Styryl quinolinium/G-quadruplex complex for dual-channel fluorescent sensing of Ag+ and cysteine

Based on the disruption and recovery of fluorescence of styryl quinolinium (SQ) dye/G-quadruplex complex, a label-free method for dual-channel detection of Ag+ and cysteine is developed. The above sensing system exhibits high sensitivity and good selectivity for Ag+, even in the presence of other metal ions (such as Na+, Mg2+, Mn2+, Ba2+, Ca2+, Pb2+, Zn2+, Cd2+, Hg2+ and Cu2+). Due to cysteine (Cys) is a strong Ag+-binder and competes with quadruplex-forming G-rich oligonucleotides for Ag+ binding, and results in the reformation of G-quadruplex structure and the recovery of fluorescence. Thus SQ dye/G-quadruplex complexes are not only a potential primary sensor toward Ag+, but also a secondary sensor toward Cys. Neither the Ag+-sensing nor the Cys-sensing systems required labeled oligonucleotides. Under the optimal conditions, this method is capable of detection of Ag+ in the range of 0.1–15.0 μM with a detection limit of 26.0 nM. In addition, the proposed method allows a good response to cysteine with a linear range of 0.008–10 μM, and the limit of detection is found to be 2.0 nM. Moreover, this method has been applied successfully to the sensing of cysteine in human urine with satisfactory recovery.