Self-assembly of rod-coil block copolymers from weakly to moderately segregated regimes

Abstract.: We report on the self-assembly behaviour of two homologue series of rod-coil block copolymers in which, the rod, a π -conjugated polymer, is maintained fixed in size and chemical structure, while the coil is allowed to vary both in molecular weight and chemical nature. This allows maintaining constant the liquid crystalline interactions, expressed by Maier-Saupe interactions, ω , while varying the tendency towards microphase separation, expressed by the product between the Flory-Huggins parameter and the total polymerization degree, χN . Therefore, the systems presented here allow testing directly some of the theoretical predictions for the self-assembly of rod-coil block copolymers in a weakly segregated regime. The two rod-coil block copolymer systems investigated were poly(DEH-p-phenylenevinylene-b-styrene), whose self-assembly takes place in the very weakly segregated regime, and poly(DEH-p-phenylenevinylene-b-4vinylpyridine), for which the self-assembly behaviour occurs under increased tendency towards microphase separation, hereby referred to as moderately segregated regime. Experimental results for both systems are compared with predictions based on Landau expansion theorie

Influence of Surface Plasmon Resonance on Fluorescence Emission of Dye-doped Nanoparticles

The influence of the surface plasmon of gold nanoparticles on the optical properties of the fluorescent nanoparticles in aqueous solution have been investigated. The fluorescence of nanoparticles can be enhanced or quenched in the presence of gold nanoparticles depending on the domination of energy transfer mechanisms: radiating surface plasmon coupling emission or F\"{o}rster energy transfer from fluorescent particles to gold particles, which exciting absorbing plasmon. The fluorescence enhancement or quenching is attributed to the increase or decrease of radiative recombination rates, respectively. The parameters of the energy transfer between fluorescent nanoparticles (dye molecules encapsulated in silica nanoparticles) and nano golds have been estimated. The results show that the interactions between nanoparticles depend on the size of both fluorophores (as donors) and gold nanoparticles (as acceptors)

Synthesis and Optical Characterization of Dye Doped in Ormosil Nanospheres for Bioapplications

Dye-doped ORMOSIL (organically modified silicate) nanoparticles (NPs) have significant advantages over single-dye labeling in signal amplification, photostability and surface modification for various biological applications. The dyes: Rhodamine 6G (R6G) and Rhodamine B (RB) were successfully incorporated into ORMOSIL nanoparticles fabricated by micellar nanochemistry from Trimethoxysilane CH$_3$Si(OCH$_3$)$_3$ precursor. The optical characterization of dye-doped ORMOSIL NPs was studied in comparison with it’s of free dye in solution. The results shown that the photostability of ORMOSIL dye doped nanospheres is much improved in comparison with it’s of dye in solution. Other studies of the photophysical properties such as anisotropy, fluorescence lifetime and energy transfer were also done

Analyzing the Distribution of Decay Constants in Pulse-Fluorimetry Using the Maximum Entropy Method

The maximum entropy method (MEM) is used to analyze time-resolved pulse-fluorescence spectrometry. The central problem in such analyses is the recovery of the distribution of exponentials describing the decay of the fluorescence (i.e., inverting the Laplace transform) which is, in turn, convolved by the shape of the excitation flash. MEM is shown to give high quality results from both computer-generated “noisy” data and experimental data from chemical and biological molecules. The use of the Shannon-Jaynes entropy function is justified and both the theoretical and practical advantages of MEM are presented. The MEM results are easy to interpret and can help to overcome some experimental limitations. In particular MEM could be a powerful tool to analyze the heterogeneity of fluorescent emission of biological macromolecules which can be correlated with their conformational dynamics in solution

Pressure effects on the physical properties of lipid bilayers detected by trans-parinaric acid fluorescence decay.

The effects of hydrostatic pressure on the physical properties of large unilamellar vesicles of single lipids dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylcholine (DMPC) and lipid mixtures of DMPC/DPPC have been studied from time-resolved fluorescence of trans-parinaric acid. Additional experiments were carried out using diphenylhexatriene to compare the results extracted from both probes. Fluorescence decays were analyzed by the maximum entropy method. Pressure does not influence the fluorescence lifetime distribution of trans-parinaric acid in isotropic solvents. However, in pressurized lipid bilayers an abrupt change was observed in the lifetime distribution which was associated with the isothermal pressure-induced phase transition. The pressure to temperature equivalence values, dT/dP, determined from the midpoint of the phase transitions, were 24 and 14.5 degrees C kbar-1 for DMPC and POPC, respectively. Relatively moderate pressures of about 500 bar shifted the DMPC/DPPC phase diagram 11.5 degrees C to higher temperatures. The effects of pressure on the structural properties of these lipid vesicles were investigated from the anisotropy decays of both probes. Order parameters for all systems increased with pressure. In the gel phase of POPC the order parameter was smaller than that obtained in the same phase of saturated phospholipids, suggesting that an efficient packing of the POPC hydrocarbon chains is hindered

Pressure effects on the lateral distribution of cholesterol in lipid bilayers: a time-resolved spectroscopy study.

The effects of hydrostatic pressure and temperature on the phase behavior and physical properties of the binary mixture palmitoyloleoylphosphatidylcholine/cholesterol, over the 0-40 molar % range of cholesterol compositions, were determined from the changes in the fluorescence lifetime distribution and anisotropy decay parameters of the natural lipid trans-parinaric acid (t-PnA). Pressurized samples were excited with a Ti-sapphire subpicosecond laser, and fluorescence decays were analyzed by the quantified maximum entropy method. Above the transition temperature (T(T) = -5 degrees C), at atmospheric pressure, two liquid-crystalline phases, alpha and beta, are formed in this system. At each temperature and cholesterol concentration below the transition pressure, the fluorescence lifetime distribution pattern of t-PnA was clearly modulated by the pressure changes. Pressure increased the fraction of the liquid-ordered beta-phase and its order parameter, but it decreased the amount of cholesterol in this phase. Palmitoyloleoylphosphatidylcholine/cholesterol phase diagrams were also determined as a function of temperature and hydrostatic pressure