Turn-On Terbium(III) Photoluminescence Enables Sensing of Methyl Salicylate by a Biomimetic Airborne Messenger Mechanism

A low cost, portable optical sensing platform enables rapid and sensitive detection of methyl salicylate (<b>MeSal</b>) in unprocessed samples. The sensing assay is based on a strong photoluminogenic response when <b>MeSal</b> forms a complex with a terbium cation (Tb³⁺) in aqueous buffer. The assay employs an airtight three-well sensing chamber that allows the volatile <b>MeSal</b> to spontaneously diffuse out of a sample well and partition into a neighboring test well where the intense green Tb-photoluminescence is detected by the naked eye or a cell phone camera. This <b>MeSal</b> vapor diffusion process mimics the biological airborne messenger mechanism used by distressed plants to warn their neighbors and prompt a communal defense. Within the assay chamber, the <b>MeSal</b> vapor diffusion acts as a purification step that separates the <b>MeSal</b> photoluminescent signal from the interfering sample background signal. Proof-of-concept experiments show that the optical assay can quickly distinguish a sample of commercial toothpaste that contains 0.48% <b>MeSal</b> from a sample that does not contain <b>MeSal</b>. This type of high-throughput semi-quantitative screening is increasingly mandated by regulatory agencies who have placed upper limits on the amount of added <b>MeSal</b> within cosmetic products. Practical utility for enzyme detection is also demonstrated by using a novel substrate molecule, <b>MeSal-Glc</b>, to detect the presence of the flavor-promoting enzyme β-glucosidase in almond flour, a representative food sample. Many other enzyme detection assays can likely be developed using bespoke substrate molecules that contain <b>MeSal</b> as a releasable reporter unit for Tb-photoluminogenic sensing.