Tang, Henry Yue Hin; Shin, David S; Hura, Gregory L.; Yang, Yue; Hu, Xiaoyu; Lightstone, Felice C; McGee, Matthew D; Padgett, Hal S; Yannone, Steven M; Tainer, John A.(Biochemistry, American Chemical Society (ACS), 2018-10-30)[Article]
Protein engineering to alter recognition underlying ligand binding and activity has enormous potential. Here, ligand binding for E. coli phosphoenolpyruvate carboxykinase (PEPCK), which converts oxaloacetate into CO2 and phosphoenolpyruvate as the first committed step in gluconeogenesis, was engineered to accommodate alternative ligands as an exemplary system with structural information. From our identification of bicarbonate binding in the PEPCK active site at the supposed CO2 binding site, we probed binding of nonnative ligands with three oxygen atoms arranged to resemble bicarbonate geometry. Crystal structures of PEPCK and point mutants with bound nonnative ligands thiosulfate and methanesulfonate along with strained ATP plus reoriented oxaloacetate intermediates and unexpected bicarbonate were solved and analyzed. The mutations successfully altered the bound ligand position and orientation, as well as its specificity: mutated PEPCKs bound either thiosulfate or methanesulfonate, but never both. Computational calculations predicted a methanesulfonate binding mutant and revealed that release of active site ordered solvent exerts a strong influence on ligand binding. Besides nonnative ligand binding, one mutant altered the Mn2+ coordination sphere: instead of the canonical octahedral ligand arrangement, the mutant in question only had a five-coordinate arrangement. From this work, critical features of ligand binding, position, and metal ion co-factor geometry required for all downstream events can be engineered with small numbers of mutations to provide insights into fundamental underpinnings of protein-ligand recognition. Through structural and computational knowledge, the combination of designed and random mutations aids robust design of predetermined changes to ligand binding and activity in order to engineer protein function.
Wongkaew, Nongnoot; Simsek, Marcel; Griesche, Christian; Baeumner, Antje J.(Chemical Reviews, American Chemical Society (ACS), 2018-09-24)[Article]
Electrochemical biosensors and associated lab-on-a-chip devices are the analytical system of choice when rapid and on-site results are needed in medical diagnostics and food safety, for environmental protection, process control, wastewater treatment, and life sciences discovery research among many others. A premier example is the glucose sensor used by diabetic patients. Current research focuses on developing sensors for specific analytes in these application fields and addresses challenges that need to be solved before viable commercial products can be designed. These challenges typically include the lowering of the limit of detection, the integration of sample preparation into the device and hence analysis directly within a sample matrix, finding strategies for long-term in vivo use, etc. Today, functional nanomaterials are synthesized, investigated, and applied in electrochemical biosensors and lab-on-a-chip devices to assist in this endeavor. This review answers many questions around the nanomaterials used, their inherent properties and the chemistries they offer that are of interest to the analytical systems, and their roles in analytical applications in the past 5 years (2013–2018), and it gives a quantitative assessment of their positive effects on the analyses. Furthermore, to facilitate an insightful understanding on how functional nanomaterials can be beneficial and effectively implemented into electrochemical biosensor-based lab-on-a-chip devices, seminal studies discussing important fundamental knowledge regarding device fabrication and nanomaterials are comprehensively included here. The review ultimately gives answers to the ultimate question: “Are they really needed or can bulk materials accomplish the same?” Finally, challenges and future directions are also discussed.
Schroeder, Vera; Swager, Timothy M.(Journal of the American Chemical Society, American Chemical Society (ACS), 2018-08-14)[Article]
Activating molecules or functional groups with high chemoselectivity in complex environments is the central goal of transition-metal-based catalysis. Promoting strong interactions between a selected substrate and a catalytic system can also be used to create highly selective and customizable sensors, and these concepts are widely recognized for enzymatic processes. We demonstrate the successful translation of organometallic reactions to sensing capability. Specifically, we have developed single-walled carbon nanotube (SWCNT) chemiresistive sensors for the highly selective detection of acrylates using conditions for the aerobic oxidative Heck reaction. The sensors mirror the catalytic processes and selectively respond to electron-deficient alkenes by adapting a catalytic reaction system to modulate the doping levels in carbon nanotubes. The sensors readily detect acrylates at parts per million (ppm) levels in untreated air. The concepts presented here are generally applicable and can guide future sensor development based upon known catalytic processes.
Aryal, Dipak; Ganesan, Venkat(The Journal of Physical Chemistry B, American Chemical Society (ACS), 2018-08-14)[Article]
The dynamics of ions and solvent molecules in polyelectrolyte desalination membranes is key to water purification technologies in which selective transport of the different components is desired. Recent experimental and our computational results have shown that nontrivial mechanisms underlie the transport properties of salt ions and water in charged polymer membranes. Explicitly, in polymer electrolytes, we found a reversal in the salt concentration dependence of the mobilities of Na+, Cl– salt ions and water molecules when compared with aqueous solutions. Motivated by such results, in this study, we have used atomistic molecular dynamics simulations to probe whether the mechanisms deduced in our earlier work apply to other salt systems and to mixtures of salts. Specifically, we report results for the ion diffusivities in aqueous KCl, MgCl2, and a 1:1 mixture of NaCl and MgCl2 salt solutions at different concentrations (ranging from 0.06 to 1 M) and investigate, at the molecular level, the mechanisms underlying the behaviors of salt and water transport properties. Our results show that diffusion of salt ions and water in charged polymer membranes are in general influenced by their association with polymer charge groups and ion pairing effects. Divalent ions are more strongly coupled with the polymeric ionic groups than monovalent salt ions and exhibit diffusivity trends that are distinct relative to monovalent salts. Further, we demonstrate that the mobilities of water molecules are influenced by coordination of water with polymer charge groups and their ion pairing tendencies and also exhibit distinct trends in monovalent and divalent salt solutions.
Kim, Taeyoung; Gorski, Christopher A.; Logan, Bruce(Environmental Science & Technology Letters, American Chemical Society (ACS), 2018-08-13)[Article]
Conventional technologies for ammonium removal from wastewaters are based on biological conversion to nitrogen gas, eliminating the possibility for ammonium recovery. A new electrochemical approach was developed here to selectively remove ammonium using two copper hexacyanoferrate (CuHCF) battery electrodes separated by an anion exchange membrane, at low applied voltages (0.1 to 0.3 V). The CuHCF battery electrodes removed NH4+ from a synthetic wastewater with a selectivity >5 (i.e., percent removed of NH4+/percent removed of Na+) when operated with a 0.1 V applied voltage, despite the much higher initial Na+ concentration in the sample (20 mM) than NH4+ (5 mM). In contrast, we observed only negligible selective removal of NH4+ over Na+ (<2) when using nonselective electrodes or ion-selective membranes (10 mM Na+, 5 mM NH4+, 0.1 V). The selectivity further increased to 9 when using equimolar concentrations of NH4+ and Na+ (10 mM). With an actual domestic wastewater, the CuHCF electrodes removed 85% of NH4+ (3.4 to 0.5 mM) with a selectivity >4 versus Na+ in the presence of other competing cations. These results demonstrate that CuHCF electrodes can be used to selectively remove NH4+ from various waters containing multiple ions.
Tu, Zhengyuan; Zachman, Michael J.; Choudhury, Snehashis; Khan, Kasim A.; Zhao, Qing; Kourkoutis, Lena F.; Archer, Lynden A.(Chemistry of Materials, American Chemical Society (ACS), 2018-07-25)[Article]
Approaches for regulating electrochemical stability of liquid electrolytes in contact with solid-state electrodes are a requirement for efficient and reversible electrical energy storage in batteries. Such methods are particularly needed in electrochemical cells in which the working potentials of the electrodes lie well outside the thermodynamic stability limits of the liquid electrolyte. Here we study electrochemical stability of liquids at electrolyte/electrode interfaces protected by a nanometer-thick, high-electrical band gap ceramic phase. We report that well-designed ceramic interphases extend the oxi-dative stability limits for both protic and aprotic liquid electrolytes, in some cases by as much as 1.5V. It is shown further that such interphases facilitate stable electrodeposition of reactive metals such as lithium at high Coulombic efficiency and in electrochemical cells subject to extended galvanostatic cycling at a high current density of 3 mA cm-2 and at capacities as high as 3 mAh cm-2. High-resolution cryo-FIB-SEM characterization reveals that solid/compact Li electrodeposits anchored by the ceramic interphase are the source of the enhanced Li deposition stability. The results enable a proof-of-concept ‘an-ode-free’ Li metal rechargeable battery in which Li initially provided in the cathode is the only source of lithium in the cell.
Ravi, Vikash Kumar; Scheidt, Rebecca A; DuBose, Jeffrey; Kamat, Prashant V.(Journal of the American Chemical Society, American Chemical Society (ACS), 2018-06-21)[Article]
The suppression of halide ion exchange between CsPbBr3 and CsPbI3 nanocrystals achieved through capping with PbSO4–oleate has enabled us to deposit different perovskite nanocrystals as aligned arrays on the electrode surfaces without intermixing of species. The electrophoretic deposition of PbSO4–oleate-capped CsPbX3 (X = Cl, Br, I) nanocrystals suspended in hexane solution on mesoscopic TiO2 films allows the design of controlled architecture with single or multiple layers of perovskite films. The hierarchy in the assembly of these nanocrystals is seen first through the linearly organized nanocrystals in hexane followed by the deposition of larger linear rods ∼500 nm in length. Since most of the photophysical properties of nanocrystals are retained in these aligned arrays, we can design films with tunable luminescence including white color. The electrophoretic deposition of layered films of perovskites in a controlled fashion opens up new ways to design tandem perovskite solar cells and tunable display devices.
Chen, Yulong; Xu, Qian; Jin, Yangfu; Qian, Xin; Liu, Li; Liu, Jun; Ganesan, Venkat(Macromolecules, American Chemical Society (ACS), 2018-05-23)[Article]
Coarse-grained molecular dynamics simulations were carried out to identify the conditions under which the nanorods (NRs) side-grafted with polymer chains can assemble in end-to-end configurations in a homopolymer matrix, a structure of significant importance for optimal property characteristics. Our results demonstrate that by adjusting the grafting density and the grafted chain length, three different NR morphologies can be obtained, viz., side-by-side aggregation, end-to-end alignment and homogeneous dispersion. To understand the underlying mechanism, the chain characteristics around the NRs were systematically investigated. We find that the transition of NR morphologies from side-by-side aggregation to others is correlated to the mushroom-to-brush transition of the grafted chain configurations. At high grafting densities corresponding to the brush regime, the entropic steric repulsions between the polymer brushes prevent the NRs from approaching in side-by-side configurations. Instead, end-to-end assembly and homogeneous dispersion are observed. Within such regimes, we observe that the splaying of the grafted polymer chains at the edges of the NRs plays a critical role in determining the occurrence of end-to-end assembly. When the extent of splaying cannot overcome the van der Waals and depletion attractions between the NR ends, which occurs at relatively short graft lengths, the end-to-end assembly is preferred. We find that this manner of self-assembly will be further promoted by increasing the NR loading but is retarded by increasing the NR aspect ratio. In general, our study identifies conditions to enable the end-to-end assembly of NRs in a homopolymer matrix, enabling significant practical applications.
Treat, Neil D.; Reid, Obadiah G.; Fearn, Sarah; Rumbles, Garry; Hawker, Craig J.; Chabinyc, Michael L; Stingelin, Natalie(ACS Applied Energy Materials, American Chemical Society (ACS), 2018-04-03)[Article]
The power conversion efficiency (PCE) of small-molecule bulk heterojunction solar cells is highly sensitive to the “ink” formulation used to produce the photoactive layer. Here we demonstrate that the addition of nucleating agents renders device fabrication notably less susceptible to the ink composition, promising a route toward more robust processing of efficient devices over large areas and enabling more facile materials screening. We selected as a model system blends of 7,7-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b]dithiophene-2,6-diyl]bis[6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole](p-DTS(FBTTh2)2) as the donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor because this is one of the small-molecule OPV blends with a device performance that is most sensitive to ink formulation, especially when used with the processing aid diiodooctane (DIO). Addition of DIO is essential to obtain high device performances; however, a notable increase in device performance is only achieved over a very narrow DIO content regime. Use of nucleating agents drastically changes this situation and leads to well-performing devices even at extreme levels of DIO. We thus start to address here one of the great challenges in organic solar cell research: the fact that, too often, only a very limited composition range leads to high efficiency devices. This means that for every new donor or acceptor a multitude of formulations have to be tested, including in combination with processing aids, to ensure that promising materials are not overlooked. The use of nucleating agents, thus, promises to render materials discovery more straightforward as this dependency of device performance with composition can be reduced.
Ravi, Vikash Kumar; Scheidt, Rebecca A.; Nag, Angshuman; Kuno, Masaru; Kamat, Prashant V.(ACS Energy Letters, American Chemical Society (ACS), 2018-04-02)[Article]
The ease of halide ion exchange in metal halide nanocrystals offers an opportunity to utilize them in a layered or tandem fashion to achieve graded bandgap films. We have now successfully suppressed the halide ion exchange by capping CsPbBr3 and CsPbI3 nanocrystals with PbSO4–oleate to create a nanostructure assembly that inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3 for several days. Furthermore, the effect of PbSO4–oleate capping on the excited state dynamics has also been elucidated. The effectiveness of PbSO4–oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid toward the tandem design of perovskite light-harvesting assemblies.
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