Development of Chemical Sensing Platforms Based on Sol−Gel-Derived Thin Films: Origin of Film Age vs Performance Trade-Offs

Abstract

We present a detailed investigation on the evolution and performance of sol−gel-derived thin films as used for chemical sensing platforms. In order to develop an understanding of how the sol−gel matrix affects the entrapped recognition chemistry and determine how and whether the analyte interacts with the sensing element, we have chosen to investigate a simple model probe−analyte system. Specifically, we use static and time-resolved fluorescence spectroscopy to report on the photophysics and O2 quenching of pyrene entrapped within sol−gel-derived thin films as a function of precursor form, processing conditions, and storage time. The results of this year-long study show that the analytical response of the pyrene-doped film/sensor to O2 decreases as a function of storage time. This response decrease results from two separate factors. First, the average bimolecular quenching constant decreases from (1.3−1.4) × 107 to (0.4−0.6) × 107 M-1 s-1 for fresh and 300-day-old films, respectively. Second, the average pyrene excited-state fluorescence lifetime, in the absence of quencher, decreases as a function of storage time. The simultaneous decrease in bimolecular quenching constant and average fluorophore lifetime are directly related to the change in analytical signal (i.e., response). These results demonstrate that single-component sol−gel-derived sensing platforms are unstable over time. However, we find that most of the observed instability occurs during the first month following film preparation.