Mussel-Inspired Polydopamine Functionalized Plasmonic

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Mussel-Inspired Polydopamine Functionalized Plasmonic Nanocomposites for Single-Particle Catalysis Jun-Gang Wang, Xin Hua, Meng Li, and Yi-Tao Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14689 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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ACS Applied Materials & Interfaces

Mussel-Inspired Polydopamine Functionalized Plasmonic Nanocomposites for Single-Particle Catalysis Jun-Gang Wang†, Xin Hua†, Meng Li†‡* and Yi-Tao Long†* †Key Laboratory for Advanced Materials, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China ‡State Environmental Protection Key Laboratory of Risk Assessment and Control on Chemical Processes, East China University of Science and Technology, Shanghai 200237, P. R. China. E-mail: [email protected]; [email protected]

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ABSTRACT Polydopamine functionalized plasmonic nanocomposites with well distributed catalytically active small gold nanoislands around large gold core were fabricated without using any chemical reductant or surfactant. The optical properties, surface molecular structures and ensemble catalytic activity of the gold nanocomposites were investigated by time-of-flight secondary ion mass spectrometry and UV-Vis spectroscopy, respectively. Moreover, the considerable catalytic activity of the nanocomposites toward 4-nitrophenol reduction was real time monitored by dark-field spectroscopy techniques at single-nanoparticle level avoiding averaging effects in bulk systems. According to the obtained plasmonic signals from individual nanocomposites, the electron charging and discharging rate for these nanocomposites during the catalytic process were calculated, respectively. Our results offer new insights into the design and synthesis of plasmonic nanocomposites for future catalytic applications, as well as a further mechanistic understanding of the electron transfer during the catalytic process at single-nanoparticle level. KEYWORDS: single-particle catalysis, polydopamine, plasmonic nanocomposites, dark-field microscopy, ToF-SIMS

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INTRODUCTION Motivated by desired structure-defined electronic, optical and catalytic properties, noble metal nanoparticles, especially gold nanoparticles, have been extensively applied in hybrid nanomaterial fabrication, heterocatalysis and biological sensing.1-3 In the field of catalysis, the fabrication of large surface-to-volume ratio and interface-dominated properties of gold nanoparticles are considered as the driving force to enhance the catalytic activity.4 However, due to high surface energy, bare gold nanoparticles have a trend to self-aggregate, leading to the decrease in their catalytic activity. One strategy to improve its stability is to modify functionalized ligands onto the surface of a gold nanoparticle, but it would also deactivate and poison the catalysts.5 In order to address these challenges, building nanocomposites structure both with catalytic efficiency and eliminating deactivation could be an effective way. Owing to the variety of morphology, size and composition of the nanoparticles, it is possible to bring about heterogeneity in catalytic activities within nanoparticle populations.6 Thus, it is significant to monitor heterogeneous catalytic processes and investigate their mechanisms on the single nanoparticle surface to avoid ensemble averaging effects in bulk systems. Peng’s group utilized the single-molecule fluorescence microscopy to study single nanoparticle catalysis to reveal and quantify heterogeneous reaction on the nanoparticles.7,8 Based on their strategy, stochastic bursts of fluorescence signals coming from highly fluorescent reaction products around single nanocatalyst were recorded and analyzed. Recently, dark-field microscopy (DFM), which enables the observation of scattering spectroscopy of single plasmonic nanoparticles, has been widely used in studying of single-particle catalysis without interference from such ensemble averaging effect.9,10 Based on this strategy, the charge transfer and storage during the catalytic reactions could be monitored directly on the single nanoparticle surface. Herein, we reported an efficient approach to fabricate polydopamine (PDA) functionalized stable gold nanocomposites without introducing any surfactant or reductant. Direct 4-nitrophenol reduction was real-time monitored on single nanoparticles by utilizing localized surface plasmon resonance (LSPR) scattering

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spectroscopy (Figure

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the

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modified

polydimethylsiloxane (PDMS) chip was fabricated and served as the dip-catalyst with excellent performance in the catalytic reduction of 4-nitrophenol, indicating the stable and efficiency of the prepared nanocomposite catalyst.

Figure 1. Schematic representation of predicted structures of polydopamine (PDA) and the synthetic process of Au@PDA@Au nanoparticles for catalytic application.

EXPERIMENTAL SECTION Materials Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, >99.0%), absolute ethanol, acetone and sodium citrate were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai China). Dopamine hydrochloride and Tris were purchased from SigmaAldrich. The 4-nitrophenol (4-NP) was purchased from J&K Scientific Ltd. (China). All reagents were used as received without any further purification. Milli-Q water with a resistivity of 18.2 MΩ cm was used in preparation of all the solutions. ITO-coated glass (sheet resistance 20-30 Ω sp-1; 1 mm thickness) was purchased from Shenzhen Laibao Technology Co. Ltd (Shenzhen China). Polydimethylsiloxane (PDMS) was fabricated from silicone eleastomer purchase from Dow Corning (Germany). Preparation of the Au NPs All glassware was immersed in an aqua regia solution (3:1 HCl/HNO3) for 12 h (Caution, aqua regia is a strong acid and is highly corrosive; it should be handled with care) and then rinsed with amount of ultrapure water before use. Au seeds with diameters of 20 nm were prepared according to a procedure that has been described 4


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previously.11,12 The TEM image showed spherical seed gold nanoparticles and the UVVis absorption peak was located at 523 nm (Figure S1, Supporting Information). In brief, 50 mL of 0.01 wt % HAuCl4 was added to a 100 mL round-bottom flask that was equipped with a condenser. The solution was brought to a rolling boil under vigorous stirring, and 5 mL of 38.8 mM sodium citrate was rapidly added to the vortex of the solution; the addition of sodium citrate caused the color to change from pale yellow to red.13 The solution was heated for 30 min and then stirred for an additional 30 min after the heating mantle had been removed. The chemical seed-mediated protocol was used to prepare larger gold nanoparticles.12 In brief, 50 mL of water, 4 mL of the solution of seed particles, and 400 μL of 0.2 M NH2OH·HCl were combined in a 100 mL beaker, and 3.0 mL of 0.1 wt % HAuCl4 was added dropwise controlled by a stepper motor under vigorous stirring. As the HAuCl4 solution was added, the color of the mixture gradually changed to red. The addition of the HAuCl4 was completed within 6 h. Then, the gold nanoparticle solutions were stored in dark bottles at 4 °C when it was not being immediately used. Preparation of the Au@PDA (AP) and Au@PDA@Au (APA) The AP nanopaticles were synthesized by self-polymerizing dopamine in basic solutions.14 In the presence of polymerization initiators (Tris) under alkaline, dopamine molecules transformed to 5, 6-dihydroxyindole, dione derivatives and other intermediates and they were held together due to π-π stacking, hydrogen bonding and charge transfer effect to build a PDA shell on gold nanoparticles.15 In brief, 1 mL of prepared Au NPs was added into 1 mL Tris buffer (10 mM) containing 0.05 mg/mL dopamine. The reaction mixture was gently stirred for 6 h. Then, the AP nanoparticles was separated by centrifugation and re-dissolved in 1 mL ultrapure water. To synthesize APA nanocomposites, 1 mL of AP nanoparticles and 40 µL of HAuCl4 (2.4 mM) were added consecutively and the obtained mixture was ultrasonificated for 10 min, followed by centrifugation and washing with ultrapure water several times. The fabricated APA nanocomposites were dispersed in 1 mL deionized water stored at 4 °C for further use. Reduction of 4-NP 5



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First, 0.2 mL of 4-NP solution (2.5 mM) was mixed with 1 mL of fresh prepared NaBH4 solution (0.1 M) in quartz cuvette. Then, 20 μL of the prepared catalysts (gold nanoparticle, AP and APA) was added into the quartz cuvette, respectively. The reduction of 4-NP was real-time monitored by UV-Vis spectrophotometer (USB2000+) at room temperature. Characterization The obtained nanostructures were characterized by a JEOL JEM-2100 transmission electron microscope operated at an acceleration voltage of 200 kV. Field emission scanning electron images was performed on a Hitachi S-4800 Field emission scanning electron microscopy (FE-SEM, Hitachi, Ltd., Japan) at accelerating voltage 15.00KV and using an in-lens ion annular electron detector. Time-of-flight secondary ion mass spectra (ToF-SIMS) of ions were obtained on secondary ion mass spectrometer ToFSIMS V (ION-TOF GmbH, Münster, Germany) equipped with a high mass resolution time-of-flight analyser of a reflectron type. Bi3+ primary ion gun was used during the mass measurement. Dark-Field Microscopy The foundation of optical dark-field spectrum measurements was a Nikon eclipse Ti-U inverted microscope that equipped with a dark-field condenser (0.8 < NA < 0.95) and a 40 × objective lens (NA = 0.6) shown in schematic S1. Illumination was provided by a 100 W halogen lamp which was used to excite the gold nanoparticles to generate the local plasmon resonance scattering light. The scattering light was focused onto the entrance port of a monochromator (Acton SP2300i, Princeton Instruments, USA) that was equipped with a grating (grating density: 300 lines/ mm; blazed wavelength: 500 nm) to disperse the scattering light. Then, the scattering light was recorded by a 400 × 1340 pixel cooled spectrograph CCD camera (Pixis 400, Princeton Instruments, USA).

RESULTS AND DISCUSSION Characterization of Mussel-Inspired Nanocomposites PDA can be facilely coated on organic or inorganic substrates with controllable 6


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thickness and durable stability.16 The abundant phenolic groups and amino groups of PDA help in immobilizing biological molecules and reducing metal ions.14,17,18 Thus, the PDA-modified nanomaterials could be served as the nanocatalyst carriers for in situ generation of nanoarchitectures with desirable plasmonic properties, enhanced stability and high catalytic efficiency.19 The gold nanoparticles with an average diameter of 60 nm are chosen as the core.12 After dopamine self-polymerized and formed a selfassembled layer around the core, HAuCl4 solution was added and gold nanoislands appeared on the surface of PDA layer 10 min later, without introducing any other reducing reagent. The transmission electron micrograph (TEM) images of Au NPs, Au@PDA (AP) and Au@PDA@Au (APA) nanoparticles indicated that Au NPs with diameters of 60 nm (Figure 2A, a and d) were coated with a 5 nm thick PDA layer (Figure 2A, b and e), and the diameter of formed gold nanoislands was around 10 nm (Figure 2A, c and f). Due to the weak reduction potential of catechols (E° = − 0.699 V vs NHE) compared to AuIII/Au0 (E° = 0.994 V vs NHE), the catechol group contained in PDA layer is an ideal reductant for the formation of gold nanoislands around gold core avoiding secondary nucleation.20,21 HR-TEM images of Figure 2A, f exhibited the inter-planar distances of Au (111) lattice planes, confirming the nanoislands are composed of Au. Moreover, the Au (111) lattice planes in nanoislands structure were parallel to the Au NPs surface, indicating that the formation of branched Au nanostructures might occur through the rapid deposition of Au atoms on Au (111) planes.22 The surface chemical composition of Au, AP and APA nanoparticles was characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The negative spectra showed the presence of m/z 197 (Au-), 393 (Au2-), 410 (Au2O-) and 591 (Au3-) in both Au and APA nanoparticles (Figure S2A and Figure 2B), indicating that the formed gold nanoislands were not encapsulated within the PDA layer.23 There were no Au related mass peaks appeared when analyzing AP particles (Figure S2C). The reason might be the case that the gold core was embedded by PDA layer and suppressed the mass signal of gold. The positive mass spectra of AP and APA nanoparticles (Figure S2D and Figure 2C) 7



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presented mass peaks at m/z 30 (CH4N+), 41 (C3H5+), 55 (C4H7+), 90 (C6H10+), 115 (C8H5N+), 147 (C8H5NO2+), 360 (C21H18N3O3+) and 376 (C21H18N3O4+), respectively. The fragments at m/z 30, 41, 55, 90, 115 and 147 originated from fragmentation and electron ionization of PDA.24 The peak at m/z 147 was attributed to the liberation of two hydrogen atoms from a 5, 6-dihydroxyindole molecule. Meanwhile, the peak at 115 (C8H5N+) was due to the release of two oxygen atoms from the C8H5NO2+ fragments. In the high-mass region, the peak at m/z 376 (C21H18N3O4+) was observed which originated from the stable building block of PDA structure.25 In comparison, these characteristic peaks of PDA were not found in the positive spectrum of Au NPs (Figure S2B). These ToF-SIMS spectra confirmed that PDA was already modified on the Au NPs for AP nanoparticles. Moreover, PDA fragments could still be detected in the APA nanocomposites, suggesting that gold nanoislands structure was formed rather than a gold shell around the gold core. The assignment of representative peaks and the proposed chemical structure of PDA was presented in Figure S3.

Figure 2. Characterizations of Au NPs, AP and APA. (A) TEM images of Au NPs (a, d), AP (b, e) and APA nanocomposites (c, f), respectively. (B) The gold species such as m/z 197 (Au-), 393 (Au2-), 410 (Au2O-) and 591 (Au3-) are marked with red stars. (C) Positive ion ToF-SIMS spectra of APA. The fragment ions of polydopamine such as m/z 30 (CH4N+), 41 (C3H5+), 55 (C4H7+), 90 (C6H10+), 115 (C8H5N+), 147 (C8H5NO2+), 360 (C21H18N3O3+) and 376 (C21H18N3O4+) are marked with green stars. 8


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The optical properties are indicated by the different colors of the nanocomposite colloids (Figure 3A inset). The solution changed from red to purple after PDA-modified on Au NPs, and subsequently changed into blue-violet during the formation of APA nanocomposites. The AP nanoparticles exhibited an UV-Vis absorption peak at around 550 nm (Figure 3A, curve b), which is red-shifted ca. 15 nm from that of bare Au NPs (535 nm) (Figure 3A, curve a). The reason is attributed to the absorption of PDA in NIR region and the change of dielectric constant ε caused by the change of local environment.20,26 The gold nanoislands around the APA nanoparticles generated strong plasmonic coupling optical signals and caused large red-shift in the absorption spectra.15 Therefore, its UV-Vis absorption spectrum exhibited a broad range absorption band at around 560 nm (Figure 3A curve c). Figure 3B and 3C represented the typical scattering spectra of single nanoparticles modified on indium tin oxide (ITO) substrates and the statistical distribution plot of scattering spectra for over 100 particles, respectively. The dark-field microscopy images were shown in Figure S4. As displayed in the plasmon scattering spectra, a red-shift of about 16 nm for AP and 33 nm for APA nanoparticles were observed in comparison with gold nanoparticles, respectively. For AP nanoparticles, the modified DPA layer on the surface of gold nanoparticle gave rise to the change of environmental refractive index. The gold nanoislands coupled with gold core produced an intense plasmon resonance coupling effect, which contributed to the plasmon resonance band redshift for APA nanoparticles.27-29 To further prove the optical properties of nanocomposites, Mie theory simulation was performed (Figure S5).30,31 The Mie theory qualitatively predicted the experimentally observed trend that λmax of the prepared nanocomposites is significantly dependent on their nanostructures.

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Figure 3. (A) UV-Vis spectra of (a) Au NPs, (b) AP and (c) APA solution. Inset: photographs of corresponding solutions. (B) Scattering spectra of a single (a) Au NPs, (b) AP and (c) APA nanoparticle and (C) corresponding statistical analysis of averaged peak wavelength over 100 nanoparticles. The Mie theory simulation of (D) absorption cross section and (E) normalised scattering cross section for (a) Au NPs, (b) AP and (c) APA in aqueous solution, respectively. (F) Normalized scattering cross section for nanoparticles modified on ITO substrate. The insets present the schematic of (I) Au NPs, (II) AP and (III) APA nanoparticles.

Catalytic Efficiency in Bulk System To evaluate the catalytic activity of APA nanocomposites at single-particle level, the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the NaBH4 (SB) aqueous solution was chosen as a model reaction (Figure S6 and S7).5 This reaction was based on the Langmuir-Hishelwood mechanism and involved the transfer of surface hydrogen species and electrons to the noble metal nanoparticles that were supplied by NaBH4.32,33 The reduction of 4-NP by Au NPs, AP and APA in ensemble system was monitored using UV-vis spectroscopy (Figure 4). As shown in Figure 4AC, the absorption of 4-NP at 400 nm decreased rapidly with a concomitant increase in the peak at 300 nm, which could be contributed to that 4-NP has been gradually reduced to the product of 4-AP. When using Au NPs as catalysts, the conversion process took 14 mins to reach 95%, while it took 60 mins for AP taking part into the reduction. In contrast, APA nanocomposites only need 6 mins to arrive at the same extent and demonstrate excellent catalytic capacity. When an excess of NaBH4 was used, the

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reduction could be considered as a pseudo first-order reaction and its reaction rate constant kapp was only related to the concentration of 4-NP.34 The kinetic constants kapp could be defined as ln(Ct/C0) =-kappt, where Ct and C0 were the concentration of 4-NP at time t and 0, kapp was the apparent first-order kinetic constants (min-1). Figure 4D showed curves that ln(Ct/C0) versus reaction time t for reducing 4-NP by Au NPs, AP and APA nanocomposites, respectively. There is no induction time observed indicating that the reduction occurred immediately after adding the catalyst and no activation or restructuring of the catalyst surface by 4-nitrophenol.35 From the linear relationship between ln(Ct/C0) and t, the kinetic constants kapp was calculated from the slope of the fitting lines. The kapp of APA nanocomposites was 0.50 min-1, which was greater than that of Au NPs (0.21 min-1) and AP (0.04 min-1). As illustrated in Figure S6, the possible mechanism of catalytic reduction of 4-NP by the Au NPs, AP and APA involved the diffusion of 4-NP and BH4- from bulk solution to the surface of nanocatalyst, and then followed with the transfer of surface hydrogen species and electrons from BH4- to 4-NP via gold surface.32 The direct contact between the reactant molecules and the catalyst is a prerequisite for the electron transfer.36 When the small gold nanoislands were formed around APA, the rate constant kapp increased about 10 and 2.4 times compared with AP and Au NPs, respectively. The size-induced high surface-to-volume ratio and high chemical potentials of gold nanoislands lead to the high catalytic ability of the catalyst.2 Moreover, the intermolecular interaction between PDA and 4-NP could be evaluated by the interaction between monomer dopamine and 4-NP and confirmed by UV-vis absorbance spectra (Figure S9). The maximum absorption wavelength of dopamine slightly shifted to red wavelength in the presence of 4-NP. The peak shift indicated that the formation of hydrogen bond interactions between the amino group (-NO2) of 4-NP and the nitrogen/oxygen atom of dopamine in the solution system.37 Moreover, the π-π stacking interaction between πrich 4-NP and dopamine was contributed to the spectral red-shifts.16 Therefore, the strong hydrogen bond interactions and π-π stacking interactions would drive 4-NP molecules to assemble onto the surface of APA and increase the concentration of 4-NP on PDA shell. Thus, the interaction between reactant molecules and the PDA shell 11



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increase the concentration of reactant molecules near the catalyst surface which contributed to the high rate constant of the heterogeneous nanocatalysts of APA (Table S1). In addition, the gold nanoislands were synthesized in situ without using any chemical surfactant or stabilizer. Hence, the gold nanoislands with higher catalytic capacity located at out layer of APA and the higher concentration of 4-NP on PDA layer lead to the enhancement of the catalytic activity of APA.2 However, when the Au NPs coated with polydopamine layer, which hindered the direct contact, the catalytic efficiency of AP is significantly lower than that of Au NPs and APA. The rate constant kapp of the AP decreased significantly about 5 and 12 times compared with bare gold nanoparticles and APA nanocomposites, respectively. It was due to that the reduction of 4-NP cannot be catalyzed by PDA layer (Figure S8 and S9).17 Nevertheless, there was still weak catalytic capacity of AP for reduction of 4-NP and it might be attributed to that the PDA shell was more likely to be a supramolecular structure of building blocks which were held together through noncovalent interaction rather than entire covalent interaction.23 Therefore, the penetration of BH4- into PDA shell cannot be completely inhibited, electrons could transfer from gold surface to 4-NP with an extremely low transfer rate (Figure 4D).

Figure 4. Time-dependent UV-Vis absorption spectra of the reduction of 0.4 mM 4-nitrophenol by NaBH4 in the presence of (A) Au NPs, (B) AP and (C) APA. (D) Absorbance ratios at 400 nm versus 12


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reaction time (ln(Ct/C0) vs t) for reducing 4-nitrophenol by NaBH4 in the presence of (a) Au NPs, (b) AP and (c) APA.

Catalytic Efficiency at Single Nanoparticle Level Direct LSPR strategy was further adopted to monitoring the 4-NP reduction on single plasmonic nanoparticles.32,38 As shown in Figure 5A and 5B top, deionized water, 10 mM SB and 0.125 mM 4-NP were served as three independent phase and sequentially injected into a micro-reactor cell (Schematic S1). The electrons were transferred from donor molecules SB to the nanoparticles and subsequently altered their intrinsic plasmon frequency, leading to a blue shift in the scattering spectra of these nanoparticles (Figure S10).39 With the injection of the acceptor molecule 4-NP, a red shift in the scattering spectra of nanoparticles was observed. It is reasonable to consider that this scattering peak shift was attributed to the electrons transfer from the nanoparticle to the acceptor 4-NP, resulting in a lower electron density in the nanoparticles. The nanoparticles in a solution couple these two redox reactions by acting as a nanoreservoir for the transferred electrons during the heterogeneous catalysis. In the control experiment, the scattering spectra of single Au NPs, AP and APA in the presence of deionized water, 4-NP and 4-AP exhibit no obvious shift and only the random fluctuations induced by the environmental factors (Figure S11). Depending on these results, the localized plasmon resonance peak shift was resulted by the electron charge and discharge during the redox reaction rather than the change of the dielectric environment.40 To investigate the catalytic reaction process on the surface of a single nanoparticles, the simultaneous dark-field scattering spectra of individual single nanoparticles were recorded as a function of the reaction time (Figure 5A and 5B, bottom). The distribution of the scattering peak shift for Au NPs and APA nanoparticles were displayed in Figure 5C and 5D. As shown in Figure 5, no noticeable change in the plasmon resonance band was observed in the deionized water (Figure 5A and 5B, the range I). With the injection of 10 mM SB solution, a blue shift ca. 3.6 nm for Au NPs was found owing to the electrons transfer from 13



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SB to gold nanoparticles (Figure 5A, the range II). Interestingly, the scattering resonance band of the APA nanoparticles blue shifted ca. 6.7 nm and a steady plateau was reached after the addition of SB (Figure 5B, the range II). These results indicated that the APA nanoparticles showed a stronger capacity to serve as the nanoreservoir for the electrons than that of Au NPs and AP nanoparticles (Figure S12) and the highest electron density was obtained on the surface of APA nanoparticles. The advanced nanoarchitecture of the APA nanoparticles contributed to this high electron density owing to the plentiful nanoislands similar to the nanoantenna with a large surface-to-volume ratio.2 Then, 0.125 mM 4-NP was added and reacted with these nanocatalysts. The gold-catalysed reduction of 4-NP was directly monitored by measuring the scattering spectra of these plasmonic nanocatalysts. The scattering peak was red shifted ca. 0.9 nm during catalysis for Au NPs, ca. 3.7 nm for APA and only ca. 0.3 nm for AP nanoparticles, respectively (Figure 5A and 5B, the range III and Figure S12). The larger redshift for the APA nanoparticles demonstrated that the APA nanoparticle showed a more excellent catalytic activity for reducing 4-NP than that of Au NPs and AP nanoparticles. According to the relationship between the plasmon resonance band shift Δλ and the change in the nanoparticle electron density ΔN, it allows us to quantify the number of transferred electrons during the heterogeneous catalysis at singleparticle level (Eq. S1-2).41 During the electron charging process, the charging rate for APA nanoparticle reaches 2307 electron s -1 which was quadruple faster than that of Au NPs (562 electron s-1) and three and a half times faster than AP nanoparticles (640 electron s-1). For the discharge of the excess electrons from the nanocatalyst to the acceptor (4-NP), the discharging rate for APA nanoparticles (1270 electron s-1) was nine and thirty times as compared with Au NPs (141 electron s-1) and AP nanoparticles (47 electron s-1), respectively. The discharge rate of the excess electrons because of 4-NP reduction corresponds to a consumption of 212, 23 and 8 p-nitrophenol molecules per second for APA, Au NPs and AP nanoparticles, respectively. These results demonstrated the higher 14


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catalytic conversion rate on the single APA nanoparticles compared with Au NPs and AP nanoparticles, which confirmed that the APA nanoparticles showed a higher catalytic activity than Au NPs and AP nanoparticles. The size-induced high chemical potentials of small gold nanoislands lead to the excellent catalytic ability of the APA nanoparticles.2 Moreover, owing to the hydrogen bond formation and π-π stacking interactions between the amino group of dopamine and nitrogen/oxygen atom of 4-NP, the 4-NP molecules were driven to assemble onto the surface of the APA nanoparticles and leading to its concentration increasement, which contributed to the high rate constant for reduction of 4-NP using APA nanoparticles as nanocatalysts.16,37

Figure 5. Plasmon shift of a single (A) Au NPs and (B) APA nanoparticles as a function of the reaction period in the presence of deionized water (Range I), 10 mM SB (Range II) and 0.125 mM 4-NP (Range III). Histograms give the distributions of the plasmon shift with a Gaussian distribution shape for (C) Au NPs and (D) APA nanoparticles in the presence of (a) deionized water, (b) 10 mM SB and (c) 0.125 mM 4-NP.

To further examine the excellent catalysis of APA, the substrate-immobilized APA nanocomposites was easily fabricated and recycled (Figure 6A). Once the APA nanoparticles anchored on PDA coated polydimethylsiloxane (PDMS) chip, the color of the chip change from transparent to purple as shown in Figure 6B and 6C, which originates from the localized surface plasmon resonance absorption of the APA nanocomposites.25 ToF-SIMS negative spectra indicated 15



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the gold related peaks such as 197 (Au-), 393 (Au2-), 410 (Au2O-) and 591 (Au3) from APA modified PDMS chip (Figure S13). Furthermore, FE-SEM images also proved that the APA nanoparticles were coated on the PDMS surface (Figure 6D and 6E). Then, the APA modified PDMS chip served as the dip-catalyst to evaluate the performance of its catalytic stability. As shown in Figure 6F, almost identical catalytic activity was obtained from ten successive cycles with a conversion efficiency of 0.95, indicating that the APA modified PDMS chips possessed the excellent stable catalytic capability and could be served as the convenient nanocatalysts.

Figure 6. (A) Schematic illustration of the interfacial assembly of APA nanocomposites and the fabrication of recyclable PDMS chip. Photographs of the PDMS chip (B) before and (C) after modified with self-assembled APA. (D) FE-SEM image and (E) corresponding HR-SEM image of APA modified PDMS chip. (F) The conversion efficiency of 4-NP in 10 mins reaction of 10 consecutive reaction cycles.

CONCLUSIONS In conclusion, mussel-inspired APA nanocomposites were synthesized without using any surfactant and reductant. Its surface structure and optical properties were characterized by ToF-SIMS, adsorption and scattering spectra, respectively. The direct LSPR monitoring of redox reactions on single nanocomposites surface using DFM provide information on catalytic processes and their mechanisms avoiding averaging effects in bulk systems. Moreover, the strategy using single nanoparticle spectroscopy techniques will facilitate rapid monitoring of dynamic processes such as mass transfer and reaction dynamics near or on the surface of plasmonic catalysts at the nanoscale.

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SUPPORTING INFORMATION TEM data, dark-filed Microscope images, Mie theory simulation, control tests. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Tel/Fax: 86-21-64252339. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by the Science Fund for Creative Research Groups (21421004), the National Natural Science Foundation of China (21327807), the Program of Shanghai Subject Chief Scientist (15XD1501200), the Programme of Introducing Talents of Discipline to Universities (B16017), and the State Key Laboratory

of

Analytical

Chemistry

for

Life

Science

(SKLACLS1512).

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