Jovice Boon Sing Ng

Lightweight and strong cellulose materials made from aqueous foams stabilized by nanofibrillated cellulose.

A lightweight and strong porous cellulose material has been prepared by drying aqueous foams stabilized with surface-modified nanofibrillated cellulose (NFC). This material differs from other dry, particle stabilized foams in that renewable cellulose is used as stabilizing particles. Confocal microscopy and high speed video imaging show that the octylamine-coated, rod-shaped NFC nanoparticles residing at the air-liquid interface prevent the air bubbles from collapsing or coalescing. Stable wet foams can be achieved at solids content around 1% by weight. Careful removal of the water results in a cellulose-based material with a porosity of 98% and a density of 30 mg cm(-3). These porous cellulose materials have a higher Youngs modulus than porous cellulose materials made from freeze-drying, at comparable densities, and have a compressive energy absorption of 56 kJ m(-3) at 80% strain. Measurement with the aid of an autoporosimeter revealed that most pores are in the range of 300 to 500 μm.

Improved enzymatic activity of Thermomyces lanuginosus lipase immobilized in a hydrophobic particulate mesoporous carrier

Lipase from Thermomyces lanuginosus has been immobilized within particulate mesoporous silica carriers, with either hydrophilic or hydrophobic supporting surfaces, produced by the newly developed emulsion and solvent evaporation (ESE) method. The Michaelis-Menten model was used to calculate the parameters related to the enzymatic activity of lipase i.e. the turnover number, k(cat), and the specific activity. The specific activity was improved by immobilization of lipase onto the hydrophobic support, compared to lipase immobilized onto the hydrophilic support and lipase free in solution. The enhanced enzymatic activity of lipase onto a hydrophobic support was attributed to interfacial activation of the Thermomyces lanuginosus lipase when it is attached to a hydrophobic surface and a reduced denaturation. Confocal scanning laser microscopy (CLSM) studies, of fluorescently tagged lipase, showed that leakage of the lipase from the mesoporous particles was limited to an initial period of only a few hours. Both the rate and the amount of lipase leached were reduced when the lipase was immobilized onto the hydrophobic support.

A Membrane-Reconstituted Multisubunit Functional Proton Pump on Mesoporous Silica Particles

We have investigated formation of a proteolipid membrane surrounding mesoporous silica particles with a diameter of 550 nm and pore sizes of 3.0 nm. A multisubunit redox-driven proton pump, cytochrome c oxidase, was incorporated into the membrane, and we show that the enzyme is functional, both with respect to catalysis of O(2) reduction to water, and charge separation across the membrane. The orientation of cytochrome c oxidase in the membrane was found to be the same ( approximately 70%) in the lipid vesicles and in the silica-particle-supported lipid membrane, which provides information on the mechanism by which the vesicles adsorb to the surface. Furthermore, cytochrome c oxidase could maintain a proton electrochemical gradient across the supported proteomembrane, that is, the membrane system was proton tight, defining an interior particle compartment that is separated from the surrounding aqueous media. Such a biofunctional cellular interface, supported onto a colloid that has a connected interior cytoskeleton-like pore structure, provides a basis for functional studies of membrane-bound transport proteins, and also for applications within pharmaceutical drug delivery.

Intraparticle transport and release of dextran in silica spheres with cylindrical mesopores

The transport of oligomeric molecules in silica spheres with cylindrical mesopores has been quantified and related to the structural features of the spherical particles and the interactions at the solid-liquid interface. An emulsion-solvent evaporation method was used to produce silica spheres having cylindrical mesopores with an average pore diameter of 6.5 nm. The transport of dextran molecules (fluorescently tagged) with molecular weights of 3000 and 10,000 g/mol was quantified using confocal laser scanning microscopy (CLSM). The intraparticle concentration profiles in the dextran-containing spheres were flat at all times, suggesting that the release is not isotropic and not limited by diffusion. The release of dextran into the solution is characterized by an initial burst, followed by long-term sustained release. The release follows a logarithmic time dependency, which was rationalized by coupling concentration-dependent effective diffusion constants with adsorption/desorption.

Spectral deconvolution of NMR cross polarization data sets

The COmponent-REsolved (CORE) strategy has been employed, for the first time to solid state NMR spectroscopy. CORE was used to extract two time-dependent spectral components in 24 (29)Si{(1)H} NMR spectra, recorded on a meso-structured silica material under conditions of cross polarization evolution. No prior assumptions were made about the component bandshapes, which were both found to be skewed to higher chemical shifts. For the silica fragments close to protons this skewness could be rationalized by a distribution of the degree of condensation in the silica network; however, for the other component the non-Gaussian shape was unexpected. We expect that the same strategy could be applied to a range of experiments in solid-state NMR spectroscopy, where spectral distributions or kinetic parameters need to be accurately extracted.

Temperature-induced formation of strong gels of acrylamide-based polyelectrolytes

Very strong physical gels have been formed by moderate temperature increases of concentrated aqueous dispersions of acrylamide-based copolymers. The results of rheometry, confocal laser scanning microscopy, and differential scanning calorimetric studies of acrylamide-based copolymers with acrylic acid (poly[AM-AA]) and the sodium salt of 2-acrylamido-2-methylpropane acid (poly[AM-NaAMPS]) suggest that the temperature-induced swelling of the polymer beads and dissolved chains creates strongly entangled polymer networks above the upper critical solution temperature. Analysis of the viscoelastic response showed that the time scale for the gelation process is about minutes to hours. The addition of high concentrations of Ca(2+) resulted in a significant reduction in the modulus.

Membrane proteins can mediate uptake and release from mesoporous spheres

Transport across membranes is mediated by membrane proteins and underpins basic functions of living cells such as sensory perception and cell recognition. The uptake of specific ions, sometimes available only at very low concentrations, is also an integral part of biomineralization processes in living organisms. Membrane proteins are situated in the cell membrane, which consists of a bilayer of lipid molecules. Any attempt to study and use the function of membrane proteins requires synthetic model systems that mimic the natural environment. Limitations posed, for example, by the stability and size of commonly used hollow lipid bilayer spheres, called vesicles, have prompted the search for other, more cell-like systems. Recent work1, 2 has shown that porous silica spheres are particularly attractive due to their cytoskeleton pore-like inner structure and ease of depositing defect-free, ion-tight (i.e., not allowing the passage of molecules such as sodium and potassium) lipid bilayers onto the other surface. The use of a rigid porous particulate substrate is of interest in systems where mechanical stresses may otherwise destroy softer protein-containing colloidosomes or proteoliposomes. Such stresses may occur in ‘cell’ sorting operations, during mechanical agitation and stirring, and in high shear flows typical in microfluidic devices. We have designed robust and versatile systems for selective uptake and release of ions from nanoporous particles sealed with ion-tight lipid bilayers of various compositions that also contain fully functional membrane proteins or peptides (protein components).2, 3 We produced porous silica spheres by two different techniques to yield particles characterized by a welldefined and connected porosity where specific target molecules4 Figure 1. Lipid-bilayer-coated mesoporous spheres are an excellent and versatile platform for designing systems for model studies of the function and response of membrane proteins, biomimetic microreactors, and protein-specific biosensing and drug delivery. A, B, and C represent different membrane proteins.