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Partial charge transfer of poly(vinylferrocene)-coated latex particles adsorbed on pyrolytic graphite electrode
Electrochemistry Communications 5 (2003) 506–510 www.elsevier.com/locate/elecom
Partial charge transfer of poly(vinylferrocene)-coated latex particles adsorbed on pyrolytic graphite electrode Cuiling Xu, Jingyuan Chen, Koichi Aoki
*
Department of Applied Physics, Fukui University, 3-9-1, Bunkyo, Fukui-shi 910-8507, Japan Received 10 April 2003; received in revised form 2 May 2003; accepted 2 May 2003
Abstract As a model of an electrode reaction of a big particle, vinylferrocene immobilized on polystyrene latex particles were synthesized by copolymerization with styrene sulfonate and styrene. They had almost mono-dispersed spheres with 1.2 lm in diameter, and each had 3.1 107 ferrocene units. The particles adsorbed on pyrolytic graphite electrode (PGE) showed the redox activity for the ferrocene unit in NaBF4 aqueous solution. Particles without the sulfonate group had no electroactivity, and hence the electroactivity needs ionic micro-environment around the ferrocene unit. From the faradaic charge of the ferrocene unit, the electroactive sites per particle were estimated to be about 8% of the whole immobilized ferrocene units. A model of this partial charge transfer was proposed, in which the particles are adsorbed in hollows of the rough surface of the PGE. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Latex; Polystyrene; Immobilized poly(vinylferrocene); Polystyrene sulfonate
1. Introduction A large redox particle is a model of electrode reactions not only for nano-particles but also proteins and enzymes. If the particle is so large that it is visualized with an optical microscope, its temporal variations and spatial distributions in electrolyte solution may provide electrochemical images of mass transport, adsorption and charge transfer [1], mimic to the conventional behavior of real electroactive species. In order to realize the model experimentally, large particles should take a common geometry without size distribution, just as a molecule takes its own conformation and size. This requirement is satisfied with polymer microspheres, called latex particles [2,3]. The particles have to contain electroactive species for electrochemical monitorings. The electroactive species is desirable to be localized on the surface of the latex particle in order to prevent complications of charge transfer within the particles [4–6]. Examples of satisfying these conditions are a polystyrene-core coated with a polyaniline-shell [4,5] and the core
coated with poly(vinylferrocene)-shell [7]. The poly(vinylferrocene)-shell is closer to the ideal model than the polyaniline-shell in that the standard redox potential of poly(vinylferrocene) is not distributed so widely [8,9] as that in conducting polymers [10,11]. Unfortunately, the poly(vinylferrocene)-coated latex particle was electroinactive in aqueous solution because of the high hydrophobicity of the particle surface owing to the property of polystyrene that was required for copolymerization of poly(vinylferrocene). It is expected that the incorporation of a hydrophilic substitute such as sulfonate makes the particle surface ionic [12,13], and hence the substituted particles may become electroactive in aqueous solutions. In this report, the latex surface is sulfonated by copolymerization of styrene sulfonate and vinylferrocene (VFc). This particle is demonstrated to be adsorbed well on pyrolytic graphite electrode and to be partially electroactive.
2. Experimentals 2.1. Chemicals
*
Corresponding author. Tel.: +81-776-27-8665; fax: +81-776-278494. E-mail address: [email protected] (K. Aoki).
Styrene and nitrobenzene were purified as was mentioned [7]. The steric stabilizer, poly(N-vinylpyrrolidone)
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00112-7
C. Xu et al. / Electrochemistry Communications 5 (2003) 506–510
(PVP), with a molar mass of ca. 360 kg mol1 , the initiators, a-azoisobutyronitrile (AIBN) and potassium persulfate, were used as received. Vinylferrocene commercially available (TCI, Tokyo) had been stored at 3 °C before use. All the solvents used were of analytical grade. Aqueous solutions were prepared using ion-exchanged and distilled water. 2.2. Synthesis of poly(VFc)-coated polystyrene latex including polystyrene sulfonate We synthesized firstly the polystyrene core, over which the copolymer of VFc, styrene sulfonate and styrene was coated secondly as a shell. The synthesis of the core was described previously [7]. A 70 cm3 suspension (11 wt.%) of the polystyrene latex core in 2propanol was diluted with 70 cm3 distilled water in a 200 cm3 round-bottomed flask equipped with a water condenser and a mechanical stirrer, and was stirred overnight under N2 . A 4 cm3 2-propanol solution including 0.47 mmol VFc, a 7 cm3 aqueous solution including 0.48 mmol styrene sulfonate and 0.08 g K2 S2 O8 were added to the diluted polystyrene latex suspension. The resulting mixture was stirred vigorously in the N2 atmosphere at room temperature for 24 h with the aim of adsorbing VFc and styrene sulfonate on the polystyrene latex surface. Temperature of the mixture was controlled at 75 °C for 8 h and then at 90 °C for 16 h in the N2 atmosphere in order to enhance the copolymerization of VFc and styrene sulfonate on the polystyrene particles [14]. The resulting suspension after cooling to room temperature was purified by centrifugation at 10 000g for 30 min and dispersed in the mixed solvent of 2propanol/water (v/v 1:1). This centrifugation/re-dispersion procedure was repeated five times. The resulting supernatant was replaced by distilled water. 2.3. Electrochemical measurements A basal plane pyrolytic graphite electrode (PGE) 3 mm in diameter was used for voltammetric measurements. The PGE surface (area A ¼ 7:07 mm2 ) was polished with alumina paste on a wet cotton and rinsed with distilled water in an ultrasonic bath. The Pt wire and the AgjAgCl (3M NaCl) electrode were used as a counter electrode and as a reference electrode, respectively. These electrodes were inserted into the poly(VFc)-coated polystyrene (VFcPS) suspension including supporting electrolyte for voltammetric measurements. Cyclic voltammetry was performed with the Potentio/Galvanostat (Model 1112, Huso, Kawasaki) under a control of a computer. The nitrobenzene film-coated PGE was used as an electrode for quantitative determination of VFc in the particles. It was prepared by syringing carefully a given volume nitrobenzene solution including 0.1 M tetrabu-
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tylammonium perchlorate (TBAClO4 ) onto the surface of the PGE, making sure that the solution covered completely the surface.
3. Results and discussion The color of the latex suspension was yellow, whereas that of the suspension before coating was white. Therefore, ferrocene moiety has been incorporated into the latex. Fig. 1 shows photographs of suspensions of the polystyrene core and the VFcPS core-shell obtained by an optical microscopy. The particles were almost spherical with common diameter, 1.2 lm. No thickness of the shell could be measured from the difference in diameters. According to the technique by means of UV spectroscopy of the dichloromethane solution of the VFcPS particles [7], the loading of the ferrocene unit on one VFcPS particle was estimated to be 3.1 107 . The surface density of the ferrocene unit was calculated to be 6.9 ferrocenes per nm2 , which corresponds to the area, 0.15 nm2 , occupied by one ferrocene unit. This value indicates that the ferrocene unit should cover the core surface in a monolayer thickness in average. The sulfonated VFcPS was not so much adsorbed on the platinum surface or the pyrolytic graphite electrode (PGE) as the sulfonate-free VFcPS. For example, the surface density of the former was 2.7 105 VFcPS particles mm2 , whereas that of the latter was 6.0 105 VFcPS particles mm2 . The smaller density for the former is ascribed to the more hydrophilicity by the incorporated sulfonate than the latter. The aqueous suspension of the VFcPS was not electroactive at platinum, gold or glassy carbon electrodes in the potential domain from 0.0 to 0.7 V. However, it was electroactive at the PGE, as is shown in Fig. 2 (curve (a)) for the cyclic voltammogram of the suspension in the 1 M NaBF4 aqueous solution. The anodic and cathodic waves near 0.25V were found with
Fig. 1. Photographs of the suspension of (a) the polystyrene-core and (b) the VFcPS core-shell obtained by an optical microscope. The substrate was a glass plate. The average diameter of the particles is 1.2 lm.
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Fig. 2. Voltammograms of the VFcPS suspension at the PGE in (a) 1 M NaBF4 and (b) in 1 M KCl aqueous solution at v ¼ 50 mV s1 . Voltammogram (c) was obtained at v ¼ 50 mV s1 on the nitrobenzene-coated PGE in which VFcPS was dissolved. The nitrobenzene included 0.1 M TBAClO4 . The concentration of the suspension was 1.60 mM for the ferrocene unit.
the peak potential difference, 20 mV. The PGE had a big capacitive current at 1 M NaBF4 without the VFcPS (not shown here but is similar to curve (b)), because of the large surface roughness. Replacement of NaBF4 by KCl, Na2 SO4 and NaClO4 showed no voltammetric peaks (curve (b) for Cl ). This fact can be explained in terms of the less hydrophobicity of Cl than BF 4 , which is quantitatively described by standard Gibbs energies for ion transfer (43.7 kJ/mol for Cl [15], 11.0 kJ/mol for BF 4 [16] in nitrobenzene/water). Consequently, BF4 facilitates the charge transfer more easily than Cl on the latex surface, as has been verified by Jureviciute et al. [17]. Kinetics of the voltammetric behavior of the suspension at the PGE was investigated by varying the scan rate, m, in 1 M NaBF4 aqueous solution. Fig. 3 shows the scan rate dependence of the anodic peak current, Ip ,
Fig. 3. Scan rate dependence of the oxidation peak currents, Ip , in the VFcPS aqueous suspension including 1 M NaBF4 at the PGE. The concentration of the suspension was 1.60 mM for the ferrocene unit.
exhibiting the excellent proportionality with v. Consequently the peak current should be controlled by a surface process, probably by electrode reactions of adsorbed species. There is negligible contribution of diffusion of the VFcPS in the suspension to and from the PGE. The adsorption can be demonstrated by the photograph (Fig. 4) of the PGE, which was immersed in the suspension and was rinsed with water. The VFcPS particles were arranged in the form of a mono-particle layer on the PGE with the coverage of 39% for the projected area. The arrangement of the adsorbed particles did not vary with voltammetric potential applications. Therefore, the adsorption is not only irreversible but also stable in location. The net charge, q1 , of the anodic peak in Fig. 2 was estimated to be 7.9 lC. According to Fig. 4, the number of the adsorbed particles on the whole PGE was about 2 106 . Thus the one adsorbed particle turned out to have 3 pC or 3 108 ferrocene units. The accurate number of the adsorbed particles could not be obtained because images of some particles in the photograph were not clear enough to be identified. In order to determine the accurate number of the adsorbed particles, we dissolved the adsorbed particles in a small amount of nitrobenzene and carried out voltammetry. Specifically, 1.2 mm3 nitrobenzene including 0.1 M TBAClO4 was dropped on the particle-adsorbed PGE to dissolve all the particles in the nitrobenzene. This nitrobenzenecoated PGE was transferred to 1 M NaBF4 aqueous solution and its voltammetry was performed. The cyclic voltammogram in Fig. 2(c) shows well-defined waves
Fig. 4. Photography of the VFcPS particles adsorbed on the PGE. The PGE was immersed in the VFcPS suspension for 4 min, rinsed with water sufficiently and mounted to the optical microscope.
C. Xu et al. / Electrochemistry Communications 5 (2003) 506–510
with 80 mV peak-to-peak potential difference. However, these peaks shifted from the peaks of curve (a) by 0.20 V. The positive potential shift can be ascribed to the higher stability of VFc in nitrobenzene than the stability of VFc in water. We evaluated the net faradaic charge of all VFcPS particles, q0 . Values of the charge were independent of scan rate, ranging from 0.005 to 0.2 V s1 . According to the previous work [7], the evaluation of the total charge depended on both the dissolving time, t, and the nitrobenzene volume, V . Values of q1 =q0 at V ¼ 1:2 mm3 decreased from 0.83 at t ¼ 2 min to about 0.1 at t P 4 min, as is shown in Fig. 5. Thus, the 4 min dissolution time can dissolve completely the particles in the NB film on the PGE. Fig. 6 shows the dependence of
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q1 =q0 on the nitrobenzene volume at t ¼ 4 min for the anodic and the cathodic waves. The plot shows a minimum value of q1 =q0 at V ¼ 1:2 mm3 . A smaller volume of nitrobenzene fails to dissolve completely all the VFcPS particles, whereas a large volume decreases the concentration of VFc unit in the nitrobenzene film. As a result, the reliable value of q1 =q0 is 0.082 0.011. In other words, only 8% of the redox charge of the adsorbed VFcPS particles can be oxidized and reduced at the PGE. We shall explain this partial charge transfer by use of a geometrical relation between the particle and the electrode surface. The simplest geometrical model is that only the redox sites available are restricted to a small portion on the particle in the vicinity of the flat electrode. It is assumed that the redox sites are restricted to the partial surface area of the sphere within the solid angle, h, as illustrated in Fig. 7(a). The area, S1 , of the partial surface is expressed by Z h S1 ¼ 2pr sin /r d/ ¼ 2pr2 ð1 cos hÞ: ð1Þ 0
Fig. 5. Variation of q1 =q0 with the dissolution time, t, for the volume of nitrobenzene was 1.2 mm3 .
Fig. 6. Dependence of the ratio of redox charge, q1 =q0 , on the volume, V , of nitrobenzene dropped on the PGE to dissolve the adsorbed VFcPS particles, where circles are for the anodic charge and triangles are for the cathodic charge. The bare PGE was immersed in the VFcPS suspension for 1 min, during which voltammetry was made at v ¼ 50 mV s1 to evaluate q1 Then the PGE was rinsed with water and dried in air. A given volume of nitrobenzene was dropped on the dried surface of PGE. The nitrobenzene-coated PGE was kept for 3 min, during which the adsorbed VFcPS particles dissolved in the nitrobenzene film. Voltammetry was carried out in 1 M NaBF4 aqueous solution at v ¼ 50 mV s1 to evaluate q0 . The concentration of the suspension was 1.60 mM for the ferrocene unit.
Then the experimental value of the partial charge implies that S1 =4pr2 ¼ 0:082, of which relation gives h ¼ 33° or the distance from the electrode surface, h ¼ rð1 cos hÞ ¼ 98 nm. These values of h and h are too large for the thickness of the shell for the poly(VFc) layer. Even if all the amount of poly(VFc) within the distance h from the adsorption point were to be oxidized without a delay by diffusion, poly(VFc) located in h > 33° should diffuse to the adsorption point. This predicted diffusion is inconsistent with the adsorption behavior in Fig. 3. Therefore, the simplest model cannot be accepted. An observation of the bare PGE through the optical microscope showed a rough surface of the micrometer
Fig. 7. Models of the VFcPS particles adsorbed on the PGE. The particle adsorbed on the flat PGE (a) has electroactivity only within the domain the solid angle, h, of which surface area is denoted as S1 . On the other hand (b) the particle adsorbed onto the inverse cone with the apex angle a has the electroactive area, S2 . Poly(VFc) is distributed non-uniformly on the surface in such an island-in-sea structure that each island has no redox interaction (c). Furthermore, every island is fixed on the polystyrene sphere surface so that each poly(VFc) strand cannot diffuse.
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order. Thus the adsorbed particle is predicted to touch the PGE surface at so many points that it may be stable at the adsorption site. The second model is that the particle is adsorbed in a recess on the inverse cone surface with the angle of the apex being p=2 a, as illustrated in Fig. 7(b). Then the area responsible for the electrode reaction is the ring-like spherical surface, S2 , which is partitioned with the solid angles, a h and a þ h. The expression for S2 is given by Z aþh 2pr sin /r d/ ¼ 4pr2 sin a sin h: ð2Þ S2 ¼ ah
The experimental value of the partial charge gives S2 =4pr2 ¼ sin a sin h ¼ 0:082. It corresponds to 9.4° and 6.7° for a ¼ 30° and 45°, respectively. These values give h ¼ 8:1 and 4.1 nm, respectively. We estimate tentatively size of poly(VFc) from the random coil model on the assumption that the polymer backbone is composed of tetrahedral bonds [18]. The root mean square separation between one end and the other end of the random coil strand is expressed as (2N )1=2 d for N monomer units each of length d [18]. Values of N corresponding to 8.1 and 4.1 nm are calculated to be 1380 and 350 for d ¼ 0:154 nm of the –C–C– length. These values of N are not so unreasonable for polymers. A linear array of adsorbed particles like spawn in Fig. 4 suggests the presence of grooves rather than recesses of the inverse cones. If grooves can be regarded as a linear assembly of inverse cones, taking a part of the circumference of S2 in Fig. 7(b) and taking a larger value of r result in the same discussion as that of the inverse cone model. The partial charge transfer seems to be inconsistent with the charge transfer of the adsorbed species in that all the amount of the adsorbed species react at the electrode. The charge left behind from the electrode reaction might participate in the electrode reaction for a long time electrolysis through the exchange of the charge between the ferrocene unit and the ferricenium unit, so-called redox conduction. A condition of prohibiting the redox conduction is to take islands-in-sea structure so that poly(VFc) islands are separated widely each other. An image of the adsorption is illustrated in Fig. 7(c), where poly(VFc) is assembled in lumps on the polystyrene sphere, each lump being so much separated that it has no redox communication.
4. Conclusion Ferrocene-immobilized latex particles, being spherical 1.2 lm in diameter and possessing 3.1 107 ferrocene units, exhibited the redox activity of the ferrocene
unit at the PGE in the NaBF4 aqueous solution when they had sulfonate on their surface. It is not only the immobilized sulfonate ion but also a relatively hydrophobic anion such as NaBF4 that plays a role in the redox activity to supply counterions to the redox site on the latex surface. The particles showing the redox activity were adsorbed on the PGE, and the voltammetric current was proportional to the potential scan rates. The charge of the ferrocene unit transferred in voltammetry was 8% of the latent redox charge on one adsorbed particle. Mass or charge transfer of a diffusion type was not observed, and hence this partial charge transfer is not due to insufficient electrolysis time but due to a geometrical difference in the adsorption states, e.g., some redox sites being in contact with the electrode and the other being not.
Acknowledgements This work was financially supported by Grants-inAid for Scientific Research (Grants 14340232 and 14540556) from the Ministry of Education in Japan.
References [1] K. Aoki, C. Wang, J. Chen, J. Electroanal. Chem. 540 (2003) 135. [2] K.S. Schmitz, in: Macroions in Solution and Colloidal Suspension, VCH, New York, 1993, pp. 1–15 (Chapter 1). [3] M. Mandel, in: H. Hara (Ed.), Polyelectrolytes, Marcel Dekker, New York, 1993 (Chapter 1). [4] T. Lei, K. Aoki, J. Electroanal. Chem. 482 (2000) 149. [5] K. Aoki, T. Lei, Langmuir 16 (2000) 10069. [6] K. Aoki, J. Chen, Q. Ke, S.P. Armes, D.P. Randall, Langmuir (2003) in press. [7] J. Chen, C. Xu, K. Aoki, J. Electroanal. Chem. 546 (2003) 79. [8] J.B. Flanagan, S. Margel, A.J. Bard, F.C. Anson, J. Am. Chem. Soc. 100 (1978) 4248. [9] R.W. Murray, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 13, Marcel Dekker, New York, 1984, p. 191. [10] K. Aoki, T. Lei, Langmuir 16 (2000) 10069. [11] K. Aoki, Electrochem. Commun. 2 (2000) 94. [12] M.W. Espenscheid, C.R. Martin, J. Electroanal. Chem. 188 (1985) 73. [13] H.R. Zumbrunnen, F.C. Anson, J. Electroanal. Chem. 152 (1983) 111. [14] H. Ahmad, L. Benee, M.J. Snowden, M.J.K. Thomas, E.A. Vidgeon, Colloids Surf. A 186 (2001) 221–228. [15] A.F. Danil de Namor, T. Hill, J. Chem. Soc., Faraday Trans. 179 (1983) 2713. [16] B. Hundhammer, T. Solomon, J. Electroanal. Chem. 157 (1983) 19. [17] I. Jureviciute, S. Bruckenstein, A.R. Hillman, J. Electroanal. Chem. 488 (2000) 73. [18] P.W. Atkins, in: Physical Chemistry, sixth ed., Oxford University Press, Oxford, 1998, p. 699.
Partial charge transfer of poly(vinylferrocene)-coated latex particles adsorbed on pyrolytic graphite electrode Cuiling Xu, Jingyuan Chen, Koichi Aoki
*
Department of Applied Physics, Fukui University, 3-9-1, Bunkyo, Fukui-shi 910-8507, Japan Received 10 April 2003; received in revised form 2 May 2003; accepted 2 May 2003
Abstract As a model of an electrode reaction of a big particle, vinylferrocene immobilized on polystyrene latex particles were synthesized by copolymerization with styrene sulfonate and styrene. They had almost mono-dispersed spheres with 1.2 lm in diameter, and each had 3.1 107 ferrocene units. The particles adsorbed on pyrolytic graphite electrode (PGE) showed the redox activity for the ferrocene unit in NaBF4 aqueous solution. Particles without the sulfonate group had no electroactivity, and hence the electroactivity needs ionic micro-environment around the ferrocene unit. From the faradaic charge of the ferrocene unit, the electroactive sites per particle were estimated to be about 8% of the whole immobilized ferrocene units. A model of this partial charge transfer was proposed, in which the particles are adsorbed in hollows of the rough surface of the PGE. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Latex; Polystyrene; Immobilized poly(vinylferrocene); Polystyrene sulfonate
1. Introduction A large redox particle is a model of electrode reactions not only for nano-particles but also proteins and enzymes. If the particle is so large that it is visualized with an optical microscope, its temporal variations and spatial distributions in electrolyte solution may provide electrochemical images of mass transport, adsorption and charge transfer [1], mimic to the conventional behavior of real electroactive species. In order to realize the model experimentally, large particles should take a common geometry without size distribution, just as a molecule takes its own conformation and size. This requirement is satisfied with polymer microspheres, called latex particles [2,3]. The particles have to contain electroactive species for electrochemical monitorings. The electroactive species is desirable to be localized on the surface of the latex particle in order to prevent complications of charge transfer within the particles [4–6]. Examples of satisfying these conditions are a polystyrene-core coated with a polyaniline-shell [4,5] and the core
coated with poly(vinylferrocene)-shell [7]. The poly(vinylferrocene)-shell is closer to the ideal model than the polyaniline-shell in that the standard redox potential of poly(vinylferrocene) is not distributed so widely [8,9] as that in conducting polymers [10,11]. Unfortunately, the poly(vinylferrocene)-coated latex particle was electroinactive in aqueous solution because of the high hydrophobicity of the particle surface owing to the property of polystyrene that was required for copolymerization of poly(vinylferrocene). It is expected that the incorporation of a hydrophilic substitute such as sulfonate makes the particle surface ionic [12,13], and hence the substituted particles may become electroactive in aqueous solutions. In this report, the latex surface is sulfonated by copolymerization of styrene sulfonate and vinylferrocene (VFc). This particle is demonstrated to be adsorbed well on pyrolytic graphite electrode and to be partially electroactive.
2. Experimentals 2.1. Chemicals
*
Corresponding author. Tel.: +81-776-27-8665; fax: +81-776-278494. E-mail address: [email protected] (K. Aoki).
Styrene and nitrobenzene were purified as was mentioned [7]. The steric stabilizer, poly(N-vinylpyrrolidone)
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00112-7
C. Xu et al. / Electrochemistry Communications 5 (2003) 506–510
(PVP), with a molar mass of ca. 360 kg mol1 , the initiators, a-azoisobutyronitrile (AIBN) and potassium persulfate, were used as received. Vinylferrocene commercially available (TCI, Tokyo) had been stored at 3 °C before use. All the solvents used were of analytical grade. Aqueous solutions were prepared using ion-exchanged and distilled water. 2.2. Synthesis of poly(VFc)-coated polystyrene latex including polystyrene sulfonate We synthesized firstly the polystyrene core, over which the copolymer of VFc, styrene sulfonate and styrene was coated secondly as a shell. The synthesis of the core was described previously [7]. A 70 cm3 suspension (11 wt.%) of the polystyrene latex core in 2propanol was diluted with 70 cm3 distilled water in a 200 cm3 round-bottomed flask equipped with a water condenser and a mechanical stirrer, and was stirred overnight under N2 . A 4 cm3 2-propanol solution including 0.47 mmol VFc, a 7 cm3 aqueous solution including 0.48 mmol styrene sulfonate and 0.08 g K2 S2 O8 were added to the diluted polystyrene latex suspension. The resulting mixture was stirred vigorously in the N2 atmosphere at room temperature for 24 h with the aim of adsorbing VFc and styrene sulfonate on the polystyrene latex surface. Temperature of the mixture was controlled at 75 °C for 8 h and then at 90 °C for 16 h in the N2 atmosphere in order to enhance the copolymerization of VFc and styrene sulfonate on the polystyrene particles [14]. The resulting suspension after cooling to room temperature was purified by centrifugation at 10 000g for 30 min and dispersed in the mixed solvent of 2propanol/water (v/v 1:1). This centrifugation/re-dispersion procedure was repeated five times. The resulting supernatant was replaced by distilled water. 2.3. Electrochemical measurements A basal plane pyrolytic graphite electrode (PGE) 3 mm in diameter was used for voltammetric measurements. The PGE surface (area A ¼ 7:07 mm2 ) was polished with alumina paste on a wet cotton and rinsed with distilled water in an ultrasonic bath. The Pt wire and the AgjAgCl (3M NaCl) electrode were used as a counter electrode and as a reference electrode, respectively. These electrodes were inserted into the poly(VFc)-coated polystyrene (VFcPS) suspension including supporting electrolyte for voltammetric measurements. Cyclic voltammetry was performed with the Potentio/Galvanostat (Model 1112, Huso, Kawasaki) under a control of a computer. The nitrobenzene film-coated PGE was used as an electrode for quantitative determination of VFc in the particles. It was prepared by syringing carefully a given volume nitrobenzene solution including 0.1 M tetrabu-
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tylammonium perchlorate (TBAClO4 ) onto the surface of the PGE, making sure that the solution covered completely the surface.
3. Results and discussion The color of the latex suspension was yellow, whereas that of the suspension before coating was white. Therefore, ferrocene moiety has been incorporated into the latex. Fig. 1 shows photographs of suspensions of the polystyrene core and the VFcPS core-shell obtained by an optical microscopy. The particles were almost spherical with common diameter, 1.2 lm. No thickness of the shell could be measured from the difference in diameters. According to the technique by means of UV spectroscopy of the dichloromethane solution of the VFcPS particles [7], the loading of the ferrocene unit on one VFcPS particle was estimated to be 3.1 107 . The surface density of the ferrocene unit was calculated to be 6.9 ferrocenes per nm2 , which corresponds to the area, 0.15 nm2 , occupied by one ferrocene unit. This value indicates that the ferrocene unit should cover the core surface in a monolayer thickness in average. The sulfonated VFcPS was not so much adsorbed on the platinum surface or the pyrolytic graphite electrode (PGE) as the sulfonate-free VFcPS. For example, the surface density of the former was 2.7 105 VFcPS particles mm2 , whereas that of the latter was 6.0 105 VFcPS particles mm2 . The smaller density for the former is ascribed to the more hydrophilicity by the incorporated sulfonate than the latter. The aqueous suspension of the VFcPS was not electroactive at platinum, gold or glassy carbon electrodes in the potential domain from 0.0 to 0.7 V. However, it was electroactive at the PGE, as is shown in Fig. 2 (curve (a)) for the cyclic voltammogram of the suspension in the 1 M NaBF4 aqueous solution. The anodic and cathodic waves near 0.25V were found with
Fig. 1. Photographs of the suspension of (a) the polystyrene-core and (b) the VFcPS core-shell obtained by an optical microscope. The substrate was a glass plate. The average diameter of the particles is 1.2 lm.
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Fig. 2. Voltammograms of the VFcPS suspension at the PGE in (a) 1 M NaBF4 and (b) in 1 M KCl aqueous solution at v ¼ 50 mV s1 . Voltammogram (c) was obtained at v ¼ 50 mV s1 on the nitrobenzene-coated PGE in which VFcPS was dissolved. The nitrobenzene included 0.1 M TBAClO4 . The concentration of the suspension was 1.60 mM for the ferrocene unit.
the peak potential difference, 20 mV. The PGE had a big capacitive current at 1 M NaBF4 without the VFcPS (not shown here but is similar to curve (b)), because of the large surface roughness. Replacement of NaBF4 by KCl, Na2 SO4 and NaClO4 showed no voltammetric peaks (curve (b) for Cl ). This fact can be explained in terms of the less hydrophobicity of Cl than BF 4 , which is quantitatively described by standard Gibbs energies for ion transfer (43.7 kJ/mol for Cl [15], 11.0 kJ/mol for BF 4 [16] in nitrobenzene/water). Consequently, BF4 facilitates the charge transfer more easily than Cl on the latex surface, as has been verified by Jureviciute et al. [17]. Kinetics of the voltammetric behavior of the suspension at the PGE was investigated by varying the scan rate, m, in 1 M NaBF4 aqueous solution. Fig. 3 shows the scan rate dependence of the anodic peak current, Ip ,
Fig. 3. Scan rate dependence of the oxidation peak currents, Ip , in the VFcPS aqueous suspension including 1 M NaBF4 at the PGE. The concentration of the suspension was 1.60 mM for the ferrocene unit.
exhibiting the excellent proportionality with v. Consequently the peak current should be controlled by a surface process, probably by electrode reactions of adsorbed species. There is negligible contribution of diffusion of the VFcPS in the suspension to and from the PGE. The adsorption can be demonstrated by the photograph (Fig. 4) of the PGE, which was immersed in the suspension and was rinsed with water. The VFcPS particles were arranged in the form of a mono-particle layer on the PGE with the coverage of 39% for the projected area. The arrangement of the adsorbed particles did not vary with voltammetric potential applications. Therefore, the adsorption is not only irreversible but also stable in location. The net charge, q1 , of the anodic peak in Fig. 2 was estimated to be 7.9 lC. According to Fig. 4, the number of the adsorbed particles on the whole PGE was about 2 106 . Thus the one adsorbed particle turned out to have 3 pC or 3 108 ferrocene units. The accurate number of the adsorbed particles could not be obtained because images of some particles in the photograph were not clear enough to be identified. In order to determine the accurate number of the adsorbed particles, we dissolved the adsorbed particles in a small amount of nitrobenzene and carried out voltammetry. Specifically, 1.2 mm3 nitrobenzene including 0.1 M TBAClO4 was dropped on the particle-adsorbed PGE to dissolve all the particles in the nitrobenzene. This nitrobenzenecoated PGE was transferred to 1 M NaBF4 aqueous solution and its voltammetry was performed. The cyclic voltammogram in Fig. 2(c) shows well-defined waves
Fig. 4. Photography of the VFcPS particles adsorbed on the PGE. The PGE was immersed in the VFcPS suspension for 4 min, rinsed with water sufficiently and mounted to the optical microscope.
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with 80 mV peak-to-peak potential difference. However, these peaks shifted from the peaks of curve (a) by 0.20 V. The positive potential shift can be ascribed to the higher stability of VFc in nitrobenzene than the stability of VFc in water. We evaluated the net faradaic charge of all VFcPS particles, q0 . Values of the charge were independent of scan rate, ranging from 0.005 to 0.2 V s1 . According to the previous work [7], the evaluation of the total charge depended on both the dissolving time, t, and the nitrobenzene volume, V . Values of q1 =q0 at V ¼ 1:2 mm3 decreased from 0.83 at t ¼ 2 min to about 0.1 at t P 4 min, as is shown in Fig. 5. Thus, the 4 min dissolution time can dissolve completely the particles in the NB film on the PGE. Fig. 6 shows the dependence of
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q1 =q0 on the nitrobenzene volume at t ¼ 4 min for the anodic and the cathodic waves. The plot shows a minimum value of q1 =q0 at V ¼ 1:2 mm3 . A smaller volume of nitrobenzene fails to dissolve completely all the VFcPS particles, whereas a large volume decreases the concentration of VFc unit in the nitrobenzene film. As a result, the reliable value of q1 =q0 is 0.082 0.011. In other words, only 8% of the redox charge of the adsorbed VFcPS particles can be oxidized and reduced at the PGE. We shall explain this partial charge transfer by use of a geometrical relation between the particle and the electrode surface. The simplest geometrical model is that only the redox sites available are restricted to a small portion on the particle in the vicinity of the flat electrode. It is assumed that the redox sites are restricted to the partial surface area of the sphere within the solid angle, h, as illustrated in Fig. 7(a). The area, S1 , of the partial surface is expressed by Z h S1 ¼ 2pr sin /r d/ ¼ 2pr2 ð1 cos hÞ: ð1Þ 0
Fig. 5. Variation of q1 =q0 with the dissolution time, t, for the volume of nitrobenzene was 1.2 mm3 .
Fig. 6. Dependence of the ratio of redox charge, q1 =q0 , on the volume, V , of nitrobenzene dropped on the PGE to dissolve the adsorbed VFcPS particles, where circles are for the anodic charge and triangles are for the cathodic charge. The bare PGE was immersed in the VFcPS suspension for 1 min, during which voltammetry was made at v ¼ 50 mV s1 to evaluate q1 Then the PGE was rinsed with water and dried in air. A given volume of nitrobenzene was dropped on the dried surface of PGE. The nitrobenzene-coated PGE was kept for 3 min, during which the adsorbed VFcPS particles dissolved in the nitrobenzene film. Voltammetry was carried out in 1 M NaBF4 aqueous solution at v ¼ 50 mV s1 to evaluate q0 . The concentration of the suspension was 1.60 mM for the ferrocene unit.
Then the experimental value of the partial charge implies that S1 =4pr2 ¼ 0:082, of which relation gives h ¼ 33° or the distance from the electrode surface, h ¼ rð1 cos hÞ ¼ 98 nm. These values of h and h are too large for the thickness of the shell for the poly(VFc) layer. Even if all the amount of poly(VFc) within the distance h from the adsorption point were to be oxidized without a delay by diffusion, poly(VFc) located in h > 33° should diffuse to the adsorption point. This predicted diffusion is inconsistent with the adsorption behavior in Fig. 3. Therefore, the simplest model cannot be accepted. An observation of the bare PGE through the optical microscope showed a rough surface of the micrometer
Fig. 7. Models of the VFcPS particles adsorbed on the PGE. The particle adsorbed on the flat PGE (a) has electroactivity only within the domain the solid angle, h, of which surface area is denoted as S1 . On the other hand (b) the particle adsorbed onto the inverse cone with the apex angle a has the electroactive area, S2 . Poly(VFc) is distributed non-uniformly on the surface in such an island-in-sea structure that each island has no redox interaction (c). Furthermore, every island is fixed on the polystyrene sphere surface so that each poly(VFc) strand cannot diffuse.
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order. Thus the adsorbed particle is predicted to touch the PGE surface at so many points that it may be stable at the adsorption site. The second model is that the particle is adsorbed in a recess on the inverse cone surface with the angle of the apex being p=2 a, as illustrated in Fig. 7(b). Then the area responsible for the electrode reaction is the ring-like spherical surface, S2 , which is partitioned with the solid angles, a h and a þ h. The expression for S2 is given by Z aþh 2pr sin /r d/ ¼ 4pr2 sin a sin h: ð2Þ S2 ¼ ah
The experimental value of the partial charge gives S2 =4pr2 ¼ sin a sin h ¼ 0:082. It corresponds to 9.4° and 6.7° for a ¼ 30° and 45°, respectively. These values give h ¼ 8:1 and 4.1 nm, respectively. We estimate tentatively size of poly(VFc) from the random coil model on the assumption that the polymer backbone is composed of tetrahedral bonds [18]. The root mean square separation between one end and the other end of the random coil strand is expressed as (2N )1=2 d for N monomer units each of length d [18]. Values of N corresponding to 8.1 and 4.1 nm are calculated to be 1380 and 350 for d ¼ 0:154 nm of the –C–C– length. These values of N are not so unreasonable for polymers. A linear array of adsorbed particles like spawn in Fig. 4 suggests the presence of grooves rather than recesses of the inverse cones. If grooves can be regarded as a linear assembly of inverse cones, taking a part of the circumference of S2 in Fig. 7(b) and taking a larger value of r result in the same discussion as that of the inverse cone model. The partial charge transfer seems to be inconsistent with the charge transfer of the adsorbed species in that all the amount of the adsorbed species react at the electrode. The charge left behind from the electrode reaction might participate in the electrode reaction for a long time electrolysis through the exchange of the charge between the ferrocene unit and the ferricenium unit, so-called redox conduction. A condition of prohibiting the redox conduction is to take islands-in-sea structure so that poly(VFc) islands are separated widely each other. An image of the adsorption is illustrated in Fig. 7(c), where poly(VFc) is assembled in lumps on the polystyrene sphere, each lump being so much separated that it has no redox communication.
4. Conclusion Ferrocene-immobilized latex particles, being spherical 1.2 lm in diameter and possessing 3.1 107 ferrocene units, exhibited the redox activity of the ferrocene
unit at the PGE in the NaBF4 aqueous solution when they had sulfonate on their surface. It is not only the immobilized sulfonate ion but also a relatively hydrophobic anion such as NaBF4 that plays a role in the redox activity to supply counterions to the redox site on the latex surface. The particles showing the redox activity were adsorbed on the PGE, and the voltammetric current was proportional to the potential scan rates. The charge of the ferrocene unit transferred in voltammetry was 8% of the latent redox charge on one adsorbed particle. Mass or charge transfer of a diffusion type was not observed, and hence this partial charge transfer is not due to insufficient electrolysis time but due to a geometrical difference in the adsorption states, e.g., some redox sites being in contact with the electrode and the other being not.
Acknowledgements This work was financially supported by Grants-inAid for Scientific Research (Grants 14340232 and 14540556) from the Ministry of Education in Japan.
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