Uv 2550 spectrometer

ZnO and TiO₂ (analytical grade, purity ≥98%) (Sinopharm, Shanghai, China) were used for synthesis of ZnONPs and TiO₂NPs, respectively. ZnO solution (0.5 M) and TiO₂ solution (0.5 M) were prepared by dissolving ZnO (4.07 g) and TiO₂ (4.00 g) separately in ethylene glycol (10 mL) (Sinopharm) and adding Millipore water to 100 mL. ZnONPs and TiO₂NPs were separately synthesized using a protocol modified from a previous study [24 (link)]. The metal oxide solution (50 mL) was mixed with the extract of lemon fruits (50 mL) at the ratio 1:1 in flasks at 100 rpm at room temperature for 4 h and became colloid. After mixing the colloid (2 mL) with Millipore water (2 mL), the colloidal NPs were identified by ultraviolet-visible spectroscopy with a Shimadzu UV-2550 spectrometer (Shimadzu, Kyoto, Japan) from 200 to 800 nm at 1 nm resolution. The colloidal NPs were centrifuged at 27,200 g for 10 min and the pellets were washed with Millipore water and then freeze-dried with an Alpha 1-2 LDplus (Martin Christ GmbH, Osterode am Harz, Germany). The freeze-dried NPs were stored at −80 °C or prepared as stock solutions (50 mg∙mL⁻¹) for further analyses.

In this study, the endophytic bacterium P. poae strain CO was used to mediate the biosynthesis of AgNPs, and this process was performed according to the method of Fouad et al. [21 (link)] with slight modification. In brief, the endophytic bacterium P. poae strain CO was inoculated in a nutrient broth (10 g of tryptone, 3 g of beef extract, 2.5 g of glucose, and 5 g of NaCl per liter; pH 7.0; all ingredients were purchased from Sangon Biotech, Shanghai, and then incubated at 30 ℃ at 200 rpm for 2 days. Ten milliliters of culture filtrates (CF) were mixed with 90 mL of 3 mM aqueous of silver nitrate (Cat. no. 10018461; Sinopharm, Shanghai, China) in a 250 mL Erlenmeyer flask, followed by shaking at 200 rpm, 30 ℃ for 4 days in darkness. A nutrient broth of the same volume was used as the control. The biosynthesis of AgNPs was noted by the change of the color from light yellow to dark brown, while the formation of AgNPs was assured by UV–visible spectrometry from 200 to 800 nm at a 1 nm resolution by using a Shimadzu UV-2550 spectrometer (Shimadzu, Kyoto, Japan). The pellets were collected by centrifuging at 10,000 g for 20 min and washing twice with double distilled water (ddH₂O). The biosynthesized AgNPs were freeze-dried for future use.

The N-GQDs (Product No. XF241) used in this study was purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China) (http://www.xfnano.com), and their physicochemical characterizations were evaluated before the study. High-resolution transmission electron microscope (HR-TEM, JEM-2100, JEOL Ltd. Japan) images were acquired on an electron microscope. The particle morphology was examined by atomic force microscopy (AFM, Dimension Icon, Bruker AXS, German). The FT-IR spectra were recorded on Nicolet iS10 spectrometer (Thermo, USA). The absorption spectra and fluorescence spectra were measured on a UV-2550 spectrometer (Shimadzu, Japan). The dynamic light scattering (DLS) and surface ξ-potential measurements were carried out on a Malvern Zetasizer Nano ZS instrument (Zetasizer Nano-ZS90, Malvern, UK).

Hydrogels were swollen in PBS at pH 7.4
for 24 h. In the case of
the oxidized hydrogels, subsequently, they were swollen in H₂O₂ 9 mM for another 24 h. Then, they were placed between
two quartz slides, and the absorbance was measured at 335 nm by using
a Shimadzu UV-2550 spectrometer equipped with a film adapter.