Formation and Self-assembly of Gold Nanoplates through an

Jun 8, 2016 - Here we present a new strategy to form 3D hierarchical gold (Au) ... Novel Strategy for One-Pot Synthesis of Gold Nanoplates on Carbon ...
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Formation and Self-assembly of Gold Nanoplates through an Interfacial Reaction for Surface-Enhanced Raman Scattering Ying Ma and Lin-Yue Lanry Yung* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore S Supporting Information *

ABSTRACT: 3D hierarchical architectures assembled from individual particles have attracted great interest because they displayed novel properties from the individual building blocks as well as their complex structures. Here we present a new strategy to form 3D hierarchical gold (Au) nanostructures via an interfacial reduction reaction. An aniline (ANI) derivative, N-(3amidino)-aniline (NAAN), and HAuCl4 were separately dissolved in toluene and water to form an organic/water interface. Au nanoplates formed at the interface and subsequently moved to the aqueous phase. As a capping agent for the nanoplate formation, the oxidized NAAN, i.e., poly(N-(3-amidino)-aniline) (PNAAN), also facilitated the self-assembly of Au nanoplates into 3D hierarchical Au nanoflowers (AuNFs) through π−π stacking. The individual AuNF exhibited good surface-enhanced Raman scattering (SERS) response both in enhancement factor and reproducibility because it integrates the SERS enhancement effects of individual Au nanoplates and their hierarchical structures. This is the first report depicting the one-pot formation and self-assembly of Au nanoplates into 3D organized hierarchical nanostructures through the molecular interaction of conducting polymer. KEYWORDS: interfacial reaction, Au nanoplate, self-assembly, Au nanoflowers, SERS

1. INTRODUCTION

Polyaniline (PANI), an electrically conducting polymer, is widely used in optical and chemical sensors, microelectronic devices, catalysts, drug delivery, and energy storage systems.15−19 Nanocomposites consisting of metallic nanoparticles and conducting polymers have great promise in a variety of applications because of the synergetic properties pertaining to the two components present.16 A series of PANI-metallic nanoparticles including Au, platinum (Pt), and palladium (Pd) have been successfully synthesized, and they have been applied to SERS, heterogeneous catalysis and electronic and sensing devices.20−25 Because PANI can spontaneously self-assemble into particles without any outside force through the π−π stacking among polymer chains,26 this property can potentially be used for the self-assembly of metallic NPs into 3D nanostructures. However, self-assembly of metallic nanoparticles by the force among PANI has rarely been reported because of the poor solubility of PANI in most of solvents. We previously synthesized a new aniline (ANI) derivative N(3-amidino)-aniline (NAAN) with an amidine pendant group at the meta-position of ANI, and the aqueous solubility of the

The focus of nanoscience is gradually shifting from the individual nanoparticle synthesis to their self-assembly into organized 3D structures with controlled sizes and shapes because individual particles cannot meet the demands of nanoelectronics, sensing, catalysis, and biomedical applications.1,2 Formation of multidimensional organized structures can either be approached in a top-down or bottom-up manner. The former approach often uses the traditional microfabrication methods such as lithography3 and microcontact printing.4 The bottom-up approach utilizes the concepts of molecular selfassembly and/or molecular recognition.5 For the self-assembly strategy, the general method has two steps: (i) synthesis of individual nanoparticles and (ii) self-assembly into superstructures via the forces between ligands or the biorecognition interaction of donor−acceptors. A series of superstructures have been fabricated using a variety of materials with isotropic or anisotropic morphologies. These include Au nanoparticles (NPs), nanorods (NRs), and nanowires (NWs),6−9 semiconductor NPs and NRs,10−12 and magnetic NPs. 13,14 However, the applications of such superstructures are very limited because of the complexity or low yield of the fabrication of such desirable structures. © 2016 American Chemical Society

Received: January 25, 2016 Accepted: May 20, 2016 Published: June 8, 2016 15567

DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573

Research Article

ACS Applied Materials & Interfaces

aqueous solution under continual stirring for 2 h. The AuNFs were subsequently washed three times by water and dispersed in 0.2 mL of water. Finally 10 μL of AuNF solution was dropped on a silicon wafer for SERS measurement after the substrate was dried in air. SERS spectra were recorded by an XploRA PLUS Raman microscope (Horiba/JY, France) using a 785 nm laser excitation source. The incident laser power was kept at 1 mW and total accumulation times of 10 s were employed.

resultant poly(N-(3-amidino)-aniline (PNAAN) polymer was greatly improved.27 In this work, we reported a simple one-pot method to spontaneously form and self-assemble Au nanoplates into 3D hierarchical AuNFs by interfacial reduction reaction with HAuCl4 in the aqueous phase and NAAN in the organic phase. The individual Au nanoplates formed at toluene/water interface and subsequently self-assembled in aqueous phase into 3D hierarchical structures, making use of the π−π stacking among PNAAN polymer chains. By tuning the reaction conditions such as pH, temperature, and NAAN/HAuCl4 ratio, a variety of nanostructures with different morphologies were acquired. The acquired individual AuNF exhibited good SERS response with enhancement factor (EF) of 108 and standard derivation (SD) of only 9.3% due to the presence of large number of edges, tips and especially gaps among the individual Au nanoplates.

3. RESULTS AND DISCUSSION 3.1. Formation of Au Nanoflowers. Interfacial reaction occurs at the phase boundary of an immiscible aqueous/organic biphasic system28 and is an effective method to suppress overreaction of reactants. Here NAAN and HAuCl4 were initially separated by the boundary between toluene and aqueous phases, and reduction reaction only occurred at the toluene/water interface. The as-synthesized Au nanoparticles then migrated away from the interface into the aqueous phase after formation. This controlled the size and morphology of Au nanoparticles because the overgrowth of the particles was inhibited. Spherical nanoparticles with the mean size of 723 ± 43 nm were observed in the SEM image after 4 h of interfacial reaction (Figure 1a). The enlarged image of a single

2. EXPERIMENTAL SECTION 2.1. Materials. HAuCl4·3H2O was purchased from Sigma. All other reagents are analytical-grade and employed as received. Deionized water (resistivity up to 18.2 MΩ cm) was used throughout the experiment. N-(3-Amidino)-aniline (NAAN) hydrochloric acid was synthesized according to our reported method.27 The hydrochloric acid was removed by neutralizing NAAN hydrochloric acid with 1 M NaOH in ethanol. After removal of the white precipitate by filtration, ethanol was removed by rotary evaporator. The collected reactant was dissolved into chloroform. After filtration, the organic phase was washed by saline and dried over anhydrous Na2SO4. The final yellowish product was collected after removing chloroform by rotary evaporator and drying in vacuum overnight. Yield (95%). 1H NMR (CDCl3) 1.91 ppm (3H, CH3), 3.03 (6H), 6.3−7.2 (4H, m, phenyl). 2.2. Synthesis of Au Nanostructures. The general procedure for the synthesis of Au nanostructures as follows: An 8 μL aliquot of 10% HAuCl4 (2.64 μmol) was added to a 20 mL glass vial containing 3 mL of 1 mM HCl. After vigorous shaking of the mixture for 20 s, the glass vial was put into a 4 °C fridge. In another vial, 2 mg of NAAN (11.2 μmol) was dissolved in 3 mL of toluene and cooled in the fridge for 30 min. Subsequently, the NAAN toluene solution was gently added on top of HAuCl4 solution. The two-layer mixture was incubated in a 4 °C fridge for the growth of Au nanostructures. To study the evolution of Au nanostructure, the interfacial reaction was quenched by removing the top organic layer at a specific time. The aqueous phase was then collected and centrifuged at 7000 rpm for 5 min, and the corresponding pellet was redissolved in water for the scanning electron microscopy (SEM) or transmission electron microscopy (TEM) sample preparation. The same reactions were conducted at 25, 45, and 70 °C to investigate the temperature effect on the Au nanostructures. To study the pH effect on the resulting nanostructures, different pH media (0.1 M and 1 mM HCl, water, and 1 mM NaOH) were used as aqueous phase with same reactant concentrations and procedures mentioned above. 2.3. Characterization. TEM (JEOL JEM 2100F TEM) and SEM (JEOL JSM-6700F field emission SEM) were used to study the structures and morphologies of nanoparticles. The samples were prepared by drop-casting 20 μL of dispersed nanoparticle solution onto 200 mesh copper grids (for TEM) or silicon wafers (for SEM) and allowing for subsequent evaporation in air. For the TEM elementary mapping, element Au and N signals were collected. Fourier-transformed infrared (FTIR) spectra were recorded in transmission mode using a Shimadzu 8400 spectrophotometer. Pellets of AuNFs in KBr were prepared to acquire the FTIR spectra. X-ray photoelectron spectroscopy (XPS) data was collected using a Kratos Axis UltraDLD spectrometer (Kratos Analytical, Ltd.) equipped with a monochromatized Al Kα X-ray source (1486.71 eV photons). 2.4. Surface-Enhanced Raman Scattering Measurement. To evaluate the SERS performance, 10 μL of the acquired AuNF solution was mixed with 0.5 mL of 1 mM 4-mercaptobenzoic acid (4-MBA)

Figure 1. (a) SEM and (b) TEM image of AuNFs acquired via interfacial reaction. Inset in a, enlarged SEM image of a single NF. (c and d)TEM-EDX mapping of AuNFs. (e) Lattice structure and (f) SAED pattern of a single petal. The boxed spot and circled spot correspond to the Au (220) and 1/3 (422) diffractions. The interfacial reaction was done at 4 °C with 3.74 mM NAAN and 0.88 mM HAuCl4 dissolved in 3 mL of toluene and 3 mL of 1 mM HCl aqueous solution, respectively.

nanoparticle showed the blooming-flower-like structures with individual nanoplates as petals (inset of Figure 1a). The morphology of the Au nanoflowers (AuNFs) was also investigated by TEM, and each single AuNF was found to consist of overlapped Au nanoplates (Figure 1b). EDX mappings of AuNFs showed the well-dispersed elemental Au and N within AuNF (Figure 1c,d, respectively) and confirmed the capping of PNAAN on the Au nanoplates. The corresponding fast Fourier transform (FFT) pattern in Figure 1e further revealed a clear Au crystal lattice. The fringes with a d-spacing of 0.24 nm was found to be the 1/3 (422) lattice spacing of the face-centered cubic (fcc) Au crystal. A selected area electron diffraction (SAED) pattern (Figure 1f), obtained by aligning the electron beam perpendicular to the plane of a selected petal, showed a series of diffraction spots with a 6-fold symmetric characteristic, which were ascribed to the (220) and 1/3 (422) reflections of (111)-oriented single-crystal Au nanoplate.29,30 15568

DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573

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ACS Applied Materials & Interfaces

(ii) nanoplate growth, (iii) self-assembly, and (iv) AuNF formation steps (Figure 2c): When NAAN comes into contact with HAuCl4 at the toluene/water interface, it reduces HAuCl4 to form Au nucleus while NAAN itself is oxidized to form PNAAN. PNAAN then adsorbs onto the nanoparticle surface as a capping agent. Au nanoparticles undergo further growth into Au nanoplates at the interface because of the preferential binding of PNAAN on the Au (111) facet. This is in agreement with the reported Au nanoplate formation when ANI35 and ophenylenediamine (o-PD)36 were used as reducing agents. The Au nanoplates stop growing after they migrate away from interface into aqueous phase because of the lack of reducing agent in aqueous phase. Therefore, the eventual size of Au nanoplates is dependent on the reduction rate of HAuCl4 and diffusion rate of the Au nanoplates from interface to aqueous phase. In the earlier stage of the reaction, only individual Au nanoplates were formed in aqueous phase. However, once the Au nanoplate concentration becomes higher, they are prone to self-assemble into AuNFs via the π−π stacking among the capping agent PNAAN on the nanoplate surface. PNAAN thus functions as a “glue” to paste the Au nanoplates together. To verify whether the Au nanoplates were formed at the toluene/water interface or in aqueous solution, a control interface reaction was carried out with gentle shaking of reaction vial without destroying the biphasic interface. In this case, the nanoparticles formed at the interface were allowed to move into the bulk aqueous phase to quench the growth of nanoparticles into nanoplates. This led to the formation of the red color solution in the aqueous phase immediately and subsequent formation of dark red as time increased. At the end of 4 h reaction, spherical large particles doped with small nanoparticles instead of nanoplates were observed (Figure S3a). This supports our assumption that the growth of Au nanoplate is inhibited once it migrates away from the toluene/ water interface. To understand better the role of interfacial reaction, a homogeneous reaction was performed by simple mixing of HAuCl4 and NAAN in aqueous solution at 4 °C. Irregular structures composed of aggregated PNAAN polymer and Au nanoparticles were observed (Figure S3b), confirming the importance of the interfacial reaction in AuNF formation. 3.2. Effects of Reaction Conditions on Interfacial Reactions. According to the growth mechanism of AuNFs shown above, the reaction conditions affecting the reduction rate of HAuCl4 and diffusion rate of Au nanoplates from interface to bulk aqueous phase have determined effects on the resulting Au nanostructures. First, mild reduction rate is important because a fast reaction may result in a large amount of Au nanoparticles, which may deplete the HAuCl4 precursor concentration at the interface prior to further growth of nanoplates. A very slow reaction can also be undesirable because it may produce insufficient Au atoms for nanoplate growth at interface. Second, sufficient residence time at interface and slower diffusion rate of Au nanoparticles to aqueous phase are necessary because growth of Au nanoplates is quenched once they enter the bulk aqueous phase. Thus, we next investigated the effects of the reaction conditions including aqueous phase pH, ratio of NAAN/HAuCl4, and reaction temperature on the interfacial reactions. To study the pH effect on the interfacial reaction, reactions with aqueous phase from strongly acidic to weakly basic pHs (i.e., 100 mM HCl, 1 mM HCl, water, and 1 mM NaOH, corresponding pHs are 1, 3, 7, and 11) were investigated. The

The chemical composition of AuNFs was investigated via FT-IR and XPS. FT-IR spectrum showed the characteristic IR absorption bands of PNAAN (Figure S1a): 1520 cm−1 band assigned to CC stretching of quinoid and benzenoid rings, 1385 cm−1 band assigned to C−N stretching of typical PANI base in the neighborhood of a quinonoid ring and indicative of a PANI backbone, 841, 767, and 690 cm−1 bands assigned to the para-disubstituted rings and the monosubstituted benzene ring revealing that the short PNAAN oligomers were present,27 and a strong peak at 1636 cm−1 assigned to the CN stretching of amiding group.31 The binding energy peaks of 84.0 eV (Au 4f7/2) and 399.5 eV (N 1s) in the XPS spectrum (Figure S1b) indicated the presence of elemental Au32 and PNAAN polymer, respectively, in AuNFs.33 The mass ratio of N/Au was calculated to be 3.8/96.2. To investigate the formation mechanism of AuNFs, timedependent interfacial reactions were conducted by quenching the reactions at specific time. As shown in Figure 2a, two

Figure 2. (a) Photographs of the interface reactions and (b) SEM images of corresponding aqueous products at different reaction time. (c) Schematic illustrating the formation of Au NFs via the interfacial reaction. The interfacial reaction was done at 4 °C with 3.74 mM NAAN and 0.88 mM HAuCl4 dissolved in 3 mL of toluene and 3 mL of 1 mM HCl aqueous solution, respectively.

separated phases were formed after layering the NAAN organic phase on top of the aqueous phase. After 10 min, the color of the aqueous solution near the interface became slightly red, indicating the formation of Au nanoparticles. The red color product slowly diffused into aqueous phase after 30 min and filled the whole aqueous phase after 1 h. After 4 h, the entire aqueous phase was dark red. SEM images of corresponding samples showed small spherical Au nanoparticles of 35.0 ± 3.1 nm after 10 min of reaction time, and mixtures of spherical particles and individual and aggregated Au nanoplates after 30 min and 1 h (Figure 2b). After 4 h, the collected sample was entirely spherical AuNFs self-assembled by Au nanoplates. After incubation with N-methyl-2-pyrrolidone (NMP) (a good solvent of PANI used for removing PANI from Au/PANI nanocomposite to obtain the individual AuNPs34), the size of AuNFs dramatically decreased, and a large amount of individual Au nanoplates were observed (Figure S2), indicating that the AuNFs were self-assembled via the π−π stacking of PNAAN. On the basis of the above-mentioned experimental result, a reaction mechanism can be proposed via (i) nucleus formation, 15569

DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573

Research Article

ACS Applied Materials & Interfaces color change of the aqueous phase showed pH-dependent interfacial reactions. After 30 min of reaction, no color change (100 mM HCl), slight color change (1 mM HCl and water), and an obvious color change (1 mM NaOH) were observed in aqueous phase (Figure 3a). SEM images of the corresponding 4

growing into nanoplates (as shown by the quick color change after 30 min of reaction in Figure 3a). Thus, spherical aggregations of small Au nanoparticles and nanoplates were observed. In 1 mM HCl and water, the reduction rate of HAuCl4 is sufficiently modest, whereas the diffusion rate of Au nanoparticles is slow enough to allow the growth of Au nanoplates to result eventually in AuNFs. The most direct way to control morphology of nanostructures was found to change the ratio of NAAN/HAuCl4. At a fixed HAuCl4 concentration of 0.88 mM, low NAAN concentrations of 1.87 and 3.74 mM (NAAN/HAuCl4 molar ratio of 2 and 4) yielded only slightly red aqueous phase after 30 min of reaction. However, the aqueous solution became dark red when the NAAN concentration exceeded 7.48 mM (Figure S4a). The SEM images of aqueous products after 4 h reaction showed that a low concentration of NAAN (1.87 mM) yielded AuNFs containing micrometer-sized Au nanoplate (Figure 4a),

Figure 3. (a) Photographs of 30 min interfacial reaction using 100 mM HCl, 1 mM HCl, H2O, and 1 mM NaOH as aqueous phase. (b) Corresponding SEM images of aqueous products obtained after 4 h interfacial reactions. Reaction condition: 0.88 mM HAuCl4 in 3 mL of pH media and 3.74 mM NAAN in 3 mL of toluene.

Figure 4. (a−d) SEM images of aqueous products obtained after 4 h interface reactions at 4 °C with varying NAAN concentrations (mM): (a) 1.87, (b) 3.74, (c) 7.48, and (d) 14.96. HAuCl4 concentration is 0.88 mM. (e−h) SEM images of aqueous products after 4 h interface reaction at 4 °C with varying HAuCl4 concentrations (mM): (e) 0.22, (f) 0.44, (g) 0.88, and (h) 1.76. NAAN concentration is 3.74 mM.

h reaction products (Figure 3b) showed irregular structures doped with tiny Au nanoparticles in strongly acidic media (100 mM HCl). AuNFs were observed in 1 mM HCl and DI water aqueous conditions, with 1 mM HCl yielding evenly size distributed AuNFs with longer edge petals. Spherical particles composed of Au nanoplates and tiny nanoparticles were mainly observed when the aqueous media is 1 mM NaOH. These observations indicate that weakly acidic and neutral pH conditions favor the formation of AuNFs. This pH-dependent interfacial interaction can be explained as follows: (i) Acidic pH increases hydrophilicity of PNAAN because of the protonation of the amidine group into amidinium via the acid−base reaction37 (the pKa of amidine connected to phenyl group was reported to be 8.32).38 This increases the diffusion rate of the acquired nanoparticles into the bulk aqueous phase. (ii) Strong acid protonates the aniline group of NAAN into anilinium, which has higher redox potential26 and lower reducing ability compared with those of neutral aniline. In strong acid (100 mM HCl), the reduction of Au3+ to Au is slower, and the diffusion of Au nanoparticles from the interface to the bulk phase is faster because of the increased hydrophilicity of protonated PNAAN. Once the particles are formed, they rapidly move into the aqueous bulk phase before further growth into nanoplates occurs, resulting in a mixture of PNAAN and tiny Au nanoparticles. In 1 mM NaOH, because the reducing ability of nonprotonated NAAN is higher, a large amount of Au nanoparticles is formed at the interface within a short time, and some of them directly migrate into the aqueous phase before

and higher NAAN concentrations of 3.74 and 7.48 mM resulted in AuNFs consisting of smaller Au nanoplates (Figure 4b,c). At an even higher NAAN concentration of 14.96 mM, spherical aggregations of small Au nanoparticles were observed (Figure 4d). Conversely, varying the HAuCl4 concentration while fixing the NAAN concentration of 3.74 mM gave similar results. After 30 min of reaction, a lower HAuCl4 concentration gave a redder solution (0.22 and 0.44 mM), whereas the slight color change was observed for higher HAuCl4 concentration case (0.88 and 1.76 mM, Figure S4b). By fixing the NAAN concentration and varying the HAuCl4 concentration, we found that at lower concentrations (0.22 and 0.44 mM), aggregations were mainly composed of tiny spherical Au nanoparticles and small size nanoplates (Figure 4e,f). At a higher concentration of 0.88 mM, most of the building blocks were Au nanoplates (Figure 4g), and AuNFs composed of even bigger Au nanoplates were observed at the HAuCl4 concentration of 1.76 mM (Figure 4h). On the basis of the above observations, the NAAN/HAuCl4 ratio affects the morphology of resulting products in the following way: Higher NAAN/HAuCl4 ratios results in a faster reduction rate of HAuCl4; thus, some Au nanoparticles directly migrate into the aqueous phase before they can grow into nanoplates and subsequently form spherical aggregations of small Au nanoparticles. In contrast, lower NAAN/HAuCl4 ratios allow the mild reduction of HAuCl4, and most of the Au nanoparticles grow into nanoplates before migrating into aqueous phase, eventually resulting in AuNFs. 15570

DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573

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Figure 5. (a) Optical microscope image of isolated AuNFs (Inset, SEM image of an AuNF, with the circled regions corresponding to the nanogaps formed at the junctions. (b) SERS spectra of (red curve) unmodified AuNFs, (blue curve) 4-MBA-modified AuNF, and (black curve) normal Raman spectrum of pure 4-MBA. (c) SERS spectra of 10 individual 4-MBA modified AuNFs.

(σ(N−H) in semiquinonoid ring), 1345 (ν(C−N+) of polarons with shorter conjugation lengths), 1229 (ν(C−N) of quinonoid ring), and 1160 cm−1 (σ(C−H) in semiquinonoid ring) were observed for unmodified AuNF, indicating the adsorption of PNAAN on AuNFs (red curve in Figure 5b).40 As for the pure 4-MBA sample (black curve in Figure 5b), two peaks at 1080 and 1590 cm−1 due to the benzene ring stretches were observed in 4-MBA-modified AuNF or 4-MBA-AuNF (blue curve in Figure 5b). The great SERS enhancement of individual 4-MBAAuNF was observed from the intense peak intensity. As shown in Figure 5c, the SERS spectra of individual 4-MBA-AuNF also exhibited good reproducibility for 10 separated measurements with a SD of 9.3%. This good reproducibility can be attributed to the good size distribution of AuNFs and their uniform structures. The enhancement factor (EF) was calculated to be 2.1−3.0 × 108 by comparing the peak intensity of the 4-MBAAuNF with that of 4-MBA powder at 1080 cm−1 according to the reported method.41 The full details for this calculation can be found in the Supporting Information. This EF is 3 orders of magnitude higher than that of the self-assembled Au nanoplates (EF ≈ 105),42 indicating that the nanogaps generated through the formation of AuNFs play more important role in this SERS enhancement. Because of the excellent SERS enhancement and reproducibility, such individual AuNFs may have potential applications as a SERS substrate for detections at the singlemolecule level because EFs on the order of 107 or greater is necessary for single molecule detection.43

Not surprisingly, we found that higher temperatures were unfavorable for the AuNF formation when the interfacial reaction was studied at 25, 45, and 70 °C because increasing reaction temperature increases reduction rate of HAuCl4 and diffusion rate of Au nanoparticles into aqueous phase. Within a couple of minutes, the color of aqueous phase changed to red for all three cases, indicating the fast reduction of HAuCl4 at higher temperature (Figure S5a−c). For all three samples, spherical aggregations of nanoplates and small nanoparticles were observed after 4 h reaction, whereas AuNFs were the main products under the same reaction conditions at 4 °C (Figure 4g). 3.3. Interfacial Reaction Using ANI as Reducing Agent. To investigate further the role of NAAN in AuNF formation, a control experiment was carried out using commercial ANI as the reducing agent. As shown in Figure S6a, some irregular PANI particles doped with tiny Au nanoparticles were observed, which was very different from the AuNFs using NAAN as reducing agent. The difference between NAAN and ANI is the existence of pendant amidine group at meta position of aniline for NAAN (Figure S6b,c). This affects the reaction in the following ways: (i) Steric hindrance of the amidine group reduces the polymerization rate of NAAN and reduction rate of HAuCl4. (ii) Presence of amidine group increases the nanoparticle solubility in water, and the electrostatic repulsion among the positive-charged amidinium partially decreases the possibility of the PNAAN nanoparticle formation. These conditions allow Au nanoplate growth and self-assembly to occur in a well-controlled manner. However, the reduction rate of HAuCl4 is higher using ANI as reducing agent, and the resulting small Au nanoparticles may directly migrate into water and bind together via the strong π−π stacking interaction of PANI, resulting in the eventual irregular aggregations. 3.4. SERS Performance of Individual AuNF. The AuNFs are expected to exhibit intense SERS performance owing to the presence of a large number of sharp tips and edges from the individual Au nanoplates. More importantly, many gaps in the nanometer range are formed among the junctions of Au nanoplates during their self-assembly (inset of Figure 5a), which create more electromagnetic “hot spots” for SERS excitation.39 The SERS response of the AuNFs was evaluated using 4-MBA as a SERS reporter. 4-MBA was chosen for SERS quantification because it provides an intense SERS signal and strongly binds to the AuNF surface via Au−S bond. By dispersing individual AuNFs on a silicon wafer surface (Figure 5a), we were able to obtain the SERS spectra of individual AuNFs. The Raman peaks located at 1622 (ν(CO) of pbenzoquinone), 1573 (ν(CC) of quinonoid ring), 1515

4. CONCLUSIONS We have developed a novel method to form and self-assemble Au nanoplates into AuNFs making use of the interfacial reaction. In comparison with a one-phase reaction, the interfacial reaction provided a way to better control the Au nanoplate growth. The NAAN synthesized by us played dual roles as reducing agent and corresponding oxidized product PNAAN as capping agent. More importantly, the PNAAN polymer coated on Au nanoplate surface assembled them together to form AuNFs. This method provides a new concept to directly synthesize and self-assemble Au nanoparticles into 3D hierarchical structures and can be used to construct 3D nanostructure for other noble metallic ions that can be reduced by NAAN such as platinum (Pt) or palladium (Pd).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01015. FTIR and XPS spectra of AuNFs; SEM images of AuNFs after incubation with NMP; SEM images of aqueous 15571

DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573

Research Article

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products through interfacial reaction under slight shaking and one-phase reaction; photographs of interfacial reaction at varied NAAN/HAuCl4 ratio; SEM images of aqueous products via interfacial reaction conducted at different temperature; SEM images of interfacial reaction product using ANI as reducing agent; calculation of SERS enhancement factor (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Singapore Millennium Foundation for the research funding support for this work. We also thank Dr. Weiqing Zhang and Professor Xianmao Lu for the technical assistance in SERS measurements.

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DOI: 10.1021/acsami.6b01015 ACS Appl. Mater. Interfaces 2016, 8, 15567−15573