Caffeic Acid-Driven Green Synthesis of Rhenium Nanoparticles Embedded in Self-Templating Double-Shelled ZnMn ₂ O ₄ Hollow Microspheres for Ultrasensitive Epinephrine Detection in Biofluids

In the present work, we report an electrochemical sensor based on rhenium (Re) nanoparticles embedded within a double-shelled ZnMn₂O₄ hollow microsphere (Re@ZnMn₂O₄) for ultrasensitive detection of epinephrine (EP) in biofluids. The Re@ZnMn₂O₄ material was synthesized via a coprecipitation and annealing route, followed by a green, caffeic acid (CA)-assisted chemical reduction. Structural and morphological analyses, including spectrophotometry, confirmed the high purity, crystallinity, and integrity of the material. Electrochemical performance was evaluated using voltammetry and impedance spectroscopy. The Re@ZnMn₂O₄-modified electrode exhibited superior electrochemical activity attributed to its high conductivity, large surface area, abundant active sites, and efficient charge transfer enabled by the hollow architecture. EP oxidation followed a diffusion-controlled 2e^-/2H⁺ transfer mechanism. The sensor demonstrated a broad linear detection range (0.5-1951.3 μM), a low detection limit (0.21 μM), and good sensitivity (0.282 μA μM^-1 cm^-2). Furthermore, it showed remarkable reproducibility, long-term stability, and strong resistance to common interferents. Its practical potential was validated by accurate EP quantification in human serum and urine, highlighting its applicability in clinical diagnostics and biomedical monitoring.