Biomedical subjects

Guillermo C Bazan

Publications and source records attributed to Guillermo C Bazan.

At least 19 recordsLinked to original sources

PubMed2006

Silver nanocrystal-modified silicon nanowires as substrates for surface-enhanced Raman and hyper-Raman scattering.

Metal catalyzed, CVD-grown silicon nanowires decorated by chemical assembly of closely spaced Ag nanocrystals were modified with the well-known "silver mirror" reaction and investigated as substrates for surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) spectroscopy. Four chromophores were examined: Rhodamine 6G, crystal violet, a cyanine dye, and a cationic donor-acceptor substituted stilbene. After soaking the substrates overnight in 10(-4) M aqueous chromophore solutions, all four chromophores gave good-quality SERS spectra in < or =60 s using <1 microW of 458-nm cw laser power, and SEHRS spectra are obtained in < or =120 s using <1 mW of mode-locked 916-nm laser power. Results from this substrate are compared with those on colloidal silver nanoparticles deposited as a film, as well as surfaces grown by the silver mirror reaction.

Microscopy, Electron, Scanning↗

PubMed2006

Single-molecule charge-transport measurements that reveal technique-dependent perturbations.

We compare scanning tunneling microscopy (STM) imaging with single-molecule conductive atomic force microscopy (C-AFM) measurements by probing a series of structurally related thiol-terminated oligo(phenylenevinylene)s (OPVs) designed to have unique charge-transport signatures. When one or two methylene spacers are inserted between the thiol points of attachment and the OPV core, a systematic reduction in the imaged molecular transconductance and the current transmitted through a metal-molecule-metal junction containing the molecule is observed, indicating good agreement between STM and C-AFM measurements. However, a structure where the OPV backbone is interrupted by a [2.2]paracyclophane core has a low molecular transconductance, as determined from STM images, and a high measured single-molecule conductance. This apparent disconnect can be understood by comparing the calculated molecular orbital topology of the OPV with one thiol bound to a gold surface (the geometry in the STM experiment) with the topology of the molecule with both thiol termini bound to gold (relevant to C-AFM). In the former case, a single contact splits low-lying molecular orbitals into two discrete fragments, and in the latter case, molecular orbitals that span the entire molecule are observed. Although the difference in observed conductance between the two different measurements is resolved, the overall set of observations highlights the importance of using combined techniques to better characterize charge-transport properties relevant to molecular electronics.

Journal Article↗

PubMed2006

Solvent effects on resonant first hyperpolarizabilities and Raman and hyper-Raman spectra of DANS and a water-soluble analog.

The two-photon-resonant first hyperpolarizabilities associated with hyper-Rayleigh and hyper-Raman scattering are reported for 4-dimethylamino-4-nitrostilbene in 1,4-dioxane, dichloromethane, acetonitrile, and methanol, and for an ionic analog, 4-N,N-bis(6-(N,N,N-trimethylammonium)-hexyl)amino-4-nitrostilbene dibromide in methanol and water. Resonance Raman and hyper-Raman excitation profiles are also measured and modeled. The resonance Raman and hyper-Raman spectra show very similar relative intensities which do not vary much as the excitation frequency is tuned across the lowest-energy strong linear absorption band, suggesting that a single resonant electronic state dominates the one- and two-photon absorptions in this region. The absorption, resonance Raman, and hyper-Raman profiles can be simulated reasonably well with a common set of parameters. The peak resonant (absolute value of beta)2, measured by hyper-Rayleigh scattering, varies by about 50% over the range of solvents examined and shows a weak correlation with the linear absorption maximum, with the redder-absorbing systems exhibiting larger peak hyperpolarizabilities. The experimental hyper-Rayleigh intensities are higher than those calculated, possibly reflecting contributions from nonresonant electronic states.

Journal Article↗

PubMed2006

PNA/dsDNA complexes: site specific binding and dsDNA biosensor applications.

The ability of peptide nucleic acids (PNA) to form specific higher-order (i.e., three- and four-stranded) complexes with DNA makes it an ideal structural probe for designing strand-specific dsDNA biosensors. Higher-order complexes are formed between a dye-labeled charge-neutral PNA probe and complementary dsDNA. Addition of a light-harvesting cationic conjugated polymer (CCP) yields supramolecular structures held together by electrostatic forces that incorporate the CCP and the dye-labeled PNA/DNA complexes. Optimization of optical properties allows for excitation of the CCP and subsequent fluorescence resonance energy transfer (FRET) to the PNA-bound dye. In the case of noncomplementary dsDNA, complexation between the probe and target does not occur, and dye emission is weak. The binding between PNA and noncomplementary and complementary dsDNA was examined by several methods. Gel electrophoresis confirms specificity of binding and the formation of higher-order complexes. Nano-electrospray mass spectrometry gives insight into the stoichiometric composition, including PNA/DNA, PNA(2)/DNA, PNA/DNA(2), and PNA(2)/DNA(2) complexes. Finally, structural characteristics and binding-site specificity were examined using ion mobility mass spectrometry in conjunction with molecular dynamics. These results give possible conformations for each of the higher-order complexes formed and show exclusive binding of PNA to the complementary stretch of DNA for all PNA/DNA complexes. Overall, the capability and specificity of binding indicates that the CCP/PNA assay is a feasible detection method for dsDNA and eliminates the need for thermal denaturing steps typically required for DNA hybridization probe assays.

Biosensing Techniques↗

PubMed2006

Quadruplex-to-duplex transition of G-rich oligonucleotides probed by cationic water-soluble conjugated polyelectrolytes.

G-quartet DNA converts to duplex form in the presence of its complementary strand. This conformational change can be detected in real time by a homogeneous assay method based on the signal amplification of conjugated polyelectrolytes and the specific interaction of intercalating dyes with double-stranded DNA (dsDNA). The probe solution contains a cationic, conjugated polymer (CCP), G-quadruplex labeled with a fluorescein at the 5'-terminus (G-quadruplex-Fl), and ethidium bromide (EB). The addition of a complementary target results in the transition from G-quadruplex to duplex (dsDNA-Fl) and EB intercalation within the duplex structure. Excitation of the CCP leads to energy transfer from CCP to dsDNA-Fl (FRET-1) and then energy transfer from dsDNA-Fl to EB (FRET-2). Increasing the number of mismatched bases discourages dsDNA formation, which is detected in the assay.

Cations↗

PubMed2006

Optical coherence and theoretical study of the excitation dynamics of a highly symmetric cyclophane-linked oligophenylenevinylene dimer.

Optoelectronic properties of a polyphenylenevinylene-based oligomer and its paracylophane-linked dimer are studied using a variety of experimental and theoretical techniques. Despite the symmetrical structure and redshifted absorption of the dimer versus the monomer, an exciton picture is not the most appropriate. Electronic structure calculations establish changes in charge density upon optical excitation and show localized excitations that cannot be accounted for by a simple Frenkel exciton model. Visible frequency pump-probe anisotropy measurements suggest that the dimer should be considered as a three-level system with a fast, approximately 130 fs, internal conversion from the higher to lower energy excited electronic state. Signatures of nuclear relaxation processes are compared for electric field-resolved transient grating and two-dimensional photon echo spectra. These measurements reveal that nuclear relaxation occurs on similar time scales for the monomer and dimer. The connection between the spectral phase of four-wave mixing signals and the time dependent width of a nuclear wave packet is discussed. Semiempirical electronic structure and metropolis Monte Carlo calculations show that the dominant line broadening mechanisms for the monomer and dimer are associated with inter-ring torsional coordinates. Together, the theoretical calculations and electric field-resolved four-wave mixing experiments suggest that while the structure of dimer is more rigid than that of monomer, the difference in their rigidities is not sufficient to slow down excited state relaxation of dimer with respect to the monomer.

Journal Article↗

PubMed2006

Simulation of single molecule inelastic electron tunneling signals in paraphenylene-vinylene oligomers and distyrylbenzene[2.2]paracyclophanes.

Inelastic resonances in the electron tunneling spectra of several conjugated molecules are simulated using the nonequilibrium Greens function formalism. The vibrational modes that strongly couple to the electronic current are different from the infrared and Raman active modes. Spatially resolved inelastic electron tunneling (IET) intensities are predicted. The simulated IET intensities for a large distyrylbenzene paracyclophane molecule are in qualitative agreement with recent experimental results.

Journal Article↗

PubMed2006

Red electrophosphorescence from a soluble binaphthol derivative as host and iridium complex as guest.

The investigation of the optical properties, carrier injection, and transport into a soluble small molecule, 6,6'-dicarbazolyl-2,2'-dihexyloxy-1,1'-binaphthol (BA), was reported. The results demonstrated that BA is a blue-emitting molecule, which can be used as a host for the fabrication of electrophosphorescent light-emitting diodes (LEDs). The single-layer electrophosphorescent LEDs fabricated from toluene solution containing BA with tris[2,5-bis-2'-(9',9'-dihexylfluorene)pyridine-kappa(2)NC(3)(')]iridium(III) [Ir(HFP)(3)] emitted red light from Ir(HFP)(3) triplet emission. The results from photoluminescence (PL) and electroluminescence (EL) demonstrated that the dominated operational mechanism in EL was charge trapping rather than Förster transfer, which was the dominated mechanism in PL. The single-layer OLEDs with 1wt % of Ir(HFP)(3) have a luminance (L) of 1000 cd/m(2) at 22 V and a luminous efficiency (LE) of 0.88 cd/A at 11 mA/cm(2). Double-layer electrophosphorescent LEDs fabricated by casting the emitting layer from a solution of BA blended with Ir(HFP)(3) and subsequently thermally depositing tris(8-hydroxyquinoline) aluminum (Alq(3)) film as an electron injection and transport layer yielded L = 1830 cd/m(2) at 30 V and LE = 2.47 cd/A at 18 mA/cm(2). These results demonstrated that electrophosphorescent LEDs can be fabricated from BA via solution processing and that L and LE can be enhanced by changing the device architecture with the goal of better balancing the electron and hole currents.

Journal Article↗

PubMed2006

Optimization of the molecular orbital energies of conjugated polymers for optical amplification of fluorescent sensors.

Cationic water-soluble poly(fluorene-co-phenylene)s with electron withdrawing or donating substituents on the conjugated backbone were designed and synthesized. Fluorescence resonance energy transfer (FRET) experiments between these conjugated polymers and dye-labeled single-stranded DNA (ssDNA-C*) reveal the importance of matching donor and acceptor orbital energy levels to improve the sensitization of C* emission. Quenching of polymer fluorescence with ssDNA-C* and differences in C* emission suggest involvement of photoinduced charge transfer (PCT) as an energy wasting mechanism. The HOMO and LUMO energy levels of the conjugated polymers and C serve as a preliminary basis to understand the competition between FRET and PCT. Dilution of C in polymer/ssDNA-C complexes by addition of ssDNA yields insight into C*...C self-quenching. Under optimized conditions, where there is no probe self-quenching and minimum PCT, efficient signal amplification is demonstrated despite poor spectral overlap between polymer and C.

Biosensing Techniques↗

PubMed2005

Matrix-induced intensity fluctuations in the fluorescence from single oligo(phenylenevinylene) molecules.

Single molecule spectroscopy on oligo(phenylenevinylene) (OPV) chromophores shows that the fluorescence intermittency strongly correlates to the rigidity of the environment surrounding the molecules. For OPV single molecules, environmental rigidity inhibits twisting about the vinyl linkages, the molecular motion associated with the observed "off" (nonabsorbing) state. By increasing the rigidity of a single molecule's environment, we can tune its room temperature fluorescence from rapid, sub-millisecond "blinking" fluctuations (fluid polymer environment) to completely "on" with no blinking observed (molecules adsorbed to a rigid bare glass substrate). The difference in fluorescence intermittency from environment to environment is immediately apparent and explicit in single molecule intensity trajectories under cw (continuous wave) excitation, demonstrating the sensitivity of these chromophores to their surroundings and emphasizing the importance of morphological control in real-world applications involving phenylenevinylene-based materials.

Journal Article↗

PubMed2005

Solvent effects on the two-photon absorption of distyrylbenzene chromophores.

A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modulate intramolecular charge transfer (ICT). These molecules allow an examination of solvent effects on the TPA cross section (delta) and on the TPA action cross section. In toluene, the delta values follow the order of ICT strength. The effect of solvent on delta is nonmonotonic: maximum delta was measured in an intermediate polarity solvent (THF) and was lowest in water. We failed to find a correlation between the observed solvent effect and previous theoretical predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are not included in calculational analysis, may be responsible for the discrepancies between experimental results and theory.

Absorption↗

PubMed2005

Chelate bis(imino)pyridine cobalt complexes: synthesis, reduction, and evidence for the generation of ethene polymerization catalysts by Li+ cation activation.

Treatment of the bis(iminobenzyl)pyridine chelate Schiff-base ligand 8 (ligPh) with FeCl2 or CoCl2 yielded the corresponding (ligPh)MCl2 complexes 9 (Fe) and 10 (Co). The reaction of 10 with methyllithium or "butadiene-magnesium" resulted in reduction to give the corresponding (ligPh)Co(I)Cl product 11. Similarly, the bis(aryliminoethyl)pyridine ligand (ligMe) was reacted with CoCl2 to yield (ligMe)CoCl2 (12). Reduction to (ligMe)CoCl (13) was effected by treatment with "butadiene-magnesium". Complex 13 reacted with Li[B(C6F5)4] in toluene followed by treatment with pyridine to yield [(ligMe)Co+-pyridine] (15). The reaction of the Co(II) complexes 10 or 12 with ca. 3 molar equiv of methyllithium gave the cobalt(I) complexes 16 and 17, respectively. Treatment of the (ligMe)CoCH3 (17) with Li[B(C6F5)4] gave a low activity ethene polymerization catalyst. Likewise, complex 16 produced polyethylene (activity = 33 g(PE) mmol(cat)(-1) h(-1) bar(-1) at room temperature) upon treatment with a stoichiometric amount of Li[B(C6F5)4]. A third ligand (lig(OMe)) was synthesized featuring methoxy groups in the ligand backbone (22). Coordination to FeCl2 and CoCl2 yielded the desired compounds 23 and 24. Reaction with MeLi gave (ligOMe)CoMe (25/26). Treatment of 25/26 with excess B(C6F5)3 gave the eta6-arene cation complex 27, where one Co-N linkage was cleaved. Activation of 25/26 with Li[B(C6F5)4] again gave a catalytically active species.

Catalysis↗

PubMed2005

Two-photon absorption in aqueous micellar solutions.

Addition of the surfactant sodium dodecyl sulfate at concentrations above the critical micelle concentration increases the fluorescence quantum yield and the two-photon absorption cross-section of charged [2.2]paracyclophane chromophores containing pairs of D-pi-D chromophores. The resulting spectra in the micellar solutions are very similar to those obtained for neutral isostructural analogues in toluene. The measured etadelta values are 1300 GM for 1C and 1920 GM for 2C, which are comparable or larger to those in toluene. These results highlight possible misleading interpretation of two-photon-induced emission for evaluating the concentration of labeled substrates used in two-photon microscopy and provide guidelines for designing molecular structures with optimized two-photon action cross-sections in water.

Acetates↗

PubMed2005

Solid-state optical properties of linear polyconjugated molecules: pi-stack contra herringbone.

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F(12)DSB) as well as the DSB:F(12)DSB cocrystals prefer cofacial pi-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant k(F). Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F(12)DSB and DSB:F(12)DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.

Journal Article↗

PubMed2005

Probing pi-coupling in molecular junctions.

Charge transport characteristics for metal-molecule-metal junctions containing two structurally related pi-conjugated systems were studied to probe pi-pi interactions in molecular junctions. The first molecule contains a typical pi-conjugated framework derived from phenylene vinylene units, whereas the second has the phenylene vinylene structure interrupted by a [2.2]paracyclophane (pCp) core. Electrochemical investigations were used to characterize the defects and packing density of self-assembled monolayers of the two molecules on gold surfaces and to enable quantitative comparison of their transport characteristics. Current-voltage measurements across molecular junctions containing the two species demonstrate that the pCp moiety yields a highly conductive break in through-bond pi-conjugation. The observed high conductivity is consistent with density functional theory calculations, which demonstrate strong through-space pi-pi coupling across the pCp moiety.

Electrochemistry↗

PubMed2005

Theoretical study of the effects of solvent environment on photophysical properties and electronic structure of paracyclophane chromophores.

We use first-principles quantum-chemical approaches to study absorption and emission properties of recently synthesized distyrylbenzene (DSB) derivative chromophores and their dimers (two DSB molecules linked through a [2.2]paracyclophane moiety). Several solvent models are applied to model experimentally observed shifts and radiative lifetimes in Stokes nonpolar organic solvents (toluene) and water. The molecular environment is simulated using the implicit solvation models, as well as explicit water molecules and counterions. Calculations show that neither implicit nor explicit solvent models are sufficient to reproduce experimental observations. The contact pair between the chromophore and counterion, on the other hand, is able to reproduce the experimental data when a partial screening effect of the solvent is taken into account. Based on our simulations we suggest two mechanisms for the excited-state lifetime increase in aqueous solutions. These findings may have a number of implications for organic light-emitting devices, electronic functionalities of soluble polymers and molecular fluorescent labels, and their possible applications as biosensors and charge/energy conduits in nanoassemblies.

Journal Article↗

PubMed2005

Solvatochromism of distyrylbenzene pairs bound together by [2.2]paracyclophane: evidence for a polarizable "through-space" delocalized state.

A series of compounds were designed and synthesized to examine how through-space and through-bond electron delocalization respond to solvent effects. The general strategy involves the study of "dimers" of the distyrylbenzene chromophore held in close proximity by the [2.2]paracyclophane core and a systematic dissection of the chromophore into components with through-space and through-bond electronic delocalization. Steady state and time-resolved fluorescence spectroscopy in a range of solvents reveals a red-shift in emission and an increase in the intrinsic fluorescence lifetime for the emitting state in polar solvents when donor substituents are absent. We propose that through-space delocalization across the [2.2]paracyclophane core is more polarizable in the excited state, relative to the through-bond (distyrylbenzene based) excited state. When strong donors are attached to the distyrylbenzene chromophore, the charge transfer character of the distyrylbenzene-based excited state dominates fluorescence properties.

Journal Article↗

PubMed2005

Synthesis of cationic water-soluble light-harvesting dendrimers.

Four generations of phenylenefluorene (1F)- and phenylenebis(fluorene) (2F)-terminated polyamidoamine (PAMAM) dendrimers were synthesized by coupling activated esters with commercially available PAMAM precursors. Treatment of Boc-terminated pendant groups on the optically active units with 3 M HCl in dioxane yields cationic water-soluble dendrimers. Fluorescence resonance energy transfer (FRET) experiments with the cationic dendrimers as the donor and double stranded DNA containing a fluorescein label as the acceptor reveal cooperative optical behavior.

DNA↗