Au@Pt Dendrimer Encapsulated Nanoparticles As Model Electrocatalysts for Comparison of Experiment and Theory

In this paper we report the electrochemical synthesis of core@shell dendrimer-encapsulated nanoparticles (DENs) consisting of cores containing 147 Au atoms (Au-147) and Pt shells having similar to 54 or similar to 102 atoms (Au-147@Pt-n (n = 54 or 102)). The significance of this work arises from the correlation of the experimentally determined structural and electrocatalytic properties of these particles with density functional theory (DFT) calculations. Specifically, we describe an experimental and theoretical study of Pb underpotential deposition (UPD) on Au-147 DENs, the structure of both Au-147@Pb-n and Au-147@Pt-n DENs, and the activity of these DENs for the oxygen reduction reaction (ORR). DFT calculations show that Pb binding is stronger on the (100) facets of Au as compared to (111), and the calculated deposition and stripping potentials are consistent with those measured experimentally. Galvanic exchange is used to replace the surface Pb atoms with Pt, and a surface distortion is found for Au-147@Pt-n particles using molecular dynamics simulations in which the Pt-covered (100) facets shear into (111) diamond structures. DFT calculations of oxygen binding show that the distorted surfaces are the most active for the ORR, and that their activity is similar regardless of the Pt coverage. These calculations are consistent with rotating ring-disk voltammetry measurements.Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy DE-FG02-09ER16090Robert A. Welch Foundation F-0032, F-1601Institute of Computational and Engineering Sciences at UT-AustinChemistr

Dendrimer-Encapsulated Nanoparticles: New Synthetic and Characterization Methods and Catalytic Applications

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1-2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.U.S. Department of Energy, Office of Basic Energy Sciences DE-FG02-09ER16090U.S. National Science Foundation 0847957Robert A. Welch Foundation F-0032Chemistr

Method for the detection and quantification of analytes using three-dimensional paper-based devices

Described herein are three-dimensional (3-D) paper fluidic devices. The entire 3-D device is fabricated on a support layer formed from a single sheet of material and assembled by folding the support layer. The folded structure may be enclosed in an impermeable cover or package. Chemically sensitive particles may be disposed in the support layer for use in detecting analytes.Board of Regents, University of Texas Syste

Microfluidic biosensor based on an array of hydrogel-entrapped enzymes,”

Here we show that a microfluidic sensor based on an array of hydrogel-entrapped enzymes can be used to simultaneously detect different concentrations of the same analyte (glucose) or multiple analytes (glucose and galactose) in real time. The concentration of paraoxon, an acetylcholine esterase inhibitor, can be quantified using the same approach. The hydrogel micropatch arrays and the microfluidic systems are easy to fabricate, and the hydrogels provide a convenient, biocompatible matrix for the enzymes. Isolation of the micropatches within different microfluidic channels eliminates the possibility of cross talk between enzymes. In this paper, we describe a microfluidic biosensor that uses an array of hydrogel-entrapped enzymes to quantitatively determine the concentration of an analyte and simultaneously detect multiple analytes. The approach relies on the presence of active enzymes within hydrogel micropatches photolithographically defined within microfluidic channels. The enzymes are sufficiently large that they are unable to escape the hydrogel matrix, but the targets are small enough to enter the hydrogel, encounter the enzyme, and be converted into detectable products. By using discrete micropatches contained within multiple channels, and in some cases multiple enzymes within a single hydrogel micropatch, it is possible to detect multiple, structurally similar analytes in parallel and in real time. The apparatus necessary to carry out the assay is straightforward to fabricate. [1][2][3] The performance of biosensors incorporating capture probes is directly linked to the approach used for probe immobilization. In this regard, the following issues are important: (1) the biomaterial must remain active on or within the support, (2) nonspecific adsorption should be minimized, (3) the number of probe receptors should be optimized to provide maximum signal, (4) it should be easy to place the immobilized probes in a desired location, and (5) mass transfer of the target from solution to the probe should be rapid. To address these points, we have focused our recent studies on two families of biomolecular supports that are particularly adaptable to the microfluidic environment: polymeric microbeads 4-6 and monolithic hydrogels. 3,7 For example, we reported that photopolymerized hydrogel micropatches could be used for immobilizing enzymes 3 and bacteria 7 within the channels of microfluidic devices. These relatively large biomaterials are physically entrapped within the photo-cross-linked hydrogel matrix, but analytes are able to diffuse through nanopores within the gel and encounter the probes. Importantly, both enzymes and bacteria retain their activity within the gel, which means that the composite gel/biomaterial can be used as a sensor unit or microbioreactor. Here, we expand upon our earlier findings by demonstrating that an array of hydrogel-entrapped enzymes can be used to simultaneously detect multiple analytes or quantitatively determine the concentration of a single analyte. In addition to our own work, others have shown that hydrogels can be used to immobilize proteins, 8,9 cells, 10-12 and DNA [13][14][15] within microfluidic devices and on planar supports. The size and shape of the gel can be defined by photolithography, 16,17 a mold, 3,10,12 or a robotic spotter. 18 The smallest hydrogel features reported are in the range of tens of micrometers. 16 Unlike array sensors that rely on surface immobilization of DNA or protein monolayers, which are usually designed to bind specific targets, hydrogel micropatches containing enzymes are essentially microbioreactors that consume reactants and generate products. It is important, therefore, to minimize cross-talk between elements of the array. This issue has been addressed by several groups. For example, Arenkov and co-workers fabricated gel pads on hydrophobic surfaces that were covalently linked to enzymes. The hydrophobic surface prevented sample droplets from spreading to nearby gel pads. 8 McDevitt and co-workers developed an * To whom correspondence should be addressed. Voice: 979-845-5629. Fax: 979-845-1399. E-mail: [email protected]. † Present address: Department of Chemistry and Biochemistry, The University of Texas, 1 University Station A5300, Austin, TX 78712-0165. (

Determination of the Intrinsic Proton Binding Constants for Poly(amidoamine) Dendrimers via Potentiometric pH Titration

ABSTRACT: Protonation of fourth-generation poly(amidoamine) dendrimers terminated with hydroxyl and amine functional groups has been studied by potentiometric pH titration. The titration data are analyzed using a multishell structural model and a Frumkin adsorption isotherm to approximate protondendrimer binding equilibria. Site-to-site correlation is ignored, and counterions are treated according to the standard Debye-Hü ckel theory. This analysis yields two binding parameters: the intrinsic proton binding constant and a constant that characterizes the strength of electrostatic interactions among occupied binding sites. For the hydroxyl-terminated dendrimers, the internal tertiary amines have an average binding constant (pK ) 6.30) 1-2 pH units lower than the value expected for a single, isolated binding site. This shift in pK is attributed to a hydrophobic microenvironment within the dendrimer interior. In contrast, no significant shift has been observed in the binding constant (pK ) 9.23) for the peripheral primary amines in the amine-terminated dendrimer because the microenvironment around the primary amines is more hydrophilic. The strength of electrostatic interactions obtained from titration data is 3 times (primary amines) and 8 times (tertiary amines) smaller than the calculated values based on the multishell model. We hypothesize that the diminished interaction strength results from ion pairing between bound protons and counterions. In addition to the Debye-Hü ckel contribution from mobile ions, ion pairing provides extra Coulomb charge screening. Introduction Because of their unique structural topology and chemical versatility, dendrimers have found applications related to catalysis, drug delivery, energy transfer, and molecular recognition. We recently developed a theoretical approach, which we refer to as the "shell model", to quantify iondendrimer binding. 12 Some of the characteristics of this model are summarized here. First, electrostatic interactions are assumed to be the sole source of site-to-site interactions. The total energy is calculated by adopting a multishell dendrimer model, and discrete charges within each shell are summed and approximated as a shell of continuous charge. This procedure makes it possible to solve the linearized Poisson-Boltzmann (PB) equation analytically within the limit of the Debye-Hü ckel approximation (i.e., a dilute electrolyte solution). Second, no distinction is made between binding configurations (or microstates) that have the same set of intrashell proton binding numbers. Instead, all degenerate configurations are averaged (mean-field approximation) so that site-to-site correlations are not considered. In contrast, such a correlation is a key aspect of the Ising model that has been used previously for modeling dendrimer binding equilibria

NMR Methods for Determining Lipid Turnover via Stable Isotope Resolved Metabolomics

Lipids comprise diverse classes of compounds that are important for the structure and properties of membranes, as high-energy fuel sources and as signaling molecules. Therefore, the turnover rates of these varied classes of lipids are fundamental to cellular function. However, their enormous chemical diversity and dynamic range in cells makes detailed analysis very complex. Furthermore, although stable isotope tracers enable the determination of synthesis and degradation of complex lipids, the numbers of distinguishable molecules increase enormously, which exacerbates the problem. Although LC-MS-MS (Liquid Chromatography-Tandem Mass Spectrometry) is the standard for lipidomics, NMR can add value in global lipid analysis and isotopomer distributions of intact lipids. Here, we describe new developments in NMR analysis for assessing global lipid content and isotopic enrichment of mixtures of complex lipids for two cell lines (PC3 and UMUC3) using both 13C6 glucose and 13C5 glutamine tracers

A Multilayer Surface Temperature, Surface Albedo, and Water Vapor Product of Greenland from MODIS

A multilayer, daily ice surface temperature (IST)-albedo-water vapor product of Greenland, extending from March 2000 through December 2016, has been developed using standard MODerate-resolution Imaging Spectroradiometer (MODIS) data products from the Terra satellite. To meet the needs of the ice sheet modeling community, this new Earth Science Data Record (ESDR) is provided in a polar stereographic projection in NetCDF format, and includes the existing standard MODIS Collection 6.1 IST and derived melt maps, and Collection 6 snow albedo and water vapor maps, along with ancillary data, and is provided at a spatial resolution of ~0.78 km. This ESDR enables relationships between IST, surface melt, albedo, and water vapor to be evaluated easily. We show examples of the components of the ESDR and describe some uses of the ESDR such as for comparison with skin temperature, albedo, and water vapor output from Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). Additionally, we show validation of the MODIS IST using in situ and aircraft data, and validation of MERRA-2 skin temperature maps using MODIS IST and in situ data. The ESDR has been assigned a DOI and will be available through the National Snow and Ice Data Center by the summer of 2018