Real-time liquid crystal-based glutaraldehyde sensor

We develop a liquid crystal (LC)-based optical sensor for detecting glutaraldehyde vapor by using real-time orientational responses of 4-cyano-4′-pentylbiphenyl (5CB) to glutaraldehyde. The sensor comprises a thin layer of 5CB (∼20 μm) supported on an amine-decorated surface. When a glutaraldehyde molecule diffuses through the layer of 5CB, it reacts with a surface amine group and forms an imine bond. Subsequently, the resulting free aldehyde group from the glutaraldehyde molecule triggers an orientational transition of 5CB, which manifests as an optical signal visible to the naked eye. Because the performance of the LC-based sensor critically depends on the diffusion of glutaraldehyde in 5CB, we also measured the solubility and diffusivity of glutaraldehyde in 5CB. Based on the diffusivity, an optimal LC sensor with a 20-μm-thick film of 5CB can respond to 207 ppmv glutaraldehyde within 20 s and shows a full response in 4 min. It also shows good specificity. Only a very weak response is observed when this sensor is exposed to acetic acid, and no response is recorded for methanol, ethanol or formaldehyde.