Microfluidic Devices Containing ZnO Nanorods with Tunable Surface Chemistry and Wetting-Independent Water Mobility.

Interest in polydimethylsiloxane (PDMS) microfluidic devices has grown dramatically in recent years, particularly in the context of improved performance lab-on-a-chip devices with decreasing channel size enabling more devices on ever smaller chips. As channels become smaller, the resistance to flow increases and the device structure must be able to withstand higher internal pressures. We report herein the fabrication of microstructured surfaces that promote water mobility independent of surface static wetting properties. The key tool in this approach is the growth of ZnO nanorods on the bottom face of the microfluidic device. We show that water flow in these devices is similar whether the textured nanorod-bearing surface is hydrophilic or superhydrophobic; that is, the device tolerates a wide range of surface wetting properties without changing the water flow within the device. This is not the case for smooth surfaces with different wetting properties, wherein hydrophilic surfaces result in slower flow rates. The ability to create monolayer-coated ZnO nanorods in a PDMS microfluidic device also allows for a variety of surface modifications within standard mass-produced devices. The inorganic ZnO nanorods can be coated with alkyl phosphonate monolayers. These monolayers can be used to convert hydrophilic surfaces into hydrophobic and even superhydrophobic surfaces that provide a platform for further surface modification. We also report photopatterned biomolecule immobilization within the channels on the monolayer-coated ZnO rods.