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

Abstract

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.

Superomniphobic Papers for On‐Paper pH Sensors

“…To overcome this issue, in this work, we developed a superomniphobic paper through a simple technique of growing nanofilaments on the microfibers of the paper. Unlike prior work, our superomniphobic paper displays very low roll‐off angle, indicative of ultra‐high droplet mobility, even with low surface tension liquids (e.g., n ‐hexadecane). Further, the required hierarchical texture is formed using a grow‐from approach on inherent microfibers of the paper, without noticeably altering the microscale features (i.e., diameter and distance of the microfibers).…”

From kirigami to three-dimensional paper-based micro-analytical device: cut-and-paste fabrication and mobile app quantitation

“…The hydrophobic pads were prepared with Whatman Grade 3, as it can be modied to be superhydrophobic when treated with a binary silane solution (OTS/MTS in toluene, 0.2% v/v). 31 All other untreated paper channels and discs were prepared with Whatman Grade 4 lter paper to simplify the fabrication procedure.…”

“…The bottom le picture shows a water droplet on the modied paper base, whose contact angle was determined to be 155 AE 2 (which is superhydrophobic). 31 In the case of wax or photoresist was used to pattern the paper base (to build hydrophobic barriers), [14][15][16] typically water contact angles of slightly above 90 were achieved. Clearly, a device with patterned superhydrophobic substrate and unaltered paper disc shows optimized renement in the reaction zone yet maintains the ideal liquid ow properties in the 3D conguration.…”

“…The stability and solvent-resistance of these kPADs are guaranteed by the silanization treatment, which has been demonstrated to produce a structurally robust, superhydrophobic surface coating. 31 The next step is to test the analytical performance of the kPADs fabricated via the cut-and-paste protocol for running quantitative assays that have potential applications. As the trial experiments, two environmental contaminants Cr(VI) and NO 2 À , representative heavy metal and inorganic salt, were chosen as they are typical sanitary contaminants in water.…”

“…A pair of scissors and a hole puncher were used to cut the hydrophilic channels, hollow pads, and discs from the printed lter paper by carefully following the printed lines (to ensure the correct size and shape). Then the hollow pads (I, II, and III) were immersed in the binary solution of OTS and MTS (v/v, 3 : 7) in hexane at room temperature for 7 min and dried in an oven for 5 min at 40 C, to achieve surface superhydrophobicity, 31 followed by embedding its holes with pre-cut paper discs (d ¼ 3 mm). Discs, embedded in the pad III, were cut from Grade 4, and others are cut from Grade 3 lter paper.…”