Smart Optical Materials by Sol-Gel Method
by R. Reisfeld
Dept. of Inorganic and Analytical Chemistry , The Hebrew University, Jerusalem 91904, Israel

Tunable Laser samples prepared by the sol-gel method

Sensors based on Sol-Gel Technology Stable and sensitive sensors for in-situ detection of trace quantities of environmental and biological materials often rely on organic compounds indicators that show specificity, sensitivity and stability through color change.  To effectively design such sensors, the detailed mechanism responsible for the color changes must be understood – for example , it is already known that many indicators deposited on solids without an intermediate medium form dimmers and higher aggregates which are easily photodecomposed.

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However, the photodecomposition can be significantly diminished by introducing the indicator molecules into a medium in which the dispersion of the molecules is complete and aggregates are not formed.

For example sol-gel and PMMA films containing oxazine-170 covering capillary tubes can be used as reversible indicators for ammonia and acid vapors. The mechanism of the color change arises from the diffusion of ammonia or of acid into the films and is accelerated by the presence of water vapor. The color changes may be monitored by illuminating the samples by light of appropriate wavelength and detection of either emission or absorption changes withy solid-state detectors.

Using the sol-gel method, oxazine-170 is incorporated into a glass. Several geometries are prepared: bulk glass, glass tubes coated on the inside and glass tubes coated on the outside. It has been shown that the color of the oxazine incorporated into the glasses changes reversibly when exposed to atmospheric ammonia or acid. Waveguides based on this material may serve as optical sensors for ammonia or acids. Comparable results are also obtained when oxazine-170 is incorporated into films of polymethylmethacrylate (PMMA); however, this latter material is less photostable. The response time of color change depends on the thickness of the films and on the velocity of diffusion of ammonia or protons.

The principles of optical and electrochemical sensors based on the use of the sol-gel technique, in particular their fabrication, working principles, and various device configuration have been extensively investigated by our group. Potentional applications of these devices are in environmental and clinical analysis, gas sensing, and bioprocess monitoring.