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

Abstract

Surface enhanced Raman spectroscopy (SERS) is a very powerful tool for the identification of chemical and biological species, owing to its recognition capabilities, typical of Raman spectroscopy, and to its high sensitivity arising from the field amplification that occurs at the surface of suitable plasmonic substrates upon excitation.[1,2] In this seminar, we shall provide an overview of the main characteristics and applications of SERS spectroscopy; then, we shall delve into a more specific topic, which is how the SERS enhancement of silver nanoparticle aggregates varies as a function of the excitation wavelength, in particular in the first transparency window of biological tissues. Our experiments show that the SERS enhancement of silver nanoparticle aggregates extends from 680 to 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.[3] 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 detection experiment.