Monolayers redox properties

From this discussion it is clear, that, independently of their redox properties, suitably modified electrodes offer themselves for the introduction of diastereo- or enantioselectivity into electrochemistry. Early reports of chiral inductions at modified electrodes include reactions at graphite and SnO surfaces derivatized with monolayers of (S)-(—)-phenylalanine. Asymmetric inductions at the chiral graphite electrode could, however, not be verified in other laboratories even after great efforts... 

The results presented here seem to indicate that 1) the local order about ruthenium centers in the polymers is essentially unchanged from that in the monomer complex and 2) that the interaction with the electrode surface occurs without appreciable electronic and structural change. This spectroscopic information corroborates previous electrochemical results which showed that redox properties (e.g. as measured by formal potentials) of dissolved species could be transferred from solution to the electrode surface by electrodepositions as polymer films on the electrode. Furthermore, it is apparent that the initiation of polymerization at these surfaces (i.e. growth of up to one monolayer of polymer) involves no gross structural change. 

The supramolecular assembly of (3-cyclodextrin-capped AuNPs on ITO previously coated with a monolayer of ferrocene molecules was shown to exhibit interesting redox properties, as presented in Figure 8.5.25 This is a fine example of the functionalization of a nanoparticle with a functionalized recognition element to create a unique supramolecular scaffold with tailor-made properties. Specifically, the electrocatalytic determination of ascorbic acid was demonstrated by this surface assembly with excellent discrimination over dopamine. 

In the first case where metal surfaces provide active oxygen species to the support contact structure is not critical. The second case is often observed when supported metal catalysts are prepared by coprecipitation or sol-gel methods. Noble metals whose oxides are more stable than Pt oxides such as Pd and Ir are more readily buried in the bulk of metal oxide supports, and the metal oxide overlayers of a thickness of about a few monolayers are modified in their electronic and redox properties by underlying noble metal nanoparticles to become active at lower temperatures. 

The signal-triggered functions of these molecular assemblies have to be first characterized in bulk solution. Then, extensive efforts have been directed to integrate these photoswitchable chemical assemblies with transducers in order to tailor switchable molecular devices. The redox properties of photoisomerizable mono-layers assembled on an electrode surface are employed for controlling interfadal electron transfer [16]. Specifically, electrical transduction of photonic information recorded by photosensitive monolayers on electrode supports can be used in developing monolayer optoelectronic systems [16-19]. Electrodes with receptor sites exhibiting controlled binding of photoisomerizable redox-active substrates from the solution [20] also allow the construction of molecular optoelectronic devices. 

Roth K.M., Rajeev N.D., Dabke B., Gryko D.T., Clausen C., Lindsey J.S., Bocian D.F. and Kuhr W.G. 2000. Molecular approach toward information storage based on the redox properties of porphyrins in self-assembled monolayers, J. Vac. Sci. Technol. B Microelectron. Process. 

On the other hand, the electrodes attaching electroactive polymer films and polymer CMEs having ionic, electronically conductive, or redox property have been widely studied. Electronically conducting polymers, which are usually more conductive than redox polymers, can be doped to increase their intrinsic conductivity. Such films are now popular because they are technically easier to modify by applying polymer onto an electrode compared to covalent monolayer formation. Polymer films generally adhere satisfactorily to electrodes simply by forces of chemisorption or by being insoluble in the contacting solvent. 

The thiolated j -CD forms monolayers with substantial defect densities due to the lack of favorable lateral interactions between the CD molecules. In order to patch the holes in these monolayer assemblies without clogging the CD cavities, we devised procedures for the preparation of mixed monolayers containing both the thiolated P-CD receptors and pentanethiol molecules. These monolayers were characterized by reductive desorption and electrode capacitance measurements. The monolayer blocking properties were also investigated, i.e., the voltammetric response of solution redox probes such as Fe(CN) and Ru(NH3)6 at the monolayer-covered electrodes were recorded. Our data supported the monolayer structure depicted in the Scheme 3 [26]. 

In humid air, the metal surface will be covered by several monolayers of water as long as no hygroscopic impurities are adsorbed. Under these circumstances, standard electrochemistry will not be valid anymore, as no electrolytic double layer is formed and the reaction products will not be transported away from the surface. Therefore, the rate of any electrochemical reaction will be rather small and the electrode potential , which is measured by the Kel-vinprobe cannot be defined in the usual terminology of electrochemistry but will reflect, e.g., the redox properties of the oxide scale. In particular, the electrode potential ... 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

Another method of immobilizing enzyme and mediator is the attachmait of a mediator-containing monolayer to an electrode surface. If this monolayer is also functionalized with an enzyme cofactor meant to bind the enzyme, a catalytic electrode results. For example, gold electrode surfaces ean be eovered with a PQQ monolayer. The monolayer retains the redox properties of PQQ and ean be functionalized further. If FAD eofaetors are then covalently bound, GOx apoenzyme can bind the FAD centers. A covalent molecular chain results and electrons may hop from FAD to PQQ to the electrode surface, thus mediating from the enzyme [87]. PQQ monolayers can also link to covalently bound NAD" ", for mediation to NAD" -dependent dehydrogenase enzymes [88,89]. Functionalizing with monolayers of cytochrome c has also been demonstrated [90]. 

The present review is concerned mainly with the electrochemical formation and redox behavior of the hydrous oxides of those transition metals centered within and around Group VIII of the periodic table. There have been a number of recent reviews of monolayer oxide growth on these metals so that this area will not be treated here in an exhaustive manner. Structural data for many of the systems (especially direct evidence obtained by investigation of hydrous films themselves) are very sparse at the present time. However, some idea of the type of material involved can be obtained from structural studies of oxide battery materials a useful introduction to the structural complexities in this area in general is Alwitt s account of the aluminium oxide system. An important feature of hydrous oxides, not normally as evident with their anhydrous analogs, is their acid-base behavior and in particular the influence of the latter on the redox properties of the hydrous material. Because of its central role in many oxide (especially hydrous oxide) processes, and its relative neglect in the electrochemistry of these systems until quite recently, this add-base character of oxide systems will be reviewed here in some detail. 

Geddes NJ, Sambles JR, Jarvis DJ, Parker WG, Sandman DJ (1992) The electrical properties of metal-sandwiched Langmuir-Blodgett multilayers and monolayers of a redox-active organic molecular compound. J Appl Phys 71 756-768... 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Fe2(III,III)2+ states. By considering that organometallic dendrimers based on conjugated ferrocene units are of special importance since mixed-valence states have interesting electrical, redox, and magnetic properties, recently three generations of polypropylene amine) dendrimers, decorated at their periphery with 4, 8, and 16 (compound 6, Fig. 6.6a) BFc units, respectively, have been synthesized and the electrochemical behavior of the dendrimers complexed with (3-cyclodextrins ([3-CD) and adsorbed at self-assembled monolayers (SAMs) of heptathioether-functionalized [3-CD on gold (molecular printboard) has been studied.40... 

Similar monolayers have been prepared with a diversity of electroactive units with the ultimate goal of elucidating the subtle balance between the structural and the electronic factors that regulate interfacial electron transfer.5,9,10 In particular, these studies have focused their attention on the rationalization of the influence that the distance between the electrode surface and the redox centers as well as the nature of the linkers between them have on the rates of electron transfer. In parallel to these fundamental investigations, the ability of thiols to anchor electroactive units on metallic electrodes has also been exploited to fabricate a wealth of nanostructured materials with tailored functions and properties.6-8 Indeed, these convenient building... 

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