A Ti₃C₂ MXene@TiO₂/Co_2.7Ni_0.3O₄ Z-scheme heterojunction as a photoelectrochemical sensing platform for H₂O₂ from Hela cell mitochondria

In this work, we have constructed a Ti₃C₂ MXene@TiO₂/Co_2.7Ni_0.3O₄ Z-type heterojunction with excellent conductivity and efficient electron transport (23 Ω charge transfer resistance, 0.79 electron transfer coefficient, and 0.29 s^-1 heterogeneous electron transfer constant, based on the redox marker [Fe(CN)₆]^4-), which have all contributed to effectively minimising recombination of photogenerated electrons and holes, making it a superior electrode material for developing a photoelectrochemical sensor. Experimentally, low-temperature annealing was exploited to partially oxidise Ti₃C₂ to TiO₂ to establish an intimate contact between them, before being loaded on a Co_2.7Ni_0.3O₄ nanowire array to form a Ti₃C₂ MXene@TiO₂/Co_2.7Ni_0.3O₄ heterojunction. After microscopically and spectroscopically characterising the material, electrochemical studies demonstrated matching conduction band voltage of Co_2.7Ni_0.3O₄ (0.66 eV) and valence band voltage of TiO₂ (-0.4 eV) that facilitate electron transport. The results have allowed us to propose a detection mechanism for H₂O₂ at the photoelectrochemical sensor. The photoelectrochemical sensor was then applied to detecting H₂O₂ enzymatically converted by superoxide dismutase from superoxide radicals released in mitochondria extracted from cancerous HeLa cells. In this respect, a 0.05-50,000 nM linear range, a 5.93 μA sensitivity, and a 0.03 nM H₂O₂ limit of detection were accomplished.