Repeated monitoring of low-level formaldehyde in air using an amperometric flow-cell biosensor with electrolyte circulation

A highly selective amperometric biosensor was developed for direct monitoring of low-level formaldehyde in air. The sensor consists of a flow-cell reaction unit with gas and liquid compartments separated by a porous PTFE diaphragm membrane. Formaldehyde dehydrogenase, NAD+, and ferrocenecarboxylic acid were used in the electrolyte solution, and the enzymatic reaction was monitored amperometrically at a carbon working electrode at a low applied potential of 80 mV versus Ag/AgCl. The electrolyte solution was circulated through the liquid compartment after each measurement to shorten the recovery time and maintain the activity of the enzyme-containing liquid phase. The sensor measured formaldehyde gas over the range of 1 ppb to 1.1 ppm within 5 min, covering concentrations relevant to indoor air monitoring. Formaldehyde concentrations down to 1 ppb were experimentally detected, and the sensor showed negligible responses to acetaldehyde, methanol, ethanol, benzene, and acetone at 100 ppb. The sensor also exhibited long-term operational stability, retaining approximately 80% of its initial response after 56 days when electrolyte circulation was used. Application to indoor air demonstrated that the proposed method could track transient changes in formaldehyde concentration associated with ventilation cycles. These results indicate that the present flow-cell biosensor provides a practical analytical method for repeated monitoring of low-level airborne formaldehyde with high sensitivity, high selectivity, and improved operational stability.