Area-selective hydrophobicity modification of plasmonic surfaces for enriched SERS analysis

Abstract

Detection of trace amount analytes is essential for the inspection of environmental contaminants in water systems. Various chromatographic and spectroscopic methods have been used for detecting pollutants, but surface-enhanced Raman spectroscopy (SERS) stands out for the obtainment of molecular-specific signals through rapid and non-destructive processes. Diverse nanostructures and fabrication methods have been reported for detection up to attomolar concentration. Yet, trace-amount detection remains problematic because the delivery of sufficient amounts of analytes to the proximity of “hot spots” is required for sufficient enhancement effects. That is, a lack of molecules near a given dense region of “hot spots” may hinder the identification of weak Raman signals within spectra of low signal-to-noise ratios. Therefore, a platform that gathers analytes dispersed over a large area to a specific target region is in need to obtain signals from a greater number of molecules near the same group of “hot spots.” Here, we introduce an area-selective hydrophobicity-modified SERS substrate for focusing analyte molecules towards a small-area hydrophilic pinning point during evaporation of water-based solvents. An Ag nanowire (AgNW) substrate of cross-stacked parallel arrays of sub-20 nm spaced nanowires is functionalized with 1H,1H,2H,2H-perfluorodecanethiol for increased hydrophobicity while the substrate center is coated with SiO$_2$ for hydrophilicity. Raman measurements at the hydrophilic center show peaks of molecules at extremely low concentrations that are difficult to detect on bare AgNW substrates with up to ten-fold increase in peak intensities.