Modular Electrochemical Sensing via In Situ Reduced Carbon Quantum Dot-Nanotube Molecular Hybrids

Electrochemical sensing is a promising route to non-invasive, point-of-care health monitoring, yet practical multiplexing remains limited by electrode materials that cannot simultaneously deliver high conductivity, high densities of electrochemically active sites, and modular surface chemistry for targeted analyte specificity. Here, we introduce a hybrid nanocarbon architecture that decouples charge transport from surface reactivity by grafting oxygen-rich carbon quantum dots (CQDs) as a chemically active "skin" onto a conductive multiwalled carbon nanotube (CNT) scaffold (CQD@CNT), then applying in situ electrochemical reduction to strengthen interfacial electronic coupling while preserving functional groups that promote analyte adsorption and catalysis. By systematically tuning the reduction potential, we identify an optimal regime that maximizes CQD-CNT interfacial coupling and charge transport while avoiding over-reduction that compromises sensor activity. Spectroscopic and electrochemical characterization reveals partial deoxygenation and restoration of conjugation within the hybrid material, increased electrochemically active surface area, and a pronounced decrease in charge-transfer resistance after in situ reduction. Using differential pulse voltammetry, the reduced molecular hybrids exhibit enhanced sensitivity toward uric acid and support multiplexed detection of uric acid, dopamine, and ascorbic acid within a single voltammetric scan. Sensor modularity is further demonstrated by covalently grafting 18-crown-6 to CQDs to enable Pb2+ and Cd2+ detection via anodic stripping voltammetry while preserving the same conductive CNT scaffold. This result illustrates how molecular recognition motifs can be integrated into the CQD skin without re-engineering the electron-transport network. We further validate the CQD@CNT platform on screen-printed carbon electrodes (SPCEs), demonstrating its compatibility with low-cost, disposable electrode formats relevant to deployable sensing. Together, these results establish in situ electrochemical reduction as a scalable, post-fabrication strategy to overcome the conductivity-activity trade-off in electrochemical sensing and provide a generalizable CQD@CNT platform for multiplexed biosensing and environmental monitoring across multiple electrode platforms.