Assembly of Gold Nanoparticles on DNA Strands
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Assembly of Gold Nanoparticles on DNA Strands Michael Noyong, Kirsten Gloddek, Ulrich Simon, RWTH Aachen, Institute of Inorganic Chemistry, Professor-Pirlet-Str. 1, 52074 Aachen, Germany. ABSTRACT In this work we report first results on a direct deposition of 2-4nm gold nanoparticles onto DNA strands. In a two step protocol first cis-Pt (cis-diamminedichloroplatinum(II), cisPt(NH3)2Cl2) is intercalated into a DNA duplex. In a second step cysteamine stabilized gold nanoparticles are immobilized on DNA due to a ligand exchange (NH3 vs. NH2-R) at the Pt center which was done in solution as well as on surface bound oligonucleotides. The DNAnanoparticle assemblies are prepared on mica and silicon and are imaged by AFM.
INTRODUCTION Metallization of DNA in many ways and means has evolved to a great deal of interest from the scientific as well as from the technological point of view. The strategies pursue different concepts: Some works show the deposition of metal ions (Ag, Pt, Pd) on DNA strands from a metal salt solution followed by reduction [1-4]. Coffer et. al. form mesoscaled structures of CdS clusters with DNA as a template [5,6]. Another way to bind metal clusters to DNA uses short oligonucleotides, functionalized nanoparticles [7] or with protein-nanoparticle-aggregates [8] which recognize a target sequence on a long DNA single strand. By using several of this oligonucleotides the DNA strands become sequentially decorated with nanoparticles. However, these preparation techniques typically lead to a broad particle size distribution between 7 and 50nm, which might be a disadvantage especially in studies on the physical properties of these assemblies. In this work, we report on the binding of small gold nanoparticles of defined size via an intercalator directly to DNA (figure 1). H2N
NH3
Cl
H2N
Pt Cl
NH3 Pt
NH3 NH3
Pt
H2N H2N
Cytosine Thymine Adenine Guanine
Figure 1. Scheme for deposition of amino-functionalized gold nanoparticles on plasmid DNA by using cis-Pt.
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The intercalation properties of cis-Pt into DNA strands had been known since 1845, whereas medical research has focused on its use as an anti cancer drug in the last 30 years [9]. The platinum complex has got a high affinity to nitrogen donor sites in oligonucleotides. After splitting of the nucleofuge chlor it binds mainly to intrastrand neighbored nucleobases guanine-guanine (65% [cis-Pt(NH3)2 5’-d(GpG)-3’]) and adenosine-guanine pairs (25% [cisPt(NH3)2 5’-d(ApG)-3’]), respectively [10-14]. In this position the Pt2+ is accessible for further ligand exchange. Thus, the two remaining ligands can be replaced by the amino functionalized gold nanoparticles (figure 1). In these experiments circular DNA strands (plasmids) with a full length of 1.5µm were used. The resulting DNA-cis-Pt-nanoparticle structures were imaged by AFM.
EXPERIMENTAL DETAILS Preparation of gold nanoparticles The nanoparticles are synthesized by reduction of tetrachloroauric acid (HAuCl4) with sodiumborohydride (NaBH4) in presence of cysteamine. The particle di
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