Red pitaya peels-based carbon dots for real-time fluorometric and colorimetric assay of Au3+, cellular imaging, and antioxidant activity.

The synthesis of fascinating multifunctional carbon dots (CDs) attracted immense attention. Here, a facile solvothermal treatment of red pitaya peels in acetic acid produced CDs (designated as ACDs, excitation/emission wavelengths at 357/432 nm). ACDs with high sp2-hybridized carbon and carboxylic group contents can rapidly and selectively reduce Au3+ to Au0, and stabilize produced Au nanoparticles (AuNPs). The synergetic effect of electron transfer from ACDs to Au3+ and inner filter effect (IFE) from ACDs to AuNPs quenches the fluorescence within 30 s. Simultaneously, the resulting AuNPs have a purple color with a maximum absorption at 545 nm for visual detection. Therefore, for the first time, we reported a fluorometric and colorimetric dual-mode sensing system for real-time, highly sensitive and selective detection of Au3+. The fluorescence quenching ratio and absorbance change linearly with the increase of Au3+ concentration in the range of 0.3-8.0 μM and 3.3-60.0 μM with limits of detection (LODs) at 0.072 μM and 2.2 μM, respectively. The assay was applied for Au3+ determination in spiked real water samples with recoveries from 95.5 to 105.0%, and relative standard deviation (RSD) of less than 6.5%. Furthermore, ACDs with good photostability, low cytotoxicity, and excellent biocompatibility were successfully applied for intracellular Au3+ sensing and imaging. In addition, ACDs exhibited an extraordinarily high antioxidant activity, with an IC50 value for DPPH radical scavenging (0.70 μg mL-1) much lower than that of ascorbic acid (4.34 μg mL-1). The proposed strategy demonstrates the outstanding properties of ACDs in chemical and biomedical analysis. Graphical abstract.