Electromagnetic Effect Of Surface Enhanced Raman For Bioanalytical Applications

According to electromagnetic theory of SERS, the enhancement factor is proportional to d-12 for a single molecule located at distance d from the surface of substrate, and also the aggregation of nanoparticles could give stronger SERS signals than those isolated particles. The paper presented here is focused on nanometal surface energy transfer, cocaine detection by using aptamer and synthesis of gold-aggregated, dye-embedded, polymer-protected nanoparticles and described as follows:1. The validity of fluorescent behavior of rhodamine 6G at discrete distance from Ag@Au core-shell nanoparticle surface which follows a d-4 distance dependence was investigated by Surface Enhanced Raman Scattering (SERS). The result is in contradiction to the literature;2. The proof-of-principle of a reagentless aptameric sensor based on surface-enhanced Raman scattering (SERS) spectroscopy with"signal-on"architecture using a model target of cocaine was reported. This novel aptameric sensor was based on the conformational change of the surface-tethered aptamer on binding target that draws a certain Raman reporter in close proximity to the SERS substrate, thereby increasing the Raman scattering signal due to the local enhancement effect of SERS. To improve the response performance, the sensor is fabricated by a cocaine-templated mixed self-assembly of 3'-terminal tetramethylrhodamine (TMR)-labeled DNA aptamer on a silver colloid film via an alkanethiol moiety at 5'end. This immobilization strategy optimizes the orientation of aptamer on the surface and facilitates the folding on binding target. Under optimized assay conditions, one can determine cocaine at a concentration of 1μM, which compares favorably with analogous aptameric sensors based on electrochemical and fluorescence techniques. The sensor can be readily regenerated through washing with a buffer. These results suggested that the SERS-based tranducer might create a new dimension for future development of novel aptameric sensors for sensitive determination in biochemical and biomedical studies;3. Gold-aggregated, dye-embedded, polymer-protected nanoparticles (GDPNs) are unusual aggregates that involve charge-driven aggregation of gold nanoparticles/poly(l-lysine) functionalized with Raman dye and a poly(l-lysine) (PLL) outer layer. Discrete, submicrometer-sized spherical aggregates are formed through three-step layer-by-layer assembly of polymer/nanoparticle. Unlike most of traditional Raman tags, aggregative gold nanoparticles are used in GDPNs as the active substrate for increasing the signal of surface enhanced Raman scattering (SERS). Furthermore, the outer PLL layer could lock in Au NPs modified with rhodamine B isothiocyanate (RBITC) and lock out external dye molecules. Specifically, PLL as the outer layer exposes an amino-group rich surface to the surrounding environment that should facilitate the conjugation of GDPNs with biomolecules (antibody/antigen, nucleic acid). These advantages suggest the potential of using GDPNs as Raman tags for multiplex and ultrasensitive detection of biomolecules. The Raman tags were characterized using transmission electron microscopy (TEM), UV-Vis absorption spectrometry, and Raman spectrometry.