<p>Electrochemical sensors utilizing molecularly imprinted polymers (MIPs) for metoprolol (MTP) often rely on its direct oxidation, which suffers from limited sensitivity due to the low electroactivity of MTP. Herein, a label-free electrochemical sensing platform was developed by electropolymerizing o-phenylenediamine (o-PD) on the carbon nano-onion (CNOs)-modified glassy carbon electrode (GCE). This MIPs/CNOs nanocomposite synergistically enhances the&#xa0;electron transfer rate and creates specific recognition sites. Consequently, the occupation of these sites by MTP hinders the diffusion of the redox probe, enabling sensitive quantification via a signal-off mechanism. Under optimized conditions, the sensor exhibits a wide linear range from 4.0 × 10⁻⁸ to 8.0 × 10⁻⁶&#xa0;mol/L with a detection limit (LOD) of 1.33 × 10⁻⁸&#xa0;mol/L. Notably, this LOD is well below the minimum required performance level (MRPL) set by the World Anti-Doping Agency. The platform exhibits high selectivity against interfering substances and successfully determines MTP in commercial tablets and fortified urine samples with satisfactory recoveries (96.5–103.0%). These results demonstrate the practical applicability of the proposed label-free MIPs/CNOs sensor for sensitive MTP determination in complex matrices and provide a promising strategy for designing label-free electrochemical sensors.</p> Graphical Abstract <p></p>