[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Colloid and Interface Science 11/2009; 343(1):359-65. DOI:10.1016/j.jcis.2009.11.014 · 3.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Polyelectrolyte multilayers are receiving much attention due to their insolubility and due to structural responses induced in them by changes in solution properties such as, for example, temperature, pH or ionic strength. These characteristics make polyelectrolyte multilayers highly interesting in encapsulation technology and controlled delivery applications. However, producing dry, biocompatible formulations for storage of low molecular weight substances poses a challenge. One possibility is to make use of a well-defined mesoporous inorganic carrier material as host for the model substance (in our case ibuprofen). Control over release properties are enabled by polyelectrolyte encapsulation of the inorganic carrier. We built such multilayers through consecutive deposition of PAH and PSS on top of a first layer ofr PEI. These layers were adsorbed either in presence or absent of ibuprofen. The influence of long time storage, 2 years, of the samples was also investigated. The polyelectrolyte multilayer structure was investigated in detail by Dual Polarization Interferometry (DPI), and we use these data to interpret the measured release profiles.
Journal of Dispersion Science and Technology 06/2009; 30(6-6):892-902. DOI:10.1080/01932690802644095 · 0.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study we demonstrate the opportunities controlling internal structure as well as exterior morphology of surfactant templated mesostructured materials through the newly developed emulsion and solvent evaporation (ESE) method. In particular, we consider the control of synthesis temperature and map the influence upon both internal structure and surface morphology of particles templated by the temperature sensitive Pluronic block copolymer F127. Furthermore, we vary compositions, by adding poly(propylene glycol) acting as a swelling agent, as well as by controlling the moisture content. Both of these are having an impact on the internal mesostructure as well as the pore size. Apart from probing internal structure by scattering techniques, the accessibility of the mesoscopic pores of these materials are investigated by measuring the adsorption of a cationic dye, Janus Green B, into the materials. This method shows that accessibility varies dramatically with internal structure.Further, by carefully controlling the moisture content when using the cationic surfactant C16TAB as template, a well ordered 3D hexagonal closed packed (P63/mmc) material with large surface area as well as pore volume was prepared. This further indicates the versatility of the new preparation technique.
[Show abstract][Hide abstract] ABSTRACT: Expansion of the mesostructure in aerosol-generated particles was performed through incorporation of polypropylene glycol (PPG), a non-volatile swelling agent. TEOS was used as silica source and the Pluronic block copolymer, F127, as template. The ratio of TEOS to F127 was kept constant during synthesis, while varying the weight ratio of PPG to F127 systematically. The impact of the PPG on the expansion of the structure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption. Different methods were used to calculate the pore size distributions, the BJH, the BdB-FHH, the KJS and the NLDFT method. Simple geometrical models of the expansion were derived to interpret the experimental data and establish their accuracy. Experimental data showed a roughly linear expansion of the unit cell and pore size, consistent with that expected by modelling the swelling of a hexagonal (p6mm) structure assuming constant wall thickness. The expansion is increasing as a function of increasing PPG/F127 ratio by about 25 Å. An expression of the density of the silica wall was calculated from the models resulting in a density of 1.95 ± 0.2 g/cm3. At a PPG/F127 ratio of approximately 0.31, the p6mm structure (found at lower PPG/F127 ratios) transforms to a microemulsion-templated foam structure. At an even higher PPG/F127 ratio (0.63–1.56), phase separation of the oil from the swollen template occurred, yielding a two-phase system of coexisting foam and large vesicles.
[Show abstract][Hide abstract] ABSTRACT: We introduced the novel technique of AFM-porosimetry and applied it to measure the total pore volume of porous particles with a spherical geometry. The methodology is based on using an atomic force microscope as a balance to measure masses of individual particles. Several particles within the same batch were measured, and by plotting particle mass versus particle volume, the bulk density of the sample can be extracted from the slope of the linear fit. The pore volume is then calculated from the densities of the bulk and matrix materials, respectively. In contrast to nitrogen sorption and mercury porosimetry, this method is capable of measuring the total pore volume regardless of pore size distribution and pore connectivity. In this study, three porous samples were investigated by AFM-porosimetry: one ordered mesoporous sample and two disordered foam structures. All samples were based on a matrix of amorphous silica templated by a block copolymer, Pluronic F127, swollen to various degrees with poly(propylene glycol). In addition, the density of silica spheres without a template was measured by two independent techniques: AFM and the Archimedes principle.