Chitosan functionalization with tin oxide nanocomposites as a biosensor for the detection of cysteine and dihydronicotinamide adenine dinucleotide biological molecules.

The early and accurate detection of biologically important molecules is vital for biomedical diagnostics and environmental monitoring. However, developing detection platforms that are both sensitive and biocompatible remains a significant challenge. In this work, we synthesized chitosan-functionalized tin oxide (CS/SnO₂) nanocomposites (NCs) using a simple solution-based approach and explored their potential as optical biosensors for cysteine and dihydronicotinamide adenine dinucleotide (NADH). X-ray diffraction (XRD) analysis confirmed well-defined crystalline peaks with an average crystallite size of ∼14 nm. Fourier-transform infrared (FTIR) validated the successful incorporation of CS and SnO₂ through characteristic functional groups. UV-Vis spectra showed clear absorbance decreases at 270 and 290 nm, indicating strong interactions between biomolecules and the NCs, attributed to electrostatic attractions, hydrogen bonding, and coordination involving chitosan's amine and hydroxyl groups. The appearance of isosbestic points during spectral changes confirmed stable binding mechanisms. Compared to bare SnO₂ nanoparticles, CS/SnO₂ NCs exhibited significantly higher sensitivity, with a detection limit of 10 μM and a strong linear response in the 10-30 μM range. Overall, these results demonstrate that CS/SnO₂ NCs are a reliable, eco-friendly, and cost-effective platform for biomolecule detection, offering promising applications in bioanalytical and clinical diagnostics.