Graphene oxide-modified ZnO particles: synthesis, characterization, and antibacterial properties

Abstract

Nanosized ZnO particles with diameters of 15 nm were prepared with a solution precipitation method at low cost and high yield. The synthesis of the particles was functionalized by the organic solvent dimethylformamide, and the particles were covalently bonded to the surface of graphene oxide. The morphology of the graphene oxide sheets and ZnO particles was confirmed with field emission scanning electron microscopy and biological atomic force microscopy. Fourier transform infrared spectroscopy and X-ray diffraction were used to analyze the physical and chemical properties of the ZnO/graphene oxide composites that differed from those of the individual components. Enhanced electrochemical properties were detected with cyclic voltammetry, with a redox peak of the composites at 0.025 mV. Excellent antibacterial activity of ZnO/graphene oxide composites was observed with a microdilution method in which minimum inhibitory concentrations of 6.25 µg/mL for Escherichia coli and Salmonella typhimurium, 12.5 µg/mL for Bacillus subtilis, and 25 µg/mL for Enterococcus faecalis. After further study of the antibacterial mechanism, we concluded that a vast number of reactive oxygen species formed on the surface of composites, improving antibacterial properties.

Keywords: ZnO; antibacterial property; characterization; graphene oxide.

Figure 1

Illustration for the synthesis of the ZnO/graphene oxide composites. Abbreviations: ZnO, zinc oxide; NaOH, sodium hydroxide; APTS, 3-aminopropyltriethoxysilane; DMSO, dimethyl sulfoxide; DMF, dimethylformamide.

Figure 2

Illustration for the preparation of the Au-PCB/ZnO/graphene oxide platform. Abbreviations: Au-PCB, gold printed circuit board; ZnO, zinc oxide.

Figure 3

FE-SEM images and EDX result. Notes: (A) ZnO particles are round in shape and disperse on the substrate; (B) a big ZnO particle contains some small nanoparticles; (C) graphene oxide has grooves and wrinkles on the edges; (D) ZnO particles were anchored onto the surface of graphene oxide via the covalent bonds; (E) EDX image clearly show that the sample has pure ZnO phases. Abbreviations: FE-SEM, field emission scanning electron microscopy; EDX, energy dispersive X-ray spectroscopy; ZnO, zinc oxide.

Figure 4

AFM images of ZnO particles, graphene oxide, and ZnO/graphene oxide composites. Notes: (A) ZnO particles were round; (B) graphene oxide sheets exhibited some grooves and wrinkles on the surface; (C) ZnO particles were anchored onto the surface of graphene oxide sheets. The arrows show the part of ZnO particles and (D)–(F) are 3D pictures of the three particle parts. Abbreviations: AFM, atomic force microscopy; ZnO, zinc oxide; 3D, three-dimensional.

Figure 5

UV-visible absorbance spectra. Notes: A peak appeared at 352 nm in spectra of ZnO. Graphene oxide exhibited an absorption peak centered at 235 nm and a shoulder at ~300 nm. In composites, one peak appeared at 352 nm related to ZnO, and the other peak corresponding to graphene oxide at 235 nm blueshifts to 225 nm. All peaks in the figure are represented by the correspondingly colored arrows. Abbreviations: UV, ultraviolet; ZnO, zinc oxide.

Figure 6

XRD patterns. Notes: (A) ZnO particles; (B) graphene oxide; (C) ZnO/graphene oxide composites. Abbreviations: XRD, X-ray diffraction; ZnO, zinc oxide.

Figure 7

FT-IR spectrum. Notes: (A) the peak of graphene oxide at 1,000 cm⁻¹, 1,150 cm⁻¹, 1,650 cm⁻¹, 1,750 cm⁻¹, and 3,350 cm⁻¹ represented C–O, C–OH, C=C, C=O, and O–H, respectively; (B) ZnO/graphene oxide composites at 650 cm⁻¹, a special peak attributed to the Zn–O vibration. Abbreviations: FT-IR, Fourier transform infrared; ZnO, zinc oxide.

Figure 8

CV curves. Notes: (A) Bare Au-PCB; (B) Au-PCB/graphene oxide; (C) Au-PCB/ZnO/graphene oxide hybrids. Abbreviations: CV, cyclic voltammetry; Au-PCB, gold printed circuit board; ZnO, zinc oxide.

Figure 9

Representative Petri plate images showing the variable number of Gram-negative and Gram-positive bacteria colonies with different amounts of ZnO/graphene oxide. Notes: (A) Escherichia coli; (B) Salmonella typhimurium; (C) Bacillus subtilis; (D) Enterococcus faecalis. Abbreviation: ZnO, zinc oxide.

Figure 10

Inhibitory zones of ZnO/graphene oxide composites against (A) Escherichia coli; (B) Salmonella typhimurium; (C) Bacillus subtilis; (D) Enterococcus faecalis. Abbreviation: ZnO, zinc oxide.

Figure 11

SEM images of (A) normal Escherichia coli cells with integrated membrane; (B)–(D) ZnO particles, graphene oxide, and ZnO/graphene oxide composites treated E. coli, respectively. Note: After 2 hours, by comparison, ZnO/graphene oxide composites cause more damage than ZnO particles and graphene oxide sheets. Abbreviations: SEM, scanning electron microscopy; ZnO, zinc oxide.

Figure 12

Fluorescence intensity spectra of Escherichia coli broth with ZnO, graphene oxide, and ZnO/graphene oxide composites. Abbreviation: ZnO, zinc oxide.

Figure 13

Fluorescence images of Escherichia coli cell. Notes: (A) with only DCFH-DA for control; (B) with DCFH-DA in ZnO; (C) with DCFH-DA in graphene oxide; (D) with DCFH-DA in ZnO/graphene oxide. Abbreviations: DCFH-DA, 2′,7′-dichlorodihydrofluorescein diacetate; ZnO, zinc oxide.

Figure 14

Flow cytometric analysis. Notes: (A) normal Escherichia coli; (B)–(D) E. coli were treated with ZnO particles, graphene oxides, and ZnO/graphene oxide composites, respectively. P₁ and P₂ regions stand for dead bacteria and live bacteria. We can estimate antibacterial property through the number of bacteria in P₁ and P₂ regions. Abbreviation: ZnO, zinc oxide.