Catalytic, antibacterial and thermal characteristics of VO₂ nanoparticles derived from dextrose

Abstract

The current research investigates the characteristics and application of vanadium oxide nanoparticles (VO₂ NPs) synthesized by the reduction of vanadyl sulphate (VOSO₄) by dextrose. They have been characterized by ultraviolet–visible (UV–vis), Fourier transform infrared (FTIR), Raman, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), zeta potential and thermal studies. An absorption peak at 227 nm in the UV–vis region suggests the presence of pure VO₂ NPs. Two peaks in the FTIR spectrum at 500 and 1071 cm⁻¹ assigned to V–O and V=O stretching frequencies, respectively, show the presence of cis- and trans structural isomers of VO₂ NPs which is also supported by Raman spectral data. Average crystallite size was found to be 5 nm and showed two different phases for the NPs. EDX profile showed two sharp peaks for vanadium which refers to the purity of the sample. Zeta potential value suggests that VO₂ NPs are stable and neutral in nature. SEM images exhibited irregular shape and two different phases for VO₂. They exhibited 97% photocatalytic degradation of methylene blue (MB) in basic medium in daylight. It has great potential for the degradation of polluted water from textile industries under ambient conditions. VO₂ NPs displayed significant antibacterial activity against both gram positive and gram negative bacterial strains. The VO₂ NPs were found to be far more effective in eliminating the P. vulgaris bacteria than the standard antibiotic, Amikacin. Heat flow vs temperature plot showed an increase in heat flow between 353 and 513 °K after which it remained stable up to 1250 °K.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s11671-026-04504-x.

Introduction

Nanotechnology has become a subject of common interest in all branches of science and offers solution to many problems in medicine, engineering, agriculture and industry [1–3]. VO₂ nanowires, nanotubes, nanobelts, nanodots, nanoflowers of different morphologies and geometrical arrangements have been synthesized. Their physical, chemical, electrical and biological properties depend on their shape and size [4]. Vanadium compounds have attracted attention due to their multiple oxidation states ranging from 2–5, although vanadium forms highly stable compound even in zero oxidation state, such as vanadium hexacarbonyl, V(CO)₆. Their reduction to lower oxidation states with a consequent change in colour can therefore, be easily identified. The vanadium compounds are used as reducing agent and also as catalyst. Besides, oxovanadium (iv) has been detected in rat liver treated with pentavalent vanadium which suggests that V⁵⁺ has undergone one electron reduction which has been ascertained by EPR spectroscopy [5].

1

Vanadium pentoxide (V₂O₅) mimics catalase activity because its crystallite structure changes with temperature. It has also been checked that small size and large surface area enhances the activity of the NPs [6]. The NPs can be synthesized using a reducing agent, microbes or plant extract [7]. It is essential to have reducing sugar, phenol, alkaloid, flavonoid or a polymer in the plant extract for green synthesis of NPs of different shapes and size. They can also be synthesized by hydrothermal or sol–gel method but they require heating. If the VO₂ is hydrolysed and reduced with a mild reducing agent and finally heated to about 100 °C, its NPs are obtained. The green synthesis of NPs is safe because the biomolecules present in the plant extract often act both as reducing and capping agent. Nano belts of V₂O₅ have been synthesized using chemicals [8] and also from tannic acid [9]. The biomolecule has been shown to contain two hydroxyl groups which have been suggested to reduce vanadium ion to give V₂O₅.

2

If V⁴⁺ is reduced by capturing two electrons from hydroxyl group it would give V²⁺ whereas V⁵⁺ has been obtained which means that the reaction is one electron oxidation of V⁴⁺ to V⁵⁺ as shown below.

3

The product has been shown as a complex of V₂O₅ rather than V₂O₅ NPs. The laser deposition process (laser ablation) gives V₂O₅ NPs of different morphology in pure state. Spherical and flower like yellow coloured V₂O₅ NPs had different α and β phases in 3:1 ratio [10]. V₂O₅ NPs of 10–20 nm have also been synthesized from Fusarium oxysporum [11] and ammonium vanadate (NH₄VO₃) as starting material. They were spherical, face centred and crystalline. Besides being active against a wide range of pathogenic fungi they were fairly effective against breast cancer cell lines, MCF.7. Patients treated with these NPs showed a reduction in diseased breast cancer cells [11]. The NPs absorb at 410 nm in the UV–vis region. High value of zeta potential ( − 35.09 mV) suggested that the biomolecules in the Fusarium oxysporum, capping the NPs, have negatively charged surface. Recently, synthesis of VO₂ NPs from V₂O₅ has been reported [12]. The process involved the reduction of V₂O₅ by sugar by heating it between 180 to 300 °C for several hours. The temperature range was so high that the sugar may have been charred. It has been shown that addition of small amount of sugar leads to the formation of dispersed NPs of small size (40–60 nm). When uncontrolled sugar was added, it yielded both, monoclinic VO₂ (M) and VO₂ (R) phases. Even though, the morphology may be controlled by regulating the quantity of sugar, the ultimate phase depends not only on the concentration of sugar but also on the temperature and duration of time. Ag-loaded TiO₂/rGO nanocomposite has also been shown to degrade MB very efficiently up to 99.76% under laboratory conditions [13]. Similarly NiO NPs obtained from biosynthesis have been used in the degradation of dyes [14]. High photocatalytic activity in the degradation of MB has been observed similar to that found for TiO₂-polymer (PANI) NPs [15].

Even though the VO₂ NPs of different morphology have been synthesized from V₂O₅ by heating it at high temperature using reducing sugar, the fabrication of VO₂ NPs from VOSO₄ has not been reported thus far. We are therefore, reporting green fabrication of VO₂ NPs in one step by the reduction of VOSO₄ by dextrose in alkaline medium (pH 9) at relatively low temperature. We have also discovered cis and trans isomerism, for the first time, in these highly stable VO₂ NPs. We have characterized them by UV–vis, FTIR, Raman spectroscopy, XRD, SEM, EDX, Zeta potential and thermal analyses. Their catalytic activity against the degradation of MB using ascorbic acid as reducing agent has also been studied. In vitro investigation of NPs as antibacterial agent against ten bacterial strains has been done according to CLSI guidelines.

Experimental

Materials and chemicals

Dextrose (C₆H₁₂O₆, 99.9%, M. W. 180, Rankem, India), vanadyl sulphate (VOSO₄, 97%, M. W.163, SRL, India), ascorbic acid (C₆H₈O₆, 99%, M.W. 176.1, CDH, India), methylene blue (C₁₆H₁₈ClN₃.2H₂O, M.W. 319.86, Thomas Baker, India) and sodium hydroxide (NaOH, 82%, M.W. 40, CDH, India) were used as received.

Synthesis

Ten mL aqueous solution of VOSO₄ (0.01 M) was added to dextrose solution of the same concentration and was made alkaline with sodium hydroxide. This bluish turbid mixture was then heated for 30 min at nearly 100 °C with continuous stirring. Black VO₂ NPs thus formed were washed several times with distilled water until the washings became neutral and showed no trace of dextrose or VOSO₄. They were centrifuged at 2000 rpm and the solid was then dried in an oven below 100 °C and stored in a glass container.

It was observed that when a slightly acidic VOSO₄ was neutralized with NaOH, the solution turned dark blue and gave a precipitate. But when it was made highly alkaline (pH 12) it turned light orange and on keeping for 24 h the colour disappeared. However, the yield of VO₂ NPs increased when the mixture of VOSO₄ + Dex + NaOH was only slightly alkaline. When Dex was added to VOSO₄ in an alkaline medium, vanadium (v) was reduced to vanadium (iv) leading to the formation of VO₂ NPs as shown in the following equations;

4

5

Materials characterizations

UV–vis spectrum of the NPs was scanned with Motras UV Plus double beam spectrophotometer working at a resolution of 1 nm. FTIR spectrum was recorded with PerkinElmer spectrometer version 10.400 in 500–4000 wave number region. Size distribution of VO₂ NPs was determined with MALVERN Dynamic Light Scattering instrument (DLS) with Zetasizer version 6.20 at room temperature. SEM micrograph was recorded with a JEOL, JSM6510LV equipment. EDX analysis was done with an Oxford instrument, Inca model. The spectrum was recorded in the spot profile mode by focusing electron beam on to a region on the surface coated with NPs. XRD pattern of the sample was recorded using Shimadzu Lab-X XRD-6100 advanced diffractometer with an accelerating voltage of 40 kV and current 30 mA in the 2θ scan range of 20 to 70° at a scan rate of 6 degrees/min. Zeta potential of VO₂ NPs was determined at 25 °C at 23.7 Kcps. Thermal analysis of the sample was carried with Simultaneous Thermal Analw3yzer STA-8000 Perkin Elmer instrument, in the temperature range of 30 to 1000 °C (303 to 1273 ^oK) at a heating rate of 20 °C/min in nitrogen atmosphere. Catalytic activity of the VO₂ NPs has been studied in the degradation of MB by ascorbic acid as reducing agent.

Results and discussion

UV–vis spectrum

A sharp absorption peak at 227 nm (Fig. 1) suggests the formation of VO₂ NPs. This peak has been ascribed to d-d charge transfer of one electron in the lower energy t₂g level to higher energy eg level due to excitation of an electron under ultraviolet light as shown in Fig. 2.

Fig. 1.

UV–vis spectrum of VO₂ NPs

Fig. 2.

Excitation energy of electron under UV light

Guha and co-workers [16] have also obtained UV–vis spectrum of VO₂ NPs synthesized from Azadirachta indica leaf extract at 261 nm. It has been assigned to π to π transition between vanadium and oxygen. It should be noted that it is simply a charge transfer from one d orbital to another d orbital of vanadium which is also the characteristic of transition metal ions containing at least one electron in its d orbital. Also, they have suggested the presence of V⁵⁺ from IR and UV–vis spectra which is fundamentally incorrect because V⁵⁺ ion is devoid of any electron in its d orbital and should be colourless. However, if any colour appears due to crystal defect it would appear in the visible region only. It is therefore, dubious if the NPs are VO₂ or V₂O₅.

VO₂ NPs derived from plant extract contain some organic substances which influence their absorption in UV–vis region. Since we have synthesized VO₂ NPs from dextrose and VOSO₄ in alkaline medium it contains no material other than VO₂ NPs. When its colloidal solution was left over night, it settled at the bottom of the container. Various people have reported absorption of VO₂ NPs at different wavelengths but all of them contain some bio-molecule [17].

Derkaoui et al. [18] have reported the UV–vis spectrum of hydrothermally prepared VO₂ nanobelts which displays a peak at 370 nm. They have suggested that VO₂ NPs are a mixture of tetrahedral and octahedral species. However, a single absorption peak cannot be ascribed to different structures. Deepika et al. [19]. have also reported a peak in UV–vis region at 366 nm for VO₂ nano rods of 10–40 nm synthesized from Tinospora cordifolia. However, their composition has not been well established.

FTIR spectrum

On the basis of selection rules the VO₂ NPs should exhibit four normal vibrations in the lower region of FTIR spectrum but all of them may not be infrared active. Deepika et al. [19] have suggested that peaks in 530–470 cm⁻¹ region are due to V–O–V and V–O bending vibrations on the basis of their study on VO nano rods [19]. However, they have not mentioned the number of allowed vibrations. VO species cannot exhibit V–O–V vibrations because it requires two vanadium atoms bridged through oxygen atom as in V₂O where vanadium exists in + 1 oxidation state. In our case we have observed only one prominent vibration [Fig. 3] exactly at 500 cm.⁻¹

Fig. 3.

FTIR spectrum of VO₂ NPs

which has been assigned to V=O stretching frequency of VO₂ NPs. Two of the four normal modes of VO₂ are doubly degenerate similar to O–C–O molecule and therefore, only three vibrations are expected to appear. However, only two vibrations are infrared active because VO₂ is a linear molecule similar to CO₂. The two vibrations appearing at 500 and 1071 cm⁻¹ have been assigned to V–O and V=O stretching frequencies [20].

Generally, the metal oxides such as VO₂⁺, MoO₂⁺ and WO₂²⁺ display sharp peaks in 1000- 850 cm⁻¹ range [21]. Only one M=O vibration is observed if oxygen is trans to each other but two bands appear if they are cis – dioxo compounds. It has been reported that cis VO₂⁺, shows two peaks, one (V=O) antisymmetric band in 907–676 cm⁻¹ and a symmetric vibration in 922–910 wave number region which are closely spaced [22].

The bending and stretching vibrations reported by other workers for VO₂ NPs are not certainly due to V–O of VO₂, V₂O₃ or V₂O₅ alone. Since the NPs synthesized from plant extract contain organic ingredients which display vibrations in the above range but mistaken for VO₂ vibration. We have not used plant extract for the synthesis of VO₂ NPs and hence we observed only two peaks. It has also been supported by a sharp vibration at 500 cm⁻¹ and another one of medium intensity at 1071 cm⁻¹ assigned to V=O stretching frequency. We are therefore, encountering cis and trans isomerism, for the first time, in the case of VO₂ NPs which has seldom been reported (shown below).

Raman spectroscopy

Raman spectroscopy is a method to study vibrational spectra of molecules. A vibrational mode in a sample is said to be Raman active if the polarizability of the molecule is changed under electromagnetic radiation. According to the selection rule for a linear molecule, four vibrations should appear in the Raman spectrum. In Raman scattering incident photons impart a part of energy to excite the molecules in the sample and scattered with reduced energy. The energy of scattered photons was recorded as Raman shift of the VO₂ NPs at room temperature [Fig. 4]. The most intense peak appeared at 57.00 cm⁻¹, while the other peaks observed at 706.83 cm⁻¹, 905.46 cm⁻¹, and 1336.20 cm⁻¹ are weak. The energies of the scattered photons corresponding to these peaks are; 7.06, 87.64, 112.27 and 165.68 meV. The first one corresponds to microwave region and the remaining ones yield vibrational excitation energies in IR region. Low frequency peak has been suggested to be due to vibration between layers of NPs. The antisymmetric spectral peak at 905.46 cm⁻¹ is also IR active [23, 24].

Fig. 4.

Raman spectrum of VO₂ NPs

XRD

The XRD pattern [Fig. 5] reveals peaks around 24°, 28°, 33° and 50° corresponding to VO₂ (B) phase [Table 1]. These peaks have been well matched with JCPDS card no. 65–7960, and exhibited monoclinic structure with space group C2 /m.

Fig. 5.

XRD of VO₂ NPs

Table 1.

Peak position, Miller indices hkl and interplanar spacing of VO₂ (B) nanoparticles

Peak positions (2θ) Degree	(hkl) Miller indices	Interplanar spacing (d) (Å)
24.54	(110)	205
27.97	(002)	180
32.84	(310)	154
41.38	(212) -VO2(M)	122
49.84	(020)	101

An additional peak at 41.38° corresponds to VO₂ (M) polymorph which matched with JCPDS card no. 72–0514. They also exhibited monoclinic structure with space group P2₁/c. The highest intensity peak at 24.547° was used to calculate the crystallite size of the nanoparticles employing the Debye–Scherrer’s equation;

6

where constant = 0.93, = 1.541 nm, is the Bragg’s diffraction angle and (1.667) is the full width at half maximum intensity of the peak in radian. The average crystallite size was found to be about 5 nm. The strongest peak with the highest intensity and narrow width suggested the high purity and crystalline nature of the NPs. Small size NPs are excellent antibacterial agent. They also work as catalyst in the degradation of commercial dyes without losing their activity as has been found in several cases [15, 25].

SEM

Colloidal solution of VO₂ NPs was dropped on a carbon coated grid and images were recorded at different resolutions. The flake like NPs with irregular shape and uneven boundaries are staked together. SEM images of VO₂ recorded at 10,000 resolution looks like a corrugated mica sheet [Fig. 6] which does not have a definite shape.

Fig. 6.

A, B, C, D: SEM images of VO₂ NPs

We have observed only two phases with similar morphology of VO₂ NPs/crystal similar to those reported by Li et al. [26]. They have also obtained granular and plate like morphology for VO₂ NPs at different temperatures for thermochromic application. At higher temperature (280 °C) they obtained only granular shaped VO₂ NPs of 20–50 nm. TiO₂ NPs similar to VO₂ NPs have also been found to act as excellent photocatalyst for the degradation of industrial dyes [15]. It has been noted that irregular shape and roughness of the VO₂ NPs indicates larger surface area compared to those with smooth surface. Such nano particles are more effective in catalytic reaction, drug delivery and electrical conductivity.

EDX

The NPs, in our case are a mixture of two phases which has also been confirmed from XRD profile [Fig. 7]. The per cent oxygen is relatively higher than expected for pure VO₂ NPs which is probably due to adsorption of atmospheric oxygen on the surface of VO_2. VO₂ undergoes reversible metal to insulator phase transition at near ambient temperature (340 °K). Upon phase transition VO₂ (M) with a monoclinic crystal structure changes to VO₂ (R) with a rutile type tetragonal one occupied with drastic changes in optical properties [27]. The nano films of smaller VO₂ NPs are better candidate for semiconductor to metal transition than larger microcrystals of 290 nm thickness. Khan and Bilal [28] have also obtained EDX pattern for VO₂ similar to that observed by us. In such cases optical band gap widens with a consequent shift of a wavelength of 443 nm in the visible region to 360 nm in the UV region. Presence of carbon shown in the EDX spectrum may be due to accumulation of CO₂ as impurity [29].

Fig. 7.

EDX profile of VO₂ NPs

Zeta potential

Zeta potential is a measure of electric charge on the surface of nano particles. It is known that its value in − 10 mV to + 10 mV range is indicative of a neutral nature while a value greater than + 30 mV or less than − 30 mV is considered to be anionic and cationic, respectively. In our case [Fig. 8], zeta potential of NPs, as measured by DLS technique was found to be − 3.10 mV and hence the VO₂ NPs are considered to be neutral. They are thus not agglomerated.

Fig. 8.

Zeta potential distribution of VO₂

Thermal analysis and specific heat

The heat flow through the sample was recorded with respect to the temperature. The plot showed very insignificant peaks [Fig. 9]. Utilizing the difference in heat flow, the specific heat capacity (C_p) was determined by the following equation,

7

where, ɸ_meas-ɸ_base is the heat flow through the sample, m is the mass of the sample and β is rate of heat flow at 293 °K/min. There is an increase in heat flow between 353–513 °K after which it becomes stable showing nearly a straight line. It obviously suggests very high stability for VO₂ NPs up to 1250  K. It has been observed that the specific heat of the sample increases with increase in temperature [Fig. 10].

Fig. 9.

VO₂ heat flow versus temperature plot

Fig. 10.

Specific heat versus temperature plot of VO₂ NPs

Catalysis

Metal oxide NPs are frequently used as photocatalyst to scavenge toxic metals and dyes from polluted water. MB is a cationic dye commonly used in textile industries.

Since untreated effluent from factories pollute the water bodies, it is essential to decompose them before they are released in the environment. Biodegradation of MB by ascorbic acid (AA) catalyzed by VO₂ NPs has been studied at room temperature. UV–vis spectra of 10 ppm aqueous solution of (1) MB (2) MB + AA and (3) MB + AA + VO₂NPs were scanned in 200–700 nm [Fig. 11]. The dye concentration, its volume and amount of catalyst (5 mg) were kept constant. MB + AA solution was slightly acidic (pH 6) and showed 60% decolouration after10 min while MB + AA + VO₂ NPs showed 79% reduction at the same pH. However, the colour reappears after shaking for 10 s, although the intensity was low. When the two solutions were left for 24 h, they were decolourised again but on shaking, they regained the blue color. However, MB was completely degraded in highly alkaline medium. It was observed that when a change in pH from acid to basic was made, the rate of degradation was enhanced quickly which is contrary to the observation made by Modi et al. [30]. The decolourization was reported to be enhanced regardless of the pH after 72 h. In our case, the decolourization occurred in a few seconds when the pH was raised to 12. It is quite obvious that the degradation is pH-dependent and consumes less time [31].

Fig. 11.

a: UV–vis spectrum of MB, b: UV–vis spectrum of MB + AA, c: UV–vis spectrum of MB + AA + VO₂ NPs

All the solutions below pH 12 were not completely degraded even after 24 h [30]. The intensity of absorption maxima, in each case, was diminished with time with a negligible shift in wavelength of absorption. It is ascribed to the degradation of MB with the formation of colorless species [32]. Tamoradi and co-workers [31] have also used VO₂ NPs in the oxidation of sulfides. It is highly effective as a heterogeneous catalyst. Also, the MB + AA + VO₂ showed some turbidity at pH 12. It seems that precipitation of some fragments of MB occurred similar to that reported by Houas et al. [33]. The solutions (1) MB + AA and (2) MB + AA + VO₂ did not show further change in colour even when they were left for up to 10 days.

TiO₂ NPs incorporating PANI polymer have been shown to have excellent photo catalytic activity (98.77%) against the degradation of MB dye. It has been observed that porosity of TiO₂ NPs enhances the rate of degradation of dye [15]. Decomposition of MB by ZnS and ZnS-PVP has been studied under UV light. It was observed that PVP-ZnS is a better photocatalyst than ZnS alone as it takes less time. PVP-ZnS NPs have also been shown to be antibacterial against some bacterial strains. The activity increases with increasing concentration of NPs [34]. Biosynthesized ZnO NPs of 20.87 nm with Wurtzite crystalline phase have shown 99.8% degradation of Malachite green in 160 min under UV radiation. They also exhibited antimicrobial activity against gram positive and gram negative bacteria [35]. Small TiO₂ NPs (15.6 nm) fabricated from plant Carica papaya leaf extract has been demonstrated to act as photocatalyst for the degradation of RO-4 dye in 180 min. under UV light. However, it is not as efficient as VO₂ NPs for the degradation of MB under day light [36].

Mechanism of photodegradation of MB

The photodegradation occurs in the following three steps.

1. Light absorption and charge carrier generation: When the MB is irradiated with sunlight, it absorbs photons (hν) with energy equal to or greater than its band gap. This energy excites electron from valence band (VB) to conduction band (CB), creating electron-hole (e⁻/h⁺) pair. The ejection of electrons leaves hole (h⁺) in the valence band.

Reaction

This step is essential as it initiates the photocatalytic activity.

• (2)Generation of reactive oxygen species (ROS): The photogenerated charge carriers (e⁻ and h⁺) migrate to the surface of catalyst where they react with adsorbed species (O₂ and H₂O) to generate the ROS. This ROS plays a key role in the degradation of dye.

Reduction of oxygen (O₂): The conduction band electrons reduce molecular oxygen in the aqueous solution to form superoxide radical (·O₂⁻).

e⁻ + O₂ → ·O₂ ⁻

Oxidation of water : The valence band holes (h⁺) oxidize water to hydroxide ion, (OH⁻) to form highly reactive hydroxyl radical (·OH)

These radicals are powerful oxidants capable of attacking complex dye molecules.

• (3)Dye degradation:

The generated ROS initiate the oxidative degradation of MB through,

Cleavage of chromophores : The conjugated structure of the dye responsible for its color is broken down.

Fragmentation: The dye molecules are further oxidized into smaller and less harmful inorganic compounds such as CO_2, H₂O and mineral acids.

Overall reaction

The degradation ultimately leads to decolorization and mineralization of the dye, contributing to the purification of waste water.

The degradation of MB is proportional to its absorption on the surface of the catalyst (VO₂ NPs). It produces ROS upon exposure to sunlight or UV light which breaks down MB to nontoxic material such as CO₂ and H₂O (Fig. 12).

Fig. 12.

Excitation of electrons from valence to conduction band

Electron–hole (e⁻/h⁺) pairs are generated during irradiation of MB solution containing VO₂ as catalyst. The excited electron in the conduction band reduces dissolved oxygen to superoxide radical and the valence band hole oxidize water molecules to hydroxyl radicals (). These two species thus generated break down of the MB which is evident from the discoloration of the dye. We have found from Tauc’s factor versus photon energy plot (Fig. 13) a large band gap between valence and conduction band. The tangent drawn showed E_g = 3.3 eV. This band gap indicates very small probability of recombination of electrons which increases the efficiency of the catalyst. In the present work the high degradation of MB (97.23%) catalyzed by VO₂ NPs is an evidence of increased efficiency of catalyst owing to large band gap (Fig. 13).

Fig. 13.

Tauc’s factor versus incident photon energy plot

A single absorption band in the UV–vis spectrum (Fig. 1) also corroborates that exposure of the VO₂ to sunlight/UV-light accelerates the degradation of MB. However, the degradation of the dye is faster under UV light than in sunlight. This property can be utilized in waste water purification and antimicrobial application as has been reported in a recent paper using TiO₂ NPs as catalyst in the breakdown of MB [37]. Since TiO₂ and VO₂ are identical they are expected to follow the same mechanism.

Antibacterial screening of VO₂ NPs

Antimicrobial study was done using Agar well and Disk Diffusion method on six gram positive bacteria viz. Staphylococcus aureus (S. aurues), Streptococcus viridans (S. viridans), Staphylococcus pyogenes (S. pyogenes), Bacillus cereus (B. cereus), Streptococcus epidermidis (S. epidermidis) and Corynebacterium xerosis (C. xerosis) and four gram negative bacterial strains, Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Proteus vulgaris (P. vulgaris) and Pseudomonas aeruginosa (P. aeruginosa). The efficacy of the VO₂ NPs was compared with the standard drug Amikacin (30 µg). The solid media, namely Nutrient Agar No.2 (NA) (M 1269S-500G), was used for preparing nutrient plates. Nutrient Broth (NB) M002- 500G, was used for the liquid culture media.

The micro-organisms were grown in NB-agar by pour plate technique. The VO₂ NPs were dissolved in 1% dimethyl sulfoxide (DMSO) and made stock solution with the concentration of 10 mg/mL. They were sterilized by filtration through syringe filter PES 0.22 µm Millipore express filter. The microorganisms were seeded in nutrient agar medium. Wells of 6 mm diameter were punched on the solidified NB-agar plates with a sterilized cork borer and 100 µL of controls and VO₂ NPs were loaded on to them. About 1% DMSO solution was taken as control and the Amikacin (30 µg)/disk was kept as standard. The inoculated plates were incubated at 37 °C for 24 h. Zone of Inhibition (ZoI) after 24 h of incubation was taken as the end parameter to assess the antibacterial effect of sample. The ZoI values less than 7 mm was defined as no inhibition zone (NIZ). All experimental procedures were done in triplicate using fresh cultures of the clinical isolates.

The toxicity of VO₂ NPs towards the bacterial strains was found to be highly significant [Fig. 14]. The growth inhibition among gram positive strains was maximum for S. epidermidis (14 mm) followed by C. xerosis (13 mm), B. cereus (12 mm), S. aureus (11 mm), S. viridans (11 mm), and S. pyogenes (8 mm). The highest zone of inhibition of gram negative bacteria was recorded against P. vulgaris (22 mm) which is indicative of greater efficacy of NPs than even the standard antibiotic, Amikacin.

Fig. 14.

Growth inhibition zone of bacterial colony by VO₂ NPs

Mechanism of antibacterial action

The NPs disrupt the activity of microbial cell by forming a complex with proteins and enzymes present within the cell which leads to cell death [38]. It has been shown [16] that VO₂ NPs inhibit the activity of many bacterial species which is dependent on its concentration. In our case, the highest zone of inhibition was recorded at 5 mg/mL which appears to be the optimum concentration to inhibit the replication of microbes. Fukuda et al [39]. have suggested that the NPs disrupt the transport channel of bacterial cells and enhance K⁺ efflux. Natalio et. al. [40]. have already demonstrated that the NPs induce oxidation of halides and deactivate microbial cells. It is known that gram negative bacterium has negatively charged thin peptidoglycan layer and lipo-polysaccharide layer which have great affinity for positively charged VO₂ NPs. It leads to enhanced interaction between microbial cells and NPs leading to their death (Table 2). On the other hand the gram positive bacteria have layer of organic acids which interact with the NPs covering their entire surface. The cell wall is thus damaged causing their death. We need NPs which are toxic to pathogens and inhibit their growth but are harmless to normal host cells.

Table 2.

Antibacterial Screening of VO₂ NPs against bacterial strains

S.No	Bacterial strain	Test sample (S)	Negative control (C)	Positive control
		Zone of inhibition (in mm)
1.	S. aureus	11	6	14
2.	S. pyogenes	8	6	15
3.	S. epidermidis	14	6	16
4.	S. viridans	11	6	18
5.	B. cereus	12	6	15
6.	C. xerosis	13	6	14
7.	E. coli	8	6	14
8.	K. pneumoniae	9	6	18
9.	P. vulgaris	22	6	15
10.	P. aeruginosa	11	6	15

Antimicrobial activity of smaller NPs is always greater than the larger ones owing to their large surface area. However, it is worth noting that the VO₂ NPs in our case are smaller than the virus/bacterium (60–140 nm) and therefore, they can easily penetrate the microbial cell wall and inhibit their growth by arresting the activity of metabolic functions [41]. It is also known that the efficiency of VO₂ NPs to inhibit the multiplication of microbes is enhanced if it is doped with any other oxide NPs such as ZnO [42].

Limitation and future prospect

Biologically or chemically synthesized VO₂ NPs are promising functional materials with applications in optical devices, antibacterial systems, and catalysis. Their characteristic semiconductor-to-metal transition at approximately 68 °C enhances their utility in thermally responsive technologies, making them attractive for temperature-dependent optical switching. However, despite their advantages at ambient conditions, challenges related to particle size, morphology control, and long-term stability, limit their broader practical use. Nevertheless, there remains substantial potential for VO₂ NPs in advanced optical systems and thermochromic smart coatings, particularly for energy-efficient windows and adaptive optical filters. Additionally, owing to the multiple oxidation states of vanadium (V²⁺ to V⁵⁺), VO₂ NPs hold promise for redox-driven applications, expanding their relevance in catalytic and energy-conversion processes.

Conclusion

VO₂ NPs of 5 nm were fabricated from vanadyl sulfate and dextrose as reducing agent in slightly alkaline medium by heating the mixture below 100 °C. Dilute solutions were taken to prevent their aggregation. They were stable up to 1200 °K and exhibited monoclinic and rutile type structure. They were packed in layers with rough surface. The NPs exhibited Cis- and Trans isomerism which is a rare stereochemical phenomenon. The VO₂ NPs acted as heterogeneous catalyst and were exploited in the degradation of MB under sunlight. About 97% MB was decomposed within few seconds and the rate was accelerated with increasing pH attaining the highest value at pH 12. Photocatalytic decomposition of dyes and industrial wastes may therefore, be made with VO₂ NPs to prevent environmental pollution and soil erosion. Further effort may be made to explore new application in agriculture, medicine and industries. Photocatalytic potential of VO₂ depends on the size, morphology, surface area and pH of the solution. While studying the degradation of dyes, such factors may always be considered to increase the efficiency of the catalyst. The NPs showed significant antibacterial activity against both gram positive and gram negative bacteria. Application of 5 mg/mL of VO₂ showed highest zone of inhibition. In the case of P. vulgaris, the NPs showed better activity than the standard antibiotic. They may therefore, be used to inhibit their growth in infected cells. The thermal analysis showed that the NPs were highly stable and can be stored even at high temperature. Effort may be made to synthesize VO₂ NPs of different shapes and size to explore more applications in medicine agriculture and industries.

Author contributions

KSS:Conceptualized the idea and wrote and edited the manuscript MS: Conducted thermal analysis, analysed the results and edited the manuscript. ZF and MSz: Conducted the catalysis experiment AR: Recorded the UV–vis spectra and analyzed them. SR: Conducted the anti-bacterial study.

Funding

The authors declare that no funds or grants were received from any funding agency during the course of experimental work and preparation of the manuscript.

Data availability

All the data generated in this study have been used and is available in the present manuscript.

Declarations

Ethics approval and consent to participate

Not applicable — human or animal were not used in this study.

Consent for publication

All authors have read and approved the manuscript for its submission for publication.

Competing interests

The authors declare no competing interests.