FESEM) was performed to better understand the surface characteristics of the synthesized CrVO₄ NPs. The material consists of tightly packed particles with unique faceted surfaces and uneven, plate-like forms, as presented in Figure 5. Due to the high surface energy of nanomaterials, particles tend to cluster, which is common. The sharp edges and flat faces observed in the micrograph suggest that the particles have a high degree of crystallinity. Additionally, the material appears somewhat rough and textured, with smaller particles adhered to the surfaces of larger ones. To further confirm the composition, EDX elemental mapping was performed. The data verifies the presence of Cr, V, and O in the sample, with no indication of unwanted impurities. These elements are uniformly distributed throughout the material, as shown in Figure 6, indicating the development of a homogeneous CrVO₄ phase.

3.5 HRTEM analysis

The microstructural characteristics of CrVO₄ NPs were investigated using TEM and HRTEM images, as shown in Figure 7. Figure 7a clearly shows that the particles have an irregular spherical agglomerated microstructure, with a mean diameter of 43.19 nm. The particle size distribution shown in the inset discloses that the majority of NPs are in the 20–40 nm range, with a significant secondary population between 50 nm and 70 nm. While the range is wide, from 5 nm to 140 nm, the Gaussian fit indicates polydispersity, consistent with experimental expectations for these nanomaterials (Figure 7b). As depicted in Figure 7c, the HRTEM image of CrVO₄ NPs reveals lattice fringes within a single particle. The interplanar (d) spacing was determined to be 0.417 nm (4.17 Å) based on the lattice fringes. The lattice fringes are highly ordered, with a d-spacing value of 0.417 nm corresponding to the (111) plane of CrVO₄, according to ICDD card no. 00-83-0761 (Dmonte et al., 2024). Figure 7d reveals the SAED pattern of CrVO₄ NPs, confirming the polycrystalline nature of the sample.

3.6 Anticancer efficacy

The anticancer potential of the as-prepared CrVO₄ NPs was examined against the MDA-MB-231 (breast cancer) cell line and compared with the standard drug doxorubicin. The findings clearly show that both CrVO₄ NPs and doxorubicin reduce cell viability in a concentration-dependent manner, with higher concentrations leading to greater inhibition of cancer cell growth, as presented in Figure 8. Cell viability gradually decreased with increasing CrVO₄ NP dose, indicating cytotoxicity toward the examined cells.

Doxorubicin, on the other hand, demonstrated notably greater activity at significantly lower concentrations when compared directly, consistent with its well-known therapeutic efficacy. This difference is further supported by the IC₅₀ values, where CrVO₄ NPs showed a substantially higher IC₅₀ (118.98 μg mL^-1) compared to doxorubicin (3.46 μg mL^-1), indicating that a higher dose of the NPs is required to achieve 50% growth inhibition. In contrast, the very low IC₅₀ value of doxorubicin highlights its high potency against MDA-MB-231 cells.

Although the NPs are less potent than the standard chemotherapeutic agent, their dose-dependent cytotoxicity suggests that CrVO₄ NPs possess intrinsic anticancer activity. Taken together, these findings suggest that although CrVO₄ NPs are not as effective as doxorubicin, they exhibit encouraging biological activity and could be viable options for further anticancer applications.

3.7 Antitubercular activity

The antitubercular potential of the as-prepared CrVO₄ NPs was evaluated against Mycobacterium tuberculosis H37Rv, with an MIC of 3.12 μg/mL, indicating effective growth inhibition at a relatively low concentration. This result demonstrates that the NPs possess meaningful activity against the tested strain.

When the MIC obtained for CrVO4 NPs is compared with those of standard antitubercular drugs, CrVO4 NPs show activity very close to that of pyrazinamide, suggesting nearly comparable inhibitory performance. However, the NPs are less potent than rifampicin and streptomycin, which exhibit stronger effects at much lower concentrations. Similarly, isoniazid and ethambutol exhibit greater efficacy than CrVO₄ NPs, as evidenced by their lower MIC values. The results are summarised in Table 1. Although they do not outperform the most potent first-line medications, the NPs’ promising antitubercular potential is demonstrated by MICs around those of pyrazinamide. Figure 9 shows the colorimetric shift in the MABA assay from blue to pink, indicating metabolic activity.

TABLE 1. MIC of CrVO₄ NPs against Mycobacterium tuberculosis-H37Rv compared with standard antitubercular drugs.

3.8 Metal-ion sensor

The interaction between green-synthesized CrVO₄ NPs and several metal ions (Mg²⁺, Ba²⁺, Ni²⁺, Ca²⁺, Cu²⁺, Zn²⁺, and Cd²⁺) was extensively studied for heavy metal ion detection (Figure 10). UV-Vis spectroscopy proved to be a trustworthy and effective method for investigating these interactions. The optical sensor study was performed using a PerkinElmer instrument to investigate the recognition of cations by CrVO₄ NPs. 2 mL of a 0.012 mM solution of each metal perchlorate (Mg²⁺, Ca²⁺, Ba²⁺, Ni²⁺, Zn²⁺, Cu²⁺, and Cd²⁺) was prepared in conductivity water. Recognition was performed by adding 2 mL of a 0.0133 mM CrVO₄ NP solution.

The UV-Vis absorption spectra of CrVO₄ NPs and their interaction with several metal ions (Mg²⁺, Ca²⁺, Ba²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺) were recorded from 200 to 800 nm (Ahmed et al., 2022; Janani et al., 2020). The clean CrVO₄ sample absorbs strongly in the ultraviolet (200–300 nm) and gradually lowers to the visible region. This behavior is due to ligand-to-metal charge transfer (LMCT) transitions between O^2- and V⁵⁺/Cr⁶⁺ centers. When various metal ions were added, only Cu²⁺ ions caused a distinct change in absorption behaviour. The absorbance intensity increased, and the characteristic absorption band shifted slightly to higher wavelengths, indicating a strong interaction between CrVO₄ NPs and Cu²⁺ ions. Additionally, a broad, weak absorption feature was observed at a higher wavelength, possibly due to charge-transfer interactions or surface complex formation. Mg²⁺, Ca²⁺, and Ba²⁺ ions caused negligible changes in the absorption spectra (Figure 10), indicating minimal interaction with the nanoparticle surface. Ni²⁺, Zn²⁺, and Cd²⁺ produced only slight changes in intensity without significant peak shifts (Ibrahim et al., 2024). The strong response to Cu²⁺ ions is due to their partially filled d⁹ electronic configuration, which enables stronger coordination and charge transfer with CrVO₄ NPs (Gerdan et al., 2022).

The efficacy of nanoparticles for metal-ion sensing has been demonstrated in prior research. For instance, green-synthesized Ag NPs exhibit strong colorimetric detection of heavy metal ions, including Hg²⁺, Cu²⁺, Pb²⁺, and Co²⁺, according to (Ibrahim et al., 2024). Similarly, Wang et al. prepared CrVO₄ NPs via co-precipitation and investigated their use for high-temperature ammonia detection using potentiometric gas sensing (Wang et al., 2021). These findings support the strong selectivity of synthetic CrVO₄ NPs for Cu²⁺ ions over other metal ions under investigation.

4 Conclusion

In conclusion, this study presents a green and efficient route for synthesizing CrVO₄ NPs using Saussurea obvallata flower extract, offering a practical alternative to conventional synthetic techniques. The environmentally benign process not only minimizes chemical hazards but also imparts functional surface characteristics derived from plant biomolecules, thereby enhancing the nanoparticles’ performance. The synthesized CrVO₄ nanoparticles demonstrated significant anticancer, antitubercular, and metal-ion sensing capabilities, highlighting their versatility across multiple application domains. These results emphasize the potential of plant-mediated synthesis in designing advanced nanomaterials with improved functionality and reduced environmental impact. Overall, the work contributes to the emerging field of sustainable nanotechnology and provides a foundation for further exploration of eco-friendly nanomaterials in biomedical and environmental systems.

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Footnotes

Edited by: Daniela Šojić Merkulov, University of Novi Sad, Serbia

Reviewed by: Nur Ashmalina Haji Abd Rahman, Universiti Brunei Darussalam, Brunei

Sadia Muzammal, Government College University, Faisalabad, Pakistan