Dynamics of a tunable superfluid junction

We study the population dynamics of a Bose-Einstein condensate in a double-well potential throughout the crossover from Josephson dynamics to hydrodynamics. At barriers higher than the chemical potential, we observe slow oscillations well described by a Josephson model. In the limit of low barriers, the fundamental frequency agrees with a simple hydrodynamic model, but we also observe a second, higher frequency. A full numerical simulation of the Gross-Pitaevskii equation giving the frequencies and amplitudes of the observed modes between these two limits is compared to the data and is used to understand the origin of the higher mode. Implications for trapped matter-wave interferometers are discussed.Comment: 8 pages, 7 figures; v3: Journal reference added, minor changes to tex

Efficient harvesting and storage of solar energy of an all-vanadium solar redox flow battery with a MoS2@TiO2 photoelectrode

Solar redox flow batteries constitute an emerging technology that provides a smart alternative for the capture and storage of discontinuous solar energy through the photo-generation of the discharged redox species employed in traditional redox flow batteries. Here, we show that a MoS2-decorated TiO2 (MoS2@TiO2) photoelectrode can successfully harvest light to be stored in a solar redox flow battery using vanadium ions as redox active species in both the catholyte and anolyte, and without the use of any bias. The MoS2@TiO2 photoelectrode achieved an average photocurrent density of ∼0.4 mA cm−2versus 0.08 mA cm−2 for bare TiO2, when tested for the oxidation of V4+ to V5+, attributed to a more efficient light harvesting and charge separation for the MoS2@TiO2 relative to TiO2. The designed solar redox flow cell exhibited an optimal overall solar-to-output energy conversion efficiency (SOEE) of ∼4.78%, which outperforms previously reported solar redox flow batteries. This work demonstrates the potential of the MoS2@TiO2 photoelectrode to efficiently convert solar energy into chemical energy in a solar redox flow battery, and it also validates the great potential of this technology to increase reliability in renewable energies

Open-circuit dissolution of platinum from the cathode in polymer electrolyte membrane water electrolysers

Platinum is the state-of-the-art catalyst for hydrogen evolution in polymer electrolyte membrane (PEM) water electrolysers; however, its stability has only been characterized to a limited extent in situ. This study measures platinum dissolving from the cathode during intermittent operation of a 3-electrode PEM electrolyser cell, using a differential pulse voltammetry technique that provided detection limits for platinum of less than 2 ng L−1. Water samples were periodically taken during on-off current cycling, and during periods of open-circuit voltage (OCV) platinum dissolution was detected when the cathode potential rose above 0.85 V NHE due to diffusion of oxygen from the anode. This reached a maximum dissolution rate at the highest cathode potential of 1.02 V NHE, and gradually decayed over a 90 h period. The average total amount of platinum dissolved per 90 h OCV period was estimated to be 152 ng cm−2 or 0.005% of the initial electrode catalyst mass. The dissolution mechanism was predicted to be the same as that occurring in PEM fuel cell cathodes, although being kinetically hindered in PEM electrolysers by the slow diffusion of oxygen from the anode to the cathode

Dehydropeptide supramolecular hydrogels and nanostructures as potential peptidomimetic biomedical materials

Supramolecular peptide hydrogels are gaining increased attention, owing to their potential in a variety of biomedical applications. Their physical properties are similar to those of the extracellular matrix (ECM), which is key to their applications in the cell culture of specialized cells, tissue engineering, skin regeneration, and wound healing. The structure of these hydrogels usually consists of a di- or tripeptide capped on the N-terminus with a hydrophobic aromatic group, such as Fmoc or naphthalene. Although these peptide conjugates can offer advantages over other types of gelators such as cross-linked polymers, they usually possess the limitation of being particularly sensitive to proteolysis by endogenous proteases. One of the strategies reported that can overcome this barrier is to use a peptidomimetic strategy, in which natural amino acids are switched for non-proteinogenic analogues, such as D-amino acids, β-amino acids, or dehydroamino acids. Such peptides usually possess much greater resistance to enzymatic hydrolysis. Peptides containing dehydroamino acids, i.e., dehydropeptides, are particularly interesting, as the presence of the double bond also introduces a conformational restraint to the peptide backbone, resulting in (often predictable) changes to the secondary structure of the peptide. This review focuses on peptide hydrogels and related nanostructures, where α,β-didehydro-α-amino acids have been successfully incorporated into the structure of peptide hydrogelators, and the resulting properties are discussed in terms of their potential biomedical applications. Where appropriate, their properties are compared with those of the corresponding peptide hydrogelator composed of canonical amino acids. In a wider context, we consider the presence of dehydroamino acids in natural compounds and medicinally important compounds as well as their limitations, and we consider some of the synthetic strategies for obtaining dehydropeptides. Finally, we consider the future direction for this research area.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding of CQUM (UID/QUI/00686/2019). FCT, FEDER, PORTUGAL2020 and COMPETE2020 are also acknowledged for funding under research project PTDC/QUI-QOR/29015/2017 (POCI-01-0145-FEDER-029015)

Breakdown of Scaling in the Nonequilibrium Critical Dynamics of the Two-Dimensional XY Model

The approach to equilibrium, from a nonequilibrium initial state, in a system at its critical point is usually described by a scaling theory with a single growing length scale, $\xi(t) \sim t^{1/z}$, where z is the dynamic exponent that governs the equilibrium dynamics. We show that, for the 2D XY model, the rate of approach to equilibrium depends on the initial condition. In particular, $\xi(t) \sim t^{1/2}$ if no free vortices are present in the initial state, while $\xi(t) \sim (t/\ln t)^{1/2}$ if free vortices are present.Comment: 4 pages, 3 figure

Towards a mechanistic understanding of particle shrinkage during biomass pyrolysis via synchrotron X-ray microtomography and in-situ radiography

Accurate modelling of particle shrinkage during biomass pyrolysis is key to the production of biochars with specific morphologies. Such biochars represent sustainable solutions to a variety of adsorption-dependent environmental remediation challenges. Modelling of particle shrinkage during biomass pyrolysis has heretofore been based solely on theory and ex-situ experimental data. Here we present the first in-situ phase-contrast X-ray imaging study of biomass pyrolysis. A novel reactor was developed to enable operando synchrotron radiography of fixed beds of pyrolysing biomass. Almond shell particles experienced more bulk shrinkage and less change in porosity than did walnut shell particles during pyrolysis, despite their similar composition. Alkaline pretreatment was found to reduce this difference in feedstock behaviour. Ex-situ synchrotron X-ray microtomography was performed to study the effects of pyrolysis on pore morphology. Pyrolysis led to a redistribution of pores away from particle surfaces, meaning newly formed surface area may be less accessible to adsorbates

Hydrogels and nanostructures formed from ciprofloxacin–peptide conjugates

Ciprofloxacin is a broad-spectrum fluoroquinolone antibiotic that possesses potent activity against both Gram-positive and Gram-negative bacteria and is used to treat many infections. Despite its widespread use, ciprofloxacin is associated with side effects, which might be reduced by improving its pharmacokinetic properties. The chemical structure of ciprofloxacin is the source of some of its limitations, which include: (1) Poor membrane permeability due to lipophobicity caused by the presence of polar groups; and (2) poor transportation and absorption due to poor water solubility caused by the flat aromatic structure. Previous methods for improving the pharmacokinetic properties of ciprofloxacin have involved the synthesis of conjugates. Issues related to poor membrane permeability, transportation and absorption of drugs can also be improved by employing nanocarriers and nanomaterials. Encapsulation within nanocarriers allows targeted drug delivery and reduced side effects as lower doses of the drug can be administered. Nanocarriers that can be used for this purpose include nanoparticles and hydrogels. Our research group is interested in supramolecular hydrogels as drug delivery systems. Short amphiphilic peptides are often able to form hydrogels through self-assembly. This present work describes the synthesis of a ciprofloxacin–dehydropeptide conjugate with the aim of forming hydrogels and related nanostructures to be used for the ‘self-delivery’ of antibacterial compounds. We assessed the hydrogelation ability, antibacterial activity, and pharmacokinetic properties. TEM microscopy revealed nanotubes and nanospheres. The conjugate was unable to form hydrogels alone but was able to form hydrogels as the major component of a co-gel with another peptide gelator. Although the conjugate retained antibacterial activity at 400 µM, activity diminished at lower concentrations. Thus, future work should focus on more hydrolysable pro-drug versions of the conjugate or versions where the peptide is connected at an alternate position.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding of CQUM (UID/QUI/00686/2019). FCT, FEDER, PORTUGAL2020, and COMPETE2020 are also acknowledged for funding under research project PTDC/QUI-QOR/29015/2017 (POCI-01-0145-FEDER-029015)

Excimer laser surface modification: Process and properties

Surface modification can improve materials for structural, tribological, and corrosion applications. Excimer laser light has been shown to provide a rapid means of modifying surfaces through heat treating, surface zone refining, and mixing. Laser pulses at modest power levels can easily melt the surfaces of many materials. Mixing within the molten layer or with the gas ambient may occur, if thermodynamically allowed, followed by rapid solidification. The high temperatures allow the system to overcome kinetic barriers found in some ion mixing experiments. Alternatively, surface zone refinement may result from repeated melting-solidification cycles. Ultraviolet laser light couples energy efficiently to the surface of metallic and ceramic materials. The nature of the modification that follows depends on the properties of the surface and substrate materials. Alloying from both gas and predeposited layer sources has been observed in metals, semiconductors, and ceramics as has surface enrichment of Cr by zone refinement of stainless steel. Rapid solidification after melting often results in the formation of nonequilibrium phases, including amorphous materials. Improved surface properties, including tribology and corrosion resistance, are observed in these materials

The origin of overpotential in lithium-mediated nitrogen reduction

The verification of the lithium-mediated nitrogen reduction system in 2019 has led to an explosion in the literature focussing on improving the metrics of faradaic efficiency, stability, and activity. However, while the literature acknowledges the vast intrinsic overpotential for nitrogen reduction due to the reliance on in situ lithium plating, it has thus far been difficult to accurately quantify this overpotential and effectively analyse further voltage losses. In this work, we present a simple method for determining the Reversible Hydrogen Electrode (RHE) potential in the lithium-mediated nitrogen reduction system. This method allows for an investigation of the Nernst equation and reveals sources of potential losses. These are namely the solvation of the lithium ion in the electrolyte and resistive losses due to the formation of the solid electrolyte interphase. The minimum observed overpotential was achieved in a 0.6 M LiClO4, 0.5 vol% ethanol in tetrahydrofuran electrolyte. This was −3.59 ± 0.07 V vs. RHE, with a measured faradaic efficiency of 6.5 ± 0.2%. Our method allows for easy comparison between the lithium-mediated system and other nitrogen reduction paradigms, including biological and homogeneous mechanisms

Evaluation of a model photo-caged dehydropeptide as a stimuli-responsive supramolecular hydrogel

Short peptides capped on the N-terminus with aromatic groups are often able to form supramolecular hydrogels, via self-assembly, in aqueous media. The rheological properties of these readily tunable hydrogels resemble those of the extracellular matrix (ECM) and therefore have potential for various biological applications, such as tissue engineering, biosensors, 3D bioprinting, drug delivery systems and wound dressings. We herein report a new photo-responsive supramolecular hydrogel based on a “caged” dehydropeptide (CNB-Phe-ΔPhe-OH 2), containing a photo-cleavable carboxy-2-nitrobenzyl (CNB) group. We have characterized this hydrogel using a range of techniques. Irradiation with UV light cleaves the pendant aromatic capping group, to liberate the corresponding uncaged model dehydropeptide (H-Phe-ΔPhe-OH 3), a process which was investigated by 1H NMR and HPLC studies. Crucially, this cleavage of the capping group is accompanied by dissolution of the hydrogel (studied visually and by fluorescence spectroscopy), as the delicate balance of intramolecular interactions within the hydrogel structure is disrupted. Hydrogels which can be disassembled non-invasively with temporal and spatial control have great potential for specialized on-demand drug release systems, wound dressing materials and various topical treatments. Both 2 and 3 were found to be non-cytotoxic to the human keratinocyte cell line, HaCaT. The UV-responsive hydrogel system reported here is complementary to previously reported related UV-responsive systems, which are generally composed of peptides formed from canonical amino acids, which are susceptible to enzymatic proteolysis in vivo. This system is based on a dehydrodipeptide structure which is known to confer proteolytic resistance. We have investigated the ability of the photo-activated system to accelerate the release of the antibiotic, ciprofloxacin, as well as some other small model drug compounds. We have also conducted some initial studies towards skin-related applications. Moreover, this model system could potentially be adapted for on-demand “self-delivery”, through the uncaging of known biologically active dehydrodipeptides.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding of CQUM (UID/QUI/00686/2019) and of IPC (UID/CTM/50025/2019). L.H. acknowledges funding from the FCT Investigator Programme through grant IF/00606/2014. FCT, FEDER, PORTUGAL2020 and COMPETE2020 are also acknowledged for funding under research project PTDC/QUI-QOR/29015/2017 (POCI-01-0145-FEDER-029015)