Silver-Incorporated Mussel-Inspired Polydopamine Coatings on

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Silver-Incorporated Mussel-Inspired Polydopamine Coatings on Mesoporous Silica as an Efficient Nanocatalyst and Antimicrobial Agent Yiyan Song,† Huijun Jiang,‡ Bangbang Wang,§ Yan Kong,§ and Jin Chen*,†,∥ †

School of Public Health, ‡School of Pharmacy, and ∥The Key Laboratory of Modern Toxicology, Ministry of Education, School of Public Health, Nanjing Medical University, Nanjing 211166, China § State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 210009 Nanjing, China S Supporting Information *

ABSTRACT: To tackle severe environmental pollution, a search for materials by economical and eco-friendly preparations is demanding for public health. In this study, a novel in situ method to form silver nanoparticles under mild conditions was developed using biomimetic reducing agents of polydopamine coated on the rodlike mesoporous silica of SBA-15. The synthesized SBA-15/polydopamine (PDA)/Ag nanocomposites were characterized by a combination of physicochemical and electrochemical methods. 4-Nitrophenol (4-NP) and methylene blue (MB) were used as models for the evaluation of the prepared nanocatalysts of SBA-15/PDA/Ag in which the composite exhibited enhanced catalytic performance toward degrading 4-NP in solution and MB on the membrane, respectively. Additionally, compared with that of solid core−shell SiO2/PDA/Ag, tubular SBA15/PDA/Ag showed the prolonged inhibitory effect on microbial growth as typified by Escherichia coli (60 h), Staphylococcus aureus (36 h), and Aspergillus f umigatus (60 h), which demonstrated efficient control of silver nanoparticles release from the mesopores. The constructed dual-functional SBA-15/PDA/Ag as the long-term antimicrobial agent and the catalyst of industrial products provides an integrated nanoplatform to deal with environmental concerns. KEYWORDS: mesoporous silica, polydopamine, Ag nanoparticles, nanocatalyst, antimicrobial mesopores with diameters ranging from 2 to 50 nm.16 SBA15 with manipulable tubular structure contains surface silicon hydroxyls ready for postmodifications, which is desirable for controlled delivery of targeted molecules.25,26 The mesoporous silica scaffold for the incorporation of antimicrobials such as metal nanoparticles may possess long-term efficacy while reducing the harmfully excessive remediation.27,28 Recently, metal nanoparticles, especially gold and silver ones, show advantageous features for industrial as well as biomedical applications because of their multifunctions in electrochemical, optical, magnetic, and catalytic aspects.29−35 Particularly, silver nanoparticles of nontoxicity exhibit a broad spectrum of antimicrobial effects36−38 as a promising chemotherapy toward multiple drug resistance (MDR).39 Nevertheless, silver nanoparticles of good dispersion and devoid of harmful byproduct formation40−42 are challenging during the synthesis process in which regular silica mesopores28 and biopolymers43 were utilized to produce nanoparticles of refined physiochemical properties.

1. INTRODUCTION As a mimic of the adhesive foot proteins secreted from deep-sea mussels, polydopamine (PDA) has attracted increasing attention because of its versatile surface adaptability.1−3 Upon polymerization of its dopamine precursors under alkaline conditions reminiscent of marine environments, PDA in essence can form a thick nanosized coatings on diverse types of inorganic and organic surfaces,2,4−9 which contributes to a broad implementation including surface modification, protein immobilization, and cell adhesion.4,10−13 Besides its remarkable mechanical reinforcement as adhesive materials attributed to the structural composition of multiple amine and/or catechols,1 PDA with aromatic hydroxyl moieties can act as the mild reducing agent to form stable and homogeneous metal nanoparticles.4,14 Because of their tunable mesostructures and improved thermal stabilities, mesoporous silica materials such as MCM41 (Mobil Composition of Matter-41),15 SBA-15 (Santa Barbara Amorphous-15),16,17 and KIT-6 (Korea Advanced Institute of Science and Technology-6)18,19 have been extensively studied as catalysts, sorbents, and biomedicines.20−24 In particular, SBA-15 possesses highly ordered large pore with thick pore walls and hexagonal arrayed © XXXX American Chemical Society

Received: November 29, 2017 Accepted: December 27, 2017

A

DOI: 10.1021/acsami.7b18136 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

30 mL of 10 mM Tris-HCl solution (pH 8.5) using an ultrasonic bath. After the 24 h incubation, the mixture was centrifuged at 8000 rpm for 5 min to collect the precipitate. The precipitate was washed three times with ultrapure water and dried prior to the assay. The obtained composite was denoted as SBA-15/PDAx, where the subscript x stands for the initial mass ratios of dopamine versus SBA-15 (x = 0.2, 0.6, and 1.2, respectively, in this text). 2.3. In Situ Formation of Ag Nanoparticles on SBA-15/PDA. SBA-15/PDA0.6 (40 mg) was mixed with AgNO3 (6 mL, 32 mM) at 80 °C and pH 8.5 for 12 h under stirring. After centrifugation at 8000 rpm for 5 min, the precipitate was washed three times with ultrapure water and dried to obtain the final product. As a control group, solid core−shell SiO2/PDA/Ag was synthesized using solid SiO2 mentioned above in our laboratory. 2.4. Characterization. The microscopic properties of samples were recorded on a scanning electron microscope (SEM, Hitachi SU1510, Japan) operated at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were obtained on an electron microscope (JEOL-1010, Japan) operated at 200 kV. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe) was recorded to evaluate the elementary composition. The XRD patterns of the samples were acquired on Smartlab TM 9 kW (Rigaku Corporation, Tokyo, Japan) equipped with a rotating anode and Cu Kα radiation (λ = 0.154 nm). The nitrogen sorption isotherms were collected on Micromeritics volumetric adsorption analyzer (ASAP2020, USA). The specific surface areas and pore size distribution of samples were calculated based on Brunauer−Emmett−Teller (BET) and Barret−Joyner−Halender (BJH) methods, respectively. Raman spectroscopy was performed by using DXR Smart Raman Spectrometer (Thermo Scientific, USA). Electrochemical data were obtained using an integrated electrochemical analyzer (CHI760D, China) with a three-electrode system including a platinum wire as an auxiliary electrode, a saturated calomel electrode as a reference, and a glassy carbon electrode (GCE) as a working electrode. UV−vis spectra were recorded on a UH5300 UV−vis spectrophotometer (Hitachi, Japan). 2.5. Evaluation of Catalytic Activity of Materials. The chemical reduction of 4-nitrophenol (4-NP) and methylene blue (MB) were used as model reactions to test the catalytic activity of SBA-15/PDA0.6/Ag nanocomposites. Typically, in the case of 4-NP, H2O (1100 μL), 4-NP (200 μL, 20 mM), and NaBH4 (660 μL, 3 M) were mixed in a quartz cuvette with a path length of 1 cm before the addition of SBA-15/PDA0.6/Ag nanocomposites (40 μL, 0.5 mg/mL) and the resulting absorbance signal at 400 nm was immediately recorded on a UV−vis spectrophotometer. For the catalytic reaction of MB, 5 mg of SBA-15/PDA0.6/Ag was fixed on a commercial filter membrane (0.45 μm) first by simply passing the suspended solution of nanocomposite through the membrane. Then the MB solution flew through the filter membrane in the presence of NaBH4 and the visible color change of MB solution was used to evaluate the catalytic performance of nanocomposite. 2.6. Antimicrobial Assay. Before the test, all the reagents and tubes in the experiments were sterilized. Antimicrobial activity was evaluated based on a spread plate method45 using Staphylococcus aureus (S. aureus ATCC 6538), Escherichia coli (E. coli ATCC 25922), and Aspergilus f umigatus (A. f umigatus ATCC 1160) as models. Typically, the dispersions of SBA-15, SBA-15/PDA0.6, SBA-15/ PDA0.6/Ag, and SiO2/PDA0.6/Ag of 400 μL, 10 mg/mL, each were added to agar plates (85 mm in diameter), respectively, and dried for 30 min. The amounts of microbes used for the assay were optimized to better visualize the antimicrobial effect caused by the composite materials. Therefore, ∼105 CFU mL for both S. aureus (10 μL) and E. coli (200 μL) and ∼107 CFU/mL for A. f umigatus (10 μL) were transferred into the agar plates, respectively, which was followed by the incubation at 37 °C for 12−60 h. Optical images of incubated plates were documented with a digital camera. Plates for each were prepared in triplicate and the experiment was repeated three times. To determine the amount of reactive oxygen species (ROS) involved with the nanomaterials-induced cell death, the intracellular ROS indicator of 2′,7′-dichlorofluorescin-diacetate (DCFDA) was employed. The

In this study, we tried to in situ reduce silver salts to produce well-distributed silver nanoparticles based on self-polymerized dopamine coatings on mesoporous SBA-15 in a green manner (Scheme 1). Because of the formed nanosized film of PDA in Scheme 1. Dopamine Polymerization and in Situ Reduction of Silver Ions to Silver Nanoparticles Deposited on the Mesopores of SBA-15

the mesopores, considerable Ag nanoparticles were encapsulated into the silica channels and a controlled release of Ag+ was achieved. Meanwhile, PDA-decorated SBA-15 of regular mesoporosity provided an integrated platform for the homogeneous distribution of Ag nanoparticles to exert their catalytic as well as antimicrobial function. The physicochemical in combination with electrochemical methods were used to characterize the SBA-15/PDA/Ag nanocomposites.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraethyl orthosilicate (TEOS), silver nitrate (AgNO3), and sodium borohydride (NaBH4) were purchased from the Sinopharm Chemical Reagent Co., Ltd. A triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(-ethylene glycol) (Pluronic P123 with a molecular weight of 5800, EO20PO70EO20) was from Sigma-Aldrich (Germany) and used as the structure-directing template. Dopamine hydrochloride and 4nitrophenol were obtained from Aladdin Industrial Corporation (Shanghai, China). All chemicals were used as received. 2.2. Synthesis of Polydopamine-Coated SBA-15 (SBA-15/ PDAx). Mesoporous SBA-15 was prepared following the previous protocol.44 Briefly, 4.0 g of P123 was first dissolved in 80 mL of ultrapure water and the solution was stirred for 3 h before 70 mL of HCl (3.4 M) was added. Then 9.14 mL of TEOS was added to the solution and mixed for another 24 h. For all the procedures, the reaction temperature was maintained at 36 °C. The solution was transferred in a Teflon autoclave and heated at 100 °C for 24 h. The product was filtered and dried at 80 °C for 24 h in an air oven and the surfactant was removed by calcination at 550 °C for 6 h. Solid SiO2 was synthesized without calcination at 550 °C for 6 h. To produce PDA-decorated SBA-15 (SBA-15/PDA), 50 mg of SBA-15 was mixed with a certain amount of dopamine (10−60 mg) in B


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Figure 1. SEM images of SBA-15 (A), SBA-15/PDA0.6 (B), SBA-15/PDA0.6/Ag (C) composites, and (D) X-ray energy dispersive spectrometry of SBA-15/PDA0.6/Ag composites. HRTEM images of SBA-15 (E), SBA-15/PDA0.6 (F), and SBA-15/PDA0.6/Ag composites (G) with corresponding TEM images inset.

Figure 2. XPS spectra of (A) SBA-15/PDA0.6/Ag and (B) high-resolution spectra of Ag 3d of SBA-15/PDA0.6/Ag. (C) Raman spectra of (a) SBA15/PDA0.6 and (b) SBA-15/PDA0.6/Ag. Inset: a magnified view of (b). oxidation of nonfluorescent DCFH to highly fluorescent 2′,7′dichlorofluorescin (DCF) gives a quantitative evaluation of ROS formation. After a 2 h incubation in the presence of nanocomposites, E. coli samples were stained with 10 μM DCFDA and the percentage of bacteria exhibiting fluorescent signal was estimated by manual counting under a fluorescence microscope (ZEISS MTB2004, Germany).

polydopamine (PDA) layer, the rodlike structure of SBA-15/ PDA0.6 was similar to that of SBA-15 (Figure 1B), indicating the thin PDA film does not alter the surface morphology of supporting mesopores. PDA has the ability to bind and reduce ions of Ag+ to form metal silver (Ag) and meanwhile the catechol and the hydroxyl groups of PDA were oxidized to quinone and carbonyl groups, respectively (as shown in Scheme 1),4 which offers an opportunity to decorate the composite SBA-15/PDAX with silver nanoparticles. Therefore, when SBA-15/PDA0.6 were incubated in the Ag+ solution at 80 °C and pH 8.5, Ag nanoparticles were successfully formed and distributed on the mesopores (Figure 1C). Notably, the positively charged PDA helped to prevent the aggregation of Ag nanoparticles by providing reciprocal electrostatic repulsion. Although the homogeneous distribution of silver nanoparticles

3. RESULTS AND DISCUSSION 3.1. Morphology Characterization. The surface morphology of the composites was examined by SEM and elemental compositions of SBA-15/PDA0.6/Ag were studied by X-ray energy dispersive spectrometry (EDS). As shown in Figure 1A, the mesoporous silica SBA-15 forms short rodlike structure at the length of about 1 μm. After being coated with C


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Figure 3. Small (A) and wide (B) angle X-ray diffraction patterns for SBA-15, SBA-15/PDA0.6, and SBA-15/PDA0.6/Ag powders. (C) N2 adsorption/desorption isotherms and (D) pore size distribution from Barret−Joyner−Hallender (BJH) desorption of SBA-15, SBA-15/PDA0.6, and SBA-15/PDA0.6/Ag.

was achieved before,24,46 the composite SBA-15/PDA0.6/Ag prepared under mild conditions offers an alternative green method without using relatively toxic chemical reagents. The elemental compositions of SBA-15/PDA0.6/Ag were confirmed by EDS (Figure 1D) and their distributions were further verified by the mapping result. As shown in Figure S1, the elements of C, N, O, Si, and Ag exist and distribute uniformly. The content of Ag on the outer surface was about 3.24 wt % as

Table 1. Mesoscale Properties of SBA-15, SBA-15/PDA0.6, and SBA-15/PDA0.6/Ag sample SBA-15 SBA-15/PDA0.6 SBA-15/ PDA0.6/Ag

BET surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

364.137 799.000 340.603

0.632 0.674 0.612

7.771 5.722 3.898

Figure 4. (A) Time-dependent UV−vis absorbance for the catalytic reduction of 4-NP by NaBH4 in the presence of SBA-15/PDA0.6/Ag. The inset shows the color change of the reaction. Assay conditions: 1100 μL of H2O, 200 μL of 20 mM 4-NP, 660 μL of 3 M NaBH4, and 40 μL of 0.5 mg/mL SBA-15/PDA0.6/Ag. (B) Influence of SBA-15/PDA0.6/Ag concentration on the reduction of 4-NP. Assay conditions: C4‑NP = 2 mM, CNaBH4 = 1 M, C SBA‑15/PDA/Ag = 0.01 or 0.02 mg/mL; inset: −ln(C/C0) as a function of reaction time. C0 is the initial concentration of 4-NP and C is the concentration at time t. (C) Photograph and illustration of the continuous flow catalysis of MB on a commercial filter membrane coated with SBA15/PDA0.6/Ag in the presence of NaBH4. D


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observed in the channels compared with those on the outer surface. (shown in Figure 1G). 3.2. Surface Properties Studied by XPS, Raman Spectroscopy, and Electrochemical Impedance Measurement. X-ray photoelectron spectroscopy (XPS) is an efficient tool to analyze the surface elemental configurations of materials. As shown in Figure 2 and Figure S2, the surface elements of SBA-15/PDA/Ag mainly include Si, O, Ag, N, and a trace amount of absorbed C originating from PDA and ambient atmosphere. A strong Ag signal peak at 370 eV could be found in XPS analysis (Figure 2A) and the high-resolution XPS spectra exhibits two peaks at 365.6 and 371.5 eV assigned to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively (Figure 2B).47 The Ag and Si contents in the composites were 0.85 and 10.99 at. % calculated by XPS, respectively, corresponding to a molar ratio of 3.4 of Si/Ag, which was comparable to that of 2.6 as calculated by EDS analysis. Raman spectroscopy as a powerful nondestructive assay has been widely used to explore the crystal orderliness of carbon materials.48 As shown in Figure 2C, two peaks at 1406 and 1580 cm−1 were aroused from the stretching and deformation of aromatic rings, which confirmed the presence of PDA layer.49 The Raman signals were significantly improved as for the composite SBA-15/PDA0.6/Ag, which was attributed to the existence of Ag nanoparticles on the coated PDA. The electrochemical properties of Ag nanoparticles on the PDA film were studied by electrochemical impedance spectroscopy (EIS) on the electrodes modified with constructed nano-

Table 2. Reported Apparent Rate Constants (kapp) of Degrading 4-NP Using Ag-Based Catalystsa catalyst

concentration (mg/mL)

kapp (min−1)

reference

Ag−AgBr/γ-Al2O3 Au@Ag NP Fe3O4@NS/Ag GO/Ag−Fe3O4 SBA-15/PDA0.6/Ag SBA-15/PDA0.6/Ag

1 0.132 0.047 0.033 0.01 0.02

0.1515 0.2982 0.16 1.56 0.4404 0.8946

53 54 55 56 this work this work

Ag−AgBr/γ-Al2O3, hollow flower-like composite; Au@Ag NP, Au@ Ag core−shell nanoparticle; Fe3O4@NS/Ag, Ag and nickel silicate coated Fe3O4 microsphere; GO/Ag−Fe3O4, graphene oxide/Ag nanparticle−Fe3O4 nanocomposite. a

calculated, suggesting some Ag nanoparticles may be encapsulated into the silica channels based on the amount of AgNO3 used for the synthesis. To further validate the formation of PDA film and Ag nanoparticles, TEM was used to record the morphologies of obtained materials. As shown in Figure 1E, the as-prepared SBA-15 sample possessed high ordered hexagonal mesostructure. After the decoration of PDA film, the ordered mesoporous structure of SBA-15 was retained with visible thin film of PDA on the mesopore surface (Figure 1F). The Ag nanoparticles that were stabilized by PDA distributed homogeneously on the surfaces of rodlike SBA-15 (shown in Figure 1G, inset), in which Ag nanoparticles of relatively small size were evidently

Figure 5. Optical images of agar plates characteristic of the antimicrobial activities of SBA-15, SBA-15/PDA0.6, SBA-15/PDA0.6/Ag, and SiO2/ PDA0.6/Ag against E. coli. Assay conditions: 400 μL of 10 mg/mL SBA-15 and nanocomposite dispersions were added to the plate, respectively, and 200 μL of E. coli (∼105 CFU/mL) was inoculated. Scale bar: 20 mm. E


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Figure 6. Optical images of agar plates characteristic of the antimicrobial activities of SBA-15, SBA-15/PDA0.6, SBA-15/PDA0.6/Ag, and SiO2/ PDA0.6/Ag against S. aureus. Assay conditions: 400 μL of 10 mg/mL SBA-15 and nanocomposite dispersions were added to the plate, respectively, and 10 μL of S. aureus (∼105 CFU/mL) was inoculated. Scale bar: 20 mm.

Figure 7. Optical images of agar plates characteristic of the antimicrobial activities of SBA-15, SBA-15/PDA0.6, SBA-15/PDA0.6/Ag, and SiO2/ PDA0.6/Ag against A. fumigatus. Assay conditions: 400 μL of 10 mg/mL SBA-15 and nanocomposite dispersions were added to the plate, respectively, and 10 μL of A. f umigatus (∼107 CFU/mL) was inoculated. Scale bar: 20 mm.

composites. The diameter of the semicircle extrapolated in the Nyquist diagram represents the electron-transfer resistance of the redox probe at the electrode surface. Thus, the higher resistance at the electrode surface will result in the larger

diameter of the semicircle. As such, the polished GCE showed a near straight line (Figure S3) because of its good conductivity. Despite the superior conductivity of Ag nanoparticles, the recorded Nyquist impedance plots of both SBA-15/PDA0.6 and F


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Figure 8. (A) Fluorescence microscopic images (left) and bright field images (right) of DCF+ labeled E. coli stained with DCFDA after the incubation with LB liquid media (negative control), SiO2/PDA0.6/Ag, and SBA-15/PDA0.6/Ag. (B) The ratios of DCF+ cells incubated with LB liquid media (negative control), SiO2/PDA0.6/Ag, and SBA-15/PDA0.6/Ag calculated by manually counting under a microscope.

were decreased for SBA-15/PDA0.6/Ag (Table 1), which accorded well with small angle XRD measurement. These observations may reveal a fine coating of PDA on SBA-15 with retained mesoscale regularity favorable for autoreducing AgNO3 at basic pH. 3.4. Catalytic Activity of Nanocomposites. The uniform distribution of Ag nanoparticles on SBA-15/PDA0.6 was related to the possible improved catalytic activity of nanocomposite. Herein, the reduction of 4-NP to 4-aminophenol in the presence of NaBH4 was used as a model reaction. When 4-NP was mixed with NaBH4, a new absorbance peak at 400 nm (Figure 4A) was observed due to the conversion of 4-NP to 4nitrophenolate ions52 with the absorbance intensity being decreased significantly after adding the catalysts, which was accompanied by the visible color change from yellow to colorless (Figure 4A, inset). Notably, as shown in Figure 4A, the reduction of 4-NP was completed within 7 min at a concentration as low as 0.01 mg/mL SBA-15/PDA0.6/Ag in comparison with no observable catalytic reaction in the absence of SBA-15/PDA0.6/Ag (Figure 4B). The catalytic reaction rate also showed a dependence on the concentration of SBA-15/ PDA0.6/Ag. As shown in Figure 4B, with a higher catalyst usage of 0.02 mg/mL, the reduction of 4-NP was completed within 3 min, which is relatively faster than that of 0.01 mg/mL. When −ln(C/C0) was plotted as a function of time (Figure 4B, inset), a linear relationship was obtained supporting a first-order kinetics (−dC/dt = kappt) was involved for the overall reaction process. The calculated apparent rate constants of the catalytic reaction of 4-NP using SBA-15/PDA0.6/Ag were listed in Table 2. In comparison with silver-based catalysts reported previously, the constructed SBA-15/PDA0.6/Ag nanocomposites exhibited a desirable catalytic performance within the concentration range of 2.5−20 mM of 4-NP that was tested (Figure S4A). Moreover, SBA-15/PDA0.6/Ag showed a nice stability and recyclability being able to reuse for at least 7 times without pronounced decrease of catalytic activities (Figure S4B).

SBA-15/PDA0.6/Ag showed a relatively large semicircle in comparison with the nearly straight line of bare GCE, which suggested the blockage of electron transfer on the PDA film with the organic semiconductor nature. 3.3. Mesostructual Characterization. The small angle XRD patterns (Figure 3A) showed that the ordered mesoporosity of SBA-15 was maintained after the adhesion of PDA film42 but such mesostructural regularity was greatly reduced after incorporation of Ag nanoparticles in SBA-15/ PDA0.6. The wide angle XRD patterns of SBA-15, SBA-15/ PDA0.6, and SBA-15/PDA0.6/Ag were shown in Figure 3B. The broad reflection peaks of SBA-15 and SBA-15/PDA0.6 revealed the amorphous nature of the produced materials. For SBA-15/ PDA0.6/Ag composites, four peaks at 38.139°, 44.319°, 64.460°, and 77.400° represented the Bragg reflections from (111), (200), (220), and (311) planes of Ag (JCPDS Card No. 040783), respectively, demonstrating the existence of the cubic structure of metallic Ag. The other peaks at 27.840°, 32.259°, 46.242°, 54.838°, 57.480°, 67.459°, 74.460°, and 76.722° represented the (111), (200), (220), (311), (222), (400), (331), and (420) planes of AgCl (JCPDS Card No. 06-0480), respectively, which pointed out the existence of AgCl particles. The crystallite size (average size of the coherent scattering region) for the Ag component was found to be 67.5 nm as calculated by the Scherrer formula.50 The mesostructual parameters of produced materials were derived from the N2 sorption isotherms with their pore size distribution from BJH adsorption as shown in Figure 3C,D. The isotherms of SBA-15, SBA-15/PDA0.6, and SBA-15/ PDA0.6/Ag exhibited type IV with a N2 hysteresis loop.51 Figure 3D indicated there was an obvious decrease in the pore size of materials, 7.77 nm for SBA-15, 5.72 nm for as-prepared SBA-15/PDA0.6, and 3.90 nm for as-prepared SBA-15/PDA0.6/ Ag, suggesting the successful encapsulation of Ag nanoparticles in the mesopores. Accordingly, the BET surface area and pore volume of SBA-15/PDA0.6 increased due to the PDA coating on the mesoporous siliceous support while these parameters G


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of 6.2% for the negative control (Ag-free) (Figure 8). These observations may indicate the formation of ROS involved with the nanocomposite-mediated cell death. It should also be noted that the percentage of DCF+-labeled cells treated by SBA-15/ PDA0.6/Ag was more than that of solid SiO2/PDA0.6/Ag, which corroborated the refined mesostructured SBA-15/PDA0.6/Ag nanocomposite. Nevertheless, further studies using the ROS quencher can be conducted to identify the direct correlation existing between ROS generation and antibacterial activity of the composite material. In addition, the antibacterial activities of Ag nanoparticles were ascribed to the physical effects like the damage of cell wall or the denaturation of cell wall proteins. Considering the structural difference of cell wall and composition, there might be different effects between E. coli (G−) and S. aureus (G+) caused by silver nanoparticles. For example, E. coli possessed the negatively charged lipopolysaccharides (LPS) capable of interacting with the weak positively charged silver nanoparticles,59 which was more sensitive toward SBA-15/PDA0.6/Ag composites than S. aureus.

The sufficient adhesion of silver nanoparticles on the PDAcoated mesoporous silica SBA-15 prevented greatly the leaking of catalytic Ag components regardless of their nanoscale sizes.1 Therefore, the efficient catalytic activities of the constructed nanocomposite were revealed and applicable for recyclable use, which may enable the continuous flow catalysis for practical purposes. When catalysts were coated on commercial filter membranes, the degradation of MB in the presence of NaBH4 could be accomplished simply by passing the MB-contained solution through the filter (Figure 4C). Compared with the untreated filter (Figure S4C), the filter with SBA-15/PDA0.6/ Ag composites coating on the membrane could sufficiently degrade MB into a colorless filtrate solution in a fast manner. Notably, the membrane immobilized by SBA-15/PDA0.6/Ag also showed desirable recyclability. As we checked, the membranes that were fixed with 4 mg of SBA-15/PDA0.6/Ag could be used continuously to deal with at least 30 mL of methylene blue (10 mg/L) and could be repeatedly used for over 10 times. Therefore, the produced composite materials coated on the commercial membrane were potentially useful for the online catalysis once equipped with a pump. 3.5. Antimicrobial Activity of Nanocomposites. Besides the industrial role as the catalyst, the mesoporous composites of SBA-15/PDA0.6/Ag also hold potential as antimicrobial materials. As silver nanoparticles normally exhibit widespectrum antimicrobial properties, we used two typical bacteria including G− E. coli and G+ S. aureus and fungus A. f umigatus as models in the assay. To evaluate the antimicrobial activity of the nanocomposite, E. coli, S. aureus, and A. f umigatus were used as models using agar plates. As shown in Figure 5, the presence of SBA-15/PDA0.6/Ag resulted in almost no visible E. coli bacterial colonies within the incubation time of 60 h in contrast to weak antimicrobial activities of SBA-15 and SBA15/PDA0.6. The solid core−shell composite of SiO2/PDA0.6/Ag showed antimicrobial properties for E. coli within 12 h incubation (Figure 5). For S. aureus, the antimicrobial properties of mesoporous SBA-15/PDA0.6/Ag were demonstrated for at least 36 h, while solid core−shell SiO2/PDA0.6/Ag was about 24 h and both SBA-15 and SBA-15/PDA0.6 did not show any pronounced antimicrobial effects (shown in Figure 6). There were no visible colonies within 12 h for all the plates due to the relatively slow growth of A. f umigatus. After further incubation of 24 h, compared with that of solid core−shell SiO2/PDA0.6/Ag, there were no observable colonies of A. f umigatus for SBA-15/PDA0.6/Ag (Figure 7). We concluded that mesoporous SBA-15/PDA0.6/Ag as compared with solid SiO2/PDA0.6/Ag led to improved long-term antimicrobial activity, which was attributed to the advantageous channeled mesostructure of SBA-15/PDA favorable for the controlled release of Ag nanoparticles. The prolonged effect of widespectrum antimicrobial activity of built nanocomposite indicated a role of adhesive PDA coating on mesopores for the homogeneous distribution of Ag nanoparticles to exert their function. The generation of free radicals has been considered as the direct cause of silver nanoparticles-induced cytotoxicity in bacteria.57,58 In the present study, to determine the burst of free radicals and reactive oxygen species (ROS), DCFDA was used as an indicator of ROS for the nanocomposite-treated cells. E. coli was first incubated with the composites for 2 h and the bacteria were stained with DCFDA for 30 min. We found that 42.4% of SBA-15/PDA0.6/Ag- and 29.6% of SiO2/PDA0.6/Agtreated bacteria became DCF+, which was compared with that

4. CONCLUSIONS Nowadays, environmental pollution is becoming a severe threat to public health because of compositional complexity such as industrial discharge of highly toxic dyes and proliferation of infectious microbia,60−64 which boosts continuous efforts in search of safe, economical, and green nanomaterials of functional versatility. In this study, dual-functional Ag nanoparticles as catalysts and antimicrobials were facilely in situ synthesized under mild conditions aided by mussel-inspired polydopamine adhesive to mesoporous supports of SBA-15. The as-prepared SBA-15/PDA/Ag nanocomposites preserved the mesoporous structure and exhibit superior catalytic and antimicrobial properties. The study provided an alternative green route for the fabrication of SBA-15/PDA/Ag nanocomposite as an efficient catalyst applicable for practical continuous flow catalysis of MB. Besides, compared with solid core−shell SiO2/PDA/Ag nanocomposites, mesoporous SBA-15/PDA/Ag nanocomposites exhibited the extended inhibitory growth of E. coli, S. aureus, and A. f umigatus. The dual functional SBA-15/PDA/Ag nanocomposites as catalysts and antimicrobial agents possess wide environmental as well as biomedical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18136. Experimental details; EDS mapping results; highresolution XPS spectra of Si 2p, C 1s, N 1s, and O 1s; EIS results; reaction time as a function of the concentration of 4-NP, reusability test of obtained catalyst, and a control flow catalysis experiment using an untreated filter (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Jin Chen: 0000-0001-8377-2708 Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS The work was supported by National Natural Science Foundation of China (U1703118), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Jiangsu Shuangchuang Program, Open Funds of the State Key Laboratory for Chemo/ Biosensing and Chemometrics (2016015), the National Laboratory of Biomacromolecules (2017kf05), and Jiangsu Specially-Appointed Professor project, China.

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