Spatially confined assembly of nanoparticles

Nanoparticle assembly by transient topography induced by applying soft lithography to block copolymer films

We present a simple approach for patterning metal nanoparticles into periodic superstructures on flat films spanning centimeter-square areas. Our approach is based on capillary force lithography, a soft lithography method that is used to impart topography to molten polymer films, and applies it to block copolymer films to obtain substrates featuring both topographic and chemical contrasts that can serve as templates for the selective deposition of nanoparticles. Here we show that flattening the films by exposure to solvent vapour prior to nanoparticle deposition not only retains chemical heterogeneity but also provides access to unique hierarchically-organized nanoparticle superstructures that are unattainable by other methods. Such structures could be useful for optical, sensor, and catalytic applications.

Hierarchically Structured Nanocomposites via Mixed-Graft Block Copolymer Templating: Achieving Controlled Nanostructure and Functionality

Integrating inorganic and polymerized organic functionalities to create composite materials presents an efficient strategy for the discovery and fabrication of multifunctional materials. The characteristics of these composites go beyond a simple sum of individual component properties; they are profoundly influenced by the spatial arrangement of these components and the resulting homo-/hetero-interactions. In this work, we develop a facile and highly adaptable approach for crafting nanostructured polymer-inorganic composites, leveraging hierarchically assembling mixed-graft block copolymers (mGBCPs) as templates. These mGBCPs, composed of diverse polymeric side chains that are covalently tethered with a defined sequence to a linear backbone polymer, self-assemble into ordered hierarchical structures with independently tuned nano- and mesoscale lattice features. Through the coassembly of mGBCPs with diversely sized inorganic fillers such as metal ions (ca. 0.1 nm), metal oxide clusters (0.5-2 nm), and metallic nanoparticles (>2 nm), we create three-dimensional filler arrays with controlled interfiller separation and arrangement. Multiple types of inorganic fillers are simultaneously integrated into the mGBCP matrix by introducing orthogonal interactions between distinct fillers and mGBCP side chains. This results in nanocomposites where each type of filler is selectively segregated into specific nanodomains with matrix-defined orientations. The developed coassembly strategy offers a versatile and scalable pathway for hierarchically structured nanocomposites, unlocking new possibilities for advanced materials in the fields of optoelectronics, sensing, and catalysis.

Magnetically controlled assembly: a new approach to organic integrated photonics

Hierarchical self-assembly of organic molecules or assemblies is of great importance for organic photonics to move from fundamental research to integrated and practical applications. Magnetic fields with the advantages of high controllability, non-contact manipulation, and instantaneous response have emerged as an elegant way to prepare organic hierarchical nanostructures. In this perspective, we outline the development history of organic photonic materials and highlight the importance of organic hierarchical nanostructures for a wide range of applications, including microlasers, optical displays, information encoding, sensing, and beyond. Then, we will discuss recent advances in magnetically controlled assembly for creating organic hierarchical nanostructures, with a particular focus on their potential for enabling the development of integrated photonic devices with unprecedented functionality and performance. Finally, we present several perspectives on the further development of magnetically controlled assembly strategies from the perspective of performance optimization and functional design of organic integrated photonics.

Core-Shell Particles: From Fabrication Methods to Diverse Manipulation Techniques

Core-shell particles are micro- or nanoparticles with solid, liquid, or gas cores encapsulated by protective solid shells. The unique composition of core and shell materials imparts smart properties on the particles. Core-shell particles are gaining increasing attention as tuneable and versatile carriers for pharmaceutical and biomedical applications including targeted drug delivery, controlled drug release, and biosensing. This review provides an overview of fabrication methods for core-shell particles followed by a brief discussion of their application and a detailed analysis of their manipulation including assembly, sorting, and triggered release. We compile current methodologies employed for manipulation of core-shell particles and demonstrate how existing methods of assembly and sorting micro/nanospheres can be adopted or modified for core-shell particles. Various triggered release approaches for diagnostics and drug delivery are also discussed in detail.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Wearable SERS Sensor Based on Omnidirectional Plasmonic Nanovoids Array with Ultra-High Sensitivity and Stability

Surface-enhanced Raman spectroscopy (SERS) is a promising technology for wearable sensors due to its fingerprint spectrum and high detection sensitivity. However, since SERS-activity is sensitive to both the distribution of "hotspots" and excitation angle, it is profoundly challenging to develop a wearable SERS sensor with high stability under various deformations during movements. Herein, inspired by omnidirectional light-harvesting of the compound eye of Xenos Peckii, a wearable SERS sensor is developed using omnidirectional plasmonic nanovoids array (OPNA), which is prepared by assembling a monolayer of metal nanoparticles into the artificial plasmonic compound-eye (APC). Specifically, APC is an interconnected frame containing omnidirectional "pockets" and acts as an "armour", not only rendering a broadband and omnidirectional enhancement of "hotspots" in the delicate nanoparticles array, but also maintaining an integrity of the "hotspots" against external mechanical deformations. Furthermore, an asymmetry super-hydrophilic pattern is fabricated on the surface of OPNA, endowing the hydrophobic OPNA with the ability to spontaneously extract and concentrate the analytes from sweat. Such an armored SERS sensor can enable the wearable and in situ analysis with high sensitivity and stability, exhibiting great potential in point-of-care analysis.

Barrier-free liquid condensates of nanocatalysts as effective concentrators of catalysis

Traditional methods of molecular confinement have physicochemical barriers that restrict the free passage of substrates/products. Here, we explored liquid-liquid phase separation as a method to restrain protein-metal nanocomposites within barrier-free condensates. Confinement within liquid droplets was independent of the protein's native conformation and amplified the catalytic efficiency of metal nanocatalysts by one order of magnitude.

Influence of anisotropy on heterogeneous nucleation of gold nanorod assemblies

In this study, we analysed for the first time heterogeneous nucleation with anisotropic nanoparticles as a model system for non-spherical building units on the nanoscale. Gold nanorods were synthesised and assembled to investigate the phenomenon of heterogeneous nucleation. To determine the influence of the particle shape on heterogeneous nucleation, we utilised gold nanorods with varying aspect ratios, ranging from 3.00 and 2.25 to 1.75, while keeping the surface chemistry constant. First, the nucleation of the gold nanorod assemblies in solution and the process kinetics were analyzed with UV-vis-NIR spectroscopy followed by a microscopic examination of the gold nanorod-based superstructures formed heterogeneously on substrates. Here, positively charged cetyltrimethylammonium bromide (CTAB)-functionalized gold nanorods and negatively charged polystyrene sulfonate (PSS) functionalized substrates ensured the directed heterogeneous nucleation on the substrates. A combination of light microscopy with simultaneous UV-vis-NIR spectroscopy allowed us to observe the gold nanorod-based superstructure formation on the substrates in situ and to determine the nucleation rates of the process. We analysed the resulting data with the classical nucleation theory, which revealed a dominating kinetic term and a negligible thermodynamic term in contrast to ionic systems like calcium carbonate. Our studies consistently exhibit an influence of the aspect ratio on the nucleation behaviour resulting in faster nucleation of superstructures as the aspect ratio decreases. Hence our studies show unprecedented insight into the influence of particle anisotropy on the nucleation and growth of nanorod-based superstructures and reveal significant differences in the nucleation of nanoparticle building units compared to the nucleation of atoms or molecules as building units.

Flexible 2D nanocellulose-based SERS substrate for pesticide residue detection

Nanocellulose holds considerable promise as an effective surface-enhanced Raman scattering (SERS) substrate for sensitive detection of trace targets. Flexible and high-sensitivity two-dimensional (2D) SERS substrates based on nanocrystalline cellulose (CNC) film were successfully developed via self assembly of two plasma nanoparticles: gold nanoflowers (AuNFs) and silver-coated gold nanocubes (Au@AgNCs). The loading process allows the precise control of nanoparticle distribution density and uniformity on CNC film, which are closely related to the plasma coupling effect between particles. The obtained CNC/Au@AgNC flexible two-dimensional substrate could sensitively detect pesticide residues on apple surface, and the detection limits (LOD) of dimethoate and acetamiprid were 4.1 and 10.7 μg/L, respectively. In addition, Raman signal intensity showed a good linear relationship with pesticide concentration in the range of 10-100 μg/L, which provided great potential for high sensitivity and field detection of dangerous targets.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Self-assembly of colloidal inorganic nanocrystals: nanoscale forces, emergent properties and applications

The self-assembly of colloidal nanoparticles has made it possible to bridge the nanoscopic and macroscopic worlds and to make complex nanostructures. The nanoparticle-mediated assembly enables many potential applications, from biodetection and nanomedicine to optoelectronic devices. Properties of assembled materials are determined not only by the nature of nanoparticle building blocks, but also by spatial positions of nanoparticles within the assemblies. A deep understanding of nanoscale interactions between nanoparticles is a prerequisite to controlling nanoparticle arrangement during assembly. In this review, we present an overview of interparticle interactions governing their assembly in a liquid phase. Considerable attention is devoted to examples that illustrate nanoparticle assembly into ordered superstructures using different types of building blocks, including plasmonic nanoparticles, magnetic nanoparticles, lanthanide-doped nanophosphors, and quantum dots. We also cover the physicochemical properties of nanoparticle ensembles, especially those arising from particle coupling effects. We further discuss future research directions and challenges in controlling self-assembly at a level of precision that is most crucial to technology development.

Deterministic Assembly of Single Sub-20 nm Functional Nanoparticles Using a Thermally Modified Template with a Scanning Nanoprobe

A deterministic assembly technique for single sub-20 nm functional nanoparticles is developed based on nanostructured templates fabricated by hot scanning nanoprobes. With this technique, single nanoparticles including quantum dots, polystyrene fluorescent nanobeads, and gold nanoparticles are successfully assembled into 2D arrays with high yields. Experimental and theoretical analyses show that the key for the high yields is the hot-probe-based template fabrication technique, which creates geometrical nanotraps and modifies their surface energy simultaneously. In addition to single nanoparticle patterning, further experiments demonstrate that this technique is also capable of building complex nanostructures, such as nanoparticle clusters with well-defined shapes and heterogeneously integrated nanostructures consisting of quantum dots and silver nanowires. It opens the door to many important applications.

Shaping Metallic Nanolattices: Design by Microcontact Printing from Wrinkled Stamps

A method for the fabrication of well-defined metallic nanostructures is presented here in a simple and straightforward fashion. As an alternative to lithographic techniques, this routine employs microcontact printing utilizing wrinkled stamps, which are prepared from polydimethylsiloxane (PDMS), and includes the formation of hydrophobic stripe patterns on a substrate via the transfer of oligomeric PDMS. Subsequent backfilling of the interspaces between these stripes with a hydroxyl-functional poly(2-vinyl pyridine) then provides the basic pattern for the deposition of citrate-stabilized gold nanoparticles promoted by electrostatic interaction. The resulting metallic nanostripes can be further customized by peeling off particles in a second microcontact printing step, which employs poly(ethylene imine) surface-decorated wrinkled stamps, to form nanolattices. Due to the independent adjustability of the period dimensions of the wrinkled stamps and stamp orientation with respect to the substrate, particle arrays on the (sub)micro-scale with various kinds of geometries are accessible in a straightforward fashion. This work provides an alternative, cost-effective, and scalable surface-patterning technique to fabricate nanolattice structures applicable to multiple types of functional nanoparticles. Being a top-down method, this process could be readily implemented into, e.g., the fabrication of optical and sensing devices on a large scale.

Multiplexed Assembly of Plasmonic Nanostructures Through Charge Inversion on Substrate for Surface Encoding

Plasmonic nanomaterials are excellent and promising building blocks for information encoding and decoding. However, the positioning of multiplexed nanomaterials into recognizable structures remains a major challenge in nanotechnology. Herein, we developed a novel method for fabricating diversified nanostructures through surface charge inversion from amino-modified substrates to carboxyl-modified ones, as well as the corresponding electrostatic-induced assembly of metal nanoparticles. Under optimal conditions, the selected gold nanospheres (NSs) and peanut-like gold nanorods were successively located into patterns of spaced lines on the same substrate. Due to their unique optical properties, these two types of designed nanoarrays exhibited distinct color contrast and spectrum difference under dark-field scattering microscopy. Furthermore, this general strategy can be extended to wide ranges of nanoparticles with different morphologies and compositions for other multifunctional and high-demanding encoding applications.

Direct Wavelength-Selective Optical and Electron-Beam Lithography of Functional Inorganic Nanomaterials

Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is a photoresist-free method for high-resolution patterning of inorganic nanocrystals (NCs) that has been demonstrated using deep UV (DUV, 254 nm) photons. Here, we expand the versatility of DOLFIN by designing a series of photochemically active NC surface ligands for direct patterning using various photon energies including DUV, near-UV (i-line, 365 nm), blue (h-line, 405 nm), and visible (450 nm) light. We show that the exposure dose for DOLFIN can be ∼30 mJ/cm², which is small compared to most commercial photopolymer resists. Patterned nanomaterials can serve as highly robust optical diffraction gratings. We also introduce a general approach for resist-free direct electron-beam lithography of functional inorganic nanomaterials (DELFIN) which enables all-inorganic NC patterns with feature size down to 30 nm, while preserving the optical and electronic properties of patterned NCs. The designed ligand chemistries and patterning techniques offer a versatile platform for nano- and micron-scale additive manufacturing, complementing the existing toolbox for device fabrication.

Strongly confined localized surface plasmon resonance (LSPR) bands of Pt, AgPt, AgAuPt nanoparticles

Multi-metallic alloy nanoparticles (NPs) can enable the advanced applications in the energy, biology, electronics, optics and catalysis due to their multi-functionality, wide tunable range and electronic heterogeneity. In this work, various mono-, bi- and tri-metallic nanostructures composed of Ag, Au and Pt are demonstrated on transparent c-plane sapphire (0001) substrates and the corresponding morphological and optical characteristics are thoroughly investigated. The resulting Pt and AuPt NPs in this study demonstrate much enhanced LSPR responses as compared to the pure Pt NPs from the previous studies, which was contributed by the synergistic effect of Au and Pt and improved surface morphology. These results are sharply distinct in terms of surface morphology and elemental variability from those obtained by the dewetting of monometallic Ag, Au and Pt films under the similar growth conditions, which is due to the distinct dewetting kinetics of the bi-layer and tri-layer films. These NPs exhibit strongly enhanced localized surface plasmon resonance (LSPR) bands in the UV-VIS wavelengths such as dipolar, quadrupolar, multipolar and higher order resonance modes depending upon the size and elemental composition of NPs. The LSPR bands are much stronger with the high Ag content and gradually attenuated with the Ag sublimation. Furthermore, the VIS region LSPR bands are readily blue shifted along with the reduction of NP size. The Ag/Pt bi-layers and Ag/Au/Pt tri-layers are systematically dewetted and transformed into various AgPt and AgAuPt nanostructures such as networked, elongated and semispherical configurations by means of enhanced surface diffusion, intermixing and energy minimization along with the temperature control. The sublimation of Ag atoms plays a significant role in the structural and elemental composition of NPs such that more isolated and semispherical Pt and AuPt NPs are evolved from the AgPt and AgAuPt NPs respectively.

Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes

We demonstrate the formation of three morphologies relevant for integration with miniaturized devices-microscale pillars, conformal coatings, and self-supported membranes-via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle-particle and particle-template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model "building blocks," remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 μm × 1 μm × 0.6 μm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.

Combining dielectrophoresis and concentration polarization-based preconcentration to enhance bead-based immunoassay sensitivity

Ionic concentration-polarization (CP)-based biomolecule preconcentration is an established method for enhancing the detection sensitivity of a target biomolecule immunoassay. However, its main drawback lies in its inability to directly control the spatial overlap between the preconcentrated plug of biomolecules and the surface immobilized antibodies. To overcome this, we simultaneously preconcentrated freely suspended, surface functionalized nanoparticles and target molecules along the edge of a depletion layer, thus, increasing the binding kinetics and avoiding the need to tune their relative locations to ensure their spatial overlap. After the desired incubation time, the nanoparticles were dielectrophoretically trapped for postprocessing analysis of the binding signal. This novel combination of CP-based preconcentration and dielectrophoresis (DEP) was demonstrated through binding of avidin and biotin-conjugated particles as a model bead-based immunoassay, wherein increased detection sensitivity was demonstrated compared to an immunoassay without CP-based preconcentration. The DEP trapping of the beads following binding is important not only for an enhanced detection signal due to the preconcentration of the beads at the electrode edges but also for controlling their location for future applications integrating localized sensors. In addition, DEP may be important also as a preprocessing step for controlling the number of beads participating in the immunoassay.

Softening gold for elastronics

Gold, one of the noble metals, has played a significant role in human society throughout history. Gold's excellent electrical, optical and chemical properties make the element indispensable in maintaining a prosperous modern electronics industry. In the emerging field of stretchable electronics (elastronics), the main challenge is how to utilize these excellent material properties under various mechanical deformations. This review covers the recent progress in developing "softening" gold chemistry for various applications in elastronics. We systematically present material synthesis and design principles, applications, and challenges and opportunities ahead.

Improved control on the morphology and LSPR properties of plasmonic Pt NPs through enhanced solid state dewetting by using a sacrificial indium layer

Platinum (Pt) nanoparticles (NPs) are important nano-material components in various catalytic, photonic and electronic applications, yet face challenges in the fabrication of desired morphology and uniformity with the conventional solid-state dewetting approach. Specifically, the necessity of high annealing temperatures, typically above 800 °C due to the low diffusivity of Pt atoms, limits the morphological and functional tunability of Pt NPs. In this work, the fabrication of Pt NPs with an improved configuration, spacing and uniformity is demonstrated through the enhancement of solid state dewetting by using a sacrificial indium (In) layer on sapphire (0001). The well-defined Pt NPs demonstrate the dynamic localized surface plasmon (LSPR) bands in the visible range between ∼400 and 700 nm depending on the size and spacing of NPs. The LSPR peak intensity and width are also varied depending on the uniformity of Pt NPs. The overall dewetting magnitude is significantly enhanced through the inter-mixing of In and Pt atoms at the In/Pt interface that eventually results in the formation of an In–Pt alloy. During the dewetting process the In atoms desorb from the NP matrix by atomic sublimation, which gives rise to pure Pt NP fabrication. In sharp contrast to the pure Pt film dewetting, the Pt NPs in this approach demonstrate significantly improved spatial arrangement with well-defined configuration and uniformity. In addition, the ratio of In can be readily controlled along with the thickness of the Pt layer to alter the dewetting kinetics and thereby the surface morphology of Pt NPs. Specifically, large hexagonal, semi-spherical and small hexagonal Pt NPs are obtained through the dewetting of In_{75 nm}/Pt_{25 nm}, In_{20 nm}/Pt_{20 nm} and In_{2.5 nm}/Pt_{7.5 nm} bilayers respectively.

Fabrication of Pt NPs with the improved configuration, spacing, uniformity and localized surface plasmon resonance (LSPR) response is demonstrated.

Programmable Negative Differential Resistance Effects Based on Self-Assembled Au@PPy Core-Shell Nanoparticle Arrays

The negative differential resistance (NDR) effect observed in conducting polymer/Au nanoparticle composite devices is not yet fully clarified due to the random and disordered incorporation of Au nanoparticles into conducting polymers. It remains a formidable challenge to achieve the sequential arrangement of various components in an optimal manner during the fabrication of Au nanoparticle/conducting polymer composite devices. Here, a novel strategy for fabricating Au nanoparticle/conducting polymer composite devices based on self-assembled Au@PPy core-shell nanoparticle arrays is demonstrated. The interval between the two Au nanoparticles can be precisely programmed by modulating the thickness of the shell and the size of the core. Programmable NDR is achieved by regulating the spacer between two Au nanoparticles. In addition, the Au/conducting polymer composite device exhibits a reproducible memory effect with read-write-erase characteristics. The sequentially controllable assembly of Au@PPy core-shell nanoparticle arrays between two microelectrodes will simplify nanodevice fabrication and will provide a profound impact on the development of new approaches for Au/conducting polymer composite devices.

Directed Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates

The assembly of nanoscale materials into arbitrary, organized structures remains a major challenge in nanotechnology. Herein, we report a general method for creating 2D structures by combining top-down lithography with bottom-up chemical assembly. Under optimal conditions, the assembly of gold nanoparticles was achieved in less than 30 min. Single gold nanoparticles, from 10 to 100 nm, can be placed in predetermined patterns with high fidelity, and higher-order structures can be generated consisting of dimers or trimers. It is shown that the nanoparticle arrays can be transferred to, and embedded within, polymer films. This provides a new method for the large-scale fabrication of nanoparticle arrays onto diverse substrates using wet chemistry.

Vapor-Phase Nanopatterning of Aminosilanes with Electron Beam Lithography: Understanding and Minimizing Background Functionalization

Electron beam lithography (EBL) is a highly precise, serial method for patterning surfaces. Positive tone EBL resists enable patterned exposure of the underlying surface, which can be subsequently functionalized for the application of interest. In the case of widely used native oxide-capped silicon surfaces, coupling an activated silane with electron beam lithography would enable nanoscale chemical patterning of the exposed regions. Aminoalkoxysilanes are extremely useful due to their reactive amino functionality but have seen little attention for nanopatterning silicon surfaces with an EBL resist due to background contamination. In this work, we investigated three commercial positive tone EBL resists, PMMA (950k and 495k) and ZEP520A (57k), as templates for vapor-phase patterning of two commonly used aminoalkoxysilanes, 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyldiisopropylethoxysilane (APDIPES). The PMMA resists were susceptible to significant background reaction within unpatterned areas, a problem that was particularly acute with APTMS. On the other hand, with both APTMS and APDIPES exposure, unpatterned regions of silicon covered by the ZEP520A resist emerged pristine, as shown both with SEM images of the surfaces of the underlying silicon and through the lack of electrostatically driven binding of negatively charged gold nanoparticles. The ZEP520A resist allowed for the highly selective deposition of these alkoxyaminosilanes in the exposed areas, leaving the unpatterned areas clean, a claim also supported by contact angle measurements with four probe liquids and X-ray photoelectron spectroscopy (XPS). We investigated the mechanistic reasons for the stark contrast between the PMMA resists and ZEP520A, and it was found that the efficacy of resist removal appeared to be the critical factor in reducing the background functionalization. Differences in the molecular weight of the PMMA resists and the resulting influence on APTMS diffusion through the resist films are unlikely to have a significant impact. Area-selective nanopatterning of 15 nm gold nanoparticles using the ZEP520A resist was demonstrated, with no observable background conjugation noted in the unexposed areas on the silicon surface by SEM.

Tailoring two-dimensional nanoparticle arrays into various patterns

A simple and effective technique has been developed to fabricate patterns of nanoparticle arrays. Lithographically fabricated structures in resists serve as scissors to tailor two-dimensional nanoparticle arrays on a flat poly(dimethylsiloxane) (PDMS) stamp. The desired patterns of nanoparticle arrays remaining on the PDMS stamp after tailoring can be printed onto solid substrates. Various regular nanoparticle patterns, such as squares, triangles, disks, and pentagons, can be easily prepared using this technique. Arbitrary nanoparticle patterns as complex as Chinese characters have been successfully demonstrated. Moreover, nanoparticle stripes with width ranging from micrometers to quasi single nanoparticle diameter have also been achieved. Nanoparticle stripes have been integrated into electronic devices for transport measurements.

Unexpected formation of gold nanoflowers by a green synthesis method as agents for a safe and effective photothermal therapy

Star fruit (Averrhoa carambola) juice rich in vitamin C and polyphenolic antioxidants was used to synthesize branched gold nanoflowers. These biocompatible and stable gold nanoflowers show strong near-infrared absorption. They are successfully demonstrated to be highly efficient for both in vitro and in vivo photothermal therapy by using an 808 nm laser.

Evaporative Optical Marangoni Assembly: Tailoring the Three-Dimensional Morphology of Individual Deposits of Nanoparticles from Sessile Drops

We have recently devised the evaporative optical Marangoni assembly (eOMA), a novel and versatile interfacial flow-based method for directing the deposition of colloidal nanoparticles (NPs) on solid substrates from evaporating sessile drops along desired patterns using shaped UV light. Here, we focus on a fixed UV spot irradiation resulting in a cylinder-like deposit of assembled particles and show how the geometrical features of the single deposit can be tailored in three dimensions by simply adjusting the optical conditions or the sample composition, in a quantitative and reproducible manner. Sessile drops containing cationic NPs and a photosensitive surfactant at various concentrations are allowed to evaporate under a single UV beam with a diameter much smaller than that of the drop. After complete evaporation, the geometrical characteristics of the NP deposits are precisely assessed using optical profilometry. We show that both the volume and the radial size of the light-directed NP deposit can be adjusted by varying the diameter or the intensity of the UV beam or alternatively by changing the concentration of the photosensitive surfactant. Notably, in all these cases, the deposits display an almost constant median height corresponding to a few layers of particles. Moreover, both the radial and the axial extent of the patterns are tuned by changing the NP concentration. These results are explained by the correlation among the strength of Marangoni flow, the particle trapping efficiency, and the volume of the deposit, and by the role of evaporation-driven flow in strongly controlling the deposit height. Finally, we extend the versatility of eOMA by demonstrating that NPs down to 30 nm in diameter can be effectively patterned on glass or polymeric substrates.

Assembly of 1D Granular Structures from Sulfonated Polystyrene Microparticles

Being able to systematically modify the electric properties of nano- and microparticles opens up new possibilities for the bottom-up fabrication of advanced materials such as the fabrication of one-dimensional (1D) colloidal and granular materials. Fabricating 1D structures from individual particles offers plenty of applications ranging from electronic sensors and photovoltaics to artificial flagella for hydrodynamic propulsion. In this work, we demonstrate the assembly of 1D structures composed of individual microparticles with modified electric properties, pulled out of a liquid environment into air. Polystyrene particles were modified by sulfonation for different reaction times and characterized by dielectric spectroscopy and dipolar force measurements. We found that by increasing the sulfonation time, the values of both electrical conductivity and dielectric constant of the particles increase, and that the relaxation frequency of particle electric polarization changes, causing the measured dielectric loss of the particles to shift towards higher frequencies. We attributed these results to water adsorbed at the surface of the particles. With sulfonated polystyrene particles exhibiting a range of electric properties, we showed how the electric properties of individual particles influence the formation of 1D structures. By tuning applied voltage and frequency, we were able to control the formation and dynamics of 1D structures, including chain bending and oscillation.

Dynamics of Templated Assembly of Nanoparticle Filaments within Nanochannels

Nanoparticles (NPs) can self-assemble into complex, organized superstructures on patterned surfaces through fluid-mediated interactions. However, the detailed mechanisms for such NP assemblies are largely unknown. Here, using in situ transmission electron microscopy, the stepwise self-assembly dynamics of hydrophobic gold NPs into long filaments formed on the surfaces of water-filled patterned nanochannel templates is observed. First, the formation of a meniscus between the nanochannel walls, during the slow drying of water, causes accumulation of the NPs in the middle of the nanochannels. Second, owing to the strong van der Waals attraction between the NP ligands, the NPs condense into filaments along the centers of the nanochannels. Filaments with highly fluctuating longitudinal NP densities are also observed to fragment into separated structures. Understanding the intermediate stages of fluid-mediated NP self-assembly on patterned surfaces will have important implications for the controlled formation of templated NP assemblies with numerous applications.

When lithography meets self-assembly: a review of recent advances in the directed assembly of complex metal nanostructures on planar and textured surfaces

One of the foremost challenges in nanofabrication is the establishment of a processing science that integrates wafer-based materials, techniques, and devices with the extraordinary physicochemical properties accessible when materials are reduced to nanoscale dimensions. Such a merger would allow for exacting controls on nanostructure positioning, promote cooperative phenomenon between adjacent nanostructures and/or substrate materials, and allow for electrical contact to individual or groups of nanostructures. With neither self-assembly nor top-down lithographic processes being able to adequately meet this challenge, advancements have often relied on a hybrid strategy that utilizes lithographically-defined features to direct the assembly of nanostructures into organized patterns. While these so-called directed assembly techniques have proven viable, much of this effort has focused on the assembly of periodic arrays of spherical or near-spherical nanostructures comprised of a single element. Work directed toward the fabrication of more complex nanostructures, while still at a nascent stage, has nevertheless demonstrated the possibility of forming arrays of nanocubes, nanorods, nanoprisms, nanoshells, nanocages, nanoframes, core-shell structures, Janus structures, and various alloys on the substrate surface. In this topical review, we describe the progress made in the directed assembly of periodic arrays of these complex metal nanostructures on planar and textured substrates. The review is divided into three broad strategies reliant on: (i) the deterministic positioning of colloidal structures, (ii) the reorganization of deposited metal films at elevated temperatures, and (iii) liquid-phase chemistry practiced directly on the substrate surface. These strategies collectively utilize a broad range of techniques including capillary assembly, microcontact printing, chemical surface modulation, templated dewetting, nanoimprint lithography, and dip-pen nanolithography and employ a wide scope of chemical processes including redox reactions, alloying, dealloying, phase separation, galvanic replacement, preferential etching, template-mediated reactions, and facet-selective capping agents. Taken together, they highlight the diverse toolset available when fabricating organized surfaces of substrate-supported nanostructures.

Morphological effects on the selectivity of intramolecular versus intermolecular catalytic reaction on Au nanoparticles

It is hard for metal nanoparticle catalysts to control the selectivity of a catalytic reaction in a simple process. In this work, we obtain active Au nanoparticle catalysts with high selectivity for the hydrogenation reaction of aromatic nitro compounds, by simply employing spine-like Au nanoparticles. The density functional theory (DFT) calculations further elucidate that the morphological effect on thermal selectivity control is an internal key parameter to modulate the nitro hydrogenation process on the surface of Au spines. These results show that controlled morphological effects may play an important role in catalysis reactions of noble metal NPs with high selectivity.

Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics

Molecular plasmonics uses and explores molecule-plasmon interactions on metal nanostructures for spectroscopic, nanophotonic, and nanoelectronic devices. This review focuses on tailored surfaces/assemblies for molecular plasmonics and describes active molecular plasmonic devices in which functional molecules and polymers change their structural, electrical, and/or optical properties in response to external stimuli and that can dynamically tune the plasmonic properties. We also explore an emerging research field combining molecular plasmonics and molecular electronics.

Formation of printable granular and colloidal chains through capillary effects and dielectrophoresis

One-dimensional conductive particle assembly holds promise for a variety of practical applications, in particular for a new generation of electronic devices. However, synthesis of such chains with programmable shapes outside a liquid environment has proven difficult. Here we report a route to simply ‘pull' flexible granular and colloidal chains out of a dispersion by combining field-directed assembly and capillary effects. These chains are automatically stabilized by liquid bridges formed between adjacent particles, without the need for continuous energy input or special particle functionalization. They can further be deposited onto any surface and form desired conductive patterns, potentially applicable to the manufacturing of simple electronic circuits. Various aspects of our route, including the role of particle size and the voltages needed, are studied in detail. Looking towards practical applications, we also present the possibility of two-dimensional writing, rapid solidification of chains and methods to scale up chain production.

Conductive colloidal chains are promising for electronics but difficult to synthesize outside of a liquid environment. Here, the authors use field-directed assembly and capillary effects to pull conductive particle chains out of a suspension, which remain held together by flexible liquid bridges even after the external field is turned off.

Colloidal Synthesis and Applications of Plasmonic Metal Nanoparticles

Plasmonic metal nanoparticles attract intense research attention because of their fascinating surface plasmon resonance properties and their potential applications in diverse fields. Here, some of the recent research efforts on the synthesis and applications of plasmonic metal nanoparticles are highlighted. Starting from the colloidal synthesis of metal nanoparticles, various shaped silver and gold nanostructures are discussed. The applications of plasmonic nanoparticles in photocatalysis, surface-enhanced Raman spectroscopy (SERS), and devices are used as excellent examples showcasing the advantages of these nanoparticles. The report closes with a brief summary and discussion on the challenges and future direction in this research field.

Scalable Fabrication of Multiplexed Plasmonic Nanoparticle Structures Based on AFM Lithography

A controllable and scalable strategy is developed to fabricate multiplexed plasmonic nanoparticle structures by mechanical scratching with AFM lithography, which exhibit multiplex plasmonic properties and surface-enhanced Raman scattering responses. It offers an intuitive way to explore the plasmonic effects on the performance of an organic light-emitting diode device integrating with multiplexed plasmonic nanostructures.

Electron microscopy reveals a soluble hybrid network of individual nanocrystals self-anchored by bifunctional thiol fluorescent bridges

Today, nanochemistry research of hybrid materials growth in liquid media represents a new challenge for tailoring specific nano-sized materials directly related to the hybrid electron-optical properties. Distinctive assumptions about the origin, the growth, and the functionalization of hybrid nanoparticles have recently been proposed by scientific research to attend the different aspects of observable behaviors. Therefore, appropriate morpho-structural observation of the hybrid nanoparticles is the most important factor for controlling the chemical and physical properties. Here, we report how the gold nanocrystals (Au-NCs) structurally covered by an outer layer material of 9,9-didodecyl-2,7-bisthiofluorene (FL) bifunctional stabilizer evolve into a self-organized 2D-network as a function of different nano-structural features. Detailed morpho-structural investigation of this hybrid material through electron microscopy techniques has been performed from the atomic-scale to hundreds of nanometers. The experimental information gathered allowed us to figure out the evolution growth of the gold-FL nanoparticles (AuFL-NPs) from the early stage of the gold-organic nucleation to the final assembled bi-dimensional network. The reported results represent a valuable background toward the full comprehension of growth mechanisms of organic-inorganic materials responsible for the final chemical and physical properties.

Surface-initiated ring-opening metathesis polymerization (SI-ROMP) to attach a tethered organic corona onto CdSe/ZnS core/shell quantum dots

Core-shell CdSe/ZnS quantum dots (QDs) are useful as tunable photostable fluorophores for multiple applications in industry, biology, and medicine. However, to achieve the optimum optical properties, the surface of the QDs must be passivated to remove charged sites that might bind extraneous substances and allow aggregation. Here we describe a method of growing an organic polymer corona onto the QD surface using the bottom-up approach of surface-initiated ring-opening metathesis polymerization (SI-ROMP) with Grubbs catalyst. CdSe/ZnS QDs were first coated with mercaptopropionic acid by displacing the original trioctylphosphine oxide layer, and then reacted with 7-octenyl dimethyl chlorosilane. The resulting octenyl double bonds allowed the attachment of ruthenium alkylidene groups as a catalyst. A subsequent metathesis reaction with strained bicyclic monomers (norbornene-dicarbonyl chloride (NDC), and a mixture of NDC and norbornenylethylisobutyl-polyhedral oligomeric silsesquioxane (norbornoPOSS)) allowed the construction of tethered organic homo-polymer or co-polymer layers onto the QD. Compounds were characterized by FT-IR, 1H-NMR, X-ray photoelectron spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Atomic force microscopy showed that the coated QDs were separate and non-aggregated with a range of diameter of 48-53 nm.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Recent Advances in TiO2 -Based Nanostructured Surfaces with Controllable Wettability and Adhesion

Bioinspired surfaces with special wettability and adhesion have attracted great interest in both fundamental research and industry applications. Various kinds of special wetting surfaces have been constructed by adjusting the topographical structure and chemical composition. Here, recent progress of the artificial superhydrophobic surfaces with high contrast in solid/liquid adhesion has been reviewed, with a focus on the bioinspired construction and applications of one-dimensional (1D) TiO2-based surfaces. In addition, the significant applications related to artificial super-wetting/antiwetting TiO2-based structure surfaces with controllable adhesion are summarized, e.g., self-cleaning, friction reduction, anti-fogging/icing, microfluidic manipulation, fog/water collection, oil/water separation, anti-bioadhesion, and micro-templates for patterning. Finally, the current challenges and future prospects of this renascent and rapidly developing field, especially with regard to 1D TiO2-based surfaces with special wettability and adhesion, are proposed and discussed.

Enhanced Photoresponse of Conductive Polymer Nanowires Embedded with Au Nanoparticles

A conductive polymer nanowire embedded with a 1D Au nanoparticle chain with defined size, shape, and interparticle distance is fabricated which demonstrates enhanced photoresponse behavior. The precise and controllable positioning of 1D Au nanoparticle chain in the conductive polymer nanowire plays a critical role in modulating the photoresponse behavior by excitation light wavelength or power due to the coupled-plasmon effect of 1D Au nanoparticle chain.

Orientational nanoparticle assemblies and biosensors

Assemblies of nanoparticles (NPs) have regional correlated properties with new features compared to individual NPs or random aggregates. The orientational NP assembly contributes greatly to the collective interaction of individual NPs with geometrical dependence. Therefore, orientational NPs assembly techniques have emerged as promising tools for controlling inorganic NPs spatial structures with enhanced interesting properties. The research fields of orientational NP assembly have developed rapidly with characteristics related to the different methods used, including chemical, physical and biological techniques. The current and potential applications, important challenges remain to be investigated. An overview of recent developments in orientational NPs assemblies, the multiple strategies, biosensors and challenges will be discussed in this review.

Deterministic Construction of Plasmonic Heterostructures in Well-Organized Arrays for Nanophotonic Materials

Plasmonic heterostructures are deterministically constructed in organized arrays through chemical pattern directed assembly, a combination of top-down lithography and bottom-up assembly, and by the sequential immobilization of gold nanoparticles of three different sizes onto chemically patterned surfaces using tailored interaction potentials. These spatially addressable plasmonic chain nanostructures demonstrate localization of linear and nonlinear optical fields as well as nonlinear circular dichroism.

Polymorphic transformation towards formation of nanotubes by self-assembly of an achiral molecule

In this paper, nanotubes with a uniform diameter were prepared by self-assembly of an achiral azobenzene-containing fatty acid. The polymorphic transformation of the assemblies during the cooling process was systematically studied. By controlling the incubation temperature, different morphologies, such as membranes, stripes, helical ribbons and tubes, were all obtained in our experiment. These elements were all predicted by Selinger et al. in the theoretical model of the formation of nanotubes. To the best of our knowledge, this is the first experimental example to fully support their theory.

Scaffold and scaffold-free self-assembled systems in regenerative medicine

Self-assembly in tissue engineering refers to the spontaneous chemical or biological association of components to form a distinct functional construct, reminiscent of native tissue. Such self-assembled systems have been widely used to develop platforms for the delivery of therapeutic and/or bioactive molecules and various cell populations. Tissue morphology and functional characteristics have been recapitulated in several self-assembled constructs, designed to incorporate stimuli responsiveness and controlled architecture through spatial confinement or field manipulation. In parallel, owing to substantial functional properties, scaffold-free cell-assembled devices have aided in the development of functional neotissues for various clinical targets. Herein, we discuss recent advancements and future aspirations in scaffold and scaffold-free self-assembled devices for regenerative medicine purposes. Biotechnol. Bioeng. 2016;113: 1155-1163. © 2015 Wiley Periodicals, Inc.