| Abstract
| - Thin films of alternating DNA and poly-l-lysine (PLL) layers were fabricated onto various planar surfacesby the layer-by-layer (LbL) self-assembly technique. For both the polydeoxynucleotide (PDN, polyguanylicacid or poly[G]) and the oligodeoxynucleotide (ODN, a 30-mer) investigated, UV−visible spectrometry andcontact angle measurements showed that a uniform layer of DNA can be fully adsorbed onto each alternatePLL layer. Various parameters affecting the DNA loading into the composite film were investigated with aparticular emphasis placed on the effect of the ionic strength in the DNA solution used for the film preparation.For the PLL/poly[G] film, it was found that 150 μg/mL of poly[G] solution containing 0.5 M NaCl attainsthe maximum loading for every poly[G] layer. Atomic force microscopy was utilized to measure the DNAsurface density and structure at the topmost layers of several representative composite films. While the DNAfilm thickness increases with the ionic strength of the DNA solution, the shape of the DNA molecules adsorbedthrough the LbL assembly was found, for the first time, to undergo a transition from extended linear structureto a more circular or coiled configuration. The AFM images, together with results accumulated from theUV−visible spectrometric and quartz crystal microbalance experiments, helped to unravel the relationshipbetween the DNA surface structure/loading and the ionic strength of the DNA solution. Our study clarifieda possible misconception about the proportionality between DNA thickness and DNA incorporation/loading.It also provided a fundamental understanding about and the practical guidance for the utilization of the LbLmethod to construct DNA multilayers or polymer-encapsulated DNA molecules for gene-delivery applications.
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