Superhydrophobic Substrates from Off-the-Shelf Laboratory Filter Paper: Simplified Preparation, Patterning, and Assay Application.

Off-the-shelf laboratory filter paper of different pore sizes and thicknesses can be modified with fluorine-free organosilanes to be superhydrophobic, patternable, and ready for quantitative assay applications. In particular, we have demonstrated that the cellulose filter paper treated with a binary hexane solution of short (methyltrichlorosilane) and long (octadecyltrichlorosilane) organosilanes exhibits remarkably high water contact angles (>150°) and low wetting hysteresis (∼10°). Beyond the optimized ratio between the two organosilanes, we have discovered that the thickness rather than the pore size dictates the resulting superhydrophobicity. Scanning electron microscopy images showed that silanization does not damage the cellulose microfibers; instead, they are coated with uniform, particulate nanostructures, which should contribute to the observed surface properties. The modified filter paper is chemically stable and mechanically durable; it can be readily patterned with UV/ozone treatment to create hydrophilic regions to prepare chemical assays for colorimetric pH and nitrite detections.