New plasmonic sensor platform using a spatially controlled photochemical reaction and enhanced fluorescence

Abstract

A plasmonic chip with a wavelength-size periodic structure coated with silver and silica layers has been applied to sensitive fluorescence observation under a microscope. In this study, to achieve more highly sensitive fluorescence detection in a spatially controlled area, an electric field enhanced with the Bull’s eye-type plasmonic chip was applied to not only the fluorescence enhancement but also the photochemical reaction field. Detecting the fluorescence signals simultaneously in-and-out of areas exposed to UV light in the single chip can improve the detection sensitivity and the accuracy. In order to construct the new sensor chip based on this concept, the surface of the chip was modified with a photoresponsive molecule 3-(2-formyl-3-methylphenyl)thio)propanoic acid (O-MBA). A plasmonic area comprising Bull’s eye patterns modified with O-MBA was exposed to ultraviolet [UV] light in the transmitted light system with an upright-inverted fluorescence microscope, and the plasmonic chip enhancement factor for the fluorescence (Ep) and the photochemical reaction promotion rate (Rp) were individually evaluated using a Cy5-Maleimide (Cy5-M), in order to indicate the essential functionality of our chip in the point of the new sensor chip. In Cy5-M, a maleimide substituent can react with intermediate of O-MBA formed photochemically and a fluorophore of Cy5 makes a roll of an indicator for extent of reaction. As the experimental results, Ep was determined to be 12.8, and the Bull’s eye pattern was found to have a twofold Rp compared with the nonpatterned area of the flat metal. The total enhancement factor above 20 and the spatial control of the area promoting a photochemical reaction will produce a highly sensitive biosensor platform.