Fluorous membrane ion-selective electrodes for perfluorinated surfactants: trace-level detection and in situ monitoring of adsorption.

Ion-selective electrodes (ISEs) with fluorous anion-exchanger membranes for the potentiometric detection of perfluorooctanoate (PFO(-)) and perfluorooctanesulfonate (PFOS(-)) were developed. Use of an anion-exchanger membrane doped with the tetraalkylphosphonium derivative (Rf8(CH2)2)(Rf6(CH2)2)3P(+) and an optimized measurement protocol resulted in detection limits of 2.3 × 10(-9) M (1.0 ppb) for PFO(-) and 8.6 × 10(-10) M (0.43 ppb) for PFOS(-). With their higher selectivity for PFO(-) over OH(-), membranes containing the alternative anion exchanger (Rf6(CH2)3)3PN(+)P((CH2)3Rf6)3 with a bis(phosphoranylidene)ammonium group further improved the detection limit for PFO(-) to 1.7 × 10(-10) M (0.070 ppb). These values are comparable with results obtained using well-established techniques such as gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and liquid chromatography-tandem mass spectrometry (LC-MS-MS), but the measurement with ISEs avoids lengthy sample preconcentration, can be performed in situ, and is less costly. Even when eventual spectrometric confirmation of analyte identity is required, prescreening of large numbers of samples or in situ monitoring with ISEs may be of substantial benefit. To demonstrate a real-life application of these electrodes, in situ measurements were performed of the adsorption of PFOS(-) onto Ottawa sand, which is a standard sample often used in environmental sciences. The results obtained are consistent with those from an earlier LC-MS study, validating the usefulness of these sensors for environmental studies. Moreover, PFOS(-) was successfully measured in a background of water from Carnegie Lake.

Advantages and limitations of reference electrodes with an ionic liquid junction and three-dimensionally ordered macroporous carbon as solid contact.

Liquid-junction-free reference electrodes that contact the sample through an ionic-liquid-doped, hydrophobic polymer membrane have attracted attention because they offer an alternative to reference electrodes with conventional salt bridges. In this work, liquid-junction-free reference electrodes were developed using plasticized poly(vinyl chloride) membranes doped with the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide. Three-dimensionally ordered macroporous (3DOM) carbon substrates infused with this ionic liquid phase were used as solid contacts for these reference membranes. As in prior work with ionophore-based 3DOM carbon-contact ion-selective electrodes, the long-term stability of the liquid-junction-free reference electrodes was excellent, with potential drifts as low as 42 µV/h over 26 days. Successful measurements of pH in milk were performed and, to the best of our knowledge, are the first example of the use of liquid-junction-free reference electrodes in complex real-life samples. A thorough analysis of their performance at low pH revealed protonation of the ionic liquid anion (L(-)) and formation of LHL(-) type of associates in the reference electrode membrane, effects not observed in prior work. Also, when reference membranes were mounted into conventional electrode bodies with inner filling solutions that contained no ionic liquid ions, zero-current ion fluxes across the sample/membrane interface occurred, as previously only seen for ionophore-doped ion-selective membranes. Understanding these effects will be crucial to the design of liquid-junction-free reference electrodes suitable for other applications.

Highly selective detection of silver in the low ppt range with ion-selective electrodes based on ionophore-doped fluorous membranes.

Ionophore-doped sensing membranes exhibit greater selectivities and wider measuring ranges if their membrane matrixes are noncoordinating and solvate interfering ions poorly. This is particularly true for fluorous phases, which are the least polar and polarizable condensed phases known. In this work, fluorous membrane matrixes were used to prepare silver ion-selective electrodes (ISEs). Sensing membranes composed of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl) phenyl]borate, and one of four fluorophilic Ag(+)-selective ionophores with one or two thioether groups were investigated. All electrodes exhibited Nernstian responses to Ag(+) in a wide range of concentrations. Their selectivities for Ag(+) over interfering ions were found to depend on host preorganization and the length of the -(CH(2))(n)- spacers separating the coordinating thioether group from the strongly electron withdrawing perfluoroalkyl groups. ISEs based on the most selective of the four ionophores, that is, 1,3-bis(perfluorodecylethylthiomethyl)benzene, provided much higher selectivities for Ag(+) over many alkaline and heavy metal ions than most Ag(+) ISEs reported in the literature (e.g., log K(Ag,J)(pot) for K(+), -11.6; Pb(2+), -10.2; Cu(2+), -13.0; Cd(2+), -13.2). Moreover, the use of this ionophore with a linear perfluorooligoether as membrane matrix and solid contacts consisting of three-dimensionally ordered macroporous (3DOM) carbon resulted in a detection limit for Ag(+) of 4.1 ppt (3.8 × 10(-1)1 M).

Cation-Coordinating Properties of Perfluoro-15-Crown-5.

The coordinative properties of perfluoro-15-crown-5 with monocations were investigated using (19)F NMR spectroscopy and ion-selective electrodes with perfluoro-15-crown-5 as the matrix of their sensor membranes and the fluorophilic tetrakis[3,5-bis(perfluorohexyl)phenyl]borate as ion exchanger site. The results show that perfluoro-15-crown-5 interacts weakly but significantly with Na(+) and K(+). Assuming 1:1 stoichiometry, the formal complexation constants were determined to be 5.5 and 1.7 M(-1), respectively. This weak binding is consistent with the strong electron withdrawing nature of the many fluorine atoms in the perfluorocrown ether. While perfluorinated crown ethers have been known to form host-guest complexes with the anions O(2) (-) and F(-) in the gas phase, this is the first study that quantitatively confirms cation binding to a perfluorocrown ether.

Effects of architecture and surface chemistry of three-dimensionally ordered macroporous carbon solid contacts on performance of ion-selective electrodes.

The effects of the architecture and surface chemistry of three-dimensionally ordered macroporous (3DOM) carbon solid contacts on the properties of ion-selective electrodes (ISEs) were examined. Infiltration of the plasticized poly(vinyl chloride) (PVC) membrane into the pores of the carbon created a large interfacial area between the sensing membrane and the solid contact, as shown by cryo-scanning electron microscopy (cryo-SEM) and elemental analysis. This large interfacial area, along with the high capacitance of the 3DOM carbon solid contacts (as determined by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy) results in an excellent long-term stability of the potentiometric response, with drifts as low as 11.7 muV/h. The comparison of 3DOM carbon solid contacts with an untemplated carbon solid contact shows that the pore structure is an essential feature for the excellent electrode performance. However, the surface chemistry of the 3DOM carbon cannot be ignored. While there is no evidence for an aqueous layer forming between the sensing membrane and unoxidized 3DOM carbon, electrodes based on oxidized 3DOM carbon exhibit potentiometric responses with the typical hysteresis indicative of a water layer. A comparison of the different techniques to characterize the solid contacts confirms that constant-current charge-discharge experiments offer an intriguing approach to assess the long-term stability of solid-contact ISEs but shows that their results need to be interpreted with care.

Fluorous polymeric membranes for ionophore-based ion-selective potentiometry: how inert is Teflon AF?

Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (87% dioxole monomer content; known as Teflon AF2400) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+ ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability, and maintained their selectivity over at least 4 weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 10(3.5), 10(1.8), 10(6.8), and 10(4.4) M(-1), respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride (COF) groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(=O)F groups can be eliminated chemically, or they can be used to readily introduce new functionalities.

Subnanomolar Detection Limit Application of Ion-Selective Electrodes with Three-Dimensionally Ordered Macroporous (3DOM) Carbon Solid Contacts.

Solid-contact ion-selective electrodes (SC-ISEs) can exhibit very low detection limits and, in contrast to conventional ISEs, do not require an optimization of the inner filling solution. This work shows that subnanomolar detection limits can also be achieved with SC-ISEs with three-dimensionally ordered macroporous (3DOM) carbon contacts, which have been shown recently to exhibit excellent long-term stabilities and good resistance to the interferences from oxygen and light. The detection limit of 3DOM carbon-contacted electrodes with plasticized poly(vinyl chloride) as membrane matrix can be improved with a high polymer content of the sensing membrane, a large ratio of ionophore and ionic sites, and conditioning with a low concentration of analyte ions. This permits detection limits as low as 1.6×10(-7) M for K(+) and 4.0×10(-11) M for Ag(+).

Ion-selective electrodes with three-dimensionally ordered macroporous carbon as the solid contact.

Electrodes with three-dimensionally ordered macroporous (3DOM) carbon as the intermediate layer between an ionophore-doped solvent polymeric membrane and a metal contact are presented as a novel approach to solid-contact ion-selective electrodes (SC-ISEs). Due to the well-interconnected pore and wall structure of 3DOM carbon, filling of the 3DOM pores with an electrolyte solution results in a nanostructured material that exhibits high ionic and electric conductivity. The long-term drift of freshly prepared SC-ISEs with 3DOM carbon contacts is only 11.7 microV/h, and does not increase when in contact with solution for 1 month, making this the most stable SC-ISE reported so far. The electrodes show good resistance to the interference from oxygen. Moreover, in contrast to previously reported SC-ISEs with conducting polymers as the intermediate layer, 3DOM carbon is an electron conductor rather than a semiconductor, eliminating any light interference.

Inhibition and enhancement by organic compounds of luminol-KIO4-H2O2 chemiluminescence.

The effects of 36 organic compounds on luminol-KIO(4)-H(2)O(2) chemiluminescence (CL) were studied. It was found that most of the tested compounds could inhibit or enhance the CL intensity. The activities of such inhibitors or enhancers were related to the pH of the CL system and the number and position of functional groups such as -OH and -NH(2) on aromatic rings. The mechanism of the CL inhibition and enhancement was considered. Based on the CL inhibition or enhancement, the possibility of analytical applications was explored. The results demonstrated that numerous compounds were detectable at the ng/mL level using the CL system.

Gold nanoparticle-catalyzed luminol chemiluminescence and its analytical applications.

Gold colloids with nanoparticles of different sizes were found to enhance the chemiluminescence (CL) of the luminol-H2O2 system, and the most intensive CL signals were obtained with 38-nm-diameter gold nanoparticles. UV-visible spectra, X-ray photoelectron spectra, and transmission electron microscopy studies were carried out before and after the CL reaction to investigate the CL enhancement mechanism. The CL enhancement by gold nanoparticles of the luminol-H2O2 system was supposed to originate from the catalysis of gold nanoparticles, which facilitated the radical generation and electron-transfer processes taking place on the surface of the gold nanoparticles. The effects of the reactant concentrations, the size of the gold nanoparticles. and some organic compounds were also investigated. Organic compounds containing OH, NH2, and SH groups were observed to inhibit the CL signal of the luminol-H2O2-gold colloids system, which made it applicable for the determination of such compounds.

Flow injection determination of p-aminophenol at trace level using inhibited luminol-dimethylsulfoxide-NaOH-EDTA chemiluminescence.

A novel flow injection procedure was developed for the determination of p-aminophenol (PAP) based on the inhibition by PAP of the chemiluminescence from luminol-dimethylsulfoxide (DMSO)-NaOH-EDTA system. The method has merits of higher sensitivity, wider linear range, simpler procedure, and a more rapid analyzing speed. It is applicable for the determination of PAP in the range of 2.5 x 10(-10)-5.0 x 10(-8) g mL(-1) with a detection limit of 1.9 x 10(-10) g mL(-1). The relative standard deviation (RSD) for 5.0 x 10(-9) g mL(-1) PAP is 0.78% (n=15). The method has been successfully used to determine PAP in industrial wastewaters and environmental waters. Additionally, the inhibition mechanism was also discussed briefly.

Effect of pH on inhibition and enhancement of luminol-H2O2-Co2+ chemiluminescence by phenolic compounds and amino acids.

The effect of pH on inhibition and enhancement of luminol-H2O2-Co2+ chemiluminescence (CL) by 18 phenolic compounds and 20 amino acids was studied. It was found that most of the tested compounds showed an inhibiting effect at lower pH and an enhancing effect at higher pH. At a midrange pH, for some phenolic compounds with two ortho-position -OH, both an inhibiting and an enhancing peak were simultaneously observed. UV-visible spectra of the tested phenolic compounds at different pH values were studied. The mechanism for CL inhibition and enhancement was proposed. It is likely that the competition of the -OH or the -NH2 group and other reducing groups in the molecules with luminol for O2*- led to the CL inhibition. A reaction of -COO(-) and quinone or ketone formed by phenolic compounds at higher pH via deprotonation with O2*- also resulted in the CL enhancement.

A novel chemiluminescent method for determination of phloroglucinol.

It was found that the inhibition and enhancement by phloroglucinol of the chemiluminescence from the luminol-K3Fe(CN)6 system were dependent on the pH of luminol solution and the concentration of phloroglucinol. In Na2CO(3)-NaHCO3 buffer, phloroglucinol exhibited strong chemiluminescent enhancement at pH 9.4. On this basis, a flow injection method was developed for the determination of phloroglucinol. The method was simple, rapid, convenient and sensitive, with a detection limit of 2.0 x 10(-9) mol/L. It is effective for determining phloroglucinol in the range of 1.0 x 10(-5)-5.0 x 10(-9) mol/L. The relative standard deviation is 1.3% within one day and 3.2% between days for the determination of 5.0 x 10(-7) mol/L phloroglucinol. The method has been successfully used to determine phloroglucinol in environmental water, with satisfactory results.