Tunable Osmium Complexes as pH-Insensitive Redox Reporters for Electrochemical Biosensing

The lack of robust and tunable redox reporters continues to hinder the development of high-performance electrochemical biosensors. In this work, we present a combined computational and experimental investigation of a family of water-compatible Os(II) complexes as pH-insensitive alternatives to the gold-standard methylene blue (MB). We systematically engineered, synthesized, and characterized, both computationally and electrochemically, four Os(II) polypyridyl complexes. We achieved a broad formal potential (E0') range from 0.0 to +0.34 V vs Ag/AgCl, while maintaining fast, quasi-reversible one-electron behavior. These trends were reproduced by theoretical calculations and linked to systematic changes in the highest occupied molecular orbital (HOMO) energy/metal-ligand charge distribution. Covalent immobilization of the complexes onto carboxylated self-assembled monolayers on gold via amide coupling underscored their suitability as redox reporters in electrochemical biosensors. The redox potential remained pH-invariant from acidic to basic conditions, in contrast to the pronounced shifts observed for MB-derived probes. Stability under rapid and continuous electrochemical pulses by square-wave voltammetry was also assessed, with complexes 1, 2, and 4 displaying high stability after 200 scans in buffer compared to complex 3 and MB-based probes. Collectively, these results provide design rules and establish Os(II) complexes as modular redox reporters with on-demand features for advanced electrochemical sensors, including ratiometric and multiplexed sensing.