A New Electrochemiluminescence Emission of Mn2+-Doped ZnS

Oct 21, 2008 - Yet more excitation energy during its electrochemiluminescence (ECL) process should be applied because of the wide energy gap (∼3.7 e...
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J. Phys. Chem. C 2008, 112, 17581–17585

17581

A New Electrochemiluminescence Emission of Mn2+-Doped ZnS Nanocrystals in Aqueous Solution Xiao-Fei Wang, Jing-Juan Xu,* and Hong-Yuan Chen Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: August 10, 2008; ReVised Manuscript ReceiVed: September 1, 2008

ZnS nanocrystals are less toxic than cadmium chalcogenide materials, which might have more applications in biological detection. Yet more excitation energy during its electrochemiluminescence (ECL) process should be applied because of the wide energy gap (∼3.7 eV) of ZnS. Here, ECL of ZnS doped with Mn2+ nanocrystals (ZnS:Mn2+ NCs) modified on glassy carbon electrode was investigated in aqueous solution with the coreactant hydrogen peroxide (H2O2) for the first time. Besides the ECL process of pure ZnS NCs at a negative potential of more than -2.0 V vs SCE, a new ECL emission peak was observed at ca. -1.50 V vs SCE on the ZnS: Mn2+ NCs-modified glassy carbon electrode in pH 9.0 phosphate buffer solution. This new ECL emission was attributed to the excited state of Mn2+ lying in the Zn2+ sites. The concentration of Mn2+ doped in ZnS NCs played a key role in this new ECL process. Furthermore, this specific ECL emission was dependent on the concentration of H2O2. The largely decreased excitation energy of this new ECL emission would promote the application of doped ZnS NCs in the fields of biological and environmental analysis without the interference of hydrogen bubble in the ECL process. 1. Introduction Electrochemiluminescence (ECL) involves the generation of species at electrode surfaces that then undergo electron-transfer reactions to form excited states that emit light.1 As an important detection method, ECL has many promising advantages such as simplicity, high sensitivity, rapidity, and no background from unnecessary photoexcitation. It has been proven to be a useful technique in analytical applications including DNA-probe assays,2 ECL-based sensors,3,4 and remote imaging application.5 ECL studies concerning semiconductor nanomaterials have attracted great attention for their unique electronic and optical properties, especially II-VI semiconductor nanocrystals (NCs) containing CdO,6 CdS,7-9 CdSe,10-12 and CdTe.13,14 Although these semiconductor NCs have been accepted as the ECL indicators for their effective and valuable ECL behaviors, one challenge of these cadmium chalcogenide materials for extensive application in biological systems is related to their toxicity that results from decomposition and release of cadmium ions, which are extremely toxic.15-20 As another typical representative of II-VI semiconductor NCs, ZnS is less toxic and more environmentally safe than are cadmium chalcogenide materials, which might have more potential applications in biological detection. However, because of the wide energy gap (∼3.7 eV) of ZnS NCs, it would require high energy to generate excited species during the ECL process in aqueous system, which would induce more by-reactions such as the generation of bubbles for the reduction of H2O and interfere with the ECL process. In recent years, photoluminescence (PL) of ZnS doped with different activating metal ions has been extensively investigated due to the tunable PL properties to a large extent, which have promising applications in optoelectronic devices.21,22 Among these doped materials, ZnS NCs doped with Mn2+ ions (ZnS: * Corresponding author. Phone: +86 25 83597294. Fax: +86 25 83594862. E-mail: [email protected].

Mn2+) are of special interest due to the highly effective luminescence.23 In ZnS:Mn2+ NCs, the Mn2+ ions distribute on Zn2+ lattice sites and are tetrahedrally coordinated by S2-. A specific orange emission around 600 nm was observed, which originates from energy transfer from ZnS states to the 4T1-6A1 Mn2+-based transition.24-26 The various types of luminescence differ from the source of energy to obtain an excited state that can relax into a ground state with the emission of light. The required excitation energy is supplied either by absorption of light giving rise to PL or by electrical injection of an electron-hole pair to generate ECL.11,27 While more and more research has been focused on the exploration of ZnS:Mn2+ as PL sensors,25,26,28 much less attention has been paid to the ECL property of ZnS:Mn2+ and their potential as ECL sensors. Herein, we synthesized ZnS NCs doped with Mn2+ (ZnS: Mn2+) and investigated the ECL phenomena of GCE modified with ZnS:Mn2+ NCs film in aqueous solution containing the coreactant H2O2 for the first time. A new ECL emission peak was observed at about -1.50 V in the ZnS:Mn2+ NCs, which did not appear in the pure ZnS NCs. This new ECL emission was attributed to the excited state of Mn2+ lying in the Zn2+ sites. This specific ECL emission was sensitive to the concentration of hydrogen peroxide (H2O2), which could promote the development of ECL applications based on the ZnS NCs. 2. Experimental Methods Analytically pure sodium sulfide (Na2S · 9H2O), zinc acetate dehydrate (Zn(CH3COO)2 · 2H2O), and manganese acetate tetrahydrate (Mn(CH3COO)2 · 4H2O) were used as received from Shanghai Chemicals Ltd. 0.10 M phosphate buffer solution used in ECL experiments was made up of KH2PO4 and K2HPO4 and adjusted to the desired pH by adding 0.10 M KOH. Ultrapure water (Millipore) was used throughout all of the experiments. The procedure used to prepare the ZnS:Mn2+ NCs was similar to that described in the literature.24,29 All steps of the synthesis

10.1021/jp807136s CCC: $40.75  2008 American Chemical Society Published on Web 10/22/2008

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Wang et al. trodes were polished and ultrasonically cleaned in acetone and water subsequently before use. Electrochemical and ECL behaviors were monitored with a MPI-A electrochemiluminescence analyzer (Remax Electronic Co. Ltd., Xi’an, China). The photomultiplier tube (PMT) was biased at 600 V.

Figure 1. X-ray diffraction patterns of pure ZnS and ZnS:Mn2+ (1.9 wt %) NCs.

were performed at room temperature and under ambient conditions. Sodium sulfide was used at the mole amount equal to that of zinc acetate dehydrate. Typically, 20 mL of solution containing 0.10 M zinc acetate dehydrate and manganese acetate tetrahydrate at the desired ratio (5, 10, 15, and 20 wt %) was stirred in a flask under nitrogen for 30 min, and then 20 mL of 0.10 M sodium sulfide was added dropwise into the above solution, and the mixture was vigorously stirred for 5 h. After the resulting precipitate was centrifuged and washed several times in water and ethanol, the resultant was dried at 50 °C in a vacuum to get a white powder. For comparison, ZnS NCs were also synthesized as in the above process without adding Mn2+. 0.01 M ZnS and ZnS:Mn2+ aqueous solutions were obtained by dispersing corresponding resultants into water under ultrasonication for 30 min. Glassy carbon electrodes (GCEs) with diameter of 3.0 mm were polished and ultrasonically cleaned in acetone and water subsequently before use. After, 5 µL of ZnS and ZnS:Mn2+ solutions were dropped on the surface of GCEs, respectively, and were dried in the dark at room temperature. X-ray diffraction (XRD) patterns were carried out on a SHIMADZU XRD-6000 instrument (Japan) using Cu KR radiation. Absorbance spectra were measured using a SHIMADZU UV-3600 spectrophotometer (Japan). Emission spectra were recorded with a SHIMADU RF-5301PC spectrophotometer (Japan). The size and morphology of nanocrystals were examined by a JEOL 2100 transmission electron microscope (Japan). The concentration of Mn2+ ions was measured with inductively coupled plasma analysis (ICP-OES, Thermo Jarrell-Ash JA1100, American). A three-electrode system containing a glassy carbon working electrode (3 mm diameter), a saturated calomel electrode (SCE) reference electrode, and a platinum wire auxiliary electrode was used throughout. Glassy carbon elec-

Figure 2. Transmission electron microscope image of ZnS:Mn2+ NCs.

3. Results and Discussion 3.1. Characterization of ZnS:Mn2+ NCs. X-ray powder diffraction analysis of the as-prepared ZnS and ZnS:Mn2+ NCs is shown in Figure 1. Three main diffraction patterns matched with the JCPDS No. 77-2100, confirming that these NCs were cubic zinc blende structure. It could be seen that XRD lines were branded with three main peaks corresponding to the (111), (220), and (311) planes. The slight broadening of the XRD line of ZnS:Mn2+ NCs was probably caused by the substitution of Zn2+ by Mn2+ because the diameter of Mn2+ (0.83 nm) was larger than that of Zn2+ (0.74 nm).30 According to the Bragg equation,26 the lattice constant of pure ZnS NCs is calculated to be 0.5413 nm, and a slight increase is obtained in the lattice constant of ZnS:Mn2+ NCs, which is 0.5417 nm. A TEM image of the ZnS:Mn2+ shown in Figure 2 displayed that the ZnS: Mn2+ NCs were agglomerated with a mean particle size of about 2.5-4 nm. Figure 3 showed the absorption spectrum and photoluminescence (PL) spectrum with an excitation wavelength of 320 nm of the Mn2+-doped ZnS NCs. The blue PL emission of ZnS: Mn2+ around 400 nm (Figure 3 c) was attributed to the defectrelated emission of ZnS, which could also be observed in the PL curve of pure ZnS NCs (Figure 3 b). The orange emission with a symmetric peak around 580 nm was characteristic for the 4T1-6A1 transition of Mn2+ ions in ZnS NCs, shown in Figure 3c.23,31 The obvious peak in the absorption spectrum at ∼300 nm was from the ZnS NCs, which was blue-shifted from ∼320 nm for the bulk because of the well-known quantum size effect.32 According to Brus formula,33 the diameter of ZnS:Mn2+ was about 3.0 nm, which is in agreement with that obtained from the TEM image, although the particles were partially agglomerated. 3.2. Electrochemical and ECL Behaviors of ZnS:Mn2+ NCs. It was reported that ZnS NCs could emit ECL at a negative scanning range from 0 to -2.0 V in 0.10 M NaOH solution containing 0.01 M K2S2O8.34,35 Herein, ECL behaviors of ZnS NCs and ZnS:Mn2+ NCs were studied in combination with cyclic voltammetry (CV) in 0.10 M NaOH solution containing 0.01 M K2S2O8. ECL of ZnS NCs also started at about -1.0 V and achieved the highest value at -2.0 V, while the ECL peak

Electrochemiluminescence Emission of ZnS Nanocrystals

Figure 3. UV absorption of ZnS:Mn2+ (doping percent: 1.9 wt %) NCs (a) and photoluminescence spectra of ZnS NCs (b) and ZnS:Mn2+ (doping percent: 1.9 wt %) NCs (c) in aqueous solutions. PL excitation wavelength is 320 nm.

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Figure 5. Dependence of Mn2+ doping level in ZnS NCs on ECL emission in 0.10 M, pH 9.0 PBS containing 5.0 mM H2O2. The potential was scanned in the negative direction from 0 to -2.5 V with a scan rate of 100 mV/s.

to Mn2+ ions doped in ZnS NCs. It was reported that a chemical oxidation of sulfur atoms at the nanocrystal surface would occur in PBS containing H2O211 and Mn2+ ion could trap a hole and transfer to Mn3+.21,36,37 In this case, the following reaction also occurs: -

S2 + H2O2fS-+ OH · + OHThe formed OH · radical could then easily inject a hole in the Mn2+ due to the high standard redox potential (∼2.84-0.06 pH V vs SHE) of the OH · /OH- couple:11 Figure 4. Electrochemical and ECL behaviors of (a) bare GCE, (b) GCE modified with pure ZnS NCs, and (c) GCE modified with ZnS: Mn2+ (doping percent: 1.9 wt %) NCs in pH 9.0 PBS containing 5.0 mM H2O2. Inset: The corresponding cyclic voltammograms. The potential scanned in the negative direction from 0 to -2.5 V with a scan rate of 100 mV/s.

of ZnS:Mn2+ was observed at a more positive value than that of pure ZnS NCs (not shown), indicating that the excitation energy of Mn2+ doped in ZnS NCs was lower. However, such strong alkaline condition with strong coreactant K2S2O8 may go against its further application in biological analysis. Hence, we conducted the ECL experiments in the moderate PBS solution with the coreactant H2O2, whose accurate and rapid determination is of essential importance in bioanalytical and environmental fields. Figure 4 showed the electrochemical and ECL behaviors of the bare GCE, GCE modified with ZnS NCs, and GCE modified with ZnS:Mn2+ (doping percent: 1.9 wt %) NCs in pH 9.0 PBS containing 5.0 mM H2O2. Only slight ECL was observed at the bare GCE (Figure 4a). After the GCE was modified with 5 µL of ZnS NCs solution, a strong ECL emission started from -1.83 V and achieved the peak value at -2.36 V (Figure 4b). At the CV curve (inset b), a reduction process was observed around -1.80 V. This implied that ZnS NCs were reduced during the negative scan. The possible ECL mechanism of ZnS NCs reacted with H2O2 could be expressed as follows:12,35

ZnS + e-f ZnSZnS-+ H2O2 f ZnS* + 2OHZnS* f ZnS + hν Two ECL emissions were observed during the negative scan of the GCE modified with ZnS:Mn2+ NCs in PBS solution containing H2O2, shown in Figure 4c. The ECL emission with a peak value lying at -2.38 V (ECL-2) was ascribed to ZnS NCs, which was consistent with the ECL peak of pure ZnS. The other new ECL emission (ECL-1) started from -1.20 V and achieved the peak value around -1.50 V, which is related

Mn2+ + OH · f Mn3++ OHSubsequent recombination with trapped electron resulted in an excited state of (Mn2+)*;24 this process could be illustrated by the reduction peak around -1.50 V during the negative scan, shown in Figure 4c. While the Mn2+ returned from the excited state to ground state, light was emitted:

Mn3+ + e- f (Mn2+)* (Mn2+)* f Mn2+ + hν When pure ZnS NCs-modified electrode was in PBS containing H2O2 and Mn2+ ions, no ECL-1 emission was observed, indicating that only the Mn doped in the lattice of ZnS crystal can result in the new emission. 3.3. Effect of Mn2+ Doping Level in ZnS NCs on the ECL. The concentration of Mn2+ doped into ZnS NCs had a great effect on the ECL-1 intensity. ICP analysis revealed that with the increase of the initial concentration of Mn2+ in the reaction solution, the actual doping percent of Mn2+ in ZnS NCs also increased. The doping percents of 0.21, 1.9, 2.13, and 3.6 wt % corresponded to the initial Mn2+ concentrations of 5, 10, 15, and 20 wt %. Figure 5 showed the effect of Mn2+ doping level on the ECL emissions. The peak intensity of ECL-2 around -2.38 V remained almost unaltered with the incorporation of Mn2+ into ZnS NCs. However, the intensity of ECL-1 became stronger with increasing doping level from 0.21% to 1.9% and did not change greatly at 2.13%. The intensity then decreased with further doping (3.6%), showing the concentration quenching effect. The initial increase in the intensity of the ECL-1 peak with Mn2+ concentration was due to the formation of more and more Mn2+ luminescent centers, whereas the quenching of the ECL intensity at higher Mn2+ doping level (3.6%) might be due to the interaction of the neighboring Mn2+ ions in ZnS.25 Although the ECL intensity of ZnS was stronger than that generated from the doping Mn2+, its ECL potential was too negative to be used in analytical application. Bubbles would be produced from H2O reduction reaction on the surface of GCEs under such a negative potential, which would affect the

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Figure 6. Effect of (A) pH value (with 5.0 mM H2O2) and (B) scan rate (in pH 9.0 PBS with 5.0 mM H2O2) on ECL-1 intensity. The potential was scanned in the negative direction from 0 to -2.0 V.

stability of the ZnS:Mn2+ NCs film and further interfere with ECL process inevitably. Thus, we carried out our next experiments within the negative potential scan from 0 to -2.0 V to research the ECL-1 of Mn2+ in ZnS NCs to explore its potential application. 3.4. Effect of Scan Rate and pH on the ECL-1 of ZnS: Mn2+ NCs. Two important factors, pH of solution and scan rate, which could influence the ECL-1 intensity of ZnS:Mn2+ (1.9 wt %) NCs greatly, were investigated (shown in Figure 6). When the pH was lower than 6.0, no emission was observed. Increasing pH values from 6.0 to 9.0 enhanced the ECL greatly. However, beyond the latter point, the ECL intensity did not change obviously. Bard’s group had demonstrated that ECL emission of semiconductor NCs was very sensitive to their surface states.38 Increasing solution pH could alter surface states of ZnS:Mn2+ NCs due to adsorption of Lewis bases,12,39 which resulted in the enhanced ECL intensity. Furthermore, the resulting OH · species were more stable at high pH, which was favorable to the ECL emission and led to the ECL intensity increasing. Thus, the ECL measurements were performed in 0.10 M PBS with pH 9.0, considering both the ECL-1 intensity and the application potential in analytical fields. Figure 6B showed that the ECL-1 intensity increased with increasing scan rate and tended to a constant value after the scan rate of 100 mV/s. Ju et al. reported that ECL efficiency significantly depended on the rate of generation/annihilation of the excited state.40 At a higher scan rate, the excited state of (Mn2+)* could be enriched in a short time span and led to the enhanced ECL intensity. The constant ECL intensity implied that the emitted photos came to saturation after a scan rate of 100 mV/s. As a result, ECL experiments were carried out at a scan rate of 100 mV/s. 3.5. Effect of H2O2 Concentration on ECL-1 of ZnS:Mn2+ NCs. Figure 7 showed the ECL response of the modified GCE with ZnS:Mn2+ (1.9 wt %) NCs to the concentration of H2O2 in 0.10 M pH 9.0 PBS solution. H2O2 could enhance the intensity of ECL-1 obviously (curves a-c). A linear response between the intensity of ECL-1 and H2O2 concentration up to 15 mM was obtained, shown in Figure 7, inset. Although the analytical performance of ZnS NCs doped with Mn2+ for H2O2 at the present stage is not so excellent as that of the CdSe12and CdS41-based ECL system, improved performance will be achieved by extensively optimizing the structure, size, and the quantity of Mn2+ doped in ZnS NCs influencing the ECL of ZnS:Mn2+, which is being carried out in our laboratory. 4. Conclusions In summary, this work elucidates a novel ECL emission of the ZnS:Mn2+ NCs-modified electrode in aqueous solution containing H2O2 as a coreactant. Also, a reasonable mechanism

Figure 7. Dependence of the ECL-1 intensity of ZnS:Mn2+ NCs on the concentration of H2O2 (a) 1.0 mM, (b) 5.0 mM, and (c) 10 mM in 0.10 M pH 9.0 PBS solution with a scan rate of 100 mV/s. Inset: Calibration curve for H2O2 detection.

was proposed for the new ECL emission. The level of Mn2+ doping in ZnS NCs, pH of solution, scan rate, and the concentration of H2O2 had a great effect on the ECL-1 intensity. This preliminary work would develop the ECL research of doped NCs and promote the application of more semiconductors in biological detection with more environmentally friendly conditions. Acknowledgment. This work was supported by the National Natural Science Foundation (No. 20890020, 20775033, 20635002), the National Natural Science Funds for Creative Research Groups (20521503), the program for New Century Excellent Talents in University (NCET), and the 973 Program (2007CB936404, 2006CB93301) of China. References and Notes (1) Richter, M. M. Chem. ReV. 2004, 104, 3003. (2) Cao, W. D.; Ferrance, J. P.; Demas, J.; Lander, J. P. J. Am. Chem. Soc. 2006, 128, 7572. (3) Zhang, L.; Dong, S. Anal. Chem. 2006, 78, 5119. (4) Zhan, W.; Alvarez, J.; Crooks, R. M. Anal. Chem. 2003, 75, 313. (5) Chovin, A.; Ganigue, P.; Vianatier, P.; Sojic, N. Anal. Chem. 2004, 76, 357. (6) Wang, X. F.; Xu, J. J.; Chen, H. Y. J. Phys. Chem. C 2008, 112, 7151. (7) Ren, T.; Xu, J. Z.; Tu, Y. F.; Zhu, J. J. Electrochem. Commun. 2005, 7, 5. (8) Miao, J. J.; Ren, T.; Li, D.; Zhu, J. J.; Chen, H. Y. Small 2005, 1, 1. (9) Ding, S. N.; Xu, J. J.; Chen, H. Y. Chem. Commun. 2006, 34, 3631. (10) Myung, N.; Ding, Z. F.; Bard, A. J. Nano Lett. 2002, 2, 1315. (11) Poznyak, S. K.; Talapin, D. V.; Shevchenko, E. V.; Weller, H. Nano Lett. 2004, 4, 693. (12) Zou, G. Z.; Ju, H. X. Anal. Chem. 2004, 76, 6871. (13) Bae, Y.; Myung, N.; Bard, A. J. Nano Lett. 2004, 4, 1153. (14) Liu, X.; Jiang, H.; Lei, J. P.; Ju, H. X. Anal. Chem. 2007, 79, 8055. (15) Simpson, S. L.; Apte, S. C.; Batley, G. E. EnViron. Sci. Technol. 1998, 32, 620.

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