Sensors, electrochemical anionic

Ferrocene appended 2,5-diamidopyrole receptors were subsequently developed in order to produce electrochemical sensors for anions (.Figure 7).8 The anion complexation of these compounds was reported through the results of both H NMR titration and cyclic voltammetry techniques. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

Preorganisation of redox anion sensors on electrode surfaces is a promising new technique for electrochemical anion sensing. Self-assembled monolayers or thin polymer films of metal-based receptors have the potential to generate an amplified response to anion binding akin to the dendritic effect. 

Supramolecules containing metal-polypyridine units, especially the Ru(dpp)-based dendrimers, could be used as electron reservoirs or components of molecular-electronic devices. Supramolecules in which an electroactive M(N,N) group is attached to a receptor capable of molecular recognition (crown ethers, calixarenes, cryptands etc.) can work as electrochemical sensors. Electrochemical recognition of cations as well as anions has been reported [33-35, 257, 263]. 

It should be noted that 86 also functions as an electrochemical anion sensor, where the Zn(II) porphyrin-based oxidation potential is sensitive to... 

As described in the previous section, the electrochemical characteristics of the self-assembly of MNC is dependent on the anion present in the supporting electrolyte and is independent of the solution pH, which makes the self-assembly of MNC an excellent and versatile electrochemical anion sensor. The addition of millimolar amounts of different kinds of anions was found to induce different magnitudes of negative shift in the of the SAM of MNC (Table 10-3) [103]. 

Gale. P.A. Hursthouse. M.B. Light, M.E. Sessler, J.L. Wamner, C.N. Zimmerman. R.S. Ferrocene-substituted calix[4]pyrole A new electrochemical sensor for anions involving CH... anion hydrogen bonds. Tetrahedron Lett. 

Shinkai and coworkers reported that ferroceneboronic acid 17 (Fig. 3) acts as an electrochemical anion sensor.It exhibits excellent selectivity for fluoride ions in the presence of other halides and anions such as SCN , S04, and H2PO4 . A i ox value of 1000 M in MeOH/H20 was found for fluoride, compared to values of less than 2 M for chloride and bromide. The fluoride ion is a hard base and. therefore, interacts considerably with the hard boron atom. Oxidation results in the ferrocene group becoming more electron withdrawing. The electron density of the boron atom decreases therefore, the strength of the fluoride complex increases. A biden-tate Z7/ s(boronate) Lewis acid was also shown to act as a fluoride sensor. 

Gale. P.A. Chen. Z. Drew. M.G.B. Heath. J.A. Beer. P.D. Lower-rim ferrocenyl substituted calixarenes New electrochemical sensors for anions. Polyhedron 1998. 17, 405. 

This interest may be attributed to the biological role played by anions. For example, enzymatic binding and reactions with ATP are the basis of the free energy in metabolism and the majority of enzymes act on anionic substrates. Several classes of artificial anion receptors have been reported, capable of not only binding but also of electrochemically recognizing anions. Such receptors have a potential application in the production of electrochemical anion sensors. 

Sap P, Anghel DF, Luca C (1983) Electrochemical sensors for anionic surfactants. I. The electrode based on bis[2,4,6-tris(2-pyridyl)-l,3,5-triazine]iron(II) dodecyl sulfate. RevRoum Chim 28 883-890... 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

The copper flow-through CL sensor comprised an anion-exchange column having luminol and cyanide coimmobilized on the resin, while copper was temporarily retained by electrochemical preconcentration on a Au electrode placed in an anodic stripping voltammetric cell [64], Injection of 0.1 mol/L NaOH through the column eluted the reagents, which then reacted with copper, stripped from the electrode to produce a CL signal. The response was linear in the 0.01-10-pg/L... 

Zhang s group [82] recently presented a novel CL sensor combined with FIA for ammonium ion determination. It is based on reaction between luminol, immobilized electrostatically on an anion-exchange column, and chlorine, electrochem-ically generated online via a Pt electrode from hydrochloric acid in a coulometric cell. Ammonium ion reacts with the chlorine and decreases the produced CL intensity. The system responds linearly to ammonium ion concentration in a range of 1.0-100 pM, with a detection limit of 0.4 pM. A complete analysis can be performed in 1 min, being satisfactorily applied to the analysis of rainwater. 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Sensing of chlorine is possible with a phthalocyanine-based optode that is elec-trochemically reset [101]. Also a direct electrochemical Clark-type sensor employing carbon electrodes has been investigated [102]. For this type of sensor, the various types of carbon gave different responses and the edge-plane sites of graphitic electrodes were identified as electrochemically active. Both chlorine reduction and chlorine evolution were studied and the effects of the trichloride anion, Ch", were highlighted. 

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