Spectroscopic studies of molecules in sol-gel silica monoliths and thin films

Abstract

This article about sol-gel research in North America is focused on transparent sol-gel silica monoliths and nano-structured thin films containing encapsulated molecules. The research was a collaborative effort carried out at the University of California Los Angeles (UCLA). Our research was focused on two goals: using optical spectroscopy to understand the chemical and physical changes occurring during the sol to the xerogel transformation; and using spectroscopy to understand what was happening to the molecules themselves after encapsulation. The first section is a brief discussion of molecular probes used to monitor in real time the hydrolysis, condensation, gelation and drying of tetraethoxysilane (TEOS) formed in optical cuvettes. In the second section, we introduce dip coating of one hundred nanometer thin films and methods for measuring thickness and chemical changes in real time during film pulling. We include surfactant templated mesostructured films and real time observation of the structure development. Third, we design molecules to add to the initial sol such that they are placed in specific regions of the ordered structure of the templated film. We describe spectroscopic methods that prove their placement and use pairs of molecules for physical studies including intermolecular energy transfer. Finally, we describe “gentle” synthesis methods for encapsulating enzymes and other proteins that retain their optical and enzymatic properties. Spectroscopy provided quantitative information about the sol-gel processes, enzymatic activity, and optical sensor applications.

Graphical Abstract

The Graphical Abstract depicts an octagonal array of sol-gel silica monoliths containing enzymes. The largest monoliths have dimensions of approximately 5 × 5 × 15 mm. The original sol was placed in an optical cuvette and hydrolysis, condensation and slow, controlled drying in the cuvette produced the parallelepiped cuboid-shaped monoliths without cracking. The colors are caused by encapsulated metalloenzymes and the absorption spectra are the same as those of the enzymes in solution. In the center of the array is an artist’s depiction of an enzyme surrounded by the amorphous silica glass.