Red pitaya peels-based carbon dots for real-time fluorometric and colorimetric assay of Au, 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 sp-hybridized carbon and carboxylic group contents can rapidly and selectively reduce Au to Au, and stabilize produced Au nanoparticles (AuNPs). The synergetic effect of electron transfer from ACDs to Au 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 Au. The fluorescence quenching ratio and absorbance change linearly with the increase of Au 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 Au 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 Au sensing and imaging. In addition, ACDs exhibited an extraordinarily high antioxidant activity, with an IC value for DPPH radical scavenging (0.70 μg mL) much lower than that of ascorbic acid (4.34 μg mL). The proposed strategy demonstrates the outstanding properties of ACDs in chemical and biomedical analysis. Graphical abstract.