Tannic Acid Induced Self-Assembly of Three-Dimensional Graphene

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Tannic acid induced self-assembly of three-dimensional graphene with good adsorption and antibacterial properties Jing Luo, jianping lai, nan zhan, yanbin liu, ren liu, and xiaoya liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01407 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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ACS Sustainable Chemistry & Engineering

Tannic acid induced self-assembly of three-dimensional graphene with good adsorption and antibacterial properties Jing Luo∗, Jianping Lai, Nan Zhang, Yanbing Liu, Ren Liu and Xiaoya Liu The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu, China 214122

Abstract: In this paper, a green one-step strategy is developed to fabricate three-dimensional (3D) graphene-based multifunctional material with the aid of tannic acid. Tannic acid (TA), a typical plant polyphenol widely present in woods, reduced GO and induced the self-assembly of reduced graphene oxide into graphene hydrogel. The preparation process was carried out in aqueous media under atmosphere pressure without using any toxic reducing agent or special instrument, which is a facile, green and low-cost method. The as-prepared monolithic 3D graphene exhibits high porosity, low density, hydrophobicity, good mechanical performance and thermal stability. In addition, it shows excellent adsorption toward dyes, oils and organic solvent, which should be a promising candidate for efficient adsorbents in water purification. Moreover, the tannic acid retained in the skeleton of 3D graphene functions as a biofunctional component, which endows the TA-GH with good antibacterial capability. Keywords: Three-dimensional (3D) graphene; Hydrogel; Aerogel; Tannic acid; Adsorption; Antibacterial capability;

*

Corresponding author. Tel: Telephone: 86-510-85917763. Fax: 86-510-85917763. E-mail: [email protected] (J.Luo). 1

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Introduction Three-dimensional (3D) graphene macrostructures, such as hydrogel and aerogel, have attracted great attention for their unique properties in recent years.1-3 3D graphene materials not only preserve the large accessible surface area of graphene sheets, but also possess abundant interconnected micropores or mesopores. Benefiting from the large surface area and porous structure, 3D graphene hydrogels and aerogels demonstrate many promising applications, for example, super adsorpbents,4-11 energy storage and conversion,12-18 sensors and catalysis19-23. Up to now, several synthetic strategies for fabricating 3D graphene architechtures, such as chemical vapor deposition (CVD), 24,25 hydrothermal treatment,26-28 and in situ reducing-assembly method,29-32 have been reported. For the CVD method, graphene sheet is deposited directly on the sacrificed template (nickel foam) under high temperature (700~900 oC) and harsh atmosphere conditions, which is thus of high cost and has great limitations. Hydrothermal process is an attractive technique to produce 3D graphene materials, which was first reported by Shi and coworkers.26 The obtained graphene hydrogel showed excellent mechanical and electrical properties. However, still high temperatures (~200 oC) and pressure conditions are required. In addition, the size of the obtained 3D graphene is greatly limited by the size of the hydrothermal reactors, which is difficult to realize large-scale production. Recently, the self-assembly of graphene sheet into 3D macrostructure by chemical reducing, which is called in situ reducing-assembly, has received intensive research interest. The chemical-reduction-induced

self-assembly

is

usually

performed

under

low-temperature heating below 100 oC at atmospheric pressure, which is free from a severe hydrothermal process. During this strategy, graphene oxide (GO) was reduced in the presence of a great deal of reducing agent and the in situ reduced graphene sheet can spontaneously self-assemble into 3D graphene architecture through π-π interactions. This in situ reducing-assembly method has the advantages of mild synthetic conditions, low requirement of instruments and convenience for large-scale production. However the often used reducing agent, such as hydrazine, hydroiodic 2


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acid (HI), hypophosphorous acid, NaBH4, and Na2S are dangerous chemicals and are harmful to the environment. Very recently, bio-compatible molecules, such as vitamin C (also known as ascorbic acid),

32-34

amino acids

35,36

and dopamine

37

, have been

reported to successfully reduce GO and fabricate 3D graphene materials. For example, Zhang and coworkers reported the synthesis of graphene hydrogel and aerogel through simultaneous reduction and assembly of GO with l-cysteine as both reducing and templating agent.35 But these biomolecules are relatively expensive, which significantly raise the preparation costs. Therefore it is still a challenge to develop a green synthetic strategy to prepare environmental-friendly 3D graphene in a large scale under mild conditions with low cost. Tannic substance, widely distributed in plants, is the third largest class of plant components only after cellulose and lignin. 38 The plentiful catechol and pyrogallol units in tannic molecules endow them reducing capabilities, which has been frequently employed as a reducer for metal ions. 39 Tannic acid (TA), one of typical hydrolysable tannins, is also a mild reducing agent. It has been reported that tannic acid could be simultaneously used as a reducer and stabilizer for GO.40,41 It is therefore expected that tannic acid could be an excellent chemical that could reduce GO and induce the self-assembly of graphene sheet to construct graphene hydrogel under proper conditions. In this paper, we report a simple, green and low-cost method for preparing 3D graphene materials, which is based on the chemical reduction and self-assembly of GO with the aid of an environmental-friendly and cheap biomolecule TA. TA plays multifunctional roles in the preparation of 3D graphene. Firstly, TA functions as reducing agent to reduce GO. In contrast with other reported reducing agents, TA is a wide-spread sustainable natural resource. Secondly, TA can interact with multiple graphene nanosheets via π-π interactions to form a network in solution, which possibly function as a template during the self-assembly of graphene nanosheets into a 3D architechture. So the use of TA enables a one-pot and one-step preparation of 3D graphene. In addition, the tannic acid retained in the skeleton of 3D graphene also 3



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functions as a biofunctional component, which endows the TA-GH with good antibacterial capability. The as-prepared three-dimensional graphene product is hydrophobic, mechanically strong and thermally stable. It shows excellent adsorption toward dyes, oils and organic solvent, which should be a promising candidate for efficient adsorbents in water purification. Experimental Chemicals Graphite powder and tannic acid (TA) were bought from Shanghai Alladin Chemical Reagents Company (China). Methylene blue, neutral red, rhodamine B, ponceau S, bismarck brown Y, kerosene, n-hexane, maize oil, petroleum ether, chloroform, dichloromethane methylene chloride (DCM), liquid paraffin, soybean oil and sudan red are purchased from Sinopharm chemical reagent company. Olive oil, castor oil, corn oil are purchased from Xiya Reagent Co., Ltd. All other chemical reagents were of analytical grade and used as received without further purification. Preparation of 3D graphene architectures Graphene oxide was firstly prepared from natural graphite according to previous literature42. GO (20 mg) was firstly dispersed in 10 mL deionized water by ulrasonication to form a homogeneous dispersion. Then a certain amount of tannic acid was added to the aqueous dispersion of GO and mixed up under ulrasonication. The obtained mixture was heated under atmospheric pressure without stirring. The as-prepared graphene hydrogel (TA-GH) was dialyzed with deionized water and freeze-dried to yield the corresponding graphene aerogel (TA-GA). Oil and organic solvent adsorption experiments The TA-GA sample was put in various organic solvents or oils and taken out until the adsorption equilibrium was achieved. Using a filter paper to remove the surface solvents or oils with, the sample was weighed. The absorption capacity (Q) was calculated by the following equation: Q=Ws/Wi

(1)

Where Wi (g) and Ws (g) is the initial and final (at adsorption equilibrium) weight of 4


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the TA-GA sample. Dye adsorption experiments A variety of water-soluble dyes, such as methylene blue, neutral red, rhodamine B, ponceau S, bismarck brown Y, were used to explore the adsorption performance of the obtained TA-GH. The above dyes were dissolved in deionized water to prepare solutions with the initial concentration of 0.5 mg mL-1. TA-GH was then added into the above aqueous dye solution (10 mL), followed by stirring at room temperature until achieving the adsorption equilibrium. The concentration of the dye solution was measured by UV-vis spectrometry at the maximum absorbance of each dye. The removed quantity of dye (Qep, mg g-1) was calculated with the following equation:

(2) where C0 (mg L-1) and Ceq (mg L-1) is the initial and final (at adsorption equilibrium) concentration of dye solution. V (L) is the volume of dye solution. m (g) is the weight of TA-GH sample after drying. Antibacterial Measurement Shaking flask method was employed to investigate the antibacterial property of TA-GH. The bacterial strains used in these studies were Gram-negative Escherichia coli (denoted as E. coli) ATCC 25922 and Gram-positive Staphylococcus aureus (denoted as S. aureus) ATCC 6538. The two strains were inoculated in Luria-Bertani (LB) medium, cultured at 37 °C for 12 h on a rotary shaker at 200 rev min-1, and were then diluted to 105~106 CFU mL-1 with a sterile 0.5% saline solution. During this process all the experimental conditions were kept sterile. As a typical process, 0.5 g of TA-GH was placed in a flask with 5 mL bacteria suspension (105~106 CFU/mL). The time the hydrogels added into flasks was the initial time, and was set as 0 h. After culturing at 37 °C with shaking at 100 rpm for 1, 2，3, and 4 h, the bacteria suspension was diluted to the appropriate concentration by using the double broth dilution method. Then, 100 µL bacteria suspension was extracted and put into the sterilized agar culture medium. After the suspension well-spread by using a sterile spreader, the 5



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plates were turned over and incubated for 24 h at 37°C to yield colony units. The grown S. aureus and E. coli colonies were clearly visible and counted. The survival rate of bacteria is calculated as % survival = A/B × 100 (where A and B are the numbers of surviving bacteria colonies in the tested sample and in the control group, respectively) Characterization A Fourier transform infrared (FTIR) spectrometer was used to record the FTIR spectrum (Shimidazu, Japan). Ultraviolet-visible (UV-vis) absorption spectra were recorded on a TU-1901 spectro-photometer (Beijing Purkinje General Instrument Co., Ltd.). Scanning electron microscopy (SEM) measurements were performed on a field-emission scanning electron microscope. X-ray diffraction (XRD) patterns were recorded from an AXS-D8 (Bruker, German) diffractometer with high-intensity Cu Kα radiation. XPS measurement was made on a VG ESCALAB MkII spectrometer with a Mg Ka X-ray source (1253.6 eV photos). The X-ray source was operated at 14 kV and 20 mA. Nitrogen adsorption/desorption analysis was carried out on an Autosorb Station (Quantachrome, USA). Thermogravimetric analysis (TGA) was conducted on a METTLER TOLEDO 1100SF instrument from room temperature to 800 oC with a heating rate of 5 oC min-1 in the nitrogen flow (20 mL min-1).

Results and discussion Synthesis and characterization of 3D graphene macrostructure The graphene hydrogel (TA-GH) was prepared by a one-step chemical reduction induced assembly of GO with the aid of TA, which was carried out at 90 oC under atmosphere pressure without stirring. The proposed formation mechanism was illustrated in Scheme 1. Owing to the reduction of GO by TA, the π-π interactions between graphene sheets increase, which will lead to the formation of compact porous graphene hydrogel. In addition, TA has multi-catechol groups in its molecular structure, which could interact with adjacent GO sheets and promote the crosslinking of GO sheets. The whole preparation process was depicted by Fig.1A. GO was easily dispersed in water under ultrasonic treatment to form a homogeneous dispersion with 6


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brown color owing to plenty of oxygen-containing hydrophilic groups on GO. When tannic acid was added into the GO dispersion and heated at 90 oC, the dispersion turned to black, indicating the effective reduction of GO to graphene by TA. After heating for 3h without stirring, a black monolith was formed and it floated in the clear residual solution as shown in Fig. 1A. With prolonging reaction time, the black monolith began to shrink and a well-defined black column-shaped hydrogel was produced after 8 h, indicating that the assembly of the hydrophobic reduced graphene oxide into 3D macroscopic structure.

Scheme 1. The schematic illustration of the proposed formation mechanism for TA-GA

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Fig. 1 Photographs of 3D graphene synthesized under different conditions: (A) different reaction time at 90 oC; (B) different reaction temperature. The reaction time was set as 8 h; (C) different TA:GO mass ratio; (D) different concentration of GO. For C and D, the reaction temperature was set at 90oC and the reaction time was set as 8 h. To find the optimal preparation conditions, the influencing factors such as reaction temperature, the feeding mass ratio of TA:GO as well as the concentration of GO on the final structure of TA-GH were investigated. As shown in Fig. 1B, when the heating temperature is too low (below 80 oC), 3D graphene could not be formed from the dispersion of GO and TA. When the temperature was increased to 90 oC, a well-defined graphene hydrogel was obtained. The possible reason is that GO was not sufficiently reduced at low temperatures and the hydrophobicity of the partially 8


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reduced GO is not strong enough to induce the self-assembly into 3D architechture. To investigate the indispensible role of TA in the formation process, different amounts of TA was added to the GO solution. It can be clearly observed that no black monolith can be found without the addition of TA, which fully confirmed the role in the preparation of 3D graphene. A mass ratio of TA:GO as low as 0.1 could induce the formation of 3D graphene hydrogel. With the increasing mass ratio of TA:GO to 0.5, the obtained black monolith became more compact. But further increasing mass ratio of TA:GO to 0.75 and 1.0 led to the over-dose of TA in the solution which can be detected from the brown color of clear residual solution. The concentration of GO also shows a great influence on the gelation process. As shown in Fig. 1D, when the concentration of GO was low (e.g., 0.5 mg mL-1), only a black dispersion was produced. However, when its concentration increased to 1.0 mg mL-1, a relatively well-formed 3D graphene hydrogel was obtained. Similar phenomina have been reported by previous literatures. The reason was attributed to that high concentration of GO could increase the contact opportunity between graphene nanosheets and increase the cross-linking degree of graphene nanosheets, resulting in the formation of a compact 3D framework35 Considering the above results and discussion, graphene hydrogel used in the characterization and adsorption performance was prepared according to the following conditions: aqueous dispersion of GO (2.0 mg mL-1) and TA (1.0 mg mL-1 ) was heated to 90 oC for 8 h.

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Fig. 2 (a) Digital image of a 12.6 mg TA-GA supporting a 200 g weight; (b) Electrical conductive performance of TA-GA. (c) Digital image of water droplets on the surface of a TA-GA sample. (d) Water contact measure of the TA-GA surface, the contact angle is 120.7o. The as-prepared graphene hydrogels contain about 98.6 wt% of water which can be removed by freeze-drying method to generate the corresponding aerogel (TA-GA). The digital photograph of the cylindrical graphene aerogel is exhibited in Scheme 1. It can be observed that TA-GA could stand on a dandelion without destroying its fluffs, suggesting that this graphene aerogel is very light. In fact, it has a very low density which is in the range of 0.019-0.025 g/cm3. Although being ultralight, the obtained graphene aerogel shows a remarkable mechanical strength. As shown in Fig. 2a, 12.5 mg of TA-GA can support a 200 g weight which is almost 20000 times its own weight without causing any obvious deformation. The obtained graphene aerogel was also electrically conductive as shown from Fig. 2b. In addition, it was found that the external surface of our graphene aerogel was water resistant. The hydrophobicity of the graphene aerogel was evaluated by water contact angle. As shown from the optical image of a water droplet on the TA-GA surface (Fig. 2d), the water contact angle of TA-GA was measured to be 120.7o.

10


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Fig.3 SEM images of graphene sheet (a), surface (b) and cross-section (c, d) of TA-GA The SEM image of graphene sheet reduced by hydrazine hydrate was also provided for comparison. It can be observed that graphene sheet showed a typical wrinkled and crumbled two-dimensional structure (Fig. 3a). In contrast, from both the surface (Fig. 3b) and cross-section images (Fig. 3c), TA-GA shows an interconnected hierarchical porous network architechture with pore sizes ranging from several hundred nanometers to several micrometers, which were distributed in a random order. The high magnification SEM image (Fig. 3d) shows that the pore walls are composed of curly and crumpled graphene sheets, and the partial overlapping or coalescing of graphene nanosheets contributes to the formation of a 3D framework. The morphology of the TA-GA was further investigated using TEM. As shown in Fig. S1A, TA-GA shows a typical wrinkled and crumbled structure, indicating that graphene sheet has been exfoliated. The high resolution TEM image (Fig. S1B) suggests that the thin walls typically consisted of several layers of graphene sheets.

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Fig. 4 N2 adsorption–desorption isotherms of TA-GA The porous properties and specific surface area of the as-prepared TA-GA were determined by the nitrogen adsorption/desorption experiments. The adsorption– desorption isotherms of TA-GA exhibited a typical hysteresis loop, suggesting a great number of mesopores in the framework of graphene aerogel (Fig. 4). By fitting the isotherm curves with the Brunauer–Emmett–Teller (BET) model, the specific surface area of TA-GA is calculated to be 136 m2 g-1, which is comparable to that of the previously reported graphene aerogel.33 The distribution curve of the pore size measured by the Barrett–Joyner–Halenda (BJH) method shows that most of the pore volumes of TA-GA are within a diameter of 3.5-50 nm (Fig. S2). Besides, the total pore volume is determined to be 0.483 cm3 g-1 from the nitrogen adsorption– desorption test. The above results further confirm the porous structure of the prepared three-dimensional TA-GA.

12


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Fig.5 FTIR spectra (A) and XRD patterns (B) of TA, GO, and TA-GA Fig. 5A shows the FTIR spectra of TA, GO, and TA-GA. GO exhibits several absorption bands at 3300, 1730, 1627, 1385, 1218, and 1062 cm-1, which are attributed to the O–H stretching vibration, the C=O stretching vibration, aromatic C=C stretching vibration, O–H deformation vibration, C–O stretching vibration, and C–O stretching vibration, respectively. For TA-GA, several new peaks at 870 and 754 cm-1 were observed, which were assigned to the characteristic bands of tannic acid, indicating that TA molecules have gone into the skeleton of graphene hydrogel. The reducing agents were often found to be attached on graphene sheets in the process of reducing graphene oxide.31 Considering the aromatic structure of TA (as shown in Scheme 1), TA molecules could be easily adsorbed to the surface of graphene sheet 13



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owing to the π-π interaction. In previous literatures, the reduction of GO can be directly reflected from the remarkable decrease in band intensity at 3300, 1730, 1380, and 1050 cm-1 with the removal of the most oxygen-containing group during chemical reduction. In our system, owing to the overlap of absorption bands of TA with those of GO, the reduction of GO could not get direct support from the FTIR spectrum. XRD and XPS results provides the evidence for the successful reduction of GO by TA. The XRD profiles of GO and TA-GA are shown in Fig. 5B. GO shows an intense peak at 10.6°, suggesting an interlayer distance (d-spacing) of 0.83 nm. For TA-GA, this peak completely disappeared, indicating the removal of most of the oxygen-containing groups of GO. Instead, a new broad diffraction peak has appeared at 24.6° with a d-spacing of 0.35 nm, which is similar to that of graphite, indicating that GO has been successfully reduced to graphene. In addition, in contrast with the narrow intensive peak of pristine graphite, this small broad diffraction peak indicates that the framework of TA-GA is composed of disorderly stacked graphene sheets, which is consistent with those obtained in SEM analysis.

Fig. 6 (A) XPS survey spectra of GO and TA-GA. High-resolution XPS C1s spectra 14


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of GO (B) and TA-GA (C). The XPS survey spectra of GO and TA-GA are shown in Fig. 6A. The ratio of C and O of TA-GA was much smaller than that of GO, indicating most of the oxygen-containing groups have been removed during the reduction process. Fig. 6B and C present the deconvoluted C1s XPS spectra of GO and TA-GA, respectively. The C1s spectrum of GO can be deconvoluted into four components centred at 289.0, 287.7, 286.6, and 285.6 eV, corresponding to O–C=O, C=O, C–O, and C–C/C=C, respectively.34 After reaction with TA at 90oC for 8 h, the intensities of the peaks of the oxygenated carbon, especially the peak centered at 286.6 eV, decreases significantly, accompanied by the great enhancement of peak associated with C– C/C=C (284.6 eV), demonstrating the efficient reduction of GO.

Fig. 7 TGA curves of TA, GO and TA-GA. The thermal stability of GO and TA-GA was investigated by TGA. TA is quite stable at relatively low temperatures and a drastic weight loss occurred at 250 oC. GO exhibited a two-step weight loss with the first step weight-loss (about 10%) below 100 oC, which was associated with the removal of absorbed water.37 The second weight-loss of GO (about 30%) was at 100–250 oC, due to the decomposition of labile oxygen containing groups on GO. By contrast, TA-GA shows a relatively smooth weight-loss curve. It should be noted that almost no weight loss was observed below 15



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200 oC, which is in contrast with 25% weight loss for GO at this temperature. This result suggests the improved thermal stability of the prepared graphene aerogel, which is beneficial for its applications. Adsorption properties of 3D graphene macrostructure Because of their light weight, high porosity structure, hydrophobicity, and mechanically stable graphene skeleton, the as-prepared 3D graphene architectures are expected as a promising candidate for adsorbing dyes, organic solvents and oil–water separation. The adsorption performance of TA-GA was investigated by absorption experiments for a series of dyes, oils and organic solvents from water.

Fig. 8 (A) Photographs of the absorption process of n-dodecane (dyed with sudan red) from water by TA-GA over time. (B) Absorption capacities (saturated) of different oils by TA-GA. Fig. 8A shows the typical oil-water separation experiments. N-dodecane was 16


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chosen as the oil which was dyed with Sudan Black B for easy observation. As shown in Fig.8A, once the TA-GA was placed on the surface of the hexane–water mixture, the n-dodecane layer immediately began to shrink immediately, indicating the adsorption of TA-GA toward n-dodecane. The n-dodecane was completely suctioned within 30 s. This quick and strong absorption capability can be attributed to the porous structure and large hydrophobic surface area of TA-GA, which provides numerous attaching sites for n-dodecane molecules. The oil filled TA-GA could float on the water without the water penetrating into its structure or the release of the oils. The TA-GA after oil adsorption could be regenerated by burning the oil and it was found that this graphene aerogel was also fire-resistant. As shown in Fig. 8A, TA-GA remained almost intact after being burned under an ambient atmosphere. In addition to n-dodecane, the adsorption for TA-GA toward other organic liquids, e.g., vegetable oil, commercial petroleum products and chemical agents with different carbon chain lengths were also investigated. The adsorption capability can be referred to the absorption capacity, i.e., the ratio of the final weight after full adsorption to the initial weight of TA-GA. Fig. 8B showed the absorption capacities of TA-GA for various oils and organic solvents. The absorption capacities are in the range of 15 to 30 times the weight of the TA-GA for the different organic liquids, which was possibly attributed to the different density of different oils and organic solvents. The absorption capacities of our GA towards organic liquids is comparable to previously reported graphene aerogel (13–37 times) 24, 43-45, spongy graphene (20–86 times),46 the graphene–CNT aerogel (30 times)47, carbon nanofiber/carbon foam (16 g/g)48 and graphene-FeCOOH (13-27 times)

49

, but lower than N-doped graphene framework

(200–600 times) 50 and hybrid graphene/carbon nanotube foams (80–130 times) 28. Owing to their toxicity to microorganism, the presence of dyes puts great limit in the subsequent usage of the water and reduces the efficacy of microbial treatment. Five kinds of dyes (rhodamine B, bismarck brown Y, methylene blue, neutral red and ponceau S) were employed to test the capability of TA-GH to absorb dyes from water. TA-GH cylinder was added to dye aqueous solutions (10 mL, 0.5 mg mL) and the 17



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mixture was allowed to stand with agitation for 8 h. Fig. 9A shows the photoimages of dye aqueous solutions before and after absorption by TA-GH for 8 h. It can be seen that the color of these dye solutions almost faded (except for ponceau S), indicating the good adsorption of TA-GH for these dyes. The possible mechanism is that there exit π–π interactions between the TA-GH and the aromatic organic dyes which drive the dyes molecules adsorbed into the hydrogels. Typical dynamic adsorption experiments were conducted to gain further insight into the adsorption performance of TA-GH towards these dyes. As shown in Fig. 9, the adsorption of dyes by TA-GH basically achieved equilibrium after 400 min. The total adsorption capacities of TA-GH towards rhodamine B, bismarck brown Y, methylene blue, neutral red and ponceau S were calculated to be 380, 250, 195, 175, and 75 mg g-1. It is quite obvious that the adsorption towards ponceau S is much weaker than the other four dyes, which is in accordance with the adsorption photograph. The possible reason is that ponceau S is an anionic dye which carries negative charge, and TA-GH also bears negative charge owing to the presence of TA in the skeleton of TA-GH. The electrostatic repulsion between ponceau S and TA-GH hinders the adsorption of TA-GH towards ponceau S. In contrast, rhodamine B, bismarck brown Y, methylene blue all bear positive charges51, thus the electrostatic interaction combined with the π–π interactions drive the dye molecules into TA-GH, leading to their relatively high adsorption capacities. As for neutral red, it is a neutral dye 51, so only π–π interactions contributed to its adsorption, which explains its relatively lower adsorption capacity than rhodamine B, bismarck brown Y and methylene blue. To further investigate the adsorption mechanism, pseudo-first-order and pseudo-second-order kinetic models were employed to analyze the adsorption kinetics of dyes onto TA-GH, which can be described as follows:

(3)

(4) 18


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where k1, k2 are the pseudo-first-order and pseudo-second-order adsorption rate constants, Qe and Qt are the adsorption capacity of dyes onto the hydrogels at equilibrium and at time t, respectively. Fig.S3 shows the pseudo-first-order (A) and pseudo-second-order (B) kinetic plots for the adsorption of various dyes onto TA-GH. It was found that the adsorption process of dyes with TA-GH fitted well with the pseudo-second-order kinetic model as shown in Fig. S3B. The values of k2 calculated from the slope are given in Table S1. High correlation coefficients (R2) for different dyes were obtained. These facts show that the pseudo-second-order adsorption mechanism is predominant, indicative of the important roles of chemisorption reaction (electrostatic interaction, π–π interactions) in the adsorption process of dyes with for TA-GH.52

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Fig. 9 (A) Photographs of dye aqueous solutions (0.5 mg/ml) before and after absorption by TA-GH for 8 h. (B) Absorption capacities of TA-GH for different dyes versus the adsorption time. Antibacterial ability Except for organic dyes, microbial contamination of water is also an important problem in environmental protection and human health. Many previous literatures have shown that TA owns good contact antibacterial properties.53,54 In recent years, graphene and GO have also been demonstrated to show strong antibacterial performance

55,56

. Considering the good antibacterial properties of both graphene and

TA, it is quite expected that the as-prepared 3D TA-GH would possess good antibacterial property. We thus further investigated whether the obtained 3D graphene material could be used for sterilization during water purification.

Fig. 10 Antibacterial activities of TA-GH against (a) E. coli and (b) S. aureus To investigate antibacterial properties of the three-dimensional TA-GH, Gram-negative bacterium (E. coli) and Gram-positive bacterium (S. aureus) were used as model bacteria in our experiment. Shaking flask method is adopted to evaluate their antibacterial efficiencies. The antibacterial performance of TA-GH against E. coli (Fig. 10a) and S. aureus (Fig. 10b) is presented in Fig. 10. The bacteria solution without the addition of TA-GH is set as blank group. And for the bacteria solution containing TA-GH, the number of surviving bacterial colonies of both E. coli and S. aureus declined significantly with the increasing incubating time, indicating that TA-GH is powerful in inhibiting the growth of E. coil and S. aureus. In addition, the 20


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survival of E. coli was more obvious than that of S. aureus, indicating more S. aureus was killed by TA-GH. For example, after incubating with TA-GH for 4 h, 41.88% of E. coli survived, i.e., 58.12% was killed by TA-GH. In great contrast, 0.01% of S. aureus survived at the same conditions, i.e., 99.99% of S. aureus was killed by TA-GH. The possible reason for the different antibacterial activities of TA-GH against S. aureus and E. coli is that S. aureus is gram-positive bacteria which are more sensitive to TA than the gram-negative bacteria (E. coli) 57. Our experimental results indicate that the three-dimensional TA-GH can inhibit bacteria effectively and possess remarkable antibacterial activities. To investigate the role of TA in the antibacterial property of TA-GH, the antibacterial activity of pure graphene sheets toward E. coli and S. aureus was also investigated. As shown from the red column of Fig. 10, only 30.7% and 31.6% of E. coli and S. aureus were killed by graphene after incubation for 4h, respectively, which are significantly lower than those killed by TA-GH. The above results clearly showed that the presence of tannic acid in the skeleton of 3D graphene make a great contribution to the outstanding antibacterial property of TA-GH.

Conclusions In summary, we report a facile, environmental-friendly and economical approach for the synthesis of three-dimensional multifunctional graphene macrostructure with tannic acid as both reducing agent and physical crosslinking sites. The preparation conditions such as reaction time, temperature, concentration as well as feeding ratio were investigated in detail. The obtained graphene aerogel has low density, good mechanical strength and thermal stability, with high specific surface area. It also shows high absorption capacity towards organic solvents and oils as well as dyes, which should be a promising material for efficient adsorbents in water purification. More importantly, good antibacterial capability was demonstrated owing to the presence of tannic acid retained in the skeleton of 3D graphene. 99.99% of S. aureus could be killed by TA-GH after 4 h’s incubation. The present work promotes the use 21



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of renewable natural resources for the self-assembly of functionalized graphene into 3D mascrostructure, offering an alternative way to fabricate multifunctional materials for a wide range of applications.

Acknowledgement We acknowledge financial support from the National Natural Science Foundation of China (under Grant Nos. 51573072), the Enterprise-university-research prospective program Jiangsu Province (BY2013015-08), the Fundamental Research Funds for the Central Universities (JUSRP 51305A), MOE & SAFEA for the 111 Project (B13025).

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Table of Contents Graphic

Tannic

acid

induced

self-assembly

of

three-dimensional

graphene with good adsorption and antibacterial properties Jing Luo, Jianping Lai, Nan Zhang, Yanbing Liu, Ren Liu and Xiaoya Liu

Synopsis: A green one-step strategy is developed to fabricate three-dimensional (3D) graphene-based multifunctional material with the aid of tannic acid.

29


Tannic Acid Induced Self-Assembly of Three-Dimensional Graphene

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