Supramolecular nesting of cyclic polymers

Advances in template-directed synthesis make it possible to create artificial molecules with protein-like dimensions, directly from simple components. These synthetic macromolecules have a proclivity for self-organization that is reminiscent of biopolymers. Here, we report the synthesis of monodisperse cyclic porphyrin polymers, with diameters of up to 21 nm (750 C–C bonds). The ratio of the intrinsic viscosities for cyclic and linear topologies is 0.72, indicating that these polymers behave as almost ideal flexible chains in solution. When deposited on ​gold surfaces, the cyclic polymers display a new mode of two-dimensional supramolecular organization, combining encapsulation and nesting; one nanoring adopts a near-circular conformation, thus allowing a second nanoring to be captured within its perimeter, in a tightly folded conformation. Scanning tunnelling microscopy reveals that nesting occurs in combination with stacking when nanorings are deposited under vacuum, whereas when they are deposited directly from solution under ambient conditions there is stacking or nesting, but not a combination of both

Directing Cluster Formation of Au Nanoparticles from Colloidal Solution

Discrete clusters of closely spaced Au nanoparticles can be utilized in devices from photovoltaics to molecular sensors because of the formation of strong local electromagnetic field enhancements when illuminated near their plasmon resonance. In this study, scalable, chemical self-organization methods are shown to produce Au nanoparticle clusters with uniform nanometer interparticle spacing. The performance of two different methods, namely electrophoresis and diffusion, for driving the attachment of Au nanoparticles using a chemical cross-linker on chemically patterned domains of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films are evaluated. Significantly, electrophoresis is found to produce similar surface coverage as diffusion in 1/6th of the processing time with an ~2-fold increase in the number of Au nanoparticles forming clusters. Furthermore, average interparticle spacing within Au nanoparticle clusters was found to decrease from 2-7 nm for diffusion deposition to approximately 1-2 nm for electrophoresis deposition, and the latter method exhibited better uniformity with most clusters appearing to have about 1 nm spacing between nanoparticles. The advantage of such fabrication capability is supported by calculations of local electric field enhancements using electromagnetic full-wave simulations from which we can estimate surface-enhanced Raman scattering (SERS) enhancements. In particular, full-wave results show that the maximum SERS enhancement, as estimated here as the fourth power of the local electric field, increases by a factor of 100 when the gap goes from 2 to 1 nm, reaching values as large as 10(10), strengthening the usage of electrophoresis versus diffusion for the development of molecular sensors

Étude de quelques polyisoprènes cycliques par fractionnement et par chromatographie de partage en phase liquide

Les polyisoprènes cycliques ont des propriétés et des structures très différentes de celles des polyisoprènes linéaires. Nous avons fractionné trois échantillons par précipitation sur colonne à l’aide d’un appareil préparatif. Les fractions ainsi que les échantillons non fractionnés ont été étudiés par chromatographie sur gel. Comme nous avions mesuré leurs masses moléculaires par diffusion de la lumière et ébulliométrie nous avons pu tracer la courbe log M en fonction du volume de rétention. En connaissant les valeurs des viscosités intrinsèques, nous avons pu aussi tracer la courbe d’étalonnage universelle qui s’applique aussi à ce type de polyisoprène. A partir de la courbe d’étalonnage classique corrigée de la polydispersité des fractions, on a déterminé les rapports Mw/Mn qui passent de la valeur 1,3 pour une fraction de Mw = 2.103 jusqu’à la valeur 2,7 pour une fraction de Mw = 7,5.105. Les chromatogrammes des fractions de Mw > 106 présentent deux pics; une explication de cette double distribution a été proposée. Les chromatogrammes des échantillons non fractionnés présentent soit un seul maximum (M de l’ordre de 103, Mw/Mn ≤ 10), soit deux maxima (M de l’ordre de 103 et de 105, Mw/Mn compris entre 10 et 100), soit un maximum (M de l’ordre de 103) suivi d’une queue qui s’étend vers les masses moléculaires élevées de l’ordre de 106 et 107 ; pour ces échantillons le rapport Mw/Mn est supérieur à 100. Les courbes cumulatives de répartition en masse ont été construites à partir des chromatogrammes ainsi qu’à partir du fractionnement; leur bon accord démontre une efficacité satisfaisante de la colonne de fractionnement. A partir de ces résultats il est possible d’apporter quelques précisions sur le mécanisme de la polymérisation

Covid-19 pandemic: Is the crypto market a safe haven? the impact of the first wave

The present study investigated whether the crypto market is a safe haven. The study argues that during the first wave of the COVID-19 crisis, gold and oil, as typical global commodities, could have been diversifiers. The study developed a unique COVID-19 global composite index that measures COVID-19 pandemic time-variant movements on each day. The study used OLS (ordinary least squares), quantile, and robust regressions to check whether the COVID-19 crisis has had any significant direct influence on the crypto market. The OLS, quantile, and robust regressions estimates confirmed that there was no statistically significant direct influence of the COVID-19 crisis on the crypto market in the first wave period. However, the study found spillovers from risky assets (S&P 500) on the crypto market, with Tether as an exception. Due to this special characteristic, Tether might present a safe haven within the crypto market. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Correction: Vukovic et al. COVID-19 pandemic: Is the crypto market a safe haven? the impact of the first wave. Sustainability 2021, 13, 8578

The authors would like to make the following corrections to the published paper [1]. The changes are as follows: (1) To clearly indicate, the authors wish to add a citation in “Section 2. Theoretical Background”. The study by [15] forecasts the impact of COVID-19 will be huge, both economically and socially. with The study by [15,16] forecasts the impact of COVID-19 will be significant, both economically and socially. (2) Replacing the citation in “Section 2. Theoretical Background”: In the study of [16], wavelet methods were applied to examine the impact of COVID-19 on Bitcoin prices. with In the study of [17], wavelet methods were applied to examine the impact of COVID-19 on Bitcoin prices. (3) To clearly indicate, the authors wish to add a citation in “Section 2. Theoretical Background”. By using specifications that can account for structural breaks in GARCH, namely Markov switching GARCH models, the authors of [20] analyzed Bitcoin daily log returns exhibiting regime changes in their volatility dynamics. with By using specifications that can account for structural breaks in GARCH, namely Markov switching GARCH models, the authors of [20,21] analyzed Bitcoin daily log returns exhibiting regime changes in their volatility dynamics. (4) Replacing the citation in “Section 2. Theoretical Background”: The study by [21] tested two hypotheses regarding the role of a Tether in the crypto world, especially on Bitcoin. © 2021 by the authors. Licensee MDPI, Basel, Switzerland