Up: Porous Media Characterisation
Sol-Gel Materials for Chemical Sensors
Dr J.D. Wright
In this group, which has been active in new chemical sensor development for over 20 years, sol-gel materials are developed for application as optical and semiconductor chemical sensors.
The group leader has also recently published a new text on "Sol-Gel Materials: Chemistry and Applications" (Gordon & Breach; ISBN 90-5699-326-7; £19.95 cloth) for details see: http://catalog.gbhap-us.com/fc3/catalog?/books/TITLE_REC_0014635
We have shown that when organic colour reagents for metal ions are entrapped in porous silica sol-gel matrices, their selectivity changes. This is due to the influence of pore size and pore-wall character on the overall reaction thermodynamics. Small pores restrict translational freedom leading to changes in entropy terms. Interactions between surface hydroxy groups and water modify the solvent structure, which affects both enthalpy and entropy terms in the complexation reactions. These effects can be controlled by varying pore size and pore-wall character using sol-gel chemistry, and can be used to fine-tune the sensing properties of the resulting composite materials.
See, for example:
Sol-gel materials for optical sensing of solvents and metal ions. S.A. Wallington, T. Labayen, A. Poppe, N.A.J.M. Sommerdijk and J.D. Wright, Sensors and Actuators, 1997, 38, 48-52;
Matrix effects on selective chemical sensing by sol-gel entrapped complexing agents. N.A.J.M. Sommerdijk and J.D. Wright, J. Sol-Gel Sci. Tech., 1998, 13, 565-568; Unexpected complexation behaviour of a sol-gel immobilised dye: the development of an optical copper (II) sensor. N.A.J.M. Sommerdijk, A. Poppe, C.A. Gibson and J.D. Wright, J. Mater. Chem. 1998, 8, 565-7
Sol-gel entrapped colour reagents for a range of toxic gases and vapours, deposited as thin layers onto piezoelectric polymer films, provide a convenient and versatile personal monitoring system. The films are worn as badges, and measured before and after exposure. The measurement involves illuminating the reagent spot with light of a suitable wavelength , which generates heat proportional to the colour change that has been generated chemically within the reagent layer. This heat expands the piezoelectric film, producing an electric charge which is measured in a commercially-available reader. The sol-gel matrix protects the reagent, provides a controlled (hydrophilic or hydrophobic) environment for the sensing reaction, and is transparent and porous so provides access to the reagent both by the target analyte and by the light used for measurement.
See, for example:
Development of a Piezo-Optical Chemical Monitoring System. John D. Wright, Florence Colin, Raoul M. Stöckle, Paul D. Shepherd, Telmo Labayen and Timothy J.N. Carter, Sensors and Actuators B, 1998, 51, 121-130;
Kinetic Factors in the Response of Piezo-Optical Chemical Monitoring Devices. Ceri A. Gibson, Timothy J.N. Carter, Paul D. Shepherd and John D. Wright, Sensors and Actuators B,.1998, 51, 238-243;
Optical transduction of chemical sensing by thin films of colour reagents and molecular receptors using piezo optical and surface plasmon resonance methods. J.D. Wright, C. von Bültzingslöwen, T.J.N. Carter, F. Colin, P.D. Shepherd, J.V. Oliver, S.J. Holder and R.J.M. Nolte, J. Mater. Chem. 2000, 10, 175-182.
Details of the commercial system are at http://ftp.itl.co.uk/www/piezoptic/
Tin oxide is well known as a semiconductor sensor material for flammable gases. The conductivity of this n-type semiconductor material is reduced by adsorption of the electron-accepting oxygen of air. Surface reaction with flammable gases removes adsorbed oxygen, returning charge carriers to the metal oxide semiconductor and increasing the conductivity to an extent proportional to the concentration of the flammable gas. We have shown that the sensitivity and selectivity of such sensors is improved if nanocrystalline material is used. This can be prepared by sol-gel processes, either in a highly-pure state or doped in a controlled manner. Care is needed to preserve nanocrystalline structure, and we have explored both the sintering and dopant diffusion in such nanocrystalline materials as a function of temperature, and the consequent effects on the sensing process.
See for example:
Sol-gel materials for Gas Sensing Applications. A. Wilson, J.D. Wright, J.J. Murphy, M.A.M. Stroud and S.C. Thorpe, Sensors and Actuators B, 1994, 19, 506-510;
A combined EXAFS and Diffraction Study of Pure and Doped Nanocrystalline Tin Oxide. S.R. Davis, A.V. Chadwick and J.D. Wright, J. Phys. Chem. A, 1997, 101, 9901;
The effects of particle growth and dopant migration on the carbon monoxide sensing characteristics of nanocrystalline tin oxide based sensor materials. Steven R. Davis, Alan V. Chadwick and John D. Wright, J.Mater. Chem., 1998, 8, 2065-2071.
The expansion and changes in optical pathlength occurring in a porous silica sol-gel film or monolith on exposure to humidity or various chemical substances can be measured very sensitively using low-coherence interferometry. We have demonstrated humidity sensing systems based on this principle, and shown that aging processes in sol-gel materials can be followed over extended time periods using such a system.
See for example:
A multiplexed low coherence interferometric system for humidity sensing. S. McMurtry, J.D. Wright and D.A. Jackson, Sensors and Actuators B, 2000, 67, 52-56.