The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis.

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs (18 nm, 26 nm, 39 nm) drive the electrochemical CO2 reduction reaction (CO2RR) while remaining well-separated from each other. CO is generated with a maximum faradaic efficiency of around 30% at -0.7 VRHE, which is similar to that of bulk Ga. The combination of electrochemical, microscopic, and spectroscopic techniques, including operando X-ray absorption, indicates that the native oxide skin of the Ga NPs is still present during CO2RR and provides a barrier to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

Copper Phosphonate Lamella Intermediates Control the Shape of Colloidal Copper Nanocrystals.

Understanding the structure and behavior of intermediates in chemical reactions is the key to developing greater control over the reaction outcome. This principle is particularly important in the synthesis of metal nanocrystals (NCs), where the reduction, nucleation, and growth of the reaction intermediates will determine the final size and shape of the product. The shape of metal NCs plays a major role in determining their catalytic, photochemical, and electronic properties and, thus, the potential applications of the material. In this work, we demonstrate that layered coordination polymers, called lamellae, are reaction intermediates in Cu NC synthesis. Importantly, we discover that the lamella structure can be fine-tuned using organic ligands of different lengths and that these structural changes control the shape of the final NC. Specifically, we show that short-chain phosphonate ligands generate lamellae that are stable enough at the reaction temperature to facilitate the growth of Cu nuclei into anisotropic Cu NCs, being primarily triangular plates. In contrast, lamellae formed from long-chain ligands lose their structure and form spherical Cu NCs. The synthetic approach presented here provides a versatile tool for the future development of metal NCs, including other anisotropic structures.

Real-time Monitoring Reveals Dissolution/Redeposition Mechanism in Copper Nanocatalysts during the Initial Stages of the CO2 Reduction Reaction.

Size, morphology, and surface sites of electrocatalysts have a major impact on their performance. Understanding how, when, and why these parameters change under operating conditions is of importance for designing stable, active, and selective catalysts. Herein, we study the reconstruction of a Cu-based nanocatalysts during the startup phase of the electrochemical CO2 reduction reaction by combining results from electrochemical in situ transmission electron microscopy with operando X-ray absorption spectroscopy. We reveal that dissolution followed by redeposition, rather than coalescence, is the mechanism responsible for the size increase and morphology change of the electrocatalyst. Furthermore, we point out the key role played by the formation of copper oxides in the process. Understanding of the underlying processes opens a pathway to rational design of Cu electro (re)deposited catalysts and to stability improvement for catalysts fabricated by other methods.

Oxygen Evolution Reaction in Ba0.5Sr0.5Co0.8Fe0.2O3-δ Aided by Intrinsic Co/Fe Spinel-Like Surface.

Among the perovskites used to catalyze the oxygen evolution reaction (OER), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) exhibits excellent activity which is thought to be related to dynamic reconstruction at the flexible perovskite surface due to accommodation of large amount of oxygen vacancies. By studying the local structure and chemistry of BSCF surfaces, in detail, via a range of transmission electron microscopy (TEM) methods, we show that the surfaces of the as-synthesized BSCF particles are Co/Fe rich, and remarkably, adopt a spinel-like structure with a reduced valence of Co ions. Post-mortem and identical location TEM analyses reveal that the Co/Fe spinel-like surface retains a stable chemical environment of the Co/Fe ions, although its structure weakens after electrochemical processing. Further, it is verified that prior to the onset of OER, the Co/Fe spinel-like surface promotes the formation of the highly active Co(Fe)OOH phase, which enhances the OER electrocatalytic properties of the underlying conductive BSCF perovskite. This study provides a detailed understanding of the fundamental transformations that oxide catalysts undergo during electrochemical processes and can aid in the development of novel oxide catalysts with enhanced activity.

Stability and Degradation Mechanisms of Copper-Based Catalysts for Electrochemical CO2 Reduction.

To date, copper is the only monometallic catalyst that can electrochemically reduce CO2 into high value and energy-dense products, such as hydrocarbons and alcohols. In recent years, great efforts have been directed towards understanding how its nanoscale structure affects activity and selectivity for the electrochemical CO2 reduction reaction (CO2 RR). Furthermore, many attempts have been made to improve these two properties. Nevertheless, to advance towards applied systems, the stability of the catalysts during electrolysis is of great significance. This aspect, however, remains less investigated and discussed across the CO2 RR literature. In this Minireview, the recent progress on understanding the stability of copper-based catalysts is summarized, along with the very few proposed degradation mechanisms. Finally, our perspective on the topic is given.

Nanocrystal/Metal-Organic Framework Hybrids as Electrocatalytic Platforms for CO2 Conversion.

The tunable chemistry linked to the organic/inorganic components in colloidal nanocrystals (NCs) and metal-organic frameworks (MOFs) offers a rich playground to advance the fundamental understanding of materials design for various applications. Herein, we combine these two classes of materials by synthesizing NC/MOF hybrids comprising Ag NCs that are in intimate contact with Al-PMOF ([Al2 (OH)2 (TCPP)]) (tetrakis(4-carboxyphenyl)porphyrin (TCPP)), to form Ag@Al-PMOF. In our hybrids, the NCs are embedded in the MOF while still preserving electrical contact with a conductive substrate. This key feature allows the investigation of the Ag@Al-PMOFs as electrocatalysts for the CO2 reduction reaction (CO2 RR). We show that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO2 RR. We also demonstrate a minor contribution of mass-transfer effects imposed by the porous MOF layer under the chosen testing conditions. Furthermore, we find an increased morphological stability of the Ag NCs when combined with the Al-PMOF. The synthesis method is general and applicable to other metal NCs, thus revealing a new way to think about rationally tailored electrocatalytic materials to steer selectivity and improve stability.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications.

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λmax ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF6- salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities ß0 range from moderate to large: (9-140) × 10-30 esu from HRS in MeCN and (44-580) × 10-30 esu from the Stark data in PrCN. The magnitude of ß0 increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The ßtot values predicted by using DFT at the same level of theory are large ((472-1443) × 10-30 esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response.

Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses.

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities ß0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured ß0 values are similar to or substantially larger than that of the commercial chromophore E-4'-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT) and time-dependent DFT calculations on two of the new cations are used to probe their molecular electronic structures and optical properties. Related molecules are expected to show bulk second-order NLO effects in even nonpolar media, overcoming a key challenge in developing useful materials.

Intense chiroptical switching in a dicationic helicene-like derivative: exploration of a viologen-type redox manifold of a non-racemic helquat.

Two-step redox switching in enantiopure helquat system [P-1](2+) ⇌ [P-1](•+) ⇌ [P-1](0) is demonstrated. The viologen-type electroactive unit embedded directly in the helical scaffold of 1 is responsible for the prominent chiroptical switching at 264 nm. This process is associated with a marked sign-reversal of Cotton effect ramping between Δε = +35 M(-1) cm(-1) for [P-1](2+) and Δε = -100 M(-1) cm(-1) for [P-1](0). This helically chiral system features the most intense chiroptical switch response documented in the field of helicenoids.

An insight into market and non-market alternative food networks in Czechia during Covid-19 and beyond.

Introduction: The Covid-19 pandemic affected food systems in many countries and emphasized a lot of already existing social, economic and environmental agri-food problems. Alternative food networks (AFNs), praised for their ability to improve the food systems, were under stress, however, at the same time, the changed conditions may have opened new possibilities. In this paper we address the importance of AFNs during the pandemic and investigate how households have changed their participation in AFNs. Our research is novel by simultaneously focusing on both market and non-market AFNs which are often studied separately. Methods: A representative questionnaire study of Czech households was carried out in Autumn 2021 to provide a case study of food and consumption behavior of the European country after several waves of Covid-19. Results and discussion: Based on the responses of 515 participants, the results show that 68% of Czech households participate in some form of AFNs, be it shopping or food self-provisioning, i.e., non-market food procurement in the form of gardening. Focusing on the market AFNs, farmers' markets and farm gate sales are the most popular. Covid-19 and 2021 emerging economic pressures led to a decrease of consumption of organic food (22% of respondents) as well as fresh fruit and vegetables in general (10% of respondents) and a noticeable occurrence of food insecurity (18% of households). Based on these findings, the paper discusses the ability of AFNs to support food and nutritional resilience. Problems which may endanger market-oriented AFNs are discussed as well. Conclusion: By addressing both market and non-market AFNs, the paper brings new knowledge into the food environment and agri-food policies research.

Search for conglomerate in set of [7]helquat salts: multigram resolution of helicene-viologen hybrid by preferential crystallization.

Investigation of a set of 12 [7]helquat salts by X-ray crystal diffraction led to identification of conglomerate behavior in bis(trifluoroacetate) salt [2][CF(3)CO(2)](2). This is to demonstrate that a systematic search for conglomerates can be performed for a given helicenoid enabling straightforward multigram resolution via preferential crystallization. Subsequently, preferential crystallization of this chiral helicene-viologen hybrid has been established to obtain pure P and M enantiomers on a multigram scale, 5 g each. Furthermore, preparation of nonracemic samples of [7]helquat 2 via diastereomeric (R,R)-dibenzoyltartrate salts is described, and determination of absolute configuration and racemization barrier is also reported.

Helquat-induced chiroselective aggregation of Au NPs.

Au nanoparticles (NPs) are functionalized with chiral (R) or (S) binaphthol phenylboronic acid ligands, (1a) or (1b). The (R)- or (S)-binaphthol phenylboronic acid ligands form donor-acceptor complexes with the chiral dicationic helicene, helquat (P)-HQ(2+) or (M)-HQ(2+), (2a) or (2b). The association constants between (1a)/(2a) and (1a)/(2b) correspond to (7.0 ± 0.5) × 10(5) M(-1) and (2.5 ± 0.3) × 10(5) M(-1), respectively, whereas the association constants between (1b)/(2b) and (1b)/(2a) correspond to (4.0 ± 0.5) × 10(5) M(-1) and (1.8 ± 0.3) × 10(5) M(-1), respectively. Chiroselective aggregation of chiral binaphthol phenylboronic acid-capped Au NPs triggered by the chiral helquats, is demonstrated.

Metastable Brominated Nanodiamond Surface Enables Room Temperature and Catalysis-Free Amine Chemistry.

Bromination of high-pressure, high-temperature (HPHT) nanodiamond (ND) surfaces has not been explored and can open new avenues for increased chemical reactivity and diamond lattice covalent bond formation. The large bond dissociation energy of the diamond lattice-oxygen bond is a challenge that prevents new bonds from forming, and most researchers simply use oxygen-terminated NDs (alcohols and acids) as reactive species. In this work, we transformed a tertiary-alcohol-rich ND surface to an amine surface with ∼50% surface coverage and was limited by the initial rate of bromination. We observed that alkyl bromide moieties are highly labile on HPHT NDs and are metastable as previously found using density functional theory. The strong leaving group properties of the alkyl bromide intermediate were found to form diamond-nitrogen bonds at room temperature and without catalysts. This robust pathway to activate a chemically inert ND surface broadens the modalities for surface termination, and the unique surface properties of brominated and aminated NDs are impactful to researchers for chemically tuning diamond for quantum sensing or biolabeling applications.

Chiral analysis of helquats by capillary electrophoresis: resolution of helical N-heteroaromatic dications using randomly sulfated cyclodextrins.

Enantiomers of helical N-heteroaromatic dications, helquats, were separated by CE. An acidic 22/35 mM sodium/phosphate background electrolyte, pH 2.4, with addition of randomly sulfated α-, ß- and γ- cyclodextrins allowed enantioresolution of a series of helquats, which comprised 5, 6 and 7 fused rings participating in the helical backbone. In general, at least one of the chiral selectors was found to provide baseline separation for 22 out of 24 helquats and partial separation for the remaining two. Individually, the sulfated γ-cyclodextrin turned out to separate 79% of the helquats, followed by the ß- and α-congeners with 54 and 42% of the resolved compounds, respectively. Migration order of enantiomers was inspected for selected helquats and a relation of molecular size of the analytes to a cavity of the cyclodextrin selectors is discussed.

Elucidating the structure-dependent selectivity of CuZn towards methane and ethanol in CO2 electroreduction using tailored Cu/ZnO precatalysts.

Understanding the catalyst compositional and structural features that control selectivity is of uttermost importance to target desired products in chemical reactions. In this joint experimental-computational work, we leverage tailored Cu/ZnO precatalysts as a material platform to identify the intrinsic features of methane-producing and ethanol-producing CuZn catalysts in the electrochemical CO2 reduction reaction (CO2RR). Specifically, we find that Cu@ZnO nanocrystals, where a central Cu domain is decorated with ZnO domains, and ZnO@Cu nanocrystals, where a central ZnO domain is decorated with Cu domains, evolve into Cu@CuZn core@shell catalysts that are selective for methane (∼52%) and ethanol (∼39%), respectively. Operando X-ray absorption spectroscopy and various microscopy methods evidence that a higher degree of surface alloying along with a higher concentration of metallic Zn improve the ethanol selectivity. Density functional theory explains that the combination of electronic and tandem effects accounts for such selectivity. These findings mark a step ahead towards understanding structure-property relationships in bimetallic catalysts for the CO2RR and their rational tuning to increase selectivity towards target products, especially alcohols.

Ligand-mediated formation of Cu/metal oxide hybrid nanocrystals with tunable number of interfaces.

Combining domains of different chemical nature within the same hybrid material through the formation of heterojunctions provides the opportunity to exploit the properties of each individual component within the same nano-object; furthermore, new synergistic properties will often arise as a result of unique interface interactions. However, synthetic strategies enabling precise control over the final architecture of multicomponent objects still remain scarce for certain classes of materials. Herein, we report on the formation of Cu/MO x (M = Ce, Zn and Zr) hybrid nanocrystals with a tunable number of interfaces between the two domains. We demonstrate that the organic ligands employed during the synthesis play a key role in regulating the final configuration. Finally, we show that the synthesized nanocrystals serve as materials platforms to investigate the impact of the Cu/metal oxide interfaces in applications by focusing on the electrochemical CO2 reduction reaction as one representative example.

N-Substituted Prodrugs of Mebendazole Provide Improved Aqueous Solubility and Oral Bioavailability in Mice and Dogs.

Mebendazole (MBZ) was developed as a broad-spectrum anthelmintic but has recently shown efficacy as an anticancer agent. The use of MBZ for cancer, however, is challenging due to its poor solubility leading to poor bioavailability. Herein, we developed a prodrug approach with various N-linked promoieties including acyloxymethyl, aminoacyloxymethyl, and substituted phosphonooxymethyl in attempt to improve these characteristics. Compound 12, containing an (((((isopropoxycarbonyl)oxy)methoxy)phosphoryl)oxy)methyl promoiety, showed a >10 000-fold improvement in aqueous solubility. When evaluated in mice, 12 displayed a 2.2-fold higher plasma AUC0- t and a 1.7-fold improvement in brain AUC0- t with a calculated oral bioavailability of 52%, as compared to 24% for MBZ-polymorph C (MBZ-C), the most bioavailable polymorph. In dogs, 12 showed a 3.8-fold higher plasma AUC0- t with oral bioavailability of 41% compared to 11% for MBZ-C. In summary, we have identified a prodrug of MBZ with better physicochemical properties and enhanced bioavailability in both mice and dog.

Anchored but not internalized: shape dependent endocytosis of nanodiamond.

Nanoparticle-cell interactions begin with the cellular uptake of the nanoparticles, a process that eventually determines their cellular fate. In the present work, we show that the morphological features of nanodiamonds (NDs) affect both the anchoring and internalization stages of their endocytosis. While a prickly ND (with sharp edges/corners) has no trouble of anchoring onto the plasma membrane, it suffers from difficult internalization afterwards. In comparison, the internalization of a round ND (obtained by selective etching of the prickly ND) is not limited by its lower anchoring amount and presents a much higher endocytosis amount. Molecular dynamics simulation and continuum modelling results suggest that the observed difference in the anchoring of round and prickly NDs likely results from the reduced contact surface area with the cell membrane of the former, while the energy penalty associated with membrane curvature generation, which is lower for a round ND, may explain its higher probability of the subsequent internalization.

Discovery of a para-Acetoxy-benzyl Ester Prodrug of a Hydroxamate-Based Glutamate Carboxypeptidase II Inhibitor as Oral Therapy for Neuropathic Pain.

4-Carboxy-α-[3-(hydroxyamino)-3-oxopropyl]-benzenepropanoic acid 1 is a potent hydroxamate-based inhibitor of glutamate carboxypeptidase II. In an attempt to improve its poor oral pharmacokinetics, we synthesized a series of prodrugs by masking its hydrophilic hydroxamate group. Prodrugs were evaluated for oral availability in mice and showed varying degree of plasma exposure to 1. Of these, para-acetoxybenzyl-based, 4-(5-(((4-acetoxybenzyl)oxy)amino)-2-carboxy-5-oxopentyl)benzoic acid, 12, provided 5-fold higher plasma levels of 1 compared to oral administration of 1 itself. Subsequently, para-acetoxybenzyl-based prodrugs with additional ester promoiety(ies) on carboxylate(s) were examined for their ability to deliver 1 to plasma. Isopropyloxycarbonyloxymethyl (POC) ester 30 was the only prodrug that achieved substantial plasma levels of 1. In vitro metabolite identification studies confirmed stability of the ethyl ester of benzoate while the POC group was rapidly hydrolyzed. At oral daily dose-equivalent of 3 mg/kg, 12 exhibited analgesic efficacy comparable to dose of 10 mg/kg of 1 in the rat chronic constrictive injury model of neuropathic pain.

Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores.

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV-vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities ß are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF6- salt, revealing a centrosymmetric packing in the triclinic space group P1[combining macron]. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical ß values are reasonably large. Oxidation of the Fc unit is predicted to cause the ßtot value to decrease by more than 80% in one of the complexes.