CORRELATION OF STRUCTURAL AND OPTICAL PROPERTIES OF SiO 2 FILM MATRICES — INDICATORS OF GAS AMMONIA A. N. Murashkevich, * T. V. Kamlyuk, V. G. Vashina, A. G. Yalg, and I. M. Zharskii UDC 681.587.712.002:66.01 Detection of ammonia in an ammonia-air mixture by means of SiO 2 film matrices doped with an optically ab- sorbing component sensitive to the determined gas and changing reversibly its optical characteristics with a change in the gas concentration in the range 0–500 ppm is demonstrated. It is shown that the structural characteristics of SiO 2 matrices can be regulated by introducing modifying additives and varying the number of film layers to improve the service properties of ammonia-sensitive sensor elements based on them. Keywords: sol-gel technology, SiO2 film, optical absorption, modifier, porous matrix, sensor. Introduction. In view of the high toxicity of ammonia, the determination of its concentration is an important problem for a number of industries and ecology. A possible solution of this problem is the use of optical sensors, the principle of operation of which is based on reversible interaction of a determined component with a sensitive layer re- sulting in a change of the optical characteristics of the porous matrix forming the base of the sensor. Of particular in- terest is the use of the sol-gel process for obtaining optical sensor materials due to its adaptability and the possibility of being carried out at room temperature and obtaining matrices with specified optical properties [1]. The object of an extensive work is a porous SiO 2 matrix formed by the immersion or centrifuging method with the use of the sol-gel process. The properties of the matrices obtained can be varied by changing the reaction conditions and also by intro- ducing organic and inorganic compounds into the SiO 2 structure. For example, the introduction of glutaraldehyde, sili- cylaldehyde, and glycolic acid leads to the formation of a gel matrix with a less branched structure on account of the blocking of the sites of the cross bonds Si–O–Si as a result of the formation of hydrogen bridges between the surface O–H groups of silica gel and the hydrogen of the modifier [2]. This suggested the conclusion on the absence of a chemical bond between the SiO 2 matrix and the modifier. It has been established that these compounds decrease the rate of the hydrolysis and condensation processes which influence the properties of films. The sol-gel method makes it possible to obtain porous matrices in the form of films and blocks. In [3], block- type gels were obtained from aqueous-ethanol solutions of alkoxy silanes with fluoride-containing additives, and the possibility of applying xerogel powders, modified with chromogenic reagents, for solid-phase spectroscopic and test de- termination of metals, halogenide ions, and organic compounds was shown. However, block-type xerogels are difficult to produce because of their considerable shrinkage in dehydration and sintering, resulting in higher brittleness of these materials or their destruction. Due to the extended specific surface of block matrices, the processes of adsorption and desorption of the determined gas component become more complicated, thus considerably increasing the time of attain- ment of the useful response. For example, according to [4], the response time of 250-nm-thick silica films is of the order of several minutes, and for monoliths of size 1 × 1 × 2 cm it is of the order of 1 h. From this viewpoint, ma- trices of the film type are more promising, because they can be made fairly thin by the sol-gel method, which makes it possible to produce miniature sensor devices based on them. Belarusian State Technological University, 13a Sverdlov Str., Minsk, 220630, Belarus; e-mail: root@bstu.uni- bel.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 69, No. 4, pp. 523–528, July–August, 2002. Original ar- ticle submitted January 24, 2002. Journal of Applied Spectroscopy, Vol. 69, No. 4, 2002 0021-9037/01/6904-0606$27.00 2002 Plenum Publishing Corporation 606 * To whom correspondence should be addressed.