Functional Materials Derived from DNA - Functional Materials and Biomaterials - page 153

Many methods have been used to stretch and align DNA molecules, including
molecular combing [3, 4], electrophoretic stretching [5, 6], and hydrodynamic
stretching [7]. The simplest and most widely used protocol is molecular
combing, schematically drawn in Fig. 3. In this method, DNA molecules are
attached by their ends to a glass surface, while suspended in a droplet of
solution. This droplet is then allowed to evaporate, causing the retreating
meniscus to exert a traction on the molecule and resulting in its extension as
it becomes bound to the glass surface.
Following stretching and positioning, the DNA molecule is generally met-
allized to enhance its conductivity. The most common method for metallizing
DNA is electroless plating involving the reduction of metal ions electrostat-
ically bound to the DNA backbone. For example, Braun et al. [8] prepared
silver nanoparticles along the DNA to enhance the conductivity of the DNA
wire. The schematic process is shown in the Fig. 4. After DNA stretching
and positioning achieved by hydrodynamic flow, Ag ions were bound to the
DNA molecule by an Ag - Na ion exchange treatment. Hydroquinone reduces
DNA-bound Ag ions to Ag(0) metallic clusters, which then autocatalyze fur-
ther reduction of Ag ions from solution. The AFM image (Fig. 4) shows an
Fig. 3 Schematic mechanism of DNA molecular combing. The meniscus generates a sur-
face tension during evaporation, which stretches DNA. Based on [4]
Fig. 4 Construction of a silver wire connecting two gold electrodes. The left images show
the process in the experiments. The right photo is the AFM image. Note the granular
morphology of the conductive wire. Reprinted with permission from [8]