Anal. Chem. 1997, 69, 703-710
Electrochemical Behavior of Graphite Electrodes Modified by Spin-Coating with Sol-Gel-Entrapped Ionomers Maria D. Petit-Dominguez,† Hong Shen, William R. Heineman,* and Carl J. Seliskar*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172
Tetraethoxysilane sol-gel doped with poly(dimethyldiallylammonium chloride) (PDMDAAC) and poly(vinylsulfonic acid, sodium salt) (PVSA) has been prepared to provide a simple method to produce electrodes coated with new ion-exchange glasses. Electrochemical sensors prepared by spin-coating spectroscopic grade graphite rods with these sol-gel ionomers have been developed. Sol-gel-modified electrodes were evaluated with Ru(bipy)32+ and Fe(CN)64- as analytes using square wave voltammetry. The results indicate a porous coating where diffusion of the analyte through the sol-gel film to the electrode surface occurred. Analyte preconcentration within the polymer-modified sol-gel network resulted in an improvement in detection limits of 1-2 orders of magnitude compared to bare electrodes. The sol-gelPDMDAAC-modified electrodes give a linear calibration curve for Fe(CN)64- from 1 × 10-6 to 1 × 10-4 M and a detection limit of 7 × 10-7 M. The response could be reproduced at different electrodes with an 18% relative standard deviation at a concentration level of 1 × 10-6 M. At the sol-gel-PVSA-modified electrodes, the calibration curve for Ru(bipy)32+ was linear from 2.8 × 10-7 to 2.8 × 10-4 M, the detection limit was 2 × 10-7 M, and the relative standard deviation was 10% for different electrodes at a concentration level of 2.8 × 10-7 M. Organic ion-exchangers incorporated into a silicate matrix combine the physical properties of the glass, such as thermal stability, negligible swelling effects, tunable porosity, and polar microenvironment, with the ion-exchanging properties of the organic functional group. The sol-gel method is a simple process to prepare glassy materials at room temperature that can support the immobilization of different reagents. The sol-gel reaction proceeds by hydrolysis of an alkoxide precursor under acidic or basic conditions, followed by polycondensation of the hydroxylated monomers to form a porous gel.1,2 Traditionally, organic reagents have been immobilized in silica gel matrices by impregnation or by using the high reactivity of surface silanol groups to anchor the reagents by covalent bonding. In recent years, a novel doping method has been introduced3,4 where dopants are incorporated in the sol† Current address: Department of Analytical Chemistry and Instrumental Analysis, Autonoma University of Madrid, 28049 Madrid, Spain. (1) Buckley, A. H.; Greenblatt, M. J. Chem. Educ. 1994, 71 (7), 599-602. (2) Haruvy, Y.; Webber, S. E. Chem. Mater. 1991, 3, 501-507. (3) Zusman, R.; Rottman, C.; Ottolenghi, M.; Avnir, D. J. Non-Cryst. Solids 1990, 122, 107-109.
S0003-2700(96)00839-6 CCC: $14.00
© 1997 American Chemical Society
gel glass at the early stages (or even before initiation) of the polymerization step. Thus, when the dry xerogel is formed, dopants remain physically encapsulated within the porous glass matrix but maintain their ability to interact with species that diffuse into the matrix. Zusman et al.3 were among the first to entrap organic reagents in sol-gel glasses and to report the qualitative determination of metals. For example, a sol-gel glass doped with dimethylglyoxime detects Ni2+ by promptly developing the characteristic deep red color of the nickel dimethylglyoxime complex. Many other solid phases allow the selective determination of metals in solution, such as silica glass detectors doped with 1-nitroso-2-naphthol for detection of cobalt ions,5 o-phenanthroline for determination of divalent iron,5,6 and pyoverdin, a natural fluorescent pigment, entrapped in sol-gel glasses and used as a reactive solid phase for the determination of trace amounts of Fe(III).7 Additionally, sol-gel-derived glasses have found many applications as laser or solar light guides;2,8,9 pH,10,11 oxygen,12-14 ammonia,15 and nitrite/ nitrogen dioxide16 sensors; and biosensors.13-17 Several sol-gel polymer hybrid materials have been developed,18-24 consisting of an alkoxide precursor polymerized in the presence of such polymers as polyacrylonitrile,18 poly(sodium (4) Braun, S.; Rappoport, S.; Zusman, R.; Avnir, D. J.; Ottolenghi, M. Mater. Lett. 1990, 10, 1-5. (5) Losefzon-Kuyavskaya, B.; Gigozin, I.; Ottolenghi, M.; Avnir, D. J. Non-Cryst. Solids 1992, 147 & 148, 808-812. (6) Lev, O.; Losefzon-Kuyavskaya, B.; Gigozin, I.; Ottolenghi, M.; Avnir, D. Fresenius’ J. Anal. Chem. 1992, 343, 370-372. (7) Barrero, J. M.; Camara, C.; Perez-Conde, M. C.; San Jose, C.; Fernandez, L. Analyst 1995, 120, 431-435. (8) Avnir, D. J.; Kaufman, V.; Reisfeld, R. J. Non-Cryst. Solids 1985, 74, 395406. (9) Reisfeld, R. J. Non-Cryst. Solids 1990, 121, 254-266. (10) Yang, L.; Saavedra, S. Anal. Chem. 1995, 67, 1307-1314. (11) Badini, G. E.; Grattan, K. T. V.; Tseung, A. C. C. Analyst 1995, 120, 10251028. (12) Chung, K. E.; Lau, E. H.; Davidson, M. S.; Dunn, B. S.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1995, 67, 1505-1509. (13) Ishiji, T.; Kaneko, M. Analyst 1995, 120, 1633-1638. (14) MacCraith, B. D.; McDonagh, C. M.; O’Keeffe, G.; Keyes, E. T.; Vos, J. G.; O’Kelly, B.; McGilp, J. F. Analyst 1993, 118, 385-388. (15) Werner, T.; Klimant, I.; Wolfbeis, O. Analyst 1995, 120, 1627-1631. (16) Raman, V.; Bahl, O. P.; Parashar, V. K. J. Mater. Sci. Lett. 1994, 13, 579581. (17) Narang, V.; Prasad, P. N.; Bright, F. V. Anal. Chem. 1994, 66, 31393144. (18) Wei, Y.; Wang, W.; Yang, D.; Tang, L. Chem. Mater. 1994, 6, 1737-1741. (19) Nakanishi, K.; Soga, N. J. Am. Ceram. Soc. 1991, 74 (10), 2518-2530. (20) David, I. A.; Scherer, G. W. ACS Polym. Prepr. 1991, 32, 530-531. (21) Mah, S. K.; Chung, I. J. J. Non-Cryst. Solids. 1995, 183, 252-259. (22) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1995, 7, 130-136. (23) Landry, C. J. T.; Coltrain, B. K. ACS Polym. Prepr. 1991, 32, 514-515. (24) Motakef, S.; Suratwala, T.; Roncone, R. L.; Boulton, J. M.; Teowee, G.; Neilson, G. F.; Uhlmann, D. R. J. Non-Cryst. Solids. 1994, 178, 31-43.
Analytical Chemistry, Vol. 69, No. 4, February 15, 1997 703
styrenesulfonate),19 poly(ethyloxazoline),20 and poly(methyl methacrylate).23 The dynamic properties, mechanical properties, composite morphology, optical transparency,24 and phase separation25 of these materials have been studied. A process based on the doping of sol-gel glasses with nonpolymeric organic anion-exchange compounds during the polymerization of the glasses has been described.26 In this work, a discussion of the advantages and limitations of sol-gel glasses doped with methyltrioctylammonium chloride, cetyltrimethylammonium bromide, or cetylpyridinium bromide as new silica-based ion-exchangers has been presented. Recently, a porous glass film has been fabricated where octyl groups were incorporated into the glass matrix during the glass formation process27 and the hybrid material was cast onto the inner walls of fused silica capillaries to function as the stationary phase for reversed-phase open-tubular liquid chromatography and open-tubular electrochromatography. Sintered sol-gel electrodes have been studied for several applications,28 such as indicator, reference, selective, and biocatalytic ceramic electrodes. This article demonstrates the usefulness of a new class of electrodes prepared by a hydrolysis-condensation reaction of tetraethoxysilane (TEOS) doped with poly(vinylsulfonic acid, sodium salt) (PVSA) or poly(dimethyldiallylammonium chloride) (PDMDAAC), structures shown below, as ion-exchangers. A spincoating technique, widely used to produce thin films,29-32 allows coverage of the electrode surface with the doped sol-gel and produces sensitive electrochemical sensors. The sol-gel-ionomermodified electrodes doped with negatively charged PVSA were characterized with positively charged Ru(bipy)32+, and the solgel-ionomer modified electrodes doped with positively charged PDMDAAC were characterized with negatively charged Fe(CN)64as ion-exchanging analytes in solution. Osteryoung square wave voltammetry (OSWV) was used to detect the electroactive analytes in order to study properties of the modified electrodes that relate to their potential use as sensors.
EXPERIMENTAL SECTION Materials. Tetraethoxysilane (TEOS) was purchased from United Chemical Technologies, Inc. (Bristol, PA). Poly(dimethyldiallylammonium chloride) (PDMDAAC, MW 240 000) was purchased from Polysciences, Inc. (Warrington, PA), and poly(vinylsulfonic acid, sodium salt) (PVSA, MW 4000-6000) and tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate were purchased from Aldrich Chemicals (Milwaukee, WI). Potassium ferrocyanide (trihydrate) was purchased from Fisher Scientific (Fair Lawn, NJ). All reagents were analytical grade. Spectroscopic grade graphite rods (type AXF 5QBG1, surface area A ) 13.5 mm2) were obtained from Unocal Poco (Dallas, TX), and Fibrmet polishing disks (Buehler, Lake Bluff, IL) were used to polish the electrode surface. Water was purified with a Barnstead organic pure water system. Apparatus. A spin-coating machine, constructed in our laboratory, was used to coat the electrode surfaces. The spincoating assembly consisted of a power supply and a dc motor, where spin rate was controlled between 0 and 3500 rpm. The motor is directly fitted with an aluminum collet with a hole bored on-center for insertion of the graphite rod electrode. OSWV experiments were conducted with a BAS-100B/W system (Bioanalytical Systems, West Lafayette, IN). The electrochemical cell consisted of a BAS Ag/AgCl saturated NaCl reference electrode, a platinum auxiliary electrode, and a bare or sol-gel-modified spectroscopic graphite working electrode. Sol-Gel Preparation. Sol-gel solutions were prepared with and without dopant. In a low-temperature process, a solution of TEOS and water was mixed with the solution containing the dopant (PDMDAAC or PVSA). The optimal molar ratio TEOS: H2O, found to avoid cracking of the films and also to shorten the gel time, was 1:4, using hydrochloric acid as the catalyst. In the ionomer-doped sol-gel, the typical weight percentages of the ionomer were 2% and 1% for PDMDAAC and PVSA, respectively. In both cases, we used sonication to prepare the homogeneous sol-gel solutions without phase separation. The viscous reaction liquid obtained was left to gel several hours for the undoped and PVSA-doped sol-gel and 1 h for the PDMDAAC-doped sol-gel before being applied to the electrode surface. Electrode Preparation. The graphite rod electrodes were first coated with an acrylic spray to insulate the rods, and then the end of each rod was exposed by polishing on emery paper (No. 420 P2) and then on a 3 µm silicon carbide disk, followed by a 0.3 µm aluminum oxide disk. Ten microliters of the sol-gel solution was placed on the end of the polished graphite rod electrode and spun at 3500 rpm for 120 s for the undoped solgel and for the ionomer-doped sol-gel. All the sol-gel electrodes were left to dry under room conditions overnight before use. The film thickness of the sol-gel film is estimated to be 0.1-1 µm. This range is based on analogy with films prepared on glass slides by an identical procedure, the thickness of which was measured by an optical interference fringe method. (25) Nakanishi, K.; Soga, N. J. Non-Cryst. Solids. 1992, 139, 1-24. (26) Levy, D.; Gigozin, I.; Zamir, I.; Kukavskaya, B.; Ottlenghi, M.; Avnir, D. J.; Lev, O. Sep. Sci. Technol. 1992, 27 (5), 589-597. (27) Guo, Y.; Colo´n, L. A. Anal. Chem. 1995, 67, 2511-2516. (28) Tsionsky, M.; Gun, G.; Glezer, V.; Lev, O. Anal. Chem. 1994, 66, 17471753. (29) Nam, C. W.; Woo, S. I. Thin Solid Films 1994, 237, 314-319. (30) Hershcovitz, M.; Lein, I. E. Microelectron. Reliab. 1993, 33 (6), 869-880. (31) Extrand, C. W. Polym. Eng. Sci. 1994, 34 (5), 390-394. (32) Sukanek, P. C. J. Electrochem. Soc. 1989, 136 (10), 3019-3026.
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Electrode Evaluation. The electrode surfaces were conditioned by soaking in supporting electrolyte overnight before electrochemical measurements were made. All experiments were conducted under ambient conditions. Solutions were purged with argon before measurements and blanketed with argon during measurements. Peak current ip was measured as the distance along a vertical line from the peak to intersection with a baseline that was drawn to intersect tangentially with the voltammogram on either side of the wave. Detection limits were calculated according to the IUPAC method. The stability of the electrode surfaces was assessed daily, and the same electrode could be used for up to a week with negligible change in performance. Electrode Regeneration. After each electrochemical measurement, the analyte was removed from the doped sol-gel electrode surface by soaking the analyte-loaded film in supporting electrolyte solution to reverse the ion-exchange reaction. RESULTS AND DISCUSSION The electrochemical behavior of Ru(bipy)32+ and ferrocyanide was first evaluated at bare and undoped sol-gel-modified graphite electrodes to establish a baseline for comparison with the solgel-polymer-modified electrodes. Spectroscopic graphite was used as the electrode material because its slightly porous nature helps the adhesion of films to its surface. We have used this graphite extensively in our studies of electrodes modified with ionic polymer networks formed by cross-linking with γ radiation.33-35 Although adhesion of film to electrode is enhanced by the porous structure, larger charging currents result from the increased surface area compared to smooth gold, platinum, or glassy carbon electrodes.36 Ru(bipy)32+ at Bare, Sol-Gel-Modified, and Sol-GelPVSA-Modified Electrodes. (i) Characterization at Bare Electrodes. A typical square wave voltammogram is shown in Figure 1A-1 for the reduction of Ru(bipy)32+ at a bare graphite working electrode. A well-developed wave for the Ru(bipy)32+/ Ru(bipy)3+ couple is observed with a peak at +1.04 V. The following supporting electrolytes were tested: 0.1 M KCl, 0.1 M NaCl, 0.1 M Na2HPO4 (pH 5.0 and 9.2), 0.1 M NaOAc (pH 5.0 and 7.0). No significant differences in peak potential and peak current were found with different supporting electrolytes, and 0.1 M KCl was chosen for the following work. (ii) Characterization at Sol-Gel-Modified Electrode. Extrand31 has reported that the final spin-coat film thickness of solgel on an electrode surface is independent of the volume of solution initially applied onto the surface. However, we have found that different spin rates and spin times affect the quality of the sol-gel film. Cracks in the sol-gel film, resulting in exposure of part of the electrode surface, are a major problem. Cracking is related to the drying of the film, which is affected by both spin rate and spin time. At least a 3500 rpm spin rate was found to be necessary to obtain a uniform electrode surface and to avoid cracking of the film. The effect of spin time at 3500 rpm on peak height for reduction of Ru(bipy)32+ at a sol-gel-modified graphite electrode is shown in Table 1. Uncracked electrodes that gave reasonably consistent peak heights were obtained for spin times (33) DeCastro, E. S.; Huber, E. W.; Vallarroel, D.; Galiatsatos, C.; Mark, J. E.; Heineman, W. R.; Murray, P. T. Anal. Chem. 1987, 59, 134-139. (34) Huber, E. W.; Heineman, W. R. Anal. Chem. 1988, 60, 2467-2472. (35) Coury, L. A.; Birch E. M.; Heineman, W. R. Anal. Chem. 1988, 60, 553560. (36) Coury, L. A.; Heineman, W. R. J. Electroanal. Chem. 1988, 256, 327-341.
Figure 1. (A) Square wave voltammograms for 2.8 mM Ru(bipy)32+, 0.1 M KCl at (1) bare graphite, (2) sol-gel-modified electrode, and (3) sol-gel-PVSA-modified electrode; exposure time, 10 min. (B) Removal of the analyte from sol-gel-PVSA-modified electrode (1) after 10 min in 0.1 M KCl, (2) after 60 min in 0.1 M KCl, and (3) after 10 min in 3.0 M NaCl. Film coating at 3500 rpm for 2 min. OSWV: scan from +1.2 to +0.8 V; scan rate, 4 mV s-1; SW amplitude, 25 mV; frequency, 14 Hz. Table 1. Effect of the Spin Time on Peak Current for Reduction of 2.8 mM Ru(bipy)32+ in 0.1 M KCla time (s)
peak current (µA)b
30 60 120 180
22 ( 5 53 ( 5 66 ( 6 57 ( 4
a Spin rate, 3500 rpm. Exposure time, 1 min. b Peak current is the average for three different electrodes.
between 1 and 3 min. A spin rate of 3500 rpm and a spin time of 2 min were chosen for subsequent experiments. When the sol-gel-modified electrodes were immersed in an aqueous solution containing Ru(bipy)32+, diffusion of the analyte into the sol-gel film could be observed by an increase in peak current for repetitive voltammograms. Well-defined voltammograms were obtained at the sol-gel-modified electrodes (Figure 1A-2). As shown in Figure 2, the peak current after immersion in 2.8 mM Ru(bipy)32+ increases rapidly within the first few minutes and then more slowly until about 10 min, and then it plateaus, indicating that the analyte reaches an equilibrium in the sol-gel film after about 10 min of exposure. When an equilibrated sol-gel electrode was removed from the solution of Ru(bipy)32+ after 60 min of immersion, rinsed with water, and placed in supporting electrolyte, expulsion of the analyte from the sol-gel film could be observed by the peak Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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Figure 2. Peak current response for uptake and expulsion of Ru(bipy)32+ at sol-gel-modified electrode. Working electrode: sol-gelmodified electrode; film coating at 3500 rpm for 2 min; exposure time (peak heights in the figure are increasing in this order): 10, 20, 30 s and 1, 5, 10, 20, 30, 40, 50, and 60 min. Electrode immersed in (×) 2.8 mM Ru(bipy)32+, 0.1 M KCl for 60 min and then (b) transferred to 0.1 M KCl. Electrodes immersed in (2) 28 µM, (9) 2.8 µM, and ([) 2.8 nM Ru(bipy)32+, 0.1 M KCl. Error bars show standard deviation for four different electrodes. OSWV settings as in Figure 1.
current decrease with exposure time in the supporting electrolyte solution. The decrease in peak current is fast, as shown in Figure 2. Figure 2 also shows the behavior of sol-gel-modified electrodes in solutions of lower Ru(bipy)32+ concentrations. Immersion in 28 µM Ru(bipy)32+ also shows a rapid increase in peak current within the first few minutes. This initial rapid increase is followed by a slow increase during the remainder of the 1 h of the experiment. This behavior is different from the 2.8 mM Ru(bipy)32+ response, which peaks at about 20 min and then slowly decreases. Although this decrease is barely distinguishable above the experimental error of these experiments, it was consistently observed for all of the electrodes tested. For the 2.8 µM Ru(bipy)32+ solution, a distinguishable peak slowly developed as the electrode continued to soak in solution. No signal for 2.8 nM Ru(bipy)32+ solution was observed (i.e., the voltammogram was essentially indistinguishable from a voltammogram of supporting electrolyte only). (iii) pH Effects on Sol-Gel Film. Comparison of the bare electrode response to Ru(bipy)32+ with the sol-gel electrode response shows that Ru(bipy)32+ preconcentrates into the solgel film to enhance the current. We attribute this behavior to the interaction of positively charged Ru(bipy)32+ with negatively charged SiO- sites on the surface of the sol-gel film. Since these sites result from deprotonation of SiOH sites, this electrostatic interaction between sol-gel and analyte should be pH dependent in a range determined by the pKa of SiOH. To verify this, a pH study was carried out to determine the effect of pH on the uptake of Ru(bipy)32+. Sol-gel-modified graphite electrodes were immersed into 2.8 mM Ru(bipy)32+, 0.1 M KCl solutions of varying pH for 10 min for uptake. The pH values for the solutions had been adjusted from 1 to 11 using concentrated HCl or NaOH. A voltammogram was taken for each electrode after waiting 10 min for equilibration to occur. Four new electrodes were used for each pH. Figure 3 shows the peak current measured from the voltammograms vs the solution pH. The shape of the plot is that of a pH titration curve, with a sharp increase in peak current in the 706 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Figure 3. Effect of solution pH on pure sol-gel-modified electrodes in 2.8 mM Ru(bipy)32+, 0.1 M KCl. Working electrode: sol-gelmodified graphite; film coating at 3500 rpm, 2 min; immersion time, 10 min. Error bars show standard deviation for four different electrodes. OSWV settings as in Figure 1.
pH 3-4 range and a more gradual increase in the pH 4-8 range. Between pH 1 and 3, the peak current is small, indicating that, under such acidic conditions, the silanol groups were essentially all protonated and the electrode process was predominately controlled by diffusion of Ru(bipy)32+ ions in and out of the neutral sol-gel film. The peak current is about the same as that at a bare graphite electrode. Between pH 3 and 5, there is a sharp upturn in the curve, and the peak current increased 4 times higher. Apparently, a certain chemical form of the surface silanol groups in the sol-gel films was deprotonated, and the preconcentration process by electrostatic attraction between the negatively charged silanol groups and positively charged Ru(bipy)32+ ions increased as more silanol groups were deprotonated by the increasing basicity. The pKa for this type of silanol group, as calculated from Figure 3, is 3.8. The current continues to increase in the pH range 4-8, albeit less dramatically. This behavior is consistent with a variety of chemically different silanol functionalities with a range of pKa values.37 The deprotonation of surface silanol groups has been studied in the field of capillary zone electrophoresis (CZE), since the negatively charged SiO- groups are the main driving force for electroosmotic flow. It is interesting to compare our results with those published by Lukacs and Jorgenson, who studied the effect of pH on the rate of electroosmotic flow driven by silica surface silanol groups.38 They report a similarly shaped “titration curve” for flow rate vs pH. However, the pKa of the silica surface silanol groups contributing to the main break in their curve was 6, two units higher than our results. Presumably this is due to the different morphology between our macroporous sol-gel film and fused silica glass.37 However, this is a phenomenon that requires further investigation. Nonetheless, it is apparent that pH is an important factor in determining the role that the sol-gel itself plays in preconcentrating cations. (iv) Sol-Gel Electrodes Doped with PVSA. The sol-gel electrodes doped with PVSA were evaluated with Ru(bipy)32+ as analyte in 0.1 M KCl solution. Figure 1A-3 shows a voltammogram for a sol-gel-PVSA-modified electrode after immersion in 2.8 mM Ru(bipy)32+. A well-defined cathodic wave for the reduction of (37) Poole, C. F.; Poole, S. K. Chromatography Today; Elsevier: New York, NY, 1991; p 311. (38) Lukacs, K. D.; Jorgenson, J. W. J. High Resolut. Chromatogr. Chromatogr. Commun. 1985, 8, 407.
Figure 4. Peak current response for uptake and expulsion of Ru(bipy)32+ at sol-gel-PVSA-modified electrode. Electrode immersed in (×) 2.8 mM Ru(bipy)32+, 0.1 M KCl for 60 min and then (b) transferred to 0.1 M KCl. Electrodes immersed in (2) 28 µM, (9) 2.8 µM, and ([) 2.8 nM Ru(bipy)32+, 0.1 M KCl. Film coating at 3500 rpm for 2 min. Error bars show standard deviation for four different electrodes. OSWV settings as in Figure 1.
Ru(bipy)32+ was obtained at the sol-gel-PVSA-modified electrode, with a peak potential (+1.03 V) close to that for the solgel electrode (+1.03) and bare electrode (+1.04 V). The sensitivity achieved with the sol-gel-PVSA electrodes was higher than that possible with the bare and sol-gel-modified electrodes, indicating that PVSA enhances the partitioning of Ru(bipy)32+ into the film. Figure 4 shows a plot of peak current vs the exposure time for three different concentrations of Ru(bipy)32+ solutions at sol-gel-PVSA-modified electrodes. The general behavior is a rapid increase in peak current in the first few minutes and then a leveling off with, perhaps, a very slow increase during the rest of the hour. The plateaus are in proportion to the concentration of Ru(bipy)32+. The voltammogram for the 2.8 nM solution is essentially indistinguishable from background. Regeneration of the sol-gel-PVSA electrode was attempted by soaking the electrode in supporting electrolyte, 0.1 M KCl. Figure 4 shows a sharp decrease in peak current within a few minutes and then a slow decrease over the rest of the hour. It is apparent that, even after 1 h of soaking, a substantial amount of Ru(bipy)32+ is still in the film. To rapidly expel more of the Ru(bipy)32+, a higher concentration salt solution was tried. The voltammograms in Figure 1B show how the peak current for the Ru(bipy)32+/Ru(bipy)3+ couple decreased after 10 min in 0.1 M KCl (voltammogram 1), 60 min in 0.1 M KCl (voltammogram 2), and 10 min in 3.0 M NaCl (voltammogram 3). It is apparent that even exposure to high concentrations of salt does not rapidly remove all of the Ru(bipy)32+. About 16% of the Ru(bipy)32+ remains in the film after 10 min of soaking in 3.0 M NaCl, as determined from the peak heights for voltammogram 3 in Figure 1A and voltammogram 3 in Figure 1B. (v) Calibration Plots. The analytical characteristics of the bare and modified electrodes were evaluated by measuring peak current as a function of Ru(bipy)32+ concentration. The modified electrodes were immersed in each solution for 10 min before recording a voltammogram, which allowed plenty of time to reach the maximum current. Log-log calibration plots are shown in Figure 5, and pertinent information from the experiments is listed in Table 2. The solid line defines the linear region for each plot in Figure 5. It can be seen that the range for the bare electrode is from ∼2 × 10-5 to 2 × 10-3 M, and the sensitivity, which is
Figure 5. Calibration curves for Ru(bipy)32+ at (2) bare electrode, (9) sol-gel-modified electrode, and (×) sol-gel-PVSA-modified electrode (exposure time, 10 min). Film coating at 3500 rpm for 2 min. OSWV settings as in Figure 1.
taken as the slope of the linear part of the peak current vs concentration plot, is 0.05 A/M. The calibration curve for bare electrodes levels off at higher concentration (0.01 M). Comparison with the plot for the undoped sol-gel-modified electrode shows a 4-fold increase in sensitivity and a substantial improvement in detection limit. It is apparent that the sol-gel film enhances electrode performance at the lower concentration levels. This is due to the effect of negatively charged SiO- groups present in the sol-gel film. This surface charge would tend to attract positively charged Ru(bipy)32+. Thus, the undoped sol-gel film would exhibit an ion-exchange interaction with Ru(bipy)32+ that would increase its concentration in the film (Figure 2). As the dashed line shows, the linear response of the sol-gel-modified electrode is lost above ∼2 × 10-4 M Ru(bipy)32+. Presumably, this is due to saturation of SiO- sites with Ru(bipy)32+. In fact the sol-gel calibration curve crosses the bare electrode curve at higher concentration, so the film has a deleterious effect on sensitivity at sufficiently high concentrations, where it becomes saturated. This loss of sensitivity is attributed to the physical structure of the sol-gel impeding mass transport of Ru(bipy)32+ to the electrode surface by blocking some of the electrode surface and/or diminishing the effective diffusion coefficient of Ru(bipy)32+ in the sol-gel film. Doping the sol-gel with PVSA further enhances the sensitivity within the linear range (2.8 × 10-7-2.8 × 10-4 M, R2 ) 0.99) and improves the detection limit by about another order of magnitude. This improvement is attributed to the additional negatively charged ion-exchange sites in the film provided by the added PVSA and, perhaps, a stronger interaction of Ru(bipy)32+ with SO3compared with SiO-. Comparison in Table 2 of the three types of electrodes shows a sensitivity enhancement of 9 between a bare electrode and a sol-gel-PVSA electrode and an improvement in detection limit of 2 orders of magnitude. Reproducibility of the electrode fabrication procedure was reasonably good. For example, the peak current could be reproduced at different electrodes with a precision of 10% at a concentration of 2.8 × 10-7 M. The electrodes were also reasonably rugged. Results for a given electrode could be reproduced 1 week later. This suggests that PVSA may not be leaching out of the sol-gel matrix of the film on this time scale. Electrode response begins to drop off after one week. Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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Table 2. Detection Limits, Sensitivity, and Enhancement Factor for Different Analytes at Bare, Sol-Gel, and Sol-Gel-Ionomer Electrodes analyte
exposure time (min)
electrode
DLa (M)
sensitivityb (A/M)
enhancementc
Ru(bipy)32+/Ru(bipy)3+
10
Fe(CN)64-/ Fe(CN)63-
10
bare sol-gel sol-gel-PVSA bare sol-gel sol-gel-PDMDAAC
1 × 10-5 2 × 10-6 2 × 10-7 2 × 10-5 3 × 10-5 7 × 10-7
0.052 ( 0.001 0.23 ( 0.01 0.45 ( 0.02 -0.014 ( 0.001 -0.0028 ( 0.0000 -0.87 ( 0.04
1 4 9 1 0.2 62
a Detection limit calculated by IUPAC method. b Calculated as the slope of the linear portion of peak current vs concentration plots. c Ratio of sensitivity to bare electrode.
Figure 7. Peak current response for uptake and expulsion of Fe(CN)64- at sol-gel-modified electrode. (9) Electrode immersed in 1.0 mM Fe(CN)64-, 0.1 M KCl for 60 min and then (b) transferred to 0.1 M KCl. Film coating at 3500 rpm for 2 min. Error bars show standard deviation for four different electrodes. OSWV settings as in Figure 6.
Figure 6. (A) Square wave voltammograms for 1.0 mM Fe(CN)64-, 0.1 M KCl at (1) bare graphite electrode, (2) sol-gel-modified electrode, and (3) sol-gel-PDMDAAC-modified electrode; exposure time, 10 min. (B) Removal of the analyte from sol-gel-PDMDAACmodified electrode (1) after 10 min in 0.1 M KCl, (2) after 60 min in 0.1 M KCl, and (3) after 10 min in 3.0 M NaCl. Film coating at 3500 rpm for 2 min. OSWV: scan from -0.4 to +0.7 V; scan rate, 4 mV s-1; SW amplitude, 25 mV; frequency, 14 Hz.
Ferrocyanide at Bare, Sol-Gel-Modified, and Sol-GelPDMDAAC-Modified Electrodes. (i) Characterization at Bare Electrodes. A typical voltammogram of the bare graphite electrode in a 1.0 mM solution of Fe(CN)64- is shown in Figure 6A-1. A well-defined anodic wave for oxidation of Fe(CN)64- was obtained at +0.21 V. (ii) Characterization at Sol-Gel-Modified Electrodes. Immersion of a sol-gel modified electrode into this same solution gave a barely detectable peak after 10 min (Figure 6-A-2). Subsequent repetitive voltammograms recorded showed an increase in peak height as Fe(CN)64- slowly diffused through the sol-gel film (Figure 7). Equilibration of Fe(CN)64- in the solgel film had not been reached after 60 min immersion, in comparison to the equilibration times of only a few minutes for Ru(bipy)32+. This different behavior between Ru(bipy)32+ and 708 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Fe(CN)64- is attributed to the effect of negatively charged SiOgroups present in the sol-gel film. Whereas this surface charge would attract positively charged Ru(bipy)32+, it would repel negatively charged Fe(CN)64-. The peak current for Fe(CN)64- decreased very slowly when the sol-gel modified electrode was rinsed with de-ionized water and immersed in supporting electrolyte solution, after having been immersed in the analyte solution for 60 min (Figure 7). The peak current decreases only about 20% during the first 10 min and then plateaus. In contrast, 85% of Ru(bipy)32+ was expelled from the sol-gel film within 10 min of immersion of the analyte-containing sol-gel film in supporting electrolyte. (iii) Sol-Gel Electrodes Doped with PDMDAAC. The effect of addition of PDMDAAC to the sol-gel film is illustrated in Figure 6A, which shows a voltammogram for a sol-gelPDMDAAC-modified electrode after a 10 min immersion in 1.0 mM Fe(CN)64-. Peak currents for the bare, sol-gel-modified, and sol-gel-PDMDAAC-modified electrodes are -12.8, -4.94, and -222 µA, respectively. The peak current for the sol-gelPDMDAAC electrode is significantly greater than those for the other two electrodes. This substantial increase in sensitivity for the sol-gel-PDMDAAC electrode is attributed to partitioning of Fe(CN)64- into the film by anion-exchange with the positively charged PDMDAAC. Figure 8 shows a plot of peak current vs the exposure time for PDMDAAC-doped sol-gel electrodes in 1 mM Fe(CN)64- solution. Peak current increased rapidly and had reached a plateau by about 10 min, at which time it essentially leveled off. This behavior is similar to that observed for the Ru(bipy)32+-PVSA system.
Figure 8. Peak current response for uptake and expulsion of Fe(CN)64- at sol-gel-PDMDAAC-modified electrodes. Electrode immersed in (9) 1.0 mM Fe(CN)64- 0.1 M KCl for 60 min, then (b) transferred to 0.1 M KCl for 60 min, and then transferred to 3 M NaCl for 10 min. Film coating at 3500 rpm for 2 min. Error bars show standard deviation for four different electrodes. OSWV settings as in Figure 6.
Regeneration of the sol-gel-PDMDAAC film was attempted by immersing the electrodes in the 0.1 M KCl supporting electrolyte solution to reverse the ion-exchange reaction. As shown in Figure 8, about a 5% decrease in current occurred in the first 10 min, but the current then leveled off at a relatively high value. Even after immersion of the electrode in the supporting electrolyte solution for 60 min, the peak current was almost the same as that after only 10 min, as shown by the voltammograms in Figure 6B. Apparently, the ionic strength of the solution was not high enough to expel Fe(CN)64- from the sol-gel-PDMDAAC electrode. When the ionic strength was increased to 3.0 M NaCl, a substantial decrease in peak current occurred rapidly as Cl- was now able to replace Fe(CN)64-, as shown in Figures 6B and 8. However a small peak current (-15 µA) persisted, indicating that even this high concentration of Clwas unable to completely displace the Fe(CN)64- from the film. (iv) Calibration Plots. The analytical capabilities of the bare and modified electrodes were evaluated by measuring peak current as a function of Fe(CN)64- concentration. The modified electrodes were immersed in each solution for 10 min before recording a voltammogram, which allowed plenty of time for the sol-gel-PDMAAC electrodes to reach the maximum current. Log-log calibration plots are shown in Figure 9, and pertinent information from the experiments is listed in Table 2. The solid line defines the linear region for each plot in Figure 9. The peak current for the sol-gel-PDMDAAC-modified electrode increased linearly with the Fe(CN)64- concentration from 1 × 10-6 to 1 × 10-4 M (R2 ) 0.98), as shown in Figure 9. Above that concentration, response dropped off, presumably due to saturation of ionexchange sites in the film. The improvement in sensitivity by addition of PDMDAAC to the sol-gel is clearly seen by comparison with the calibration plots for the bare and the sol-gel-modified electrodes. Table 2 gives the sensitivities of the three electrodes and an enhancement factor, which is normalized relative to the bare electrode. The sensitivity for the sol-gel-PDMDAAC electrode shows an enhancement of 62 and an improvement in detection limit of almost 2 orders of magnitude compared to those of the bare and sol-gel-modified electrodes. It is apparent from a comparison of the sensitivities of the linear regions of the plots that the effect of the undoped sol-gel film is
Figure 9. Calibration curves for Fe(CN)64- at (2) bare electrodes, (9) sol-gel-modified electrodes (exposure time, 10 min), and (×) solgel-PDMDAAC-modified electrodes (exposure time, 10 min). Film coating at 3500 rpm for 2 min. OSWV settings as in Figure 6.
to diminish the current response in comparison with a bare electrode. This is due to the slow electrode response caused by electrostatic repulsion by SiO- on the anionic analyte. It is clear from these results that this decrease in sensitivity caused by coating the sol-gel on a bare electrode is more than compensated for by incorporation of the anion-exchange polymer PDMDAAC into the sol-gel film. Apparently, the number of active anionexchange sites from the polymer is greater than the number of negatively charged sites from SiO-. In fact, some of the PDMDAAC may serve to essentially neutralize the SiO- sites. Reproducibility of the electrode fabrication procedure was reasonably good. The peak current could be reproduced at different electrodes with an 18% relative standard deviation at a concentration of 1 × 10-6 M. The electrodes were also reasonably rugged. Results for a given electrode could be reproduced a week later. Thus, as was also the case with PVSA, it appears that PDMDAAC may not leach out of the sol-gel matrix on this time scale. It is interesting to compare results for the sol-gel-PDMDAAC electrodes with earlier results on spectroscopic graphite electrodes coated with pure PDMDAAC polymer networks formed by γ radiation cross-linking.32 These electrodes showed substantial uptake of Fe(CN)63- by cyclic voltammetry when immersed in 0.4 mM Fe(CN)63-, 0.2 M KNO3. A sensitivity enhancement of 26 compared to that of a bare electrode was obtained, which is less than that obtained at the sol-gel-PDMDAAC electrodes. However, this concentration of Fe(CN)63- may be above the linear range of a peak current vs concentration plot for the polymer electrode, which was not investigated in that study. Consequently, the sensitivity enhancement of the pure polymer electrode may be larger than 26 at lower concentrations, if it behaves like the sol-gel-PDMDAAC coatings do. A shift in E°′ from +0.22 to +0.08 V vs Ag/AgCl was observed at the pure polymer electrode and was interpreted as a stabilization of the oxidized form by the polymer film. A similar shift in peak potential (from +0.21 V at the bare electrode to +0.12 V vs Ag/AgCl for the sol-gelPDMDAAC electrode) is observed in the present study. This provides evidence that the PDMDAAC in the sol-gel matrix is behaving in a manner similar to that of PDMDAAC in a pure polymer network. In the case of the polymer network, which was a flexible hydrogel, substantial shrinkage of the film was observed as it accumulated Fe(CN)63-. However, no shrinkage of the solAnalytical Chemistry, Vol. 69, No. 4, February 15, 1997
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gel-PDMDAAC films, which are rigid, occurred. The absence of shrinking/swelling could be advantageous for applications in which structural rigidity of a sensor is required. CONCLUSIONS In the sol-gel network doped with polymers, organic and inorganic components can be combined on a molecular level, and the properties of these hybrid materials can be easily tailored by using functionalized polymers. In this work, sol-gel doped with PDMDAAC and PVSA served as ion-exchangers to produce ionexchange polymer-modified electrodes that may be useful as sensors for electroactive ions. The doped sol-gel films can preconcentrate ions, causing the electrochemical detection limit for an analyte at a doped sol-gel-modified electrode to be orders of magnitude lower than the detection limit at analogous bare and undoped sol-gel-modified electrodes. Detection limits of 10-7 M could be achieved with the sol-gel-ionomer-modified electrodes for Ru(bipy)32+. This detection limit compares favorably with those for several polymer-modified electrodes, with ion-exchange properties, that have been reported in the literature. Detection (39) Whiteley, L. D.; Martin, C. R. Anal. Chem. 1987, 59, 1746-1751. (40) Moussy, F.; Harrison, D. J.; Rajotte, R. V. Int. J. Artif. Organs 1994, 2, 88. (41) Gorsky, W.; Cox, J. A. Anal. Chem. 1992, 64, 2706. (42) Hernandez, P.; Alda, E.; Hernandez, L. Fresenius J. Anal. Chem. 1987, 327, 676. (43) Abruna, H. D. In Electroresponsive Molecular and Polymeric System; Skotheim, T. A., Ed.; Marcel Dekker: New York, NY, 1988; Vol. 1, p 1. (44) Kalcher, K. Analyst 1986, 111, 625. (45) Doug, K. L.; Kryger, L.; Christensen, J. K.; Thomson, K. N. Talanta 1991, 8, 101.
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limits of about 10-9 M were achieved by using carbon fibers or glassy carbon electrodes modified with Nafion39-41 in the determination of different analytes in solution. Detection limits of about 10-7 M were achieved with carbon paste electrodes modified with zeolites,42-44 and a detection limit of 10-10 M was achieved for the determination of Bi(II) in reference materials using a carbon paste electrode modified with Dowex 50-X8.45 The response times of the sol-gel-PVSA- and the sol-gelPDMDAAC-modified electrodes were about 5 and 10 min, respectively. This is considerably slower than the response at a bare electrode, which is faster than can be measured by the simple immersion techniques employed here (i.e., less than a few seconds). Since the response time is determined primarily by mass transport within the film, thinner films should give faster response times, which would be important for certain applications. Furthermore, these sol-gel-polymer-modified electrodes offer greater structural rigidity compared with pure polymer network films, which exhibit substantial shrinking/swelling due to the hydrophilicity of PVSA and PDMDAAC. ACKNOWLEDGMENT Support provided by the Department of Energy (Grant No. DEFG07-96ER 62311) is gratefully acknowledged. Received for review August 16, 1996. Accepted November 27, 1996.X AC960839Q X
Abstract published in Advance ACS Abstracts, January 1, 1997.
