Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2001, 40 (16), 3069 - 3071
Writing with a Microscope
to precisely arrange individual particles
Strictly speaking, atomic force microscopes are used for the examination of surface structures with atomic resolution. Chad A. Mirkin and his co-workers have been making use of the technique for a different purpose: they don’t just "read" the surface, they use the atomic force microscope to "write". Their goal is to arrange nanoparticles on a substrate in a defined pattern. This method is called dip-pen nanolithography, in analogy to the writing process with which it has a few things in common.
A capillary delivers an aqueous solution to the extremely fine tip of the atomic force microscope, where it forms a tiny meniscus. As the tip scans the surface line by line, the "molecular ink" travels via the meniscus to specific locations on the surface of the substrate. Mirkin and his colleagues are working with gold surfaces, to which the "ink" adheres well. The "ink" consists of hydrocarbon chains with a sulfur atom at one end and an acid group at the other. The sulfur binds the chain securely to the gold. In this way, patterns of tiny spots are generated. Just as in true lithography, the surface that remains uncovered is passivated. This is done by binding hydrocarbon chains without acid ends to the gold - once again by using sulfur atoms. This renders the "stencil" ready for the application of the particles. For their first experiments, the researchers used tiny spheres of polystyrene, to which positively charged amino groups had been attached. Through electrostatic attraction, these bind to the negatively charged acid groups in the "ink" - attaching the spheres to the gold surface. If the proper size ratios are used, a single sphere will stick to a single glue-like spot. Balls with a diameter of barely one micrometer (0.0001 mm) go well with glue spots with a diameter of 0.5 to 0.75 micrometers. Arrangements of these particles into well-defined crystals could lead to materials that can be used in the optical communications industry for controlling the movement of light.
"What’s special about our technique is that it’s fast, effective, and yet very flexible. We can vary the chemical composition, size, form, and spacing of the ink spots," explains Mirkin. "And because we can also later go back and write on the passivated regions, we can place different types of particles into a single pattern, one after the other." Dip-pen nanolithography could be useful in the production of molecular electronic devices, understanding the mechanism behind pharmaceutical-biomolecule interactions, and constructing new types of catalysts for chemical transformations important to the oil and pharmaceutical industries.