Simple optical sensor for amine vapors based on dyed silica microspheres

A new optical sensing method for sensitive detection of amine vapors down to ppb levels is described. The sensor is based on the pH indicator Bromocresol green, adsorbed onto a silica sphere matrix. When the amines adsorb onto the matrix, the color changes from orange to blue. The color change is detected with a fiber optic spectrometer. Sensor performance is demonstrated for the aliphatic amines tert-butylamine, diethylamine and triethylamine and also for pyridine and aniline. The response for the aliphatic amines is linear with concentration up to 2 ppm with a detection limit of approximately 1.4 ppb for tert-butylamine. The microsphere sensor is more sensitive than other optical amine sensor designs described in the literature. The sensor response varies with temperature, with lower sensitivity and faster response at higher temperatures allowing for adjustment to prioritize sensitivity or speed. At 80 °C the response time is 2–20 s, with maximum signal after approximately 2 min and a recovery time of 2–10 min depending on concentration of amine vapor. Sensor response is highly reproducible and fully reversible.