Cationic Carbon Dots for Modification-Free Detection of Hyaluronidase via an Electrostatic-Controlled Ratiometric Fluorescence Assay

Carbon dots (CDs) emerge as excellent fluorescent nanomaterials, but the full exploitation and application of their exceptional properties in the development of fluorescence assay are still rare. In this work, cationic carbon dots (C-CDs) covered with plenty of positive charges on the surface were synthesized through a facile ultrasonic method. Negatively charged hyaluronic acid (HA) caused the aggregation of positively charged C-CDs and neutral red (NR) along its linear chain via electrostatic adsorption, leading to a remarkable Förster resonance energy transfer (FRET) from C-CDs to NR. However, the presence of hyaluronidase (HAase) resulted in the enzymolysis of HA, as well as the liberation of C-CDs and NR. The corresponding change of fluorescence color from red to green-yellow afforded a reliable ratiometric assay for HAase. Also the ratio of fluorescence intensity for C-CDs (I525) to that for NR (I630) was used for quantitative detection of HAase. The proposed sensing system was easily operated in aqueous media with a detection limit of 0.05 U/mL. This strategy provides a new approach for the wider application of some special CDs in detecting biomolecules.