Controlled assembly of monolayer-protected gold clusters by dissolved DNA

One-dimensional chains of gold nanoparticles coated with cationic trimethyl(mercaptoundecyl)ammonium (TMA) monolayers have been electrostatically assembled along DNA molecules in solution by careful control of the relative molar quantities of nanoparticles and DNA base pairs. The nanoparticles are monolayer-protected Au clusters (MPCs) with average diameter 4.4 nm. Transmission electron microscopy (TEM) images of dried solution droplets in which the DNA base pair/MPC mole ratio was about 20:1 display chains of nanoparticles a few microns long in which the Au cores are lined up with nearly contacting cationic monolayer edges along the anionic DNA template. The gold core edge-edge distance in the TEM image is 2.9+/-0.9 nm, which is roughly twice the TMA monolayer thickness. The linear structures formed at a 10:1 DNA base pair/MPC mole ratio appear to persist stably in solutions. The Au surface plasmon resonance band in UV-vis spectra intensifies and red-shifts as the DNA/MPC mole ratio is increased; precipitation occurs at higher ratios. Other and more complex patterns of DNA/MPC binding are observed in TEM images prepared from solutions with different DNA/MPC mole ratios. We believe entropic effects are significant in the assembly process and lead to strong binding constants.