(68d) Continuous Monitoring and Removal of Formaldehyde Vapor in Ambient Air Using Polymer Catalyst Membranes

In 2011, formaldehyde was recognized by the US National Toxicology Program as a human carcinogen.  The sources of formaldehyde in ambient air are wide-ranging, including pressed-wood products, glues &adhesives, and even air fresheners.   Formaldehyde is also a byproduct of inefficient direct methanol fuel cell operation.  Human toxicity is presently believed to occur at formaldehyde exposures as low as 0.5 ppm.  We here report on the results of a portable, unobtrusive method of continuously monitoring ambient formaldehyde that is both highly sensitive and selective.  Measurement is based on changes in the visible region of the electromagnetic spectrum as light is passed through an immobilized dye molecule (resorcinol) when it reacts with a target molecule (formaldehyde) in the ambient gas in the presence of catalyst. The novelty of the technology is two-fold. First, a polymer is used as both catalyst and supporting membrane for the dye in the sensing element.  This approach provides substantially enhanced durability relative to previous approaches based on nanoparticle or molecular catalysts that are temporarily supported on non-catalytic polymer scaffolds and that elute over time. Second, the unique chemical product that is formed as a result of the reaction ensures a highly selective optical response and avoids the multi-sensor arrays required to provide selectivity for even a single target molecule in the case of more generic chemical processes (i.e.,  protonation/deprotonation and metalloporphyrin ligand binding). Such improved selectivity using visible light is the basis for the high degree of miniaturization that is possible utilizing this approach and is essential for portable measurement.   Furthermore, reaction product immobilization prevents otherwise rapid formaldehyde transport through polymeric membranes.  This method of ambient formaldehyde detection and removal is here shown to be very fast, with the intense purple color of the reaction product observed after less than 1 min. formaldehyde exposure at concentrations ranging from ppb to ppm.