Novel Cu/CuO/ZnO hybrid hierarchical nanostructures for non-enzymatic glucose sensor application

Highlights

• A novel CuO nanoleaf/ZnO NRs electrode was fabricated by chemical oxidation and hydrothermal method.

• Structure and morphology of the as prepared electrodes were examined by XRD and FE-SEM.

• The sensor showed excellent electrocatalytic activity towards glucose oxidation.

• The sensitivity and low detection limit of the sensor is 408 μA mM⁻¹ cm⁻² and 18 μM.

Abstract

In this report, we have presented a novel CuO nanoleaf/ZnO nanorods (NRs) hierarchical architecture for non-enzymatic glucose sensor applications. The XRD pattern revealed the presence of monoclinic phase for CuO nanoleaf and hexagonal phase for ZnO NRs. FE-SEM images showed the ZnO NR arrays are densely covered on the leaf-shaped CuO nanostructure. The amperometric response indicated that the Cu/CuO/ZnO hybrid electrode has a good response for glucose with a linear range from 0.1 mM to 1 mM with a sensitivity of 408 μA mM⁻¹ cm⁻² and a low detection limit of 18 μM. The CuO nanoleaf/ZnO NRs structure exhibited higher electrocatalytic activity towards glucose oxidation. The greatly enhanced electrochemical catalytic reactivity is mainly attributed to the lower over-potential and larger electroactive surface area of the CuO nanoleaf structure and the one-dimensional ZnO NRs structures.

Introduction

In recent years, biosensors including glucose sensor are most important and essential devices in the day-to-day clinical applications, environmental monitoring, biological and chemical analyses [1], [2], [3]. Among the numerous reports glucose oxidase (GOx) has been widely used to construct various amperometric biosensors, due to its high sensitivity and selectivity to glucose [4], [5]. A majority of glucose biosensors is based on the enzymatic sensors because of their simplicity, relatively low-cost, and high-sensitivity [6], [7], [8]. However, the greatest drawback of enzymatic sensors is their lack of stability due to the intrinsic nature of enzymes [9], [10]. To overcome the above problems, lately a number of studies have been carried out to develop non-enzymatic glucose sensors based on transition metal oxides with high sensitivity, good stability, and low cost, such as NiO, ZnO, CoO, MnO₂, and CuO [10], [11], [12], [13], [14], [15]. But most of these electrodes have the disadvantages like poor sensitivity and low selectivity, which cause the surface poisoning of adsorbed intermediates and chloride, which greatly limits their applications.

Therefore, the development of a cost-effective, highly selective, fast and reliable non-enzymatic glucose sensor is still greatly demanded. Among the transition metal oxides, copper oxide (CuO) is a p-type semiconductor with a narrow-band gap of 1.2 eV has been extensively studied because of its numerous applications in catalysis, semiconductors, batteries, gas sensors, biosensors, solar cells, magnetic storage media, and field transistors [4], [5], [16], [17], [18]. Further, the nanostructured CuO exhibits a high specific surface area, good electrochemical activity, and the possibility of promoting electron transfer reactions at a lower over-potential [19], [20].

On the other hand, zinc oxide (ZnO) has become one of the most important semiconductor materials used for various applications such as, optics, optoelectronics, sensors, and actuators due to their semiconducting, piezoelectric, and pyroelectric properties [21], [22], [23], [24], [25]. A well-aligned ZnO nanorod array with a large specific surface area, higher electron conductivity, many active sites, low-cost and long-term stability has attracted extensive interest applications for photocatalysis, light-emitting diodes (LEDs), dye-sensitized solar cells and sensors [26], [27]. Recently, ZnO nanostructures have considerable attention for the biosensors applications due to the following advantages; biocompatibility, high surface area to volume ratios, chemical stability, facile preparation, low-cost, non-toxicity, bio-safety, fast electron transfer rates, enhanced electrochemical response, and increased sensitivity [8], [28], [29]. In addition, ZnO has a high isoelectric point (IEP) of about 9.5, which makes it suitable for the absorption of protein with low IEPs (GOx = 4.2), as the protein immobilization is primarily driven by the electrostatic interaction [8], [21], [30].

Recently, hybrid nanostructured materials with a high surface area, good conductivity and permeability have been considered to be one of the most significant functional materials for the various emerging research fields, such as, sensor, energy conversion and environmental remediation [31], [32], [33]. The performance of the hybrid nanostructured materials mainly depends upon their size, morphology, composition, structure, crystal phases and crystal facets [34]. Therefore, the controlled synthesis of hybrid metal oxide nanostructures is of considerable interest to achieve the required size, morphology and structure of the hybrid nanostructure. To the best of our knowledge, there are very few reports on the CuO/ZnO nanocomposites for the non-enzymatic glucose sensing applications [35], [36].

In this regard, first time we have reported, a facile, low temperature and cost effective synthesis method in the preparation of CuO nanoleaf/ZnO NRs hierarchical nanostructure on Cu substrate for the non-enzymatic glucose sensor applications. The formation of these highly active CuO nanoleaf/ZnO NRs hierarchical structure could serve as a promising electrode material for the enhancing non-enzymatic glucose sensing due to their high surface area and electrical conductivity.