Ratiometric fluorescence and smartphone dual-mode detection of glutathione using carbon dots coupled with Ag + -triggered oxidation of o-phenylenediamine.

Developing ratiometric fluorescence and smartphone dual-mode bioanalysis methods is important but challenging. A ratiometric fluorescence method for determining glutathione (GSH) using carbon dots (CDs) and Ag ⁺ -triggered o-phenylenediamine (OPD) oxidation is described here. Ag ⁺ oxidizes OPD to give 2,3-diaminophenazine (oxOPD), which effectively quenches CD fluorescence at 436 nm through the inner filter effect and causes a new emission peak at 561 nm. GSH chelates with Ag ⁺ and prevents the Ag ⁺ oxidizing OPD and therefore effectively preserves CD emission at 436 nm (blue) and allows only weak oxOPD fluorescence at 561 nm (orange) to occur. The oxOPD to CD fluorescence intensity ratio decreased linearly as the GSH concentration increased in the range 0-150 nM, and the detection limit was 15 nM. The ratiometric fluorescence probe lit with an ultraviolet lamp clearly changed color from orange to blue as the GSH concentration increased. An image was acquired using a smartphone camera and converted into digital values. The blue and red channel ratio was calculated and used to quantify GSH. The method therefore allows dual-mode detection of GSH.
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