Generation of local concentration gradients by gas-liquid contacting.

We present a generic concept to create local concentration gradients, based on the absorption of gases or vapors in a liquid. A multilayer microfluidic device with crossing gas and liquid channels is fabricated by micromilling and used to generate multiple gas-liquid contacting regions, separated by a hydrophobic membrane. Each crossing can acts as both a microdosing and microstripping region. Furthermore, the liquid and gas flow rate can be controlled independently of each other. The focus of this conceptual article is on the generation of pH gradients, by locally supplying acidic or basic gases/vapors, such as carbon dioxide, hydrochloric acid, and ammonia, visualized by pH-sensitive dyes. Stationary and moving gradients are presented in devices with 500-microm channel width, depths of 200-400 microm, and lengths of multiple centimeters. It is shown that the method allows for multiple consecutive switching gradients in a single microchannel. Absorption measurements in a microcontactor with the model system CO2/water are presented to indicate the dependence of gas absorption rate on channel depth and residence time. Achievable concentration ranges are ultimately limited by the solubility of used components. The reported devices are easy to fabricate, and their application is not limited to pH gradients. Two proof of principles are demonstrated to indicate new opportunities: (i) local crystallization of NaCl using HCl vapor and (ii) consecutive reactions of ammonia with copper(II) ions in solution.