Numerous studies have demonstrated the bottom-up assembly of complex structures such as colloidal crystals, close-packed aggregates, and even rings and tetramers. In this paper we produce a simple localized and nanoscale charge distribution on the surfaces of individual colloidal microspheres using our technique of "particle lithography". In this technique parts of the microspheres are masked off, while polyelectrolytes (or other molecules) cover the remaining portions of the microspheres. The effectiveness of this process is demonstrated by the accurate and reproducible production of colloidal heterodoublets composed of oppositely charged microspheres. These "colloidal molecules" have the potential for significantly higher information content than previous attempts in the literature. The particle lithography technique is advantageous because it is not limited by the resolution of photolithography or by functionalizing chemistries, and the technique opens the door for complex site-specific functionalization of particles.

Individual colloidal particles are locally functionalized with nanoscale control. Here we use the particle lithography technique to mask one region of a silica or polystyrene particle (size 3.0 mum down to 170 nm), while the remaining 95% or more of the particle is coated with various sized nanocolloids. The images and data show precise and predictable control over the size of the region, with fine-tuned patch size control obtainable by changing the ionic strength of the solution. The coating on the particles remains stable even when subjected to sonication for 5 min. Both single regions and multilayer annuluses are readily formed. Particle lithography provides a general, reliable, stable, controllable, and scalable method for placing site-specific functionalizations on individual particles, opening the way to more complex particle patterning and the bottom-up assembly of more complex structures.

It is well-known that high ionic strength promotes colloid aggregation. Here we show that, by controlling this aggregation process, we can produce high yields of homodoublet and heterodoublet polymer colloids. The aggregation process is started by increasing the ionic strength to roughly 250 mM KCl. After approximately the rapid flocculation time, we quench the "reaction" by mixing in a large quantity of deionized water, which dilutes the ionic strength and prevents further aggregation. At this point, the suspension consists primarily of singlet and doublet particles. Through heating above the glass transition temperature of the polymers, the doublets are fused together and remain intact even after sonication. It is also shown that heterodoublets can include a silica particle together with a polymer colloid. The salting out-quenching-fusing technique is a rapid, easy-to-perform, repeatable process for fabricating colloidal doublets from polymers and other materials.

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