Salting Out on a Surface: A Versatile Method for Conjugating Carbon Dots With Proteins, Nucleic Acids, and Polysaccharides.

Carbon quantum dots (CQD) constitute a unique class of materials for biomedical applications due to their inherent optical properties and biocompatibility. However, currently, there is no generalized universal procedure for conjugation of CQD with biopolymers, which significantly reduces their practical value. Here we show an ultrafast "salting out on a surface" (SOS) method for the conjugation of CQD with a wide range of biomacromolecules, including proteins, nucleic acids, and polysaccharides. This method relies on the salting out process, which forms a biopolymer layer on the CQD surface that persists even after ionic strength is restored. The salting out also causes the formation of a transient precipitate phase of CQD conjugates, allowing for their easy separation from the unbound biopolymers. Importantly, the biopolymers retain their biological function after the procedure. Applying the SOS, we demonstrate the successful use of CQD conjugates in lateral flow assay (conjugation with anti-chloramphenicol antibody), in enzyme-linked immunosorbent assay (simultaneous conjugation with either NanoLuc luciferase or horseradish peroxidase and anti-chloramphenicol antibody), in cell targeting (conjugation with trastuzumab), and in gene delivery (conjugation with polyethylenimine and plasmid DNA). Owing to its versatility, rapidity, and simplicity of the purification step, the suggested procedure can significantly expand the spectrum of biomedical applications of CQD.