Green synthesis of carbon quantum dots and the preparation of bio-nanocomposites based on chitosan with modified refractive indices and optoelectronic properties.

Carbon quantum dots (CQDs) were synthesized via a green hydrothermal route from Nerium oleander flower dye. XRD and FTIR spectroscopic investigations were used to identify the CQD particles. The CQD particles were embedded into a chitosan host biopolymer to produce polymer composites with enhanced optical properties. FTIR results verified -OH/-COOH/-NH surface functionalities on the CQDs and their hydrogen-bonding interactions with the CS host polymer. XRD results showed the principal CS diffraction peaks moving from 20.56° (CS) to ∼21.0° (CSCD20) and 21.42° (CSCD40), evidencing matrix rearrangement and crystallinity suppression upon CQD doping. UV-vis-NIR spectra revealed new absorption features at ∼240, ∼301, and ∼402 nm with a pronounced redshift of the edge. Compared with pristine CS (high UV transmittance), the doped films exhibited stronger UV absorption while preserving high transmittance toward the near-IR region. Optical band-gap analyses (Tauc/ASF) demonstrated a substantial narrowing from ∼4.95 eV (CS) to ∼2.45 eV (CSCD20) and ∼2.22 eV (CSCD40), indicating enhanced visible-light harvesting. Concomitantly, the refractive index increased and remained comparatively flat across the UV-visible range (up to ∼2.7), and both the real and imaginary parts of the dielectric function rose at low photon energies, consistent with higher electronic polarizability and charge-transfer capacity. Drude/Wemple-DiDomenico evaluations indicated an increased carrier density-to-effective-mass ratio and reduced optical resistivity with CQD content, while sheet resistance and thermal emissivity decreased across 190-1100 nm. Thermal analyses (TGA/DSC) showed improved robustness: peak decomposition temperatures rose from ∼240 °C (CS) to ∼270 °C/295 °C (CSCD20/CSCD40) and T g from ∼95 °C to ∼110 °C/120 °C. Overall, Nerium-derived CQDs provide an eco-friendly, effective route to tailor the optical constants and stability of CS films for photonic coatings, optical sensing, and sustainable bio-optoelectronic components.