Improving Stability and Sensitivity of SERS-Based Biosensors

Abstract

Detection and monitoring of disease biomarkers increases probability of successful disease treatment. Among available readout strategies utilized in biosensors - surface enhanced Raman scattering (SERS) has several advantages over conventional techniques. In particular, multiplexing capabilities of SERS readout is a very attractive feature. Our improvements to SERS readout include protection of SERS labels and temperature assisted plasmonic coupling of DNA modified AuNP contributing to better reproducibility and higher sensitivity of biosensors. We have introduced several protection strategies to overcome SERS signal deterioration under prolonged exposure to intense laser light. Simple protection strategy involves using either polymer or graphene monolayer as a thin protective layer applied on top of the assay that renders signal to be more stable against photo-induced damage. Another protection strategy involves coadsorption of thiolated polyetheleneglycol (PEG) together with Raman reporter molecule (RRM): 4 nitrobenzenethiol (NBT) onto AuNP. These protection strategies improve stability of the assay measured as the time dependence of the Raman signal intensity at 1336 cm−1 (the most intense band for NBT) on duration of sample exposure to laser light. Protection also reduces RRM desorption from the surface of AuNP, photodamage, and catalytic photoconversion of NBT to diazobenzene. We discuss possible mechanisms of such a conversion and effectiveness of PEG protection against it. We have evaluated the size and amount of PEG molecules for optimal protection of SERS labels. Higher sensitivity of the biosensor was achieved via DNA assisted coupling between individual nanoparticles. Our design allowed for controllable plasmonic coupling which resulted in further several fold enhancement of Raman signal, thus improving sensitivity of the biosensor. Practical applications of SERS-based biosensor for in vivo and multiplex detection of biomarkers are also discussed.