Glucose Oxidation Using Copper Hydroxide Nanosheet Electrode

Abstract

Non-enzymatic electrocatalysts have recently received much attention as a glucose sensor. These catalysts basically contain transition metals such as Pt, Au, Ni, and Cu. The glucose adsorbs onto the surface of the catalytic electrode and is oxidized by hydroxyl radicals. Smaller structured materials make the surface area larger, thus nanostructured electrode is expected to show efficient oxidation of glucose. As a nanomaterial, nanosheets have recently received much attention as a new material with an ultimate two-dimensional anisotropy. Inorganic nanosheets have been prepared by delamination of layered materials. We synthesized copper hydroxide in which dodecylbenzene sulfonate intercalated and delamination of these compounds to monolayer nanosheets. Copper oxide and hydroxide are viable candidates for the non-enzymatic electrochemical glucose sensor, thus copper hydroxide nanosheets are expected to make novel non-enzymatic electrochemical glucose sensor. In this study, we synthesized copper hydroxide nanosheet and investigated its electrochemical oxidation of glucose. The precursor of the nanosheet was a layered copper hydroxide synthesized by the ion exchange of dodecylbenzene sulfonate with acetate in Cu2(OH)3(CH3COO)·H2O. The Cu2(OH)3(CH3COO)·H2O was synthesized by hydrolysis of Cu(CH3COO)2·H2O solution (0.1 M) by heating at 65 °C for several days until crystalline product formed. The Cu(OH)3(CH3COO)·H2O (0.042 g) was added to the solution of NaDBS (DBS– = dodecylbenzene sulfonate, 26.6 mM, 40 mL) and shaken at 30 °C for 18 h. The precipitate was separated by centrifugation and washed with water and air dried. The yielded product (0.053 g) is named as Cu-DBS. The nanosheet was prepared by delamination of Cu-DBS by dispersion in 1-butanol. To make dispersion, the Cu-DBS (0.03 g) in 1-butanol (45 mL) was shaken and let stand for 2-4 weeks. The supernatant of the dispersion was used to further measurements. To make a working electrode, the 15 g of the dispersion of the copper hydroxide nanosheet was concentrated by an evaporator and dried after dropped on the ϕ 3 mm glassy carbon (GC) electrode. Electrochemical measurements of this working electrode were carried out in a standard one-compartment cell equipped with an additional bare GC working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode in 0.1 M NaOH solution. The morphology and size of nanosheets in the dispersion were examined by atomic force microscopy (AFM), which revealed nanosheet structure. The examined samples were prepared by deposition of a droplet of the dispersion on a silicon wafer. Intermittent contact mode AFM images showed two-dimensional ultrathin sheets with the lateral dimensions of ca. 2 μm. Some aggregates were occasionally observed. The height profile reveals that the sheets have a fairly flat terrace with a thickness of 4.33-6.30 nm with 320-460 aspect ratio and some sheets are stacked step by step with keeping the height. If DBS– ions place perpendicular to the hydroxide layer, the minimum thickness of the sheets should be ca. 5 nm because DBS– ions (22–25 Å) coordinate the hydroxide layer both on surfaces of the copper hydroxide sheet. Therefore DBS chains would align closely to the hydroxide layers at an angle of ca. 40 °. These copper hydroxide nanosheets are thin enough to be considered as a monolayer. Cyclic voltammetry (CV) was measured at 20 mV/s scan rate after addition of glucose solution to 0-2 mmol/l concentration (Fig. 1). The CV of the nanosheet coated electrode showed oxidation current peak at around +0.6 V vs. Ag/AgCl and this peak current increased as the concentration of glucose increased. This peak was not detected with only the GC electrode thus current peak was caused by the oxidation of glucose. Amperometry was measured at +0.6V vs. Ag/AgCl with successive addition of glucose solution (Fig. 2). The concentration of glucose increased to 0.1 to 27.8 mM, the current of the nanosheet coated electrode increased in a stepwise when glucose solution was added. The current was not detected with only the GC electrode. Glucose concentration and catalytic current were almost proportional. When the linear range is 0.1 to 4.9 mM, the sensitivity was 1.16 mA mM-1cm-2 from the slope. This value is comparable to the other glucose electrolytic oxidation electrodes using copper. Figure 1