Ion-Selective Potentiometry with Plasma-Initiated Covalent Attachment of Sensing Membranes onto Inert Polymeric Substrates and Carbon Solid Contacts.

The physical delamination of the sensing membrane from underlying electrode bodies and electron conductors limits sensor lifetimes and long-term monitoring with ion-selective electrodes (ISEs). To address this problem, we developed two plasma-initiated graft polymerization methods that attach ionophore-doped polymethacrylate sensing membranes covalently to high-surface-area carbons that serve as the conducting solid contact as well as to polypropylene, poly(ethylene-co-tetrafluoroethylene), and polyurethane as the inert polymeric electrode body materials. The first strategy consists of depositing the precursor solution for the preparation of the sensing membranes onto the platform substrates with the solid contact carbon, followed by exposure to an argon plasma, which results in surface-grafting of the in situ polymerized sensing membrane. Using the second strategy, the polymeric platform substrate is pretreated with argon plasma and subsequently exposed to ambient oxygen, forming hydroperoxide groups on the surface. Those functionalities are then used for the initiation of photoinitiated graft polymerization of the sensing membrane. Attenuated total reflection-Fourier transform infrared spectroscopy, water contact angle measurements, and delamination tests confirm the covalent attachment of the in situ polymerized sensing membranes onto the polymeric substrates. Using membrane precursor solutions comprising, in addition to decyl methacrylate and a cross-linker, also 2-(diisopropylamino)ethyl methacrylate as a covalently attachable H+ ionophore and tetrakis(pentafluorophenyl)borate as ionic sites, both plasma-based fabrication methods produced electrodes that responded to pH in a Nernstian fashion, with the high selectivity expected for ionophore-based ISEs.

Microporous Ag/AgCl on a Titanium Scaffold for Use in Capillary Reference Electrodes.

AgCl-coated silver fabricated with the thermal-electrolytic method can be used to prepare more reproducible reference electrodes than Ag/AgCl prepared with alternative methods such as electrolytic and chemical AgCl deposition or thermal fabrication. However, thermal-electrolytic fabrication requires a scaffold material upon which to build the layers upon. Platinum and rhodium have been used for this purpose as they are mechanically strong and chemically inert, but their cost is prohibitive for wider application. Herein, we report the stability of Ag/AgCl reference electrodes built atop a titanium scaffold using the thermal-electrolytic method and the use of these Ti/Ag/AgCl constructs in capillary-based reference electrodes. Electrochemical characterization shows that the probable presence of small amounts of oxygen at the Ti/Ag interface does not affect the reference electrode performance; in particular, over a wide pH range, the half-cell potential is pH independent. The electrical resistance of the Ti/Ag/AgCl/KCl system is dominated by the charge transfer resistance at the interface of the AgCl to KCl solution but is kept very small by the large AgCl surface area and a high solution concentration of chloride. The resulting high exchange current minimizes the effect of system impurities on the reference half-cell potential. Capillary-based reference electrodes comprising Ti/Ag/AgCl show exceptionally low potential drifts (as low as 0.03 ± 2.01 µV/h) and standard deviations of the potential at or below ±0.5 mV over a 60 h period. These capillary-based reference electrodes are suitable for very small sample volumes while still providing a free-flowing liquid junction that prevents reference electrode contamination.

Functionalizing Carbon Substrates with a Covalently Attached Cobalt Redox Buffer for Calibration-Free Solid-Contact Ion-Selective Electrodes.

With a view to potentiometric sensing with minimal calibration requirements and high long-term stability, colloid-imprinted mesoporous (CIM) carbon was functionalized by the covalent attachment of a cobalt redox buffer and used as a new solid contact for ion-selective electrodes (ISEs). The CIM carbon surface was first modified by electroless grafting of a terpyridine ligand (Tpy-ph) using diazonium chemistry, followed by stepwise binding of Co(II) and an additional Tpy ligand to the grafted ligand, forming a bis(terpyridine) Co(II) complex, CIM-ph-Tpy-Co(II)-Tpy. Half a molar equivalent of ferrocenium tetrakis(3-chlorophenyl)borate was then used to partially oxidize the Co(II) complex. Electrodes prepared with this surface-attached CIM-ph-Tpy-Co(III/II)-Tpy redox buffer as a solid contact were tested as K+ sensors in combination with valinomycin as the ionophore and Dow 3140 silicone or plasticized poly(vinyl chloride) (PVC) as the matrixes for the ion-selective membrane (ISM). This solid contact is characterized by a redox capacitance of 3.26 F/g, ensuring a well-defined interfacial potential that underpins the transduction mechanism. By use of a redox couple as an internal reference element to control the phase boundary potential at the interface of the ISM and the CIM carbon solid contact, solid-contact ion-selective electrodes (SC-ISEs) with a standard deviation of E° as low as 0.3 mV for plasticized PVC ISMs and 3.5 mV for Dow 3140 silicone ISMs were obtained. Over 100 h, these SC-ISEs exhibit an emf drift of 20 µV/h for plasticized PVC ISMs and 62 µV/h for silicone ISMs. The differences in long-term stability and reproducibility between electrodes with ISMs comprising either a plasticized PVC or silicone matrix offer valuable insights into the effect of the polymeric matrix on sensor performance.

Improvement of the Upper Detection Limit of Ionophore-Based H+-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses.

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H+-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions. This work shows that coextraction of H+ and a sample anion into the sensing membrane alone does not explain these super-Nernstian responses, even when membrane-internal diffusion potentials are taken into account. Instead, these super-Nernstian responses are explained by the formation of complexes between that anion and at least two protonated ionophore molecules. As demonstrated by experiments and explained with quantitative phase boundary models, the apparently super-Nernstian responses at low pH can be eliminated by restricting the molecular ratio of ionophore and ionic sites. Notably, this conclusion results in recommendations for the optimization of sensing membranes that, in some instances, will conflict with previously reported recommendations from the ionic site theory for the optimization of the lower detection limit. This mechanistic insight is key to maximizing the response range of these ionophore-based ISEs.

Effect of Ion Identity on Capacitance and Ion-to-Electron Transduction in Ion-Selective Electrodes with Nanographite and Carbon Nanotube Solid Contacts.

The use of large surface area carbon materials as transducers in solid-contact ion-selective electrodes (ISEs) has become widespread. Desirable qualities of ISEs, such as a small long-term drift, have been associated with a high capacitance that arises from the formation of an electrical double layer at the interface of the large surface area carbon material and the ion-selective membrane. The capacitive properties of these ISEs have been observed using a variety of techniques, but the effects of the ions present in the ion-selective membrane on the measured value of the capacitance have not been studied in detail. Here, it is shown that changes in the size and concentration of the ions in the ion-selective membrane as well as the polarity of the polymeric matrix result in capacitances that can vary by up to several hundred percent. These data illustrate that the interpretation of comparatively small differences in capacitance for different types of solid contacts is not meaningful unless the composition of the ion-selective membrane is taken into account.

Covalently attached ionophores extend the working range of potentiometric pH sensors with poly(decyl methacrylate) sensing membranes.

The pH working range of solid-contact ion-selective electrodes (ISEs) with plasticizer-free poly(decyl methacrylate) sensing membranes is shown to be expanded by covalent attachment of H+ ionophores to the polymeric membrane matrix. In situ photopolymerization not only incorporates the ionophores into the polymer backbone, but at the same time also attaches the sensing membranes covalently to the underlying inert polymer and nanographite solid contact, minimizing sensor drift and preventing failure by membrane delamination. A new pyridine-based H+ ionophore, 3-(pyridine-3-yl)propyl methacrylate, has lower basicity than trialkylamine ionophores and expands the upper detection limit. This reduces in particular the interference from hydrogen phthalate, which is a common component of commercial pH buffers. Moreover, the lower detection limit is improved by replacing the CH2CH2 spacer of previously reported dialkylaminoethyl methacrylates with a (CH2)10 spacer, which increases its basicity. Notably, for the more basic and highly cation-selective ionophore 10-(diisopropylamino)decyl methacrylate, the extent of counterion interference from hydrogen phthalate shifted the upper detection limit to lower pH by nearly one pH unit when the crosslinker concentration was decreased.

Influence of the Composition of Plasticizer-Free Silicone-Based Ion-Selective Membranes on Signal Stability in Aqueous and Blood Plasma Samples.

Solid-contact ion-selective electrodes (SC-ISEs) in direct long-term contact with physiological samples must be biocompatible and resistant to biofouling, but most wearable SC-ISEs proposed to date contain plasticized poly(vinyl chloride) (PVC) membranes, which have poor biocompatibility. Silicones are a promising alternative to plasticized PVC because of their excellent biocompatibility, but little work has been done to study the relationship between silicone composition and ISE performance. To address this, we prepared and tested K+ SC-ISEs with colloid-imprinted mesoporous (CIM) carbon as the solid contact and three different condensation-cured silicones: a custom silicone prepared in-house (Silicone 1), a commercial silicone (Dow 3140, Silicone 2), and a commercial fluorosilicone (Dow 730, Fluorosilicone 1). SC-ISEs prepared with each of these polymers and the ionophore valinomycin and added ionic sites exhibited Nernstian responses, excellent selectivities, and signal drifts as low as 3 µV/h in 1 mM KCl solution. All ISEs maintained Nernstian response slopes and had only very slightly worsened selectivities after 41 h exposure to porcine plasma (log KK,Na values of -4.56, -4.58, and -4.49, to -4.04, -4.00, and -3.90 for Silicone 1, Silicone 2, and Fluorosilicone 1, respectively), confirming that these sensors retain the high selectivity that makes them suitable for use in physiological samples. When immersed in porcine plasma, the SC-ISEs exhibited emf drifts that were still fairly low but notably larger than when measurements were performed in pure water. Interestingly, despite the very similar structures of these matrix polymers, SC-ISEs prepared with Silicone 2 showed lower drift in porcine blood plasma (-55 µV/h, over 41 h) compared to Silicone 1 (-495 µV/h) or Fluorosilicone 1 (-297 µV/h).

Ion-Selective Electrodes With Sensing Membranes Covalently Attached to Both the Inert Polymer Substrate and Conductive Carbon Contact.

The use of solid-contact ion-selective electrodes (ISEs) is of interest to many clinical, environmental, and industrial applications. However, upon extended exposure to samples and under thermal and mechanical stress, adhesion between these membranes and underlying substrates often weakens gradually. Eventually, this results in the formation of a water layer at the interface to the underlying electron conductor and in delamination of the membrane from the electrode body, both major limitations to long-term monitoring. To prevent these problems without increasing the complexity of design with a mechanical attachment, we use photo-induced graft polymerization to simultaneously attach ionophore-doped crosslinked poly(decyl methacrylate) sensing membranes covalently both to a high surface area carbon as ion-to-electron transducer and to inert polymeric electrode body materials (i.e., polypropylene and poly(ethylene-co-tetrafluoroethylene)). The sensors provide high reproducibility (standard deviation of E0 of 0.2 mV), long-term stability (potential drift 7 µV h-1 over 260 h), and resistance to sterilization in an autoclave (121 °C, 2.0 atm for 30 min). For this work, a covalently attached H+ selective ionophore was used to prepare pH sensors with advantages over conventional pH glass electrodes, but similar use of other ionophores makes this approach suitable to the fabrication of ISEs for a variety of analytes.

The Effect of Paper on the Detection Limit of Paper-Based Potentiometric Chloride Sensors.

While paper is an excellent material for use in many other portable sensors, potentiometric paper-based sensors have been reported to perform worse than conventional rod-shaped electrodes, in particular in view of limits of detection (LODs). Reported here is an in-depth study of the lower LOD for Cl- measurements with paper-based devices comprising AgCl/Ag transducers. Contamination by Cl- from two commonly used device materials─a AgCl/Ag ink and so-called ashless filter paper─was found to increase the concentration of Cl- in paper-contained samples far above what is expected for the spontaneous dissolution of the transducer's AgCl, thereby worsening lower LODs. In addition, for the case of Ag+, the commonly hypothesized adsorption of metal cations onto filter paper was found not to significantly affect the performance of AgCl/Ag transducers. We note that in the context of chemical analysis, metal impurities of paper are often mentioned in the literature, but Cl- contamination of paper has been overlooked.

Hydrogels Doped with Redox Buffers as Transducers for Ion-Selective Electrodes.

Solid-contact ion-selective electrodes (ISEs) with an unintentional water layer between the sensing membrane and underlying electron conductor are well known to suffer from potential drift caused by the instability of the phase boundary potential between the sensing membrane and the water layer with its uncontrolled ionic composition. The reproducibility and long-term emf stability of ISEs with a miniaturized inner filling solution comprising a hydrogel and a hydrophilic electrolyte have not been studied as thoroughly. Here, such devices are discussed with a view to electrode-to-electrode reproducibility, using both hydrophilic ion-exchange and plasticized PVC membranes, along with a hydrophilic redox buffer composed of ferrocyanide and ferricyanide to control the potential between the hydrogel and the underlying electron conductor. With plasticized PVC sensing membranes, these electrodes showed an E0 reproducibility of ±1.1 mV or better, while with hydrophilic ion-exchange membranes, this variability was slightly larger. Long-term drifts were also assessed with both membranes, and the effect of osmotic pressure on drift was shown to be insignificant for the PVC membranes and very small at most for the hydrophilic membranes.

Solid-Contact pH Sensor with Covalent Attachment of Ionophores and Ionic Sites to a Poly(decyl methacrylate) Matrix.

With a view to improving the sensor lifetime, solid-contact ion-selective electrodes (ISEs) were prepared with a plasticizer-free and cross-linked poly(decyl methacrylate) matrix, to which only the ionic sites, only the ionophore, or both the ionic sites and ionophore were covalently attached. In earlier work with covalently attached ionophores or ionic sites, it was difficult to discount the presence of ionophores or ionic site impurities that were not covalently attached to the polymer backbone because the reagents used to introduce the ionophore or ionic sites had high hydrophobicities. In this work, we deliberately chose readily available hydrophilic reagents for the introduction of covalently attached H+ ionophores with tertiary amino groups and covalently attached sulfonate groups as ionic sites. This simplified the synthesis and made it possible to thoroughly remove ionophores and ionic sites not covalently attached to the polymer backbone. Our results confirm the expectation that hydrophobic ISE membranes with both covalently attached ionophores and ionic sites have impractically long response times. In contrast, ISEs with either covalently attached H+ ionophores or covalently attached ionic sites responded to pH with quick Nernstian responses and high selectivity. Both conventional plasticized poly(vinyl chloride) (PVC)-based ISEs and the new poly(decyl methacrylate) membranes were exposed to 90 °C heat for 2 h, 10% ethanol for 1 day, or undiluted blood serum for 5 days. In all three cases, the poly(decyl methacrylate) ISEs exhibited properties superior to conventional PVC-based ISEs, confirming the advantages of the covalent attachment.

Solid-Contact Ion-Selective and Reference Electrodes Covalently Attached to Functionalized Poly(ethylene terephthalate).

Numerous ion-selective and reference electrodes have been developed over the years. Following the need for point-of-care and wearable sensors, designs have transitioned recently from bulky devices with an aqueous inner filling solution to planarizable solid-contact electrodes. However, unless the polymeric sensing and reference membranes are held in place mechanically, delamination of these membranes from the underlying solid to which they adhere physically limits sensor lifetime. Even minor external mechanical stress or thermal expansion can result in membrane delamination and, thereby, device failure. To address this problem, we designed a sensing platform based on poly(ethylene terephthalate) substrates to which polyacrylate-based sensing and polymethacrylate-based reference membranes are attached covalently. Ion-selective membranes with covalently attached or freely dissolved ionophore- and ionic-liquid-doped reference membranes can be directly photopolymerized onto surface-functionalized poly(ethylene terephthalate), resulting in the formation of covalent bonds between the underlying substrate and the attached membranes. H+- and K+-selective electrodes thus prepared exhibit highly selective responses with the theoretically expected (Nernstian) response slope, and reference electrodes provide sample-independent reference potentials over a wide range of electrolyte concentrations. Even repeated mechanical stress does not result in the delamination of the sensing and reference membranes, leading to electrodes with much improved long-term performance. As demonstrated for poly(ethylene-co-cyclohexane-1,4-dimethanol terephthalate) (PETG), this approach may be expanded to a wide range of other polyester, polyamide, and polyurethane platform materials. Covalent attachment of sensing and reference membranes to an inert plastic platform material is a very promising approach to a problem that has plagued the field of ion-selective electrodes and field effect transistors for over 30 years.

More than a Liquid Junction: Effect of Stirring, Flow Rate, and Inward and Outward Electrolyte Diffusion on Reference Electrodes with Salt Bridges Contained in Nanoporous Glass.

The Henderson equation is usually used to calculate liquid-junction potentials between miscible electrolyte solutions. However, the potentials of reference electrodes that comprise an electrolyte-filled nanoporous glass frit may also be affected by charge screening. As reported previously, when the Debye length approaches or surpasses the glass pore diameter, reference potentials depend on the composition of the bridge electrolyte, the pore size of the frit, and the concentration of electrolyte in the sample. We report here that stirring of samples may alter the reference potential as it affects the electrolyte concentration in the section of the nanoporous glass frit that is facing the sample solution. When the flow rate of bridge electrolyte into the sample is small, convective mass transport of sample into the nanoporous frit occurs. The depth of penetration into the frit is only a few nanometers but, despite the use of concentrated salt bridges, this is enough to affect the extent of electrostatic screening when samples of low ionic strength are measured. Mixing of sample and salt bridge solutions-and in particular penetration of sample components into the frit-was optically monitored by observation of a deeply colored Fe[(SCN)(H2O)5]2+ complex that formed in situ exclusively in the region where the sample and salt bridge mixed. Importantly, because flow through nanoporous frits is very slow, mass transport through these frits is dominated by diffusion. Consequently, over as little as 1 h, reference electrode frits with low flow rates become contaminated with sample components and undergo depletion of electrolyte within the frit to a depth of several millimeters, which can negatively affect subsequent experiments.

Potentiometric Selectivities of Ionophore-Doped Ion-Selective Membranes: Concurrent Presence of Primary Ion or Interfering Ion Complexes of Multiple Stoichiometries.

The selectivities of ionophore-doped ion-selective electrode (ISE) membranes are controlled by the stability and stoichiometry of the complexes between the ionophore, L, and the target and interfering ions (I zi and J zj, respectively). Well-accepted models predict how these selectivities can be optimized by selection of ideal ionophore-to-ionic site ratios, considering complex stoichiometries and ion charges. These models were developed for systems in which the target and interfering ions each form complexes of only one stoichiometry. However, for a few ISEs, the concurrent presence of two primary ion complexes of different stoichiometries, such as IL zi and IL2 zi, was reported. Indeed, similar systems were probably often overlooked and are, in fact, more common than the exclusive formation of complexes of higher stoichiometry unless the ionophore is used in excess. Importantly, misinterpreted stoichiometries misguide the design of new ionophores and are likely to result in the formulation of ISE membranes with inferior selectivities. We show here that the presence of two or more complexes of different stoichiometries for a given ion may be inferred experimentally from careful interpretation of the potentiometric selectivities as a function of the ionophore-to-ionic site ratio or from calculations of complex concentrations using experimentally determined complex stabilities. Concurrent formation of JL zj and JL2 zj complexes of an interfering ion is shown here to shift the ionophore-to-ionic site ratio that provides the highest selectivities. Formation of IL n-1 zi and IL n zi complexes of a primary ion is less of a concern because an optimized membrane typically contains an excess of ionophore, but lower than expected selectivities may be observed if the stepwise complex formation constant, KILn, is not sufficiently large and the ionophore-to-ionic site ratio does not markedly exceed n.

Redox Buffer Capacity of Ion-Selective Electrode Solid Contacts Doped with Organometallic Complexes.

While ion-selective electrodes (ISEs) with inner filling solutions are used widely, solid-contact ISEs are better suited for miniaturization and mass manufacturing. Calibration-free measurements with such electrodes require the reproducible control of the phase boundary potential between the ion-selective membrane and the underlying electron conductor. The most promising approach to achieve this goal is based on redox buffers incorporated into the ion-selective membrane. Here we introduce the theory and present experimental data for Co(III), Co(II), Ru(II), Fe(II), and Os(II) compounds that show quantitatively how the phase boundary potential at a solid contact doped with redox-active compounds is affected by weighing errors, reagent impurities, and redox-active interferents. Perhaps surprisingly, theory predicts that there is only a minimal dependence of the phase boundary potential on the ratio of the concentrations of a pure oxidized and a pure reduced compounds if those two compounds are not a redox couple. However, theory predicts that even small redox-active impurities of those compounds shift the phase boundary potential drastically. Experimentally, a surprisingly good in-batch reproducibility was observed by us and others for solid contacts prepared to contain either only the reduced or only the oxidized species of a redox couple. This can be explained by redox-active impurities and is unlikely to be repeatable when different suppliers of reagents are used or long-term experiments are performed. This work confirms that the preferred approach to calibration-free sensing is based on redox buffers that comprise the reduced and oxidized species of a redox couple in well-controlled concentrations.

Electrochemical Impedance Spectroscopy of Ion-Selective Membranes: Artifacts in Two-, Three-, and Four-Electrode Measurements.

Electrochemical impedance spectroscopy is frequently used to characterize, optimize, and monitor ion-selective membranes. However, because of the relatively high resistance of ion-selective membranes, their impedance spectra often contain artifacts that can cause misinterpretation. While in the high-frequency range artifacts are often readily identifiable by the occurrence of inductive features or negative resistances, artifacts are easy to overlook in the low-frequency range, where telltale characteristics are typically missing. Some artifacts can be avoided by the use of two-electrode cells, but this experimental design makes it hard to distinguish the impedance of the ion-selective membrane from that of the measuring electrodes. This work shows that experimental data can be analyzed accurately with the use of models that account for the capacitive leakage present in the reference channels of the impedance spectrometer. To test these models, valinomycin-doped K+-selective membranes were studied by electrochemical impedance spectroscopy with two-, three-, and four-electrode cells, using several measuring electrodes with low to high impedances. The models were found to correctly predict experimental data and provide an intuitive understanding of the cause of the impedance artifacts. This understanding can be applied to design electrochemical impedance spectroscopy experiments of ion-selective membranes with three- and four-electrode cells that minimize artifacts.

Avoiding Errors in Electrochemical Measurements: Effect of Frit Material on the Performance of Reference Electrodes with Porous Frit Junctions.

In many commercially available and in-house-prepared reference electrodes, nanoporous glass frits (often of the brand named Vycor) contain the electrolyte solution that forms a salt bridge between the sample and the reference solution. Recently, we showed that in samples with low ionic strength, the half-cell potentials of reference electrodes comprising nanoporous Vycor frits are affected by the sample and can shift in response to the sample composition by more than 50 mV (which can cause up to 900% error in potentiometric measurements). It was confirmed that the large potential variations result from electrostatic screening of ion transfer through the frit due to the negatively charged surfaces of the glass nanopores. Since the commercial production of porous Vycor glass was recently discontinued, new materials have been used lately as porous frits in commercially available reference electrodes, namely frits made of Teflon, polyethylene, or one of two porous glasses sold under the brand names CoralPor and Electro-porous KT. In this work, we studied the effect of the frit characteristics on the performance of reference electrodes, and show that the unwanted changes in the reference potential are not unique to electrodes with Vycor frits. Increasing the pore size in the glass frits from the <10 nm into the 1 µm range or switching to polymeric frits with pores in the 1 to 10 µm range nearly eliminates the potential variations caused by electrostatic screening of ion transport through the frit pores. Unfortunately, bigger frit pores result in larger flow rates of the reference solution through the pores, which can result in the contamination of test solutions.

One-dimensional ionic self-assembly in a fluorous solution: the structure of tetra-n-butylammonium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate in perfluoromethylcyclohexane by small-angle neutron scattering (SANS).

Fluorous liquids are the least polarizable condensed phases known, and their nonpolar members form solutions with conditions the closest to being in vacuo. A soluble salt consisting of a large fluorophilic anion, tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and its counterion, tetra-n-butylammonium, dissolved in perfluoromethylcyclohexane produces ionic solutions with extremely low conductivity. These solutions were subjected to small-angle neutron scattering (SANS) to ascertain the solute structure. At concentrations of 9% mass fraction, the fluorophilic electrolyte forms straight, long (>160 Å) self-assembled structures that are, in essence, long, homogeneous cylinders. Molecular models were made assuming a requirement for electroneutrality on the shortest length scale possible. This shows a structure formed from a stack of alternating anions and cations, and the structures fit the experimental scattering well. At the lower concentration of 1%, the stacks of ion pairs are shorter and eventually break up to form solitary ion pairs in the solution. These characteristics suggest such conditions provide an interesting new way to form long, self-assembling ionic nanostructures with single-molecule diameters in free solution onto which various moieties could be attached.

A Disposable Planar Paper-Based Potentiometric Ion-Sensing Platform.

Ion-selective electrodes (ISEs) are widely used tools for fast and accurate ion sensing. Herein their design is simplified by embedding a potentiometric cell into paper, complete with an ISE, a reference electrode, and a paper-based microfluidic sample zone that offer the full function of a conventional ISE setup. The disposable planar paper-based ion-sensing platform is suitable for low-cost point-of-care and in-field testing applications. The design is symmetrical and each interfacial potential within the cell is well defined and reproducible, so that the response of the device can be theoretically predicted. For a demonstration of clinical applications, paper-based Cl(-) and K(+) sensors are fabricated with highly reproducible and linear responses towards different concentrations of analyte ions in aqueous and biological samples. The single-use devices can be fabricated by a scalable method, do not need any pretreatment prior to use, and only require a sample volume of 20 µL.

Hierarchically Porous Polymer Monoliths by Combining Controlled Macro- and Microphase Separation.

The ability to tune polymer monolith porosity on multiple length scales is desirable for applications in liquid separations, catalysis, and bioengineering. To this end, we have developed a facile synthetic route to nanoporous polymer monoliths based on controlled polymerization of styrene and divinylbenzene from a poly(lactide) macro-chain transfer agent in the presence of nonreactive poly(ethylene oxide) (PEO). Simple variations in the volume fraction and/or molar mass of PEO lead to either polymerization-induced microphase separation or simultaneous macro- and microphase separation. These processes dictate the resultant morphology and allow for control of the macro- and microstructure of the monoliths. Subsequent selective etching produces monoliths with morphologies that can be tailored from mesoporous, with control over mesopore size, to hierarchically meso- and macroporous, with percolating macropores. This convenient synthetic route to porous polymer monoliths has the potential to be useful in applications where both rapid mass transport and a high surface area are required.