(The) preparation of LB films for sensing materials and its application to QCM gas sensor

Abstract

A gas sensor system for olfactory sensing and environmental monitoring was investigated with a quartz crystal microbalance(QCM), one of the piezo-electric devices with high mass-sensitivity. Langmuir-Blodgett(LB) technique was adapted for the coating of a sensing membrane on the QCM. Target gases were volatile materials such as alcohol for olfactory sensing, and $NO_{2}$ and $SO_{2}$ for environmental monitoring. As the sensing membrane of olfactory sensor, various lipids were used in order to mimic the biomembranes in human olfactory cell. In the environmental monitoring, alkyl-substituted metallophthalocyanines (MPcs) and polythiophenes were examined because of their selectivity for some gases such as $NO_{2}$ and their superior thermal and mechanical properties. To build a sensing membrane, a constant-perimeter type Langmuir trough equipped with a vertical dipping unit was constructed. All the lipids used were purchased, but derivatives of MPc or polythiophene were synthesized. Alkyl-substituted MPcs had symmetric 2-ethylhexyloxy side chain, and their complex central metal atoms were dihydrogen, copper, cobalt, platinum, and lead. Also, the derivatives of polythiophenes were poly(3-octylthiophene)(POT), poly(3-propanoyloxy-2-ethylthiophene)(PPET), and poly(3-decanoyloxy-2-ethylthio-phene)(PDET). The target gases were generated by the dynamic method of saturation or multi-dilution using a flow system, and calibrated accurately. The QCM showed a stable oscillation and high sensitivity at room temperature, but required accurate temperature control at high temperatures. The monolayer behavior of membrane materials at the air/water interface was studied. From the surface pressure-area(Π-A) isotherm, evaluated were the cross-sectional area of an oriented molecule, phase transition, and miscibility of mixed monolayers. In the case of lipid monolayers, the monolayer stability established by added subphase salts was observed. The ionization of phosphatic acids could...