In this study we used KOH-modified Ni electrodes (films and nanowires) for the electro-oxidation of formaldehyde (HCHO), which are promising to be used as effective electrochemical receptor element. Ni ‘’films’’ were deposited on Au substrates by the electro-deposition from acidic and neutral NiSO4-based solutions with subsequent modification in 1 M KOH. From the SEM results we found out that pH of Ni2+ solution affects the morphology of the deposited films. Under acidic conditions homogeneous Ni film was formed, but under neutral conditions porous Ni film was deposited. Ni nanowires (200 nm diameter, 1 μm length) were prepared with template-assisted electrodeposition into alumina templates. After the electrodeposition of the nanowires, the template was removed in 10 M solution of NaOH in order to get free standing wires.

The modification step with KOH is very important for the further investigations of HCHO oxidation, because NiO(OH)/Ni(OH)2 redox couple exhibits high catalytic activity towards HCHO. The electrocatalytic activity of modified Ni nanowires and Ni films (Ni-OOH) for formaldehyde detection in alkaline media was investigated via a series of electrochemical measurements. The potential range for modification in KOH has to be chosen in the region where oxygen does not form (the maximum potential 0.6V), because molecular oxygen can be adsorbed on the electrode and inhibits further oxidation of HCHO [1].

Experimental results show that the 2D and 1D Ni-based electrode (nanowires and films) displays a remarked electro-catalytic activity for the oxidation of HCHO and exhibit a linear relationship in a concentration range from 1 mM to 0.5 M. An oxidation peak was obtained at potentials around 0.5 V vs. Ag/AgCl for concentrations 1 mM – 0.1 M and around 0.8 V vs. Ag/AgCl for concentrations above 0.1 M. Results showed that the reaction of the electro-oxidation most probably proceeds by a chemical reaction with NiO(OH) as shown on Scheme 1 [2]. The experimental data further reveal that Ni nanowires exhibits a higher sensitivity (approximately 7x higher) compared to the homogeneous and porous Ni electrode, because the slope of the curve current signals vs. concentrations reached higher value. The detection limit was 0.1 mM for Ni nanowires, 0.4 mM for porous Ni film and 0.5 mM for homogeneous Ni film. It was concluded from results that Ni nanowires exhibit a higher catalytic activity due to the more surface active sites and connected higher amounts of adsorbed –OOH groups on the surface that promote the electron transfer between receptor elements and HCHO molecules. These advantages of Ni nanowires make them promising for providing a low cost and simple method in real samples.