Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

In the past decade, mesoporous silica nanoparticles (MSNs) have attracted more and more attention for their potential biomedical applications. With their tailored mesoporous structure and high surface area, MSNs as drug delivery systems (DDSs) show significant advantages over traditional drug nanocarriers. In this review, we overview the recent progress in the synthesis of MSNs for drug delivery applications. First, we provide an overview of synthesis strategies for fabricating ordered MSNs and hollow/rattle-type MSNs. Then, the in vitro and in vivo biocompatibility and biotranslocation of MSNs are discussed in relation to their chemophysical properties including particle size, surface properties, shape, and structure. The review also highlights the significant achievements in drug delivery using mesoporous silica nanoparticles and their multifunctional counterparts as drug carriers. In particular, the biological barriers for nano-based targeted cancer therapy and MSN-based targeting strategies are discussed. We conclude with our personal perspectives on the directions in which future work in this field might be focused.

Mesoporous Silica Nanoparticles: Synthesis, Biocompatibility and Drug Delivery

Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications

Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms’ 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance. Downsizing platinum based nanocatalysts has the twin advantages of lower platinum usage and increased activity per platinum atom. Here, the authors report an atomic layer deposition technique for single platinum atom catalyst fabrication and assess their hydrogen evolution activity.

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Platinum single-atom and cluster catalysis of the hydrogen evolution reaction

Developing highly active single-atom catalysts for electrochemical reactions is a key to future renewable energy technology. Here we present a universal design principle to evaluate the activity of graphene-based single-atom catalysts towards the oxygen reduction, oxygen evolution and hydrogen evolution reactions. Our results indicate that the catalytic activity of single-atom catalysts is highly correlated with the local environment of the metal centre, namely its coordination number and electronegativity and the electronegativity of the nearest neighbour atoms, validated by available experimental data. More importantly, we reveal that this design principle can be extended to metal–macrocycle complexes. The principle not only offers a strategy to design highly active nonprecious metal single-atom catalysts with specific active centres, for example, Fe-pyridine/pyrrole-N4 for the oxygen reduction reaction; Co-pyrrole-N4 for the oxygen evolution reaction; and Mn-pyrrole-N4 for the hydrogen evolution reaction to replace precious Pt/Ir/Ru-based catalysts, but also suggests that macrocyclic metal complexes could be used as an alternative to graphene-based single-atom catalysts.

A universal principle for a rational design of single-atom electrocatalysts

The carbon-coated Ni(C) nanocapsules were prepared by a modified arc-discharge method in methane atmosphere. Its electromagnetic parameters were measured at 2–18GHz. It is observed that the natural resonance which appeared at 5.5GHz is dominant among microwave absorption properties of Ni(C) nanocapsules, as the consequence of the increased surface anisotropic energy for nanosized particles. The measured relative complex permittivity indicates that a high resistivity existed in Ni(C) nanocapsules samples. The maximum reflection loss of Ni(C) nanocomposites can reach 32dB at 13GHz with 2mm in thickness. The microwave absorptive mechanisms of Ni(C) nanocapsule absorbent were discussed.

Microwave absorption properties of the carbon-coated nickel nanocapsules

Carbon-coated Fe [Fe(C)] nanocapsules were synthesized by a modified arc-discharge method, and their microstructure and electromagnetic (EM) properties (2–18 GHz) were investigated by means of transmission electron microscopy, Raman spectroscopy and a network analyser. The reflection loss R of less than −20 dB was obtained in the frequency range 3.2–18 GHz. A minimum reflection loss of −43.5 dB was reached at 9.6 GHz with an absorber thickness of 3.1 mm. The in-depth study of relative complex permittivity and permeability reveals that the excellent microwave absorption properties are a consequence of a proper EM match in microstructure, a strong natural resonance, as well as multi-polarization mechanisms, etc.

https://iopscience.iop.org/article/10.1088/0022-3727/40/17/056/pdf

Microstructure and microwave absorption properties of carbon-coated iron nanocapsules

Ni/polyaniline (PANi) nanocomposites were prepared by chemical polymerization, and electromagnetic characteristics were then studied at 2–18GHz. The permittivity of the Ni/PANi nanocomposite presents dual dielectric relaxations with increasing content of PANi to over 15.6wt%, which is ascribed to a cooperative consequence of the core/shell interfaces and the dielectric PANi shells. Additionally, the permeability presents a strong natural resonance around 2–8GHz, which is dominant among microwave magnetic loss. The proper matching of the permittivity and the permeability contributes to enhanced microwave absorption.

Enhanced microwave absorption in Ni/polyaniline nanocomposites by dual dielectric relaxations

Spin precession with frequencies up to 280 GHz is observed in Mn(3-δ)Ga alloy films with a perpendicular magnetic anisotropy constant K(u)∼15  M erg/cm(3). The damping constant α, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the δ=1.46 (0.88) film. Those are about one-tenth of α values for known materials with large K(u). First-principles calculations well describe both low α and large K(u) for these alloys.

Xuefeng Zhang

Papers

Microwave absorption properties of the carbon-coated nickel nanocapsules

Microstructure and microwave absorption properties of carbon-coated iron nanocapsules

Enhanced microwave absorption in Ni/polyaniline nanocomposites by dual dielectric relaxations

Long-lived ultrafast spin precession in manganese alloys films with a large perpendicular magnetic anisotropy.

Microwave absorption properties of the core/shell-type iron and nickel nanoparticles