Fabrication of High-Adhesion Hydrophobic Filter Paper and Its Application as a Device for Microscale Synthesis

Microscale synthesis in droplets enables the precise production of materials in tiny, isolated volumes, offering benefits such as high throughput, reduced reagent use, and improved reaction control. Inspired by the rose petal effect, hydrophobic surfaces with high water adhesion have shown potential for facilitating controlled synthesis within individual microdroplets. In this study, a novel hydrophobic filter paper (HFP) with high adhesion properties was fabricated and applied to carry out reactions in microscale settings. The fabrication process involves a straightforward two-step procedure and utilizes environmentally friendly chemicals. In the first step, iron hydroxide nanoparticles were deposited via a precipitation reaction, endowing the modified filter paper with hierarchical surface roughness. In the second step, a fatty acid was used to lower the surface energy and produce a hydrophobic surface (WCA ≈ 146°). The hydrophobic nature of the filter paper repels water-based liquids, while its adhesive properties enable microdroplet manipulations, such as transfer and mixing, without the use of external devices. The oxidative polymerization of aniline was demonstrated as a model reaction for the proposed microscale synthetic methodology. To the best of our knowledge, this is the first report of microscale synthesis achieved on hydrophobic paper with strong adhesion properties. This approach aligns with green chemistry principles by minimizing chemical consumption and reducing chemical waste. The fabricated HFP has potential application as a microreactor device for microscale synthesis and for reactions involving microdroplet transfer through controlled wettability and adhesion.