Polymeric Membrane Ion-selective Electrodes Research Articles

Influences of marine biofouling on the potential responses of polymeric membrane ion-selective electrodes

Using polymeric membrane ion-selective electrodes (ISEs) for reliable long-term marine environmental observations is still a challenge due to the formation of biofilms on electrode surfaces. Unfortunately, the contributions of the specific biofilm components to sensor failure have not been studied. Herein, the influences of the specific biofilm components on the performance of the neutral carrier-based polymeric membrane ISEs are systematically investigated in terms of sensitivity, selectivity, and stability. More importantly, the spatial distributions of these foulants in the fouled sensing membrane phase are studied by confocal laser scanning microscopy for the first time to give insight into the fouling mechanisms. Three typical biochemicals (i.e., proteins, polysaccharides, microbial lipids) in biofilms are selected as the representative foulants. The poly (vinyl chloride)-based CO32--ISEs and K+-ISEs/Ca2+-ISEs have been selected as the model sensors for detection of anions and cations, respectively. Experiments show that depending on the type of the biofoulants, the adsorption of the foulants on the polymeric membrane surface and/or their extraction into the organic membrane phase could occur and induce a change in the potential responses. This study provides a deep insight into the influences of the specific biofilm components on the polymeric membrane ISEs, which is of importance for development of the polymeric membrane marine sensors with high antifouling capabilities for long-term marine monitoring.

Potentiometric/time resolved chronopotentiometric sensing for an all-solid-state ion-selective electrode based on MXene/MWCNTs as solid contact

Solid-contact polymeric membrane ion-selective electrodes (ISEs) have been an attractive tool for potentiometric sensors. Sufficient transducing performance evaluation and multiple potentiometric analysis are critical to evaluate whether the solid contact transducer material used in ISE is promising. Herein, we describe sandwich-like MXene/MWCNTs-based polymeric membrane ISEs with three-readout strategies of potential response, chronopotentiometric response, and transition time response. The MXene/MWCNTs can be readily prepared by the electrostatic self-assembly strategy, which not only prevented the aggregation of MXene nanosheets and facilitated charge-transfer, but also significantly contributed to the large double-layer capacitance and water-layer free from interference. With Ca2+ as a model, two kinds of Ca2+-ISEs based on thermodynamic response and dynamic response by modulating the polymeric membrane components with three readouts were explored. Moreover, the proposed Ca2+-ISEs under different background electrolytes exhibit the similar Nernstian responses in potentiometry, variable super-Nernstian ranges in chronopotentiometry, and the consistent transition time readout. This study offers a facile MXene/MWCNTs transducer to develop a highly reliable polymeric membrane ISE.

Maintenance-free antifouling polymeric membrane potentiometric sensors based on self-polishing coatings.

Polymeric membrane ion-selective electrodes (ISEs) have been widely used in environmental monitoring. However, in complicated marine environments, marine biofouling usually becomes a sticky problem for these electrodes. Herein, for the first time, a novel maintenance-free antifouling potentiometric marine sensor based on a self-polishing coating (SPC) is proposed. The SPC is synthesized by using the seeded emulsion polymerization method based on the triisopropylsilyl methacrylate monomer as the regulator of the self-renewal rate. This coating can be simply modified onto the electrode surface by drop-casting. The silyl acrylate side groups of the obtained SPC on the sensor surface can be hydrolyzed in the marine alkaline medium. The shear movement of seawater driven by sea waves, wind, gravity, or vibration removes the leftover (fouled) brittle polymer backbone and thus the fouling marine microorganisms. As a proof-of-concept experiment, a polymeric membrane Ca2+-ISE is chosen as a model. Compared to the unmodified electrode, the SPC-coated Ca2+-ISE exhibits remarkable improved antifouling properties in terms of superior anti-adhesive abilities towards marine microorganisms, such as bacterial cells and algae and excellent long-term stability even in the presence of high levels of marine microorganisms. Since no additional manual maintenance is required for maintaining the antifouling abilities of the sensor, the proposed self-polishing sensor may lay an important foundation for construction of unattended long-term potentiometric monitoring systems in real marine environments.

Modern Potentiometric Biosensing Based on Non-Equilibrium Measurement Techniques.

Modern potentiometric sensors based on polymeric membrane ion-selective electrodes (ISEs) have achieved new breakthroughs in sensitivity, selectivity, and stability and have extended applications in environmental surveillance, medical diagnostics, and industrial analysis. Moreover, nonclassical potentiometry shows promise for many applications and opens up new opportunities for potentiometric biosensing. Here, we aim to provide a concept to summarize advances over the past decade in the development of potentiometric biosensors with polymeric membrane ISEs. This Concept article articulates sensing mechanisms based on non-equilibrium measurement techniques. In particular, we emphasize new trends in potentiometric biosensing based on attractive dynamic approaches. Representative examples are selected to illustrate key applications under zero-current conditions and stimulus-controlled modes. More importantly, fruitful information obtained from non-equilibrium measurements with dynamic responses can be useful for artificial intelligence (AI). The combination of ISEs with advanced AI techniques for effective data processing is also discussed. We hope that this Concept will illustrate the great possibilities offered by non-equilibrium measurement techniques and AI in potentiometric biosensing and encourage further innovations in this exciting field.

Solid-contact polymeric membrane ion-selective electrodes using a covalent organic framework@reduced graphene oxide composite as ion-to-electron transducer.

A solid-contact ion-selective electrode (SC-ISE) based on a covalent organic framework@reduced graphene oxide (rGO) composite is proposed. The composite can be synthesized through the polycondensation of 1,3,5-triformylphloroglucinol (TFP) and 2,6-diaminoanthraquinone (DAAQ) on the rGO nanosheets, which shows high capacitance and good redox-active properties. By applying Cd2+-ISE as a model, the electrode exhibits a Nernstian slope of 29.7±0.4 mV/decade in the activity range of 1.0×10-7 - 7.9×10-4M and the limit of detection is 6.8×10-8M. Particularly, the electrode based on DAAQ-TFP@rGO exhibits a low potential drift of 1.2±0.2μV/h over 70h due to the large capacitance of 2.0mF. Moreover, the DAAQ-TFP@rGO-based Cd2+-ISE shows good reproducibility and the standard deviations of the standard potentials for single batch and batch-to-batch are 0.28 (n=4) and 0.30mV (n=4), respectively. The developed SC-Cd2+-ISE is not disturbed by light or gas and no aqueous layer occurs between the sensing membrane and DAAQ-TFP@rGO layer. The DAAQ-TFP@rGO composite is highly promising for construction of calibration-free SC-ISEs.

Anti-fouling TiO2-Coated Polymeric Membrane Ion-Selective Electrodes with Photocatalytic Self-Cleaning Properties

Nowadays, using a polymeric membrane ion-selective electrode (ISE) to achieve reliable ion sensing in complex samples remains challenging because of electrode fouling. To address this challenge, we describe a polymeric membrane ISE with excellent anti-fouling and self-cleaning properties based on surface covalent modification of an anatase TiO2 coating. Under ultraviolet illumination, the reactive oxygen species produced by photocatalytic TiO2 can not only kill microorganisms but also degrade organic foulants into carbon dioxide and water, and a formed superhydrophilic film can effectively prevent the adsorption of foulants, thus inhibiting the occurrence of biofouling and organic fouling of the sensors. More importantly, residual foulants could be fully self-cleaned through the flow of water droplets. By using Ca2+-ISE as a model, an anti-fouling polymeric membrane potentiometric sensor has been developed. Compared to the unmodified electrode, the TiO2-coated Ca2+-ISE exhibits remarkably improved anti-biofouling properties with a low bacterial adhesion rate of 4.74% and a high inhibition rate of 96.62%. In addition, the proposed electrode displays unique properties of anti-organic dye fouling and a superior self-cleaning ability even after soaking in a concentrated bacterial suspension of 109 CFU mL-1 for 60 days. The present approach can be extended to improve the fouling resistance of other electrochemical or optical membrane sensors and is promising for the construction of contamination-free sensors.

Towards potentiometric detection in nonaqueous media: Evaluation of the impacts of organic solvents on polymeric membrane ion-selective electrodes

Polymeric membrane ion-selective electrodes (ISEs) have been widely used in various fields including clinical diagnosis, environmental monitoring and industrial analysis. Although most samples of analytical interest measured by the ISEs are aqueous solutions, the applications of these electrodes in nonaqueous media are often inevitable. Unfortunately, so far, little has been known about the extent to which the properties of the ISEs could be affected by the organic solvents. Herein, the feasibility for the applications of the polymeric membrane ISEs in nonaqueous media has been investigated. A polymeric membrane Ca2+-ISE is chosen as a model of potentiometric sensors. Four typical water miscible organic solvents (three protic solvents: ethanol, acetic acid, and methanol, and one aprotic dipolar solvent: acetonitrile) are used as the representative examples. Experiments show that the aprotic solvent acetonitrile has the strongest destructive ability towards the sensing performance of the ISE in terms of Nernstian slope and selectivity coefficient. Moreover, the effect on the sensing performance depends on the kind of the protic solvent, the immersion time and the polarity of the membrane plasticizer. We believe that the obtained results could promote further applications of the polymeric membrane ISEs in the organic solvent-containing samples, which could significantly extend the application scope of the ISEs.

All-Solid-State Polymeric Membrane Ion-Selective Electrodes Based on NiCo2S4 as a Solid Contact.

The performance criteria for the design of all-solid-state ion-selective electrodes mainly include high electrode-to-electrode reproducibility and a low potential drift. Here, we introduce nickel cobalt sulfide (NiCo2S4) as a solid contact for ion-to-electron transduction based on multiple redox couples. NiCo2S4 materials with different morphologies can be prepared through a facile hydrothermal/solvothermal method. A NiCo2S4-based solid-contact Ca2+-ISE has been developed, which exhibits a Nernstian slope of 27.5 ± 0.2 mV/dec in the activity range from 1.0 × 10-6 to 2.9 × 10-2 M with a detection limit of 5.0 × 10-7 M. A variation of the standard potential E° for eight individual solid-contact electrodes can be obtained as low as 0.35 mV. Due to the synergistic effect of cobalt and nickel ions in the ternary sulfide, an excellent redox capacitance (565 μF) of the buried solid contact coated with the ion-selective membrane can be achieved and is much larger than those obtained from other redox solid-contact materials reported so far, thus yielding a high potential stability of 2.2 ± 0.4 μV/h. In addition, the NiCo2S4-based solid-contact Ca2+-ISE shows a reduced water layer at the sensing membrane/NiCo2S4 interface and provides an excellent resistance to the interferences from light, O2, and CO2. The proposed strategy utilizing NiCo2S4 as a solid contact is a promising alternative for the fabrication of calibration-free ASS-ISEs.

Enhancing the Oil-Fouling Resistance of Polymeric Membrane Ion-Selective Electrodes by Surface Modification of a Zwitterionic Polymer-Based Oleophobic Self-Cleaning Coating

Due to the frequent oil spill accidents and pollution of industrial oily wastewater, oil fouling has become a great challenge to polymeric membrane ion-selective electrodes (ISEs) for applications in oil-contaminated areas. Herein, a simple approach is proposed to enhance the oil-fouling resistance of polymeric membrane ISEs by surface modification of a zwitterionic polymer-based underwater oleophobic coating. As a proof-of-concept, a classical poly(vinyl chloride) membrane-based calcium ion-selective electrode (Ca2+-ISE) is chosen as a model sensor. The zwitterionic polymer-based coating can be readily modified on the sensor's surface by immersion of the electrode into a mixture solution of dopamine and a zwitterionic acrylate monomer (i.e., sulfobetaine methacrylate, SBMA). The formed poly(SBMA) (PSBMA) coating alters the oleophilic membrane surface to an oleophobic one, which endows the surface with excellent self-cleaning properties without loss of the sensor's analytical performance. Compared to the pristine Ca2+-ISE, the PSBMA-modified Ca2+-ISE exhibits an improved analytical stability when exposed to oil-containing wastewater. The proposed approach can be explored to enhance the oil-fouling resistance of other polymeric membrane-based electrochemical sensors for use in the oil-polluted environment.

Polymeric membrane ion-selective electrodes with anti-biofouling properties by surface modification of silver nanoparticles

Polymeric membrane ion-selective electrodes (ISEs) have been widely applied in environmental analysis. In many cases of practical relevance, however, when these sensors are applied in complex samples, they could suffer from problems of biofouling (related to microorganisms and biofilms) which may shorten the sensors’ lifetimes and cause the measurement errors. Herein, we describe a robust and effective method to endow the ISE membranes with anti-biofouling properties based on surface modification of silver nanoparticles (Ag NPs). The ISE membrane is modified with the hydrophilic polydopamine (PDA) followed by in-situ formation of Ag NPs. The PDA modification enhances anti-adhesive properties of the ISE membrane by increasing the surface hydrophilicity, while Ag NPs impart strong anti-microbial properties to the membrane. A classical polymeric membrane K+-ISE has been chosen as a model. Compared to the unmodified K+-ISE, the Ag NPs modified K+-ISE exhibits significantly improved anti-biofouling properties in terms of the low viability and adhesion rates of bacteria while retaining its original potentiometric ion response properties. Moreover, Ag NPs on the sensor surface show an excellent long-term stability. We believe that the proposed approach can be extended to improve the anti-fouling abilities of other polymeric membrane-based electrochemical sensors for detection in complex environmental samples.

Thin polymeric membrane ion-selective electrodes for trace-level potentiometric detection

In this work, we describe a novel method to improve the detection limits of the non-classical polymeric membrane ion-selective electrodes (ISEs) which are conditioned with highly discriminated ions instead of primary ions. It is based on a thin-layer ISE membrane with a thickness of 5 μm, which is coated on ordered mesoporous carbon used as solid contact. The diffusion of the primary ion from the surface of the sensing membrane to the bulk of the membrane could be avoided by the proposed thin membrane configuration. Since the detection sensitivity of the non-classical ISEs depends on the accumulation of the primary ion in the interfacial layer of the sensing membrane, a lower detection limit can be obtained. By using the copper ion as a model, the present potentiometric sensor shows a significantly improved detection sensitivity compared to the conventional ISE with a membrane thickness of ca. 200 μm. Low detection limits of 0.29 and 0.53 nM can be obtained in 0.01 and 0.5 M NaCl, respectively. In addition, the proposed sensor exhibits an excellent reversibility by using a neutral proton-selective ionophore incorporated in the thin membrane.

Photoelectric current as a highly sensitive readout for potentiometric sensors.

We report here a general strategy to read out potentiometric signals via a photoelectrochemical method. The photocurrent at a working electrode coated with a ZnSe/r-GO composite can be modulated by a polymeric membrane ion-selective electrode that works as a reference electrode.

Ionic liquid-based reference electrodes for miniaturized ion sensors: What can go wrong?

Ionic liquid-based reference electrodes, especially those containing 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (MeOctIm TFSI), are increasingly used in potentiometric measurements. They replace the conventional salt bridges and establish the reference potential through local partitioning of the ions of the ionic liquid between the reference membrane and the sample solution. Unless the electrochemical cell and the measurement protocol are designed appropriately, these ions can interfere with the response of ion-selective electrodes (ISEs) with polymeric membranes. This work characterizes the effect of MeOctIm TFSI on the response of K+, Na+, and Cl− ISEs with polymeric membranes. The leaching of MeOctIm TFSI from the reference membrane to the sample solution was monitored with UV–vis spectroscopy. The concentration of MeOctIm TFSI in the aqueous phase increased gradually and plateaued at approximately 1.0 mM. Concentrations as low as 100 μM of MeOctIm TFSI caused large changes to the emf (50–150 mV) of K+, Na+, and Cl− ISEs. The presence of 10 μM of TFSI− was enough to cause Donnan failure of K+ ISEs with valinomycin as ionophore (that is, co-extraction of K+ and TFSI− into the sensing membrane). Use of less lipophilic anions such as tetrafluoroborate (BF4-) or triflate (OTf-) postponed the onset of Donnan failure of cation-selective ISEs to higher concentrations of the anion, but decreased the stability in the reference potential and lifetime of the reference electrode. These results imply that although MeOctIm TFSI-based reference electrodes provide sample-independent and stable electrical potentials, they should be used with caution for measurements with polymeric-membrane ISEs, due to strong interference of both MeOctIm+ and TFSI− with measured values of emf.

Solid-Contact Ion-Selective Electrodes Comprising Hydrophobic Redox Buffers

A calibration-free measurement with an ionophore-doped polymeric membrane ion-selective electrode requires that the phase boundary potential at the sample/sensing membrane interface is controlled by the activity of the target ion in the sample of interest, while all other phase boundary potentials in the electrochemical cell are constant and long term stable. Historically, the biggest difficulty lies in establishing a reproducible phase boundary potential at the interface of the sensing membrane and the underlying electron conductor. Efforts over several decades to use conducting polymers as an interlayer between the ion-selective membrane and an underlying electron conductor, such as a metal or carbon, have had limited success. While the performance of such devices has been much improved in terms of light sensitivity and hydrophobicity of the conducting polymer layer, devices that can be considered calibration-free are still elusive. To that end, hydrophobic redox buffers have been introduced as an alternative to conducting polymers. While hydrophilic redox buffers play central roles in all living organisms, control many geological and environmental processes, and are often utilized in the laboratory, buffering of redox potentials in hydrophobic media is a topic that has in the past been overlooked. This presentation will address principles for the use of hydrophobic redox buffers, and it will discuss recent examples of hydrophobic redox buffers suitable for the fabrication of ion-selective electrode membranes that are calibration-free. (1) Redox Buffer Capacity of Ion-Selective Electrode Solid Contacts Doped with Organometallic Complexes, Zhen, X. V.; Rousseau, C. R.; Buhlmann, P. Anal. Chem., 2018, 90, 11000-11007. (2) Paper-Based All-Solid-State Ion-Sensing Platform with a Solid Contact Comprising Colloid-Imprinted Mesoporous Carbon and a Redox Buffer, Hu, J.; Zhao, W.; Bühlmann, P.; Stein, A., ACS Appl. Nano Mat. 2018, 1, 293–301.

Label-Free Potentiometric Ion Flux Immunosensor for Determination of Recombinant Human Myelin Basic Protein: Application to Downstream Purification from Transgenic Milk.

Recombinant human myelin basic protein (rhMBP) produced in the milk of transgenic cows was found exclusively associated with milk caseins. This hindered its direct determination without extensive sample pretreatment. Here, a label-free potentiometric immunosensor was developed and validated for the determination of rhMBP. An ion flux was generated under zero-current based on surface blocking of the polymeric membrane ion-selective electrode by anti-hMBP antibody and tetrabutylammonium bromide as a marker ion. The immunosensor was successfully employed in the quantitative determination of hMBP in the range of 0.10-20.00 μg/mL with a limit of detection of 50.00 ng/mL. The applicability of the passive ion flux immunosensor for determination of target analyte in complex matrices was investigated. Downstream purification of rhMBP from the milk of transgenic cows was achieved using cation exchange chromatography, immobilized metal affinity chromatography, and immunoaffinity chromatography. The specificity of the immunosensor along with matrix effect of milk proteins were demonstrated. Results obtained using the rhMBP immunosensor were further cross-validated using an orthogonal testing protocol assembled of RP-HPLC and SE-HPLC. It should be noted that the proposed ion flux immunosensor provided a feasible and specific tool for monitoring rhMBP concentration/purity, immunogenic activity, and stability. Such approach provides an attractive economic alternative to sophisticated biosensors required for in-process quality control of biopharmaceutical products.

Potentiometric Detection of Listeria monocytogenes via a Short Antimicrobial Peptide Pair-Based Sandwich Assay

Peptide-based sandwich assays are promising tools in molecular detection, but may be restricted by the availability of "pairs" of affinity peptides. Herein, a new potentiometric sandwich assay for bacteria based on peptide pairs derived from an antimicrobial peptide (AMP) ligand is demonstrated. As a model, the original AMP with a well-defined structure for Listeria monocytogenes (LM) can be split into two fragments to serve as the peptide pairs for the sandwich assay. The recognition and binding of the short peptide pairs to the target can be verified by circular dichroism, flow cytometry, fluorometry, and optical microscopy. The potentiometric magnetic bead-based sandwich assay is designed by using horseradish peroxidase as a label. The enzyme can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine with H2O2 to induce a potential change on a polymeric membrane ion-selective electrode. Under optimal conditions, the concentration of LM can be determined potentiometrically in a linear range of 1.0 × 102 to 1.0 × 106 CFU mL-1 with a detection limit of 10 CFU mL-1 (3σ). The proposed sensing strategy expands the applications of peptides in the field of bioassays.

Sequential and Selective Detection of Two Molecules with a Single Solid-Contact Chronopotentiometric Ion-Selective Electrode.

A polymeric membrane ion-selective electrode (ISE) is typically designed for the determination of one specific ion using a conventional method. In this work, we demonstrate a simple, versatile, and sensitive platform for simultaneous detection of two molecules with a single ISE. Under a series of periodic galvanostatic polarization, a solid-contact ISE without ion exchanger properties under zero-current conditions has been successfully used for simultaneous detection of two opposite charged ions with high sensitivity, good selectivity, and fast reversibility. By integration of biorecognition elements with the potentiometric measurement, highly sensitive and selective detection of a broad range of different molecular targets can be predicted. As a proof of concept, a potentiometric genosensor based on magnetic beads-enzyme sandwich assay has been designed for sensitive and selective detection of pathogenic bacteria Escherichia coli O157:H7 and Staphylococcus aureus. Under optimal conditions, two bacteria nucleic acid sequences can be detected simultaneously with high sensitivity and good selectivity by using a single solid-contact potentiometric ISE. The detection limits of Escherichia coli O157:H7 DNA and Staphylococcus aureus DNA are 120 and 54 fM (3σ), respectively. Because of its simplicity, this potentiometric technique based on ISE can be an attractive tool or detector to perform two analyte measurements.

Optical Ion Sensing Platform Based on Potential-Modulated Release of Enzyme.

We report here on an optical ion sensing platform, in which a polymeric membrane ion-selective electrode (ISE) serves as not only a potentiometric transducer for ion activities in the sample solution but also a reference electrode for the potential-modulated release of enzyme from an iron-alginate-horseradish peroxidase (HRP) thin film modified working electrode. The ISE and working electrode are physically separated by a salt bridge. The dissolution of the HRP-embedded thin film can be triggered by the reduction of Fe3+, which is modulated by the potential response of the ISE to the target ion in the sample. The released enzyme induces the oxidation of its substrate mediated by H2O2 to produce a visual color change. With this setup, an optical ion sensing platform for both cations (e.g., NH4+) and anions (e.g., Cl-) can be obtained. The proposed platform provides a general and versatile visual-sensing strategy for ions and allows optical ion sensing in colored and turbid solutions.

English

Surfactants (surface active agents) are substances which incorporate a hydrophobic non-polar and a hydrophilic polar group, and have the tendency to accumulate at the phase boundary. This feature causes the wide application of surfactants. In 2012, the world surfactant market amounted to 27,040 million USD, and with average annual increase of 6.18 %, this amount in 2017 is estimated at 36,518 million USD2. ASs are used as components in cosmetic formulations, household products, industrial cleaners. Their wide application leads to water pollution, and thus endangers the flora and fauna3. The standard method for the determination of anionic surfactants (ASs) in wastewater is MBAS4 (Methylene Blue Active Substances), while twophase titration5 is used for determining AS in commercial products. Both standard methods have many drawbacks (use of carcinogenic organic solvents, difficulties in determination of anionic surfactants in turbid solution, the subjectivity of the person performing the analysis, inability of auto mation...). The limitations of the standard methods for the determination of AS impose the need for continuous development of new surfactant sensors, which are less expensive, miniaturized, easier to use. The Environmental Protection Act of Croatia does not allow the use of surfactants whose biodegradability is less than 90 %6, thus, the determination of low concentrations of AS in effluents is of great importance. Today, there are a number of techniques that are able to detect, identify, and quantify the surfactant in samples of a complex matrix7. The first methods for determination of ASs were developed from simple volumetric methods5, followed by spectrophotometric methods8,9, chromatography10–12, microfluidic techniques such as flow-injection analysis (FIA)13, capillary electrophoresys14, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR)15. The polymeric membrane ion-selective electrodes (ISEs)16–19 successfully replace standard methods for determination of ASs in various samples. ISEs are a simpler and cost-effective alternative to standard methods. ISEs have evolved from the coated-wire electrodes and liquid polymeric membrane electrodes containing ion-pair based sensing material. The widest use have found ISEs with PVC liquid membrane. Low concentrations of ASs can be determined by ISFET sensors20, biosensors21 or ISEs associated with FIA system. The aim of this study was the application of a newly developed screen-printed microsensor, based on dimethyldioctadecylammonium-tetraphenylborate (DDA-TPB) ion pair as a sensing material, for determination of ASs in industrial wastewater. Particular attention was paid to the determination of low concentrations of ASs. Application of a New Potentiometric Sensor for Determination of Anionic Surfactants in Wastewater

All-solid-state polymeric membrane ion-selective miniaturized electrodes based on a nanoporous gold film as solid contact.

A new type of all-solid-state polymeric membrane ion-selective electrodes (ISEs) is developed by using a nanoporous gold (NPG) film as solid contact. The NPG film is in situ formed on the surface of a gold wire electrode by the multicyclic electrochemical alloying/dealloying method. The characteristics of the NPG film, such as the large surface area, high double layer capacitance, and good conductivity, have been demonstrated by cyclic voltammetry and electrochemical impedance spectroscopy. The NPG film offers a well-defined interface between the electronic conductor and the ion-selective membrane. The NPG film-based all-solid-state K(+) ISE shows a stable Nernstian response within the concentration range from 10(-6) to 10(-2) M, and the detection limit is 4.0 × 10(-7) M. The proposed electrode exhibits an improved potential stability with a reduced water layer in comparison with the coated-wire K(+)-ISE, which is due to the bicontinuous electron- and ion-conducting properties of the ionophore-doped polymeric membrane/NPG film interlayer. Unlike the additionally coated intermediate layers as single-use solid contacts, the in situ formed NPG film as solid contact is reusable. This work provides a versatile method for fabricating the robust, reliable, and low-maintenance miniaturized ISEs.