Glucose electrooxidation in alkaline solution was examined using glassy carbon electrodes modified with Au nanoparticles. Au nanoparticles were prepared following the two-phase protocol and characterized by transmission electron microscopy (TEM), UV-Vis spectroscopy, X-ray diffraction spectroscopy (XRD), and cyclic voltammetry (CV). It was found that, under the study conditions, it is possible to obtain nanoparticles between 1 and 5 nm; also it was found that the crystallographic orientation is strongly influenced by the ratio metal/thiol and to a lesser extent by the synthesis temperature. The voltammetric response for the electrocatalytic oxidation of glucose at carbon Au nanoparticle-modified electrode shows an increasing activity with nanoparticles size. Electroactivity and possibly selectivity are found to be nanoparticles' crystallographic orientation dependent. Classical electrochemical analysis shows that glucose electrooxidation is a diffusion-controlled process followed by a homogenous reaction.
Glucose electrooxidation has been extensively studied for glucose fuel cells, glucose sensor for medical applications, and food industry [
Gold nanoparticles were prepared according to a previously published procedure [
Au nanoparticle-modified electrodes were prepared as follows: 1
Synthesized Au nanoparticles were characterized using TEM, UV-Vis, XRD, and CV. TEM characterizations were performed on a Philips CM-200 microscope. Images and statistical treatment were performed using the SIMM software developed by one of us. Nanoparticle samples dissolved in hexane were cast onto a carbon-coated copper grid sample holder followed by natural evaporation at room temperature.
UV-Vis measurements were carried out on a HP spectrophotometer model 8453.
XRD measurements were obtained using a Bruker model D8 Advance diffractometer. Spectra were collected from 10 to 50° at a speed of 0.0025°seg−1.
Cyclic voltammetric measurements were performed using a BAS Epsilon potentiostat/galvanostat (Bioanalytical Systems), with a conventional three electrode cell. An Hg/HgO was used as reference electrode and a Pt wire as the counter electrode. All potentials were referred to this electrode. The electrolyte solution was purged for twenty minutes with high-purity nitrogen before taking measurements. Glucose was used at various concentrations ranging from 0.00625 to 0.1 M in 0.1 M NaOH.
Figures
Summary of sizes and plane orientation ratio as a function of synthesis conditions.
Addition time of reducing agent (sec) | 10 | 60 | ||||||
---|---|---|---|---|---|---|---|---|
Synthesis temperature (°C) | 10 | 50 | 10 | 50 | ||||
Au : thiol ratio | 3 : 1 | 1 : 1/16 | 3 : 1 | 1 : 1/16 | 3 : 1 | 1 : 1/16 | 3 : 1 | 1 : 1/16 |
Size (nm) | 3.6 | 1.4 | 4.6 | 1.2 | 1.7 | 1.4 | 1 | |
(111) : (200) ratio | 1 | 2.25 | 1.87 | 2.12 | 1.4 | 1.59 | 1.2 | |
Sample | M1 | M3 | M2 | M4 | M5 | M7 | M8 |
(a) TEM micrograph and distribution size of M1. (b) TEM micrograph and distribution size of M2. (c) TEM micrograph and distribution size of M3. (d) TEM micrograph and distribution size of M4. (e) TEM micrograph and distribution size of M5. (f) TEM micrograph of M7. (g) TEM micrograph of M8.
It is known that the addition rate of reducing agent affects the nanoparticles size [
For the 10-second reducing agent addition, it was found that the greater the Au/thiol ratio, the smallest core size is obtained regardless of synthesis temperature; at least temperature seems to have less influence than Au/thiol ratio on nanoparticles size. Increasing temperature tends to reduce nanoparticles size. Core size for the highest Au/thiol ratio is about three times smaller than the lowest Au/thiol ratio (Table
Nanoparticle size versus Au : thiol ratio, reducing agent addition rate, and synthesis temperature.
Taking into account the measurements uncertainty, there is no significant size variation at the 60 seconds reducing agent addition time (Table
It is well known that Au nanoparticles have surface plasmon (SP) resonance absorption bands in the visible region. SP resonance bands are strongly dependent on the size, shape, composition, and dielectric properties of nanoparticles and their local environment. Figure
UV-Visible spectra for Au nanoparticles.
Figure
(a) XRD spectra for the Au nanoparticles. (b) (111) : (200) ratio versus Au : thiol ratio.
We have used cyclic voltammetry for the Au nanoparticles electrochemical characterization. Figure
(a) Cyclic voltammetry of Au and carbon-modified electrodes with Au nanoparticles in 0.1 M NaOH, 50 mVs−1. (b) Cyclic voltammetry of carbon modified electrode with Au nanoparticles in 0.1 M
Figure
Cyclic voltammetry of carbon modified electrodes with different size Au nanoparticles in 0.1 M NaOH 50 mVs−1 in presence of glucose.
(a) Peak current versus particle size. (b) Peak current versus (111) : (200) ratio.
Figure
Plots of (a)
It was found that Au nanoparticles supported on glassy carbon presented a catalytic activity and selectivity towards glucose oxidation, depending on the particle size and on the crystallographic orientation.
Results also suggest that oxidation process in these conditions is taking place with lower poisoning of the surface in the case of the Au nanoparticles than for massive gold, and that this process is irreversible, with perhaps some chemical reactions involved in the overall oxidation process.
Nafion is a trade mark, and the authors do not have any financial relation. The SIMM software was developed by Dr. Ivan Terol-Villalobos in behalf of CIDETEQ (Centro de Investigación y Desarrollo Tecnológico en Electroquímica S. C.), owner of the registered mark. The authors thank the Mexican Council for Science and Technology (CONACYT) for financial support through Fomix-Guanajuato, Grant GTO-2006-C01-23776. M. Guerra-Balcázar thanks the Mexican Council for Science and Technology (CONACYT) for graduate fellowship.