Chemical Sensing System Using Plasma Polymer Films and Pattern Recognition Algorithm

Abstract

A chemical sensing system using a sensor array with sensitive but durable plasma polymer films is developed. Plasma polymer films have unsaturated bonds and radical sites which cause several unique characteristics. These films contain high concentrations of unsaturated bonds and radical sites, which act as interactive sites. These sites, scattered throughout an inert fluorocar bon framework, are believed to induce specific interactions with small molecules through π and spin interactions. We have tried to apply our knowledge of these interactions to molecular recognition. For sensing small molecules, these films are deposited on both sides of an AT-cut quartz crystal microbalance (QCM) with a resonant frequency of 9 MHz by radio-frequency (rf) sputtering of polymers such as polychlorotrifluoroethylene. The QCM is connected to an oscillator circuit and its resonant shift is proportional to the mass of the adsorbed molecules. The affinity of plasma polymer films can be shifted by changing sputtering conditions such as the target materials, temperature, or rf power. The chemical sensing system studied here uses a sensor array having modified films with various sensitivities. Because the sensor films have an affinity for several kinds of gases, a pattern recognition algorithm is needed to discern unique gas information from sensors that have overlap ping selectivities. The equilibrium mass of adsorbed gas and a time constant are first extracted from the time-dependent sensor outputs, which show that the adsorption process resembles Langmuir ad sorption, and then the parameters are mapped to a classification space and used for classification. The addition of a time constant increases the selectivity of our sensor system for single-gas analysis and mixture analysis.