Eco-friendly synthesis of carbon quantum dots from lemons for enhanced uric acid detection using gold-integrated surface plasmon resonance

In this study, the preparation of carbon quantum dots (CQDs) derived from lemon juice as a precursor using hydrothermal method is described. The optical properties of the CQDs were characterized using photoluminescence spectroscopy, which exhibited a maximum emission peak at 450 nm, with the highest intensity recorded for the sample treated at 180 °C. Fourier transform infrared spectroscopy confirmed the presence of amino, carboxylic acid, carboxyl, and hydroxyl groups in the CQDs. Transmission electron microscopy analysis revealed that the CQDs were predominantly spherical in shape, with an average particle diameter of approximately 4.73 nm. The integration of the gold/CQDs thin film with surface plasmon resonance spectroscopy enabled the detection of uric acid. Using Winspall software, the refractive index of the gold/CQDs thin film was determined to be n + ik = 1.6014 + 3.7i, with a thickness of 12.00 nm. The sensor demonstrated a linear response to uric acid concentrations up to 1 µM, achieving sensitivity values of 3.968° µM^-1 and 0.4098° µM^-1, along with a binding affinity constant of 6.94754 MM^-1. These findings demonstrate that the gold/CQDs thin film possesses strong potential for the detection of uric acid, achieving quantitation and detection limits as low as 0.05 µM and 0.002 µM, respectively.