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From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design
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Accounts of Chemical Research

Cite this: Acc. Chem. Res. 2019, 52, 5, 1400–1408
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https://doi.org/10.1021/acs.accounts.9b00056
Published April 24, 2019

Copyright © 2019 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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Conspectus

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading.

Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities.

We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion–ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO2 in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane.

In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Copyright © 2019 American Chemical Society

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Ion Sensing Probes

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When one imagines a molecular ion sensor, a purposefully and carefully designed chemical receptor comes to mind. It should reversibly recognize its analyte ion with the appropriate selectivity within the relevant concentration range, even in a sample environment of high complexity. The binding event should give rise to a change in optical signal, often by fluorescence and ideally in a ratiometric fashion. Such reversible ion probes do exist and are indeed well-established as indicator dyes. Among those, Fura-2 and Calcium Green are perhaps best known as they are routinely used to image intracellular calcium fluctuations by biologists.
In many cases, however, the water solubility of a reported molecular sensing probe is limited and its binding behavior must be studied in an organic solvent. Indeed, the detection of hydrophilic ionic species is often very difficult in an aqueous phase because water is such a strong competitor, resulting in very weak complex formation constants. This is well-established for the electrically neutral, nonchromogenic potassium ionophore valinomycin (see Figure 1 for the structure), for which the 1:1 complex formation constant in water has been determined to be negligibly small (log β = 0.43 (1)) while its value rises quite dramatically to log β = 11.6 (2) in a hydrophobic environment such as plasticized poly(vinyl chloride). Stable complexation and recognition selectivity are therefore very difficult to achieve in an aqueous solvent, even though water remains the practically relevant solvent where the measurement must be performed.

Figure 1

Figure 1. Structures of selected ionophores: valinomycin as one of the first biological ionophores used as a receptor for potassium ion sensing; cryptand 222 as a macrocyclic compound that inspired the structural development of the highly successful calcium ionophore I, also called ETH 1001; tweezer-type carbonate ionophore of high selectivity, a rare example of a receptor reversibly binding an ion with a covalent bond.

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Let us use Figure 2 as a molecular ion sensor example, (3) a chromogenic receptor molecule that encapsulates potassium ions. The resulting conformational change gives rise to an approach of the two pyrene units, thereby increasing the fluorescence response. As shown in Figure 2, this interaction is reversible and quite selective but requires a hydrophobic solvent environment. To make these molecular probes practically useful, one may choose to incorporate them into a separate solvent, immiscible with the aqueous sample phase. One such approach involves the use of emulsified nanoscale droplets that contain the receptor of interest. In such a manner, the receptor molecule is adequately solubilized and retains its high affinity and selectivity to the target ion, while the aqueous phase to be probed is not unduly modified. Such systems, however, must now be understood on the basis of heterogeneous phase transfer equilibria. With the data shown in Figure 2, potassium sensing is accomplished by doping the emulsified organic phase with the sodium salt of a lipophilic cation-exchanger. A potassium ion in the sample is then exchanged with these sodium ions in order to interact with the chromogenic receptor. Sensor behavior then depends on the relative concentrations (strictly, activities) of the two competing ions and the relative affinities of the two ions with the sensing phase and receptor molecule. In Figure 2, the potassium ions from the sample are quantitatively taken up by the emulsified phase, resulting in the observed linear calibration curve. (3)

Figure 2

Figure 2. (a) A conformational change from potassium binding to pyreneamide-functionalized 18-crown-6 ether, triggering a fluorescence response. (b) Observed normalized fluorescence at 406 nm as a function of the concentration of different cations (λexcitation = 346 nm). Adapted with permission from ref (3). Copyright 2016 The Royal Society of Chemistry.

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Droplets (or particles) of 100 nm in diameter that make up a microemulsion can be also understood as confined chemical reactors with a volume of 4 aL that may be doped with multiple selective sensing reagents to accomplish an ion sensing task. In contrast to the former described example, organic microemulsions, part of the bulk optode family, can be doped with two competitive receptor molecules together with an appropriate ion-exchanger. One serves as the indicator dye, typically for pH, while the other is an optically silent but highly selective receptor for the ion of interest. An unsuspecting reader might interpret the pH response of such an emulsified indicator dye in complete analogy to the classical pH buffer behavior, relating signal change directly to sample pH. This would be incorrect. Hydrogen ions partition into the emulsified phase by competitive ion-exchange and bind only then to the indicator. If the ion of interest increases in concentration, it competes more favorably with the hydrogen ion, which is exchanged out of the sensing phase. This results in a visible deprotonation of the lipophilic pH indicator dye (see Figure 3a). The response of the indicator therefore depends on sample pH, sample concentration and nature of the competing ion, and the binding strength of the indicator in the emulsified phase. If no ion-exchanger is present, protonation of the indicator may still occur but requires the coextraction of hydrogen ions together with a solution anion.

Figure 3

Figure 3. Sensing mechanism of ionophore-based ion-selective optical nanosensors containing (a) a pH indicator (chromoionophore) or (b) a charged lipophilic solvatochromic dye as an optical reporter. The ion exchange with an analyte cation results in protonation/deprotonation of the indicator or a localized repartitioning of the solvatochromic dye bearing a positive charge.

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Thus, this sensing principle is highly flexible because the characteristics of ion binding and spectral change can be designed separately and easily adjusted to a particular sensing problem. The main drawback of this sensing approach is that the ion of interest cannot be quantified without prior knowledge or control of the solution pH. More recently, this limitation has been overcome with lipophilic solvatochromic dyes, where competitive ion-exchange and signal reporting involve an ionic dye. (4−7) The chromogenic unit of the dye molecule changes color or fluorescence when expelled into the aqueous phase, while the dye itself remains anchored onto the sensing particle thanks to a lipophilic side chain (see Figure 3b).

Ionophores

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The sensing strategies discussed above make it possible to design optical sensors with numerous optically silent receptors, such as so-called ionophores well established in potentiometric sensing probes. While the first studied ionophores, such as valinomycin, were isolated from living cells, a wide range of new ionophore structures were subsequently synthesized that typically rely on multitopic interactions with the guest ion on the basis of ion-dipole, metal ligation, hydrogen, and covalent bonds.
Macrocyclic ionophores were originally developed independently by different research groups, with Pedersen introducing crown ethers, (8) circular macrocyclic compounds containing ether groups for the recognition of cations and Jean-Marie Lehn presenting the cryptands (“cryptates” in French), (9) a class of very effective synthetic polycyclic multidentate ligands where metal cations can be encapsulated, as in a crypta (Latin for cavity) (see Figure 1 for structures). Later, a different class of versatile macrocyclic ionophores formed from p-hydrocarbylphenols and formaldehyde was presented. David Gutsche called them calixarenes, (10) inspired by the shape of a Greek vase called “calyx krater”. As with valinomycin, the specificity of the macrocycle molecule binding is driven by the size of the cavity and the number of oxygen/nitrogen atoms available for complexation.
To be practically useful, ionophores should also provide rapid and reversible binding characteristics. For cryptands, their very strong and almost irreversible complexing ability has limited their application in chemical analysis. Still, the synthetic pathway for the realization of cryptands served as the inspiration for the development of highly successful noncyclic amide ionophores for the recognition of calcium, essentially by eliminating the macrocyclic structure to weaken the interaction. As a case in point, the structural similarity of calcium ionophore I (ETH 1001) (Figure 1) to cryptands is rather evident. This ionophore is used with great success in a wide range of calcium-selective sensors for blood analysis and exhibits exquisite selectivity.
Early electrochemical blood analyzers were reported to include a trifluoroacetophenone-based plasticized membrane that showed preference for bicarbonate, even though interference from blood chloride had to be corrected. (11) Owing to the electron-withdrawing character of the fluorinated substituent, trifluoroacetyl groups are understood to form hydrates by nucleophilic attack of water in aqueous solution. Stabilization of bicarbonate was therefore thought to occur via hydrogen bonding with this hydrated form. (11) A careful spectroscopic study by Meyerhoff et al. suggested that the nucleophilic attack occurs not by water, but actually by the carbonate ion itself. (12) This appears to be a rare occurrence of reversible recognition by covalent bond formation. The structure shown in Figure 1 was introduced by the groups of Nam and Cha (13) and is based on a cholic acid molecular frame that contains two axial trifluoroacetophenone substituents that interact with carbonate in a tweezer action. The selectivity of the resulting sensors to carbonate is exquisite and allows for carbonate detection in samples as complex as seawater without interference from chloride. (14)

Design Features of Optical Sensors

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So, what happens when this carbonate-selective ionophore is used in conjunction with an added lipophilic indicator to make an optical sensor of the type discussed above? Here, the sensor responds by extracting carbonate together with hydrogen ions from the sample to the sensing phase, with each of the two ions interacting with their respective molecular receptor (see Figure 4). As carbonate is a divalent ion, two hydrogen ions are extracted for each anion. The resulting sensor is therefore sensitive to the carbonate ion activity, multiplied with the square of the hydrogen ion activity. A glance at the appropriate dissociation equilibria for carbon dioxide and carbonic acid, however, shows that this is directly proportional to the carbon dioxide partial pressure. This optical sensor therefore measures carbon dioxide by the direct molecular recognition of carbonate (see Figure 4). (15)

Figure 4

Figure 4. (a) Illustration of a CO2 optode sensing mechanism. The carbonate ion is extracted together with hydrogen ions into organic sensing film containing the anion exchanger, an electrically charged pH indicator and the carbonate ionophore shown in Figure 1. The optical response is proportional to the product of carbonate ion activity and the square of hydrogen ion activity, which is directly linked to the activity of carbon dioxide. (b) Calibration curves of the CO2 optode to CO2 in a hydrogen carbonate solution, in the gas phase equilibrated with hydrogen carbonate solution, and a dry CO2 + N2 gas mixture with different levels of PCO2. For proper sensor function, humid air samples are required. Adapted with permission from ref (15). Copyright 2012 American Chemical Society.

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Another attractive approach for ion optodes operates them in a so-called exhaustive sensing mode. (16−18) While in a classic equilibrium mode, the sample concentration is usually considered to be unchanged before and after the contact with the sensor; the analyte here is completely consumed by the sensor, but this works only if the sensing phase contains more binding sites than analyte ions available in the sample. This makes the response dependent only on the amount of analyte in the sample but not that of the reference ion. The latter means that the response of optodes based on pH indicator dye will be pH-independent when they are operated in the exhaustive sensing mode (compare Figure 5a and 5b). In Figure 5, it is apparent that another important feature of the exhaustive sensing mode is that the response is linear with the analyte concentration.

Figure 5

Figure 5. Theoretical response curves of the calcium-selective optode used in (a) an equilibrium or (b) an exhaustive sensing mode. The response of optodes operated by an exhaustive sensing mode is linear with the analyte concentration and sufficiently independent of the pH of the sample. Adapted with permission from ref (18). Copyright 2015 Springer-Verlag Berlin Heidelberg.

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Emulsions of ion optodes may not only function as conventional optodes but also offer new analytical opportunities. Nanosphere emulsions have been used for light-controlled ion concentration perturbation when doped with photoresponsive compounds such as spiropyran. (19) In another application, a nanosphere emulsion was applied with a wax printed paper-based microfluidic device for the quantification of potassium in aqueous and human serum samples, using colorimetric distance-based analysis methods. (20)
Ion optode emulsions prepared without a lipophilic pH indicator were recently proposed as a new promising class of complexometric titration reagents, chiefly for Ca2+ and Pb2+. (21−23) This versatile toolbox allows one to use ionophores as highly selective chelators despite their poor solubility in water. In comparison with traditional water-soluble chelators such as EDTA, these new reagents possess a significantly reduced pH dependence, and control of pH is often not necessary. (21) Such chelating emulsions can also serve as optical indicators for complexometric titrations, using a small amount of lipophilic pH indicator or ionic solvatochromic dye as an optical reporter. (22,23) Only at the end point, when the chelating nanospheres become saturated with analyte, the indicating nanospheres will be interacting with the analyte and result in a change of chromoionophore protonation or repartitioning of solvatochromic dye, giving a color change. The titrimetric detection of alkali metal ions has recently also been demonstrated (K+, Na+, and Li+). (24)

Membrane Electrodes

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The optically silent molecular ion receptors discussed above and shown in Figure 1 have their origin in ion-selective membrane electrodes (ISEs) that are typically interrogated by potentiometry, at zero current, just as one would operate an established pH glass electrode. In many ways, they are complementary to optical ion sensors. The sensing material containing the ionophore and added ion-exchanger is termed a membrane because of its permselective properties, which mean that only cations or anions may pass through the membrane upon application of a small current. This property is essential to observing a Nernstian electrode response (at 25 °C, 59.2 mV/zi change for each 10-fold ion activity change; zi is the charge of the analyte ion) that characterizes these sensors.
The first ion-selective membranes were prepared as a solution of an electrically neutral ionophore in an organic solvent infused into porous glass filters as a support, starting with the group of Simon. (25) In 1967, Bloch and co-workers introduced the first ionophore-based solvent polymeric membrane made of polyvinyl chloride, (26) which is still commonly used for ISE fabrication despite the fact that many other polymers, such as silicone rubbers, polyurethanes, acrylates, and perfluoropolymers, have been suggested over the years. In order to lower the glass transition temperature below room temperature and form homogeneous elastic films, polyvinyl chloride and other polymers are often blended with plasticizers, such as 2-nitrophenyl octyl ether, bis(2-ethylhexyl) sebacate, or dibutyl phthalate. The choice of plasticizer influences the polarity of the membrane phase and subsequently the ISE response.
An important feature of ionophore-based membrane electrodes is that the composition of an ideally functioning membrane electrode does not change as the analyte ion concentration varies in the contacting sample solution. The potential response has its origin in a free energy balance of the interfacial concentration gradient and electrical potential difference where the ion to be measured may freely transfer into and out of the membrane. If this process is sufficiently selective and no other ions can compete with the ion of interest, a Nernstian response slope is observed. Unlike most other sensing methods, such potentiometric probes are quite robust, and changes in size and shape, in principle, bear no influence on the potential readout.
Membrane electrodes can also be used to conveniently characterize the binding properties of ion–ionophore complexes. For this purpose, membrane symmetry must be overcome, which otherwise cancels the contribution of the ion binding event within the membrane. One method relies on the potentiometric characterization of ion-exchange selectivity if a reference ion can be assumed not to interact with the ionophore. (27) For cations, this is often the tetrabutylammonium ion, which is water-soluble but typically too bulky for complexation. An ion-exchange selectivity study with and without ionophore gives the desired information. Another technique performs potentiometric measurements by fusing two membranes of a different composition, one with and one without ionophore. (27) It can also be used to evaluate to what extent any reference ion in the first method binds to the ionophore.
Ionophore-based ISEs have found applications in many different fields including environmental science, medicine, biochemistry, neuroscience, and corrosion studies, but indisputably their largest success has been in clinical diagnostics where blood-gas analyzers with ionophore-based potentiometric sensors for detection of Na+, K+, Ca2+, and Cl– have completely displaced the previously used flame photometers from the market. Owing to their simplicity, ease of miniaturization, and low energy consumption, ISEs are also used for aquatic in situ analysis. Several examples of developing and deploying submersible potentiometric multisensor platforms for in situ monitoring of ion species in natural waters have been reported by our group. (28−32)
Let us use the tweezer-type carbonate ionophore discussed above as an example to develop an environmental sensor. As mentioned, the recognition selectivity of membranes doped with this receptor is so high that it may detect carbonate directly in environmental samples, without concern for interference. However, to use such materials for submersible in situ probes, the inner aqueous solution at the back side of the membrane should be replaced with a solid support, owing to the associated pressure fluctuations. All-solid-state membrane electrodes require a lipophilic ion-to-electron transducing layer, which are now well established for cationic analytes but less so for anionic analytes. The conducting polymer poly(octylthiophene), for example, placed between electrode and ion-selective membrane, has been shown to partially oxidize with cation-selective membranes. (33) For complete oxidation, the cation-exchanger in the membrane must migrate to the transducing layer to form the counterion of the oxidized, anionic form, but it is not able to do so, chiefly for reasons of steric hindrance. The polymer therefore exhibits a mix of reduced and oxidized forms, giving the desired redox buffer. With anion-selective membranes, however, this mechanism is not possible because the membrane contains an anion-exchanger that cannot play this role. A stable potentiometric response behavior for both cationic and anionic analytes was, however, achieved with highly capacitive and lipophilic nanomaterials, making it now possible to fabricate potentiometric probes that can be deployed in situ.
An interesting characteristic of potentiometric probes is that the reference electrode can be replaced by another indicator electrode to gain a different type of information from the sample. A carbonate probe, for example, can be combined with a pH electrode. In some analogy to the PCO2 optode discussed above, by considering the acid dissociation constants of CO2/carbonic acid, such a detection cell has been shown to directly measure carbon dioxide partial pressure. (34) Compared to other known CO2 probes that require gas-permeable separation membranes, one expects dramatically faster sensor responses. This was recently confirmed during deployment in a stratified Swiss lake, see Figure 6, where a traditional membrane-based Severinghaus probe failed to provide the same level of detail in the depth region where steep gradients of dissolved gases occur. (32)

Figure 6

Figure 6. In situ profiles of dissolved CO2 in Lake Rotsee obtained with a commercial Severinghaus CO2 probe and the coupled hydrogen ion and carbonate selective electrodes with a profiling speed of 5 cm/s. In contrast to the Severinghaus CO2 probe exhibiting a response time of minutes, the setup with two coupled ISEs provides a response time of a few seconds. This allows for CO2 measurements at a high temporal and spatial resolution. Adapted with permission from ref (32). Copyright 2018 American Chemical Society.

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Other Dynamic Electrochemistry Examples

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Ionophore-based membrane electrodes can also be placed under dynamic electrochemistry control by applying a current or potential. This allows one to obtain complementary information about the sample and result in new readout modes that expand the scope of application of these chemically selective materials. One example is the use of chronopotentiometry, where an applied current imposes a net ion flux across the membrane that can in many cases be understood as an imposed concentration gradient of the analyte ion near the membrane surface. The diffusion theory predicts that the diffusion distance over which the gradient occurs will grow with time, eventually resulting in ion depletion at the membrane surface at a so-called transition time that manifests itself by a potential change (Figure 7a). The square root of transition time is ideally proportional to concentration and can therefore be used to determine the analyte concentration in the sample.

Figure 7

Figure 7. Different readout modes of membrane ISEs placed under dynamic electrochemistry control: (a) chronopotentiometry; (b) thin-layer coulometry; (c) ion transfer voltammetry at a high concentration of the analyte; (d) ion transfer voltammetry at a low concentration of the analyte; (e) coulometry with a capacitive layer; (f) colorimetry.

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ISEs operated by chronopotentiometry are attractive for speciation analysis because they give information about the kinetically labile fraction of ion concentration, while zero current potentiometry measures so-called free ion concentration (strictly, activity). Recent examples include the speciation analysis of calcium in blood samples to distinguish the total calcium from the so-called ionized calcium. (35) A sequential zero current potentiometric and chronopotentiometric readout with a single H+-selective electrode can give information about pH and acidity/alkalinity in water samples. (36,37)
In contrast to the above-described examples, a current pulse with a reversed sign should impose an ion flux in the direction of the sample phase. Thus, if the membrane contains a significant concentration of the analyte ion, it can be used as an electrochemical titration pump where the amount of delivered titrant is controlled by the imposed charge in agreement with Faraday’s law. This was demonstrated for the coulometric delivery of calcium (38) for EDTA titrations and protons (39) for the determination of alkalinity in natural water samples. Here, the electrochemical titration can be considered as nearly nondestructive because chemical perturbation occurs only in the diffusion layer of the aqueous phase where the analyte concentration can be re-equilibrated after stirring the sample.
If one reduces the sample volume to a thin layer with dimensions of the diffusional layer, exhaustive turnover becomes possible. In this case, the ion transfer current observed upon applying an appropriate potential can be integrated to give the overall charge of the ion transfer process (Figure 7b). This charge is used to calculate the analyte concentration for a known sample volume according to the Faraday’s law. An indisputable advantage of this sensing mode is a calibration-free application of ISEs. An electrochemical protocol for compensating non-Faradaic charging currents by performing a second excitation pulse whose charge was subtracted from that of the first pulse was proposed. (40) The successful coulometric determination of potassium, (40−43) calcium, (44) and nitrite (45) with ionophore-based ISEs was demonstrated. Different approaches have been proposed to decrease the sample phase dimensions, such as sampling with a paper support (43) or microfluidic devices with a tubular shaped ISE membrane. (42,44,45)
Alternatively, membranes thinner than 1 μm have allowed for the establishment of ion transfer voltammetry as an alternative readout of ionophore-based ISEs. It was shown that, by sweeping the applied potential, the oxidation/reduction of the conducting polymer layer (46,47) placed underneath the sensing membrane or redox molecular probes (48,49) incorporated into the membrane phase may trigger ion transfer across the membrane/sample interface. Such voltammetric ISEs can be exploited in two sensing modes depending on the analyte concentration level. (50) At relatively high concentrations, the position of the ion transfer peak becomes the analytical signal that is related to the logarithmic analyte activity in complete analogy to potentiometric measurements at zero current (Figure 7c). Interestingly, it was shown that ISE membranes doped with three different ionophores in this sensing mode allows one to detect and quantify the sample activity of three ions (Li+, K+, and Na+) within one potential scan. (47) Instead, at low concentration levels, the peak current linearly increases with analyte concentration in complete analogy with stripping voltammetry measurements (Figure 7d).
Another interesting coulometric readout principle was recently proposed for solid-contact ISEs with a capacitive transducer layer. For an electrochemical cell held at a constant potential, any change of the indicator electrode potential (caused by a sample concentration change) is compensated with an opposite change at a capacitive layer placed beneath the ion-selective membrane. This results in a transient current response where the accumulated charge is proportional to the logarithmic analyte activity change (Figure 7e). The applicability of this sensing mode has been demonstrated for potassium (51) and chloride (52) detection.
It was recently shown that a closed bipolar electrode configuration with an ISE on one pole and an indium tin oxide electrode covered with a redox indicator in a thin-layer cell on another pole allows one to convert the potential change at the ISE into a color change in the detection compartment (Figure 7f). (53) An electrical potential imposed across the bipolar electrode is used to trigger a color change within the desired concentration change and can be tuned to match the application. In this manner, optical ion sensors can be realized with membrane electrodes, fundamentally and physically separating optical detection from chemical sensing. This approach was successfully applied for calcium detection. (53)
To conclude, molecular ion recognition for chemical sensing may preferentially occur in a separate sensing phase, rather than in a homogeneous sample solution. This means that phase transfer equilibria must also be taken into account to understand how such sensors must work. This gives rise to a rich world of sensing principles and materials that can be used to design a wide range of exciting sensing systems, from nanoscale optical probes to membranes that can be addressed by dynamic electrochemistry. This is a field where advances in molecular recognition can be quite readily translated into practically useful and novel sensing probes, as shown here with the carbonate-selective ionophore as a guiding example.

Author Information

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  • Corresponding Authors
    • Elena Zdrachek - University of Geneva, Department of Inorganic and Analytical Chemistry, Quai Ernest Ansermet 30, Geneva 1211, Switzerland;  Email: [email protected]
    • Eric Bakker - University of Geneva, Department of Inorganic and Analytical Chemistry, Quai Ernest Ansermet 30, Geneva 1211, Switzerland;  Orcidhttp://orcid.org/0000-0001-8970-4343;  Email: [email protected]
    • Author Contributions

      E.Z. and E.B. contributed equally.

    • Notes
      The authors declare no competing financial interest.

    Biographies

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    Elena Zdrachek received both her B.Sc. (2010) and M.Sc. (2011) degrees in chemistry from the Belarusian State University in Minsk. In 2015, she obtained her Ph.D. from the same university, working with Prof. Egorov V. V. Currently, she is a postdoctoral researcher at the University of Geneva. Her research focuses on new electrochemical sensing concepts based on membrane electrodes.

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    Eric Bakker was educated at ETH Zurich in Switzerland (Ph.D. in 1993), pursued postdoctoral studies at the University of Michigan and was on the faculty at Auburn University (AL), Purdue University (West Lafayette, IN), and Curtin University (Perth, Western Australia) before assuming his current position in 2010.

    Acknowledgments

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    The authors thank all of the co-workers and collaborators who contributed to the cited research. The authors gratefully appreciate the financial support of the Swiss National Science Foundation for supporting research in this field.

    References

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    This article references 53 other publications.

    1. 1
      Feinstein, M. B.; Felsenfeld, H. The Detection of Ionophorous Antibiotic-Cation Complexes in Water with Fluorescent Probes. Proc. Natl. Acad. Sci. U. S. A. 1971, 68, 2037– 2041,  DOI: 10.1073/pnas.68.9.2037
      Google Scholar
      1
      Detection of ionophorous antibiotic-cation complexes in water with fluorescent probes
      Feinstein, M. B.; Felsenfeld, H.
      Proceedings of the National Academy of Sciences of the United States of America (1971), 68 (7), 2037-41CODEN: PNASA6; ISSN:0027-8424.
      The binding of alkali cations by the ionophorous antibiotics valinomycin, nigericin, alamethicin, and the macrotetralide actins has been shown to occur, in aq. media, by the use of the fluorescent probes 1-anilino-8-naphthalenesulfonate and 2-p-toluidinyl-6-naphthalenesulfonate. The interaction of the ionophore-cation complexes with the fluorescent dyes produced enhanced fluorescence emission, increased lifetime and polarization, and a significant blue-shift of the emission maxima of the fluorescence spectrum. At const. antibiotic and fluorophore concns. in water, the intensity of the fluorescence emission is a function of the cation concn. This permitted relative cation affinities to be detd. for alamethicin (Na+ ≈ K+), valinomycin (Rb+ > K+ > Cs+), nigericin (K+ > Rb+ > Na+ > Cs+) and trinactin (NH4+ > K+ > Rb+ > Cs+).
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE3MXlt1Wgsb4%253D&md5=48a678602d171d25b3f7ccd48abbb279
    2. 2
      Qin, Y.; Mi, Y.; Bakker, E. Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether. Anal. Chim. Acta 2000, 421, 207– 220,  DOI: 10.1016/S0003-2670(00)01038-2
      Google Scholar
      2
      Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether
      Qin, Y.; Mi, Y.; Bakker, E.
      Analytica Chimica Acta (2000), 421 (2), 207-220CODEN: ACACAM; ISSN:0003-2670. (Elsevier Science B.V.)
      A segmented sandwich membrane method is used to det. complex formation consts. of 18 elec. neutral ionophores in situ in solvent polymeric sensing membranes. These ionophores are commonly used in potentiometric and optical sensors, and knowledge of such binding information is important for ionophore and sensor design. In this method, two membrane segments are fused together, with only one contg. the ionophore, to give a concn.-polarized sandwich membrane. Unlike other approaches, this method does not require the use of a ref. ion in the sample and/or a 2nd ionophore in the membrane, and is typically pH insensitive. The following ionophores responsive for the common cations lithium, sodium, potassium, magnesium and calcium are characterized and discussed: valinomycin, BME-44, bis[(benzo-15-crown-5)-4'-ylmethyl]pimelate, ETH 157, ETH 2120, bis[(12-crown-4)methyl]dodecylmethylmalonate, 4-tert-butylcalix [4] arene tetraacetic acid tetra-Et ester, ETH 149, ETH 1644, ETH 1810, 6,6-dibenzyl-14-crown-4, N,N,N',N',N'',N''-hexacyclohexyl-4,4',4''-propylidyne tris(3-oxabutyramide), ETH 1117, ETH 4030, ETH 1001, ETH 129, ETH 5234, and A23187. The logarithmic complex formation consts. range from 4.4 to 29 and compare well to published data for ionophores that were characterized earlier. From the obsd. complex formation consts., max. possible selectivities are calcd. that would be expected if interfering ions show no binding affinity to the ionophore, and the values are compared with exptl. findings. Each ionophore is characterized in poly(vinyl chloride) membranes plasticized either with a polar (NPOE) or a nonpolar plasticizer (DOS). Membranes based on NPOE always show larger complex formation consts. of the embedded ionophore.
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    3. 3
      Jarolimova, Z.; Vishe, M.; Lacour, J.; Bakker, E. Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles. Chem. Sci. 2016, 7, 525– 533,  DOI: 10.1039/C5SC03301B
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      3
      Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles
      Jarolimova, Zdenka; Vishe, Mahesh; Lacour, Jerome; Bakker, Eric
      Chemical Science (2016), 7 (1), 525-533CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      These compds. were successfully synthesized by the reaction of α-diazo-β-ketoesters with cyclic ethers of the desired size in the presence of dirhodium complexes followed by a stereo-selective tandem amidation-transposition process and characterized by 1H-NMR, 13C-NMR, IR, HR-ESI-MS, UV-VIS and fluorescence spectroscopy and potentiometry. Their unique structure consisting of a crown ether ring linked to pyrene moieties through amide groups exhibits on-off switchable behavior upon binding of specific cations and allows one to incorporate these chemosensors as fluorescent ionophores into ion-exchange nanospheres. The nanosphere matrix is composed of bis(2-ethylhexyl)sebacate , poly(ethylene glycol), sodium tetrakis 3,5-bis(trifluoromethyl)phenyl borate and pyreneamide functionalized 18-crown-6 ether (18C6). These optode nanoparticles exhibit a strong affinity to the potassium cation over other metal ions up to the millimolar concn. range in an exhaustive detection mode. The logarithmic complex formation const. was detd. using the segmented sandwich membrane method and was found to be 6.5 ± 0.3 (SD) in PVC membrane plasticized with NPOE and 5.3 ± 0.3 in DOS with a 1 : 1 complex stoichiometry. The nanosensors were characterized in broad range of pH from 4 to 10 and the same linear calibration curves were obtained in the concn. range from 10-7 M to 10-5 M and thus the pH dependent response was largely overcome.
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    4. 4
      Xie, X.; Zhai, J.; Bakker, E. Potentiometric Response from Ion-Selective Nanospheres with Voltage-Sensitive Dyes. J. Am. Chem. Soc. 2014, 136, 16465– 16468,  DOI: 10.1021/ja5107578
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      4
      Potentiometric Response from Ion-Selective Nanospheres with Voltage-Sensitive Dyes
      Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
      Journal of the American Chemical Society (2014), 136 (47), 16465-16468CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Potentiometric sensors require the implementation of conducting wires for signal transduction, but this is impractical for the readout of individual nanoparticles. It is here demonstrated that the potentiometric response of ion-selective nanospheres can be obsd. with voltage-sensitive dyes, thereby converting nanoscale electrochem. signals into an optical readout. No ref. electrode is needed since the readout is by fluorescence. The results strongly support the potentiometric origin for the fluorescence response. The ion-selective nanospheres exhibit excellent selectivity and respond to ion concn. changes independent of sample pH, providing a new platform of potentiometric nanosensors with optical readout compatible with optical imaging equipment.
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    5. 5
      Xie, X.; Gutierrez, A.; Trofimov, V.; Szilagyi, I.; Soldati, T.; Bakker, E. Charged Solvatochromic Dyes as Signal Transducers in pH Independent Fluorescent and Colorimetric Ion Selective Nanosensors. Anal. Chem. 2015, 87, 9954– 9959,  DOI: 10.1021/acs.analchem.5b02566
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      5
      Charged Solvatochromic Dyes as Signal Transducers in pH Independent Fluorescent and Colorimetric Ion Selective Nanosensors
      Xie, Xiaojiang; Gutierrez, Agustin; Trofimov, Valentin; Szilagyi, Istvan; Soldati, Thierry; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2015), 87 (19), 9954-9959CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Ionophore-based ion selective optical nanosensors that operate independently of the sample pH are developed here using elec. charged solvatochromic dyes as signal transducers. A series of dye mols. with a D-π-A structure was synthesized and characterized in various solvents and incorporated into ion selective nanospheres for K+, Na+, and H+. Since dye leakage was greatly suppressed when the solvatochromic dyes were encapsulated in the nanosphere core, ion sensing nanospheres were explored for cellular ion imaging in Dictyostelium discoideum live cells but spontaneous dye loss resulted in undesired staining of cells. The in vitro anal. of potassium in human plasma was successfully demonstrated with this approach. A theor. model was developed for the response of the ion selective nanosensors contg. charged solvatochromic dyes. The nanosensors exhibited a tunable response range, high sensitivity, and good stability.
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    6. 6
      Wang, L.; Xie, X.; Zhai, J.; Bakker, E. Reversible pH-independent optical potassium sensor with lipophilic solvatochromic dye transducer on surface modified microporous nylon. Chem. Commun. 2016, 52, 14254– 14257,  DOI: 10.1039/C6CC07841A
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      6
      Reversible pH-independent optical potassium sensor with lipophilic solvatochromic dye transducer on surface modified microporous nylon
      Wang, Lu; Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
      Chemical Communications (Cambridge, United Kingdom) (2016), 52 (99), 14254-14257CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A reversible and pH-independent fluorescent ion optode is introduced with an ionophore and surface confined solvatochromic dye transducer doped onto microporous nylon membranes. The resulting film responds to K+ with excellent selectivity over the range of 10-7 to 10-2 M and a response time of t95 < 60 s above 10-6 M.
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    7. 7
      Xie, X.; Szilagyi, I.; Zhai, J.; Wang, L.; Bakker, E. Ion-Selective Optical Nanosensors Based on Solvatochromic Dyes of Different Lipophilicity: From Bulk Partitioning to Interfacial Accumulation. ACS Sens 2016, 1, 516– 520,  DOI: 10.1021/acssensors.6b00006
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      7
      Ion-Selective Optical Nanosensors Based on Solvatochromic Dyes of Different Lipophilicity: From Bulk Partitioning to Interfacial Accumulation
      Xie, Xiaojiang; Szilagyi, Istvan; Zhai, Jingying; Wang, Lu; Bakker, Eric
      ACS Sensors (2016), 1 (5), 516-520CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      The sensing mechanism of fluorescent ion-selective nanosensors incorporating solvatochromic dyes (SDs), with K+ as model ion, is shown to change as a function of dye lipophilicity. Water-sol. SDs obey bulk partitioning principles where the sensor response directly depends on the lipophilicity of the SD and exhibits an influence on the phase vol. ratio of nanosensors to aq. soln. (diln. effect). A lipophilization of the SDs is shown to overcome these limitations. An interfacial accumulation mechanism is proposed and confirmed with F.ovrddot.orster resonance energy transfer (FRET) with a ratiometric near-IR fluorescence FRET pair. This work lays the foundation for operationally more robust ion-selective nanosensors incorporating SDs.
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    8. 8
      Pedersen, C. J. Cyclic polyethers and their complexes with metal salts. J. Am. Chem. Soc. 1967, 89, 2495– 2496,  DOI: 10.1021/ja00986a052
      Google Scholar
      8
      Cyclic polyethers and their complexes with metal salts
      Pedersen, Charles J.
      Journal of the American Chemical Society (1967), 89 (10), 2495-6CODEN: JACSAT; ISSN:0002-7863.
      Two cyclic polyethers, 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxocyclo-2,11-octadecadiene (I) and its hydrogenation product (over RuO2) 2,5,8,15,18,21-hexaoxatricylo[20.4.0.09,24] hexacosane (II), were prepd. which have the unusual property of forming relatively stable complexes with alkali and alk. earth metal ions. Stable complexes of I and II have been made for the following cationic compds.: Li, Na, NH4, RNH3, K, Rb, Cs, Ag(I), Ca, Sr, Ba, Cd, Hg(I), Hg(II), La(III), Tl(I), Ce(III), and Pb(II).
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    9. 9
      Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Diaza-polyoxa-macrocycles et macrobicycles. Tetrahedron Lett. 1969, 10, 2885– 2888,  DOI: 10.1016/S0040-4039(01)88299-X
      Google Scholar
      There is no corresponding record for this reference.
    10. 10
      Gutsche, C. D.; Muthukrishnan, R. Calixarenes. 1. Analysis of the product mixtures produced by the base-catalyzed condensation of formaldehyde with para-substituted phenols. J. Org. Chem. 1978, 43, 4905– 4906,  DOI: 10.1021/jo00419a052
      Google Scholar
      10
      Calixarenes. 1. Analysis of the product mixtures produced by the base-catalyzed condensation of formaldehyde with para-substituted phenols
      Gutsche, C. David; Muthukrishnan, Ramamurthi
      Journal of Organic Chemistry (1978), 43 (25), 4905-6CODEN: JOCEAH; ISSN:0022-3263.
      The title reactions of phenols with HCHO gave mixts. of at least two components, none of which are cyclic tetramers (calixarenes). The product from p-cresol is a mixt. of cyclic heptamer (major product) and cyclic octamer (minor product) and the product from p-Me3CC6H4OH is a mixt. of cyclic octamer (major product) and cyclic pentamer or hexamer (minor product).
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    11. 11
      Scott, W. J.; Chapoteau, E.; Kumar, A. Ion-selective membrane electrode for rapid automated determinations of total carbon dioxide. Clin. Chem. 1986, 32, 137– 141
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      11
      Ion-selective membrane electrode for rapid automated determinations of total carbon dioxide
      Scott, W. James; Chapoteau, Eddy; Kumar, Anand
      Clinical Chemistry (Washington, DC, United States) (1986), 32 (1, Pt. 1), 137-41CODEN: CLCHAU; ISSN:0009-9147.
      Previously described ion-selective membrane electrodes useful for detn. of total CO2 in biol. fluids in automated analyzers show interference from several sources e.g., fatty acids, keto acids, salicylate, and heparin. Judicious selection of membrane components has produced a membrane with superior performance characteristics; short conditioning time, long lifetime in storage, rapid and stable response, low drift, and significantly less susceptibility to interference than other electrodes thus far reported. Samples can be analyzed at ≥240 samples/h and results correlate well with those by e.g., the Technicon SMAC II method for total CO2. Mounted on the Technicon RA-1000, the CO2 sensor is arranged in tandem with Na and K ion-selective electrodes. When all 3 species are measured in the same buffered sample stream, the sampling rate becomes 720 tests/h, done on 25 μL samples. Prepn. of the membranes and optimization of their performance is described in detail.
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    12. 12
      Meyerhoff, M. E.; Pretsch, E.; Welti, D. H.; Simon, W. Role of trifluoroacetophenone solvents and quaternary ammonium salts in carbonate-selective liquid membrane electrodes. Anal. Chem. 1987, 59, 144– 150,  DOI: 10.1021/ac00128a030
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      12
      Role of trifluoroacetophenone solvents and quaternary ammonium salts in carbonate-selective liquid membrane electrodes
      Meyerhoff, Mark E.; Pretsch, Ernoe; Welti, Dieter H.; Simon, Wilhelm
      Analytical Chemistry (1987), 59 (1), 144-50CODEN: ANCHAM; ISSN:0003-2700.
      UV absorption and 13C NMR spectroscopic techniques are used in conjunction with potentiometric studies to characterize the interaction of carbonate ions with trifluoroacetyl-p-butylbenzene, a solvent often utilized to prep. liq./polymeric membrane carbonate-selective electrodes. Single-phase spectroscopic expts. in cyclohexane suggest that carbonate ions react with the carbonyl C atom of the fluorinated ketone to form lipophilic carbonate dianion adducts. Similar adduct formation is obsd. in 2-phase expts. in which aq. carbonate solns. are equilibrated with an org. phase contg. the ketone and tridodecylammonium chloride. The direct interaction between carbonate and the ketone enables the prepn. of functional PVC-type carbonate electrodes with <1 wt. % of ketone and little or no quaternary ammonium species in the membrane; however, such membranes display significant cation interference. From these spectral and potentiometric findings, the roles of the fluorinated ketone and quaternary ammonium species in carbonate-selective membrane electrodes are established.
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    13. 13
      Lee, H. J.; Yoon, I. J.; Yoo, C. L.; Pyun, H.-J.; Cha, G. S.; Nam, H. Potentiometric Evaluation of Solvent Polymeric Carbonate-Selective Membranes Based on Molecular Tweezer-Type Neutral Carriers. Anal. Chem. 2000, 72, 4694– 4699,  DOI: 10.1021/ac991212l
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      13
      Potentiometric Evaluation of Solvent Polymeric Carbonate-Selective Membranes Based on Molecular Tweezer-Type Neutral Carriers
      Lee, Han Jin; Yoon, In Joon; Yoo, Choong Leol; Pyun, Hyung-Jung; Cha, Geun Sig; Nam, Hakhyun
      Analytical Chemistry (2000), 72 (19), 4694-4699CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Potentiometric properties of the ion-selective electrodes based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (CA) derivs. have been measured. The carbonate-selective neutral carriers have been prepd. by coupling one to three trifluoroacetobenzoyl (TFAB) groups to a cholic acid deriv. 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 derivs. with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(TFAB)CA] exhibited remarkably improved carbonate selectivity, indicating that the bis(TFAB)CAs behave like mol. 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-, HPO42-, 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-Å range at the AM1 level semiempirical calcn., 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 compds. 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 anal. 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.
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    14. 14
      Choi, Y. S.; Lvova, L.; Shin, J. H.; Oh, S. H.; Lee, C. S.; Kim, B. H.; Cha, G. S.; Nam, H. Determination of Oceanic Carbon Dioxide Using a Carbonate-Selective Electrode. Anal. Chem. 2002, 74, 2435– 2440,  DOI: 10.1021/ac0108459
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      14
      Determination of Oceanic Carbon Dioxide Using a Carbonate-Selective Electrode
      Choi, Yong Suk; Lvova, Larisa; Shin, Jae Ho; Oh, Seong Hee; Lee, Chang Suk; Kim, Byeong Hyo; Cha, Geun Sig; Nam, Hakhyun
      Analytical Chemistry (2002), 74 (10), 2435-2440CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Potentiometric properties of PVC membrane-based electrodes prepd. with mol. 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 obsd. that deoxy-3,12-bis(TFAB)CA-based electrode provided greatly enhanced CO32- selectivity over Cl- (log KCO32-,Cl-POT ∼-6) and other minor anions present in seawater. Thus, the possibility of applying this carbonate-selective electrode for direct detn. of oceanic CO2 was assessed. Total CO2 (TCO2) concn. in Yellow Sea surface water was detd. with the deoxy-3,12-bis(TFAB)CA-based electrode, Severinghaus-type CO2 gas sensor, and traditional potentiometric titrn. methods. Results showed the carbonate-selective electrode provided accurate oceanic TCO2 detn. comparable to that obtained with the other methods. The anal. procedure based on a carbonate-selective electrode is clearly advantageous over conventional methods; it does not require sample pretreatment or extra reagents other than the std. calibration solns., while providing measured results directly and immediately.
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    15. 15
      Xie, X.; Pawlak, M.; Tercier-Waeber, M. L.; Bakker, E. Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore. Anal. Chem. 2012, 84, 3163– 3169,  DOI: 10.1021/ac2030046
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      15
      Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore
      Xie, Xiaojiang; Pawlak, Marcin; Tercier-Waeber, Mary-Lou; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2012), 84 (7), 3163-3169CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A novel optical method for the detn. of CO2 concn. in aq. and gaseous samples of plasticized PVC film is presented. The detection principle makes use of a direct mol. recognition of the carbonate ion by a mol. tweezer-type ionophore, which was previously demonstrated to exhibit excellent carbonate selectivity. The carbonate ion is extd. together with hydrogen ions into a polymeric film that contains the anion exchanger tridodecylmethylammonium chloride, a lipophilic, elec. charged, and highly basic pH indicator, which is used for the readout in absorbance mode, in addn. to the lipophilic carbonate ionophore. According to known bulk optode principles, such an optical sensor responds to the product of the carbonate ion activity and the square of hydrogen ion activity. This quantity is thermodynamically linked to the activity of carbon dioxide. This allows one to realize a direct carbon dioxide sensor that does not make use of the traditional Severinghaus sensing principle of measuring a pH change upon CO2 equilibration across a membrane. A selectivity anal. shows that common ions such as chloride are sufficiently suppressed for direct PCO2 measurements in freshwater samples at pH 8. Chloride interference, however, is too severe for direct seawater measurements at the same pH. This may be overcome by placing a gas-permeable membrane over the optode sensing film. This is conceptually confirmed by establishing that the sensor is equally useful for gas-phase PCO2 measurements. As expected, humid air samples are required for proper sensor functioning, as dry CO2 gas will not cause any signal change. The sensor showed acceptable response times and good reproducibility under both conditions.
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    16. 16
      Xie, X.; Zhai, J.; Bakker, E. pH Independent Nano-Optode Sensors Based on Exhaustive Ion-Selective Nanospheres. Anal. Chem. 2014, 86, 2853– 2856,  DOI: 10.1021/ac403996s
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      16
      pH Independent Nano-Optode Sensors Based on Exhaustive Ion-Selective Nanospheres
      Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2014), 86 (6), 2853-2856CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Bulk optode-based ion selective optical sensors work from extn. equil., and their response toward the analyte ion is known to dependent on the sample pH. This pH dependence was one of the major disadvantages that have hampered the broad acceptance of bulk optodes in chem. sensing. The authors present here for the 1st time the use of exhaustive Ca2+-selective nanosensors that may overcome this pH dependent response. The nanosensors were characterized at different pH and the same linear calibration was obtained in the Ca2+ concn. range from 10-7 M to 10-5 M.
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    17. 17
      Xie, X.; Zhai, J.; Crespo, G. A.; Bakker, E. Ionophore-Based Ion-Selective Optical NanoSensors Operating in Exhaustive Sensing Mode. Anal. Chem. 2014, 86, 8770– 8775,  DOI: 10.1021/ac5019606
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      17
      Ionophore-Based Ion-Selective Optical NanoSensors Operating in Exhaustive Sensing Mode
      Xie, Xiaojiang; Zhai, Jingying; Crespo, Gaston A.; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2014), 86 (17), 8770-8775CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Ion selective optical sensors are typically interrogated under conditions where the sample concn. is not altered during measurement. The authors describe here an alternative exhaustive detection mode for ion selective optical sensors. This exhaustive sensor concept is demonstrated with ionophore-based nanooptodes either selective for calcium or the polycationic heparin antidote protamine. In agreement with a theor. treatment presented here, linear calibration curves were obtained in the exhaustive detection mode instead of the sigmoidal curves for equil.-based sensors. The response range can be tuned by adjusting the nanosensor loading. The nanosensors showed av. diams. of <100 nm and the sensor response is dramatically faster than that for film-based optodes. Due to the strong binding affinity of the exhaustive nanosensors, total calcium concn. in human blood plasma was successfully detd. Optical detn. of protamine in human blood plasma using the exhaustive nanosensors was attempted, but is less successful.
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    18. 18
      Xie, X. J.; Bakker, E. Ion selective optodes: from the bulk to the nanoscale. Anal. Bioanal. Chem. 2015, 407, 3899– 3910,  DOI: 10.1007/s00216-014-8413-4
      Google Scholar
      18
      Ion selective optodes: from the bulk to the nanoscale
      Xie, Xiaojiang; Bakker, Eric
      Analytical and Bioanalytical Chemistry (2015), 407 (14), 3899-3910CODEN: ABCNBP; ISSN:1618-2642. (Springer)
      This review describes recent advances in the miniaturization of ion selective optodes into microscale and nanoscale sensors. The topics include a comparison between film-based and miniaturized ion optodes, equil. and exhaustive detection modes, recent prepn. methodologies and applications of microscale and nanoscale ion optodes, criteria for the design of optode sensors, and other future perspectives. [Figure not available: see fulltext.].
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    19. 19
      Xie, X.; Bakker, E. Light-Controlled Reversible Release and Uptake of Potassium Ions from Ion-Exchanging Nanospheres. ACS Appl. Mater. Interfaces 2014, 6, 2666– 2670,  DOI: 10.1021/am4049805
      Google Scholar
      19
      Light-Controlled Reversible Release and Uptake of Potassium Ions from Ion-Exchanging Nanospheres
      Xie, Xiaojiang; Bakker, Eric
      ACS Applied Materials & Interfaces (2014), 6 (4), 2666-2670CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Here, we report for the first time on photoswitchable nanospheres contg. spiropyran (Sp) for reversible release and uptake of metal ions. K+ is used as a model ion to demonstrate the chem. principle of this approach. Valinomycin is incorporated in the nanospheres to stabilize K+. Upon UV illumination, Sp transforms to the more basic ring-opened merocyanine form, which takes up H+ from the surrounding aq. soln. and expels K+ from the nanospheres. The process can be reversed by irradn. with visible light to reduce the surrounding K+ concn.
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    20. 20
      Gerold, C. T.; Bakker, E.; Henry, C. S. Selective Distance-Based K+ Quantification on Paper-Based Microfluidics. Anal. Chem. 2018, 90, 4894– 4900,  DOI: 10.1021/acs.analchem.8b00559
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      20
      Selective Distance-Based K+ Quantification on Paper-Based Microfluidics
      Gerold, Chase T.; Bakker, Eric; Henry, Charles S.
      Analytical Chemistry (Washington, DC, United States) (2018), 90 (7), 4894-4900CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      In this study, paper-based microfluidic devices (μPADs) capable of K+ quantification in aq. samples, as well as in human serum, using both colorimetric and distance-based methods are described. A lipophilic phase contg. potassium ionophore I (valinomycin) was utilized to achieve highly selective quantification of K+ in the presence of Na+, Li+, and Mg2+ ions. Successful addn. of a suspended lipophilic phase to a wax printed paper-based device is described and offers a soln. to current approaches that rely on org. solvents, which damage wax barriers. The approach provides an avenue for future alkali/alk. quantification utilizing μPADs. Colorimetric spot tests allowed for K+ quantification from 0.1-5.0 mM using only 3.00 μL of sample soln. Selective distance-based quantification required small sample vols. (6.00 μL) and gave responses sensitive enough to distinguish between 1.0 and 2.5 mM of sample K+. μPADs using distance-based methods were also capable of differentiating between 4.3 and 6.9 mM K+ in human serum samples. Distance-based methods required no digital anal., electronic hardware, or pumps; any steps required for quantification could be carried out using the naked eye.
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    21. 21
      Zhai, J.; Xie, X.; Bakker, E. Ionophore-based ion-exchange emulsions as novel class of complexometric titration reagents. Chem. Commun. 2014, 50, 12659– 12661,  DOI: 10.1039/C4CC05754F
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      21
      Ionophore-based ion-exchange emulsions as novel class of complexometric titration reagents
      Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (84), 12659-12661CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Complexometric titrns. rely on a drastic change of the pM value at the equivalence point with a water sol. chelator forming typically 1 : 1 complexes of high stability. The available chem. toolbox of suitable chelating compds. is unfortunately limited because many promising complexing agents are not water sol. The authors introduce here a novel class of complexometric titrn. reagents, a suspension of polymeric nanospheres whose hydrophobic core is doped with lipophilic ion-exchanger and a selective complexing agent (ionophore). The emulsified nanospheres behave from heterogeneous ion exchange equil. where the initial counterion of the ion-exchanger is readily displaced from the emulsion for the target ion that forms a stable complex in the nanosphere core. Two different examples are shown with Ca2+ and Pb2+ as target ions. The lack of protonatable groups on the calcium receptor allows one to perform Ca2+ titrn. without pH control.
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    22. 22
      Zhai, J.; Xie, X.; Bakker, E. Ion-Selective Optode Nanospheres as Heterogeneous Indicator Reagents in Complexometric Titrations. Anal. Chem. 2015, 87, 2827– 2831,  DOI: 10.1021/ac504213q
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      22
      Ion-Selective Optode Nanospheres as Heterogeneous Indicator Reagents in Complexometric Titrations
      Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2015), 87 (5), 2827-2831CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Traditionally, optical titrns. of inorg. ions are based on a rapid and visible color change at the end point with water-sol. org. dyes as indicators. Adequate selectivity is required for both the indicator and the complexing agent, which is often limited. The authors present here alternative, heterogeneous ionophore-based ion-selective nanospheres as indicators and chelators for optical titrns. The indicating nanospheres rely on a weaker extn. of the analyte of interest by ion-exchange, owing to the addnl. incorporation of a lipophilic pH indicator in the nanosphere core. Ca2+ titrn. was demonstrated as a proof-of-concept. Both the chelating and the indicating nanospheres showed good selectivity and a wide working pH range.
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    23. 23
      Zhai, J.; Xie, X.; Bakker, E. Solvatochromic Dyes as pH-Independent Indicators for Ionophore Nanosphere-Based Complexometric Titrations. Anal. Chem. 2015, 87, 12318– 12323,  DOI: 10.1021/acs.analchem.5b03663
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      23
      Solvatochromic Dyes as pH-Independent Indicators for Ionophore Nanosphere-Based Complexometric Titrations
      Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2015), 87 (24), 12318-12323CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      For half a century, complexometric titrns. of metal ions have been performed with water-sol. chelators and indicators that typically require careful pH control. Very recently, ion-selective nanosphere emulsions were introduced that exhibit ion-exchange properties and are doped with lipophilic ionophores originally developed for chem. ion sensors. They may serve as novel, highly selective and pH independent complexometric reagents. While ion optode emulsions have been demonstrated as useful indicators for such titrns., they exhibit a pH cross-response that unfortunately complicates the identification of the end point. Here, we present pH-independent optode nanospheres as indicators for complexometric titrns., with calcium as an initial example. The nanospheres incorporate an ionic solvatochromic dye (SD), ion exchanger and ionophore. The solvatochromic dye will be only expelled from the core of the nanosphere into the aq. soln. at the end point at which point it results in an optical signal change. The titrn. curves are demonstrated to be pH-independent and with sharper end points than with previously reported chromoionophore-based optical nanospheres as indicator. The calcium concn. in mineral water was successfully detd. using this method.
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    24. 24
      Zhai, J.; Xie, X.; Cherubini, T.; Bakker, E. Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum. ACS Sens 2017, 2, 606– 612,  DOI: 10.1021/acssensors.7b00165
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      24
      Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum
      Zhai, Jingying; Xie, Xiaojiang; Cherubini, Thomas; Bakker, Eric
      ACS Sensors (2017), 2 (4), 606-612CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      While the titrimetric assay is one of the most precise anal. techniques available, only a limited list of complexometric chelators is available, as many otherwise promising reagents are not water-sol. Recent work demonstrated successful titrimetry with ion-exchanging polymeric nanospheres contg. hydrophobic complexing agents, so-called ionophores, opening an exciting avenue in this field. However, this method was limited to ionophores of very high affinity to the analyte and exhibited a relatively limited titrn. capacity. To overcome these two limitations, the authors report here on solvent based titrn. reagents. This heterogeneous titrn. principle is based on the dissoln. of all hydrophobic recognition components in a solvent such as dichloromethane (CH2Cl2) where the ionophores are shown to maintain a high affinity to the target ions. HSV (hue, satn., value) anal. of the images captured with a digital camera provides a convenient and inexpensive way to det. the end point. This approach is combined with an automated titrn. setup. The titrns. of the alkali metals K+, Na+, and Li+ in aq. soln. are successfully demonstrated. The potassium concn. in human serum without pretreatment was precisely and accurately detd. as 4.38 mM ± 0.10 mM (automated titrn.), which compares favorably with at. emission spectroscopy (4.47 mM ± 0.20 mM).
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    25. 25
      Štefanac, Z.; Simon, W. Ion specific electrochemical behavior of macrotetrolides in membranes. Microchem. J. 1967, 12, 125– 132,  DOI: 10.1016/0026-265X(67)90014-8
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      25
      Ion-specific electrochemical behavior of macrotetrolides in membranes
      Stefanac, Zlata; Simon, Wilhelm
      Microchemical Journal (1967), 12 (1), 125-32CODEN: MICJAN; ISSN:0026-265X.
      cf. CA 66, 61872v. The emf. measurements were obtained using the following electrochem. cell: Ag; AgCl, inner soln.//membrane//sample/0.1M NH4NO3/KCl standard, Hg2Cl2; Hg. The aq. inner soln. was made 0.1M in the chlorides of all cations tested and buffered (pH 8) with 1M N(C2H4OH)3 and 0.5M HOAc. Supersatd. solns. of the nonactin homologs (20% nonactin, 35% monactin, 35% dinactin, 10% trinactin) in CCl4 were transferred onto sintered glass disks to form the membrane. Cells of this type show specificity (K+ over Na+) far superior to other ion-specific glass electrodes and have cation selectivity consts. up to 750 as compared with ∼30 for glass electrodes. This value is comparable to the ion-specific behavior observed in rat liver mitochondria.
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    26. 26
      Bloch, R.; Shatkay, A.; Saroff, H. A. Fabrication and Evaluation of Membranes as Specific Electrodes for Calcium ions. Biophys. J. 1967, 7, 865– 877,  DOI: 10.1016/S0006-3495(67)86626-8
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      26
      Fabrication and evaluation of membranes as specific electrodes for calcium ions
      Bloch, Rene; Shatkay, Adam; Saroff, H. A.
      Biophysical Journal (1967), 7 (6), 865-77CODEN: BIOJAU; ISSN:0006-3495.
      A technique was developed to construct membranes with the bulk transport for specific metallic ions. Membranes were fabricated from an inert poly(vinyl chloride) matrix impregnated with Bu3PO4 or Bu3PO4 plus thenoyltrifluoroacetone, and these were tested as electrodes specific for Ca ions. Both types of membrane exhibited a high specificity for Ca ions in the presence of Na, Mg, and Ba ions. In the presence of perturbing ions, the usefulness of the membranes is limited by both the transport of the current and development of potentials by the interfering ion. The potential developed across the membrane was measured in 2 different cells. In the 1st cell the membrane is clamped between 2 halves of a Perspex cell, the solns. are poured into the 2 chambers, and then calomel electrodes are introduced into the chambers. In the 2nd cell, the membrane is clamped between 2 small glass cups (vol. ∼3 ml.), and each cup is connected to a calomel electrode with a 5-way stopcock. This cell allows a convenient and frequent change of solns. of both sides of the membrane and of the liq. junctions between solns. and the calomel electrodes. Also, this cell gives more accurate and reproducible results, although the 1st one is easier to operate. An exptl. method is given for evaluating the interfering quantities. 17 references.
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    27. 27
      Bakker, E.; Pretsch, E. Modern Potentiometry. Angew. Chem., Int. Ed. 2007, 46, 5660– 5668,  DOI: 10.1002/anie.200605068
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      27
      Modern potentiometry
      Bakker, Eric; Pretsch, Ernoe
      Angewandte Chemie, International Edition (2007), 46 (30), 5660-5668CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. A silent revolution of ion-selective electrodes (ISEs) took place during the past decade. Polymeric membrane electrodes are now routinely used to det. complex formation consts. between lipophilic guests and ionic hosts. Ultratrace level measurements have become possible even in samples of very small vols., with detection limits in the attomole range.
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    28. 28
      Pankratova, N.; Crespo, G. A.; Afshar, M. G.; Crespi, M. C.; Jeanneret, S.; Cherubini, T.; Tercier-Waeber, M. L.; Pomati, F.; Bakker, E. Potentiometric sensing array for monitoring aquatic systems. Environ. Sci.-Process Impacts 2015, 17, 906– 914,  DOI: 10.1039/C5EM00038F
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      28
      Potentiometric sensing array for monitoring aquatic systems
      Pankratova, Nadezda; Crespo, Gaston A.; Afshar, Majid Ghahraman; Crespi, Miquel Coll; Jeanneret, Stephane; Cherubini, Thomas; Tercier-Waeber, Mary-Lou; Pomati, Francesco; Bakker, Eric
      Environmental Science: Processes & Impacts (2015), 17 (5), 906-914CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
      Since aquatic environments are highly heterogeneous and dynamic, there is the need in aquatic ecosystem monitoring to replace traditional approaches based on periodical sampling followed by lab. anal. with new automated techniques that allow one to obtain monitoring data with high spatial and temporal resoln. We report here on a potentiometric sensing array based on polymeric membrane materials for the continuous monitoring of nutrients and chem. species relevant for the carbon cycle in freshwater ecosystems. The proposed setup operates autonomously, with measurement, calibration, fluidic control and acquisition triggers all integrated into a self-contained instrument. Exptl. validation was performed on an automated monitoring platform on lake Greifensee (Switzerland) using potentiometric sensors selective for hydrogen ions, carbonate, calcium, nitrate and ammonium. Results from the field tests were compared with those obtained by traditional lab. anal. A linear correlation between calcium and nitrate activities measured with ISEs and relevant concns. measured in the lab. was found, with the slopes corresponding to apparent single ion activity coeffs. [Formula Omitted] and [Formula Omitted]. Good correlation between pH values measured with ISE and CTD probes (SD = 0.2 pH) suggests adequate reliability of the methodol.
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    29. 29
      Cuartero, M.; Pankratova, N.; Cherubini, T.; Crespo, G. A.; Massa, F.; Confalonieri, F.; Bakker, E. In Situ Detection of Species Relevant to the Carbon Cycle in Seawater with Submersible Potentiometric Probes. Environ. Sci. Technol. Lett. 2017, 4, 410– 415,  DOI: 10.1021/acs.estlett.7b00388
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      29
      In Situ Detection of Species Relevant to the Carbon Cycle in Seawater with Submersible Potentiometric Probes
      Cuartero, Maria; Pankratova, Nadezda; Cherubini, Thomas; Crespo, Gaston A.; Massa, Francesco; Confalonieri, Fabio; Bakker, Eric
      Environmental Science & Technology Letters (2017), 4 (10), 410-415CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      We report on the development of a submersible probe for the simultaneous potentiometric detection of carbonate, calcium, and pH in seawater. All-solid-state electrodes incorporating nanomaterials provide an adequate response time (<10 s), stability (drifts of <0.9 mV h-1), reproducibility (calibration parameter deviation of <0.7%), and accuracy (deviation of <8% compared to ref. techniques) for real-time monitoring of seawater using a flow system. The functioning of the deployable prototype was checked in an outdoor mesocosm and via long-term monitoring in Genoa Harbor. The electrodes worked properly for 3 wk, and the system demonstrated the capability to autonomously operate with routines for repetitive measurements, data storage, and management. In situ profiles obsd. in Genoa Harbor and Arcachon Bay were validated using on site and ex situ techniques. The validation of in situ-detected carbonate is a challenge because both re-equilibration of the sample with atm. CO2 and the use of apparent thermodn. consts. for speciation calcns. lead to some differences (<20% deviation). The submersible probe is a promising tool for obtaining rapid and trustworthy information about chem. levels in marine systems. Moreover, the fluidic approach allows for the integration of other ion sensors that may require sample pretreatment.
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    30. 30
      Cuartero, M.; Crespo, G.; Cherubini, T.; Pankratova, N.; Confalonieri, F.; Massa, F.; Tercier-Waeber, M. L.; Abdou, M.; Schafer, J.; Bakker, E. In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors. Anal. Chem. 2018, 90, 4702– 4710,  DOI: 10.1021/acs.analchem.7b05299
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      30
      In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors
      Cuartero, Maria; Crespo, Gaston; Cherubini, Thomas; Pankratova, Nadezda; Confalonieri, Fabio; Massa, Francesco; Tercier-Waeber, Mary-Lou; Abdou, Melina; Schafer, Jorg; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2018), 90 (7), 4702-4710CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A new submersible probe for the in situ detection of nitrate, nitrite, and chloride in seawater is presented. Inline coupling of a desalination unit, an acidification unit, and a sensing flow cell contg. all-solid-state membrane electrodes allows for the potentiometric detection of nitrate and nitrite after removal of the key interfering ions in seawater, chloride and hydroxide. Thus, the electrodes exhibited attractive anal. performances for the potentiometric detection of nitrate and nitrite in desalinated and acidified seawater: fast response time (t95 < 12 s), excellent stability (long-term drifts of <0.5 mV h-1), good reproducibility (calibration parameter deviation of <3%), and satisfactory accuracy (uncertainties <8%Diff compared to ref. technique). The desalination cell, which can be repetitively used for about 30 times, may addnl. be used as an exhaustive, and therefore calibration-free, electrochem. sensor for chloride and indirect salinity detection. The detection of these two parameters together with nitrate and nitrite may be useful for the correlation of relative changes in macronutrient levels with salinity cycles, which is of special interest in recessed coastal water bodies. The system is capable of autonomous operation during deployment, with routines for repetitive measurements (every 2 h), data storage and management, and computer visualization of the data in real time. In situ temporal profiles obsd. in the Arcachon Bay (France) showed valuable environmental information concerning tide-dependent cycles of nitrate and chloride levels in the lagoon, which are here obsd. for the first time using direct in situ measurements. The submersible probe based on membrane electrodes presented herein may facilitate the study of biogeochem. processes occurring in marine ecosystems by the direct monitoring of nitrate and nitrite levels, which are key chem. targets in coastal waters.
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    31. 31
      Athavale, R.; Dinkel, C.; Wehrli, B.; Bakker, E.; Crespo, G. A.; Brand, A. Robust Solid-Contact Ion Selective Electrodes for High-Resolution In Situ Measurements in Fresh Water Systems. Environ. Sci. Technol. Lett. 2017, 4, 286– 291,  DOI: 10.1021/acs.estlett.7b00130
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      Robust Solid-Contact Ion Selective Electrodes for High-Resolution In Situ Measurements in Fresh Water Systems
      Athavale, Rohini; Dinkel, Christian; Wehrli, Bernhard; Bakker, Eric; Crespo, Gaston A.; Brand, Andreas
      Environmental Science & Technology Letters (2017), 4 (7), 286-291CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      Biogeochem. processes are often confined to very narrow zones in aquatic systems. Therefore, highly resolved in situ measurements are required to study these processes. Potentiometric solid-contact ion selective electrodes (SC-ISEs) are promising tools for such measurements. SC-ISEs show good performance in analyses under controlled exptl. conditions. Very few sensor designs, however, can sustain the challenges of natural water matrixes and external environmental conditions during in situ applications. We fabricated ammonium and pH selective SC-ISEs with functionalized multiwalled carbon nanotubes (f-MWCNT) as a solid contact. Their functionality was tested in the lab. and applied in situ for vertical profiling in a eutrophic lake. Sensors were insensitive to strong redox changes, high sulfide concns., and bright daylight conditions during the application in the lake. In addn., sensors are easily fabricated and exhibit short response times (<10 s). The proposed design of SC-ISEs based on f-MWCNTs is quite suitable for high-resoln. in situ profiling of ionic species in fresh water lakes.
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    32. 32
      Athavale, R.; Pankratova, N.; Dinkel, C.; Bakker, E.; Wehrli, B.; Brand, A. Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems. Environ. Sci. Technol. 2018, 52, 11259– 11266,  DOI: 10.1021/acs.est.8b02969
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      32
      Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems
      Athavale, Rohini; Pankratova, Nadezda; Dinkel, Christian; Bakker, Eric; Wehrli, Bernhard; Brand, Andreas
      Environmental Science & Technology (2018), 52 (19), 11259-11266CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      We present a new potentiometric sensor principle and a calibration protocol for in situ profiling of dissolved CO2 with high temporal and spatial resoln. in fresh water lakes. The sensor system is based on the measurement of EMF between two solid-contact ion selective electrodes (SC-ISEs), a hydrogen ion selective and a carbonate selective sensor. Since it relies on SC-ISEs, it is insensitive to changes in pressure, thus suitable for in situ studies. Also, as it offers a response time (t95%) of <10 s, it allows for profiling applications at high spatial resoln. The proposed optimum in situ protocol accounts for the continuous drift and change in offset that remains a challenge during profiling in natural waters. The fast response resolves features that are usually missed by std. methods like the classical Severinghaus CO2 probe. In addn., the insensitivity of the presented setup to dissolved sulfide allows also for measurements in anoxic zones of eutrophic systems. Highly resolved CO2 concn. profiles obtained by the novel and robust SC-ISE setup along with the developed optimum in situ protocol allow investigating hotspots of biogeochem. processes, such as mineralization and primary prodn. in the water column and help improving ests. for CO2 turnover in freshwater systems.
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    33. 33
      Veder, J.-P.; De Marco, R.; Patel, K.; Si, P.; Grygolowicz-Pawlak, E.; James, M.; Alam, M. T.; Sohail, M.; Lee, J.; Pretsch, E.; Bakker, E. Evidence for a Surface Confined Ion-to-Electron Transduction Reaction in Solid-Contact Ion-Selective Electrodes Based on Poly(3-octylthiophene). Anal. Chem. 2013, 85, 10495– 10502,  DOI: 10.1021/ac4024999
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      33
      Evidence for a surface confined ion-to-electron transduction reaction in solid-contact ion-selective electrodes based on poly(3-octylthiophene)
      Veder, Jean-Pierre; De Marco, Roland; Patel, Kunal; Si, Pengchao; Grygolowicz-Pawlak, Ewa; James, Michael; Alam, Muhammad Tanzirul; Sohail, Manzar; Lee, Junqiao; Pretsch, Erno; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2013), 85 (21), 10495-10502CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The ion-to-electron transduction reaction mechanism at the buried interface of the electrosynthesized poly-(3-octylthiophene) (POT) solid-contact (SC) ion-selective electrode (ISE) polymeric membrane has been studied using synchrotron radiation-XPS (SR-XPS), near edge X-ray absorption fine structure (NEXAFS), and electrochem. impedance spectroscopy (EIS)/neutron reflectometry (NR). The tetrakis-[3,5-bis-(triflouromethyl)-phenyl]-borate (TFPB-) membrane dopant in the polymer ISE was transferred from the polymeric membrane to the outer surface layer of the SC on oxidn. of POT but did not migrate further into the oxidized POT SC. The TFPB- and oxidized POT species could only be detected at the outer surface layer (≤14 ) of the SC material, even after oxidn. of the electropolymd. POT SC for an hour at high anodic potential demonstrating that the ion-to-electron transduction reaction is a surface confined process. Accordingly, this study provides the first direct structural evidence of ion-to-electron transduction in the electropolymd. POT SC ISE by proving TFPB- transport from the polymeric ISE membrane to the oxidized POT SC at the buried interface of the SC ISE. It is inferred that the performance of the POT SC ISE is independent of the thickness of the POT SC but is instead contingent on the POT SC surface reactivity and/or elec. capacitance of the POT SC. In particular, the results suggest that the electropolymd. POT conducting polymer may spontaneously form a mixed surface/bulk oxidn. state, which may explain the unusually high potential stability of the resulting ISE. It is anticipated that this new understanding of ion-to-electron transduction with electropolymd. POT SC ISEs will enable the development of new and improved devices with enhanced anal. performance attributes.
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    34. 34
      Xie, X. J.; Bakker, E. Non-Severinghaus Potentiometric Dissolved CO2 Sensor with Improved Characteristics. Anal. Chem. 2013, 85, 1332– 1336,  DOI: 10.1021/ac303534v
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      34
      Non-Severinghaus Potentiometric Dissolved CO2 Sensor with Improved Characteristics
      Xie, Xiaojiang; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2013), 85 (3), 1332-1336CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A new type of carbon dioxide sensor comprising a pH glass electrode measured against a carbonate-selective membrane electrode based on a tweezer type carbonate ionophore is presented here for the 1st time. No cumbersome liq. junction based ref. element is used in this measurement. The sensor shows an expected Nernstian divalent response slope to dissolved CO2 over a wide range covering the routine environmental and physiol. PCO2 levels. Unlike the conventional Severinghaus CO2 probe for which the response is substantially delayed to up to 10 min due to diffusion of carbon dioxide into the internal compartment, the ion-selective CO2 sensor proposed here shows a response time (t95%) of 5 s. When used together with a traditional ref. electrode, the sensor system is confirmed to also monitor sample pH and carbonate along with carbon dioxide. A selectivity anal. suggests that Cl- does not interfere even at high concns., allowing one to explore this type of sensor probe for use in seawater or undiluted blood samples. The CO2 probe was used in an aquarium to monitor the CO2 levels caused by the diurnal cycles caused by the metab. of the aquatic plants and shows stable and reproducible results.
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    35. 35
      Afshar, M. G.; Crespo, G. A.; Bakker, E. Direct Ion Speciation Analysis with Ion-Selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode. Anal. Chem. 2012, 84, 8813– 8821,  DOI: 10.1021/ac302092m
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    36. 36
      Crespo, G. A.; Afshar, M. G.; Bakker, E. Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes. Anal. Chem. 2012, 84, 10165– 10169,  DOI: 10.1021/ac302868u
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      36
      Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes
      Crespo, Gaston A.; Ghahraman Afshar, Majid; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2012), 84 (23), 10165-10169CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      An electrochem. sensing protocol based on supported liq. ion-selective membranes for the direct detection of total alky. of a sample that contains a weak base such as Tris (pKa = 8.2) is presented here for the 1st time. Alky. is detd. by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane-sample interface depletes owing to diffusion limitation is related to sample alky. in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alky. (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ detn. of these characteristics in complex samples such as natural waters.
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    37. 37
      Afshar, M. G.; Crespo, G. A.; Xie, X. J.; Bakker, E. Direct Alkalinity Detection with Ion-Selective Chronopotentiometry. Anal. Chem. 2014, 86, 6461– 6470,  DOI: 10.1021/ac500968c
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      37
      Direct Alkalinity Detection with Ion-Selective Chronopotentiometry
      Afshar, Majid Ghahraman; Crespo, Gaston A.; Xie, Xiaojiang; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2014), 86 (13), 6461-6470CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The authors explore the possibility to directly measure pH and alky. in the sample with the same sensor by imposing an outward flux of hydrogen ions from an ion-selective membrane to the sample soln. by an applied current. The membrane consists of a polypropylene-supported liq. membrane doped with a hydrogen ionophore (chromoionophore I), ion exchanger (KTFBP), and lipophilic electrolyte (ETH 500). While the sample pH is measured at zero current, alky. is assessed by chronopotentiometry at anodic current. Hydrogen ions expelled from the membrane undergo acid-base soln. chem. and protonate available base in the diffusion layer. With time, base species start to be depleted owing to the const. imposed hydrogen ion flux from the membrane, and a local pH change occurs at a transition time. This pH change (potential readout) is correlated to the concn. of the base in soln. As in traditional chronopotentiometry, the obsd. square root of transition time (τ) is linear in the concn. range of 0.1 mM to 1 mM, using the bases tris(hydroxymethyl)aminomethane, ammonia, carbonate, hydroxide, hydrogen phosphate, and borate. Numerical simulations were used to predict the concn. profiles and the chronopotentiograms, allowing the discussion of possible limitations of the proposed method and its comparison with volumetric titrns. of alky. Finally, the P-alky. level is measured in a river sample to demonstrate the anal. usefulness of the proposed method. As a result of these preliminary results, probably this approach becomes useful for the in situ detn. of P-alky. in a range of matrixes.
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    38. 38
      Afshar, M. G.; Crespo, G. A.; Bakker, E. Coulometric Calcium Pump for Thin Layer Sample Titrations. Anal. Chem. 2015, 87, 10125– 10130,  DOI: 10.1021/acs.analchem.5b02856
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    39. 39
      Afshar, M. G.; Crespo, G. A.; Bakker, E. Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection. Angew. Chem., Int. Ed. 2015, 54, 8110– 8113,  DOI: 10.1002/anie.201500797
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      39
      Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection
      Afshar, Majid Ghahraman; Crespo, Gaston A.; Bakker, Eric
      Angewandte Chemie, International Edition (2015), 54 (28), 8110-8113CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors report a general concept based on a selective electrochem. ion pump used for creating concn. perturbations in thin layer samples (∼40 μL). As a 1st example, hydrogen ions are released from a selective polymeric membrane (proton pump) and the resulting pH is assessed potentiometrically with a 2nd membrane placed directly opposite. By applying a const. potential modulation for 30 s, an induced proton concn. of up to 350 mΜ may be realized. This concept may become an attractive tool for in situ titrns. without the need for sampling, because the thin layer eventually reequilibrates with the contacting bulk sample. Acid-base titrns. of NaOH and Na2CO3 are demonstrated. The detn. of total alky. in a river water sample is carried out, giving levels (23.1 mΜ) comparable to that obtained by std. methods (23.6 mΜ). The concept may be easily extended to other ions (cations, anions, polyions) and may become attractive for environmental and clin. applications.
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    40. 40
      Grygolowicz-Pawlak, E.; Numnuam, A.; Thavarungkul, P.; Kanatharana, P.; Bakker, E. Interference Compensation for Thin Layer Coulometric Ion-Selective Membrane Electrodes by the Double Pulse Technique. Anal. Chem. 2012, 84, 1327– 1335,  DOI: 10.1021/ac202273k
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      40
      Interference Compensation for Thin Layer Coulometric Ion-Selective Membrane Electrodes by the Double Pulse Technique
      Grygolowicz-Pawlak, Ewa; Numnuam, Apon; Thavarungkul, Panote; Kanatharana, Proespichaya; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2012), 84 (3), 1327-1335CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Ion-selective membranes operated in a thin layer coulometric detection mode were previously demonstrated to exhibit attractive characteristics in view of realizing sensors without the need for frequent recalibration. In this methodol., the analyte ion is exhaustively removed across an ion-selective membrane by an applied potential, and the resulting current is integrated to yield the coulomb no. and hence the amt. of analyte originally present in the sample. This exhaustive process, however, places greater demands on the selectivity of the membrane compared to direct potentiometry, since the level of interference will increase as the analyte depletes. The authors evaluate here a double pulse protocol to reduce the level of interference, in which the sample is electrolyzed once again after the initial coulometric detection pulse. Since the analyte ion is no longer present at significant concns. during the 2nd pulse, but an interfering ion of high concn. did not appreciably deplete, the 2nd electrolysis step may be used to partially compensate for undesired interference. These processes are here evaluated by numerical simulation for ions of the same charge, demonstrating that the resulting coulomb no. may indeed be reduced for systems of limited selectivity. The improvement in operational selectivity relative to uncompensated coulometry is ∼6-fold. The methodol. is successfully demonstrated exptl. with a calcium selective membrane and tetraethylammonium as a model interfering agent, and the obsd. relative errors after background compensation can be favorably compared to that in direct potentiometry where no sample depletion occurs.
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    41. 41
      Grygolowicz-Pawlak, E.; Bakker, E. Thin layer coulometry ion sensing protocol with potassium-selective membrane electrodes. Electrochim. Acta 2011, 56, 10359– 10363,  DOI: 10.1016/j.electacta.2011.02.053
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      41
      Thin layer coulometry ion sensing protocol with potassium-selective membrane electrodes
      Grygolowicz-Pawlak, Ewa; Bakker, Eric
      Electrochimica Acta (2011), 56 (28), 10359-10363CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The authors recently presented a calcium-selective thin-layer coulometric system in view of realizing recalibration free ion sensors. A multi-pulse protocol allowing for a significant background current correction was further modified here by a numerical evaluation of the electrolysis end-point. A potassium-selective system was studied here within the 10 μM to 1 mM KCl concn. range with a 10 mM LiCl as a background soln. A residual 20 nA s-1 signal change was used to indicate the end of exhaustive ion transfer from the sample across the ion-selective membrane. Optimized electrolysis times were found from 13.4 s for a 10 μM sample to 270 s for a 1 mM sample. The intercept of the calibration curve of obsd. ion transfer charge vs. concn. is just 2.3 ± 2.0 μC. The obsd. slope of 1.18 ± 0.02 C M-1 was very similar to the one calcd. based on the known vol. of the sample, 1.19 ± 0.12 C M-1, suggesting that the method is promising as an abs. measurement tool.
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    42. 42
      Shvarev, A.; Neel, B.; Bakker, E. Detection Limits of Thin Layer Coulometry with lonophore Based Ion-Selective Membranes. Anal. Chem. 2012, 84, 8038– 8044,  DOI: 10.1021/ac301940n
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      42
      Detection Limits of Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes
      Shvarev, Alexey; Neel, Bastien; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2012), 84 (18), 8038-8044CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The authors report here on a significant improvement in lowering the low detection limit of thin layer coulometric sensors based on liq. ion-selective membranes, using a potassium-selective system as a model example. Various possible processes that may result in an elevated residual current reading after electrolysis were eliminated. Self-dissoln. of AgCl on the Ag/AgCl inner element may result in a residual ion flux that could adversely affect the lower detection limit. It was here replaced with an Ag/AgI inner pseudo-ref. electrode where the self-dissoln. equil. is largely suppressed. Possible residual currents originating from a direct contact between inner element and ion-selective membranes were eliminated by introducing an inert PVDF separator of 50 μm diam. that was coiled around the inner element by a custom-made instrument. Finally, the influence of electrolyte fluxes from the outer soln. across the membrane into the sample was evaluated by altering its lipophilic nature and reducing its concn. This last effect is most likely responsible for the obsd. residual current for the potassium-selective membranes studied here. For the optimized conditions, the calibration curves demonstrated a near zero intercept, thereby paving the way to the coulometric calibration-free sensing of ionic species. A linear calibration curve for the coulometric cell with valinomycin potassium-selective membrane was obtained in the range of 100 nM to 10 μM potassium in the presence of a 10 μM sodium background. In the presence of a higher (100 μM) concn. of sodium, a reliable detection of 1-100 μM of potassium was achieved.
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    43. 43
      Ding, J.; Cherubini, T.; Yuan, D.; Bakker, E. Paper-supported thin-layer ion transfer voltammetry for ion detection. Sens. Actuators, B 2019, 280, 69– 76,  DOI: 10.1016/j.snb.2018.10.046
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      44
      Paper-supported thin-layer ion transfer voltammetry for ion detection
      Ding, Jiawang; Cherubini, Thomas; Yuan, Dajing; Bakker, Eric
      Sensors and Actuators, B: Chemical (2019), 280 (), 69-76CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      We report here on paper-supported thin sample layer voltammetry for the detn. of ions. To achieve this goal, a simple setup for the coupling of a com. available electrode to a silver rod electrode was designed and evaluated for paper-supported thin-layer voltammetry. Linear scan ion transfer voltammetry was explored here for ion-selective membranes doped with an ionophore. The ion-transfer processes and electrochem. behaviors of the system are here evaluated and confirmed by numerical simulation. In the proof-of-concept expts. described, the ions tetrabutylammonium chloride (TBA+) and potassium (K+) were studied as model analytes at membranes without and with ionophore, resp. A linear relationship from 0.1 mM to 1.0 mM K+ was obtained between the charge and ion concn. The coexistence of background sodium ions did not give appreciable interference, but the background wave was not completely isolated from the analyte wave, as also confirmed by the model. The methodol. was successfully demonstrated for detn. of K+ in mineral water. It is anticipated that this paper-supported thin-layer detection approach may provide an attractive readout protocol for disposable paper-based anal. devices as the methodol. does not place strict demands on ref. electrode performance.
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    44. 44
      Dorokhin, D.; Crespo, G. A.; Afshar, M. G.; Bakker, E. A low-cost thin layer coulometric microfluidic device based on an ion-selective membrane for calcium determination. Analyst 2014, 139, 48– 51,  DOI: 10.1039/C3AN01715J
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      43
      A low-cost thin layer coulometric microfluidic device based on an ion-selective membrane for calcium determination
      Dorokhin, Denis; Crespo, Gaston A.; Afshar, Majid Ghahraman; Bakker, Eric
      Analyst (Cambridge, United Kingdom) (2014), 139 (1), 48-51CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
      A prototype of a low-cost and easy-to-use thin layer coulometric microfluidic device based on an ion-selective membrane for calcium detection is described. The microfluidic device was fabricated and consequently assembled with inexpensive materials without using sophisticated and centralized fabrication lab. facilities. The linear range of the device is 10-100 μM for a 60 s current integration time. Preliminary validations showed that the microfluidic device is suitable for the quantification of calcium in mineral water.
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    45. 45
      Afshar, M. G.; Crespo, G. A.; Dorokhin, D.; Neel, B.; Bakker, E. Thin Layer Coulometry of Nitrite with Ion-Selective Membranes. Electroanalysis 2015, 27, 609– 615,  DOI: 10.1002/elan.201400522
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      45
      Thin Layer Coulometry of Nitrite with Ion-Selective Membranes
      Afshar, Majid Ghahraman; Crespo, Gaston A.; Dorokhin, Denis; Neel, Bastien; Bakker, Eric
      Electroanalysis (2015), 27 (3), 609-615CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We report on a thin layer coulometric detection method based on an ion-selective membrane in view of a quant. detn. of nitrite in undiluted human urine and tap water. A cobalt(II) tert-Bu salophen compd., previously characterized by our group in ion-selective membrane electrodes, is used here as ionophore. The four orders of magnitude discrimination of chloride permits the detection of nitrite in samples that contain a large excess of chloride. The approach developed here allows one to det. nitrite within a concn. range of 20-100 μM for a 120 s current integration time. A modification of the reported electrochem. protocol to reduce the capacitive contribution diminishes the intercept of the calibration to -8.6 μC. This is esp. important in view of reaching calibration free systems. Initial inhouse validation exhibits a limit of detection of 10 μM in undiluted urine and in tap water. These values correspond to the regulation limits established in European Union and are adequate for the detn. of nitrite for medical applications.
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    46. 46
      Crespo, G. A.; Cuartero, M.; Bakker, E. Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection. Anal. Chem. 2015, 87, 7729– 7737,  DOI: 10.1021/acs.analchem.5b01459
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      46
      Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection
      Crespo, Gaston A.; Cuartero, Maria; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2015), 87 (15), 7729-7737CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A concept is introduced that allows one to detect the activity of multiple ions simultaneously and selectively with a single ion-selective membrane. This is demonstrated with ∼300 nm thin plasticized PVC membranes contg. up to two ionophores in addn. to a lipophilic cation-exchanger, overlaid on an electropolymd. poly-3-octylthiophene (POT) film as the electron to ion transducer. The ion-selective membranes are formulated under ionophore depleted conditions (avoiding excess of ionophore over ion-exchanger), which is purposely different from common practice with ion-selective electrodes. Cyclic voltammetry is used to interrogate the films. An anodic scan partially oxidizes the POT underlayer, which results in the expulsion of cations from the membrane at an appropriate potential. During the scan of a membrane contg. multiple ionophores, the least bound ion is expelled 1st, giving distinct Gaussian peak shaped ion transfer voltammetric waves that are analyzed in terms of their peak potential. These potentials change with the logarithm of the ion activity, in complete analogy to ion-selective electrodes, and multiple such waves are obsd. with multiple ionophores that exhibit no obvious interference from the other ionophores present in the membrane. The concept is established with lithium and calcium ionophores and accompanied by a response model that assumes complete equilibration of the membrane at every applied potential. From the model, diffusion coeffs. in the membrane or aq. phase bear no influence on the peak potentials as long as thin layer behavior is obsd., further confirming the analogy to a potentiometric expt. Idealized ion transfer waves are narrower than exptl. findings, which is explained by a broader than expected anodic peak for the oxidn. of conducting polymer. The correspondence between expt. and theory is otherwise excellent in terms of thin layer behavior and Nernstian shift of the peaks with analyte concn.
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    47. 47
      Cuartero, M.; Crespo, G. A.; Bakker, E. Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes. Anal. Chem. 2016, 88, 1654– 1660,  DOI: 10.1021/acs.analchem.5b03611
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      Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes
      Cuartero, Maria; Crespo, Gaston A.; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2016), 88 (3), 1654-1660CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      As shown in recent work, thin layer ion-selective multi-ionophore membranes can be interrogated by cyclic voltammetry to detect the ion activity of multiple species simultaneously and selectively. Addnl. fundamental evidence is put forward on ion discrimination with thin multi-ionophore-based membranes with thicknesses of 200 ± 25 nm and backside contacted with poly-3-octylthiophene (POT). An anodic potential scan partially oxidizes the POT film (to POT+), thereby initiating the release of hydrophilic cations from the membrane phase to the sample soln. at a characteristic potential. Varying concn. of added cation-exhanger demonstrates that it limits the ion transfer charge and not the deposited POT film. Voltammograms with multiple peaks are obsd. with each assocd. with the transfer of one type of ion (lithium, potassium, and sodium). Exptl. conditions (thickness and compn. of the membrane and concn. of the sample) are chosen that allow one to describe the system by a thermodn. rather than kinetic model. As a consequence, apparent stability consts. for sodium, potassium, and lithium (assuming 1:1 stoichiometry) with their resp. ionophores are calcd. and agree well with the values obtained by the potentiometric sandwich membrane technique. As an anal. application, a membrane contg. three ionophores was used to det. lithium, sodium, and potassium in artificial samples at the same location and within a single voltammetric scan. Lithium and potassium were also detd. in undiluted human plasma in the therapeutic concn. range.
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    48. 48
      Jarolimova, Z.; Bosson, J.; Labrador, G. M.; Lacour, J.; Bakker, E. Ion Transfer Voltammetry at Thin Films Based on Functionalized Cationic 6 Helicenes. Electroanalysis 2018, 30, 650– 657,  DOI: 10.1002/elan.201700669
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      48
      Ion Transfer Voltammetry at Thin Films Based on Functionalized Cationic [6]Helicenes
      Jarolimova, Zdenka; Bosson, Johann; Labrador, Geraldine M.; Lacour, Jerome; Bakker, Eric
      Electroanalysis (2018), 30 (4), 650-657CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors describe a new family of mol. ion-to-electron redox probes based on cationic diaza, azaoxa, and dioxa [6]helicenes and their derivs. Their unique structure combines, in a single framework, two privileged families of mols. - helicenes and triaryl Me carbenium moieties. These cationic [6]helicenes exhibit reversible and reproducible oxidn./redn. behavior and facilitate the ion transfer into thin layer sensing films composed of bis(2-ethylhexyl)sebacate (DOS), polyurethane (PU), sodium tetrakis 3.5-bis(trifluoromethyl)phenyl borate, sodium ionophore X and diaza+(C8)2Br2 for cation transfer. Cyclic voltammetry was used to interrogate the thin films. The cationic response can be tuned by adjusting the membrane loading. Addn. of lipophilic cation exchanger into the membrane film results in transfer waves of Gaussian shape for cations. A peak sepn. of 60 mV and peak width of 110 mV are near the theor. values for a surface confined process. While Nernstian shifts of the peak potentials with analyte concn. was obtained for membranes based on cationic [6]helicenes and doped with sodium-selective ionophore X, this ionophore was found to promote a gradual loss of redox active species from the ionophore-based membranes into the sample soln.
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    49. 49
      Jarolimova, Z.; Bosson, J.; Labrador, G. M.; Lacour, J.; Bakker, E. Ion Transfer Voltammetry in Polyurethane Thin Films Based on Functionalised Cationic 6 Helicenes for Carbonate Detection. Electroanalysis 2018, 30, 1378– 1385,  DOI: 10.1002/elan.201800080
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      49
      Ion Transfer Voltammetry in Polyurethane Thin Films Based on Functionalised Cationic [6]Helicenes for Carbonate Detection
      Jarolimova, Zdenka; Bosson, Johann; Labrador, Geraldine M.; Lacour, Jerome; Bakker, Eric
      Electroanalysis (2018), 30 (7), 1378-1385CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We explore here mol. ion-to-electron redox probes based on cationic diaza, azaoxa, and dioxa [6]helicenes and their derivs. as ion-to-electron transducers for the electrochem. detection of anions. These cationic [6]helicenes exhibit reversible and reproducible oxidn./redn. behavior and facilitate the anion transfer of Gaussian shape into polymeric thin layer sensing films. Films composed of bis(2-ethylhexyl) sebacate (DOS), polyurethane (PU), tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH 500) and [6]helicenes were interrogated by cyclic voltammetry. Even though the peak sepn. of 90 mV is larger than ideal, the obsd. peak width at half max. of 130 mV and the linear relationship between current and scan rate are near theor. values, confirming a surface confined process. A Nernstian shift of the peaks with increasing carbonate concn. is obtained in the presence of carbonate ionophore VII incorporated into the thin sensing film. The concn. of carbonate was detd. in an unfiltered sample of the Arve river (flowing from Chamonix to Geneva) and compared to a ref. method (automatic titrator with potentiometric detection). The results suggested that cationic diaza [6]helicene functionalized with two bromine atoms is an attractive mol. ion-to-electron transducer for anion-selective electrodes.
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    50. 50
      Yuan, D. J.; Cuartero, M.; Crespo, G. A.; Bakker, E. Voltammetric Thin-Layer lonophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations. Anal. Chem. 2017, 89, 586– 594,  DOI: 10.1021/acs.analchem.6b03354
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      50
      Voltammetric Thin-Layer Ionophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations
      Yuan, Dajing; Cuartero, Maria; Crespo, Gaston A.; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2017), 89 (1), 586-594CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Voltammetric thin layer (∼200 nm) ionophore-based polymeric films of defined ion-exchange capacity have recently emerged as a promising approach to acquire multi-ion information about the sample, in analogy to performing multiple potentiometric measurements with individual membranes. They behave under two different regimes that are dependent on the ion concn. A thin layer control (no mass transport limitation of the polymer film or soln.) is identified for ion concns. of >10 μM, in which case the peak potential serves as the readout signal, in analogy to a potentiometric sensor. However, ion transfer at lower concns. is chiefly controlled by diffusional mass transport from the soln. to the sensing film, resulting in an increase of peak current with ion concn. This concn. range is suitable for electrochem. ion transfer stripping anal. Here, the transition between the two mentioned scenarios is explored exptl., using a highly Ag-selective membrane as a proof-of-concept under different conditions (variation of ion concn. in the sample from 0.1 μM to 1 mM, scan rate from 25 mV s-1 to 200 mV s-1, and angular frequency from 100 rpm to 6400 rpm). Apart from exptl. evidence, a numerical simulation is developed that considers an idealized conducting polymer behavior and permits one to predict exptl. behavior under diffusion or thin-layer control.
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    51. 51
      Vanamo, U.; Hupa, E.; Yrjana, V.; Bobacka, J. New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes. Anal. Chem. 2016, 88, 4369– 4374,  DOI: 10.1021/acs.analchem.5b04800
      Google Scholar
      51
      New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes
      Vanamo, Ulriika; Hupa, Elisa; Yrjana, Ville; Bobacka, Johan
      Analytical Chemistry (Washington, DC, United States) (2016), 88 (8), 4369-4374CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A novel approach to signal transduction concerning solid-contact ion-selective electrodes (SC-ISE) with a conducting polymer (CP) as the solid contact was studied. The method presented here is based on const. potential coulometry, where the potential of the SC-ISE vs. the ref. electrode is kept const. using a potentiostat. The change in the potential at the interface between the ion-selective membrane (ISM) and the sample soln., due to the change in the activity of the primary ion, is compensated with a corresponding but opposite change in the potential of the CP solid contact. This enforced change in the potential of the solid contact results in a transient reducing/oxidizing current flow through the SC-ISE. By measuring and integrating the current needed to transfer the CP to a new state of equil., the total cumulated charge that is linearly proportional to the change of the logarithm of the primary ion activity was obtained. Different thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contact. Also, coated wire electrodes (CWEs) were included in the study to show the general validity of the new approach. The ISM employed was selective for K+ ions, and the selectivity of the membrane under implementation of the presented transduction mechanism was confirmed by measurements performed with a const. background concn. of Na+ ions. A unique feature of this signal readout principle is that it allows amplification of the anal. signal by increasing the capacitance (film thickness) of the solid contact of the SC-ISE.
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    52. 52
      Jarolimova, Z.; Han, T. T.; Mattinen, U.; Bobacka, J.; Bakker, E. Capacitive Model for Coulometric Readout of Ion-Selective Electrodes. Anal. Chem. 2018, 90, 8700– 8707,  DOI: 10.1021/acs.analchem.8b02145
      Google Scholar
      52
      Capacitive Model for Coulometric Readout of Ion-Selective Electrodes
      Jarolimova, Zdenka; Han, Tingting; Mattinen, Ulriika; Bobacka, Johan; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2018), 90 (14), 8700-8707CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      We present here a capacitive model for the coulometric signal transduction readout of solid-contact ion-selective membrane electrodes (SC-ISE) with a conducting polymer (CP) as an intermediate layer for the detection of anions. The capacitive model correlates well with exptl. data obtained for chloride-selective SC-ISEs utilizing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with chloride as the ion-to-electron transducer. Addnl., Prussian blue is used as a simple sodium capacitor to further demonstrate the role of the transduction layer. The influence of different thicknesses of PEDOT as a conducting polymer transducer, different thicknesses of the overlaying ion-selective membranes deposited by drop casting and spin coating, and different compns. of the chloride-selective membrane are explored. The responses are evaluated in terms of current-time, charge-time, and charge-chloride activity relationships. The utility of the sensor with coulometric readout is illustrated by the monitoring of very small concn. changes in soln.
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    53. 53
      Jansod, S.; Cuartero, M.; Cherubini, T.; Bakker, E. Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes. Anal. Chem. 2018, 90, 6376– 6379,  DOI: 10.1021/acs.analchem.8b01585
      Google Scholar
      53
      Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes
      Jansod, Sutida; Cuartero, Maria; Cherubini, Thomas; Bakker, Eric
      Analytical Chemistry (Washington, DC, United States) (2018), 90 (11), 6376-6379CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      We present here a general strategy to translate potential change at a potentiometric probe into a tunable color readout. It is achieved with a closed bipolar electrode where the ion-selective component is confined to one end of the electrode while color is generated at the opposite pole, allowing one to phys. sep. the detection compartment from the sample. An elec. potential is imposed across the bipolar electrode by soln. contact such that the potentiometric signal change at the sample side modulates the potential at the detection side. This triggers the turnover of a redox indicator in the thin detection layer until a new equil. state is established. The approach is demonstrated in sep. expts. with a chloride responsive Ag/AgCl element and a liq. membrane based calcium-selective membrane electrode, using the redox indicator ferroin in the detection compartment. The principle can be readily extended to other ion detection materials and optical readout principles.
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    • Abstract

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      Figure 1

      Figure 1. Structures of selected ionophores: valinomycin as one of the first biological ionophores used as a receptor for potassium ion sensing; cryptand 222 as a macrocyclic compound that inspired the structural development of the highly successful calcium ionophore I, also called ETH 1001; tweezer-type carbonate ionophore of high selectivity, a rare example of a receptor reversibly binding an ion with a covalent bond.

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      Figure 2

      Figure 2. (a) A conformational change from potassium binding to pyreneamide-functionalized 18-crown-6 ether, triggering a fluorescence response. (b) Observed normalized fluorescence at 406 nm as a function of the concentration of different cations (λexcitation = 346 nm). Adapted with permission from ref (3). Copyright 2016 The Royal Society of Chemistry.

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      Figure 3

      Figure 3. Sensing mechanism of ionophore-based ion-selective optical nanosensors containing (a) a pH indicator (chromoionophore) or (b) a charged lipophilic solvatochromic dye as an optical reporter. The ion exchange with an analyte cation results in protonation/deprotonation of the indicator or a localized repartitioning of the solvatochromic dye bearing a positive charge.

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      Figure 4

      Figure 4. (a) Illustration of a CO2 optode sensing mechanism. The carbonate ion is extracted together with hydrogen ions into organic sensing film containing the anion exchanger, an electrically charged pH indicator and the carbonate ionophore shown in Figure 1. The optical response is proportional to the product of carbonate ion activity and the square of hydrogen ion activity, which is directly linked to the activity of carbon dioxide. (b) Calibration curves of the CO2 optode to CO2 in a hydrogen carbonate solution, in the gas phase equilibrated with hydrogen carbonate solution, and a dry CO2 + N2 gas mixture with different levels of PCO2. For proper sensor function, humid air samples are required. Adapted with permission from ref (15). Copyright 2012 American Chemical Society.

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      Figure 5

      Figure 5. Theoretical response curves of the calcium-selective optode used in (a) an equilibrium or (b) an exhaustive sensing mode. The response of optodes operated by an exhaustive sensing mode is linear with the analyte concentration and sufficiently independent of the pH of the sample. Adapted with permission from ref (18). Copyright 2015 Springer-Verlag Berlin Heidelberg.

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      Figure 6

      Figure 6. In situ profiles of dissolved CO2 in Lake Rotsee obtained with a commercial Severinghaus CO2 probe and the coupled hydrogen ion and carbonate selective electrodes with a profiling speed of 5 cm/s. In contrast to the Severinghaus CO2 probe exhibiting a response time of minutes, the setup with two coupled ISEs provides a response time of a few seconds. This allows for CO2 measurements at a high temporal and spatial resolution. Adapted with permission from ref (32). Copyright 2018 American Chemical Society.

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      Figure 7

      Figure 7. Different readout modes of membrane ISEs placed under dynamic electrochemistry control: (a) chronopotentiometry; (b) thin-layer coulometry; (c) ion transfer voltammetry at a high concentration of the analyte; (d) ion transfer voltammetry at a low concentration of the analyte; (e) coulometry with a capacitive layer; (f) colorimetry.

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    • References

      This article references 53 other publications.

      1. 1
        Feinstein, M. B.; Felsenfeld, H. The Detection of Ionophorous Antibiotic-Cation Complexes in Water with Fluorescent Probes. Proc. Natl. Acad. Sci. U. S. A. 1971, 68, 2037– 2041,  DOI: 10.1073/pnas.68.9.2037
        1
        Detection of ionophorous antibiotic-cation complexes in water with fluorescent probes
        Feinstein, M. B.; Felsenfeld, H.
        Proceedings of the National Academy of Sciences of the United States of America (1971), 68 (7), 2037-41CODEN: PNASA6; ISSN:0027-8424.
        The binding of alkali cations by the ionophorous antibiotics valinomycin, nigericin, alamethicin, and the macrotetralide actins has been shown to occur, in aq. media, by the use of the fluorescent probes 1-anilino-8-naphthalenesulfonate and 2-p-toluidinyl-6-naphthalenesulfonate. The interaction of the ionophore-cation complexes with the fluorescent dyes produced enhanced fluorescence emission, increased lifetime and polarization, and a significant blue-shift of the emission maxima of the fluorescence spectrum. At const. antibiotic and fluorophore concns. in water, the intensity of the fluorescence emission is a function of the cation concn. This permitted relative cation affinities to be detd. for alamethicin (Na+ ≈ K+), valinomycin (Rb+ > K+ > Cs+), nigericin (K+ > Rb+ > Na+ > Cs+) and trinactin (NH4+ > K+ > Rb+ > Cs+).
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        https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE3MXlt1Wgsb4%253D&md5=48a678602d171d25b3f7ccd48abbb279
      2. 2
        Qin, Y.; Mi, Y.; Bakker, E. Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether. Anal. Chim. Acta 2000, 421, 207– 220,  DOI: 10.1016/S0003-2670(00)01038-2
        2
        Determination of complex formation constants of 18 neutral alkali and alkaline earth metal ionophores in poly(vinyl chloride) sensing membranes plasticized with bis(2-ethylhexyl)sebacate and o-nitrophenyloctylether
        Qin, Y.; Mi, Y.; Bakker, E.
        Analytica Chimica Acta (2000), 421 (2), 207-220CODEN: ACACAM; ISSN:0003-2670. (Elsevier Science B.V.)
        A segmented sandwich membrane method is used to det. complex formation consts. of 18 elec. neutral ionophores in situ in solvent polymeric sensing membranes. These ionophores are commonly used in potentiometric and optical sensors, and knowledge of such binding information is important for ionophore and sensor design. In this method, two membrane segments are fused together, with only one contg. the ionophore, to give a concn.-polarized sandwich membrane. Unlike other approaches, this method does not require the use of a ref. ion in the sample and/or a 2nd ionophore in the membrane, and is typically pH insensitive. The following ionophores responsive for the common cations lithium, sodium, potassium, magnesium and calcium are characterized and discussed: valinomycin, BME-44, bis[(benzo-15-crown-5)-4'-ylmethyl]pimelate, ETH 157, ETH 2120, bis[(12-crown-4)methyl]dodecylmethylmalonate, 4-tert-butylcalix [4] arene tetraacetic acid tetra-Et ester, ETH 149, ETH 1644, ETH 1810, 6,6-dibenzyl-14-crown-4, N,N,N',N',N'',N''-hexacyclohexyl-4,4',4''-propylidyne tris(3-oxabutyramide), ETH 1117, ETH 4030, ETH 1001, ETH 129, ETH 5234, and A23187. The logarithmic complex formation consts. range from 4.4 to 29 and compare well to published data for ionophores that were characterized earlier. From the obsd. complex formation consts., max. possible selectivities are calcd. that would be expected if interfering ions show no binding affinity to the ionophore, and the values are compared with exptl. findings. Each ionophore is characterized in poly(vinyl chloride) membranes plasticized either with a polar (NPOE) or a nonpolar plasticizer (DOS). Membranes based on NPOE always show larger complex formation consts. of the embedded ionophore.
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      3. 3
        Jarolimova, Z.; Vishe, M.; Lacour, J.; Bakker, E. Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles. Chem. Sci. 2016, 7, 525– 533,  DOI: 10.1039/C5SC03301B
        3
        Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles
        Jarolimova, Zdenka; Vishe, Mahesh; Lacour, Jerome; Bakker, Eric
        Chemical Science (2016), 7 (1), 525-533CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
        These compds. were successfully synthesized by the reaction of α-diazo-β-ketoesters with cyclic ethers of the desired size in the presence of dirhodium complexes followed by a stereo-selective tandem amidation-transposition process and characterized by 1H-NMR, 13C-NMR, IR, HR-ESI-MS, UV-VIS and fluorescence spectroscopy and potentiometry. Their unique structure consisting of a crown ether ring linked to pyrene moieties through amide groups exhibits on-off switchable behavior upon binding of specific cations and allows one to incorporate these chemosensors as fluorescent ionophores into ion-exchange nanospheres. The nanosphere matrix is composed of bis(2-ethylhexyl)sebacate , poly(ethylene glycol), sodium tetrakis 3,5-bis(trifluoromethyl)phenyl borate and pyreneamide functionalized 18-crown-6 ether (18C6). These optode nanoparticles exhibit a strong affinity to the potassium cation over other metal ions up to the millimolar concn. range in an exhaustive detection mode. The logarithmic complex formation const. was detd. using the segmented sandwich membrane method and was found to be 6.5 ± 0.3 (SD) in PVC membrane plasticized with NPOE and 5.3 ± 0.3 in DOS with a 1 : 1 complex stoichiometry. The nanosensors were characterized in broad range of pH from 4 to 10 and the same linear calibration curves were obtained in the concn. range from 10-7 M to 10-5 M and thus the pH dependent response was largely overcome.
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      4. 4
        Xie, X.; Zhai, J.; Bakker, E. Potentiometric Response from Ion-Selective Nanospheres with Voltage-Sensitive Dyes. J. Am. Chem. Soc. 2014, 136, 16465– 16468,  DOI: 10.1021/ja5107578
        4
        Potentiometric Response from Ion-Selective Nanospheres with Voltage-Sensitive Dyes
        Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
        Journal of the American Chemical Society (2014), 136 (47), 16465-16468CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
        Potentiometric sensors require the implementation of conducting wires for signal transduction, but this is impractical for the readout of individual nanoparticles. It is here demonstrated that the potentiometric response of ion-selective nanospheres can be obsd. with voltage-sensitive dyes, thereby converting nanoscale electrochem. signals into an optical readout. No ref. electrode is needed since the readout is by fluorescence. The results strongly support the potentiometric origin for the fluorescence response. The ion-selective nanospheres exhibit excellent selectivity and respond to ion concn. changes independent of sample pH, providing a new platform of potentiometric nanosensors with optical readout compatible with optical imaging equipment.
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      5. 5
        Xie, X.; Gutierrez, A.; Trofimov, V.; Szilagyi, I.; Soldati, T.; Bakker, E. Charged Solvatochromic Dyes as Signal Transducers in pH Independent Fluorescent and Colorimetric Ion Selective Nanosensors. Anal. Chem. 2015, 87, 9954– 9959,  DOI: 10.1021/acs.analchem.5b02566
        5
        Charged Solvatochromic Dyes as Signal Transducers in pH Independent Fluorescent and Colorimetric Ion Selective Nanosensors
        Xie, Xiaojiang; Gutierrez, Agustin; Trofimov, Valentin; Szilagyi, Istvan; Soldati, Thierry; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2015), 87 (19), 9954-9959CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Ionophore-based ion selective optical nanosensors that operate independently of the sample pH are developed here using elec. charged solvatochromic dyes as signal transducers. A series of dye mols. with a D-π-A structure was synthesized and characterized in various solvents and incorporated into ion selective nanospheres for K+, Na+, and H+. Since dye leakage was greatly suppressed when the solvatochromic dyes were encapsulated in the nanosphere core, ion sensing nanospheres were explored for cellular ion imaging in Dictyostelium discoideum live cells but spontaneous dye loss resulted in undesired staining of cells. The in vitro anal. of potassium in human plasma was successfully demonstrated with this approach. A theor. model was developed for the response of the ion selective nanosensors contg. charged solvatochromic dyes. The nanosensors exhibited a tunable response range, high sensitivity, and good stability.
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      6. 6
        Wang, L.; Xie, X.; Zhai, J.; Bakker, E. Reversible pH-independent optical potassium sensor with lipophilic solvatochromic dye transducer on surface modified microporous nylon. Chem. Commun. 2016, 52, 14254– 14257,  DOI: 10.1039/C6CC07841A
        6
        Reversible pH-independent optical potassium sensor with lipophilic solvatochromic dye transducer on surface modified microporous nylon
        Wang, Lu; Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
        Chemical Communications (Cambridge, United Kingdom) (2016), 52 (99), 14254-14257CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
        A reversible and pH-independent fluorescent ion optode is introduced with an ionophore and surface confined solvatochromic dye transducer doped onto microporous nylon membranes. The resulting film responds to K+ with excellent selectivity over the range of 10-7 to 10-2 M and a response time of t95 < 60 s above 10-6 M.
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      7. 7
        Xie, X.; Szilagyi, I.; Zhai, J.; Wang, L.; Bakker, E. Ion-Selective Optical Nanosensors Based on Solvatochromic Dyes of Different Lipophilicity: From Bulk Partitioning to Interfacial Accumulation. ACS Sens 2016, 1, 516– 520,  DOI: 10.1021/acssensors.6b00006
        7
        Ion-Selective Optical Nanosensors Based on Solvatochromic Dyes of Different Lipophilicity: From Bulk Partitioning to Interfacial Accumulation
        Xie, Xiaojiang; Szilagyi, Istvan; Zhai, Jingying; Wang, Lu; Bakker, Eric
        ACS Sensors (2016), 1 (5), 516-520CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
        The sensing mechanism of fluorescent ion-selective nanosensors incorporating solvatochromic dyes (SDs), with K+ as model ion, is shown to change as a function of dye lipophilicity. Water-sol. SDs obey bulk partitioning principles where the sensor response directly depends on the lipophilicity of the SD and exhibits an influence on the phase vol. ratio of nanosensors to aq. soln. (diln. effect). A lipophilization of the SDs is shown to overcome these limitations. An interfacial accumulation mechanism is proposed and confirmed with F.ovrddot.orster resonance energy transfer (FRET) with a ratiometric near-IR fluorescence FRET pair. This work lays the foundation for operationally more robust ion-selective nanosensors incorporating SDs.
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      8. 8
        Pedersen, C. J. Cyclic polyethers and their complexes with metal salts. J. Am. Chem. Soc. 1967, 89, 2495– 2496,  DOI: 10.1021/ja00986a052
        8
        Cyclic polyethers and their complexes with metal salts
        Pedersen, Charles J.
        Journal of the American Chemical Society (1967), 89 (10), 2495-6CODEN: JACSAT; ISSN:0002-7863.
        Two cyclic polyethers, 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxocyclo-2,11-octadecadiene (I) and its hydrogenation product (over RuO2) 2,5,8,15,18,21-hexaoxatricylo[20.4.0.09,24] hexacosane (II), were prepd. which have the unusual property of forming relatively stable complexes with alkali and alk. earth metal ions. Stable complexes of I and II have been made for the following cationic compds.: Li, Na, NH4, RNH3, K, Rb, Cs, Ag(I), Ca, Sr, Ba, Cd, Hg(I), Hg(II), La(III), Tl(I), Ce(III), and Pb(II).
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      9. 9
        Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Diaza-polyoxa-macrocycles et macrobicycles. Tetrahedron Lett. 1969, 10, 2885– 2888,  DOI: 10.1016/S0040-4039(01)88299-X
        There is no corresponding record for this reference.
      10. 10
        Gutsche, C. D.; Muthukrishnan, R. Calixarenes. 1. Analysis of the product mixtures produced by the base-catalyzed condensation of formaldehyde with para-substituted phenols. J. Org. Chem. 1978, 43, 4905– 4906,  DOI: 10.1021/jo00419a052
        10
        Calixarenes. 1. Analysis of the product mixtures produced by the base-catalyzed condensation of formaldehyde with para-substituted phenols
        Gutsche, C. David; Muthukrishnan, Ramamurthi
        Journal of Organic Chemistry (1978), 43 (25), 4905-6CODEN: JOCEAH; ISSN:0022-3263.
        The title reactions of phenols with HCHO gave mixts. of at least two components, none of which are cyclic tetramers (calixarenes). The product from p-cresol is a mixt. of cyclic heptamer (major product) and cyclic octamer (minor product) and the product from p-Me3CC6H4OH is a mixt. of cyclic octamer (major product) and cyclic pentamer or hexamer (minor product).
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      11. 11
        Scott, W. J.; Chapoteau, E.; Kumar, A. Ion-selective membrane electrode for rapid automated determinations of total carbon dioxide. Clin. Chem. 1986, 32, 137– 141
        11
        Ion-selective membrane electrode for rapid automated determinations of total carbon dioxide
        Scott, W. James; Chapoteau, Eddy; Kumar, Anand
        Clinical Chemistry (Washington, DC, United States) (1986), 32 (1, Pt. 1), 137-41CODEN: CLCHAU; ISSN:0009-9147.
        Previously described ion-selective membrane electrodes useful for detn. of total CO2 in biol. fluids in automated analyzers show interference from several sources e.g., fatty acids, keto acids, salicylate, and heparin. Judicious selection of membrane components has produced a membrane with superior performance characteristics; short conditioning time, long lifetime in storage, rapid and stable response, low drift, and significantly less susceptibility to interference than other electrodes thus far reported. Samples can be analyzed at ≥240 samples/h and results correlate well with those by e.g., the Technicon SMAC II method for total CO2. Mounted on the Technicon RA-1000, the CO2 sensor is arranged in tandem with Na and K ion-selective electrodes. When all 3 species are measured in the same buffered sample stream, the sampling rate becomes 720 tests/h, done on 25 μL samples. Prepn. of the membranes and optimization of their performance is described in detail.
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      12. 12
        Meyerhoff, M. E.; Pretsch, E.; Welti, D. H.; Simon, W. Role of trifluoroacetophenone solvents and quaternary ammonium salts in carbonate-selective liquid membrane electrodes. Anal. Chem. 1987, 59, 144– 150,  DOI: 10.1021/ac00128a030
        12
        Role of trifluoroacetophenone solvents and quaternary ammonium salts in carbonate-selective liquid membrane electrodes
        Meyerhoff, Mark E.; Pretsch, Ernoe; Welti, Dieter H.; Simon, Wilhelm
        Analytical Chemistry (1987), 59 (1), 144-50CODEN: ANCHAM; ISSN:0003-2700.
        UV absorption and 13C NMR spectroscopic techniques are used in conjunction with potentiometric studies to characterize the interaction of carbonate ions with trifluoroacetyl-p-butylbenzene, a solvent often utilized to prep. liq./polymeric membrane carbonate-selective electrodes. Single-phase spectroscopic expts. in cyclohexane suggest that carbonate ions react with the carbonyl C atom of the fluorinated ketone to form lipophilic carbonate dianion adducts. Similar adduct formation is obsd. in 2-phase expts. in which aq. carbonate solns. are equilibrated with an org. phase contg. the ketone and tridodecylammonium chloride. The direct interaction between carbonate and the ketone enables the prepn. of functional PVC-type carbonate electrodes with <1 wt. % of ketone and little or no quaternary ammonium species in the membrane; however, such membranes display significant cation interference. From these spectral and potentiometric findings, the roles of the fluorinated ketone and quaternary ammonium species in carbonate-selective membrane electrodes are established.
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      13. 13
        Lee, H. J.; Yoon, I. J.; Yoo, C. L.; Pyun, H.-J.; Cha, G. S.; Nam, H. Potentiometric Evaluation of Solvent Polymeric Carbonate-Selective Membranes Based on Molecular Tweezer-Type Neutral Carriers. Anal. Chem. 2000, 72, 4694– 4699,  DOI: 10.1021/ac991212l
        13
        Potentiometric Evaluation of Solvent Polymeric Carbonate-Selective Membranes Based on Molecular Tweezer-Type Neutral Carriers
        Lee, Han Jin; Yoon, In Joon; Yoo, Choong Leol; Pyun, Hyung-Jung; Cha, Geun Sig; Nam, Hakhyun
        Analytical Chemistry (2000), 72 (19), 4694-4699CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Potentiometric properties of the ion-selective electrodes based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (CA) derivs. have been measured. The carbonate-selective neutral carriers have been prepd. by coupling one to three trifluoroacetobenzoyl (TFAB) groups to a cholic acid deriv. 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 derivs. with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(TFAB)CA] exhibited remarkably improved carbonate selectivity, indicating that the bis(TFAB)CAs behave like mol. 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-, HPO42-, 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-Å range at the AM1 level semiempirical calcn., 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 compds. 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 anal. 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.
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      14. 14
        Choi, Y. S.; Lvova, L.; Shin, J. H.; Oh, S. H.; Lee, C. S.; Kim, B. H.; Cha, G. S.; Nam, H. Determination of Oceanic Carbon Dioxide Using a Carbonate-Selective Electrode. Anal. Chem. 2002, 74, 2435– 2440,  DOI: 10.1021/ac0108459
        14
        Determination of Oceanic Carbon Dioxide Using a Carbonate-Selective Electrode
        Choi, Yong Suk; Lvova, Larisa; Shin, Jae Ho; Oh, Seong Hee; Lee, Chang Suk; Kim, Byeong Hyo; Cha, Geun Sig; Nam, Hakhyun
        Analytical Chemistry (2002), 74 (10), 2435-2440CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Potentiometric properties of PVC membrane-based electrodes prepd. with mol. 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 obsd. that deoxy-3,12-bis(TFAB)CA-based electrode provided greatly enhanced CO32- selectivity over Cl- (log KCO32-,Cl-POT ∼-6) and other minor anions present in seawater. Thus, the possibility of applying this carbonate-selective electrode for direct detn. of oceanic CO2 was assessed. Total CO2 (TCO2) concn. in Yellow Sea surface water was detd. with the deoxy-3,12-bis(TFAB)CA-based electrode, Severinghaus-type CO2 gas sensor, and traditional potentiometric titrn. methods. Results showed the carbonate-selective electrode provided accurate oceanic TCO2 detn. comparable to that obtained with the other methods. The anal. procedure based on a carbonate-selective electrode is clearly advantageous over conventional methods; it does not require sample pretreatment or extra reagents other than the std. calibration solns., while providing measured results directly and immediately.
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      15. 15
        Xie, X.; Pawlak, M.; Tercier-Waeber, M. L.; Bakker, E. Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore. Anal. Chem. 2012, 84, 3163– 3169,  DOI: 10.1021/ac2030046
        15
        Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore
        Xie, Xiaojiang; Pawlak, Marcin; Tercier-Waeber, Mary-Lou; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2012), 84 (7), 3163-3169CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        A novel optical method for the detn. of CO2 concn. in aq. and gaseous samples of plasticized PVC film is presented. The detection principle makes use of a direct mol. recognition of the carbonate ion by a mol. tweezer-type ionophore, which was previously demonstrated to exhibit excellent carbonate selectivity. The carbonate ion is extd. together with hydrogen ions into a polymeric film that contains the anion exchanger tridodecylmethylammonium chloride, a lipophilic, elec. charged, and highly basic pH indicator, which is used for the readout in absorbance mode, in addn. to the lipophilic carbonate ionophore. According to known bulk optode principles, such an optical sensor responds to the product of the carbonate ion activity and the square of hydrogen ion activity. This quantity is thermodynamically linked to the activity of carbon dioxide. This allows one to realize a direct carbon dioxide sensor that does not make use of the traditional Severinghaus sensing principle of measuring a pH change upon CO2 equilibration across a membrane. A selectivity anal. shows that common ions such as chloride are sufficiently suppressed for direct PCO2 measurements in freshwater samples at pH 8. Chloride interference, however, is too severe for direct seawater measurements at the same pH. This may be overcome by placing a gas-permeable membrane over the optode sensing film. This is conceptually confirmed by establishing that the sensor is equally useful for gas-phase PCO2 measurements. As expected, humid air samples are required for proper sensor functioning, as dry CO2 gas will not cause any signal change. The sensor showed acceptable response times and good reproducibility under both conditions.
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      16. 16
        Xie, X.; Zhai, J.; Bakker, E. pH Independent Nano-Optode Sensors Based on Exhaustive Ion-Selective Nanospheres. Anal. Chem. 2014, 86, 2853– 2856,  DOI: 10.1021/ac403996s
        16
        pH Independent Nano-Optode Sensors Based on Exhaustive Ion-Selective Nanospheres
        Xie, Xiaojiang; Zhai, Jingying; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2014), 86 (6), 2853-2856CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Bulk optode-based ion selective optical sensors work from extn. equil., and their response toward the analyte ion is known to dependent on the sample pH. This pH dependence was one of the major disadvantages that have hampered the broad acceptance of bulk optodes in chem. sensing. The authors present here for the 1st time the use of exhaustive Ca2+-selective nanosensors that may overcome this pH dependent response. The nanosensors were characterized at different pH and the same linear calibration was obtained in the Ca2+ concn. range from 10-7 M to 10-5 M.
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      17. 17
        Xie, X.; Zhai, J.; Crespo, G. A.; Bakker, E. Ionophore-Based Ion-Selective Optical NanoSensors Operating in Exhaustive Sensing Mode. Anal. Chem. 2014, 86, 8770– 8775,  DOI: 10.1021/ac5019606
        17
        Ionophore-Based Ion-Selective Optical NanoSensors Operating in Exhaustive Sensing Mode
        Xie, Xiaojiang; Zhai, Jingying; Crespo, Gaston A.; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2014), 86 (17), 8770-8775CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Ion selective optical sensors are typically interrogated under conditions where the sample concn. is not altered during measurement. The authors describe here an alternative exhaustive detection mode for ion selective optical sensors. This exhaustive sensor concept is demonstrated with ionophore-based nanooptodes either selective for calcium or the polycationic heparin antidote protamine. In agreement with a theor. treatment presented here, linear calibration curves were obtained in the exhaustive detection mode instead of the sigmoidal curves for equil.-based sensors. The response range can be tuned by adjusting the nanosensor loading. The nanosensors showed av. diams. of <100 nm and the sensor response is dramatically faster than that for film-based optodes. Due to the strong binding affinity of the exhaustive nanosensors, total calcium concn. in human blood plasma was successfully detd. Optical detn. of protamine in human blood plasma using the exhaustive nanosensors was attempted, but is less successful.
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      18. 18
        Xie, X. J.; Bakker, E. Ion selective optodes: from the bulk to the nanoscale. Anal. Bioanal. Chem. 2015, 407, 3899– 3910,  DOI: 10.1007/s00216-014-8413-4
        18
        Ion selective optodes: from the bulk to the nanoscale
        Xie, Xiaojiang; Bakker, Eric
        Analytical and Bioanalytical Chemistry (2015), 407 (14), 3899-3910CODEN: ABCNBP; ISSN:1618-2642. (Springer)
        This review describes recent advances in the miniaturization of ion selective optodes into microscale and nanoscale sensors. The topics include a comparison between film-based and miniaturized ion optodes, equil. and exhaustive detection modes, recent prepn. methodologies and applications of microscale and nanoscale ion optodes, criteria for the design of optode sensors, and other future perspectives. [Figure not available: see fulltext.].
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      19. 19
        Xie, X.; Bakker, E. Light-Controlled Reversible Release and Uptake of Potassium Ions from Ion-Exchanging Nanospheres. ACS Appl. Mater. Interfaces 2014, 6, 2666– 2670,  DOI: 10.1021/am4049805
        19
        Light-Controlled Reversible Release and Uptake of Potassium Ions from Ion-Exchanging Nanospheres
        Xie, Xiaojiang; Bakker, Eric
        ACS Applied Materials & Interfaces (2014), 6 (4), 2666-2670CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
        Here, we report for the first time on photoswitchable nanospheres contg. spiropyran (Sp) for reversible release and uptake of metal ions. K+ is used as a model ion to demonstrate the chem. principle of this approach. Valinomycin is incorporated in the nanospheres to stabilize K+. Upon UV illumination, Sp transforms to the more basic ring-opened merocyanine form, which takes up H+ from the surrounding aq. soln. and expels K+ from the nanospheres. The process can be reversed by irradn. with visible light to reduce the surrounding K+ concn.
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      20. 20
        Gerold, C. T.; Bakker, E.; Henry, C. S. Selective Distance-Based K+ Quantification on Paper-Based Microfluidics. Anal. Chem. 2018, 90, 4894– 4900,  DOI: 10.1021/acs.analchem.8b00559
        20
        Selective Distance-Based K+ Quantification on Paper-Based Microfluidics
        Gerold, Chase T.; Bakker, Eric; Henry, Charles S.
        Analytical Chemistry (Washington, DC, United States) (2018), 90 (7), 4894-4900CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        In this study, paper-based microfluidic devices (μPADs) capable of K+ quantification in aq. samples, as well as in human serum, using both colorimetric and distance-based methods are described. A lipophilic phase contg. potassium ionophore I (valinomycin) was utilized to achieve highly selective quantification of K+ in the presence of Na+, Li+, and Mg2+ ions. Successful addn. of a suspended lipophilic phase to a wax printed paper-based device is described and offers a soln. to current approaches that rely on org. solvents, which damage wax barriers. The approach provides an avenue for future alkali/alk. quantification utilizing μPADs. Colorimetric spot tests allowed for K+ quantification from 0.1-5.0 mM using only 3.00 μL of sample soln. Selective distance-based quantification required small sample vols. (6.00 μL) and gave responses sensitive enough to distinguish between 1.0 and 2.5 mM of sample K+. μPADs using distance-based methods were also capable of differentiating between 4.3 and 6.9 mM K+ in human serum samples. Distance-based methods required no digital anal., electronic hardware, or pumps; any steps required for quantification could be carried out using the naked eye.
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      21. 21
        Zhai, J.; Xie, X.; Bakker, E. Ionophore-based ion-exchange emulsions as novel class of complexometric titration reagents. Chem. Commun. 2014, 50, 12659– 12661,  DOI: 10.1039/C4CC05754F
        21
        Ionophore-based ion-exchange emulsions as novel class of complexometric titration reagents
        Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
        Chemical Communications (Cambridge, United Kingdom) (2014), 50 (84), 12659-12661CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
        Complexometric titrns. rely on a drastic change of the pM value at the equivalence point with a water sol. chelator forming typically 1 : 1 complexes of high stability. The available chem. toolbox of suitable chelating compds. is unfortunately limited because many promising complexing agents are not water sol. The authors introduce here a novel class of complexometric titrn. reagents, a suspension of polymeric nanospheres whose hydrophobic core is doped with lipophilic ion-exchanger and a selective complexing agent (ionophore). The emulsified nanospheres behave from heterogeneous ion exchange equil. where the initial counterion of the ion-exchanger is readily displaced from the emulsion for the target ion that forms a stable complex in the nanosphere core. Two different examples are shown with Ca2+ and Pb2+ as target ions. The lack of protonatable groups on the calcium receptor allows one to perform Ca2+ titrn. without pH control.
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      22. 22
        Zhai, J.; Xie, X.; Bakker, E. Ion-Selective Optode Nanospheres as Heterogeneous Indicator Reagents in Complexometric Titrations. Anal. Chem. 2015, 87, 2827– 2831,  DOI: 10.1021/ac504213q
        22
        Ion-Selective Optode Nanospheres as Heterogeneous Indicator Reagents in Complexometric Titrations
        Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2015), 87 (5), 2827-2831CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Traditionally, optical titrns. of inorg. ions are based on a rapid and visible color change at the end point with water-sol. org. dyes as indicators. Adequate selectivity is required for both the indicator and the complexing agent, which is often limited. The authors present here alternative, heterogeneous ionophore-based ion-selective nanospheres as indicators and chelators for optical titrns. The indicating nanospheres rely on a weaker extn. of the analyte of interest by ion-exchange, owing to the addnl. incorporation of a lipophilic pH indicator in the nanosphere core. Ca2+ titrn. was demonstrated as a proof-of-concept. Both the chelating and the indicating nanospheres showed good selectivity and a wide working pH range.
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      23. 23
        Zhai, J.; Xie, X.; Bakker, E. Solvatochromic Dyes as pH-Independent Indicators for Ionophore Nanosphere-Based Complexometric Titrations. Anal. Chem. 2015, 87, 12318– 12323,  DOI: 10.1021/acs.analchem.5b03663
        23
        Solvatochromic Dyes as pH-Independent Indicators for Ionophore Nanosphere-Based Complexometric Titrations
        Zhai, Jingying; Xie, Xiaojiang; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2015), 87 (24), 12318-12323CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        For half a century, complexometric titrns. of metal ions have been performed with water-sol. chelators and indicators that typically require careful pH control. Very recently, ion-selective nanosphere emulsions were introduced that exhibit ion-exchange properties and are doped with lipophilic ionophores originally developed for chem. ion sensors. They may serve as novel, highly selective and pH independent complexometric reagents. While ion optode emulsions have been demonstrated as useful indicators for such titrns., they exhibit a pH cross-response that unfortunately complicates the identification of the end point. Here, we present pH-independent optode nanospheres as indicators for complexometric titrns., with calcium as an initial example. The nanospheres incorporate an ionic solvatochromic dye (SD), ion exchanger and ionophore. The solvatochromic dye will be only expelled from the core of the nanosphere into the aq. soln. at the end point at which point it results in an optical signal change. The titrn. curves are demonstrated to be pH-independent and with sharper end points than with previously reported chromoionophore-based optical nanospheres as indicator. The calcium concn. in mineral water was successfully detd. using this method.
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      24. 24
        Zhai, J.; Xie, X.; Cherubini, T.; Bakker, E. Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum. ACS Sens 2017, 2, 606– 612,  DOI: 10.1021/acssensors.7b00165
        24
        Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum
        Zhai, Jingying; Xie, Xiaojiang; Cherubini, Thomas; Bakker, Eric
        ACS Sensors (2017), 2 (4), 606-612CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
        While the titrimetric assay is one of the most precise anal. techniques available, only a limited list of complexometric chelators is available, as many otherwise promising reagents are not water-sol. Recent work demonstrated successful titrimetry with ion-exchanging polymeric nanospheres contg. hydrophobic complexing agents, so-called ionophores, opening an exciting avenue in this field. However, this method was limited to ionophores of very high affinity to the analyte and exhibited a relatively limited titrn. capacity. To overcome these two limitations, the authors report here on solvent based titrn. reagents. This heterogeneous titrn. principle is based on the dissoln. of all hydrophobic recognition components in a solvent such as dichloromethane (CH2Cl2) where the ionophores are shown to maintain a high affinity to the target ions. HSV (hue, satn., value) anal. of the images captured with a digital camera provides a convenient and inexpensive way to det. the end point. This approach is combined with an automated titrn. setup. The titrns. of the alkali metals K+, Na+, and Li+ in aq. soln. are successfully demonstrated. The potassium concn. in human serum without pretreatment was precisely and accurately detd. as 4.38 mM ± 0.10 mM (automated titrn.), which compares favorably with at. emission spectroscopy (4.47 mM ± 0.20 mM).
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      25. 25
        Štefanac, Z.; Simon, W. Ion specific electrochemical behavior of macrotetrolides in membranes. Microchem. J. 1967, 12, 125– 132,  DOI: 10.1016/0026-265X(67)90014-8
        25
        Ion-specific electrochemical behavior of macrotetrolides in membranes
        Stefanac, Zlata; Simon, Wilhelm
        Microchemical Journal (1967), 12 (1), 125-32CODEN: MICJAN; ISSN:0026-265X.
        cf. CA 66, 61872v. The emf. measurements were obtained using the following electrochem. cell: Ag; AgCl, inner soln.//membrane//sample/0.1M NH4NO3/KCl standard, Hg2Cl2; Hg. The aq. inner soln. was made 0.1M in the chlorides of all cations tested and buffered (pH 8) with 1M N(C2H4OH)3 and 0.5M HOAc. Supersatd. solns. of the nonactin homologs (20% nonactin, 35% monactin, 35% dinactin, 10% trinactin) in CCl4 were transferred onto sintered glass disks to form the membrane. Cells of this type show specificity (K+ over Na+) far superior to other ion-specific glass electrodes and have cation selectivity consts. up to 750 as compared with ∼30 for glass electrodes. This value is comparable to the ion-specific behavior observed in rat liver mitochondria.
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      26. 26
        Bloch, R.; Shatkay, A.; Saroff, H. A. Fabrication and Evaluation of Membranes as Specific Electrodes for Calcium ions. Biophys. J. 1967, 7, 865– 877,  DOI: 10.1016/S0006-3495(67)86626-8
        26
        Fabrication and evaluation of membranes as specific electrodes for calcium ions
        Bloch, Rene; Shatkay, Adam; Saroff, H. A.
        Biophysical Journal (1967), 7 (6), 865-77CODEN: BIOJAU; ISSN:0006-3495.
        A technique was developed to construct membranes with the bulk transport for specific metallic ions. Membranes were fabricated from an inert poly(vinyl chloride) matrix impregnated with Bu3PO4 or Bu3PO4 plus thenoyltrifluoroacetone, and these were tested as electrodes specific for Ca ions. Both types of membrane exhibited a high specificity for Ca ions in the presence of Na, Mg, and Ba ions. In the presence of perturbing ions, the usefulness of the membranes is limited by both the transport of the current and development of potentials by the interfering ion. The potential developed across the membrane was measured in 2 different cells. In the 1st cell the membrane is clamped between 2 halves of a Perspex cell, the solns. are poured into the 2 chambers, and then calomel electrodes are introduced into the chambers. In the 2nd cell, the membrane is clamped between 2 small glass cups (vol. ∼3 ml.), and each cup is connected to a calomel electrode with a 5-way stopcock. This cell allows a convenient and frequent change of solns. of both sides of the membrane and of the liq. junctions between solns. and the calomel electrodes. Also, this cell gives more accurate and reproducible results, although the 1st one is easier to operate. An exptl. method is given for evaluating the interfering quantities. 17 references.
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      27. 27
        Bakker, E.; Pretsch, E. Modern Potentiometry. Angew. Chem., Int. Ed. 2007, 46, 5660– 5668,  DOI: 10.1002/anie.200605068
        27
        Modern potentiometry
        Bakker, Eric; Pretsch, Ernoe
        Angewandte Chemie, International Edition (2007), 46 (30), 5660-5668CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
        A review. A silent revolution of ion-selective electrodes (ISEs) took place during the past decade. Polymeric membrane electrodes are now routinely used to det. complex formation consts. between lipophilic guests and ionic hosts. Ultratrace level measurements have become possible even in samples of very small vols., with detection limits in the attomole range.
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      28. 28
        Pankratova, N.; Crespo, G. A.; Afshar, M. G.; Crespi, M. C.; Jeanneret, S.; Cherubini, T.; Tercier-Waeber, M. L.; Pomati, F.; Bakker, E. Potentiometric sensing array for monitoring aquatic systems. Environ. Sci.-Process Impacts 2015, 17, 906– 914,  DOI: 10.1039/C5EM00038F
        28
        Potentiometric sensing array for monitoring aquatic systems
        Pankratova, Nadezda; Crespo, Gaston A.; Afshar, Majid Ghahraman; Crespi, Miquel Coll; Jeanneret, Stephane; Cherubini, Thomas; Tercier-Waeber, Mary-Lou; Pomati, Francesco; Bakker, Eric
        Environmental Science: Processes & Impacts (2015), 17 (5), 906-914CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
        Since aquatic environments are highly heterogeneous and dynamic, there is the need in aquatic ecosystem monitoring to replace traditional approaches based on periodical sampling followed by lab. anal. with new automated techniques that allow one to obtain monitoring data with high spatial and temporal resoln. We report here on a potentiometric sensing array based on polymeric membrane materials for the continuous monitoring of nutrients and chem. species relevant for the carbon cycle in freshwater ecosystems. The proposed setup operates autonomously, with measurement, calibration, fluidic control and acquisition triggers all integrated into a self-contained instrument. Exptl. validation was performed on an automated monitoring platform on lake Greifensee (Switzerland) using potentiometric sensors selective for hydrogen ions, carbonate, calcium, nitrate and ammonium. Results from the field tests were compared with those obtained by traditional lab. anal. A linear correlation between calcium and nitrate activities measured with ISEs and relevant concns. measured in the lab. was found, with the slopes corresponding to apparent single ion activity coeffs. [Formula Omitted] and [Formula Omitted]. Good correlation between pH values measured with ISE and CTD probes (SD = 0.2 pH) suggests adequate reliability of the methodol.
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      29. 29
        Cuartero, M.; Pankratova, N.; Cherubini, T.; Crespo, G. A.; Massa, F.; Confalonieri, F.; Bakker, E. In Situ Detection of Species Relevant to the Carbon Cycle in Seawater with Submersible Potentiometric Probes. Environ. Sci. Technol. Lett. 2017, 4, 410– 415,  DOI: 10.1021/acs.estlett.7b00388
        29
        In Situ Detection of Species Relevant to the Carbon Cycle in Seawater with Submersible Potentiometric Probes
        Cuartero, Maria; Pankratova, Nadezda; Cherubini, Thomas; Crespo, Gaston A.; Massa, Francesco; Confalonieri, Fabio; Bakker, Eric
        Environmental Science & Technology Letters (2017), 4 (10), 410-415CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
        We report on the development of a submersible probe for the simultaneous potentiometric detection of carbonate, calcium, and pH in seawater. All-solid-state electrodes incorporating nanomaterials provide an adequate response time (<10 s), stability (drifts of <0.9 mV h-1), reproducibility (calibration parameter deviation of <0.7%), and accuracy (deviation of <8% compared to ref. techniques) for real-time monitoring of seawater using a flow system. The functioning of the deployable prototype was checked in an outdoor mesocosm and via long-term monitoring in Genoa Harbor. The electrodes worked properly for 3 wk, and the system demonstrated the capability to autonomously operate with routines for repetitive measurements, data storage, and management. In situ profiles obsd. in Genoa Harbor and Arcachon Bay were validated using on site and ex situ techniques. The validation of in situ-detected carbonate is a challenge because both re-equilibration of the sample with atm. CO2 and the use of apparent thermodn. consts. for speciation calcns. lead to some differences (<20% deviation). The submersible probe is a promising tool for obtaining rapid and trustworthy information about chem. levels in marine systems. Moreover, the fluidic approach allows for the integration of other ion sensors that may require sample pretreatment.
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      30. 30
        Cuartero, M.; Crespo, G.; Cherubini, T.; Pankratova, N.; Confalonieri, F.; Massa, F.; Tercier-Waeber, M. L.; Abdou, M.; Schafer, J.; Bakker, E. In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors. Anal. Chem. 2018, 90, 4702– 4710,  DOI: 10.1021/acs.analchem.7b05299
        30
        In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors
        Cuartero, Maria; Crespo, Gaston; Cherubini, Thomas; Pankratova, Nadezda; Confalonieri, Fabio; Massa, Francesco; Tercier-Waeber, Mary-Lou; Abdou, Melina; Schafer, Jorg; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2018), 90 (7), 4702-4710CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        A new submersible probe for the in situ detection of nitrate, nitrite, and chloride in seawater is presented. Inline coupling of a desalination unit, an acidification unit, and a sensing flow cell contg. all-solid-state membrane electrodes allows for the potentiometric detection of nitrate and nitrite after removal of the key interfering ions in seawater, chloride and hydroxide. Thus, the electrodes exhibited attractive anal. performances for the potentiometric detection of nitrate and nitrite in desalinated and acidified seawater: fast response time (t95 < 12 s), excellent stability (long-term drifts of <0.5 mV h-1), good reproducibility (calibration parameter deviation of <3%), and satisfactory accuracy (uncertainties <8%Diff compared to ref. technique). The desalination cell, which can be repetitively used for about 30 times, may addnl. be used as an exhaustive, and therefore calibration-free, electrochem. sensor for chloride and indirect salinity detection. The detection of these two parameters together with nitrate and nitrite may be useful for the correlation of relative changes in macronutrient levels with salinity cycles, which is of special interest in recessed coastal water bodies. The system is capable of autonomous operation during deployment, with routines for repetitive measurements (every 2 h), data storage and management, and computer visualization of the data in real time. In situ temporal profiles obsd. in the Arcachon Bay (France) showed valuable environmental information concerning tide-dependent cycles of nitrate and chloride levels in the lagoon, which are here obsd. for the first time using direct in situ measurements. The submersible probe based on membrane electrodes presented herein may facilitate the study of biogeochem. processes occurring in marine ecosystems by the direct monitoring of nitrate and nitrite levels, which are key chem. targets in coastal waters.
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      31. 31
        Athavale, R.; Dinkel, C.; Wehrli, B.; Bakker, E.; Crespo, G. A.; Brand, A. Robust Solid-Contact Ion Selective Electrodes for High-Resolution In Situ Measurements in Fresh Water Systems. Environ. Sci. Technol. Lett. 2017, 4, 286– 291,  DOI: 10.1021/acs.estlett.7b00130
        31
        Robust Solid-Contact Ion Selective Electrodes for High-Resolution In Situ Measurements in Fresh Water Systems
        Athavale, Rohini; Dinkel, Christian; Wehrli, Bernhard; Bakker, Eric; Crespo, Gaston A.; Brand, Andreas
        Environmental Science & Technology Letters (2017), 4 (7), 286-291CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
        Biogeochem. processes are often confined to very narrow zones in aquatic systems. Therefore, highly resolved in situ measurements are required to study these processes. Potentiometric solid-contact ion selective electrodes (SC-ISEs) are promising tools for such measurements. SC-ISEs show good performance in analyses under controlled exptl. conditions. Very few sensor designs, however, can sustain the challenges of natural water matrixes and external environmental conditions during in situ applications. We fabricated ammonium and pH selective SC-ISEs with functionalized multiwalled carbon nanotubes (f-MWCNT) as a solid contact. Their functionality was tested in the lab. and applied in situ for vertical profiling in a eutrophic lake. Sensors were insensitive to strong redox changes, high sulfide concns., and bright daylight conditions during the application in the lake. In addn., sensors are easily fabricated and exhibit short response times (<10 s). The proposed design of SC-ISEs based on f-MWCNTs is quite suitable for high-resoln. in situ profiling of ionic species in fresh water lakes.
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      32. 32
        Athavale, R.; Pankratova, N.; Dinkel, C.; Bakker, E.; Wehrli, B.; Brand, A. Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems. Environ. Sci. Technol. 2018, 52, 11259– 11266,  DOI: 10.1021/acs.est.8b02969
        32
        Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems
        Athavale, Rohini; Pankratova, Nadezda; Dinkel, Christian; Bakker, Eric; Wehrli, Bernhard; Brand, Andreas
        Environmental Science & Technology (2018), 52 (19), 11259-11266CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
        We present a new potentiometric sensor principle and a calibration protocol for in situ profiling of dissolved CO2 with high temporal and spatial resoln. in fresh water lakes. The sensor system is based on the measurement of EMF between two solid-contact ion selective electrodes (SC-ISEs), a hydrogen ion selective and a carbonate selective sensor. Since it relies on SC-ISEs, it is insensitive to changes in pressure, thus suitable for in situ studies. Also, as it offers a response time (t95%) of <10 s, it allows for profiling applications at high spatial resoln. The proposed optimum in situ protocol accounts for the continuous drift and change in offset that remains a challenge during profiling in natural waters. The fast response resolves features that are usually missed by std. methods like the classical Severinghaus CO2 probe. In addn., the insensitivity of the presented setup to dissolved sulfide allows also for measurements in anoxic zones of eutrophic systems. Highly resolved CO2 concn. profiles obtained by the novel and robust SC-ISE setup along with the developed optimum in situ protocol allow investigating hotspots of biogeochem. processes, such as mineralization and primary prodn. in the water column and help improving ests. for CO2 turnover in freshwater systems.
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      33. 33
        Veder, J.-P.; De Marco, R.; Patel, K.; Si, P.; Grygolowicz-Pawlak, E.; James, M.; Alam, M. T.; Sohail, M.; Lee, J.; Pretsch, E.; Bakker, E. Evidence for a Surface Confined Ion-to-Electron Transduction Reaction in Solid-Contact Ion-Selective Electrodes Based on Poly(3-octylthiophene). Anal. Chem. 2013, 85, 10495– 10502,  DOI: 10.1021/ac4024999
        33
        Evidence for a surface confined ion-to-electron transduction reaction in solid-contact ion-selective electrodes based on poly(3-octylthiophene)
        Veder, Jean-Pierre; De Marco, Roland; Patel, Kunal; Si, Pengchao; Grygolowicz-Pawlak, Ewa; James, Michael; Alam, Muhammad Tanzirul; Sohail, Manzar; Lee, Junqiao; Pretsch, Erno; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2013), 85 (21), 10495-10502CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        The ion-to-electron transduction reaction mechanism at the buried interface of the electrosynthesized poly-(3-octylthiophene) (POT) solid-contact (SC) ion-selective electrode (ISE) polymeric membrane has been studied using synchrotron radiation-XPS (SR-XPS), near edge X-ray absorption fine structure (NEXAFS), and electrochem. impedance spectroscopy (EIS)/neutron reflectometry (NR). The tetrakis-[3,5-bis-(triflouromethyl)-phenyl]-borate (TFPB-) membrane dopant in the polymer ISE was transferred from the polymeric membrane to the outer surface layer of the SC on oxidn. of POT but did not migrate further into the oxidized POT SC. The TFPB- and oxidized POT species could only be detected at the outer surface layer (≤14 ) of the SC material, even after oxidn. of the electropolymd. POT SC for an hour at high anodic potential demonstrating that the ion-to-electron transduction reaction is a surface confined process. Accordingly, this study provides the first direct structural evidence of ion-to-electron transduction in the electropolymd. POT SC ISE by proving TFPB- transport from the polymeric ISE membrane to the oxidized POT SC at the buried interface of the SC ISE. It is inferred that the performance of the POT SC ISE is independent of the thickness of the POT SC but is instead contingent on the POT SC surface reactivity and/or elec. capacitance of the POT SC. In particular, the results suggest that the electropolymd. POT conducting polymer may spontaneously form a mixed surface/bulk oxidn. state, which may explain the unusually high potential stability of the resulting ISE. It is anticipated that this new understanding of ion-to-electron transduction with electropolymd. POT SC ISEs will enable the development of new and improved devices with enhanced anal. performance attributes.
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      34. 34
        Xie, X. J.; Bakker, E. Non-Severinghaus Potentiometric Dissolved CO2 Sensor with Improved Characteristics. Anal. Chem. 2013, 85, 1332– 1336,  DOI: 10.1021/ac303534v
        34
        Non-Severinghaus Potentiometric Dissolved CO2 Sensor with Improved Characteristics
        Xie, Xiaojiang; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2013), 85 (3), 1332-1336CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        A new type of carbon dioxide sensor comprising a pH glass electrode measured against a carbonate-selective membrane electrode based on a tweezer type carbonate ionophore is presented here for the 1st time. No cumbersome liq. junction based ref. element is used in this measurement. The sensor shows an expected Nernstian divalent response slope to dissolved CO2 over a wide range covering the routine environmental and physiol. PCO2 levels. Unlike the conventional Severinghaus CO2 probe for which the response is substantially delayed to up to 10 min due to diffusion of carbon dioxide into the internal compartment, the ion-selective CO2 sensor proposed here shows a response time (t95%) of 5 s. When used together with a traditional ref. electrode, the sensor system is confirmed to also monitor sample pH and carbonate along with carbon dioxide. A selectivity anal. suggests that Cl- does not interfere even at high concns., allowing one to explore this type of sensor probe for use in seawater or undiluted blood samples. The CO2 probe was used in an aquarium to monitor the CO2 levels caused by the diurnal cycles caused by the metab. of the aquatic plants and shows stable and reproducible results.
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      35. 35
        Afshar, M. G.; Crespo, G. A.; Bakker, E. Direct Ion Speciation Analysis with Ion-Selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode. Anal. Chem. 2012, 84, 8813– 8821,  DOI: 10.1021/ac302092m
        There is no corresponding record for this reference.
      36. 36
        Crespo, G. A.; Afshar, M. G.; Bakker, E. Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes. Anal. Chem. 2012, 84, 10165– 10169,  DOI: 10.1021/ac302868u
        36
        Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes
        Crespo, Gaston A.; Ghahraman Afshar, Majid; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2012), 84 (23), 10165-10169CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        An electrochem. sensing protocol based on supported liq. ion-selective membranes for the direct detection of total alky. of a sample that contains a weak base such as Tris (pKa = 8.2) is presented here for the 1st time. Alky. is detd. by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane-sample interface depletes owing to diffusion limitation is related to sample alky. in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alky. (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ detn. of these characteristics in complex samples such as natural waters.
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      37. 37
        Afshar, M. G.; Crespo, G. A.; Xie, X. J.; Bakker, E. Direct Alkalinity Detection with Ion-Selective Chronopotentiometry. Anal. Chem. 2014, 86, 6461– 6470,  DOI: 10.1021/ac500968c
        37
        Direct Alkalinity Detection with Ion-Selective Chronopotentiometry
        Afshar, Majid Ghahraman; Crespo, Gaston A.; Xie, Xiaojiang; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2014), 86 (13), 6461-6470CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        The authors explore the possibility to directly measure pH and alky. in the sample with the same sensor by imposing an outward flux of hydrogen ions from an ion-selective membrane to the sample soln. by an applied current. The membrane consists of a polypropylene-supported liq. membrane doped with a hydrogen ionophore (chromoionophore I), ion exchanger (KTFBP), and lipophilic electrolyte (ETH 500). While the sample pH is measured at zero current, alky. is assessed by chronopotentiometry at anodic current. Hydrogen ions expelled from the membrane undergo acid-base soln. chem. and protonate available base in the diffusion layer. With time, base species start to be depleted owing to the const. imposed hydrogen ion flux from the membrane, and a local pH change occurs at a transition time. This pH change (potential readout) is correlated to the concn. of the base in soln. As in traditional chronopotentiometry, the obsd. square root of transition time (τ) is linear in the concn. range of 0.1 mM to 1 mM, using the bases tris(hydroxymethyl)aminomethane, ammonia, carbonate, hydroxide, hydrogen phosphate, and borate. Numerical simulations were used to predict the concn. profiles and the chronopotentiograms, allowing the discussion of possible limitations of the proposed method and its comparison with volumetric titrns. of alky. Finally, the P-alky. level is measured in a river sample to demonstrate the anal. usefulness of the proposed method. As a result of these preliminary results, probably this approach becomes useful for the in situ detn. of P-alky. in a range of matrixes.
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      38. 38
        Afshar, M. G.; Crespo, G. A.; Bakker, E. Coulometric Calcium Pump for Thin Layer Sample Titrations. Anal. Chem. 2015, 87, 10125– 10130,  DOI: 10.1021/acs.analchem.5b02856
        There is no corresponding record for this reference.
      39. 39
        Afshar, M. G.; Crespo, G. A.; Bakker, E. Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection. Angew. Chem., Int. Ed. 2015, 54, 8110– 8113,  DOI: 10.1002/anie.201500797
        39
        Thin-Layer Chemical Modulations by a Combined Selective Proton Pump and pH Probe for Direct Alkalinity Detection
        Afshar, Majid Ghahraman; Crespo, Gaston A.; Bakker, Eric
        Angewandte Chemie, International Edition (2015), 54 (28), 8110-8113CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
        The authors report a general concept based on a selective electrochem. ion pump used for creating concn. perturbations in thin layer samples (∼40 μL). As a 1st example, hydrogen ions are released from a selective polymeric membrane (proton pump) and the resulting pH is assessed potentiometrically with a 2nd membrane placed directly opposite. By applying a const. potential modulation for 30 s, an induced proton concn. of up to 350 mΜ may be realized. This concept may become an attractive tool for in situ titrns. without the need for sampling, because the thin layer eventually reequilibrates with the contacting bulk sample. Acid-base titrns. of NaOH and Na2CO3 are demonstrated. The detn. of total alky. in a river water sample is carried out, giving levels (23.1 mΜ) comparable to that obtained by std. methods (23.6 mΜ). The concept may be easily extended to other ions (cations, anions, polyions) and may become attractive for environmental and clin. applications.
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      40. 40
        Grygolowicz-Pawlak, E.; Numnuam, A.; Thavarungkul, P.; Kanatharana, P.; Bakker, E. Interference Compensation for Thin Layer Coulometric Ion-Selective Membrane Electrodes by the Double Pulse Technique. Anal. Chem. 2012, 84, 1327– 1335,  DOI: 10.1021/ac202273k
        40
        Interference Compensation for Thin Layer Coulometric Ion-Selective Membrane Electrodes by the Double Pulse Technique
        Grygolowicz-Pawlak, Ewa; Numnuam, Apon; Thavarungkul, Panote; Kanatharana, Proespichaya; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2012), 84 (3), 1327-1335CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Ion-selective membranes operated in a thin layer coulometric detection mode were previously demonstrated to exhibit attractive characteristics in view of realizing sensors without the need for frequent recalibration. In this methodol., the analyte ion is exhaustively removed across an ion-selective membrane by an applied potential, and the resulting current is integrated to yield the coulomb no. and hence the amt. of analyte originally present in the sample. This exhaustive process, however, places greater demands on the selectivity of the membrane compared to direct potentiometry, since the level of interference will increase as the analyte depletes. The authors evaluate here a double pulse protocol to reduce the level of interference, in which the sample is electrolyzed once again after the initial coulometric detection pulse. Since the analyte ion is no longer present at significant concns. during the 2nd pulse, but an interfering ion of high concn. did not appreciably deplete, the 2nd electrolysis step may be used to partially compensate for undesired interference. These processes are here evaluated by numerical simulation for ions of the same charge, demonstrating that the resulting coulomb no. may indeed be reduced for systems of limited selectivity. The improvement in operational selectivity relative to uncompensated coulometry is ∼6-fold. The methodol. is successfully demonstrated exptl. with a calcium selective membrane and tetraethylammonium as a model interfering agent, and the obsd. relative errors after background compensation can be favorably compared to that in direct potentiometry where no sample depletion occurs.
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      41. 41
        Grygolowicz-Pawlak, E.; Bakker, E. Thin layer coulometry ion sensing protocol with potassium-selective membrane electrodes. Electrochim. Acta 2011, 56, 10359– 10363,  DOI: 10.1016/j.electacta.2011.02.053
        41
        Thin layer coulometry ion sensing protocol with potassium-selective membrane electrodes
        Grygolowicz-Pawlak, Ewa; Bakker, Eric
        Electrochimica Acta (2011), 56 (28), 10359-10363CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
        The authors recently presented a calcium-selective thin-layer coulometric system in view of realizing recalibration free ion sensors. A multi-pulse protocol allowing for a significant background current correction was further modified here by a numerical evaluation of the electrolysis end-point. A potassium-selective system was studied here within the 10 μM to 1 mM KCl concn. range with a 10 mM LiCl as a background soln. A residual 20 nA s-1 signal change was used to indicate the end of exhaustive ion transfer from the sample across the ion-selective membrane. Optimized electrolysis times were found from 13.4 s for a 10 μM sample to 270 s for a 1 mM sample. The intercept of the calibration curve of obsd. ion transfer charge vs. concn. is just 2.3 ± 2.0 μC. The obsd. slope of 1.18 ± 0.02 C M-1 was very similar to the one calcd. based on the known vol. of the sample, 1.19 ± 0.12 C M-1, suggesting that the method is promising as an abs. measurement tool.
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      42. 42
        Shvarev, A.; Neel, B.; Bakker, E. Detection Limits of Thin Layer Coulometry with lonophore Based Ion-Selective Membranes. Anal. Chem. 2012, 84, 8038– 8044,  DOI: 10.1021/ac301940n
        42
        Detection Limits of Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes
        Shvarev, Alexey; Neel, Bastien; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2012), 84 (18), 8038-8044CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        The authors report here on a significant improvement in lowering the low detection limit of thin layer coulometric sensors based on liq. ion-selective membranes, using a potassium-selective system as a model example. Various possible processes that may result in an elevated residual current reading after electrolysis were eliminated. Self-dissoln. of AgCl on the Ag/AgCl inner element may result in a residual ion flux that could adversely affect the lower detection limit. It was here replaced with an Ag/AgI inner pseudo-ref. electrode where the self-dissoln. equil. is largely suppressed. Possible residual currents originating from a direct contact between inner element and ion-selective membranes were eliminated by introducing an inert PVDF separator of 50 μm diam. that was coiled around the inner element by a custom-made instrument. Finally, the influence of electrolyte fluxes from the outer soln. across the membrane into the sample was evaluated by altering its lipophilic nature and reducing its concn. This last effect is most likely responsible for the obsd. residual current for the potassium-selective membranes studied here. For the optimized conditions, the calibration curves demonstrated a near zero intercept, thereby paving the way to the coulometric calibration-free sensing of ionic species. A linear calibration curve for the coulometric cell with valinomycin potassium-selective membrane was obtained in the range of 100 nM to 10 μM potassium in the presence of a 10 μM sodium background. In the presence of a higher (100 μM) concn. of sodium, a reliable detection of 1-100 μM of potassium was achieved.
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      43. 43
        Ding, J.; Cherubini, T.; Yuan, D.; Bakker, E. Paper-supported thin-layer ion transfer voltammetry for ion detection. Sens. Actuators, B 2019, 280, 69– 76,  DOI: 10.1016/j.snb.2018.10.046
        44
        Paper-supported thin-layer ion transfer voltammetry for ion detection
        Ding, Jiawang; Cherubini, Thomas; Yuan, Dajing; Bakker, Eric
        Sensors and Actuators, B: Chemical (2019), 280 (), 69-76CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
        We report here on paper-supported thin sample layer voltammetry for the detn. of ions. To achieve this goal, a simple setup for the coupling of a com. available electrode to a silver rod electrode was designed and evaluated for paper-supported thin-layer voltammetry. Linear scan ion transfer voltammetry was explored here for ion-selective membranes doped with an ionophore. The ion-transfer processes and electrochem. behaviors of the system are here evaluated and confirmed by numerical simulation. In the proof-of-concept expts. described, the ions tetrabutylammonium chloride (TBA+) and potassium (K+) were studied as model analytes at membranes without and with ionophore, resp. A linear relationship from 0.1 mM to 1.0 mM K+ was obtained between the charge and ion concn. The coexistence of background sodium ions did not give appreciable interference, but the background wave was not completely isolated from the analyte wave, as also confirmed by the model. The methodol. was successfully demonstrated for detn. of K+ in mineral water. It is anticipated that this paper-supported thin-layer detection approach may provide an attractive readout protocol for disposable paper-based anal. devices as the methodol. does not place strict demands on ref. electrode performance.
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      44. 44
        Dorokhin, D.; Crespo, G. A.; Afshar, M. G.; Bakker, E. A low-cost thin layer coulometric microfluidic device based on an ion-selective membrane for calcium determination. Analyst 2014, 139, 48– 51,  DOI: 10.1039/C3AN01715J
        43
        A low-cost thin layer coulometric microfluidic device based on an ion-selective membrane for calcium determination
        Dorokhin, Denis; Crespo, Gaston A.; Afshar, Majid Ghahraman; Bakker, Eric
        Analyst (Cambridge, United Kingdom) (2014), 139 (1), 48-51CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
        A prototype of a low-cost and easy-to-use thin layer coulometric microfluidic device based on an ion-selective membrane for calcium detection is described. The microfluidic device was fabricated and consequently assembled with inexpensive materials without using sophisticated and centralized fabrication lab. facilities. The linear range of the device is 10-100 μM for a 60 s current integration time. Preliminary validations showed that the microfluidic device is suitable for the quantification of calcium in mineral water.
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      45. 45
        Afshar, M. G.; Crespo, G. A.; Dorokhin, D.; Neel, B.; Bakker, E. Thin Layer Coulometry of Nitrite with Ion-Selective Membranes. Electroanalysis 2015, 27, 609– 615,  DOI: 10.1002/elan.201400522
        45
        Thin Layer Coulometry of Nitrite with Ion-Selective Membranes
        Afshar, Majid Ghahraman; Crespo, Gaston A.; Dorokhin, Denis; Neel, Bastien; Bakker, Eric
        Electroanalysis (2015), 27 (3), 609-615CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
        We report on a thin layer coulometric detection method based on an ion-selective membrane in view of a quant. detn. of nitrite in undiluted human urine and tap water. A cobalt(II) tert-Bu salophen compd., previously characterized by our group in ion-selective membrane electrodes, is used here as ionophore. The four orders of magnitude discrimination of chloride permits the detection of nitrite in samples that contain a large excess of chloride. The approach developed here allows one to det. nitrite within a concn. range of 20-100 μM for a 120 s current integration time. A modification of the reported electrochem. protocol to reduce the capacitive contribution diminishes the intercept of the calibration to -8.6 μC. This is esp. important in view of reaching calibration free systems. Initial inhouse validation exhibits a limit of detection of 10 μM in undiluted urine and in tap water. These values correspond to the regulation limits established in European Union and are adequate for the detn. of nitrite for medical applications.
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      46. 46
        Crespo, G. A.; Cuartero, M.; Bakker, E. Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection. Anal. Chem. 2015, 87, 7729– 7737,  DOI: 10.1021/acs.analchem.5b01459
        46
        Thin Layer Ionophore-Based Membrane for Multianalyte Ion Activity Detection
        Crespo, Gaston A.; Cuartero, Maria; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2015), 87 (15), 7729-7737CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        A concept is introduced that allows one to detect the activity of multiple ions simultaneously and selectively with a single ion-selective membrane. This is demonstrated with ∼300 nm thin plasticized PVC membranes contg. up to two ionophores in addn. to a lipophilic cation-exchanger, overlaid on an electropolymd. poly-3-octylthiophene (POT) film as the electron to ion transducer. The ion-selective membranes are formulated under ionophore depleted conditions (avoiding excess of ionophore over ion-exchanger), which is purposely different from common practice with ion-selective electrodes. Cyclic voltammetry is used to interrogate the films. An anodic scan partially oxidizes the POT underlayer, which results in the expulsion of cations from the membrane at an appropriate potential. During the scan of a membrane contg. multiple ionophores, the least bound ion is expelled 1st, giving distinct Gaussian peak shaped ion transfer voltammetric waves that are analyzed in terms of their peak potential. These potentials change with the logarithm of the ion activity, in complete analogy to ion-selective electrodes, and multiple such waves are obsd. with multiple ionophores that exhibit no obvious interference from the other ionophores present in the membrane. The concept is established with lithium and calcium ionophores and accompanied by a response model that assumes complete equilibration of the membrane at every applied potential. From the model, diffusion coeffs. in the membrane or aq. phase bear no influence on the peak potentials as long as thin layer behavior is obsd., further confirming the analogy to a potentiometric expt. Idealized ion transfer waves are narrower than exptl. findings, which is explained by a broader than expected anodic peak for the oxidn. of conducting polymer. The correspondence between expt. and theory is otherwise excellent in terms of thin layer behavior and Nernstian shift of the peaks with analyte concn.
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      47. 47
        Cuartero, M.; Crespo, G. A.; Bakker, E. Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes. Anal. Chem. 2016, 88, 1654– 1660,  DOI: 10.1021/acs.analchem.5b03611
        47
        Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes
        Cuartero, Maria; Crespo, Gaston A.; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2016), 88 (3), 1654-1660CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        As shown in recent work, thin layer ion-selective multi-ionophore membranes can be interrogated by cyclic voltammetry to detect the ion activity of multiple species simultaneously and selectively. Addnl. fundamental evidence is put forward on ion discrimination with thin multi-ionophore-based membranes with thicknesses of 200 ± 25 nm and backside contacted with poly-3-octylthiophene (POT). An anodic potential scan partially oxidizes the POT film (to POT+), thereby initiating the release of hydrophilic cations from the membrane phase to the sample soln. at a characteristic potential. Varying concn. of added cation-exhanger demonstrates that it limits the ion transfer charge and not the deposited POT film. Voltammograms with multiple peaks are obsd. with each assocd. with the transfer of one type of ion (lithium, potassium, and sodium). Exptl. conditions (thickness and compn. of the membrane and concn. of the sample) are chosen that allow one to describe the system by a thermodn. rather than kinetic model. As a consequence, apparent stability consts. for sodium, potassium, and lithium (assuming 1:1 stoichiometry) with their resp. ionophores are calcd. and agree well with the values obtained by the potentiometric sandwich membrane technique. As an anal. application, a membrane contg. three ionophores was used to det. lithium, sodium, and potassium in artificial samples at the same location and within a single voltammetric scan. Lithium and potassium were also detd. in undiluted human plasma in the therapeutic concn. range.
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      48. 48
        Jarolimova, Z.; Bosson, J.; Labrador, G. M.; Lacour, J.; Bakker, E. Ion Transfer Voltammetry at Thin Films Based on Functionalized Cationic 6 Helicenes. Electroanalysis 2018, 30, 650– 657,  DOI: 10.1002/elan.201700669
        48
        Ion Transfer Voltammetry at Thin Films Based on Functionalized Cationic [6]Helicenes
        Jarolimova, Zdenka; Bosson, Johann; Labrador, Geraldine M.; Lacour, Jerome; Bakker, Eric
        Electroanalysis (2018), 30 (4), 650-657CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
        The authors describe a new family of mol. ion-to-electron redox probes based on cationic diaza, azaoxa, and dioxa [6]helicenes and their derivs. Their unique structure combines, in a single framework, two privileged families of mols. - helicenes and triaryl Me carbenium moieties. These cationic [6]helicenes exhibit reversible and reproducible oxidn./redn. behavior and facilitate the ion transfer into thin layer sensing films composed of bis(2-ethylhexyl)sebacate (DOS), polyurethane (PU), sodium tetrakis 3.5-bis(trifluoromethyl)phenyl borate, sodium ionophore X and diaza+(C8)2Br2 for cation transfer. Cyclic voltammetry was used to interrogate the thin films. The cationic response can be tuned by adjusting the membrane loading. Addn. of lipophilic cation exchanger into the membrane film results in transfer waves of Gaussian shape for cations. A peak sepn. of 60 mV and peak width of 110 mV are near the theor. values for a surface confined process. While Nernstian shifts of the peak potentials with analyte concn. was obtained for membranes based on cationic [6]helicenes and doped with sodium-selective ionophore X, this ionophore was found to promote a gradual loss of redox active species from the ionophore-based membranes into the sample soln.
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      49. 49
        Jarolimova, Z.; Bosson, J.; Labrador, G. M.; Lacour, J.; Bakker, E. Ion Transfer Voltammetry in Polyurethane Thin Films Based on Functionalised Cationic 6 Helicenes for Carbonate Detection. Electroanalysis 2018, 30, 1378– 1385,  DOI: 10.1002/elan.201800080
        49
        Ion Transfer Voltammetry in Polyurethane Thin Films Based on Functionalised Cationic [6]Helicenes for Carbonate Detection
        Jarolimova, Zdenka; Bosson, Johann; Labrador, Geraldine M.; Lacour, Jerome; Bakker, Eric
        Electroanalysis (2018), 30 (7), 1378-1385CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
        We explore here mol. ion-to-electron redox probes based on cationic diaza, azaoxa, and dioxa [6]helicenes and their derivs. as ion-to-electron transducers for the electrochem. detection of anions. These cationic [6]helicenes exhibit reversible and reproducible oxidn./redn. behavior and facilitate the anion transfer of Gaussian shape into polymeric thin layer sensing films. Films composed of bis(2-ethylhexyl) sebacate (DOS), polyurethane (PU), tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH 500) and [6]helicenes were interrogated by cyclic voltammetry. Even though the peak sepn. of 90 mV is larger than ideal, the obsd. peak width at half max. of 130 mV and the linear relationship between current and scan rate are near theor. values, confirming a surface confined process. A Nernstian shift of the peaks with increasing carbonate concn. is obtained in the presence of carbonate ionophore VII incorporated into the thin sensing film. The concn. of carbonate was detd. in an unfiltered sample of the Arve river (flowing from Chamonix to Geneva) and compared to a ref. method (automatic titrator with potentiometric detection). The results suggested that cationic diaza [6]helicene functionalized with two bromine atoms is an attractive mol. ion-to-electron transducer for anion-selective electrodes.
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      50. 50
        Yuan, D. J.; Cuartero, M.; Crespo, G. A.; Bakker, E. Voltammetric Thin-Layer lonophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations. Anal. Chem. 2017, 89, 586– 594,  DOI: 10.1021/acs.analchem.6b03354
        50
        Voltammetric Thin-Layer Ionophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations
        Yuan, Dajing; Cuartero, Maria; Crespo, Gaston A.; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2017), 89 (1), 586-594CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        Voltammetric thin layer (∼200 nm) ionophore-based polymeric films of defined ion-exchange capacity have recently emerged as a promising approach to acquire multi-ion information about the sample, in analogy to performing multiple potentiometric measurements with individual membranes. They behave under two different regimes that are dependent on the ion concn. A thin layer control (no mass transport limitation of the polymer film or soln.) is identified for ion concns. of >10 μM, in which case the peak potential serves as the readout signal, in analogy to a potentiometric sensor. However, ion transfer at lower concns. is chiefly controlled by diffusional mass transport from the soln. to the sensing film, resulting in an increase of peak current with ion concn. This concn. range is suitable for electrochem. ion transfer stripping anal. Here, the transition between the two mentioned scenarios is explored exptl., using a highly Ag-selective membrane as a proof-of-concept under different conditions (variation of ion concn. in the sample from 0.1 μM to 1 mM, scan rate from 25 mV s-1 to 200 mV s-1, and angular frequency from 100 rpm to 6400 rpm). Apart from exptl. evidence, a numerical simulation is developed that considers an idealized conducting polymer behavior and permits one to predict exptl. behavior under diffusion or thin-layer control.
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      51. 51
        Vanamo, U.; Hupa, E.; Yrjana, V.; Bobacka, J. New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes. Anal. Chem. 2016, 88, 4369– 4374,  DOI: 10.1021/acs.analchem.5b04800
        51
        New Signal Readout Principle for Solid-Contact Ion-Selective Electrodes
        Vanamo, Ulriika; Hupa, Elisa; Yrjana, Ville; Bobacka, Johan
        Analytical Chemistry (Washington, DC, United States) (2016), 88 (8), 4369-4374CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        A novel approach to signal transduction concerning solid-contact ion-selective electrodes (SC-ISE) with a conducting polymer (CP) as the solid contact was studied. The method presented here is based on const. potential coulometry, where the potential of the SC-ISE vs. the ref. electrode is kept const. using a potentiostat. The change in the potential at the interface between the ion-selective membrane (ISM) and the sample soln., due to the change in the activity of the primary ion, is compensated with a corresponding but opposite change in the potential of the CP solid contact. This enforced change in the potential of the solid contact results in a transient reducing/oxidizing current flow through the SC-ISE. By measuring and integrating the current needed to transfer the CP to a new state of equil., the total cumulated charge that is linearly proportional to the change of the logarithm of the primary ion activity was obtained. Different thicknesses of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) were used as solid contact. Also, coated wire electrodes (CWEs) were included in the study to show the general validity of the new approach. The ISM employed was selective for K+ ions, and the selectivity of the membrane under implementation of the presented transduction mechanism was confirmed by measurements performed with a const. background concn. of Na+ ions. A unique feature of this signal readout principle is that it allows amplification of the anal. signal by increasing the capacitance (film thickness) of the solid contact of the SC-ISE.
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      52. 52
        Jarolimova, Z.; Han, T. T.; Mattinen, U.; Bobacka, J.; Bakker, E. Capacitive Model for Coulometric Readout of Ion-Selective Electrodes. Anal. Chem. 2018, 90, 8700– 8707,  DOI: 10.1021/acs.analchem.8b02145
        52
        Capacitive Model for Coulometric Readout of Ion-Selective Electrodes
        Jarolimova, Zdenka; Han, Tingting; Mattinen, Ulriika; Bobacka, Johan; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2018), 90 (14), 8700-8707CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        We present here a capacitive model for the coulometric signal transduction readout of solid-contact ion-selective membrane electrodes (SC-ISE) with a conducting polymer (CP) as an intermediate layer for the detection of anions. The capacitive model correlates well with exptl. data obtained for chloride-selective SC-ISEs utilizing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with chloride as the ion-to-electron transducer. Addnl., Prussian blue is used as a simple sodium capacitor to further demonstrate the role of the transduction layer. The influence of different thicknesses of PEDOT as a conducting polymer transducer, different thicknesses of the overlaying ion-selective membranes deposited by drop casting and spin coating, and different compns. of the chloride-selective membrane are explored. The responses are evaluated in terms of current-time, charge-time, and charge-chloride activity relationships. The utility of the sensor with coulometric readout is illustrated by the monitoring of very small concn. changes in soln.
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      53. 53
        Jansod, S.; Cuartero, M.; Cherubini, T.; Bakker, E. Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes. Anal. Chem. 2018, 90, 6376– 6379,  DOI: 10.1021/acs.analchem.8b01585
        53
        Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes
        Jansod, Sutida; Cuartero, Maria; Cherubini, Thomas; Bakker, Eric
        Analytical Chemistry (Washington, DC, United States) (2018), 90 (11), 6376-6379CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
        We present here a general strategy to translate potential change at a potentiometric probe into a tunable color readout. It is achieved with a closed bipolar electrode where the ion-selective component is confined to one end of the electrode while color is generated at the opposite pole, allowing one to phys. sep. the detection compartment from the sample. An elec. potential is imposed across the bipolar electrode by soln. contact such that the potentiometric signal change at the sample side modulates the potential at the detection side. This triggers the turnover of a redox indicator in the thin detection layer until a new equil. state is established. The approach is demonstrated in sep. expts. with a chloride responsive Ag/AgCl element and a liq. membrane based calcium-selective membrane electrode, using the redox indicator ferroin in the detection compartment. The principle can be readily extended to other ion detection materials and optical readout principles.
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