Non-enzymatic screen printed sensor based on Cu₂O nanocubes for glucose determination in bio-fermentation processes

Highlights

• Non-enzymatic electrochemical sensor based on cuprous oxide nanocubes (Cu₂O-NC).

• Monitoring of glucose in bio-fermentation processes by means of electrochemical sensors.

• Cu₂O-NC modified SPCE exhibits good sensing performance toward glucose monitoring.

• Satisfactory stability and selectivity against other sugars and ethanol.

• Simple way to fabricate inexpensive and reliable glucose sensors in industrial processes.

Abstract

A non-enzymatic electrochemical sensor based on cuprous oxide nanocubes (Cu₂O-NC) has been developed. Cu₂O-NC samples have been synthesized using a wet precipitation technique under different precipitation temperatures (25 and 60 °C). The samples were characterized by XRD (X-ray Diffraction), UV–visible Diffuse Reflectance Spectroscopy (UV–Vis DRS) and Scanning Electron Microscopy (SEM). Screen-printed carbon electrodes (SPCE) have been modified by casting the synthesized Cu₂O-NC. The fabricated Cu₂O-NC-SPCE sensor prepared at 25 °C provided the best sensing performance toward glucose, with a sensitivity of 1040 μA/mM cm⁻² in the linear range from 0.007 to 4.5 mM, and a detection limit of 3.1 μM (S/N = 3). Cu₂O-NC-SPCE electrodes were also evaluated for the amperometric determination of glucose. Testing results showed a satisfactory stability toward glucose sensing and selectivity against other sugars and ethanol, suggesting that the SPCE modification with Cu₂O-NC could be a simple way to fabricate inexpensive and reliable sensors to monitor glucose in bio-fermentation processes.

Introduction

The determination of glucose level is important in many industrial applications such as bio-fermentation processes. Among these, alcoholic fermentation is one of the most important. Alcoholic fermentation process consists in the transformation of sugars (in particular glucose, fructose and sucrose) into ethanol and carbon dioxide, which occurs thanks to the unicellular yeasts present on the grape's skin. This process is also applied industrially in the production of bioethanol from agricultural commodities for blending into gasoline, very helpful nowadays to decrease the actual dependence on crude oil [1].

Monitoring of glucose in these industrial processes by means of electrochemical sensors is, without doubt, the easiest and cheapest way [[2], [3], [4]]. Electrochemical glucose biosensors based on enzymes have been widely investigated due to their high sensitivity, specificity and low detection limit [[4], [5], [6]]. However, they present some drawbacks arising from their thermal/chemical instabilities and the complex fabrication procedures, which limit their further progress [7,8]. The catalytic activity of biosensor is also affected by the operating conditions like temperature and pH. Furthermore, most of them are developed for biomedical applications and cannot be used for long-term monitoring in bioreactors.

The development of non-enzymatic electrochemical sensors for glucose monitoring may be an effective alternative to overcome the above issues. In the last years, noble metals [[9], [10], [11], [12]], transition metals [[13], [14], [15], [16]], bimetallic systems [[17], [18], [19], [20]], metal oxides [21,22] and their hybrid with carbon-based nanomaterials [23] have been reported as efficient materials for glucose non-enzymatic sensing. However, many issues still exist, which are mainly attributed to complex synthetic procedures, low sensibility and poor versatility in real application. Electrochemical sensors modified with metal nanoparticles usually reveal enhanced electrochemical performance, due to very large surface-to-volume ratio and dimensions analogous to the extension of surface charge region, increasing mass transport and catalytic performance [24]. In the case of noble metal nanoparticles (e.g. Pt, Au) and their alloys, high cost, low selectivity and poisoning are the main factors that hinder them from a real implementation [25,26].

Copper oxide represents a special case. Electrochemical glucose sensors based on CuO, [27] and Cu₂O nanoparticles [[28], [29], [30]], in fact, offer many advantages compared to enzyme-based glucose sensors, and can also be less expensive compared with the ones based on noble metal nanoparticles. Copper-based sensors show a low detection limit, quick and reproducible amperometric response, as well as a wide linear range and low overpotential, due to their great ability to perform electron-transfer reactions [31,32]. The high sensitivity of copper with respect to noble metals can be addressed to a low oxidation potential in alkaline solutions, despite its stability is not great in acidic electrolyte [33,34]. Copper-based materials with different morphology have been designed so far for glucose amperometric detection, i.e. Cu/Cu₂O hollow microspheres, CuO nanowalls on Cu substrate, Cu micropuzzles, CuO nanobelt arrays, CuO/TiO₂ nanotubes, Cu₂O/straight multi-walled carbon nanotubes [35]. These nanoarchitectures were prepared by different techniques including electrodeposition, sol-gel, hydrothermal and wet chemical processes [36]. However, the issue related to their high cost of preparation still exists.

In this context, here we present a very cheap strategy for the fabrication of non-enzymatic glucose sensors based on Cu₂O-nanocubes modified carbon working electrodes, prepared by an easy wet precipitation method. The samples, synthesized using two precipitation temperatures (25 and 60 °C) to explore the effect of nanoparticle size, have been characterized by XRD, UV–visible Reflectance Spectroscopy and SEM analysis, while their electrochemical behaviour was evaluated by cyclic voltammetry (CV) and chronoamperometry. The developed Cu₂O-based sensor has many advantages such as simple fabrication procedure, low cost, good sensitivity, great stability and reproducibility and high selectivity for a reliable detection of glucose in fermentation processes.