Supported NiOx Nanocatalysts on Graphene for Nonenzymatic Lactate Sensing

Electrodeposition of nickel nanoparticles, achieved through potentiostatic amperometry followed by thermal annealing at 200–400 °C, has produced nickel oxide nanoparticles on flexible graphene substrates. Using transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, we demonstrate that the resulting NiOx nanoparticles exhibit several Ni oxidation states and a core–shell heterostructure, with a metallic Ni crystalline core and a NiO crystalline shell with a mixed crystalline–amorphous NiOOH/Ni(OH)2 skin. The composition of the NiOOH/Ni(OH)2 redox couple relative to NiO is found to vary with the annealing temperature and annealing time. The use of a highly conductive graphene substrate enhances electron transfer. The NiOx nanoparticle samples obtained with selected annealing temperature–time combinations are used for lactate detection, with the best sample showing an excellent linear range of 0.02–65.1 mM, a high sensitivity of 80.0 μA mM–1 cm–2, and an impressive limit of detection of 0.00015 mM. The NiOx nanoparticle sample is also tested for lactate sensing in an artificial sweat electrolyte, and it exhibits a reduced linear range of 0.02–53.1 mM and a lower limit of detection of 0.00013 mM while maintaining the same high sensitivity of 80.0 μA mM–1 cm–2. This sensing performance can be optimized by controlling the relative composition NiOOH + Ni(OH)2 to NiO, with the sample obtained with a higher relative content at a lower annealing temperature found to provide more reactive sites for lactate detection and therefore higher sensitivity.