Green Synthesis of CuO/CQD Nanocomposites for Electrochemical Glucose Sensing: From Drop-Casting to Microplotting

In this study, we report the first nonenzymatic glucose (NEG) sensor based on CuO/carbon quantum dot (CQD) nanocomposites synthesized via a green hydrothermal method using Aloe arborescens extract as the reducing agent and carbon source. The use of this phytochemical source and the application of microplotting for fabricating green-synthesized nanomaterial-based NEG sensor electrodes have not been previously explored. Structural characterization (SEM, EDS, TEM/SAED, FT-IR, Raman, XRD, XPS, and TGA/DTA) confirmed the formation of quasi-spherical CuO nanostructures decorated with CQDs. Electrochemical characterization of pristine CuO and CuO/CQD films on fluorine-doped tin oxide (FTO) substrates revealed the superior performance of the CuO/CQD/FTO platform. A transition from drop-casting to precision microplotting was implemented to improve film uniformity and electrode reproducibility. Comparative evaluation showed that microplotting produced CuO/CQD electrodes with more application-relevant electrochemical behavior. Accordingly, CuO/CQD ink was microplotted onto screen-printed gold electrodes (SPGEs) and evaluated for glucose detection in 0.1 M NaOH. The resulting CuO/CQD/SPGE platform exhibited a wide linear range of 0.9–17.1 mM (R2 ≥ 0.995), a detection limit of 0.33 mM, and sensitivities of 0.131–0.0826 mA·mM–1·cm–2, with a rapid steady-state response (<6 s). This linear range spans hypoglycemic to hyperglycemic states, confirming the clinical relevance of the platform. The CuO/CQD sensor also demonstrated excellent repeatability, reproducibility, stability, and selectivity under the influence of common interferents, chelating agents, and physiologically relevant chloride concentrations. Comparative benchmarking with other CuO-based NEG sensors indicates that electrode geometry, deposition precision, and material compatibility contribute to extending the linear range. Proof-of-concept testing in serum showed <6% deviation from a commercial glucose meter, affirming the clinical potential of the CuO/CQD/SPGE platform. These findings illustrate how green-synthesized nanomaterials and precision microfabrication can enable sustainable, high-performance glucose sensing platforms for clinical and point-of-care applications.