Engineering substrates for SERS : fundamentals and applications

Abstract

This work describes studies on sphere segment void (SSV) substrates for surface-enhanced Raman and resonant Raman scattering (SER(R)S) and their application in detecting and differentiating mutations in DNA sequences. SSV substrates are fabricated using the colloidal crystal templating strategy which has been optimized, improved and extended during this thesis work. Successful fabrication of substrates with a range of metals such as rhodium, nickel, platinum, palladium and gold was carried out although the bulk of the work presented in this thesis deals with gold SSV substrates. SSV substrates have unique optical and plasmonic properties, which can be manipulated controllably using the sphere size used for templating and film height during electrodeposition; allowing their tuning for SERS suiting a particular excitation laser. This tailoring of substrates for visible to infrared range of laser wavelengths is demonstrated establishing the plasmon engineering approach for optimum SERS with different media, metal and laser wavelengths. Not only the SERS peak intensities but also the spectral continuum (background) observed on SSV substrates has been found to be correlated to the strength of plasmons on the surface and nature of molecules adsorbed on the surface. The substrates have also been found to be suitable for SERRS with enhancements ~ 103 stronger than for SERS. All these studies permitted the development of a biosensing application based on SER(R)S. Using gold SSV substrates a SER(R)S application for detecting DNA sequences has been developed wherein the stability and reusability of substrates as well as the extreme sensitivity of detection is demonstrated. The detection methodology has been extended further by developing a method for distinguishing mutations in DNA sequences by carrying out denaturation (melting) of surface-bound duplexes. The melting can be induced thermally or electrochemically and monitored by SER(R)S. With this method, termed SERS-melting, it has been possible to discriminate mutations using sequences in the C FTR gene with model synthetic oligonucleotides as well as with unpurified PCR amplicons. Flexibility in the choice of the label, which can be either non-resonant, partially resonant or resonant with the laser employed and electro active at the same time is demonstrated. The electrochemically induced SERS-melting method has been found to be dependent on pH and ionic strength of the buffer solution.

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