Electrical and dynamic properties of non-plasticized potassium selective membranes

Biocompatible and Na+-sensitive thin-film transistor for biological fluid sensing

In this study, we develop a Na⁺-sensitive thin-film transistor (TFT) for a biocompatible ion sensor and investigate its cytotoxicity. A transparent amorphous oxide semiconductor composed of amorphous In-Ga-Zn-oxide (a-InGaZnO) is utilized as a channel of the Na⁺-sensitive TFT, which includes an indium tin oxide (ITO) film as the source and drain electrodes and a Ta₂O₅ thin-film gate, onto which a Na⁺-sensitive membrane is coated. As one of the Na⁺-sensitive membranes, the polyvinyl chloride (PVC) membrane with bis(12-crown-4) as the ionophore used on the TFT sensors shows good sensitivity and selectivity to changes in Na⁺ concentration but has high cytotoxicity owing to the leaching of its plasticizer to the solution; the plasticizer is added to solve and entrap the ionophore in the PVC membrane. On the other hand, a plasticizer-free Na⁺-sensitive membrane, the fluoropolysilicone (FPS) membrane with the bis(12-crown-4) ionophore, also reduces cell viability owing to the leaching of the ionophore. However, the FPS membrane with calix[4]arene as the ionophore on the gate of TFT sensors exhibits not only favorable electrical properties but also the lack of cytotoxicity. Thus, considering structural flexibility of TFTs, a platform based on TFT sensors coated with the Na⁺-sensitive FPS membrane containing calix[4]arene is suitable as a biocompatible Na⁺ sensing system for the continuous monitoring of ionic components in biological fluids such as sweat and tears.

Charge transport in conducting polymers: insights from impedance spectroscopy

This tutorial review gives a brief introduction to impedance spectroscopy and discusses how it has been used to provide insight into charge transport through conducting polymers, particularly when the polymers are used as electrodes for solution studies or the design of electrodes for biomedical applications. As such it provides both an introduction to the topic and references to both classic and contemporary work for the more advanced reader.

Current-polarized ion-selective membranes: The influence of plasticizer and lipophilic background electrolyte on concentration profiles, resistance, and voltage transients

Lipophilic background electrolytes consisting of a lipophilic cation and a lipophilic anion, such as tetradodecylammonium tetrakis(4-chlorophenyl) borate (ETH 500), or bis(triphenylphosphoranylidene) ammonium tetrakis[3,5bis(trifluoromethyl) phenyl] borate (BTPPATFPB) are incorporated into the membranes of ion-selective electrodes (ISEs) to improve the detection limit and selectivity of the electrodes and decrease the resistance of the sensing membrane. In this work, spectroelectrochemical microscopy (SpECM) is used in conjunction with chronopotentiometry to quantify the effects of a lipophilic background electrolyte on the concentration profiles induced inside current-polarized membranes and on the measured voltage transients in chronopotentiometric experiments. In agreement with the theoretical model, the lipophilic background electrolyte incorporated into o-NPOE or DOS plasticized membranes decreases the membrane resistance and thus the contribution of migration in the overall transport across ion-selective membranes. Consequently, it has a significant influence on the changing concentration profiles of the ion-ionophore complex during chronopotentiometric experiments.

Membrane characterization of anion-selective CHEMFETs by impedance spectroscopy

Impedance spectroscopy can be used to determine the influence of several membrane parameters on the membrane resistance of anion selective CHEMFETs. The concentration of the ammonium sites in the membrane, the anion-receptor complex stoichiometry, and the polarity of the membrane matrix are of particular importance. In general the resistance of polysiloxane membranes is higher than that of PVC membranes. However, in polysiloxane membranes the membrane polarity can be influenced by the type or concentration of polar substituents on the polysiloxane chain. Polysiloxane ion-exchange membranes with 25 mol% of polar sulfone substituents exhibit the same conductance as NPOE plasticized PVC membranes. Remarkably, the membrane resistance of cation-selective polysiloxane membranes is much lower and is much less dependent on the substituents.

Ion-selective membranes with low plasticizer content: electroanalytical characterization and biocompatibility studies

High molecular weight poly(vinyl chloride) and aliphatic polyurethane (Tecoflex)-based ion selective membranes, with normal and reduced amounts of plasticizer, as well as without plasticizer, were tested with respect to their analytical properties, their biocompatibility, and cellular responses. The analytical properties of the membranes did not change significantly within a wide range of polymer to plasticizer ratios. However, the membranes with reduced plasticizer content had better adhesive properties, less anion interference, extended life time, and better biocompatibility. Using the cage implant system, the results showed that an increase of plasticizer weight percent in Tecoflex membranes correlated positively with the increase in host inflammatory response up to 14 days of implantation. The results also demonstrated that both PVC and Tecoflex-based ion-selective membranes with the most common membrane composition (1:2 polymer to plasticizer ratio) exhibited a similar acute inflammatory response, but the PVC-based membrane elicited a reduced chronic inflammatory response when compared with the Tecoflex-based membrane.

Impedances of thin and layered systems: cells with even or odd numbers of interfaces

Impedance data, e.g., system responses, from perturbing small amplitude applied sinusoid signals of near DC to high kilohertz frequencies, give chemical information. Analysis of frequency-dependent imaginary and real impedance proceeds from equivalent analog circuit elements to chemical and physical significance determined from many model systems. Already, it is possible to interpret bulk transport processes, surface kinetic effects, diffusion phenomena, and dependencies on the type of contacts: symmetric ion contact, symmetric metal contact or asymmetric metal-ion interfaces, and cell design; even (battery or sensor) and odd numbered (constrained junction or immiscible liquid) interfaces in a system. These analyses cover the chemical origins, locations and meanings of the lumped resistances, capacitances and transmission lines that are introduced by engineers in their strict analog interpretations of the impedance data. Examples cover simple ohmic, simple diffusive behavior, complex behavior with surface interfacial kinetics or surface resistances, and with finite (nonblocking) or infinite (blocking) DC impedance. High and low frequency responses may show so-called constant phase element character that suggests fractal behavior. Low frequency data occasionally appear in the second quadrant of impedance plane plots. These results are caused by negative capacitances and resistances. In this paper, chemical interpretations of analog circuit elements are mainly based on theory and observations of thin cells of electrolytes and solid and liquid films (membranes) that are ionic or mixed ionic/electronic conductors. The information should carry over into thickened, gelled, and tissue electrolyte phases and serve as a basis for medically-oriented, perhaps diagnostic impedance measurement applications already pioneered by Herman Schwan.