Self Assembly of Nanospheres into Reversible Nanoclusters By Tuning Colloidal Interactions

A major challenge in nanoparticle engineering is to devise a flexible and robust synthetic strategy to pack sufficient multifunctionality into nanoparticles smaller than ~100 nm.  A general self assembly concept is presented to form nanoparticle clusters from primary nanoparticles, instead of from atoms and molecules.  The nanocluster size and particle spacing within the clusters is controlled by manipulation of the particle concentration pathways and the colloidal interaction potential. This “bricks and mortar” interfacial dynamic assembly technique provides great flexibility in choosing and mixing nanoparticle building blocks to engineer desired function.  

A variety of colloidal interactions are tuned to control the nanocluster size, as well as optical properties for Au nanoclusters, as a function of the charge of stabilizing ligands on the primary particle surfaces.   The close spacings of the Au nanoparticles in the clusters produce strong NIR extinction, which is of practical interest in biomedical imaging and therapy.  The spacing between the Au primary particles in the nanoclusters is examined with small angle X-ray scattering. The Au nanoparticles are stabilized by ligands with buried charges that unexpectedly resist the adsorption of protein molecules, despite the substantial surface charge. Au nanoclusters with controlled size reversible dissociate back to primary particles upon biodegradation of the stabilizing polymer.