Younan Xia

Bio: Younan Xia is an academic researcher from The Wallace H. Coulter Department of Biomedical Engineering. The author has contributed to research in topics: Nanocages & Nanowire. The author has an hindex of 216, co-authored 943 publications receiving 175757 citations. Previous affiliations of Younan Xia include Washington University in St. Louis & University of Texas at Dallas.

Pd‐Catalyzed Growth of Pt Nanoparticles or Nanowires as Dense Coatings on Polymeric and Ceramic Particulate Supports

Abstract: This Communication describes an approach that can control the growth of Pt into shells that consist of nanoparticles or nanowires on colloidal spheres. Core/shell particles have been extensively studied largely because of their attractive properties (optical, mechanical, magnetic, or catalytic) that are often different from their bulk counterparts. As a result of their unique features, core/shell particles hold promise in potential applications such as controlled delivery, catalysis, magnetic information storage, optical sensing, and confinement of reactions. Many research efforts have been directed towards the development of new techniques for “engineering” such materials with well-controlled properties. In general, the properties of core/shell particles can be tailored precisely by varying the composition, dimension, and structure of the cores or shells. One-dimensional (1D) nanostructures, especially nanowires, have attracted much attention due to their potential use as interconnects in fabricating electronic devices. However, with respect to the procedure and cost effectiveness, producing nanowires is far from being trivial. It remains a grand challenge to develop a plausible method for generating large quantities of nanowires from various metals. Here we present a versatile approach capable of generating core/ shell particles, with the cores being polymer or silica beads and the shells being dense, uniform coatings of Pt nanoparticles or quasiradial Pt nanowires. We recently discovered that the introduction of a trace amount of iron species (Fe or Fe) to the polyol process could induce the formation of Pt nanowires or multipods by significantly reducing the net reduction rate of the salt precursor. We have also shown that these nanowires could be catalytically activated to grow from the surface of micrometersized aggregates consisting of Pt nanoparticles. Herein, we demonstrate a more affordable and practical method for growing Pt nanowires by using Pd-coated colloidal spheres to imitate the catalytic property of the Pt aggregates. Moreover, this procedure can be modified to grow thick, uniform shells composed of Pt nanoparticles. The key to the success of these syntheses are Pd nanoparticles (2–4 nm in size) that can be readily generated in situ as sub-monolayers on colloidal spheres terminated by an amino functional group by reducing a palladium precursor with ethanol under sonication. No growth of Pt nanoparticles or nanowires was observed when there were no Pd nanoparticles on the surface of the colloidal spheres. We note that such Pd nanoparticles have been widely used as a catalyst in the electroless deposition of thin films of metals such as Ni, Cu, and Ag on various substrates. In the present work, Pt nanowires with aspect ratios of up to 30 could be controllably grown through an iron-mediated polyol reduction when there was a catalytic metal exposed on the substrate surface. Furthermore, shells of Pt nanoparticles with thicknesses of up to 100 nm could be formed by reducing the amount of iron species added to the reaction solution. Figure 1 illustrates the multistep protocol designed to grow Pt nanoparticles or nanowires as thick coatings on colloidal spheres. In the first step, amino-functionalized melamine beads were sonicated in a solution of [PdCl4] 2– and ethanol for 1 h, resulting in the direct attachment of Pd nanoparticles to the polymer surface as a sub-monolayer (product A in Fig. 1). Here, ethanol acts as a reducing agent to produce Pd from Pd. The amino-terminated coupling agents on the substrate surface might also act as a primer to attract the Pd nanoparticles, which nucleated in the solution phase. Dokoutchaev et al. have reported that the surface of amino-derivatized polymer beads showed a large affinity for Pd nanoparticles. The resulting beads were recovered from the reaction solution by centrifugation and washed several times with ethanol and water. In the next step, the Pd-coated beads were dispersed in ethylene glycol (EG) and heated to 110 °C. After heating for 1 h to activate the immobilized Pd nanoparticles and to decompose some EG to aldehyde, specific amounts of H2PtCl6/EG and poly(vinyl pyrrolidone)/EG (PVP/EG) were added dropwise to the reaction solution. At this stage, H2PtCl6 was reduced by the aldehyde to form a Pt II intermediate, which could remain in this state at room temperature for more than one month without being reduced further to Pt. The solution was continuously heated for another 2 h (or until the solution turned yellow-green) to ensure C O M M U N IC A IO N

Seed-mediated synthesis of single-crystal gold nanospheres with controlled diameters in the range 5-30 nm and their self-assembly upon dilution.

Abstract: Single-crystal gold nanospheres with controlled diameters in the range 5-30 nm were synthesized by using a facile approach that was based on successive seed-mediated growth. The key to the success of this synthesis was the use of hexadecyltrimethylammonium chloride (CTAC) as a capping agent and a large excess of ascorbic acid as a reductant to ensure fast reduction and, thus, single crystallinity and a spherical shape of the resultant nanoparticles. The diameters of the gold nanospheres could be readily controlled by varying the amount of seeds that were introduced into the reaction system. The gold nanospheres could be produced with uniform diameters of up to 30 nm; thus, their localized surface plasmon resonance properties could be directly compared with the results that were obtained from theoretical calculations. Interestingly, we also found that these gold nanospheres self-assembled into dimers, larger aggregates, and wavy nanowires when they were collected by centrifugation, dispersed in deionized water, and then diluted to different volumes with deionized water.

One-Pot Synthesis of Penta-twinned Palladium Nanowires and Their Enhanced Electrocatalytic Properties

Abstract: This article reports the design and successful implementation of a one-pot, polyol method for the synthesis of penta-twinned Pd nanowires with diameters below 8 nm and aspect ratios up to 100. The key to the success of this protocol is the controlled reduction of Na2PdCl4 by diethylene glycol and ascorbic acid through the introduction of NaI and HCl. The I– and H+ ions can slow the reduction kinetics by forming PdI42– and inhibiting the dissociation of ascorbic acid, respectively. When the initial reduction rate is tuned into the proper regime, Pd decahedral seeds with a penta-twinned structure appear during nucleation. In the presence of I– ions as a selective capping agent toward the Pd(100) surface, the decahedral seeds can be directed to grow axially into penta-twinned nanorods and then nanowires. The Pd nanowires are found to evolve into multiply twinned particles if the reaction time is extended beyond 1.5 h, owing to the involvement of oxidative etching. When supported on carbon, the Pd nanowires s...

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346