Multiwalled Carbon Nanorings Found in Raw Single-Walled Carbon Nanotubes and Applied for Electrochemiluminescent Immunoassay

Graphene quantum dots (GQDs) prepared by the top-down route have promising applications in chemical sensing and biosensing for their easy preparation, strong electrochemiluminescence (ECL), low cytotoxicity, easy labeling, and favorable biocompatibility. However, the extraordinary water solubility of GQDs results in poor immobilization of GQDs on the sensing interfaces and then low sensing sensitivity and stability, which limit their extensive sensing applications. In order to solve the problem of poor immobilization of GQDs on the sensing interfaces and improve their sensing performances, we in situ prepared and loaded single-layered GQDs onto multiwalled carbon nanorings (MWCNRs) by a simple one-step acid treatment of raw single-walled carbon nanotubes (SWCNTs) with side products of MWCNRs. It is for the first time that a large number of MWCNRs with well crystallization nanostructures, good electroconductivity, and excellent chemical stability were found in raw SWCNTs and applied for ECL sensing. During the HNO₃-treatment of the raw SWCNTs (nanohybrids of SWCNTs/MWCNRs), the component of SWCNT was oxidized into single-layered GQDs, while the component of MWCNR was essentially kept unchanged. The resultant GQDs were in situ and stably immobilized on the surfaces of MWCNRs by acid-enhanced π-π stacking interactions, in the absence of electrostatic repulsion. The synthesized MWCNRs@GQDs exhibit a strong and stable ECL activity on the sensing interface and show promising applications in ECL immunoassay of biomarkers, such as human chorionic gonadotropin.