Solid-state synthesis of self-functional carbon quantum dots for detection of bacteria and tumor cells

Abstract

We have developed a simple solid-state synthesis procedure to functionalize fluorescent carbon quantum dots (CQDs), using ammonium citrate as a carbon source and appropriate molecules as recognition ligands. Mannose and folic acid were used to modify CQDs to selectively label Escherichia coli (E. coli) and tumor cells, respectively. First, fluorescent-core CQDs (approximate size: 3nm) were synthesized through carbonization of ammonium citrate via dry heating. In the second step, CQDs were heated with mannose and folic acid to prepare mannose-functionalized CQDs (Man–CQDs) and folic acid-functionalized CQDs (FA–CQDs), respectively, through a dehydration reaction in the solid state. Solid-state synthesis of the self-functional CQDs is achievable without a coupling agent. We optimized the labeling efficiencies of self-functional Man–CQD and FA–CQD to cells by controlling the ratio of mannose or folic acid to CQDs, as well as the reaction temperature during synthesis. The solid-state synthesized Man–CQDs and FA–CQDs exhibited excitation-dependent fluorescence with excitation and emission maxima of 365 and 450nm, respectively, and a fluorescence quantum yield of approximately 9%. Man–CQDs can be used for selective labeling of E. coli and detection at concentrations as low as 100 colony forming units mL−1 in real samples (e.g., drinking water, apple juice, urine). Furthermore FA–CQDs are highly selective for labeling of folate receptor-overexpressing tumor cells. The synthesis of self-functional CQDs is simple, cost effective, and easily scaled up, and can be extended to the synthesis of various functional carbon nanomaterials, such as graphene oxide nanosheets, carbon nanotubes, fullerene nanoparticles and carbon nanodiamonds, with different ligands for other biolabeling applications and targeted therapies.