Nanomaterial of rare earth orthovanadates (RVO4) have been broadly considered in recent years due to their imperative properties. Amongst them, CeVO4 NPs have a tetragonal zircon-type structure belonging to the I41/amd space group. Nowadays, CeVO4 nanoparticles are utilized for potential applications in various fields such as photoelectric , gas sensor , luminescence  electrochromic material [4, 5], electrochemical sensor , photocatalyst , antibacterial agent , lubricating additives , and batteries fields . Considering the magnificent chemical and physical properties of CeVO4 nanoparticles, it was critical to seek a facile, rapid, efficient and energy saving preparation method. Heretofore, some notable methods were used for the synthesis of CeVO4NPs. Accordingly, a variety of techniques, such as electrospinning , sol-gel method , microwave assisted synthesis , precipitation method , hydrothermal method [2, 14], solid-state reaction method , sonochemical method , and ultrasound method  have been reported. Therewith, mixed metal oxide NPs can be used as an anti-biotic, antioxidant, pesticide formulation, antimicrobial and antifungal agent [18-24].
Sol–gel auto combustion is a swift and economically affordable synthetic route for the fabrication of nanomaterial and has been widely used for the fabrication of a variety of metal and metal oxide NPs, forming nano-sized, homogeneous, and highly reactive powders through mixing different elements at the atomic level.
In this work, we rapidly synthesized CeVO4 NPs by simple and efficient sol-gel auto combustion method using urea as a fuel agent. These synthesized nanoparticles were assessed for antifungal activities by employing against some selected fungal strains. It was found that efficiently synthesized CeVO4 NPs manifested good biomedical application in the field of nanomedicine.
Synthesis of CeVO4 NPs
All analytical purity grade reagents were used as received without any purification. In this investigation, CeVO4 powder was effectively synthesized by sol-gel auto combustion method using the precursor as Ce(NO3)3.6H2O, V(NO3)3.6H2O and urea as a fuel agent. Urea is an organic fuel providing a platform for redox reactions during the course of auto combustion because it possesses a high heat of combustion. Initially vanadium nitrate, cerium nitrate and urea are taken in the 1:1:4 stoichiometric ratio and homogenous paste was made. The paste formed was evaporated on hot plate at about 70 to 80 0C to get thick gel. This kept on a hot plate for auto combustion and heated at 180 to 190 0C. To obtain nanocrystalline CeVO4 powder, this was sintered at 800 0C for 4 h (Fig. 1). A fine dark brown colored powder was obtained and this was carefully collected for further characterization purposes.
The crystal phases and crystallinity of CeVO4 NPs were characterized by X-ray diffraction (XRD, Brukar, D8-Advanced Diffractometer) pattern measured with Cu-Kα Radiation (λ= 1.5406 Å) in the range of 10–80o. The morphology and composition of the synthesized CeVO4 NPs were examined by field emission scanning electron microscopy (JEOL JSM-6701), and FESEM coupled energy-dispersive X-ray spectroscopy (EDX, Bruker, XFlash 6I30). UV-vis DRS absorption spectra of CeVO4 NPs were performed using JASCO spectrophotometer V-770. Spectral analysis of photoluminescence was measured on FP-8200 spectrofluorometer. Functional group and structural properties was observed by Fourier transform Infrared (FT-IR) spectrum (FT-IR 4600).
In-vitro antifungal activity of the synthesized CeVO4 NPs
Antifungal activity of the synthesized CeVO4 NPs was examined against fungal strains (Candida albicans MTCC 227, Aspergillus niger MTCC 282, Aspergillus clavatus MTCC 1323, Trichophyton rubrum MTCC 296, Trichophyton mentographytes MTCC 8476, Epidermophyton floccosum MTCC 7880 and Microsporum gypseum MTCC 2819) using the agar dilution protocol . Determine the minimum inhibitory concentration (MIC), a stock solution of the synthesized CeVO4 NPs was prepared in dimethyl sulfoxide and then incorporated in a specified quantity of molten sterile agar, i.e., dextrose agar for antifungal screening. The inoculums was prepared by taking a stock culture to about 100 mL of nutrient broth, in 250 mL clean and sterilized conical flasks. The flasks were incubated at 27 °C for 24 h before use. The plates were kept in aseptic condition at room temperature (at least 2 h) to allow diffusion of the solution properly into potato-dextrose-agar medium. Then, the plates were incubated at 25 °C for 48 h. The highest dilution showing at least 99 % inhibition zone is taken as MIC. Greseofulvin was used as a reference drug for antifungal activity. The experiments were performed in triplicate in order to minimize the errors of whole method.
RESULTS AND DISCUSSION
Structural and crystallographic analyses
The synthesized CeVO4 NPs formation was affirmed by the characteristic peaks observed in the XRD profile, as shown in Fig. 2. Powder XRD of fabricated CeVO4 NPs was carried out using monochromatic CuKα1 radiation (wavelength 1.5406Å) in the angular range 2θ of 10-80 deg. XRD profile exhibited a series of diffraction peaks at 18.16º, 24.02º, 30.26º, 32.40º, 34.20º, 36.78º, 39.02º, 43.50º, 46.32º, 47.82º, 49.22º, 55.52º, 60.16º, 62.38º, 67.82º and 71.10º, corresponding to (101), (200), (211), (112), (220), (202), (301), (103), (321), (312), (400), (420), (332), (204), (224) and (512) crystal planes of tetragonal CeVO4 nanostructures (JCPDS No. 12-0757). The sharp XRD peaks indicated that the synthesized CeVO4 nanoparticles are good crystalline in nature. The size of the CeVO4 NPs formed were calculated using Debye–Scherrer’s equation which was around 45-95 nm.
Morphology of the synthesized CeVO4 NPs was examined by the FESEM technique. It can be seen that the average crystal grain size of the CeVO4 NPs was mainly 50-95 nm having quasi-hexagonal shape except slightly agglomeration (Figs. 3). This result exceeded to the literature result in which tetragonal structure of CeVO4 NPs was prepared by precipitation method .
The elemental composition of the synthesized CeVO4 NPs was analyzed by investigating the EDX, as shown in Fig. 4. This was carried out to understand the elemental composition of the vanadium, cerium and oxygen in the fabricated nanomaterial. There was no unidentified peak are observed in EDX. This quantitative data affirms the NPs purity, composition and formation of CeVO4 NPs.
UV–Vis diffuse reflectance spectrum and photoluminescence of CeVO4 NPs
Fig. 5 demonstrates UV-Vis DRS of CeVO4 NPs. It can be seen that the nanomaterial has good absorption capacity in the visible region (400–700 nm). Additionally, the band gap energy is the criteria of material selectivity for the photocatalyst. The plot of (αhʋ)2 versus photon energy (hʋ) was obtained to determine band gap of CeVO4 NPs (Fig. 6). The band gap was found to be 3.2 eV suggesting that the synthesized material using sol-gel auto combustion method is useful for photocatalytic applications. Fig. 7 indicates fluorescence spectrum with an excitation wavelength of 290 nm. The spectrum exhibited broad band peak of emission at 288 nm and 578 nm (yellow color).
To further affirm the formation of the CeVO4 crystal structure using FT-IR spectroscopy as shown in Fig. 8. The broad IR band at 810 cm-1 is attributed to the V-O-V vibrations of ReVO4. Residual -OH group and water are detected around 3465 cm-1, corresponding to the O-H stretching frequency due to the bending vibrational frequency of associated water. It suggested that the CeVO4 NPs was successfully fabricated by simply sol-gel auto combustion method.
Antifungal activity of CeVO4 NPs
The results of antifungal activity of the synthesized CeVO4 NPs are presented in Table 1. The antifungal activity of the synthesized CeVO4 NPs was determined in-vitro using an Agar plate method against selected strains viz. C. albicans, A. niger, A. clavatus, T. rubrum, T. mentographyte, E. floccosum and M. gypseum at different concentrations ranging between 100 μg/ml to 1250 μg/ml. Fabricated CeVO4 NPs exhibited a moderate activity against T. rubrum, T. mentographyte and evinced excellent activity against C. albicans at concentration of 250 μg/ml reference standard Griseofulvin at concentration 500 μg/mL.
In summary, we have successfully fabricated tetragonal CeVO4 NPs via sol-gel auto combustion method to obtain biologically active nanomaterial. The synthesized CeVO4 NPs were quasi-hexagonal in shape as observed in FESEM analysis. The DRS spectrum confirmed that the synthesized CeVO4 NPs have a high absorption with 3.2 eV band gap. The synthesized CeVO4 NPs has shown excellent antifungal activity against Candida albicans fungal strain and hence it may be useful for the treatment of Candidiasis. This result provides useful information of CeVO4 NPs having no any side effects and play significant role in nanomedicine.
Author SG are thankful to Jaysingpur College, Jaysingpur, Savitribai Phule Pune University and Microcare Laboratory Gujrat for providing the technical, instrumental and biological activities supports.
CONFLICT OF INTEREST
The author declare that there is no conflict of interests regarding the publication of this manuscript.