Porphyrin-based anion sensors

Recently a porphyrin unit has been incorporated into an anion receptor (Fig. 46) (Beer et al., 1995g). H nmr titration experiments with this compound demonstrated the formation of 1 1 stoichiometric complexes with tetrabutylammonium halides, nitrate, hydrogensulfate and dihydrogenphosphate. 

The electrochemical properties were investigated by CV and SWV. The effects of adding anions to solutions of the porphyrin are summarized in Table 20. Interestingly, the porphyrin reduction waves are not significantly perturbed by any anionic guest, but the cobaltocenium moieties do show cathodic shifts of up to 225 mV with the dihydrogenphosphate anions. 

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Studies of synthetic porphyrin-based anion receptors should form the basis for more effective sensors. Metallocene-substituted porphyrins examined by Beer and coworkers have proven successful in the solution-phase binding of ions such as chloride, bromide and nitrate." The cobaltocenium-substituted and ferrocene-substituted porphyrins (Figure 134) bind ions in solution, as shown by H NMR and electrochemical studies. The latter measurements reveal that the porphyrin and ferrocene redox... 

A. Preparation of Porphyrin-Based Potentiometric Sensors for Anions.252... 

A. PREPARATION OF PORPHYRIN-BASED POTENTIOMETRIC SENSORS FOR ANIONS... 

Very recently Beer and co-workers have developed a surface-enhanced optical anion sensor based on gold nanoparticles [78]. Dodecanethiol-stablised gold nanoparticles were modified by ligand substitution with a disulphide-substituted zinc porphyrin 113 to provide 30 and 80 receptors per nanoparticle. Titration of both the free receptor and the modified nanoparticles with various... 

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

Metalloporphyrins, one carbon larger but structurally analogous to the corrin ring of vitamin B12, exhibited unique anion ionophore properties when incorporated into a polymer membrane. Mn(III), Co(III), Ru(II), and Sn(IV) porphyrin-based ion-selective sensors exhibit high selectivity for thiocyanate, nitrite/thiocyanate, thiocyanate and siicylate, respectively. doo-io7,i 10-114 nature of the... 

Sapphyrin had a very sharp, strong Soret band and very weak Q bands in the visible region, and the color was quite different from that of porphyrin [98]. Sapphyrin was named after its clear sapphire-like blue color, which was closely related to the characteristics in the electronic absorption spectra. There is no other obvious absorption than the strong Soret band in the visible region. Because this band was known to be quite sensitive to pH and coexisting anions, sapphyrins have attracted attention as an anion sensor [99]. Preparation of sapphyrins was mostly achieved by the [3 + 2], [3 + 1 + 1], and [4+1] methods [99]. Scheme 15.21 shows the preparation of BCOD-fused sapphyrins based on the [3 + 2] condensation. Precursors of mono-, di-, tri-, and penta-benzosapphytins were obtained (Scheme 15.21) [25]. Mono- and di-BCOD-fiised sapphyrins were obtained as di-protonated forms of 95a and 95b by treatment with hydrochloric acid for the sake of easy isolation. On the other hand. 

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. 

Compared to systems based on a single interaction, cooperative sensors based on multiple, concerted, inter-molecular interactions between the components exhibit higher discrimination capabilities and enhanced functions. As examples for this class of sensors, two systems, both based on the absorption/luminescence properties of the porphyrin nucleus are illustrated in Fig. 5. Structure (a) represents a ditopic receptor where cooperative binding of a cation by the amide-appended calixarene and of an anion by the Zn-porphyrin allows recognition of complete binary metal salts. Structure (b) incorporates two cyclodextrin cavities and relies on the modification of the Fe(III) complex spectroscopic properties by inclusion of guests in these hydrophobic sites.This species is able to sense the presence of benzylmercaptane and 1-adamantanecarboxylate in four... 

But the most smdied electropolymerizable porphyrins are the tetra-amino-and tetrahydroxyphenyl-substituted ones. Indeed, both of them were developed to elaborate electrodes having potentiometric responses to several kinds of anions, such as iodide for example . Co(II) tetrakis(p-hydroxyphenyl)porphyrin 53-based films were used for the elaboration of potentiometric and fibre optic pH sensors with minimal interferences from anions. Zn(II)-53-based films were also used and characterized for kinetics studies of hydrogen evolution at their surface . Zn(II)-53 as well as Pd(II)-53 and free base tetra(4-methyl pyridinium) porphyrin (H2-54) were electropolymerized on indium tin oxide substrates, leading thus to starting materials for the making of donor/acceptor... 

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... 

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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