Halogen Doping Selectively Induces a High-Spin State in Octahedral Cobalt Sites for Boosting Electrochemiluminescence

Abstract

In luminol-dissolved oxygen electrochemiluminescence (ECL) systems, endogenous O2 serves as a co-reactant, generating reactive oxygen species (ROS) in situ through cathodic oxygen reduction reaction (ORR). However, this process is constrained by spin-restricted kinetics arising from the spin-forbidden transition from triplet O2 to singlet H2O, which requires the unpaired d electrons of the metal to undergo spin-exchange interaction with the π* electrons of oxygen. Herein, we propose a spin barrier synergy strategy via halogen substitution at oxygen sites of octahedral cobalt, where bromine doping induces a noticeable lattice distortion that increases d-electron occupancy, thereby alleviating the spin-forbidden transition in ORR. Magnetic measurements and theoretical calculations reveal that bromine substitution induces a spin-state transition from intermediate-spin (t2g5eg1) to high-spin (t2g4eg2) state, increasing dz2 orbital occupancy and shifting the d-band center closer to the Fermi level. Consequently, precise spin-mediated control reduces the energy change for the conversion of adsorbed O2 to 0.21 eV, compared to 0.87 eV without spin regulation. This energy reduction redirects the ORR pathway toward hydroxyl radical generation and ultimately boosts luminol ECL performance by nearly 3-fold relative to Co3O4, with total emission originating from the combined contributions of ROS generated at the working electrode and luminol oxidation mediated by the Pt counter electrode. As an application, an aptamer-based ECL sensor with bromine-doped Co3O4 as a catalyst achieves sensitive and selective detection of isocarbophos. This study provides fundamental insights into electron spin catalysis and offers a rational strategy for designing high-efficiency spin-modulated ECL systems.