Electronic Supplementary Material
An enzymatic glucose biosensor based on a glassy carbon electrode modified with manganese dioxide nanowires
Li Zhang, Sheng-mei Yuan, Li-ming Yang, Zhen Fang, Guang-chao Zhao*
College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, No. 1 East Beijing Road, Wuhu 241000, P. R. China
*Corresponding author: Tel.: 86-553-3869303; Fax: 86-553-3869303, E-mail address:
Electrochemical characterization of the modified electrodes
Figure S1A displays the cyclic voltammetric responses of various electrodes in 0.1 M phosphate buffer solution (pH 7.0). No redox peak was observed when the Naf/GCE was used as the working electrode (curve a, Figure S1A). While the Naf/MnO2/GCE was employed as the work electrode, a pair of broad but weak waves between 0.4 and 0.7 V was obtained (curve b, Figure S1A). This could be assigned to the reduction of MnO2 to Mn (II, or III) and the reoxidation of Mn (II, or III) back to MnO2 [1, 2]. Figure S1B depicts the cyclic voltammograms of the Naf/MnO2/GCE in 0.1 M phosphate buffer solution (pH 7.0) at different scan rates. As can be seen, both cathodic and anodic peak currents increased linearly with the square root of scan rate (v) from 0.1 to 0.4 V·s-1. This result reveals a typical diffusion-controlled process.
Figure S1. (A) Cyclic voltammograms (CVs) recorded in 0.1 M phosphate buffer solution (pH 7.0) at the scan rate of 0.05 V/s, using various modified electrodes: (a) Naf/GCE and (b) Naf/MnO2/GCE. (B) CVs of the Naf/MnO2/GCE in 0.1 M phosphate buffer solution (pH 7.0) at different scan rates (from inside to outside: a → m: 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36, 0.40 V/s, respectively).
The amperometric response of H2O2 at the Naf/MnO2/GCE
Figure S2 illustrates the current–time plots for the Naf/MnO2/GCE and Naf/GCE with successive changes of the H2O2 concentration. As H2O2 was injected into the stirring phosphate buffer solution, the Naf/MnO2/GCE exhibited much higher electrocatalytic activity toward H2O2 than Naf/GCE, and the steady-state currents reached another steady state value (95% of the maximum) in less than 3 s. Such a fast response implies that MnO2 can promote the oxidation of H2O2. The linear relationship between the catalytic current and the concentration is shown in inset of Figure S2. As can be seen, the Naf/MnO2/GCE displays the linear response range of 0.02 mM to 4.42 mM. The linear regression equation was obtained as ipa/μA = - 0.4667- 6.668 c (mM), with a correlation coefficient of 0.9999. The sensitivity of the Naf/MnO2/GCE to the oxidation of H2O2 was 388.9 μA·mM-1·cm-2, which was comparable to the results of 2.66 × 105 μA·M-1·cm−2 at +0.65 V using a Nano-MnO2/DHP modified electrode [3] and 31.4 μA·mM-1·cm−2 at +0.65 V for a MnO2–Na-montmorillonite modified GC electrode [4]. The repeated use of electrodes did not affect the long-term stability in the case that the measurement was not performed at high concentrations of H2O2 (>10 mM). For instance, the reproducibility of the current signal for six repeated injections of 120 μM hydrogen peroxide was within 4.65 %.
Figure S2. Typical amperometric response of Naf/MnO2/GCE (a) and Naf/GCE (b) upon the successive addition of H2O2 at 0.7 V. Calibration curve of H2O2 concentration at the sensor was shown in the inset.
Optimization of glucose detection conditions
Figure S3 (A) shows the influence of applied potential for the amperometric detection hydrogen peroxide at Naf/MnO2/GCE. The response increases with the detection potential from 0.50 to 0.90 V and then reaches a plateau at higher potentials. While the signal is highest at 0.75 V, there is an increased noise over 0.70 V, which is disadvantageous for measuring low H2O2 concentrations. Thus, a detection potential of 0.70 V is selected.
The activity of GOx in phosphate buffer solution during the biosensor fabrication procedure is important. Therefore, the effects of the loading amount of GOx on the performance of the biosensor were investigated. The biosensor was constructed with various GOx loadings between 0 and 25 mg·mL-1, and the response of the prepared biosensor toward the oxidation of 2.0 mM glucose was recorded. The results were presented in Figure S3 (B). Increasing the enzyme loading led to an enhancement in the response current of the glucose oxidation and the response reaches the maximum at 15 mg·mL-1 of GOx. The response of the biosensor has a slight decrease at a higher concentration of GOx. This may be due to the blockage created by the GOx overloading on the Naf/MnO2/GCE. Therefore, the substrate and product could not freely move into and out of the film. Therefore, a concentration of 15 mg·mL-1 for GOx was chosen as the enzyme loading for the enzyme biosensor preparation.
Figure S3. (A) Effect of the applied potential value on the amperometric detection of 0.1 mM H2O2 obtained at Naf/MnO2/GCE using 10 μg of MnO2; (B) Optimization of GOx loadings on the detection of 2 mM glucose at 0.70 V; and (C) Optimization of MnO2 amount on the diction of 2 mM glucose at 0.70 V.
At the optimized detection potential and the activity of GOx, the effect of β-MnO2 nanowires loaded onto the GC electrode on the current response of the biosensor was evaluated by preparing electrodes in the 2-16 μg β-MnO2 nanowires range, and the response of the prepared biosensor toward the oxidation of 2.0 mM glucose was recorded. Figure S3 (C) shows that the steady-state current increased quickly and reached a constant at a loading of 10 μg, then slightly decreases with increased MnO2 loadings. This is probably due to blocking of the electrode active surface, because the catalytical current depends on not only the concentration of H2O2 but also the concentration of activated MnO2 loading on the electrode. Sensitivity is lower for the film containing insufficient MnO2 nanowires, because too much MnO2 on the electrode decreases conductivity and slows mass transport through the film. The amount of 10 μg assured complete electrode modification and, therefore, was selected for further work.
Effect of pH and temperature
Figure S4. The dependence of the response of the GOx/Naf/MnO2/GCE biosensor toward the oxidation of 1 mM glucose on the pH of buffer (A), and the temperature of system (B).
Comparison of Sensitivity of H2O2 and Glucose
Figure S5. Calibration curves of H2O2 and glucose concentrations.
Table S1 Comparison of the amperometric glucose biosensors based on various glucose oxidase modified nanomaterials
electrode material sensitivity detection limit response time reference(µA·mM-1·cm-2 ) (µM) (s)
β-MnO2 NWs 38.20 25.56 ＜5 this work
ZnO NWs 19.5 ＜50 ＜5 [5]
1DHS TiO2 9.9 1.29 ＜5 [6]
NiO hollow spheres 3.4 µA·mM-1 47 ~8 [7]
MnO2 1 nA·mmol-1 0.8×103 ＜40 [1]
MnO2 0.55 µA·mM-1 0.085 µg·mL-1 - [2]
Pt-SWCNTs 2.11 µA·mM-1 0.5 μM - [8]
Osmium complex 1.11 µA·mM-1 - 2-3 [9]
CNT/Os-complex 826.3 0.056 ＜5 [10]
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