Synthesis, Surface Assembly, Characterization and Electrochemistry of Gold Nanoparticles

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Outi Toikkanen Name of the doctoral dissertation Synthesis, Surface Assembly, Characterization and Electrochemistry of Gold Nanoparticles Publisher School of Chemical Technology Unit Department of Chemistry Series Aalto University publication series DOCTORAL DISSERTATIONS 19/2011 Field of research Physical Chemistry Manuscript submitted 12.01.2011 Manuscript revised Date of the defence 31.03.2011 Language English Monograph Article dissertation (summary + original articles) Abstract Gold nanoparticles are intensively studied due to their interesting optical, electronic, catalytic and biological applications. Monolayer protected clusters are particles stabilized by a thiolate layer that have high stability, size-dependent charging behaviour and well established structures. This thesis reports developments in their synthesis and characterization as well as use of electrochemical methods in the field. A synthesis method and full characterization for very small clusters having 38 atoms in their core and a diameter of 1.1 nm is presented. The method is based on the special stability of the Au38 clusters in excess thiol relative to other core sizes formed during the first minutes of the synthesis method used. In this size range, the transition from metallic to molecular behaviour is observed in electrochemical experiments at room temperature. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is used to count the atoms in the cluster core. Electrochemical methods are used to characterize small clusters as each cluster size has its own charging signature. Electrochemical methods are also used to probe ion permeability of monolayers on nanoparticle surfaces, charging reaction mechanisms and cluster stability. Ion penetration to the monolayer can increase the capacitance and thus the charging energy; this effect can be tuned by the charge of the particle or the properties of the ion and the solvent. Reduction of the Au38 cluster is shown to be irreversible due to desorption of the protecting monolayer. Long-term stability as well as the charge-dependent stability of the Au38 is shown to be solvent dependent. The desorbing species are Au-thiolate oligomers that correlate well with the Au38 structure. Surface density gradients are structures where the area density of material is continuously varied along the surface. Such structures are of interest in nanotechnology applications in the fields of electronics, sensing and biology. A method for continuously controlling the density of a self-assembled monolayer and nanoparticles attached to it is presented. The molecular area density on the surface is controlled by varying the parameters on electron beam patterning of the self-assembled monolayer, and citrate-stabilized gold nanoparticles are used as labels to visualize the structures. The results are relevant for fundamental understanding, practical handling and choosing the conditions for applications of gold nanoparticles.Gold nanoparticles are intensively studied due to their interesting optical, electronic, catalytic and biological applications. Monolayer protected clusters are particles stabilized by a thiolate layer that have high stability, size-dependent charging behaviour and well established structures. This thesis reports developments in their synthesis and characterization as well as use of electrochemical methods in the field. A synthesis method and full characterization for very small clusters having 38 atoms in their core and a diameter of 1.1 nm is presented. The method is based on the special stability of the Au38 clusters in excess thiol relative to other core sizes formed during the first minutes of the synthesis method used. In this size range, the transition from metallic to molecular behaviour is observed in electrochemical experiments at room temperature. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is used to count the atoms in the cluster core. Electrochemical methods are used to characterize small clusters as each cluster size has its own charging signature. Electrochemical methods are also used to probe ion permeability of monolayers on nanoparticle surfaces, charging reaction mechanisms and cluster stability. Ion penetration to the monolayer can increase the capacitance and thus the charging energy; this effect can be tuned by the charge of the particle or the properties of the ion and the solvent. Reduction of the Au38 cluster is shown to be irreversible due to desorption of the protecting monolayer. Long-term stability as well as the charge-dependent stability of the Au38 is shown to be solvent dependent. The desorbing species are Au-thiolate oligomers that correlate well with the Au38 structure. Surface density gradients are structures where the area density of material is continuously varied along the surface. Such structures are of interest in nanotechnology applications in the fields of electronics, sensing and biology. A method for continuously controlling the density of a self-assembled monolayer and nanoparticles attached to it is presented. The molecular area density on the surface is controlled by varying the parameters on electron beam patterning of the self-assembled monolayer, and citrate-stabilized gold nanoparticles are used as labels to visualize the structures. The results are relevant for fundamental understanding, practical handling and choosing the conditions for applications of gold nanoparticles.