Electrodeposition of Cu-based nanoarrays with tailored hierarchical nanostructure and their application for electrochemical nitrate sensing

Excessive nitrate content in the ecosystem has raised environmental concerns as a contaminant in soil and water bodies. Therefore, there is a consistent demand for rapid and continuous detection technologies with simple operational processes. In this work, we present a Cu-based nanoarray fabricated by a template-assisted electrodeposition process. By varying the deposition current density, the nanoarray exhibited a hierarchical nanoneedle structure with a sharp-needle terminal. Structural analysis confirmed that the metallic Cu nanoneedle has a unique crystal structure compared to the control samples of foil or wire structures. The possible deposition mechanism of nanoneedle was discussed based on the initial crystal formation step during electrodeposition. Furthermore, we explored the sensing performance of the Cu nanoarrays through electrochemical nitrate detection by using cyclic voltammetry (CV). The effect of the needle length on sensing performance was evaluated and the 7 μm nanoneedle array showed the highest sensitivity of 1.22 μA μM⁻¹ cm⁻² and a wide detection range of 0.25 mM to 16 mM, about 41-fold enhancement compared to the Cu foil. The square-wave voltammetry (SWV) method was further utilized for the low nitrate content detection (0.02 mM to 0.3 mM) and the nanoneedle array achieved an improved sensitivity of 6.99 μA μM⁻¹ cm⁻² and an LOD of 26 μM. The interference study indicated a good tolerance to various interference species. Multiple cycle scans and long term stability tests were conducted using the nanoneedle electrode, which maintained 80 % sensing response after 25 scan tests and close to 95 % sensing response after 21 testing days. The nanoneedle electrode also demonstrated sensing capability in analyzing nitrate species in a real water sample (tap water). Our work provides a convenient method to obtain a hierarchical metallic nanostructure, and the Cu-based nanoarrays show potential for sensitive detection of nitrate across a wide range of applications.