Electrophoretic preconcentration for highly intense surface-enhanced raman spectroscopy using plasmonic nanopillar arrays

Abstract

Surface-enhanced Raman spectroscopy (SERS) is a surface-sensitive spectroscopic technique based on Raman scattering, one of the inelastic scatterings of photon depending on molecular vibrational, rotational, and other low frequency modes. Recently, many researchers have tried to get highly intense SERS signal due to many unique features of SERS; non-destructive detection, label-free detection, and easiness to identify an individual molecule that emits Raman signal depending its energy states. However, many biomolecules such as neurotransmitters are still hard to measure SERS signals due to their low Raman activity. Raman signal can be strongly enhanced near hot spots of localized surface plasmon resonance (LSPR), collective oscillation of free electrons between metal and dielectric surfaces. Highly intense SERS signals can be achieved by SERS substrates with rich hot spots, LSPR tuning, and alignment of molecules near the hot spots. Among those approached, the alignment of molecules near hot spots can be effectively conducted with electrokinetic preconcentration, which can be done by applying electric fields between two electrodes in solution. However, metal nanostructures on top of a conductive layer have much smaller LSPR intensity due to less confinement of free electrons in the nanostructures. This work presents electrophoretic preconcentration on plasmonic nanopillar arrays for highly intense SERS. This SERS substrate is featured as nanogap-rich 3D nanostructures and strong confinement of free electron in metal nanoislands, which results in enhancement factor over 109 with high uniformity. Also, highly intense SERS device including microfluidic reservoir and top and bottom electrodes locally concentrates bio-molecules near hot spot rich nanostructures to enhance SERS signals of biomolecules with extremely low Raman activity. SERS signals were measured under applied electric field using spectrometer coupled with inverted confocal laser scanning microscope ...