Synthesis, photocatalytic and antidiabetic properties of ZnO/PVA nanoparticles

A series of ZnO and ZnO/poly(vinyl alcohol) (PVA) catalysts were prepared using sol–gel method. An X-ray diffraction analysis confirmed the existence of the wurtzite ZnO phase, and scanning electron microscopy (SEM) observation revealed the formation of spherical ZnO and ZnO/PVA nanoparticles. The decomposition of methylene blue (MB) and methyl orange (MO) induced by the synthesized pure ZnO and ZnO/PVA nanoparticles was studied under ultraviolet–visible irradiation. Among the catalysts evaluated, ZnO/5PVA was the most active in the decomposition of MB, whereas ZnO/7PVA was the most active catalyst in the decomposition of MO. Moreover, an investigation of the biological activity of pure ZnO and ZnO/PVA indicated that ZnO/5PVA exhibited the best performance in lowering the glucose level in diabetic rats.

Characterization. X-ray diffraction. X-ray diffraction (XRD) patterns were recorded on a PW 150 diffractometer (Philips) using Ni-filtered Cu Kα radiation in a 2θ range of 10°-80° to investigate the formation of ZnO, the ZnO/PVA nanoparticle phases, and the crystal size in a nondestructive manner 23 .
Ultraviolet-visible spectrometry. The wavelength and absorbance values of the prepared photocatalysts obtained using an ultraviolet-visible (UV-Vis) reflectance spectrophotometer was utilized to determine the bandgap values from Kubelka-Munk equation 24,25 .
Scanning electron microscopy. The sizes of the ZnO and ZnO/PVA particles were determined using scanning electron microscopy (SEM) analysis on the JSM-5900 LV microscope with an accelerating voltage of 20 kV 26 .
Fourier transform infrared spectroscopy. The Brönsted and Lewis acid sites of ZnO and ZnO/PVA were investigated through Fourier transform infrared spectroscopy (FTIR) analysis on a Bruker spectrophotometer.
Potentiometric titration. Nonaqueous potentiometric titration was used to determine the total number of acid sites of the samples 27 . First, 0.1 g of a catalyst was activated by heating under vacuum for 2 h. Then, it was immersed into 10 mL acetonitrile for 3 h to be adsorbed on the active sites, followed by the titration of this suspension against a 0.01 N n-butyl amine solution (0.05 mL/min). An Orion 420 digital model device was utilized to determine the variation of the electrode potential.
The MB and MO degradation in water using UV-Vis light irradiation. The photocatalytic activities of the ZnO and ZnO/PVA nanoparticles were estimated by conducting degradation experiments at 20 °C using an external lamp (Halogen Mercury lamp, 400 W UV/Vis lamps).
Antidiabetic activity. Before the experiment, 12 albino rats (100-120 g) were kept for adaptation under normal laboratory conditions for 7 days. All rats were fed on a balanced basal diet and allowed free access of water.
Induced of diabetes mellitus. The experimental was then induced on the rats, which were divided randomly into two major groups: Group 1 (control diabetic rats): Four rats received a normal diet for 30 days without any treatment. Group 2: Eight rats were fasted for 24 h and then intraperitoneally injected with streptozocin freshly prepared in a 0.1 M citrate buffer (pH 4.5) at a dose of 4.5 mg/100 g of body weight to induce diabetes mellitus according to a reported method 28 . Then, the rats were fasted for 18 h before determination of the serum glucose level. Rats with serum glucose levels over 250 mg/dL were considered as streptozocin diabetic rats and ready for treatment with ZnO and ZnO/PVA.
The eight diabetic rats of Group 2 were divided randomly into two series (four rats each) as follows: www.nature.com/scientificreports/ Heparinized tubes with blood samples were collected from the eye canthus of the rats every 5 days after starting the administration of the extracts. Then, each blood sample was centrifuged to obtain a clear serum and determine immediately glucose levels of fasting animals.
• All experiments were performed in agreement with regulations of the Institutional Animal Ethics Committee of Mansoura University, Mansoura, Egypt, which are in accordance with the "Guide for the Care and Use of Laboratory Animals" published by the National Academy of Sciences. • The study was conducted in compliance with the ARRIVE guidelines.

Results and discussions
X-ray analysis. The effect of the calcination temperature and the PVA concentration on the crystal phase and the crystal size of the synthetized samples was determined on the basis of the XRD patterns (Figs. 2 and 3). All samples exhibited typical hexagonal structures (JCPDS card no.   29 , and no phases attributable to PVA were observed. As can be seen in Fig. 2, the intensity of the (101), (002), and (100) peaks at 2θ = 36.1°, 34.3°, and 31.6°, respectively, decreased with increasing PVA content as a result of the increase in the crystal deformation. The (101) ZnO peak at 2θ = 36.1° was used to measure the crystal size of the ZnO/PVA samples. By increasing the quantity of PVA from 35-18 to 21-18 nm, a decrease in the crystallinity and the crystal size of ZnO was observed. In addition, increasing the PVA content led to a decrease in the mean grain size because PVA covered the crystalline surface of ZnO, causing a decrease in the diffraction intensity. Subsequently, further reduction in 10     www.nature.com/scientificreports/ the crystal size occurred as the PVA substitution with Zn 2+ ions increased 30,31 . Figure 3 indicates that at 300 °C, a peak appearing at 2θ = 25.1° can be assigned to Zn oxalate hydrate with a monoclinic structure, and that at 2θ = 37.7° corresponds to Zn oxalate with a monoclinic structure. By increasing the calcination temperature of ZnO/5PVA to 400 °C and 500 °C, the intensity of the (101), (002), and (100) peaks at 2θ = 36.1°, 34.3°, and 31.6°, respectively, corresponding to an hexagonal structure increased because the PVA content decreased upon the increase of temperature. SEM analysis. Figure 5 displays SEM images of selected samples of ZnO and ZnO/PVA photocatalysts with various PVA contents (5%, 7%, and 10%), in which agglomeration can be observed for all the samples to some extent. This particle aggregation may lead to the difference observed in the particle size obtained through the XRD and SEM analyses for ZnO and ZnO/PVA 33 . The SEM images revealed that the hexagonal phase increased with increasing PVA content. Small crystals with various phases were interwoven with each other, creating strongly bound nanoclusters 34 . SEM was also used to confirm the distribution and size of ZnO in the polymeric matrix. ZnO appeared as white spots. It is obvious that the ZnO particles were well spread in the polymeric matrix regardless of the type of ZnO. This revealed that good adhesion between the surface of the ZnO nanoparticles and the polymeric matrix was achieved by modifying the organic surface of the ZnO nanoparticles. Acidity test. Potentiometric titration was utilized to estimate the number of surface acid sites of solid catalysts 35,36 . Neutralization of the surface acid sites was conducted by adding n-butylamine, and the electrode potential (mV) was evaluated as a function of the increasing concentration of n-butylamine (mmol/g catalyst) 37 . As can be extracted from Fig. 6, when the PVA content was increased up to 10%, the total number of acid sites increased. The amount of n-butyl amine/g used for the neutralization of the surface acid sites of the solid catalysts as a function of the PVA content at 500 °C and the total number of acid sites/g are summarized in Table 1.

UV-Vis absorption spectroscopy.
The total number of acid sites of the solid catalysts/g was determined using the following equation: N, Avogadro's number.
Total number of acid sites/g = mL equivalent/g × N × 1000,      Table 1 reveal that ZnO exhibited lower photocatalytic activity and higher number of acid sites than those of ZnO/PVA. For the MB cationic dye, the low photocatalytic activity of ZnO was probably due to (1) fast recombination of electrons and holes and (2) participation of a small part of these electrons and holes in the photocatalytic reaction 39 . Among the ZnO/PVA catalysts, ZnO/5PVA proved to have the optimum doping level leading to the maximum photocatalyst activity, which stems from the influence of PVA in decreasing the recombination rate and creating a new energy level in ZnO 24 . The photocatalytic activity decreased with the increase in the PVA content beyond 5%, as illustrated in Fig. 7. This is due to the high PVA content increasing the electron-hole recombination 40,41 .
Effect of the calcination temperature on the MB degradation. The SEM, XRD, and UV-Vis analyses proved that the temperature of calcination has a significant effect on the bandgap and structure of the catalysts, eventually affecting the photocatalytic performance. Figure 8 reveals the photocatalytic activity of ZnO/5PVA for the photodegradation of the MB cationic dye at calcination temperatures of 300 °C, 400 °C, and 500 °C. The ZnO/5PVA nanocomposite exhibited the best photocatalyst activity at a calcination temperature of 300 °C, as confirmed by the SEM and XRD results.
Effect of PVA concentration on MO degradation. Figure 9 indicates the activity of the different catalysts prepared from ZnO and diverse PVA concentrations calcined at 300 °C for the degradation of the MO anionic dye. www.nature.com/scientificreports/ Among the ZnO/PVA nanocomposites, ZnO/7PVA exhibited the optimum doping level to obtain the maximum photocatalytic activity. It was also found that the photocatalytic activity decreased upon increasing the PVA content beyond 7%.
Effect of the calcination temperature on the MO degradation. Figure 10 reveals that ZnO/7PVA calcined at 300 °C exhibited the best photocatalytic activity for photodegradation of the MO anionic dye among the photocatalysts evaluated.

Biological activity
The effect of the catalysts on the blood glucose levels of the two series of rats described in Sect. 2.5 was evaluated considering the following factors 28 .
Blood glucose level and the number of days at a constant PVA content. The results summarized in Fig. 11 and Table 2 reveal that the lowest glucose level was achieved by the ZnO/5PVA catalyst calcined at 300 °C for the two series.  www.nature.com/scientificreports/ Blood glucose level and PVA content at a fixed number of days. Figure 12 reveals that ZnO/5PVA afforded the lowest glucose level after 20 days for the two series.

Conclusion
ZnO and ZnO/PVA nanoparticles were prepared using the sol-gel method. The formation of the hexagonal phase of polycrystalline ZnO and ZnO/PVA was confirmed by an XRD analysis. Meanwhile, an SEM analysis indicated that the ZnO and ZnO/PVA nanocomposites have a spherical shape. Photocatalytic experiments indicated that ZnO/5PVA is the most photocatalytically active catalyst for degradation of the MB cationic dye, whereas ZnO/7PVA is the most active for degradation of the MO anionic dye. The best results were obtained at a calcination temperature of 300 °C. Potentiometric titration proved that ZnO/10PVA had the highest number of total acid sites. As revealed by a biological activity study, ZnO/5PVA calcined at 300 °C was the best catalyst for decreasing the glucose level in diabetic rats. Therefore, it can be concluded that loading PVA onto the surface of ZnO improved the photocatalytic and biological properties of pure ZnO.