Electrochemical detection, with porphyrins

This is a typical example of the unique application of the electrochemical sensor for detection of NO release ftom normotensive and hypertensive rats." - It has been reported based on spectroscopic measurements, that endothelium of hypertensive rats produced more NO2/NO3 than endothelium of normotensive rats. However these reports were in contradiction to data obtained on smooth-muscle relaxation, hindered in hypertensive rats. This means that the endothelium of hypertensive rats should produce less NO. Electrochemical measurements with porphyrinic sensor clearly show that net concentration of NO produced by endothelium of SHR rats is lower than that produced by the endothelium of WKY rats. These results correlate well with previously reported smooth-muscle relaxation data. Total production of NO by the endothelium of hypertensive rats is slightly higher than in normotensive rats. However, the endothelium of SHR rats also generated... 

B. Electrochemical Detection of Azanone with a Cobalt Porphyrin 131... 

Irradiation of iron and cobalt porphyrins (13 Fe(TPP), 14 Co(TPP), H2TPP = 5,10,15,20-tetraphenyl-21/f,23/f-porphyrin) in the presence of triethylamine (TEA) using > 320-nm tight caused the photocatalytic reduction of CO2 [15, 16, 27-31]. When 13 was used as a photocatalyst, CO was detected with TNco = 70 after 180-h irradiation [15]. Formic acid was the main product when 14 was employed as a photocatalyst [16]. The reaction mechanism proposed on the basis of UV-vis absorption changes during photolysis and radiolysis, and electrochemical measurements are shown in Scheme 3. M (TPP) is reduced to M (TPP) by photoinduced electron transfer from TEA, which subsequently disproportionates to M°(TPP), the proposed catalyticaUy-active species. 

Nitric oxide has been of considerable recent interest because of its signaling properties and its importance in pathophysiology [112, 113]. Methods for the electrochemical detection of NO have been reviewed [114,115]. The two principal am-perometric methods have recently been compared for in vivo NO detection direct oxidation at Pt/Ir electrodes, with selectivity provided by membrane coatings, or oxidation at Ni-porphyrin modified carbon fiber microelectrodes coated with Nafion for anion rejection of these, only the second method yielded signals that could he attributed to NO [116]. 

On a glassy carbon electrode modified with nanowires of polytetrakis (o-aminophenyl)porphyrin, SWCNT and Nation as binders resulted in an efficient electrochemical detection of hydrogen peroxide [228], while detection of H2O2 in beverages was achieved with the picket-fence porphyrin, FeTpivPP, on MWCNTs [229]. 

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. 

In summary, the position of the frontier electron energy levels (HOMO and LUMO) of porphyrins and phthalocyanines can be finely tuned by the appropriate combination of central metal and substituted ligand as detected in photoelectron spectroscopy or in the redox potentials. A rich redox chemistry in interplay with a number of reactants and counterions has been estabhshed. A systematic consideration of the electrochemical and photoelectrochemical characteristics of porphyrins and phthalocyanines as individual molecules in solution, as molecules adsorbed at surfaces, and as molecular thin films serving to mimic the characteristics of molecular aggregates is of much relevance to the choosing or designing of optimized porphyrin or phthalocyanine sensitizers for DSSCs. 

It has been described the formation of porphyrin- and sapphyrin-containing self-assembled monolayers on electrochemically prepared Au surfaces by a multistep approach After electrochemical preparation the Au substrate was characterized by SERS and cyclic voltammetry. SERS characterization was also used to study the basic architecture and properties of the porphyrin- and sapphyrin modified SAMs and their interactions with fluoranthene, 1,10-phenantroline and adenine. In general, it can be proposed that macrocycle oligopyrrole functionalized SAMs could be useful in the analysis and detection of polyaromatic and heterocyclic compounds. 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

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