Fine-Tuning Ni/Co Ratio to Elucidate the Coordination Structure-Activity Relationship of MOF-derived Bimetallic Layered Double Hydroxide 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 hydroxide (LDH) 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 the α-phase hydroxide with superior catalytic activity, while a high Co ratio led to β-phase hydroxide formation. Advanced X-ray absorption spectroscopy (XAS) and Raman analyses revealed that the optimized Ni-rich LDH exhibited a unique mixed octahedral (Oh)/tetrahedral (Td) 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 (vs. Ag/AgCl/3 M KCl) in alkaline medium. Furthermore, as proposed biosensor exhibited excellent repeatability, maintaining 85.70% of its initial response after 18 days under 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.