Search for new particles decaying to t&tmacr; in p&pmacr; collisions at radicals = 1.8 TeV

We use 106 pb (-1) of data collected with the Collider Detector at Fermilab to search for narrow-width, vector particles decaying to a top and an antitop quark. Model independent upper limits on the cross section for narrow, vector resonances decaying to t&tmacr; are presented. At the 95% confidence level, we exclude the existence of a leptophobic Z' boson in a model of top-color-assisted technicolor with mass M(Z')<480 GeV/c(2) for natural width gamma = 0.012M(Z'), and M(Z')<780 GeV/c(2) for gamma = 0.04M(Z').     

Search for second and third generation leptoquarks including production via technicolor interactions in p&pmacr; collisions at radicals = 1.8 TeV

We report the results of a search for second and third generation leptoquarks using 88 pb(-1) of data recorded by the Collider Detector at Fermilab. Color triplet technipions, which play the role of scalar leptoquarks, are investigated due to their potential production in decays of strongly coupled color octet technirhos. Events with a signature of two heavy flavor jets and missing energy may indicate the decay of a second (third) generation leptoquark to a charm (bottom) quark and a neutrino. As the data are found to be consistent with standard model expectations, mass limits are determined.     

Transmission mode ion/ion reactions in the radiofrequency‐only ion guide of hybrid tandem mass spectrometers

Transmission mode ion/ion reactions have been performed within the first quadrupole, the Q0 radiofrequency (RF)‐only quadrupole, of two types of hybrid tandem mass spectrometers (viz., triple quadrupole/linear ion trap and QqTOF instruments). These transmission mode reactions involved the storage of either the reagent species and the transmission of the analyte species through the Q0 quadrupole for charge inversion reactions or the storage of the analyte ions and transmission of the reagent ions as in charge reduction experiments. A key advantage to the use of transmission mode ion/ion reactions is that they do not require any instrument hardware modifications to provide interactions of oppositely charged ions and can be implemented in any instrument that contains a quadrupole or linear ion trap. The focus of this work was to investigate the potential of using the RF‐only quadrupole ion guide positioned prior to the first mass‐resolving element in a tandem mass spectrometer for ion/ion reactions. Two types of exemplary experiments have been demonstrated. One involved a charge inversion reaction and the other involved a charge reduction reaction in conjunction with ion parking. Ion/ion reactions proved to be readily implemented in Q0 thereby adding significantly greater experimental flexibility in the use of ion/ion reaction experiments with hybrid tandem mass spectrometers. Copyright © 2009 John Wiley & Sons, Ltd.     

Millisecond kinetics of nanocrystal cation exchange using microfluidic X-ray absorption spectroscopy

We describe the use of a flow-focusing microfluidic reactor to measure the kinetics of the CdSe-to-Ag2Se nanocrystal cation exchange reaction using micro-X-ray absorption spectroscopy (microXAS). The small microreactor dimensions facilitate the millisecond mixing of CdSe nanocrystals and Ag+ reactant solutions, and the transposition of the reaction time onto spatial coordinates enables the in situ observation of the millisecond reaction using microXAS. Selenium K-edge absorption spectra show the progression of CdSe nanocrystals to Ag2Se over the course of 100 ms without the presence of long-lived intermediates. These results, along with supporting stopped-flow absorption experiments, suggest that this nanocrystal cation exchange reaction is highly efficient and provide insight into how the reaction progresses in individual particles. This experiment illustrates the value and potential of in situ microfluidic X-ray synchrotron techniques for detailed studies of the millisecond structural transformations of nanoparticles and other solution-phase reactions in which diffusive mixing initiates changes in local bond structures or oxidation states.     

Anchoring molecular chromophores to colloidal gold nanocrystals: surface-enhanced Raman evidence for strong electronic coupling and irreversible structural locking

High-affinity anchoring groups such as isothiocyanate (ITC, -N═C═S) are often used to attach organic chromophores (reporter molecules) to colloidal gold nanocrystals for surface-enhanced Raman scattering (SERS), to atomically smooth gold surfaces for tip-enhanced Raman scattering, and to scanning tunneling microscopy probes (nanosized electrodes) for single-molecule conductance measurements. However, it is still unclear how the attached molecules interact electronically with the underlying surface, and how the anchoring group might affect the electronic and optical properties of such nanoscale systems. Here we report systematic surface-enhanced Raman studies of two organic chromophores, malachite green (MG) and its ITC derivative (MGITC), that have very different functional groups for surface binding but nearly identical spectroscopic properties. A surprise finding is that, under the same experimental conditions, the SERS signal intensities for MGITC are nearly 500-fold higher than those of MG. Correcting for the intrinsic difference in scattering cross sections of these two dyes, we estimate that the MGITC enhancement factors are ~200-fold higher than for MG. Furthermore, pH-dependent studies reveal that the surface structure of MGITC is irreversibly stabilized or "locked" in its π-conjugated form and is no longer responsive to pH changes. In contrast, the electronic structure of adsorbed MG is still sensitive to pH and can be switched between its localized and delocalized electronic forms. These results indicate that ITC is indeed an unusual anchoring group that enables strong electronic coupling between gold and the adsorbed dye, leading to more efficient chemical enhancement and higher overall enhancement factors.     

Quantum Dot Photoluminescence Quenching by Cr(III) Complexes. Photosensitized Reactions and Evidence for a FRET Mechanism

Reported are quantitative studies of the energy transfer from water-soluble CdSe/ZnS and CdSeS/ZnS core/shell quantum dots (QDs) to the Cr(III) complexes trans-Cr(N(4))(X)(2)(+) (N(4) is a tetraazamacrocycle ligand, X(-) is CN(-), Cl(-), or ONO(-)) in aqueous solution. Variation of N(4), of X(-), and of the QD size and composition allows one to probe the relationship between the emission/absorption overlap integral parameter and the efficiency of the quenching of the QD photoluminescence (PL) by the chromium(III) complexes. Steady-state studies of the QD PL in the presence of different concentrations of trans-Cr(N(4))(X)(2)(+) indicate a clear correlation between quenching efficiency and the overlap integral largely consistent with the predicted behavior of a F?rster resonance energy transfer (FRET)-type mechanism. PL lifetimes show analogous correlations, and these results demonstrate that spectral overlap is an important consideration when designing supramolecular systems that incorporate QDs as photosensitizers. In the latter context, we extend earlier studies demonstrating that the water-soluble CdSe/ZnS and CdSeS/ZnS QDs photosensitize nitric oxide release from the trans-Cr(cyclam)(ONO)(2)(+) cation (cyclam = 1,4,8,11-tetraazacyclotetradecane) and report the efficiency (quantum yield) for this process. An improved synthesis of ternary CdSeS core/shell QDs is also described.     

Design and development of a field-deployable single-molecule detector (SMD) for the analysis of molecular markers

Single-molecule detection (SMD) has demonstrated some attractive benefits for many types of biomolecular analyses including enhanced processing speed by eliminating processing steps, elimination of ensemble averaging and single-molecule sensitivity. However, it's wide spread use has been hampered by the complex instrumentation required for its implementation when using fluorescence as the readout modality. We report herein a simple and compact fluorescence single-molecule instrument that is straightforward to operate and consisted of fiber optics directly coupled to a microfluidic device. The integrated fiber optics served as waveguides to deliver the laser excitation light to the sample and collecting the resulting emission, simplifying the optical requirements associated with traditional SMD instruments by eliminating the need for optical alignment and simplification of the optical train. Additionally, the use of a vertical cavity surface emitting laser and a single photon avalanche diode serving as the excitation source and photon transducer, respectively, as well as a field programmable gate array (FPGA) integrated into the processing electronics assisted in reducing the instrument footprint. This small footprint SMD platform was tested using fluorescent microspheres and single AlexaFluor 660 molecules to determine the optimal operating parameters and system performance. As a demonstration of the utility of this instrument for biomolecular analyses, molecular beacons (MBs) were designed to probe bacterial cells for the gene encoding Gram-positive species. The ability to monitor biomarkers using this simple and portable instrument will have a number of important applications, such as strain-specific detection of pathogenic bacteria or the molecular diagnosis of diseases requiring rapid turn-around-times directly at the point-of-use.