Fe Single-Atom and Fe Cluster-Coupled N, S Co-doped Carbon Nanomaterial-Based Flexible Electrochemical Sweat Biosensor for the Real-Time Analysis of Uric Acid and Tyrosine

Fe single-atom and Fe cluster-coupled N, S co-doped carbon nanomaterials (Fe_SA–FeO_NC-NSC) were prepared through a two-step high-temperature pyrolysis process using Gelidium corneum enriched with C, Fe, O, N, and S as precursors. The analysis by aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy revealed the presence of single-atom Fe in Fe–N₄ coordination structures, along with small clusters as Fe–O-coordinated Fe₂O₃. Single-atom Fe in the form of Fe²⁺/Fe³⁺ provides more electrocatalytic active sites, which synergistically accelerates the charge migration process in the assembly of Fe_SA–FeO_NC-NSC with Fe₂O₃ clusters. The flexible nonenzymatic sensor, based on Fe_SA–FeO_NC-NSC and fabricated using a polydimethylsiloxane substrate, exhibited excellent catalytic activity for both uric acid (UA) and tyrosine (Tyr). Low detection limits for UA (0.14 μmol L^–1) and Tyr (0.03 μmol L^–1) were observed by using chronoamperometry in artificial sweat. The in situ detection of sweat was performed in combination with an integrated circuit board affixed to human skin, and the results were generally consistent with those of the high-performance liquid chromatography method. Therefore, Fe_SA–FeO_NC-NSC serves as a good modifier for wearable electrochemical sweat sensor applications.