Assembling and positioning latex nanoparticles via controlled evaporative self-assembly†

By capitalizing on a simple, robust, and fast approach based on controlled evaporative self-assembly in a cylinder-on-flat geometry, two kinds of polystyrene (PS) latex nanoparticles with different diameters (i.e., D 1⁄4 50 nm and 500 nm, respectively) were deposited on the Si substrate, forming ordered ‘‘coffee rings’’ composed of PS nanoparticles. Within a ‘‘coffee ring’’, the packing of PS nanoparticles was found to be dependent upon the particle diameter. More intriguingly, when the aqueous solution of mixed two PS nanoparticles was used, ‘‘coffee rings’’ of small particles were positioned at the edge of three-phase contact line (D 1⁄4 50 nm), followed by the adjacent deposition of large particles (D 1⁄4 500 nm). These assembled deposits consisting of monolayeror multilayer-thick latex nanoparticles over large areas may offer potential for applications in electronic devices, photonic bandgap materials, biosensors, particle sorting, and disease diagnosis.