Subwavelength metal nanogap surface for surface-enhanced Raman spectroscopy

Abstract

Over the last 40 years since its discovery, Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical technique in various fields of interest such as chemistry, biology and other scientific areas, which provides very large Raman signals of molecules/adsorbates on (or in close proximity to) metal surfaces. SERS is an ultrasensitive detection technique that combines all advances of Raman spectroscopy; for example, provide molecular structural information, as well as surface chemistry of adsorbates on metallic nanostructured surfaces. The widely accepted mechanism of surface-enhanced Raman scattering is electromagnetic mechanism, which is based on extremely large electric field due to highly localized surface plasmon resonances created in metal nanogaps. Making good SERS/plasmonic substrates is a key step toward SERS as a standard analytical technique. Our research group recently reported metallic nanowire array surfaces with tunable sub-20 nm separation nanogaps that are uniform and highly dense hotspot sites over large area (using SEM, AFM and TEM imaging), which was presented in Chapter 3. These substrates were then characterized by using reflectance measurements, which showed that these surfaces are optically uniform and act as very good nanoantennas with which the surface plasmon resonance can be easily and precisely tuned within visible wavelength range of interest; the characterization of these substrates used surface-enhanced Raman spectroscopy was carried out using benzenethiol self-assembled monolayer (SAM) as probed molecules, which showed the reproducible and largely enhanced Raman signal, the average enhancement factor reached up to approximately 107, as reported in Chapter 2 and Chapter 4. Another good point for this structure that the silicon nitride templates on silicon wafer created by electron beam lithography, which used for generating the metal nanowire arrays, can be used multiple times by replacing metal thin films (Chapter 3). The developed SERS substrates were applied to single molecules detection of pairs of pyridine/deuterated-pyridine using the bi-analyte method; access to surface conformation characteristics of SAMs, as described in Chapter 2; and studying the well known SERS background continuum, which was described in detail in Chapter 5 with respect to nanostructured gold surfaces and a red laser source. In collaboration with the Analytical Biochemistry Group at the University of Groningen, an integrated microfluidic SERS spectroelectrochemical analysis system for studying Hemin-modified electrodes, this small volume SERS spectroelectrochemical cell can further be utilized to access to the orientation of molecules, or monitor surface reaction dynamics and surface conformation, as presented in Chapter 6. Furthermore, it is commonly known that only molecules in the hotspot regions in nanogaps that mainly contribute to the SERS signals, about 1% of entire surface coverage in this case, the SERS enhancement factor is estimated up to 109 when the surface plasmon resonance is tuned to coincide with excitation laser wavelength, this allows regular SERS measurements, not resonant SERS, and therefore these substrates can be used for wider range of molecules of interest and make the measurements more simply, as seen in Chapters 3, 4 and 6.