![]() The successful formation of Fe-HISs was verified by various characterization methods, such as confocal laser-scanning microscopy (CLSM) with differential interference contrast (DIC) mode, field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The open voids of MH-HISs facilitate substrate diffusion, enhancing their potential applications in enzymatic cascade reactions. Specifically, we report that solid CaCO 3 particles gradually transform into CaCO 3/MH core-shell, CaCO 3/MH yolk-shell, and MH hollow topologies by controlled exchange of Ca 2+ and metal ions (e.g., Fe 3+, Ru 3+, and V 3+), serving as biocompatible sacrificial templates for the construction of biocatalytic MH-HISs that contain multiple enzymes in the shells. Our strategy is based on the chemical evolution of calcium carbonate (CaCO 3) particles into MH-HISs under physiologically relevant conditions (Fig. In this work, we have developed a highly biocompatible method for fabricating metal hydroxide HISs (MH-HISs), which encapsulate multiple enzymes for cascade reactions through efficient substrate channeling. ![]() As an alternative, metal-organic frameworks (MOFs) have emerged as soft templates and/or shell components for hollow shell formation 29, 30, 31, although the preparation of MOFs still faces issues with harsh reaction conditions and instability in an aqueous solution. Recent examples include the fabrication of Fe 2O 3-zeolite HISs, catalytically active in gasoline production by the Fischer-Tropsch synthesis, which involves calcination at 550 ☌ and hydrothermal treatment at 190 ☌ 28. However, the conventional chemical methods for HIS fabrication 1, 2, 21, 22, 23, 24, 25, 26, 27―based on template removal, Kirkendall effect, Ostwald ripening, and galvanic replacement―unavoidably require harsh synthetic conditions, such as high temperatures (e.g., for calcination or solvothermal processes) and extreme acidity/basicity (e.g., HF, HCl, or NaOH), in addition to time-consuming, cumbersome operations, so far limiting their applications to abiotic compounds and materials. Note that compartmentalization of bioentities in inorganic architectures is rather not a new concept the iron sulfide (FeS) membrane has been proposed as the first proto-membrane for life, serving as a protective barrier against environmental changes, in natural evolution 18, 19, 20. Tandem-biocatalysis reactors could be constructed, based on HISs, by spatially compartmentalizing enzymes and co-factors in the HIS structures 17. The mechanical durability of inorganic shells would ensure the structural and functional integrity of the biological entities (e.g., enzymes, biotherapeutic agents, and even cells), encapsulated in the porous shells and internal voids, during storage, transport, and use 9, 10, 11, 12, 13, 14, 15, 16. In addition to the intensively exploited, energy-related potential, HISs would advance the bio-related fields, including biocatalysis, biomedicine, and systems biology, were established the methods for biocompatibly confining biochemically functional entities to the HIS’s voids and/or shells. Moreover, the thin shells effectively shorten diffusion pathways for both charges and ions, leading to high-rate capacity 8. For example, HISs have intensively been used as electrode materials for the construction of lithium-ion batteries and supercapacitors, in which their commodious interior spaces alleviate the structural deformations that occur during charge and discharge processes, such as destructive volume expansion and contraction, and improve the device’s mechanical and electrochemical stability 7. Their intrinsic physical properties, such as low mass density, large surface area, short mass-/charge-transport length, and mechanical stability, make HISs promising candidates in various technological areas, particularly in the area of energy storage, conversion, and production 1, 2, 4, 5, 6. ![]() Hollow inorganic spheres (HISs), composed of metals, metal oxides, metal sulfides, metal hydroxides, or others, constitute functional materials uniquely featured by large empty spaces inside distinct shells 1, 2, 3. ![]()
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