Using Hybrid Coating to Fabricate Highly Stable and Expandable Transparent Liquid Marbles

Liquid marbles (LMs) are microliter-scale droplets coated with hydrophobic solid particles. The particle size and hydrophobicity of the surface coating determine their properties, such as transparency, expandability, and resistance to evaporation and coalescence, one or more of which can be critical to their application as microreactors. This study reports the use of a mixture of two different hydrophobic powders for fabrication of LMs for colorimetric assays: trichloro(1H,1H,2H,2H-perfluorooctyl) silane-linked silica gel (modified silica gel (MSG), particle size: 40-75 μm) and hexamethyldisilazane-linked fumed silica (modified fumed silica (MFS), average aggregate length: 200-300 nm). The hybrid coating mixture (MIX) prepared by mixing these MSG and MFS powders in a ratio of 3:7 (w/w), respectively, contained particles of different sizes as well as different hydrophobicity as the silane linked to MSG is more hydrophobic than the one linked to MFS. LMs fabricated using MIX as the surface coating were characterized and compared to LMs coated with MSG or MFS alone. It was observed that MIX LMs were comparable to the MFS LMs in transparency (higher than the MSG LMs), expandability (more than 20 times their initial volume), and stability against evaporation (for more than 4 h at 78% relative humidity at 26 °C). However, in terms of resistance to coalescence, the MIX LMs showed a resistance comparable to that of MSG LMs, much higher than that of MFS LMs. Further experiments demonstrated that it is the presence of the particles of different sizes (MSG particles are ∼100 times larger than MFS) that improves the resistance to coalescence rather than the higher hydrophobicity of the MSG. Three different colorimetric assays were performed in the MIX LMs, and the results obtained were comparable in accuracy and precision to those obtained in a standard polystyrene microwell plate system. Low quantities of the analytes could be detected and quantified, as evidenced by the limit of detection (alkaline phosphatase (AP): 0.18 μg/mL; bovine serum albumin (BSA): 2.28 μg/mL; and chymotrypsin: 3.69 μM) and limit of quantification (AP: 0.59 μg/mL; BSA: 12.29 μg/mL; and chymotrypsin: 7.59 μM) values. Color intensities in LMs were quantified using a smartphone application, which provides the added benefit of an instrument-free approach. These findings highlight the potential of using LMs stabilized with mixtures of nano- and microparticles as robust, versatile microreactors for portable and sensitive colorimetric assays, paving the way for more accessible and efficient diagnostic tools.