Adsorption at electrode surfaces

The study of the adsorption of DNA at electrode surfaces is of fundamental interest, since the interaction of DNA with charged surfaces can be expected in biological systems. In fact, a number of studies of the adsorption of DNA were conducted at mercury electrodes [43-51] and carbon electrodes [52] and in general weaker adsorption was observed with native DNA than with denatured DNA. 

Osmium tetroxide is an electroactive marker of the polynucleotide chain and is a good probe of the DNA structure since dsDNA is modified by osmium to a much lesser extent than single-stranded polynucleotides. The limit of detection of osmium-labelled DNA was below 5 ng cm-3 after 2 min accumulation time [66]. Adsorptive stripping linear sweep voltammetry of osmium tetroxide labelled DNA at a mercury electrode [66, 67] was shown to be a good sensor for hybridization of DNA. 

Some similar features were observed concerning the adsorption and electrochemical oxidation of DNA on glassy carbon and tin oxide electrodes [68]. Differential pulse voltammograms were recorded in buffer solution without DNA after adsorption of DNA onto the electrode surface during a predetermined time at a fixed potential suggesting the possibility of using adsorption to preconcentrate DNA on solid electrode surfaces and use this DNA-modified electrode for analytical purposes. 

A recently developed small-volume sono-voltammetric cell [69] in which the glassy carbon electrode is exposed to high intensity ultrasound was used in order to permit reliable simultaneous voltammetric determinations 

Spectroscopic techniques such as surface enhanced Raman spectroscopy (SERS) have also been used to investigate the adsorption of DNA onto the electrode surfaces [76, 77]. 

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Application of the foregoing relations to the study of adsorption at electrode surfaces requires an understanding of the electrochemical processes at electrode-solution interfaces. Consider an electrode in contact with a solution containing electroactive species along with supporting electrolytes. Two important processes occur at the electrode surface a faradaic process in which electrons are transferred across the electrodesolution interface (oxidation-reduction reaction). As a result of these reactions current flows through the medium. Adsorption-desorption is... 

Lipkowski, J. and Ross RN. (eds) (1992) Adsorption at Electrode Surface, VCH, New York. Schmickler, W. and Parsons, R. (1997) Interfacial Electrochemistry, Oxford University Press, London. 

Variation of Dissociation Constants of Acids and Bases during Adsorption at Electrode Surface... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

Com, R. M., In situ second harmonic generation studies of molecular orientation at electrode surfaces, in Adsorption of Molecules at Metal Electrodes, J. Lipkowski and P. N. Ross, Eds., VCH, New York, 1992, p. 391. 

The thermodynamic redox potential of NAD+/NADH is —0.56 V vs SCE at neutral pH. The NADH cofactor itself is not a useful redox mediator because of the high overpotential and lack of electrochemical reversibility for the NADH/NAD+ redox process, and the interfering adsorption of the cofactor at electrode surfaces. 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

The determination of the Gibbs energy of adsorption at zero surface coverage AGg=o nd of the interaction parameter A as a function of an electrical variable, may become a valuable source of information on the interactions at the interface. The value of AG°can be considered as the energy required to replace n monomolecularly adsorbed solvent molecules from a fully solvent-covered electrode surface by one monomeric molecule of the solute... 

Several approaches have been described for the immobilization of NPs at electrode surfaces. The simplest is direct adsorption of the NP from solution onto an electrode surface. This method takes advantage of favorable electrostatic interactions between the surface and the NPs. This has been used, for example, to immobilize Ti02 NPs... 

Adsorbate Molecular Orientation at Electrode Surface. Adsorption of some molecules from solution produces an oriented adsorbed layer. For example, nicotinic acid (NA, or 3-pyridinecarboxylic acid, niacin, or vitamin B3) is attached to a Pt(lll) surface primarily or even exclusively through the N atom with the ring in a (nearly) vertical orientation (12) (Fig. 10.5a). 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

Refs. [i] Inzelt G (2005) / Solid State Electrochem 9 245 [ii] Horanyi G (1980) Electrochim Acta 25 45 [iii] Horanyi G (2004) In Horanyi G (ed) Radiotracer studies of interfaces. Elsevier, Amsterdam, chapters 1,2,4,6 [iv] Horanyi G (2002) State of art present knowledge and understanding. In Bard AJ, Stratmann M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, Weinheim, Chap. 3 [v] Horanyi G (1999) Radiotracer studies of adsorption/sorption phenomena at electrode surfaces. In Wieckowski A (ed) Interfacial electrochemistry. Marcel Dekker, New York, pp 477 [vi] Horanyi G, Inzelt G (1978) / Electroanal Chem 87 423 [vii] Horanyi G, Inzelt G, Szetey E (1977) / Electroanal Chem 81 395 [viii] Vertes G, Horanyi G (1974) / Electroanal Chem 52 47 [ix] Horanyi G (1994) Catal Today19 285 [x] Horanyi G (2003) Electrocatalysis - heterogeneous. In Horvath IT (ed) Encyclopedia of catalysis, vol. 3. Wiley Interscience, Hoboken, pp 115-155 [xi] Inzelt G, Horanyi G (2006) The nickel group (nickel, palladium, and platinum). In Bard AJ, Stratmann M, Scholz F, Pickett CJ (eds) Inorganic chemistry. Encyclopedia of electrochemistry, vol 7a. Wiley-VCH, Weinheim, chap. 18... 

Refs. [i] Horanyi G (1999) Radiotracer studies of adsorption/sorption phenomena at electrode surfaces. In Wieckowski A(ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 477-491 [ii] Kalman E, Lakatos M, Karman FH, Nagy F, Klenc-sar Z, Virtes A (2005) Mossbauer spectroscopy for characterization of corrosion products and electrochemically formed layers. In Freund HE, Zewi I (eds) Corrosion reviews. Freund Publishing House, Tel Aviv, pp 1-106 [iii] Horanyi G, Kalman E (2005) Recent developments in the application of radiotracer methods in corrosion studies. In Marcus PH, Mansfeld F (eds) Analytical methods in corrosion science and engineering, CRC Press, Boca Raton, pp 283-333 [iv] Stivegh K, Horanyi TS, Vdrtes A (1988) Electrochim Acta 33 1061... 

Electrode potential influences organic adsorption at Pt surfaces from aqueous solutions primarily by bringing to the surface competing adsorbates such as OH or H [61] (Fig. 30) and by oxidation or reduction of the adsorbate. Based upon the limited reliable data available, it appears that these redox and chemisorption effects greatly outweigh the electrostatic effects. 

While there have been many SHG studies at the solid electrode/liquid interface as both an in situ probe of the electrode interface and the influence of adsorption at the surface of the electrode, there have been far fewer studies of electrochenucal processes occurring at the boundary between two immiscible electrolyte solutions. 

The principal aims of this review are to indicate the role of chemisorbed intermediates in a number of well-known electrocatalytic reactions and how their behavior at electrode surfaces can be experimentally deduced by electrochemical and physicochemical means. Principally, the electrolytic gas evolution reactions will be covered thus, the extensive work on the important reaction of O2 reduction, which has been reviewed recently in other literature, will not be covered. Emphasis will be placed on methods for characterization of the adsorption behavior of the intermediates that are the kinetically involved species in the main pathway of the respective reactions, rather than strongly adsorbed by-products that may, in some cases, importantly inhibit the overall reaction. The latter species are, of course, also important as they can determine, in such cases, the rate of the overall reaction and its kinetic features, even though they are not directly involved in product formation. 

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