Direct monitoring of organic vapours with amperometric enzyme gas sensors

In this study, amperometric enzyme gas sensors for direct monitoring of organic vapours (formaldehyde, ethanol and phenol) are presented using exemplarily different sensing strategies: NADH detection, H2O2 detection and direct substrate recycling, respectively. The presented sensor configurations allow the selective, continuous, online monitoring of organic vapours without prior accumulation or sampling of the analyte. The gaseous samples are provided as headspace above aqueous solutions. The concentration in the gas phase was calculated from the concentration in solution at room temperature according to the respective Henry constants given in the literature. The enzymes employed are NAD-dependent formaldehyde dehydrogenase [EC 1.2.1.46] from Pseudomonas putida, alcohol oxidase [EC 1.1.3.13] from Pichia pastoris, and tyrosinase [EC 1.14.18.1] from mushroom. The gas diffusion working electrodes used in the sensors are based on a porous, hydrophobic PTFE membrane (exposed geometric electrode area: 1.77 cm2) covered with a porous layer of gold, platinum or graphite/Teflon. Detection limit, sensitivity, and measuring range are 34 μM (6.5 ppb), 117 nA/mM, and 0.46–66.4 mM for formaldehyde, 9.9 μM (55 ppb), 3.43 μA/mM, and 0.1–30 mM for ethanol, and 0.89 μM (0.36 ppb), 2.4 μA/mM, and 0.01–1 mM for phenol, respectively. Further sensor characteristics such as response time and stability are also determined: t90% (formaldehyde: 4.5 min; ethanol: 69 s; phenol: 27 min), stability at permanent exposure (formaldehyde: 63%, 15 h @ 2.62 mM; ethanol: 86%, 18 h @ 1 mM; phenol: 86%, 16.5 h @ 0.1 mM).