J. Phys. Chem. C 2009, 113, 7003–7008
7003
Fast Preparation, Characterization, and Property Study of r-Fe2O3 Nanoparticles via a Simple Solution-Combusting Method Xinghong Wang,† Li Zhang,† Yonghong Ni,*,† Jianming Hong,‡ and Xiaofeng Cao† College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China, and Centers of Modern Analysis, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: NoVember 23, 2008; ReVised Manuscript ReceiVed: March 12, 2009
In the present article, we report the successful synthesis of R-Fe2O3 nanoparticles via a simple solutioncombusting method, employing a mixture of ethanol and ethyleneglycol (V/V ) 60/40) as the solvent, iron chloride (FeCl3) as the iron source, and O2 in the atmosphere as the oxygen source. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometry (EDS). The magnetic property of the product was investigated. The electrochemical experiments showed that the as-prepared R-Fe2O3 nanoparticles could promote the electrochemical reduction of H2O2. Simultaneously, control experiments showed that the as-prepared product owned better the photocatalytic ability for the degradation of organic dyes than that prepared by the solvothermal route. Introduction In the past decade, nanostructures of iron oxides, such as R-Fe2O3, γ-Fe2O3, and Fe3O4, have evoked remarkable interest from both theoretical aspects and their wide range of potential applications in nanodevices.1-3 Among the above iron oxides, hematite (R-Fe2O3) is an environment-friendly n-type semiconductor (Eg ) 2.1 eV) and the most thermodynamically stable phase of iron oxide under ambient conditions. Also, hematite has extensive applications in many fields including catalysis, gas sensors, magnetic recording media, lithium-ion batteries, anticorrosive agents, water treatment, and pigments.4-7 As stimulated by the promising applications of hematite, the synthesis of R-Fe2O3 has attracted much attention. A variety of methods have been reported for the synthesis of R-Fe2O3 nanostructures, including vapor-solid growth technique,8 sol-gel process,9 hydrothermal approach,10 chemical precipitation,11 and highenergy ball milling.12 Nevertheless, it is still a great challenge to search for a simple and cost-effective route to prepare R-Fe2O3 nanoparticles with a high yield. Herein, we report a simple solution-combusting method to prepare R-Fe2O3 nanoparticles, using a mixture of ethanol and ethyleneglycol as the solvent, FeCl3 as the iron source, and oxygen gas in the atmosphere as the oxygen source. The combustion reaction was carried out in air, and R-Fe2O3 nanoparticles were obtained in one step. As compared to the above-mentioned methods, the present route has some obvious advantages: (1) it is very cost-effective and convenient because of not requiring any sophisticated experimental setups and complicated operation; (2) the reaction is safe, quick, and the experiments can be easily repeated; and (3) the yield is high above 95%. Moreover, the heat produced during combustion can be also utilized fully. The magnetic, photocatalytic, and electrochemical properties of the as-prepared R-Fe2O3 nano* Correspondingauthor.Fax:+86-553-3869303.E-mail:
[email protected]. † Anhui Normal University. ‡ Nanjing University.
particles were studied. Experiments showed that the as-prepared hematite nanoparticles could accelerate electrochemical reduction of H2O2. Experimental Section 30% H2O2 solution was purchased from Beijing Chemical Reagent Factory (Beijing, China). Chitosan was obtained from Hefei BoMei Biotechnology Co. All other chemicals were analytical grade, purchased from Shanghai Chemical Co, and used without further purification. Double-distilled water was used for preparation of buffer and standard solutions. H2O2 solution was diluted before the electrochemical measurements. In a typical synthesis process, 0.02 mol of FeCl3 · 6H2O was dissolved in 100 mL of mixed solvents of ethanol and ethyleneglycol with the volume ratio of 60/40. After the solution was transferred into a spirit lamp with an absorbent cotton lampwick, the spirit lamp was fired. For a moment, some red substances were produced on the top of the lampwick. When the lampwick was extinguished, the red product was collected and repeatedly washed with distilled water to remove the impurities. Finally, the product was separated by a magnet and dried at 50 °C in air for 5 h. As a control, R-Fe2O3 nanoparticles were also synthesized via a simple solvothermal route. The preparation procedure was as follows: 0.543 g of FeCl3 · 6H2O was dissolved in 25 mL of DMF. Next, the solution was transferred into a Teflon-lined stainless steel autoclave of 30 mL capacity and heated at 180 °C for 3 h. After that, the autoclave was cooled to room temperature naturally. The red precipitates were collected by centrifugation, washed with deionized water and ethanol several times, and finally dried in air at the above same conditions. Powder X-ray diffraction (XRD) of the products was carried out on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.154060 nm), employing a scanning rate of 0.02 deg s-1 and 2θ ranges from 15° to 80°. SEM images and energy dispersive spectrum (EDS) of the product were obtained on a Hitachi S-4800 field-emission scanning electron
10.1021/jp810296v CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
7004 J. Phys. Chem. C, Vol. 113, No. 17, 2009 microscope, employing an operating voltage of 5 or 15 kV. TEM images were carried out on a JEOL 2010 high-resolution transmission electron microscope, employing an accelerating voltage of 200 kV. The magnetic properties were measured on a vibrating sample magnetometer (VSM) at room temperature. The optical property changes of dyes were recorded on an F-4500 spectrofluorometer with a quartz cell of 1 cm. Electrochemical experiments were performed with a CHI 440A electrochemical analyzer (CHI, USA) with a conventional threeelectrode cell. The bare glass carbon electrode (GCE) and modified GCE were used as the working electrode. An Ag/AgCl electrode and a platinum electrode were used as the reference and the auxiliary electrode, respectively. Experiments were carried out under the nitrogen atmosphere at room temperature. To investigate the photocatalytic property of R-Fe2O3 nanoparticles (HeNPs) for the degradation of organic dyes, 15 mg of HeNPs was dispersed into 40 mL of aqueous solutions containing Safranine T or Fluorescein with a concentration of 10 mg L-1, respectively. The suspension was dispersed by the ultrasonic wave for 15 min and then magnetically stirred in the dark for 30 min to ensure adsorption/desorption equilibrium prior to UV irradiation. After the suspension was irradiated by 254 nm UV light under continuous stirring for given times, the catalysts were removed by centrifugation, and the PL spectra of the as-obtained solutions were measured. The above experiments were repeated via using the product synthesized by the solvothermal method as the photocatalyst instead of HeNPs. The GCE modified by HeNPs was fabricated in the following way: First, the GCE (Φ ) 3 mm) was polished with a 1700# diamond paper, washed successively with double distilled water and ethanol in an ultrasonic bath, and scanned for 15 cycles in the solution of 1.0 mol/L H2SO4 with the potential of 2.0 to -2.0 V (vs SCE). Second, a Chitosan (0.50%) solution was prepared by dissolving Chitosan of 25 mg into 50 mL of acetate buffer solution (0.05 M, pH 4.2). Third, the calculated amount of HeNPs was dispersed in the Chitosan at the mass ratio of 1:5 by sonication at room temperature (it was found that 1:5 was the optimal ratio between HeNPs and Chitosan (Chi), which exhibited the maximum amperometric current). Finally, 20 µL of HeNPs-Chi solution was cast on the surface of the GCE and dried in air. Results and Discussion Figure 1 shows the XRD patterns of two products prepared through a simple solution-combusting method and a conventional solvothermal route, respectively. No obvious difference is found in two diffraction patterns besides the peak intensities. All diffraction peaks can be indexed as the hexagonal-phase R-Fe2O3 by comparison with JCPDS card files no. 86-0550 (a ) 5.035 Å and c ) 13.74 Å). No characteristic peaks of other iron oxides such as Fe3O4 and γ-Fe2O3 are detected. Hematite can be synthesized by the above two approaches. Because the diffraction peaks shown in Figure 1a are stronger and narrower than those shown in Figure 1b, the product prepared by the solution-combusting method has higher crystallinity and larger size. To further confirm the formation of R-Fe2O3 under the present experimental conditions, we employed the energy dispersive X-ray spectrometry (EDS) for the composition analysis of the product. Figure 2 shows the EDS analysis of the as-prepared product. The peaks of Fe and O can be easily seen, and the atomic ratio of Fe/O is very close to the stoichiometry of Fe2O3 based on the calculation of peak areas. The weak C peak should be attributed to the carbon film on the support.
Wang et al.
Figure 1. XRD patterns of the products prepared by different methods: (a) solution-combusting route, and (b) conventional solvothermal route.
Figure 2. EDS analysis of the product prepared by the solutioncombusting route.
The morphology of the as-prepared HeNPs was characterized by SEM and TEM. SEM observations showed that the product was comprised of abundant irregular nanoparticles with a mean size of ∼100 nm (see Figure 3a). The presence of some bigger particles should be attributed to the aggregating of smaller particles. A typical TEM image is shown in Figure 3b, from which the average size of nanoparticles is measured to be ∼100 nm. This is in good agreement with the SEM result. Moreover, the investigations showed that the as-prepared HeNPs could be easily dispersed in polar solvents (such as water and ethanol) to form homogeneous solutions, and under the extra magnetic field, the HeNPs could be separated from solutions rapidly (see Figure 3c). However, the HeNPs could be redispersed under vibration after removing the extra magnetic field. The above fact implies that the surfaces of as-prepared HeNPs probably contain abundant hydrophilic groups (e.g., -OH), which come from the incomplete combustion of alcohols. Figure 3d is a representative TEM image of the product prepared by the simple solvothermal route. Highly aggregated particles were presented. On the basis of the present experimental facts and our previous work,13 a possible formation process of HeNPs can be suggested. In this simple combustion process, alcohols combusted to produce CO2 and H2O and to free abundant heat; then, iron ions dissolved in alcohols reacted with O2 in air under
Fast Preparation of R-Fe2O3 Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7005
Figure 3. (a) A representative SEM image, (b) TEM image of the product, (c) the dispersion and separation of R-Fe2O3 nanoparticles in aqueous solution, and (d) a typical SEM image of R-Fe2O3 nanoparticles prepared by a conventional solvothermal method.
the assistance of heat. As a result, R-Fe2O3 nanoparticles were prepared. Some related reactions are described below:
C2H5OH + 3O2 f 2CO2+ 3H2O + Q
(1)
C2H6O2 + 2.5O2 f 2CO2+ 3H2O + Q
(2)
-
4Fe + 12Cl + 3O2 + Q f 2Fe2O3+ 6Cl2 3+
(3)
Recently, the magnetic property of R-Fe2O3 nanoparticles is attracting much attention.14 Usually, R-Fe2O3 is an antiferromagnetic material, and the two antiferromagnetic spins are along the [001] direction below the Morin temperature (TM ) 263 K).15 However, R-Fe2O3 can also exhibit a weak ferromagnetic behavior when the temperature locates between 263 and 960 K (Ne´el temperature).16 At 263 K, bulk hematite has a Morin transition from the low-temperature antiferromagnetic phase to a weakly ferromagnetic phase.17 Below TM, spins are oriented in antiparallel directions along the trigonal [111] axis (c-axis), and the material behaves as a uniaxial antiferromagnet. Above TM, spins show slight canting with respect to the basal (111) plane, and a weak ferromagnetism appears.18 It is well-known that the size of the product is related to its property. As the size of the hematite particle decreases to the nanometer scale, due to the nanoscale confinement,19 materials can exhibit unusual magnetic behaviors that are quite different from those of conventional bulk materials (antiferromagnetic at low temperature and ferromagnetic above TM). New phenomena appear. The Morin temperature decreases with the decrease of the particle size and tends to disappear for particles smaller than about 8-20 nm.20 If the particles become small enough, the magnetic moment in a single domain fluctuates in a certain direction due to thermal agitation, leading to superparamagnetic behavior above the blocking temperature TB, and to spatial freezing of these moments below TB.3 During the measurements, to avoid the interference of magnetic signals from the Fe-based alloy substrate, all of the nanoparticles were dispersed on a Si substrate. Figure 4 gives the result of magnetic measurements
Figure 4. The room-temperature magnetic hysteresis loop of the assynthesized R-Fe2O3 nanoparticles.
on the as-prepared R-Fe2O3 nanoparticles. A symmetric hysteresis loop is exhibited. This fact implies that the product has a weak ferromagnetic property at room temperature. The saturation magnetization (Ms), remanent magnetization (Mr), and coercive force (Hc) of the as-synthesized R-Fe2O3 nanoparticles are 11.05 emu g-1, 1.798 emu g-1, and 155.76 Oe, respectively. As compared to the works reported in literature,14a,15 the present product had a higher value of Ms, which should be ascribed to the influence of small amounts of Fe3O4 produced during the combustion of alcohols. Ethanol and ethyleneglycol are weak reducing reagents, so it is probable that a small amount of Fe3+ is reduced to Fe2+ under the high temperature due to the reduction of alcohols. However, the produced Fe3O4 was too small to be detected by XRD. Usually, nanomaterials can be modified on the electrode for uses as chemical/biological sensors.21-24 To study the electrochemical property of HeNPs, we measured the cyclic voltam-
7006 J. Phys. Chem. C, Vol. 113, No. 17, 2009
Figure 5. Cyclic voltammograms of the HeNPs-Chi/GCE in an oxygen-free 0.1 M KNO3 solution over a range of scan rates (a-j: 10-100 mV/s). Inset: CVs of (1) bare GCE and (2) Chi/GCE in an oxygen-free 0.1 M KNO3 solution with the scan rate of 50 mV/s.
metric response of the HeNPs-Chi/GCE in an oxygen-free 0.1 M KNO3 solution at the scan rate of 10-100 mV/s (Figure 5). The GCE modified by HeNPs-Chi composite film gives a welldefined redox peak with an average formal potential (Em ) (Epa + Epc)/2) of -0.28 V (vs Ag/AgCl) at a scan rate of 50 mV/s, which is in good agreement with the previous results reported on the direct electron transfer of R-Fe2O3 onto ITO and nanotube Fe2O3 onto Au electrode.25,26 The redox peaks can be ascribed to the redox reaction of R-Fe2O3 nanoparticles entrapped in the chitosan composite film. Both the cathodic and the anodic peak currents increase linearly with the increase of the scan rate from 10 to 100 mV/s. This phenomenon reveals the electron transfer between HeNPs-Chi complex and the GCE in a surfaceconfined electrochemical process because the bare GCE and Chi/ GCE show no obvious voltammetric peaks within the same potential range in 0.1 M KNO3 solution (the inset in Figure 5, 1 and 2). It is found that HeNPs-Chi composite film exhibits good electrocatalytic behavior toward the reduction of H2O2. Figure 6 shows the cyclic voltammograms of HeNPs-Chi/GCE in the absence or presence of H2O2 at the scan rate of 50 mV/s in an oxygen-free 0.1 M KNO3 solution. When the system did not contain any H2O2, a reduction peak at -0.370 V could be observed; also, the reduction peak current obviously enhanced with the increase of H2O2 concentration from 0.2, 0.4, to 0.8 mM (see b, c, and d in Figure 6). The inset in Figure 6 depicts the CV curves of Chi/GCE and HeNPs-Chi/GCE in the presence of H2O2 of 0.8 mM. Distinctly, the response at Chi/ GCE is barely detectable, except the slightly cathodic polarization of H2O2 (Figure 6e). Contrarily, H2O2 exhibits a strong catalytic reduction behavior at the HeNPs-Chi/GCE (Figure 6f). The above facts clearly show that HeNPs entrapped in the composite film can effectively promote the reduction of H2O2.26 Figure 7 shows an amperometric response for the HeNPs-Chi/ GCE with successive step changes of H2O2 concentration. Upon the addition of H2O2, the HeNPs-Chi/GCE reaches the maximum steady-state response within 5 s, indicating a very rapid amperometric response. The linear relationship was obtained in the concentration range from 5.0 × 10-6 to 2.0 × 10-4 M (correlation coefficient: 0.999, n ) 20), with a detection limit of 1.5 µM. The stability of the electrode was also tested. When
Wang et al.
Figure 6. Cyclic voltammograms of the HeNPs-Chi/GCE in the presence of different H2O2 concentrations: (a) 0.0, (b) 0.2, (c) 0.4, and (d) 0.8 mM. The inset shows CVs of different work electrodes in the presence of H2O2 of 0.8 mM: (e) Chi/GCE and (f) HeNPs-Chi/GCE. Scan rate: 50 mV/s.
Figure 7. Amperometric response for HeNPs-Chi/GCE upon successive addition of 5 µM H2O2 into gently stirred 0.1 M KNO3 at -0.37 V.
the electrode was stored at 4 °C for 1 week, the CV peak currents still retained 95%, and in the next 3 weeks the response still retained 90% of the initial value. As for various materials, one of the challenging and intriguing problems is to investigate their potential properties and thus to achieve application in life-relating fields. At present, water pollution control and rapid detection of biomolecules are considered to be two of the most troublesome embarrassments. Applications of nanomaterials in these fields may be a direction of materials research. To investigate the photocatalytic degradation property of the as-obtained R-Fe2O3 nanoparticles, we studied the optical property changes of several organic dyes such as Safranine T and Fluorescein in the absence/presence of R-Fe2O3 nanoparticles under the irradiation of 254 nm UV light. Figure 8 depicts the PL spectra of two dyes irradiated by UV light of 254 nm for 0-90 min in the presence/absence of R-Fe2O3 nanoparticles, respectively. A similar change trend can be found: under the absence of R-Fe2O3 nanoparticles, the intensities of all PL peaks slightly reduced after irradiation for
Fast Preparation of R-Fe2O3 Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7007
Figure 8. PL spectra of various dyes under the irradiation of 254 nm UV light for various times in the absence/presence of R-Fe2O3 nanoparticles: (a) Safranine T and (b) Fluorescein.
90 min, while the peak intensities obviously decreased when the systems containing R-Fe2O3 nanoparticles were irradiated only for 15 min. After 90 min, the decrease of the peak intensity was very prominent. Furthermore, the experiments showed that the PL peak decreases of dyes caused by adsorption of dyes on the surfaces of R-Fe2O3 nanoparticles could be neglected. The above facts indicate that the as-prepared HeNPs can accelerate the degradation of Safranine T and Fluorescein. The HeNPs prepared by the present solution-combustion route possessed good catalytic degradation activity for some organic dyes. As controls, we also investigated the ability of R-Fe2O3 nanoparticles synthesized via the solvothermal route to degrade the above two dyes under the same conditions. Figure 9 depicts the absorbance-time curves of two dyes in the presence of different catalysts. When no catalyst was used, two dyes could be weakly degraded under UV irradiation, and the degradation ratio was less than 30% in 90 min. While catalysts were used, two dyes could be markedly degraded in the above same time. The degradation ratios reached 52% and 66% (Figure 9a), and 65% and 80% (Figure 9b) within 90 min, respectively. As seen in Figure 9, HeNPs prepared by the present solution-combusting route possess better photocatalytic property for the degradation of Safranine T and Fluorescein than do R-Fe2O3 nanoparticles synthesized via the solvothermal route. Generally, hydroxyl radicals are considered to play crucial roles in the photocatalytic degradation of organic dye solutions.27 That more hydroxyl radicals are produced will be available to the degradation of organic dyes. Because Fe2O3 is a semiconductor with the band gap energy of 2.1 eV,4,5 some electrons in the valence band can be excited to the conductance band under
Figure 9. The absorbance-time curves of (a) Safranine T and (b) Fluorescein in the presence of two different samples.
the UV irradiation, which leads to the formation of electron-hole pairs.28 Sequentially, the above carriers rapidly combine with O2 in the solution to produce a great deal of highly oxidative • OH groups.29 Thus, organic dyes are degraded. Furthermore, Safranine T and Fluorescein molecules contain bare -NH2 and -OH groups with strong coordinating ability, respectively, so they can coordinate with Fe atoms in Fe2O3 to form intermediaries, which effectively reduce the activation energy of the above photochemistry reaction. However, because the activation energies for degrading two dyes were different, two dyes had different degradation ratios. Additionally, HeNPs prepared by the solution-combusting route may have larger surface area than the product synthesized by the solvothermal route, because the former possessed stronger degradation ability. Conclusions R-Fe2O3 nanoparticles have been successfully prepared by a simple solution-combusting method without any requirement of a calcination step at high temperature. The as-prepared product owned weak ferromagnetic property at room temperature and was a good photocatalyst for degradation of organic dyes such as Safranine T and Fluorescein under irradiation of 254 nm UV light. The experiments showed that R-Fe2O3 nanoparticles presented good electrochemical response in oxygen-free KNO3 solution of 0.1 M and could promote the reduction of H2O2. This property can be used as sensors for the detection of H2O2 (the detection limit is 1.5 µM). Furthermore,
7008 J. Phys. Chem. C, Vol. 113, No. 17, 2009 this synthetic approach can also be applied for the synthesis of other inorganic oxides with improved performances. However, it is worth noting that the present method cannot precisely control the size and morphology of the product. Acknowledgment. We thank the National Natural Science Foundation of China (20771005 and 20571002), the Science and Technological Fund of Anhui Province for Outstanding Youth (08040106834), and the Education Department of Anhui Province (no. 2006KJ006TD) for fund support. References and Notes (1) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941. (2) Pascal, C.; Pascal, J. L.; Favier, F.; Moubtassim, M. L. E.; Payen, C. Chem. Mater. 1999, 11, 141. (3) Tadic´, M.; Markovic´, D.; Spasojevic´, V.; Kusigerski, V.; Remsˇkar, M.; Pirnat, J.; Jaglicˇic´, Z. J. Alloys Compd. 2007, 441, 291. (4) Zhan, S. H.; Chen, D. R.; Jiao, X. L.; Liu, S. S. J. Colliods Interface Sci. 2007, 308, 265. (5) Chen, J.; Xu, L.; Li, W.; Gou, X. AdV. Mater. 2005, 17, 582. (6) Cao, S. W.; Zhu, Y. J. J. Phys. Chem. C 2008, 112, 6253. (7) Zhang, X. L.; Sui, C. H.; Gong, J.; Su, Z. M.; Qu, L. Y. J. Phys. Chem. C 2007, 111, 9049. (8) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A. V.; Zhang, H. Chem. Phys. Lett. 2003, 379, 373. (9) Gong, C. R.; Chen, D. R.; Jiao, X. L.; Wang, Q. L. J. Mater. Chem. 2004, 14, 905. (10) Jia, C. J.; Sun, L. D.; Yan, Z. G.; You, L. P.; Luo, F.; Han, X. D.; Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 4328.
Wang et al. (11) Su, X. Q.; Yan, B. Mater. Chem. Phys. 2005, 93, 552. (12) Wang, L. L.; Jiang, J. S. Physica B 2007, 390, 23. (13) Ni, Y. H.; Cao, X. F.; Wu, G.; Hu, G.; Yang, Z.; Wei, X. W. Nanotechnology 2007, 18, 155603. (14) (a) Giri, S.; Samanta, S.; Maji, S.; Ganguli, S.; Bhaumik, A. J. Magn. Magn. Mater. 2005, 285, 296. (b) Tang, B.; Wang, G. L.; Zhuo, L. H.; Ge, J. C.; Cui, L. J. Inorg. Chem. 2006, 45, 5196. (15) Liu, X. M.; Fu, S. Y.; Xiao, H. M.; Huang, C. J. J. Solid State Chem. 2005, 178, 2798. (16) Besser, P. J.; Morrish, A. M.; Searle, C. W. Phys. ReV. 1967, 153, 632. (17) Morin, F. J. Phys. ReV. 1950, 78, 819. (18) Shull, C. G.; Strauser, W. A.; Wollan, E. O. Phys. ReV. 1951, 83, 333. (19) Arnim, H. Chem. ReV. 1989, 89, 1861. (20) Amin, N.; Arajs, S. Phys. ReV. B 1987, 35, 4810. (21) Palecek, E.; Fojta, M.; Tomschik, M.; Wang, J. Biosens. Bioelectron. 1998, 13, 621. (22) Wang, J. Anal. Chim. Acta 2002, 469, 63. (23) Lucarelli, F.; Marrazza, G.; Mascini, M. Biosens. Bioelectron. 2005, 20, 2001. (24) Lucarelli, F.; Tombelli, S.; Minunni, M.; Marrazza, G.; Mascini, M. Anal. Chim. Acta 2008, 609, 139. (25) Mckenzie, K. J.; Marken, F. Pure Appl. Chem. 2001, 73, 1885. (26) Gong, J. M.; Wang, L. Y.; Zhao, K.; Song, D. D. Electrochem. Commun. 2008, 10, 123. (27) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemannt, D. W. Chem. ReV. 1995, 95, 69. (28) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (29) Dang, S. N.; Lu, S. X.; Xu, W. G.; Sa, J. J. Non-Cryst. Solids 2008, 354, 5018.
JP810296V
