Chitin-Based Carbon Dots with Tunable Photoluminescence for Fe³⁺ Detection

Carbon dots (CDs) derived from biomass have attracted increasing interest not only due to their sustainability, biocompatibility, and application in metal ion detection but also due to the valorization of waste renewable materials. Herein, tunable photoluminescent (PL) chitin-based CDs were prepared using a deep eutectic solvent (DES) made from a quaternary ammonium salt and sulfonic acid in one step. The DES played two roles during the synthesis procedure: CD formation and surface modification. The PL emission wavelength of the CDs was red-shifted via a strategy of heteroatom doping and size adjustment by changing the structure of the DES and experimental temperature. The structure of sulfonic acid in the DES was the major factor affecting the PL emission peaks of the obtained CDs. The nitrogen-, oxygen-, and sulfur-doped CDs with the highest PL emission peaks at 515 nm were prepared with ChCl/sulfamic acid (1:2 molar ratio) treatment at 150 °C for 5 h. The results demonstrated that the different sizes and surface amino and hydroxyl groups were the main reasons for the red shift of PL emission properties. Additionally, the prepared CDs exhibited high sensitivity for Fe3+ with a limit of detection of 0.43 μM, and the concentration range was 40–600 μM.