Nitrogen-Doped Carbon Quantum Dot-Anchored Hydrogels for Visual Recognition of Dual Metal Ions through Reversible Fluorescence Response

Carbon quantum dots (CQDs) are novel and promising fluorescent materials that are extensively investigated in sensing, drug delivery, and bioimaging. However, they are easy to agglomerate and difficult to recover, which reduced their sensitivity and reusability. In this work, we innovatively prepared a "smart switch sensor" hydrogel (SWSH) by incorporating nitrogen-doped carbon quantum dots (N-CQDs) into a polyacrylamide (PAM) hydrogel network with functionalities of easy handling, recyclability, and simultaneously recognizing dual metal ions. The prepared SWSH showed reversible fluorescence "off/on" response with Cu2+/Ag+ and l-cysteine (l-cys), and the limit of detection values of the SWSH for these metal ions were 0.813 × 10–3, 0.468 × 10–3, and 0.437 × 10–3 mM, respectively. The N-CQDs in the SWSH would form a N-CQD–metal complex with Cu2+ or Ag+ ions, in which the electrons on the surface of CQDs were easily transferred to the metal, resulting in fluorescence quenching (off). However, only the fluorescence of the N-CQD–Cu2+ complex could be recovered (on) by the addition of l-cys as it could form a more stable Cu2+–S bond with Cu2+, thus achieving recognition of Cu2+ and Ag+ through fluorescence change observed with naked eyes. Therefore, the SWSH avoided the agglomeration of CQDs and facilitated their fluorescence recovery and reusability, which exhibited great potential applications in identifying metal ions.