Broad Tuning of Paper Microfluidic Properties by Covalent Surface Modification for Precise Flow Control and Sensing

In an effort to innovate on-site sensing platforms for a wide range of analytes in different matrices, microfluidic paper-based devices (μPADs) are promising candidates to bring the lab to the sample, as they allow passive, capillary-action-driven flow. Their use, however, is somewhat limited by the fact that the integration of advanced functionality and flow control is difficult. Although recent progress in this area has led to the development of on/off-valving and timing control of flow by changing the chemical and physical properties of paper, precise control over flow in paper microfluidics remains challenging. Here, we propose the use of a simple covalent modification of cellulose paper to tune its surface properties, thereby introducing a broad range of functionality and applicability. For this purpose, fatty acyl chlorides with different chain lengths were used as hydrophobic reagents to change the surface properties. The modified paper was characterized by FTIR-ATR, static water contact angle measurements, and capillary flow properties (permeability, maximum flow distance, and flow rate). The produced papers were then applied in several proof-of-concept devices to demonstrate their potential in sensing and actuating for improved on-site analysis. We demonstrate how precisely modified paper can be used for surface tension measurements and multistep valving based on its wickability for solutions of varying surface tensions, for the determination of ethanol concentration in water by monitoring the maximum flow distance in a 3D-printed device, and for the optimization of on-paper liquid-liquid extraction via fine-tuned control of capillary flow rates.