Figure: Off-the-shelf laboratory filter paper, which is typically water-wicking, can be modified with fluorine-free organosilanes to be waterproof. A near-spherical water droplet (dyed with green ink) can stand on this silanized paper substrate. The scanning electron microscopy image (as the background photo) shows that the modified paper fibers are coated with uniform, particulate nanostructures, which should contribute to the remarkable switching of their wetting properties.

Repurposing the application of laboratory filter paper: Beyond filtration

The motivation The history of paper can be traced back nearly two millennia to ancient China; this natural product remains essential to our daily life. Beyond its traditional applications, paper has been adapted over the last few decades as a material for the development of analytical assays such as home pregnancy strips, blood glucose test strips, and pH test paper. The so-called paper-based microfluidic analytical devices are newly developed tools for inexpensive, rapid, and portable chemical analysis and medical diagnosis. The key task in fabrication of microfluidic analytical devices is to modify the paper substrate from its water-wicking property to a waterproof state in which different reaction zones can be created on the same piece of paper. The Yu Group at Simon Fraser University has been interested in developing a simple method to modify the property of laboratory filter papers, a goal that stems from their research interests in the surface chemistry of functional materials.

The discovery – Yu and coworkers have discovered that off-the-shelf laboratory filter paper of different pore-sizes and thicknesses can be modified to be waterproof, patternable and ready to use for a range of analytical assays if they treat it with fluorine-free organosilanes. In particular, three types of cellulose filter paper treated with a special mixture of short (methyltrichlorosilane, MTS) and long (octadeclytrichlorosilane, OTS) organosilane molecules exhibit a remarkably high level of water repellency such that the paper is completely waterproof. Furthermore, the treated filter paper is chemically stable and mechanically durable, and it can be further treated using with ultraviolet irradiation to create a pattern of water wet-able (hydrophilic) regions. The ability to create these patterned areas is important for many chemical tests that rely on colour changes (e.g., accurate pH measurements with universal indicators).

Its significance – The fabrication of paper-based microfluidic analytical devices relies on the quality of the treated paper substrates; not only does it have to be totally waterproof, but it should be chemically robust and able to be further patterned (i.e., partially converted to be hydrophilic). Methods that have been used previously to achieve these patterning results involve either cumbersome reaction conditions or the use of the environmentally unfriendly fluoroalkylsilane compounds. Unfortunately, these compounds can be accumulated in ecosystem and are acutely or chronically toxic to organisms.

The Yu lab’s simplified procedure for modifying filter paper will initially impact the researchers who are interested in the design and fabrication of microfluidic analytical devices. In another recent publication, they also demonstrated that these waterproof filter paper substrates can be used for efficient oil/water separation, which is a key environmental issue for water and soil treatment. By and large, the repurposing of the application of paper products via simple chemical treatment should be of great interest to today’s pulp and paper industry.

Read the paper“Superhydrophobic Substrates from Off-the-Shelf Laboratory Filter Paper: Simplified Preparation, Patterning, and Assay Application” by Zhang, LS; Kwok, H; Li, XC; Yu, HZ. ACS Applied Materials & Interfaces 9(45): 39728-39735 (2017). DOI: 10.1021/acsami.7b08957

Website article compiled by Jacqueline Watson with Theresa Kitos