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Langmuir 2007, 23, 8400-8405
Heavy Phosphate Adsorption on Amorphous ITO Film Electrodes: Nano-Barrier Effect for Highly Selective Exclusion of Anionic Species Dai Kato,†,§ Guobao Xu,‡,§ Yuzuru Iwasaki,‡,§ Yoshiki Hirata,† Ryoji Kurita,† and Osamu Niwa*,†,§ National Institute of AdVanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8566, Japan, NTT Microsystem Integration Laboratories, 3-1 Morinosato, Wakamiya, Atsugi, Kanagawa 243-0198, Japan, and CREST, Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed February 16, 2007. In Final Form: May 16, 2007 We prepared an amorphous indium tin oxide (ITO) film and studied it with respect to its surface characterization and the effect of phosphate adsorption on its electrochemical properties. The film was deposited using RF sputtering under ambient low-oxygen conditions at room temperature. The XPS results revealed that the amount of phosphate adsorbed on the amorphous ITO film was more than 4.6 times greater than that adsorbed on commercially available polycrystalline ITO film in spite of the smaller microscopic surface area of the former. Electrochemical responses for anionic species such as L-ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) on the phosphateadsorbed ITO film electrodes were more effectively suppressed at the amorphous ITO film electrode than at the polycrystalline ITO film electrode when a phosphate-containing electrolyte was used. Such suppression could be attributed to the electrostatic repulsion between the anionic species and more heavily adsorbed phosphate on our amorphous ITO film electrode surface. This effect is made more pronounced by increasing the phosphate concentration to 1 mM. With 1 mM phosphate, the amorphous ITO film electrode showed the highest selectivity for dopamine (DA) against the anionic species, namely, 880 for DA/AA and 330 for DA/DOPAC, respectively. In contrast, the selectivity was 120 for DA/AA and 20 for DA/DOPAC with the polycrystalline ITO film electrode.
Introduction Indium tin oxide (ITO), a degenerate n-type semiconductor, has been widely used for optoelectronic applications, e.g., solar cells,1 flat panel/liquid crystal displays (LCD),2-4 and organic light-emitting diodes (OLED),5-7 since they have certain interesting physical properties including relatively low resistivity and high transparency.5,8 ITO films have also been employed as working electrodes for the electrochemical measurement of biomolecules such as the direct electron transfer of proteins9,10 and electrochemical biosensors.11-17 Electroanalytical applica* National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki, Japan, Tel: +81-29-861-6158, Fax: +81-29-861-6177, E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ NTT Microsystem Integration Laboratories. § CREST-JST. (1) Namavar, F.; Maruska, H. P.; Kalkhoran, N. M. Appl. Phys. Lett. 1992, 60, 2514. (2) Gross, M.; Muller, D. C.; Nothofer, H. G.; Scherf, U.; Neher, D.; Brauchle, C.; Meerholz, K. Nature (London) 2000, 405, 661. (3) Wu, C. C.; Sturm, J. C.; Register, R. A.; Thompson, M. E. Appl. Phys. Lett. 1996, 69, 3117. (4) Toko, Y.; Sugiyama, T.; Katoh, K.; Iimura, Y.; Kobayashi, S. J. Appl Phys. 1993, 74, 2071. (5) Malinsky, J. E.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Richter, A. G.; Marks, T. J.; Armstrong, N. R.; Kippelen, B.; Dutta, P.; Peyghambarian, N. AdV. Mater. 1999, 11, 227. (6) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471. (7) Wu, C. C.; Wu, C. I.; Sturm, J. C.; Kahn, A. Appl. Phys. Lett. 1997, 70, 1348. (8) Tahar, R. B. H.; Ban, T.; Ohya, Y.; Takahashi, Y. J. Appl. Phys. 1998, 83, 2631. (9) Cohen, D. J.; King, B. C.; Hawkridge, F. M. J. Electroanal. Chem. 1998, 447, 53. (10) Taniguchi, I.; Watanabe, K,; Tominaga, M.; Hawkridge, F. M. J. Electroanal. Chem. 1992, 333, 331. (11) Armistead, P. M.; Thorp, H. H. Anal. Chem. 2000, 72, 3764. (12) Popovich, N. D.; Eckhardt, A. E.; Mikulecky, J. C.; Napier, M. E.; Thomas, R. S. Talanta 2002, 56, 821. (13) Chiang, M. T,; Whang, C. W. J. Chromatogr., A 2001, 934, 59.
tions have focused on the transparency of the ITO film electrodes and are widely used in combination with spectrochemical techniques such as electrochemiluminescence (ECL).13-15 Recently, ITO-coated optic waveguide (OW) systems have also been studied.16,17 Furthermore, Thorp et al. have developed a label-free detection system for a nucleic acid based on the catalytic oxidation of guanine residue by a ruthenium complex mediator. They employed this approach because ITO film electrodes have a wider potential window in the positive direction that makes them capable of oxidizing guanine residues more stably than metal electrodes such as Pt and Au.11 The adsorption of phosphonate/phosphate groups on the ITO film surface is another interesting property. Wrighton et al. have reported that a conventional ITO film surface induced adsorption for thiols, carboxylic acids, and phosphonate/phosphate groups.18 Such phosphate adsorption is also well-known on certain other metal oxides such as TiO2, Al2O3, and Fe2O3.19-21 This is of much interest with respect to the formation of self-assembled monolayers that are capable of improving the ITO film surface by modifying it with various functionalized groups. Therefore, many research groups have proposed the easy modification of ITO film surfaces by utilizing phosphonate/phosphate-function(14) Qiu, H,; Yan, J.; Sun, X.; Liu, J.; Cao, W.; Yang, X.; Wang, E. K. Anal. Chem. 2003, 75, 5435. (15) Wilson, R.; Barker, M. H.; Schiffrin, D. J.; Abuknesha, R. Biosens. Bioelectron. 1997, 12, 277. (16) Brusatori, M. A.; Van, Tassel, P. R. Biosens. Bioelectron. 2003, 18, 1269. (17) Bearinger, J. P.: Vo¨ro¨s, J.; Hubbell, J. A.; Textor, M. Biotechnol. Bioeng. 2003, 82, 465. (18) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (19) Nooney, M. G.; Campbell, A.; Murrell, T. S.; Lin, X. F.; Hossner, L. R.; Chusuei, C. C.; Goodman, D. W. Langmuir 1998, 14, 2750. (20) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. J. Phys. Chem. B 2003, 107, 11726. (21) Oh, S.Y.; Yun, Y. J.; Hyung, K. H.; Han, S. H.; New J. Chem. 2004, 28, 495.
10.1021/la700466y CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
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Table 1. Characteristics of Prepared ITO Film (Amorphous) and Commercially Available ITO Film (Polycrystalline)
amorphoous (as-prepared) polycrystalline (as-received)
sheet resistance (Ω/sq)
thickness (nm)
transmittancea (%)
rms roughnessb (nm)
C° c (µF/cm2)
O/In+Sn
Pd (at %)
120
40
90
0.55
6.74
1.21
1.71
15
100
90
7.0
13.9
1.33
0.37
a Transmittance values at 550 nm. b Surface roughness of the films was determined from AFM images with a size of 10 µm2. c The apparent capacitance value (C°) was obtained from background voltammograms at 0.45 V vs Ag/AgCl in 1.0 M KCl. d P (%) was obtained from XPS measurement of the ITO electrodes after they had been immersed in 50 mM phosphate buffer solution for 2h, rinsed with water and dried with nitrogen.
Figure 1. Photographs (a) and AFM images and the line profiles (b) of amorphous (as-prepared) and polycrystalline (as-received) ITO film surfaces.
alized groups.12,18,22-24 Popovich et al. reported that phosphate adsorption provided a more negative ITO film surface that had a significant influence on the electrochemical behavior of charged redox molecules such as Ru(bpy)32+ and Fe(CN)64-.12,25-27 Electrochemical measurements performed using an ITO film electrode in phosphate buffer revealed an electron transfer that was selective against anionic species. It has also been reported that both the adsorption of phosphate on the ITO film electrode and the internal microstructure of ITO film are closely related to the electron-transfer rate.25 In a previous study, some of the present authors found that the amorphous ITO film electrode exhibited high selectivity for positively charged dopamine (DA) against anionic species including catecholamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), and L-ascorbic acid (AA) when using phosphate buffer as an electrolyte.28-30 The selectivity is significantly greater than previously reported results obtained using conventional electrodes. The surface state and physical properties of amorphous ITO film were assumed to be different from those of polycrystalline ITO film, and further investigation of the mechanism will (22) Brewer, S. H.; Brown, D. A.; Franzen, S. Langmuir 2002, 18, 6857. (23) Koh, S. E.; McDonald, K. D.; Holt, D. H.; Dulcey, C. S.; Chaney, J. A.; Pehrsson, P. E. Langmuir 2006, 22, 6249. (24) Vercelli, B.; Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A. Langmuir 2003, 19, 9351. (25) Popovich, N. D.; Wong, S. S.; Yen, B. K.; Yeom, H. Y.; Paine, D. C. Anal. Chem. 2002, 74, 3127. (26) Popovich, N. D.; Yen, B. K.; Wong, S. S. Langmuir 2003, 19, 1324. (27) Popovich, N. D.; Wong, S. S.; Ufer, S.; Sakhrani, V.; Paine, D. J. Electrochem Soc. 2003, 150, H255. (28) Niwa, O.; Xu, G. B.; Iwasaki, Y. Electrochemistry 2006, 74, 135. (29) Xu, G. B.; Iwasaki, Y.; Niwa, O. Chem. Lett. 2005, 34, 1120. (30) Hayashi, K.; Iwasaki, Y.; Horiuchi, T.; Sunagawa, K.; Tate, A. Anal. Chem. 2005, 77, 5236.
be useful for developing various sensors such as in vivo monitoring microelectrodes or microfabricated arrays.30 This is because ITO film could be deposited on any shape of electrode such as needle-like micro- or nano electrodes for cell measurements, and it is also compatible with the microfabrication of micrometer-sized electrodes,14,30-33 since it is currently prepared by conventional RF sputtering methods. In this work, we studied the amorphous ITO film in terms of its surface characterization and phosphate adsorption property in order to clarify the surface chemical structure and quantitative adsorption performance of our amorphous ITO film electrodes. The properties of the electrodes, such as the surface ratio of indium, tin, and oxygen and the amount of adsorbed phosphate, were compared with results obtained for a commercially available polycrystalline ITO film electrode. We also investigated phosphate-adsorbed ITO film with a view to achieving the optimum selective measurement for dopamine against its interfering species such as AA and its metabolite DOPAC. Experimental Section Chemicals. All chemicals were analytical grade and were used as received. Dopamine hydrochloride (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and l-ascorbic acid (AA) were purchased from Sigma-Aldrich (St. Louis, MO). Ultrapure water (Milli-Q) was used in all the experiments. ITO Film Electrodes. Amorphous ITO films were deposited on BK7 glass from an In2O3-5% SnO2 ceramic target using RF sputter deposition equipment (SEED Lab. Kanagawa, Japan). Prior to (31) Lee, T. M. H.; Carles, M. C.; Hsing, I. M. Lab Chip 2003, 3, 100. (32) Yang, L. J.; Li, Y. B.; Erf, G. F.; Anal. Chem. 2004, 76, 1107. (33) Liu, D.; Perdue, R. K.; Sun, L.; Crooks, R. M. Langmuir 2004, 20, 5905.
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Figure 2. Schematic representation of possible phosphate-adsorbed amorphous and polycrystalline surfaces. The phosphate ion size and the film surface roughness were consistently represented on the basis of the P-O distance (1.5 Å) and line profile obtained from the AFM measurement, respectively. deposition, the chamber pressure was reduced to 2 × 10-6 Torr, and then argon was flowed into the chamber until the pressure was 0.005 Torr. The RF power was 50 W. The substrate was kept at room temperature and exposed to the plasma at a rotation rate of 10.7 rpm. The sputtering time was 90 min. A commercially available ITO film (15 Ω/sq, BAS, Tokyo, Japan) was used as a control electrode in all the experiments. For all the experiments, the ITO film electrodes were cleaned with acetone and dried with nitrogen before use. ITO Film Characterization. The sheet resistance of the ITO film was measured by the four-point probe method using a K-705RS resistivity meter (Kyowa Riken, Tokyo) at room temperature. The optical transmittance of the sample was measured with a UV-160A spectrophotometer (Shimadzu, Japan). The thickness of the ITO films was measured with a surface profilometer (DEKTAK 3030, Sloan Technology). AFM measurements were performed on a Nanoscope IIIa Multimode AFM (Veeco Digital Instruments, Santa Barbara, CA) with a 130 µm VJ-type scanner. All measurements were made using a silicon cantilever (Multi75: spring constant ∼5 N/m, resonance frequency ∼75 kHz, NanoDevices, Inc., CA). Images were recorded in the tapping mode at scan rates of 1-3 Hz with 512 × 512 pixels. XPS was conducted with a Kratos AXIS-Ultra X-ray photoelectron spectrometer (monochromatic Al KR source at 1486.6 eV) to determine the elemental composition of the film surface. Electrochemical Measurements. All electrochemical experiments were performed using an ALS/CHI 760B electrochemical analyzer (CH Instruments, Inc., USA). A platinum wire and an Ag/ AgCl (3 M NaCl) electrode were used as auxiliary and reference electrodes, respectively. Amorphous or commercial ITO film was used as the working electrode. The ITO film electrode area was defined by using masking tape with a 1 mm diameter hole in it. A 50 mM sodium phosphate buffer (PB, pH 7.0, Wako), 50 mM TrisHCl buffer (Tris, pH 7.0, Sigma), and 1.0 M KCl (Kanto Chemical, Tokyo) were used as the electrolyte solutions for the electrochemical measurements.
Results and Discussion Characterization and Phosphate Adsorption of Amorphous ITO Film Surface. The properties of the amorphous ITO film prepared in this study were reproducible with different sputtering runs. The obtained film was about 40 nm thick and the sheet resistance was about 120 Ω/sq as summarized in Table 1. The amorphous ITO film transmitted 90% of visible light at 550 nm, which is the same as the commercial ITO film, whereas low transmittance was observed at less than 500 nm. The obtained film was thus distinguishable from the commercial ITO film from its appearance as shown in Figure 1a. We characterized the surface properties of these ITO films by AFM and XPS. Figure 1b shows AFM images and line profiles of the prepared ITO film and the commercial ITO film surface. The prepared and the commercial ITO film electrodes exhibited amorphous and polycrystalline states, respectively. We obtained a typical surface roughness (rms) of 0.55 nm for our amorphous ITO film determined from the image size of 10 × 10 µm2. This is much flatter than the value of 7 nm for the polycrystalline ITO film and the values previously reported by other groups (2-4
Figure 3. Ratio of the peak currents for DOPAC oxidation on the phosphate-treated electrode to that of an untreated electrode as a function of time. The phosphate-treated ITO film electrodes were obtained by immersing them in 50 mM PB for 10 min and then rinsing them with water. The peak current for DOPAC at each electrode was obtained from DPV measurement in 50 mM Tris buffer (pH 7.0) at intervals of 5 min. The DOPAC concentration was 100 µM. DPV was performed with the following parameters: ∆E ) 5 mV, pulse amplitude ) 50 mV, pulse width ) 50 ms, pulse period ) 0.2 s.
nm),12,25,27,34 and almost the same as that of the amorphous ITO film prepared by dc magnetron sputtering reported by Armstrong’s group (0.8 nm).35 The average grain sizes for the polycrystalline ITO film were 0.3-1.2 µm, whereas no grain structure was observed for the amorphous ITO film. The apparent capacitance value (C°) of the ITO film electrode surfaces was also measured from cyclic voltammograms at 0.45 V vs Ag/AgCl in 1.0 M KCl in accordance with McCreery’s report.36 It is well-known that an ITO film electrode has a relatively low C° compared with other conventional electrodes such as a glassy carbon electrode (35 µF/cm2).37,38 The small surface roughness may contribute to the low C° values for the amorphous ITO film. Indeed, the amorphous ITO film prepared in this study exhibited a C° value of 6.74 µF/cm2, which is smaller than that of the polycrystalline (34) Kim, J. S.; Ho, P. K. H.; Thomas, D. S.; Friend, R. H.; Cacialli, F.; Bao, G. W.; Li, S. F. Y. Chem. Phys. Lett. 1999, 315, 307. (35) Donley, C. C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.; Alloway, D.; Armstrong, N. R. Langmuir 2002, 18, 450. (36) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893. (37) Xu, J.; Chen, Q.; Swain, G. M. Anal. Chem. 1998, 70, 3146. (38) Krysinski, P.; Blanchard, G. J. Langmuir 2003, 19, 3875.
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Figure 4. Barrier effect of phosphate adsorption during DPV measurements of AA and DOPAC at the amorphous and the polycrystalline ITO film electrodes in 50 mM Tris buffer (pH 7.0) containing 0, 10, 100, and 1000 µM phosphate (solid lines a-d). The dotted line shows a background scan. The concentration of each sample was 100 µM. The DPV conditions are the same as those in Figure 3.
ITO film (13.9 µF/cm2). Such flatness should be very useful in terms of fabricating nanosized electrodes for use in electroanalysis or in improving the ITO film surface with the formation of selfassembled monolayers of various functionalized groups. In addition, a low C° value is advantageous with regard to suppressing the background current and achieving a lower detection limit. The ITO film surfaces were investigated by XPS measurements.22,23,34,35,39-41 As summarized in Table 1, there was a subtle difference between the O/In+Sn values of the amorphous and polycrystalline ITO film, indicating that the amorphous ITO film prepared in this study had a relatively low oxygen content compared with that of the polycrystalline ITO film, since our amorphous ITO film was prepared under ambient low-oxygen (39) Donley, C. L.; Dunphy, D.; Peterson, R. A. P.; Zangmeister; Nebesny, K. W.; Armstrong, N. R. Indium-Tin-Oxide Thin Films for Characterization of Electrochemical Processes in Molecular Assemblies. In Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications; Salaneck, W. R., Seki, K., Kahn, A., Pireaux, J. J., Eds.; Marcel Dekker: New York, 2002; Chapter 9, p 269. (40) Nu¨esch, F.; Rothberg, L. J.; Forsythe, E. W.; Le, Q. T.; Gao, Y. L. Appl. Phys. Lett. 1999, 74, 880. (41) Satoh, T.; Fujikawa, H.: Taga, Y. Appl. Phys. Lett. 2005, 87, 143503.
conditions as described above. We also conducted XPS measurements with both ITO films after they had been immersed in PB solution for 2 h and then rinsed with water and dried with nitrogen. The results for phosphorus 2p XPS (133 eV)22,23 clearly indicated that the phosphate was adsorbed onto both ITO films. The values were 1.71 and 0.37 at % for the amorphous and the polycrystalline ITO films, respectively. Interestingly, the amount of phosphate adsorbed on the amorphous ITO film was 4.6 times larger than that adsorbed on the polycrystalline ITO film, even though the amorphous ITO film had a flatter surface. It is reported that phosphates/phosphonates would react with hydroxyl groups on an ITO film surface.12.24.27,35 Armstrong et al. reported that the surface of the amorphous ITO film produced by dc magnetron sputtering at room temperature has more hydroxyl groups (54% of total surface oxygen) than that of as-received polycrystalline ITO film (25%).35 Satoh et al. also reported that amorphous ITO film deposited at room temperature involved many more hydroxyl groups near the surface than polycrystalline ITO film deposited at 573 K.41 Also in our case, high surface concentrations of hydroxyl groups may lead to high concentrations of adsorbed phosphate on the amorphous ITO film surface. In fact, the content
8404 Langmuir, Vol. 23, No. 16, 2007
ratios of (In-O-In/In-OH) obtained from deconvoluted indium 3d5/2 XPS spectra35 were 1.87 and 2.54 for the amorphous and the polycrystalline ITO films, respectively. This result also suggests that the amount of surface hydroxyl groups for the amorphous ITO film was greater than that for the polycrystalline ITO film. Furthermore, the amorphous ITO film used in this study had a smaller roughness than the polycrystalline ITO film, suggesting that the phosphates were adsorbed at higher densities on the amorphous ITO film surface. From these considerations, Figure 2 shows the possible phosphate-adsorbed amorphous and polycrystalline ITO film surfaces based on actual dimensions. Electrochemical Behavior of Phosphate-Adsorbed ITO Film Electrode. As described above, an amorphous ITO film electrode with a high density of adsorbed phosphates can induce a noticeable barrier effect against anionic species during their electrochemical measurement. In fact, one of the present authors has previously observed that an amorphous ITO film electrode can suppress the oxidation of anionic species such as AA and DOPAC when the PB solution is used as a supporting electrolyte.28-30 In this work, we were able to clarify that the electrochemical behavior at the phosphate-adsorbed amorphous ITO film electrode was more efficient than results for polycrystalline ITO film electrode. Therefore, it is also important to study how long the phosphates were adsorbed on the amorphous ITO film surface compared with those on the polycrystalline ITO film surface. Figure 3 shows the sustained effect of the adsorbed phosphate on given electrodes by measuring the oxidation peak currents of differential pulse voltammograms (DPVs) for DOPAC in Tris buffer solution. The Y-axis in Figure 3 is the ratio of the DOPAC response at the phosphate-treated electrode to that of an untreated electrode. For the first measurements, smaller peak currents were obtained than those of the untreated electrodes, due to electrostatic repulsion between DOPAC and the phosphate remaining on both ITO film electrode surfaces. The peak current of DOPAC at the polycrystalline ITO film electrode was gradually increased during the measurement period at intervals of 5 min and reached a 100% response after 30 min. In addition, the peak current of DOPAC at the polycrystalline ITO film electrode became larger than that without the treatment after 30 min. This current increase is attributed to the adsorption of DOPAC onto the polycrystalline ITO film surface after the removal of the phosphates from the polycrystalline ITO film surface. On the other hand, the amorphous ITO film electrode also exhibited a gradual current increase. However, the current value saturated at about a 60% response 30 min after the start of the measurement, indicating that the amorphous ITO film electrode exhibited a long-lasting effect of the adsorbed phosphate that resulted in DOPAC suppression even at very low phosphate concentrations. This result supports the view that a large amount of the phosphate adsorbed on the amorphous ITO film surface remained at a relatively high density that was sufficient for the repulsion of DOPAC compared with that on the polycrystalline ITO film surface. The effect of the added phosphate concentration on the electrochemical oxidation of anionic AA and DOPAC was investigated by measuring oxidation potentials and peak heights. As shown in Figure 4, the phosphate-adsorbed amorphous ITO film electrode had a noticeable effect on the electrochemical behavior that greatly suppressed the oxidation of these anionic species. The anodic peak currents decreased from 21 to 0.3 nA for AA and from 360 to 0.8 nA for DOPAC as the concentration of the added phosphate increased. Such suppression was also observed for other anionic species including Fe(CN)63-/4- (not
Kato et al.
Figure 5. Effects of phosphate concentration on selective DA measurements for the anionic species at the amorphous (solid lines) and polycrystalline (dotted lines) ITO film electrodes in 50 mM Tris buffer (pH 7.0). The selectivity was calculated from the peak current (0.32 V) of DPVs for 100 µM DA to that of the 100 µM anionic species at the same potential. The DPV conditions are the same as those in Figure 3.
shown). On the other hand, the polycrystalline ITO film electrode scarcely suppressed the oxidation of these anionic species, although their oxidation peak potential shifted to a higher value as the concentration of the added phosphate increased due to the decrease in the electron transfer of these anionic species. These results suggest that the polycrystalline ITO film electrode is insufficient for achieving the selective detection of other biomolecules with higher oxidation potential such as NADH, since the oxidation peak of the interfering molecules overlaps with that of the analyte. We also estimated the selectivity for DA against these anionic species at the same concentration (100 µM) by comparing peaks obtained by DPV. Figure 5 demonstrates the selectivity for DA against these anionic species at the amorphous and polycrystalline ITO film electrodes. For both electrodes, the current ratios were increased as the concentration of the added phosphate increased. This effect was more pronounced at the amorphous ITO film electrode. Current ratios of 880 and 330 were achieved for DA/ AA and DA/DOPAC at the amorphous ITO film electrode, whereas these values were increased to 20 for DA/AA and 120 for DA/DOPAC, respectively, at the polycrystalline ITO film electrode. Such high selectivity without any surface modification is particularly advantageous with regard to measuring DA with high temporal resolution. In the electrochemical detection of DA, electrodes modified with anionic polymers such as Nafion have often been used to remove the anionic species.42-44 However, the electrode response usually becomes slower than that of an unmodified electrode due to the lower diffusion coefficient of DA in the modified films, and it is difficult to monitor fast events such as variations in neurotransmitter concentration near a cultured cell. Therefore, our amorphous ITO film electrode could be useful for such purposes, since high selectivity can be achieved while largely maintaining the response of the bare electrode. When the (42) Yavich, L.; Tiihonen, J. J. Neurosci. Methods 2000, 104, 55. (43) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180. (44) Capella, P.; Ghasemzadeh, B.; Mitchell, K.; Adams, R. N. Electroanalysis 1990, 2, 175.
Phosphate Adsorption on ITO Film Electrodes
phosphate concentration is over 1 mM (e.g., pure 50 mM phosphate buffer), there were no observable changes in the peak current of DPVs for the anionic species. However, the peak current of DA also decreased, since even the electron transfer between DA and the electrode was suppressed. This is presumably due to the increase in the average distance of the closest approach between DA and the electrode.26 Therefore, the addition of 1 mM of phosphate is sufficient for selective measurements of DA at the amorphous ITO film electrode.
Conclusion We prepared amorphous ITO film under ambient low-oxygen conditions and studied the surface characterization and phosphate adsorption property of the film surface. The obtained amorphous ITO film exhibited intrinsic performance comparable to that of commercially available polycrystalline ITO film such as high transparency for visible light. The amorphous ITO film showed a much larger degree of phosphate adsorption than polycrystalline
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ITO film. The phosphate-adsorbed amorphous ITO film electrode could suppress the oxidation of anionic interfering AA and DOPAC that resulted in high selectivity for DA (up to 880 against AA and 330 against DOPAC) without any surface modification, unlike a polycrystalline ITO film electrode (DA selectivity of less than 120 against AA and 20 against DOPAC). Since the amorphous ITO film electrodes in this study have a much flatter surface and less background noise than polycrystalline ITO film electrodes, they have the potential for use in fabricating nanosized electrodes for application to scanning electrochemical microscopy integrated with scanning near-field optical microscopy based on their good transparency. We are now preparing amorphous ITO film sputtered nanoelectrodes based on glass fiber for use in the selective detection of DA released from a single nerve cell. Acknowledgment. This work is supported by the CREST project of the Japan Science and Technology Agency (JST). LA700466Y
