Catecholamine-Induced Electroless Metallization of Silver on Silica

Feb 4, 2014 - School of Chemistry and Chemical Engineering, Southeast University, .... International Journal of Pharmaceutics 2017 533 (1), 73-83 ... ...
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Catecholamine-Induced Electroless Metallization of Silver on Silica@Polymer Hybrid Nanospheres and Their Catalytic Applications Li Qun Xu,† Beatrice Swee Min Yap,† Rong Wang,† Koon-Gee Neoh,† En-Tang Kang,*,† and Guo Dong Fu*,‡ †

Department of Chemical & Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 117576 School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province, P.R. China 211189

‡

S Supporting Information *

ABSTRACT: Narrowly dispersed raspberry-like SiO2@poly(dopamine acrylamide-co-methacrylic acid-co-ethylene glycol dimethacrylate)/Ag (or SiO2@PDA/Ag) composite nanospheres were synthesized via a combination of sol−gel reaction, distillation-precipitation polymerization, reactive ester-amine reaction, and electroless metallization. In this approach, SiO2@ poly(pentafluorophenyl acrylate-co-methacrylic acid-co-ethylene glycol dimethacrylate) (or SiO2@PPFA) core−shell nanospheres were first prepared by distillation-precipitation polymerization, using the SiO2-3-(trimethoxysilyl)propyl methacrylate (SiO2-MPS) nanospheres from sol−gel reaction as seeds. The reactive pentafluorophenyl (PFP) ester moieties on SiO2@PPFA nanospheres can readily react with dopamine hydrochloride to form amide linkages with no side reaction under a mild condition. The catecholamine moieties in the resulting SiO2@PDA nanospheres were utilized for simultaneous reduction of Ag+ ions and coordinative binding of the metal nanoparticles. The SiO2-MPS, SiO2@PPFA, and SiO2@PDA nanospheres as well as the raspberry-like SiO2@PDA/Ag composite nanospheres were characterized by field-emission transmission electron microscopy (FETEM), thermogravimetric analysis (TGA), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), and FT-IR spectroscopy. Furthermore, the as-synthesized SiO2@PDA/Ag composite nanospheres were explored for the catalytic reduction of p-nitrophenol and degradation of rhodamine 6G (R6G) dye.

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INTRODUCTION Composite particles composed of colloidal spheres and noble metal nanoparticles have attracted considerable research interest because of their potential for advanced applications in surface coating, photonics, electronics, separations, catalysis, pharmacology, and diagnostics.1−3 Raspberry-like particles, characterized by a discontinuous or rough inorganic shell on the larger colloidal particles,4 are an important class of composite particles. The raspberry-like particles have been widely used in surface-enhanced Raman spectroscopy (SERS), superhydrophobic/superhydrophilic coatings, separation, nanoscale electronics and molecular devices, antibacterial agents, and catalysts.5−10 A variety of chemical and physicochemical methods have been explored for depositing small inorganic nanoparticles on the larger colloidal particles to produce raspberry-like particles. A classic synthetic strategy of raspberrylike particles consists of two steps: i) activating the surface of large particles and ii) coating small inorganic nanoparticles on the surface of these large particles.10−13 Alternatively, the raspberry-like particles may be fabricated by in situ assembly of building blocks of different size and composition on the larger colloidal particles.14−17 The protein-based adhesives, found in marine mussel Mytilus edulis, are receiving growing attention in the context of biomimetics.18,19 The essential component of these adhesive proteins has been identified as the unusual amino acid 3,4dihydroxyphenylalanine (DOPA).20 Inspired by the salient features of adhesive proteins, interest in integrating DOPA and its analogs into organic and polymeric materials as active © 2014 American Chemical Society

adhesive constituents has increased substantially in recent years. 21−25 In addition, the catechol-containing DOPA derivates are redox active and therefore can function as a reducing agent to reduce metal ions and form strong coordination interactions with metal surfaces through two adjacent hydroxyls.26−29 For example, Lee et al. have demonstrated that Ag+ and Cu2+ ions can undergo spontaneous electroless metallization from self-polymerized polydopaminecoated substrates.30 In another study, Black et al. have reported that the catecholamine-modified poly(ethylene glycol) (PEG) polymers can reduce Au3+ or Ag+ ions, leading to the formation of noble metal nanoparticle cores surrounded by PEG shells and catecholamine oxidation induced cross-linked cores.31 Inspired by the redox activity and strong coordination interactions of the catecholamine-containing molecules with metals, catecholamine-coated colloidal spheres can be prepared and utilized to construct raspberry-like nanostructures. This strategy allows the simultaneous in situ reduction of metal ions and anchoring of the formed metal nanoparticles on the surface of catecholamine-coated colloidal spheres. In this study, the catecholamine-coated SiO2 nanospheres have been prepared via combined sol−gel reaction, distillation-precipitation polymerization, and reactive ester-amine reaction. The Ag satellite nanoparticles are formed by coupling of Ag+ reduction with Received: Revised: Accepted: Published: 3116

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in tetrahydrofuran (THF). The SiO2-MPS nanospheres with CC double bonds on the surface were dried under reduced pressure for 12 h. Synthesis of SiO2@poly(PFA-co-MAA-co-EGDMA) (SiO2@ PPFA) Nanospheres via Distillation-Precipitation Polymerization. SiO2-MPS nanospheres (0.2 g) were dispersed into acetonitrile (40 mL) via sonication for 30 min. PFA (0.51 mL), MAA (0.10 mL, a dispersing agent), EGDMA (0.28 mL, a cross-linking agent), and AIBN (8.0 mg) were added. The mixture was heated to reflux, and the copolymerization reaction was carried out for 2 h. The resulting SiO2@PPFA nanospheres were purified by three centrifugation/redispersion cycles, twice in ethanol and once in THF. The SiO2@PPFA nanospheres were dispersed in ethanol for subsequent use. Postfunctionalization of SiO2@PPFA Nanospheres with Dopamine via Reactive Ester-Amine Reaction. SiO2@PPFA nanospheres (0.18 g) were dispersed into ethanol (100 mL) via sonication for 30 min. Dopamine hydrochloride (100 mg) and triethylamine (74 μL) was then added into the reaction mixture. The mixture was stirred at 50 °C for 24 h. After reaction, the SiO2@poly(dopamine acrylamide-co-MAA-coEGDMA) (SiO2@PDA) nanospheres were purified by three centrifugation/redispersion cycles in ethanol. The SiO2@PDA nanospheres were redispersed in distilled water for subsequent use. Synthesis of SiO2@PDA/Ag Composite Nanospheres. In a typical reaction, SiO2@PDA nanospheres (1 mg/mL, 5 mL) and AgNO3 solution (0.1, 0.2, or 0.5 mg/mL, 5 mL) were mixed in 15 mL test tubes. The mixtures were placed on the vortex genie for reaction overnight at room temperature. After reaction, the SiO2@PDA/Ag composite nanospheres were purified by three centrifugation/redispersion cycles in doubly distilled water. The resulting SiO2@PDA/Ag composite nanospheres from AgNO3 additions of 0.1, 0.2, and 0.5 mg/ mL are referred to as SiO2@PDA/Ag1, SiO2@PDA/Ag2, and SiO2@PDA/Ag3 composite nanospheres, respectively. The amount of Ag deposited on the surface of SiO2@PDA nanospheres was determined using a Thermal Jarrell Ash Duo Iris inductively coupled plasma-mass spectrometer (ICPMS), after extraction in aqua regia. Quantification of the amount of Ag reduced was performed by direct injection of ionic silver standard and sample solutions into the ICP-MS system, integration of the corresponding peaks, and linear fitting. The fitted calibration curve is [y (ppm) = 687556.6 × x (CPS) − 9610.1]. Reduction of p-Nitrophenol Catalyzed by the SiO2@PDA/ Ag Composite Nanospheres. SiO2@PDA/Ag3 composite nanospheres in doubly distilled water (9.5 × 10−4 g/mL, 0.5 mL) was added to freshly prepared NaBH4 (0.06 M, 1.0 mL) in a quartz cell and mixed well. p-Nitrophenol (3.4 × 10−5 M, 1.5 mL) was added quickly to the quartz cell. The pale yellow coloration of the mixture faded with reaction time, indicating a gradual reduction of p-nitrophenol. The reduction process was monitored by UV−visible absorption spectroscopy at intervals of 2 min. For comparison purposes, reduction was also carried out at another concentration (1.9 × 10−4 g/mL) of SiO2@ PDA/Ag3 composite nanospheres. Degradation of R6G Catalyzed by SiO2@PDA/Ag Composite Nanospheres. SiO2@PDA/Ag3 composite nanospheres in doubly distilled water (9.5 × 10−4 g/mL, 0.5 mL) was added to R6G (2 × 10−5 M, 10 mL) in a test tube and mixed well. Freshly prepared NaBH4 (0.01 M, 1.0 mL) was added quickly to the test tube. A sample (3 mL) was drawn into a quartz cell.

catecholamine oxidation and subsequent immobilization of the metal nanoparticles on the outer surface of the catecholaminecoated SiO2 nanospheres (Scheme 1). Scheme 1. Schematic Illustration of the Preparation of Reactive SiO2@PPFA and Catecholamine-Containing SiO2@PDA Nanospheres and the in Situ Electroless Metallization of Ag+ on the Surface of SiO2@PDA Nanospheres

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EXPERIMENTAL SECTION Materials. Silver nitrate (AgNO3, 99%), tetraethyl orthosilicate (TEOS, 98%), 3-(trimethoxysilyl)propyl methacrylate (MPS, 98%), dopamine hydrochloride (99%), rhodamine 6G (R6G, 99%), ethylene glycol dimethacrylate (EGDMA, 98%), and p-nitrophenol (98%) were used as received from SigmaAldrich Chem. Co., Inc. Methacrylic acid (MAA, 99%) from Sigma-Aldrich Chem. Co. was purified by vacuum distillation. 2,2′-Azobisisobutyronitrile (AIBN), also from Sigma-Aldrich Chem. Co., was recrystallized in methanol. Sodium borohydride (NaBH4), ammonia (25 wt %), and acetonitrile (HPLC grade) were obtained from Merck Chem. Co. All other reagents and solvents were purchased either from Sigma-Aldrich or Merck Chem. Co. and were used as received. Pentafluorophenyl acrylate (PFA) was prepared according to the method reported in the literature.32 Synthesis of Silica-3-(trimethoxysilyl)propyl Methacrylate (SiO2-MPS) Nanospheres via Sol−Gel Chemistry. TEOS (9 mL) was added into a mixture of ethanol (150 mL), doubly distilled water (15 mL), and 25 wt % ammonia solution (3 mL). The reaction mixture was stirred for 12 h at room temperature. MPS (3 mL) was introduced into the SiO2 sol, and the reaction was allowed to proceed for another 24 h. The resulting SiO2-MPS nanospheres were purified by three centrifugation/redispersion cycles, twice in ethanol and once 3117

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Figure 1. FETEM images of the (a,a′) SiO2-MPS, (b,b′) SiO2@PPFA, and (c,c′) SiO2@PDA nanospheres.

merization of pentafluorophenyl acrylate (PFA), methacrylic acid (MAA), and ethylene glycol dimethacrylate (EGDMA) from SiO2-3-(trimethoxysilyl)propyl methacrylate (SiO2-MPS) nanospheres, SiO2 nanospheres with reactive PFP esters on their surface could be fabricated to allow the introduction of dopamine via reactive ester-amine reaction (Scheme 1). It is also not feasible to perform distillation-precipitation copolymerization of PFA, MAA, and EGDMA on the surface of bare SiO2 nanospheres, because of the difficulty in anchoring the monomers and newly formed oligomers.34 The SiO2-MPS nanospheres with CC double bonds on the surface were synthesized a priori via the modified Stöber process.36 The successful preparation of SiO2-MPS nanospheres was confirmed by field-emission transmission electron microscopy (FETEM), FT-IR, and thermogravimetric analysis (TGA) analyses. The FETEM image of the SiO2-MPS nanospheres is shown in Figure 1a. The SiO2-MPS nanospheres are narrowly distributed with an average diameter of 149 nm. The characteristic absorption peak at 1097 cm−1 in the FT-IR spectrum of SiO2-MPS nanospheres (Figure 2a) is assigned to the Si−O−Si stretching vibration. The absorption band at 1631 cm−1 is associated with the vinyl group of MPS on the surface of SiO2 nanospheres.37 The TGA curve of SiO2-MPS nanospheres is shown in Figure 3a. The weight loss of SiO2MPS nanospheres is about 8.5 wt %, which is attributed to the loss of the MPS layer and allows the amount of CC double bonds on the silica nanospheres to be deduced. These CC double bonds serve as the anchoring and initiation sites for the subsequent distillation-precipitation polymerization from the nanosphere surface.33 Narrowly dispersed SiO2@poly(PFA-co-MAA-co-EGDMA) (SiO2@PPFA) nanospheres were synthesized by distillationprecipitation polymerization of PFA, in the presence of EGDMA (a cross-linking agent) and MAA (a dispersing agent), from the SiO2-MPS nanospheres. The FETEM image of SiO2@PPFA nanospheres is shown in Figure 1b. The core− shell structure is discernible from the contrast between SiO2 core and the poly(PFA-co-MAA-co-EGDMA) cross-linked shell. The average thickness of the poly(PFA-co-MAA-co-EGDMA) shell in the SiO2@PPFA nanospheres is about 14 nm. The FTIR spectrum of SiO2@PPFA nanospheres (Figure 2b) reveals

The pink coloration of the mixture faded with reaction time, indicating a gradual reduction of R6G. The reduction process was monitored by UV−visible absorption spectroscopy at intervals of 1 or 2 min. For comparison purposes, the reduction was carried out at two other concentrations (1.9 × 10−4 g/mL and 1.9 × 10−5 g/mL) of SiO2@PDA/Ag3 composite nanospheres. Characterization. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos AXIS Ultra HSA spectrometer equipped with a monochromatized Al Kα X-ray source (1486.71 eV photons). The binding energy (BE) of the photoelectrons can be calculated using the following relationship: BE = 1486.71 eV − (KE + φ)

In the above equation, KE is the kinetic energy of emitted photoelectrons, and ϕ (−4.05 eV) is the work function of the spectrometer. Field emission transmission electron microscopy (FETEM) images were obtained from a JEOL JEM-2010 FETEM. The UV−visible absorption spectra were obtained from a Hitachi U2800 spectrophotometer. FT-IR spectroscopy was carried out on a Bio-Rad FTS-135 spectrophotometer. Thermogravimetric analysis (TGA) was conducted on a Shimadzu TGA-60 analyzer under a N2 atmosphere at a heating rate of 15 °C/min. The dynamic light scattering (DLS) measurements were performed on a Brookhaven 90 plus laser light scattering spectrometer at the scattering angle of 90°.

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RESULTS AND DISCUSSION Distillation-precipitation polymerization has been explored for the fabrication of core−shell colloidal spheres with controllable morphology and tailored functionality.33 However, it is difficult to prepare the catecholamine-coated colloidal nanospheres directly via distillation-precipitation polymerization of dopamine methacrylamide, because the synthesis of dopamine acrylamide requires protection and deprotection of the catechol group.34 The innovative work of Theato et al.35 has prompted us to investigate the use of reactive pentafluorophenyl (PFP) ester groups in preparing the catecholamine-coated colloidal nanospheres. By utilizing the distillation-precipitation copoly3118

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Figure 2. FT-IR spectra of the (a) SiO2-MPS, (b) SiO2@PPFA, and (c) SiO2@PDA nanospheres.

Figure 4. XPS wide-scan spectra of the (a) SiO2-MPS, (b) SiO2@ PPFA, (c) SiO2@PDA, and (d) SiO2@PDA/Ag3 composite nanospheres. Inset of (c) is the N 1s core-level spectrum of the SiO2@PDA nanospheres.

the FETEM image (Figure 1c). The FETEM image reveals that a well-defined core−shell structure of the SiO2@PDA nanospheres is retained after the ester-amine reaction. The average thickness of poly(DA-co-MAA-co-EGDMA) shell in the SiO2@ PDA nanospheres is about 18 nm, which is slightly larger than that (∼14 nm) of the poly(PFA-co-MAA-co-EGDMA) shell in the SiO2@PPFA nanospheres. The SiO2@PDA nanospheres were subsequently characterized by FT-IR spectroscopy (Figure 2c). The presence of amide bands at 1652 cm−1 and the barely discernible reactive PFP ester stretching vibration at 1788 cm−1 confirm the successful conversion of reactive PFP esters into dopamine groups. The TGA curve of SiO2@PDA nanospheres (Figure 3c) shows a decrease in weight loss in comparison to that of the SiO2@PPFA nanospheres. The reduced weight loss is probably due to the decrease in molecular weight of dopamine. XPS analysis was carried out to confirm the successful postfunctionalization of dopamine on the SiO2 surface. The appearance of N 1s signal at the BE of about 400 eV and the decrease in F 1s signal intensity at the BE of about 690 eV in the XPS wide-scan spectrum of SiO2@PDA nanospheres (Figure 4c) are consistent with the successful postfunctionalization by dopamine. The catecholamine-containing SiO2@PDA nanospheres can be utilized to reduce metal ions and coordination binding of the reduced metal nanoparticles.30,31 Ag+ ion was chosen as the metal ion model for the in situ preparation of raspberry-like SiO2@PDA/Ag composite nanospheres. Addition of AgNO3 into a Tris-HNO3 buffer solution (pH = 8.5) of SiO2@PDA nanospheres induces a color change over the span of a minute. The color changes from gray to silver-gray to brown for the addition of 5 mL of the respective 0.1, 0.2, and 0.5 mg/mL AgNO3 aqueous solutions into 5 mL of the SiO2@PDA nanospheres (1 mg/mL) Tris-HNO3 buffer solution. The SiO2@PDA/Ag composite nanospheres resulting from the addition of 0.1, 0.2, and 0.5 mg/mL of the AgNO3 solutions are referred to as SiO2@PDA/Ag1, SiO2@PDA/Ag2, and SiO2@ PDA/Ag3 composite nanospheres, respectively. To characterize this color transformation, UV−visible spectroscopy was

Figure 3. TGA curves of the (a) SiO2-MPS, (b) SiO2@PPFA, (c) SiO2@PDA, and (d) SiO2@PDA/Ag3 composite nanospheres.

an absorption band at around 1788 cm−1, which can be attributed to the ester stretching vibration typically found in PFP-based esters.35 As shown in Figure 3b, the weight loss of SiO2@PPFA nanospheres is higher than that of the SiO2-MPS nanospheres in the temperature range of 230 to 700 °C. This provides further evidence to the grafting of a poly(PFA-coMAA-co-EGDMA) shell onto the SiO2-MPS surface. Based on the final weight loss of SiO2@PPFA nanospheres (34 wt %), the amount of grafted poly(PFA-co-MAA-co-EGDMA) shell in the SiO2@PPFA nanospheres is calculated to be about 25.6 wt %. In comparison to the XPS wide-scan spectrum of SiO2-MPS nanospheres (Figure 4a), the spectrum of SiO2@PPFA nanospheres (Figure 4b) shows a unique F 1s peak at the binding energy (BE) of about 690 eV, attributable to the PFP moieties of the grafted copolymer shell. The SiO2@PPFA nanospheres with reactive PFP ester groups can undergo ester-amine reaction under a mild condition to allow the direct formation of dopamine acrylamide (DA). The morphology of the formed SiO2@poly(DA-coMAA-co-EGDMA) (SiO2@PDA) nanospheres was revealed by 3119

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performed before and after the addition of AgNO3 into the solution of SiO2@PDA nanospheres (Figure 5). Before the

Figure 6. Dh distribution (from DLS) of the (a) SiO2-MPS, (b) SiO2@PPFA, (c) SiO2@PDA nanospheres and (d) SiO2@PDA/Ag3 composite nanospheres. Figure 5. UV−visible absorption spectra of the SiO2@PDA and SiO2@PDA/Ag3 composite nanosheres in doubly distilled water.

EGDMA) shell obtained from FETEM image, probably due to swelling of the copolymer shell in dispersion medium. Upon transformation the PFA repeat units to dopamine moieties, the SiO2@PDA nanospheres in doubly distilled water have an average Dh of 187 ± 7 nm. The Dh of the SiO2@PDA/Ag3 composite nanospheres in doubly distilled water increases further to 224 ± 13 nm after deposition of the Ag nanoparticles. The effect of AgNO3 concentration on the formation of SiO2@PDA/Ag composite nanospheres was revealed by the FETEM images. Figure 7 shows the respective FETEM images of SiO2@PDA/Ag1, SiO2@PDA/Ag2, and SiO2@PDA/Ag3 composite nanospheres from the initial AgNO3 concentrations of 0.1, 0.2, and 0.5 mg/mL. When the concentration of AgNO3 added into the reaction mixture is low, only a small amount of Ag satellite nanoparticles is deposited on the surface of SiO2@ PDA nanospheres. As the concentration of AgNO3 is increased to 0.5 mg/mL, the amount of Ag satellite nanoparticles on the SiO2@PDA nanospheres is significantly increased. However, some large metal aggregates also appear after the formation of Ag satellite nanoparticles, because of the fast redox reaction between catecholamine and Ag+ ions to result in uncontrolled particle deposition.31 The amount of Ag deposited on the surface of SiO2@PDA nanospheres was determined from a Thermal Jarrell Ash Duo Iris inductively coupled plasma-mass spectrometer (ICP-MS) to be about 1.3, 3.7, and 5.6 wt %, respectively, for the SiO2@PDA/Ag1, SiO2@PDA/Ag2, and SiO2@PDA/Ag3 composite nanospheres. Ag nanoparticles have been extensively investigated for their catalytic reduction of nitrophenols and nitroanilines and degradation of organic dyes.9,43−46 The introduction of Ag nanoparticles onto the surface of SiO2@PDA nanospheres will endow good catalytic reactivity to the composites. The reduction of p-nitrophenol by NaBH4 has been widely employed to investigate the catalytic performance of metal nanocomposites.43,47,48 The original p-nitrophenol aqueous solution is slightly yellowish in color and shows typical absorption at 317 nm. Upon the addition of NaBH4, the absorption maximum shifts to 400 nm due to the formation of 4-nitrophenolate.49 There was no significant change in the absorption intensity at 400 nm after leaving the reaction mixture alone or even in the presence of SiO2@PDA nanospheres at room temperature for 24 h, indicating that it

AgNO3 addition, no obvious absorption is detected in the 250− 600 nm region of the UV−visible absorption spectrum of SiO2@PDA nanospheres. After addition of AgNO3 and overnight shaking at room temperature, the UV−visible absorption spectrum of SiO2@PDA/Ag3 composite nanospheres displays a significant surface plasmon band at about 413 nm, which is characteristic of metallic silver colloids.38,39 The chemical state of Ag on the surface of SiO2@PDA nanospheres was investigated by XPS analysis. The XPS widescan spectra of the SiO 2 @PDA and SiO 2 @PDA/Ag3 composite nanospheres are shown in Figures 4c and 4d, respectively. The appearance of a strong Ag 3d signal at the BE of about 370 eV (Ag 3d5/2 at 368.6 eV and Ag 3d3/2 at 374.6 eV, Figure S1, Supporting Information)40 for the SiO2@PDA/ Ag3 composite nanospheres indicates that a layer of Ag species has been deposited on the surface of SiO2@PDA nanospheres. The Ag 3d BE’s for metallic and oxidized Ag are quite similar, making identification of the Ag species difficult.41 To distinguish the oxidation state of Ag, the Auger parameter (AP) of the SiO2@PDA/Ag3 composite nanospheres was determined by measuring the kinetic energy (KE) of the Ag M5N45N45 and M4N45N45 Auger photoelectrons (Figure S1, Supporting Information). AP’s for Ag 3d5/2−M5N45N45 and 3d5/2−M4N45N45 of the SiO2@PDA/Ag3 composite nanospheres are determined to be 720.0 and 725.6 eV, respectively. These AP’s are close to that of Ag in its metallic form (720.5 and 726.5 eV for Ag0) and show a clear difference from Ag in other oxidation states: 717.8 and 723.9 eV for Ag2O; 718.3 and 723.8 eV for AgNO3; 718.1 and 724.2 eV for AgO.41,42 Thus, the Ag species on the surface of SiO2@PDA nanospheres should be in its metallic form. The surface-functionalized SiO2 nanospheres were further characterized by dynamic light scattering (DLS) measurements. Figure 6 shows the hydrodynamic diameter (Dh) of the assynthesized SiO2-MPS, SiO2@PPFA, SiO2@PDA, and SiO2@ PDA/Ag3 composite nanospheres. The SiO2-MPS nanospheres have an average Dh of 155 ± 4 nm in ethanol. After distillationprecipitation copolymerization of PFA, MAA, and EGDMA, the Dh of the as-synthesized SiO2@PPFA nanospheres in ethanol increases to 200 ± 5 nm. The increase in Dh is slightly larger than the thickness (∼14 nm) of poly(PFA-co-MAA-co3120

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Figure 7. FETEM images of the (a,a′) SiO2@PDA/Ag1, (b,b′) SiO2@PDA/Ag2, and (c,c′) SiO2@PDA/Ag3 composite nanospheres.

Figure 8. (a) Catalytic reduction of p-nitrophenol (3.4 × 10−5 M, 1.5 mL) by NaBH4 (0.06 M, 1.0 mL) in the presence of SiO2@PDA/Ag3 composite nanospheres (1.9 × 10−4 g/mL, 0.5 mL) as monitored by time-dependent UV−visible absorption spectra. (b) Changes in p-nitrophenol concentration with time for the reactions catalyzed by different amounts of the SiO2@PDA/Ag3 composite nanospheres.

is difficult for the reduction to proceed without a catalyst and the SiO2@PDA nanospheres have negligible catalytic effect on the reduction of p-nitrophenol. Figure 8a shows the changes in UV−visible absorption spectrum of the p-nitrophenol (3.4 × 10−5 M, 1.5 mL) and NaBH4 (0.06 M, 1.0 mL) reaction mixtures in the presence of SiO2@PDA/Ag3 composite nanospheres (1.9 × 10−4 g/mL, 0.5 mL). As the reduction reaction proceeds, p-nitrophenol is converted to p-aminophenol, resulting in a decrease in the absorption intensity at 400 nm.37 Figure 8b shows the changes in p-aminophenol concentration (Ct/C0, C0 is the initial concentration, Ct is the concentrations after reaction time t, and Ct/C0 is obtained from the ratio of time-dependent and initial UV−visible absorption maximum) with time for the reaction catalyzed by different amounts of the SiO2@PDA/Ag3 composite nanospheres. With the increase in concentration of the SiO 2@PDA/Ag3 composite nanospheres to 9.5 × 10−4 g/mL, the reduction rate of p-nitrophenol is increased. However, the SiO2@PDA/ Ag3 composite nanospheres have an absorption maximum at about 413 nm (Figure 5), which interferes with the determination of reactant concentration. As shown in Figure 8b, it is not feasible to determine the final reactant

concentration directly by UV−visible absorption spectroscopy, even after the successful reduction. To solve this problem, the reduced solution was centrifuged at 8000 rpm for 20 min, and the supernatant was passed through a 0.22 μm filter membrane to completely remove the SiO2@PDA/Ag3 composite nanospheres. The filtered solution was subjected to UV−visible absorption measurement. The absorption intensity at 400 nm was negligible. The filtered solution was also measured by high performance liquid chromatography (HPLC). No p-nitrophenol was detected by either the UV or mass spectrometry detectors. Figure S2 (Supporting Information) also shows the changes in p-nitrophenol concentration with time for the reaction catalyzed by the same amount of SiO2@PDA/Ag1, SiO2@ PDA/Ag2 and SiO2@PDA/Ag3 composite nanospheres. The highest reduction rate of p-nitrophenol is achieved by SiO2@ PDA/Ag3 composite nanospheres, followed by SiO2@PDA/ Ag2 and SiO2@PDA/Ag1 composite nanospheres. This phenomenon is probably due to the highest coverage of Ag nanoparticles on the surface of SiO2@PDA/Ag3 composite nanospheres, which provide sufficient reactive sites for the catalytic reduction of p-nitrophenol.50 Since NaBH4 can 3121

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Figure 9. (a) Catalytic degradation of R6G (2 × 10−5 M, 10 mL) by NaBH4 (0.01 M, 1.0 mL) in the presence of SiO2@PDA/Ag3 composite nanospheres (9.5 × 10−4 g/mL, 0.5 mL) as monitored by time-dependent UV−visible absorption spectra. (b) Changes in R6G concentration with time for the reactions catalyzed by different amounts of the SiO2@PDA/Ag3 composite nanospheres.

as a catalyst for the reduction of p-nitrophenol and degradation of R6G.

undergo hydrolysis in water to produce hydrogen gas, it is desirable to utilize a catalyst with higher reduction rates. Thus, the SiO2@PDA/Ag3 composite nanospheres were selected to study the catalytic performance. The rhodamine 6G (R6G) dye with the absorption maximum located at about 525 nm was also selected as a model for degradation reaction.9,46,51 After a small amount of SiO2@PDA/Ag3 composite nanospheres is added, the pink R6G and NaBH4 reaction mixture faded with the reaction time. Figure 9a displays the time-dependent UV−visible absorption spectra of the R6G (2 × 10−5 M, 10 mL) and NaBH4 (0.01 M, 1.0 mL) aqueous solution upon addition of the SiO2@PDA/ Ag3 composite nanospheres (9.5 × 10−4 g/mL, 0.5 mL). The UV−visible absorption spectrum of the reaction mixture undergoes a gradual decrease in peak intensity at 525 nm with negligible changes in peak shape and position, indicating degradation of the R6G dyes. Figure 9b shows the changes in R6G concentration (Ct/C0) with time for reactions catalyzed by different amounts of SiO2@PDA/Ag3 composite nanospheres. As anticipated, the degradation rate increases with the increase in concentration of the SiO2@PDA/Ag3 composite nanospheres. The Ct/C0 values are close to zero after a time period, indicating the complete degradation of R6G. Thus, the assynthesized SiO2@PDA/Ag3 composite nanospheres exhibit good catalytic performance. The SiO2@PDA/Ag3 composite nanospheres can be easily recovered by centrifugation from the reaction mixture. Figure S3 (Supporting Information) shows the FETEM images of SiO2@PDA/Ag3 composite nanospheres recovered from the reaction mixtures after the reduction of p-nitrophenol and degradation of R6G. The core−shell and raspberry-like structures are retained after the catalytic reactions, and the surface coverage of Ag nanoparticles does not change appreciably, suggesting that the composite structure has good stability. The stability and reusability of the SiO2@PDA/Ag3 composite nanospheres was investigated in repeated reduction and degradation cycles. Figure S4 (Supporting Information) shows the change in efficiency of reduction of p-nitrophenol and degradation of R6G catalyzed by the SiO2@PDA/Ag3 composite nanospheres over four successive cycles. The reduction of p-nitrophenol and degradation efficiency of R6G within 30 min remain at about 89% and 91%, respectively, after the fourth cycle. These results indicate that the as-synthesized SiO2@PDA/Ag3 composite nanospheres have good reusability

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CONCLUSION The synthesis of raspberry-like SiO2@PDA/Ag composite nanospheres with a SiO2 nanocore, PDA shell, and Ag satellite nanoparticles was described. The SiO2@PPFA nanospheres with reactive PFP esters were first prepared via distillationprecipitation polymerization from SiO2-MPS nanospheres. Postfunctionalization of reactive PFP ester groups with dopamine produced the catecholamine-containing SiO2@PDA nanospheres. Dopamine-promoted reduction of Ag+ ions resulted in concurrent electroless metallization and coordination binding of the metal nanoparticles on the surface of SiO2@ PDA nanospheres. Surface coverage of the Ag satellite nanoparticles on the SiO2@PDA nanospheres can be easily tuned by varying the AgNO3 salt concentration for reduction. Moreover, the SiO2@PDA/Ag composite nanospheres exhibit good catalytic performance and good reusability in the reduction of p-nitrophenol and degradation of R6G dye. Future work will utilize the SiO2@PDA nanospheres for in situ reduction of other heavy and precious metal ions (e.g., Cu2+ and Au3+) for coordination binding.

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ASSOCIATED CONTENT

S Supporting Information *

XPS Ag 3d and Ag MNN core-level spectra of the SiO2@PDA/ Ag3 composite nanosheres (Figure S1). Changes in pnitrophenol concentration with time for the reactions catalyzed by SiO2@PDA/Ag1, SiO2@PDA/Ag2, and SiO2@PDA/Ag3 composite nanospheres (Figure S2). FETEM images of the SiO2@PDA/Ag3 composite nanospheres recovered by centrifugation from the reaction mixtures of the reduction of pnitrophenol and degradation of R6G (Figure S3). Catalytic activities of SiO2@PDA/Ag3 composite nanospheres for the reduction of p-nitrophenol and degradation of R6G over four successive reuse cycles (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.T.K.). *E-mail: [email protected] (G.D.F.). 3122

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Notes

(20) Sagert, J.; Sun, C.; waite, J. H. Chemical Subtleties of Mussel and Polychaete Holdfasts; Springer-Verlag Berlin Heidelberg: Berlin, 2006. (21) Tahir, M. N.; Zink, N.; Eberhardt, M.; Therese, H. A.; Kolb, U.; Theato, P.; Tremel, W. Overcoming the insolubility of molybdenum disulfide nanoparticles through a high degree of sidewall functionalization using polymeric chelating ligands. Angew. Chem., Int. Ed. 2006, 45, 4809. (22) Saxer, S.; Portmann, C.; Tosatti, S.; Gademann, K.; Zurcher, S.; Textor, M. Surface assembly of catechol-functionalized poly(L-lysine)graf t-poly(ethylene glycol) copolymer on titanium exploiting combined electrostatically driven self-organization and blomimetic strong adhesion. Macromolecules 2010, 43, 1050. (23) Ryu, S.; Lee, Y.; Hwang, J. W.; Hong, S.; Kim, C.; Park, T. G.; Lee, H.; Hong, S. H. High-strength carbon nanotube fibers fabricated by infiltration and curing of mussel-inspired catecholamine polymer. Adv. Mater. 2011, 23, 1971. (24) Malisova, B.; Tosatti, S.; Textor, M.; Gademann, K.; Zurcher, S. Poly(ethylene glycol) adlayers immobilized to metal oxide substrates through catechol derivatives: Influence of assembly conditions on formation and stability. Langmuir 2010, 26, 4018. (25) Ye, Q.; Zhou, F.; Liu, W. M. Bioinspired catecholic chemistry for surface modification. Chem. Soc. Rev. 2011, 40, 4244. (26) Fullenkamp, D. E.; Rivera, J. G.; Gong, Y. K.; Lau, K. H. A.; He, L.; Varshney, R.; Messersmith, P. B. Mussel-inspired silver-releasing antibacterial hydrogels. Biomaterials 2012, 33, 3783. (27) Zhang, L.; Wu, J. J.; Wang, Y. X.; Long, Y. H.; Zhao, N.; Xu, J. Combination of bioinspiration: A general route to superhydrophobic particles. J. Am. Chem. Soc. 2012, 134, 9879. (28) Chien, H. W.; Kuo, W. H.; Wang, M. J.; Tsai, S. W.; Tsai, W. B. Tunable micropatterned substrates based on poly(dopamine) deposition via microcontact printing. Langmuir 2012, 28, 5775. (29) Alvarez-Paino, M.; Marcelo, G.; Munoz-Bonilla, A.; FernandezGarcia, M. Catecholic chemistry to obtain recyclable and reusable hybrid polymeric particles as catalytic systems. Macromolecules 2013, 46, 2951. (30) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426. (31) Black, K. C. L.; Liu, Z. Q.; Messersmith, P. B. Catechol redox induced formation of metal core-polymer shell nanoparticles. Chem. Mater. 2011, 23, 1130. (32) Eberhardt, M.; Mruk, R.; Zentel, R.; Théato, P. Synthesis of pentafluorophenyl (meth)acrylate polymers: New precursor polymers for the synthesis of multifunctional materials. Eur. Polym. J. 2005, 41, 1569. (33) Li, G. L.; Xu, L. Q.; Tang, X.; Neoh, K. G.; Kang, E. T. Hairy hollow microspheres of fluorescent shell and temperature-responsive brushes via combined distillation-precipitation polymerization and thiol−ene click chemistry. Macromolecules 2010, 43, 5797. (34) Ji, H. F.; Wang, X. X.; Zhang, X.; Yang, X. L. Preparation of silica/poly(methacrylic acid)/poly(divinylbenzene-co-methacrylic acid) tri-layer microspheres and the corresponding hollow polymer microspheres with movable silica core. Chinese J. Polym. Sci. 2010, 28, 807. (35) Roth, P. J.; Jochum, F. D.; Forst, F. R.; Zentel, R.; Theato, P. Influence of end groups on the stimulus-responsive behavior of poly[oligo(ethylene glycol) methacrylate] in water. Macromolecules 2010, 43, 4638. (36) Stö ber, W.; Fink, A.; Bohn, E. Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 1968, 26, 62. (37) Pradhan, N.; Pal, A.; Pal, T. Catalytic reduction of aromatic nitro compounds by coinage metal nanoparticles. Langmuir 2001, 17, 1800. (38) Behrens, S.; Wu, J.; Habicht, W.; Unger, E. Silver nanoparticle and nanowire formation by microtubule templates. Chem. Mater. 2004, 16, 3085.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support for this study from National Environmental Agency of Singapore under ETRP Grant no. 0910 149 (NUS WBS no. R279-000-314-490).

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REFERENCES

(1) Caruso, F. Nanoengineering of particle surfaces. Adv. Mater. 2001, 13, 11. (2) Bourgeat-Lami, E.; Lansalot, M. Organic/Inorganic Composite Latexes: The Marriage of Emulsion Polymerization and Inorganic Chemistry; Springer-Verlag Berlin Heidelberg: Berlin, 2010. (3) Caruso, F.; Caruso, R. A.; Möhwald, H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998, 282, 1111. (4) Agrawal, M.; Gupta, S.; Stamm, M. Recent developments in fabrication and applications of colloid based composite particles. J. Mater. Chem. 2011, 21, 615. (5) Sun, Y.; Yin, Y.; Chen, M.; Zhou, S.; Wu, L. One-step facile synthesis of monodisperse raspberry-like P(S−MPS−AA) colloidal particles. Polym. Chem. 2013, 4, 3020. (6) Shin, J. Y.; Lee, B. S.; Jung, Y.; Kim, S. J.; Lee, S. G. Palladium nanoparticles captured onto spherical silica particles using a urea crosslinked imidazolium molecular band. Chem. Commun. 2007, 5238. (7) Li, Y.; Pan, Y.; Zhu, L.; Wang, Z.; Su, D.; Xue, G. Facile and controlled fabrication of functional gold nanoparticle-coated polystyrene composite particle. Macromol. Rapid Commun. 2011, 32, 1741. (8) Hu, J.; Zhou, S. X.; Sun, Y. Y.; Fang, X. S.; Wu, L. M. Fabrication, properties and applications of Janus particles. Chem. Soc. Rev. 2012, 41, 4356. (9) Deng, Z. W.; Zhu, H. B.; Peng, B.; Chen, H.; Sun, Y. F.; Gang, X. D.; Jin, P. J.; Wang, J. L. Synthesis of PS/Ag nanocomposite spheres with catalytic and antibacterial activities. ACS Appl. Mater. Interfaces 2012, 4, 5625. (10) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Superhydrophobic films from raspberry-like particles. Nano Lett. 2005, 5, 2298. (11) Gandra, N.; Abbas, A.; Tian, L.; Singamaneni, S. Plasmonic planet−satellite analogues: Hierarchical self-assembly of gold nanostructures. Nano Lett. 2012, 12, 2645. (12) Xu, S.; Song, X.; Guo, J.; Wang, C. Composite microspheres for separation of plasmid DNA decorated with MNPs through in situ growth or interfacial immobilization followed by silica coating. ACS Appl. Mater. Interfaces 2012, 4, 4764. (13) Kim, J.; Lee, J. E.; Lee, J.; Jang, Y.; Kim, S. W.; An, K.; Yu, J. H.; Hyeon, T. Generalized fabrication of multifunctional nanoparticle assemblies on silica spheres. Angew. Chem., Int. Ed. 2006, 45, 4789. (14) Li, X.; He, J. In situ assembly of raspberry- and mulberry-like silica nanospheres toward antireflective and antifogging coatings. ACS Appl. Mater. Interfaces 2012, 4, 2204. (15) Liu, Y.; Li, M.; Chen, G. A new type of raspberry-like polymer composite sub-microspheres with tunable gold nanoparticles coverage and their enhanced catalytic properties. J. Mater. Chem. A 2013, 1, 930. (16) Deng, Z.; Zhu, H.; Peng, B.; Chen, H.; Sun, Y.; Gang, X.; Jin, P.; Wang, J. Synthesis of PS/Ag nanocomposite spheres with catalytic and antibacterial activities. ACS Appl. Mater. Interfaces 2012, 4, 5625. (17) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Formanek, P.; Jehnichen, D.; Stamm, M. Tailored growth of In(OH)3 shell on functionalized polystyrene beads. Langmuir 2009, 26, 526. (18) Sparks, B. J.; Hoff, E. F. T.; Hayes, L. P.; Patton, D. L. Musselinspired thiol−ene polymer networks: Influencing network properties and adhesion with catechol functionality. Chem. Mater. 2012, 24, 3633. (19) Matos-Pérez, C. R.; White, J. D.; Wilker, J. J. Polymer composition and substrate influences on the adhesive bonding of a biomimetic, cross-linking polymer. J. Am. Chem. Soc. 2012, 134, 9498. 3123

dx.doi.org/10.1021/ie403840p | Ind. Eng. Chem. Res. 2014, 53, 3116−3124

Industrial & Engineering Chemistry Research

Article

(39) Radziuk, D.; Shchukin, D. G.; Skirtach, A.; Möhwald, H.; Sukhorukov, G. Synthesis of silver nanoparticles for remote opening of polyelectrolyte microcapsules. Langmuir 2007, 23, 4612. (40) Briggs, D.; Seah, P. Practical Surface Analysis: Auger and X-ray photoelectron spectroscopy; Wiley: New York, 1990. (41) Ferraria, A. M.; Carapeto, A. P.; Botelho do Rego, A. M. X-ray photoelectron spectroscopy: Silver salts revisited. Vacuum 2012, 86, 1988. (42) Wagner, C.; Naumkin, A.; Kraut-Vass, A.; Allison, J.; Powell, C.; Rumble, J., Jr. NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.4 (Web Version); U.S. Department of Commerce: 2003. (43) Xiong, R.; Lu, C.; Wang, Y.; Zhou, Z.; Zhang, X. Nanofibrillated cellulose as the support and reductant for the facile synthesis of Fe3O4/Ag nanocomposites with catalytic and antibacterial activity. J. Mater. Chem. A 2013, 1, 14910. (44) Solanki, J. N.; Murthy, Z. V. P. Reduction of nitro aromatic compounds over Ag/Al2O3 nanocatalyst prepared in water-in-oil microemulsion: Effects of water-to-surfactant mole ratio and type of reducing agent. Ind. Eng. Chem. Res. 2011, 50, 7338. (45) Kumar, P.; Govindaraju, M.; Senthamilselvi, S.; Premkumar, K. Photocatalytic degradation of methyl orange dye using silver (Ag) nanoparticles synthesized from Ulva lactuca. Colloids Surf., B 2013, 103, 658. (46) Ko, W. B.; Oh, Y. J.; Cho, B. H. Preparation of Ag-SiO2 nanocomposites and photocatalytic degradation of organic dyes. Asian J. Chem. 2013, 25, 4657. (47) Li, Y.; Wu, Y.; Gao, Y.; Sha, S.; Hao, J.; Cao, G.; Yang, C. A facile method to fabricate polystyrene/silver composite particles and their catalytic properties. RSC Adv. 2013, 3, 26361. (48) Liang, M.; Wang, L.; Su, R.; Qi, W.; Wang, M.; Yu, Y.; He, Z. Synthesis of silver nanoparticles within cross-linked lysozyme crystals as recyclable catalysts for 4-nitrophenol reduction. Catal. Sci. Technol. 2013, 3, 1910. (49) An, Q.; Yu, M.; Zhang, Y.; Ma, W.; Guo, J.; Wang, C. Fe3O4@ carbon microsphere supported Ag−Au bimetallic nanocrystals with the enhanced catalytic activity and selectivity for the reduction of nitroaromatic compounds. J. Phys. Chem. C 2012, 116, 22432. (50) Li, M. L.; Chen, G. F. Revisiting catalytic model reaction pnitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres. Nanoscale 2013, 5, 11919. (51) Zhang, T.; Li, X.; Kang, S.-Z.; Qin, L.; Li, G.; Mu, J. Facile assembly of silica gel/reduced graphene oxide/Ag nanoparticle composite with a core-shell structure and its excellent catalytic properties. J. Mater. Chem. A 2014, DOI: 10.1039/C3TA14307D.

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dx.doi.org/10.1021/ie403840p | Ind. Eng. Chem. Res. 2014, 53, 3116−3124