Molecular Recognition-Driven Reaction-Based Sensing of Catecholamines in a Lipid Nanoreactor

Reaction-based sensing is a rapidly expanding principle in the design of fluorescent probes. Although effective for sensing highly reactive species, it is less suitable for more complex small molecules, including neurotransmitters (NTs). The latter are essential for the functioning of the nervous system and serve as biomarkers for numerous neurological disorders. Here, to provide reaction-based sensing with molecular specificity to catecholamines, we coupled molecular recognition with an irreversible reaction in a lipid nanoreactor. In the obtained nanosensors, lipophilic boronic acids encapsulated in the lipid nanoreactors play the role of the molecular recognition ligands, specifically capturing catecholamines, while the pyrylium-based dyes react with the amino groups of the captured analytes, transforming them to the corresponding pyridinium derivatives. This irreversible reaction ensures a strong fluorescence turn-on response of the nanosensors, together with covalent labeling of the analyte. Rational design of the fluorogenic dyes featuring optimal reactivity with amines and chemical stability inside the nanoreactor is essential for the good performance of the nanosensors. A limit of detection of 11 nM for dopamine, with a large dynamic range, is achieved for one of them. Its lipid nanoreactor ensures high selectivity to catecholamines and operation in complex media (including porcine urine) by excluding other hydrophilic biogenic amines not captured by the recognition ligand in the hydrophobic oil core. Owing to the high modularity of the system, the development of new recognition ligands and reactive fluorogenic modules will lead to the expansion of the concept toward new nanosensors and labeling reagents for a broad variety of small molecules.