Electrolyte diodes with weak acids and bases. I. Theory and an approximate analytical solution

Iontronics

Iontronics is an emerging technology based on sophisticated control of ions as signal carriers that bridges solid-state electronics and biological system. It is found in nature, e.g., information transduction and processing of brain in which neurons are dynamically polarized or depolarized by ion transport across cell membranes. It suggests the operating principle of aqueous circuits made of predesigned structures and functional materials that characteristically interact with ions of various charge, mobility, and affinity. Working in aqueous environments, iontronic devices offer profound implications for biocompatible or biodegradable logic circuits for sensing, ecofriendly monitoring, and brain-machine interfacing. Furthermore, iontronics based on multi-ionic carriers sheds light on futuristic biomimic information processing. In this review, we overview the historical achievements and the current state of iontronics with regard to theory, fabrication, integration, and applications, concluding with comments on where the technology may advance.

Ion flow crossing over a polyelectrolyte diode on a microfluidic chip

Key evidences are reported for the rectification mechanism of an aqueous ion diode based on polyelectrolytic plugs on a microfluidic chip by monitoring the ion flow crossing over the junction. The ion flow penetrating the polyelectrolyte junction is visualized by employing a fluorescent chemodosimeter, rhodamine B hydrazide and the pH-dependent dye, carboxy-fluorescein. How hysteresis phenomena, exhibited through the nonlinear behavior of the polyelectrolyte diode, are affected by a variety of parameters (e.g., switching potential, scan rate, and electrolyte concentration) is also investigated. The insights and knowledge from this study provide a crucial foundation for ion control at the iontronic diode in the aqueous phase, leading to more advanced aqueous organic computing devices and more diverse applications for microfluidic logic devices.

Micropatterned polyvinyl butyral membrane for acid-base diodes

Until now, polyvinyl alcohol (PVA) gel cylinders have been used in electrolyte diodes as a connecting element between the acidic and alkaline reservoirs. In this paper, a new connecting element is reported: a breath figure templated polyvinyl butyral (PVB) membrane prepared with dip-coating from a dichloromethane solution of the polymer in a humid atmosphere. The procedure gives a 1.5-2 μm thick membrane with a hexagonal pattern, the average characteristic length of which is 1 μm. After an acidic etching, it was found to be a good connecting element. The voltage-current characteristics and dynamic properties of PVA and PVB were measured and compared. The PVB membrane has a faster response to voltage changes than the PVA gel, but in both cases, there was a slow drift in the current that prevented it from reaching a steady state. Reproducible characteristics can be obtained, however, after the current reaches a well-defined quasi-steady state.

Transient behavior of an electrolytic diode

The transient behavior of an electrolytic diode system was studied. A gel-like electrolytic diode was incorporated in a capillary microfluidic chip. The microfluidic platform guaranteed a constant composition of solutions on the diode boundaries. The current responses of the electrolytic diode to step-like changes of the imposed DC electric voltage were measured. Some of these transients were accompanied by a short-time overshoot of electric current density. In order to explain this phenomenon, a mathematical model of the electrolytic diode system was developed. Dynamical analysis of the model equations confirmed the existence of the electric current overshoots. Because the results of the experimental and the numerical transient studies were quite similar, we have explained the physical meaning of three selected overshoots by means of an analysis of the reaction-transport processes inside the electrolytic diode system. The transient experiments carried out in this study indicate that our physical concept of the electrolytic diode system presented in previous papers is correct.

Electrolyte diodes with weak acids and bases. II. Numerical model calculations and experiments

This is the second part of our work dealing with electrolyte diodes with weak acids and bases. In the first part an approximative analytical solution was derived for the steady-state current-voltage characteristic (CVC) of a reverse-biased diode (a quasi-one-dimensional gel connecting an acidic and an alkaline reservoir), applying either strong or weak electrolytes. An approximative analytical solution is compared here with a numerical solution free of any approximations and with CVCs measured experimentally with both strong and weak electrolytes. It is shown that the deviations between the numerical and analytical solutions are mostly due to assumptions made for the fixed charge concentration profiles. The concept of optimal analytical solution is introduced which does not use such assumptions and applies only the quasielectroneutrality and quasiequilibrium approximations. It is proven that the slope of the CVC based on the optimum analytical solution can be calculated without the complicated derivation of that solution itself. The calculation of that slope is based on the fact that in the optimum analytical solution all currents are inversely proportional to the length if the boundary conditions are held constant and realizing that in the middle part of the gel the only mobile counterions of the fixed ionized groups are hydrogen ions. In the experimental part the apparatus and the preparation of the gel are described together with the CVCs measured with strong and weak electrolytes. From these CVCs the fixed ion concentration in the middle part of the gel can be determined. That fixed ion concentration is 1.96 x 10(-4)M measured with weak electrolytes and 3.48 x 10(-4)M measured with strong electrolytes. The deviation indicates that the strong base causes some hydrolysis of the gel. Finally, possible applications of weak acid-weak base diodes are discussed.