Light-activated Single Molecule SERS Substrates

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CBET-0756791, KalkanThe proposed research will develop a high throughput and reliable SM-SERS substrate on the basis of an effect recently discovered by the PI: light-induced electrochemical Ostwald ripening (LIECOR) of metal nanoparticles on a nanotextured semiconductor. LIECOR will be exploited for the creation of nanoscopic gaps in between nanoparticles which are giant electromagnetic enhancement sites. The proposed effort aims at overcoming the current reliability and low throughput issues in SERS by relying on 3 key features: surfactant-free nanoparticles permitting quick analyte adsorption; creation of the hot spots by the Raman laser at the onset of the spectral acquisition so that minimum contamination and oxidation are allowed at the hot spots prior to SERS; self-inhibiting feature of LIECOR yielding uniformity. The specific goals targeted and how they will be addressed are as follows. 1) Elucidation and control of the LIECOR. Possible enabling or driving mechanisms like ?photoconductance? and ?formation of nano-photocells? will be tested experimentally. The impact of semiconductor dark conductivity and surface defect density, as well as laser intensity will be studied. Confocal transmission spectroscopy will be employed at the LIECOR micro-spot. In particular, LIECOR kinetics will be monitored real time from the spectral changes in nanoparticle plasmon modes for a set of laser intensity levels. 2) Searching for experimental evidence whether surface-enhanced optical forces expedite molecules find hot spots. Transient SERS due to sudden changes in laser excitation (e.g., on/off and change of polarization) will be explored (after the LIECOR is complete). Such transients suggest molecules concentrate around hot spots under the action of optical forces. In addition, millisecond-resolved SM-SERS will be carried out to reveal any optically-biased surface diffusion on the nanoparticles. 3) Demonstration of the utility of the SM-SERS substrates in biophotonics. The SM-SERS substrates will be employed to elucidate the difference in the structures of photo-active yellow protein between the receptor state and the signaling state at the single molecule level. 4) Development of SERS substrates, which are selective for cationic or anionic molecules by the control of metal nanoparticle charge.