Screening of polymer coatings for surface plasmon resonance sensing of ammonia vapor

Abstract

Several polymeric membranes were evaluated for their potential to improve the sensitivity and impart chemical selectivity to surface plasmon resonance (SPR)-based sensors. The membranes tested encompass a variety of deposition methods, providing an insight of the contact requirements between polymers and the plasmon supporting metal. Among the membranes evaluated, preliminary results utilizing polyelectrolyte multilayer membranes displayed reliable detection of vapor-phase ammonia at ~40 ppm levels. Chemically synthesized polyaniline also presented encouraging results, responding to ammonia gas at 48 ppm. This is in sharp contrast to the electropolymerized counterpart, which showed minor wavelength shifts even at elevated ammonia levels (4 %). SPR has been adopted by the bioanalytical community to probe biomolecular interactions and obtain information relating to binding kinetics. Similarly, modifying plasmon-supporting surfaces with bioreceptors enables access to biosensing applications. Gas-phase sensing with SPR has largely remained unexplored primarily due to the small changes in refractive index from low molecular weight molecules. Coating SPR sensors with tailored polymers has been discussed as a viable approach to amplifying refractive index changes related to low molecular weight analytes. Ammonia is a low molecular weight analyte that is ubiquitously present in the gas phase. Industrial and medical interest in ammonia at low ppm level yielded numerous scientific contributions describing diverse sensing approaches. Hence, ammonia is a good candidate to provide a baseline for immediate comparison with other approaches for evaluation of the polymers with regards to their susceptibility to undergo changes in dielectric properties and chemical affinity for the analyte.