Silver nanoparticle aggregates: Wavelength dependence of their SERS properties in the first transparency window of biological tissues

Abstract

Silver nanoparticle aggregates are very common substrates for surface enhanced Raman scattering (SERS) due to their simplicity of preparation and good optical enhancement. In this paper, their SERS properties are investigated from 680 to 920 nm, region that spans most of the first transparency window of biological tissues.Our experiments show that the SERS enhancement extends as far as 920 nm, without a significant wavelength dependence: the observed very broad optical response has been interpreted with the formation of large sized aggregates, possibly possessing a partial fractal character. On the other hand, the SERS signal, normalized by the laser power and by the integration time, quickly diminishes from 680 to 920 nm. This happens because the SERS signal depends on the enhancement but also on the instrument sensitivity and on the molecular cross section: for dispersive Raman instruments equipped with silicon CCD detectors and for non resonant molecules, these two parameters tend to diminish towards longer excitation wavelengths.The relevance of these results is twofold. a) Simple silver nanoparticle aggregates are good plasmonic substrates in most of the spectral region in which biological tissues present low absorption. b) The enhancement of the substrate and the normalized SERS signal (that is related to the limit of detection) are not necessarily correlated: they carry complementary pieces of information, both needed for properly designing a SERS experiment.