Using Hybrid Coating\nto Fabricate Highly Stable and\nExpandable Transparent Liquid Marbles

Liquid marbles (LMs) are microliter-scale droplets coated\nwith\nhydrophobic solid particles. The particle size and hydrophobicity\nof the surface coating determine their properties, such as transparency,\nexpandability, and resistance to evaporation and coalescence, one\nor more of which can be critical to their application as microreactors.\nThis study reports the use of a mixture of two different hydrophobic\npowders for fabrication of LMs for colorimetric assays: trichloro(1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorooctyl) silane-linked silica gel (modified silica gel (MSG),\nparticle size: 40–75 μm) and hexamethyldisilazane-linked\nfumed silica (modified fumed silica (MFS), average aggregate length:\n200–300 nm). The hybrid coating mixture (MIX) prepared by mixing\nthese MSG and MFS powders in a ratio of 3:7 (w/w), respectively, contained\nparticles of different sizes as well as different hydrophobicity as\nthe silane linked to MSG is more hydrophobic than the one linked to\nMFS. LMs fabricated using MIX as the surface coating were characterized\nand compared to LMs coated with MSG or MFS alone. It was observed\nthat MIX LMs were comparable to the MFS LMs in transparency (higher\nthan the MSG LMs), expandability (more than 20 times their initial\nvolume), and stability against evaporation (for more than 4 h at 78%\nrelative humidity at 26 °C). However, in terms of resistance\nto coalescence, the MIX LMs showed a resistance comparable to that\nof MSG LMs, much higher than that of MFS LMs. Further experiments\ndemonstrated that it is the presence of the particles of different\nsizes (MSG particles are ∼100 times larger than MFS) that improves\nthe resistance to coalescence rather than the higher hydrophobicity\nof the MSG. Three different colorimetric assays were performed in\nthe MIX LMs, and the results obtained were comparable in accuracy\nand precision to those obtained in a standard polystyrene microwell\nplate system. Low quantities of the analytes could be detected and\nquantified, as evidenced by the limit of detection (alkaline phosphatase\n(AP): 0.18 μg/mL; bovine serum albumin (BSA): 2.28 μg/mL;\nand chymotrypsin: 3.69 μM) and limit of quantification (AP:\n0.59 μg/mL; BSA: 12.29 μg/mL; and chymotrypsin: 7.59 μM)\nvalues. Color intensities in LMs were quantified using a smartphone\napplication, which provides the added benefit of an instrument-free\napproach. These findings highlight the potential of using LMs stabilized\nwith mixtures of nano- and microparticles as robust, versatile microreactors\nfor portable and sensitive colorimetric assays, paving the way for\nmore accessible and efficient diagnostic tools.