Polymer-Inorganic Crystalline Nanocomposite Materials via Nanoparticle Occlusion

Spatioselective Occlusion of Copolymer Nanoparticles within Calcite Crystals Generates Organic-Inorganic Hybrid Materials with Controlled Internal Structures

Efficient occlusion of particulate additives into a single crystal has garnered an ever-increasing attention in materials science because it offers a counter-intuitive yet powerful platform to make crystalline nanocomposite materials with emerging properties. However, precisely controlling the spatial distribution of the guest additives within a host crystal remains highly challenging. We herein demonstrate a unique, straightforward method to engineer the spatial distribution of copolymer nanoparticles within calcite (CaCO3) single crystals by judiciously adjusting initial [Ca2+] concentration used for the calcite precipitation. More specifically, polymerization-induced self-assembly is employed to synthesize well-defined and highly anionic poly(3-sulfopropyl methacrylate potassium)41-block-poly(benzyl methacrylate)500 [PSPMA41-PBzMA500] diblock copolymer nanoparticles, which are subsequently used as model additives during the growth of calcite crystals. Impressively, such guest nanoparticles are preferentially occluded into specific regions of calcite depending on the initial [Ca2+] concentration. These unprecedented phenomena are most probably caused by dynamic change in electrostatic interaction between Ca2+ ions and PSPMA41 chains based on systematic investigations. This study not only showcases a significant advancement in controlling the spatial distribution of guest nanoparticles within host crystals, enabling the internal structure of composite crystals to be rationally tailored via a spatioselective occlusion strategy, but also provides new insights into biomineralization.

Controlling Adsorption of Diblock Copolymer Nanoparticles onto an Aldehyde-Functionalized Hydrophilic Polymer Brush via pH Modulation

Sterically stabilized diblock copolymer nanoparticles with a well-defined spherical morphology and tunable diameter were prepared by RAFT aqueous emulsion polymerization of benzyl methacrylate at 70 °C. The steric stabilizer precursor used for these syntheses contained pendent cis-diol groups, which means that such nanoparticles can react with a suitable aldehyde-functional surface via acetal bond formation. This principle is examined herein by growing an aldehyde-functionalized polymer brush from a planar silicon wafer and studying the extent of nanoparticle adsorption onto this model substrate from aqueous solution at 25 °C using a quartz crystal microbalance (QCM). The adsorbed amount, Γ, depends on both the nanoparticle diameter and the solution pH, with minimal adsorption observed at pH 7 or 10 and substantial adsorption achieved at pH 4. Variable-temperature QCM studies provide strong evidence for chemical adsorption, while scanning electron microscopy images recorded for the nanoparticle-coated brush surface after drying indicate mean surface coverages of up to 62%. This fundamental study extends our understanding of the chemical adsorption of nanoparticles on soft substrates.

Interfacial Supra-Assembly of Copolymer Nanoparticles Enables the Formation of Nanocomposite Crystals with a Tunable Internal Structure

It is highly desirable but technically challenging to precisely control the spatial composition and internal structure of crystalline nanocomposite materials, especially in a one-pot synthetic route. Herein, we demonstrate a versatile pathway to tune the spatial distribution of guest species within a host inorganic crystal via an incorporation strategy. Specifically, well-defined block copolymer nanoparticles, poly(methacrylic acid)x-block-poly(styrene-alt-N-phenylmaleimide)y [PMAAx-P(St-alt-NMI)y], are synthesized by polymerization-induced self-assembly. Such anionic nanoparticles can supra-assemble onto the surface of larger cationic nanoparticles via an electrostatic interaction, forming colloidal nanocomposite particles (CNPs). Remarkably, such CNPs can be incorporated into calcite single crystals in a spatially controlled manner: the depth of CNPs incorporation into calcite is tunable. Systematic investigation indicates that this interesting phenomenon is governed by the colloidal stability of CNPs, which in turn is dictated by the PMAAx-P(St-alt-NMI)y adsorption density and calcium ion concentration. This study opens up a general and efficient route for the preparation of a wide range of crystalline nanocomposite materials with a controlled internal composition and structure.

Multi-Functionalization of Single crystals Mediated by Gel-Incorporation: A Bioinspired Strategy

Biominerals are inherently organic-inorganic crystal composites. Drawing inspiration from this biomineral structure, functionalized single crystals can be synthesized using the gel-grown method, resulting in the incorporation of gel-networks into the host crystals. By incorporating gel-networks, diverse guest materials, such as nanoparticles and dye molecules, can be uniformly and isotropically distributed within the crystals, thereby imparting non-intrinsic optical or magnetic properties to the host crystals. Additionally, gel-incorporation enhances the toughness and stability of the crystals as the incorporated gel-fibers and accompanying guest materials act as bridges to prevent crack propagation. Furthermore, gel-incorporation enables protein crystals to exhibit self-healing properties, which can be attributed to the dynamic bonding interaction between gel-networks and crystals. Notably, recent research has demonstrated that the incorporation of zwitterionic gel-networks enhances the charge effects on crystal morphology evolution as the charged groups become bound to the developing crystal surfaces, and their detachment is impeded by the interconnected gel-networks. Therefore, preparing single crystals with gel-incorporation is a remarkable strategy for synthesizing functionalized crystal materials.

Synergistic Effect of Hydroxyl and Carboxyl Groups on Promoting Nanoparticle Occlusion within Calcite

Direct occlusion of guest nanoparticles into host crystals enables the straightforward preparation for various of nanocomposite materials with emerging properties. Therefore, it is highly desirable to elucidate the 'design rules' that govern efficient nanoparticle occlusion. Herein, a series of sterically-stabilized nanoparticles are rationally prepared, where the surface stabilizer chains of such nanoparticles are composed of either poly(methacrylic acid), or poly(glycerol monomethacrylate), or poly((2-hydroxy-3-(methacryloyloxy)propyl)serine). Systematic investigation reveals that hydroxyl groups and carboxyl groups play a synergistic role in driving nanoparticle incorporation into calcite crystals, where the hydroxyl groups enhance colloidal stability of the nanoparticles and the carboxyl groups provide binding sites for efficient occlusion. The generality of these findings is further validated by extending it to polymer-stabilized gold nanoparticles. This study demonstrates that precision synthesis of polymer stabilizers comprising of synergistic functional groups can significantly promote nanoparticle occlusion, thus enabling the efficient construction of organic-inorganic hybrid materials via nanoparticle occlusion strategy.

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.

Metal-Organic Frameworks@Calcite Composite Crystals

The direct incorporation of guest crystals into another type of host crystals during the formation of the latter is technically challenging due to the large difference in surface energy for different crystalline components. Nevertheless, we herein demonstrate that metal-organic frameworks (MOFs, UiO-66-NH₂ as a model guest crystal) after postsynthetic modification with poly(methacrylic acid) can be efficiently incorporated into calcite single crystals, forming a unique composite structure where the MOF crystals are uniformly distributed throughout the whole calcite host crystals. Remarkably, such MOF@calcite composite crystals exhibit superior performance in fluoride removal compared with the MOF or calcite alone. Moreover, this incorporation strategy is general as it can be extended to other guest particles. In principle, this study opens up a versatile avenue for the rational design and preparation of a wide range of hybrid functional materials with controllable compositions and enhanced physicochemical properties.