Water scarcity threatens over half of the world population, yet over 141 billion liters of fresh water is used globally each day for toilet flushing. This is nearly 6 times the daily water consumption of the population in Africa. The toilet water footprint is so large primarily because large volumes of water are necessary for the removal of human feces; human feces is viscoelastic and sticky in nature, causing it to adhere to conventional surfaces. Here, we designed and fabricated the liquid-entrenched smooth surface (LESS), a sprayable non-fouling coating that can reduce cleaning water consumption by ~90% compared to untreated surfaces due to its extreme repellency towards liquids, bacteria, and viscoelastic solids. Importantly, LESS-coated surfaces can repel viscoelastic solids with dynamic viscosities spanning over 9 orders of magnitude, i.e., three orders of magnitude higher than previously reported for other repellent materials. With an estimated >~1 billion toilets and urinals worldwide, incorporating LESS coating into sanitation systems will have significant implications for global sanitation and large-scale wastewater reduction for sustainable water management.

Immobilizing enzymes on nanoparticles (NPs) enhances the cost-efficiency of biocatalysis; however, when the substrates are large, it becomes difficult to separate the enzyme@NP from the products while avoiding leaching or damage of enzymes in the reaction medium. Metal–organic framework (MOF)-coated magnetic NPs (MNPs) offer efficient magnetic separation and enhanced enzyme protection; however, the involved enzymes/substrates have to be smaller than the MOF apertures. A potential solution to these challenges is coprecipitating metal/ligand with enzymes on the MNP surface, which can partially bury (protect) the enzyme below the composite surface while exposing the rest of the enzyme to the reaction medium for catalysis of larger substrates. Here, to prove this concept, we show that using Ca2+ and terephthalic acid (BDC), large-substrate enzymes can be encapsulated in CaBDC-MOF layers coated on MNPs via an enzyme-friendly, aqueous-phase one-pot synthesis. Interestingly, we found that using MNPs as the nuclei of crystallization, the composite size can be tuned so that nanoscale composites were formed under low Ca2+/BDC concentrations, while microscale composites were formed under high Ca2+/BDC concentrations. While the microscale composites showed significantly enhanced reusability against a non-structured large substrate, the nanoscale composites displayed enhanced catalytic efficiency against a rigid, crystalline-like large substrate, starch, likely due to the improved diffusivity of the nanoscale composites. To our best knowledge, this is the first report on aqueous-phase one-pot synthesis of size-tunable enzyme@MOF/MNP composites for large-substrate biocatalysis. Our platform can be applied to immobilize other large-substrate enzymes with enhanced reusability and tunable sizes.

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