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Highly active, super-stable, and biocompatible Ag/polydopamine/g-C3N4 bactericidal photocatalyst: Synthesis, Characterization, and Mechanism Yunyan Wu, Yazhou Zhou, Han Xu, Qinqin Liu, Yi Li, Lili Zhang, Hanqing Liu, Zhigang Tu, Xiaonong Cheng, and Juan Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02620 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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ACS Sustainable Chemistry & Engineering

Highly

active,

super-stable,

and

biocompatible

Ag/polydopamine/g-C3N4 bactericidal photocatalyst: Synthesis, Characterization, and Mechanism Yunyan Wu,

†⊥

Yazhou Zhou,

§

*,†⊥

‡

†

†

†

Han Xu, Qinqin Liu, Yi Li, Lili Zhang, Hanqing Liu,

†

Zhigang Tu, Xiaonong Cheng, Juan Yang,

*,‡

*,†

†

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

‡

School of Pharmacy, Jiangsu University, Zhenjiang 212013, P. R. China

§

Institute of Life Sciences, Jiangsu University, Zhenjiang 212013, P. R. China

*Email address corresponding author Juan Yang: [email protected] Yazhou Zhou: [email protected] Hanqing Liu: [email protected]

ABSTRACT:

The

goal

of

this

paper

is

developing

bactericidal

photocatalyst

silver/polydopamine/graphitic carbon nitride (Ag/PDA/g-C3N4) as a new type of antibacterial materials with high activity, super stability, and excellent biocompatibility. Small Ag NPs (3.6-10.5 nm) disperse uniformly on the PDA-modified g-C3N4 sheets. This biophotocatalyst has excellent antibacterial activity against Escherichia coli, including low MIC100%Ag of 9.5 ppm, MBC100%Ag of 6.3 ppm, but also a low cytotoxicity for human umbilical vein endothelial cells (HUVECs) because of biocompatible PDA. After 30 days in air environment, only 0.18% of Ag+ was detected, which indicated the super stability of this biophotocatalyst comparing with the state-of the-art antibacterial materials. The possible bactericidal mechanism is the synergistic effect between photocatalytic 1

ACS Paragon Plus Environment

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PDA-modified g-C3N4 and active Ag NPs. The Ag NPs themselves have strong antibacterial activity due to their small sizes. Importantly, under light irradiation, the surface plasmonic effect of Ag NPs and the incorporation of PDA enhance the photocatalytic activity of g-C3N4 significantly by increasing its light absorption, improving photoconductivity for photo-generated electrons and holes, and inhibiting their recombination. As a result, the sufficient radicals such as •OH and O2•radicals can be formed. The radicals, particularly •OH, together with holes can accelerate the death of bacterial through the destruction of biomolecules. KEYWORDS: Graphitic carbon nitride, Silver nanoparticles (Ag NPs), Photocatalytic performance, Antibacterial property, Biocompatibility INTRODUCTION Silver nanoparticles (Ag NPs) are currently one of the most widely commercialized nanomaterials because of their unique physical-chemical properties.1 They are utilized for a variety of applications in many technological fields such as nanoelectronics, sensors, catalysts, and particularly, health care.2-4 As a broad-spectrum antimicrobial agent, Ag NPs can be widely utilized in medical and consumer products, including hand sanitizers, antiseptic sprays, wound dressings, kitchen utensils, and the coating for medical apparatus.5-7 It is well known that the Ag ions (Ag+) released from Ag NPs contribute the most to the antimicrobial activity of Ag NPs.8 Specifically, more Ag+ release makes Ag NPs more active. This antimicrobial activity, however, is undesirable for utilization of Ag NPs in health care because of their non-negligible toxicity for human treatments. It is because that the active Ag NPs are easily oxidized and dissolved. The leaching Ag+ can not only act against microorganisms, but also dictate the biocompatibility problem. In addition, the accelerating release of Ag+ also brings the instable issue, and shortens the useful life of Ag NPs. Decorating Ag NPs on the supports (such as carbon materials, zeolite, calcium phosphate, polymer materials, and so on) has been commercialized to improve the antibacterial activity, stability, and broad application areas

2


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of Ag NPs.9-11 However, it is still a challenge to fundamentally enhance both antibacterial activity and stability of Ag NPs. The photocatalytic materials have attracted a lot of attention as a new class of antibacterial materials. In previous works, bactericidal photocatalysts were mainly constructed by the decoration of Ag-based NPs onto photocatalytic NPs such as TiO2, AgCl,12 and ZnO.13 For example, Hoek and co-workers deposited Ag NPs onto TiO2 NPs to obtain the hybrid Ag-TiO2 nanomaterials using either wet-impregnation or UV photo-reduction methods.14 This hybrid Ag-TiO2 activated by UV light exhibited a significant enhancement in antibacterial activity compared with that of Ag/UV. Li and co-workers reported a ternary Ag/AgBr/TiO2 array electrode with a high antibacterial activity towards Escherichia coli (E. coli) under visible light irradiation.15 Owing to the synergistic effect of Ag/AgBr NPs and TiO2 nanotubes, the prepared Ag/AgBr/TiO2 electrode showed a high photocatalytic activity, resulting in high efficiency of reactive oxygen species (ROS) generation. Those ROS can attack the cell wall and membrane of E. coli, and eventually caused the cell to die. In our recent work, we decorated ultrafine Ag/AgCl NPs onto the surface of graphene to obtain the material with excellent antibacterial activity, super-stability, biocompatibility, and burn wound healing performance.16 The antibacterial mechanism of this material is not released Ag+, but generated ROS. Therefore, studies on the design of biophotocatalyst for antibacterial materials and investigation of its antibacterial mechanism are beneficial for the development of noble antibacterial materials with both high activity and stability. Graphitic carbon nitride (g-C3N4), the first metal-free polymeric semiconductor, has attracted considerable concern owing to its visible-light absorption, excellent chemical and thermal stability, low cost, and environmentally friendly.17 Due to these excellent properties, g-C3N4 exhibits a variety of promising applications in energy storage,18 large-band-gap semiconductor,19 heterogeneous catalyst,20 photocatalysts, and biomaterials.21,22 Recently, a few research works have demonstrated that g-C3N4 can be one of the candidates for bactericidal photocatalysts. Although pure g-C3N4 is inactive against bacteria, it can be enhanced by combining with active 3



nanomaterials. For example, Yu and co-workers reported the improved bactericidal activity of gC3N4 photocatalyst through combining with reduced graphene oxide and α-Sulfur crystals.23 Kim and co-workers prepared Ag-ZnO/g-C3N4 photocatalyst with a great antibacterial property.24 However, the antibacterial activity of g-C3N4-based photocatalysts has to be further improved, and their biocompatibility, stability, and antibacterial mechanism also have to be investigated. Therefore, the goal of our research is to develop a g-C3N4-based photocatalyst for expected antibacterial materials, and study its antibacterial mechanism. Due to the excellent surface plasmon resonance effect, Ag NPs are usually employed to improve the activity of photocatalysts. Li and co-workers deposited Ag NPs onto the surface of gC3N4 to obtain improved photocatalytic activity towards hydrogen evolution reaction and degradation of organic pollutants.25 Similarly, Ge and co-workers demonstrated the Ag NPs/g-C3N4 composite with enhanced photocatalytic efficiency.26 Therefore, functionalization of g-C3N4 with Ag NPs is a promising approach to obtain the excellent biophotocatalyst for antibacterial applications. In order to achieve above goal, two issues have to be addressed. One is uniform deposition of Ag NPs onto the surface of g-C3N4 sheets with small size. Another is improving compatibility. It is well-known that the g-C3N4 is fabricated by pyrolysis of nitrogen-rich precursors, including melamine, cyanamide, triazine, urea and so on.27-29 The g-C3N4 prepared by above method has rare functional groups existing on the surface of g-C3N4, thus leading to insolubility or difficult dispersion of g-C3N4 in any solvents. The insufficient solubility or dispersibility makes NPs modified g-C3N4 difficult, resulting in NPs aggregate and big particle size. The particle size, particle dispersion, and binding force of Ag NPs on the surface of g-C3N4 have a strong influence on the photocatalytic performance of Ag NPs/g-C3N4 composites.30 Although acid treatment is a useful process to improve the solubility and dispersion of g-C3N4 significantly, it also decreases the compounds’ photocatalytic activity.31 For the second issue, the g-C3N4 in previous work showed an excellent compatibility,32 however we found that g-C3N4 exhibits non-negligible toxicity for human cells in our experiments. 4


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Herein, we demonstrate the decoration of Ag NPs onto the polydopamine (PDA) modified gC3N4 sheets to form Ag/PDA/g-C3N4 biophotocatalyst. In previous work, PDA as a polymeric nanocoating can be utilized to modify the g-C3N4 to improve photocatalytic activity of g-C3N4 owing to its excellent sunlight absorption ability, photoconductivity for accelerating photogenerated electrons (e-) and holes (h+), and abundant catechol groups for effective transferring and separating e- and protons. In doing so, there are several important benefits in this work. (a) The solubility and dispersibility of g-C3N4 can be improved by modification with PDA significantly, which is favorable for decorating Ag NPs with small size and highly-dispersive. (b) The deposition of PDA nanocoating and the desirable Ag NPs is beneficial for enhancement in photocatalytic activity for g-C3N4, resulting in active and stable Ag/PDA/g-C3N4 antibacterial material. (c) The PDA nanocoating prepared by self-polymerization of biomolecular dopamine contributes an excellent biocompatibility for Ag/PDA/g-C3N4 material. EXPERIMENTAL SECTION Reagents. Urea (A.R, 99%), sodium borohydride (NaBH4, A.R, 98 %), ethanol (A.R, 99.7 %), and hydrochloride (HCl, 36.5-38.0%), potassium dichromate (K2Cr2O7, A.R, 99.8%), sodium oxalate (A.R, 99.8%), isopropyl alcohol (IPA, A.R, 99.5 %), ethylene glycol (A.R, 98 %), Nafion solution (5 wt %) were purchased from Aladdin Reagent Company (Shanghai, China). Silver nitrate (AgNO3, A.R, 99.8%), dopamine hydrochloride (HDA, A.R, 98%), and tris (hydroxymethyl) aminomethane (Tris, A.R, 99%), 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPOL, 97%) were purchased from Sigma-Aldrich (St. Louis, MO USA). Synthesis of g-C3N4. The g-C3N4 was prepared by thermal treatment of urea in the tube furnace with a temperature of 550 oC for 240 min according to the previous work.17, 25 The heating rate was 2 oC min-1. After the thermal treatment procedure, the creamy white powder was ground to obtain the g-C3N4. Synthesis of Ag/PDA/g-C3N4 biophotocatalysts. Tris-HCl buffer with a pH value of 8.5 was 5



prepared firstly as a solvent for the synthesis of Ag/PDA/g-C3N4. 60.57 mg Tris was dispersed in 500 mL deionized water, and its pH was adjusted by 0.2 mM HCl. The 30 mg of g-C3N4 powder was added to 60 mL of Tris-HCl buffer, and the milky white suspension was obtained after 0.5 h vigorous stirring and 1.5 h sonication. Then, 40 mg HDA was added into the g-C3N4 Tris-HCl solution to form the mixture with vigorous stirring. At this time, the color of mixture was changed from milky white to brown. After 12 h polymerization, the mixture showed the dark color, and the PDA/g-C3N4 was obtained. A certain volume of AgNO3 aqueous solution (60 mg mL-1) was then added into PDA/g-C3N4 solution with 1 h vigorous stirring. After that, the glass beaker containing the above mixture was put into the ice-water bath, and 1 mL NaBH4 aqueous solution (1.4 mM) was quickly added to the mixture under continuous vigorous stirring. The Ag/PDA/g-C3N4 with dark color was obtained after 2 h reduction reaction. After washing with water and alcohol for five times, and freeze-drying, the Ag/PDA/g-C3N4 powder was prepared. The final products prepared with the weight ratios of AgNO3/g-C3N4 of 1:1, 1:2, and 1:4 were labeled as Ag/PDA/g-C3N4 (1:1), Ag/PDA/g-C3N4 (1:2), and Ag/PDA/g-C3N4 (1:4), respectively. The Ag/PDA, Ag/g-C3N4(1:2) with the same silver precursors were prepared for comparisons (see Supporting Information). Characterization. The crystal structure of products was characterized by X-ray powder diffraction (XRD) on Rigaku D/max 2500 powder X-ray with high-intensity CuKα. The surface functional groups and chemical compositions of products were investigated by using Fourier transform infrared spectroscopy (FT-IR, Nicolet Avatar-330) by KBr pellet pressing method and Xray photoelectron spectroscopy (XPS) by ESCALAB250Xi where Mg/Al X-ray was used as the excitation source. The morphological structures of the product were measured by scanning electron microscope (SEM, JSM-7001F) and transmission electron microscope (TEM, JEOL 2011). The Ag contents of Ag/PDA/g-C3N4 biophotocatalysts were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) (VISTAMPX, Varian Inc.). Zeta potential values of samples in pure water were recorded on Nano ZS90 (Malvern Instruments). The optical properties of products were tested by UV-vis diffuse-reflectance spectroscopy (UV-2550, SHIMADZU) and 6


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photoluminescence spectrophotometer spectra (PL, QuantaMasterTM 40) at room temperature. Reactive oxygen species (ROS) including O2•-, •OH from composites solution were identified and quantified with electron spin resonance spectroscopy (ESR, JESFA200). Photoelectrochemical experiments. Electrochemical impedance spectroscopy (EIS) and transient photocurrent were tested on CHI 660E (Chenhua Instrument, Shanghai, China) with a typical three-electrode cell. The electrode contains a working electrode (prepared sample), a counter electrode (platinum foil), and a reference electrode (Ag/AgCl). 0.2 M Na2SO4 aqueous solution (pH 5.8) was employed as electrolyte, and a 300 W Xe lamp equipped with a 420 nm cut-off filter was utilized as visible light source. The working electrode was prepared according to the following procedure: 5 mg of the as-prepared photocatalyst was dispersed into ethanol (250 µL), ethylene glycol (250 µL), and Nafion (40 µL) mixed solution. The above solution (80µL) was then dropped onto a pre-cleaned fluorine tin oxide (FTO) glass with an exposed area of 1 cm2. The photocurrent responses of the photocatalysts to light switching on and off were measured with 1.2 V bias voltage. EIS spectra were recorded in the range from 0.01 Hz to105 Hz at an ac voltage of 10 mV. In vitro antimicrobial activity evaluation. Bacteria culture. E. coli bacteria (E. coli ATCC 117) were used as a model microorganism to estimate the antibacterial activity of Ag/PDA/g-C3N4. E. coli were initially transferred from the -79 oC refrigerator and cultured in Luria-Bertani (LB) medium (Tryptone 10 g L-1, Yeast extract 5 g L-1, and NaCl 10 g L-1) at 37 oC with shaking of 200 rpm for 12 h to revitalize and breed until OD600 ≈ 1. All results are expressed as the mean of three replicates, and each experiment was repeated at least once on a different day with essentially same results. Inhibition zone testing. The antimicrobial activity of Ag/PDA/g-C3N4 was firstly investigated by measuring inhibition zone using agar-well diffusion assay. Initially, LB-agar plate was smeared with E. coli suspension (100 µL, OD600 = 0.02). Then, the bottom of each well (8 mm) drilled by an Oxford cup on the inoculated agar plate was sealed with molten agar medium. A certain amount of 7



Ag/PDA/g-C3N4 composites were dropped immediately into the wells. The agar plate was then incubated at 37 °C for 24 h under visible light. Finally, the inhibition zones were photographed and measured. The water and pure g-C3N4 were used as the positive and negative controls, respectively. Growth inhibition testing. Various concentrations of Ag/PDA/g-C3N4 biophotocatalysts were added into 20 mL LB medium, and then 200 µL E. coli cells (OD600 = 1) were added into above LB medium. The final concentrations of biophotocatalysts are in a range of 15 ppm to 185 ppm (ppm = mg L-1). Then, above bacterial suspensions with biophotocatalysts were incubated at 37 °C for 24 h with shaking of 200 rpm under darkness or light (300W Xe lamp), respectively. The OD values of bacterial suspensions were measured at 600 nm at 1 h intervals for 24 h during inoculation. The growth inhibition curves of E. coli were obtained according to the OD values; meanwhile, the minimum inhibitory concentration (MIC) of biophotocatalysts was obtained, which can be used to evaluate the antibacterial activity. The water and pure g-C3N4 were used as the positive and negative controls, respectively. Bacterial viability assay. The antimicrobial activity of Ag/PDA/g-C3N4 biophotocatalyst was further measured by bacterial survival through counting colony-forming units (CFUs). Initially, the E. coli cells were grown to mid-log phase at OD600=0.3 (about 3×105 CFU/mL) at 37 °C in LB medium. Cells were then collected by centrifugation, washed and diluted to OD600 = 0.1 by adding PBS buffer. After that, Ag/PDA/g-C3N4 aqueous solution was added into bacterial LB medium. The final concentrations of biophotocatalyst were determined in a range of 10-50 ppm. The above bacterial LB medium with biophotocatalyst was incubated at 37 °C with shaking of 200 rpm under light (300W Xe lamp) for 3 h. Every hour, 1 µL resultant bacterial suspension was withdrawn and diluted 100 times. Finally, the LB-agar plates are spread over a quantity of the diluted bacterial suspension (100 µL) and incubated at 37 °C for 48 h, and then the CFUs were counted. The minimum bactericidal concentration (MBC) of biophotocatalyst against E. coli was obtained. Cell toxicity assay. Human umbilical vein endothelial cells (HUVECs) were cultured in RPMI 8


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1640 (Thermo Fisher Scientific) medium containing fetal bovine serum, biotin, vitamin B12, and PABA in a humidified atmosphere containing 5% CO2 at 37 °C. At the beginning of cell toxicity assay, HUVECs were counted and plated at a density of 4000 cells per well in 96-well plates. After incubation of 24 h, Ag/PDA/g-C3N4 with indicated concentration was added to the wells. After 24 h culture, the supernatant was removed. 10 µL of Thiazolyl Blue Tetrazolium Bromide solution (MTT, 0.5 mg mL-1) was added to each well and the cells were further incubated for 4 h. After replaced with 100 µL of dimethyl sulfoxide (DMSO), the solutions were slightly shaken for 5 min. Absorbance values were measured at 550 nm using a Varioskan Flash (Thermo Fisher Scientific). The viability of cells was obtained. Cells treated with water, g-C3N4, PDA/g-C3N4, Ag/g-C3N4, and AgNO3 were used as the controls. Ag+ release. The Ag+ release was tested to investigate the stability and antibacterial mechanism of Ag/PDA/g-C3N4. A 10 mL of 1 mg mL-1 Ag/PDA/g-C3N4 aqueous solution was exposed at the outdoor environment with the slow magnetic stirring. After a given time interval, 1 mL solution was fetched out, wherein the Ag/PDA/g-C3N4 was removed out by centrifugation. The amount of release Ag+ can be measured by ICP-OES to evaluate the stability of the materials. The AgNO3, Ag NP colloid, and Ag/rGO were used as controls. Active species trapping experiments. To further understand the antibacterial mechanism of such photocatalyst, the active species trapping experiments are conducted according to previous work.33 The procedures are similar to those in “Bacterial viability assay” section. However, before the incubation, we added the corresponding species trapping agents such as 1 mM sodium oxalate (h+), 1 mM IPA (•OH), 0.5 mM TEMPOL (O2•-), and 0.1 mM K2Cr2O7 (e-) into the bacterial LB medium. The effect of each active species can be obtained by counting CFU. The toxicities of species trapping agents against bacteria were also demonstrated as the controls. RESULTS Preparation, crystal structure, and chemical states analysis. The strategy for synthesis of 9



Ag/PDA/g-C3N4 biophotocatalysts is illustrated in Figure 1, and the digital photographs of asprepared powder and their aqueous solution are shown in Figure 1e. First of all, the PDA/g-C3N4 was prepared using in situ polymerization of DA to form the PDA nanocoating onto the surface of g-C3N4 sheets (Figure 1b). The Ag+ then can be highly absorbed onto the surface of PDA/g-C3N4 (Figure 1c). Finally, the Ag/PDA/g-C3N4 biophotocatalysts were obtained by in situ-reduction of Ag+ (Figure 1d). The colors of powders obtained in different procedures are obviously different (Figure 1e). After PDA modification, the color of g-C3N4 is changed from milky white to brown, and is further changed to black after Ag reduction. The zeta potential values of the solutions at different steps can be seen in Table S1 (see Supporting Information). After PDA modification, the zeta potential value of g-C3N4 solution increased from -5.8 mV to 23.7 mV though incorporating more H+ onto the surface of g-C3N4 sheets.38 The improved solubility of g-C3N4 by PDA is not only beneficial for highly dispersive of Ag NPs with small size onto the surface of g-C3N4 through providing Ag+ absorbing spots, but also is useful for improving the solubility and dispersibility of biophotocatalysts in water (Zeta potential value: 25.8 mV), which is important for the antibacterial activity of materials. The crystal structure of Ag/PDA/g-C3N4 composites was investigated by XRD, which can be seen in Figure 2(a). For the pure g-C3N4, two distinct peaks located at 2θ=27.4° and 12.7 °27.4° can be indexed as (100), (002) diffraction planes of g-C3N4, respectively. Compared with pure g-C3N4, the XRD peaks of g-C3N4 in Ag/PDA/g-C3N4 biophotocatalysts have no obvious change, except disappeared (100) peak, which is probably attributed to the modified PDA and dispersed Ag NPs. At the same time, four characteristic peaks were found at 2θ of 38.1°, 44.4°, 64.4°, and 77.3°, which can be attributed to the (111), (200), (220), and (311) lattice planes of metallic Ag, respectively.34 In addition, the intensity of Ag XRD peaks increased with the increment of AgNO3 amount in the experiments, indicating the increase of Ag NPs loading. The FT-IR of the as-prepared material was shown to detect surface functional groups in Figure S1. Pure g-C3N4 exhibits several broad absorbing bands in the range of 1200-1650 cm-1 and 3300-3600cm-1, which may be ascribed 10


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to the typical stretching vibrations of CN heterocycles and N-H bonds. Also, the absorbing band at 808 cm-1 corresponds to the breathing mode of triazine units.35 Compared with pure g-C3N4, all typical absorbing bands of Ag NPs appear in the spectra of the Ag/PDA/g-C3N4 biophotocatalysts, whereas the absorption bands of PDA are, apparently, unable to be detected by FT-IR because there are little PDA on the surface. The XPS characterization was employed to further confirm the structure and chemical components of Ag/PDA/g-C3N4 (1:2). Figure 2(b) shows the XPS survey spectra of pure g-C3N4 and Ag/PDA/g-C3N4(1:2). Compared with C, N elements in pure g-C3N4, a new element of Ag can be found in the Ag/PDA/g-C3N4(1:2). Moreover, the content of O element is much higher than that of pure g-C3N4, implying the existing of PDA in Ag/PDA/g-C3N4(1:2). The XPS spectra of C 1s in Figure 2(c) show that three peaks occurred at 284.6 eV, 286.2 eV, and 288.0 eV can be accredited to C-C, C-O, and N-C=N species which belongs to the aromatic nitrogen heterocycle structure of gC3N4, hydroxyl, and amidogen of DA, respectively. XPS spectra of N 1s can be fitted into three component peaks in Figure 2(d). The main peak at 398.7 eV is ascribed to C=N-C which derives from the sp2-bonded N in triazine rings; the peak at 400.1 eV is attributed to N-(C)3 groups; and the peak at 401.1 eV is related to C-N-H caused by polymerization. Figure 2(e) shows the O 1s spectra of pure g-C3N4 and Ag/PDA/g-C3N4 biophotocatalyst. For Ag/PDA/g-C3N4 biophotocatalyst, the O 1s XPS spectra can be fitted well with two peaks at 531.2 eV and 532.7 eV, corresponding to OC=O and C-O, respectively. Those oxygen-containing groups arose from hydroxyl and amidogen of PDA. In contrast, two weak peaks were found at 531.9 eV and 533.2 eV for pure g-C3N4, which should be attributed to adsorbed H2O or CO2 on the surface of g-C3N4. Thus, the existence of PDA in biophotocatalysts is confirmed by the XPS results. It is highly consistent with the reported research work in literature.36,38 The typical Ag 3d spectrum of Ag/PDA/g-C3N4 biophotocatalyst is displayed in Figure 2(f). Two individual peaks can be found at approximately 368.3 eV and 374.3 eV, corresponding to Ag 3d3/2 (Ag0) and 3d5/2 (Ag0) binding energies, respectively.37 The resulting Ag 3d spectrum is close to the reported data, which further proves that Ag NPs have been fabricated 11



successfully on the g-C3N4. The XPS together with XRD and FT-IR results can confirm that Ag NPs and PDA are decorated on the g-C3N4 sheets successfully. Morphology analysis. The microscopic morphologies of pure g-C3N4 and Ag/PDA/g-C3N4(1:2) biophotocatalyst were characterized by SEM. As shown in SEM images (Figure S2), both pure gC3N4 and biophotocatalyst have the typical porous structure of constructing 2D g-C3N4 sheets. Compared with pure g-C3N4, many Ag NPs can be seen on the g-C3N4 for Ag/PDA/g-C3N4(1:2) biophotocatalyst. The morphologies of g-C3N4 and Ag/PDA/g-C3N4 biophotocatalysts were further investigated by TEM analysis. As shown in Figure S3, the pure g-C3N4 shows the typical layer structure. For Ag/PDA/g-C3N4 biophotocatalysts, many particles can be seen on the surface of gC3N4 sheets after decoration of Ag NPs. We also found that particle dispersity and particle size are affected by initial weight ratio of AgNO3/g-C3N4 (Figure 3). When the ratio is 1:4, ultrafine sized Ag NPs in Ag/PDA/g-C3N4(1:4) are uniformly dispersed on the g-C3N4 sheets (Figure 3a and Figure 3d). The average particle size is around 3.6 nm (Figure 3g). With the increase of the weight ratio to 1:2, more Ag NPs are observed and they are highly dispersed on the g-C3N4 sheets for Ag/PDA/g-C3N4(1:2) (Figure 3b and Figure 3e). Furthermore, no Ag NP aggregates and uncoated Ag NPs are found. The particle size of Ag grew to the bigger size of 8.0 nm (Figure 3h). When the weight ratio increased to 1:1, the Ag NPs loading on the g-C3N4 sheets increased for Ag/PDA/gC3N4(1:1) (Figure 3c and Figure 3f). However, some Ag NP aggregates can be found on g-C3N4 sheets, resulting in the bigger size and broader size distribution (10.5±1.6 nm) (Figure 3i). The Ag contents in Ag/PDA/g-C3N4 biophotocatalysts were measured by ICP-OES, which were 12.8 wt% for Ag/PDA/g-C3N4(1:4), 21.3 wt% for Ag/PDA/g-C3N4(1:2) and 30.2 wt% for Ag/PDA/gC3N4(1:1), respectively. The Ag/g-C3N4 without modification of PDA, and Ag/PDA without gC3N4, respectively were also synthesized as controls. The Ag NPs in resulting samples have the lager sizes and serious particle aggregates (Figure S4 and Figure S5, see Supporting Information). Those experiments further prove that the synergistic effect of PDA and g-C3N4 played an important role in the synthesis of Ag/PDA/g-C3N4 biophotocatalysts. 12


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Photoelectrochemical properties of Ag/PDA/g-C3N4 biophotocatalysts. The optical property of material was firstly measured by UV-vis diffuse reflectance spectroscopy. Figure 4(a) displays the UV-vis spectra of g-C3N4 and Ag/PDA/g-C3N4 biophotocatalysts with different Ag NP loadings. Compared with pure g-C3N4, the absorption edges of the Ag/PDA/g-C3N4 biophotocatalysts have remarkable red shifts and significantly enhanced absorption intensities of light, indicating more visible light harvesting by g-C3N4 owing to the unique optimal property of Ag NPs. The corresponding band gaps of the pure g-C3N4 and Ag/PDA/g-C3N4 biophotocatalysts were calculated by the Kubelka-Munk transformation (Figure 4b). The band gaps are 2.82 eV for pure g-C3N4, 2.74 eV for Ag/PDA/g-C3N4(1:4), 2.72 eV for Ag/PDA/g-C3N4(1:2), and 2.52 eV for Ag/PDA/g-C3N4(1:1), respectively. Thus, Ag NPs play an important role in the enhancement of photocatalytic activity of g-C3N4 through increasing its light absorption and reducing its band gap. The PL analysis is an important technique to demonstrate the photocatalytic activity through revealing the migration, transfer, and separation of photogenerated charge carriers. Figure 4(c) shows the PL spectra of pure g-C3N4 and Ag/PDA/g-C3N4 biophotocatalysts with different Ag loading. A strong PL emission spectrum can be seen for pure g-C3N4 at 455 nm, which is attributed to the high radiative recombination of photogenerated e- and h+. However, all the three kinds of Ag/PDA/g-C3N4 biophotocatalyst have the significantly lower emission intensities than that of pure g-C3N4. It is well known that Ag NPs that have an excellent conductivity can be used as the eacceptor to effectively transfer the photogenerated e- from g-C3N4, resulting in less charge recombination. With the increase of Ag NP loading, the PL emission intensity of g-C3N4 was further weakened (Figure 4d), which further proved the effect of Ag NPs in the enhancement of photocatalytic activity of g-C3N4. In addition, previous works have proven that PDA is also helpful for transfer of e-.38 Therefore, the Ag NPs and PDA enable g-C3N4 to be much more active in the biophotocatalyst through enhancing light absorption and lowering radiative recombination of ewith h+. The electronic interaction between Ag NPs and g-C3N4 was demonstrated by transient 13



photocurrent measurements, as shown in Figure 4e. Both Ag/PDA/g-C3N4 and pure g-C3N4 showed a sensitive photocurrent response during the on/off visible light irradiation. The photocurrent value for Ag/PDA/g-C3N4 (1:1, 1:2, 1:4) is 11.6, 10.2, 7.6 times higher than that of g-C3N4. With the increase of Ag NP loading, the photocurrent response was further enhanced. The higher photocurrent for Ag/PDA/g-C3N4 revealed a better visible-light response and more efficient photoexcited separation of e-/h+ pairs compared with pure g-C3N4. The separation process of charge carriers of Ag/PDA/g-C3N4 biophotocatalysts was further investigated by EIS. Figure 4f shows the EIS Nyquist plots of Ag/PDA/g-C3N4 and pure g-C3N4 electrodes under visible-light irradiation. The Ag/PDA/g-C3N4 has a much smaller arc radius on EIS Nyquist plot than that of pure g-C3N4, suggesting a more effective separation of photo-excited e- and h+ pairs and faster interfacial charge transfer of Ag/PDA/g-C3N4 compared with pure g-C3N4. The photocurrent measurements and EIS results indicate the dramatically enhanced separation and transfer efficiency of charge carriers by a synergistic effect between Ag NPs and g-C3N4, which were also highly consistent with PL results. Antimicrobial properties. The antibacterial activity of Ag/PDA/g-C3N4 biophotocatalyst was qualitatively investigated by agar-well diffusion assay. The inhibition zones of materials against E. coli were measured to determine the antibacterial activity of materials (Figure S6a, see Supporting Information). It’s clear that there is no obvious inhibition zone for positive control, indicating that visible light did not affect the survival of E. coli. For negative control, a small inhibition zone with a diameter of 4 mm is found for g-C3N4 with a high concentration of 400 ppm. In contrast, obvious inhibition zones can be seen for Ag/PDA/g-C3N4(1:2) against E. coli. The sizes of zones are increases by 12.5 mm and 22 mm for 40 ppm and 100 ppm of Ag/PDA/g-C3N4(1:2) biophotocatalysts, respectively (Figure S6b). The results proved that Ag/PDA/g-C3N4 (1:2) possesses a good antibacterial activity. The antibacterial activities of Ag/PDA/g-C3N4 biophotocatalysts were quantifiably investigated by the measurement of MIC. Figure 5 shows the growth curves of E. coli treated with different 14


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Ag/PDA/g-C3N4(1:2) biophotocatalysts under the dark (left column) and visible light (right column). The E. coli treated with water, pure g-C3N4, PDA/g-C3N4, and Ag/PDA were used as controls. When incubating in a dark environment, the growth curve of E. coli shows a slight lag phase when the concentration of g-C3N4 and PDA/ g-C3N4 increased to 1400 and 1000 ppm, respectively (Figure 5a, Figure 5c), indicating the negligible antibacterial activity of pure g-C3N4 and PDA/g-C3N4 without light irradiation. However, both g-C3N4 and PDA/g-C3N4 have significantly improved antibacterial activities under the visible light irradiation (Figure 5b, Figure 5d). The MIC100% of PDA/g-C3N4 is ~ 600 ppm, which is much lower than that of pure g-C3N4 (~ 800 ppm). Therefore, the PDA can obviously enhance the antibacterial activity of pure g-C3N4 under the light irradiation. For Ag/PDA/g-C3N4(1:2), the MIC100% decreased to 75 ppm in the darkness, which is attributed to the activity from Ag NPs. Importantly, MIC100% of Ag/PDA/gC3N4(1:2) further decreased to 45 ppm under the visible light irradiation, which indicated that approximately 40% enhancement in antibacterial activity is provided by photocatalytic effect (Table S3, see Supporting Information). After careful comparisons with g-C3N4, PDA/g-C3N4, Ag/PDA (Figure S7a, b), and Ag/g-C3N4 (Figure S7c, d), we can conclude that the PDA and Ag NPs play an important role in development of antibacterial activity of g-C3N4 under the visible light irradiation. In addition, the antibacterial activity of Ag/PDA/g-C3N4 is obviously affected by Ag NPs loading, and it is strengthened with the increase of Ag NPs loading (Figure S6, see Supporting Information). Here, the MIC100%Ag is employed to evaluate the cost effectiveness of antibacterial materials, and the Ag/PDA/g-C3N4(1:2) is the optimal antibacterial material because of the lowest MIC100%Ag of 9.5 ppm (Table S2, see Supporting Information). The bactericidal activity of Ag/PDA/g-C3N4 biophotocatalyst was further tested through a timekill curve. Based on MIC results, the time-kill curves of E. coli treated with different concentrations of Ag/PDA/g-C3N4(1:2) biophotocatalyst were investigated, and shown in Figure 6a. 99.2% of E. coli can be killed within 2 h by 30 ppm Ag/PDA/g-C3N4(1:2) biophotocatalyst (Figure 6b). The E. coli can be completely killed by this biophotocalysts within 2.5 h. The MBC100%Ag value of 15



Ag/PDA/g-C3N4(1:2) biophotocatalyst is about 6.3 ppm, indicating its high antibacterial activity. The Ag/PDA/g-C3N4 also has better or comparable antibacterial activity with some state-of-the-art antibacterial materials in the previous works (Table 1),39-47 which indicates a great potential of Ag/PDA/g-C3N4 in antibacterial applications. The change in E. coli’s morphology after treatment with 30 ppm of the biophotocatalyst was investigated by SEM and TEM. As shown in Figure 6c and Figure 6d, the untreated E. coli cells show the typical ellipsoid morphology with intact cell walls and flagellum. However, the bacteria cell structure was destroyed after incubating with Ag/PDA/g-C3N4 (1:2) for 2 h (Figure 6e and Figure 6f). We also find that the bacteria cells are wrapped by Ag/PDA/g-C3N4 sheets, which demonstrates that Ag/PDA/g-C3N4 sheets can strongly interact with bacteria cells through electrostatic adsorption between positively charged Ag/PDA/g-C3N4 and negatively charged bacteria cells to affect the cells directly and accelerate the death of bacteria. Biocompatibility and stability of Ag/PDA/g-C3N4 biophotocatalysts. The biocompatibility and potential cytotoxicity of Ag/PDA/g-C3N4 biophotocatalyst were evaluated by MTT assays. Pure gC3N4, AgNO3, PDA/g-C3N4, and Ag/g-C3N4 were used as controls. The ratios of survived HUVEC cells after treatment with indicated materials were measured and shown in Figure 7a. The ordering of the samples’ biocompatibility is as follow: PDA/g-C3N4 > Ag/PDA/g-C3N4 > g-C3N4 > Ag/gC3N4 > AgNO3. According to this ordering, we can draw three conclusions. First, the cytotoxicity of pure g-C3N4 should not be ignored, particularly in visible-light irradiation. Second, Ag decoration significantly increases the cytotoxicity of samples. However, the Ag/PDA/g-C3N4 shows much better biocompatibility than those of other Ag-based samples. Third, the PDA plays an important role in the improvement of biocompatibility of g-C3N4 and Ag NPs. The PDA itself, synthesized by polymerization of biomolecular DA, has an excellent biocompatibility. After being modified by PDA, we can observed an increase in cell spreading area, which results in a notable effect in cell proliferation support and apoptosis suppression in the prolonged culture.50-51 In details, 90% of HUVEC cells survived after treatment with 60 ppm Ag/PDA/g-C3N4; meanwhile, the 16


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bacteria can be completely killed with Ag/PDA/g-C3N4 around 60 ppm. Compared with the state-of the-art antibacterial materials, Ag/PDA/g-C3N4 is the superior antibacterial material with an excellent biocompatibility (Table S4). The stability of Ag/PDA/g-C3N4 was demonstrated by detection of dissolved Ag+ from biophotocatalyst within the air exposure, which is shown in Figure 7b. The pure Ag NPs and Ag NPs/rGO were prepared according to our previous works for comparisons.16,52 After exposure in the air for 30 days, only 0.375 ppm of Ag+ was detected, which means that 0.18% Ag+ released out from the biophotocatalyst. In contrast, 95% and 88.7% Ag+ dissolved from Ag NPs and Ag/rGO, respectively. The Ag/PDA/g-C3N4 showed super stability compared with those of the state-of the-art antibacterial materials (Table S4, see Supporting Information). The three-cycle experiments of photocatalytic disinfection (Figure 7c) and the negligible change in structure (Figure 7d) and morphology (Figure S8, see Supporting Information) of Ag/PDA/g-C3N4 prior to and after the three-cycle experiments strongly confirm its excellent stability. The released Ag+ can be chelated by PDA, and then be reduced to Ag0 by PDA and photoexcited e-, which results in its super stability. 38,50 DISCUSSION The goal of this paper is developing biophotocatalysts as a new type of antibacterial materials. The prepared Ag/PDA/g-C3N4(1:2) biophotocatalyst exhibited high activity, super stability, and excellent biocompatibility against E. coli. The super stability of Ag/PDA/g-C3N4 biophotocatalysts in long-term exposure to air environment implies that the released Ag+ is not the key for its high antibacterial activity. A possible bactericidal mechanism for this biophotocatalyst is combination of active Ag NPs and a synergistic photocatalytic performance between Ag NPs and PDA modified gC3N4, which is shown in Figure 9. Without visible-light irradiation, the Ag/PDA/g-C3N4 biophotocatalyst also showed a good antibacterial activity; meanwhile pure g-C3N4 was nontoxic for bacteria. Thus, the antibacterial activity of Ag/PDA/g-C3N4 is attributed to the active Ag NPs due to their small size. In addition, after PDA modification, the Ag/PDA/g-C3N4 can dissolve into water easily (Figure. 1), and affect the bacteria directly through electrostatic adsorption (SEM 17



image, Figure 6e). Under the visible-light irradiation, the antibacterial activity of biophotocatalysts has been significantly improved, which is attributed to the improved photocatalytic activity of gC3N4 by Ag NPs and PDA. It is well known that the g-C3N4-based photocatalysts have a good photodegradation performance due to photogenerated h+ and radicals. These h+ and radicals such as •OH, O2•- etc, also have a strong photocytotoxicity against bacteria cells by damaging all types of organic biomolecules and causing biomembrane oxidation and degradation, which is called ROS mechanism.53 The photo-excited radicals such as •OH, O2•- from biophotocatalysts were investigated by the ESR spin-trap technique with DMPO (Figure 8). It can be clearly seen that both •OH and O2•- can be found in Ag/PDA/g-C3N4(1:2) solution under the visible-light irradiation, increasing with longer irradiated time, while no •OH, O2•- can be detected under the dark condition (Figure 8a and 8b). The intensities of •OH in ESR spectra are much higher than those of O2•-, indicating that the •OH radicals are dominant photo-excited radicals. The photo-excited radicals from g-C3N4, and different Ag/PDA/g-C3N4 biophotocatalysts are also investigated (Figure 8c and 8d). All the Ag/PDA/g-C3N4 biophotocatalysts can photo-excited a large amount of •OH and O2•radicals compared with g-C3N4, and the Ag/PDA/g-C3N4(1:2) is most active. It means that when the photocatalysts have the photo-generated radicals with higher efficiency, they have better antibacterial activity. In this work, the electronic structure of g-C3N4 was modified by PDA and Ag NPs. The decoration of Ag NPs and PDA not only can extend the absorption edge of g-C3N4 (Figure 4a), and the band gap decreased (Figure 4b), which mean that the Ag/PDA/g-C3N4 can harvest visible light and has improved photocatalytic activity. Importantly, the decorated Ag NPs and PDA can act as e- acceptors, which is beneficial for promoting the transfer and separation of charge carriers, and decreasing the recombination of charge carriers more efficiently than pure gC3N4 (Figure 4c and 4d). In doing so, the photo-excited efficiency and the number of radicals are much higher than that of pure g-C3N4, which contributes to the improved antibacterial activity of Ag/PDA/g-C3N4 under the visible light irradiation. The active species trapping experiments are carried out to confirm the dominate active species 18


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for photocatalytic disinfection and further understand the antibacterial mechanism. As shown in Figure 8e, the antibacterial activity of Ag/PDA/g-C3N4(1:2) is changed after adding corresponding active specie capturer. The ordering of species’ effects on antibacterial activity can be obtained (Figure 8f), which is as follow: h+ (50.5%) > •OH (42.6%) > O2•- (32.2%) >> e- (11.8%). Clearly, the h+ and •OH radicals are dominant active species for antibacterial performance because of their strong oxidative activity.54 Importantly, the main formation of •OH is the oxidation of H2O with h+.55 Although O2•- is a modest oxidant, the effect of O2•- still cannot be ignored because they can be transformed to high oxidative •OH radicals.56 In addition, the e- has the minuscule effect in antibacterial activity through being transformed to •OH radicals eventually. Thus, we can confirm that the photo-excited h+ and •OH radicals are dominant active species for antibacterial performance of Ag/PDA/g-C3N4 biophotocatalyst. Therefore, we confirm that the bactericidal mechanism for this biophotocatalyst is combination of active Ag NPs and improved photocatalytic activity of gC3N4 with the synergistic effect of Ag NPs and PDA modified g-C3N4. The strategy of the facile in situ construction of Ag NPs with biophotocatalyst of g-C3N4 could be used to develop a new type of antibacterial materials with high activity, long life and excellent biocompatibility.

CONCLUSIONS In summary, a new type of antibacterial materials of Ag/PDA/g-C3N4 biophotocatalysts has been synthesized by situ reduction method. The biophotocatalyst exhibited excellent antibacterial activity against E. coli, wherein the MIC100%Ag and MBC100%Ag values of Ag/PDA/g-C3N4(1:2) are 9.5 ppm and 6.3 ppm, respectively. Small sized Ag NPs (3.6-10.5 nm) contribute to its antibacterial activity; meanwhile, the visible-light irradiation stimulates significant enhancement in antibacterial activity through photo-excited ROS. Thus, its possible antibacterial mechanism is a combination of active Ag NPs and improved photocatalytic property by a synergistic effect between Ag NPs and PDA modified g-C3N4. Specifically, the Ag NPs and PDA can significantly improve the photocatalytic activity of Ag/PDA/g-C3N4 through promoting the transfer and separation of charge carriers owing 19



to their e- acceptor effects, extending the visible light absorption. Therefore, the h+ and sufficient radicals (particularly •OH), can be photo-generated by Ag/PDA/g-C3N4 efficiently, causing the improved antibacterial activity. Arming with its unique antibacterial mechanism, the Ag/PDA/gC3N4 biophotocatalysts also have excellent biocompatibility and super stability, which hold a great potential in antibacterial applications. ASSOCIATED CONTENT Supporting Information The following files are available free of charge. FT-IR, SEM, TEM images of g-C3N4 nanosheets, Ag/PDA, Ag/g-C3N4 and Ag/PDA/g-C3N4, Zeta potential, inhibition zones, Visible light efficiency and MIC100% values of comparisons, Biocompatibility and stability of Ag NPs-based antibacterial materials AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]; [email protected]; [email protected]

Author Contributions ⊥Y.

Wu and Y.Z Zhou contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The National Science Foundation of China (Grant 51702129 and 51572114) supported this work. YZZ wants to thank supporting from Postdoctoral Science Foundation (2018M630527) and China Scholarship Council (201708320150). ZN thanks to the Key University Science Research Project of Jiangsu province (16KJB430009). We would also like to express our thanks to Kangmin Chen and Li Pan for their help on TEM, SEM and PL studies in Jiangsu University Analysis and Test Center.

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Figures & Tables

Figure 1. Schematic illustration for synthesis of Ag/PDA/g-C3N4 biophotocatalysts: (a) dispersing g-C3N4 in Tris-HCl buffer solution, (b) coating PDA on the surface of g-C3N4 sheets by in situ polymerization of DA, (c) adsorbing Ag+ on the surface of PDA/g-C3N4, (d) reducing Ag+ to Ag by NaBH4 in ice-water condition, (e) the digital photographs of powders in different experimental procedures and their aqueous solutions. 169.8 x 68.3 mm (300 x 300 DPI)

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Figure 2. (a) XRD patterns of g-C3N4 and three kinds of Ag/PDA/g-C3N4 biophotocatalysts, (b) XPS survey spectra, (c) C 1s, (d) N 1s and (e) O 1s spectra of g-C3N4 and biophotocatalyst, and (f) Ag 3d XPS spectrum of the biophotocatalyst. 169.9 x 91.6 mm (300 x 300 DPI)

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Figure 3. TEM images and particle size distributions for (a, d, g) Ag/PDA/g-C3N4(1:4), (b, e, h) Ag/PDA/g-C3N4(1:2), and (c, f, i) Ag/PDA/g-C3N4(1:1). 129.3 x 110.5 mm (300 x 300 DPI)

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Figure 4. UV-vis (a, b), PL (c, d), Transient photocurrent response (e) and EIS (f) spectra of gC3N4 and Ag/PDA/g-C3N4 composites. 165.3 x 180.7 mm (300 x 300 DPI)

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Figure 5. Bacterial growth curves of E. coli treated with serial concentrations of pure g-C3N4, PDA/g-C3N4, Ag/PDA/g-C3N4(1:2) biophotocatalysts in the dark (left column) and under visible light irradiation (right column) respectively. 154.6 x 169.1 mm (300 x 300 DPI)

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Figure 6. (a) Bacteria time-kill curve profiles of E. coli treated with different concentrations of Ag/PDA/g-C3N4(1:2) biophotocatalysts within 3 h, (b) Inhibition of colonies of E. coli after 30 ppm Ag/PDA/g-C3N4(1:2) treatments with incubation time, SEM and TEM images of E. coli cells before (c, d) and after (e, f) the treatment with 30 ppm Ag/PDA/g-C3N4 (1:2) for 2 h under the visible light irradiation. 169.8 x 85.8 mm (300 x 300 DPI)

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Figure 7. (a) Cytotoxicity of Ag/PDA/g-C3N4(1:2), AgNO3, g-C3N4, PDA/g-C3N4 and Ag/gC3N4(1:2) against HUVEC cells, (b) Temporal evolution of dissolved Ag+ from AgNO3, Ag NPs, Ag/rGO and Ag/PDA/g-C3N4(1:2) detected by ICP-OES within the air exposure time (CAg=200 ppm for original solution), (c) Three-cycle run experiments of photocatalytic disinfection of 30 ppm Ag/PDA/g-C3N4 (1:2) under visible light irradiation, (d) XRD patterns of the composite before and after three-cycle experiments. 148.8 x 112.8 mm (300 x 300 DPI)

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Figure 8. DMPO-•OH (a,c) and DMPO-O2•- (b,d) of Ag/PDA/g-C3N4 (1:2) for various time of irradiation, Ag/PDA/g-C3N4 (1:1, 1:2, 1:4) and g-C3N4 under visible light irradiation, (e) Photocatalytic disinfection ability of 30 ppm Ag/PDA/ g-C3N4（1:2） with different scavengers under visible light irradiation, (f) Effect of scavengers on photocatalytic antibacterial efficiency of 30 ppm Ag/PDA/g-C3N4 (1:2) under visible light irradiation. 131.2 x 137.9 mm (300 x 300 DPI)

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Figure 9. Bactericidal mechanism for Ag/PDA/g-C3N4 biophotocatalyst. 144.2 x 93.4 mm (300 x 300 DPI)

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Table 1. Comparison of the bactericidal performance of the state-of the-art antibacterial materials Sample type

MIC

MBC

MBC Ag

Light source

GO-PEGa-Ag

>5 ppm (100%)

>5 ppm (100%)

>1.2 ppm (100%)

No light

39

silver nanoparticles

>30ppm (30%)

>17.5 ppm (100%)

>17.5ppm (100%)

No light

40

Ag/CuO Microspheres

NR

>100 ppm (92%)

>70 ppm (92%)

No light

41

2-4 ppm (100%)

>2 ppm (100%)

≈1.5ppm (100%)

Graphene quantum

NIR light

dot/silver

Z-scheme AgI/BiVO4

Ref

42 (808 nm cutoff filter)

NRb

>36 ppm (100%)

300 W Xenon lamp

(9.09% AgI)

(420 nm UV cutoff filter)

>404 ppm

43

300 W Xenon lamp Ag nanoparticles /g-C3N4

NR

>50 ppm (98%)

>18 ppm (98%)

44 (400 nm UV cut-off filter)

>0.85 mg/mL

NIR light

Silver/molybdenum oxide

>1.12mg/mL (99.2%)

>37.5 mgL-1

(97%)

45 (780 nm cutoff filter)

300 W xenon lamp Graphene Oxide/g-C3N4

>100 ppm (100%)

>100 ppm (98.9%)

NR


40 W fluorescent Ag/TiO2

NR

>400 ppm

>40 ppm (100%)


Ag/AgCl/ZnO

NR

>90 ppm (100%)

>27 ppm (100%)

α‑S Graphene/g‑C3N4

NR

>100 ppm (100%)

NR

300 W xenon lamp

48

300 W Xenon lamp 49 (400 nm UV cut-off filter)

300 W Xenon lamp Ag/PDA/g-C3N4(1:2)

>45 ppm (100%)

>30 ppm (100%)

>6.3 ppm (100%)

Our work (420 nm UV cut-off filter)

a

PEG: polyethylene glycol; b NR: not reported;

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For Table of Contents Use Only

84.7 x 47.6 mm (600 x 600 DPI)

Brief Synopsis Ag/PDA/g-C3N4 that a photocatalyst shows an excellent and sustainable bactericidal performance under visible light.

39


g

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