Microfluidic patterning of close-packed nanoparticle monolayer

Herein, a microfluidic-assisted dynamic assembly technique for close-packed nanoparticle monolayer patterning with high feature edge acuity and large-area uniformity is developed. Temporary PDMS microchannels are reversibly bonded to an oxidized silicon substrate whose surface is immobilized with 3- Aminopropyltriethoxysilane (APTES) to achieve a positive-potential surface. When the silica nanoparticle suspension flows through the microchannels, nanoparticles with a negative surface potential can be attracted onto the surface due to electrostatic attraction. Significantly, the shear force induced by the flow exerts a dual function: adjusting the first-layer nanoparticles to a uniform monolayer and cleaning up the excess nanoparticles above the first layer, which guarantees a monolayer. By tuning the substrate surface potential, the suspension concentration and the flow rate, a more close-packed nanoparticle monolayer can be achieved. Experimental results indicated that the best coverage obtained up to now is about 0.65. This work proposed a simple, versatile and cost-effective technique to pattern a close-packed nanoparticle monolayer out of cleanroom.