Triple emission ratiometric fluorescence and colorimetric dual-mode sensing of glutathione based on multi-function Cu-CDs.

Herein, a method was developed for triple emission ratiometric fluorescence/colorimetric dual-mode detection of glutathione (GSH) based on multifunctional copper-doped carbon dots (Cu-CDs). Cu-CDs displayed dual emission at 465 nm and 650 nm, along with oxidase-like (OXD) and peroxidase-like (POD) activities. The Cu-CDs effectively catalyzed the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP), which emitted yellow fluorescence at 550 nm and selectively quenched the blue fluorescence of Cu-CDs at 465 nm through an inner filter effect (IFE), while the red fluorescence at 650 nm unaffected. GSH could effectively chelate Cu in the Cu-CDs, inhibiting DAP formation, led to the recovery of the 465 nm fluorescence, enabling ratiometric fluorescence detection of GSH. The triple-emission system enabled the discrimination of structurally similar substances while improved the sensitivity of the detection. The fluorescence detection demonstrated a limit of detection (LOD) of 1.13 μM for GSH. For colorimetric detection, Cu-CDs catalyzed HO-mediated 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product (oxTMB) with absorption peak at 652 nm, which further can be reduced by GSH to colorless TMB. Thus, the colorimetric detection of GSH was achieved with LOD of 3.78 μM. The recoveries of the two methods for detecting GSH in fruits and vegetables were found to be 89.67-106.32 % with RSDs as low as 0.6 % (fluorescence) and 1.2 % (colorimetric), respectively.