Novel ratiometric fluorescence and colorimetric dual-mode biosensors for Cu2+ and biothiols detection based on norepinephrine modified carbon dots

Novel norepinephrine modified carbon dots (NECDs) were prepared by one-step method. Compared with unmodified carbon dots (CDs), the fluorescence emission peak of NECDs was red shifted and the fluorescence intensity and quantum yield were improved. A ratiometric fluorescence and colorimetric dual-mode sensing platform has been developed for detecting Cu2+ and biothiols based on NECDs. O-phenylenediamine (OPD) works as chromogenic agent in the sensing system, and OPD is oxidized to 2, 3-diaminobenzoazine (oxOPD) in the presence of Cu2+. Due to the fluorescence resonance energy transfer (FRET) between NECDs and oxOPD, the fluorescence of CDs at 450 nm is effectively quenched, but a new emission peak of oxOPD at 563 nm is formed. By adding L-cysteine (Cys) and glutathione (GSH), their sulfhydryl groups bind to Cu2+, thus inhibiting the oxidation of OPD and retaining the fluorescence of CDs. In addition, Cu2+, GSH and Cys can be quantitatively detected by the absorption change of solution. Moreover, the color change is visible. The dual-mode detection method has high sensitivity and selectivity for the determination of Cu2+ and biothiols. The biosensors are successfully applied in actual samples, providing a new way for the detection of ions and biothiols in the environment and biology in the future.