Nanopore-Based Electrodes for Quinotrione Detection: Host-Guest-Induced Electrochemical Signal Switching.

Nanopore-based detection techniques, with a wide range of transport properties, exhibit impressive selectivity and sensitivity for analytes. To expand the application of nanoporous sensors, real-time and fast detection of targets, all within a portable device, is highly desired for nanopore-based sensors. In addition, to improve the accuracy of the output signal, more appropriate readout methods also need to be explored. In this manuscript, we describe a nanopore-based electrode, regarded as NAC-P6-PC@AuE, prepared by coupling a pillararene-based nanoporous membrane with an electrochemical impedance measurement method. The fabricated device is demonstrated by exposing pillararene-based receptors to trace amounts of pesticide molecules. NAC-P6-PC@AuE devices exhibit distinguished selectivity to quinotrione, as well as the ability to quantify quinotrione with a limit of quantitation (LOQ) of 10 nM. The mechanism that allows sensing was verified using finite-element simulations and may be explained as host-guest-induced surface charge shielding, which influences the electrochemical response of probe molecules. The applications of this nanopore-based electrode may be extended toward other target molecules by decorating the nanopore surfaces with specifically chosen receptors.

Tunable Nanopore Arrays as the Basis for Ionic Circuits.

There has been considerable interest in preparing ionic circuits capable of manipulating ionic and molecular transport in a solution. This direction of research is inspired by biological systems where multiple pores with different functionalities embedded in a cell membrane transmit external signals and underlie all physiological processes. In this manuscript, we describe the modeling of ion transport through small arrays of nanopores consisting of 3, 6, and 9 nanopores and an integrated gate electrode placed on the membrane surface next to one pore opening. We show that by tuning the gate voltage and strategically placing nanopores with nonlinear current-voltage characteristics, the local signal at the gate affects ionic transport through all nanopores in the array. Conditions were identified when the same gate voltage induced opposite rectification properties of neighboring nanopores. We also demonstrate that an ionic diode embedded in a nanopore array can modulate transport properties of neighboring pores even without a gate voltage. The results are explained by the role of concentration polarization and overlapping depletion zones on one side of the membrane. The modeling presented here is intended to become an inspiration to future experiments to create nanopore arrays that can transduce signals in space and time.

Ionic amplifying circuits inspired by electronics and biology.

Integrated circuits are present in all electronic devices, and enable signal amplification, modulation, and relay. Nature uses another type of circuits composed of channels in a cell membrane, which regulate and amplify transport of ions, not electrons and holes as is done in electronic systems. Here we show an abiotic ionic circuit that is inspired by concepts from electronics and biology. The circuit amplifies small ionic signals into ionic outputs, and its operation mimics the electronic Darlington amplifier composed of transistors. The individual transistors are pores equipped with three terminals including a gate that is able to enrich or deplete ions in the pore. The circuits we report function at gate voltages < 1 V, respond to sub-nA gate currents, and offer ion current amplification with a gain up to ~300. Ionic amplifiers are a logical step toward improving chemical and biochemical sensing, separations and amplification, among others.

Electrokinetic Phenomena in Organic Solvents.

Solid/liquid interfaces play a key role in separation processes, energy storage devices, and transport in nanoscale systems. Nanopores and mesopores with well-defined geometry and chemical characteristics have been a valuable tool to unravel electrochemical properties of interfaces, but the majority of studies have been focused on aqueous solutions. Here, we present experiments and numerical modeling aimed at characterizing effective surface charge of polymer pores in mixtures of water and alcohols as well as in propylene carbonate and acetone. The charge properties of pore walls are probed through analysis of current-voltage curves recorded in the presence of salt concentration gradients. The presence and direction of electro-osmotic flow lead to asymmetric current-voltage curves, with rectification characteristics determined by the polarity of surface charge. The results suggest that the effective surface charge of the pore walls depends not only on the type of solvent but also on the concentration of the electrolyte and voltage. We identified conditions at which polymer pores that are negatively charged in aqueous solutions become positively charged in propylene carbonate and acetone. The findings are of importance for nonaqueous separations, fundamental knowledge on solid/liquid interfaces in organic media, and preparation of porous devices with tunable surface charge characteristics.

Viscosity and Conductivity Tunable Diode-like Behavior for Meso- and Micropores.

Rectifying pores, which transport ions mainly in one direction blocking the ionic flow in the other, were shown to be important in the preparation of chemical sensors, components of ionic circuits, and mimics of biological channels. Ionic rectification has been shown with various engineered systems, but pores with similar opening diameters often rectify to a various uncontrolled extent. In this Letter we present a system of single meso-pores, whose current-voltage curves and rectification can be tuned with great precision via viscosity and conductivity gradients of solutions placed on both sides of the membrane. The mechanism of rectification is based on electroosmotically induced flow, which fills the entire volume of the pore with a single solution from either side of the membrane. The highly predictable rectifying system can find various applications, including measuring viscosity of unknown media and tuning electrokinetic passage of particles.