Synthesis and SERS Application of SiO2@Au Nanoparticles

Abstract

In this letter, we report a chemical route for synthesizing SiO 2 @Au core-shell nanoparticles. The process includes four steps: i) preparation of the silica cores, ii) grafting gold nanoparticles over SiO 2 cores, iii) priming of the silica-coated gold nanoparticles with 2 and 10 nm gold colloids and finally iv) formation of complete shell. The optical extinction spectra were experimentally measured and compared to numerical calculations in order to confirm the dimensions deduced from SEM images. Finally, the potential of such coreshell nanoparticles for biosensing was probed by means of Surface Enhanced Raman Scattering measurements and revealed higher sensitivities with much lower gold quantity of such core-shell nanoparticles compared to Au nanoparticles exhibiting similar diameters.SiO 2 core-Au shell nanoparticles have attracted much attention for the past ten years due to potential applications in various fields such as cancer imaging and treatment (Loo et al., 2004), Surface-Enhanced Raman Spectroscopy (SERS) detection of molecules (Wang et al., 2006, Maurer et al., 2013, catalytic degradation of environmental pollutants (Ma et al., 2009) or even fabrication of SPASER (Surface Plasmon Amplification by Stimulated Emission of radiation) devices when the silica is dye-doped (Stockman, 2008). However, the growth of a complete gold shell around silica nanoparticles is very hard to achieve. Recently, most reports have been limited to the grafting of many Au small colloids onto silica cores (Hiramatsu and Osterloh, 2003, Osterloh et al., 2004, Shi and Asefa, 2007, Zhang et al., 2007. In this case, the more Au colloidal particles surround silica cores, the more red-shifted are their Localized Surface Plasmon Resonances (LSPR), which are typically between 500 nm and 550 nm. The optical signature of a complete gold shell depends in fact on the SiO 2 core diameter. When the SiO 2 cores are large enough (typically larger than 150 nm), the completion of the gold shell is evidenced by the apparition of two LSPR modes in the visible and near-infrared range (Oldenburg et al., 1998), which both blue-shift when the gold shell thickness increases. However, when the SiO 2 cores exhibit lower diameters, only one LSPR peak can be observed in the extinction spectra, which also blue shifts with larger shell thickness. Surprisingly, it seems that there has been no successful report of complete shell growth since the Halas and coworkers study fifteen years ago (Oldenburg et al., 1998). The basics of the synthesis process was indicated but not detailed: i) synthesis of the SiO 2 cores via the Stöber method (Stöber et al., 1968), ii) grafting of previously prepared very small gold colloids (Loo et al., 2004) via organosilane molecules and iii) growth of additional gold onto the colloids via chemical reduction. Therefore, the complete gold shell synthesis around silica cores is far from being easy to achieve.In this paper, we report a detailed process that allows growing a complete gold shell around silica cores. This process presen...

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Toward Ultrathin Gold Nanoshells: Exploring Seed Density, Polymer Additives and Microwave Radiation Effects

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