Carbonate-selective Electrode Research Articles

High-redox-capacity solid contact based on ferrocenyl self-assembled monolayer functionalized macroporous gold for an all-solid-state carbonate-selective electrode

Solid-contact ion-selective electrodes (ISEs) are a promising tool for direct detection of CO32– activity/concentration in seawater. Herein, a stable solid-contact CO32–-ISE was fabricated based on ferrocenyl self-assembled monolayer (SAM) functionalized macroporous gold (m-PG). Instead of the commonly used planar electrode, the m-PG film, prepared by using an electrochemical self-templating method involving the gold electrodeposition and hydrogen bubbling generation, is used as the conductive substrate. The redox capacity of the ferrocenyl SAM on the m-PG film is much larger than that on the planar Au electrode, due to the high surface area and good conductivity of m-PG. The solid-contact CO32–-ISE based on ferrocenyl SAM modified m-PG shows a Nernstian potential response in the activity range from 2.6 × 10-5 to 5.3 × 10-4 M with a slope of 28.3 ± 0.4 mV/dec and a detection limit of 1.1 × 10-5 M. The proposed electrode also exhibits a good electrode-to-electrode reproducibility with a standard variation of the standard potential (E0) of 1.5 mV (n = 7), an improved potential stability and a long lifetime up to at least 120 days. The strategy for electrode substrate transformation from planar gold substrate to porous gold substrate provides an alternative way to improve the redox capacities of ferrocenyl SAMs for preparing the stable and reliable solid-contact ISEs.

Potentiometric Sensor Array with Multi-Nernstian Slope.

The sensitivity of potentiometric sensors functioning in equilibrium mode is limited by the value predicted according to the Nernst equation and inversely proportional to the charge in the analyte ion. Therefore, an increased ion charge results in a dramatic decrease in the sensor sensitivity. We propose an approach to allow one to increase the sensitivity of the potentiometric measurements by using a combined electrochemical cell composed of several identical ion-selective electrodes immersed into separate sample solutions of equal composition. The combination of n electrodes, demonstrating individually a Nernstian slope in one electrochemical cell allows to amplify the signal and associated response slope by n times. The proposed approach is shown to provide a double and triple Nernstian slope for potassium-, calcium-, nitrate-, and carbonate-selective electrodes by combining two or three identical electrodes, correspondingly. Each ion-selective electrode functions in an equilibrium mode, hence, ensuring response stability and reproducibility.

EFFECT OF NATURE OF PLASTICIZER ON CHARACTERISTICS OF ION-SELECTIVE ELECTRODES REVERSIBLE TO DOUBLY CHARGED INORGANIC ANIONS

A study of the effect of the nature of the plasticizer on the characteristics (lower detection limit and selectivity) of electrodes that are reversible to carbonate, hydrogen phosphate, sulfate, selenate, selenite, sulfite, molybdate, tungstate, and thiosulfate ions was carried out. It was found that the membranes of carbonate and hydrophosphate selective electrodes based on quaternary ammonium salts are preferable to plasticize with o-nitrophenyl decyl ether, membranes of sulfate, selenate, selenite-selective electrodes - 1-bromonaphthalene, sulfite, molybdate, tungstate, and thiosulfate selective electrodes - dibutyl phthalate. For sulfate, selenate, and selenite-selective electrodes, there is an improvement in selectivity and a decrease in the detection limit in the series: o-nitrophenyldecyl ether – dibutylphthalate ≈ didecylphthalate ≈ bis (ethylhexyl) sebacinate -1-bromonaphthalene. The replacement of o-nitrophenyldecyl ether with 1-bromonaphthalene leads to a decrease in logKPot for these electrodes by 0.2–1.0 orders of magnitude depending on the interfering ion. The detection limit for sulfate, selenate, and selenite-selective electrodes is reduced by 0.2–0.65 orders of magnitude. For carbonate and hydrogen phosphate selective electrodes from o-nitrophenyldecyl ether to 1-bromonaphthalene, the detection limit decreases, for example, for a carbonate selective electrode by 0.6 orders of magnitude, for a hydrogen phosphate selective electrode by 1 order; the values of logKPot (CO32–, j) decrease by 0.1–0.9 orders of magnitude, and the values of logKPot (HPO42–, j) decrease by 0.6–1.25 orders of magnitude depending on the interfering ion. The results are explained in terms of the theories of Aigen-Denison-Ramzi-Fuoss and Born. The plasticizer that is optimal for carbonate and hydrophosphate selective electrodes has a large dielectric constant ε = 24; for all other electrodes, the optimal plasticizers have lower ε values (ε = 5–6). The use of dibutyl phthalate and 1-bromonaphthalene membranes as plasticizers is consistent with the Aigen-Denison-Ramzi-Fuoss theory, and the use of o-nitrophenyldecyl ether with the Born theory.

Substantiation of the Selection of Trifluoroacetophenone Derivatives for the Manufacture of Membranes of Sulfate- and Carbonate-Selective Electrodes

The distribution of trifluoroacetophenone (TFAP) and its derivatives—p-methyl trifluoroacetophenone (p-MTFAP), 2,4-dimethyl trifluoroacetophenone (DMTFAP), 2,4,6-trimethyl trifluoroacetophenone (TMTFAP), and heptyl p-trifluoroacetylbenzoate (H-p-TFAB)—in a hexane–water system, which simplifies a polyvinylchloride membrane of selective electrodes, is studied by UV spectrophotometry and chromatography. These substances are used as neutral carriers (NCs) in membranes of ion-selective electrodes reversible to doubly charged carbonate and sulfate ions. The hydration of TFAP and some of its derivatives is systematically investigated. It is found that TFAP has higher solubility in water (partition coefficient D = 415) compared to those of p-MTFAP, DMTFAP, TMTFAP, and H-p-TFAB (D = 1360–2700), which makes it unsuitable as a neutral carrier for manufacturing membrane electrodes. H-p-TFAB is most strongly hydrated in an alkaline medium. It is found that p-MTFAP and H-p-TFAB form crystalline hydrates. The selectivity coefficients for the carbonate- and sulfate-selective electrodes are determined for all the neutral carriers studied; the selectivity of the electrodes increases in the series TFAP < p-MTFAP < DMTFAP < TMTFAP < p-BTFAP (p-butyl trifluoroacetophenone) < H-p-TFAB.

Carbonate-Selective Electrodes Based on Higher Quaternary Ammonium Salts with Increased Steric Accessibility of the Exchange Site

A film polyvinyl chloride (33 wt %) carbonate-selective electrode based on a higher quaternary ammonium salt with increased steric accessibility of the exchange site, 3,4,5-tris(dodecyloxy)benzyl( oxyethyl)3trimethylammonium chloride (5 wt %), was developed using o-nitrophenyl decyl ether (52 wt %) as a plasticizer and heptyl p-trifluoroacetylbenzoate (10 wt %) as a solvating agent. The limit of detection of hydrocarbonate was 1.9 × 10−9 M, lifetime was 2.5 months, and the slope of the electrode function was 32.3 ± 0.6 mV/decade. The electrode is selective in the presence of interfering SO42-, Cl−, C2O42-, Br−, HPO42-, and NO3- ions, which allows its application for the determination of HCO3- ions in mineral water.

All-solid-state carbonate-selective electrode based on screen-printed carbon paste electrode

A novel disposable all-solid-state carbonate-selective electrode based on a screen-printed carbon paste electrode using poly(3-octylthiophene-2,5-diyl) (POT) as an ion-to-electron transducer has been developed. The POT was dropped onto the reaction area of the carbon paste electrode covered by the poly(vinyl chloride) (PVC) membrane, which contains N,N-Dioctyl-3α,12α-bis(4-trifluoroacetylbenzoyloxy)-5β-cholan-24-amide as a carbonate ionophore. The electrode showed a near-Nernstian slope of −27.5 mV/decade with a detection limit of 3.6 * 10−5 mol l−1. Generally, the detection time was 30 s. Because these electrodes are fast, convenient and low in cost, they have the potential to be mass produced and used in on-site testing as disposable sensors. Furthermore, the repeatability, reproducibility and stability have been studied to evaluate the properties of the electrodes. Measurement of the carbonate was also conducted in a human blood solution and achieved good performance.

Alkalinization of Thin Layer Samples with a Selective Proton Sink Membrane Electrode for Detecting Carbonate by Carbonate-Selective Electrodes.

Potentiometry is known to be sensitive to so-called free ion activity and is a potentially valuable tool in environmental speciation analysis. Here, the direct detection of free and total carbonate is demonstrated by alkalinization of a thin layer sample (∼100 μm), which is electrochemically triggered at a pH responsive membrane placed opposite a carbonate-selective membrane electrode. The concept may serve as a promising future methodology for in situ environmental sensing applications where traditional sampling and pretreatment steps are no longer required. The possibility of increasing the pH of the sample was demonstrated first with a proton selective membrane (pH readout at zero current) placed opposite the thin layer gap. An optimal applied potential (600 mV) for 300 s resulted in a pH increase of 4 units in an artificial sample, with a relative standard deviation (RSD) of ∼2%. The pH probe was subsequently replaced by a solid contact carbonate selective electrode for the determination of carbonate species (4.17 μM) in a sample of 1 mM NaHCO3. Increasing the pH to 12.1 by the electrochemically controlled proton sink allowed one to convert bicarbonate to the detectable carbonate species. Initial bicarbonate concentration (∼1 mM) was obtained as the difference between the converted bicarbonate and the initial carbonate concentration. An initial application of this concept was illustrated by the speciation analysis of an unfiltered sample from the Arve river (12.3 ± 0.2 μM and 22.5 ± 0.3 mM carbonate and bicarbonate, respectively). The values were confirmed by volumetric titration.

Urea-functionalized calix[4]arenes as carriers for carbonate-selective electrodes

A new PVC membrane electrode for CO 3 2− anion based on urea-functionalized calix[4]arenes (I–IV) as neutral ionophores is prepared. Of the various membranes prepared, the membrane based on bis-urea calix[4]arene I exhibits a linear stable response over a concentration range (5.0 × 10 −4 to 1.0 × 10 −1) with a slope of −29.2 mV per decade and a detection limit of 1.2 × 10 −4 M, and the best selectivity for CO 3 2− anion [ K T C O 2 J pot = 1.9 × 10 − 2 ( sa l − ) and 2.1 × 10 − 2 ( I − ) ] over salicylate and iodide.

A lipophilic sol-gel matrix for the development of a carbonate-selective electrode.

Organic-inorganic hybrid sol-gel matrixes were used as hosts for trifluoroacetyl-p-decylbenzene (TFADB), a traditional ionophore for carbonate. The sol-gel precursor was prepared by the reaction of (3-isocyanopropyl)triethoxysilane with ethylene glycol. Hexadecyltrimethoxysilane (HDTMOS) was employed as a co-precursor. An appropriate amount of tridodecylmethylammonium chloride (TDMAC) and 2-nitrophenyloctyl ether (NPOE) were used as membrane components. On mixing with an acidic catalyst, the sol-state precursors slowly gelled, yielding a membrane in which the active components, TFADB and TDMAC, were encapsulated. Infrared, (1)H, and (29)Si MAS NMR spectrometers were employed to monitor the sol-gel process and the degree of polymerization. The performances of the sol-gel membrane-based electrodes were compared to those of TFADB-based poly(vinyl chloride) (PVC) membrane electrodes. Membranes with a molar ratio of TFADB:TDMAC (1:0.14) showed extended lifetime and stable baseline potential. The response slope toward carbonate was approximately 27 mV/decade between 10(-)(5) and 10(-)(3.5) M at 18 degrees C. Interestingly, selectivity toward carbonate over salicylate and other lipophilic anions was improved, clearly deviating from the Hofmeister selectivity pattern. Responses toward small inorganic anions including chloride and sulfate were negligible. The selectivity coefficients measured by the matched potential method in 0.1 M tris-sulfuric acid buffer, pH 8.75, were log = -0.3, log = -4.2, and log = -2.5.

New conducting poly(1-alkyl-3,4-dimethyl-2,5-pyrrolylene) (alkyl: butyl, hexyl): synthesis, characterization and properties

The synthesis and characterization of two soluble poly(3,4-dimethylpyrrole) substituted with butyl and hexyl on the nitrogen moiety is reported. The poly(1-alkyl-3,4-dimethyl-2,5-pyrrolylene) (alkyl: butyl, hexyl) were synthesized by oxidative coupling polymerization of 1-alkyl-3,4-dimethylpyrrole (alkyl: butyl, hexyl) derived from cis-1,4-dibromo-2,3-dimethyl-2-butene and alkylamines. The highest yield was obtained when the polymers were synthesized by a monomer/oxidant mole ratio of 1:1. Characterization of the polymers includes IR, 1 H and 13 C NMR, and UV-Vis spectroscopy as well as TGA and molecular weight studies. Specially, polymer 1 excited within the UV wavelength produces very bright blue emission, promising its application in blue luminescence device. In particular, polymer 1b is utilized as the intermediary layer of the all-solid-state carbonate-selective electrodes (ASSE) with solvent polymeric ion-selective membranes.

Improved selectivity and detection limit of the carbonate-selective electrode.

The properties of the carbonate neutral carrier 4-( n-hexadecyl)-3-nitro-1-trifluoroacetylbenzene were compared with those of a similar carrier, without a nitro group, studied previously. In spite of differences in the Hammett constant of the carbonyl group responsible for interaction with the analyte, the analytical characteristics of both carriers, measured under the same conditions, were comparable. Special care was taken to avoid the presence of an excessive carbon dioxide level in the diffusion layer at the membrane-solution interface. The internal reference solution was prepared without carbonate components; the external solution was protected from contact with atmospheric carbon dioxide. Under such conditions the detection limit of both electrodes was extended to 10(-11 )mol L(-1), and the selectivity towards salicylate, chloride, and acetate was significantly improved.

Carbonate ion-selective electrode with reduced interference from salicylate

A carbonate ion-selective electrode for determination of total carbon dioxide species such as carbon dioxide, bicarbonate and carbonate with reduced interference from salicylate is described. Derivatives of trifluoroacetophenone were used as neutral carriers for carbonate. A polymer-free liquid carbonate-selective membrane with a cellophane outer membrane was found to give a carbonate-selective electrode with a negligible response to salicylate. The electrical contact was obtained by insertion of a silver/silver chloride electrode directly into the liquid membrane. The electrode does not require any aqueous filling solution and is therefore maintenance-free. The response to carbon dioxide species was found to be highly reproducible with a response time of 1–2 min at total carbon dioxide concentrations in the range from 5 to 50 mM. The lifetime of the electrode was at least 3 months. The electrode is regarded as very promising for clinical analysis of carbon dioxide species in body fluids such as plasma and serum.

Determination of oceanic carbon dioxide using a carbonate-selective electrode.

Potentiometric properties of the PVC membrane-based electrodes prepared with molecular tweezer-type neutral carriers, 3,12-bis(TFAB)CA and deoxy-3,12-bis(TFAB)CA, and trifluoroacetyl-p-decylbenzene (TFADB) were measured in buffered electrolytes (0.1 M Tris-H2SO4, pH 8.6 and 8.0) and artificial seawater. It was observed that the deoxy-3,12-bis(TFAB)CA-based electrode provides greatly enhanced carbonate selectivity over chloride (log K(CO3(2-), Cl-)POT approximately -6) and other minor anions present in seawater. Thus, we explored the possibility of applying this new carbonate-selective electrode for direct determination of oceanic carbon dioxide. The total carbon dioxide (TCO2) level in surface Yellow Sea water was determined with the deoxy-3,12-bis(TFAB)CA-based electrode, Severinghaus-type CO2 gas sensor, and the traditional potentiometric titration methods. The results showed that the carbonate-selective electrode provides accurate oceanic TCO2 determination comparable to that obtainable with the other two methods. The analytical procedure based on a carbonate-selective electrode is clearly advantageous over other conventional methods: it does not require any sample pretreatment and extra reagents other than the standard calibration solutions, while providing the measured results directly and immediately.

All-solid-state carbonate-selective electrode based on a molecular tweezer-type neutral carrier with solvent-soluble conducting polymer solid contact

Carbonate-selective membranes were prepared by incorporating a molecular tweezer-type carbonate-selective neutral carrier [ N, N-dioctyl-3α,12α-bis(4-trifluoroacetylbenzyloxy)-5β-cholan-24-amide] into a room temperature vulcanizing-type silicone rubber (3140 RTV-SR) matrix, and deposited on the planar-type electrodes (Pt containing Ag/AgCl electrodes formed on a ceramic plate) with and without an intermediary conducting polymer layer. Two types of solvent-soluble conducting polymers [poly(1-hexyl-3,4-dimethyl-2,5-pyrrolylene) or poly(3-octylthiophene-2,5-diyl)] have been examined as the solid contact material. Potentiometric properties of the resultant all-solid-state electrodes were evaluated in terms of their carbonate selectivity, response slope, potential stability and reproducibility. The sensitivity and carbonate selectivity of the SR membrane-based all-solid-state electrodes with conducting polymer solid contact were comparable to those of conventional electrodes. Experimental results also showed that the intermediary conducting polymer layer used in the all-solid-state electrodes greatly reduces the interference from dissolved oxygen.

Potentiometric evaluation of solvent polymeric carbonate-selective membranes based on molecular tweezer-type neutral carriers

Potentiometric properties of the ion-selective electrodes) based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (CA) derivatives have been measured. The carbonate-selective neutral carriers have been prepared by coupling one to three trifluoroacetobenzoyl (TFAB) groups to a cholic acid derivative which has three hydroxyl linkers lining on the C3, C7, and C12 positions of its rigid steroidal ring structure. The membranes based on cholic acid derivatives with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(T-FAB)CA] exhibited remarkably improved carbonate selectivity, indicating that the bis(FAB)CAs behave like molecular tweezers for the carbonate ion. For example, 3,12-bis(TFAB)CA resulted in 10-300-fold-enhanced carbonate selectivity over other anions (e.g., salicylate, ClO4-, SCN-, (HPO4)2-, NO3-, NO2-, Br-, and Cl-) compared to that of the neutral carriers with a single TFAB group. The distances between the carbonate binding centers of bis(TFAB)CAs, i.e., the carbonyl carbons of the two TFAB groups, are in the 7.3-7.9-A range at the AM1 level semiempirical calculation, which is too far for the carbonate ion to form direct covalent bonding. The fast atom bombardment mass spectra of bis(TFAB)CAs show that significant fractions of the compounds are either mono- or dihydrated before complexing the carbonate ion. These findings seem to suggest that bis(TFAB)CAs recognize the incoming carbonate ion by forming both covalent and hydrogen bonding between the hydrated and unhydrated TFAB groups. The analytical utility of the carbonate-selective electrode based on 3,12-bis(TFAB)-CA has been demonstrated by measuring the total carbon dioxide in human serum in the presence of lipophilic anion interferents, e.g., salicylate.

Trifluoroacetophenone derivative-based carbonate ion-selective electrodes: effect of diluent pH on the anion selectivity

Potentiometric properties of carbonate-selective electrodes were examined from both theoretical and experimental viewpoints, evaluating the relationship between dilution buffer pH and anion selectivity, and the effect of the type of buffer reagents (i.e. basic or acidic nature) on the carbonate measurement. The Nikolskii equation expressed in terms of the activity of hydroxide describes the potentiometric behavior of trifluoroacetophenone derivative (TFA)-based carbonate-selective electrodes well. Also investigated were the response characteristics of PVC-based carbonate-selective membranes doped with four different TFAs (e.g. trifluoroacetyl- p -decylbenzene (TFADB), 1,7-bis(4′-trifluoroacetophenyl)-4-dodecyl-1,7-dioxo-2,6-dioxy-heptane (compound I), 1,2-bis(4-trifluoroacetylbenzoyloxymethyl) benzene (compound II), and 2,7-diaza-1,8-bis(4′-trifluoroacetophenyl)-2,7-di(1′′-hexyl)-1,8-dioxo-octan (compound III)). Of these electrodes, the one based on compound III exhibited the best carbonate selectivity, response slope, and detection limit in a high pH (e.g. pH>9.0) dilution buffer.

Photocurable carbonate-selective membranes for chemical sensors containing lipophilic additives

The potentiometric properties of carbonate sensors with photocurable membranes based on polyurethane and polyvinyl chloride (PVC) membranes containing tetradecylammonium carbonate and hexyl- p-trifluoroacetylbenzoate are examined. The properties of these membranes with different concentrations of plasticizer (dioctylphtalate) are investigated. The difference between the electrodes based on photocurable polymer and PVC membranes is shown. In the case of low concentrations of plasticizer, the comparison of the photocurable membranes with PVC ones shows higher slope and selectivity. The influence of lipophilic additive is studied. It is shown that the presence of anionic sites inside the membrane improves the potentiometric properties of photocurable carbonate-selective electrodes.