Gold nanoparticle thin films fabricated by electrophoretic deposition method for highly sensitive SERS application
© Zhu et al.; licensee Springer. 2012
Received: 3 September 2012
Accepted: 14 October 2012
Published: 6 November 2012
We report an electrophoretic deposition method for the fabrication of gold nanoparticle (GNP) thin films as sensitive surface-enhanced Raman scattering (SERS) substrates. In this method, GNP sol, synthesized by a seed-mediated growth approach, and indium tin oxide (ITO) glass substrates were utilized as an electrophoretic solution and electrodes, respectively. From the scanning electron microscopy analysis, we found that the density of GNPs deposited on ITO glass substrates increases with prolonged electrophoresis time. The films possess high mechanical adhesion strength and exhibit strong localized surface plasmon resonance (LSPR) effect by showing high SERS sensitivity to detect 1 × 10−7 M rhodamine 6 G in methanol solution. Finally, the relationship between Raman signal amplification capability and GNP deposition density has been further investigated. The results of our experiment indicate that the high-density GNP film shows relatively higher signal amplification capability due to the strong LSPR effect in narrow gap regions between the neighboring particles on the film.
Films composed of noble metal nanoparticles (typically, Au or Ag) currently have attained wide popularity and aroused intense research interest in nanotechnology due to the intriguing optical properties introduced by localized surface plasmon resonances (LSPRs). LSPRs, which are optical phenomenona arising from the collective oscillation of conduction electrons in noble metal nanoparticles when the electrons are disturbed from their equilibrium positions, lead to enormous optical local-field enhancement at the nanoscale and obtain potential applications in many fields such as chemical or biosensors[1–4], solar cell designs[5–8], and surface-enhanced Raman scattering (SERS)[9–11].
Due to the exceptional optical properties of noble metal nanoparticle films, many available methods for the fabrication of these films have been proposed in the past two decades. In order to obtain a strong LSPR effect, it is often necessary to engineer the particle deposition density on substrates for practical application because the deposition density can greatly affect the optical property of the films. The most recently available methods for the fabrication of these films include electron beam lithography and nanoimprint lithography, both can completely control the micromorphology of the nanostructures for the design with unique LSPR spectrum[13, 14]. However, these methods require sophisticated fabrication equipment and are limited by either expensive cost or small sample size in practical applications. Instead, some simpler bottom-up approaches based on self-assembly, e.g., Langmuir-Blodgett, dip coating, and electrochemical deposition, have shown great conveniency in large-scale fabrication and much less defectivity[15–19]. Such techniques can surely produce noble metal nanoparticle thin films with large areas, but the resulting films usually lack mechanical adhesion strength of nanoparticles to substrate materials required for device construction.
In this paper, we report a rapid, simple, and room-temperature electrophoretic deposition method using gold nanoparticle (GNP) sol as an electrophoretic solution to fabricate sensitive GNP films with high mechanical adhesion strength. It is found that the GNP deposition density on the films can be adjusted by changing the electrophoresis time, and the sub-10-nm gaps can be formed between the neighboring particles on the film for the relatively longer electrophoresis time. Finally, we demonstrate the excellent signal enhancement ability of these GNP thin films as substrates in SERS measurements and find that rhodamine 6 G (R6G) molecules can be detected at very low concentrations using these films.
Results and discussion
The mechanical adhesion strength of the GNP thin films was tested by ultrasonication in deionized water (>18.0 MΩ cm). For comparison, an extra sample was prepared by directly depositing a droplet of the GNP sol onto an ITO glass substrate, and GNPs remained on the surface of the substrate after evaporating the sol under ambient conditions. FESEM observations (not shown in figure) were carried out at the same positions on the substrates both before and after applying 5-min ultrasonication on the GNP thin films and the reference one. We found that the GNPs on thin films fabricated by electrophoretic deposition method were resistant to the ultrasonication process. The micromorphology had little change on the surface of the thin film after applying ultrasonication. In contrast, GNPs on the reference sample were not able to resist the ultrasonication process, and a retention rate below 35% was observed. This contrasting result indicates that the GNP thin films fabricated by electrophoretic deposition method possess superior mechanical adhesion strength.
Raman spectroscopy is not only a powerful analytical technique in composition analysis, but also a testing tool to examine the LSPR effect of GNP thin films. The Raman spectra have been intensively used in the field of identification of organic molecules from their vibration spectra at very low concentrations. In the past two decades, many studies have revealed that noble metal nanoparticle films with a strong LSPR effect can amplify the Raman signals in biological detection process[9–11]. In order to evaluate the Raman-enhancing capability of the GNP thin films fabricated by electrophoretic deposition method, various R6G solutions with different concentrations were prepared because they have been extensively studied in prior literatures[1, 24]. R6G is a strongly fluorescent xanthene derivative that is a yellowish heterocyclic compound and shows a molecular resonance Raman effect when excited into its visible absorption band. In this experiment, a confocal Raman microscope/spectrometer with a × 50 objective was used for the Raman measurements. An Ar laser with a wavelength of 514 nm was employed for the excitations. The laser power focused on the samples was measured to be 1.5 mW/cm2. The laser beam size focused on the samples was 2 μm in diameter, and the acquisition time was 10 s. For comparison, R6G in methanol solutions with different concentrations were dropped onto GNP thin films and bare ITO glass substrates with no nanoparticles. Raman spectra were recorded after drying the solvent. For bare ITO glass substrates, the resulting spectra show no discernable Raman peaks of R6G molecules at the concentration of 10−4 M (not shown in figure). Until the concentration increased to 10−3 M, a few weak peaks can be observed. Therefore, when the R6G concentration is equal to or less than 10−4 M, peaks associated to R6G molecules cannot be detected in the Raman measurement.
In previous works[9, 11], a plasmon resonance theory is widely used to explain SERS enhancement. In this theory, the LSPR effect of metal nanoparticles plays an important role in Raman signal amplification because the electromagnetic field strength on the metal particle surface becomes strongest when LSPR is excited. Metal films that serve as SERS substrates often need high particle deposition density to form ‘hot spots’ in the narrow gap regions between neighboring particles, which can harvest high electromagnetic field to enhance the Raman signal. From Figure3d, one can find that many narrow gaps with sub-10-nm distance are clearly formed between neighboring particles on the film. These regions can produce a strong LSPR effect on the surface of the thin film when GNPs are excited by the incident light and enable the Raman characteristic peaks of R6G at a very low concentration to be easily detected. Compared with the previous works[17, 27], Raman signal enhancement is also observed in similar GNP films because of the presence of narrow gaps. For instance, in, R6G at an extremely low concentration of 10−9 M was successfully identified using nanosphere arrays with sub-10-nm gaps as sensitive SERS substrates.
In summary, a rapid, simple, and room-temperature electrophoretic deposition method has been proposed to fabricate GNP thin films with high mechanical adhesion strength. Sub-10-nm gaps between neighboring particles have been formed when GNP deposition density increases. The films made by this method exhibit a high Raman signal due to the strong LSPR effect, which is produced in the narrow gap regions. This design of GNP thin films with a highly sensitive SERS-active property may provide a new framework for the fabrication of large-area SERS-based sensors.
This work is supported by NSFC under grant nos. 60977038 and 21173041, the Doctoral Fund of Ministry of Education of China under grant no. 20110092110016, Graduate Innovation Program of Jiangsu Province under grant no. CXLX_0114, and the National Basic Research Program of China (973 Program) under grant no. 2011CB302004.
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