Green synthesized multiple fluorescent nitrogen-doped carbon quantum dots as an efficient label-free optical nanoprobe for in vivo live-cell imaging

In this work, nitrogen-doped carbon quantum dots (N-CQDs) have been synthesized successfully by a simple hydrothermal method and demonstrated its application for multicolor imaging in Caenorhabditis elegans (C. elegans) as an in vivo model. The synthesized N-CQDs were characterized by various physicochemical techniques such as XRD, Raman spectroscopy, ATR-FTIR spectroscopy, XPS, HRTEM, UV–vis spectroscopy, and fluorescence spectroscopy. The synthesized N-CQDs exhibited a strong fluorescence due to the uniform size distribution with nitrogen-containing and oxygen-containing functional groups onto the surface of N-CQDs which induce the excellent dispersibility in aqueous media. The N-CQDs has an excitation-dependent fluorescence behavior and the strongest fluorescence appeared at 411 nm (emission peak position) under the excitation of 340 nm. Also, the N-CQDs displayed a high quantum yield (QY) of 12.5. The fluorescence behaviour of the aqueous N-CQDs suspension retains for a long time up to 1 year. The prolonging fluorescent N-CQDs was utilized as a staining agent for bioimaging and toxicity of N-CQDs on C. elegans that was conducted by killing assay. In-vivo studies suggested that the N-CQDs displayed excellent biocompatibility and successfully used for high-contrast imaging of N-CQDs in living and dead C. elegans. Based on the strongest fluorescence along with excellent aqueous dispersibility and biocompatibility, the green synthesized N-CQDs would be an ideal candidate for many biological applications.