Manipulation of gold nanoparticles in liquids using MAC ModeTM Atomic Force Microscopy

P recise control of the structure of matter at the na-nometer scale will have revolutionary implications for science and technology. Nanoelectromechanical systems (NEMS) will be extremely small and fast, and have applications that range from cell repair to ultrastrong materials. One approach for the construction of such nanostructures is the assembly from molecular-sized components. Although regular, symmetric patterns of nanoparticles can be constructed by self-assembly, many of the possible applications require asymmetric shapes. Several methods have been proposed to construct such structures. Among these, a promising strategy is using the SFM (Scanning Force Microscope) as a manipulation tool to move nanoparticles and nanostructures without restrictions imposed by the physics of self-assembly (1-5). However, manipulation of nanoparticles with the SFM has been mostly limited until now to clearing areas on a surface, or to moving single particles sequentially to create 2-D patterns. By connecting individual nanoparticles (6), it may be possible to construct relatively rigid structures, " primi-tives " , of arbitrary (pla-nar) shapes. These, in turn, may serve as components for building complex nano-electro-mechanical systems (NEMS). By using organic molecules, e.g. dithiols, and gold nanoparticles, scientists at the Laboratory for Molecular Robotics at USC have recently successfully linked these nanoparticles and manipulated them as primitives consisting of up to four particles (7). This is a major breakthrough in developing a " bottom-up " approach for building nanostructures. The capability to perform this precise manipulation in air illustrates the potential of this approach. Nanomanipulation in biochemical and medical areas, however, will require that most experiments be performed in a liquid environment. Atomic force micros-copy has already proven its capabilities in liquid environments. But in nanomanipulation only dynamic force mi-croscopy DFM (with an oscillating cantilever) will likely satisfy the need to minimize the influence of the tip on the sample. In the study described in this report, we have used the PicoSPM™ from Molecular Imaging operated in the MAC (Magnetic AC) mode to perform precise and controlled manipulation of gold nanoparticles in aqueous solutions (deionized water) for the first time. The sample was prepared by depositing gold colloidal particles (EM.GC15; Ted Pella Inc.) with a diameter of 15 nm from aqueous solutions on a mica substrate that had been coated previously with a poly-L-lysine film. The PicoSPM was operated with an Autoprobe CP (Park Scientific Instruments, PSI) controller. Manipulation of the nanoparticles was performed by utilizing the Probe Control Software (PCS) developed …