New developments in liquid/liquid interface transport

Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes

The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E_1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k⁰), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i-t amperometry. The E_1/2 of ACh transfer at the NPOE/water nanoITIES was -0.342 ± 0.009 V versus the E_1/2 of tetrabutylammonium (TBA⁺). The limit of detection of ACh at the NPOE/water nanoITIES was 37.1 ± 1.5 μM for an electrode with a radius of ∼127 nm. We also determined the ion-transfer kinetics parameters, α and k⁰, of TBA⁺ at the NPOE/water nanoITIES by fitting theoretical cyclic voltammograms to experimental voltammograms. This work lays the basis for future cellular studies using ACh detection at the nanoscale and for studies to detect other analytes. The NPOE/water ITIES offers a potential window distinct from that of the 1,2-dichloroethane (DCE)/water ITIES. This unique potential window would offer the ability to detect analytes that are not easily detected at the DCE/water ITIES.

Selectivity behavior and multianalyte detection capability of voltammetric ionophore-based plasticized polymeric membrane sensors

The current response features ofvoltammetric ion-selective polymeric membranes doped with neutral ionophores in view of practical sensor development are elucidated. The membranes are designed to extract ions only under applied external potentials and interrogated by normal-pulse voltammetry and pulsed amperometry. They contain two polarizable interfaces to avoid loss of lipophilic ions at the sample side and to maximize the available potential window. A simple theoretical model is developed that describes the observed current at the end of an uptake pulse to the applied membrane potential, which is the sum of both boundary potentials (at the sample and inner electrolyte side) and the membrane internal iR drop. The results describe how the selectivity of the resulting sensor must be dependent on the applied potential. Evidently, the role of the applied potential is akin to incorporating lipophilic cationic and anionic sites with potentiometric ionophore-based membranes, which are well known to considerably affect membrane selectivity and to define the charge type of the assessed ions. This has important implications for sensor design, as the applied cell potential can be used to tune sensor selectivity. Theory also explains the role of the inner electrolyte on sensor behavior. A maximum measuring range is expected with ions in the inner electrolyte that are difficult to extract into the membrane. This corresponds to Kihara's experimental results and contrasts to common ion-selective electrode practice, where a salt of the analyte ion is normally present in the inner electrolyte. Separate and mixed solution experiments with membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and the lithium ionophore ETH 1810 agree very well with theoretical expectations. Multianalyte detection capability with a single sensing membrane is demonstrated in a selectivity-modifying pulsed amperometric detection mode, where each applied voltage yields a different practical selectivity of the sensor. The sensor is altered from being sodium to potassium selective as the magnitude of the applied potential is repetitively varied within the pulse sequence. The sensors show high long-term stability under continuous measuring conditions over 15 h.

Voltammetric and amperometric transduction for solvent polymeric membrane ion sensors

This paper describes basic response features of solvent polymeric membrane ion sensors with voltammetric and amperometric transduction. The model systems used here contain no ionophore for simplicity reasons. Reasonable simplifications of the theory are introduced that allow one to understand the response mechanism in view of a practical application of these sensors. It is shown that ion-sensing membranes preferentially contain no ion-exchanger properties in order to function optimally in a voltammetric mode. As with the systems studied by Kihara, both liquid-polymer interfaces of the membrane are preferably polarizable. Specifically, they contain the highly lipophilic electrolyte tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) in the membrane to improve lifetime, increase the magnitude of the potential window, and prohibit exchange reactions with sample ions. An ohmic behavior that is associated with an assisted electrolyte-transfer process is observed only above a threshold potential which can be quantitatively predicted by theory. The threshold potential depends on the nature and activity of sample anions and cations in the sample and inner filling solution of the membrane electrode. Within the experimental conditions discussed in this paper, these sensors seem to measure sample ion activities, not concentrations, since the rate-limiting step is the diffusion of extracted ions away from the interface into the membrane bulk. Similarly, no effect of sample stirring on the measured current is observed. This contrasts to work done on liquid-liquid electrolyte-transfer reactions, where large diffusion coefficients in the organic phase often lead to substantial sample depletion effects. The detection of anions and cations with the same membrane is demonstrated in a cyclic voltammetric mode. Direct continuous detection of one type of anion is accomplished by pulsed amperometry to ensure a rapid, repetitive renewal of the membrane composition between measurements.