Gold University of Minnesota M. Skip to main content.University of Minnesota. Home page.
News & Events
Graduate Program
Undergraduate Program
Course Offerings


Research News


Stringing up nanobeads

Recent research from the research group of Professor Andreas Stein.

Spherical colloids with diameters of a few hundred nanometers (several hundred times smaller than the thickness of a human hair) are useful building blocks for self-assembly into functional structures. They can be likened to atoms but on a larger scale. Just like metal atoms that assemble into close-packed structures, they can also pack into regular patterns. Opals are natural examples of close packed colloidal spheres, and their brilliant colors ("opalescence") arise from the interaction of light with the periodic sphere structures. Many examples of close-packed colloids can be found in the literature, but more open structures or analogs of molecules, in which "colloidal atoms" are connected with specific directionality are more difficult to obtain.

In a recent paper published in the Journal of the American Chemical Society 2009, 131, 9920, graduate student Fan Li and Professor Andreas Stein describe a novel approach to connect colloids into conjugated clusters and chains of various geometries, using a process called "capillary condensation" to provide the glue and to impart clusters with specific directional interactions. Capillary condensation involves selective condensation of a vapor below its saturation vapor pressure in cavities or between surfaces. Dichloromethylvinylsilane vapor was deposited between spin-coated aggregates of polymer spheres. The vapor condensed primarily between sphere contacts and reacted to form a thin coating in these regions that linked the spheres together and stabilized the clusters. This process occurred even if spheres were slightly separated from each other.

The figure below shows a schematic drawing of a colloidal cluster with three spheres that are linked by the capillary condensation process. When fluorescent polymer spheres are connected, the "nanoglue" shows up as dark regions in the fluorescence image. In the scanning electron images of a linked chain and a seven-particle cluster, the glue appears as lighter regions. Either the coated or the non-coated regions can then be selectively functionalized to provide contact points for further reactions, for example, if clusters are intended to be linked up into more extended structures via directional interactions. This method provides a new tool toward mimicking molecular assembly at the colloidal scale and toward the realization of rational colloid assembly processes by true designer pathways.


The University of Minnesota is an equal opportunity educator and employer.