Near-Infrared Surface-Enhanced Raman Scattering (NIR SERS) on Colloidal Silver and Gold

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Abstract
Near-infrared surface-enhanced Raman scattering (NIR SERS) was detected with high sensitivity for two model compounds, crystal violet and the DNA base adenine, adsorbed on silver and gold colloidal particles in aqueous solutions. The NIR SERS spectra were measured with a fiber-optic probe with the use of a Ti:sapphire laser at 850 nm as the excitation source and a charge-coupled device (CCD) as the detection system. In a step to achieve the optimal electromagnetic SERS-enhancement conditions, silver and gold sols were modified to shift their extinction spectra more to the NIR region. Surface enhancement factors for both crystal violet and adenine were estimated to be on the order of 106 and 108 for gold and silver, respectively. A larger enhancement factor was observed for adenine with NIR excitation than with visible excitation. The benefits of NIR SERS and its potential applications are discussed.