Engineering FeOOH/Fe₂O₃@Carbon Interfaces With Biomass-Derived Carbon Nanodot/Iron Colloids for Efficient...

The Fe²⁺/Fe³⁺ redox couple is effective for voltammetric detection of trace dopamine (DA). However, achieving adequate concentrations with high electroactive surface area (ECSA), DA affinity, and fast interfacial charge transfer is challenging. Consequently, most reported Fe-based sensors have a high nanomolar range detection limit (LOD). Herein, we address these limitations by manipulating the phase and morphology of FeOOH/Fe₂O₃ heterojunctions anchored on sp²-carbon. FeOOH/Fe₂O₃ is synthesized by variable temperature aging of unique Fe₅H₉O₁₅/Fe₂O₃@sp²-carbon colloidal nanoparticles, which form via chelation between biomass-derived carbon nanodots (CNDs) and Fe²⁺ ions. At 27 °C and 120 °C, Fe₅H₉O₁₅/Fe₂O₃@sp²-carbon transforms into β-FeOOH/Fe₂O₃ nanoparticles and α-FeOOH/Fe₂O₃ nanosheet, respectively. The β-FeOOH/Fe₂O₃ interface exhibits higher e_g orbital electron occupancy than α-FeOOH/Fe₂O₃, thereby facilitating oxygen adsorption and the generation of Fe²⁺/Fe³⁺ sites near the polarization potential of DA. This facilitates interfacial electron transfer between Fe³⁺ and DA. Moreover, its nanoparticle morphology enhances ECSA and DA adsorption compared to α-FeOOH/Fe₂O₃ nanosheets. With a LOD of ~3.11 nM, β-FeOOH/Fe₂O₃ surpasses the lower threshold in humans (~10 nM) and matches noble-metal sensors. Furthermore, it exhibits selective detection of DA over 10 biochemicals in urine. Therefore, the β-FeOOH/Fe₂O₃@sp²-C platform holds promise as a low-cost, easy-to-synthesize, and practical voltammetric DA monitor.