Preparing nitrogen and sulfur-codoped carbon quantum dots to achieve labeling of bovine serum albumin

Labeling of proteins with carbon quantum dots (CQDs) has always been limited by their complex and cumbersome modifications, which leads to the poor reproducibility of results. In this paper, a one-step solvothermal synthesis approach was developed to prepare nitrogen and sulfur-codoped CQDs using citric acid and thiourea as precursors. Three components emitting blue, yellow and red fluorescence were isolated, and the isothiocyanate functional group was demonstrated to be attached to red emissive CQDs (R-CQDs) by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Bovine serum albumin (BSA) was successfully labeled with R-CQDs at a mass ratio of 1 : 1 via a reaction of the isothiocyanate functional group and the lysine residue of BSA. The labeling of R-CQDs with BSA follows a pseudo-second order kinetic model with a rate constant of 0.0796 h^-1 and a maximum labeling efficiency of 31.22%. A Förster resonance energy transfer effect was found between BSA and R-CQDs during labeling. The apparent dissociation constant of the R-CQD-labeled BSA and the Hill coefficient are 1.35 × 10^-2 mg mL^-1 and 1.05, respectively, while the binding constant and the number of binding sites are 3.98 × 10² and 1.35, respectively. This labeling method can also be used for quantification of proteins with a detection limit of 7.0 μg mL^-1 and is promising to be applied in the integration of optical diagnosis and photothermal therapy due to the outstanding photothermal conversion efficiency (60.6%) of R-CQDs.