Turning Waste into Treasure: Multicolor Carbon Dots Synthesized from Waste Leather Scrap and their Application in Anti-Counterfeiting

Using biomass materials to prepare carbon dots (CDs) has received significant interest. As a cheap and environmentally friendly material, waste leather scrap can be used for the preparation of CDs and performance tuning. Herein, waste leather scrap is used as a precursor reactant to fabricate multicolor (red, green, and blue) carbon dots (R-CDs, G-CDs, and B-CDs) with a tunable emission wavelength via a simple one-step hydrothermal/solvothermal strategy using only a nitrogen source (p-phenylenediamine or o-phenylenediamine) for doping. The morphology, structure, and optical properties of the CDs were investigated by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, ζ-potential measurements, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–vis), spectroscopy, photoluminescence (PL), and UV-differential reflectance spectroscopy (UV-DRS). Results showed that the sizes of R-CDs, G-CDs, and B-CDs are 10.0, 7.4, and 2.5 nm, respectively, and all have a typical crystal plane of graphitic carbon structure rich in chemical groups such as C═O, −OH, C–N, etc. The detailed characterization and comparison of the three types of CDs confirm that the size and functional groups can effectively add other energy levels for electron transitions and tuning of luminescent properties of CDs. The emission wavelengths of R-CDs, G-CDs, and B-CDs are 612, 535, and 429 nm, and at the same time, the energy levels were calculated by UV–vis DRS to be 2.11, 2.43, and 2.58 eV, respectively. Because of the stable optical properties and excellent chemical performance, CDs can be employed as ink and flexible films. The utilization of renewable waste leather scrap, along with convenient hydrothermal/solvothermal methods, paves the way to the sustainable, cost-effective synthesis of CDs for practical applications in anti-counterfeiting.