Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering
Introduction
Surface-enhanced Raman scattering (SERS) technique is a powerful analytical tool in the fields of surface science [1], electrochemistry [2], [3], biology [4], [5], and materials research [6]. The excellent sensitivity and selectivity of SERS allow for the determination of chemical information from single monolayer on planar surfaces and extend the possibilities of surface vibrational spectroscopy to solve a wide array of problems. To obtain appreciable enhancement of Raman scattering, a certain metal surface with both large scale and atomic scale roughness is necessary, so appropriately prepared substrates are essential for the enhancement of SERS signal.
The most commonly used substrate is silver because it has suitable plasmon resonance frequencies and adsorption characteristic to give effective enhancement with visible excitation. A great deal of effort have been put into performing methods of forming silver surfaces to obtain strong SERS signals. The most commonly used SERS-active substrates are aggregated silver colloids. However, the stability and the reproducibility of the aggregated colloids are two major problems. Another commonly used substrate is a roughened silver electrode [7], [8], [9], [10]. Although this substrate is more stable than colloids, it is not as sensitive. In our previous study, we used a simple method for preparing a SERS active substrate [11]. An elaborately devised U-shape capillary device was used to form the Ag particle films quickly from the Ag particles at the air–water interface. The Ag particle film method for SERS shows high enhancement factor and high stability. Recently, we reported a new method for preparing Ag colloid and SERS-active solid substrate [12]. The Ag colloid was prepared by reduction of AgI with NaBH4 using HSCH2COOH as stabilizer and the Ag nanoparticles can be transferred from the colloid to the solid substrate by a self-assembled technique. The advantages of the new approach lie in the ease in the preparation of Ag particle films and in the fact that the surface morphology can be easily controlled by the different number of depositions.
In this paper, we will show a new SERS substrate derived from the enlargement of silver seeds directly on glass surface nanoparticles based on the colloid silver surface-catalyzed reduction of silver nitrate by citrate under light irradiation. The morphology changes of the silver nanoparticles were characterized by ultraviolet-visible (UV-vis) spectra, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The relationship between SERS intensity and the reduction time was investigated so that we can find the optimum conditions to get the largest enhancement. The SERS activity of the silver film was tested by using pyridine as a molecular probe; the enhancement factor (EF) was estimated according to the formalism of Tian and coworkers.
Section snippets
Sample preparation
AgNO3 (99.5%) was obtained from Wako Pure Chemical Industries, Ltd. NaBH4 (98%) was obtained from Sigma Chemical Co. Poly(diallydimethylammonium chloride) (PDDA) with medium molecular weight (200,000-350,000) was purchased from Aldrich Chemical Co. Inc. 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BVPP) was provided by another group of our lab [13]. Pyridine, H2SO4 and 30% H2O2 were obtained from Beijing Chemical Plant.
For the preparation of silver seeds, deionized water was deoxygenated by bubbling
Characterization of silver films
The procedure for the fabrication of silver films is outlined in Fig. 1. The time course of the formation of silver film on a quartz slide was investigated by UV-vis spectra. Fig. 2 shows the UV-vis spectra recorded from the quartz slide as a function of reaction time. Curves a–f correspond to the spectra recorded after 0–8 h of reaction, respectively. The increase in the integral intensity of the surface plasmon resonance with time indicates that the density of the silver nanoparticles on the
Conclusion
A method for enlargement of silver seeds directly on glass surface has been developed. Based on the colloidal Ag surface-catalyzed reduction of Ag+ by citrate under light irradiation, the silver seeds can grow into bigger nanoparticles with different sizes and shapes by varying the reduction time. The Uv-vis spectra and SEM results show that some triangular and hexagonal nanoparticles were formed at the first 3 h of reaction, with the reaction proceeding, the shapes became irregular and the
Acknowledgement
This research was supported by the National Natural Science Foundation (Grant Nos. 20473029, 20375014, 20173019, 20273022) of the People's Republic of China, Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), Program for New Century Excellent Talents in University (NCET) and the Innovative Scholars of Jilin University (2004CX035).
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