Assessments of Surface Coverage after Nanomaterials are Drop Cast onto Electrodes for Electroanalytical Applications

The drop-casting method for the suspension of nanomaterials on conducting surfaces is a commonly used procedure for evaluating the electrochemical properties of the drop-cast materials. In this study, we pinpoint a key limitation of the method, which may lead to misinterpretation of the obtained data, especially when evaluating heterogeneous electron-transfer rates. The electrochemical responses recorded at 1mm-diameter copper electrodes modified with porous layers of drop-cast multiwalled carbon nanotubes (MWCNTs) in 0.1M Na₂SO₄ aqueous solutions were examined. Standard amounts of the MWCNTs that are typically used for the drop-casting procedure (1mg MWCNT in 1mL of dimethylformamide) were deposited drop wise on the surface of the copper electrodes. Layers of MWCNTs were progressively built up on the electrode surface by varying the number of drops from 0 to 10. The ability of the MWCNTs to cover and prevent diffusion to the base copper electrode was assessed by performing oxidative cyclic (CV) and linear-sweep voltammetry (LSV) experiments in the presence of aggressive SO₄^2- supporting electrolyte, where a large oxidative current indicated the occurrence of corroding copper metal. It was demonstrated that a total of 10 drops of the coating solution (equivalent to 640μgcm^-2 of MWCNTs per unit area) was still insufficient in providing complete coverage over the underlying electrode surface (as a corrosion current was still observed), even though considerably lower CNT loadings have been applied in many literature reports. The electrochemical results indicate that, for experiments that utilize the drop-casting procedure to modify electrode surfaces, it cannot be assumed that the base electrode, nor the pore structure of the coating material, does not significantly contribute to the overall observed voltammetric response.