Cation Radicals of Covalently Linked Porphyrin Dimers and their Monomeric Constituents: An ESR and ENDOR Study

The spin density distributions in the cation radicals of various covalently linked porphyrin dimers have been studied by liquid phase ESR and ENDOR methods to find out whether these systems show intramolecular electron delocalization. Such a delocalization is known to occur in the bacteriochlorophyll dimer (“special pair”) in the photosynthetic reaction center. The dimers that were studied in this work were derived from zinc mesotetratolylporphyrin (ZnTTP) and linked at the ortho or para positions of one phenyl ring with varying bridge lengths. ¹H and ^I4N hyperfine coupling constants could be measured for the dimer cation radicals and compared with those of monomeric ZnTTP as well as ZnTTP derivatives that carry alkoxy or hydroxy substituents to mimic the bridges of the dimers. By comparing the hyperfine data of the monomer and dimer cation radicals it is concluded that in the present dimers the unpaired electron is localized on one porphyrin unit.     

Attention-based dynamic visual search using inner-scene similarity: algorithms and bounds

A visual search is required when applying a recognition process on a scene containing multiple objects. In such cases, we would like to avoid an exhaustive sequential search. This work proposes a dynamic visual search framework based mainly on inner-scene similarity. Given a number of candidates (e.g., subimages), we hypothesize is that more visually similar candidates are more likely to have the same identity. We use this assumption for determining the order of attention. Both deterministic and stochastic approaches, relying on this hypothesis, are considered. Under the deterministic approach, we suggest a measure similar to Kolmogorov's epsilon-covering that quantifies the difficulty of a search task. We show that this measure bounds the performance of all search algorithms and suggest a simple algorithm that meets this bound. Under the stochastic approach, we model the identity of the candidates as a set of correlated random variables and derive a search procedure based on linear estimation. Several experiments are presented in which the statistical characteristics, search algorithm, and bound are evaluated and verified.     

Nano Spray‐Dried Block Copolymer Nanoparticles and Their Transformation into Hybrid and Inorganic Nanoparticles

A novel combination of block copolymer (BCP) nano spray‐drying (NSD), solvent annealing, and selective metal oxide growth is utilized to create functional polymer nanoparticles, polymer‐metal‐oxide hybrid nanoparticles, and templated metal oxide nanoparticles with tunable composition, internal morphology, and porosity. NSD of BCPs from chloroform and toluene solutions results in porous and nonporous nanoparticles, respectively, with various degrees of phase separation. Further tuning of the nanoparticle internal morphology is performed by solvent annealing the spray‐dried particles with judicious choice of the nonsolvent dispersion medium and the surfactant, yielding assembly of both blocks at the surface of the nanoparticles. Finally, ZnO and Al₂O₃ are grown inside the polar blocks of phase‐ordered nanoparticles using a sequential infiltration synthesis method, in a post‐assembly process, resulting in hybrid BCP‐ZnO particles and BCP‐templated Al₂O₃ nanoparticles, as demonstrated by scanning transmission electron microscopy tomography. These structure engineering methods open new ways to direct and template functional nanoparticles.     

Metal Oxide Heterostructure Array via Spatially Controlled–Growth within Block Copolymer Templates

Nanofabrication is continuously searching for new methodologies to fabricate 3D nanostructures with 3D control over their chemical composition. A new approach for heterostructure nanorod array fabrication through spatially controlled–growth of multiple metal oxides within block copolymer (BCP) templates is presented. Selective growth of metal oxides within the cylindrical polymer domains of polystyrene‐block‐poly methyl methacrylate is performed using sequential infiltration synthesis (SIS). Tuning the diffusion of trimethyl aluminum and diethyl zinc organometallic precursors in the BCP film directs the growth of AlO_x and ZnO to different locations within the cylindrical BCP domains, in a single SIS process. BCP removal yields an AlO_x‐ZnO heterostructure nanorods array, as corroborated by 3D characterization with scanning transmission electron microscopy (STEM) tomography and a combination of STEM and energy‐dispersive X‐ray spectroscopy tomography. The strategy presented here will open up new routes for complex 3D nanostructure fabrication.     

Co-Doped MoSe2 Nanoflowers as Efficient Catalysts for Electrochemical Hydrogen Evolution Reaction (HER) in Acidic and Alkaline Media

Transition metal dichalcogenides (TMDCs) based materials are considered as highly active alternatives to the precious Pt-based catalysts for the hydrogen evolution reaction (HER). In particular, MoSe₂ emerges as a promising catalyst due to its abundance and electrochemical stability, but further modifications are still required to enhance its performance, specifically in alkaline conditions. Here, we developed a method to obtain MoSe₂ with two cobalt doping patterns: homogeneously doped and edge doped nanoflowers, with abundant edge sites and extended surface area. The results show that low concentration of doping enhances the catalytic activity toward HER. Incorporation of cobalt as a substituent dopant within the layered structure of MoSe₂ appears to have two major contributions: it changes the chemical environment providing more active sites with favored hydrogen adsorption properties, and improves the charge transfer resistance and thus facilitates the HER kinetics. Moreover, the homogeneous and edge-doped nanoflowers show different pH-dependence of HER activity. Edge-doped samples exhibit significantly improved performance in acidic medium, while the overpotential increases in alkaline environment upon doping. A mechanistic explanation of the observed effect is proposed. This work opens up an additional path for improving the catalytic activity of TMDCs in acidic or alkaline medium using a simple and facile method with only small quantities of dopants.     

Effective time step restrictions for explicit MPM simulation

Time steps for explicit MPM simulation in computer graphics are often selected by trial and error due to the challenges in automatically selecting stable time step sizes. Our time integration scheme uses time step restrictions that take into account forces, collisions, and even grid-to-particle transfers calculated near the end of the time step. We propose a novel set of time step restrictions that allow a time step to be selected that is stable, efficient to compute, and not too far from optimal. We derive the general solution for the sound speed in nonlinear isotropic hyperelastic materials, which we use to enforce the classical CFL time step restriction. We identify a single-particle instability in explicit MPM integration and propose a corresponding time step restriction in the fluid case. We also propose a reflection-based boundary condition for domain walls that supports separation and accurate Coulomb friction while preventing particles from penetrating the domain walls.     

Biologically Relevant Molecular Finite Automata

Bio-Molecular Computing (BMC) has been rapidly evolving as an independent field at the interface between computer science, mathematics, chemistry, and biology. Over the years, numerous architectures of autonomous molecular computing devices have been developed in the lab on the basis of opportunities offered by molecular biology techniques. This account focuses mainly on the realization of programmable DNA-based finite-state automata that can compute autonomously upon mixing all their components in solution.The main advantage of autonomous BMC devices over electronic computers arises from their ability to interact directly with biological systems and even with living organisms without any interface. Indeed, it has been demonstrated that appropriately designed computing machines can produce output signals in the form of a specific biological function via direct interaction with living cells. Additional topics are briefly included to point at interesting opportunities in the field and to describe some of the potential applications and extension of the basic concepts. These include logic evaluators and logic gates that operate in cells, applications in developmental biology, as well as chemical encoding and processing of alphanumeric information.