Underwater superoleophobic materials have attracted increasing attention because of their remarkable potential applications, especially antifouling, self-cleaning and oil-water separation. A limitation of most superoleophobic materials is that they are non-transparent and have limited mechanical stability underwater. Here, we report a protocol for preparing a transparent and robust superoleophobic film that can be used underwater. It is formed by a hydrogel layer prepared by the superspreading of chitosan solution on a superhydrophilic substrate and biomimetic mineralization of this layer. In contrast to conventional hydrogel-based materials, this film exhibits significantly improved mechanical properties because of the combination of high-energy, ordered, inorganic aragonite (one crystalline polymorph of calcium carbonate) and homogeneous external hierarchical micro/nano structures, leading to robust underwater superoleophobicity and ultralow oil adhesion. Moreover, the mineralized film is suitable for neutral and alkaline environments and for containing organic solvent underwater and can be coated on different transparent materials, which has promising applications in underwater optics, miniature reactors and microfluidic devices. In this protocol, the time for the whole biomimetic mineralization process is only ~6 h, which is significantly shorter than that of traditional methods, such as gas diffusion and the Kitano method. The protocol can be completed in ~2 weeks and is suitable for researchers with intermediate expertise in organic chemistry and inorganic chemistry.
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