| The research of self-assembled multilayer films has attracted attention of scientists from all areas in recent years. Numerous materials, including charged and uncharged materials have been incorporated into multilayers by exploiting various techniques. In 1991, G. Decher developed a way to fabricate multilayer structures which makes full use of the electrostatic interaction between cationic and anionic groups as driving force. This technique has been proven to be a rapid and experimentally very simple way to produce complex layered structures with precise control of layer composition and thickness. So polyelectrolyte multilayers (PEMs), fabricated by electrostatic layer-by-layer (LBL) self-assembly, have seen tremendous progress in the last two decades. Besides their versatility, ease of fabrication, and tuneable functionality, PEMs also offer the advantage of compatibility with functional species without loss of their specific functions. Functional PEMs have potential applications in tuneable surface wettability, controlled drug release, and switchable permeability.In this article we studied a special type of PEM system, consisting of two functional components, i.e. high molar mass semi-flexible double-stranded DNA and a low molar mass, flexible polycation (strong polyelectrolyte or weak polyelectrolyte).we find that PEM system, fabricated by DNA and strong polyelectrolyte, i.e.PFS and pdda, shows macroporous architectures without using any further post treatment following fabrication. However, PEM film, consisting of DNA and weak polyelectrolyte, PAH, is more flate without pore structure in its surface. We know that negative charged DNA can formed packed network structure in positive charged and hard substrate by electrostatic interaction, which offers possibility of preparation of polyelectrolyte multilayer film with pore structure. The formation of the peculiar porous structure is considered to result from an interplay of thepersistence length mismatch, chain length/molar mass mismatch and the hydrophobic/hydrophilic nature of the two components in the DNA/PFS multilayer film. under the relatively high salt concentrations (0.5 M) of our experimental conditions, the charges on the flexible PFS polycations will be completely shielded, forcing them to adopt a collapsed coiled conformation. In contrast, DNA is an extended anionic polyelectrolyte, with an Lp of about 50 nm, which is relatively insensitive to ionic strength. [12] The large persistence length of DNA will tend to"direct"subsequent bilayer depositions, giving rise to porous structure. The same mechanism can be used to interpreted the cause of formation of macroporous DNA/PDDA film as well. At the same time, the salt concentration of DNA solution is crucial for the porous structure , pore size and growth of PEM films.The interplay between DNA and weak polyelectrolyte such as PAH is weaker than that between DNA and weak polyelectrolyte such as PFS,PDDA, which cause PAH can not be wrapped and adsorbed the DNA backbone closely. After PAH layer deposited, PAH molecular have ability to move in local place and cover the preformed DNA network so that the film become flat. Although we change the PH in PAH solution in order to enhance the interplay between DNA and PAH, the DNA/film is still flat without porous structure in film.In conclusion, we introduce a new method to prepare porous films, which are fabricated by DNA and strong polyelectrolyte based on LBL assembly method. Then we try to interpret this phenoment.
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