Origin of non-Nernstian anion response slopes of metalloporphyrin-based liquid/polymer membrane electrodes

The Long-Lasting Story of One Sensor Development: From Novel Ionophore Design toward the Sensor Selectivity Modeling and Lifetime Improvement

The metalloporphyrin ligand bearing incorporated anion-exchanger fragment, 5-[4-(3-trimethylammonium)propyloxyphenyl]-10,15,20-triphenylporphyrinate of Co(II) chloride, CoTPP-N, has been tested as anion-selective ionophore in PVC-based solvent polymeric membrane sensors. A plausible sensor working mechanism includes the axial coordination of the target anion on ionophore metal center followed by the formed complex aggregation with the second ionophore molecule through positively charged anion-exchanger fragment. The UV-visible spectroscopic studies in solution have revealed that the analyte concentration increase induces the J-type porphyrin aggregation. Polymeric membranes doped with CoTPP-N showed close to the theoretical Nernstian response toward nitrite ion, preferably coordinated by the ionophore, and were dependent on the presence of additional membrane-active components (lipophilic ionic sites and ionophore) in the membrane phase. The resulting selectivity was a subject of specific interaction and/or steric factors. Moreover, it was demonstrated theoretically and confirmed experimentally that the selection of a proper ratio of ionophore and anionic additive can optimize the sensor selectivity and lifetime.

Si-corrole-based fluoride fluorometric turn-on sensor

We present here a new type of fluoride ion optode, constituted by a highly lipophilic PVDF porous membrane modified with a liquid receptor layer containing the emission-active Si corrole F[Formula: see text] selective ionophore. For the optimized composition of the receptor layer, in acidic solutions an increase of Si-corrole emission was observed by increasing fluoride ion concentration, a behavior different from most porphyrinoid-based optical sensors. An observed linear dependence of the Si corrole emission intensity (read at 635 nm) was within the range 10[Formula: see text] to 10[Formula: see text] M of fluoride ions.

Gallium(III) and Indium(III) Complexes with meso-Monophosphorylated Porphyrins: Synthesis and Structure. A First Example of Dimers Formed by the Self-Assembly of meso-Porphyrinylphosphonic Acid Monoester

The synthesis and structural characterization, both in solution by means of ¹H and ³¹P NMR and UV-vis spectroscopies and in the solid state by X-ray diffraction on single crystal, of a series of gallium(III) and indium(III) meso-mono(diethoxyphosphoryl)porphyrins bearing different peripheral substituents as well as the corresponding monoesters and phosphonic acids are reported. This work describes the first example of the X-ray structure of a self-assembled dimer formed via strong binding between the oxygen atom of the phosphonate substituent and the gallium(III) cations of adjacent porphyrin molecules [Ga-O = 1.9708(13) Å].

Porphyrinoids for Chemical Sensor Applications

Porphyrins and related macrocycles have been intensively exploited as sensing materials in chemical sensors, since in these devices they mimic most of their biological functions, such as reversible binding, catalytic activation, and optical changes. Such a magnificent bouquet of properties allows applying porphyrin derivatives to different transducers, ranging from nanogravimetric to optical devices, also enabling the realization of multifunctional chemical sensors, in which multiple transduction mechanisms are applied to the same sensing layer. Potential applications are further expanded through sensor arrays, where cross-selective sensing layers can be applied for the analysis of complex chemical matrices. The possibility of finely tuning the macrocycle properties by synthetic modification of the different components of the porphyrin ring, such as peripheral substituents, molecular skeleton, coordinated metal, allows creating a vast library of porphyrinoid-based sensing layers. From among these, one can select optimal arrays for a particular application. This feature is particularly suitable for sensor array applications, where cross-selective receptors are required. This Review briefly describes chemical sensor principles. The main part of the Review is divided into two sections, describing the porphyrin-based devices devoted to the detection of gaseous or liquid samples, according to the corresponding transduction mechanism. Although most devices are based on porphyrin derivatives, seminal examples of the application of corroles or other porphyrin analogues are evidenced in dedicated sections.

Corrole-based ion-selective electrodes

Different corrole derivatives have been exploited as ionophores for the development of ion-selective electrodes (ISEs). The compounds used are free-base 5,10,15-triphenylcorrole, the corresponding Mn and Fe chloride complexes, the Cu complex of 5,10,15-tris(4-tert-butylphenyl)-corrole and the Fe chloride complex of 2,3,17,18-tetraethyl,7,8,12,13-tetramethylcorrole. Corroles have been dispersed in the polyvinyl chloride polymeric matrix and different plasticizers have been used for the preparation of selective membranes. The developed ISEs have been tested for the detection of model analytes and the working mechanism has been investigated in detail. In the case of free-base corrole, the results showed that it is difficult to obtain high selective ISEs, due to the macrocycle high sensitivity to the background solution pH. On the other hand, Mn derivative are shown to be highly selective for chloride ion detection, with performances superior to the corresponding Mn porphyrin-based ISEs. Finally both Cu and Fe derivatives are very promising ionophores for the detection of hydrophilic anions, such as carbonate and monohydrogen phosphate ion.

Ion-selective electrodes with unusual response functions: simultaneous formation of ionophore-primary ion complexes with different stoichiometries

It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here, we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes and how this results in what looks like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs(+) but also the smaller ions Li(+), Na(+), K(+), and NH(4)(+) form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 10(15.0) and 10(21.0) for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare but have lacked in the past an adequate discussion in the literature, remaining ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries.

Potentiometric sensors based on fluorous membranes doped with highly selective ionophores for carbonate

Manganese(III) complexes of three fluorophilic salen derivatives were used to prepare ion-selective electrodes (ISEs) with ionophore-doped fluorous sensing membranes. Because of their extremely low polarity and polarizability, fluorous media are not only chemically very inert but also solvate potentially interfering ions poorly, resulting in a much improved discrimination of such ions. Indeed, the new ISEs exhibited selectivities for CO(3)(2-) that exceed those of previously reported ISEs based on nonfluorous membranes by several orders of magnitude. In particular, the interference from chloride and salicylate was reduced by 2 and 6 orders of magnitude, respectively. To achieve this, the selectivities of these ISEs were fine-tuned by addition of noncoordinating hydrophobic ions (i.e., ionic sites) into the sensing membranes. Stability constants of the anion-ionophore complexes were determined from the dependence of the potentiometric selectivities on the charge sign of the ionic sites and the molar ratio of ionic sites and the ionophore. For this purpose, a previously introduced fluorophilic tetraphenylborate and a novel fluorophilic cation with a bis(triphenylphosphoranylidene)ammonium group, (R(f6)(CH(2))(3))(3)PN(+)P(R(f6)(CH(2))(3))(3), were utilized (where R(f6) is C(6)F(13)). The optimum CO(3)(2-) selectivities were found for sensing membranes composed of anionic sites and ionophore in a 1:4 molar ratio, which results in the formation of 2:1 complexes with CO(3)(2-) with stability constants up to 4.1 × 10(15). As predicted by established theory, the site-to-ionophore ratios that provide optimum potentiometric selectivity depend on the stoichiometries of the complexes of both the primary and the interfering ions. However, the ionophores used in this study give examples of charges and stoichiometries previously neither explicitly predicted by theory nor shown by experiment. The exceptional selectivity of fluorous membranes doped with these carbonate ionophores suggests their use not only for potentiometric sensing but also for other types of sensors, such as the selective separation of carbonate from other anions and the sequestration of carbon dioxide.

Potentiometric Polymeric Film Sensors Based on 5,10,15-tris(4-aminophenyl) Porphyrinates of Co(II) and Cu(II) for Analysis of Biological Liquids

Novel carbonate-selective potentiometric sensors based on 5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinates of Cu(II) and Co(II) have been developed. Ionophore functioning mechanism and possible source of carbonate sensitivity have been evolved. Potentiometric properties of Co(II)- and Cu(II)TATPP-based sensors were compared with common carbonate-ISEs containing trifluoroacetophenone derivatives. The analytical utility of newly developed sensors has been demonstrated by measuring the bicarbonate content in human blood plasma.

Polymeric membrane electrodes with high nitrite selectivity based on rhodium(III) porphyrins and salophens as ionophores

Several porphyrin and salophen complexes with Rh(III) are examined as ionophores to prepare nitrite selective polymeric membrane electrodes. All ionophores tested exhibit preferred selectivity toward nitrite anion. Enhanced potentiometric nitrite selectivity is observed in the presence of either lipophilic anionic as well as cationic sites within the membranes, suggesting that the ionophores can function via either a charged or a neutral carrier response mechanism. Among a range of complexes and membrane formulations examined, optimal nitrite selectivity and reversible response down to 5 x 10(-6) M is achieved using Rh(III)-tetra(t-butylphenylporphyrin) as the ionophore in the presence of lipophilic cationic sites in plasticized poly(vinyl chloride) membrane. Response times are substantially longer than typical membrane electrodes apparently because of a slow nitrite ligation reaction with Rh(III); however, a significant improvement in dynamic EMF response can be realized by optimizing the membrane formulation and increasing the temperature. The selectivity observed with these membranes is greater than the best nitrite selective electrodes reported to date in the literature based on lipophilic Co(III)-corrin complexes, allowing the new nitrite electrodes to be utilized to determine the level of nitrite in meats with good correlation to the colorimetric Griess assay method.

Polymethacrylate polymers with appended aluminum(III)-tetraphenylporphyrins: Synthesis, characterization and evaluation as macromolecular ionophores for electrochemical and optical fluoride sensors

The synthesis and characterization of a novel polymethacylate polymer with covalently linked Al(III)-tetraphenylporphyrin (Al(III)-TPP) groups is reported. The new polymer is examined as a potential macromolecular ionophore for the preparation of polymeric membrane-based potentiometric and optical fluoride selective sensors. To prepare the polymer, an Al(III) porphyrin monomer modified with a methacrylate functionality is synthesized, allowing insertion into a polymethacrylate block copolymer (methyl methacrylate and decyl methacrylate) backbone. The resulting polymer can then be incorporated, along with appropriate additives, into conventional plasticized poly(vinyl chloride) films for testing electrochemical and optical fluoride response properties. The covalent attachment of the Al(III)-TPP ionophore to the copolymer matrix provides potentiometric sensors that exhibit significant selectivity for fluoride ion with extended lifetimes (compared to ion-selective membrane electrodes formulated with conventional free Al(III)-TPP structure). However, quite surprisingly, the attachment of the ionophore to the polymer does not eliminate the interaction of Al(III)-TPP structures to form dimeric species within the membrane phase in the presence of fluoride ion. Such interactions are confirmed by UV/visible spectroscopy of the blended polymeric films. Use of the new polymer-Al(III)-TPP conjugates to prepare optical fluoride sensors by co-incorporating a lipophilic pH indicator (4',5'-dibromofluorescein octadecyl ester; ETH7075) is also examined and the resulting optical sensing films are shown to exhibit excellent selectivity for fluoride, with the potential for prolonged operational lifetime.

Polymeric membrane electrodes with improved fluoride selectivity and lifetime based on Zr(IV)- and Al(III)-tetraphenylporphyrin derivatives

Novel aluminum(III)- and zirconium(IV)-tetraphenylporhyrin (TPP) derivatives are examined as fluoride-selective ionophores for preparing polymer membrane-based ion-selective electrodes (ISEs). The influence of t-butyl- or dichloro-phenyl ring substituents as well as the nature of the metal ion center (Al(III) versus Zr(IV)) on the anion complexation constants of TPP derivative ionophores are reported. The anion binding stability constants of the ionophores are characterized by the so-called "sandwich membrane" method. All of the metalloporphyrins examined form their strongest anion complexes with fluoride. The influence of plasticizer as well as the type of lipophilic ionic site additive and their amounts in the sensing membrane are discussed. It is shown that membrane electrodes formulated with the metalloporphyrin derivatives and appropriate anionic or cationic additives exhibit enhanced potentiometric response toward fluoride over all other anions tested. Since selectivity toward fluoride is enhanced in the presence of both anionic and cationic additives, the metalloporphyrins can function as either charged or neutral carriers within the organic membrane phase. In contrast to previously reported fluoride-selective polymeric membrane electrodes based on metalloporphyrins, nernstian or near-nernstian (-51.2 to -60.1 mV decade(-1)) as well as rapid (t < 80 s) and fully reversible potentiometric fluoride responses are observed. Moreover, use of aluminum(III)-t-butyltetraphenylporphyrin as the ionophore provides fluoride sensors with prolonged (7 months) functional lifetime.

Organotin compounds: An ionophore system for fluoride ion recognition

Ion-selective properties were established for membrane electrodes prepared by using organotin compounds of type (L(CN)RSnF2)n, (R=n-Bu (I),=Ph (II)) and (L(CN)SnF3)n (III) (L(CN)=C(6)H4(CH2NMe2)-2). Electrodes formulated with the optimized membranes containing the organotin compounds I-III as ionophores and sodium tetraphenylborate (10-30%) exhibited high selectivity for fluoride over other anions. An electrode prepared with ionophore II using dibutyl phthalate as the plasticizer and 15% sodium tetraphenylborate (NaTPB) as anion additive, possesses the best potentiometric response characteristics. It shows a detection limit of 7.9 x 10(-7) M with a slope of 62.7 mV decade(-1) of activity in buffer solutions of pH 5.5. The interference from other anions is suppressed under this optimized measurement conditions. An entirely non-Hofmeister selectivity sequence (F->CH3COO->Cl->I- approximately Br->ClO4->NO2->NO3->SCN-) with remarkable preference towards fluoride is obtained. The influence on the electrode performances by anion additive was studied, and the possible response mechanism was investigated by UV-vis spectra. The electrode has been used for direct determination of fluoride in drinking mineral water with satisfactory results.

Potentiometric anion selectivity of polymer-membrane electrodes based on cobalt, chromium, and aluminum salens

Metallo-salens of cobalt(II) (Co-Sal), chromium(III) (Cr-Sal), and aluminum(III) (Al-Sal) are used as the active ionophores within plasticized poly(vinyl chloride) membranes. It is shown that central metal-ion plays a critical role in directing the ionophore selectivity. Polymer-membrane electrodes based on Co-Sal, Cr-Sal, and Al-Sal are demonstrated to exhibit enhanced responses and selectivity toward nitrite/thiocyanate, thiocyanate, and fluoride anions, respectively. The improved anion selectivity of the three ionophore systems is shown to deviate significantly from the classical Hofmeister pattern that is based only on ion lipophilicity. For example, optimized membrane electrodes for nitrite ion based on Co-Sal exhibit logK(Nitrite,Anion)(pot) values of -5.22, -4.66, -4.48, -2.5 towards bromide, perchlorate, nitrate, and iodide anions, respectively. Optimized membrane electrodes based on Co-Sal and Cr-Sal show near-Nernstian responses towards nitrite (-57.9+/-0.9 mV/decade) and thiocyanate (-56.9+/-0.8 mV/decade), respectively, with fast response and recovery times. In contrast, Al-Sal based membrane electrodes respond to fluoride ion in a super-Nernstian (-70+/-3 mV/decade) and nearly an irreversible mode. The operative response mechanism of Co-Sal, Cr-Sal, and Al-Sal membrane electrodes is examined using the effect of added ionic sites on the potentiometric response characteristics. It is demonstrated that addition of lipophilic anionic sites to membrane electrodes based on the utilized metallo-salens enhances the selectivity towards the primary ion, while addition of cationic sites resulted in Hofmeister selectivity patterns suggesting that the operative response mechanism is of the charged carrier type. Electron spin resonance (ESR) data indicates that Co(II) metal-ion center of Co-Sal ionophore undergoes oxidation to Co(III). This process leads to formation of a charged anion-carrier that is consistent with the response behavior obtained for Co-Sal based membrane electrodes.

Elimination of dimer formation in InIIIporphyrin-based anion-selective membranes by covalent attachment of the ionophore

The spontaneous hydroxy-bridged dimer formation of metalloporphyrins in ion-selective membranes gives rise to a short sensor lifetime (typically days), triggered by solubility problems, the occurrence of a super-Nernstian response slope, and a pH cross response. This dimer formation is eliminated here by covalent attachment of the ionophore to the polymer matrix. Specifically, two different indium(III)porphyrins containing polymerizable groups, the chloride-selective chloro(3-[18-(3-acryloyloxypropyl)-7,12-bis(1-methoxyethyl)-3,8,13,17-tetramethylporphyrin-2-yl]propyl ester)indium(III) and the nitrite-selective Chloro(5-(4-acryloyloxyphenyl)-10,15,20-triphenylporphyrinato)indium(III), were synthesized and copolymerized with methyl methacrylate and decyl methacrylate. The covalent attachment of the ionophore to the polymer matrix indeed prevents the metalloporphyrin from forming dimeric species, as confirmed by UV/visible spectroscopy. The ion-selective membranes with grafted indium porphyrin showed Nernstian response slopes to chloride, nitrite, perchlorate, and thiocyanate anions, with a selectivity comparable to membranes with freely dissolved or underivatized metalloporphyrin. The membranes containing grafted ionophores showed a lifetime of at least two months, apparently since crystallization of the poorly soluble dimeric species may no longer occur. This is one of the first examples where the covalent attachment of an ionophore drastically improves on a number of important sensor characteristics.

Effects of two-phase reactions on composition and potential response of zirconium(IV)-tetraphenylporphyrin complexes as carrier for citrate-selective electrode

The reactions between zirconium(IV)-tetraphenylporphyrin(tpp)-hydroxide complexes ([ZrIV2(OH)4(tpp)2] and [ZrIV(OH)2(tpp)]) in chlorobenzene and various acids (HX: HClO4, HCl, HNO3, CH3COOH) in water were studied. Three species [ZrIV2(OH)3(tpp)2],X, [ZrIV(OH)X(tpp)], and [ZrIVX2(tpp)] were identified, and their stability constants were determined. The extent of formation of each species depends on the coordination ability and lipophilicity of X-. The performance of these complexes was evaluated as a carrier of an anion-selective electrode. The cationic complex [ZrIV2(OH)3(tpp)2],X exhibited selectivity following the Hofmeister series to weakly coordinating anions, while showing a super-Nernstian response to strongly coordinating citrates. The latter suggests the cleavage of the dimeric structure. The dichloro complex [ZrIVCl2(tpp)] was prone to hydrolyze in contact with an aqueous solution and gave drifting potentials. The monochloro complex [ZrIV(OH)Cl(tpp)] was more stable and strongly responded to acetates, which may be ascribed to substitution of a weakly bound chloride. A slow hydrolysis giving [ZrIV(OH)2(tpp)], however, caused gradual deterioration of this strong response to acetates. Conditioning of the membrane in 1 M HCl regenerated the once-deteriorated carrier [ZrIV(OH)Cl(tpp)], while continuous conditioning extended its operating life. This carrier was effectively used to continuously monitor the acetate in a flow system. The dihydroxo complex [ZrIV(OH)2(tpp)] showed lower responses to common anions due to strong Zr-OH bonding and was thus successfully used for selective potentiometry of citrates.

Fluoride-selective optical sensor based on aluminum(III)-octaethylporphyrin in thin polymeric film: further characterization and practical application

More detailed analytical studies of a new fluoride-selective optical sensor based on the use of aluminum(III)-octaethylporphyrin and a lipophilic pH indicator (4',5'-dibromofluorescein octadecyl ester; ETH-7075) within a thin plasticized poly(vinyl chloride) film are reported. The sensor exhibits extraordinary optical selectivity for fluoride over a wide range of other anions, including anions with far more positive free energies of hydration (e.g., perchlorate, thiocyanate, nitrate, etc.). UV-visible spectrophotometric studies of the sensing films indicate that fluoride interacts with the Al(III) center of the porphyrin structure, yielding both a change in the Soret band lambda(max) of the porphyrin and a change in the protonation state of the pH indicator within the film. The same change in spectral properties of the metalloporphyrin occurs in the absence of added pH indicator or with added tetraphenylborate derivative anionic sites, but optical responses to fluoride in these cases are shown to be irreversible. The presence of the pH indicator and the simultaneous fluoride/proton coextraction equilibrium chemistry is shown to greatly enhance the reversibility of fluoride binding to the Al(III) porphyrin. Optical response toward fluoride can be observed in the range of 0.1 microM-1.6 mM. Optical selectivity coefficients of <10(-6) for common anions (e.g., sulfate, chloride, nitrate, etc.) and <10(-4) for perchlorate and thiocyanate are obtained. Measurements of fluoride in drinking water via the new optical sensor are shown to correlate well with values obtained for the same samples using a classical LaF3-based fluoride ion-selective electrode method.

Polymeric Sensor Materials: Toward an Alliance of Combinatorial and Rational Design Tools?

Increased selectivity, response speed, and sensitivity in the chemical and biological determinations of gases and liquids are of great interest. Particular attention is paid to polymeric sensor materials, which are applicable to sensors exploiting various energy transduction principles, such as radiant, electrical, mechanical, and thermal energy. Ideally, numerous functional parameters of sensor materials can be tailored to meet specific needs using rational design approaches. However, increasing the structural and functional complexity of polymeric sensor materials makes it more difficult to predict the desired properties. Combinatorial and high-throughput methods have had an impact on all areas of research on polymer-based sensor materials including homo- and copolymers, formulated materials, polymeric structures with engineered morphology, and molecular shape-recognition materials. Herein we report on the state-of-the-art, the development trends, and the remaining knowledge gaps in the area of combinatorial polymeric sensor materials design.