Carbon Quantum Dots Derived from Fresh Natural Dyes to Improve the Optoelectronic Properties of Chitosan Biopolymer: Structural, Spectroscopic and Optical Studies

In this study an eco-friendly, one-pot hydrothermal processes use to synthesize water-soluble CQDs from natural dye extracted from Bougainvillea flowers (DBG). Carbon quantum dots (CQDs) offer outstanding optical properties and low toxicity, making them attractive for sustainable optoelectronic materials. The resulting bougainvillea carbon quantum dots (BQDs) were integrated with chitosan (CS) biopolymer to deliver composite films with high optical properties. Structural, morphological, and optical analyses, supported by NMR, FTIR, XRD, HRTEM, XPS, and UV-vis spectroscopy, confirmed the production of amorphous BQDs and strong CQD–CS interactions with reduced crystallinity. Notably, the optical band gap narrowed significantly from 5.23 eV in pure CS to 2.42 eV in BQDs-doped composites. The BQDs exhibited excitation-dependent photoluminescence with a quantum yield of 2.74%, while Urbach energy analysis indicated increased localized states with higher BQD content. Enhanced refractive indices, dielectric constants, and non-linear optical properties further demonstrate the composites’ potential for photonic and optoelectronic applications. This work establishes a green, scalable strategy to boost biopolymer functionality using natural carbon sources.