Publications by authors named "Thomas A Moore"

158 Publications

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst.

J Am Chem Soc 2021 Aug 11;143(33):13034-13043. Epub 2021 Aug 11.

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF)ppy)(bpy)][PF] (dF(CF)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF)ppy)(BIP-bpy)][PF] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.
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http://dx.doi.org/10.1021/jacs.1c01701DOI Listing
August 2021

Electron-Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer.

J Am Chem Soc 2021 03 18;143(8):3104-3112. Epub 2021 Feb 18.

Department of Chemistry, University of California, Berkeley, California 94720, United States.

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic-vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.
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http://dx.doi.org/10.1021/jacs.0c10626DOI Listing
March 2021

Tuning the redox potential of tyrosine-histidine bioinspired assemblies.

Photosynth Res 2021 Jan 11. Epub 2021 Jan 11.

School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA.

Photosynthesis powers our planet and is a source of inspiration for developing artificial constructs mimicking many aspects of the natural energy transducing process. In the complex machinery of photosystem II (PSII), the redox activity of the tyrosine Z (Tyr) hydrogen-bonded to histidine 190 (His190) is essential for its functions. For example, the Tyr-His190 pair provides a proton-coupled electron transfer (PCET) pathway that effectively competes against the back-electron transfer reaction and tunes the redox potential of the phenoxyl radical/phenol redox couple ensuring a high net quantum yield of photoinduced charge separation in PSII. Herein, artificial assemblies mimicking both the structural and redox properties of the Tyr-His190 pair are described. The bioinspired constructs contain a phenol (Tyr model) covalently linked to a benzimidazole (His190 model) featuring an intramolecular hydrogen bond which closely emulates the one observed in the natural counterpart. Incorporation of electron-withdrawing groups in the benzimidazole moiety systematically changes the intramolecular hydrogen bond strength and modifies the potential of the phenoxyl radical/phenol redox couple over a range of ~ 250 mV. Infrared spectroelectrochemistry (IRSEC) demonstrates the associated one-electron, one-proton transfer (E1PT) process upon electrochemical oxidation of the phenol. The present contribution provides insight regarding the factors controlling the redox potential of the phenol and highlights strategies for the design of futures constructs capable of transporting protons across longer distances while maintaining a high potential of the phenoxyl radical/phenol redox couple.
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http://dx.doi.org/10.1007/s11120-020-00815-xDOI Listing
January 2021

Role of Intact Hydrogen-Bond Networks in Multiproton-Coupled Electron Transfer.

J Am Chem Soc 2020 12 18;142(52):21842-21851. Epub 2020 Dec 18.

School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287-1604, United States.

The essential role of a well-defined hydrogen-bond network in achieving chemically reversible multiproton translocations triggered by one-electron electrochemical oxidation/reduction is investigated by using pyridylbenzimidazole-phenol models. The two molecular architectures designed for these studies differ with respect to the position of the N atom on the pyridyl ring. In one of the structures, a hydrogen-bond network extends uninterrupted across the molecule from the phenol to the pyridyl group. Experimental and theoretical evidence indicates that an overall chemically reversible two-proton-coupled electron-transfer process (E2PT) takes place upon electrochemical oxidation of the phenol. This E2PT process yields the pyridinium cation and is observed regardless of the cyclic voltammogram scan rate. In contrast, when the hydrogen-bond network is disrupted, as seen in the isomer, at high scan rates (∼1000 mV s) a chemically reversible process is observed with an characteristic of a one-proton-coupled electron-transfer process (E1PT). At slow cyclic voltammetric scan rates (<1000 mV s) oxidation of the phenol results in an overall chemically irreversible two-proton-coupled electron-transfer process in which the second proton-transfer step yields the pyridinium cation detected by infrared spectroelectrochemistry. In this case, we postulate an initial intramolecular proton-coupled electron-transfer step yielding the E1PT product followed by a slow, likely intermolecular chemical step involving a second proton transfer to give the E2PT product. Insights into the electrochemical behavior of these systems are provided by theoretical calculations of the electrostatic potentials and electric fields at the site of the transferring protons for the forward and reverse processes. This work addresses a fundamental design principle for constructing molecular wires where protons are translocated over varied distances by a Grotthuss-type mechanism.
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http://dx.doi.org/10.1021/jacs.0c10474DOI Listing
December 2020

Fecal Microbiota Transplantation Is Highly Effective in Real-World Practice: Initial Results From the FMT National Registry.

Gastroenterology 2021 01 1;160(1):183-192.e3. Epub 2020 Oct 1.

Division of Gastroenterology, University of Miami Miller School of Medicine, Miami, Florida.

Background & Aims: Fecal microbiota transplantation (FMT) is used commonly for treatment of Clostridioides difficile infections (CDIs), although prospective safety data are limited and real-world FMT practice and outcomes are not well described. The FMT National Registry was designed to assess FMT methods and both safety and effectiveness outcomes from North American FMT providers.

Methods: Patients undergoing FMT in clinical practices across North America were eligible. Participating investigators enter de-identified data into an online platform, including FMT protocol, baseline patient characteristics, CDI cure and recurrence, and short and long-term safety outcomes.

Results: Of the first 259 participants enrolled at 20 sites, 222 had completed short-term follow-up at 1 month and 123 had follow-up to 6 months; 171 (66%) were female. All FMTs were done for CDI and 249 (96%) used an unknown donor (eg, stool bank). One-month cure occurred in 200 patients (90%); of these, 197 (98%) received only 1 FMT. Among 112 patients with initial cure who were followed to 6 months, 4 (4%) had CDI recurrence. Severe symptoms reported within 1-month of FMT included diarrhea (n = 5 [2%]) and abdominal pain (n = 4 [2%]); 3 patients (1%) had hospitalizations possibly related to FMT. At 6 months, new diagnoses of irritable bowel syndrome were made in 2 patients (1%) and inflammatory bowel disease in 2 patients (1%).

Conclusions: This prospective real-world study demonstrated high effectiveness of FMT for CDI with a good safety profile. Assessment of new conditions at long-term follow-up is planned as this registry grows and will be important for determining the full safety profile of FMT.
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http://dx.doi.org/10.1053/j.gastro.2020.09.038DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8034505PMC
January 2021

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center.

iScience 2020 Aug 15;23(8):101366. Epub 2020 Jul 15.

School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA. Electronic address:

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the MnCaO cluster, with participation of the redox-active tyrosine residue (Y) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between Y and D1-His190 likely renders Y kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at Y remains elusive owing to the transient nature of its intermediate states involving Y⋅. Herein, we employ a combination of high-resolution two-dimensional N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the Y residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.
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http://dx.doi.org/10.1016/j.isci.2020.101366DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7394912PMC
August 2020

Latency of subthalamic nucleus deep brain stimulation-evoked cortical activity as a potential biomarker for postoperative motor side effects.

Clin Neurophysiol 2020 06 12;131(6):1221-1229. Epub 2020 Mar 12.

Department of Neurology, University of Alabama at Birmingham, Birmingham, AL, USA.

Objective: Here, we investigate whether cortical activation predicts motor side effects of deep brain stimulation (DBS) and whether these potential biomarkers have utility under general anesthesia.

Methods: We recorded scalp potentials elicited by DBS during surgery (n = 11), both awake and under general anesthesia, and in an independent ambulatory cohort (n = 8). Across a range of stimulus configurations, we measured the amplitude and timing of short- and long-latency response components and linked them to motor side effects.

Results: Regardless of anesthesia state, in both cohorts, DBS settings with capsular side effects elicited early responses with peak latencies clustering at <1 ms. This early response was preserved under anesthesia in all participants (11/11). In contrast, the long-latency components were suppressed completely in 6/11 participants. Finally, the latency of the earliest response could predict the presence of postoperative motor side effects both awake and under general anesthesia (84.8% and 75.8% accuracy, awake and under anesthesia, respectively).

Conclusion: DBS elicits short-latency cortical activation, both awake and under general anesthesia, which appears to reveal interactions between the stimulus and the corticospinal tract.

Significance: Short-latency evoked cortical activity can potentially be used to aid both DBS lead placement and post-operative programming.
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http://dx.doi.org/10.1016/j.clinph.2020.02.021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7214089PMC
June 2020

Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic "Special Pair".

J Phys Chem Lett 2020 May 20;11(9):3443-3450. Epub 2020 Apr 20.

IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133 Milan, Italy.

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates.
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http://dx.doi.org/10.1021/acs.jpclett.0c00856DOI Listing
May 2020

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires.

Chem Sci 2020 Mar 20;11(15):3820-3828. Epub 2020 Mar 20.

School of Molecular Sciences, Arizona State University Tempe Arizona 85287-1604 USA

Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BIP) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 p units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BIP series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular "dry proton wires" with these moieties, which can transfer protons a Grotthuss-type mechanism over long distances without the intervention of water molecules.
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http://dx.doi.org/10.1039/c9sc06010cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8152432PMC
March 2020

A microfluidic mammary gland coculture model using parallel 3D lumens for studying epithelial-endothelial migration in breast cancer.

Biomicrofluidics 2019 Nov 5;13(6):064122. Epub 2019 Dec 5.

Department of Mechanical & Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada.

In breast cancer development, crosstalk between mammary epithelial cells and neighboring vascular endothelial cells is critical to understanding tumor progression and metastasis, but the mechanisms of this dynamic interplay are not fully understood. Current cell culture platforms do not accurately recapitulate the 3D luminal architecture of mammary gland elements. Here, we present the development of an accessible and scalable microfluidic coculture system that incorporates two parallel 3D luminal structures that mimic vascular endothelial and mammary epithelial cell layers, respectively. This parallel 3D lumen configuration allows investigation of endothelial-epithelial crosstalk and its effects of the comigration of endothelial and epithelial cells into microscale migration ports located between the parallel lumens. We describe the development and application of our platform, demonstrate generation of 3D luminal cell layers for endothelial cells and three different breast cancer cell lines, and quantify their migration profiles based on number of migrated cells, area coverage by migrated cells, and distance traveled by individual migrating cells into the migration ports. Our system enables analysis at the single-cell level, allows simultaneous monitoring of endothelial and epithelial cell migration within a 3D extracellular matrix, and has potential for applications in basic research on cellular crosstalk as well as drug development.
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http://dx.doi.org/10.1063/1.5123912DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6894982PMC
November 2019

Proton-Coupled Electron Transfer Drives Long-Range Proton Translocation in Bioinspired Systems.

J Am Chem Soc 2019 09 13;141(36):14057-14061. Epub 2019 Aug 13.

School of Molecular Sciences , Arizona State University , Tempe , Arizona 85287-1604 , United States.

Proton-coupled electron transfer (PCET) combines the movement of fundamental charged species to form an essential link between electron- and proton-transport reactions in bioenergetics and catalysis in general. The length scale over which proton transport may occur within PCET processes and the thermodynamic consequences of the resulting proton chemical potential to the oxidation reaction driving these PCET processes have not been generally established. Here we report the design of bioinspired molecules that employ oxidation-reduction processes to move reversibly two, three, and four protons via a Grotthuss-type mechanism along hydrogen-bonded networks up to ∼16 Å in length. These molecules are composed of benzimidazole moieties linking a phenol to the final proton acceptor, a cyclohexylimine. Following electrochemical oxidation of the phenol, the appearance of an infrared band at 1660 cm signals proton arrival at the terminal basic site. Switching the electrode potential to reducing conditions reverses the proton translocation and resets the structure to the initial species. In addition to mimicking the first step of the iconic PCET process used by the Tyr-His190 redox relay in photosystem II to oxidize water, this work specifically addresses theoretically and experimentally the length scale over which PCET processes may occur. The thermodynamic findings from these redox-driven, bioinspired "proton wires" have implications for understanding and rationally designing pumps for the generation of proton-motive force in artificial and reengineered photosynthesis, as well as for management of proton activity around catalytic sites, including those for water oxidation and oxygen reduction.
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http://dx.doi.org/10.1021/jacs.9b06978DOI Listing
September 2019

Implementation of an Innovative, Multiunit, Postevent Debriefing Program in a Children's Hospital.

Pediatr Emerg Care 2020 Jul;36(7):345-346

Department of Pediatrics, Division of Emergency Medicine, Children's Hospital of the King's Daughters, Eastern Virginia Medical School, Norfolk, VA.

Background: Postevent debriefing has been associated with improved resuscitation outcomes and is recommended by the American Heart Association and the American Academy of Pediatrics to improve clinical performance.

Objective: Despite the benefits of postevent debriefing, published debriefing programs have focused on single areas within a hospital. We are unaware of any hospital-wide debriefing programs implemented in a pediatric setting.

Methods: We established a multidisciplinary, interprofessional debriefing collaborative at the Children's Hospital of Philadelphia to implement postevent debriefings in multiple areas of the hospital. The collaborative created a standardized debriefing form to capture data about the postevent debriefings.

Results: From July 23, 2015 to December 31, 2017, the emergency department performed 153 debriefings (18%) for 850 resuscitations. The neonatal intensive care unit conducted 10 debriefings (9%) for 107 resuscitations, and the pediatric intensive care unit performed 5 debriefings (7%) for 73 resuscitations.

Conclusions: Several departments at the Children's Hospital of Philadelphia have incorporated hot and cold debriefings into their clinical practice as part of their continuous quality improvement programs. By disseminating the tools and lessons learned from the implementation process, the collaborative hopes that other institutions will benefit from their lessons learned to successfully create their own debriefing programs. Widespread adoption of debriefing programs will enable a more scientific approach to studying the outcomes of debriefing.
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http://dx.doi.org/10.1097/PEC.0000000000001898DOI Listing
July 2020

Biomicrofluidic Systems for Hematologic Cancer Research and Clinical Applications.

SLAS Technol 2019 10 7;24(5):457-476. Epub 2019 Jun 7.

Department of Mechanical & Industrial Engineering, University of Toronto, Toronto, ON, Canada.

A persistent challenge in developing personalized treatments for hematologic cancers is the lack of patient specific, physiologically relevant disease models to test investigational drugs in clinical trials and to select therapies in a clinical setting. Biomicrofluidic systems and organ-on-a-chip technologies have the potential to change how researchers approach the fundamental study of hematologic cancers and select clinical treatment for individual patient. Here, we review microfluidics cell-based technology with application toward studying hematologic tumor microenvironments (TMEs) for the purpose of drug discovery and clinical treatment selection. We provide an overview of state-of-the-art microfluidic systems designed to address questions related to hematologic TMEs and drug development. Given the need to develop personalized treatment platforms involving this technology, we review pharmaceutical drugs and different modes of immunotherapy for hematologic cancers, followed by key considerations for developing a physiologically relevant microfluidic companion diagnostic tool for mimicking different hematologic TMEs for testing with different drugs in clinical trials. Opportunities lie ahead for engineers to revolutionize conventional drug discovery strategies of hematologic cancers, including integrating cell-based microfluidics technology with machine learning and automation techniques, which may stimulate pharma and regulatory bodies to promote research and applications of microfluidics technology for drug development.
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http://dx.doi.org/10.1177/2472630319846878DOI Listing
October 2019

A Case of Extrapulmonary Tuberculosis Two-Ways.

Kans J Med 2019 Feb 26;12(1):20-21. Epub 2019 Feb 26.

University of Kansas School of Medicine-Wichita, Department of Internal Medicine.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6396956PMC
February 2019

Tuning spin-orbit torques at magnetic domain walls in epitaxial Pt/Co/Pt Au trilayers.

Nanotechnology 2019 Jun 19;30(23):234003. Epub 2019 Feb 19.

School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom.

Magnetic domain walls (DWs) in perpendicularly magnetised thin films are attractive for racetrack memories, but technological progress still requires further reduction of the operationing currents. To efficiently drive these objects by the means of electric current, one has to optimize the damping-like torque which is caused by the spin Hall effect (SHE). This not only requires a high net spin Hall angle but also the presence of a Dzyaloshinskii-Moriya interaction (DMI) to produce magnetic textures sensitive to this type of the torque. In this work, we explore the coexistence and importance of these two phenomena in epitaxial Pt/Co/Pt Au films in which we control the degree of inversion symmetry-breaking between the two interfaces by varying x. Gold is used as a material with negligible induced magnetic moment and SHE and the interface between Co/Au as a source of a small DMI. We find no current-induced DW motion in the symmetric Pt/Co/Pt (x = 0) trilayer. By fitting a one-dimensional model to the DW velocity as a function of drive current density and in-plane applied field in samples with non-zero values of x, we find that both net DMI strength and spin Hall angle rise monotonically as Au is introduced. They reach values of 0.75 ± 0.05 mJ m and 0.10 ± 0.01, respectively, for Pt/Co/Au (x = 1).
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http://dx.doi.org/10.1088/1361-6528/ab087bDOI Listing
June 2019

Domain-wall motion and interfacial Dzyaloshinskii-Moriya interactions in Pt/Co/Ir( )/Ta multilayers.

Phys Rev B 2019 ;99

School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a Pt/Co/Ta multilayer between the Co and Ta layers. There is a marked increase in magnetic moment, due to the suppression of the dead layer at the interface with Ta, but the perpendicular anisotropy is hardly affected. All samples show a universal scaling of the field-driven DW velocity across the creep and depinning regimes. Asymmetric bubble expansion shows that DWs in all of the samples have the left-handed Néel form. The value of in-plane magnetic field at which the creep velocity shows a minimum drops markedly on the introduction of Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in Brillouin light scattering (BLS) measurements. Despite this qualitative similarity, there are quantitative differences in the DMI strength given by the two measurements, with BLS often returning higher values. Many features in bubble expansion velocity curves do not fit simple models commonly used, namely a lack of symmetry about the velocity minimum and no difference in velocities at high in-plane fields. These features are explained by the use of a new model in which the depinning field is allowed to vary with in-plane field in a way determined from micromagnetic simulations. This theory shows that the velocity minimum underestimates the DMI field, consistent with BLS giving higher values. Our results suggest that the DMI at an Ir/Co interface has the same sign as the DMI at a Pt/Co interface.
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http://dx.doi.org/10.1103/PhysRevB.99.094409DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7739563PMC
January 2019

Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays.

J Am Chem Soc 2018 11 31;140(45):15450-15460. Epub 2018 Oct 31.

School of Molecular Sciences , Arizona State University , Tempe , Arizona 85287-1604 , United States.

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the Tyr-His190 redox relay in photosystem II. These compounds provide a platform to theoretically and experimentally explore and expand proton-coupled electron transfer (PCET) processes. The models feature H-bonds between the phenol and the nitrogen at the 3-position of the benzimidazole and between the 1 H-benzimidazole proton and the imine nitrogen. Protonation of the benzimidazole and the imine can be unambiguously detected by infrared spectroelectrochemistry (IRSEC) upon oxidation of the phenol. DFT calculations and IRSEC results demonstrate that with sufficiently strong electron-donating groups at the para-position of the N-phenylimine group (e.g., -OCH substitution), proton transfer to the imine is exergonic upon phenol oxidation, leading to a one-electron, two-proton (E2PT) product with the imidazole acting as a proton relay. When transfer of the second proton is not sufficiently exergonic (e.g., -CN substitution), a one-electron, one-proton transfer (EPT) product is dominant. Thus, the extent of proton translocation along the H-bond network, either ∼1.6 Å or ∼6.4 Å, can be controlled through imine substitution. Moreover, the H-bond strength between the benzimidazole NH and the imine nitrogen, which is a function of their relative p K values, and the redox potential of the phenoxyl radical/phenol couple are linearly correlated with the Hammett constants of the substituents. In all cases, a high potential (∼1 V vs SCE) is observed for the phenoxyl radical/phenol couple. Designing and tuning redox-coupled proton wires is important for understanding bioenergetics and developing novel artificial photosynthetic systems.
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http://dx.doi.org/10.1021/jacs.8b09724DOI Listing
November 2018

Discrete Hall resistivity contribution from Néel skyrmions in multilayer nanodiscs.

Nat Nanotechnol 2018 12 1;13(12):1161-1166. Epub 2018 Oct 1.

School of Physics and Astronomy, University of Leeds, Leeds, UK.

Magnetic skyrmions are knot-like quasiparticles. They are candidates for non-volatile data storage in which information is moved between fixed read and write terminals. The read-out operation of skyrmion-based spintronic devices will rely on the electrical detection of a single magnetic skyrmion within a nanostructure. Here we present Pt/Co/Ir nanodiscs that support skyrmions at room temperature. We measured the Hall resistivity and simultaneously imaged the spin texture using magnetic scanning transmission X-ray microscopy. The Hall resistivity is correlated to both the presence and size of the skyrmion. The size-dependent part matches the expected anomalous Hall signal when averaging the magnetization over the entire disc. We observed a resistivity contribution that only depends on the number and sign of skyrmion-like objects present in the disc. Each skyrmion gives rise to 22 ± 2 nΩ cm irrespective of its size. This contribution needs to be considered in all-electrical detection schemes applied to skyrmion-based devices. Not only the area of Néel skyrmions but also their number and sign contribute to their Hall resistivity.
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http://dx.doi.org/10.1038/s41565-018-0268-yDOI Listing
December 2018

Electronic Structure and Triplet-Triplet Energy Transfer in Artificial Photosynthetic Antennas.

Photochem Photobiol 2019 01 11;95(1):211-219. Epub 2018 Aug 11.

School of Molecular Sciences, Arizona State University, Tempe, AZ.

Three Pd(II) phthalocyanine-carotenoid dyads featuring chromophores linked by amide bonds were prepared in order to investigate the rate of triplet-triplet (T-T) energy transfer from the tetrapyrrole to the covalently attached carotenoid as a function of the number of conjugated double bonds in the carotenoid. Carotenoids having 9, 10 and 11 conjugated double bonds were studied. Transient absorption measurements show that intersystem crossing in the Pd(II) phthalocyanine takes place in 10 ps in each case and that T-T energy transfer occurs in 126, 81 and 132 ps in the dyads bearing 9, 10 and 11 double bond carotenoids, respectively. To identify the origin of this variation in T-T energy transfer rates, density functional theory (DFT) was used to calculate the T-T electronic coupling in the three dyads. According to the calculations, the primary reason for the observed T-T energy transfer trend is larger T-T electronic coupling between the tetrapyrrole and the 10-double bond carotenoid. A methyl group adjacent to the amide linker that connects the Pd(II) phthalocyanine and the carotenoid in the 9 and 11-double bond carotenoids is absent in the 10-double bond carotenoid, and this difference alters its electronic structure to increase the coupling.
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http://dx.doi.org/10.1111/php.12979DOI Listing
January 2019

Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems.

Acc Chem Res 2018 02 8;51(2):445-453. Epub 2018 Jan 8.

School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.

Artificial photosynthetic constructs can in principle operate more efficiently than natural photosynthesis because they can be rationally designed to optimize solar energy conversion for meeting human demands rather than the multiple needs of an organism competing for growth and reproduction in a complex ecosystem. The artificial photosynthetic constructs described in this Account consist primarily of covalently linked synthetic chromophores, electron donors and acceptors, and proton donors and acceptors that carry out the light absorption, electron transfer, and proton-coupled electron transfer (PCET) processes characteristic of photosynthetic cells. PCET is the movement of an electron from one site to another accompanied by proton transfer. PCET and the transport of protons over tens of angstroms are important in all living cells because they are a fundamental link between redox processes and the establishment of transmembrane gradients of proton electrochemical potential, known as proton-motive force (PMF), which is the unifying concept in bioenergetics. We have chosen a benzimidazole phenol (BIP) system as a platform for the study of PCET because with appropriate substitutions it is possible to design assemblies in which one or multiple proton transfers can accompany oxidation of the phenol. In BIP, oxidation of the phenol increases its acidity by more than ten pK units; thus, electrochemical oxidation of the phenol is associated with a proton transfer to the imidazole. This is an example of a PCET process involving transfer of one electron and one proton, known as electron-proton transfer (EPT). When the benzimidazole moiety of BIP is substituted at the 4-position with good proton acceptor groups such as aliphatic amines, experimental and theoretical results indicate that two proton transfers occur upon one-electron oxidation of the phenol. This phenomenon is described as a one-electron-two-proton transfer (E2PT) process and results in translocation of protons over ∼7 Å via a Grotthuss-type mechanism, where the protons traverse a network of internally H-bonded sites. In the case of the E2TP process involving BIP analogues with amino group substituents, the thermodynamic price paid in redox potential to move a proton to the final proton acceptor is ∼300 mV. In this example, the decrease in redox potential limits the oxidizing power of the resulting phenoxyl radical. Thus, unlike the biological counterpart, the artificial construct is thermodynamically incapable of effectively advancing the redox state of a water oxidation catalyst. The design of systems where multiple proton transfer events are coupled to an oxidation reaction while a relatively high redox potential is maintained remains an outstanding challenge. The ability to control proton transfer and activity at defined distances and times is key to achieving proton management in the vicinity of catalysts operating at low overpotential in myriad biochemically important processes. Artificial photosynthetic constructs with well-defined structures, such as the ones described in this Account, can provide the means for discovering design principles upon which efficient redox catalysts for electrolysis and fuel cells can be based.
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http://dx.doi.org/10.1021/acs.accounts.7b00491DOI Listing
February 2018

Two-Photon Spectra of Chlorophylls and Carotenoid-Tetrapyrrole Dyads.

J Phys Chem B 2017 11 24;121(43):10055-10063. Epub 2017 Oct 24.

Technische Universität Braunschweig , Institute for Physical and Theoretical Chemistry, Department of Biophysical Chemistry, Gaußstraße. 17, 38106 Braunschweig, Germany.

We present a direct comparison of two-photon spectra of various carotenoid-tetrapyrrole dyads and phthalocyanines (Pc) as well as chlorophylls (Chl) in the spectral range between 950 and 1360 nm, corresponding to one-photon spectra between 475 and 680 nm. For carotenoids (Car) with 8, 9, or 10 conjugated double bonds, the two-photon absorption cross section of states below the optical allowed carotenoid S is at least about 3-10 times higher than that of Pc or chlorophyll a and b at 550/1100 nm. A quantitative comparison of spectra from Pc with and without carotenoids of eight and nine conjugated double bonds confirms energy transfer from optically forbidden carotenoid states to Pc in these dyads. When considering that less than 100% efficient energy transfer reduces the two-photon contribution of the carotenoids in the spectra, the actual Car two-photon cross sections relative to Chl/Pc are even higher than a factor of 3-10. In addition, strong spectroscopic two-photon signatures at energies below the optical allowed carotenoid S state support the presence of additional optical forbidden carotenoid states such as S*, S, or, alternatively, contributions from higher vibronic or hot S states dominating two-photon spectra or energy transfer from the carotenoids. The onset of these states is shifted about 1500-3500 cm to lower energies in comparison to the S states. Our data provides evidence that two-photon excitation of the carotenoid S*, S, or hot S states results in energy transfer to tetrapyrroles or chlorophylls similar to that observed with the Car S two-photon excitation.
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http://dx.doi.org/10.1021/acs.jpcb.7b08502DOI Listing
November 2017

Integrating Population Heterogeneity Indices with Microfluidic Cell-Based Assays.

SLAS Discov 2018 06 19;23(5):459-473. Epub 2017 Oct 19.

1 Department of Mechanical & Industrial Engineering, Institute of Biomaterials & Biomedical Engineering, University of Toronto, Toronto, ON, Canada.

Recent advances in cell-based assays have involved the integration of single-cell analyses and microfluidics technology to facilitate both high-content and high-throughput applications. These technical advances have yielded large datasets with single-cell resolution, and have given rise to the study of cell population dynamics, but statistical analyses of these populations and their properties have received much less attention, particularly for cells cultured in microfluidic systems. The objective of this study was to perform statistical analyses using Pittsburgh Heterogeneity Indices (PHIs) to understand the heterogeneity and evolution of cell population demographics on datasets generated from a microfluidic single-cell-resolution cell-based assay. We applied PHIs to cell population data obtained from studies involving drug response and soluble factor signaling of multiple myeloma cancer cells, and investigated effects of reducing population size in the microfluidic assay on both the PHIs and traditional population-averaged readouts. Results showed that PHIs are useful for examining changing population distributions within a microfluidic setting. Furthermore, PHIs provided data in support of finding the minimum population size for a microfluidic assay without altering the heterogeneity indices of the cell population. This work will be useful for novel assay development, and for advancing the integration of microfluidics, cell-based assays, and heterogeneity analyses.
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http://dx.doi.org/10.1177/2472555217738533DOI Listing
June 2018

Multiple Myeloma Cell Drug Responses Differ in Thermoplastic vs PDMS Microfluidic Devices.

Anal Chem 2017 Nov 17;89(21):11391-11398. Epub 2017 Oct 17.

Department of Mechanical & Industrial Engineering and the Institute of Biomaterials & Biomedical Engineering, University of Toronto , Toronto, ON M5S 3G8, Canada.

Poly(dimethylsiloxane) (PDMS) is a commonly used elastomer for fabricating microfluidic devices, but it has previously been shown to absorb hydrophobic molecules. Although this has been demonstrated for molecules such as estrogen and Nile Red, the absorption of small hydrophobic molecules in PDMS specifically used to treat cancer and its subsequent impact on cytotoxicity measurements and assays have not been investigated. This is critical for the development of microfluidic chemosensitivity and resistance assay (CSRA) platforms that have shown potential to help guide clinical therapy selection and which rely on the accuracy of the readout involving interactions between patient-derived cells and cancer drugs. It is thus important to address the issue of drug absorption into device material. We investigated drug absorption into microfluidic devices by treating multiple myeloma (MM) tumor cells with two MM drugs (bortezomib (BTZ) and carfilzomib (CFZ)) in devices fabricated using three different materials (polystyrene (PS), cyclo-olefin polymer (COP), and PDMS). Half-maximal inhibitory concentrations (IC) were obtained for each drug-material combination, and an increase in IC of ∼4.3× was observed in PDMS devices compared to both thermoplastic devices. Additionally, each MM drug was exposed to polymer samples, and samples were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) to characterize adsorption and absorption of the drugs into each material. ToF-SIMS data showed the bias observed in IC values found in PDMS devices was directly related to the absorption of drug during dose-response experiments. Specifically, BTZ and CFZ absorption in both PS and COP were all in the range of ∼100-300 nm, whereas BTZ and CFZ absorption in PDMS was ∼5.0 and ∼3.5 μm, respectively. These results highlight the biases that exist in PDMS devices and the importance of material selection in microfluidic device design, especially in applications involving drug cytotoxicity and hydrophobic molecules.
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http://dx.doi.org/10.1021/acs.analchem.7b02351DOI Listing
November 2017

Triplet-triplet energy transfer in artificial and natural photosynthetic antennas.

Proc Natl Acad Sci U S A 2017 07 26;114(28):E5513-E5521. Epub 2017 Jun 26.

Institute of Integrative Biology of the Cell, UMR 9891, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, CNRS, Université Paris Sud, 91191 Gif sur Yvette, France;

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.
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http://dx.doi.org/10.1073/pnas.1614857114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5514699PMC
July 2017

Understanding iridium oxide nanoparticle surface sites by their interaction with catechol.

Phys Chem Chem Phys 2017 Jun;19(24):16151-16158

Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA.

Iridium oxide (IrO) is one of the best water splitting electrocatalysts, but its active site details are not well known. As with all heterogeneous catalysts, a strategy for counting the number of active sites is not clear, and understanding their nature and structure is remarkably difficult. In this work, we performed a combined study using optical spectroscopy, magnetic resonance and electrochemistry to characterize the interaction of IrO nanoparticles (NPs) with a probe molecule, catechol. The catalyst is heterogeneous given that the substrate is in a different phase, but behaves as a homogeneous catalyst from the point of view of electrochemistry since it remains in colloidal suspension. We find two types of binding sites: centers A which bind catechol irreversibly making up 21% of the surface, and centers B which bind catechol reversibly making up 79% of the surface. UV-vis absorption spectroscopy shows that the A sites are responsible for the characteristic blue color of the NPs. Electrochemical experiments indicate that the B sites are catalytically active and we give the number of active sites per nanoparticle. We conclude by performing a survey of ligands used in solar cell architectures and show which ones bind well to the surface and which ones inhibit the catalytic activity when doing so, presenting quantitative guidelines for the correct handling of IrO nanoparticles during their incorporation into multifunctional solar energy harvesting architectures. We suggest ligands binding on the surface oxygen atoms allow for large bound ligand densities with no detrimental effect on the catalytic activity.
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http://dx.doi.org/10.1039/c7cp01516jDOI Listing
June 2017

Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II.

ACS Cent Sci 2017 May 9;3(5):372-380. Epub 2017 May 9.

School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States.

Nature employs a Tyr-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole-phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the Tyr-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox potential of the phenol by ∼300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.
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http://dx.doi.org/10.1021/acscentsci.7b00125DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5445534PMC
May 2017

Synthetic ferrimagnet nanowires with very low critical current density for coupled domain wall motion.

Sci Rep 2017 05 9;7(1):1640. Epub 2017 May 9.

School of Physics & Astronomy, University of Leeds, Leeds, LS2 9JT, United Kingdom.

Domain walls in ferromagnetic nanowires are potential building-blocks of future technologies such as racetrack memories, in which data encoded in the domain walls are transported using spin-polarised currents. However, the development of energy-efficient devices has been hampered by the high current densities needed to initiate domain wall motion. We show here that a remarkable reduction in the critical current density can be achieved for in-plane magnetised coupled domain walls in CoFe/Ru/CoFe synthetic ferrimagnet tracks. The antiferromagnetic exchange coupling between the layers leads to simple Néel wall structures, imaged using photoemission electron and Lorentz transmission electron microscopy, with a width of only ~100 nm. The measured critical current density to set these walls in motion, detected using magnetotransport measurements, is 1.0 × 10 Am, almost an order of magnitude lower than in a ferromagnetically coupled control sample. Theoretical modelling indicates that this is due to nonadiabatic driving of anisotropically coupled walls, a mechanism that can be used to design efficient domain-wall devices.
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http://dx.doi.org/10.1038/s41598-017-01748-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5431626PMC
May 2017

Design of porphyrin-based ligands for the assembly of [d-block metal : calcium] bimetallic centers.

Dalton Trans 2017 Mar;46(13):4199-4208

School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA.

The association of different metals in stable, well-defined molecular assemblies remains a great challenge of supramolecular chemistry. In such constructs, the emergence of synergism, or cooperative effects between the different metal centers is particularly intriguing. These effects can lead to uncommon reactivity or remarkable physico-chemical properties that are not otherwise achievable. For example, the association of alkaline or alkaline-earth cations and transition metals is pivotal for the activity of several biomolecules and human-made catalysts that carry out fundamental redox transformations (water oxidation, nitrogen reduction, water-gas shift reaction, etc.). In many cases the precise nature of the interactions between the alkaline-earth cations and the redox-active transition metals remains elusive due to the difficulty of building stable molecular heterometallic assemblies that associate transition metals and alkaline or alkaline-earth cations in a controlled way. In this work we present the rational design of porphyrin-based ligands possessing a second binding site for alkaline-earth cations above the porphyrin macrocycle primary complexation site. We demonstrate that by using a combination of crown ether and carboxylic acid substituents suitably positioned on the periphery of the porphyrin, bitopic ligands can be obtained. The binding of calcium, a typical alkaline-earth cation, by the newly prepared ligands has been studied in detail and we show that a moderately large binding constant can be achieved in protic media using ligands that possess some degree of structural flexibility. The formation of Zn-Ca assemblies discussed in this work is viewed as a stepping stone towards the assembly of well defined molecular transition metal-alkaline earth bimetallic centers using a versatile organic scaffold.
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http://dx.doi.org/10.1039/c6dt04647aDOI Listing
March 2017

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices.

J Vis Exp 2017 01 17(119). Epub 2017 Jan 17.

Department of Mechanical & Industrial Engineering, University of Toronto; Institute of Biomaterials and Biomedical Engineering (IBBME), University of Toronto;

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.
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http://dx.doi.org/10.3791/55175DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5352265PMC
January 2017
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