2D Binary Plasmonic Nanoassemblies with Semiconductor n/p-Doping-Like Properties

The electronic, optical, thermal, and magnetic properties of an extrinsic bulk semiconductor can be finely tuned by adjusting its dopant concentration. Here, it is demonstrated that such a doping concept can be extended to plasmonic nanomaterials. Using two-dimensional (2D) assemblies of Au@Ag and Au nanocubes (NCs) as a model system, detailed experimental and theoretical studies are carried out, which reveal collective semiconductor n/p-doping-like plasmonic properties. A threshold doping concentration of Au@Ag NCs is observed, below which p-doping dominates and above which n-doping prevails. Furthermore, Au@Ag NC dopants can be converted into corresponding Au seed "voids" dopants by selectively removing Ag without changing the overall structural integrity. The results show that the plasmonic doping concept may serve as a general design principle guiding synthesis and assembly of plasmonic metamaterials for programmable optoelectronic devices.

Two-dimensional gold trisoctahedron nanoparticle superlattice sheets: self-assembly, characterization and immunosensing applications

Nanoparticles were called "artificial atoms" about two decades ago due to their ability to organize into regular lattices or supracrystals. Their self-assembly into free-standing, two-dimensional (2D) nanoparticle arrays enables the generation of 2D metamaterials for novel applications in sensing, nanophotonics and energy fields. However, their controlled fabrication is nontrivial due to the complex nanoscale forces among nanoparticle building blocks. Here, we report a new type of 2D plasmonic superlattice from high-index gold trisoctahedron (TOH) nanoparticles. TOH is an anisotropic polyhedron with 24 facets and 14 vertices. By using polymer ligands in conjunction with drying-mediated self-assembly, we obtained highly ordered 2D superlattices as quantified by synchrotron based grazing-incidence small-angle X-ray scattering (GISAXS). The plasmonic properties were optimized by adjusting the ligand length and particle size. The excellent surface-enhanced Raman scattering (SERS) performance enables us to demonstrate TOH superlattices as uniform SERS immunosubstrates with a detection limit down to 1 pg ml^-1 and a dynamic range from 1 pg ml^-1 to 100 ng ml^-1.

Shape Transformation of Constituent Building Blocks within Self-Assembled Nanosheets and Nano-origami

Self-assembly of nanoparticles represents a simple yet efficient route to synthesize designer materials with unusual properties. However, the previous assembled structures whether by surfactants, polymer, or DNA ligands are "static" or "frozen" building block structures. Here, we report the growth of transformable self-assembled nanosheets which could enable reversible switching between two types of nanosheets and even evolving into diverse third generation nanosheet structures without losing pristine periodicity. Such in situ transformation of nanoparticle building blocks can even be achieved in a free-standing two-dimensional system and three-dimensional origami. The success in such in situ nanocrystal transformation is attributed to robust "plant-cell-wall-like" ion-permeable reactor arrays from densely packed polymer ligands, which spatially define and confine nanoscale nucleation/growth/etching events. Our strategy enables efficient fabrication of nanocrystal nanosheets with programmable building blocks for innovative applications in adaptive tactile metamaterials, optoelectronic devices, and sensors.

Nanoparticle Superlattices: The Roles of Soft Ligands

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine-tune the interparticle potential as well as program lattice structures and interparticle distances - the two key parameters governing superlattice properties. This article aims to review the up-to-date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft-ligand-based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Bifunctional plasmonic-magnetic particles for an enhanced microfluidic SERS immunoassay

Surface-Enhanced Raman Scattering (SERS) is emerging as a promising strategy for the quantification of immunoglobulin G (IgG) due to its inherent high sensitivity and specificity; however, it remains challenging to integrate SERS detection with a microfluidic system in a simple, efficient and low-cost manner. Here, we report on a novel bifunctional plasmonic-magnetic particle-based immunoassay, in which plasmonic nanoparticles act as soluble SERS immunosubstrates, whereas magnetic particles are for promoting micromixing in a microfluidic chip. With this novel SERS immunosubstrate in conjunction with the unique microfluidic system, we could substantially reduce the assay time from 4 hours to 80 minutes as well as enhance the detection specificity by about 70% in comparison to a non-microfluidic immunoassay. Compared to previous microfluidic SERS systems, our strategy offers a simple microfluidic chip design with only one well for mixing, washing and detection.

Two-Dimensional Bipyramid Plasmonic Nanoparticle Liquid Crystalline Superstructure with Four Distinct Orientational Packing Orders

Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.

Free-Standing Bilayered Nanoparticle Superlattice Nanosheets with Asymmetric Ionic Transport Behaviors

Natural cell membranes can directionally and selectively regulate the ion transport, which is critical for the functioning of living cells. Here, we report on the fabrication of an artificial membrane based on an asymmetric nanoparticle superlattice bilayered nanosheet, which exhibits similar ion transport characteristics. The superlattice nanosheets were fabricated via a drying-mediated self-assembly of polystyrene-capped gold nanoparticles at the liquid-air interface. By adopting a layer-by-layer assembly process, an asymmetric nanomembrane could be obtained consisting of two nanosheets with different nanoparticle size. The resulting nanomembranes exhibit an asymmetric ion transport behavior, and diode-like current-voltage curves were observed. The asymmetric ion transport is attributed to the cone-like nanochannels formed within the membranes, upon which a simulation map was established to illustrate the relationship between the channel structure and the ionic selectivity, in consistency with our experimental results. Our superlattice nanosheet-based design presents a promising strategy for the fabrication of next-generation smart nanomembranes for rationally and selectively regulating the ion transport even at a large ion flux, with potential applications in a wide range of fields, including biosensor devices, energy conversion, biophotonics, and bioelectronics.

Tattoolike Polyaniline Microparticle-Doped Gold Nanowire Patches as Highly Durable Wearable Sensors

Wearable and highly sensitive strain sensors are essential components of electronic skin for future biomonitoring and human machine interfaces. Here we report a low-cost yet efficient strategy to dope polyaniline microparticles into gold nanowire (AuNW) films, leading to 10 times enhancement in conductivity and ∼8 times improvement in sensitivity. Simultaneously, tattoolike wearable sensors could be fabricated simply by a direct "draw-on" strategy with a Chinese penbrush. The stretchability of the sensors could be enhanced from 99.7% to 149.6% by designing curved tattoo with different radius of curvatures. We also demonstrated roller coating method to encapusulate AuNWs sensors, exhibiting excellent water resistibility and durability. Because of improved conductivity of our sensors, they can directly interface with existing wireless circuitry, allowing for fabrication of wireless flexion sensors for a human finger-controlled robotic arm system.

Plasmonic core-shell nanoparticles for SERS detection of the pesticide thiram: size- and shape-dependent Raman enhancement

We systematically investigated the size- and shape-dependent SERS activities of plasmonic core-shell nanoparticles towards detection of the pesticide thiram. Monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) were synthesized and their Ag shell thickness was precisely adjusted from ∼1 nm to ∼16 nm. All these nanoparticles were used as SERS substrates for thiram detection, and the Raman intensities with three different lasers (514 nm, 633 nm and 782 nm) were recorded and compared. Our results clearly show that: (1) the excitation wavelength discriminated particle shapes regardless of particle sizes, and the maximized Raman enhancement was observed when the excitation wavelength approaches the SERS peak (provided there is significant local electric field confinement on the plasmonic nanostructures at that wavelength); (2) at the optimized laser wavelength, the maximum Raman enhancement was achieved at a certain threshold of particle size (or silver coating thickness). By exciting particles at their optimized sizes with the corresponding optimized laser wavelengths, we achieved a detection limit of roughly around 100 pM and 80 pM for NCs and NBs, respectively.

Giant plasmene nanosheets, nanoribbons, and origami

We introduce Plasmene- in analogy to graphene-as free-standing, one-particle-thick, superlattice sheets of nanoparticles ("meta-atoms") from the "plasmonic periodic table", which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75,000. In conjunction with top-down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.

DNA based strategy to nanoparticle superlattices

Over more than 20 years of development has led to the substantial progress made in the wet chemical synthesis of elementary nanoparticle building blocks including metal nanoparticles, quantum dots, and magnetic particles. However, it remains challenging to rationally assemble them into well-defined molecule-like architectures. DNA was first used to program nanomaterials synthesis in 1996, and more recently highly-ordered structures have emerged, including finite-number assemblies (nanoparticle molecules), regularly spaced nanoparticle chains (nanoparticle polymers) and extended two- and three-dimensional ordered arrays (nanoparticle superlattices). In this review, we largely focus on the use of DNA to grow nanoparticle superlattices. First, typical synthetic approaches and characterization methodologies for monodisperse nanoparticle building blocks used in DNA-based nanoparticle superlattices are described; secondly, the viable conjugation and characterization methods are discussed; finally, the three representative self-assembly strategies are introduced in detail.