Continuous free-flow electrophoresis and scanning electrochemical microscopy investigations of monolayer-protected nanoclusters

Abstract

Monolayer-protected clusters (MPCs) have become a focus of interest due to their unique optical, electronic, and electrochemical properties rendering them good candidates for use in nano- or molecular electronics. Water-soluble nanoparticles are of particular interest as platforms for biological and immunological sensing. The focus of this dissertation is to develop new techniques to isolate more monodisperse fractions of gold MPCs and to investigate the electron transfer properties of gold MPCs. Scanning electrochemical microscopy (SECM) approach curves of organic soluble gold MPCs revealed the following electron transfer rates (kf): 0.098 ± 0.025 cm/s for hexanethiol, 0.024 ± 0.004 cm/s for octanethiol, 0.011 ± 0.002 cm/s for decanethiol, 0.0048 ± 0.0008 cm/s for dodecanethiol, and 0.035 ± 0.001 cm/s for 2-phenylethane thiol. Surprisingly, these rates were not dependent on the electrode potential, as expected with the Butler-Volmer kinetic model. The kf was very slow for the water-soluble glutathione and N,N,N-trimethyl(11-mercaptoundeceyl) ammonium MPCs and found to be pH dependent with kf of 0.054 ± 0.008 cm/s at pH 3 and 0.0064 ± 0.0013 cm/s at pH 9 for the tiopronin MPCs. At a high pH the MPC is more deprotonated increasing the layer of negative charge around the nanoparticle and retards the transfer of electrons. It was also found that the water-soluble MPCs do not store charge in the metal core as with the organic soluble MPCs, but form gold oxide on the metallic core. Tiopronin MPCs were also successfully used as a novel redox mediator for SECM imaging. Water-soluble tiopronin MPCs were fractionated by continuous free-flow electrophoresis (CFE) resulting in the isolation of greater quantities of MPCs than previous methodologies with complete recovery. The particle dispersity of the MPC fractions neared the resolving power of the TEM. A novel CFE instrument was constructed in order to fractionate additional MPCs varieties. The novel CFE consists of an inverted annular separation chamber composed of stainless steel which decreases the effects of gravity, reduces the complexity of the apparatus, and allows the CFE to be used to fractionate non-aqueous samples.