Electrochemical Potential Gradient Research Articles

Addressing the challenge of solution gating in biosensors based on field-effect transistors

Transistor-based biosensing (BioFET) is a long-enduring vision for next generation medical diagnostics. The study addresses a challenge associated with the BioFET solution gating. The standard BioFET sensing measurement involves sweeping of the solution gate (Vsol) with a concurrent measurement of the source-drain current (IDS). This IDS-Vsol sweep poses a great challenge, as Vsol does not only determine IDS, but also determines the pH levels, ion concentrations, and electric fields at the sensing area double layer accommodating the biomolecules. Therefore, inevitably, an IDS-Vsol sweep implies that the sensing area double layer is not in an electrochemical equilibrium, but rather in a continuous transient state as electrochemical potential gradients induce transient ion currents continuously affecting double layer hosting the biomolecules and the biological interactions. This challenge calls for a BioFET design which permits IDS sweeping from an off-state to an on-state while keeping Vsol constant and the double layer sensing area in electrochemical equilibrium. The study explores a BioFET design addressing this challenge by decoupling the solution potential from IDS gating. Specific and label-free sensing of ferritin in 0.5 μL drops of 1:100 diluted plasma is pursued. We show an excellent sensing performance once the solution potential and IDS gating are decoupled, with a limit-of-detection of 10 fg/ml, a dynamic range of 10 orders of magnitude in ferritin concentration and excellent linearity and sensitivity. In contrast, a poor sensing performance is recorded for the conventional Vsol sweep performed in parallel to the above. Extensive control measurements quantifying the non-specific signals are reported.

Measuring Charge Selective Contacts on Semiconducting Photocatalyst Particles

Water splitting could replace steam-methane reforming, a carbon intensive process, to generate hydrogen for energy storage, fertilizer production, and many other industrial processes. Photoelectrochemical (PEC) overall water splitting (OWS), for example, produces hydrogen from water, without wires or applied voltages, via a clean and renewable energy source, sunlight. To do so, PEC particles or photocatalysts (PC) must absorb light, and selectively collect the generated carriers, either electrons or holes, at specific sites which drive reduction or oxidation reactions, respectively. While theoretical solar to hydrogen (STH) efficiencies are at least 18%, state-of-the-art PCs are below 3%. Despite the extreme efforts screening and tuning semiconductor photocatalysts, a thorough understanding and control of electronic properties of the semiconductor-electrocatalysts (sem|cat) interface is limited.Here, we use advanced electrochemical methods (dual-working electrode), scanning probe microscopy (conductive-AFM) and ambient-pressure XPS to measure and understand the origins of the electrochemical driving force (gradient in electrochemical potential) developed at complimentary electrocatalysts on photocatalyst particles. We use the dual-working electrode geometry on a high-quantum-yield wide-bandgap perovskite oxide, SrTiO3, to demonstrate the adaptive nature of electrocatalysts, Pt and CoOx, as they accumulate electrons and holes respectively. The cooperativity of half-reaction kinetics and adaptive sem|cat barrier height will be demonstrated along with the influence of crystal termination, which we argue is commonly overemphasized. Finally, a new contactless method of measuring the photovoltage at chemically distinct catalysts sites will be presented. Measured photovoltage induced shifts to element specific core levels in ambient pressure XP spectra in the dark and under illumination directly probe interfacial mechanisms operando. In sum, the introduced techniques and their findings demonstrate the underlying mechanisms and the degree that they play, enabling directed research of photocatalyst systems under one dominant engineering condition, charge selective Figure 1

(Invited) Understanding Carrier-Selective Catalyst/Semiconductor Contacts in Water-Splitting Photoelectrodes and Photocatalyst Particles

Heterogeneous electrochemical processes, including photoelectrochemical water splitting to evolve hydrogen and oxygen using electrocatalyst-coated semiconductors, are driven by the accumulation of charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. Conventional electrochemical techniques measure/control potentials at the conductive substrate or semiconductor ohmic contact, but are unable to isolate processes and electrochemical potentials at the surface during operation. I will discuss how the nanoelectrode tip of an atomic-force-microscope cantilever can effectively sense the surface electrochemical potential of electrocatalysts coating semiconductor photoelectrodes during operation. This data can be combined with ex-situ electrical measurements, spectroscopy, and materials characterization approaches to provide a comprehensive physical picture of how excited electrons and holes can be effectively separated and catalyst/semiconductor contacts. This new understanding is particularly important for the design of photocatalyst particles where direct electrical measurements of catalyst/semiconductor interface properties is not possible. In these systems we are applying spectroscopic techniques, for example ambient pressure x-ray photoelectron spectroscopy and Stark-effect spectroscopy to understand electric and electrochemical potential distribution and how nanoscale, adaptive, heterogeneous contacts can be controlled to design higher performance photochemical systems.

Experimental determination of stray currents in parallel operated cells exemplified on alkaline water electrolysis

Avoiding undesired electric field-driven cell-to-cell ion conduction is a common issue in electrochemical bipolar stacks affecting many processes such as alkaline water electrolysis, chlor-alkali electrolysis, and redox flow batteries. In bipolar stack systems, they are known as ionic shunt or stray currents and flow alongside a cell stack through the electrolyte manifolds bypassing the bipolar plates. A similar phenomenon occurs if multiple single cells are operated in parallel with a common electrolyte supply, even though each individual cell is galvanically isolated. If the parallel cells are operated at different voltages, an electrochemical potential gradient is formed across the electrolyte tube manifold, causing stray currents to flow from one cell to the other. It is important to determine and reduce stray currents and its implications to ensure high system efficiency and maximum system lifetime. This work presents a new approach with which stray currents between parallel operated, galvanically isolated electrochemical cells has been determined for the first time. For this purpose, an ionic 4-point measurement was implemented into an experimental setup with two electrolyzer flow cells with a common electrolyte supply using reference electrodes. This approach was used to measure the potential differences ΔE between both electrolyzers exemplarily for alkaline water electrolysis. Combined with the measured ionic resistances of the electrolyte within the polymeric tube manifold, stray currents were accurately determined. The presented results show that the ionic 4-point measurement could be successfully implemented and validated to experimentally determine stray currents between parallelly operated cells. The method presented enabled stray current measurements with high precision in the mA range. Due to the high cell currents and high ionic tube resistance, the stray current is less than 1 % in the introduced electrochemical setup. The demonstrated systematic measurement approach can be easily transferred to other electrochemical systems or processes.

Gradient Porous Graphene-Based Ion-Selective Electrode for Real-Time Monitoring of Sweat Electrolyte

Human sweat, a biomarker-rich physiological fluid, is an ideal candidate for non-invasive and real-time health monitoring. State-of-the-art electrochemical sweat sensors, that detect concentrations of analytes, such as electrolytes, through ion-selective membranes (ISM) in ion-selective electrodes (ISEs), show great potential for monitoring bodily conditions with high selectivity and affordability. Ion detection can be accomplished by utilizing the electrochemical potential gradient across an ISM within the solid-contact ISE. The solid-contact interface, located between the sensing membrane and electron-conducting substrate, plays a crucial role in ion-to-electron transduction. This critical layer serves to minimize the formation of the water layer, enabling stable potential responses and maintaining long-term stability.This work focuses on the use of chemical vapor deposition-grown, hydrophobic, gradient porous graphene (3D-G) material as the solid-contact layer in the ISE, offering a wider detection range, enhanced sensitivity, and improved long-term stability. The highly porous structure promotes ion recognition, diffusion, and adsorption onto the enhanced electrode surface, resulting in long-term stability, rapid analyte diffusion, and enhanced ion-to-electron transduction for sweat electrolyte sensing. A three-dimensionally interconnected porous electrode design can significantly enlarge the sensing areas in comparison to a 2D planar electrode, thereby improving the overall sensing performance of the sweat sensor. Owing to their large interface area, coupled with large double-layer capacitance in all-solid-state ISEs, hydrophobic nanostructured materials have been demonstrated to facilitate the ion-to-electron transduction process. Employing the 3D-G ISEs for health assessment has been exemplified through an on-body sweat sensing experiment, wherein the obtained results exhibit notable similarity with those generated by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Figure 1

Catalytic Proton-Hydroxide Recombination for Forward-Bias Bipolar Membranes

Bipolar membranes (BPMs) uniquely generate steady state pH gradients in electrochemical cells, enabling half reactions to occur efficiently in different pH environments, and are thus of broad interest. Forward-bias BPMs further enable new fuel-cell, redox-flow batteries, and CO2-electrolyzer concepts. In forward bias, the gradient in electrochemical potential drives ionic charge carriers toward the bipolar junction where they can recombine. We use a H2-pump electrochemical cell to study H+ / OH− recombination at the bipolar junction. We discover that metal oxide nanoparticles substantially catalyze the recombination reaction in the bipolar junction under forward bias and find evidence that H+ / OH− recombination occurs via a surface mechanism on the oxide catalyst. We propose a rate equation to describe the catalytic H+ / OH− recombination mechanism, supported by numerical simulations. Membrane sensing experiments further reveal the nature of the oxide catalyst layers and provide insight into the potential and temperature dependence of H+ / OH− recombination at the bipolar junction. This work thus elucidates materials-design strategies for recombination catalysts to advance forward-bias BPM technologies.

The Na+,K+-ATPase and its stoichiometric ratio: some thermodynamic speculations.

Almost seventy years after its discovery, the sodium-potassium adenosine triphosphatase (the sodium pump) located in the cell plasma membrane remains a source of novel mechanistic and physiologic findings. A noteworthy feature of this enzyme/transporter is its robust stoichiometric ratio under physiological conditions: it sequentially counter-transports three sodium ions and two potassium ions against their electrochemical potential gradients per each hydrolyzed ATP molecule. Here we summarize some present knowledge about the sodium pump and its physiological roles, and speculate whether energetic constraints may have played a role in the evolutionary selection of its characteristic stoichiometric ratio.

Fabrication and Performance Evaluation of a Cation Exchange Membrane Using Graphene Oxide/Polyethersulfone Composite Nanofibers.

Ion exchange membranes, especially cation exchange membranes (CEMs), are an important component in membrane-based energy generation and storage because of their ability to transport cations via the electrochemical potential gradient while preventing electron transport. However, developing a CEM with low areal resistance, high permselectivity, and stability remains difficult. In this study, electrospun graphene oxide/polyethersulfone (GO/PES) composite nanofibers were prepared with varying concentrations of GO. To fabricate a CEM, the pores of the electrospun GO/PES nanofiber substrates were filled with a Nafion ionomer. The pore-filled PES nanofiber loaded with 1% GO revealed a noticeable improvement in hydrophilicity, structural morphology, and mechanical properties. The 1% GO/PES pore-filled CEM was compared to a Nafion membrane of a varying thickness and without a nanofiber substrate. The CEM with a nanofiber substrate showed permselectivity of 85.75%, toughness of 111 J/m3, and areal resistance of 3.7 Ω cm2, which were 12.8%, 4.3 times, and 4.0 times better, respectively, than those of the Nafion membrane at the same thickness. The development of a reinforced concrete-like GO/PES nanofiber structure containing stretchable ionomer-enhanced membrane surfaces exhibited suitable areal resistance and reduced the thickness of the composite membrane without compromising the mechanical strength, suggesting its potential application as a cation exchange membrane in electrochemical membrane-based systems.

Observation of the Coupling Current During Stress Corrosion Cracking of Aluminum Alloy 5083

A unique fracture mechanics device was created to allow for simultaneously measuring the load and localized electrochemical potential gradients in AA5083 bend bars. For the first time, periodic Scanning Vibrating Probe (SVP) maps were collected in order to obtain a full view of the coupling current, (which can be deduced from measured potential gradients) that emanates from an anodic crack tip. As a result of mapping the current density over a wide surface area compared to the area of the notch and crack tip, clarification is given on whether or not the external surfaces of an alloy or any other metal that experiences Stress Corrosion Cracking (SCC) play a role in crack growth. The SVP maps clearly show the anodic and cathodic potential gradients of a growing crack (resulting from SCC) in an AA5083 fracture mechanics specimen while the stress intensity increases. The notch and crack-tip remained anodic throughout most of the obtained scans for sensitized and as-received specimens. Furthermore, a coupling effect is observed on sensitized specimens even at low fracture toughness values in sensitized specimens under load for longer periods of time (> 1 week). These higher resolution maps captured during the crack growth process give more insight to the SCC phenomenon and the electrochemical mechanisms of crack growth in AA5083 and other alloys.

A bioelectrochemical approach based on a solid supported membrane to evaluate the effect of natural products on Ca2+-ATPase: The case of 6-gingerol

The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) plays an essential role in maintaining the low cytosolic Ca2+ level that enables a variety of cellular processes. SERCA couples ATP hydrolysis to the transport of two Ca2+ ions against their electrochemical potential gradient from the cytoplasm into the lumen of the sarco/endoplasmic reticulum (SR/ER). Because of its central role in regulating cytoplasmic Ca2+ concentration, SERCA dysfunction has been associated with several pathological conditions. Stimulation of SERCA activity may represent a potential therapeutic strategy in various disease states connected with dysfunctional SERCA. The natural phenolic compound 6-gingerol, the most abundant and the major biologically active compound of ginger, was reported to activate the SERCA enzyme. The present study aimed at investigating the effect of 6-gingerol on SERCA transport activity using a bioelectrochemical approach based on a solid supported membrane (SSM). We first performed a voltammetric characterization of 6-gingerol to better understand its electrochemical behavior. We then studied the interaction of 6-gingerol with SR vesicles containing SERCA adsorbed on the SSM electrode. The measured current signals indicated that ATP-dependent Ca2+ translocation by SERCA was remarkably increased in the presence of 6-gingerol at low micromolar concentration. We also found that 6-gingerol has a rather high affinity for SERCA (EC50 of 1.8 ± 0.3 µM), and SERCA activation by 6-gingerol is reversible. The observed stimulatory effect of 6-gingerol on SERCA Ca2+-translocating activity may be beneficial in the prevention and/or treatment of pathological conditions related to SERCA dysfunction.

Analysis of cation permeation mechanism of ChR2 by molecular dynamics simulation.

The biomembranes separate intra- and extra-cellular. Permeation of small molecules via the biomembrane is mainly regulated by transport proteins. The ion channel transports ions along an electrochemical potential gradient. Channelrhodopsin-2 (ChR2) is a light-activated cation channel and the first widely adopted optogenetic tool. The structure of ChR2 is a homo-dimer of 7-transmembrane G-protein with covalently bound retinal. ChR2 absorbs blue light with maximum absorption and action spectrum at 480 nm. When the all-trans-retinal complex absorbs a photon, it changes into 13-cis-retinal and induces a conformation change of the protein to permeate cations. It is also known that a mutant ChR2 changes ion permeability. However, the ion permeation mechanism of ChR2 has not been clarified yet. In this study, we have performed molecular dynamics simulations of ChR2 with different conditions and compared them to examine the mechanism of ion permeability.

Sustainable nutrient recovery from synthetic urine by Donnan dialysis with tubular ion-exchange membranes

To address interconnected issues related to deteriorated water quality and unsustainable fertilizer production, Donnan dialysis was employed for nutrient recovery from model waste solutions and synthetic urine. Conventional Donnan dialysis reactors with flat-sheet, ion-exchange membranes and 500-mL solutions were deployed to determine the impact of the synthetic urine matrix on nutrient recovery relative to model wastes that only contained sodium phosphate (NaH2PO4) or ammonium chloride (NH4Cl). Compared to the model wastes, the competing ions in the synthetic urine caused orthophosphate (P(V)) and ammonium (NH4+) removal to decrease from 90.4 % to 55.2 % and from 87.8 % to 84.8 %, respectively. To improve reactor design for future scale up, an innovative Donnan dialysis system was established by placing tubular anion- and cation-exchange membranes into a 30-L waste solution and continuously recirculating 5 L of an NaCl-based draw solution through the inside of the membranes. P(V), NH4+, and other nutrients were simultaneously recovered, and the concentrations in the draw solution exceeded those in the initial synthetic urine in accordance with Donnan equilibrium for unequal solution volumes. Over 75 % P(V) and 74 % NH4+ were removed from the synthetic urine. MgCl2 and NaOH were added to the nutrient-enriched draw solution to precipitate solids and reset the electrochemical potential gradient, enabling enhanced nutrient recovery. Chemical equilibrium modeling and solids characterization confirmed that the recovered precipitates were ∼73–90 % struvite. The proof-of-concept tubular reactor represents a promising strategy for scaling up Donnan dialysis systems for selective nutrient recovery from urine and other wastes.

Algal PETC-Pro171-Leu suppresses electron transfer in cytochrome b6f under acidic lumenal conditions.

Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH varies, cyclic electron flow (CEF) between Photosystem I and the cytochrome b6f complex (b6f) produces extra ATP. b6f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b6f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 (proton gradient regulation 1) mutant of the vascular plant Arabidopsis (Arabidopsis thaliana), which contains a Pro194Leu substitution in the b6f Rieske Iron-sulfur protein Photosynthetic Electron Transfer C (PETC) subunit. In this work, we introduced the equivalent pgr1 mutation in the green alga Chlamydomonas reinhardtii to generate PETC-P171L. Consistent with the pgr1 phenotype, PETC-P171L displayed impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provide evidence that the ΔpH component in PETC-P171L depends on oxygen availability. Only under low oxygen conditions was the ΔpH component sufficient to trigger a phenotype in algal PETC-P171L where the mutant b6f was more restricted to oxidize the plastoquinol pool and showed diminished electron flow through the b6f complex. These results demonstrate that photosynthetic control of different stringency are established in C. reinhardtii depending on the cellular metabolism, and the lumen pH-sensitive PETC-P171L was generated to read out various associated effects.

Catalytic Proton–Hydroxide Recombination for Forward-Bias Bipolar Membranes

Bipolar membranes (BPMs) can generate steady-state pH gradients in electrochemical cells, enabling half-reactions to occur in different pH environments, and are thus of broad interest. Forward-bias BPMs further enable new approaches to fuel cells, redox-flow batteries, and CO2 electrolyzers. In forward bias, the gradient in electrochemical potential drives ionic charge carriers toward the bipolar junction where they can recombine. We use a H2-pump electrochemical cell to study H+/OH– recombination at the bipolar junction. We discover that metal-oxide nanoparticles catalyze the recombination reaction in the bipolar junction under forward bias and find evidence that H+/OH– recombination occurs via a surface mechanism on the oxide catalyst. We propose a rate equation to describe the catalytic H+/OH– recombination mechanism, supported by numerical simulations. This work thus elucidates materials-design strategies for recombination catalysts to advance forward-bias BPM technologies.

Sustained unidirectional rotation of a self-organized DNA rotor on a nanopore

Flow-driven rotary motors such as windmills and water wheels drive functional processes in human society. Although examples of such rotary motors also feature prominently in cell biology, their synthetic construction at the nanoscale has remained challenging. Here we demonstrate flow-driven rotary motion of a self-organized DNA nanostructure that is docked onto a nanopore in a thin solid-state membrane. An elastic DNA bundle self-assembles into a chiral conformation upon phoretic docking onto the solid-state nanopore, and subsequently displays a sustained unidirectional rotary motion of up to 20 rev s−1. The rotors harness energy from a nanoscale water and ion flow that is generated by a static chemical or electrochemical potential gradient in the nanopore, which are established through a salt gradient or applied voltage, respectively. These artificial nanoengines self-organize and operate autonomously in physiological conditions, suggesting ways to constructing energy-transducing motors at nanoscale interfaces.

Design principles for water dissociation catalysts in high-performance bipolar membranes

Water dissociation (WD, H2O → H+ + OH−) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO2 as an earth-abundant WD catalyst, and generate O2 and H2 at 500 mA cm−2 with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.

Strategy for Measuring Thermodynamic Factors in Lithium Electrolytes with Two Neutral Solvents

Thermodynamic factors, which convert species concentration gradients (∇ci) into electrochemical-potential gradients (∇μi), are essential quantities within concentrated-solution transport theory. Originally suggested by Darken [1], thermodynamic factors (χij) are defined through thermodynamic derivatives, asRT χij = yi ∂μi/∂yj,in which yi is the particle fraction of species i, R the universal gas constant, and T the absolute temperature. Accurate parameterisation of thermodynamic factors is important for the practical modelling of electrochemical systems, because they quantify how concentration polarization translates into concentration overpotential.The most common electrochemical approach to thermodynamic-factor characterisation is the concentration-cell method, whereby the steady-state voltage difference between two reservoirs containing different salt concentrations – i.e., the liquid-junction potential – is measured. A series of such measurements over a range of salt concentrations suffices to produce a functional form of χij, provided that the transference number is known from an auxiliary experiment. This method is well-established for binary electrolytes (i.e., a single salt in a neutral solvent), and has been used to produce ample data in the lithium-battery literature, for example by Nyman et al. [2], Landesfeind and Gasteiger [3], and Wang et al. [4].In practice, however, many electrochemical devices – especially lithium batteries – use electrolytes that incorporate a mixture of solvents. The relationship between voltage readings and thermodynamic factors in concentration-cell experiments becomes immediately unclear in these more complicated systems. In this talk we consider the thermodynamics of multi-solvent systems in the simplest case: that of two neutral solvents and a binary electrolyte, such as the LiPF6:EC:EMC solutions commonly used in Li-ion battery applications. Experimental procedures for characterising independent thermodynamic factors in two-solvent systems will be discussed.

Unassisted Photoelectrochemical Cell with Multimediator Modulation for Solar Water Splitting Exceeding 4% Solar-to-Hydrogen Efficiency

Photoelectrochemical overall water splitting has been considered as a promising approach for producing chemical energy from solar energy. Although many photoelectrochemical cells have been developed for overall water splitting by coupling two semiconductor photoelectrodes, inefficient charge transfer between the light-harvesters and electron acceptor/donor severely restricts the solar energy conversion efficiency. Inspired by natural photosynthesis, we assembled a photoelectrochemical platform with multimediator modulation to achieve unassisted overall water splitting. Photogenerated electrons are transferred in order through multimediators driven by the electrochemical potential gradient, resulting in efficient charge separation and transportation with enhanced charge transfer rate and reduced charge recombination rate. The integrated system composed of inorganic oxide-based photoanode (BiVO4) and organic polymer-based photocathode (PBDB-T:ITIC:PC71BM) with complementary light absorption, exhibits a solar-to-hydrogen conversion efficiency as high as 4.3%. This work makes a rational design possible by constructing an efficient charge-transfer chain in artificial photosynthesis systems for solar fuel production.

Phosphorus recovery by Donnan dialysis: Membrane selectivity, diffusion coefficients, and speciation effects

Donnan dialysis occurs when an electrochemical potential gradient exists for ions on either side of an ion-exchange membrane. We posit that this phenomenon can be leveraged to develop a sustainable process for nutrient recovery from wastewater. In this work, we conducted a fundamental study of the key parameters that control orthophosphate (P(V)) removal and recovery by Donnan dialysis. First, a new variable, namely the minimum draw ion concentration ratio between the draw and waste solutions (Rd/w), was established as the principal design parameter for Donnan dialysis. Then, the following variables were evaluated for their effects on P(V) removal and recovery: waste solution composition; draw anion type and concentration; and, membrane selectivity, thickness, and hydration. The waste solution pH controlled P(V) sorption to the anion-exchange membrane, with HPO42− exhibiting a higher affinity than H2PO4−. For an Rd/w of 10, 90.7% of H2PO4− and 98.4% of HPO42− were removed from a 10 mM P(V) waste solution using 100 mN and 218 mN NaCl draw solutions, respectively. The P(V) removal efficiency was dependent on draw solution concentration, and 77.1%, 95.3%, and 98.4% HPO42− removal was achieved with 48 (Rd/w = 2), 115 (Rd/w = 5), and 218 mM (Rd/w = 10) NaCl draw solutions, respectively. The rate of P(V) recovery was faster with HCOO− draw anions than with Cl− due to (i) the higher separation factor for P(V) over HCOO− (7.28) compared to Cl− (1.27) and (ii) the greater extent of membrane hydration with HCOO− draw solutions. Overall, this work established a new design parameter (Rd/w) and applied that parameter to optimize the draw solution chemistry for phosphorus recovery by Donnan dialysis.

Function Trumps Form in Two Sugar Symporters, LacY and vSGLT.

Active transport of sugars into bacteria occurs through symporters driven by ion gradients. LacY is the most well-studied proton sugar symporter, whereas vSGLT is the most characterized sodium sugar symporter. These are members of the major facilitator (MFS) and the amino acid-Polyamine organocation (APS) transporter superfamilies. While there is no structural homology between these transporters, they operate by a similar mechanism. They are nano-machines driven by their respective ion electrochemical potential gradients across the membrane. LacY has 12 transmembrane helices (TMs) organized in two 6-TM bundles, each containing two 3-helix TM repeats. vSGLT has a core structure of 10 TM helices organized in two inverted repeats (TM 1–5 and TM 6–10). In each case, a single sugar is bound in a central cavity and sugar selectivity is determined by hydrogen- and hydrophobic- bonding with side chains in the binding site. In vSGLT, the sodium-binding site is formed through coordination with carbonyl- and hydroxyl-oxygens from neighboring side chains, whereas in LacY the proton (H3O+) site is thought to be a single glutamate residue (Glu325). The remaining challenge for both transporters is to determine how ion electrochemical potential gradients drive uphill sugar transport.