J. M. G. Martinho

One Step Synthesis of the Smallest Photoluminescent and Paramagnetic PVP-Protected Gold Atomic Clusters

Gold atomic clusters of only two and three atoms were prepared by a simple electrochemical technique based on the anodic dissolution of a gold electrode in the presence of PVP, and subsequent electroreduction of the Au-PVP complexes. These clusters show stable photoluminescent and magnetic properties, which make them the smallest and most elemental gold (0) building blocks in nature (after atoms) bringing new possibilities to construct novel nano/microstructures with large potential interest in biomedicine, catalysis, and so forth.

Synthesis and luminescence properties of three novel polyfluorene copolymers

We report on the synthesis and characterization (including structural, optical, electrochemical and electroluminescence properties) of three alternating F-alt-X copolymers, where F is 9,9-bis(2 0 -ethylhexyl)fluorene unit and the X comonomer varies from a phenylene, to a thiophene and to a thiophene-S,S-dioxide unit. Among these X comonomers, the phenylene group is at the origin of a blue-emitting copolymer with unitary luminescence efficiency in solution, while thiophene-S,S-dioxide promotes the highest electron affinity. These copolymers are also used in the fabrication of light-emitting diodes. q 2003 Elsevier Science Ltd. All rights reserved.

Dynamic properties of humic matter by dynamic light scattering and voltammetry

The diffusion coefficients of humic matter samples (Purified Peat (PP) Moss and Fluka) in solution were obtained by both dynamic light scattering (DLS) and voltammetric methods. The diffusion coefficients from voltammetric measurements are greater than those determined by DLS, due to the different way each technique responds to the size distribution of polydisperse samples. Large aggregates are present in both PP humic acid (HA) and Fluka samples (diameters larger than 30 nm), their size depending on the origin and/or preparation procedure of the sample, as well as on the experimental conditions of pH and ionic strength. The influence of ionic strength on the diffusion coefficient of PPHA was analysed. Similar results were obtained by DLS and voltammetry in samples with low polydispersity up to ionic strength of 0.1 M. For higher ionic strengths aggregation may occur, leading to an increase in the apparent molecular size. The influence of pH on the diffusion coefficient of PPHA shows one population formed by large aggregates (185 nm diameter) at pH 5 in the absence of added salt. When pH is lowered below 2.5, coagulation occurs quickly leading to the precipitation of PPHA. Some irreversible disagreggation (120 nm diameter) is promoted by pH increase up to 10. PPHA samples, initially set to pH 10 and then adjusted to pH between 3 and 5, present somehow different characteristics from samples directly set to the required pH, which stresses the influence of the preparation method on final sample properties.

A V-shaped cationic dye for nonlinear optical bioimaging

A symmetric cationic molecule with D-π-A(+)-π-D architecture was synthesized with high two-photon absorption cross-section (σ(2) ≈ 1140 GM). Application as a marker in fluorescence microscopy of living cells revealed its presence inside the cell staining vesicular shape organelles in the cytoplasm. Fluorescence lifetime imaging microscopy shows that it is also able to penetrate within the nucleus.

p-Dimethylaminobenzonitrile in polar solution.: II. Quantum yield measurements and quasi-stationary kinetic study of the electron transfer reaction

Abstract The temperature influence of the quantum yield of the planar b* stete of DMABIN in various polar solvents has been interpreted within the framework of a model of deexcitation by charge transfer reaction in the absence of a potential barrier. Rate parameters characterizing the TICT state have been deduced from the kinetic study of the temporal evolution of the two emission bands: the rate constant for the backward b* ← a* reaction has been determined as well as its activation energy which is correlated to the stabilization of the TICT state by dipolar interaction with the medium.

Diffusion‐influenced excimer formation kinetics

A model for reversible monomer–excimer kinetics is developed, taking into account that different distributions of monomers around the excited ones are created by light absorption and excimer dissociation. The excimer formation rate coefficient departs from the Collins–Kimball equation owing to reversibility, originating significant deviations to Birks’ kinetics in the monomer and excimer decays. The contribution of the geminate pair created by excimer dissociation on the overall kinetics is significant for low monomer concentrations and high viscosities. Simultaneous analysis of the monomer and excimer decay curves according to the model developed should allow to extract all the relevant information concerning the excimer formation diffusion controlled process.

Test of a model for reversible excimer kinetics : pyrene in cyclohexanol

A model for reversible monomer–excimer kinetics that considers the time dependence of the excimer formation rate coefficient is proposed and tested for pyrene in cyclohexanol from 25 up to 85 °C. Simultaneous analysis of the monomer and excimer experimental decay curves allows the determination of all the relevant parameters for this reaction. The diffusion coefficient follows an Arrhenius plot with activation energy of 36±1 KJ mol−1, the encounter radius varies between 7.3 and 8.9 A, and the intrinsic rate constant for excimer formation varies between 2.4×109 and 1.1×1010 M−1 s−1. The intrinsic reciprocal lifetime of the excimer follows an Arrhenius plot with activation energy of 12±1 KJ mol−1, and the rate constant for excimer dissociation, determined for high temperatures (T>55 °C) when reversibility is important, has different values depending whether geminate pair effects are considered or not in the analysis. The binding energy of pyrene excimer obtained considering pair effects (ΔH=32±2 KJ mol−1) a...

Accurate determination of the limiting anisotropy of rhodamine 101. Implications for its use as a fluorescence polarization standard.

The S1-S0 limiting anisotropy of a widely used fluorophore, rhodamine 101, is determined with unprecedented accuracy. From time-resolved and steady-state fluorescence measurements in several solvents, it is shown that the limiting anisotropy of rhodamine 101 is for all practical purposes equal to the theoretical one-photon fundamental anisotropy value of 2/5, both in rigid and in fluid media. This fact, along with the favorable chemical and photophysical properties of rhodamine 101, point to its use as a standard for fluorescence polarization measurements. It is also shown that if the excitation pulse can be considered a delta impulse with respect to the time scale of the anisotropy decay (but not necessarily to the time scale of the intensity decay), then no deconvolution procedure is needed for anisotropy decay analysis.

The effect of radiative transport on fluorescence emission

A model for radiative transport of electronic excitation energy in solution is presented and applied to the time‐resolved and steady‐state fluorescence of DPA (9, 10‐diphenylanthracene) in benzene. The model predicts a nonexponential and wavelength‐dependent decay at high concentrations, in agreement with experimental results. Recovered parameters, along with the time‐resolved emission spectrum, are interpreted on the basis of a progression of the excitation with time farther into the cell, after the excitation pulse.

Activity, conformation and dynamics of cutinase adsorbed on poly(methyl methacrylate) latex particles

The adsorption of a recombinant cutinase from Fusarium solani pisi onto the surface of 100 nm diameter poly(methyl methacrylate) (PMMA) latex particles was evaluated. Adsorption of cutinase is a fast process since more than 70% of protein molecules are adsorbed onto PMMA at time zero of experiment, irrespective of the tested conditions. A Langmuir-type model fitted both protein and enzyme activity isotherms at 25 degrees C. Gamma(max) increased from 1.1 to 1.7 mg m(-2) and U(max) increased from 365 to 982 U m(-2) as the pH was raised from 4.5 to 9.2, respectively. A decrease (up to 50%) in specific activity retention was observed at acidic pH values (pH 4.5 and 5.2) while almost no inactivation (eta(act) congruent with 87-94%) was detected upon adsorption at pH 7.0 and 9.2. Concomitantly, far-UV circular dichroism (CD) spectra evidenced a reduction in the alpha-helical content of adsorbed protein at acidic pH values while at neutral and alkaline pH the secondary structure of adsorbed cutinase was similar to that of native protein. Fluorescence anisotropy decays showed the release of some constraints to the local motion of the Trp69 upon protein adsorption at pH 8.0, probably due to the disruption of the tryptophan-alanine hydrogen bond when the tryptophan interacts with the PMMA surface. Structural data associated with activity measurements at pH 7.0 and 9.2 showed that cutinase adsorbs onto PMMA particles in an end-on orientation with active site exposed to solvent and full integrity of cutinase secondary structure. Hydrophobic interactions are likely the major contribution to the adsorption mechanism at neutral and alkaline pH values, and a higher amount of protein is adsorbed to PMMA particles with increasing temperature at pH 9.2. The maximum adsorption increased from 88 to 140 mg cutinase per g PMMA with temperature raising from 25 to 50 degrees C, at pH 9.2.