Phthalocyanine modified surfaces

Thiols can be oxidised at a variety of solid electrodes, such as noble metals, carbon and carbon with chemically modified surfaces. Mefford and Adams found that relatively high voltages, greater than -1-1 V, were required to oxidise GSH and cysteine at glassy carbon electrodes. Chemically modified surfaces can reduce the oxidation potentials required and hopefully increase specificity and sensitivity. Halbert and Baldwin used cyclic voltammetry to study the electrochemistry of cysteine, homocysteine, A -acetylcysteine and GSH at both unmodified and cobalt phthalocyanine-modified carbon paste electrodes. This non-chromatographic technique was used to measure the relatively high concentrations of GSH in whole blood. The electrochemistry of thiols has been reviewed. ... 

Bravo-Diaz C —> Gonzalez-Romero E Briscoe WH, Horn RG Electrical double layer interactions in a non-polar liquid measured with a modified surface force apparatus 147 Brunner M, Bechinger C Colloidal systems in intense, two-dimensional laser fields 156 Buckin V Lehmann L Burrows HD, Kharlamov AA About energy and electron transfer processes in Cgo/phthalocyanine films 52 Burrows HD Kharlamov AA Burrows HD Hungerford G... 

Porphyrin-modified electrodes were widely utilized for quantification of low concentrations of metals, pharmaceutical products, and species of environmental and industrial importance in association with flow techniques. This strategy enhances the amperometric response and sensitivity (the capacitive current is virtually reset), while reducing significantly the time necessary for analyses. Flow injection analysis (FIA) has been extensively explored for this purpose and a revision on the application of porphyrins and derivatives was published some years ago [203]. Alternatively, batch injection analysis (BIA) can be an interesting way for fast analyses utilizing a similar mass transport process to the electrode surface [204]. For example, citric acid was electrochemically determined by BIA using cobalt phthalocyanine modified carbon paste electrodes [205]. 

XPS can be used to characterize the modified surfaces [43]. XPS has been used to analyze thin films containing azide groups fi om as early as 1989 [44, 45]. For the SAM formation using aryl- or alkyl-thio (SR) phthalocyanines, the question as to whether the R group remains intact following SAM formation was resolved using XPS, Fig. 10, which showed that some RS bond remains intact [46]. 

The penetration of ions from the subphase into the shell of spread particles is a general phenomenon and can be used to modify and functionalize the particle surface. For example, metal ions, such as Ba and Fe, or cationic polyelectrolytes, such as the polycation of polyallylamine, can be adsorbed at anionic particles, while anionic water-soluble dyes, such as phthalocyanine tetrasulfonic acid and 1.4-diketo-3.6-diphenylpyrrolo[3.4-c]pyrrole-4, 4 -disulfonic acid (DPPS) [157], can be adsorbed at cationic particles. However, since only a monolayer of the dye is adsorbed, a deep coloration of the particles is not obtained unless a dye with very high absorption coefficient is used [156],... 

Phthalocyanines have attracted particular attention as potential surface modifiers due to their stability and tendency to form ordered structures directed by dispersion forces. They are inherently host-guest structures with a readily interchangeable coordinating metal ion, which in the solid state results in a tunable bandgap. At a surface, in addition to possibly interesting electronic... 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

MWCNTs were functionalized with iron phthalocyanines (FePc) to improve the sensitivity towards hydrogen peroxide. A highly sensitive glucose sensor with an FePc-MWCNT electrode based on the immobilization of GOx on poly(o-amino-phenol) (POAP)-electropolymerized electrode surface [219]. A hemin-modified MWCNT electrode to be used as a novel 02 sensor was obtained by adsorption of hemin at MWCNTs and the electrochemical properties of the electrode were characterized by cyclic voltammetry [220]. 

The surface chemical effects of interest do not go as far as those induced in (extensively) modified carbon electrodes [248], e.g., by pyrolyzed phthalocyanines or macrocycles [249-255], by anthraquinone or its derivatives [126,247,256-259], or by aryl groups [125], or those of stable and efficient sonoelectrocatalysts by modifying GC electrodes with 9,10-phenanthraquinone or 1,2-naphthoquinone [260], Instead, it is explored here whether and how a seemingly simple but crucial issue has been addressed or resolved what makes 02 adsorption in ORR nondissociative The isotopic labeling evidence for this experimental fact has been presented half a century ago [261], and it has not been challenged [262], The implication, based on the equally noncontrover-sial literature that 02 chemisorption on carbons (even at room temperature) is dissociative, is summarized below ... 

Figure 3.11 shows the chemical deposition process of GOx enzyme assembled 2-mercaptoethanol on gold that was previously modified by cobalt tetracarboxylic acid chloride phthalocyanine self-assembled monolayer gold. Electrochemical biosensor was introduced in the following steps pretreatment hydrolysis functionalized surface -> enzyme modification. Shervedani and his coworkers reported a similar... 

Electrochemical NO sensors based on platinized or electrocatalyst-modified electrodes often in combination with a permselective and charged membrane for interference elimination were proposed. Although the catalytic mechanism is still unknown, it can be assumed that NO is co or dinative ly bound to the metal center of porphyrin or phthalocyanine moieties immobilized at the electrode surface. The coordinative binding obviously stabilizes the transition state for NO oxidation under formation of NO+. Typically, sub-pM concentrations of NO can be quantified using NO sensors enabling the detection of NO release from individual cells. 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

As shown in this symposium, interest in chemical modification of electrode surfaces has been extended in many directions, including the study of light-assisted redox reactions, and the use of modified electrodes in electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (A,5). 

An electrode covered with several molecular layers of dye could be made to adsorb all of the visible light, and obviate the need for the multielectrode stack. Very thick dye layers have tended not to be conductive or highly photoconductive so that their photoelectrochemical efficiencies are no better and perhaps worse than those seen on electrodes modified with very thin dye films. Molecular disorder of the dye appears to be the dominant reason for lack of conductivity in thick films of fluorescein-type, cyanine-type, and phthalocyanine-type dyes (12). It has been shown however that ordered molecular systems (mainly conjugated, highly unsaturated hydrocarbons) have considerable potential as conductive media, and that these ordered systems may be used to chemically modify electrode surfaces (12, 15). 

Figure 4. Various semiconductor electrodes, modified with monolayers (covalently attached) of phthalocyanine tethered to the electrode surface (a) or multilayers (adsorbed or sublimed) which aggregate to leave a semiporous surface layer (b) and a uniform phthalocyanine film leading to a p-type semiconductor layer adjacent to the n-type semiconductor substrate (c).

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