Versatile Tetrablock Copolymer Scaffold for Hierarchical Colloidal Nanoparticle Assemblies: Synthesis, Characterization, and Molecular Dynamics Simulation.

A unique combination of molecular dynamics (MD) simulation and detailed size exclusion chromatography-multiangle light scattering (SEC-MALS) analysis is used to provide important a priori insights into the solution self-assembly of a well-defined and symmetric tetrablock copolymer with two acrylic acid (AA) outer blocks, two polystyrene (PS) inner blocks, and a trithiocarbonate (TTC) central group, prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. SEC-MALS experiments show that the copolymer forms aggregates in both tetrahydrofuran and N,N-dimethylformamide (DMF), even in the presence of different salts, but not in 1,4-dioxane (dioxane). Combined with MD simulations, these results indicate that the AA units are the main cause of aggregation through intermolecular hydrogen bonding, with additional stabilization by the central TTC. The block copolymer chains self-assemble in dioxane by adding cadmium acetate, originating flowerlike inverse micelles with a cadmium acrylate core and the TTC groups in the outer surface of the PS corona. The micelles were used as nanoreactors in the templated synthesis of a single cadmium selenide (CdSe) quantum dot (QD) in the core of each micelle, whereas the shell TTC groups can be converted into thiol functions for further use of these units in hierarchical nanostructures. Only in dioxane where simulations and SEC-MALS suggest an absence of copolymer aggregates prior to cadmium acetate addition do well-dispersed and highly luminescent CdSe QDs form by templated synthesis. These results provide valuable insights into the self-assembly of RAFT copolymers in different solvent systems as it relates to the preparation of emissive QDs with polymer-spaced thiol functionality for binding to gold nanostructures.

Polymer-coated nanoparticles by adsorption of hydrophobically modified poly(N,N-dimethylacrylamide).

We prepared a reactive random copolymer of N-acryloxysuccinimide and N,N-dimethylacrylamide (DMA) by reversible addition-fragmentation chain transfer polymerization, with Mn ≈ 50k and 23 mol % reactive NAS groups. This copolymer was subsequently modified with hydrophobic (dodecyl) and fluorescent (pyrene, PY; phenanthrene, PHE; or anthracene, AN) side groups, to obtain fluorescent amphiphilic polymers with the same backbone and different substituents. These polymers were adsorbed onto model (ca. 130 nm diameter) poly(butyl methacrylate) nanoparticles, and the size and structure of the adsorbed layer were evaluated using a combination of fluorescence techniques and light scattering. The total diameter increases very fast with polymer concentration up to ca. 140 nm, and then more slowly to 154 nm, stabilizing at this value which corresponds to a polymer shell thickness of ca. 12 nm. In order to evaluate the distribution of hydrophobic groups on the adsorbed polymer layer, we used Förster resonance energy transfer between PHE- and AN-labeled poly(DMA) chains. The obtained concentration profile of the adsorbed polymer corresponds to a coated particle radius which is only slightly smaller than the hydrodynamic radius measured in the same conditions, indicating that the dyes are not located at the particle interface but mostly distributed across the adsorbed layer. Finally, we observed that hydrophobically modified PHE-labeled poly(DMA) chains adsorbed to the nanoparticles were very efficiently displaced by identical hydrophobically modified chains with five times their molecular weight (Mn ≈ 250k) but labeled with PY.

The influence of nanoparticle architecture on latex film formation and healing properties.

We present a study of chain interdiffusion in films formed by specially architectured PBMA nanoparticles by Förster Resonance Energy Transfer -FRET. Polymer nanoparticles contained linear chains with narrower molecular weight distributions than other previous reports, allowing a more detailed study. Apparent fractions of mixing and diffusion coefficients, determined from the quantum efficiency of energy transfer, were used to characterize the interdiffusion mechanism in the different films. The resistance of the films to dissolution by a good solvent was finally correlated with the interdiffusion results, in order to get information about film healing. We concluded that whenever interdiffusion occurs between nanoparticles containing linear chains and fully cross-linked nanoparticles, healing becomes more effective in spite of showing slower interdiffusion. We also observed that particles with longer chains are more effective for film healing. Finally, we concluded that interdiffusion occurs both ways across interfaces in blends formed by particles swollen with linear chains of different molecular weights.

Biochemical and structural characterisation of cutinase mutants in the presence of the anionic surfactant AOT.

The reactivity, stability and unfolding of wild-type (WT) Fusarium solani pisi cutinase and L153Q, S54D and T179C variants were studied in the absence and presence of the dioctyl sulfosuccinate sodium salt (AOT) surfactant. In the absence of surfactant the S54D variant catalytic activity is similar to that of the WT cutinase, whereas L153Q and T179C variants show a lower activity. AOT addition induces an activity reduction for WT cutinase and its variants, although for low AOT concentrations a small increase of activity was observed for S54D and T179C. The enzyme deactivation in the presence of 0.5 mM AOT is relatively slow for the S54D and T179C variants when compared to wild-type cutinase and L153Q variant. These results were correlated with secondary and tertiary structure changes assessed by the CD spectrum and fluorescence of the single tryptophan and the six tyrosine residues. The WT cutinase and S54D variant have similar secondary and tertiary structures that differ from those of T179C and L153Q variants. L153Q, S54D and T179C mutations prevent the formation of hydrophobic crevices responsible for the unfolding by anionic surfactants, with the consequent decrease of the AOT-cutinase interactions.

Interfacial behavior of poly(isoprene-b-methyl methacrylate) diblock copolymers and their blends with polystyrene at the air-water interface.

The interfacial behavior of poly(isoprene-b-methyl methacrylate) diblock copolymers (PI-b-PMMA), with similar PMMA blocks but differing in the percentage of PI segments, SP19 (5% PI) and SP38 (52% PI), was studied at the air-water interface. The surface pressure-area (pi-A) isotherms, compression-expansion cycles, and relaxation curves were compared with those of the PMMA homopolymer. The short hydrophobic PI block of SP19 does not contribute to the mean molecular area at low surface pressures and yet has a negative contribution (condensing effect) when the surface pressure increases. On the contrary, the long PI block of SP38 contributes considerably to the surface area from low to high surface pressures. The A-t relaxation curves compare well with those of PMMA at low surface pressures (pi = 2 mN.m-1), but not at intermediate and high pressures (pi = 10, 30 mN.m-1), where a clear dependence on the length of the PI block was observed. The quantitative analysis of the relaxation curves at high pressures shows both a fast and slow component, attributed mostly to the local and middle-to-long-range reorganization of PMMA chains, respectively. PI-b-PMMA diblocks and PMMA were further blended with PS. The PS and PMMA are immiscible at the air-water interface. The addition of PS does not change the pi-A isotherm of PMMA, but the copolymers blended with PS form films that are more condensed at low pressures. The Langmuir-Blodgett (LB) films transferred onto mica substrates were analyzed by atomic force microscopy (AFM). The LB films of single diblocks are uniform, while those of PI-b-PMMA and PMMA blended with PS show aggregates with variable patterns.

Preparation and surface characterization of polymer nanoparticles designed for incorporation into hybrid materials.

We prepared water dispersions of poly(n-butyl methacrylate-st-butyl acrylate) crosslinked core-shell nanoparticles functionalized with different amounts of trimethoxisilane (TMS) groups in the outer shell. The purpose of the TMS groups is to chemically bind the rubbery particles to a nanostructured silica network, using sol-gel copolymerization. Here, we present nanoparticles containing 13 mol % and 30 mol % of TMS groups in the outer shell and compare their surface morphology with particles that do not contain TMS. The particles are prepared by a two-step seeded emulsion polymerization technique at neutral pH. In the first step, we obtained crosslinked seed particles (44 nm in diameter) by a batch process. In the second step, we used a semi-continuous emulsion polymerization technique under starved feed conditions to obtain monodispersed particles of controlled composition and size (ca. 100 nm in diameter). Fluorescence decay measurements were performed in situ on the dispersions, using a pair of cationic dyes adsorbed onto the surface of the nanoparticles: rhodamine 6G as the energy transfer donor and malachite green carbinol hydrochloride as the acceptor. The kinetics of Förster resonance energy transfer (FRET) between the dyes is sensitive to the donor-acceptor distance, allowing us to obtain the binding distribution of the dyes at the nanoparticle surface. For the unmodified nanoparticles, we found a dye distribution that corresponds to an average interface thickness of delta = (5.2 +/- 0.2) nm. For the samples containing 13 mol % and 30 mol % of TMS groups in the outer shell we obtained broader interfaces, with widths of delta = (6.2 +/- 0.2) nm and delta = (6.5 +/- 0.1) nm respectively. This broadening of the distribution with the surface modification is interpreted in terms of the increase in free volume of the shell caused by the TMS groups. Finally, we studied the effect of temperature on the water-polymer interface fuzziness, in order to evaluate the accessibility of the TMS groups during the sol-gel synthesis of nanostructured hybrid materials.

Photonic superdiffusive motion in resonance line radiation trapping Partial frequency redistribution effects.

The relation between the jump length probability distribution function and the spectral line profile in resonance atomic radiation trapping is considered for partial frequency redistribution (PFR) between absorbed and reemitted radiation. The single line opacity distribution function [M. N. Berberan-Santos et al., J. Chem. Phys. 125, 174308 (2006)] is generalized for PFR and used to discuss several possible redistribution mechanisms (pure Doppler broadening; combined natural and Doppler broadening; and combined Doppler, natural, and collisional broadening). It is shown that there are two coexisting scales with a different behavior: the small scale is controlled by the intricate PFR details while the large scale is essentially given by the atom rest frame redistribution asymptotic. The pure Doppler and combined natural, Doppler, and collisional broadening are characterized by both small- and large-scale superdiffusive Levy flight behaviors while the combined natural and Doppler case has an anomalous small-scale behavior but a diffusive large-scale asymptotic. The common practice of assuming complete redistribution in core radiation and frequency coherence in the wings of the spectral distribution is incompatible with the breakdown of superdiffusion in combined natural and Doppler broadening conditions.

Photonic superdiffusive motion in resonance radiation trapping.

In this work we consider the relation between the jump length probability density function and the line shape function in resonance radiation trapping in atomic vapors. The two-sided jump length probability density function suitable for a unidimensional formulation of radiative transfer is also derived. As a side result, a procedure to obtain the Maxwell distribution of velocities from the Maxwell-Boltzmann distribution of speeds was obtained. General relations that give the asymptotic jump length behavior and the Levy flight parameter mu for any line shape are obtained. The results are applied to generalized Doppler, generalized Lorentz, and Voigt line shape functions. It is concluded that the lighter the tail of the line shape function, the less heavy the tail of the jump length probability density function, although this tail is always heavy, with mu < or =1.

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide).

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.

Adsorption of oligonucleotides on PMMA/PNIPAM core-shell latexes: polarity of the PNIPAM shell probed by fluorescence.

The adsorption of a rhodamine X labeled oligonucleotide composed of 25-mers of thymine (dT(25)-ROX) onto the thermosensitive shell of PMMA/PNIPAM core-shell latex particles was studied at 22 and 40 degrees C, below and above the T(VPT) (volume phase transition temperature) of the PNIPAM shell, respectively. The experimental binding isotherms were well fitted with the cooperative Hill model. The Hill coefficient is lower than 1 at both temperatures showing that the adsorption is anticooperative. The polarity of the shell was probed by both the lifetimes and solvatochromic shifts of the zwitterionic form of rhodamine X. For temperatures below the shell T(VPT) has a polarity similar to that of water, while for temperatures above the transition the polarity is equivalent to that of a water/dioxane mixture with 30% (v/v) water.

Fluorescence of the single tryptophan of cutinase: temperature and pH effect on protein conformation and dynamics.

The cutinase from Fusarium solani pisi is an enzyme with a single L-tryptophan (Trp) involved in a hydrogen bond with an alanine (Ala) residue and located close to a cystine formed by a disulfide bridge between two cysteine (Cys) residues. The Cys strongly quenches the fluorescence of Trp by both static and dynamic quenching mechanisms. The Trp fluorescence intensity increases by about fourfold on protein melting because of the disruption of the Ala-Trp hydrogen bond that releases the Trp from the vicinity of the cystine residue. The Trp forms charge-transfer complexes with the disulfide bridge, which is disrupted by UV light irradiation of the protein. This results in a 10-fold increase of the Trp fluorescence quantum yield because of the suppression of the static quenching by the cystine residue. The Trp fluorescence anisotropy decays are similar to those in other proteins and were interpreted in terms of the wobbling-in-cone model. The long relaxation time is attributed to the Brownian rotational correlation time of the protein as a whole below the protein-melting temperature and to protein-backbone dynamics above it. The short relaxation time is related to the local motion of the Trp, whose mobility increases on protein denaturation.

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.