Fluorescent turn-off competitive immunoassay for biotin based on hydrothermally synthesized carbon dots

Blue fluorescent carbon dots (C-Dots) were synthesized by a hydrothermal process using citric acid and polyethylene-polyamines as the precursors. The C-dots display strong fluorescence, excellent water solubility, good biocompatibility, and possess many amino groups on their surface. Biotin was linked to the amino groups, and the resulting biotinylated C-dots were applied in a binding assay where they compete with free biotin (vitamin B7; coenzyme R) for binding to streptavidin immobilized on superparamagnetic Dynabeads®. After incubation and magnetic separation, the fluorescence intensity of immobilized biotinylated C-dots released from the beads is inversely proportional to the concentration of biotin in the 0.5 to 100 ng·mL−1 concentration range. The detection limit is 0.1 ng·mL−1. Conceivably, the method may be further extended to the determination of biotinylated biomolecules.