A Novel and Simplified Procedure for Patterning Hydrophobic and Hydrophilic SAMs for Microfluidic Devices by Using UV Photolithography

Abstract

This work describes how selective patterning of hydrophobic and hydrophilic areas inside microchannels of microfluidic devices can be achieved by combining well-known chemical protocols and standard photolithography equipment (365 nm). Two techniques have been performed and compared. The first technique is based on the preparation of self-assembled monolayers of photocleavable organosilane and the second one on photoassisted grafting (365 nm) of self-assembled monolayers (SAMs) on a silicon or glass substrate. In the first case, we begin with monolayers carrying an o-nitrobenzyl function (hydrophobic area) that is photochemically cleaved, revealing a carboxylic acid group (hydrophilic area). The problem is that the energy necessary to cleave this monolayer is too high and the reaction time is more than 1 h with 50 mW/cm(2) irradiation flux. To overcome this practical disadvantage, we propose another approach that is based on the thiol-ene reaction with benzophenone as photoinitiator. In this approach, a monolayer of mercaptopropyltrimethoxysilane (MPTS) is prepared first. Subsequently, a hydrocarbon chain is photografted locally onto the thiol layer, forming a hydrophobic surface while the reminding unmodified thiol surface is oxidized into sulfonic acid (hydrophilic area). We demonstrated the feasibility of this approach and synthesized high-quality self-assembled monolayers by UV grafting with an irradiation time of 30 s at 365 nm (50 mW/cm(2)). The modified surfaces have been characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), AFM, and multiple internal reflection infrared spectroscopy (MIR-FTIR). The difference in the contact angles on the hydrophilic and hydrophobic surfaces reached a remarkable 77 degrees. We have also demonstrated that this method is compatible with selective surface grafting inside microfluidic channels.