Light-Enhanced Antibacterial Activity of Graphene Oxide, Mainly via

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Light-enhanced Antibacterial Activity of Graphene Oxide Mainly via Accelerated Electron Transfer Yu Chong, Cuicui Ge, Ge Fang, Renfei Wu, He Zhang, Zhifang Chai, Chunying Chen, and Jun-Jie Yin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00663 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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Environmental Science & Technology

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Light-enhanced Antibacterial Activity of Graphene Oxide

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Mainly via Accelerated Electron Transfer

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Yu Chong,†,⊥ Cuicui Ge,*,†,⊥ Ge Fang,† Renfei Wu,† He Zhang,† Zhifang Chai,†

4

Chunying Chen*,‡ and Jun-Jie Yin*,⊥

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†

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Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University,

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Suzhou 215123, China

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‡

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Excellence in Nanoscience, National Center for Nanoscience and Technology of China, Chinese

School for Radiological and Interdisciplinary Sciences (RAD-X), Collaborative Innovation

Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety & CAS Center for

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Academy of Sciences, Beijing 100190, China

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⊥

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Regulatory Science, Center for Food Safety and Applied Nutrition, U.S. Food and Drug

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Administration, College Park, Maryland 20740, United States

Division of Bioanalytical Chemistry and Division of Analytical Chemistry, Office of

ACS Paragon Plus Environment


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ABSTRACT:

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Before graphene derivatives can be exploited as next generation antimicrobials, we

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must understand their behavior under environmental conditions. Here we demonstrate

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how exposure to simulated sunlight significantly enhances the antibacterial activity of

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graphene oxide (GO) and reveal the underlying mechanism. Our measurements of

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reactive oxygen species (ROS) showed only singlet oxygen (1O2) is generated by GO

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exposed to simulated sunlight, which contributes only slightly to the oxidation of

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antioxidant biomolecules. Unexpectedly, we find the main cause of oxidation is

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light-induced electron-hole pairs generated on the surface of GO. These light-induced

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electrons promote the reduction of GO, introducing additional carbon-centered free

24

radicals which may also enhance the antibacterial activities of GO. We conclude that

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GO-mediated oxidative stress mainly is ROS-independent; simulated sunlight

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accelerates the transfer of electrons from antioxidant biomolecules to GO, thereby

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destroying bacterial antioxidant systems and causing the reduction of GO. Our

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insights will help support the development of graphene for antibacterial applications.

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Table of Contents

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INTRODUCTION The threat of bacterial infections that are untreatable due to antibiotic resistance

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is a deepening crisis worldwide. Therefore, it is highly desirable to develop novel

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antibacterial agents, particularly those which have mechanisms of action difficult for

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bacteria to overcome. The extraordinary electronic, mechanic, and optical

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characteristics of graphene structures and graphene derivatives suggest these could

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become the next generation of antimicrobial materials.1-3 The antibacterial action of

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graphene oxide (GO) arises from both physical and chemical activities.4-10 Physical

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damage is caused by the sharp edges of GO that shred bacterial membranes, damage

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RNA, and cause the destructive extraction phospholipid molecules.5-7 The primary

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type of chemical damage is believed to be oxidative stress, initiated by either

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reactive oxygen species (ROS) or by charge transfer.9,10

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To date, most reports have focused on the close relationship between

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antimicrobial activities and the physicochemical properties of GO.1,11,12 For instance,

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the antibacterial activity of GO is influenced by the size of the graphene sheets.4,13

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Data also indicate that the antimicrobial activity of GO can be enhanced by

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increasing the number of defects in GO nanosheets. Other characteristics which can

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alter GO antibacterial properties include layer number, morphology, dispersibility,

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surface charge, and oxygen content.14-17

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However, environmental conditions might also transform the physicochemical

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features of GO in ways that would alter their antibiotic effects.18,19 Several studies

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have shown that GO can be photochemically reduced by exposure to high-energy


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ultraviolet or laser light.20,21 Yang et al.22 reported such reduced GO could be used

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for in vivo photothermal therapy on tumors, due to its strong optical absorbance in

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the near-infrared (NIR) region. A study by Hou et al. in 2015 demonstrated that GO

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could behave as a semiconductor photocatalyst, generating electron−hole pairs able

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to reduce the oxygen content of GO structures.23 These findings leave open the

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question of whether exposure to sunlight could affect the antibacterial activity of GO.

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Further, it is not known whether data from previous explorations of GO exposed to

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specific durations of high intensity laser or other artificial light sources can provide

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adequate predictions about how photoreactions will occur under conditions of

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natural or simulated sunlight.

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Our current project examines these factors for the first time, systematically

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evaluating changes in the antibacterial activity of GO upon exposure to simulated

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sunlight. Then, we studied the oxidative stress pathway, including the generation of

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ROS, using electron spin resonance (ESR) techniques, to identify the mechanism

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responsible for that enhanced antibacterial activity. We investigated the possibility of

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ROS-independent oxidative stress and determined the production of light-induced

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electron-hole pairs. By using UV-vis spectrum and X-ray photoelectron

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spectroscopy (XPS) we were able to characterize the chemical transformation of GO

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in the presence of both simulated sunlight and reducing agents. Insights from these

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multiple approaches, enable us to provide a clear explanation for the observed

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enhancement of antibacterial activity of GO after exposure to simulated sunlight.

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MATERIALS AND METHODS



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Materials. Graphene oxide (GO) was purchased from Chengdu Organic

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Chemical Company, Chinese Academy of Science. The E. coli strain ATCC-25922

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was obtained from the American Type Culture Collection (ATCC, Rockville, MD,

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USA). 5,5'-Dithiobis (2-nitrobenzoic acid) (Ellman’s reagent, DTNB), 5,5-Dimethyl-

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1-pyrroline-N-oxide (DMPO), 5-tert-Butoxycarbonyl-5-methyl-1-pyrroline-N-oxide

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(BMPO) were obtained from Dojindo Laboratories (Kumamoto, Japan).

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1-Hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) and 2,2,6,6-

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tetramethylpiperidine-1-oxyl (TEMPO) were purchased from Alexis Biochemicals

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(Enzo Life Sciences, Farmingdale, NY). All solutions were prepared using Milli-Q

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water (18 MΩ cm). Other chemicals were obtained from Sigma Aldrich (St. Louis,

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MO) and used as received. A 450 W Xenon lamp, filtered by an air mass (AM) 1.5 G

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solar simulation bandpass filter was used to provide simulated sunlight irradiation.

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The light path length was about 40 cm and the spectral irradiance of this source,

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monitored with a Newport Optical Meter, was determined to be 380 mW/cm2.

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Characterization of GO. The as-received sample was characterized by various

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techniques. Atomic force microscope (AFM) images were acquired from an AFM

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(Dimension Icon, Bruker) operating in the ScanAsyst mode. The UV-vis spectra

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were recorded on a Shimadzu UV-3600 UV-vis spectrophotometer. Raman spectra

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were obtained on a JY HR800 spectrometer with 532 nm wavelength incident laser

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light. We used a Horiba Fluorolog 3-221 spectrofluorometer with a 1 cm light path

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quartz cuvette to characterize photoluminescence of GO with or without antioxidants.

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The scans of excitation-emission matrix and its light scattering were corrected for


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the background subtraction and instrument configurations. The measurements were

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conducted at room temperature (about 22℃) in air (240 μM O2 concentration and

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pH=7.4) Assay for Antibacterial Activity of GO. The E. coli strain ATCC-25922 was

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cultured overnight in LB agar (Luria-Bertani) medium at 37ºC to promote an

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exponential growth phase. Bacterial suspensions were centrifuged (5000 rpm, 5 min)

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and washed twice with sterile saline to remove residual media components. Finally,

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each suspension was diluted with saline solution to 106-107 CFU/mL for the

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following experiments.

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First, GO was added to the bacterial suspension to reach final concentrations of

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25, 50, 75 and 100 𝜇g/mL, respectively. These suspensions were incubated at 37℃

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at 250 rpm shaking speed for 2 h. To assess the effects of simulated sunlight on the

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behavior of GO, 25 μg/mL GO exposed to simulated sunlight for periods ranging

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between 0-30 min under the following conditions: with either NaN3 or histone

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(which can scavenge singlet oxygen), or with no further addition (Control). Then,

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each bacterial suspension was plated on LB agar media and incubated overnight at

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37℃ for CFU enumeration.

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The antibacterial activity in each GO concentration/duration of light exposure

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was measured using LIVE/DEAD fluorescent staining as follows. After 2 h exposure

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to GO either with or without exposure to simulated sunlight, the cells were stained

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by adding SYTO9 and propidium iodide (PI) to each suspension. These samples

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were incubated for 30 min in the dark before 5 μL was pipetted to a microscope slide



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and then covered with a coverslip. Confocal laser microscopy (FV1200, OLYMPUS,

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Japan) was then used to take luminescence images.

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Morphological changes to the bacteria after treatment were visualized using a

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scanning electron microscope (SEM). After GO treatments, the cells were collected

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and fixed in a glutaraldehyde solution, dehydrated by a sequential series of ethanol

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solutions (25, 50, 75, 85, 95 and 100%) and dried in a desiccator to remove ethanol.

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Each sample was mounted onto an aluminum stub and imaged by SEM (S-4700,

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Hitachi, Japan).

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Direct ESR Detection of ROS. All ESR measurements were carried out at

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ambient temperature using an ESR spectrometer (EMX, Bruker, USA). The spin

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adducts were detected at the following settings, unless otherwise stated: 20 mW

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microwave power, 1 G field modulation and 100 G scan range. Fifty microliter

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aliquots of the sample solutions were put in separate quartz capillary tubes with

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internal diameters of 0.9 mm. The spin trap 4-oxo-TEMP was used to verify the

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presence of any singlet oxygen generated by GO upon exposure to simulated

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sunlight. The superoxide and hydroxyl radicals were determined using BMPO. The

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presence of holes and electrons were detected using the spin labels CPH and

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TEMPO.

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Assay for GSH and AA Oxidation. As described in our previous publications,

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UV-vis spectroscopy was used to determine the oxidation of glutathione (GSH) and

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ascorbic acid (AA) by GO exposed to simulated sunlight, as follows.28 50 µM GSH

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were added to the suspensions of GO (25 µg/mL) and exposed to simulated sunlight


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for 30 min. Adding 100 µM DNTB to the mixtures yielded a yellow product,

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quantified spectrophotometrically by measuring absorbance at 412 nm. To examine

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the effects of irradiated GO on AA, 25 µg/mL GO were mixed with 100 µM AA.

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During these 30 minutes of exposure to simulated sunlight, samples were collected

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every five minutes and the amount of remaining AA was determined by measuring

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the absorbance of AA at the 265 nm. We used NaN3 and histone separately to

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scavenge the singlet oxygen during photoexcitation of GO to further distinguish the

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effects of singlet oxygen on the oxidation of either AA or GSH.

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Characterization of GO Exposed to Simulated Sunlight in Presence of

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Antioxidants. Fluorescence spectra of 0.1 mg/mL GO in the presence or absence of

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0.2 mg/mL AA were acquired using an Edinburgh Instruments FLS 980

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spectrophotometer with a 1 cm light path quartz cuvette. Absorption spectra were

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recorded on a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan). Milli-Q

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water (18 MΩ cm) was used as reference. The XPS measurements were performed

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on an X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Scientific, USA)

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using a monochromatic Al Ka (1486.6 eV) source.

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RESULTS AND DISCUSSION

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Characterization of GO. GO used in the present study was purchased from

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Chengdu Organic Chemical Company, Chinese Academy of Science. The

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characterization of this nanomaterial was shown in Figure S1, including Atomic

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force microscope (AFM) images, UV-vis spectra and Raman spectra. From the AFM

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height images, we observed GO have lateral dimension of several micrometers and



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thickness around 1 nm. The UV−vis absorption spectra showed that there are two

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peaks centered at around 231 and 280 nm in the UV region. Raman analysis

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displayed large D and G bands at 1345 and 1592 cm-1, respectively. All these

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characteristics agree well with other reports [13-15] and clearly demonstrate that the

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as-received sample is a fully exfoliated GO nanosheet.

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Antimicrobial Activity of GO Exposed to Simulated Sunlight. First, we

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determined the toxicity of GO to bacteria without exposure to simulated sunlight. As

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shown in Figure S2, E. coli cells (106 to 107 CFU/mL) were incubated with various

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concentrations of GO for 2 h; colony count results indicated a considerable

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cytotoxicity, which increased in direct proportion to concentrations of GO. Scanning

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electron microscopy (SEM) images (Figure S3) clearly showed that GO treatment

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resulted in loss of bacterial membrane integrity. At this point we did not detect any

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trace of an ROS signal (Figure S4) using ESR, which is considered the most reliable

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and direct method to detect short-lived free radicals. However, found upon addition

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of GO, noticeable time-dependent GSH oxidation was observed (Figure S5), perhaps

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as a consequence of a direct charge transfer from GSH to GO. These findings

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indicate the antibacterial activity of GO may be caused by both direct membrane

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damage and ROS-independent oxidative stress; these observations are consistent

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with several earlier reports.1,4,14

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Next, we studied the effect of exposure to simulated sunlight on the antibacterial

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activities of GO. As shown in Figure 1a, 30 min simulated sunlight exposure alone

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had a negligible effect on E. coli cells compared to the set of control cells. Cell


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cultures treated with 25 μg/mL GO had a survival percentage of 68.8±9.2%;

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however when cultures containing that same amount of GO were exposed to

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simulated sunlight, the survival percentage was reduced to 24.9±5.9%,

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demonstrating that simulated sunlight significantly enhanced the antibacterial

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activity of GO (p < 0.01). We also investigated how duration of exposure to

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simulated sunlight could affect the antibiotic activity of GO. As shown in Figure 1b,

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as the length of exposure to simulated sunlight increased, the viability of E. coli in

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the presence of GO decreased in a stepwise fashion. To confirm this phenomenon we

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performed a fluorescence-based Live/Dead assay. As expected, no obvious cell

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apoptosis could be found in cultures exposed to just the simulated sunlight alone or

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to just the GO treatment alone. However, simultaneous treatment with GO and

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simulated sunlight markedly reduced viability, as indicated by a remarkable increase

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in the number of PI-permeable, red fluorescent cells (Figure 1c).

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ROS Generated by Light-irradiated GO. Previous research has established

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that, graphene-based nanomaterials (e.g., GO, rGO) exhibit a photothermal effect

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when irradiated by an NIR laser. This effect can kill both bacteria and tumor

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cells.18,22,25 However, exposure to simulated sunlight for 0 – 30 minutes does not

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significantly raise the temperature of GO-treated bacterial suspensions (data not

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shown); therefore photothermal effects can be excluded. Instead, we inferred that the

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light-enhanced antibacterial activities of GO might be a consequence of oxidative

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stress, resulting from light-induced generation of reactive oxygen species. To test

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this hypothesis, we used ESR spectroscopy to measure whether GO was able to



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produce ROS after exposure to simulated sunlight. We employed DMPO and BMPO

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as spin trap for the hydroxyl radical and superoxide anion, respectively, and

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4-oxo-TEMP to detect singlet oxygen. Based on previous research, we used an •OH

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generating Fenton reaction, enzymatic O2•− generating system, and the 1O2 generating

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compound Rose Bengal as references.26-28 Neither hydroxyl radical nor superoxide

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anion signals were observed (Figure S4), although a triplet spectrum, characteristic

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for the reaction between the 4-oxo-TEMP and singlet oxygen, was noted during the

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exposure of GO to simulated sunlight (Figure 2a). As a spin trap, 4-oxo-TEMP itself

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is ESR silent, but it can specifically capture 1O2 to yield a nitroxide radical,

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TEMPONE, which has an observable and stable ESR spectrum. In addition, only a

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negligible amount of hydrogen peroxide was generated during photoexcitation of GO

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by using the hydrogen peroxide assay kit (data not shown).

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These results demonstrate singlet oxygen could be generated by GO exposed to

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sunlight. To confirm this conclusion, we added sodium azide (NaN3), a scavenger

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capable of consuming generated 1O2, during the test. After adding 1 mM NaN3, only

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a negligible TEMPO signal could be detected (Figure 2a), indicating the triplet

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spectrum can be ascribed to the presence of singlet oxygen generated by GO. As

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superoxide anions cannot be generated under anaerobic conditions, we infer that this

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singlet oxygen must be generated by energy transfer from GO to the ground-state

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oxygen.

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GO-mediated Oxidative Stress and its Relationship with ROS. The overproduction of singlet oxygen will predictably impair the antioxidant defense


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systems of bacteria.29 To document this, we examined in vitro glutathione (GSH)

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oxidation by GO with or without exposure to simulated sunlight. GSH is an

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endogenous bacterial antioxidant, present in concentrations ranging between 0.1 and

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10 mM.14,30,31 The thiol groups in GSH, which can be quantified by the Ellman’s

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assay, can be oxidized to a disulfide bond, yielding the oxidized form, glutathione

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disulfide (GSSG). As shown in Figure 2b, 30 min exposure to either simulated

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sunlight or to GO alone caused extremely limited oxidation of glutathione, which

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agrees well with earlier findings.13,14,23 However, the extent of GSH oxidation was

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remarkably enhanced upon exposure to GO in combination with simulated sunlight;

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these effects increased as a function of exposure time, indicating that sunlight could

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accelerate GSH oxidation by GO. Next, we used ascorbic acid (AA) as another

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indicator of oxidative stress. AA is a water-soluble antioxidant which has an

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important biologic role regulating the intracellular redox state, through its interaction

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with GSH.32-34 The characteristic absorption maximum for AA is located at 265 nm.

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We consistently found that extremely significant AA oxidation occurred when GO

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and simulated sunlight were introduced; however, samples which did not contain GO

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or were not exposed to simulated sunlight only exhibited a minor depletion of AA

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(Figure 2c). These observations verify that GO exposed to sunlight could cause

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significant oxidation of biological antioxidants.

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The aforementioned results have suggested that only 1O2 could be determined,

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therefore we inferred the light-enhanced oxidation capacity of GO might arise

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primarily from the generated 1O2. In order to verify our hypothesis, we add NaN3 and



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histone (to scavenge the generated 1O2) into the GO dispersion before exposure to

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simulated sunlight. We had expected that the extent of GSH and AA oxidation by GO

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would be greatly inhibited, because the inducer, 1O2, would have been consumed.

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Surprisingly, the majority of GSH and AA still could be oxidized by GO that had been

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exposed to simulated sunlight even after the addition of scavengers. These findings

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suggest that generated 1O2 is not the dominant factor in the light-enhanced oxidation

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capacity of GO (Figure 2d). To confirm this hypothesis, we assessed the antibacterial

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activity of GO (after exposure to simulated sunlight) in the presence of 1O2

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scavengers. As expected, those conditions produced data comparable to those

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acquired without adding 1O2 scavengers (Figure S6). Given these results, we must

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conclude that oxidative stress caused by ROS is not the primary mechanism

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responsible for light-enhanced oxidation capacity of GO.

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Exploring the ROS-Independent oxidation capacity of GO to simulated

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sunlight. In order to identify a more likely mechanism, we investigated the

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photochemical fate of GO under biologically-relevant conditions. Previous research

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has shown GO has a large energy gap between π-state from its sp2 carbon sites and

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σ-state of its sp3-bonded carbons.35 Therefore, it is possible that electrons and holes

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are generated on the surface of GO when it is exposed to simulated sunlight. To

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explore this possibility, we investigated the light-induced formation of electron-hole

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pairs in situ, using ESR spectroscopy.28,36 Specifically, we used CPH to examine the

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oxidizing activity of photogenerated holes and used TEMPO to verify photogenerated

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electron. As shown in Figure 3a, CPH is ESR silent, therefore no reaction between


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CPH and GO was anticipated. Surprisingly, when the GO was exposed to simulated

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sunlight, an ESR spectrum of three lines with intensity ratios of 1:1:1 was observed.

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As the duration of exposure was extended, more CPH was oxidized to CP-nitroxide.

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In contrast to the standard behavior of CPH, TEMPO normally exhibits a stable

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triplet ESR spectrum, with relative intensities of 1:1:1, although it can be reduced to

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TEMPOH, which is ESR silent. We observed that the TEMPO signal intensity was

280

unchanged in the presence of either GO alone or light alone. When both GO and

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simulated sunlight were present, we observed an obvious reduction in the ESR signal

282

intensity, and that signal intensity continued to decrease as light exposure time was

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increased. These results confirm that GO exposed to simulated sunlight induces the

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generation of electron-hole pairs. However, due to the confinement of redox potential,

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those light-induced electron-hole pairs cannot react with surrounding H2O and O2 to

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respectively form •OH and O2•−.

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Our results demonstrate that 1O2, the only type of ROS determined in these

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experiments, could not be the primary cause of GSH oxidation. The oxidation activity

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of the light-induced holes led us to speculate that these holes might be the main factor

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driving the oxidation of antioxidant biomolecules. To verify this, we used

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fluorescence excitation-emission spectra characterization. As shown in Figure 3b, GO

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exhibited an inhomogeneous broadened fluorescence in the range of 410-750 nm,

293

with a maximum at about 550 nm and excitation at 470 nm. Previous research has

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demonstrated that photoluminescence of GO is a consequence of radiative

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electron-hole pair recombination.35,37,38 Nonetheless, when we introduced AA, a



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remarkable decrease in photoluminescence intensity, even though the

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photoluminescence wavelength characteristics remained unchanged. This

298

phenomenon demonstrates how antioxidant biomolecules could consume the

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light-induced holes, inhibit electron-hole pair recombination, and thereby suppress the

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photoluminescence of GO.

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Chemical Transformation of GO upon Exposure to Simulated Sunlight. The

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generation of light-induced holes could cause the oxidation of vital cellular

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components which mediate ROS-independent oxidative stress. This leads us to

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another question: if holes are responsible for oxidization of antioxidant biomolecules,

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what is the role of light-induced electrons? To investigate the fate of light-induced

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electrons, we added antioxidant biomolecules (GSH, AA) to GO dispersions, thereby

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consuming holes and leaving residual excited electrons. We characterized the

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changes of GO under a variety of conditions (including the presence or absence of

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antioxidants and with or without exposure to simulated sunlight) using UV-vis

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spectrum and X-ray photoelectron spectroscopy. Figure 4a illustrates a series of GO

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dispersions: no obvious color change was observed under conditions when only

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simulated sunlight or only antioxidant molecules were present, which confirms the

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slow reduction process of GO observed in previous reports.23,39,40 However, when

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both antioxidant molecules and simulated sunlight were present, the GO dispersions

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underwent very obvious color changes (from yellow to black), which indicate the

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tremendous reduction of GO under these conditions. These results demonstrate how

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the presence of antioxidants could induce sunlight-exposed GO to create significant


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amounts of excited residual electrons, which, in turn, would greatly accelerate the

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reduction of GO.

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Our next assays monitored the reduction of GO using UV-vis spectroscopy. As

321

with previous assays, the spectroscopic profile of GO did not change significantly

322

when AA was added by itself, or if the GO was only exposed to simulated sunlight

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(Figure 4b). However, when both these factors were present, the maximum

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absorption peak of the GO dispersion gradually red-shifted from 231 nm to 263 nm

325

and the optical absorption in the NIR region also markedly increased. We used XPS

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to characterize the reduction of GO under various conditions. As shown in Figure 4c,

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we detected three different peaks, centered at 284.5, 286.4 and 289.2 eV,

328

corresponding to C=C/C–C in the aromatic rings, C–O of the epoxy, or the alkoxy

329

groups and the COOH groups, respectively. Upon addition of AA, the intensities of

330

all C 1s peaks of the carbons binding to the oxygen of the light-exposed GO

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decreased dramatically, indicating that most of the oxygen-containing functional

332

groups had been removed. Because those light-induced electrons could be trapped by

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the oxygen functional groups, it is possible for oxygen-centered radicals or

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carbon-centered radicals to be introduced during the process of removing oxygen on

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the GO surface. As indicated in Figure 5, GO nanosheets exhibit a unique,

336

symmetrical ESR signal at g = 2.002, close to the free electron g-value of 2.0023,

337

representing carbon-centered radicals.41 We found that neither simulated sunlight

338

alone nor antioxidant alone had any noticeable effects on the ESR signal intensity

339

from the paramagnetic defects of GO. However, adding antioxidants in the presence



340

of simulated sunlight resulted in a marked enhancement of the ESR signal. Notably,

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the ESR spectrum for the AA radical indicated that the oxidation of AA and the

342

reduction of GO were simultaneous.

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Based on all the data, we conclude that exposure to simulated sunlight accelerate

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the electron transfer from antioxidant biomolecules to GO, and as a result, the

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antioxidant system is destroyed and GO itself is reduced accompanied by the

346

introduction of carbon-centered free radicals. The oxidative stress, reduced GO and

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introduced carbon-centered free radicals co-contribute to the overall antibacterial

348

efficacy.

349

Implications. The antibacterial properties of graphene-based nanomaterials,

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such as GO, may offer specific advantages that surpass those of inorganic and

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polymeric nanomaterials.42-45 Yet, these cannot be exploited until we understand their

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behavior under environmental and other biologically-relevant conditions. Here we

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have demonstrated how GO exhibit impressive antibacterial upon exposure to

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simulated sunlight, resulting in oxidative stress that is mainly independent of ROS.

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Exposure to simulated sunlight accelerates the electron transfer from the innate

356

antioxidant systems of E. coli to GO, thereby destroying the biomolecules that

357

ordinarily provide protection from oxidative stressors. Meanwhile, the light-induced

358

electrons promoted the reduction of GO, inducing additional carbon-centered free

359

radicals, which may augment the antibacterial activities of GO. Our work provides

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the first full description of the antibacterial mechanisms of GO and offers guidance

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for developing highly-efficient graphene-based antibacterial materials.


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ASSOCIATED CONTENT

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Supporting Information Available: Additional experimental details and figures

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(Figures S1−S6), as described in the text. This material is available free of charge at

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http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Authors

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*E-mail: [email protected] (C. Y. Chen); [email protected] (J. J. Yin);

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[email protected] (C. C. Ge); Tel: +86-10-82545560.

370

Notes

371

The authors declare no competing financial interests.

372

ACKNOWLEDGMENTS

373

This work is partially supported by the National Basic Research Program of China

374

(973 Program Grant No. 2014CB931900 and 2016YFA0201600), National Natural

375

Science Foundation of China (11575123, 11621505 and 21320102003), Jiangsu

376

Provincial Key Laboratory of Radiation Medicine and Protection, a project funded

377

by the Priority Academic Program Development of Jiangsu Higher Education

378

Institutions (PAPD), and a regulatory science grant under the FDA Nanotechnology

379

CORES Program. C. Chen appreciates the support from the NSFC Distinguished

380

Young Scholars (11425520). C. Ge appreciates the support from the Open Project

381

Program of Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety,

382

Chinese Academy of Sciences (NSKF201611). Y. Chong appreciates the support

383

from the China Scholarship Council (no. 1410100007). The authors thank Dr. Lili



384

Fox Vélez for scientific writing and editing support. This article is not an official U.S.

385

FDA guidance or policy statement. No official support or endorsement by the U.S.

386

FDA is intended or should be inferred.


Page 20 of 31

Page 21 of 31


b 120

120 **

100 *

80 60 40

**

20 0 Control

c

Control

Light

GO GO+Light

Light

E.coli Survival (%)

E.coli Survival (%)

a

GO+light

100

** *

80 60 40 20 0

Control

GO

0 10 20 30 Irradiation time (min)

GO+Light

387 388

Figure 1. Evidence of GO nanosheets killing E. coli with or without exposure to

389

simulated sunlight. (a) Antibacterial activity by GO under various conditions as

390

assessed by numbers of colony-forming units. Cultured E.coli cells were treated by

391

isotonic saline as control, simulated sunlight, or GO with or without exposure to

392

simulated sunlight. (b) Antibacterial activities of GO are influenced by duration of

393

exposure to simulated sunlight. The data shown are mean values and standard

394

deviations from a representative of three independent experiments. P values were

395

calculated by the student's test: *p < 0.05, **p < 0.01. (c) Representative fluorescence

396

images of live (green) and dead (red) cells after different treatments. Scale bars = 20

397

μm.

398



13000000

a

12000000

b

Control

11000000 10000000 9000000

GO

8000000 7000000 6000000 5000000 4000000

GO+NaN3

3000000

50

Loss of GSH (%)

14000000

2000000 1000000

c Loss of AA (%)

80

3320

Light3340 3360 GO GO with light

3380

3400

3420

40 20 0 5

10

15

20

Time (min) 399

30 20 10

5

10

15

25

30

20

25

30

Time (min)

d

AA GSH *

3440

100

X [G]

60

0

40

0

Loss of Antioxidant (%)

10 G 1003300

Light GO GO with light

0

GO+N2

0 -1000000

Page 22 of 31

80 60 40

*

20

0 GO+Light NaN3 Histone

+ -

+ + -

+ +

400

Figure 2. The generation of ROS and enhanced oxidizing activity by GO nanosheets

401

exposed to simulated sunlight. (a) ESR spectra were obtained from samples

402

containing spin label (4-oxo-TEMP) and GO under different conditions, including

403

GO alone, addition of NaN3, and anaerobic conditions. ESR spectra were recorded

404

after 5 min of exposure to simulated sunlight. (b) In vitro GSH oxidation under three

405

conditions: simulated sunlight alone, GO alone, and GO in the presence of simulated

406

sunlight. (c) In vitro AA oxidation under the three conditions as above. (d) Loss of

407

antioxidant agents (GSH or AA) by GO during exposure to simulated sunlight in

408

either the absence or presence of NaN3 or histone (singlet oxygen scavengers). These

409

data present the means and standard deviation from three experiments. P values

410

comparing differences between GO in the presence of simulated sunlight and other

411

conditions were calculated by the Student's T test: *p < 0.05.


Page 23 of 31


10000000

8000000

6000000

TEMPO

CPH

a Control

Control

1 min

1 min

5 min

5 min

10 min

10 min

4000000

2000000

0

800

800

700.0

700

550.0 400.0 600

250.0 100.0 -50.00

500

400 350

412

400

450

Excitation wavelength(nm)

500

Emission wavelength (nm)

Emission wavelength(nm)

b

700.0

700

550.0 400.0

600

250.0 100.0 -50.00

500

400 350

400

450

500

Excitation wavelength (nm)

413

Figure 3. The generation of electron-hole pairs by GO exposed to simulated sunlight.

414

(a) ESR spectra were obtained from samples containing different spin probes (CPH

415

and TEMPO) and GO exposed to simulated sunlight for different periods. The

416

control represents the sample contained the spin probe alone, or exposed to

417

simulated sunlight, or the sample containing spin probe and GO before exposure to

418

simulated sunlight. (b) Excitation−emission matrices (EEM) characterize the

419

photoluminescence properties of GO by itself (left column) and GO upon addition of

420

AA (right column).

421



422 423

Figure 4. The chemical reduction of GO. (a) Photographs of GO dispersions under

424

various conditions: sunlight alone, antioxidant (AA or GSH) alone, or antioxidant

425

combined with sunlight. (b) UV-vis spectra of GO dispersions under different

426

conditions as above. The inset is an irradiation time-dependent UV-vis spectra of GO.

427

Items 4A through 4G correspond to a range of exposure times (0-60 min). (c) XPS

428

spectra of C 1s of GO in the presence of AA before and after exposure to simulated

429

sunlight.

430


Page 24 of 31

Page 25 of 31


b

a 6

1x10 0

GO

6

g value=2.002

6

6

6

6

GO+GSH 30 min g value=2.002

6

1x10 0

GO+GSH+Light 30 min g value=2.002

6

-1x10

GO+AA 30 min g value=2.002

6

GO+AA+Light 30 min

1x10 0 6

g value=2.002

-1x10

-1x10

431

g value=2.002

-1x10

-1x10

1x10 0

GO+Light

6

6

-1x10

1x10 0

1x10 0

3350 3360 3370 3380 3390 3400 Magnetic field (G)

3350 3360 3370 3380 3390 3400 Magnetic field (G)

432

Figure 5. The characterization of the carbon-centered free radicals formed by GO

433

nanosheets under various conditions. (a) ESR spectra were obtained from samples

434

containing GO in the absence or presence of an antioxidant (GSH or AA). (b) ESR

435

spectra were obtained from samples containing GO in the absence or presence of an

436

antioxidant (GSH or AA) then exposed to simulated sunlight for 30 min.



437

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