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

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 (muCP). 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.