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Electrochemical Studies of Surface-Modified Glass Bead/Nafion Composites Sam J. Brancato and Shelley D. Minteer* Department of Chemistry, Saint Louis University, St. Louis, Missouri 63103 Received March 22, 2001. In Final Form: June 7, 2001 In the past decade, electrochemists have begun modifying electrode surfaces with polymer composites. Each of those composites had advantages over the standard polymer modified electrode. Usually, the major advantage is increased sensitivity. However, years of chromatographic research have proven that tailoring the surface of glass beads can increase the selectivity of a system. The same principle should be applicable to polymer film composites. This research is based upon making glass bead/ion exchange polymer composites with unique interfaces that alter the selectivity of redox species on the basis of physical and chemical characteristics. This is accomplished by modifying the surface of glass beads with organosilanes of varying chain lengths and functional groups. The significance of this research lies in its ability to alter the selectivity of polymer modified electrodes. A surface-modified glass bead/Nafion composite was designed that excludes Ru(bpy)32+ from the film. The surface-modified glass bead/Nafion composites were able to alter electrochemical flux for hydroquinone, methylviologen, and Ru(bpy)32+. Composites have no significant effects on the electrochemical flux of Fe3+.

Introduction Composites are formed by mixing two or more immiscible materials. Two-component composites have three distinct regions. Each region has its own distinct chemical and physical properties. Two of the regions will be characteristic of the two respective materials. The third region is an interfacial region between the two materials. This region has chemical and physical properties which are distinctly different than those of the other two component materials. These unique properties are capable of altering the chemical selectivity, extraction properties, mass transport, and conduction of the material. In recent decades, electrodes have been modified to increase selectivity, to preconcentrate the electrode surface with redox species, and/or to act as electron-transfer mediators between the solution and the electrode surface.1 Modified electrodes are fabricated by adsorption, electroadsorption, physical placement, or chemical bonding of a species to the electrode surface. Electrodes are commonly modified with polymers (ion exchange, conducting, etc.), sol-gels, zeolites, or electroactive species.1 In recent years, researchers have started modifying electrodes with composite films. Composite modified electrodes have been composed of a variety of materials, including polystyrene bead/ion exchange polymer composites,2 neutron etched polycarbonate filter/ion exchange polymer composites,3conducting polymer/ion exchange polymer composites,4,5 sol-gel/ion exchange polymer composites,6,7 and other polymer/polymer composites.8-10 * Corresponding author. Phone: (314) 977-3624. Fax: (314)977-2521. E-mail: [email protected]. (1) Martin, C. R.; Foss, C. A. Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1996; Chapter 13. (2) Zook, L. A.; Leddy, J. J. Phys. Chem. B 1998, 102, 10013. (3) Fang, Y.; Leddy, J. J. Phys. Chem. 1995, 99, 6064. (4) Chang, C. M.; Huang, H. J. Anal. Chim. Acta 1995, 300, 15. (5) Li, G. C.; Pickup, P. G. J. Phys. Chem. B 1999, 103, 10143. (6) Hu, Z. M.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1998, 70, 5230. (7) Khramov, A. N.; Collinson, M. M. Anal. Chem. 2000, 72, 2943. (8) Gao, L.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1999, 71, 4061-4068.

Modifying electrodes with composites has been used to increase the mass transport through a modifying layer11 and to increase the physical stability and optical characteristics of the modifying layer.8 Up until now, composite modified electrodes have been used to increase electrochemical flux. However, composite modified electrodes can also enhance chemical selectivity if the interfacial region is tailored specifically to the redox species. The optimum composite modified electrode would increase mass transport of the analyte through the modifying layer, while selectively eliminating interferences from all other electroactive species in solution. This research involved analyzing polymer/glass bead composites to determine if surface modification of the glass bead can alter the flux of redox species through the modifying layer, thereby altering the selectivity of the modified electrode. The polymer used in this study was Nafion, a perfluorinated cation-exchange polymer. The glass beads were modified with commercially available organosilanes of different functional groups, including amino, chloro, methacryloxy, and phenyl functional groups. The silanol groups on the surface of the glass bead react with the organosilanes to form siloxane bonds. The process is relatively simple and has been used by chromatographic researchers for decades.12The terminal groups of the silanes interact differently with the ion-exchange polymer. This interaction will form distinctly different interfacial regions in the composite. These unique interfacial regions will interact differently with each redox couple, depending on the physical and chemical properties of the redox couple, thereby altering the extraction of the redox couple into the polymer and the mass transport of the redox couple to the electrode surface. (9) Benmakroha, Y.; Christie, I.; Desai, M.; Vadgama, P. Analyst 1996, 121, 521-526. (10) Dasenbrock, C. O.; Ridgway, T. H.; Seliskar, C. J.; Heineman, W. R. Electrochim. Acta 1998, 43, 3497-3502. (11) Leddy, J. Langmuir 1999, 15, 710. (12) Poole, C. F.; Poole, S. K. Chromatogr. Today; Elsevier: New York, 1991.

10.1021/la010431a CCC: $20.00 © 2001 American Chemical Society Published on Web 09/05/2001

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Experimental Section Microparticles. Glass microspheres were used as received from Polysciences, Inc. The microspheres were 3-10 µm in diameter. The glass microspheres were coated with organosilanes as per the procedure described in ref 13.13 This was accomplished by suspending the glass microspheres in a 50/50 solution of an organosilane/toluene and stirring for 4 h. The glass microspheres were then rinsed several times with toluene and dried in a vacuum desiccator. The organosilanes employed were (3-aminopropyl)trimethoxysilane (APTMOS; Aldrich), (3-methacryloxypropyl)trimethoxysilane (MAOTMOS, Kodak), (2-phenylethyl)trimethoxysilane (PETMOS, Fluka), and (3-chloropropyl)trimethoxysilane (CPTMOS, Fluka). Reagents. The redox couples were tris(2,2′-bipyridyl) ruthenium (II) chloride hexahydrate (Aldrich), hydroquinone(Aldrich), iron(III) perchlorate(Aldrich), tris(2,2′-bipyridyl) iron (III) hexafluorophosphate (Aldrich), tris(1,10-phenathroline) iron (II) hexafluorophosphate (Aldrich), and methyl viologen dichloride (Aldrich). All solutions were made in Nanopure water. The concentration of the redox species was 1.0 mM with 0.10 M electrolyte. Nitric acid electrolyte was used for hydroquinone. Sodium sulfate electrolyte was used for the other redox couples. Electrode Preparation. Glass bead/Nafion composites were formed by co-casting the silane coated glass beads with 5% by wt. suspension of Nafion 1100 (Solution Technologies). Glassy carbon working electrodes (area ) 0.1986 cm2) were polished successively with 1.0 and 0.05 µm alumina on polishing cloths (Buehler). The electrodes were soaked in concentrated nitric acid and thoroughly rinsed with Nanopure water. Nafion films were prepared by pipetting a 2.5-µL volume of 5% by weight Nafion suspension onto the electrode surface. This will produce Nafion films that are 3.5 µm thick after solvent evaporation. The glass bead/Nafion composites were prepared by adding the appropriate fraction of 5% by weight Nafion suspension and dry, silane-coated glass beads together. The fractions are calculated so that after solvent evaporation the composite film is composed of 15 vol % glass beads and 85 vol % Nafion. A 2.5-µL volume of the glass bead/Nafion casting solution was pipetted onto the electrode surfaces. All modified electrodes were dried in a vacuum desiccator before use. Electrochemical Measurement. Flux of the redox species through the composite layer to the electrode surface was studied by using cyclic voltammetry. Nafion films and glass bead/Nafion composite modified working electrodes were equilibrated in 1.0 mM redox species and 0.1 M electrolyte before measurements were made. The reference electrode was a saturated calomel electrode. The counter electrode was a large platinum gauze. Data were collected and analyzed on a Pentium computer interfaced to a CH Instruments Potentiostat model 650 A. Cyclic voltammetry was performed at five scan rates ranging from 50 to 250 mV/s. These scan rates are fast enough that diffusion is confined inside the film. In cyclic voltammetry, electrochemical flux is determined from peak currents (ip), which are a function of the extraction coefficient (K) and the apparent diffusion coefficient (D) of the redox probe in the modifying layer:

Flux )

ip nFA

ip ) (2.69 x 105)n3/2AD1/2v1/2KC

(1) (2)

where n is the number of electron transferred, F is Faradays constant, A is the area of the electrode, v is the scan rate, and C is the concentration of redox species in solution. At least three replicates of each cyclic voltammetric experiment were performed; reported uncertainties correspond to one standard deviation. Rotating disk experiments were not performed due to the instability of the composite films over the long equilibration times.

Results and Discussion Selectivity in electrochemical methods is more complicated than chromatographic methods, because the current (13) Emoto, K.; Harris, J. M.; VanAlstine, J. M. Anal. Chem. 1996, 68, 3751.

Table 1. Summary of KD1/2 Values for Each Redox Couple in Each Composite

APTMOS MAOTMOS CPTMOS PETMOS Nafion

Ru(bpy)32+, 10-3 cm2/s

Fe3+, 10-2 cm2/s

H2Q, 10-3 cm2/s

MV2+, 10-2 cm2/s

1.75 ( 0.47 0.00 ( 0.00 1.54 ( 0.47 1.32 ( 0.35 1.46 ( 0.01

4.21 ( 0.85 4.47 ( 0.65 3.91 ( 1.29 5.01 ( 0.52 5.20 ( 0.25

1.34 ( 0.26 2.36 ( 0.79 3.71 ( 0.92 1.80 ( 0.49 3.00 ( 0.25

1.81 ( 0.37 2.13 ( 0.55 2.58 ( 0.19 1.78 ( 0.14 2.06 ( 0.29

densities are a function of both the extraction coefficient (K) and the diffusion coefficient (D) of the redox species. The selectivity factor (R) in chromatographic science is the ratio of the extraction coefficient for species A (KA) to the extraction coefficient for species B (KB). If we were able to construct a composite with a chromatographic selectivity factor of 5 for species A vs species B, it would only be useful in cyclic voltammetry if the ratio of D1/2C of species A over D1/2C of species B is larger than 0.2.

DA1/2CA DB1/2CB

g 0.2

(3)

where CA is the bulk concentration of species A in the solution and CB is the bulk concentration of species B in the solution. Since we are not solely affecting the extraction coefficient by making a composite, we do not determine the chromatographic selectivity factor. These redox couples were not used to test their electrochemical selectivity among each other; they were used to illustrate the alteration of the electrochemical flux through the polymer film by addition of surface-modified glass beads. Illustrating the alteration of the electrochemical flux of a single redox couple is the first step in studying the glass bead/ ion exchange polymer modified electrode’s ability to alter electrochemical selectivity of multicomponent systems. Each of the four redox couples was studied with a Nafion modified electrode and each of the four organosilanemodified glass bead/Nafion composite modified electrodes. The KD1/2 values for each redox couple/organosilane are shown in Table 1. The KD1/2 values were determined from the slope of the peak current (ip) versus the square root of the scan rate (v1/2) plot. The flux of each redox couple through a film or a composite is directly proportional to the KD1/2 values, provided the bulk concentration is fixed in solution. The uncertainties in these measurements are large due to the complex nature of the composite and the variability of temperature/humidity in the casting and measurement conditions. The composites show visible variations in the clumping of beads in the center of the composite. Figure 1 shows the KD1/2 values of each redox couple. These data shows that Ru(bpy)32+/3+ can be electrochemically excluded from a methacryloxypropyltrimethoxysilane-modified glass bead/Nafion composite (MAOTMOS) electrochemistry. The three other organosilane-modified glass bead/Nafion composites show no more or less selectivity toward Ru(bpy)32+/3+ than a Nafion film does. Figure 2 shows representative cyclic voltammograms of Ru(bpy)32+/3+ in a MAOTMOS composite and a Nafion film. The MAOTMOS composite is without a forward or reverse peak. Even after equilibrating the MAOTMOS composite for several days, no visible peaks were detected across the potential window. Upon inspection of the electrode, the MAOTMOS composite does not have the dark orange color that a Nafion film has after it has been equilibrated in 1.0 mM Ru(bpy)32+/3+ and electrolyte. Results are consistent with the MAOTMOS composite excluding Ru(bpy)32+/3+ from the film.

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Figure 1. Summary of KD1/2 values for each redox couple in each composite.

Figure 2. Cyclic voltammograms of a MAOTMOS composite and a Nafion film in 1.0 mM Ru(bpy)32+ and 0.1 M nitric acid. Scan rate: 100 mV/s.

All previously discussed MAOTMOS composites were 15 vol % beads in Nafion. Five percent by volume MAOTMOS/Nafion composites were studied and they showed 48.2 ( 6.8% of the flux of Ru(bpy)32+/3+ through the film. One percent by volume MAOTMOS/Nafion composites showed 88.2 ( 8.02% of the flux of Ru(bpy)32+/3+ through the film. These data are consistent with a gradual decrease in the flux of Ru(bpy)32+/3+ through the film as the volume of the MAOTMOS coated beads is increased from 0% to 15%. This is expected because the volume of the interface is increasing with increasing glass bead volume. Previous studies of polystyrene bead/Nafion composites have shown that electrochemical flux increases as the volume fraction of microparticles in a Nafion matrix increases.2 This study is the first system where the electrochemical flux decreases as the volume fraction of microparticles in the Nafion matrix increases. Tris(2,2′-bipyridyl)ruthenium is the largest redox species studied. The estimated diameter of the ion is 13.6 x 10-8 cm. The literature values for the apparent diffusion coefficient of Ru(bpy)32+/3+ through a pure Nafion film range from 4 x 10-10 cm2/s14 to 1.2 x 10-8 cm2/s.15 Although an exact apparent diffusion coefficient has not been agreed upon, it is agreed that hydrophobic Ru(bpy)32+/3+ is strongly bound in Nafion, which significantly decreases its mass transport through the film.16 However, extraction coefficients of Ru(bpy)32+/3+ into the film are large, ranging (14) White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4811-4817. (15) Buttry, D. A.; Anson, F. C. J. Electroanal. Chem. 1981, 130, 333-338. (16) Shi, M.; Anson, F. C. Anal. Chem. 1997, 69, 2653-2660.

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from 500 to 700 for 1 and 0.5 mM solutions.14 Since there are no oxidation or reduction peaks for Ru(bpy)32+/3+ in a MAOTMOS composite and no characteristic color change of the film, it can be said that the MAOTMOS composite significantly suppresses the extraction of Ru(bpy)32+/3+ into the film. This significant decrease is likely due in part to size exclusion. The cyclic voltammetry of Fe(phen)32+ in a Nafion film and all four composites showed no oxidation or reduction peaks. This is contrary to the results of Martin and Dollard17 because the Nafion casting solutions and procedures were different in this study. Kaneko et al. has shown that the flux of redox species through Nafion films is critically dependent on the casting solutions and procedures.18 This study used commercially available 5.0% by weight Nafion suspensions that were dried in a vacuum desiccator, whereas Martin and Dollard used recast Nafion sheets in 0.6% by weight Nafion suspensions (in 50:50 ethanol-water solvent) that were dried in air. This is the reason for these contrary results. Fe(phen)32+ is only slightly larger than Ru(bpy)32+/3+ and has the same hydrophobic characteristics. The cyclic voltammetric results show that a small decrease in the pore size (or interface width) will likely have a dramatic decrease in the extraction coefficient of large moieties. Since Ru(bpy)32+/3+ and Fe(bpy)33+/2+ are approximately the same size, the cyclic voltammetry of Fe(bpy)33+/2+ is similar in a Nafion film and a MAOTMOS composite. Interestingly, the electrochemical flux of Fe(bpy) 33+/2+ through a MAOTMOS composite is 40.9 ( 20.8% of the flux through a Nafion film. Since the formal potential of Fe(bpy)33+/2+ is 300 mV farther from the solvent limit than that of Ru(bpy)32+/3+, there are far fewer background problems in examining the electrochemical flux through Nafion films and MAOTMOS composites. It should be noted that there is a visible color change between the Nafion film and the MAOTMOS composite after the extraction of Fe(bpy)33+. These data support the previous data that MAOTMOS composites can diminish the flux of tris(2,2′-bypyridyl) composites through the film. The KD1/2 values for Fe3+/2+ show that the organosilanemodified glass bead/Nafion composites and Nafion are not statistically different at the 90% confidence level using the t test. Iron is the most hydrophilic of all the redox couples. Since hydrophilic redox couples have a tendency to extract to high degree in hydrophilic films, we would not expect to significantly decrease the flux of Fe3+/2+ through the film, unless the interface became significantly more hydrophobic. Further studies are being done on Fe3+/2+ in surface-modified glass bead/polymer composites of less hydrophilic cation exchange polymers to see if fluxes could be altered. The hydroquinone data are by far the most interesting. The aminopropyltrimethoxysilane-modified glass bead/ Nafion composite (APTMOS) decreases the flux of hydroquinone to 45% of the flux of hydroquinone in a Nafion film. Figure 3 is a representative cyclic voltammogram of hydroquinone in an APTMOS composite and a Nafion film. The phenylethyltrimethoxysilane-modified glass bead/ Nafion composite (PETMOS) decreases the flux of hydroquinone to 60% of the flux of hydroquinone in a Nafion film. Figure 4 is a representative cyclic voltammogram of hydroquinone in a PETMOS composite and a Nafion film. A close study of Figures 3 and 4 shows that the PETMOS composite has lower flux than the Nafion film, but the (17) Martin, C. R.; Dollard, K. A. J. Electroanal. Chem. 1983, 159, 127-135. (18) Zhan, J.; Zhao, F.; Kaneko, M. Electrochem. Acta 1999, 44, 33673375.

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Figure 5. Relative electrochemical fluxes of redox couples through composites versus a Nafion film. Figure 3. Cyclic Voltammograms of a PETMOS composite and a Nafion film in 1.0 mM hydroquinone and 0.1 M nitric acid. Scan rate: 100 mV/s.

exchange polymer composites may not be able to electrochemically exclude methylviologen as a chemical interference, these data show that an electrode array of different surface-modified glass bead/ion exchange polymer modified electrodes may be useful. Figure 5 is plot of percent relative flux of each redox couple in each composite. The relative fluxes are determined by using the following equation and the data in Table 1:

(KD1/2)comp

% Fluxcomp ) 100%

Figure 4. Cyclic Voltammograms of an APTMOS composite and a Nafion film in 1.0 mM hydroquinone and 0.1 M nitric acid. Scan rate: 100 mV/s.

peaks retain the same shape. The APTMOS composite also has lower flux than a Nafion film, but the peak shapes have been altered significantly. Upon titration of the number of available exchange sites (SO3-) using the procedure describe by Chen and Leddy,19 we found that the APTMOS composites have only 49.7 ( 0.9% of the available exchange sites of a Nafion film. This is likely due to a strong interaction between the amino function group on the organosilane and the sulfonic acid group on Nafion. Since hydroquinone has very pH dependent electrochemistry,20 this shape change is probably due to a change in the pH environment due to changes in available exchange sites. Within the error in the experiment, there is no statistical difference (at the 90% confidence level) between the KD1/2 values of methylviologen in any of the composites and in a Nafion film. However, the flux of methylviologen in a CPTMOS is statistically different at the 99% confidence level than the flux of methylviologen in an APTMOS composite and a PETMOS composite. The flux of methylviologen in an APTMOS composite is 70% of the flux of methylviologen in a CPTMOS composite. The flux in a PETMOS composite is 69% of the flux in a CPTMOS composite. Although our surface-modified glass bead/ion (19) Chen, T. Y. Langmuir 2000, 16, 2866-2871. (20) Laviron, E. J. Electroanal. Chem. 1984, 164, 213.

(KD1/2)Naf

(4)

where (KD1/2)comp is the KD1/2 value of the redox couple in a composite and (KD1/2)Naf is the KD1/2 value of the redox couple in a Nafion film. A close examination of Figure 5 shows that each organosilane composite has measurable effects on the electrochemical flux of at least one redox couple through the modifying layer. For instance, the APTMOS composites significantly lower the flux of hydroquinone through the film. The MAOTMOS composites electrochemically exclude Ru(bpy)32+ from the system. The CPTMOS composite enhances the flux of methylviologen through the film, while decreasing the flux of Fe3+. The PETMOS composites show differing flux effects on hydroquinone than on Fe3+. Conclusions The electrochemical flux of hydroquinone, methylviologen, and Ru(bpy)3+2 through polymer-modified electrodes can be altered by incorporating surface-modified glass beads into the ion exchange polymer matrix. The electrochemical flux effects are not due solely to charge, size, or simple hydrophobicity. We have been able to design a cation-exchange polymer modified electrode that electrochemically excludes Ru(bpy)32+ from the polymer film. The significance of this research lies in the added selectivity of the electrochemical measurements provided by incorporating glass beads into polymer film modified electrodes. The surface of the glass beads can be modified with a limitless variety of silanes. This can offer ample versatility in designing selective electrodes for sensor applications. The goal is to create a passive sensor that can be employed for redox species without the use of other separation techniques. Surface-modified glass bead/ion exchange polymer composites offer a method to electrochemically exclude all other redox species from the electrode surface except for the species of interest. This

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allows for a mechanism based solely on altering diffusion and extraction. Further investigations of anion-exchange polymers and more hydrophobic cation-exchange polymers are needed to completely understand this phenomena. Investigation of multicomponent mixtures is necessary to determine

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the electrochemical selectivity effects of surface-modified glass bead/ion-exchange polymer composites. Acknowledgment. Financial support provided by Saint Louis University is greatly appreciated. LA010431A