Discriminatory Detection of Cysteine and Homocysteine Based on Dialdehyde-Functionalized Aggregation-Induced Emission Fluorophores

We demonstrate a concept-proof work of using fluorescence (FL) turn-on probes for the discriminatory detection of cysteine (Cys) over homocysteine (Hcy). The fluorogens are provided with aggregation-induced emission (AIE) characteristic and functionalized with two aldehyde-groups (DMTPS-ALD and TPE-ALD). All the detections were carried out in a biocompatible medium (10 mm HEPES buffer and DMSO, pH 7.4). In principle, the formation of thiazinane/thiazolidine through the chemical reaction of aldehydes on the probe molecules and the residue of Cys/Hcy determines the selective recognition of Cys and Hcy over other amino acids and glucose. The FL responses originate from the AIE property of thiazinane/thiazolidine resultants, which have low solubility and precipitate (aggregate) in the detection medium. The discrimination between Cys and Hcy comes from the difference in reaction kinetics of TPE-ALD/DMTPS-ALD with Cys and Hcy, thereby the FL responses show different time courses and intensity enhancement. It is worth noting that TPE-ALD outshined the other two probes in performance with fast response, a high FL enhancement up to 16-fold, high sensitivity, and good specificity and selectivity. Moreover, its FL response threshold at 250 mu m is very close to the lower limit of the normal level of Cys in human plasma, which implies that TPE-ALD could be applied as a potential indicator of Cys deficiency.