Colorimetric and near-infrared fluorescence turn-on molecular probe for direct and highly selective detection of cysteine in human plasma

A highly efficient colorimetric and fluorescence turn-on molecular probe for sensitive and selective detection of biologically important cysteine (Cys) in human plasma is demonstrated using a new zwitterionic dye (M1). The sensing mechanism involves release of the blue-colored and fluorescent M1 from its non-fluorescent, pale-yellow copper complex (M1–Cu) upon exposure to Cys. The new dye has the desired properties suitable for bio-sensing applications, such as distinct blue color, near-infrared (NIR) fluorescence at the wavelength of 875 nm, a large Stokes shift (195 nm), high solubility and stability. This new probe shows both colorimetric and fluorescent responses specifically to Cys over other naturally occurring amino acids, amines and some bioactive thiols and allows for detection of Cys in aqueous solution with a low detection limit of 4.07 × 10−6 M and in human plasma at the physiologically relevant levels.