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Ordered mesoporous carbon-supported Prussian blue: Characterization and electrocatalytic properties
Microporous and Mesoporous Materials 119 (2009) 193–199
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Ordered mesoporous carbon-supported Prussian blue: Characterization and electrocatalytic properties Jing Bai, Bin Qi, Jean Chrysostome Ndamanisha, Li-ping Guo * Faculty of Chemistry, Northeast Normal University, Renmin Street 5268, Changchun 130024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 July 2008 Received in revised form 17 October 2008 Accepted 20 October 2008 Available online 7 November 2008 Keywords: Ordered mesoporous carbon Prussian blue Electrocatalysis Hydrogen peroxide Ascorbic acid
a b s t r a c t Ordered mesoporous carbon (OMC) was successfully modified with one kind of electroactive inorganic compounds: Prussian blue (PB). The composites were characterized by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM) and electrochemical methods. The results show that, after depositing PB on the OMC, PB remains electroactivity. The new material obtained (PB/OMC) was used to modify glassy carbon electrode (GC). Compared with Prussian blue (PB) modified glassy carbon electrode (GC), the PB/OMC/GC electrode exhibits much better electrochemical stability, much wider pH range and larger response currents to the reduction of hydrogen peroxide (H2O2). Additionally, the PB/OMC/GC electrode also exhibits remarkably strong and stable electrocatalytic response toward the oxidation of ascorbic acid (AA) compared with PB/OMC. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Recently, a series of porous materials such as clayminerals [1], montmorillonite [2], porous aluminosilicates, carbon nanotubes (CNTs) and sol–gel matrix etc. [3–5] have been shown to be promising as immobilization matrices. These new materials have large pores, thick pore walls and high hydrothermal stability. Mesoporous crystalline materials exhibit extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. In addition, the unique structural and catalytic properties of molecular sieves for structuring an electrochemical/electron-transfer environment and resistance to biodegradation have attracted considerable attention [6–8]. Besides these materials mentioned above, highly ordered mesoporous carbon (OMC) has been receiving much attention owing to the fact that SBA-15 template is inexpensive [9] and easy to synthesize [10]. With their considerable properties, such as uniform and tailored pore structure, high specific surface areas, large pore volume and chemical inertness [11,12], ordered mesoporous carbons (OMCs) are of great interest for the fabrication of new classes of advanced materials [13]. In addition, the ability of ordered mesoporous carbons (OMCs) to promote the electron-transfer reactions of important molecules, such as L-cysteine [14], dopamine [15], and epinephrine [16], have made them attractive for the construction of various electrochemical sensors. While immobilization
* Corresponding author. Tel.: +86 431 85099762; fax: +86 413 85099762. E-mail address: [email protected] (L.-p. Guo). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.030
of electroactive substance on mesoporous materials was also reported, such as the grafting of ferrocene on mesoporous silica materials [17], the immobilization of ferrocene on mesoporous carbon [18], and the hybrid materials exhibit a high catalytic activity. Hence, ordered mesoporous carbon could have more interests and potential advantages for many advanced applications than other mesoporous materials [3–5]. PB is an inorganic polycrystal with well-known electrochromic and electrocatalytical properties [19]. Since its deposition on solid electrodes by Neff [20], it has been the subject of many studies. Due to the unique properties, synthetic versatility and ability of the bridging cyanide ligands to efficiently mediate its properties, PB and its analogues have been employed intensively in many fields, such as in the electrochemical [21,22], electrochromic [23], magnetic [24] and potential analytic applications [25]. The electrochemical stability of PB layer has also been a matter of investigation since its first use. The effect of pH on PB activity and stability is still a subject of interest for researchers involved in the characterization of PB. Thus, how to efficiently improve the electrochemical stability of PB film becomes desirable. To solve this problem, multi-walled carbon nanotubes (MWNTs) [26], Screenprinted electrodes (SPEs) [27], polyaniline (PANI) [28] have been used to immobilize PB. There were the oxygen-containing functional groups on the surface of OMC, which have been confirmed by FT-IR [29]. So we deposited PB on the OMC, which can stabilize PB film. To the best of our knowledge, there are few reports on deposition of PB on OMC substrate. Taking the advantage of the unique properties of both OMC [30] and PB, it would greatly broaden the applications of OMC and PB.
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In this paper, PB was efficiently deposited on the ordered mesoporous carbons (OMC) modified glassy carbon (GC) electrode and thus formed PB/OMC composite film. With the introduction of OMC, the PB/OMC hybrid composites show synergistic augmentation of the response current for H2O2 and AA detection. Compared with PB film, the PB/OMC composite film shows much better electrochemical stability, much wider pH adaptive range and larger response current to the reduction of H2O2.
2. Experimental 2.1. Reagents OMC was synthesized according to the previous reported work [31]. Iron chloride hexahydrate (FeCl3 6H2O), potassium ferricyanide (K3Fe(CN)6) and H2O2 were purchased from Chemical Reagent Company of Shanghai (China). Ascorbic acid was purchased from Beijing Chemical Reagent Company. All other chemicals not mentioned here were of analytical reagent grade and were used as received. Double-distilled water was used throughout. 0.1 M phosphate buffer solution (PBS) was used as supporting electrolyte solution, which was made from K2HPO4, KH2PO4, and adjusting the pH with H3PO4 or KOH.
3. Results and discussion 3.1. Characterization of OMC and PB/OMC The presence of the oxygen-containing functional groups on the surface of OMC was confirmed by FT-IR in Fig. 1. The band around 1628 cm1 is attributed to C@O stretch vibration, and the bands around 1560 and 1522 cm1 are ascribed to COO stretch asymmetric vibration. The band around 1185 cm1 is assignable to O– H vermicular vibration, while the carboxylic acid O–H stretch vibration is responsible for the band around 3438 cm1. Fig. 2 shows the Raman spectra of OMC (a) and PB/OMC (b). Both spectra exhibit the presence of D and G bands, located at 1384 cm1 (disorder mode) and 1595 cm1 (tangential) [32]. The D band at around 1348 cm1 is associated with the presence of defects in the graphite layer. The peak at 1595 cm1 corresponds to the Raman-active E2g, which resulted from the vibration mode corresponding to the movement in opposite directions of two neighboring carbon atoms in a single crystal graphite sheet. Furthermore, the relative intensity ratio of the D and G bands (ID/IG ratio) is proportional to the number of defect sites in the graphite carbon [33]. As can be clearly seen from Fig. 2 the ID/IG ratio of PB/OMC is much larger than OMC, indicating that there are more significant edge-plane-like defective sites existing on the surface of PB/OMC than on the surface of OMC. Researchers have demonstrated that
2.2. Apparatus All the electrochemical experiments were performed with a CHI 830b Electrochemical Analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional threeelectrode cell. The working electrode (WE) used was glassy carbon (GC) electrode (Model CHI104, 3 mm diameter) or the modified electrode. A platinum foil as the counter electrode (CE), and an Ag/AgCl (KCl saturated) electrode as reference electrode (RE). All potentials in this paper were measured and reported versus Ag/ AgCl. The sample solutions were purged with purified nitrogen for at least 15 min to remove oxygen prior to the beginning of a series of experiments. All measurements were carried out at room temperature (20 ± 2 °C). The micrographs of the PB and PB/OMC films were obtained with SEM. The SEM images were determined with a Philips XL-30 ESEM operating at 3.0 kV. Raman spectras were recorded at ambient temperature on a Renishaw Raman system model 1000 spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. Infrared spectrum of the sample was recorded with Nicolet Magna 560 FT-IR Spectrometer with KBr plate. 2.3. Preparation of the modified electrodes Prior to use, the GC electrode was polished with 1, 0.3 and 0.05 lm alumina powder, respectively, then ultrasonically cleaned with ethanol and double-distilled water and dried in nitrogen. The specific procedure of preparation of modified electrode was as followed: 5 mg of the OMC was dispersed in 10 mL of N,N-dimethylformamide (DMF) with the aid of ultrasonic oscillation to give a 0.5 mg/mL black suspension, then casting 3 lL of OMC suspension on the surface of GC electrode and the solvent was allowed to dry under an infrared lamp. PB film was prepared by a electro-deposition method as follows: the GC or OMC/GC was immersed into an unstirred fresh solution containing 2 mM FeCl3 6H2O, 2 mM K3Fe(CN)6, 0.1 M KCl and 0.1 M HCl, where a constant potential of +0.4 V was applied for 200 s. Then the electrode was carefully washed with water and transferred into a solution containing 0.1 M KCl + 0.1 M HCl and activated through electrochemical cycling between +0.4 V and 0.5 V (20 cycles) at a scan rate of 50 mV s1. Finally the electrode was rinsed with double-distilled water and dried in air.
Fig. 1. FT-IR spectrum of OMC.
Fig. 2. Raman spectras of OMC (a) and PB/OMC (b).
J. Bai et al. / Microporous and Mesoporous Materials 119 (2009) 193–199
edge plane graphite sites/defects may generally show much more reactive than those at the basal-plane graphite toward electron transfer [30]. In the microstructure of PB, the ferric ions are coordinated to the nitrogen atoms, and the ferrous ions connect strongly to the carbon atoms, of the bridging cyanide ligands. It is noticed that both carbon atoms in the OMC and the –CN of PB are conjugated, and then they could act as electron donor and acceptor, respectively. It may be that p–p stacking interactions occur between the carbon atoms of OMC and –CN groups of PB. It was reported that single wall carbon nanotubes (SWNTs) could be chemically functionalized by PB nanoparticles [34]. In the present paper, PB was firstly modified onto the surface of OMC by electro-deposition. SEM was exploited to gain insights into the particle size and distribution of PB particles assembled on OMC. Fig. 3 shows the SEM images for OMC (a) and PB/OMC (b). The SEM images reveal that the OMC is well distributed on the surface and that most of the OMC are in the form of small bundles (Fig. 3a). After electro-deposition of PB film on the OMC, the nano-PB particles (diameter, ca. 50 nm) were deposited along the sidewalls of OMC bundles and this could be attributed to the p– p stacking interaction between carbon atoms in the OMC and the –CN group of PB. This is in agreement with the result from the Raman spectra (Fig. 2). The kinetics of the OMC modified electrode reaction was also investigated. We explored the response of the OMC/GC modified electrode (Fig. 4a) and bare GC electrode (Fig. 4b) toward the
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Fig. 4. Cyclic voltammograms obtained at the OMC modified electrode (a) and bare GC electrode (b) in 5 mM K3Fe(CN)6/0.1 M KCl solution at a scan rate of 50 mV s1.
one-electron aqueous redox probe potassium ferricyanide. Fig. 4 shows cyclic voltammograms obtained at the OMC modified electrode and bare GC electrode in the presence of 5 mM K3Fe(CN)6 + 0.1 M KCl solution. The CVs show that the difference in potential between the anodic and cathodic peaks for ferricyanide (DEp) is 64 mV for the OMC modified electrode and 97 mV for bare GC electrode at a scan rate of 50 mV s1. The peak-to-peak separation DEp was a measure of the standard rate constant for electron transfer, the lower DEp, the higher electron-transfer rate. The results indicate that the OMC modified electrode shows a faster electron-transfer rate compared to bare GC electrode. Furthermore, the electrochemical response current at the OMC modified electrode is much larger compared to bare GC electrode. According to the Randles–Sevcik equation, the peak currents were proportional to the area of the electroactive surface area. So the electroactive surface area for OMC/GC electrode is more than that of bare GC electrode, which has been reported previously [29]. All these indicate that OMC/GC electrode has relativity better electrochemical reacting ability. The greatly enhanced electrochemical behavior of OMC may be attributed to a large number of edge plane defect sites on the surface of OMC accessible for the electrolyte [30]. 3.2. Cyclic voltammetric behavior of the PB/OMC/GC electrode
Fig. 3. SEM images of OMC (a) and PB/OMC (b).
The cyclic voltammograms of the bare GC (a), OMC/GC (b), PB/ GC (c) and PB/OMC/GC (d) electrodes in 0.1 M PBS (pH 5.00) are shown in Fig. 5. It can be seen that no obvious redox peaks are observed at the bare GC (Fig. 5a) and OMC/GC (Fig. 5b) electrodes in the potential range of 0.40 to +1.20 V. However, two pairs of the redox peaks appear at +0.2 (peak I) and +0.9 V (peak II) at the PB/ GC (Fig. 5c) and PB/OMC/GC (Fig. 5d) electrodes. The PB/OMC retains the redox activity of PB. The responses around +0.2 V result from the reduction/oxidation of high spin Fe3+/2+, while another couple around +0.9 V corresponding to the redox reaction of low spin FeðCNÞ63=4 . This is similar with the previous reports [35]. One can note that the peak current of the PB/OMC/GC electrode is approximately five fold higher than that of the PB/GC electrode (Fig. 5). We also found that the background current at the OMC/ GC electrode is larger than that at the bare GC electrode. It is clear that the presence of OMC promotes the charge transfer of PB due to the existence of a large mount of edge-plane-like defective sites in the OMC materials and high surface area of OMC. Additionally we studied the effect of the potential scan rate (mV) on the peak current (ipa and ipc) that has been investigated in the range of 20–200 mV s1 (Fig. 6). The results show that the
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Fig. 5. The Cyclic voltammograms of the bare GC (a), OMC/GC (b), PB/GC (c) and PB/ OMC/GC (d) electrodes in 0.1 M PBS (pH 5.00). Scan rate: 50 mV s1.
Fig. 6. The Cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00) at different scan rates: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 mV s1. Inset: the relation between the peak currents and scan rates (the peak I).
peak currents have a linear relationship (Fig. 6 inset) with the square root of the scan rates (peak I is used to study), the correlation coefficient is 0.9999, implying that the electrochemical kinetics is a diffusion-controlled process. For PB/OMC/GC electrode, it is found that the values of Epa and Epc shift slightly to positive and negative directions, respectively, and the peak potential separation DEp becomes larger with the increase of scan rate. This indicates that the redox peak potentials of PB/OMC/GC modified electrode depend on scan rates. But the formal potentials E1/2 (E1/2 = Epa/2 + Epc/2) for peaks I and II are almost independent on the scan rates, which is similar with the previous reports [26]. The electrochemical stability of the PB/OMC/GC electrode was investigated with the cyclic voltammogram. The PB/OMC/GC electrode shows a good stability after scanned in 0.1 M PBS (pH 5.00) for 100 cycles. The peak currents decrease by 16.5%, indicating the monolayer film of PB/OMC is stable. However, the peak currents decrease by 51.7% after 100 cycles for PB/GC electrode. Fig. 7 illustrates the dependence of the peak currents of peak II at PB/GC (a) and PB/OMC/GC (b) electrodes on cycle number. These results indicate that the presence of OMC in the PB/OMC composite film greatly enhances the electrochemical stability of PB. The remarkable stability of the PB/OMC hybrid composite film could
Fig. 7. The dependence of the peak current of peak II at PB/GC (a) and PB/OMC/GC (b) electrodes on cycle number.
be due to the pp stacking interaction between carbon atoms in the ordered mesoporous carbons and the –CN groups of PB. Besides, cations in the PB (iron ions) might also be ready to interact with anions in the OMC (carboxyl moieties) through ionic interaction and also, the hydrophobic interaction of the OMC and PB might be contributed to the stability of the composite film. This result has been reported with PB/MWNTs composite film [36,37]. We also studied the stability of the PB/OMC composite film to pH changes using cyclic voltammetry. It has been reported that the PB layer was disrupted after a few scans at neutral pH and a very low stability was observed with alkaline pH [38,39]. The reason for this behavior is probably to be ascribed to the strong interaction between ferric ions (Fe3+) and hydroxyl ions (OH) which forms Fe(OH)3 at pH higher than 6.4 [40], thus leading to the destruction of the Fe–CN–Fe bond, hence solubilising PB [41]. For many years, this instability has represented the main drawback to the use of PB modified electrodes. The cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS at different pH values are shown in Fig. 8. The effect of pH on anodic and cathodic peak potentials and currents of PB/OMC/GC electrode is shown in Table 1. In the pH range of 3.00–7.00, DEp gradually diminishes with pH increasing, but when the pH is above 7.00 DEp increases (see Table 1). The response current just a slight changes with pH changing from 3.00 to 5.00. However, the response current decreases significantly when the solution pH is further increased. We have known that the PB has good stability in acidic solution. But it shows that
Fig. 8. Cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS with scan rate 50 mV s1 with various pH 3.00, 4.00, 5.00, 6.00, 7.00, 7.50, 8.00, 8.67, 9.14, and 10.36, respectively.
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J. Bai et al. / Microporous and Mesoporous Materials 119 (2009) 193–199 Table 1 The cyclic voltammograms data of the PB/OMC/GC electrode at different pH values. pH
Epa (V)
Epc (V)
DEp (V)
ipa (104)
ipc (104)
3.00 4.00 5.00 6.00 7.00 7.50 8.00 8.67 9.14
0.267 0.266 0.263 0.264 0.253 0.246 0.241 0.237 0.239
0.161 0.162 0.161 0.163 0.173 0.151 0.142 0.133 0.081
0.106 0.104 0.102 0.101 0.080 0.095 0.099 0.104 0.158
1.473 1.478 1.400 1.320 1.026 0.749 0.541 0.332 0.182
1.986 1.906 1.837 1.545 1.079 0.737 0.583 0.395 0.194
the lower pH is not the most condition for the electrochemical character of PB. From the results above, it is observed that the effect of pH on the potentials and currents of the peaks for PB/OMC/GC electrode is more complicated. In acidic electrolytes, the oxygen-containing functional groups are presented on the surface of OMC as –COOH form. So there are few ionic interactions between PB and OMC, which was mentioned previously. As the pH increases, –COOH is transformed into –COO ion. –COO ion can interact with Fe3+ or Fe2+, which enhances the stability of the PB/OMC composite film and makes electrochemical reaction of PB much easier in weak acidic electrolytes. On the other hand, it may be that H+ ions react with FeðCN Þ3 6 and generate HCN in acidic electrolytes [42]. The chemical bond of Fe–CN–Fe bound may be weaken by H–CN bond. As the pH increases above 7.00, hydroxyl ions (OH) are in the majority of solution. The ferric ions (Fe3+) and hydroxyl ions (OH) which forms Fe(OH)3 lead to the destruction of Fe–CN–Fe bound and decreases iron ions amount. Therefore, the response currents also decrease. In Fig. 8, one can observe that two pairs of sharp redox peaks appear obviously when pH 6 6.0. However, as the pH value increases to 7.00 the peak currents I decrease. But the two pairs of peaks can be observed obviously. And even the pH value rise to 9.14, the two pairs of peaks still can be clearly seen. With the further increase of pH value (up to 10.36), both peaks I and II disappear. This pH, which is reported for the first time by our workgroup, is wider than that PB/MWNTs/PG electrode can detect [26]. Two pairs of sharp redox peaks can be observed even at weak alkaline medium. It indicates that the PB is stable in PB/OMC composite while the pH value changes. This implies that the use of OMC matrix results in the improvement of the stability of PB film. A possible explanation could be the presence of carboxyl groups on the surface of OMC which contributed to prevent the formation of Fe(OH)3. The oxygen-containing functional groups are presented on the surface of OMC, which were confirmed by FT-IR (Fig. 1). On the other hand, OMC increases the surface dimension. The porous high surface area OMC matrix provides a high loading capacity for the deposition of PB particles. It was proved previously that there are more significant edge-plane-like defective sites existing on the surface of PB/OMC than on the surface of OMC (Fig. 2). For all these reasons, a larger and deeper deposition of PB is probably obtained, and the OMC surface acts as a reservoir of wellbound PB, which minimizes the leakage due to the hydrolysis of ferric ions, and increases the operational stability even at alkaline pH [38].
Fig. 9. Cyclic voltammograms of PB/GC (a) and PB/OMC/GC (b) electrodes in 0.1 M PBS (pH 5.00) without (solid line) and with (dot line) 2.5 mM H2O2. Scan rate: 50 mV s1.
the PBS (pH 5.00) were recorded in the absence and presence of 2.5 mM H2O2 in the potential range of 0.2 to +0.6 V (vs. Ag/AgCl), as illustrated in Fig. 9a and b. It is possible to note that in the presence of H2O2, the reduction peak current increased significantly and the oxidation peak current decreased obviously for both PB/ GC and PB/OMC/GC electrodes (Fig. 9a and b). This showed that both PB/GC and PB/OMC/GC electrodes exhibited excellent electrocatalytic activity towards H2O2. But the increased reduction currents of PB/OMC/GC electrode were much higher than that of PB/ GC electrode. This indicates that PB/OMC/GC electrode possesses the strong catalysis towards H2O2. The catalytic reduction reaction can be ascribed to the reduction of H2O2 to water as previously reported [39]. The reaction of the process can be expressed as follows:
2K2 FeII ½FeII ðCNÞ6 þ H2 O2 þ 2Hþ ¼ 2KFeIII ½FeII ðCNÞ6 þ 2H2 O þ 2Kþ ; ð1Þ III
II
þ
II
II
KFe ½Fe ðCNÞ6 þ K þ e ¼ K2 Fe ½Fe ðCNÞ6 :
ð2Þ
On the basis of the voltammetric results described above, PB/ OMC/GC electrode can effectively catalyze reduction of H2O2, and it appears likely that one can detect H2O2 with PB/OMC/GC electrode. According to the relationship between applied potential and H2O2 electrocatalytic reduction current, the optimum elec-
3.3. Electrocatalytic reduction of H2O2 at the PB/OMC/GC electrode From the experimental results above, the PB/OMC not only retains the redox activity of PB but also shows good stability. Due to the unique properties of PB and OMC, PB/OMC is expected to have electrocatalytic activity to some biomolecules. The cyclic voltammograms of PB/GC and PB/OMC/GC electrodes performed in
Fig. 10. Current–time curve obtained at PB/OMC/GC electrode for successive addition of 0.4 mM H2O2. The inset shows the calibration curve for H2O2 detection at the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00). Applied potential: +0.18 V.
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trode potential was selected at +0.18 V for amperometric measurements in order to obtain good repeatability and high sensitivity. Fig. 10 illustrates a typical current–time plot for PB/OMC/GC electrode upon the successive addition of 0.4 mM H2O2 with the potential held at +0.18 V. It can be seen that the amperometric signal is stable after the addition of H2O2 and the time required to attain the 95% of the steady state response is less than 5 s. These indicate that PB/OMC/GC electrode not only has high sensitivity, but also has a fast response to H2O2 with operating potential controlled at +0.18 V in 0.1 M PBS (pH 5.00). The calibration curve of H2O2 at PB/OMC/GC electrode is shown in inset of Fig. 10. The PB/OMC/ GC electrode exhibits a good linear relationship to H2O2 with a correlation of 0.9994 and a very low detection of 1 lM (S/N = 3). Thus, PB/OMC/GC electrode can be used to detect H2O2 as an amperometric sensor. 3.4. Electrocatalytic oxidation of AA at the PB/OMC/GC electrode We also investigated the electrocatalytic activity of PB/OMC/GC electrode for the oxidation of AA. The catalytic oxidation of AA at PB/OMC/GC and PB/GC electrodes is shown in Fig. 11. With addition of AA, oxidation current of PB peak gradually increases while the corresponding reduction peak current decreases. But when 3 mM AA solution was added into the solution, the increase of the oxidation peak current of PB/OMC/GC electrode was much higher than that of PB/GC electrode, which indicates the PB/OMC shows better electrocatalytic activity for the oxidation of AA. The current–time plot could be obtained by successive addition of 0.1 mM AA to the phosphate buffer solution (pH 5.0) using an oper-
Fig. 12. Current–time curve obtained at PB/OMC/GC electrode for successive addition of 0.1 Mm AA. The inset shows the calibration curve for AA detection at the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00). Applied potential: +0.3 V.
ation potential of 0.3 V, which is pictured in Fig. 12. When an aliquot of AA solution was added into the buffer solution, the oxidation current rose steeply to reach a stable value. A good linear relationship with the concentration of AA was exhibited by chronoamperometry in inset of Fig. 12. It also provided a very low detection limit of about 0.26 lM at the signal to noise ratio of 3 (S/N = 3). These advantages could be attributed to the unique surface property of OMC. The OMC possesses large surface area and many edge-plane-like defective sites, which would be very beneficial for PB adsorption. So OMC could play an important role in electron transfer with most of its redox partners. 4. Conclusions The surface of ordered mesoporous carbon was firstly modified with Prussian blue and prepared PB/OMC composite film. The electrochemical behavior of PB/OMC was also investigated and it was found that PB retained its good redox properties. Compared to the known PB/GC electrode, the PB/OMC/GC electrode exhibited much wider pH adaptive range, much better electrochemical stability and larger response current to the reduction of H2O2. Furthermore, the PB/OMC/GC electrode also performed more obvious catalysis for oxidation of AA. The results above showed that the electrochemical and electrocatalytic properties of PB were improved as the presence of OMC. The good results can be attributed the synergistic effect between OMC and PB. Therefore it would pave a new pathway to manipulate molecular entities of OMC by cooperation with functional inorganic electroactive compounds. Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 20875012), and the Analysis and Testing Foundation of Northeast Normal University. References
Fig. 11. Cyclic voltammograms of PB/GC (a) and PB/OMC/GC (b) electrodes in 0.1 M PBS (pH 5.00) without (solid line) and with (dot line) 3.0 mM AA. Scan rate: 50 mV s1.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Ordered mesoporous carbon-supported Prussian blue: Characterization and electrocatalytic properties Jing Bai, Bin Qi, Jean Chrysostome Ndamanisha, Li-ping Guo * Faculty of Chemistry, Northeast Normal University, Renmin Street 5268, Changchun 130024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 July 2008 Received in revised form 17 October 2008 Accepted 20 October 2008 Available online 7 November 2008 Keywords: Ordered mesoporous carbon Prussian blue Electrocatalysis Hydrogen peroxide Ascorbic acid
a b s t r a c t Ordered mesoporous carbon (OMC) was successfully modified with one kind of electroactive inorganic compounds: Prussian blue (PB). The composites were characterized by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM) and electrochemical methods. The results show that, after depositing PB on the OMC, PB remains electroactivity. The new material obtained (PB/OMC) was used to modify glassy carbon electrode (GC). Compared with Prussian blue (PB) modified glassy carbon electrode (GC), the PB/OMC/GC electrode exhibits much better electrochemical stability, much wider pH range and larger response currents to the reduction of hydrogen peroxide (H2O2). Additionally, the PB/OMC/GC electrode also exhibits remarkably strong and stable electrocatalytic response toward the oxidation of ascorbic acid (AA) compared with PB/OMC. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Recently, a series of porous materials such as clayminerals [1], montmorillonite [2], porous aluminosilicates, carbon nanotubes (CNTs) and sol–gel matrix etc. [3–5] have been shown to be promising as immobilization matrices. These new materials have large pores, thick pore walls and high hydrothermal stability. Mesoporous crystalline materials exhibit extremely high surface area and well defined pore size as well as high thermal stability and flexible framework composition. In addition, the unique structural and catalytic properties of molecular sieves for structuring an electrochemical/electron-transfer environment and resistance to biodegradation have attracted considerable attention [6–8]. Besides these materials mentioned above, highly ordered mesoporous carbon (OMC) has been receiving much attention owing to the fact that SBA-15 template is inexpensive [9] and easy to synthesize [10]. With their considerable properties, such as uniform and tailored pore structure, high specific surface areas, large pore volume and chemical inertness [11,12], ordered mesoporous carbons (OMCs) are of great interest for the fabrication of new classes of advanced materials [13]. In addition, the ability of ordered mesoporous carbons (OMCs) to promote the electron-transfer reactions of important molecules, such as L-cysteine [14], dopamine [15], and epinephrine [16], have made them attractive for the construction of various electrochemical sensors. While immobilization
* Corresponding author. Tel.: +86 431 85099762; fax: +86 413 85099762. E-mail address: [email protected] (L.-p. Guo). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.030
of electroactive substance on mesoporous materials was also reported, such as the grafting of ferrocene on mesoporous silica materials [17], the immobilization of ferrocene on mesoporous carbon [18], and the hybrid materials exhibit a high catalytic activity. Hence, ordered mesoporous carbon could have more interests and potential advantages for many advanced applications than other mesoporous materials [3–5]. PB is an inorganic polycrystal with well-known electrochromic and electrocatalytical properties [19]. Since its deposition on solid electrodes by Neff [20], it has been the subject of many studies. Due to the unique properties, synthetic versatility and ability of the bridging cyanide ligands to efficiently mediate its properties, PB and its analogues have been employed intensively in many fields, such as in the electrochemical [21,22], electrochromic [23], magnetic [24] and potential analytic applications [25]. The electrochemical stability of PB layer has also been a matter of investigation since its first use. The effect of pH on PB activity and stability is still a subject of interest for researchers involved in the characterization of PB. Thus, how to efficiently improve the electrochemical stability of PB film becomes desirable. To solve this problem, multi-walled carbon nanotubes (MWNTs) [26], Screenprinted electrodes (SPEs) [27], polyaniline (PANI) [28] have been used to immobilize PB. There were the oxygen-containing functional groups on the surface of OMC, which have been confirmed by FT-IR [29]. So we deposited PB on the OMC, which can stabilize PB film. To the best of our knowledge, there are few reports on deposition of PB on OMC substrate. Taking the advantage of the unique properties of both OMC [30] and PB, it would greatly broaden the applications of OMC and PB.
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In this paper, PB was efficiently deposited on the ordered mesoporous carbons (OMC) modified glassy carbon (GC) electrode and thus formed PB/OMC composite film. With the introduction of OMC, the PB/OMC hybrid composites show synergistic augmentation of the response current for H2O2 and AA detection. Compared with PB film, the PB/OMC composite film shows much better electrochemical stability, much wider pH adaptive range and larger response current to the reduction of H2O2.
2. Experimental 2.1. Reagents OMC was synthesized according to the previous reported work [31]. Iron chloride hexahydrate (FeCl3 6H2O), potassium ferricyanide (K3Fe(CN)6) and H2O2 were purchased from Chemical Reagent Company of Shanghai (China). Ascorbic acid was purchased from Beijing Chemical Reagent Company. All other chemicals not mentioned here were of analytical reagent grade and were used as received. Double-distilled water was used throughout. 0.1 M phosphate buffer solution (PBS) was used as supporting electrolyte solution, which was made from K2HPO4, KH2PO4, and adjusting the pH with H3PO4 or KOH.
3. Results and discussion 3.1. Characterization of OMC and PB/OMC The presence of the oxygen-containing functional groups on the surface of OMC was confirmed by FT-IR in Fig. 1. The band around 1628 cm1 is attributed to C@O stretch vibration, and the bands around 1560 and 1522 cm1 are ascribed to COO stretch asymmetric vibration. The band around 1185 cm1 is assignable to O– H vermicular vibration, while the carboxylic acid O–H stretch vibration is responsible for the band around 3438 cm1. Fig. 2 shows the Raman spectra of OMC (a) and PB/OMC (b). Both spectra exhibit the presence of D and G bands, located at 1384 cm1 (disorder mode) and 1595 cm1 (tangential) [32]. The D band at around 1348 cm1 is associated with the presence of defects in the graphite layer. The peak at 1595 cm1 corresponds to the Raman-active E2g, which resulted from the vibration mode corresponding to the movement in opposite directions of two neighboring carbon atoms in a single crystal graphite sheet. Furthermore, the relative intensity ratio of the D and G bands (ID/IG ratio) is proportional to the number of defect sites in the graphite carbon [33]. As can be clearly seen from Fig. 2 the ID/IG ratio of PB/OMC is much larger than OMC, indicating that there are more significant edge-plane-like defective sites existing on the surface of PB/OMC than on the surface of OMC. Researchers have demonstrated that
2.2. Apparatus All the electrochemical experiments were performed with a CHI 830b Electrochemical Analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China) in a conventional threeelectrode cell. The working electrode (WE) used was glassy carbon (GC) electrode (Model CHI104, 3 mm diameter) or the modified electrode. A platinum foil as the counter electrode (CE), and an Ag/AgCl (KCl saturated) electrode as reference electrode (RE). All potentials in this paper were measured and reported versus Ag/ AgCl. The sample solutions were purged with purified nitrogen for at least 15 min to remove oxygen prior to the beginning of a series of experiments. All measurements were carried out at room temperature (20 ± 2 °C). The micrographs of the PB and PB/OMC films were obtained with SEM. The SEM images were determined with a Philips XL-30 ESEM operating at 3.0 kV. Raman spectras were recorded at ambient temperature on a Renishaw Raman system model 1000 spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. Infrared spectrum of the sample was recorded with Nicolet Magna 560 FT-IR Spectrometer with KBr plate. 2.3. Preparation of the modified electrodes Prior to use, the GC electrode was polished with 1, 0.3 and 0.05 lm alumina powder, respectively, then ultrasonically cleaned with ethanol and double-distilled water and dried in nitrogen. The specific procedure of preparation of modified electrode was as followed: 5 mg of the OMC was dispersed in 10 mL of N,N-dimethylformamide (DMF) with the aid of ultrasonic oscillation to give a 0.5 mg/mL black suspension, then casting 3 lL of OMC suspension on the surface of GC electrode and the solvent was allowed to dry under an infrared lamp. PB film was prepared by a electro-deposition method as follows: the GC or OMC/GC was immersed into an unstirred fresh solution containing 2 mM FeCl3 6H2O, 2 mM K3Fe(CN)6, 0.1 M KCl and 0.1 M HCl, where a constant potential of +0.4 V was applied for 200 s. Then the electrode was carefully washed with water and transferred into a solution containing 0.1 M KCl + 0.1 M HCl and activated through electrochemical cycling between +0.4 V and 0.5 V (20 cycles) at a scan rate of 50 mV s1. Finally the electrode was rinsed with double-distilled water and dried in air.
Fig. 1. FT-IR spectrum of OMC.
Fig. 2. Raman spectras of OMC (a) and PB/OMC (b).
J. Bai et al. / Microporous and Mesoporous Materials 119 (2009) 193–199
edge plane graphite sites/defects may generally show much more reactive than those at the basal-plane graphite toward electron transfer [30]. In the microstructure of PB, the ferric ions are coordinated to the nitrogen atoms, and the ferrous ions connect strongly to the carbon atoms, of the bridging cyanide ligands. It is noticed that both carbon atoms in the OMC and the –CN of PB are conjugated, and then they could act as electron donor and acceptor, respectively. It may be that p–p stacking interactions occur between the carbon atoms of OMC and –CN groups of PB. It was reported that single wall carbon nanotubes (SWNTs) could be chemically functionalized by PB nanoparticles [34]. In the present paper, PB was firstly modified onto the surface of OMC by electro-deposition. SEM was exploited to gain insights into the particle size and distribution of PB particles assembled on OMC. Fig. 3 shows the SEM images for OMC (a) and PB/OMC (b). The SEM images reveal that the OMC is well distributed on the surface and that most of the OMC are in the form of small bundles (Fig. 3a). After electro-deposition of PB film on the OMC, the nano-PB particles (diameter, ca. 50 nm) were deposited along the sidewalls of OMC bundles and this could be attributed to the p– p stacking interaction between carbon atoms in the OMC and the –CN group of PB. This is in agreement with the result from the Raman spectra (Fig. 2). The kinetics of the OMC modified electrode reaction was also investigated. We explored the response of the OMC/GC modified electrode (Fig. 4a) and bare GC electrode (Fig. 4b) toward the
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Fig. 4. Cyclic voltammograms obtained at the OMC modified electrode (a) and bare GC electrode (b) in 5 mM K3Fe(CN)6/0.1 M KCl solution at a scan rate of 50 mV s1.
one-electron aqueous redox probe potassium ferricyanide. Fig. 4 shows cyclic voltammograms obtained at the OMC modified electrode and bare GC electrode in the presence of 5 mM K3Fe(CN)6 + 0.1 M KCl solution. The CVs show that the difference in potential between the anodic and cathodic peaks for ferricyanide (DEp) is 64 mV for the OMC modified electrode and 97 mV for bare GC electrode at a scan rate of 50 mV s1. The peak-to-peak separation DEp was a measure of the standard rate constant for electron transfer, the lower DEp, the higher electron-transfer rate. The results indicate that the OMC modified electrode shows a faster electron-transfer rate compared to bare GC electrode. Furthermore, the electrochemical response current at the OMC modified electrode is much larger compared to bare GC electrode. According to the Randles–Sevcik equation, the peak currents were proportional to the area of the electroactive surface area. So the electroactive surface area for OMC/GC electrode is more than that of bare GC electrode, which has been reported previously [29]. All these indicate that OMC/GC electrode has relativity better electrochemical reacting ability. The greatly enhanced electrochemical behavior of OMC may be attributed to a large number of edge plane defect sites on the surface of OMC accessible for the electrolyte [30]. 3.2. Cyclic voltammetric behavior of the PB/OMC/GC electrode
Fig. 3. SEM images of OMC (a) and PB/OMC (b).
The cyclic voltammograms of the bare GC (a), OMC/GC (b), PB/ GC (c) and PB/OMC/GC (d) electrodes in 0.1 M PBS (pH 5.00) are shown in Fig. 5. It can be seen that no obvious redox peaks are observed at the bare GC (Fig. 5a) and OMC/GC (Fig. 5b) electrodes in the potential range of 0.40 to +1.20 V. However, two pairs of the redox peaks appear at +0.2 (peak I) and +0.9 V (peak II) at the PB/ GC (Fig. 5c) and PB/OMC/GC (Fig. 5d) electrodes. The PB/OMC retains the redox activity of PB. The responses around +0.2 V result from the reduction/oxidation of high spin Fe3+/2+, while another couple around +0.9 V corresponding to the redox reaction of low spin FeðCNÞ63=4 . This is similar with the previous reports [35]. One can note that the peak current of the PB/OMC/GC electrode is approximately five fold higher than that of the PB/GC electrode (Fig. 5). We also found that the background current at the OMC/ GC electrode is larger than that at the bare GC electrode. It is clear that the presence of OMC promotes the charge transfer of PB due to the existence of a large mount of edge-plane-like defective sites in the OMC materials and high surface area of OMC. Additionally we studied the effect of the potential scan rate (mV) on the peak current (ipa and ipc) that has been investigated in the range of 20–200 mV s1 (Fig. 6). The results show that the
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Fig. 5. The Cyclic voltammograms of the bare GC (a), OMC/GC (b), PB/GC (c) and PB/ OMC/GC (d) electrodes in 0.1 M PBS (pH 5.00). Scan rate: 50 mV s1.
Fig. 6. The Cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00) at different scan rates: 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 mV s1. Inset: the relation between the peak currents and scan rates (the peak I).
peak currents have a linear relationship (Fig. 6 inset) with the square root of the scan rates (peak I is used to study), the correlation coefficient is 0.9999, implying that the electrochemical kinetics is a diffusion-controlled process. For PB/OMC/GC electrode, it is found that the values of Epa and Epc shift slightly to positive and negative directions, respectively, and the peak potential separation DEp becomes larger with the increase of scan rate. This indicates that the redox peak potentials of PB/OMC/GC modified electrode depend on scan rates. But the formal potentials E1/2 (E1/2 = Epa/2 + Epc/2) for peaks I and II are almost independent on the scan rates, which is similar with the previous reports [26]. The electrochemical stability of the PB/OMC/GC electrode was investigated with the cyclic voltammogram. The PB/OMC/GC electrode shows a good stability after scanned in 0.1 M PBS (pH 5.00) for 100 cycles. The peak currents decrease by 16.5%, indicating the monolayer film of PB/OMC is stable. However, the peak currents decrease by 51.7% after 100 cycles for PB/GC electrode. Fig. 7 illustrates the dependence of the peak currents of peak II at PB/GC (a) and PB/OMC/GC (b) electrodes on cycle number. These results indicate that the presence of OMC in the PB/OMC composite film greatly enhances the electrochemical stability of PB. The remarkable stability of the PB/OMC hybrid composite film could
Fig. 7. The dependence of the peak current of peak II at PB/GC (a) and PB/OMC/GC (b) electrodes on cycle number.
be due to the pp stacking interaction between carbon atoms in the ordered mesoporous carbons and the –CN groups of PB. Besides, cations in the PB (iron ions) might also be ready to interact with anions in the OMC (carboxyl moieties) through ionic interaction and also, the hydrophobic interaction of the OMC and PB might be contributed to the stability of the composite film. This result has been reported with PB/MWNTs composite film [36,37]. We also studied the stability of the PB/OMC composite film to pH changes using cyclic voltammetry. It has been reported that the PB layer was disrupted after a few scans at neutral pH and a very low stability was observed with alkaline pH [38,39]. The reason for this behavior is probably to be ascribed to the strong interaction between ferric ions (Fe3+) and hydroxyl ions (OH) which forms Fe(OH)3 at pH higher than 6.4 [40], thus leading to the destruction of the Fe–CN–Fe bond, hence solubilising PB [41]. For many years, this instability has represented the main drawback to the use of PB modified electrodes. The cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS at different pH values are shown in Fig. 8. The effect of pH on anodic and cathodic peak potentials and currents of PB/OMC/GC electrode is shown in Table 1. In the pH range of 3.00–7.00, DEp gradually diminishes with pH increasing, but when the pH is above 7.00 DEp increases (see Table 1). The response current just a slight changes with pH changing from 3.00 to 5.00. However, the response current decreases significantly when the solution pH is further increased. We have known that the PB has good stability in acidic solution. But it shows that
Fig. 8. Cyclic voltammograms of the PB/OMC/GC electrode in 0.1 M PBS with scan rate 50 mV s1 with various pH 3.00, 4.00, 5.00, 6.00, 7.00, 7.50, 8.00, 8.67, 9.14, and 10.36, respectively.
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J. Bai et al. / Microporous and Mesoporous Materials 119 (2009) 193–199 Table 1 The cyclic voltammograms data of the PB/OMC/GC electrode at different pH values. pH
Epa (V)
Epc (V)
DEp (V)
ipa (104)
ipc (104)
3.00 4.00 5.00 6.00 7.00 7.50 8.00 8.67 9.14
0.267 0.266 0.263 0.264 0.253 0.246 0.241 0.237 0.239
0.161 0.162 0.161 0.163 0.173 0.151 0.142 0.133 0.081
0.106 0.104 0.102 0.101 0.080 0.095 0.099 0.104 0.158
1.473 1.478 1.400 1.320 1.026 0.749 0.541 0.332 0.182
1.986 1.906 1.837 1.545 1.079 0.737 0.583 0.395 0.194
the lower pH is not the most condition for the electrochemical character of PB. From the results above, it is observed that the effect of pH on the potentials and currents of the peaks for PB/OMC/GC electrode is more complicated. In acidic electrolytes, the oxygen-containing functional groups are presented on the surface of OMC as –COOH form. So there are few ionic interactions between PB and OMC, which was mentioned previously. As the pH increases, –COOH is transformed into –COO ion. –COO ion can interact with Fe3+ or Fe2+, which enhances the stability of the PB/OMC composite film and makes electrochemical reaction of PB much easier in weak acidic electrolytes. On the other hand, it may be that H+ ions react with FeðCN Þ3 6 and generate HCN in acidic electrolytes [42]. The chemical bond of Fe–CN–Fe bound may be weaken by H–CN bond. As the pH increases above 7.00, hydroxyl ions (OH) are in the majority of solution. The ferric ions (Fe3+) and hydroxyl ions (OH) which forms Fe(OH)3 lead to the destruction of Fe–CN–Fe bound and decreases iron ions amount. Therefore, the response currents also decrease. In Fig. 8, one can observe that two pairs of sharp redox peaks appear obviously when pH 6 6.0. However, as the pH value increases to 7.00 the peak currents I decrease. But the two pairs of peaks can be observed obviously. And even the pH value rise to 9.14, the two pairs of peaks still can be clearly seen. With the further increase of pH value (up to 10.36), both peaks I and II disappear. This pH, which is reported for the first time by our workgroup, is wider than that PB/MWNTs/PG electrode can detect [26]. Two pairs of sharp redox peaks can be observed even at weak alkaline medium. It indicates that the PB is stable in PB/OMC composite while the pH value changes. This implies that the use of OMC matrix results in the improvement of the stability of PB film. A possible explanation could be the presence of carboxyl groups on the surface of OMC which contributed to prevent the formation of Fe(OH)3. The oxygen-containing functional groups are presented on the surface of OMC, which were confirmed by FT-IR (Fig. 1). On the other hand, OMC increases the surface dimension. The porous high surface area OMC matrix provides a high loading capacity for the deposition of PB particles. It was proved previously that there are more significant edge-plane-like defective sites existing on the surface of PB/OMC than on the surface of OMC (Fig. 2). For all these reasons, a larger and deeper deposition of PB is probably obtained, and the OMC surface acts as a reservoir of wellbound PB, which minimizes the leakage due to the hydrolysis of ferric ions, and increases the operational stability even at alkaline pH [38].
Fig. 9. Cyclic voltammograms of PB/GC (a) and PB/OMC/GC (b) electrodes in 0.1 M PBS (pH 5.00) without (solid line) and with (dot line) 2.5 mM H2O2. Scan rate: 50 mV s1.
the PBS (pH 5.00) were recorded in the absence and presence of 2.5 mM H2O2 in the potential range of 0.2 to +0.6 V (vs. Ag/AgCl), as illustrated in Fig. 9a and b. It is possible to note that in the presence of H2O2, the reduction peak current increased significantly and the oxidation peak current decreased obviously for both PB/ GC and PB/OMC/GC electrodes (Fig. 9a and b). This showed that both PB/GC and PB/OMC/GC electrodes exhibited excellent electrocatalytic activity towards H2O2. But the increased reduction currents of PB/OMC/GC electrode were much higher than that of PB/ GC electrode. This indicates that PB/OMC/GC electrode possesses the strong catalysis towards H2O2. The catalytic reduction reaction can be ascribed to the reduction of H2O2 to water as previously reported [39]. The reaction of the process can be expressed as follows:
2K2 FeII ½FeII ðCNÞ6 þ H2 O2 þ 2Hþ ¼ 2KFeIII ½FeII ðCNÞ6 þ 2H2 O þ 2Kþ ; ð1Þ III
II
þ
II
II
KFe ½Fe ðCNÞ6 þ K þ e ¼ K2 Fe ½Fe ðCNÞ6 :
ð2Þ
On the basis of the voltammetric results described above, PB/ OMC/GC electrode can effectively catalyze reduction of H2O2, and it appears likely that one can detect H2O2 with PB/OMC/GC electrode. According to the relationship between applied potential and H2O2 electrocatalytic reduction current, the optimum elec-
3.3. Electrocatalytic reduction of H2O2 at the PB/OMC/GC electrode From the experimental results above, the PB/OMC not only retains the redox activity of PB but also shows good stability. Due to the unique properties of PB and OMC, PB/OMC is expected to have electrocatalytic activity to some biomolecules. The cyclic voltammograms of PB/GC and PB/OMC/GC electrodes performed in
Fig. 10. Current–time curve obtained at PB/OMC/GC electrode for successive addition of 0.4 mM H2O2. The inset shows the calibration curve for H2O2 detection at the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00). Applied potential: +0.18 V.
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trode potential was selected at +0.18 V for amperometric measurements in order to obtain good repeatability and high sensitivity. Fig. 10 illustrates a typical current–time plot for PB/OMC/GC electrode upon the successive addition of 0.4 mM H2O2 with the potential held at +0.18 V. It can be seen that the amperometric signal is stable after the addition of H2O2 and the time required to attain the 95% of the steady state response is less than 5 s. These indicate that PB/OMC/GC electrode not only has high sensitivity, but also has a fast response to H2O2 with operating potential controlled at +0.18 V in 0.1 M PBS (pH 5.00). The calibration curve of H2O2 at PB/OMC/GC electrode is shown in inset of Fig. 10. The PB/OMC/ GC electrode exhibits a good linear relationship to H2O2 with a correlation of 0.9994 and a very low detection of 1 lM (S/N = 3). Thus, PB/OMC/GC electrode can be used to detect H2O2 as an amperometric sensor. 3.4. Electrocatalytic oxidation of AA at the PB/OMC/GC electrode We also investigated the electrocatalytic activity of PB/OMC/GC electrode for the oxidation of AA. The catalytic oxidation of AA at PB/OMC/GC and PB/GC electrodes is shown in Fig. 11. With addition of AA, oxidation current of PB peak gradually increases while the corresponding reduction peak current decreases. But when 3 mM AA solution was added into the solution, the increase of the oxidation peak current of PB/OMC/GC electrode was much higher than that of PB/GC electrode, which indicates the PB/OMC shows better electrocatalytic activity for the oxidation of AA. The current–time plot could be obtained by successive addition of 0.1 mM AA to the phosphate buffer solution (pH 5.0) using an oper-
Fig. 12. Current–time curve obtained at PB/OMC/GC electrode for successive addition of 0.1 Mm AA. The inset shows the calibration curve for AA detection at the PB/OMC/GC electrode in 0.1 M PBS (pH 5.00). Applied potential: +0.3 V.
ation potential of 0.3 V, which is pictured in Fig. 12. When an aliquot of AA solution was added into the buffer solution, the oxidation current rose steeply to reach a stable value. A good linear relationship with the concentration of AA was exhibited by chronoamperometry in inset of Fig. 12. It also provided a very low detection limit of about 0.26 lM at the signal to noise ratio of 3 (S/N = 3). These advantages could be attributed to the unique surface property of OMC. The OMC possesses large surface area and many edge-plane-like defective sites, which would be very beneficial for PB adsorption. So OMC could play an important role in electron transfer with most of its redox partners. 4. Conclusions The surface of ordered mesoporous carbon was firstly modified with Prussian blue and prepared PB/OMC composite film. The electrochemical behavior of PB/OMC was also investigated and it was found that PB retained its good redox properties. Compared to the known PB/GC electrode, the PB/OMC/GC electrode exhibited much wider pH adaptive range, much better electrochemical stability and larger response current to the reduction of H2O2. Furthermore, the PB/OMC/GC electrode also performed more obvious catalysis for oxidation of AA. The results above showed that the electrochemical and electrocatalytic properties of PB were improved as the presence of OMC. The good results can be attributed the synergistic effect between OMC and PB. Therefore it would pave a new pathway to manipulate molecular entities of OMC by cooperation with functional inorganic electroactive compounds. Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 20875012), and the Analysis and Testing Foundation of Northeast Normal University. References
Fig. 11. Cyclic voltammograms of PB/GC (a) and PB/OMC/GC (b) electrodes in 0.1 M PBS (pH 5.00) without (solid line) and with (dot line) 3.0 mM AA. Scan rate: 50 mV s1.
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