Self-assembly of large-scale gold nanoparticle arrays and their application in SERS
© Zhu et al.; licensee Springer. 2014
Received: 4 December 2013
Accepted: 20 February 2014
Published: 13 March 2014
Surface-enhanced Raman scattering is an effective analytical method that has been intensively applied in the field of identification of organic molecules from Raman spectra at very low concentrations. The Raman signal enhancement that makes this method attractive is usually ascribed to the noble metal nanoparticle (NMNP) arrays which can extremely amplify the electromagnetic field near NMNP surface when localized surface plasmon resonance (LSPR) mode is excited. In this work, we report a simple, facile, and room-temperature method to fabricate large-scale, uniform gold nanoparticle (GNP) arrays on ITO/glass as SERS substrates using a promoted self-assembly deposition technique. The results show that the deposition density of GNPs on ITO/glass surface increases with prolonging deposition time, and nanochain-like aggregates appear for a relatively longer deposition time. It is also shown that these films with relatively higher deposition density have tremendous potential for wideband absorption in the visible range and exhibit two LSPR peaks in the extinction spectra because the electrons simultaneously oscillate along the nanochain at the transverse and the longitudinal directions. The SERS enhancement activity of these GNP arrays was determined using 10-6 M Rhodamine 6G as the Raman probe molecules. A SERS enhancement factor as large as approximately 6.76 × 106 can be obtained at 1,363 cm-1 Raman shift for the highest deposition density film due to the strong plasmon coupling effect between neighboring particles.
Surface-enhanced Raman scattering (SERS) has been considered as a highly sensitive and convenient analytical tool to detect chemical and biological molecules [1–7]. SERS provides an extreme signal enhancement over traditional Raman spectrum intensity due to the effect of localized surface plasmon resonances (LSPR), which is an optical phenomenon arising from the collective oscillation of conduction electrons in a noble metallic nanostructure when the electrons are disturbed from their equilibrium positions [8, 9]. The plasmonic behaviors of the structures (e.g., the position of resonant peaks, transmission pass-bands, and the magnitude of the optical-field enhancement) are highly sensitive to their size, shape, composition, and surrounding medium [10, 11]. Moreover, the distance of sub-10 nm between neighboring noble metal particles is also an important factor that affects the amplification ability of SERS signal because the plasmonic electromagnetic field obtained from interparticle plasmon coupling, known as ‘hot spots’ or ‘hot junctions', is significantly larger than that obtained from isolated particles [12, 13].
To obtain tremendous SERS signal enhancement ability, numerous available approaches such as electron beam lithography (EBL) [14, 15], nanoimprintation [16, 17], nanosphere lithography (NSL) , mask-assisted deposition (MAD) , vacuum evaporation, and other strategies have been proposed to fabricate well-ordered or random nanostructures [20, 21], which composed of uniform noble metal (Au or Ag) nanoparticles. EBL method can completely control the formation, shape and size of the nanostructures for the design of metallic films with unique LSPR spectra but is too expensive for practical applications. Nanoimprintation, NSL, and MAD methods can provide large-scale uniform noble metallic structure array, but the preparation process is complicated and the gap between particles cannot be reduced to sub-10 nm. Vacuum evaporation should be a simple, low-cost, and large-scale approach to produce nanoisland array but it cannot control the shape of the nanostructures. Currently, the self-assembly method is widely used to fabricate highly large-scale-ordered two-dimensional noble metal particle films (Au or Ag) consisting of metal nanoparticles such as nanosphere, nanorod, nanocube, and nanotriangular on ITO/glass or Si substrates [22–26]. However, such self-assembly method usually require complicated preparation processes and special substrate surface modifications. Therefore, exploring a new simple method that directly assembles large-scale NMPs on a no-special surface-treated substrate is still a formidable challenge.
In this paper, we proposed a promoted self-assembly method for fabricating gold nanoparticle (GNP) arrays onto ITO/glass substrate surface. This method has advantages of being simple, room-temperature preparation, no special modification of substrate surface, and having the capability to tune the GNP deposition density through prolonging deposition process time. Furthermore, we find that after a deposition time longer than 6 days, gold nanochains appear. This kind of nanochains has two strong LSPR peaks because electrons simultaneously oscillate along the transverse and the longitudinal directions. Due to the stronger LSPR effect of the films, we use Rhodamine 6G as probe molecules to estimate the enhancement capability of the films to SERS signal. Finally, we demonstrate the excellent Raman signal enhancement on these metallic films and found that a SERS enhancement factor as large as approximately 6.76 × 106 can be obtained. Consequently, our experiment indicates that this facile self-assembly method may be a promising strategy to prepare large-scale, inexpensive, highly sensitive SERS substrates.
Chemicals and reagents
Gold chloride trihydrate (HAuCl4 · 3H2O, >99.9%), sodium borohydride (NaBH4, >96%), cetyltrimethylammonium bromide (CTAB, >99.0%), polyvinyl-pyrrolidone (PVP, >99.0%), and ascorbic acid (AA, >99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. Ultrapure water (resistivity >18.0 MΩ cm) was used throughout the experiments.
Preparation of GNP solution
The GNP solution was prepared using a modified seed-mediated approach according to Murphy's method [27, 28]. The synthesis process of GNP solution contains two steps: seed synthesis and particle growth process. In a typical procedure, the gold seed solution was prepared by the addition of a freshly prepared ice-cold 0.3 mL of aqueous 0.01 M NaBH4 solution into an aqueous mixture solution which composed of 0.125 mL of an aqueous 0.01 M HAuCl4 solution and 3.75 mL of an aqueous 0.1 M CTAB, followed by rapid inversion mixing for 2 min. The resulting seed solution was kept at room temperature (approximately 25°C) for 1 h before use. The growth solution was prepared by the sequential addition of 8 mL of 0.1 M CTAB, 1 mL of 0.01 M HAuCl4, and 3 mL of 0.1 M AA into 38 mL of deionized water. Amount of 4.175 mL of the CTAB-stabilized seed solution was diluted to 10 mL by adding deionized water, and then 10 μL of the diluted solution was added into the growth solution. The resulting solution was gently stirred using a magneton for 10 s and then left undisturbed overnight. After finishing the growth process of the GNP solution, the solution was centrifuged at 14,000 rpm for 10 min and then redispersed into the deionized water to reduce the concentration of redundant reactants in the solution.
Self-assembly of large-scale GNP arrays on ITO/glass substrate
Large-scale GNP arrays on ITO/glass substrates were prepared according to the modified method we have previously reported . In a typical process, 2 mL of 0.01 M PVP and 1.5 mL of 0.1 M AA were added into the GNP solution and stirred strongly subsequently. Four pieces of ITO/glass substrates, treated by detergent, acetone, and deionized water in sequence, were immersed into the modified GNP solution for 2, 4, 6, and 8 days respectively. Thus, large-scale GNP arrays were consciously assembled onto the surface of the ITO/glass substrates with the different deposition density. Compared with the previous works, the main feature of this self-assembly method is to be able to directly deposit gold nanoparticles on ITO/glass substrates without special substrate surface modifications, while in previous work, GNPs usually assembled on the substrate surface functionalized with aminopropyltriethoxylsilane [23, 25], hydrofluoric acid, or C18 alkyl chains . In addition, another advantage of this method is that the density of GNPs deposited on ITO/glass can be conveniently tuned by deposition time. The four substrates with self-assembled GNP arrays (deposited for 2, 4, 6, and 8 days) are denoted as samples A, B, C, and D.
Preparation of SERS substrates
Rhodamine 6G (R6G) was used as the probe molecular for Raman detection. Amount of 10 μL of 10-6 M R6G (in ethanol) was dropped onto the surface of the four samples with different GNP arrays, respectively, and blow dried for SERS measurement.
Characterization of materials
The morphological features of the GNPs in solution and GNP arrays on ITO/glass substrates were characterized by Tecnai G2 transmission electron microscope (TEM) and Quanta 400 FEG field emission scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA). Samples for TEM were prepared by placing a drop (approximately 5 μL) of GNP solution onto a carbon-coated copper grid and dried at room temperature. Extinction spectra were collected on a UV–vis-near-infrared spectrophotometer (UV2100). Raman spectra were obtained using a confocal microprobe Raman system (HR 800) equipped with a holographic notch filter and a CCD detector. A long working distance × 50 objective was used to collect the Raman scattering signal. The laser beam size focused on the samples is 1.5 μm in diameter. An Ar laser (514 nm) and a He-Ni laser (633 nm) were used for the excitations.
Results and discussion
Synthesis and characterization of the GNPs
Micro-morphological and optical characterization of the self-assembled GNP arrays
SERS analysis of the self-assembled GNP arrays
Enhancement factor of R6G in each structure under 514 nm excitation
9.32 × 105
2.05 × 106
3.44 × 106
5.83 × 106
Enhancement factor of R6G in each structure under 633 nm excitation
8.92 × 104
4.04 × 106
6.76 × 106
In summary, we report a simple method to assemble gold nanoparticle arrays on ITO/glass substrates using two chemical ingredients of PVP and ascorbic acid. The micro-morphology varies with the increase of deposition time, and nanochain structures appear when prolonging to 6 or 8 days. These nanochain structures exhibit high Raman signals of R6G due to the strong LSPR effect in the sub-10-nm gap regions. This design of GNP arrays with a highly sensitive SERS-active property may provide a new framework for the fabrication of large-scale SERS-based sensors.
This work is supported by NSFC under grant nos. 61307066 and 21173041, Doctoral Fund of Ministry of Education of China under grant numbers 20110092110016 and 20130092120024, Graduate Innovation Program of Jiangsu Province under grant no. CXLX_0114, Natural Science Foundation of Jiangsu Province under grant number BK20130630, the National Basic Research Program of China (973 Program) under grant number 2011CB302004, and the Foundation of Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology, Ministry of Education, China under grant number 201204.
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