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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.4 Concepción dic. 2009
J. Chil. Chem. Soc., 54, N° 4 (2009), págs. 366-371.
PREPARATION OF MANGANESE DIOXIDE MODIFIED GLASSY CARBON ELECTRODE BY A NOVEL FILM PLATING/CYCLIC VOLTAMMETRY METHOD FOR H202 DETECTION
LE WEN-ZHI LIU YOU-QIN* HU GUANG-QI
Department of Applied Chemistry, College of Sciences, South China Agriculture University, Guangzhou, Guangdong 510642, PR China.e-mail: email@example.com
Manganese dioxide modified glassy carbon electrode (Mn02/GC) was prepared by a novel film plating/cyclic voltammetry method for the determination of H202. A manganese film was first cathodically deposited on the surface of glassy carbon electrode from MnCl2 solution at the potential of-1.4 V versus Ag/ AgCl (satd. KC1), and then a well defined manganese dioxide was deposited on the surface of glassy carbon electrode by cyclic voltammetry at potential range -0.6 ~ 0.6 V, scan rate 100 mV s-1 in 0.1 mol L71 NaOH solution. The resulted modified electrode was characterized by cyclic voltammetry and scanning electron microscopy (SEM), which showed excellent electrocatalytic activity for the oxidation of H202. The chronoamperometric detection of H202 was carried out at 0.6 V in phosphate buffer solution pH 7.38 containing 0.1 mol L71 KC1 and the linear relationship of response current on H202 concentration was obtained in the range from 4. Ixl0~10 to lxl0~-mol L-1 with a minimum detectable concentration of 3xl0~10 mol L71 [S/N (signal noise ratio) = 3]. The response time of the electrode to achieve 95% of the steady-state current was < 2 s. No measurable reduction in analytical performance of the modified electrode was found by storing the electrode in ambient conditions for 30 days. This modified electrode has many advantages such as simple preparation procedure, remarkable catalytic activity, good reproducibility and long term stability of signal response during hydrogen peroxide oxidation. The deposition of manganese dioxide on the surface of GC appears to be a highly efficient method for the development of a new class of sensitive, stable and reproducible hydrogen peroxide electrochemical sensor.
Key words: Manganese dioxide, Film plating/Cyclic voltammetry, Glassy carbon electrode, Electrocatalysis.
Hydrogen peroxide (H202) is a very important intermediate in environmental and biological reactions. The monitoring of H202 with a reliable, rapid and economic method is of great significance for numerous processes. Several analytical techniques such as titrimetry, spectrophotometry and chemiluminesence have been employed for its determination1. Electrochemical methods have been proved to be an effective and inexpensive way for H202 determination. The direct oxidation of H202 at bare usual electrodes is not suited for analytical application due to the slow electrode kinetics and high over-potential material, so preparation of chemical modified electrode (CME) with catalytic function is of practical significance. The methods reported for preparing CME contained sol-gel technique2, electrodeposition3, pyrolysis oxidation, self-assembly5, and so on.
Since inorganic surface mediators have the major advantages of efficient electrocatalysis and inherent stability, as needed for practical analytical work6, some workers have been devoted to the preparation and characterization of inorganic catalytic centers7,9. The reported inorganic materials included clay, zeolite, metal phthalocyanine, metalloporphyrin, transition metal particles, transitional metal oxides, oxometalates and polynuclear transition metal hexacyanometallate10. The potential applications of transition metal coating in the field of science and technology have been widely concerned by people. So far, different metal oxide particles, such as manganese oxide11, iron oxide12, tungsten oxide 13, lead oxide14, ruthenium oxide15 and cobalt oxide16, have been successfully used for immobilization of enzymes and proteins and in fabrication of H202 sensor. Manganese dioxide and manganese oxide-materials have been widely used for amperometric determination of bonded glucose17, fuel cells18, rechargeable alkaline batteries19.
For the past decades, manganese dioxide has been proven to be a suitable mediator for catalytic substance to reduce the over potential for H202 oxidation. Taba and Wang20 prepared a film of Mn02 deposited on glassy carbon electrode (GCE) from MnCl2/NaOH solution, which showed that it could be used in alkaline solution and exhibited highly stable and effective electrocatalytic oxidation of hydrogen peroxide and hydrazine compounds. Bai et al.21 fabricated a Mn02 nanoparticles modified electrode with bi-catalytic activity to H202. Since the electrocatalytic property of CME is obviously affected by physical and chemical state of the modifiers on electrode surface, designing proper film preparation method is essential. For this purpose, several methods, such as bulk-modified17, reduce potassium permanganate22, cyclic voltammetry23, have been used. But these preparation methods suffered from complex preparation process, time-consuming and uneven film that was not in close contact with electrodes, easily shed and prone to crack.
In this paper, a new film plating/cyclic voltammetry method has been developed to electrodeposit manganese dioxide film on the surface of GCE, whose most virtue consists in preventing metal ions from forming sedimentation with other components in electrolyte. Compared with above methods, the course of film plating/cyclic voltammetry method is easier to control the number of active sites on electrode surface and the film is uniform and in close contact with electrode. The factors, which affected the process of modification, electrochemical property of film, were evaluated by electrochemical techniques. Determination of H202 and interference of some substances probably coexisted with H202 were also investigated and the possible interacting mechanism of sensor was discussed. The sensor has been used to detect H202 in real water samples and exhibits some advantages, such as simple preparation procedures, high activity, good stability and reproducibility.
Analytical grade MnCl2 ■ 4H20, KC1, Na2HP04, KH2P04, H202 (30%) and so on were all purchased from Shanghai Chemical Reagent Company (Shanghai, China) and used without further purification. Stock solutions were prepared with doubly-distilled deionized water. 0.5 mol L71 H202 was standardized by KMn04. The phosphate buffer solution (PBS) with desired pH was prepared by mixing different volume of Na2HP04(l/15 mol L71) and KH2P04 (1/15 mol L-1) solutions.
Cyclic voltammetry (CV) and Chronoamperometry were carried out on a CHI 660C electrochemical analyzer (Shanghai Chenhua Co, China). A three-electrode configuration consisted of a modified glassy carbon electrode (GC ø3.7 mm) as a working electrode, a reference electrode (Ag/AgCl satd. KC1) and a counter electrode (Pt wire, length 5 mm, diameter 1 mm). Environmental Scanning Electron Microscopy image (ESEM) was obtained using a XL-30 microscope (Philips Co. Netherlands).
Mn02/GC electrode preparation
Before modification, the GC surface was carefully polished with aluminum (ø0.05 um) slurry on polishing cloth and then sonicated successively in ethanol, HN03+H20(1:1, V/V) and doubly distilled water for 2 min, respectively. Last, the water on electrode was dried with high purity N2 (99.999%).
A manganese film was first electrodeposited on GC surface by maintaining potential -1.4 V for 180 s in 1.8 μmol L-1 MnCl2 solution containing 0.6 mol L-1 KC1. Then, the electrode was rinsed with water and dried with N . After that, the manganese film on GC was put in 0.1 mol L-1 NaOH solution and scanned repetitively for 20 cycles under potential range -0.6 ~ 0.6 V at 0.05 V s-1. By this means, a well defined and adhesive Mn02 film was bound to GC surface.
During all experiments, the electrolyte was pre-purged with high purity N2 for 10 min to remove 02 and a continuous flow of N2 gas was maintained over the solution. Experiments were performed at room temperature (25 ± 1 °C).
Cyclic voltammetry (CV) and Chronoamperometry were carried out in an electrochemical cell holding 12 ml of 1/15 mol IA phosphate buffer (pH=7.38) and 0.1 mol IA KCl. In chronoamperometry, a holding potential of 0.6 V was applied to the modified electrode and the background current was allowed to decay to a constant value before H202 solution was added to the cell. The current-time curve for the amperometric experiment was recorded and calibration curve was obtained by amperometric responses when the same amount of H202 standard solution was added into the electrochemical cell.
RESULTS AND DISCUSSION
Preparation of Mn02/GC electrode Electrochemical behavior of Mn2+ on GC
Fig. 1 shows the typical cyclic voltammogram of GC electrode in 0.6 mol IA KCl electrolyte containing 1.8 umol IA MnCl2. There are four redox peaks in curve a which is the first cyclic voltammogram at scan rate of 0.02 V sA Peak A0 is very broad, which can be attributed to the oxidation of Mn(Mn→Mn2+) produced at potential lower than 1.2 V24. The anodic peak in A3 region and cathodic peak in C region are found to split into two spaced peaks in the voltammogram. The anodic peak A3 may be attributed to two distinct oxidation mechanisms represented by Eqs. (l)-(6)25:
The cathodic peak in the C region may be due to the reduction of Mn02 to MnOOH, which is further reduced to Mn (II) in the C2 potential region26. Curve b illustrates the second cyclic voltammogram of GC electrode in the same solution. There are additional anodic peak Ai at around 0.25 V and much broader anodic oxide wave A2, which didn't appear in the first cyclic voltammogram, indicating that their appearance resulted from the formation of Mn02(peak AJ. At, A2 can be attributed to the oxidation reaction of Mn(III)/ Mn(IV)and Mn(II)/Mn(IV) species, respectively21. Other peaks (A,, Ci; and C2) all increased, indicating that Mn02 film can successfully grow on GC surface by further potential scanning.
Formation of manganese dioxide film on GC
Fig. 2 shows the repetitive cyclic voltammograms of the deposition process of manganese dioxide on the surface of GC electrode in 0.1 mol IA NaOH solution. There were two broad anodic peaks (At, A2) in the first cyclic voltammogram, which was attributed to the oxidation of Mn to Mn (II) and Mn (II) to Mn (IV)6. The anodic peak currents(especially peak AJ evidently declined during the first three voltammograms, which was attributed to manganese being basically oxidated to Mn (II) and manganese dioxide formed at the first scan blocked ion transfer. With the increase of scan number, they slowly rised, indicating the formation of manganese dioxide film. The anodic peak (A2) at 0.35 V moved to 0.07 V (A3) and the cathodic peak (C ) moved from -0.36 V to -0.2 V with increasing cyclic number, suggesting the reversibility of manganese dioxide modified electrode.
After 20 cycles, the separation of anodic and cathodic peak potential (ΔEp ) basically unchanged, showing that manganese dioxide film became uniform and dense and visible oxide layer was noticed by eye in this condition. No further peak current growth was observed, so 20 cycles was adopted in all cases. Whereafter, the electrode was rinsed thoroughly with distilled water and transferred into other solutions to study its electrochemical characteristics.
Film formation could be attributed to the formation of Mn(OH)4~ and Mn(OH)42~ on the condition of high concentration of sodium hydroxide27. Mn(OH)4~ and Mn(OH)42~ were not stable and decomposited to Mn02. From the potential-pH diagram of Mn28, the anodic deposition of Mn02 from Mn2+ should occur in the potential range of 0 ~ 0.4 V at pH 13, which was in accordance with the results of this work.
Factors influencing electrocatalytic activity of Mn02/GC electrode
Fig. 3 (a)-(c) shows the optimal factors for the preparation of Mn02/GC electrode by examining its catalytic activity for H202 oxidation. From the potential-pH diagram of Mn24, the cathodic deposition of Mn from Mn2+ should occur in the potential range under 1.2 V. The deposition rate of manganese film become fast when the cathodic deposition potential is lower. The film of manganese will be thicker for the same deposition time and ultimately makes the catalytic activity of MnO /GC electrode changed.
Fig. 3(a) illustrates the changing of electrocatalytic activity of Mn02/GC electrode for H202 oxidation in the deposition potential range 1.2----1.6 V, indicating the optimal deposition potential with high catalytic activity being -1.4 V. The more negative was the deposition potential, the higher was the catalytic activity, indicating that the number of active sites of manganese dioxide film played a major role. However, the catalytic ability decreased slowly when the deposition potential was less than 1.5 V, which may be attributed to the increasing of resistance of ion transfer for charge balance in the process of film oxidation and reduction29. Fig. 3 (b) shows the influence of Mn2+ concentration for manganese film deposition on the catalytic activity of Mn02/GC electrode. The optimal Mn2+concentration was 1.8 umol L-1, which was the result of interaction of active sites with film resistance by influencing the electrocatalytic activity of Mn02/GC electrode. Fig. 3(c) shows the effect of deposited time for manganese film growth on the electrocatalytic activity of Mn02/GC electrode. The deposited time determined the manganese film thickness and indirectly influenced the manganese dioxide film thickness on GC electrode. The manganese dioxide film could not completely coat the surface of the GC electrode for deposition times less than 30 s and lead to an insufficient number of active sites. With the deposition time increasing, the manganese dioxide film became thicker, which lead the number of active sites and the transfer resistance of electron and hydrogen peroxide in the film to synchronously increase. The suitable deposition time was 180 s.
Characterization of Mn02/GC electrode
Fig. 4 shows a typical SEM image of electrodeposited manganese dioxide film adhered on GC electrode. It can be seen that a uniform film of manganese dioxide particles with average size of about 100 nm was distributed on the surface of electrode. Since the manganese dioxide film was formed by the direct oxidation of manganese on the electrode surface, the compact combination of manganese dioxide with electrode surface enhanced the transfer speed of electrons and further increased the catalytic activity of Mn02.
Fig. 5 shows the recording cyclic voltammograms of Mn02/GC electrode in 0.1 mol L-1 NaOH solution at various scan rates. As can be seen, increasing the scan rate the anodic and cathodic peak potentials shifted towards positive and negative directions, indicating charge transfer kinetics limitation. The results of the inset of Fig. 5 show that both the anodic and cathodic peak currents increase linearly with scan rates. The ratio of cathodic to anodic peak current is about unity as expected for surface bound redox sites. Thus, the overall redox process confined at the electrode surface can be considered to be relatively fast on the voltammetric time scale, which indicated that a surface confined redox process corresponded to rapid conversion of a surface film without diffusion or kinetically controlled reaction step.
For investigating the repeatability of electrode modification process, five independent modified glassy carbon electrodes were prepared. Cyclic voltammograms of the prepared modified electrodes at scan rate 100 mV s~' in 0.1 mol L-1 NaOH solution were recorded (not shown). Almost same results were found for all electrodes. By measuring the anodic peak current at 0.22 V for five independently modified electrodes, the RSD value of about 5% was obtained, indicating good repeatability of the modification process.
Electrocatalytic oxidation of H202 on Mn02/GC electrode
Fig. 6 shows the cyclic voltammograms of Mn02/GC electrode in the absence and presence of H202 (0.11 mmol L-1) at pH 7.38 buffer solution. In order to verify the eletrocatalytic activity of the modified electrode for hydrogen peroxide oxidation, the electrochemical experiments in the presence of hydrogen peroxide were carried out. As shown for the bare glassy carbon electrode, no oxidation response of H202 can be seen in the potential range from 0 to 0.9 V (curve a and curve b). However, there was a pair of obvious redox peaks between 0.2 and 0.8 V (curve c) for Mn02/GC electrode in the absence of hydrogen peroxide31, which may be assigned to the redox pair Mn02/Mn(II)32-34.
In the presence of H202, the oxidative peak current of Mn02 film greatly increased while the reductive peak disappeared (curve d). The overvoltage was decreased by about 0.35 V and the increased peak current of H202 oxidation confirmed that manganese dioxide nanoparticles had high catalytic ability for H202 oxidation16. The characteristic shape of the cyclic voltammogram in this potential region indicated that the signal was probably due to a parallel catalytic reaction. As soon as Mn02 was reduced to lower states by H202, they were electro-oxidized back to Mn02 at the electrode surface32:
Since the above reactions are fast, the parallel current is much higher than the oxidation current of MnO on the electrode surface without H202. Therefore, manganese dioxide nanoparticles are suitable as mediators to shuttle electrons between hydrogen peroxide and working electrode.
In order to optimize the electrocatalytic response of Mn02/GC electrode for hydrogen peroxide oxidation, the effect of electrolyte pH on Mn02/GC catalytic activity was investigated. The cyclic voltammograms of Mn02/GC electrode in 50 umol L-1 H202 at different pH values were recorded (Fig. 7). The Mn02/GC electrode showed electrocatalytic activity at different electrolyte (pH 4.92 ~ 9.18), but higher peak current was observed at pH 7.38 (Inset). The peak currents and potentials all changed with the changing of pH values. Peak potentials shifted positively with decreasing pH values. Since Mn02/GC electrode showed excellent response for trace amount of H202 at nearly physiological pH values of 7.0 ~ 8.0, it can be used for detection of H202 generated by enzyme catalytic reactions. Fig. 8 shows the cyclic voltammograms of Mn02/GC electrode in electrolyte containing 41 nmol L-1 H202 at different scan rates. The peak current (ipa) for the anodic oxidation of hydrogen peroxide is proportional to the square root of scan rate (v1,2) (ipa (μA) = - 4.1984 + 0.8090v1/2(mV s-1)1/2, R = 0.9992), suggesting that the process is controlled by diffusion as expected for a catalytic system.
Amperometric determination of H202 on Mn02/GC electrode
Since amperometry under stirred condition is much more sensitive than cyclic voltammetry, it is employed to estimate the detection low limit. Fig. 9 shows the steady-state catalytic current time response of Mn02/GC electrode with successive injection of H202 at the applied potential 0.6 V in buffer solution (pH 7.38). As shown in the figure, a well response was observed during the successive addition of 0.41 umol L-1 (Fig. 9 A) and 4.1 nmol L-1 (Fig. 9 C) of H202, which demonstrated an effective catalytic property of Mn02 immobilized on GC.
There was a linear relationship between response current and hydrogen peroxide concentration in the range 0.1 mmol L-1 and 0.41 nmol L-1, while the plot of current vs. analyte concentration deviated from linearity for higher concentration of H202 (Fig. 9 B). The calibration curve for H202 measurement was presented in Fig. 9 D. The linear least squares calibration curve over the range of 4.1-100 nmol L-1 (25 points) was Δi pa(μA) = 0.0193 CH 0 + 0.125 with the correlation coefficient being 0.9988, demonstrating that the regression line was well fitted with the experimental data, and the detection lower limit was estimated to be 0.3 nmol L-1 (S/N=3). Therefore, the regression equation can be used for the determination of H202 in real samples.
In addition, Fig. 9E shows the amperometric response of 41 umol L-1 H202 during prolonged 27.5 min experiment. The response remained stable throughout the experiment, indicating that Mn02 nanoparticles showed high stability for amperometric measurements of H202. Mn02/GC electrode didn't lose activity with increasing time. Mn02/GC electrode was used each day for 10 measurements and each measurement took 1 h, 97% of the initial response of Mn02/GC electrode was retained after 30 days.
Fig. 9. Amperometric response of Mn02/GC electrode at potential 0.6 V in buffer solution (pH 7.38) for successive addition of 0.41 umol L-1 (A) and 4.1 nmol L-1 (C) of H202. (B), (D): plots of chronoamperometric current vs. H202 concentrations. (E): chronoamperogram for 4.1 umol L-1 H202 during a long period of time (27.5 min).
Four groups of substances that potentially coexist with H202 were investigated for interference on H202 (table 1). Most anions (Group 1) such as chlorate, sulfate, nitrate, ferricyanide didn't interfere when present in 100-fold mass. Amino acids (Group 2) slightly reduced the current of hydrogen peroxide if their concentration was 50 times the concentration of H202. Probably H202 in the sample solution may be consumed by redox processes. Substances that potentially existed in biological liquids (Group 3) interfered strongly. The amperometric response was markedly increased by ascorbate, dopamine and uric acid when the concentration of them was 50 times the concentration of H202, which may be attributed to the redox processes. The species existing in food and drug (Group 4) had little interference toward the signals of H202. Thus, the H202 sensor can be used for the determination of H202 in water, foods, drugs and cosmetics except the biological molecules in high concentration.
A novel film plating/potential cycling method has been developed for electrodeposition of manganese dioxide nanoparticles on the surface of glassy carbon electrode. It provides an effective way for the preparation of a new class of sensitive, stable and reproducible hydrogen peroxide electrochemical sensor. The advantages of modification procedure are less expensive and more convenient than those used by others. The fast response and high sensitivity of this H202 sensor are attributed to the compaction between manganese dioxide and glassy carbon electrode for electron transmission and the high specific surface area of Mn02 for a uniform film. In addition, the electrooxidation of nanomolar hydrogen peroxide solution at reduced overpotential was performed using Mn02/GC electrode. Its applicability to practical samples was verified by measuring H202 in real water of Poyang lake, which held promise for the quantitative detection of H202, specially lower detection limit and its applicability in neutral solution, suggesting that the film plating/cyclic voltammetry method can probably act as a novel and useful method for the preparation of electrochemical sensors.
Project is supported by the Scientific Research Starting Foundation for Drs., Natural Science Foundation of Guangdong Province, China (Grant No. 5300842) and the President Science Foundation of South China Agricultural University (Grant No. 2005K119)
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(Received: December 16, 2009 - Accepted: August 31, 2009).