Integrated Optics Evanescent Wave Surface Enhanced Raman Scattering (IO-EWSERS) of Mercaptopyridines on a Planar Optical Chemical Bench: Binding of Hydrogen and Copper Ion

Abstract

Chemical interactions at molecular interfaces have been studied by integrated optics evanescent wave surface-enhanced Raman spectroscopy (IO-EWSERS). We describe methods to decorate planar glass waveguides with two-dimensional aggregates of Rayleigh limit silver particles by covalent attachment of them through a propane thiolate moiety grafted to the glass surface in a prior step. The evanescent field of the propagating transverse electric waves is enhanced by coupling with surface plasmon modes of the metal particles. Surface tethering and orientations of 2- and 4-mercaptopyridine (MPy) on Ag particles are studied through a comparative analysis of their SER spectra on silver metal liquid-like films (MELLFs), Ag hydrosols, and the colloid functionalized waveguide. Both molecules adopt roughly perpendicular orientations when bonded through the sulfur atom to silver. Overcoating the silver adlayer on the waveguide converts the construction into a sensing “optical chemical bench” (OCB). Accordingly, reversible acid−base titration of surface-bound 4-Mpy could be examined. In contrast, Cu2+ is coordinated irreversibly. X-ray photoelectron spectroscopy gives insight into the OCB fabrication and sensing process as the structure is evaluated step by step from a silica substrate surface to the copper ion at the outermost layer. The EWSER experiment is sensitive to perturbations of the vibrational structure of the heterocycle adsorbate at sub-monolayer coverage. The experiment can be conducted without photodegradation at relatively low laser power on inexpensive waveguides.