Highly sensitive and rapid gas biosensor for formaldehyde based on an enzymatic cycling system

An enzymatic cycling system for detecting formaldehyde in water and indoor environments has been developed. The system is based on both the dehydrogenation of formaldehyde and the reduction of nicotinamide adenine dinucleotide (NAD+) in the presence of formaldehyde dehydrogenase. NADH generated in the reaction then reduces 4-[3-(2-methoxy-4-nitrophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-l,3-benzene disulfonate sodium salt (WST-8) to yellow WST-8 formazan in the presence of enzyme diaphorase. The quantity of formaldehyde can be detected by the color change resulting from the formation of WST-8 formazan. The assay solution was optimized with respect to the pH, type of buffer, and enzyme and WST-8 concentrations. The limit of detection for formaldehyde in water was found to be 10 ppb after 3 min. The assay solution also displayed a highly selective response to formaldehyde over other pollutant gases present in indoor air. In addition, formaldehyde gas was detected after bubbling prepared formaldehyde/air mixtures with known formaldehyde concentrations in assay solutions. The limit of detection for formaldehyde gas was 1.5 ppb after 5 min. This system can quickly and accurately detect low concentration of formaldehyde in water and gas.