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Immobilization of Highly Dispersed Ag Nanoparticles on Carbon Nanotubes Using Electron-Assisted Reduction for Antibacterial Performance Xiaoliang Yan, Sha Li, Jiehua Bao, Nan Zhang, Binbin Fan, Ruifeng Li, Xuguang Liu, and Yun-Xiang Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03106 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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ACS Applied Materials & Interfaces

Immobilization of Highly Dispersed Ag Nanoparticles on Carbon Nanotubes Using Electron-Assisted Reduction for Antibacterial Performance

Xiaoliang Yan,† Sha Li,*, ‡ Jiehua Bao,† Nan Zhang,† Binbin Fan,† Ruifeng Li,† Xuguang Liu,† and Yun-Xiang Pan*,§

†

College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024,

China; ‡

College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, China;

§

School of Chemistry and Chemical Engineering, Hefei University of Technology, Heifei 230009,

China

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ABSTRACT: Silver nanoparticles (Ag NPs) supported on certain materials have been widely used as disinfectants. Yet, to date, the antibacterial activity of the supported Ag NPs is still far below optimum. This is mainly associated with the easy aggregation of Ag NPs on the supporting materials. Herein, an electron-assisted reduction (EAR) method, which is operated at temperature as low as room temperature and without using any reduction reagent, was employed for immobilizing highly dispersed Ag NPs on aminated-CNTs (Ag/A-CNTs). The average Ag NPs size on the EAR-prepared Ag/A-CNTs is only 3.8 nm, which is much smaller than that on the Ag/A-CNTs fabricated from the traditional thermal calcination (25.5 nm). Compared with Ag/A-CNTs fabricated from traditional thermal calcination, EAR-prepared Ag/A-CNTs shows a much better antibacterial activity to E.coli/S.aureus and antifouling performance to P. subcordiformis/T.lepidoptera. This is mainly originated from the significantly enhanced Ag+ ion releasing rate and highly dispersed Ag NPs with small size on the EAR-prepared Ag/A-CNTs. The findings from the present work are helpful for fabricating supported Ag NPs with small size and high dispersion for efficient antibacterial process.

KEYWORDS: electron-assisted reduction, Ag nanoparticles, carbon nanotubes, antibacterial activity, plasma


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INTRODUCTION Silver nanoparticles (Ag NPs) have been widely used as disinfectants, owing to their high activity at low doses against a broad range of bacteria, microbe and parasite.1–5 For example, by using Ag NPs at only a few mg mL-1, full growth inhibition of bacteria can occur.6,7 A key factor affecting the disinfecting activity of Ag NPs is their sizes. Smaller Ag NPs have higher disinfecting activity over larger ones. For example, compared with 35, 44, and 50 nm Ag NPs, 25 nm Ag NPs showed high antimicrobial and bactericidal activity against gram-positive and gram-negative bacteria.8 For a given mass of Ag NPs, well-dispersed Ag NPs with smaller size indeed have a higher surface to contact with bacteria, microbe and parasite, thereby enhancing the disinfecting activity. A good strategy to keep Ag NPs smaller is to incorporate Ag NPs into supporting materials.9–11 Besides stabilizing Ag NPs, dispersing Ag NPs on a convenient support can also obtain optimum Ag NPs utilization and reduce the amount of Ag NPs used, thus reducing the costs of the disinfectants. Carbon nanotubes (CNTs) have been reported to be one of the most appealing materials to support Ag NPs to form an efficient composite disinfectant.12-15 CNTs can be easily functionalized through acid treatment and amination process. The functionalization produces vacancies, dangling bonds and functional groups (e.g. COOH, OH and NH2) on the external surface of CNTs, which benefits for anchoring Ag NPs on CNTs, thus suppressing Ag NPs aggregation.16 In addition, CNTs have a good biocompatibility and low toxicity to organisms.17,18 A number of strategies have been developed for supporting Ag NPs on CNTs, which can be generally classified into two groups: (i) incipient wetness impregnation followed by chemical reduction of Ag+ ion using a reducing agent like NaBH4, citrate or



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ascorbate, with the protection of a stabilizer;12,13 and (ii) incipient wetness impregnation followed by thermal reduction.14,15 For chemical reduction, impurities are easily involved in the final products because so many reagents (reducing agent or stabilizer) are used. For thermal reduction, Ag NPs aggregation easily occurs at elevated temperatures. Furthermore, the functional groups on CNTs decompose and the amounts of these groups decrease because of the thermal effect. Electron-assisted reduction (EAR) has been of enormous interest in recent year, as it is fast, facile, green and economic.19-23 During the EAR process, electrons are used to reduce the metal ions in water solution at temperatures as low as room temperature, without use of any reduction reagent, stabilizer and other agent.24,25 Supported Au, Pt, Pd and Ir NPs have been synthesized by using electron-assisted reduction.26–29 For example, by using the electron-assisted reduction, Zhao et al. fabricated Al2O3-supported Ir NPs with an average size of 1.18 nm and a metal dispersion of 83.95%.29 In contrast to the abundant applications for preparing other supported metal NPs, the application of the EAR method in preparing supported Ag NPs is scarce. Herein, we applied the EAR method in preparing CNTs-supported Ag NPs. By using the EAR method, highly dispersed Ag NPs with an average size of about 3.80 nm were produced on the functionalized CNTs support. As-synthesized CNTs-supported Ag NPs exhibited excellent antibacterial activity.

EXPERIMENTAL SECTION Materials. All chemicals, including sulfuric acid, nitric acid, 1,6-hexamethylenediamine (HDA), dicyclohexylcarbodiimide (DCC), silver nitrate, ethanol and CNTs, were commercially obtained and used without further purification. 4 ACS Paragon Plus Environment

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Surface Modification of CNTs. Wet treatment procedures, i.e. oxidation and amination processes, were employed (Figure 1) to graft functional groups on the external surface of CNTs to make CNTs more active. Oxidation process was developed to introduce carbonyl and carboxyl groups. Typically, the pristine CNTs were blended with a mixed solution of H2SO4/HNO3 (volume ratio: 3/1) in a flask, followed by treatment through ultrasonic process at 70 °C for 2 h. And then, the sample was washed with distilled water, dried and grounded to give oxidized CNTs. For further modification through amination process, the oxidized CNTs were mixed with HDA and DCC in a flask, and stirred at 90 °C in a constant temperature oil bath for 2 h. As-obtained sample was washed with dichloromethane, ethanol and distilled water for three times and dried in a vacuum oven at 60 °C for 12 h. For clarity, the oxidized and aminated CNTs were denoted by O-CNTs and A-CNTs, respectively. Preparation of Ag/A-CNTs. Incipient wetness impregnation method was applied to load Ag+ ion onto A-CNTs (6 wt% Ag). A-CNTs were firstly impregnated with an aqueous solution of silver nitrate for about 12 h and dried at 60°C for 12 h. The obtained sample was treated in the following two approaches, including thermal calcination and EAR. One part was thermally decomposed in nitrogen at 400 °C for 3 h to fabricate the Ag/A-CNTs-C sample. The decomposition temperature was determined from the thermogravimetric (TG) analysis (Figure S1 in the supporting information). The TG curves in Figure S1 indicate that calcination at 400 °C is enough for obtaining Ag0. The other part was reduced through the EAR method to produce the Ag/A-CNTs-P sample. The electrons used during the EAR process were generated by radio frequency (RF) plasma. RF plasma is an ionized gas, and electrons are its main components. Figure S2 in the supporting information illustrates a schematic of the setup for the RF plasma. The RF plasma uses an SY-500W RF generator, designed by Institute of Plasma Physics Chinese Academy of Sciences. The plasma chamber as a quartz reactor (i.d. 60 mm) was surrounded by high frequency coil outside.The reactor was evacuated to 50 Pa by a vacuum pump. Ultra-high pure He 5 ACS Paragon Plus Environment


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(>99.999%) was introduced at 20 mL min-1 and acted as the plasma-forming gas. When the voltage reached 100V, bright discharge plasma was generatedin the reactor, where the sample was placed for reduction. The powder sample (about 100 mg) was put in the middle of the plasma reactor and exposed to the plasma. By using the water cooling system, the temperature of the plasma was kept at room temperature. The time of each plasma treatment was 6 min and each sample was treated for 10 times. Between each operation, the sample was manually stirred and ground. The EAR treatment of 1 h for each sample was carried out to make sure that the reduction of Ag+ ion was thoroughly.24 Characterizations. The scanning electron microscopy (SEM) images were used to study the morphology of CNTs by Hitachi field emission scanning electron microscope (S4800). The samples were sputtercoated with gold to avoid charge accumulations prior to examinations. The Fourier transform infrared spectroscopy (FTIR) measurement was carried out to understand the changes in the functional groups of CNTs by using a Tensor27 spectrometer (Bruker) with a resolution of 4 cm-1. The sample was finely pulverized and then diluted in dried KBr to form a homogeneous mixture with a sample to KBr ratio of 1/200. The thermogravimetric (TG) analysis was carried out to evaluate the amounts of functional groups on CNTs using a Netzsch STA 449 F3 system. Approximately 10 mg of sample was used for each measurement and the temperature was increased at a heating rate of 10°C min-1 in anair flow (30 mL min-1). The X-ray diffraction (XRD) analysis was performed to study the crystalline phase of Ag and CNTs by a Rigaku D/MAX-2500 V/PV using Cu-Kα radiation at a scanning speed of 4 ° min-1 over the 2θ range from 10 ° to 80 °. The voltage and the current were 40 kV and 200 mA, respectively. The phase identification was made by comparing with Joint Committee on Powder Diffraction Standards (JCPDS). UV-visible spectroscopy analysis was carried out with a Shimadzu UV-2550 spectrophotometer. The X-Ray photoelectron spectroscopy (XPS) measurement was conducted on a Perkin-Elmer PHI-1600 spectrometer with monochromatic Mg Kα (1253.6 eV) radiation 6 ACS Paragon Plus Environment

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to study the valence state of Ag. Binding energies were calibrated using the C1s peak (284.6 eV) as a reference. The transmission electron microscopy (TEM) observations were measured to illustrate the particle size and size distribution of Ag NPs by a JEM100CXII (100 kV). Ag concentration of the samples was measured by inductively-coupled plasma mass spectrometry (ICP-MS, Thermo X series 2). Antibacterial and Algal Inhibiting Tests. Gram-negative and gram-positive bacteria were chosen as the indicators for antibacterial evaluation. Escherichia coli (E. coli) refers to gram-negative bacteria and Staphylococcus aureus (S. aureus) represents for gram-positive bacteria. A nutrient agar culture medium was prepared. Bacteria were grown in the medium and incubated overnight at 37 °C. The antibacterial activities of Ag/A-CNTs-C and Ag/A-CNTs-P were performed by disk diffusion assay. 20 µL of Ag/A-CNTs (500 µg µL-1) was dropped into a 5 mm filter paper disk and placed ontoan agar plate with 106 CFU/mLE. colior S. aureus. After incubation at 36.8 °C for 12 h, the diameters of the inhibition zones were measured. Antifouling evaluation was performed against Platymonassubcordiformis (P. subcordiformis) and Tropidoneislepidoptera (T.lepidoptera). The f/2 seawater medium was filtered by 0.4 µm microporous membrane. Algae were placed in the medium and cultured at 20 °C with 3000 lx illumination in the shaking bed. 100 ppm of Ag/A-CNTs was added. The performances were observed under microscope after 24 h. Antibacterial activity was also studied by confocal microscopy on the basis of fluorescent-based cell live/dead test withE. coli DH5a. E. coli (108 CFU mL-1) was seeded on 100 µg mL-1 Ag/A-CNTs and incubated by a rotator at 60 rpm for 2 h. Then, the cells were collected by centrifugation and washed with PBS (pH= 7.4) three times. After washing, the cells were stained with propidium iodide (PI) and green fluorescent protein (GFP), where GFP was expressed in all cells, and PI only penetrated bacteria with damaged membranes. The images were collected by a laser scanning fluorescence microscope 7 ACS Paragon Plus Environment


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(Leica, SP5). Ag+ Ion Release Rate Evaluation. Ag concentration in Ag/A-CNTs solution was measured by Ag+ ion release evaluation. The initial Ag content was set at the same value of 1000 ppm. Ag/A-CNTs solution (10 mL) was put intodialysis bags (MWCO7000). The bag containing Ag solution was immersed in 100 mL of ultrapure water. During each 2 h, 1 mL of released solution was withdrawn with syringe and the whole ultrapure water was changed every 12 h.

RESULTS AND DISCUSSION Figure 2 shows the SEM images of the pristine CNTs, O-CNTs and A-CNTs. The morphologies of the three samples are similar, and a good network of CNTs with a diameter of about 30 nm can be observed for all of the samples. This indicates that the structure of CNTs is almost unchanged after the oxidation and amination processes. FTIR spectra were used to characterize the functional groups on the pristine CNTs, O-CNTs and A-CNTs (Figure 3a). A band at about 3500 cm-1 appears for the three samples, which is caused by the OH group. For O-CNTs, besides the peak at 3500 cm-1, there are still another three bands at 1400, 1634, and 1730 cm-1, which correspond to the C=O, C-O and COOH groups, respectively, indicating the formation of COOH groups from the oxidation of the pristine CNTs. For A-CNTs, the bands due to the COOH groups are absent, while four new bands at 1070, 1637, 2850 and 2924 cm-1 appear, attributed to the C-N, N-H, asymmetric vibration of C-H groupsand symmetric stretching vibration of C-H groups, respectively. This demonstrates the formation of the C-N and N-H groups on A-CNTs from the amination of O-CNTs. On the TG curves (Figure 3b), the weight losses of O-CNTs and A-CNTs are about 4.8% and 12.4%, respectively, while that of the pristine CNTs is negligible. The weight losses of O-CNTs and A-CNTs are due to the decomposition of the functional groups including oxygen- and nitrogen-containing groups. The higher weight loss on A-CNTs than that 8 ACS Paragon Plus Environment

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on O-CNTs suggests an enhanced concentration of the functional groups on A-CNTs in comparison with O-CNTs. The differential TG (DTG) profile of A-CNTs shows a maximum weight loss rate at 280 °C, which may be caused by the dissociation of the covalent bonds between CNTs and HDA. Ag NPs were supported on A-CNTs through the thermal calcination and EAR method. For clarity, the thermal-calcination- and EAR-driven samples were named as Ag/A-CNTs-C and Ag/A-CNTs-P, respectively. Figure 4a shows the XRD patterns of Ag/A-CNTs-C and Ag/A-CNTs-P. For Ag/A-CNTs-C, four diffraction peaks at 38.1°, 44.3°, 64.4°, and 77.5° are assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of metallic Ag crystalline lattice, respectively (JCPDS#04-0783). Besides the peaks of Ag NPs, two peaks at 25.8° and 42.7° can also be observed on the XRD patterns of Ag/A-CNTs-C, which are attributed to the (0 0 2) and (1 0 0) planes of graphitic carbon in CNTs. The XRD patterns of Ag/A-CNTs-P are significantly different from those of Ag/A-CNTs-C. Besides the peak at 38.1° corresponding to the Ag (1 1 1) plane, there are no other peaks for Ag NPs on the XRD patterns of Ag/A-CNTs-P. The peaks at 25.8°, 42.7°, and 44.3° are caused by the (0 0 2), (1 0 0) and (1 0 1) planes of graphitic carbon in CNTs, respectively. On the UV-visible spectra of Ag/A-CNTs-C and Ag/A-CNTs-P (Figure 4b), a peak at 292 nm and a peak locating in the range of 400−500 nm are observed. The peak at 292 nm is ascribed to the CNTs, while the peak locating in the range of 400−500 nm is assigned to theAg NPs.30 The XPS results evidence the presence of C, O, N and Ag on Ag/A-CNTs-C and Ag/A-CNTs-P (Figure 4c). On the XPS spectra of Ag on Ag/A-CNTs-C and Ag/A-CNTs-P (Figure 4d), two symmetrical peaks are observedat 368.3 and 374.3 eV, corresponding to the Ag 3d5/2 and Ag 3d3/2, respectively. The surface compositions of Ag/A-CNTs-C and Ag/A-CNTs-P determined from the XPS results are listed in Table 1. The contents of oxygen and nitrogen on Ag/A-CNTs-P are 8.21% and 3.23%, respectively, which are much higher than those on Ag/A-CNTs-C (3.30% and 0.82%, 9 ACS Paragon Plus Environment


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respectively). This indicates that the oxygen- and nitrogen-containing functional groups are well maintained during the EAR process, which could be resulted from the low operation temperature (close to room temperature) of the EPR reduction method. The oxygen- and nitrogen-containing functional groups can be easily destroyed during the thermal calcinationswhich usually use temperature higher than 100 °C, as reported in the literature.14,31 Based on the XPS results, the surface Ag content on Ag/A-CNTs-C and Ag/A-CNTs-P are 0.63% and 1.05%, respectively (Table 1), much lower than the prepared Ag loading. This indicates that parts of Ag NPs are confined in the channels of CNTs. The surface Ag/C ratio on Ag/A-CNTs-P is calculated to be 0.012, slightly higher than that for Ag/A-CNTs-C (0.007) (Table 1). The Ag content on Ag/A-CNTs-C and Ag/A-CNTs-P was investigated from the TGA analyses in Figure S3 in the supporting information. The Ag content on Ag/A-CNTs-C and Ag/A-CNTs-P, calculated from the TGA results, are 6.20% and 5.91%, respectively. The ICP-AES analyses indicate that the Ag content on Ag/A-CNTs-C and Ag/A-CNTs-P are 6.82% and 5.80%, respectively. As reflected by the TEM imagesin Figures 5a and 5b, on Ag/A-CNTs-C, Ag NPs grow large and are more diversified in size. Ag NPs with size of about 50 nm can be clearly seen on Ag/A-CNTs-C, moreover, the crystallographic planes of Ag NPs on Ag/A-CNTs-C are complicated (Figure 5b). Significantly different from those on Ag/A-CNTs-C, homogeneously and highly dispersed Ag NPs are formed on Ag/A-CNTs-P (Figures 5c,5d and 5e). Well-defined lattice fringes of the Ag (1 1 1) planes are clearly observed (Figure 5f). The size distributions of Ag NPs on Ag/A-CNTs-C and Ag/A-CNTs-P were calculated from the TEM observations, as shown in Figure 6. The size distribution of Ag NPs on Ag/A-CNTs-C is broad, from 5.0 to 55.0 nm, with an average of 25.5 nm. The size distribution of Ag NPs on Ag/A-CNTs-P is very narrow, from 2.5 nm to 5.5 nm, with an average size of 3.8 nm. The much smaller size and higher dispersion of Ag NPs could be the reason for much weaker XRD peaks for 10 ACS Paragon Plus Environment

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Ag/A-CNTs-P, as compared with those for Ag/A-CNTs-C (Figure 4a). The possible growth mechanismof Ag NPs on Ag/A-CNTs was proposed. The oxidation and amination of the pristine CNTs increases the concentrations of the oxygen- and nitrogen-containing functional groups, as reflected by the FTIR spectra and TG curves (Figure 2). The oxygen- and nitrogen-containing functional groups act as anchoring groups to capture Ag+ ion and stabilize the Ag NPs to suppress the aggregation of the Ag NPs.32,33 Therefore, for producing Ag NPs with smaller size and higher dispersion on CNTs, it is essential to keep a high concentration of the oxygen- and nitrogen-containing functional groups on CNTs. Owing to the elevated operation temperature, thermal calcination method can easily destroy the functional groups on CNTs.14,31 On the other hand, the EAR method is operated at temperature as low as room temperature, which is beneficial for keeping the functional groups on CNTs. Besides, it has been reported that the EAR method can increase the carbonyl and hydroxyl groups on the surface of carbon materials and modify the surface from hydrophobic to hydrophilic property, which is favorable for the subsequent wetness impregnation with metal ions.34 As such, the concentration of the functional groups on CNTs from the EAR process is higher than that from the thermal calcination. This is an origin for the smaller Ag NPs and higher Ag NPs dispersion on Ag/A-CNTs-P than on Ag/A-CNTs-C (Figure 5). During the EAR process, the origin for reducing Ag+ into Ag0 is the electrons with high energy.24 The possible mechanism of EAR for Ag decorated carbon materials has been proposed on the basis of FTIR and DFT studies.11 As electrons get access to the surface OH groups of carbon materials, hydrated electrons or H· radicals are formed by the dissociation and ionization of OH groups, and these strong reducing agents can reduce Ag+ into Ag0. In addition, it has been demonstrated that the nucleation of metal nanoparticles during the EAR process is much faster than that during the thermal calcination.19,24,25 A faster nucleation of metal nanoparticles is favorable for forming Ag NPs with 11 ACS Paragon Plus Environment


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smaller size.24 This is another origin for the smaller Ag NPs and higher Ag NPs dispersion on Ag/A-CNTs-P than on Ag/A-CNTs-C, in addition to the higher concentration of the functional groups on Ag/A-CNTs-P. Moreover, a faster nucleation of metal nanoparticles is favorable for forming Ag NPs with exposing facets with lower free energy.35,36 The free energies of the facets of Ag NPs are in the order of Ag(1 1 0)>Ag(1 0 0)>Ag(1 1 1).37 This could be the reason for the appearance of uniform Ag (1 1 1) on Ag/A-CNTs-P (Figure 5f). It is interesting that, as compared with the silver NPs on carbon microspheresin our previous work (10−20 nm),11 the size of Ag NPs on Ag/A-CNTs-P (3.8 nm) is much smaller under the same treatment conditions. The higher conductivity of CNTs than that of carbon microsphere is beneficial for the fast transfer of electrons, causing the formation of smaller Ag NPs on CNTs. In addition, during the EAR process, a large number of electrons surround the Ag NPs. Due to these electrons, repulsive interactions are formed among the Ag NPs. The repulsive interactions also make some contribution to the smaller and well dispersed Ag NPs on Ag/A-CNTs-P. The antibacterial activities of Ag/A-CNTs-C and Ag/A-CNTs-P were evaluated by determining the diameter ofinhibition zones (Figure 7). The blank experiment with only filter paper demonstrates almost no antibacterial activity. The inhibition zone occurs with the addition of Ag/A-CNTs samples. The diameters of inhibition zone from Ag/A-CNTs-C and Ag/A-CNTs-P against S. aureus/E.coli are 16.7/12.4 mm and 22.2/13.7 mm, respectively. Obviously, Ag/A-CNTs-P exhibits a better antibacterial performance than Ag/A-CNTs-C. The antifouling performance of Ag/A-CNTs-C and Ag/A-CNTs-P was evaluated against Platymonassubcordiformis (P. subcordiformis) and Tropidoneislepidoptera (T. lepidoptera). P. subcordiformis is active and mobile, whereas T. lepidopterais immobile and adherent onto the substrate surface. Less P. subcordiformis is available on 100 ppm Ag/A-CNTs-P in comparison with Ag/A-CNTs-C. Less T. lepidoptera is settled and adhered on 100 ppm Ag/A-CNTs-P, as can be 12 ACS Paragon Plus Environment

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seen from the low density of T.lepidoptera in Figure 7. Figure S4 in the supporting information shows the confocal fluorescent images of A-CNTs, Ag/A-CNTs-C, and Ag/A-CNTs-P. In the confocal fluorescent images, the green spots represent the live bacterial colonies, while the red spots correspond to the dead bacterialcolonies. As illustrated in Figure S4, the amount of the red spots in the confocal fluorescent image of Ag/A-CNTs-P is much more than those in the confocal fluorescent image of A-CNTs and Ag/A-CNTs-C, indicating abetter antibacterial activityof Ag/A-CNTs-P than those of A-CNTs and Ag/A-CNTs-C. The antibacterial activity of the Ag-based materials depends strongly on the Ag+ releasing rate.1,38–40 Ag+ ion can be bound to thiol groups in protein, inducing the inactivation of the bacterial proteins.1,38–41 A higher Ag+ ion releasing rate is beneficial for a better antibacterial activity. Figure 8 illustrates the Ag+ ion releasing rates on Ag/A-CNTs-C and Ag/A-CNTs-P in water. Both Ag/A-CNTs-C and Ag/A-CNTs-P show stable Ag+ ion releasing, and good repetition in five cycle test. However, the Ag+ ion releasing rateon the two samples is significantly different. The Ag+ ion releasing rate on Ag/A-CNTs-P is significantly higher than that on Ag/A-CNTs-C. This is the first origin for the higher antibacterial activity of Ag/A-CNTs-P than that of Ag/A-CNTs-C. The size of the Ag NPs is another key factor affecting the antibacterial activity of the Ag-based materials. During the antibacterial process, the Ag NPs interact with the cell membranes, andmake the cell malfunction. At a certain amount of Ag, a smaller Ag NP size will lead more Ag NPs to participate in the antibacterial process, thus beneficial for enhancing the antibacterial activity. As reflected by the TEM observations, the size of Ag NPs on Ag/A-CNTs-P (3.8 nm) is much smaller than that on Ag/A-CNTs-C (25.5 nm). This is the second origin for the enhanced antibacterial activity of Ag/A-CNTs-P, as compared with Ag/A-CNTs-C.



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CONCLUSIONS In summary, an electron-assisted reduction (EAR) method, which is operated at temperature as low as room temperature, was employed for fabricating aminated-CNTs-supported Ag NPs (Ag/A-CNTs). The average Ag NPs size on the EAR-prepared Ag/CNTs is only 3.8 nm, which is much smaller than that on the Ag/A-CNTs fabricated from the traditional thermal calcination (25.5 nm). Compared with Ag/A-CNTs fabricated from the traditional thermal calcination, the EAR-prepared Ag/A-CNTs shows much higher antibacterial activity against E.coli/S.aureus and antifouling performance against P. subcordiformis/T.lepidoptera. This is mainly originated from the significantly enhanced Ag+ ion releasing rate and highly dispersed Ag NPs with smaller size on the EAR-prepared Ag/A-CNTs. The findings from the present work are helpful for fabricating supported Ag NPs with smaller size and higher dispersion for efficient antibacterial process.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. TG analyses of AgNO3, Ag/A-CNTs in N2 and in Air. Schematic of the setup for the RF plasma. TG curves of (a) Ag/A-CNTs-C and (b) Ag/A-CNTs-P. Confocal fluorescent images of live and dead E.coli bacterial cells treated with 100 µg mL-1 of (a, d) A-CNTs, (b, e) Ag/A-CNTs-C and (e, f) Ag/A-CNTs-P andstained with GFP (green) and PI (red).The scale bar is 20 µm.


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AUTHOR INFORMATION Corresponding Authors *E-mail (S. Li): [email protected]. *E-mail (Y. –X. Pan): [email protected]. Author Contributions All authors are responsible for the originality of the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the National Natural Science Foundation of China (Nos. 21406153, 21576177 and 21503062), Shanxi Province Science Foundation for Youths (No. 2014021014-2).and the Supported by Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi.

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Table 1. Surface Atomic Composition of Ag/A-CNTs-C and Ag/A-CNTs-P Surface atomic composition （%） Sample

Ag/C atomic ratio C

O

N

Ag

Ag/A-CNTs-C

95.25

3.30

0.82

0.63

0.007

Ag/A-CNTs-P

87.51

8.21

3.23

1.05

0.012



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Figure 1. Surface modification of CNTs through oxidation and amination processes.


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Figure 2. SEM images of the pristine CNTs, O-CNTs and A-CNTs.



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(b)

(a)

Figure 3. (a) FTIR spectra and (b) TG analyses of pristine CNTs, O-CNTs, and A-CNTs (DTG profile of A-CNTs inserts in the left corner).


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(a)

(b)

(c)

(d)

Figure 4. (a) XRD patterns, (b) UV-visible spectra, (c) XPS spectra, and (d) Ag XPS spectra of Ag/A-CNTs-C and Ag/A-CNTs-P.



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Figure 5. TEM images of (a, b) Ag/A-CNTs-C and (c-f) Ag/A-CNTs-P.


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Figure 6. Size distribution of Ag NPs on (a) Ag/A-CNTs-C and (b) Ag/A-CNTs-P.



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Figure 7. Antibacterial and algal inhibiting properties of Ag/A-CNTs-C and Ag/A-CNTs-P.


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100 90 80 70 60 50 40 30

+

Ag ion concentration (ppb)

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20 nd

st

th

rd

2 cycle

1 cycle

10

th

4 cycle

3 cycle

5 cycle

0 0

6

12 18 24 0

Time (h)

6

12 18 24 0

Time (h)

6

12 18 24 0

Time (h)

6

12 18 24 0

Time (h)

6

12 18 24

Time (h)

Figure 8. The Ag+ ion release rates on Ag/A-CNTs-C (▲) and Ag/A-CNTs-P (■).



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Graphic Abstract (Table of Contents):


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Immobilization of Highly Dispersed Ag ... - ACS Publications

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