Sol-gel derived polyelectrolyte-silica composite materials: Preparation, characterization and applications in spectroscopic and spectroelectrochemical sensing

A new family of polyelectrolyte modified silica composite materials were prepared by sol-gel process, and characterized using UV-visible spectrophotometry, cyclic voltammetry, infrared spectroscopy, and scanning electron microscopy. These materials are ion-exchangeable and less brittle than the parent silica. These new composites retain the nano-scale porosity and optical transparency into the ultraviolet of the parent silica sol-gel glasses making them attractive passive sensor materials for indicator and/or biomolecule immobilization, and as active sensor materials for charge-selective-based optical sensing applications.; Using these composite materials, we have developed spectroscopic sensors for pH and transition metal ions based on electrostatic immobilization of charged indicators in the composite thin films, as well as spectroelectrochemical sensors for transition metal complexes based on charge-selective partitioning in the composite thin films. All these sensors were constructed in a planar configuration with which the sol-gel derived sensing films were formed on glass substrates (1 mm thick) by spin-coating technique. In the spectroelectrochemical sensors, three modes of selectivity, i.e., charge-selective partitioning, electrolysis potential and spectral absorption wavelength, were incorporated into the sensing devices for selective optical sensing of analyte in the presence of direct interferences. The multimode selectivity concept of this new type of sensor was demonstrated by several mixture sample systems. The response kinetics of this new type of sensor for the determination of analyte in the presence of optical and charge interferences were also investigated using both voltammetric and spectroelectrochemical techniques.; In all optical absorption-based sensing schemes described above, both single pass transmission and attenuated total reflection (ATR) methods were employed to monitor the sensor response, with the latter showing much higher sensitivity. In order to enhance further the sensitivity of the ATR-based optical sensors, we have fabricated micron-thick integrated optical waveguides (IOWs) (including slab and channel IOWs) by molten salt ion-exchange process, and used these IOWs as the ATR optical elements for optical transduction. The channel IOWs were microfabricated by standard photolithographic and wet-etching techniques followed by the molten salt ion-exchange process. Sensitivity enhancement by orders of magnitude higher than the 1 mm-thick ATR optical elements has been demonstrated for these IOW-ATR sensors.