A fast response squaraine-based colorimetric probe for detection of thiols in physiological conditions

A squaraine-based compound (USQ-1) was designed and synthesized as a new colorimetric probe for thiols, with a sensing mechanism based on the nucleophilic attack of a thiol at the electron-deficient central squaraine ring of USQ-1 in the presence of cetyltrimethylammonium bromide (CTAB). The interaction between surfactants and USQ-1 have been investigated, and the influence of type and amount of surfactants were also examined. Notably, USQ-1 exhibited a rapid response to thiols without the interference from other amino acids and bioanalytes in physiological conditions, and the linear range of the method was 0.15–8.5 μM with a detection limit of 1.0 × 10−8 M for glutathione. The whole recognition process was accompanied by a visual color change from green to yellow which allowed a naked-eye observation. More interestingly, through time controlled experiments, the sensing system may be used to discriminate cysteine from glutathione and homocysteine.