Supported NiO_x 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 NiO x 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 NiO x 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 NiO x 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.