A reversible water-soluble naphthalimide-based chemosensor for imaging of cellular copper(II) ion and cysteine

Cellular free copper ion is essential yet potentially toxic to living orgasms, thus cells have evolved mechanisms to balance global copper content including the chelation reactions by cysteine. To better understand the delicate counter-balance of free copper(II) ion and cysteine in the living cells, we developed a water-soluble naphthalimide-based derivative NC as a chemosensor for reversible dual detection of copper(II) ion and cysteine. The chemosensor NC exhibits a rapid, sensitive and quantitative response for copper(II) ion in aqueous solution due to the formation of the corresponding complex NC-Cu(II) in the range of 0–5 μM with the detection limit of 7.11 nM. Afterwards, NC is released from NC-Cu(II) complex accompanied with a “turn-on” fluorescence in the presence of cysteine ranging from 0 to 160 μM. It's also worth noting that the linear range of the NC-Cu(II) complex for the cysteine is identical with the intracellular level of cysteine under physiological conditions (30–200 μM). To achieve its practical application, we further demonstrated that the chemosensor NC exhibits good cell permeability and could be employed to monitor free copper(II) ion and cysteine in the living cells.