One-pot solid phase pyrolysis synthesis of highly fluorescent nitrogen-doped carbon dots and the interaction with human serum albumin

One kind of fluorescent nitrogen–doped carbon dots (denoted as NCDs) was successfully fabricated by a simple and facile one–pot solid phase pyrolysis process using citric acid and glycine as raw materials. The obtained NCDs under the optimal synthetic conditions have a fluorescence quantum yield up to 35.6% and show a spherical shape with an average diameter of about 3.0 nm. Moreover, NCDs can emit strong blue fluorescence and the surfaces when excited at 345 nm and their surfaces were decorated with lots of carboxyl, hydroxyl and amine groups. In order to further elucidate the effect of NCDs to protein, steady state/time-resolved fluorescence, UV–vis absorption, circular dichroism (CD), and Raman spectroscopic techniques as well as cyclic voltammetry (CV) were employed to explore the interaction of the as-synthesized NCDs with human serum albumin (HSA) in the simulated physiological environment. The experimental data reveal that NCDs could quench the intrinsic fluorescence of HSA via the formation of a ground-state complex with the association constants of the order of 104 L/mol. The values of thermodynamic parameters obtained at three different temperatures suggest that the binding reaction of NCDs with HSA mainly driven by hydrophobic forces and hydrogen bonds was spontaneous. Furthermore, the displacement experiments confirm that the binding of NCDs primarily took place in site I of HSA. The average distance between NCDs and tryptophan (Trp) residue of HSA was estimated to be 2.3 nm according to the theory of Förster non-radiation energy transfer. Finally, the analysis of synchronous fluorescence, three-dimensional fluorescence (3D), Raman, and CD spectra manifests that HSA underwent some conformational changes by unfolding the polypeptides of protein and increasing the polarity of the microenvironment surrounding both Trp and tyrosine (Tyr) residues in the presence of NCDs.