Matrix-Directed Mineralization for Bulk Structural Materials.

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Amorphous Calcium Carbonate Cluster Nanospheres in Water-Deficient Organic Solvents.

As the key intermediate phase of crystalline calcium carbonate biominerals, amorphous calcium carbonate (ACC) remains mysterious in its structures because of its long-range disorder and instability. We herein report the synthesis of ACC nanospheres in a water-deficient organic solvent system. The obtained ACC nanospheres are very stable under dry conditions. Cryo-TEM reveals that each nanospheres consists of smaller nanosized clusters. We further demonstrate that these clusters can precipitate on other substrates to form an ultrathin ACC coating, which should be an ACC cluster monolayer. The results demonstrate that the presence of small ACC clusters as the subunits of larger aggregates is inherent to ACC synthesized in water-alcohol system but not induced by polymer additives.

Seeded Mineralization Leads to Hierarchical CaCO3 Thin Coatings on Fibers for Oil/Water Separation Applications.

Like their biogenic counterparts, synthetic minerals with hierarchical architectures should exhibit multiple structural functions, which nicely bridge the boundaries between engineering and functional materials. Nevertheless, design of bioinspired mineralization approaches to thin coatings with distinct micro/nanotextures remains challenging in the realm of materials chemistry. Herein, a general morphosynthetic method based on seeded mineralization was extended to achieve prismatic-type thin CaCO3 coatings on fibrous substrates for oil/water separation applications. Distinct micro/nanotextures of the overlayers could be obtained in mineralization processes in the presence of different soluble (bio)macromolecules. These hierarchical thin coatings therefore exhibit multiple structural functions including underwater superoleophobicity, ultralow adhesion force of oil in water, and comparable stiffness/strength to the prismatic-type biominerals found in mollusk shells. Moreover, this controllable approach could proceed on fibrous substrates to obtain robust thin coatings, so that a modified nylon mesh could be employed for oil/water separation driven by gravity. Our bioinspired approach based on seeded mineralization opens the door for the deposition of hierarchical mineralized thin coatings exhibiting multiple structural functions on planar and fibrous substrates. This bottom-up strategy could be readily extended for the syntheses of advanced thin coatings with a broad spectrum of engineering and functional constituents.

Synergistic Effect of Granular Seed Substrates and Soluble Additives in Structural Control of Prismatic CaCO3 Thin Films.

In biomineralization and bioinspired mineralization, substrates and additives function synergistically in providing structural control of the mineralized layers including their orientation, polymorph, morphology, hierarchical architecture, etc. Herein, a novel type of granular aragonitic CaCO3-poly(acrylic acid) substrate guides the mineralization of prismatic CaCO3 thin films of distinct morphology and polymorph in the presence of different additives including organic compounds and polymers. For instance, weakly charged amino acids lead to columnar aragonite overlayers, while their charged counterparts and organic acids/bases inhibit the overgrowth. Employment of several specific soluble polymer additives in overgrowth instead results in calcitic overlayers with distinct hierarchical architecture, good hardness/Young's modulus, and under-water superoleophobicity. Interestingly, self-organized patterns in the CaCO3-poly(l-glutamic acid) overlayer are obtained under proper mineralization conditions. We demonstrate that the granular seed comprised of mineralized and polymeric constituents is a versatile platform for obtaining prismatic CaCO3 thin films, where structural control can be realized by the employment of different types of additives in overgrowth. We expect the methodology to be applied to a broad spectrum of bioinspired, prismatic-type crystalline products, aiming for the development of high-performance hybrids.

Charged Nanowire-Directed Growth of Amorphous Calcium Carbonate Nanosheets in a Mixed Solvent for Biomimetic Composite Films.

Bio-inspired mineralization is an effective way for fabricating complex inorganic materials, which inspires us to develop new methods to synthesize materials with fascinating properties. In this article, we report that the charged tellurium nanowires (TeNWs) can be used as biomacromolecule analogues to direct the formation of amorphous calcium carbonate (ACC) nanosheets (ACCNs) in a mixed solvent. The effects of surface charges and the concentration of the TeNWs on the formation of ACCNs have been investigated. Particularly, the produced ACCNs can be functionalized by Fe3O4 nanoparticles to produce magnetic ACC/Fe3O4 hybrid nanosheets that can be used to construct ACC/Fe3O4 composite films through a self-evaporation process. Moreover, sodium alginate-ACC nanocomposite films with remarkable toughness and good transmittance can also be fabricated by using such ACCNs as nanoscale building blocks. This mineralization approach in a mixed solvent using charged TeNWs as biomacromolecule analogues provides a new way for the synthesis of ACCNs, which can be used as nanoscale building blocks for the fabrication of biomimetic composite films.

Micrometer-Thick Graphene Oxide-Layered Double Hydroxide Nacre-Inspired Coatings and Their Properties.

Robust, functional, and flame retardant coatings are attractive in various fields such as building construction, food packaging, electronics encapsulation, and so on. Here, strong, colorful, and fire-retardant micrometer-thick hybrid coatings are reported, which can be constructed via an enhanced layer-by-layer assembly of graphene oxide (GO) nanosheets and layered double hydroxide (LDH) nanoplatelets. The fabricated GO-LDH hybrid coatings show uniform nacre-like layered structures that endow them good mechanic properties with Young's modulus of ≈ 18 GPa and hardness of ≈ 0.68 GPa. In addition, the GO-LDH hybrid coatings exhibit nacre-like iridescence and attractive flame retardancy as well due to their well-defined 2D microstructures. This kind of nacre-inspired GO-LDH hybrid thick coatings will be applied in various fields in future due to their high strength and multifunctionalities.

Hydrogels from Amorphous Calcium Carbonate and Polyacrylic Acid: Bio-Inspired Materials for "Mineral Plastics".

Given increasing environmental issues due to the large usage of non-biodegradable plastics based on petroleum, new plastic materials, which are economic, environmentally friendly, and recyclable are in high demand. One feasible strategy is the bio-inspired synthesis of mineral-based hybrid materials. Herein we report a facile route for an amorphous CaCO3 (ACC)-based hydrogel consisting of very small ACC nanoparticles physically cross-linked by poly(acrylic acid). The hydrogel is shapeable, stretchable, and self-healable. Upon drying, the hydrogel forms free-standing, rigid, and transparent objects with remarkable mechanical performance. By swelling in water, the material can completely recover the initial hydrogel state. As a matrix, thermochromism can also be easily introduced. The present hybrid hydrogel may represent a new class of plastic materials, the "mineral plastics".

Bioinspired, Ultrastrong, Highly Biocompatible, and Bioactive Natural Polymer/Graphene Oxide Nanocomposite Films.

Tough and biocompatible nanocomposite films: A new type of bioinspired ultrastrong, highly biocompatible, and bioactive konjac glucomannan (KGM)/graphene oxide (GO) nanocomposite film is fabricated on a large scale by a simple solution-casting method. Such KGM-GO composite films exhibit much enhanced mechanical properties under the strong hydrogen-bonding interactions, showing great potential in the fields of tissue engineering and food package.

Ultrathin hybrid films of polyoxohydroxy clusters and proteins: layer-by-layer assembly and their optical and mechanical properties.

The hierarchical assembly of inorganic and organic building blocks is an efficient strategy to produce high-performance materials which has been demonstrated in various biomaterials. Here, we report a layer-by-layer (LBL) assembly method to fabricate ultrathin hybrid films from nanometer-scale ionic clusters and proteins. Two types of cationic clusters (hydrolyzed aluminum clusters and zirconium-glycine clusters) were assembled with negatively charged bovine serum albumin (BSA) protein to form high-quality hybrid films, due to their strong electrostatic interactions and hydrogen bonding. The obtained hybrid films were characterized by scanning electron microscope (SEM), UV-vis, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), and X-ray diffraction (XRD). The results demonstrated that the cluster-protein hybrid films exhibited structural homogeneity, relative transparency, and bright blue fluorescence. More importantly, these hybrid films displayed up to a 70% increase in hardness and up to a 100% increase in reduced Young's modulus compared to the pure BSA film. These hybrid cluster-protein films could be potentially used as biomedical coatings in the future because of their good transparency and excellent mechanical properties.

Bioinspired High-Magnesium Calcite for Efficiently Reducing Chemical Oxygen Demand in Lake Water.

Organic matter accumulation in water can cause serious problems such as oxygen depletion and quality deterioration of waters. While calcium carbonate has been used as green and low-cost adsorbent for water treatment, its efficiency in reducing the chemical oxygen demand (COD) of water, which is a measure of organic pollution, is restrained by the limited specific surface area and chemical activity. Herein, inspired by the high-magnesium calcite (HMC) found in biological materials, a feasible method to synthesize fluffy dumbbell-like HMC with large specific surface area is reported. The magnesium inserting increases the chemical activity of the HMC moderately but without lowering its stability too much. Therefore, the crystalline HMC can retain its phase and morphology in aqueous environment for hours, which allows the establishment of adsorption equilibrium between the solution and the adsorbent that retains its initial large specific surface area and improved chemical activity. Consequently, the HMC exhibits notably enhanced capability in reducing the COD of lake water polluted by organics. This work provides a synergistic strategy to rationally design high-performance adsorbents by simultaneously optimizing the surface area and steering the chemical activity.