Ion channels, such as gramicidin A, selectively facilitate the transport of ions across biological and synthetic membranes. The conductance properties of ion channels are frequently characterized in synthetic bilayer lipid membranes (BLMs). The instability of BLMs has seriously limited the range of applications for these structures, and tethered bilayer lipid membranes (tBLMs) have addressed the problem through tethering many of the membrane components to a solid surface. In the present study, thin gold substrates have been used to tether thiol-and disulfide-terminated membrane components to form a tBLM electrode to provide a reservoir for ions. This study reports on the ion selectivity and apparent permeability of gramicidin channels in such tethered bilayer membranes. The investigations using electrical impedance spectroscopy indicated that the magnitude of ionic conductance varies substantially in reservoirs with different chemical structures. This study addressed the effect of changing ionic concentration, the effect of changing the species in the bulk solution above the membrane, and the influence of the chemical structure of the reservoir tethers. The effect of two-dimensional packing on membrane conductance was also investigated. The present observations suggested that (a) the reservoir region resistivity has a major influence on the overall conductivity of the membrane and in some instances can dominate conduction, (b) the conduction behavior is nonlinear and exhibits saturation with increasing electrolyte concentration, and (c) that ion pairing in the reduced dielectric ( ∼50) reservoir region is the likely basis for the latter effect. The inferred limiting ionic mobilities of alkali chloride species in the membrane reservoir regions were 3-4 orders of magnitude less than in aqueous solution, indicating that the reservoirs resembled hydrated polymer gels.

The use of polar linkers to tether lipid bilayer membranes to a gold substrate results in a hydrophilic layer between the membrane and the gold surface. The tethering of lipid bilayer membranes to gold substrates using tetraethylene glycol chains results in a polar layer between the membrane and the gold surface. This region may sequester ions and can act as a reservoir for ions transported across the tethered lipid membrane. In the present article, we report on the electrical properties of this ionic reservoir. In particular, the Stern model of ionic distribution is used to describe the interfacial capacitance. The model combines a surface adsorption layer (Helmholtz model) and a dynamic diffuse layer of ions (Gouy-Chapman model) to describe the interfacial capacitance. This model is used to interpret data from measurements of the interfacial capacitance obtained over a range of ionic species and concentrations. Four analogues of the sulfur-tetraethylene glycol tethers have been investigated. These studies show the effects of varying the structure of the linker group and of introducing a passivation layer adjacent to the gold. Studies were also made of the influence of spacer molecules included to vary the "in-plane" two-dimensional packing. The effect of applying a dc bias potential between an external reference electrode and the gold surface was also studied. These measurements were carried out using ac impedance spectroscopy on bilayers assembled using the method of Cornell et al. 6 Most data are successfully modeled as a constant Helmholtz capacitance in series with a diffuse region capacitance that depends on ionic concentration. The dependence on ionic concentration has been modeled by the Gouy-Chapman formalism. At low ionic concentrations (<20 mM), the model becomes inadequate. Deviation from the model also occurs at higher concentrations for more tightly packed membranes, in the absence of tethered spacer molecules. According to the model at very low concentrations of electrolyte, the ionic Debye length intrudes into the hydrocarbon region of the bilayer, violating the Gouy-Chapman assumption of a uniform dielectric medium in the diffuse double layer. The Helmholtz capacitance is insensitive to potential and ionic concentration. This is consistent with Helmholtz capacitance being defined by a hard sphere distance of closest approach of the ions to the gold interface over the range of concentrations studied here. The model suggests that the application of a dc potential alters the permittivity of the diffuse region as a result of water and ions being transported into the reservoir. However, the effective relative permittivity in the reservoir region varies only from 27 to 54, suggesting the reservoir has properties more akin to a dense hydrated gel with restricted ionic mobility than to a bulk electrolyte.

The learning analytics community has matured significantly over the past few years as a middle space where technology and pedagogy combine to support learning experiences. To continue to grow and connect these perspectives, research needs to move beyond the level of basic support actions. This means exploring the use of data to prove richer forms of actions, such as personalized feedback, or hybrid approaches where instructors interpret the outputs of algorithms and select an appropriate course of action. This paper proposes the following three contributions to connect data extracted from the learning experience with such personalized student support actions: 1) a student-instructor centred conceptual model connecting a representation of the student information with a basic set of rules created by instructors to deploy Personalized Learning Support Actions (PLSAs); 2) a software architecture based on this model with six categories of functional blocks to deploy the PLSAs; and 3) a description of the implementation of this architecture as an open-source platform to promote the adoption and exploration of this area. Notes for Practice• The report draws on research findings related to the effect of personalized feedback on student satisfaction and academic performance (Pardo, Jovanović, Dawson, Gašević, & Mirriahi, 2018).• The main contribution is the description of the design and implementation of an open source platform for researchers and practitioners to connect data with personalized learning support actions.• The area of learning analytics needs tools such as the one described in this document to serve as a vehicle to exchange insights among researchers and practitioners.• This is an example of the note for practice and research