Self-assembly of polystyrene nano particles into patterns of random-close-packed monolayers via chemically induced adsorption

Abstract

We have found that addition of 1-ethyl 3-(3-(dimethylamino)propyl)carbodiimide (EDC) to a suspension of sulfated polystyrene (PS) nanoparticles leads to formation of an irregular, dense-packed monolayer of self-assembled particles on homogeneous as well as chemically patterned surfaces. The adsorption process depends sensitively on the EDC concentration with a critical concentration of 13 mM. At higher concentrations above this critical value, random-close-packed (rcp) monolayers form irrespective of whether the substrate is native gold, a gold surface modified by either hydrophobic or hydrophilic alkanethiols, or chromium/chromium oxide. On the other hand, at lower concentrations particle adsorption is observed only on carboxy-terminated surfaces. We will describe how this phenomenon can be utilized to form 2-dimensional patterns of PS nanoparticles. First, a pattern consisting of carboxy and methyl-terminated thiols is formed on native gold by means of microcontact printing (µCP). Subsequent exposure of the pattern to a suspension of sulfated PS particles with an appropriate EDC concentration in the range 1.3 to 5.2 mM leads to a patterned rcp monolayer. In the second half of the paper we discuss various adsorption mechanisms based on observations with UV–vis spectroscopy and scanning electron microscopy (SEM). Two adsorption mechanisms governing the observed phenomena are proposed.