Quinoline-Malononitrile-Based Aggregation-Induced Emission Probe for Monoamine Oxidase Detection in Living Cells

A quinoline-malononitrile (QM)-based aggregation-induced emission probe was developed to detect MAOs in cells through an enzymatic reaction followed by β-elimination. After being incubated at 37 °C, QM-NH2 responded to the MAO enzymes with great specificity and within just 5 min. This 5 min responsive mechanism was fast, with the limit of detection (LOD) at 5.49 and 4.76 µg mL−1 for MAO-A and MAO-B, respectively. Moreover, QM-NH2 displayed high enzyme specificity even in the presence of high concentrations of biological interferences, such as oxidizing and reducing agents, biothiols, amino acids, and glucose. Furthermore, QM-NH2 demonstrated biocompatibility as the cells retained more than 70% viability when exposed to QM-NH2 at concentrations of up to 20 µM. As a result, QM-NH2 was used to detect MAO-A and MAO-B in SH-SY5Y and HepG2 cells, respectively. After 1h incubation with QM-NH2, the cells exhibited enhanced fluorescence by about 20-fold. Moreover, the signal from cells was reduced when MAO inhibitors were applied prior to incubating with QM-NH2. Therefore, our research recommends using a QM probe as a generic method for producing recognition moieties for fluorogenic enzyme probes.