ABSTRACT:The article is concerned with the preparation of polymer-iron oxide nanocomposites and the study as drug-delivery matrices under the influence of applied magnetic field. Biocompatible materials were prepared by incorporating an aqueous ferrofluid in poly(vinyl alcohol) and scleroglucan (SCL) hydrogels, loaded with theophylline as model drug for release studies. The in vitro release profile was obtained using a flat Franz cell and the kinetic parameters were derived applying a semiempirical power law. A magnetic characterization of nanoparticles contained in the ferrofluid was performed by obtaining the magnetization curve. For both systems, the observed drug release profiles decreased when a uniform external magnetic field is applied suggesting they can be used as environmental responsive matrices for biomedical applications. Dynamic rheological measurements show that a higher storage modulus and a more compact structure are obtained by incorporating the ferrofluid into the hydrogels. These rheological results and environmental electron scanning microscopy micrographs point to an understanding of release behavior once the magnetic field is applied.
Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel‐phase state. No dimer fluorescence was observed either in monopalmitoylphosphati‐dylcholine (C16PC) micelles or in liquid‐crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC‐LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC‐LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.
Dedicated to Professor Andre M. Braun on the occasion of his 60th birthdayThe absorption and emission properties of Rose Bengal (RB) have been studied in colloidal suspensions of positively charged alumina-coated silica nanoparticles (Sil). Experimental spectra can be rationalized by the existence of an equilibrium between aqueous monomers and only one adsorbed species. However, a simple partition or a Langmuir-type adsorption-aggregation equilibrium do not explain the observed results. No evidence regarding the existence of adsorbed monomers is found even at low surface coverage. Aggregation stops at the dimer level, though, at high enough dye concentrations, the surface coverage is almost complete. Comparative experiments performed on negatively charged silica nanoparticles show that monomers are the only species present in this case. Fluorescence experiments on Sil indicate that dimers are fluorescent. Laser excitation of adsorbed dye leads to the formation of RB radical cations, while the dye triplet state is not observed.Introduction. ± The study of systems composed of organic dyes supported on particulate materials is of interest both from the applied and the theoretical points of view. Photophysical and photochemical processes carried out in heterogeneous media have been the subject of different studies because of their potentiality in areas such as conversion and storage of light energy and photocatalysis [1 ± 3]. A common problem found in this kind of system is the formation of dye aggregates, as aggregation usually impairs the photochemical response. It is known that planar organic dyes are prone to aggregate strongly when constrained to restricted environments, such as the interior of a micelle or the surface of a solid, where the local concentration increases.Aggregation is normally evidenced by electronic spectroscopy, but, while straightforward methods can be used to characterize homogeneous or microheterogeneous systems, studies become complicated in heterogeneous systems for a number of reasons. The existence of different phases or microphases obscures the interpretation of spectroscopical data, as aggregation can take place in more than one phase. Furthermore, heterogeneous systems are normally complicated by the occurrence of light dispersion. The nature of the interaction between dye and solvent and between dye and solid not only determines the partition equilibrium of the dye but also the degree of aggregation. As a result, a single dye can behave in entirely different ways on different solids suspended in the same solvent, and similar dyes can show different behaviors in identical environments.
Solid polymeric matrices based on xanthan and chitosan and using KNO 3 as a model agrochemical were prepared by direct compression, with a view to evaluating their potential as controlled-release fertilizers (CRFs). The swelling behavior, surface characteristics, and durability in soil of the tablets were studied. The release data were treated with a power law model in order to understand the delivery kinetics of KNO 3 . This proved to be non-Fickian diffusion, with release exponents ranging from 0.80 to 0.88, highlighting the importance of polymer relaxation on drug release. The presence of drug-free surface layers was an important factor in modulating the release. When comparing experiments without and with stirring, the release time ratios between them were as high as 40, predicting a significantly greater release time in soils. In durability experiments in soil, the polymeric matrices lasted longer than 6 weeks. These results show that layered xanthan and xanthan-chitosan matrices perform as a promising system for developing CRFs.
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