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The layer-by-layer (LbL) assembly technique is one of the most promising approaches for the preparation of nanostructured multilayer ultrathin films based on different mechanisms. The LbL method is performed commonly in water solution, and exhibits many highlights, such as simplicity, low cost and mildness. LbL films have shown potential applications in the areas of nano-reactors, electrochemical devices, separation membranes, drug delivery, biosensors, surface modification and coatings. Among which, stimuli-responsive controlled drug release has been the focus in recent years.Here, multilayer films were prepared using LbL technique, methylene blue (MB) was used as a model drug to investigate the loading and release ability of prepared multilayer films under different pH values and ionic strength conditions. The main results were listed below.(1) To the poly(diallyldimethylammonium chloride) (PDDA)/poly(acrylic acid) (PAA) film, the loading amount of MB increased with increasing pH value. Release rate can be controlled both by ionic strength and pH value of immersing solution: high ionic strength can accelerate the release. The profiles of release at pH 9.0, 7.0 and 5.0 were similar, but the release was burst within 1 min at pH 3.0. Furthermore, PDDA/PAA multilayer film exhibited a good reversibility for the loading and release of MB. The kinetic data demonstrated that the film released MB in accordance with Fickian diffusion at pH 3.0, and followed non-Fickian transport mechanism at pH 5.0~9.0.(2) To the polyurethane (PU)/PAA film, the loading amount of MB decreased with increasing NaCl concentration, and increased when the pH value was changed from 3.0 to 5.0. The loading profiles were only a small change at pH 5.0 and 7.0, however, when the pH is increased to 9.0, the loading amount dropped down. The release rates increased with the increase of NaCl concentrations. The release rates increased when the pH values changed from 5.0 to 9.0. However, the highest release rate was observed at pH 3.0. In addition, we found the PU/PAA multilayer film swelled upon exposure to solutions with high ionic strength and pH, and the film gradually eroded when immersed in 500 mM NaCl solution and pH value higher than 7. Furthermore, the film also exhibited a good reversibility for the loading and release of MB. The kinetic data demonstrated that the release of MB followed Fickian diffusion at pH 3.0, and exhibited non-Fickian transport mechanism at pH 5.0~9.0.(3) To the hydrogen-bonded poly(vinyl pyrrolidone) (PVPON)/PAA multilayer film, the loading amount of MB increased when pH was increased from 1.0 to 7.0.When at a pH 9.0, the film would be erasable due to the breakage of hydrogen bonds, as a result, MB could not be adsorbed into the film. The release rate of MB increased with the increase of NaCl concentrations. The release rate was also influenced by pH. After 30 min, approxsimately 90% of the MB was released at pH 3.0, and less than 10% was release at pH 5.0 and 7.0. When pH was increased to 9.0, no release was observed, and the film start to erode after 10 min. It was showed that the loading and release process could be repeated many times in centain conditions. The kinetic data suggested that the release of MB followed Fickian diffusion at pH 3.0, and exhibited non-Fickian transport mechanism at pH 5.0 and 7.0.
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