Physics Faculty Publications and Presentations

Radiation-Induced Galectin-1 by Endothelial Cells: A Promising Molecular Target for Preferential Drug Delivery to the Tumor Vasculature

Meenakshi Upreti et al. — Fri, 03 May 2013 10:04:45 PDT

The present study reports on a new strategy for selective, radiation therapy-amplified drug delivery using an antiangiogenic 33-a.a., tumor vasculature-targeting ligand, anginex, to improve the therapeutic ratio for strategies developed against solid tumors. Our findings indicate that galectin-1 is (a) one of the major receptors for anginex (b) overexpressed by tumor neovasculature and (c) further specifically upregulated in endothelial cells in response to radiation exposure as low as 0.5 Gy. An investigation of [18]-F-labeled anginex biodistribution in SCK tumors indicates that anginex is an effective targeting molecule for image and radiation-guided therapy of solid tumors. An anginex-conjugated liposome capable of being loaded with drug was shown to selectively target endothelial cells post-radiation. The presence of endothelial cells in a three-dimensional co-culture system with tumor cells developed to study tumor/endothelial cell interactions in vitro led to higher levels of galectin-1 and showed a further increase in expression upon radiation exposure when compared to tumor cell spheroids alone. Similar increase in galectin-1 was observed in tumor tissue originating from the tumor‐endothelial cell spheroids in vivo and radiation exposure further induced galectin-1 in these tumors. The overall results suggest feasibility of using a clinical or subclinical radiation dose to increase expression of the galectin-1 receptor on the tumor microvasculature to promote delivery of therapeutics via the anginex peptide. This approach may reduce systemic toxicity, overcome drug resistance, and improve the therapeutic efficacy of conventional chemo/radiation strategies.

Cantilever-Based Optical Interfacial Force Microscope in Liquid Using an Optical-Fiber Tip

Byung I. Kim et al. — Mon, 22 Apr 2013 12:58:48 PDT

We developed a novel cantilever-based optical interfacial force microscope (COIFM) to study molecular interaction in liquid environments. The force sensor was created by attaching a chemically etched optical-fiber tip to the force sensor with UV epoxy, and characterized by imaging on a calibration grid. The performance of the COIFM was then demonstrated by measuring the force between two oxidized silicon surfaces in 1 mM KCl as a function of distance. The result was consistent with previously reported electrical double layer forces, suggesting that a COIFM using an optical-fiber tip is capable of measuring force in a liquid environment.

Mechanical Property Investigation of Soft Materials by Cantilever-Based Optical Interfacial Force Microscopy

Byung I. Kim et al. — Wed, 27 Mar 2013 15:45:27 PDT

Cantilever-based optical interfacial force microscopy (COIFM) was applied to the investigation of the mechanical properties of soft materials to avoid the double-spring effect and snap-to-contact problem associated with atomic force microscopy (AFM). When a force was measured as a function of distance between an oxidized silicon probe and the surface of a soft hydrocarbon ﬁlm, it increases nonlinearly in the lower force region below ∼10 nN, following the Herzian model with the elastic modulus of∼50 MPa. Above∼10 nN, it increases linearly with a small oscillatory sawtooth pattern with amplitude 1–2 nN. The pattern suggests the possible existence of the layered structure within the ﬁlm. When its internal part of the ﬁlm was exposed to the probe, the force depends on the distance linearly with an adhesive force of−20 nN. This linear dependence suggests that the adhesive internal material behaved like a linear spring with a spring constant of ∼1 N/m. Constant-force images taken in the repulsive and attractive contact regimes revealed additional features that were not observed in the images taken in the noncontact regime. At some locations, however, contrast inversions were observed between the two contact regimes while the average roughness remained constant. The result suggests that some embedded materials had spring constants different from those of the surrounding material. This study demonstrated that the COIFM is capable of imaging mechanical properties of local structures such as small impurities and domains at the nanometer scale, which is a formidable challenge with conventional AFM methods.

Identifying the Location in the Host Galaxy of the Short GRB 111117A with the Chandra Subarcsecond Position

J. P. Norris — Wed, 27 Mar 2013 14:53:43 PDT

We present our successful Chandra program designed to identify, with subarcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was discovered by Swift and Fermi. Thanks to our rapid target of opportunity request, Chandra clearly detected the X-ray afterglow, though no optical afterglow was found in deep optical observations. The host galaxy was clearly detected in the optical and near-infrared band, with the best photometric redshift of z = 1.31^+0.46 _−0.23 (90% conﬁdence), making it one of the highest known short gamma-ray burst (GRB) redshifts. Furthermore, we see an offset of 1.0 ± 0.2 arcsec, which corresponds to 8.4 ± 1.7 kpc, between the host and the afterglow position. We discuss the importance of using Chandra for obtaining subarcsecond X-ray localizations of short GRB afterglows to study GRB environments.

Studies in Astronomical Time Series Analysis. VI. Bayesian Block Representations

Jeffrey D. Scargle et al. — Wed, 27 Mar 2013 14:08:37 PDT

This paper addresses the problem of detecting and characterizing local variability in time series and other forms of sequential data. The goal is to identify and characterize statistically significant variations, at the same time suppressing the inevitable corrupting observational errors. We present a simple nonparametric modeling technique and an algorithm implementing it—an improved and generalized version of Bayesian Blocks—that finds the optimal segmentation of the data in the observation interval. The structure of the algorithm allows it to be used in either a real-time trigger mode, or a retrospective mode. Maximum likelihood or marginal posterior functions to measure model fitness are presented for events, binned counts, and measurements at arbitrary times with known error distributions. Problems addressed include those connected with data gaps, variable exposure, extension to piecewise linear and piecewise exponential representations, multivariate time series data, analysis of variance, data on the circle, other data modes, and dispersed data. Simulations provide evidence that the detection efficiency for weak signals is close to a theoretical asymptotic limit derived by Arias-Castro et al. In the spirit of Reproducible Research all of the code and data necessary to reproduce all of the figures in this paper are included as supplementary material.

Determination of the Point-Spread Function for the Fermi Large Area Telescope from On-Orbit Data and Limits on Pair Halos of Active Galactic Nuclei

J. P. Norris — Wed, 27 Mar 2013 13:54:43 PDT

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from 20 MeV to >300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of γ rays from pulsars and active galactic nuclei (AGNs). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of γ-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGNs. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low- and high-redshift BL Lac AGNs and the TeV blazars 1ES0229+200 and 1ES0347–121.

Sensing Single Protein Molecules with Solid-State Nanopores

Bradley Ledden et al. — Wed, 13 Feb 2013 11:16:20 PST

This chapter is focused on the development of experiments and theory of using solid-state nanopores for sensing single protein molecules in their native and unfolded states. Proteins serve diverse roles such as transport carriers, catalysts, molecular motors, cellular structural support, and others that make life possible. Because of these widely differing roles, proteins have an enormously diverse set of shapes, sizes, and charge structures as compared to polynucleic acids. Solid-state nanopores are particularly suitable for characterizing single protein molecules because they can be fabricated with adjustable dimensions and are stable under conditions that denature proteins. This chapter describes the nanopore experimental setup, signal recording, data analysis, and basic principles related to the experiments and the theory connecting the electrical signal with the properties of proteins. Examples of experimental results illustrate the ability of solid-state nanopores to differentiate proteins in their folded and unfolded states. Native-state protein nanopore translocation follows biased one-dimensional diffusion of charged particles that is sensitive to size and electrical charge. Due to the heterogeneous charge sequence of polypeptides, unfolded proteins obey a coupled electrophoretic and thermally activated process that is sequence specific. The chapter concludes with a discussion of future directions and open challenges for single protein characterization using solid-state nanopores.

Multiwavelength Observations of GRB 110731A: GeV Emission from Onset to Afterglow

J. P. Norris — Tue, 12 Feb 2013 12:22:58 PST

We report on the multiwavelength observations of the bright, long gamma-ray burst GRB 110731A, by the Fermi and Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that GRB 110731A shares many features with bright Large Area Telescope bursts observed by Fermi during the first three years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power-law component and temporally extended high-energy emission. In addition, this is the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favor emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal and the spectral and temporal analysis of the forward-shock afterglow emission independently lead to an estimate of the bulk Lorentz factor of the jet Γ ~ 500-550.

Broadband Study of GRB 091127: A Sub-Energetic Burst at Higher Redshift?

J. P. Norris — Tue, 15 Jan 2013 13:05:40 PST

GRB 091127 is a bright gamma-ray burst (GRB) detected by Swift at a redshift z = 0.49 and associated with SN 2009nz. We present the broadband analysis of the GRB prompt and afterglow emission and study its high-energy properties in the context of the GRB/SN association. While the high luminosity of the prompt emission and standard afterglow behavior are typical of cosmological long GRBs, its low-energy release (E _γ < 3 × 10⁴⁹ erg), soft spectrum, and unusual spectral lag connect this GRB to the class of sub-energetic bursts. We discuss the suppression of high-energy emission in this burst, and investigate whether this behavior could be connected with the sub-energetic nature of the explosion.

Constraints on the Galactic Halo Dark Matter from Fermi-LAT Diffuse Measurements

J. P. Norris — Tue, 15 Jan 2013 12:50:37 PST

We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope (LAT) in the Milky Way halo region, searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e ⁺/e ^– produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum, and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as the annihilation of dark matter.

Nanoscale Characteristics of Triacylglycerol Oils: Phase Separation and Binding Energies of Two-Component Oils to Crystalline Nanoplatelets

Colin J. MacDougall et al. — Tue, 11 Dec 2012 10:33:08 PST

Fats are elastoplastic materials with a defined yield stress and flow behavior and the plasticity of a fat is central to its functionality. This plasticity is given by a complex tribological interplay between a crystalline phase structured as crystalline nanoplatelets (CNPs) and nanoplatelet aggregates and the liquid oil phase. Oil can be trapped within microscopic pores within the fat crystal network by capillary action, but it is believed that a significant amount of oil can be trapped by adsorption onto crystalline surfaces. This, however, remains to be proven. Further, the structural basis for the solid–liquid interaction remains a mystery. In this work, we demonstrate that the triglyceride liquid structure plays a key role in oil binding and that this binding could potentially be modulated by judicious engineering of liquid triglyceride structure. The enhancement of oil binding is central to many current developments in this area since an improvement in the health characteristics of fat and fat-structured food products entails a reduction in the amount of crystalline triacylglycerols (TAGs) and a relative increase in the amount of liquid TAGs. Excessive amounts of unbound, free oil, will lead to losses in functionality of this important food component. Engineering fats for enhanced oil binding capacity is thus central to the design of more healthy food products. To begin to address this, we modelled the interaction of triacylglycerol oils, triolein (OOO), 1,2-olein elaidin (OOE) and 1,2-elaidin olein (EEO) with a model crystalline nanoplatelet composed of tristearin in an undefined polymorphic form. The surface of the CNP in contact with the oil was assumed to be planar. We considered pure OOO and mixtures of OOO + OOE and OOO + EEO with 80% OOO. The last two cases were taken as approximations to high oleic sunflower oil (HOSO). The intent was to investigate whether phase separation on a nanoscale took place. We defined an “oil binding capacity” parameter, B(Q,Q′), relating a state Q to a reference state Q′. We used atomic scale molecular dynamics in the NVT ensemble and computed averages over 1–5 ns. We found that the probability of the OOE phase separating into a layer on the surface of the CNP compared to being retained randomly in an OOO + OOE mix were approximately equal. However, we found that it was probable that the EEO component of an OOO + EEO mix would phase separate and coat the surface of the CNP. These results suggest a mechanism whereby many-component oils undergo phase separation on a nanoscale so as to create a transition oil region between the surface of the CNP and the bulk major oil component (OOO in the case considered here) so as to create the appropriate oil binding capacity for the use to which it is put.

The Fermi Large Area Telescope on Orbit: Event Classification, Instrument Response Functions, and Calibration

Jay P. Norris — Wed, 28 Nov 2012 14:11:57 PST

The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy γ-ray telescope, covering the energy range from 20 MeV to more than 300 GeV. During the first years of the mission, the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the instrument response functions (IRFs), the description of the instrument performance provided for data analysis. In this paper, we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources.

Fermi Observations of γ-Ray Emission from the Moon

Jay P. Norris — Wed, 28 Nov 2012 13:03:54 PST

We report on the detection of high-energy γ-ray emission from theMoon during the first 24 months of observations by the Fermi Large Area Telescope (LAT). This emission comes from particle cascades produced by cosmicray (CR) nuclei and electrons interacting with the lunar surface. The differential spectrum of the Moon is soft and can be described as a log-parabolic function with an effective cutoff at 2–3 GeV, while the average integral flux measured with the LAT from the beginning of observations in 2008 August to the end of 2010 August is F(>100 MeV) = (1.04 ± 0.01 [statistical error] ± 0.1 [systematic error]) × 10−6 cm−2 s−1. This flux is about a factor 2–3 higher than that observed between 1991 and 1994 by the EGRET experiment on board the Compton Gamma Ray Observatory, F(>100 MeV)≈5×10−7 cm−2 s−1, when solar activity was relatively high. The higher γ-ray flux measured by Fermi is consistent with the deep solar minimum conditions during the first 24 months of the mission, which reduced effects of heliospheric modulation, and thus increased the heliospheric flux of Galactic CRs. A detailed comparison of the light curve with McMurdo Neutron Monitor rates suggests a correlation of the trends. The Moon and the Sun are so far the only known bright emitters of γ-rays with fast celestial motion. Their paths across the sky are projected onto the Galactic center and high Galactic latitudes as well as onto other areas crowded with high-energy γ-ray sources. Analysis of the lunar and solar emission may thus be important for studies of weak and transient sources near the ecliptic

Imaging Stability in Force-Feedback High-Speed Atomic Force Microscopy

Byung I. Kim et al. — Wed, 31 Oct 2012 10:47:25 PDT

We studied the stability of force-feedback high-speed atomic force microscopy (HSAFM) by imaging soft, hard, and biological sample surfaces at various applied forces. The HSAFM images showed sudden topographic variations of streaky fringes with a negative applied force when collected on a soft hydrocarbon film grown on a grating sample, whereas they showed stable topographic features with positive applied forces. The instability of HSAFM images with the negative applied force was explained by the transition between contact and noncontact regimes in the force-distance curve. When the grating surface was cleaned, and thus hydrophilic by removing the hydrocarbon film, enhanced imaging stability was observed at both positive and negative applied forces. The higher adhesive interaction between the tip and the surface explains the improved imaging stability. The effects of imaging rate on the imaging stability were tested on an even softer adhesive Escherichia coli biofilm deposited onto the grating structure. The biofilm and planktonic cell structures in HSAFM images were reproducible within the force deviation less than ~0.5 nN at the imaging rate up to 0.2 s per frame, suggesting that the force-feedback HSAFM was stable for various imaging speeds in imaging softer adhesive biological samples.

Properties of the Soliton-Lattice State in Double-Layer Quantum Hall Systems

C. B. Hanna et al. — Wed, 05 Sep 2012 12:44:57 PDT

Application of a sufficiently strong parallel magnetic field B_∥>B_c produces a soliton-lattice (SL) ground state in a double-layer quantum Hall system. We calculate the ground-state properties of the SL state as a function of B_∥ for total filling factor ν_T=1, and obtain the total energy, anisotropic SL stiffness, Kosterlitz–Thouless melting temperature, and SL magnetization. The SL magnetization might be experimentally measurable, and the magnetic susceptibility diverges as |B_∥−B_c|⁻¹.

GeV Observations of Star-Forming Galaxies with the Fermi Large Area Telescope

Jay P. Norris — Tue, 04 Sep 2012 14:18:33 PDT

Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives. Building upon those results, we examine a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of data collected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi). Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies. We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies (conservative P-values 0.05 accounting for statistical and systematic uncertainties). The normalizations of these scaling relations correspond to luminosity ratios of log (L _{0.1-100 GeV}/L _{1.4 GHz}) = 1.7 ± 0.1_{(statistical)} ± 0.2_(dispersion) and log (L _{0.1-100 GeV}/L _{8-1000 μm}) = –4.3 ± 0.1_{(statistical)} ± 0.2_(dispersion) for a galaxy with a star formation rate of 1 M _☉ yr^–1, assuming a Chabrier initial mass function. Using the relationship between infrared luminosity and gamma-ray luminosity, the collective intensity of unresolved star-forming galaxies at redshifts 0 < z < 2.5 above 0.1 GeV is estimated to be 0.4-2.4 × 10^–6 ph cm^–2 s^–1 sr^–1 (4%-23% of the intensity of the isotropic diffuse component measured with the LAT). We anticipate that ~10 galaxies could be detected by their cosmic-ray-induced gamma-ray emission during a 10 year Fermi mission.

Gamma-Ray Observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

Jay P. Norris — Tue, 04 Sep 2012 14:06:25 PDT

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between ~100 MeV and ~100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to ~10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity-integrated CO intensity (W _CO) at a 1° × 1° pixel level. The correlation is found to be linear over a W _CO range of ~10-fold when divided in three regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The W _CO-to-mass conversion factor, X _CO, is found to be ~2.3 × 10²⁰ cm^-2(K km s^–1)^–1 for the high-longitude part of Orion A (l > 212°), ~1.7 times higher than ~1.3 × 10²⁰ found for the rest of Orion A and B. We interpret the apparent high X _CO in the high-longitude region of Orion A in the light of recent works proposing a nonlinear relation between H₂ and CO densities in the diffuse molecular gas. W _CO decreases faster than the H₂ column density in the region making the gas "darker" to W _CO.

Force-Feedback High-Speed Atomic Force Microscope for Studying Large Biological Systems

Byung I. Kim et al. — Tue, 04 Sep 2012 12:02:26 PDT

We designed and developed a high-speed atomic force microscope (HSAFM) utilizing a force-feedback scheme for imaging large biological samples. The system collects three simultaneous images: a deflection image, a topographic image, and a force image. We demonstrated that this force-feedback HSAFM is capable of acquiring large topographic images of Escherichia coli biofilms at approximately one frame per second in air. We discuss how the self-actuating cantilever and the piezo tube follow those larger biological topographic features during the HSAFM imaging process.

Fermi Large Area Telescope Study of Cosmic Rays and the Interstellar Medium in Nearby Molecular Clouds

Jay P. Norris — Fri, 24 Aug 2012 11:18:46 PDT

We report an analysis of the interstellar γ-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ~300 pc from the solar system. The γ-ray emission produced by interactions of cosmic rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained γ-ray emissivities above 250 MeV are (5.9 ± 0.1_stat ^+0.9 _–1.0sys) × 10^–27 photons s^–1 sr^–1 H-atom^–1, (10.2 ± 0.4_stat ^+1.2 _–1.7sys) × 10^–27 photons s^–1 sr^–1 H-atom^–1, and (9.1 ± 0.3_stat ^+1.5 _–0.6sys) × 10^–27 photons s^–1 sr^–1 H-atom^–1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ~20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, X _CO = N(H₂)/W _CO, is found to be (0.96 ± 0.06_stat ^+0.15 _–0.12sys) × 10²⁰ H₂-molecule cm^–2 (K km s^–1)^–1, (0.99 ± 0.08_stat ^+0.18 _–0.10sys) × 10²⁰ H₂-molecule cm^–2 (K km s^–1)^–1, and (0.63 ± 0.02_stat ^+0.09 _–0.07sys) × 10²⁰ H₂-molecule cm^–2 (K km s^–1)^–1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of X _CO in the vicinity of the solar system. From the obtained values of X _CO, the masses of molecular gas traced by W _CO in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ~5 × 10³ M _☉, ~10³ M _☉, and ~3.3 × 10⁴ M _☉, respectively. A comparable amount of gas not traced well by standard H I and CO surveys is found in the regions investigated.

Influence of Solvent on the Chiral Resolution of Organic Molecules on Au(111): EC-STM Study of Biphenyl Dicarboxylic Acid on Au(111) in an Aqueous Environment

Byung I. Kim et al. — Tue, 07 Aug 2012 15:20:51 PDT

Adsorption-induced chiral resolution of organic molecules is important due to its potential applications in stereo-selective catalysis. We studied the adsorption-induced chiral resolution using a model achiral molecule of 4,4′ biphenyl dicarboxylic acid (BPDA) on Au(111) in 0.1 M perchloric acid (HClO₄) by electrochemical scanning tunneling microscopy (EC-STM). Our experimental data showed that the BPDA molecules formed island structures with distinctive preferred orientations at the length scale of the molecular size. The molecules did not show any orientational ordering above the length scale, indicating that chiral resolution was absent in the aqueous environment. Previously, the molecules were found to have chiral resolution on Au(111) in ultra-high vacuum conditions (UHV). We calculated angle-dependent binding energy between the substrate and a BPDA molecule, the intermolecular interactions between the BPDA molecules, and their interactions with water molecules. The calculations suggest that the absence of chiral resolution in the aqueous environment originated from the decrease in the intermolecular energy of the BPDA molecules due to their hydrogen bonds with the surrounding water molecules. The strength of the hydrogen bonding between BPDA molecules was sufficient to overcome the energy barrier for chiral resolution through rotational motion in UHV, but not in an aqueous environment.