Fabrication of the Ni-based composite wires for electrochemical detection of copper(Ⅱ) ions

Abstract

Copper ions (Cu²⁺) pollution in the water environment poses a great threat to the health function of life-sustaining metabolic activities. However, the current detection methods need relatively expensive instruments, complex operation procedures and long time, so a facile and direct detection method is desired to be developed. In this work, the Ni-based composite wires with p-n junction (the Ni/NiO/ZnO/Chitosan wire) and Schottky junction (the Ni/NiO/Au/Chitosan wire) were fabricated, and the barrier driven electrochemical sensing mechanism was studied. The direct and facile detection of Cu²⁺ was achieved with a wide linear range (0–6000 nM) and a low LOD (0.81 nM). The excellent stability and recovery in real water samples made the Ni-based composite wires a promising candidate for the practical application. The interfacial barriers of semiconductor can be used as a special sensing factor to develop novel sensors.

Introduction

Cu²⁺ is an essential element in living organisms and plays an important role in body functions [1,2]. Nevertheless, excess Cu²⁺ intake can cause Wilson, Alzheimer’s and Menkes diseases [3,4]. The potential toxic effects of Cu²⁺ to human beings continue to be a global challenge as its pollution in rivers or sea. It is important for clinical research, food industry and environmental monitoring to develop an efficient method for the trace detection of Cu²⁺ [5,6].

Various techniques for detecting Cu²⁺ have been developed, including inductively coupled plasma atomic emission spectroscopy (ICP-AES) [7], flameless atomic absorption spectroscopy (FAAS) [8] and atomic absorption spectroscopy (AAS) [9]. Although they are sensitive and selective, they require relatively expensive instruments, complicated procedures and long time. Electrochemical analysis methods have attracted much attention because of its simple operation, fast response and high sensitivity. The current electrochemical method for detection of Cu²⁺ is adsorptive stripping voltammetry (ASV) [10], but it is an indirect and complex process involving enrichment and dissolution. Therefore, it is desired to develop a direct electrochemical method for online detection of Cu²⁺.

In recent years, the fabrication of semiconductor heterojunctions has become a focus of attention in improving the electronic performance in the applications of solar cells, sensors, catalytic reduction and so on [[11], [12], [13], [14]]. For example, the prepared Co₃O₄–ZnO core-shell nanofibers showed the improved gas sensing performance to formaldehyde [15]. The heavily doped Si p-n junction nanowires show a highly localized sensitive area at the interface, which can be used for intracellular recording [16]. Moreover, Wang’s research team reported that the use of Schottky contacts could obtain the highly sensitive and fast-responsive field-effect transistor devices [17]. All of these demonstrate that the formation of special semiconductor interfaces has a significant impact on the sensing function. Therefore, the rational design of semiconductor heterojunction may provide a new opportunity for heavy metals detection.

With the development of nanotechnology, the metal oxides nanostructures have become a kind of promising electrochemical sensing material due to their easy availability, excellent electronic properties and chemical stability. Among them, NiO, a wide band gap p-type semiconductor material (E_g = 3.55 eV), is widely studied in catalysts and sensors due to its well catalytic and sensing performance [18]. ZnO, a wide band gap n-type semiconductor (E_g = 3.37 eV), has been recognized as a good sensing material due to its high photosensitivity, excellent electronic transmission characteristics [19]. Since both NiO and ZnO have the wide band gap, the formed p-n junction has a large adjustable range of the electronic characteristics. In addition, Au nanoparticles decorated metal oxides can form Schottky junction and obtain the special electronic characteristics [[20], [21], [22]]. Chitosan (CS) is a polymer material with strong adhesion and film-forming ability. Because it is rich in hydroxyl and amino active functional groups, it opens up possibilities for various chemical modifications such as acylation, chelation, graft copolymerization, and crosslinking. And it shows excellent selective adsorption properties for Cu²⁺ in solution [23,24].

In this work, the Ni/NiO/ZnO/CS and Ni/NiO/Au/CS wires were successfully fabricated and used for the direct electrochemical detection of Cu²⁺. The electrochemical signal was produced by the adsorbed Cu²⁺ induced change of the barrier height and exhibited the excellent stability and recovery rate in the real water samples. It is promising to be applied in the direct detection of Cu²⁺ in the actual water environment by employing the interfacial barrier effects on electrochemistry.