Studies On Preparation And Properties Of New Structural Polyelectrolyte Microcapsules

Microcapsules have been widely used in many fields, such as pharmaceutical, cosmetic, food as well as agricultural industries. Nowadays, they are more and more used as micro-reactors, drug carriers, protective shells for cells or enzymes and transfection vectors in gene therapy. These new applications require the capsules with precisely controlled structures and properties. Recently, the layer-by-layer (LbL) self-assembly method has been used to build up polyelectrolyte multilayers on decomposable templates, followed by core removal to produce hollow microcapsules. Using this technique, capsules with well-controlled size and shape, finely tuned wall thickness and variable wall composition have been produced. This type of novel capsules is important for both of fundamental research and real applications. Here we focus on how to fabricate new structural polyelectrolyte microcapsules and regulate their properties.First, a strategy was applied to fill the capsules with polyelectrolyte. The uniform poly(styrene sulfonate) (PSS) doped CaCO₃ microparticles were fabricated as template. Multilayer microcapsules containing free PSS inside were successfully fabricated by employing these CaCO₃ particles as templates. Some of the PSS in the cores were simultaneously encapsulated during the core removal process. Scanning force microscopy (SFM), UV-Vis spectroscopy, Raman spectroscopy and (ζ-potential confirmed the existence of PSS molecules in the resultant microcapsules. Most of PSS from the templates were released out of the capsules. Part of these additionally introduced PSS molecules interacted with PAH molecules residing on the inner surface of the multilayer wall to form a stable complex, while the other part was intertwined in the capsule wall or in a free state.Capsules with this structure have highly sensitive selectivity toward the charge sign of probe molecules and their permeation behaviors can be tuned by probe charge and environmentally controlled gating. They can completely reject negatively charged probes, but attract positive ones to form a higher concentration in the capsule interior.By reversing the charge of the probes through pH change, or by suppressing charge repulsion through salt addition, the permeation can be readily switched for proteins such as albumin or small dyes such as fluorescein sodium salt.Based on these observations, a theoretical model combining the Donnan equilibrium and Manning counterion condensation was employed to describe the phenomena. Due to the semi-permeability of the capsule wall, a Donnan equilibrium between the inner solution within the capsules and the bulk solution is created. The equilibrium distribution of the negatively charged permeants was investigated as a function of ionic strength, solvent polarity and pHs. The equilibrium distribution of the negatively charged permeants can be tuned by increasing the bulk ionic strength to decrease the Donnan potential. Decreasing the solvent polarity also can enhance the permeation of FL, which induces a sudden increase of permeation when the ethanol volume fraction is higher than 0.7. This is mainly attributed to the precipitation of PSS. The effect of pH is complicated, but we still can draw a conclusion based on the experiment results: the change of small molecule's permeation behavior is mainly due to the change of their charges, while the influence of the shell is neglectable.Then we applied the chemical cross-linking to manipulate the capsule properties. It was demonstrated here that multilayer capsules assembled from PAH and PSS could be considerably stabilized by cross-linking of only the PAH component with GA. Evidenced by UV-Vis spectroscopy, the reaction is very fast at initial stage then slowed down gradually. After cross-linking by 2% GA for 2h, no alteration of the macroscopic topology of the capsules was observed after incubation in 0.1M NaOH for 24h. The cross-linking significantly improves the mechanical strength of the capsules to resist osmotic pressure induced invagination. Consequently, both the critical pressure and the elasticity modulus (680MPa) of the capsule wall are doubled compared with that of the control (290MPa). The cross-linked capsules can keep their initial size after incubated in water at 90℃ for 2 h, while the control capsules shrink greatly. The cross-linking also can greatly lower the permeability of the capsule wall. Quantitative analysis on the data of fluorescence recovery after photobleaching revealed that the permeation coefficient for dextran (Mw～250kD) was reduced by afactor of 3 after crosslinking.Stable weak polyelectrolyte microcapsules were also prepared by assembly of branched poly(ethyleneimine) (PEI) and sodium poly(acrylic acid) (PAA) followed by GA cross-linking. The cross-linked capsules can maintain their macroscopic topology at extreme low or high pH, while reorganize their microstructure to enable selective permeation or rejection of macromolecules at lower (pH6), respectively. Using this property, dextran with a molecular weight of 2,000 kDa was successfully encapsulated. Thus it is possible to produce capsules which are at the same time pH responsive as well as stable over a large pH range.Finally, single polyelectrolyte component multilayers and microcapsules, exemplified with PAH, were prepared via a method of GA mediated covalent LbL assembly. The PAH multilayers can grow linearly along with the layer number and their thickness can be controlled at the nanometer scale. The single polyelectrolyte microcapsules are stable in strong polar organic solvent and their shells are semi-permeable. Using the same strategy, the hollow PEI capsules with different molecular weight were obtained. The capsule structure as well as the cut-off molecular weight of the capsule wall can be tuned by the molecular weight of PEI.Bovine serum albumin (BSA), which has defined number of cross-linkable groups and pH sensitive groups, was used to fabricate hollow microcapsules by the GA mediated covalent LbL method. The capsules show reversible pH-responsive permeability for macromolecules. They are permeable for macromolecules at pH below 4 or above 10, while are impermeable at pH in between. It's worth mentioning that the capsule size (5.1±0.3 μm) keeps constant when pH<10, while increases to 6.1 ±0.3 μm when pH>10. The permeability change and swelling are both reversible. The mechanism of permeability transition is discussed.