Sol–gel derived mesoporous Pt and Cr-doped WO<Subscript>3</Subscript> thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties

Mesoporous Cr or Pt-doped WO₃ thin films to be employed as ammonia gas sensors were prepared by a fast one-step sol–gel procedure, based on the use of triblock copolymer as templating agent. The obtained films were constituted by aggregates of interconnected WO₃ nanocrystals (20–50 nm) separated by mesopores with dimensions ranging between 2 and 15 nm. The doping metals, Pt and Cr, resulted differently hosted in the WO₃ mesoporous matrix. Chromium is homogeneously dispersed in the oxide matrix, mainly as Cr(III) and Cr(V) centers, as revealed by EPR spectroscopy; instead platinum segregated as Pt (0) nanoparticles (4 nm) mainly included inside the WO₃ nanocrystals. The semiconductor layers containing Pt nanoclusters revealed, upon exposure to NH₃, remarkable electrical responses, much higher than Cr-doped and undoped layers, particularly at low ammonia concentration (6.2 ppm). This behavior was attributed to the presence of Pt nanoparticles segregated inside the semiconductor matrix, which act as catalysts of the N–H bond cleavage, decreasing the activation barrier in the ammonia dissociation. The role of the mesoporous structure in influencing the chemisorption and the gas diffusion in the WO₃ matrix appeared less decisive than the electronic differences between the two examined doping metals. The overall results suggest that a careful combination between mesoporous architecture and metal doping can really promote the electrical response of WO₃ toward ammonia.     

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials

The self‐assembly and self‐organization behavior of chromophoric acetylenic scaffolds bearing 2,6‐bis(acetylamino)pyridine ( 1 , 2 ) or uracyl‐type ( 3 – 9 ) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2 , thanks to a combination of solvophilic/solvophobic forces and π–π stacking interactions, undergo self‐organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide‐field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2 , a more uniform size distribution is found (80–200 nm) compared to 1 (20–1000 nm). Temperature scans in the range 283–353 K show that the self‐organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen‐bonding units 3 – 9 . Depending on the specific geometrical structure of 3 – 9 , different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3 , which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis‐uracyl derivative 4 or 5 , a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3 , which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self‐assembly was studied in parallel with modules 8 and 9 , in which the uracyl recognition sites are protected with tert‐butyloxycarbonyl (BOC) groups. This strategy allows further control of the self‐assembly/self‐organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC‐protected ditopic modules 8 self‐assemble and self‐organize with 1 into ordered linear nanostructures, whereas BOC‐protected tritopic system 9 gives rise to extended domains of circular nano‐objects in combination with 1 .     

Optimization of the antioxidant activity of hydroxy-substituted 4-thiaflavanes: a proof-of-concept study

The design and the synthesis of a new family of hydroxy-4-thiaflavanes, in which the reactive phenolic OH is ortho to the sulfur atom of the benzofused oxathiin ring, allowed to prepare antioxidants that show rate constants for the reaction with peroxyl radicals (k(inh)), and bond dissociation energies (BDE), of the ArO-H group identical to those of α-tocopherol, the main component of vitamin E and the most effective lipophilic antioxidant known in nature. The peculiar conformation of the six-membered heterocyclic ring prevents the formation of an intramolecular hydrogen bond between the OH group and the S atom, while ensuring a good stabilization by electron donation of the phenoxyl radical formed after the reaction with peroxyl radicals. The preparation of these compounds was achieved through an inverse electron demand hetero Diels-Alder reaction of styrenes with o-thioquinones, in turn prepared from accurately designed 1,3-dihydroxy arenes. Properly arranging the substitution pattern on the aromatic ring, as in derivatives 9 and 11, allowed to reach values of k(inh) up to 4.0×10(6) M(-1) s(-1) and BDE((OH)) of 77.2 kcal mol(-1). This approach represents an innovative way to obtain highly active antioxidants without using strongly electron donating alkylamino groups which are associated with adverse toxicological profiles.     

Cell adhesion on nanotextured slippery superhydrophobic substrates

In this work, the response of Saos2 cells to polymeric surfaces with different roughness/density of nanometric dots produced by a tailored plasma-etching process has been studied. Topographical features have been evaluated by atomic force microscopy, while wetting behavior, in terms of water-surface adhesion energy, has been evaluated by measurements of drop sliding angle. Saos2 cytocompatibility has been investigated by scanning electron microscopy, fluorescent microscopy, and optical microscopy. The similarity in outer chemical composition has allowed isolation of the impact of the topographical features on cellular behavior. The results indicate that Saos2 cells respond differently to surfaces with different nanoscale topographical features, clearly showing a certain inhibition in cell adhesion when the nanoscale is particularly small. This effect appears to be attenuated in surfaces with relatively bigger nanofeatures, though these express a more pronounced slippery/dry wetting character.     

The Reaction of Sulfenic Acids with Peroxyl Radicals: Insights into the Radical‐Trapping Antioxidant Activity of Plant‐Derived Thiosulfinates

Sulfenic acids play a prominent role in biology as key participants in cellular signaling relating to redox homeostasis, in the formation of protein‐disulfide linkages, and as the central players in the fascinating organosulfur chemistry of the Allium species (e.g., garlic). Despite their relevance, direct measurements of their reaction kinetics have proven difficult owing to their high reactivity. Herein, we describe the results of hydrocarbon autoxidations inhibited by the persistent 9‐triptycenesulfenic acid, which yields a second order rate constant of 3.0×10⁶ M ^?1 s^?1 for its reaction with peroxyl radicals in PhCl at 30 °C. This rate constant drops 19‐fold in CH₃CN, and is subject to a significant primary deuterium kinetic isotope effect, k_H/k_D=6.1, supporting a formal H‐atom transfer (HAT) mechanism. Analogous autoxidations inhibited by the Allium‐derived (S)‐benzyl phenylmethanethiosulfinate and a corresponding deuterium‐labeled derivative unequivocally demonstrate the role of sulfenic acids in the radical‐trapping antioxidant activity of thiosulfinates, through the rate‐determining Cope elimination of phenylmethanesulfenic acid (k_H/k_D≈4.5) and its subsequent formal HAT reaction with peroxyl radicals (k_H/k_D≈3.5). The rate constant that we derived from these experiments for the reaction of phenylmethanesulfenic acid with peroxyl radicals was 2.8×10⁷ M ^?1 s^?1; a value 10‐fold larger than that we measured for the reaction of 9‐triptycenesulfenic acid with peroxyl radicals. We propose that whereas phenylmethanesulfenic acid can adopt the optimal syn geometry for a 5‐centre proton‐coupled electron‐transfer reaction with a peroxyl radical, the 9‐triptycenesulfenic is too sterically hindered, and undergoes the reaction instead through the less‐energetically favorable anti geometry, which is reminiscent of a conventional HAT.     

RNA-based therapeutics: current progress and future prospects

Recent advances of biological drugs have broadened the scope of therapeutic targets for a variety of human diseases. This holds true for dozens of RNA-based therapeutics currently under clinical investigation for diseases ranging from genetic disorders to HIV infection to various cancers. These emerging drugs, which include therapeutic ribozymes, aptamers, and small interfering RNAs (siRNAs), demonstrate the unprecedented versatility of RNA. However, RNA is inherently unstable, potentially immunogenic, and typically requires a delivery vehicle for efficient transport to the targeted cells. These issues have hindered the clinical progress of some RNA-based drugs and have contributed to mixed results in clinical testing. Nevertheless, promising results from recent clinical trials suggest that these barriers may be overcome with improved synthetic delivery carriers and chemical modifications of the RNA therapeutics. This review focuses on the clinical results of siRNA, RNA aptamer, and ribozyme therapeutics and the prospects for future successes.     

Charge localization in multiply charged clusters and their electrical properties: some insights into electrospray droplets

The surface composition of charged Lennard-Jones clusters A(N) (n+), composed of N particles (55 ≤ N ≤ 1169) among which n are positively charged with charge q, thus having a net total charge Q = nq, is investigated by Monte Carlo with Parallel Tempering simulations. At finite temperature, the surface sites of these charged clusters are found to be preferentially occupied by charged particles carrying large charges, due to Coulombic repulsions, but the full occupancy of surface sites is rarely achieved for clusters below the stability limit defined in this work. Large clusters (N = 1169) follow the same trends, with a smaller propensity for positive particles to occupy the cluster surface at non-zero temperature. We show that these charged clusters rather behave as electrical spherical conductors for the smaller sizes (N ≤ 147) but as spheres uniformly charged in their volume for the larger sizes (N = 1169).     

Thermal and environmental effects on Oligothiophene low-energy singlet electronic excitations in dilute solution: a theoretical and experimental study

The absorption spectra in dilute dichloromethane solution at 300 K of four Oligotiophenes (OT), namely 2,2′:5′,2″-Terthiophene, 2,2′:5′,2″:5″,2″′-Quaterthiophene, 4,4″′-Didodecyl-2,2′:5′,2″:5″,2″′-quaterthiophene and 5,5′′′′′-Dihexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene, have been studied both experimentally and theoretically by using a combination of molecular dynamics simulations, time-dependent density functional theory (TD-DFT) and perturbed matrix method calculations. A deep analysis of the theoretical results, affected by a systematic although not dramatic underestimation of the absorption maxima due to the well-documented TD-DFT limitations, clearly indicates that both the environmental (solvent) and thermal effects significantly alter the Oligotiophenes photophysical properties mainly because of the wide repertoire of the S₀–S₁ energy gaps and electronic densities experienced in solution by the different conformers actually populated. In particular, all the investigated OT display a very high flexibility resulting in a very high repertoire of sampled conformations. The comparison of the calculated and experimental lineshape of the S₀–S₁ electronic transition has clearly indicated that for a correct modeling of OT spectral features, the lack of an exhaustive sampling of semiclassical configurational space of the overall system, i.e., solute in interaction with the solvent, might result in an incomplete picture even in the presence of well-documented important aspects such as reliable definition of excited electronic states and the inclusion of quantum vibronic effects.     

The reactivity of air-stable pyridine- and pyrimidine-containing diarylamine antioxidants

We recently reported a preliminary account of our efforts to develop novel diarylamine radical-trapping antioxidants (Hanthorn et al. J. Am. Chem. Soc.2012, 134, 8306-8309), wherein we demonstrated that the incorporation of ring nitrogens into diphenylamines affords compounds that display a compromise between H-atom transfer reactivity to peroxyl radicals and stability to one-electron oxidation. Herein, we report the results of thermochemical and kinetic experiments on an expanded set of diarylamines (see the accompanying paper, DOI: 10.1021/jo301013c ), which provide a more complete picture of the structure-reactivity relationships of these compounds as antioxidants. Nitrogen incoporation into a series of alkyl-, alkoxyl-, and dialkylamino-substituted diphenylamines raises their oxidation potentials systematically with the number of nitrogen atoms, resulting in overall increases of 0.3-0.5 V on going from the diphenylamines to the dipyrimidylamines. At the same time, the effect of nitrogen incorporation on their reactivity toward peroxyl radicals was comparatively small (a decrease of only 6-fold at most), which is also reflected in their N-H bond dissociation enthalpies. Rate constants for reactions of dialkylamino-substituted diarylamines with peroxyl radicals were found to be >10(7) M(-1) s(-1), which correspond to the pre-exponential factors that we obtained for a representative trio of compounds (log A ～ 7), indicating that the activation energies (E(a)) are negligible for these reactions. Comparison of our thermokinetic data for reactions of the diarylamines with peroxyl radicals with literature data for reactions of phenols with peroxyl radicals clearly reveals that diarylamines have higher inherent reactivities, which can be explained by a proton-coupled electron-transfer mechanism for these reactions, which is supported by theoretical calculations. A similar comparison of the reactivities of diarylamines and phenols with alkyl radicals, which must take place by a H-atom transfer mechanism, clearly reveals the importance of the polar effect in the reactions of the more acidic phenols, which makes phenols comparatively more reactive.