Porphyrin films, anion-selective

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

Figure 8. Selectivity pattern of an ISE based on an electropolymerized cobalt porphyrin film. The electrode was exposed to the following anions thiocyanate (1), perchlorate (2), iodide (3), nitrite (4), salicylate (5), bromide (6), chloride (7), bicarbonate (8), phosphate (9). The baseline potential was 364 mV. (Adapted from ref. 26.)...

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

Daunert, S., S. Wallace, A. Hondo, and L.G. Bachas (1991). Anion-selective electrodes based on electropolymerized porphyrin films. Anal. Chem. 63, 1676-1679. 

Several authors have also reported the potential use of electropolyme-rized metalloporphyrin films as new electrode materials for anion detection and pH measurements. For example, Daunert et al reported that anion-selective membrane electrodes can be prepared by electropolymerizing aniline-substituted cobalt tetraphenylporphyrin complexes onto a glassy carbon surface. It was shown that the resulting electrodes are highly selective towards the detection of thiocyanate and nitrite anions. The mechanism by which these metalloporphyrin electrode sensors operate is probably related to the well-known axial complexation capability of the central metal cation of the porphyrin towards different kind of anions and bases. The same authors also reported a detection limit of 5 x 10 M for these ion-selective electrodes which have lifetimes of more than 2 months... 

Meyerhoff et al have reported the use of a non-metallated porphyrin film as an ion-selective electrode. They have shown that the electropolymerized films offer significant selectivity for the detection of iodide over a wide range of anions. The lifetime of these electrodes was estimated by analyzing some of their characteristics such as sensitivity and detection limits. The effect of pH on the detection of iodide by the film-supported electrodes was also established. For pH values from 3 to 8, there is relatively little effect of the pH on the electrochemical response of the electrodes. At pH values higher than 8, the electrode response is affected by the interference by hydroxide ions. 

The anion optical sensor can also be fabricated with metalloporphyr-ins. For example, polymeric membranes doped with indium porphyrins and a lipophilic dichlorofluorescein derivative were shown to be very selective to chloride and acetate anions. The response mechanism is based on extraction of anions into the bulk organic film by indium porphyrins and a simultaneous coextraction of hydrogen ions. This results in protonation of the pH chromophore, and hence a change in the optical absorbance of the polymeric film. 

Combined physical/supramolecular methods have also been applied for the organization of porphyrins. The layer-by-layer method,based on alternating electrostatic assembly of materials, has been used to assemble porphyrins (Figure 7), and their potential as sensing elements was assessed. Also, porphyrinoids such as N-confused porphyrins have been incorporated in LB films and used as a selective sensor for iodide anions based on the incorporation of these anions in the LB films upon their transfer to a substrate. In that work, only iodide anions formed a stable... 

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