Oxygen/nitrogen-related surface states controlled carbon nanodots with tunable full-color luminescence: Mechanism and bio-imaging

Fluorescent carbon nanodots (CNDs) have exhibited attractive potential for biomedical applications due to their intriguing luminescent properties and good biocompatibility. One interesting properties of CNDs is that they typically show different emission colors upon optical excitation using different wavelengths. However, it is still a challenge to obtain emissions covering the entire visible spectrum with comparable intensity with only one type of CND. Herein, CNDs with tunable full-color luminescence is successful prepared by a solvothermal approach, show broad absorption (200–700 nm) and fluorescence emission (nearly cover the entire visible region: from 400 to 700 nm) with comparable fluorescence intensities. The oxidation (o-CNDs) and reduction (r-CNDs) of CNDs surface would lead to the shifting of photoluminescence band into blue/green and red-light region, respectively. Further spectroscopic analyses and structural characterizations of CNDs (pristine state), o-CNDs (oxidation state) and r-CNDs (reduction state) demonstrate that O-related defect states (C=O) and N-related defect states (C=N) on surface of CNDs should be responsible for blue/green and red fluorescence emissions, respectively. Moreover, the CNDs are found to be biocompatible, as verified by in vitro cells assay and in vivo histological analysis. In vivo fluorescence imaging assay demonstrates that CNDs can be excreted through intestine and bladder system. The tunable full-color emission, good biocompatibility and metabolism suggest that the CNDs are promising fluorescent probes for biomedical applications.