Red-emissive CDs (carbon dots) are synthesized from Amaranth leaves via an eco-friendly solvothermal (ST) route, exhibiting intense fluorescence with an emission peak at 610   nm under 540   nm excitation and a high quantum yield of 39%. These CDs are incorporated into polyvinyl alcohol (PVA) matrices (1-9   wt %) to fabricate multifunctional nanocomposite (NCs) with superior mechanical, thermal and optical properties. Mechanical analysis revealed enhanced flexibility and a tensile strength improvement of up to 132.48%. Thermogravimetric/Derivative thermogravimetry analysis (TGA/DTA) showed that CDs significantly influenced the degradation behaviour, with higher content resulting in earlier decomposition and up to 93.55% weight loss. Optical studies confirmed excellent UV-blocking capability (100%) with varying film thickness while maintaining high visible light transmittance. Water contact angle (WCA) measurements demonstrated increased hydrophobicity and the 3   wt % CDs-PVA NCs with thickness 200   µm exhibited durable self-cleaning behaviour, even after mechanical wear. These NCs are successfully applied in optoelectronics, food packaging and forensic science. When applied in white light emitting diodes (WLEDs) (0.61, 0.32) and red light emitting diodes (RLEDs) (0.33, 0.32), the NCs displayed emission spectra closely matching the absorption of chlorophyll, thereby promoting spinach growth under indoor lighting. In food packaging applications, the PVA/CDs (3   wt %) NCs effectively preserved the quality and shelf life of cherry fruits for over 20 days by reducing g weight loss and microbial spoilage. For forensic applications, CDs (3   wt %) are blended with Polyvinylpyrrolidone (PVP) to mitigate aggregation-induced quenching (AIQ), enabling the high-resolution visualization of latent fingerprints (LFPs), including distinct level III features. This work demonstrates the versatility and sustainability of biomass-derived CDs as multifunctional nanomaterials for advanced applications in optoelectronics, agriculture, food packing and forensic science.