Direct Ion Speciation Analysis with Ion-Selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode

Direct Energy Transfer from a pH Glass Electrode to a Liquid Crystal Display

Self-powered sensors are attractive because the lack of a dedicated battery makes them environmentally friendly and allows them to be more easily miniaturized. Unfortunately, the development of self-powered potentiometric sensors is challenging because only very limited energy can be harvested from this measurement principle. For the first time, the potential of a high impedance glass pH electrode (130 M Ω) is shown here to be directly read out optically. This is accomplished by a liquid crystal display (LCD) as the electrochromic transducer, which changes its transmission upon imposing an external voltage in the range of 2-3 V. Importantly, owing to its low capacitance of about 50 pF, this process requires a very small transient charge on the order of 100 pC, which may be spontaneously imposable even across pH glass electrodes. For the LCD to be turned on, the cell voltage is boosted by additional Zn²⁺/Zn elements placed in series. The LCD is found to give a time-dependent absorbance decrease, which is mitigated by adding a high resistance element to attenuate the associated decay. The approach gives repeatable LCD absorbance values that allows one to directly visualize pH with a precision of about 0.01 pH units. The absorbance value depends inversely on pH in a much wider range (pH 1-13) than what is normally observed with optical sensors while based on the same underlying measurement as a potentiometric pH probe.

Electrochemical modulation enhances the selectivity of peripheral neurostimulation in vivo

SignificanceBioelectronic medicine relies on electrical stimulation for most applications in the peripheral nervous system. It faces persistent challenges in selectively activating bundled nerve fibers. Here, we investigated ion-concentration modulation with ion-selective membranes and whether this modality may enhance the functional selectivity of peripheral nerve stimulation. We designed a multimodal stimulator that could control Ca²⁺ concentrations within a focused volume. Acutely implanting it on the sciatic nerve of a rat, we demonstrated that Ca²⁺ depletion could increase the sensitivity of the nerve to electrical stimulation in vivo. We provided evidence that it selectively influenced individual fascicles of the nerve, allowing selective activation by electrical current. Improved functional selectivity may improve outcomes for important therapeutic modalities.

Quantitative determination of dextran sulfate and pentosan polysulfate and their binding with protamine using chronopotentiometry with polyion-selective electrodes

We report for the first time a chronopotentiometric measurement of polyanions based on localized ion depletion at the sample/membrane interface at a characteristic transition time τ, using polymer membrane polyanion-selective electrodes. Chronopotentiometric transduction of polyions based on the measurement of transition time has analytically more attractive applications compared to the controlled-current reversible pulsed chronopotentiometric transduction based on electromotive force (emf) measurement. This is because traditional polyion-selective electrodes based on emf measurement intrinsically give nonlinear (sigmoidal) calibration curves. While these can be used for indirect determination of polyions via polyanion-polycation titrations, they are not convenient for direct quantitation. However, under chronopotentiometric measurement based on the measurement of transition time, the square root of the transition time τ is linearly related to the concentration of the polyion according to the Sand equation and can be used for a direct calibration-free rapid determination. In this work, we have measured the concentrations of dextran sulfate (DS) and pentosan polysulfate (PPS) using polyanion selective electrodes under chronopotentiometric method where the transition time was measured and controlled-current pulsed chronopotentiometric transductions, where the phase boundary potential (emf) was measured. In addition, the protamine-DS and the protamine-PPS binding ratios have been determined using both transductions. The protamine-PPS binding ratio was determined to be 1.51:1 by the titration method and 1.54:1 by chronopotentiometry. The protamine-DS binding ratio was determined to be 1.37:1 by the titration method and 1.41:1 by chronopotentiometry, showing excellent agreement between the two methods. These simple measurement methods of binding ratios between polysaccharides and polypeptides may become important tools for screening safer and more reliable antidotes for the newer and safer anticoagulants such as Low Molecular Weight Heparins(LMWHs) and also to determine the dosages of antidotes needed to neutralize the anticoagulant activity.

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design

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.

Tunable Optical Sensing with PVC-Membrane-Based Ion-Selective Bipolar Electrodes

We show here that the response of ion-selective membrane electrodes (ISEs) based on traditional PVC membranes can be directly translated to a colorimetric readout by a closed bipolar electrode (BPE) arrangement. Because the resulting optical response is based on the turnover of the redox probe, ferroin, dissolved in a thin layer compartment, it directly indicates the potential change at the ISE in combination with a reference electrode. This class of probes measures ion activity, analogous to their ISE counterparts. Unlike other ion optodes, the response is also fully tunable over a wide concentration range by the application of an external potential and occurs in a compartment that is physically separate from the sample. To allow for the electrical charge to pass across the ion-selective electrodes, the membranes are doped with inert lipophilic electrolyte, ETH 500, but otherwise have an established composition. The observed response behavior correlates well with theory. A wide range of ion-selective membranes are confirmed to work with this readout principle, demonstrating the detection of potassium, sodium, calcium, and carbonate ions. The corresponding sigmodal calibration curve is used for quantitative analysis in a range of samples including commercial beverages and river and lake samples. The data are successfully correlated with atomic emission spectroscopy and direct potentiometry.

A paper based, all organic, reference-electrode-free ion sensing platform

We present a reference-electrode free, all organic K⁺ sensitive ion sensing platform fabricated by simplest means on a plain sheet of paper.