A specific fluorescent probe for fast detection and cellular imaging of cysteine based on a water-soluble conjugated polymer combined with copper(II).

In pure water system, the specific and rapid detection of cysteine (Cys) is very important and challenging. Herein, a new optical probe was developed for the purpose based on the complex of cupric ion (Cu2+) with a water-soluble conjugated polymer, poly[3-(3-N,N-diacetateaminopropoxy)-4-methyl thiophene disodium salts] (PTCO2). The fluorescence of PTCO2 in 100% aqueous solution can almost completely extinguished by Cu2+ ions due to its intrinsic paramagnetic properties. Among various amino acids, only Cys causes immediately the efficient recovery of the Cu2+-quenched fluorescence of PTCO2 with ~31-folds fluorescence enhancement because of the stronger affinity of Cys to Cu2+ leading to the formation of Cu2+-Cys complex through Cu-S bond and separation of Cu2+ from weak-fluorescent PTCO2-Cu(II) ensemble and thereby restoring the free PTCO2 fluorescence. In tris-HCl buffer solution (2 mM, pH 7.4), the intensity of the restored fluorescence is linear with the concentration of Cys, ranging from 0 to 120 μM and the estimated detection limit of Cys is 3.3 × 10-7 M with the correlation coefficient R = 0.9981. In addition, the PTCO2-Cu(II) ensemble probe exhibits low cytotoxicity and good membrane penetration, and its application in living cell imaging of Cys has also been explored.