Fine-tuning the Ni/Co ratio to elucidate the coordination structure–activity relationship of MOF-derived bimetallic layered double hydroxides for highly sensitive enzyme-free lactate biosensors

The development of a reliable, non-enzymatic electrochemical sensor for lactate detection is crucial for real-time monitoring of muscle fatigue and human metabolism. In this work, we present a straightforward and controllable synthesis method for nickel-cobalt bimetallic layered double hydroxides (LDHs) derived from a metal-organic framework (MOF) precursor in an alkaline medium. The Ni/Co ratio was systematically tuned to induce distinct hydroxide phase transformations, where a high Ni content favored the formation of α-phase hydroxide with superior catalytic activity, while a high Co ratio led to the formation of β-phase hydroxide. Advanced X-ray absorption spectroscopy (XAS) and Raman analyses revealed that the optimized Ni-rich LDH exhibited a unique mixed octahedral (O_h)/tetrahedral (T_d) coordination, with a tetrahedral-dominant structure that enhanced charge transfer and electronic conductivity. Additionally, the Ni-rich LDH facilitated the formation of trivalent metal (Ni³⁺/Co³⁺) species, promoting stronger redox activity essential for lactate oxidation. The optimized Ni-rich LDH-modified screen-printed carbon electrode demonstrated outstanding electrochemical performance, achieving a high sensitivity of 63.66 ± 3.86 μA mM^-1 within a lactate concentration range of 0-12.5 mM at an applied potential of 0.60 V (<i>vs.</i> Ag/AgCl/3 M KCl) in an alkaline medium. Furthermore, the as-proposed biosensor exhibited excellent repeatability, maintaining 85.70% of its initial response after 18 days at room temperature, highlighting its remarkable stability for long-term applications. This study provides valuable insights into the structure-activity relationship of MOF-derived LDHs and offers a promising pathway for developing high-performance, enzyme-free lactate biosensors for non-invasive monitoring of physiological conditions.