Analysis of free chlorine in aqueous solution at very low concentration with lateral flow tests.

Test strips are convenient tools for rapid, semi-quantitative analysis of a variety of parameters by dipping them for a few seconds in a sample solution followed by a simple colorimetric read-out. Their sensitivity is mainly determined by the reactivity of the test dyes on the reaction zone and is not sufficient for some applications. The detection limit of commercially available free chlorine test strips, for example, is at present not low enough to confirm the absence of this analyte as disinfectant in rinsing solutions after disinfection or to control required residual amounts of chlorine in drinking water. Therefore, we developed a user-friendly lateral flow test which is capable to detect very low amounts of free chlorine. The latter relies on a larger sample volume passing the reaction zone as compared to simple dip test strips. An amount of as low as 0.05 ppm chlorine can, however, only be detected if oxidation stable flow test substrates are used. The eventually developed flow test reaches a 10x higher sensitivity than a commercial dip test. The result is obtained within 4-5 min flow time, whereby no action is required by the user during this analysis time.

Combining Wax Printing with Hot Embossing for the Design of Geometrically Well-Defined Microfluidic Papers.

A simple, efficient, and repeatable combination of wax printing and hot embossing is reported. This combination yields microfluidic channels in paper, where fluid transport driven by paper-intrinsic capillary forces takes place inside the noncompressed areas, whereas embossed and wax-bonded areas serve as hydrophobic barriers laterally confining the fluid flow. Lab-made paper sheets first coated with a hydrophobic wax were hot-embossed with a tailor-made metal stamp. Both paper-intrinsic (e.g., grammage, fiber type) and paper-extrinsic parameters (e.g., embossing force) were studied for their influence on the geometry of the embossed structures and the resulting redistribution of the wax within the paper matrix. Embossing of wax-printed paper at temperatures above the wax melting point was completed within 15 s. Cotton linters papers required higher embossing forces than eucalyptus papers, which can be explained by their different intrinsic mechanical properties. In summary, both paper-intrinsic and paper-extrinsic parameters were found to have strong impact on resolution and reproducibility of the channels. All in all, the approach yields microfluidic channels in a fast and robust and reproducible manner with comparably low constrains on the precision of manufacturing parameters, such as embossing time, force, or temperature. Most importantly, embossing greatly reduces the lateral spreading of the wax as seen with melting approaches and therefore allows for a much higher feature density than the latter.