Three-Dimensional Copper Foam Nanocatalysts for Nonenzymatic Electrochemical Lactate Sensing

Copper foam annealed in oxygen at a relatively low temperature has been found to exhibit remarkable catalytic activities, with enhanced electrochemically active surface area, high conductivity, and distinct surface structures, appropriate for nonenzymatic lactate sensing. X-ray photoelectron spectroscopy and X-ray diffraction data show that these annealed Cu foam exhibit several Cu-oxidation states, and a morphology consisting of a metallic Cu base, a CuO layer on the surface, and an intermediary Cu₂O layer in between. The relative compositions of Cu₂O and CuO layers are found to vary with the annealing temperature, resulting in differences in the conductivity and catalytic properties of the as-prepared samples. In particular, the Cu foam annealed in oxygen at 200 °C for 60 min exhibits high conductivity and high charge carrier concentration, as determined by electrochemical impedance spectroscopy and Hall effect measurement, respectively. High specific surface area is also shown, in accordance with the electrochemically active surface area calculation. For nonenzymatic lactate sensing, this annealed Cu foam sample is found to exhibit an excellent linear range of 1–120 mM, a high sensitivity of 800 μA mM^–1 cm^–2, and a low limit of detection of 0.367 μM. When tested for lactate sensing in an artificial sweat electrolyte, this sample shows the same excellent linear range (1–120 mM), but a higher limit of detection of 1.807 μM, while maintaining a somewhat reduced sensitivity of 680 μA mM^–1 cm^–2. This work demonstrates that manipulating Cu foam consisting of an appropriate 3D framework with an inherently high surface area is a promising strategy to develop advanced nanocatalysts for lactate and other biochemical sensing.