The ability of metal ions to interact and stick to DNA molecules has given rise to the synthesis of different kinds of 1-dimensional semi-conducting nanostructures such as nanowires, nanoropes and nanotubes. These involve the treatment of the DNA molecules with the appropriate metal ion leading to the doping of the DNA strand with the metal and subsequently reaction with the appropriate source of chalcogenide (anion) to precipitate on the metal leading to the formation of continuous coated 1-D nanostructures. This process makes use of the stiffness and metal-binding properties of DNA and is not dependent on the sequence information on the DNA.
The templating process of the metal chalcogenides on DNA involves three main stages: (i) the activation stage, which involves the creating of an active metal sites on the DNA when metal cations bind to the aromatic bases or polyanionic phosphate backbone. (ii) the nucleation stage, in which the added sulfide anion reacts with the active metal sites to form metal sulfide precipitates or nanoclusters on the DNA.(iii) the growth stage, in which the metal sulfide nanoclusters acts as the nucleation site for growth of the metal sulfide nanostructures during a process called Oswald ripening (the process of dissolution of the metal sulfide and re-precipitation that occurs in order to re-distribute material from regions of high surface energy to low surface energy) leading to smoothening of the nanowire surface61,62 .The diagrammatic representation
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Figure 1.8 Schematic representation of the steps involved in the synthesis of DNA
templated semiconducting nanowire50.
For a ternary metal sulfide, the DNA molecule is first doped with the two metals ions before treatment with the anion (sulfide) to avoid the formation of two different metal anions in solution. Different research groups have reported on the synthesis of various metal sulfides such as Zinc Sulfide (ZnS)55, Cadmium sulfide (CdS)63, Copper sulfide
(CuS)64, Lead sulfides (PbS)65, Molybdenum Sulfide (MoS2)66.
An attempt to template PbS nanoparticles on DNA molecules was carried out by Sargent and his co-researchers65. They reported a novel route for synthesis of efficient
photoluminescent PbS quantum dots on DNA template. It was prepared by dropwise addition of Pb(NO3)2 to DNA, followed by treatment with Na2S at temperatures ranging
from 20 ℃ to 100℃. It was established that the nanoparticles prepared had intensive living tissue penetration and low auto-fluorescence (fluorescence of the cells in the presence of UV light) because they emit light in the infrared wavelength region. This shows that efficient light emitting PbS nanoparticles can be synthesised by templating reaction on DNA molecules. Gao et al 55 have also explored the fabrication of ZnS
nanoparticles on DNA template from Escherichia coli. In that research, the templating reaction was done by reacting aqueous DNA solution with an aqueous solution of Zn(Ac)2 and incubating for 24 hours at 4℃. This was later followed by addition of
Na2S2O3 aqueous solution and increasing the temperature from 48℃ to 80℃ within 0.5
to 3 hours. The DNA templated ZnS nanowires prepared had zinc blende structure with diameter range of 20 – 30 nm.
Another metal sulfide previously studied for its ability to bind to DNA template was CuS64. In this particular case, CuS which is an indirect bandgap metal sulfide was
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copper complex which is further treated with Na2S to form DNA templated copper
sulfide nanowires. The DNA templated CuS nanowires was prepared both in solution and by a surface immobilised method. AFM studies showed that the solution based approach gave smoother and more continuous nanowires while surface immobilised method showed beads of the nanoparticles along the DNA strands with heights as high as 10 nm. The crystal structure of the nanowires were determined using TEM, which showed the covallite crystal lattice. Electrical properties of the nanowires were however not measured. Recently, Goswani et al 66 reported the ability of MoS2 nanocrystals to
be confined on a DNA matrix by wet chemical synthesis through the regulation of reaction conditions such as pH. The reaction process involved two stages. The first step was the reaction of an aqueous solution of DNA from salmon testes with MoCl5 at
room temperature with vigorous stirring, which was then followed by addition of NaOH solution to reduce the pH to ~9. The second step involves the addition of NaS2 to the
already synthesized DNA/Mo complex, followed by addition of HCl to reduce the pH to ~6 and stirring. The nanocrystals of DNA/MoS2 was shown to be somewhat uniform in
size and spherical in shape with an average diameter of ~5 nm. The nanowires exhibited large quantum confinement phenomena but were not photoluminescent. There have been several attempts to synthesise DNA templated CdS nanowires by templating reaction of CdS nanoparticles on DNA. The ability of DNA to stabilise nanocrystals of CdS was first reported by Coffer et al 67. Calf thymus DNA was initially
reacted with cadmium perchlorate to dope it with Cd2+ ion and subsequently treated
with NaS2 to give DNA templated CdS nanowires with a zinc blende lattice structure
with diameter in the range of 2.3 – 12 nm.