DIRECT OBSERVATION OF NATIVE AND UNFOLDED GLUCOSE-OXIDASE STRUCTURES BY SCANNING-TUNNELING-MICROSCOPY

Native and unfolded glucose oxidase (GOD) structures have been directly observed with scanning tunnelling microscopy (STM) for the first time. STM images show an opening butterfly-shaped pattern for the native GOD. When GOD molecules are extended on anodi

MEASUREMENT OF CATALYTIC ACTIVITY VARIATION FOR H2O2 OXIDATION AT A COBALT PORPHYRIN COATED ELECTRODE

A rapid rotation-scan method was used for the electrocatalytic oxidation of H2O2 at a cobalt protoporphyrin modified pyrolytic graphite electrode (CoPP/PG). The rate constant of H2O2 oxidation at the CoPP/PG electrode at different potentials and in different pH solutions was measured. The variation of catalytic activity with reaction charges (Q) passed through the electrode was analyzed. This provided a convenient electrochemical method to study the passivation and poisoning of catalytic sites with time.

A new model of phycobilisome in Spirulina platensis

Phycobilisomes (PBS) were isolated from blue-green alga Spirulina platensis. Scanning tunneling microscope was used to investigate the three-dimensional structure of PBS deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) in ambient condition at room temperature. The results showed that the rods of PBS radiated from the core to different directions in the space other than arrayed in one plane, which was different from the typical hemi-discoidal model structure. The diameter of PBS was up to 70 nm, and the rod was approximately 50 nm in length. Similar results were observed in Langmuir-Blodgett (LB) film of PBS. The dissociated PBS could reaggregate into rod-like structures and easily form two-dimensional membrane while being absorbed on HOPG, however, no intact PBS was observed. The filling-space model structure of PBS in Spirulina platensis with STM from three-dimensional real space at nanometer scale was found, which showed that this new structural model of PBS surely exists in blue-green algae and red algae. The function of this structural model of PBS was also discussed.

INVESTIGATION OF C-PHYCOCYANIN FROM BLUE-GREEN-ALGA SPIRULINA-PLATENSIS WITH SCANNING TUNNELING MICROSCOPE

C-phycocyanin (C-PC) was isolated from blue-green alga spirulina platensis. A scanning tunneling microscope (STM) has been used to investigate its three-dimensional structure. The samples were dialyzed before the STM experiment, and then deposited on highly oriented pyrolytic graphite (HOPG). The measurement was carried out in ambient condition at room temperature. STM images showed that C-phycocyanin was uniformly distributed on solid-state substrate HOPG. The shape of C-phycocyanin is disklike with a channel in the center. It is concluded that STM has great potential to observe the structure of biliproteins and phycobilisomes.

Controlled growth of uniform silver clusters on HOPG

We have investigated growth of silver clusters on three different, i.e. normally cleaved, thermally oxidized and Ar+ ion sputtered highly oriented pyrolytic graphite (HOPG), surfaces. Scanning tunneling microscopy (STM) observations reveal that uniformly sized and spaced Ag clusters only form on the sputtered surface. Ar+ sputtering introduces relatively uniform surface defects compared to other methods. These defects are found to serve as preferential sites for Ag cluster nucleation, which leads to the formation of uniform clusters. (c) 2005 Elsevier B.V. All rights reserved.

Probing polarization and dielectric function of molecules with higher order harmonics in scattering-near-field scanning optical microscopy

The idealized system of an atomically flat metallic surface [highly oriented pyrolytic graphite (HOPG)] and an organic monolayer (porphyrin) was used to determine whether the dielectric function and associated properties of thin films can be accessed with scanning-near-field scanning optical microscopy (s-NSOM). Here, we demonstrate the use of harmonics up to fourth order and the polarization dependence of incident light to probe dielectric properties on idealized samples of monolayers of organic molecules on atomically smooth substrates. An analytical treatment of light/sample interaction using the s-NSOM tip was developed in order to quantify the dielectric properties. The theoretical analysis and numerical modeling, as well as experimental data, demonstrate that higher order harmonic scattering can be used to extract the dielectric properties of materials with tens of nanometer spatial resolution. To date, the third harmonic provides the best lateral resolution (∼50 nm) and dielectric constant contrast for a porphyrin film on HOPG. © 2009 American Institute of Physics.

Wide angle crystal spectrometer for angularly and spectrally resolved x-ray scattering experiments

A novel wide angle spectrometer has been implemented with a highly oriented pyrolytic graphite crystal coupled to an image plate. This spectrometer has allowed us to look at the energy resolved spectrum of scattered x rays from a dense plasma over a wide range of angles ( ~ 30°) in a single shot. Using this spectrometer we were able to observe the temporal evolution of the angular scatter cross section from a laser shocked foil. A spectrometer of this type may also be useful in investigations of x-ray line transfer from laser-plasmas experiments.

Evolution of elastic x-ray scattering in laser-shocked warm dense lithium

We have studied the dynamics of warm dense Li with near-elastic x-ray scattering. Li foils were heated and compressed using shock waves driven by 4-ns-long laser pulses. Separate 1-ns-long laser pulses were used to generate a bright source of 2.96 keV Cl Ly-alpha photons for x-ray scattering, and the spectrum of scattered photons was recorded at a scattering angle of 120 degrees using a highly oriented pyrolytic graphite crystal operated in the von Hamos geometry. A variable delay between the heater and backlighter laser beams measured the scattering time evolution. Comparison with radiation-hydrodynamics simulations shows that the plasma is highly coupled during the first several nanoseconds, then relaxes to a moderate coupling state at later times. Near-elastic scattering amplitudes have been successfully simulated using the screened one-component plasma model. Our main finding is that the near-elastic scattering amplitudes are quite sensitive to the mean ionization state Z and by extension to the choice of ionization model in the radiation-hydrodynamics simulations used to predict plasma properties within the shocked Li.

Temperature dependence of the Raman spectra of carbon nanotubes

We report on a temperature dependence of the frequency of all the major peaks in the Raman spectra of carbon nanotubes, using different excitation laser powers at the sample. The frequency decreases with increasing temperature for all peaks, and the shifts in Raman frequencies are linear in the temperature of the sample. In comparison, a similar dependence is found in active carbon, but no shift is observed for the highly ordered pyrolytic graphite within the same range of variation in laser power. A lowering of frequency at higher temperature implies an increase in the carbon-carbon distance at higher temperature. The relatively strong temperature dependence in carbon nanotubes and active carbon may be due to the enhanced increase in carbon-carbon distance. This enhancement may originate from the heavy defects and disorder in these materials. (C) 1998 American Institute of Physics. [S0021-8979(98)05219-0].

X-ray Polarization Spectroscopy from Ultra-Intense Interactions

Detailed knowledge of fast electron energy transport following the interaction of ultrashort intense laser pulses is a key subject for fast ignition. This is a problem relevant to many areas of laser-plasma physics with particular importance to fast ignition and X-ray secondary source development, necessary for the development of large-scale facilities such as HiPER and ELI. Operating two orthogonal crystal spectrometers set at Bragg angles close to 45 degrees determines the X-ray s- and p-polarization ratio. From this ratio, it is possible to infer the velocity distribution function of the fast electron beam within the dense plasma. We report on results of polarization measurements at high density for sulphur and nickel buried layer targets in the high intensity range of 10(19) - 10(21) Wcm(-2). We observe at 45 degrees the Ly-alpha doublet using two sets of orthogonal highly-orientated pyrolytic graphite (HOPG) crystals set in 1(st) order for sulphur and 3(rd) order for nickel.

Characterization of x-ray lens for use in probing high energy density states of matter

By using polycapillary lenses to focus laser-produced x-ray sources to high intensities, an improvement in signal-to-noise ratio can be achieved. Here the He-alpha line emission produced by driving a titanium backlighter target is focused by a polycapillary lens and the output characterized. The x-ray spot is measured to have a peak intensity of 4.5 x 10(7) photons, with a total photon count of 8.8 x 10(8) in 0.13 +/- 0.01 mm(2). This setup is equivalent to placing the backlighter target 3 mm from the sample with a 600 mu m diameter pinhole. The polycapillary lens enables the placement of the backlighter target at a much larger distance from the sample to be studied and therefore has the ability to greatly improve the signal-to-noise ratio on detectors. We demonstrate this with two simple diffraction experiments using pyrolytic graphite and polycrystalline aluminium.

Comparative Raman Study of Carbon Nanotubes Prepared by D.C. Arc Discharge and Catalytic Methods

The Raman spectra of carbon nanotubes prepared by catalytic (C-CNT) and d.c. arc discharge (D-CNT) methods are reported. A previously unnoticed third-order Raman peak at ca. 4248 cm-1 was observed in the Raman spectrum of D-CNT. The Raman features of D-CNT and C-CNT are similar to those of highly oriented pyrolytic graphite (HOPG) and active carbon, respectively. The data also suggest that the increase in disorder in D-CNT compared with HOPG is due to structural defects in D-CNT. © 1997 by John Wiley & Sons, Ltd.

Supramolecular Organization of Metal Nanoparticles in Solution and of Phenyleneethynylenes on Surfaces

Most of the procedures reported for the synthesis of metal nanoparticles involve the use of strong reducing agents or elevated temperatures. This limits the possibility of developing metal nanoparticle based sensors for the in situ detection of analytes. One of the objectives of the present investigations is to (i) develop newer methodologies for the synthesis of metal nanoparticles in aqueous medium at ambient conditions and (ii) their use in the detection of metal cations by taking advantage of the unique coordination ability. Ideally, biocompatible molecules which possess both the reducing and stabilizing groups are desirable for such applications. Formation of stable supramolecular assembly, by bringing metal nanoparticles close to each other, results in plasmon coupling and this strategy can be effectively utilized for the development of metal nanoparticle based sensors.Another objective of the present study is to understand the supramolecular organization of molecules on surfaces. Various noncovalent interactions between the molecules and with surface play a decisive role in their organizations. An in-depth understanding of these interactions is essential for device fabrications. Recent photophysical studies have revealed that phenyleneethynylene based molecular systems are ideal for device application. The second objective of the thesis focuses on understanding the (i) organization of phenyleneethynylenes on highly oriented pyrolytic graphite (HOPG) surface with atomic level precision and (ii) weak intermolecular interactions which drive their organization.

On the pH dependence of electroactivity of poly(methylene blue) films

The electrochemical behavior of poly(methylene blue) on different electrodes has been investigated by electrochemical quartz crystal microbalance and in situ spectrophotometric measurements coupled to cyclic voltammetry. Polymeric films were obtained potentiodynamically and the charge transport mechanism was analyzed. The electrochemical results show that polymer electroactivity depends not only on pH but also on the substrate. Charge compensation changes with both pH and the size of the anions showing a transition in the pH range of polymer pKa. It was demonstrated by spectroelectrochemical experiments that the electroactivity of the film depends on the radical/radical cation equilibrium. The potentials where the most electroactive species are formed have been determined. (C) 2009 Elsevier Ltd. All rights reserved.

FIA determination of ascorbic acid at low potential using a ruthenium oxide hexacyanoferrate modified carbon electrode

The anodic oxidation of ascorbic acid on a ruthenium oxide hexacyanoferrate modified electrode was characterized by cyclic voltammetry. On this modified surface, the electrocatalytic process allows the determination of ascorbic acid to be performed at 0.0 V and pH 6.9 with a limit of detection of 2.2 mu M in a flow injection configuration. Under this experimental condition, no interference from glucose, nitrite and uric acid was noticed. Lower detection limit values were obtained by measuring flow injection analysis (FIA) responses at 0.4 V (0.14 mu M), but a concurrent loss of selectivity is expected at this more positive potential. Under optimal FIA operating conditions, the linear response of the method was extended up to 1 mM ascorbic acid. The repeatability of the method for injections of a 1.0 mM ascorbic acid solution was 2.0% (n=10). The usefulness of the method was demonstrated by an addition-recovery experiment with urine samples and the recovered values were in the 98-104% range. (c) 2007 Elsevier B.V. All rights reserved.