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Sep 7, 2007 - Prussian blue hollow nanostructures: Sacrificial template synthesis and application in hydrogen peroxide sensing. Qingming Shen , Jingyi...
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Ind. Eng. Chem. Res. 2007, 46, 6847-6851

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Fabrication of Prussian Blue/Multiwalled Carbon Nanotubes/Glass Carbon Electrode through Sequential Deposition Sufang Han, Yongmei Chen, Ran Pang, and Pingyu Wan* School of Science, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China

Maohong Fan* School of Materials Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332

Prussian blue (PB)/multiwalled carbon nanotubes (MWNTs)-modified glass carbon (GC) electrode (PB/ MWNTs/GC electrode) was chemically fabricated for the first time, using a sequential deposition-based method. PB crystallites are observed to be finely attached to the MWNTs of the prepared PB/MWNTs/GC electrode in which the MWNTs not only act as a carrier of PB but also as a modifier with catalytic function. Cyclic voltammetry (CV) studies demonstrate that the modified electrode has outstanding stability. The prepared PB/MWNTs/GC electrode can be used to determine the concentration of H2O2 in solution. 2. Experiment

1. Introduction 2+ Fe3+ 4 [Fe (CN)6]3,

Prussian blue (PB), which has the formula has been used as the electron-transfer mediator in the modified electrode, because of its fine redox reversibility and stability. PB with a particular structure has an outstanding catalytic effect on the redox reactions of some substances, especially hydrogen peroxide (H2O2).1 As a result, electrodes modified by PB have been widely studied in different fields.2-7 Electrochemistry deposition,4,8-10 chemistry deposition,11 and the self-assembling method12,13 have been used to fabricate PBmodified electrodes. Although the PB film can be rapidly prepared via electrochemical deposition, its properties are observed to be sensitive to preparation conditions, such as potential,8,9,14 current,15 and cyclic voltammetry (CV) scan velocity.4,10,16 Compared with the electrochemistry method, PB particles prepared via chemosynthesis is stable in a wider range of pH. However, wax or other binders, which cause poor conductivity, must be used to fix PB onto the electrode surface. In the self-assembling method, the application of polymolecules also increases the impedance of the electrode. Therefore, the study on a new method to fix chemically prepared PB without binders has great significance for expanding the application of the PB-modified electrode. Carbon nanotubes (CNTs) are considered to be a good mediator for PB-modified electrodes, because of their good electric conductivity and the property of being particle carriers. Some researchers have recently fabricated PB particles on the CNTs surface through π-π interaction17 or electrostatic interaction;18 however, the procedures are relatively complex. In the present study, PB crystallites were synthesized on the surface of multiwalled carbon nanotubes (MWNTs), using a sequential deposition method to fabricate a PB/MWNTs/glass carbon (PB/ MWNTs/GC) electrode without binders. To the best of our knowledge, this is the first introduction of the sequential deposition method for fabricating a PB/MWNTs/GC electrode, in which the MWNTs not only act as a carrier of PB but also as a modifier with catalytic function. * To whom correspondence should be addressed. Tel.: 86-10-64445917. Fax: 86-10-6444-5917. E-mail address: [email protected] (for P.W.). Tel.: 1-404-385-6725. Fax: 1-404-894-9140. E-mail address: [email protected] (for M.F.).

2.1. Purification of MWNTs. A total of 0.5 g of MWNTs (purchased from Tsinghua University) were dispersed in a 25 mL mixture of concentrated HNO3 and H2SO4 (1:3, v:v) with ultrasonic agitation for 10 min. The mixture then was refluxed at 90 °C for 0.5 h, followed by filtrating and rinsing to neutrality with distilled water. After being dried at 120 °C for 2 h, the powder was used as purified MWNTs in the following procedures. 2.2. Fabrication of a PB/MWNTs/GC Electrode via Sequential Deposition. Before each experiment, the GC electrode (4 mm in diameter) was polished with 1-3 µm R-alumina power; rinsed thoroughly with water; cleaned with ultrasonic with 1% HCl solution, 1% NaOH solution, and 95% ethanol; and then dried to obtain a GC electrode with a clean surface. The next step was to disperse 2.5 mg of purified MWNTs in 5 mL of acetone, with the aid of ultrasonic mixing, to form a stable black suspension. Twelve microliters of the MWNTs suspension was dropped on the surface of the GC electrode. The MWNTs/GC electrode was obtained just after evaporation of the solvent. FeCl3 and K4[Fe(CN)6] solutions with the same concentration were used for the deposition experiment. The MWNTs/GC electrode was first immersed in stirred FeCl3 solution for 30 min. After the electrode was rinsed and dried, it was then immersed in K4[Fe(CN)6] solution and stirred for another 30 min, followed by re-rinsing and re-drying processes to finish the first deposition cycle. The electrodes with the expected amount of PBsthe so-called “PB/MWNTs/GC electrodes”s were obtained by repeating the aforementioned procedure while their voltammetry values were examined after each cycle. 2.3. Characterization of MWNTs and PB/MWNTs. Fourier transform infrared spectra (FTIR) analyses were performed with Nicolet AVATAR 370 infrared spectroscopy equipment. The samples of purified MWNTs and PB/MWNTs films for FTIR analysis were prepared using CaF2 as the substrate, according to the aforementioned description. The field-emission scanning electron microscopy (FE-SEM) images of the prepared PB/ MWNTs films substrated with indium tin oxide (ITO) glasses were obtained using a Hitachi S-4700 FE-SEM system. The X-ray photoelectron spectra (XPS) tests were conducted with a ThermoVG ESCALAB 250 spectrometer.

10.1021/ie061511i CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2007

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Figure 1. Fourier transform infrared (FTIR) spectra of (a) purified multiwalled nanotubes (MWNTs) and (b) a combination of Prussian blue and MWNTs (PB/MWNTs). The method for preparing the PB/MWNTs was as follows: sequential deposition, using CaF2 glass as the substrate, a deposition solution comprised of 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6], a deposition cycle number of 3, and a wavenumber scan range of 40001200 cm-1.

2.4. Electrochemical Behaviors of PB/MWNTs/GC Electrode. The electrochemical behavior of the modified electrodes was studied using a three-electrode electrochemical cell with a platinum electrode as the counter electrode and a saturated calomel electrode as the reference electrode. CV curves in the range of 50-350 mV were achieved using a buffer solution (pH 6.0) that contained 5.0 mM KOH, 50.0 mM KH2PO4, and 45.0 mM KCl. The H2O2 solutions used to obtain the calibration line of current measurements were prepared by diluting the standard 54.84 mM H2O2 solutions with different volumes of the aforementioned buffer solutions and the concentrations of H2O2 were measured using KMnO4 titration. All the electrochemical measurements were conducted with a CS 300UA electrochemical test system that was developed by Huazhong University of Science and Technology (Wuhan, PRC). 3. Results and Discussion 3.1. Structure of the Fabricated PB/MWNTs. The IR spectrums of purified MWNTs and PB/MWNTs are shown in Figures 1a and 1b, respectively. There are obvious differences between Figure 1a and Figure 1b. Figure 1a shows several peaks. The peaks at 3375 cm-1 may be attributed to the physically adsorbed H-O-H. The peak at 1733 cm-1 may be attributed to the stretching vibrations of CdO groups. The two peaks observed at 2919 cm-1 and 2868 cm-1 are due to the C-H stretching vibration of saturated hydrocarbon, which have been reported by several research groups, although the stretching mechanism is still not clear.19 Those peaks indicate that some functional groups (such as -COOH groups) have been introduced to MWNTs during their purification processes. The strong peaks at 2076 cm-1 and 1733 cm-1, which resulted from the stretching vibrations of CN and COO- , respectively, and the symmetry as well as asymmetry stretching vibrations of COOat 1696 cm-1 and 1454 cm-1 in Figure 1b are the indicators that PB had been assembled on the surface of MWNTs. The functional groups on purified MWNTs can also be confirmed with the C 1s, O 1s spectra obtained from their XPS characterizations and shown in Figures 2a and 2b, respectively. Three peakssat 284.3, 286.0, and 289.6 eVsare shown on the C1s spectrum in Figure 2a, which might be caused by the sp2 carbon and COOH in MWNTs. The peak at 286.0 eV in Figure 2a results from a C atom bonded to COOH.20 The O 1s spectrum in Figure 2b also shows three peaks: two at 531.3 and 532.5 eV, which respectively correspond to OdC and O-C in the OdC-O functional group, and one at 543.5 eV, which is due to the physically adsorbed H-O-H.21 These polar groups in

Figure 2. X-ray photoelectron spectroscopy (XPS) spectra of (a,b) purified MWNTs and (c) Fe3+/MWNTs. The experimental conditions were as follows: deposition solution, 1.0 mM FeCl3; deposition time, 3; binding energy scan range, 1350.00-0.00 eV; X-ray source, Al.

purified MWNTs provide strong interaction with the cleaned GC electrode surface, which leads to the formation of the comparatively stable MWNTs/GC electrode. The Fe 2p XPS spectrum of purified MWNTs, after adsorbing Fe3+ cations, is presented in Figure 2c. Two characteristic peaks of Fe 2p3/2 and Fe 2p1/2 are observed, at 711.6 and 724.8 eV,22 respectively, which demonstrated that the Fe3+ cation has been adsorbed on the surface of MWNTs. When MWNTs along with the Fe3+ cation further reacted with K4[Fe(CN)6] solutions, the adsorbed Fe3+ cations would combine with the [Fe(CN)6]4- anions in solution to form PB crystallites. Based on the aforementioned results of the FTIR and XPS tests, it is obvious that the sequential deposition could be an effective method for fabricating PB/MWNTs electrodes. This fact was also confirmed by the SEM images of as-received MWNTs and fabricated PB/MWNTs that are shown in Figures 3a and 3b, respectively. Compared to Figure 3a, Figure 3b evidently shows that nanosized PB has been coated on the walls of MWNTs. 3.2. Effect of the Preparation Conditions on the Properties of the Prepared PB/MWNTs/GC Electrode. According to the characterization results discussed previously, the mechanism of PB/MWNTs/GC electrode fabrication through sequential deposition can be described as follows: the hydrophilicity of purified MWNTs leads to the permeation and eventually physicochemical adsorption of Fe3+ cations into the interlayer of MWNTs. After the adsorbed Fe3+ cation reacts with the [Fe(CN)6]4anion, the formed unstoichiometric precipitation of Fex[Fe(CN)6]y deposits on the interlayer of MWNTs. The unstoichiometric precipitation keeps active sites of CdN groups with

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Figure 4. Cyclic voltammetry (CV) curves of PB/MWNTs/GC electrodes prepared using different concentrations of FeCl3 and K4[Fe(CN)6]: (curve ad, 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]; curve be, 5.0 mM FeCl3 and 5.0 mM K4[Fe(CN)6]; curve cf, 10 mM FeCl3 and 10 mM K4[Fe(CN)6]. The experimental conditions were as follows: deposition cycle number, 3; potential scan range, 50-350 mV; potential scan rate, 50 mV/s.

Figure 5. Peaks current in CV curves versus the square root of scanning speed. The experimental conditions were as follows: deposition solution, 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]; deposition number, 3; potential scan range, 50-350 mV; scanning speed range, 10-100 mV/s.

Figure 3. Scanning electron microscopy (SEM) images of (a) as-received MWNTs and (b) PB/MWNTs film fabricated with the sequential deposition method, using indium tin oxide glass as substrates. The experimental conditions were as follows: deposition solution, 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]; deposition cycle number, 3.

which Fe3+ cations would further react; thus, more PB would be produced after several deposition cycles. Based on the proposed mechanism, the electrochemical properties of PB/MWNTs/GC electrodes could be controlled during their preparation procedures to meet different needs. In other words, the quantity and the crystalline property of PB on the modified electrode could be adjusted by varying the concentrations of the deposition solutions and the cycle numbers of depositions, which is confirmed with the test results illustrated in Figure 4. This figure shows that the higher the concentrations of FeCl3 and K4[Fe(CN)6] solutions used for the same times of deposition, the larger the crystallite size and the greater the amount of PB deposited on MWNTs, and, thus, the stronger the reductive and oxidative current intensity of the obtained PB/ MWNTs/GC electrode. The difference between the potentials of oxidative and reductive peaks in the CV curve (∆Ep) is an indicator for reversibility. ∆Ep is usually within 59 mV/n for reversible reactions. As shown in Figure 4, the reversibility of the modified electrode slightly decreases as the concentrations of FeCl3 and K4[Fe(CN)6] with which the electrode was prepared increase. However, ∆Ep is within 59 mV, which still expresses a satisfactory reversibility. The better reversibility of modified electrodes prepared with lower concentrations of FeCl3 and K4[Fe(CN)6] is contributed to the presence of larger-sized PB crystal grains formed during the fabrication process. The largersized PB crystallite is beneficial to the transport of K+ cations

among the PB crystal lattice, to maintain electronic balance during the electrochemical reaction. Therefore, the dilute deposition solution is preferred to the preparation of the PB/ MWNTs/GC electrodes. The lowest ∆Ep, 20 mV, was achieved when the modified electrode was prepared with the solution that contained 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]. The effects of the deposition cycle number on the intensity of oxidative and reductive peaks of the prepared PB/MWNTs/ GC electrodes were also studied. The intensity of oxidative and reductive peaks was enhanced as the deposition cycle number increased. This fact is easy to understand because the amount of PB deposited on the modified electrodes would increase with increasing deposition cycle number. The reversibility of the modified electrode slightly decreases as the deposition number increases. The lowest ∆Ep, 20 mV, was achieved when the modified electrode was prepared for three deposition cycles. The relationships between peak currents and the square root of scanning speed are presented in Figure 5. The linear characteristics of Figure 5 indicate that the reaction of the modified electrode is determined by diffusion. 3.3. Electrochemical Stability of the PB/MWNTs Film. The electrochemical stability of the PB/MWNTs film modified electrode was tested by CV scanning in a buffer solution (pH 6.0) that contained 5.0 mM KOH, 50.0 mM KH2PO4, and 45.0 mM KCl at a scan rate of 50 mV/s. The intensity of reductive peaks in Figure 6 has no obvious decay, despite a small variation of 0.02 µA, even after 5000 cycles of tests. It is obvious that the active material on the modified electrode possessed good structural and electrochemical stability. The phenomenon can be explained from the following aspects of the structure of the PB/MWNTs film: (1) The coalescence between the GC electrode and the purified hydrophilic MWNTs is strong; and

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Figure 6. Intensity of reductive peaks of CV curves versus the cycle numbers. The experimental conditions were as follows: deposition solution, 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]; deposition number, 3; potential scan range, 50-350 mV; potential scan rate, 50 mV/s.

Figure 8. Relationship between the currents of the reductive peaks in the CV curves of PB/MWNTs/GC electrode and the concentration of H2O2.

that the limit could be reduced by adjusting the amount of PB on the prepared electrode. 4. Conclusion

Figure 7. Response of the reductive peak current in CV curves of a PB/ MWNTs/GC electrode to variation of the H2O2 concentration: curve ae ˜, 0 mM H2O2; curve bf ˜, 0.088 mM H2O2; curve cg ˜, 0.439 mM H2O2; curve dh 0.878 mM H2O2. The experimental conditions were as follows: ˜, deposition solution, 1.0 mM FeCl3 and 1.0 mM K4[Fe(CN)6]; deposition cycle number, 3; potential scan rate, 50 mV/s.

(2) The adsorption of the Fe3+ cation on the interlayer of MWNTs strengthens the embedding of PB crystallites formed during deposition. 3.4. Applications in Determining the Concentration of H2O2. The oxidative and reductive peaks in CV curves of the PB/MWNTs/GC electrode should theoretically be decreased and increased, respectively, when H2O2 is added to the electrolyte solution, based on reactions R1 and R2:19

KFe3+Fe2+(CN)6 + K+ + e f K2Fe2+Fe2+(CN)6

(R1)

2K2Fe2+Fe2+(CN)6 + H2O2 + 2H+ f 2KFe3+Fe2+(CN)6 + 2H2O + 2K+ (R2) The actual relationships between the changes in the CV curves of the prepared PB/MWNTs/GC electrode and the concentrations of H2O2 in electrolyte solutions were studied, and the results are shown in Figure 7. This figure shows that the oxidative and reductive peak currents do decrease and increase, respectively, as the concentration of H2O2 increases. More importantly, the reductive peak current of the CV curves of the fabricated PB/MWNTs/GC electrode is proportional to the concentration of H2O2 in the tested range of 0.08-1.0 mM (shown in Figure 8), with a detection limit of 0.04 mM. Therefore, the prepared PB/MWNTs/GC electrodes could be applied to measure the concentration of H2O2 in water. The strength of the linear responses of the PB/MWNTs/GC electrodes to the concentration of H2O2 could be adjusted by controlling the amount of MWNTs and the PB deposition conditions. Compared to other methods,1 the detection limit of the tested electrodes is slightly higher. However, it is expected

Prussian blue (PB)/multiwalled carbon nanotubes (MWNTs)modified glass carbon (GC) electrodes (PB/MWNTs/GC electrodes) were successfully fabricated using a new sequential deposition method. The morphology and the amount of PB adsorbed on MWNTs could be regulated by varying the concentration of the deposition solutions and the deposition cycle number. The formed PB crystallites embedded into the interlayer of MWNTs are sufficiently strong when PB/MWNTs/GC electrodes are used for electrochemical measurement. The linear changes in the intensity of reductive peak currents of PB/ MWNTs/GC electrodes with the concentration of H2O2 provide the basis for their applications in analyzing the concentration of H2O2 in water. It is expected that the electrodes can also be used for the measurement of other peroxides in water. Literature Cited (1) Ricci, F.; Palleschi, G. Sensor and biosensor preparation, optimisation and applications of Prussian Blue modified electrodes. Biosens. Bioelectron. 2005, 21, 389. (2) Pan, K. C.; Chuang, C. S.; Cheng, S. H.; Su, Y. O. Electrocatalytic reactions of nitric oxide on Prussian blue film modified electrodes. J. Electroanal. Chem. 2001, 501, 160. (3) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. Amperometric Biosensor for Glutamate Using Prussian Blue-Based “Artificial Peroxidase” as a Transducer for Hydrogen Peroxide. Anal. Chem. 2000, 72, 1720. (4) Garjonyte, R.; Malinauskas, A. Amperometric glucose biosensors based on Prussian Blue- and polyaniline-glucose oxidase modified electrodes. Biosens. Bioelectron. 2000, 15, 445. (5) Ricci, F.; Arduini, F.; Amine, A.; Moscone, D.; Palleschi, G. Characterisation of Prussian blue modified screen-printed electrodes for thiol detection. J. Electroanal. Chem. 2004, 563, 229. (6) Guan, J. G.; Miao, Y. Q.; Chen, J. R. Prussian blue modified amperometric FIA biosensor: one-step immunoassay for R-fetoprotein. Biosens. Bioelectron. 2004, 19, 789. (7) Lenarczuk, T.; Gła, S.; Koncki, R. Application of Prussian bluebased optical sensor in pharmaceutical analysis. J. Pharmaceut. Biomed. 2001, 26, 163. (8) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. On the mechanism of H2O2 reduction at Prussian Blue modified electrodes. Electrochem. Commun. 1999, 1, 78. (9) Nakanishi, S. J.; Lu, G. T.; Kothari, H. M.; Bohannan, E. W.; Switzer. J. A. Epitaxial Electrodeposition of Prussian Blue Thin Films on SingleCrystal Au(110). J. Am. Chem. Soc. 2003, 125, 14998. (10) Zen, J. M.; Chen, P. Y.; Kumar, A. S. Flow Injection Analysis of an Ultratrace Amount of Arsenite Using a Prussian Blue-Modified ScreenPrinted Electrode. Anal. Chem. 2003, 75, 6017. (11) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Multilayer Membranes via Layer-by-Layer Deposition of Organic Polymer Protected Prussian Blue Nanoparticles and Glucose Oxidase for Glucose Biosensing. Langmuir 2005, 21, 9630. (12) Pyrasch, M.; Toutianoush, A.; Jin, W. Q.; Schnepf, J.; Tieke. B. Self-assembled Films of Prussian Blue and Analogues: Optical and

Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 6851 Electrochemical Properties and Applicationas Ion-Sieving Membranes. Chem. Mater. 2003, 15, 245. (13) Jin, W. Q.; Toutianoush, A.; Pyrasch, M.; Schnepf, J.; Gottschalk, H.; Rammensee, W.; Tieke, B. Self-Assembled Films of Prussian Blue and Analogues: Structure and Morphology, Elemental Composition, Film Growth, and Nanosieving of Ions. J. Phys. Chem. B 2003, 107, 12062. (14) Karyakin, A. A.; Karyakina, E. E. Prussian Blue-based ‘artificial peroxidase’ as a transducer for hydrogen peroxide detection. Application to biosensors. Sens. Actuators B 1999, 57, 268. (15) Wang, X. M.; Gershman, Z.; Kharitonov, A. B.; Katz, E.; Willner, I. Probing Electrocatalytic and Bioelectrocatalytic Processes by Contact Angle Measurements. Langmuir 2003, 19, 5413. (16) Karyakin, A. A.; Puganova, E. A.; Budashov, I. A.; Kurochkin, I. N.; Karyakina, E. E.; Levchenko, V. A.; Matveyenko, V. N.; Varfolomeyev, S. D. Prussian Blue Based Nanoelectrode Arrays for H2O2 Detection. Anal. Chem. 2004, 76, 474. (17) Zhang, Y. J.; Wen, Y.; Liu, Y.; Li, D.; Li, J. H. Functionalization of single-walled carbon nanotubes with Prussian blue. Electrochem. Commun. 2004, 6, 1180.

(18) Lei, Q.; Yang, X. Y. Assembly of Prussian blue onto SiO2 nanoparticles and carbon nanotubes by electrostatic interaction. Colloid Surf. A 2006, 278, 123. (19) Alciaturi, C. E.; Escobar, M. E.; de la Cruz, C.; Vallejo, R. Determination of chemical properties of pyrolysis products from coals by diffuse-reflectance infrared spectroscopy and partial least squares. Anal. Chim. Acta 2001, 436, 265. (20) Cecchet, F.; Marcaccio, M.; Margotti, M.; Paolucci, F.; Rapino, S.; Rudolf, P. Redox Mediation at 11-Mercaptoundecanoic Acid SelfAssembled Monolayers on Gold. J. Phys. Chem. B 2006, 110, 2241. (21) Fang, H. T.; Liu, C. G.; Chang, L.; Li, F.; Liu, F. M.; Cheng, H. M. Purification of Single-Wall Carbon Nanotubes by Electrochemical Oxidation. Chem. Mater. 2004, 16, 5744. (22) Young, J. K.; Chong, R. P. Analysis of Problematic Complexing Behavior of Ferric Chloride with N,N-Dimethylformamide Using Combined Techniques of FT-IR, XPS, and TGA/DTG. Inorg. Chem. 2002, 41, 6211.

ReceiVed for reView November 27, 2006 ReVised manuscript receiVed June 20, 2007 Accepted July 29, 2007 IE061511I