CHAPTER I Introduction Since the 1960s, electrochemists have shown interest in the occurrence and consequence of adsorption of ions and molecules on electrode surfaces. Adsorption can have both desirable and deleterious consequences, and adsorption research has been adjunct to numerous fundamental insights into the electrical double layer and the kinetics and mechanisms of electrochemical reactions. A great deal of information has accumulated from what species adsorb on various electrodes in different solvents and electrolyte media. In some instances, adsorption phenomena are easily explained based on chemical reactivity or solubility grounds, the adsorption of simple metal complexes on mercury electrodes being a case in point. However, to a substantial extent, the discovery of an adsorption phenomenon is an empirical event, and the exploitation of it for useful purposes has had few systematic or fundamental studies. Chemically modified electrodes (CMEs) as discussed here diverge sharply from the traditional field of adsorption on electrode surfaces. The most essential difference is that one deliberately seeks in some hopefully rational fashion to immobilize a chemical on an electrode surface so that the electrode thereafter displays the chemical, electrochemical, optical, and other properties of the immobilized molecule(s) [1-4]. Recently, the terminology, chemically modified electrodes, has been clearly delineated and a short lexicon of related terms provided. Chemically modified electrode is defined as "an electrode made of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic, or polymeric film of a chemical modifier and that by means of faradaic (charge transfer) reactions or interfacial potential difference (no net charge transfer) exhibits chemical, electrochemical, and/or optical properties of the film" [3]. CMEs are a relatively modern approach in electrode systems that find utility in a wide spectrum of basic electrochemical investigations, including the relationship of heterogeneous electron transfer and chemical reactivity of electrode surface chemistry, electrostatic phenomena at electrode surfaces, and electron and ionic transport phenomena in polymers; the design of electrochemical devices and systems for applications (e.g. chemical sensing, energy conversion and storage, molecular electronics, electrochromic displays, corrosion protection, and electroorganic syntheses, etc) are the ideas and motivations associated with many of the recent and current researches on electrodes bearing immobilized chemicals. In this chapter, Section I discusses the chemical and physical routes for deliberate, hopefully stable immobilization of molecular systems on electrodes and the electrochemical and other consequences of this. Section II provides a discussion of the techniques that have been used or invented to detect the electroactivity, chemical reactivity, and surface structure of electrode-immobilized molecules and films. In Section III, applications in both applied science and fundamental are presented. These descriptions are brief plus a few virtues, promises, and limitations. Finally, the objectives of this thesis are discussed. 1.1 Preparation of chemically modified electrodes Molecular species for preparing chemically modified electrodes fall into three broad categories: monomolecular layers, multimolecular layers, and spatially defined, molecularly heterogeneous layers [2]. The manners of preparation and the uses of these modifiers are generally distinctive to each category, as will be indicated in the sections that follow. 1.1.1 Electrodes modified with monomolecular layers Monomolecular layers modified electrodes can be further classified according to the principal routes used to immobilize the substrates, which can be grouped as chemisorption, covalent bonding and hydrophobic layers [1,2]. 1.1.1.1 Chemisorption Chemisorption is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface [5,6]. Chemisorption requires direct contact between the chemisorbed molecule and the electrode surface; as a result, the highest coverage achievable is usually a monomolecular layer. In addition to this coverage limitation, chemisorption is rarely completely irreversible. In most cases, the chemisorbed molecules slowly leach into the contacting solution phase during electrochemical or other investigations of the chemisorbed layer. For these reasons, electrode modification via chemisorption was quickly supplanted by other methods, most notably polymer-coating methods. 1.1.1.2 Covalent bonding Reagents can also be attached covalently to surfaces. On the electrode with oxide surfaces, metal hydroxyl is a natural terminator of the oxide phase. For carbon, the edges of basal plane sheets tend to be terminated with oxidized sites including carboxylic acid groups. These two kinds of surface functionalities lead to a fairly versatile monomolecular layer surface bonding chemistry [7-9]. Organosilane chemistry, commonly used to prepare chromatographic stationary phases, can also be used to modify surfaces containing hydroxyl groups [10,11]. The covalent schemes include procedures for attaching both monomolecular layer and (in polymeric form) multimolecular layer quantities of electroactive sites by the natural or inducible functional groups available on the electrode surface. Because of this interesting synthetic strategy, covalent attachment of functional groups remains an attractive approach to modifying electrode surfaces. 1.1.1.3 Hydrophobic Layers The "stiff" model theme has become important in recent works in which hydrophobic chain interactions have been invoked to produce structurally organized monolayers on electrodes [2]. These monolayers are of two broad types, those relying for molecular organization on hydrophobic effects plus compression as LangmuirBlodgett (LB) films [12,13] and those based on self-assembly of hydrophobic chains with a chemisorbable terminus [14,15]. In the LB experiment, a long-chain hydrophobic target molecule, which may have an electron donor or acceptor group at one terminus, is spread and compressed as a monolayer at the air-water interface in a Langmuir trough, and then transferred to the electrode. Self-assembly is also a powerful method for electrode modification. It has the virtue of operational simplicity, but lacks the molecular layer compression variable (Langmuir trough surface pressure) of the LB experiment. In self-assembled films, chemisorption occurs onto the electrode from the solutions of functionalized hydrophobic molecules, such as fatty thiols, sulfides, disulfides, silanes onto Au surfaces, and nitriles on Pt. 1.1.2 Electrodes modified with multimolecular layers Typically, the following materials are employed for multimolecular layers modified electrodes: polymer and inorganic film. 1.1.2.1 Polymers By using polymeric modifying layers, fairly thick films, containing many more electroactive sites than a monolayer, can be formed on an electrode surface. The introduction of electrochemically reactive polymer materials was an important development in molecularly designed electrode surfaces [1,2]. A further categorization for polymeric multilayers can be made with respect to the electronic character of the electron donor-acceptor sites. Polymeric multilayer species with delocalized electronic states, such as poly(aniline), are usually referred to as electronically conducting polymer; Molecular layers in which the donor-acceptor sites are electronically well defined and localized as molecular states, such as ferrocene, are referred to as redox polymer; Polymeric ion-exchange materials with redox ion, such as Nafion®, are usually referred to as ion-exchange polymer (loaded ionomer) [4]. Various methods are used to prepare polymer-modified electrodes [1,2]: A. Dip coating. This procedure consists of immersing the electrode in a solution of the polymer for a period sufficient for spontaneous film formation to occur by adsorption. The film quantity in this procedure may be augmented by withdrawing the electrode from the solution and by allowing the film of polymer solution to dry on the electrode. B. Solvent evaporation. A droplet of a solution of the polymer is applied to the electrode surface and the solvent is allowed to evaporate. A major advantage of this approach is that the polymer coverage is immediately known from the original polymer solution concentration and droplet volume. C. Spin coating. The electrode is set spinning after a drop of the polymer solution is placed on the surface. Excess solution is spun off the surface and the remaining thin polymer film is allowed to dry. Multiple layers are applied in the same way until the desired thickness is obtained. D. Electrochemical deposition. This relies on the dependence of polymer solubility with oxidation (and ionic) state, so that film formation will occur, often irreversibly when a polymer is oxidized or reduced to its less soluble state. E. Organosilanes. This is a particularly useful chemical basis for polymer films because bonding to the electrode (SnO2, Pt / PtO, etc.) as well as polymer cross-linking (-SiOSi-) can occur. Organosilane monomers can be polymerized under dip coating or droplet evaporation conditions as mentioned above, or a vinyl copolymer can first be formed between the vinyl monomer of interest (vinylferrocene) or styrene sulfonic acid and a silane monomer. F. Radiofrequency polymerization. Forming polymeric materials by exposing vapors of monomers to a radio-frequency plasma discharge is a well-known polymer-filming method. Upon exposure to air, plasma films typically take up oxygen and contain other unknown functionalities as a result of chemical damage in the RF discharge. G. Electrostatic binding of redox ion. When the film is to be employed as an ion as exchanger, scavenging ionic redox species from solutions, it is of course deposited first by one of the procedures described above. H. Electropolymerization. A solution of monomer is oxidized or reduced to intermediates which electropolymerize sufficiently rapidly as to form a polymer film directly on the electrode. Electropolymerization presents several advantages which make itself a unique tool for electrochemical studies as well as for some specific electrochemically oriented technological applications of conducting polymers. Rapidity is probably the most immediate feature of electropolymerization. The growth of a polymer film of a few hundred nanometers thickness, which is generally convenient for most electrochemical and spectroscopic characterizations, requires only a few seconds. This is of course nothing compared to the several hours and tedious work-up required by chemical methods. Simplicity is another evident advantage. Besides further time saving, this specific one-step process leads to more heavily and more homogeneously doped materials than post-polymerisation doped chemically synthesized polymers. Perhaps the most attractive feature of electropolymerization is that it represents one of the simplest and most straightforward methods for the preparation of modified electrodes. Electronically Conducting Polymer Electronically conducting polymer (ECP) is an exciting new class of modifying materials with unique electronic, electrochemical, and optical properties. Because of these unusual and useful properties, ECP is the focus of massive international research efforts [1,2,4]. One of the most interesting and potentially useful aspects of this polymer is that it can be reversible “switched” between electronically insulating state and electronically conductive state. This switching reaction involves either oxidation or reduction of a nonionic and electronically insulating parent polymer to form a conductive polycationic or polyanionic daughter polymer. Further, ECP is conjugated, and the cationic sites created upon oxidation are delocalized along the polymer chain. The delocalization in the ECP causes this polymer to be electronic conductor (i.e. similar to metals). This conductivity is imparted due to the addition of dopants in relatively large quantities into the polymer matrix [4]. The deliberate and controlled modification of the electrode surface with ECP can produce electrodes with new and interesting properties [16-19]. Redox Polymer Redox polymer consists of electronically locally electron donor and acceptor sites that are bonded to a polymer chain, or linked together to form a polymeric chain [4]. In this material, electron transfer occurs via a process of sequential electron selfexchange between neighboring redox groups. It is in contrast to ECP whose backbone is extensively conjugated, which results in considerable charge delocalization. Redox film can be preassembled and then applied as a film to the electrode surface, or it can be assembled from monomer directly as a film on the electrode. Both approaches have been widely researched and each offers certain advantages. Generally, larger amounts of materials are available through direct synthesis of the redox polymer, which allows a relatively better analytical and structural characterization. On the other hand, fabrication of very thin, uniform films from preassembled redox polymers can be difficult, since they are often multiply charged and reluctantly soluble. In situ assembly of redox polymer films by hydrolytic or electrochemical polymerization of monomers can yield superb thin film forming characteristics, but usually at the expense of a less thorough analytical characterization, since only ultra-thin film is available [2,20-27]. Ion-Exchange Polymer Ion-exchange polymer is not electroactive itself, but can incorporate electroactive guest molecules [4]. For example, Anson’s group showed that the film of poly(vinylpyridine) can incorporate electroactive coordinatively unsaturated metal complex via coordination of the metal to the polymer-bound pyridine [28]. Likewise, ion-exchange polymer incorporates electroactive counterions via an ion-exchange reaction. The most extensively investigated polymer of this type is Du Pont’s perfluorosulfonate ionomer, Nafion®. Nafion® is a strong acid ion-exchange polymer. A large number of electroactive cations can be incorporated into Nafion® films at electrode surfaces. Since the procedure for dissolving the film polymer of Nafion® was developed by Martin’ group [29], Nafion® film-coated electrodes have become the most extensively investigated modified electrodes [30-33]. 1.1.2.2 Inorganic films This section deals with the preparation of electrodes modified by forming films of inorganic materials on a conductive substrate surface. These inorganic materials are of interest because they are ion exchangers, like ion-exchange polymers; however, unlike polymers, zeolites and clays can withstand high temperatures and highly oxidizing solution environments. Furthermore, these inorganic materials have well-defined microstructures [34]. Different types of inorganic materials, such as metal oxides, zeolites and clays, can be deposited on electrode surfaces. In the following, a few examples are described. Metal oxides A wide variety of metal oxide materials can be employed to modify electrode surfaces through sol-gel technique. In this approach, metal oxides undergo the hydrolysis followed by a cascade of condensation and polycondensation reactions in solution at room temperature. Sol-gel processes can be divided into two synthetic routes: aqueous-based methods, which originate with a solution of a metal salt, and alcohol-based methods, which employs an organometallic precursor that is dissolved 10 .0 O1 i / µA .0 -5 .0 P1 -1 .0 -0 .5 -0 .2 .2 .5 .7 .0 .2 .5 E / V olt (a) 7.5 5.0 i / µA 2.5 -2.5 -5.0 -7.5 -1 0.0 -0.50 -0 .2 .25 0.50 .75 .00 E / V olt (b) 80 O1 O2 40 i / µA P1 -4 -8 -1 -0 .5 P2 -0 .2 .2 .5 .7 .0 E / V o lt O2 30 (c) O1 i / µA -3 P1 -6 -9 -0 .5 P2 -0 .2 .2 E / V o lt .5 .7 .0 (d) Continued 143 50 O2 O1 i / µA 25 P1 -2 -5 -7 - .5 P2 - .2 40 .2 E / V o lt O2 .5 .7 .0 (e) O1 20 i / µA P1 -2 -4 -6 -0 .5 P2 -0 .2 .2 E / V o lt .5 .7 .0 (f) Figure 5.7 CVs (50 mV s-1) of bare GC electrode (a), monolayer zeolite EPD (b), pure P4NoPD film (c), and the composite films: (d) submonolayer zeolite EPD coated by P4NoPD film; (e) monolayer zeolite EPD coated by P4NoPD film; (f) multilayer zeolite EPD coated by P4NoPD film. The solution is 0.10 M KNO3 (pH = 2.00, adjusted by HNO3) containing 1.00 mM Fe(CN)63-. 144 5.3.3.2 pH effect pH is expected to have a strong effect on the electrochemical reaction of the composite film. We examined this effect using KNO3. Commencing from 0.10 M in each case, the electrolyte was titrated with HNO3 to decrease the pH. All the cases were for the presence of 1.00 mM Fe(CN)63- as the analyte. Figure 5.8 shows that generally, the peak heights of both O1/P1 and O2/P2 decreased with increasing pH, until these peaks were non-existent or not observable at pH value of 4.8. It was earlier established [9] that at pH higher than about 4.12, within the potential region (from 400 mV to 800 mV) studied for P4NoPD redox activities, P4NoPD did not exhibit electroactive behavior. According to the electron-transfer mechanism of conducting polymers [17-24], the protonation of conducting polymers is accompanied by the transfer of the charge compensating ions (e.g. NO3- and Fe(CN)63-) in the oxidation reaction. Although the zeolite framework is a negatively charged electrical insulator and would hinder the transfer of anions (e.g. NO3- and Fe(CN)63-), the negative sites of zeolite in the composite film were partly neutralized with cationic P4NoPD film. Furthermore, it might be assumed that the concentration of Fe(CN)63- anions in the bulk polymer could even be higher than the solution concentration due to possible ion associates formation [18]. It predicts that the redox peaks of Fe(CN)63- may disappear after complete deprotonation of the film in the higher pH solution. This is in a complete agreement with our experiment results (see Figure 5.8(c)). Therefore, these above observations (also Figure 5.6 and 5.7) can confirm that the composite films obtained higher conductivities in the electrolyte containing Fe(CN)63- due to the mediation with the redox process of P4NoPD, and this property is highly pH dependent. This is in consistence with the previous investigations [17-22]. 145 40 O2 20 O1 i / µA P1 -2 -4 -6 -0 .5 P2 -0 .2 30 .2 E / V o lt .7 .0 (a) O2 O1 15 i / µA .5 P1 -1 -3 -4 -0 .5 P2 -0 .2 .2 E / V o lt .5 .7 .0 (b) 5.0 i / µA -5.0 -10.0 -15.0 -20.0 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 1.50 E / Volt (c) Figure 5.8 CVs (50 mV s-1) of the composite film (multilayer zeolite EPD coated by P4NoPD film) in various pH of 1.00 mM Fe(CN)63-, 0.10 M KNO3, and pH values were adjusted by HNO3: (a) pH = 2.00; (b) pH = 3.00; (c) pH = 4.80. 146 5.3.3.3 Accumulation of Fe(CN)63- In order to examine the adsorption of Fe(CN)63- in the composite films, the composite film (multilayer zeolite EPD coated by P4NoPD film) was selected and potentiostatically held at –0.40 V for different periods prior to the start of the potential scan (from –0.40 V to 0.80 V) in 1.00 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3). The CVs obtained are shown in Figure 5.9(a), and then the charges for the anodic peak of Fe(CN)63-/4- couple verus the time period are plotted in Figure 5.9(b). Figure 5.9(a) shows that although the shapes of P1 essentially remained constant, the peak areas of O1 considerably increased with increasing times. At sufficiently long time, no further increase in anodic charge occurred probably due to essentially complete equilibration of Fe(CN)63- in the composite film (see Figure 5.9(b)). Additionally, there are slight decreases in ∆Ep values of O1/P1. This is due to the accumulation of Fe(CN)64- ions in the composite film, which may make less diffusional resistance to the electrode surface and cause the redox reaction easier. On the other hand, if the composite film was either potentiostatically at dc potential ranges (between 0.40 V and 1.00 V) for a time period (up to hrs) or open-circuit, no similar accumulation of Fe(CN)63- in the composite film was observable. Therefore, these observations further support that Fe(CN)63-/4- couples can penetrate inside the composite films as compensating ions, and also participate in redox reactions of the composite films. 5.3.3.4 Effect of redox active species All the above discussions were for the presence of Fe(CN)63- as the redox active couple. It would also be meaningful to study Ru(NH3)63+ and HQ as the redox active couples (see Figure 5.10). The CVs of these three redox active couples on the 147 40 O2 O1 i / µA 20 P1 -20 -40 P2 -60 -0.50 -0.25 0.25 0.50 0.75 1.00 E / Volt (a) 200 Qp1,a / uC 150 100 50 0 1000 2000 Time / Second 3000 4000 (b) Figure 5.9 (a) CVs (50 mV s-1) of the composite film (multilayer zeolite EPD coated by P4NoPD film) in 1.00 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was kept at –0.40 V for different periods prior to scanning: 10 s (····); 100 s (----); 1800 s (⎯). (b) Plot (50 mV s-1) of the anodic peak charges Qp1,a of the composite film (multilayer zeolite EPD coated by P4NoPD film) in 1.0 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was potentiostatically at –0.40 V for different periods prior to scanning. 148 40 O2 O1 i / µA 20 P1 -20 -40 P2 -60 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 1.50 E / Volt (a) 20 O2 i / µA -20 -40 P2 -60 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 1.50 E / Volt (b) O1 40 i / µA 15 O2 -10 P1 -35 P2 -60 -0.50 -0.25 0.25 0.50 0.75 1.00 1.25 E / Volt 1.50 (c) Figure 5.10 CVs (50 mV s-1) of the composite film (multilayer zeolite EPD coated by P4NoPD film) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), containing: (a) 1.00 mM Fe(CN)63-; (b) 1.00 mM Ru(NH3)63+; (c) 1.00 mM hydroquinone; In all curves, solid line is on the composite film, and dash line is on the bare GCE. 149 bare GCE were also obtained. In contrast to Fe(CN)63- (Figure 5.10(a)), no similar redox peaks for Ru(NH3)63+/4+ couple were observed (Figure 5.10(b)). It is also true if the composite film was potentiostatically at dc potentials (between -0.40 V and 1.25 V) for a time period (up to hrs). A possible explanation is that positively multicharged redox-species seem to play no appreciable role in the course of redoxtransformations inside the bulk polymer [18]. On the other hand, even if we were not able to observe any corresponding redox peaks for Ru(NH3)63+, we cannot make sure that they were really non-existent or simply to overlay with the redox peaks of the P4NoPD film. Figure 5.10(c) shows that the anodic peak current of O1/P1 couple for HQ/Q on the composite film increased significantly compared to that on the bare electrode. Even more interesting is the decrease in ∆Ep of 280 mV for O1/P1 on the composite film, which is much less than on the bare GC electrode (∆Ep = 510 mV). This data indicates that the redox behavior of HQ/Q on the composite film was more reversible than on the bare GCE. This is in contrast to Fe(CN)63-, whose redox reaction on composite film was more difficult than on the bare GCE, due to the diffusional resistance. The values of ∆Ep for HQ/Q versus the scan rate are plotted in Figure 5.11. It shows that the magnitude of ∆Ep significantly decreased compared to bare GC electrode (e.g. values decreased from 340 mV to160 mV at the scan rate of mV s-1), and ∆Ep values were expected to increase with increasing scan rate until to the level. We also examined the adsorption of HQ on the composite films. For this purpose, CVs were recorded by holding the composite film at -0.40 V for the different time periods prior to scanning in 1.00 mM HQ, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3). It is seen from Figure 5.12 that the cathodic peak currents of HQ/Q remained constant, and the anodic peak currents increased with increasing time. Although 150 550 Ep1,a - Ep1,c / mV 450 350 250 150 20 40 60 v / mV s 80 100 120 -1 Figure 5.11 Plots of peak potential separation ∆Ep=Ep1,a – Ep1,c (mV) of HQ/Q versus scan rate v (mV s-1) on the bare GCE (•) and the composite film (multilayer zeolite EPD coated by P4NoPD film) (▲) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3) containing 1.00 mM HQ. 151 75 O1 50 O2 i / µA 25 P1 -25 -50 -0.50 P2 -0.25 0.25 0.50 0.75 1.00 1.25 1.50 E / Volt Figure 5.12 CVs (50 mV s-1) of the composite film (multilayer zeolite EPD coated by P4NoPD film) in 1.00 mM HQ, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was kept at –0.40 V for different periods prior to scanning: 10 s (····); 100 s (----); 1800 s (⎯). 152 neutral HQ species should not be involved in the electroneutrality process of the polymer, the above data demonstrates that the redox peaks of HQ/Q species on the composite film were even more reversible than on the bare GC electrode. The likely explanations for this phenomenon are in the following: (1) both HQ and Q species were able to form hydrogen bonds with the polymer. In this sense, this would lead to their penetration into the composite; (2) the redox reaction of HQ/Q was electrocatalyzed by P4NoPD. The catalytic effect of P4NoPD towards the oxidation of HQ was perhaps due to a great tendency of HQ and Q to undergo adsorption on the composite film [19,23,24]. 153 5.4 Conclusions We have shown here the anodic electropolymerization of 4NoPD on the ZMEs (fabricated by pulsed voltage EPD). The composite films thus prepared exhibit good stability for a long term, and show conductivity especially in acidic solutions. The nitro-group of the composite film is also electroactive. The redox properties of the composite films are found to be highly dependent on the amount of zeolite particles and pH. The electrochemistry of the composite films was also studied in the presence of the redox active probes. All the observations give the consistent result that the properties of the composite films can be derived from the successful combination of the characteristics of P4NoPD and zeolite 13X. 154 References: [1] P. Gomez-Romero, Adv. Mater. 13 (2001) 163. [2] R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608. [3] L. F. Nazar, Z. Zhang, D. Zinkweg, J. Am. Chem. Soc. 114 (1992) 6239. [4] J. K. Vassiliou, R. P. Ziebarth, P. J. Disalvo, Chem. Mater. (1990) 738. [5] L. L. Beecroft, C. K. Ober, Chem. Mater. (1997) 1302. [6] G. Cao, M. E. Carcia, M. Aleala, L. F. Burgess, T. E. Mallouk, J. Am. Chem. Soc. 1141 (1992) 7574. [7] K. Ogura, N. Endo, M. Nakayama, J. Electrochem. Soc. 145 (1998) 3801. [8] B. Yu, S. B. Khoo, Electrochem. Comm. (2002) 737. [9] B. Yu, S. B. Khoo, Electrochim. Acta in press [10] Eric G. Derouane (ed.) A molecular view of heterogeneous catalysis: Proceedings of the First Francqui Colloquium, 19-20 February 1996, Brussels, De Boeck Universite, Paris, (1998). [11] J. J. Harris, M. L. Bruening, Langmuir 16 (2000) 2006. [12] J. Tanguy, J. L. Baudoin, F. Chao, M. Cosia, Electrochim. Acta 37 (1992) 1417. [13] K. Martinusz, G. Lang, G. Inzelt, J. Electroanal. Chem. 433 (1997) 1. [14] T. Ohsaka, S. Kunimura, N. Oyama, Electrochim. Acta (33) 1988 639. [15] C. Barbero, J. J. Silber, L. Sereno, J. Electroanal. Chem. 291 (1990) 81. [16] M. A. Valdes Garcia, P. Tunon Blanco, A. Ivaska, Electrochim. Acta 43 (1998) 3533. [17] E. Laviron, J. electroanal. Chem. 131 (1982) 61. [18] M. D. Levi, E. Y. Pisarevskaya, Electrochim. Acta 37 (1992) 635. 155 [19] E. Buttner, R. Holze, J. electroanal. Chem. 508 (2001) 150. [20] C. Deslouis, C. Gabrielli, M. M. Musiani, B. Tribollet, J. F. Equey, O. Haas, J. Electroanal. Chem. 244 (1988) 325. [21] W. W. Focke, G. E. Wnek, Yen Wei, J. Phys. Chem. 91 (1987) 5813. [22] M. D Levi, E. Yu. Pisarevskaya, Synth. Met. 45 (1991) 309. [23] A.R. Hillman, in- R.G. Linford (Ed.), Electrochemical Science and Technology of Polymers, vol. 1, Elsevier Applied Science, London, 1987, p. 241. [24] G. P. Evans, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, vol. 1, VCH, Weinheim, Germany, 1990, p. 1. 156 Future work Firstly, future work has to deepen the understanding of ZMEs fabricated by EPD method. The solution parameters (pH, supporting electrolyte concentration) and other experimental parameters (voltage, deposition time, wave shape, and types of electrode materials) should be investigated in more detailed how to affect the fabrication condition and properties of ZMEs (e.g. surface deactivation, controllability, uniformity and reproducibility). There is no doubt that it will lead to the design of more applications in various fields, such as analytical chemistry, and energy-producing, etc. Secondly, there is a need for improved understanding in the properties of P4NoPD film using some other techniques, such as electrochemical quartz crystal microbalance (EQCM) and surface analytical techniques (e.g. SECM, STM). The studies of the more related nitro compounds and their polymers (if feasible) should be helpful to further elucidate structure of the P4NoPD film and the mechanism of the reduction of nitro-groups in polymers. There may be possible applications for this feature e.g. as electron relay or for immobilization of biomolecules, etc. Thirdly, for the practical applications in battery cathodes, microelectronics and biosensing, the vivo, fabrication of ordered organic-inorganic composite materials is promising. The availability of the composite materials will definitely ease the fabrication of new functional materials. 157 List of Publications 1. B. Yu, S. B. Khoo, Controllable Zeolite Films on Electrodes – Comparing DC Voltage Electrophoretic Deposition and a Novel Pulsed Voltage Method, Electrochemistry Communications 737 (4) 2002. 2. B. Yu, S. B. Khoo, Electropolymerization of 4-nitro-1,2-phenylenediamine and electrochemical studies of poly(4-nitro-1,2-phenylenediamine) film, Electrochim. Acta, in press 3. B. Yu, S. B. Khoo, Electrochemical impedance spectroscopic studies of poly(4nitro-1,2-phenylenediamine) film-modified electrodes, to submitted to J. Electroanal. Chem. 4. B. Yu, S. B. Khoo, Studies of electrodes modified with poly(4-nitro-1,2phenylenediamine) / zeolite composite film, in preparation. 158 [...]... poly(1, 2phenylenediamine) because of site-blockage at the para-position Of even more interest is the reduction of the nitro- group on the benzene ring Therefore, we carried out the anodic electropolymerization of 4 -nitro- 1 ,2- phenylenediamine (4NoPD) in different supporting electrolytes at different pH The feasibilities of forming the polymer poly(4- nitro- 1 ,2- phenylenediamine) (P4NoPD) on gold and glassy... control and manipulate the properties of electrode surface can meet the needs of many sensing problems This field of modified electrodes, which has experienced a period of rapid growth over the past decades, has now reached a level of maturity that allows the used of these electrodes for routine sensing applications [104-108] Chemical sensors based on modified electrodes are still in the early stages of their. .. semiconductor electrodes were exploited for the photoelectrocatalytic reduction of hydrogen ion [ 52, 53], and films on conductive electrodes for the electrocatalytic reduction of bicarbonate [54] Bilayer electrodes are prepared by coating the electrode first with a layer of a redox polymer and then with a second layer of a different redox polymer There is no contact between the electrode and the second... driving electrodes as 19 the working electrode [101] EQCM is particularly suited to the modified electrode studies where oxidation or reduction of the film on the electrode surface causes ions to enter or leave the film [1 02, 103] 20 1.3 Applications of chemically modified electrodes The applications involving modified electrodes are multiple and widespread: chemical sensing, energy conversion and storage,... mixing them with the sol-gel precursors Zeolites and Clays Zeolites are crystalline aluminosilicates organized into regular three dimensional networks with intracrystalline void spaces consisting of channels and cages which may be interconnected [38] Such pores and channels allow the ingress and egress of molecular and ionic species controlled by factors such as size, charge and shape Thus, zeolites possess... based on the diversity of potential (chemical and biological) surface modifiers It is also expectable for more powerful sensing probes based on polishable and robust modified surfaces, arrays of micoelectrodes (each coated with a different modifier), 21 multifunctional films (based on the coupling of several moieties), and intimate integration of biological and chemical entities 1.3 .2 Energy-producing... preconcentration, and ion-exchange, etc In the context of electrochemistry, the first reported instance of a zeolite modified electrode was in 1983 when Ghosh and Bard modified a tin oxide electrode with a thin layer of clay (a material related to zeolite) [39] Since then, due to the advantageous properties of zeolite, there have been keen interests in zeolite modified electrodes (ZMEs) [40] 1.1.3 Electrodes modified. .. selectivity, and the scope of such sensors are highly desirable to meet new challenges posed by clinical and environmental samples Traditionally, the utility of solid-based sensors is often hampered by a gradual fouling of the surface due to the adsorption of large organic surfactants or of reaction products This offers a great potential for alleviating the above problems; hence, the tailoring of modified electrodes. .. 86 (1988) 135 [20 ] G T R Palmore, D K Smith, M S Wrighton, J Phys Chem B 101 (1997) 24 37 [21 ] L J Kepley, A J Bard, J Electrochem Soc 1 42 (1995) 4 129 [22 ] D D Schlereth, A A Karyakin, J Electroanal Chem 395 (1995) 22 1 [23 ] B Persson, H S Lee, L Gorton, T Skotheim, P Bartlett, Electroanalysis 7 (1995) 935 [24 ] A A Karyakin, O A Bobrova, E E Karyakina, J Electroanal Chem 399 (1995) 179 [25 ] C X Cai, K... Electrochem Soc 146 (1999) 4 324 [114] C E D Chidsey, Science 25 1 (1991) 919 [115] R A Marcus, J Chem Phys 48 (1976) 124 7 34 CHAPTER II Studies of zeolite modified electrodes fabricated by electrophoretic deposition 35 2. 1 Introduction The controlled assembly of colloidal particles has received much attention in recent years because of the potential applications of nano- and micro-structured materials . 14 1 .2 Characterization and analysis of chemically modified electrodes Any discussion of attaching molecules to electrodes surfaces is incomplete without a description of how the success of. interface [2, 56-58]. For example, a typical system consists of a Pt substrate with an electrodeposited film of polymerized [Os(vbpy) 3 ] 2+ on which a film of poly[Os(vbpy) 2 (bpy) 2 ] 2+ is. properties of zeolite, there have been keen interests in zeolite modified electrodes (ZMEs) [40]. 1.1.3 Electrodes modified with spatially defined and heterogeneous layers In addition to the modified