Immobilized Seed-Mediated Growth of Two-Dimensional Array of Metallic Nanocrystals with Asymmetric Shapes

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

Vesicular self-assembly of copolymer-grafted nanoparticles with anisotropic shapes

Plasmonic nanovesicles show broad applications in areas such as cancer theranostics and drug delivery, but the preparation of nanovesicles from shaped nanoparticles remains challenging. This article describes the vesicular self-assembly of shaped nanoparticles, such as gold nanocubes grafted with amphiphilic block copolymers, in selective solvents. The nanocubes assembled within the vesicular membranes exhibit two distinctive packing modes, namely square-like and hexagonal packing, depending on the relative dimensions of the copolymer ligands and nanocubes. The corresponding optical properties of the plasmonic nanovesicles can be tuned by varying the length of the grafted copolymers and the size of the nanocubes. This work provides guidance for the fabrication of functional plasmonic vesicles for applications in catalysis, nanomedicines and optical devices.

Rigid metal/liquid metal nanoparticles: Synthesis and application for locally ablative therapy

Locally ablative therapy, as the main therapy for advanced tumors, has fallen into a bottleneck in recent years. The breakthrough of metal nanoparticles provides a novel approach for ablative therapy. Previous studies have mostly focused on the combined field of rigid metal nanoparticles and ablation. However, with the maturity of the preparation process of liquid metal nanoparticles, liquid metal nanoparticles not only have metallic properties but also have fluid properties, showing the potential to be combined with ablation. At present, there is no review on the combination of liquid metal nanoparticles and ablation. In this article, we first review the preparation, characterization and application characteristics of rigid metal and liquid metal nanoparticles in ablation applications, and then summarize the advantages, disadvantages and possible future development trends of rigid and liquid metal nanoparticles.

Synthesizable nanoparticle eigenshapes for colloidal crystals

The gulf between the complexity and diversity of colloidal crystal phases predicted to form in computer simulation and that realized to date in experiment is narrowing, but is still wide. Prior work shows that many synthesized particles are far from optimal "eigenshapes" for target superlattice structures. We use digital alchemy to determine eigenshapes for possible target colloidal crystal structures for eight families of polyhedral nanoparticle shapes already synthesized in the laboratory. Within each family we predict optimal building block shapes to obtain several target superlattice structures, as a guide for future experiments. For three target crystal structures common to multiple families, we identify which of the optimal shapes is most optimal under the same thermodynamic conditions.

Engineering Oxygen-Irrelevant Radical Nanogenerator for Hypoxia-Independent Magnetothermodynamic Tumor Nanotherapy

Tumor hypoxia substantially lowers the treatment efficacy of oxygen-relevant therapeutic modalities because the production of reactive oxygen species in oxygen-relevant anticancer modalities is highly dependent on oxygen level in tumor tissues. Here a distinctive magnetothermodynamic anticancer strategy is developed that takes the advantage of oxygen-irrelevant free radicals produced from magnetothermal decomposable initiators for inducing cancer-cell apoptosis in vitro and tumor suppression in vivo. Free-radical nanogenerator is constructed through in situ engineering of a mesoporous silica coating on the surface of superparamagnetic Mn and Co-doped nanoparticles (MnFe₂ O₄ @CoFe₂ O₄ , denoted as Mag) toward multifunctionality, where mesoporous structure provides reservoirs for efficient loading of initiators and the Mag core serves as in situ heat source under alternating magnetic field (AMF) actuation. Upon exposure to an exogenous AMF, the magnetic hyperthermia effect of superparamagnetic core lead to the rapid decomposition of the loaded/delivered initiators (AIPH) to produce oxygen-irrelevant free radicals. Both the magnetothermal effect and generation of toxic free radicals under AMF actuation are synergistically effective in promoting cancer-cell death and tumor suppression in the hypoxic tumor microenvironment. The prominent therapeutic efficacy of this radical nanogenerator represents an intriguing paradigm of oxygen-irrelevant nanoplatform for AMF-initiated synergistic cancer treatment.

Rationally designed dual-plasmonic gold nanorod@cuprous selenide hybrid heterostructures by regioselective overgrowth for in vivo photothermal tumor ablation in the second near-infrared biowindow

NIR-II plasmonic materials offer multiple functionalities for in vivo biomedical applications, such as photothermal tumor ablation, surface-enhanced Raman scattering biosensing, photoacoustic imaging, and drug carriers. However, integration of noble metals and plasmonic semiconductors is greatly challenging because of the large lattice-mismatch. This study reports the regioselective overgrowth of Cu_2-xSe on gold nanorods (GNRs) for preparation of dual-plasmonic GNR@Cu_2-xSe hybrid heterostructures with tunable NIR-II plasmon resonance absorption for in vivo photothermal tumor ablation.

Methods: The regioselective deposition of amorphous Se and its subsequent conversion into Cu_2-xSe on the GNRs are performed by altering capping agents to produce the GNR@Cu_2-xSe heterostructures of various morphologies. Their photothermal performances for NIR-II photothermal tumor ablation are evaluated both in vitro and in vivo.

Results: We find that the lateral one- and two-side deposition, conformal core-shell coating and island growth of Cu_2-xSe on the GNRs can be achieved using different capping agents. The Cu_2-xSe domain size in these hybrids can be effectively adjusted by the SeO₂ concentration, thereby tuning the NIR-II plasmon bands. A photothermal conversion efficiency up to 58-85% and superior photostability of these dual-plasmonic hybrids can be achieved under the NIR-II laser. Results also show that the photothermal conversion efficiency is dependent on the proportion of optical absorption converted into heat; however, the temperature rise is tightly related to the concentration of their constituents. The excellent NIR-II photothermal effect is further verified in the following in vitro and in vivo experiments.

Conclusions: This study achieves one-side or two-side deposition, conformal core-shell coating, and island deposition of Cu_2-xSe on GNRs for GNR@Cu_2-xSe heterostructures with NIR-II plasmonic absorption, and further demonstrates their excellent NIR-II photothermal tumor ablation in vivo. This study provides a promising strategy for the rational design of NIR-II dual-plasmonic heterostructures and highlights their therapeutic in vivo potential.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Self-assembly and characterization of 2D plasmene nanosheets

Freestanding plasmonic nanoparticle (NP) superlattice sheets are novel 2D nanomaterials with tailorable properties that enable their use for broad applications in sensing, anticounterfeit measures, ionic gating, nanophotonics and flat lenses. We recently developed a robust, yet general, two-step drying-mediated approach to produce freestanding monolayer, plasmonic NP superlattice sheets, which are typically held together by holey grids with minimal solid support. Within these superlattices, NP building blocks are closely packed and have strong plasmonic coupling interactions; hence, we termed such freestanding materials 'plasmene nanosheets'. Using the desired NP building blocks as starting material, we describe the detailed fabrication protocol, including NP surface functionalization by thiolated polystyrene and the self-assembly of NPs at the air-water interface. We also discuss various characterization approaches for checking the quality and optical properties of the as-obtained plasmene nanosheets: optical microscopy, spectrophotometry, transmission/scanning electron microscopy (TEM/SEM) and atomic force microscopy (AFM). With regard to different constituent building blocks, the key experimental parameters, including NP concentration and volume, are summarized to guide the successful fabrication of specific types of plasmene nanosheets. This protocol, from initial NP synthesis to the final fabrication and characterization, takes ~33.5 h.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

Covalent-Cross-Linked Plasmene Nanosheets

Thiol-polystyrene (SH-PS)-capped plasmonic nanoparticles can be fabricated into free-standing, one-nanoparticle-thick superlattice sheets (termed plasmene) based on physical entanglement between ligands, which, however, suffer from irreversible dissociation in organic solvents. To address this issue, we introduce coumarin-based photo-cross-linkable moieties to the SH-PS ligands to stabilize gold nanoparticles. Once cross-linked, the obtained plasmene nanosheets consisting of chemically locked nanoparticles can well maintain structural integrity in organic solvents. Particularly, arising from ligand-swelling-induced enlargement of the interparticle spacing, these plasmene nanosheets show significant optical responses to various solvents in a specific as well as reversible manner, which may offer an excellent material for solvent sensing and dynamic plasmonic display.

Symmetry-Breaking Synthesis of Multicomponent Nanoparticles

Multicomponent nanoparticles (MCNs) composed of disparate inorganic colloidal components have attracted great attention from researchers in both the academic and industrial community, because of their unique properties and diverse applications in energy conversion and storage; heterogeneous catalysis; optics and electronics; and biomedical imaging, diagnosis, and therapy. Compared with single-component nanoparticles (NPs), new or advanced properties of MCNs arise from the synergistic effect between their constituent components and the presence of nanoscale interfaces between distinct materials within the particles. Consequently, the spatial arrangement of nanoscale domains of MCNs becomes equally important in property or function control of MCNs as their size, shape, and composition, if not more. In particular, compositionally asymmetric MCNs may outperform their symmetric counterparts in many of their applications. To this end, the seed-mediated growth (SMG) method, which involves depositing a second material onto seed NPs, has been considered as the most common strategy for the synthesis of asymmetric MCNs with desired complexity. In this approach, the control of symmetry breaking during MCN growth is usually achieved by manipulating the growth kinetics or using seed NPs with asymmetric shapes or surfaces. Although great progress has been made in the past decade, there remains a challenge to control the shape, orientation and organization of colloidal components of MCNs with a high yield and reproducibility. Recently, several unconventional methods have been developed as an important addition to the synthetic toolbox for the production of complex MCNs that otherwise may not be readily attainable. This Account summarizes recent advancements on the development of unconventional synthetic strategies for breaking the growth symmetry in the synthesis of asymmetric MCNs. We start with a brief discussion of the achievements and limitations of the conventional strategies for symmetry breaking synthesis. In the subsequent section, we present three unconventional approaches toward symmetry-breaking synthesis of asymmetric MCNs, namely, surface-protected growth, interface-guided growth, and welding-induced synthesis. First, we discuss how commonly used soft agents (e.g., collapsed polymer) and hard agents (e.g., silica) can be asymmetrically coated on seed NPs to template the asymmetric growth of secondary material, generating a broad range of MCNs with complex architectures. The unique features and key factors of this surface-protected synthesis are discussed from the viewpoints of the surface chemistry of seed NPs. We further discuss the use of a solid/liquid or liquid/liquid interface to guide the synthesis of Janus or more complex MCNs through two general mechanisms; that is, selective blocking or impeding the access of precursors to one side of seed NPs and interfacial reaction-enabled generation of asymmetric seeds for further growth. Finally, we discuss a symmetry-breaking method beyond the SMG mechanism, directed welding of as-synthesized single-component NPs. Moreover, we discuss how the unique structural symmetry and compositional arrangement of these MCNs significantly alter the physical and chemical properties of MCNs, thus facilitating their performance in exemplary applications of photocatalysis and electrocatalysis. We finally conclude this Account with a summary of recent progress and our future perspective on the future challenges.

Tailoring patches on particles: a modified microcontact printing routine using polymer-functionalised stamps

Herein, we report a modified microcontact printing (μCP) routine suitable to introduce particle patches of a low molecular weight ink (LMWI) on porous SiO2 microparticles. Thereby, patch precision could be significantly improved by utilising stamps which have been surface-functionalised with grafted polymers. This improvement was evaluated by a profound software-assisted statistical analysis.

Arrays of highly complex noble metal nanostructures using nanoimprint lithography in combination with liquid-phase epitaxy

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core-shell, core-void-shell, and core-void-nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.