Sustainable hydrothermal synthesis of ultrasmall carbon quantum dots from drinking coffee and their impact on optoelectronic properties of the methyl cellulose biopolymer

Abstract

Green synthesis of carbon quantum dots (CQDs) from sustainable precursors offers a promising route toward environmentally friendly optoelectronic materials. In this work, waste coffee-derived CQDs were synthesized via a benign, water-based hydrothermal process without toxic reagents and successfully incorporated into a biodegradable methyl cellulose (MC) matrix to engineer polymer nanocomposites with tunable optical properties. The CQDs exhibit a quasi-spherical morphology with abundant surface functional groups, enabling strong interfacial interactions with the MC chains. As a result, significant modulation of the polymer's electronic structure is achieved, evidenced by a pronounced reduction in the optical band gap from 6.1 to 1.9 eV. This behavior is attributed to the formation of localized electronic states and enhanced optically inferred interfacial electronic interactions within the MC/CQD system. Concurrently, the refractive index increases from 1.15 to 1.51, indicating improved polarizability and charge carrier density. The nanocomposites also exhibit modified light propagation characteristics, including reduced group velocity, highlighting their potential in photonic applications. By integrating biomass waste valorization with biodegradable polymer engineering, this study establishes a sustainable and scalable platform for next-generation optoelectronic materials.