Self-assembly of gold nanoparticles to silver microspheres as highly efficient 3D SERS substrates
- Shouhui Chen†1, 2,
- Peng Huang†2,
- Zhihua Wang2,
- Zhe Wang2,
- Magdalena Swierczewska2,
- Gang Niu2,
- Daxiang Cui1Email author and
- Xiaoyuan Chen2Email author
© Chen et al.; licensee Springer. 2013
Received: 6 March 2013
Accepted: 17 March 2013
Published: 12 April 2013
Herein we report a simple, one-pot, surfactant-free synthesis of 3D Ag microspheres (AgMSs) in aqueous phase at room temperature. The 3D AgMSs act as supports to fix the gold nanoparticles (GNPs) in 3D space via the interaction between the carboxyl groups of GNPs and the Ag atoms of AgMSs. The ensemble of AgMSs@GNPs with high surface-enhanced Raman scattering (SERS) activity and sensitivity can be an ideal 3D substrate choice for practical SERS detection applications. The simple self-assembly strategy may be extended to other metallic materials with great potentials in SERS, catalysis, and photoelectronic devices.
Ensembles of inorganic nanoparticles, which display unique collective properties that are different from those of both the individual nanoparticles and bulk materials, are of much scientific and technological interest [1–5]. The ensembles have the following potential advantages: (1) to display new electronic, magnetic, and optical properties as a result of interactions between the excitons, magnetic moments, or surface plasmons of individual nanoparticles; (2) to improve the mechanical properties of composite materials; and (3) to allow multiple tasks to be performed simultaneously or in sequence. Typically, the self-assembly of noble-metal nanoparticles has attracted much attention because of their unique plasmon resonance and their tremendous applications in the area of optical waveguides , superlensing , photon detection , and surface-enhanced Raman scattering (SERS) [9–12]. Recently, the SERS effect based on noble-metal ensembles is of particular interest because of its extraordinary ability to detect a wide variety of chemical/biological species at extremely low concentrations even down to the single-molecule level .
Gold nanoparticles (GNPs) have been widely used as Raman active substrates because of their good biocompatibility and strong SERS enhancement [13–18]. However, it should be mentioned that the particles tend to aggregate during aging, which results in an unwanted reduction of the active surface area [19, 20]. To address this issue, the fixation of GNPs in one-dimensional (1D), 2D, or 3D spaces can avoid the aggregation of the particles as SERS substrates. Tsukruk et al. assembled GNPs onto 1D silver nanowires and 2D silver nanoplates to create bimetallic nanostructures as efficient single-nanoparticle Raman markers . Li et al. developed a 2D GNP monolayer film as SERS substrate by the self-assembly of nanoparticles at a liquid/liquid interface . Zhang et al. reported that GNPs dispersed on the grapheme oxide (GO, 2D) and reduced graphene oxide (RGO, 2D) supports exhibit excellent SERS and catalytic performance compared with the metal nanoparticles alone . Qian et al. prepared the self-assembled 3D-ordered GNP precursor composite (SiO2/GNPs) arrays as SERS nanoprobes . Choi et al. reported a highly ordered SERS-active surface that is provided by a 3D GNP array based on thermal evaporation of gold onto an indium tin oxide (ITO) surface through a nanoporous alumina mask . This SERS-active surface was applied to analyze the intracellular state. Therefore, the development of appropriate support materials to fix GNPs is very important in practical SERS detection applications.
Recently, 3D Ag microspheres (AgMSs), which contain special fine structure, large specific surface area, and micron-sized particles, have been applied as SERS substrates [19, 26]. For example, Zhao et al. prepared 3D AgMSs with nanotextured surface morphology by a simple, sonochemical, surfactant-free method. Due to their special structural features with nanoscale corrugations, the obtained 3D silver microstructures showed a structurally enhanced SERS performance . Zhang et al. developed hierarchical assemblies of silver nanostructures as highly sensitive SERS platforms by an acid-directed assembly method . Our group also used proteins  and microorganisms  as templates to synthesize AgMSs and hollow porous AgMSs, respectively. However, the controlled synthesis of AgMSs with clean rough surface is still a significant challenge.
Preparation of gold nanoparticles
Briefly, 50 mL (0.2 mg/mL) of chloroauric acid (Sigma-Aldrich) was heated to boiling point, and then 1.2 mL (10 mg/mL) of sodium citrate (Sigma-Aldrich) was added. Boiling lasted for 5 min until the solution became dark red in color. After cooling down to room temperature, 20 μL of GNPs was used for the analysis using transmission electron microscopy (TEM).
Zeta potential of the assemblies prepared at different molar ratios of Ag microspheres to gold nanoparticles
Typically, 2.5 mL of 5 mM AgNO3 aqueous solution was added to 95 mL of deionized (DI) water in a 150-mL beaker. Then, 2.5 mL of 5 mM l-AA (Sigma-Aldrich) was added into the above-mentioned solution under vigorous stirring at room temperature. The system was stirred vigorously under ambient conditions for 4 h. The color of the solution rapidly changed from colorless to gray. The resulting product was collected by centrifugation, washed three times with DI water and ethanol, and then dispersed in ethanol for further use.
Preparation of the assemblies of GNPs to AgMSs
AgMSs (10.8 mg) was dispersed in 0.9 mL of ethanol solution, then 100 μL of different concentrations of GNPs (0.4, 0.2, 0.1, 0.02, and 0.01 mg) were mixed with AgMSs solution under ultrasonic interaction, respectively. After 10 min, the resulting product was collected by centrifugation at 1,000 rpm for 5 min and washed twice with DI water and then dispersed in 1 mL DI H2O for further use.
Preparation of Raman samples
A total of 200 μL of GNPs to AgMSs (AgMSs@GNPs) was immersed in ethanol solutions containing 200 μL of 2-mercaptopyridine (2-Mpy) (10 to 7 M) under ultrasound for 10 min. After 2-Mpy molecules (Sigma-Aldrich) were adsorbed on the AgMSs@GNPs, the samples were washed twice with DI water and ethanol by centrifugation and finally dispersed in 10 μL ethanol. Then, an aliquot of 10 μL of 2-Mpy-loaded AgMSs@GNPs in ethanol solution was dropped onto a Si wafer. The dropped solution was spread evenly into a circle. After evaporation of ethanol under the dry N2, the sample was measured by a simple Raman instrument for six times. All of the experiments were carried out at room temperature.
The UV-visible spectra were recorded in a Shimadzu UV-2450 UV-visible spectrophotometer (Shimadzu Co. Ltd., Beijing, China) from 300 to 600 nm. DI water was used as the blank. SEM images were taken on a ZEISS-ULTRA 55 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). For TEM, a drop of aqueous solution containing the samples was placed on the carbon-coated copper grids and dried under an infrared lamp for 30 min. The micrographs were obtained using a JEOL JEM-2010 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV. Electron diffraction patterns were also recorded for the selected area. The surface charge of the samples was performed on NICOMP 380ZLS (Zeta potential/particle sizer; Agilent Technologies Inc., Santa Clara, CA, USA) system. SERS spectra of 2-Mpy-loaded AgMSs@GNPs were recorded by a simple Raman instrument (BWS415 B&W Tek Inc., Newark, DE, USA).
Results and discussion
In summary, we report a simple, one-pot, surfactant-free synthesis of 3D AgMSs in aqueous phase at room temperature. The 3D AgMSs act as supports to fix the GNPs in 3D space via the interaction between the carboxyl groups of GNPs and the Ag atoms of AgMSs. The ensemble of AgMSs@GNPs with high SERS activity and sensitivity can be an ideal 3D substrate choice for practical SERS detection applications. The simple self-assembly strategy may be extended to other metallic materials with great potentials in SERS, catalysis, photoelectronic devices, etc.
This work was supported in part by the Intramural Research Program (IRP), National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH), the National Key Basic Research Program (973 Project) (2010CB933902 and 2011CB933100), National 863 Hi-tech Project (2007AA022004), Important National Science & Technology Specific Projects (2009ZX10004-311), National Natural Scientific Fund (nos. 81225010, 20771075, 20803040, and 81028009), New Century Excellent Talent of Ministry of Education of China (NCET-08-0350), and Shanghai Science and Technology Fund (10XD1406100).
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