Volatile organic compound vapor detection with responsive microgel-based etalons

Optical sensors (etalons) composed of poly(N-isopropylacrylamide) (pNIPAm)-based microgels have been developed for detecting volatile organic compound (VOC) vapors such as hexane, cyclohexane, chloroform, petroleum ether and tetrahydrofuran (THF). PNIPAm-based microgels were synthesizedandcharacterized, and etalons were subsequently fabricated, which exhibited visual color and multipeak reflectance spectra. The color change of etalons in response to VOC vapor exposure was evaluated by bubbling the vapors through water containing the etalon, and the shift in the reflectance peak position monitored. The results showed that the etalons were able to respond to the VOC vapor by changing their color, as evidenced by a shift in the position of the reflectance peak. Dynamic light scattering data showed that the microgels dissolved in solutions composed of the respective VOCs exhibit sizes that vary with VOC type and concentration, which leads to the optical response observed for the microgel-based etalons. The results showed that the etalons were most responsive to THF, which we conclude is a result of cononsolvency of the microgels in THF/water mixtures. With the setup used here, we could detect 1.27mM THF vapor in 4 min, although the limit of detection, and detection time, can be tuned by changing the experimental conditions. Finally, we showed that individual etalons can be used at least 3 times to detect THF vapor without a loss in etalon function, which suggests that the etalons can be used multiple times for detecting VOC vapors.