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Dynamic preconcentration of gold nanoparticles for surface-enhanced Raman scattering in a microfluidic system.

As considerable attention was recently paid to label-free technology in bioanalysis field,[1–3] analytical techniques employing no label such as surface plasmon resonance,[4] piezoelectrics,[5] cantilevers,[6] and transistors[7] have been increasingly suggested. Surface-enhanced Raman scattering (SERS) is one of the promising candidates for label-free analysis because it can provide molecular fingerprint information in the aqueous phase. It is attractive in that it provides sufficient enhancement, as high as 1010 times that of normal Raman signals, allowing quick detection of low-concentration analytes. Currently, SERS is expected to enable even realtime monitoring of molecular secretions in biological systems[8,9] or intermediate species temporarily present during chemical/electrochemical reactions without labeling.[10,11] SERS, employing visible incident light, is observed at nanoparticles, nanoshells, and the rough surfaces of a few metals like Au, Ag, or Cu, as well as narrow interstices among them (“hot spots”). Since SERS originates from nanostructures per se, its intensity is sensitive to the geometry, size, and shape of the substrates.[12] Sophisticated engineering of nanomaterials became a hot issue in nanoscience and in due course has driven significant recent progress in the development of better SERS substrates.[13] As a consequence, many metallic nanostructures with highly elaborate morphologies have been reported, such as Au nanorods[14] and Ag