Iodine substitution compromises the aggregation-induced emission (AIE) property and enhances the biothiol sensitivity of quinoline–malononitrile-based fluorescent probes

The development of long-wavelength fluorescent probes with a pH working range from weakly acidic to neutral is important for cell-imaging studies. In this study, quinoline-malononitrile (QM)-based fluorescent probes without and with iodine substitution surrounding its phenolic ring are synthesized. The iodine substitution manner was found to compromise the aggregation-induced emission of QM whilst resulting in a significantly enhanced sensitivity for biothiols including l-cystine (L-Cys) detection in weakly acidic to neutral aqueous buffer solutions through reducing the pKa of the QM fluorogen. The probe also effectively achieved the fluorescence visualization of the fluctuation in L-Cys levels in cells.