A fiber-optic sensor to detect volatile organic compounds based on a porous silica xerogel film

Fiber-optic sensors are increasingly used for the determination of volatile organic compounds (VOCs) in air matrices. This paper provides experimental results on the sensitivity of a fiber-optic sensor that uses a film of a porous silica xerogel as the sensing element. This film was synthesized by the sol–gel process and affixed to the end of the optical fiber by the dip-coating technique. This intrinsic sensor works in reflection mode, and the transduction takes place in the light that travels through the core of the fiber. The VOCs included in this research cover a wide range of compounds with different functional groups and polarities. The highest sensitivity was for 2-propanol (13.1±1.4 M−1 nm−1), followed by toluene (11.4±1.4 M−1 nm−1), and 1-butylamine (9.5±0.4 M−1 nm−1). Acetone and cyclohexane had the lowest sensitivity of all studied VOCs. Limits of detection varied between 9.1×10−5 M for 1-butylamine and 1.6×10–3 M for ethanol. Silanol groups on the xerogel surface act as weak acids and interact strongly with molecules that contain OH groups like alcohols, π-electrons like toluene, or a lone pair of electrons like toluene. Stronger interaction of methanol and ethanol with the silanol groups on the film led to some irreversible adsorption of these analytes at room temperature.