In this feature article, the applications of layered nanomaterials as green materials are demonstrated along with their structures and properties. Due to the unique 2-dimensional structures and biocompatibility, various eco- and bio-friendly materials can be hybridized with layered nanoparticles such as layered double hydroxides (LDHs), hydroxy double salts (HDSs) and cationic clays. From small molecules such as drugs, herbicides, fertilizers, and food ingredients to large substances like deoxyribonucelotides (DNA), protein, and enzymes, various functional materials can be incorporated into layered nanoparticles via intercalation reactions to produce green materials with versatile applications. The application fields of layered nanomaterials, especially LDHs, are summarized into three categories; biomolecule reservoir, pharmaceutical and other applications.

Layered nanomaterials for green materials

Topochemical reaction strategies offer an important low-temperature (<500 °C) approach to the conscious manipulation of various inorganic host materials, allowing access to compounds that cannot be prepared by standard high-temperature methods. As the utility of these strategies continues to expand, researchers will be able to more effectively target materials with technologically significant properties. This Progress Report presents recent advances in topochemical reaction strategies as applied to perovskite and perovskite-related compounds. Emphasis is placed on structural modifications and corresponding variations in properties. Additionally, the future prospects of this approach to the rational design of intricate target compounds are discussed.

Topochemical manipulation of perovskites: low-temperature reaction strategies for directing structure and properties.

A colloidal suspension of exfoliated, layered cobalt oxide nanosheets has been synthesized through the intercalation of quaternary tetramethylammonium ions into protonated lithium cobalt oxide. According to atomic force microscopy, exfoliated nanosheets of layered cobalt oxide show a plateau-like height profile with nanometer-level height, underscoring the formation of unilamellar 2D nanosheets. The exfoliation of layered cobalt oxide was cross-confirmed by X-ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The maintenance of the hexagonal in-plane structure of the cobalt oxide lattice after the exfoliation process was evidenced by selected-area electron diffraction and Co K-edge X-ray absorption near-edge structure analysis. The zeta-potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets. Adopting the nanosheets of layered cobalt oxide as a precursor, we were able to prepare the monodisperse CoO nanocrystals with a particle size of approximately 10 nm as well as the heterolayered film composed of cobalt oxide monolayer and polycation.

Soft-chemical exfoliation route to layered cobalt oxide monolayers and its application for film deposition and nanoparticle synthesis.

The clinical efficacy of conventional chemotherapeutic agent, methotrexate (MTX), can be limited by its very short plasma half-life, the drug resistance, and the high dosage required for cancer cell suppression. In this study, a new drug delivery system is proposed to overcome such limitations. To realize such a system, MTX was intercalated into layered double hydroxides (LDHs), inorganic drug delivery vehicle, through a co-precipitation route to produce a MTX-LDH nanohybrid with an average particle size of approximately 130 nm. Biodistribution studies in mice bearing orthotopic human breast tumors revealed that the tumor-to-liver ratio of MTX in the MTX-LDH-treated-group was 6-fold higher than that of MTX-treated-one after drug treatment for 2 hr. Moreover, MTX-LDH exhibited superior targeting effect resulting in high antitumor efficacy inducing a 74.3% reduction in tumor volume compared to MTX alone, and as a consequence, significant survival benefits. Annexin-V and propidium iodine dual staining and TUNEL analysis showed that MTX-LDH induced a greater degree of apoptosis than free MTX. Taken together, our data demonstrate that a new MTX-LDH nanohybrid exhibits a superior efficacy profile and improved distribution compared to MTX alone and has the potential to enhance therapeutic efficacy via inhibition of tumor proliferation and induction of apoptosis.

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Inorganic Nanovehicle Targets Tumor in an Orthotopic Breast Cancer Model

As an emerging group of two-dimensional (2D) nanomaterials, 2D group-VA layered nanomaterials have been attracting increasing attention in recent years due to their intriguing physiochemical properties and functional structures for broad and promising applications. In contrast to phosphorene and black phosphorus, 2D group-VA pnictogen elemental (As, Sb, and Bi) nanomaterials have tunable direct bandgaps, high stability, charge-carrier mobility and unique in-plane anisotropic structures, giving them great prospects for applications in significant and extensive research areas. In this review, we systematically introduced the recent advances in 2D group-VA nanomaterials beyond black phosphorus. First, the synthesis methods were summarized and grouped into top-down and bottom-up categories. Then, their fundamental properties were described, referring to their band structures and carrier transport as well as mechanical, thermal, optical, magnetic and electronic properties. Subsequently, functional nanostructures were discussed, such as heterostructures, doping, absorption, pnictogen-containing hybrids and surface functionalization. Finally, potential applications were illustrated, involving catalysis, energy storage, field-effect transistors, topological spintronic devices, electronic devices, nonlinear photonics, light-emitting devices, gas sensors, thermoelectric materials and biomedicine. Finally, the current states, challenges and perspectives for the emerging 2D group-VA nanomaterials were discussed rationally. This timely and comprehensive review is attractive for scientists from different research fields and promotes the further development of low-dimensional nanomaterials and functionalized hybrid materials.

Two-dimensional group-VA nanomaterials beyond black phosphorus: synthetic methods, properties, functional nanostructures and applications

A coordination compound of HgI(2)(pyridine)(2) can be successfully intercalated into a single crystalline Bi(2)Sr(2)CaCu(2)O(y) high-T(c) superconductor through an interlayer complexation reaction between pyridine molecules and bismuth cuprate pre-intercalated with mercuric iodide. X-ray diffraction and X-ray absorption spectroscopic results clearly demonstrate that the single crystalline nature of the pristine bismuth cuprate remains unchanged even after the intercalation of organic complex as well as those of iodine and mercuric iodide. According to the angle-dependent dc magnetization measurements, the intercalation of bulky organic molecules completely blocks superconductive currents along the c-axis, whereas a superconducting transition along the in-plane direction still occurs in the organic intercalate. In the case of the iodine or mercuric iodide intercalates with smaller lattice expansions, an out-of-plane diamagnetic transition is not wholly quenched but significantly depressed by the intercalation, confirming the reduction of interlayer interaction. The present finding can provide straightforward evidence of the two-dimensionality of high temperature superconductivity in the present cuprate-based nanohybrid material.

Il Won Chung

Papers

Evidence of Two-Dimensional Superconductivity in the Single Crystalline Nanohybrid of Organic-Bismuth Cuprate