Size-Separation and Self-Assembly of Anisotropic Nanoparticles in a Coffee-Stain Ring

Abstract

Nanoparticles deposition via droplet drying technique generally leads to a coffee-stain-like ring. In the case of different shapes and sizes of nanoparticles, such ring-like deposits also demonstrate various phases within the self-assembly superstructures. For broad spectrum sizes in suspension, size-separation effect has not been considered before. Herein, polydisperse gold anisotropic nanoparticles have been prepared via wet chemical synthesis technique. High-resolution scanning electron microscopy (HRSEM) at various positions of a coffee-stain ring displayed both size-separation and self-assembly of nanoparticles. Especially at the outer edges of a ring-like deposit, side-by-side organized arrays of anisotropic nanoparticles have been observed. Likewise, such ordered arrays generally begin with thin nanorods and the width of nanoparticles increases as one moves inward within such arrays. Also, assembly of deposited arrays is more orderly arranged at the periphery of the ring, whereas such order significantly decreases at the interior region of the ring. Furthermore, it has been observed for a mixture of thick–thin nanorods and spheres that only thick nanorods self-assemble into ordered arrays whereas thin rod-sphere was found disordered everywhere within the ring. Nanoscale interactions are quantified for these systems to understand self-assembly behavior of such anisotropic nanoparticles.