José Paulo S. Farinha

Functional Films from Silica/Polymer Nanoparticles

High performance functional coatings, based on hybrid organic/inorganic materials, are being developed to combine the polymer flexibility and ease of processing with the mechanical properties and versatility of inorganic materials. By incorporating silica nanoparticles (SiNPs) in the polymeric matrices, it is possible to obtain hybrid polymer films with increased tensile strength and impact resistance, without decreasing the flexural properties of the polymer matrix. The SiNPs can further be used as carriers to impart other functionalities (optical, etc.) to the hybrid films. By using polymer-coated SiNPs, it is possible to reduce particle aggregation in the films and, thus, achieve more homogeneous distributions of the inorganic components and, therefore, better properties. On the other hand, by coating polymer particles with silica, one can create hierarchically structured materials, for example to obtain superhydrophobic coatings. In this review, we will cover the latest developments in films prepared from hybrid polymer/silica functional systems.

Preparation and characterization of low dispersity anionic multiresponsive core-shell polymer nanoparticles.

We prepared anionic multistimuli responsive core-shell polymer nanoparticles with very low size dispersity. By using either acrylic acid (AA) or methacrylic acid (MA) as a comonomer in the poly(N-isopropyl acrylamide) (PNIPAM) shell, we are able to change the distribution of negative charges in the nanoparticle shell. The particle size, volume phase transition temperature, and aggregation state can be modulated using temperature, pH, or ionic strength, providing a very versatile platform for applications in sensors, medical diagnostics, environmental remediation, etc. The nanoparticles have a glassy poly(methyl methacrylate) (PMMA) core of ca. 40 nm radius and a cross-linked PNIPAM anionic shell with either AA or MA comonomers. The particles, p(N-AA) and p(MA-N), respectively, have the same total charge but different charge distributions. While the p(MA-N) particles have the negative charges preferentially distributed toward the inner shell, in the case of the p(N-AA) particles the charge extends more to the particle outer shell. The volume phase transition temperature (T(VPT)) of the particles is affected by the charge distribution and can be fine-tuned by controlling the electrostatic repulsion on the particle shell (using pH and ionic strength). By suppressing the particle charge we can also induce temperature-driven particle aggregation.

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...

Oxygen-proof fluorescence temperature sensing with pristine C70 encapsulated in polymer nanoparticles

We report the first successful encapsulation of pristine fullerene C70 in polymer nanoparticles with very low size polydispersity. We obtained water-dispersed polystyrene (PS) nanoparticles with diameters from 60 nm to 190 nm using a miniemulsion polymerization technique. Contrarily to pristine fullerenes, which are insoluble in most solvents and materials, the nanoparticles containing fullerene C70 (PS-C70) are stable in water and can be easily incorporated in different materials. When blended with polyacrylonitrile (PAN), a virtually oxygen-impermeable polymer, the PS-C70 nanoparticles show a strong temperature dependence of the thermally activated delayed fluorescence (TADF) intensity and lifetimes, even when exposed to air. This is the first fluorescence temperature sensor based on TADF that can operate in the presence of oxygen. Unlike other fluorescence temperature sensors, our sensor material is insensitive to oxygen, has emission lifetimes in the millisecond range, and shows a strong emission intensity increase when the temperature increases. This sensor exhibits a very broad sensitivity in a working range from −75 °C to at least 105 °C (based on fluorescence intensity), surpassing the performance of other temperature fluorescence sensors at high temperatures.

Non‐linear least‐squares and chemical kinetics. An improved method to analyse monomer‐excimer decay data

The least squares fitting of experimental results with a non‐linear model can result in a serious loss of accuracy in the model parameters estimation if the statistical nature of the method is not correctly considered. This occurs when the experimental data is fitted to a set of functional parameters that depend in the model parameters to be estimated in the end. A realistic example can be found in the two state model of monomer‐excimer kinetics. The decay curves of the monomer and excimer are a sum and a difference of two exponentials, respectively. It is usual to fit the experimental decays in order to obtain the pre‐exponential factors and decay constants, thus using a reparametrization that is non‐linear with respect to the model parameters. This procedure is thoroughly discussed and a new method to analyse the decay curves that circumvents the problem of reparametrization is presented. The proposed method yields improved results with less than 7% bias in the recovered rate constants. Monte Carlo simulations have been performed in order to obtain confidence intervals for the fitting and model parameters.

Biotin-end-functionalized highly fluorescent water-soluble polymers

Many biosensing and imaging systems use fluorescence detection. We present the synthesis of biotin-end-functionalized highly fluorescent water-soluble polymers for potential use in biotin–avidin systems. Statistical polymers of N-acryloylmorpholine (NAM) and N-acryloxysuccinimide (NAS) were prepared by RAFT polymerization using a biotinylated chain transfer agent that ensured 95% end-functionalization of the chains. They were further labeled with a lucifer yellow (LY) dye, yielding 7 to 62 LY fluorophores per polymer chain. The resulting polymers exhibited reduced fluorescence self-quenching, with 7- to 43-fold higher brightness than free LY dye. In addition, they featured low pH sensitivity and very good photobleaching resistance. Moreover, we showed that a more extended polymer conformation was beneficial to the binding of the terminal biotin with streptavidin. This work paves the way for the development of polymers for signal amplification in biosensing assays, labeling of biotin-receptors at cell surfaces in some cancer studies, labeling of antibodies and microscopy imaging purposes.

High performance NIR fluorescent silica nanoparticles for bioimaging

We synthesized a new alkoxysilane-functionalized perylenediimide (PDI) dye with fluorescence emission in the near infrared (NIR). Incorporation of the dye into silica nanoparticles and subsequent internalization in living cells shows promising results for application in confocal laser scanning microscopy, allowing simultaneous imaging with other commonly used dyes or fluorescent proteins.

Flexible hybrid aerogels prepared under subcritical conditions

A new flexible insulator was developed, consisting of a silica–latex-fiber nanocomposite aerogel that combines the lightness and insulating properties of silica aerogels with the mechanical properties of silica–latex hybrids and the flexibility of fibers. This was accomplished by coating selected insulating fibers (polyester and glass) with a silica–polymer hybrid aerogel, synthesized in situ by a two-step sol–gel process, consisting of the acidic co-hydrolysis of TEOS and trimethoxysilyl-grafted latex nanoparticles in excess water, followed by basic co-condensation. The hybrid coating has improved mechanical properties that allow its drying under subcritical conditions. The outstanding properties of the insulating blankets produced render them extremely attractive for high-technology applications, and, due to the low energy consumption process, they may even displace the commonly used mineral wool, by presenting a best value-for-money compromise.

Schizophrenic Behavior of a Thermoresponsive Double Hydrophilic Diblock Copolymer at the Air−Water Interface

The thermoresponsive behavior of the rhodamine B end-labeled double hydrophilic block copolymer (DHBC) poly(N,N-dimethylacrylamide)-b-poly(N,N-diethylacrylamide) (RhB-PDMA(207)-b-PDEA(177)) and the 1:1 segmental mixture of PDEA and rhodamine B end-labeled PDMA homopolymers was studied over the range of 10-40 degrees C at the air-water interface. The increase in collapse surface pressure (second plateau regime) of the DHBC with temperature confirms the thermoresponsiveness of PDEA at the interface. The sum of the pi-A isotherms of the two single homopolymers weighted by composition closely follows the pi-A isotherm of the DHBC, suggesting that the behavior of each block of the DHBC is not influenced by the presence of the other block. Langmuir-Blodgett monolayers of DHBC deposited on glass substrates were analyzed by laser scanning confocal fluorescence microscopy (LSCFM), showing schizophrenic behavior: at low temperature, the RhB-PDMA block dominates the inside of bright (core) microdomains, switching to the outside (shell) at temperatures above the lower critical solution temperature (LCST) of PDEA. This core-shell inversion triggered by the temperature increase was not detected in the homopolymer mixture. The present results suggest that both the covalent bond between the two blocks of the DHBC and the tendency of rhodamine B to aggregate play a role in the formation of the bright cores at low temperature whereas PDEA thermoaggregation is responsible for the formation of the dark cores above the LCST of PDEA.

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.