A novel fluorescent silver ion biosensor based on nucleic acid molecular "light switch"

As one of nucleic acid molecular "light switch", Ru(bipy)(2)(dppx)(2+) is a good probe for the determination of double-helical DNA, which displays intense fluorescence when double-helical DNA is present. However, the fluorescence of Ru(bipy)(2)(dppx)(2+) is quenched when Ag(+) is added to the Ru(bipy)(2)(dppx)(2+)-DNA system. Based on the quenching of the fluorescence of Ru(bipy)(2)(dppx)(2+)-DNA system by Ag(+), a simple, rapid and specific method for Ag(+) determination was proposed. In the optimum conditions, Ag(+) concentration versus Ru(bipy)(2)(dppx)(2+) fluorescence intensity gave a linear response in the range from 0.2 to 6.0 microM with a detection limit (3sigma) of 3.2 x 10(-8) M. The proposed method has been applied to determine the Ag(+) in water samples and sulfadiazine silver cream successfully. Because of the intense fluorescence of Ru(bipy)(2)dppx(2+) when DNA is present, the interaction between Ag(+) and DNA was confirmed by fluorescence microscopy and the phenomenon of the fluorescence images agreed well with the results. The possible mechanism of the reaction was also discussed by circular dichroism spectra and isothermal titration calorimetry.