Scalable synthesis of self-assembling nanoparticle clusters based on controlled steric interactions

A simple one-pot method is presented for the synthesis of amphiphilic gold nanoparticles that can spontaneously self-assemble into stable clusters with controllable geometry. The technique is based on the control of colloidal interactions through sequential functionalization of the surfaces with long polyethylene glycol (PEG) chains followed by short alkane-thiol molecules. This process renders the gold nanoparticles amphiphilic and initiates the self-assembly of stable clusters in the dispersion. It is shown that the dominant structures and sizes of the clusters (i.e. singlets, doublets, triplets and others) are directly and reproducibly controlled by the concentration of PEG chains that are linked to the particle surface. Changes in the structure of the clusters are systematically characterized by small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and transmission electron microscopy (TEM). Systematic shifts in the plasmon resonance peak of the gold nanoparticle clusters demonstrate that this new method for self-assembly can be effectively used to tune optical properties with great precision. With this new approach, we show that it is possible to fabricate large numbers of amphiphilic nanoparticles and clusters with controllable structure by manipulating the repulsive steric forces that are imparted by surface bound polymers and the attractive interactions of the hydrophobic alkanethiols. The major advantage of this technique lies in its simplicity, versatility and potential for scalable production of self-assembled 'colloidal molecules'.