Hand-held optical instrument for CO_2 in gas phase based on sensing film coating optoelectronic elements

摘要

Different strategies have been tested in order to include a CO_2 optical gas sensor in a low-cost portable electronic instrument. Plastic solid-state sensor membranes are based on the quenching of a phosphorescent dye by the deprotonated form of a non-luminescent pH indicator. As a result of this study, the configuration with both optoelectronic components coated with sensing chemistry (LED with the luminophore and photodetector with the pH-active dye) presents a better CO_2 response in terms of sensitivity and reproducibility than the other configurations studied by us. The portable measurement system resulting from the integration of coated LED with the luminophore and photodetector with the pH indicator was characterized in terms of calibration, sensitivity, short and long-term stability, transient response and thermal dependence with results comparable to laboratory instrumentation and other sensing films described in literature. The sensor calibration curve has been modelled according to a theoretical model, reported in the literature, with two coefficients, which has been included in the microcontroller of the measurement system to provide a direct reading of the gas concentration in the display. The sensor's full range is from 0% to 100% CO_2 concentration. The study of the response time of the membranes was made, obtaining response t_(90) and recovery t_(10) times of 31 and 117s, respectively. Temperature dependence was successfully fitted to an Arrhenius type function that has been included in the microcontroller of the instrument, to calculate and display the CO_2 concentration, correcting the temperature dependence. The characterization demonstrated the reliability and good performance of this type of solution aimed at integrating chemical sensors in electronic and optoelectronic devices. The localization of the sensing film on the detector and on the light source makes any additional optical element unnecessary, thus reducing system costs, avoiding alignment problems, optimizing the efficiency of the signal collection and making it possible to replace the sensor easily.