Fiber-optic biochemical gas sensor (bio-sniffer) for sub-ppb monitoring of formaldehyde vapor

A high sensitive fiber-optic biochemical gas sensor (bio-sniffer) for sub-parts-per-billion (sub-ppb) monitoring of formaldehyde (FA) vapor was constructed and tested. The bio-sniffer measures formaldehyde vapor as fluorescence of reduced nicotinamide adenine dinucleotide (NADH), which is the product of formaldehyde dehydrogenase (FALDH) reaction. An ultraviolet light emitting diode (UV-LED) with peak emission of 335 nm was employed as an excitation light source. Emission of the UV-LED was introduced to the optode through an optical fiber and fluorescence of NADH was picked up coaxially at the optode. In order to achieve sub-ppb sensitivity, two types of detectors (a spectrometer and a photomultiplier tube (PMT)) were tested. The reaction system was also improved. Aldehyde dehydrogenase (ALDH), which is more active with formaldehyde, was first tested instead of FALDH. The product of ALDH reaction was again oxidized by formate dehydrogenase (FDH) to enhance production of NADH. As a result of these improvements, real-time monitoring of FA was performed with a detection limit of 0.75 ppb, which is comparative to chromatographic methods. Such a high sensitive monitoring method is expected to improve the quality of indoor air management.