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1. Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes

   https://doi.org/10.1039/c5sc00433k  

   Ma, Chaoxiong ; Zaino III, Lawrence P. ; Bohn, Paul W. ( January 2015 , Chemical Science)

   We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy) 3 2/3+ /tri- n -propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufinmore »at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH 3 ) 6 3+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.« less
2. Effects of fixed charge group physicochemistry on anion exchange membrane permselectivity and ion transport

   https://doi.org/10.1039/D0CP00018C  

   Ji, Yuanyuan ; Luo, Hongxi ; Geise, Geoffrey M. ( April 2020 , Physical Chemistry Chemical Physics)

   Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixedmore »charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes.« less
3. Review—Development of Highly Active and Durable Hybrid Compressive Platinum Lattice Catalysts for Polymer Electrolyte Membrane Fuel Cells: Mathematical Modeling and Experimental Work

   https://doi.org/10.1149/1945-7111/ab6bc6  

   Popov, Branko N. ; Lee, Jong-Won ; Kriston, Akos ; Kim, Taekeun ( February 2020 , Journal of The Electrochemical Society)

   This review provides a comprehensive overview on the development of highly active and durable platinum catalysts with ultra-low Pt loadings for polymer electrolyte membrane fuel cells (PEMFCs) through a combined mathematical modeling and experimental work. First, simulation techniques were applied to evaluate the validity of the Tafel approximation for the calculation of the mass activity (MA) and specific activity (SA). A one-dimensional agglomeration model was developed and solved to understand the effects of exchange current density, porosity, agglomerate size, Nafion^®film thickness, and Pt loading on the MA and SA. High porosity (> 60%) and agglomerations at high Pt loadings cause the loss of the Tafel approximation and consequently the decrease in MA and SA. A new structure parameter was introduced to estimate the real porous structure using the fractal theory. The volumetric catalyst density was corrected by the fractal dimension (measured by Hg porosimetry), which gave a good agreement with the experimental values. The loading-dependent Tafel equation was then derived, which contains both the utilization and the non-linear scaling factor. Second, activated carbon composite support (ACCS) with optimized surface area, porosity, pore size, and pore size distribution was developed. The hydrophilic/hydrophobic ratio, structural properties (amorphous/crystalline ratio), and the number ofmore »active sites were optimized through metal-catalyzed pyrolysis. Stability of ACCS and Pt/ACCS were evaluated using an accelerated stress test (AST). The results indicated that Pt/ACCS showed no significant loss of MA and power density after 5,000 cycles at 1.0–1.5 V, while the commercial Pt/C catalysts showed drastic losses of MA and power density. Finally, monolayers of compressed Pt (core–shell-type Pt₃Co₁) catalysts were structured by diffusing Co atoms (previously embedded in ACCS) into Pt. Compressive Pt lattice (Pt^*) catalysts were synthesized through an annealing procedure developed at the University of South Carolina (USC). The Pt^*/ACCS catalyst showed high initial power density (rated) of 0.174 g_PtkW⁻¹and high stability (24 mV loss) at 0.8 A cm⁻²after 30,000 cycles (0.6–1.0 V). The outstanding performance of Pt^*/ACCS is due to the synergistic effect of ACCS and compressive Pt^*lattice.

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4. Engineered interaction between short elastin-like peptides and perfluorinated sulfonic-acid ionomer

   https://doi.org/10.1039/C8SM00351C  

   Su, Zihang ; Pramounmat, Nuttanit ; Watson, Skylar T. ; Renner, Julie N. ( January 2018 , Soft Matter)

   Control of ionomer thin films on metal surfaces is important for a range of electrodes used in electrochemical applications. Engineered peptides have emerged as powerful tools in electrode assembly because binding sites and peptide structures can be modulated by changing the amino acid sequence. However, no studies have been conducted showing peptides can be engineered to interact with ionomers and metals simultaneously. In this study, we design a single-repeat elastin-like peptide to bind to gold using a cysteine residue, and bind to a perfluorinated sulfonic-acid ionomer called Nafion® using a lysine guest residue. Quartz crystal microbalance with dissipation monitoring and atomic force microscopy are used to show that an elastin-like peptide monolayer attached to gold facilitates the formation of a thin, phase-separated ionomer layer. Dynamic light scattering confirms that the interaction between the peptide with the lysine residue and the ionomer also happens in solution, and circular dichroism shows that the peptides maintain their secondary structures in the presence of ionomer. These results demonstrate that elastin-like peptides are promising tools for ionomer control in electrode engineering.
5. Identification of SLAC1 anion channel residues required for CO ₂ /bicarbonate sensing and regulation of stomatal movements

   https://doi.org/10.1073/pnas.1807624115  

   Zhang, Jingbo ; Wang, Nuo ; Miao, Yinglong ; Hauser, Felix ; McCammon, J. Andrew ; Rappel, Wouter-Jan ; Schroeder, Julian I. ( October 2018 , Proceedings of the National Academy of Sciences)

   Increases in CO₂concentration in plant leaves due to respiration in the dark and the continuing atmospheric [CO₂] rise cause closing of stomatal pores, thus affecting plant–water relations globally. However, the underlying CO₂/bicarbonate (CO₂/HCO₃⁻) sensing mechanisms remain unknown. [CO₂] elevation in leaves triggers stomatal closure by anion efflux mediated via the SLAC1 anion channel localized in the plasma membrane of guard cells. Previous reconstitution analysis has suggested that intracellular bicarbonate ions might directly up-regulate SLAC1 channel activity. However, whether such a CO₂/HCO₃⁻regulation of SLAC1 is relevant for CO₂control of stomatal movements in planta remains unknown. Here, we computationally probe for candidate bicarbonate-interacting sites within the SLAC1 anion channel via long-timescale Gaussian accelerated molecular dynamics (GaMD) simulations. Mutations of two putative bicarbonate-interacting residues, R256 and R321, impaired the enhancement of the SLAC1 anion channel activity by CO₂/HCO₃⁻inXenopusoocytes. Mutations of the neighboring charged amino acid K255 and residue R432 and the predicted gate residue F450 did not affect HCO₃⁻regulation of SLAC1. Notably, gas-exchange experiments withslac1-transformed plants expressing mutated SLAC1 proteins revealed that the SLAC1 residue R256 is required for CO₂regulation of stomatal movements in planta, but not for abscisic acid (ABA)-induced stomatal closing. Patch clamp analyses of guard cells show that activation ofmore »S-type anion channels by CO₂/HCO₃⁻, but not by ABA, was impaired, indicating the relevance of R256 for CO₂signal transduction. Together, these analyses suggest that the SLAC1 anion channel is one of the physiologically relevant CO₂/HCO₃⁻sensors in guard cells.

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