Electrostatic Screening Length in "Soft" Electrolyte Solutions

Disjoining Pressure Decay Length in Room-Temperature Ionic Liquids: A Molecular Simulation Study

We present all-atom molecular simulations to investigate the behavior of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) in negatively charged carbon nanopores of different widths (h = 5÷15 nm) and lengths (l = 4÷10 nm). The goal of our study was to determine how the disjoining pressure varies as a function of the pore width at different lengths and to understand the influence of edge effects. Our results show that the edge effect decreases as the pore length increases. Using an exponential function, we can approximate the disjoining pressure at large pore widths and use this approximation to estimate the decay length that can correlate with the electrostatic screening length. The results agreed well with those of previous experimental studies.

Interactions between Rigid Polyelectrolytes Mediated by Ordering and Orientation of Multivalent Nonspherical Ions in Salt Solutions

Multivalent ions in solutions with polyelectrolytes (PEs) induce electrostatic correlations that can drastically change ion distributions around the PEs and their mutual interactions. Using coarse-grained molecular dynamics simulations, we show how in addition to valency, ion shape and concentration can be harnessed as tools to control rigid like-charged PE-PE interactions. We demonstrate a correlation between the orientational ordering of aspherical ions and how they mediate the effective PE-PE attraction induced by multivalency. The interaction type, strength, and range can thus be externally controlled in ionic solutions. Our results can be used as generic guidelines to tune the self-assembly of like-charged polyelectrolytes by variation of the characteristics of the ions.

Role of a Multivalent Ion-Solvent Interaction on Restricted Mg2+ Diffusion in Dimethoxyethane Electrolytes

The diffusion behavior of Mg²⁺ in electrolytes is not as readily accessible as that from Li⁺ or Na⁺ utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting ²⁵Mg NMR. In MgTFSI₂/DME solutions, "bound" DME (coordinating to Mg²⁺) and "free" DME (bulk) are distinguishable from ¹H NMR. With the exchange rates between them obtained from 2D ¹H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg²⁺) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65-70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C-H bond length. Comparison of the diffusion behaviors of Mg²⁺, TFSI^-, DME, and Li⁺ reveals a relative restriction to Mg²⁺ diffusion that is caused by the long-range interaction between Mg²⁺ and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.

Interplay between Electrolyte-Gated Organic Field-Effect Transistors and Surfactants: A Surface Aggregation Tool and Protecting Semiconducting Layer

Molecular surfactants, which are based on a water-insoluble tail and a water-soluble head, are widely employed in many areas, such as surface coatings or for drug delivery, thanks to their capability to form micelles in solution or supramolecular structures at the solid/liquid interface. Electrolyte-gated organic field-effect transistors (EGOFETs) are highly sensitive to changes occurring at their electrolyte/gate electrode and electrolyte/organic semiconductor interfaces, and hence, they have been much explored in biosensing due to their inherent amplification properties. Here, we demonstrate that the EGOFETs and surfactants can provide mutual benefits to each other. EGOFETs can be a simple and complementary tool to study the aggregation behavior of cationic and anionic surfactants at low concentrations on a polarized metal surface. In this way, we have monitored the monolayer formation of cationic and anionic surfactants at the water/electrode interface with p-type and n-type devices, respectively. On the other hand, the operational stability of EGOFETs has been dramatically enhanced, thanks to the formation of a protective layer on top of the organic semiconductor by exposing it to a high concentration of a surfactant solution (above the critical micelle concentration). Stable performances were achieved for more than 10 and 2 h of continuous operation for p-type and n-type devices, respectively. Accordingly, this work points not only that EGOFETs can be applied to a wider range of applications beyond biosensing but also that these devices can effectively improve their long-term stability by simply treating them with a suitable surfactant.

A Two-Electrode, Double-Pulsed Sensor Readout Circuit for Cardiac Troponin I Measurement

This paper presents a pulse-stimulus sensor readout circuit for use in cardiovascular disease examinations. The sensor is based on a gold nanoparticle plate with an antibody post-modification. The proposed system utilizes gated pulses to detect the biomarker Cardiac Troponin I in an ionic solution. The characteristic of the electrostatic double-layer capacitor generated by the analyte is related to the concentration of Cardiac Troponin I in the solvent. After sensing by the transistor, a current-to-frequency converter (I-to-F) and delay-line-based time-to-digital converter (TDC) convert the information into a series of digital codes for further analysis. The design is fabricated in a 0.18-μm standard CMOS process. The chip occupies an area of 0.92 mm² and consumes 125 μW. In the measurements, the proposed circuit achieved a 1.77 Hz/pg-mL sensitivity and 72.43 dB dynamic range.