We investigate acrylodan-labeled bovine and human serum albumin (BSA-Ac and HSA-Ac) entrapped within a tetramethylorthosilane-derived biogel composite. The effects of biogel aging and drying were studied by following the acrylodan steady-state and time-resolved emission, the decay of anisotropy, and the dipolar relaxation kinetics as a function of ambient storage time. The results indicate that there is a substantial amount of nanosecond and subnanosecond dipolar relaxation within the local environment surrounding cysteine-34 in both proteins, even when they are fully encapsulated in a dry biogel. Time-resolved anisotropy experiments show that the acrylodan residue and the protein are able to undergo nanosecond motion within the biogel. The semiangle through which the acrylodan can process is the same for a freshly formed biogel and the native protein in buffer. However, once the biogel begins to dry, the semiangle increases (approximately 20 degrees and 10 degrees for BSA-Ac and HSA-Ac, respectively). This suggests that the "pocket" hosting the acrylodan reporter group opens as the biogel dries.

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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.

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A new aerosol-based deposition method, for the production of sol−gel-derived films under ambient conditions, has been developed. Tetraethyl orthosilicate (TEOS) and N-octyltriethoxysilane (TrEOS-C8)-derived sol−gel-processed films were produced using the new technique and compared to films produced by a conventional spin-casting approach. All films were characterized using scanning electron microscopy, profilometry, electron spectroscopy for chemical analysis (ESCA), diffuse-reflectance infrared Fourier transform spectroscopy, and steady-state fluorescence spectroscopy. Sol−gel-derived films produced using the aerosol-based method were uniform, and their thickness could be controlled between 0.6 and at least 3.0 μm. Spin casting of the neat sol−gel-processed solutions generally yielded more thick (2.0 ± 0.10 μm) films, but these thicker films were of poorer optical quality and very often more highly cracked. ESCA data demonstrate surface segregation of the C8 moiety within the TrEOS−C8-derived films. This segregation phenomenon is much more pronounced in the aerosol-generated films. A scenario is proposed where the distribution of and/or the dynamics/solvation of the C8 residue within the aerosol droplet vs the bulk cast film are very different and lead to the segregation. Static fluorescence experiments demonstrate that several dopant classes can be incorporated directly into the sol−gel-processed solution and aerosol deposited. All results are also consistent with films that are heterogeneous on a molecular level. The utility of aerosol-deposited, sol−gel-derived films as a chemical sensing platform is demonstrated using fluorescence quenching of entrapped pyrene by O2.