Selective turn-on fluorescence detection of formaldehyde in the gas phase

Reliable detection of formaldehyde in gas phase still remains a challenge regarding either sensitivity or selectivity or both. In this study we report on a simple, unique sensor composite, which enables efficient detection of gaseous formaldehyde with both low limit of detection (LOD) and extremely high selectivity. The primary components of the sensor composite include a hydroxylamine sulfuric acid salt ((NH2OH)2·H2SO4) and a fluorescence turn-on probe, which both are coated within the porous matrix of a cellulose plate. The fluorescence probe is based on a perylene diimide (PDI) molecule modified with an aniline group, which is non-fluorescent in its pristine state due to the photoinduced intramolecular electron transfer. Upon interaction with formaldehyde, the hydroxylamine salt will be converted to the corresponding Schiff base through aldimine condensation, releasing the sulfuric acid. The acid released will protonate the aniline group on PDI, resulting in a quick fluorescence turn-on. By measuring the fluorescence increase we can detect formaldehyde both quantitatively and specifically. Under the optimized condition, the sensor demonstrated a LOD of 3.7 ppb, which is much lower than the air quality threshold set by WHO. The sensor was also proven highly selective towards formaldehyde, with negligible response towards other common chemicals. The work reported will not only provide a new sensor for formaldehyde, but moreover open a unique design pathway for sensor composite that combines reactive components with various fluorescence indicators so as to enable highly efficient detection of many other air pollutants and toxic chemicals.