Optical and electrochemical-mediated detection of ascorbic acid using manganese porphyrin and its gold hybrids

The developments concerning new hybrids based on porphyrin derivatives and colloids destined for the detection of ascorbic acid (AA) in the relevant range for medical investigations are presented. Mn(III) tetratolylporphyrin chloride (MnTTPCl), spherical gold colloid (n-Au), and their hybrid (MnTTPCl/n-Au) were chosen to be comparatively investigated by ultraviolet–visible spectroscopy in the presence of AA. The hybrid material (MnTTPCl/n-Au) has the best capacity to detect concentrations of AA in the range of 2.6 × 10^?6–4.38 × 10^?5 M. Modified glassy carbon (GC) electrodes were obtained by thin film deposition of MnTTPCl, n-Au alone, and in successive mixed thin films, comparing their response during the electrochemical oxidation of AA. The electrocatalytic effect of the MnTTPCl on the AA oxidation is justified both by the increase in the peak current density and by the shift toward more negative potentials (0.024 V). The GC/MnTTPCl electrode has the best electrocatalytic effect for the AA oxidation and is promising for sensor applications.     

Catalyzed Vinylogous Mukaiyama Aldol Reactions with Controlled Enantio‐ and Diastereoselectivities

In control : A new catalytic vinylogous Mukaiyama aldol reaction provides products with high diastereo‐ and enantioselectivities (up to 99 % de and ee; see scheme). The relative and absolute stereochemistry of a representative product was rigorously assigned by NMR and CD spectroscopies (measured and calculated), X‐ray diffraction, and quantum‐chemical calculations.

     

Electrical control of spin relaxation in a quantum dot

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.     

Single-transverse-spin asymmetries of identified charged hadrons in polarized pp collisions at sqrt[s]=62.4 GeV

The first measurements of xF-dependent single-spin asymmetries of identified charged hadrons, pi+/-, K+/-, and protons, from transversely polarized proton-proton collisions at 62.4 GeV at RHIC are presented. Large asymmetries are seen in the pion and kaon channels. The asymmetries in inclusive pi+ production, AN(pi+), increase with xF from 0 to approximately 0.25 and AN(pi-) decrease from 0 to approximately -0.4. Observed asymmetries for K- unexpectedly show positive values similar to those for K+, increasing with xF, whereas proton asymmetries are consistent with zero over the measured kinematic range. Comparisons of the data with predictions of QCD-based models are presented.     

Parametrically forced pattern formation

Pattern formation in a nonlinear damped Mathieu-type partial differential equation defined on one space variable is analyzed. A bifurcation analysis of an averaged equation is performed and compared to full numerical simulations. Parametric resonance leads to periodically varying patterns whose spatial structure is determined by amplitude and detuning of the periodic forcing. At onset, patterns appear subcritically and attractor crowding is observed for large detuning. The evolution of patterns under the increase of the forcing amplitude is studied. It is found that spatially homogeneous and temporally periodic solutions occur for all detuning at a certain amplitude of the forcing. Although the system is dissipative, spatial solitons are found representing domain walls creating a phase jump of the solutions. Qualitative comparisons with experiments in vertically vibrating granular media are made. (c) 2001 American Institute of Physics.     

Transverse-momentum spectra in Au+Au and d+Au collisions at sqrt[s(NN)]=200 GeV and the pseudorapidity dependence of high-p(T) suppression

We present spectra of charged hadrons from Au+Au and d+Au collisions at sqrt[s(NN)]=200 GeV measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from p+(-)p collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central Au+Au collisions at eta=0 and eta=2.2 evidence a strong suppression in the high p(T) region (>2 GeV/c). In contrast, the d+Au nuclear modification factor (at eta=0) exhibits an enhancement of the high p(T) yields. These measurements indicate a high energy loss of the high p(T) particles in the medium created in the central Au+Au collisions. The lack of suppression in d+Au collisions makes it unlikely that initial state effects can explain the suppression in the central Au+Au collisions.     

Evolution of the nuclear modification factors with rapidity and centrality in d + Au collisions at (sqrt)[N(S)N]=200 GeV

We report on a study of the transverse momentum dependence of nuclear modification factors R(dAu) for charged hadrons produced in deuteron + gold collisions at sqrt[s(NN)]=200 GeV, as a function of collision centrality and of the pseudorapidity (eta=0, 1, 2.2, 3.2) of the produced hadrons. We find a significant and systematic decrease of R(dAu) with increasing rapidity. The midrapidity enhancement and the forward rapidity suppression are more pronounced in central collisions relative to peripheral collisions. These results are relevant to the study of the possible onset of gluon saturation at energies reached at BNL RHIC.     

Energy-dependent tunneling in a quantum dot

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements.     

Production of mesons and baryons at high rapidity and high p(T) in proton-proton collisions at square root[s] = 200 GeV

We present particle spectra for charged hadrons pi(+/-), K(+/-), p, and p[over] from pp collisions at square root[s] = 200 GeV measured for the first time at forward rapidities (2.95 and 3.3). The kinematics of these measurements are skewed in a way that probes the small momentum fraction in one of the protons and large fractions in the other. Large proton to pion ratios are observed at values of transverse momentum that extend up to 4 GeV/c, where protons have momenta up to 35 GeV. Next-to-leading order perturbative QCD calculations describe the production of pions and kaons well at these rapidities, but fail to account for the large proton yields and small p[over]/p ratios.