NADH-Fluorometric Biochemical Gas Sensor (Bio-Sniffer) for Evaluation of Indoor Air Quality

A fiber-optic biochemical gas sensor (bio-sniffer) for assessment of indoor formaldehyde (FA) is fabricated and tested in monitoring of FA degradation by TiO₂. The biosniffer measures FA vapor as fluorescence of reduced nicotinamide adenine dinucleotide (NADH), which is the product of enzymatic reaction of FA dehydrogenase (FALDH). Usually, an enzyme loses its specific activity in the gas phase. This makes biochemical gas monitoring difficult. We employ a microflow-cell with a FALDHimmobilized membrane to prevent the FALDH from deactivation at the optode. An ultraviolet light-emitting diode (UV-LED) with peak emission of 335 nm is employed as an excitation light source. Emission of the UV-LED is introduced to the optode through an optical fiber and fluorescence of NADH is picked up coaxially at the optode. To improve the sensitivity, a multi-LED light source is introduced instead of the previously reported system. A photomultiplier tube is utilized as a photodetector. Continuous FA monitoring with biochemical method is successfully conducted with high selectivity at subppb level. Owing to the improved excitation source, the detection limit is improved to 750 ppt. A real-sample test is also carried out using a photocatalytic titania. A standard gas is flowed into a cell, in which photocatalyst glasses are located, and the FA level is measured using the biosniffer. Therefore, degradation of FA by the catalytic reaction of titania is monitored as reduction of FA in real time. According to the results, it is expected to be useful in fast and convenient monitoring of indoor FA.