Metallized DNA: Synthesis, Analysis and Properties

Metallic nanoparticles have been studied intensively during the last decades because of their intriguing optical properties: Due to collective oscillations of the conducting electrons - the so called plasmonic oscillations - they absorb light in the visible spectrum. The resonance frequency thereby sensitively depends on parameters such as the particle size and shape as well as the dielectric constant of the medium. DNA exhibits outstanding recognition properties and can be modified easily. Thus, template-directed material synthesis along synthetic DNA is a promising route to grow nanoparticles of defined shape and size and with defined interparticle-spacing. In this study, two different methods are used to deposit silver on oligonucleotides of different lengths, ranging from 23 to 96 basepairs, in order to synthesize metallic nanorods of controlled aspect ratios. The first method involves the specific labeling of nucleotides with aldehyde groups, followed by exposure to a Tollens reagent and a developer. The second method relies on the photoinduced deposition of silver onto unmodified DNA samples. Several preparation parameters such as the DNA sequence, buffer salt type, silver concentration and UV illumination time are varied systematically. The metallized DNA molecules are characterized concerning their optical and structural properties. Absorption spectra show plasmon peaks around 420nm. Peak positions, intensities and bandwidths are analyzed. Dynamic Light Scattering studies in solution provide information about the particle sizes as well as their structural asymmetry. Both optical techniques are used to observe the temporal evolution of the nanoparticle growth in the Tollens metallization process. Structural information is inferred from Atomic Force Microscopy; for that purpose, the particles are deposited on single-crystalline silicon substrates.