Polymer‐Based Photonic Crystals

In this report, we highlight the development of polymers as 1D photonic crystals and subsequently place special emphasis on the activities in self‐assembled block copolymers as a promising platform material for new photonic crystals. We review recent progress, including the use of plasticizer and homopolymer blends of diblock copolymers to increase periodicity and the role of self‐assembly in producing 2D and 3D photonic crystals. The employment of inorganic nanoparticles to increase the dielectric contrast and the application of a biasing field during self‐assembly to control the long‐range domain order and orientation are examined, as well as in‐situ tunable materials via a mechanochromic materials system. Finally, the inherent optical anisotropy of extruded polymer films and side‐chain liquid‐crystalline polymers is shown to provide greater degrees of freedom for further novel optical designs.     

Responsive Assembly of Upconversion Nanoparticles for pH‐Activated and Near‐Infrared‐Triggered Photodynamic Therapy of Deep Tumors

Upconversion nanoparticle (UCNP)‐mediated photodynamic therapy has shown great effectiveness in increasing the tissue‐penetration depth of light to combat deep‐seated tumors. However, the inevitable phototoxicity to normal tissues resulting from the lack of tumor selectivity remains as a major challenge. Here, the development of tumor‐pH‐sensitive photodynamic nanoagents (PPNs) comprised of self‐assembled photosensitizers grafted pH‐responsive polymeric ligands and UCNPs is reported. Under neutral pH conditions, photosensitizers aggregated in the PPNs are self‐quenched; however, upon entry into a tumor microenvironment with lower pH, the PPNs not only exhibit enhanced tumor‐cell internalization due to charge reversal but also are further disassembled into well‐dispersed nanoparticles in the endo/lysosomes of tumor cells, enabling the efficient activation of photosensitizers. The results demonstrate the attractive properties of both UCNP‐mediated deep‐tissue penetration of light and high therapeutic selectivity in vitro and in vivo.     

Single‐Mode Lasing from Colloidal Water‐Soluble CdSe/CdS Quantum Dot‐in‐Rods

Core–shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution‐phase synthesis is based on surface‐stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core–shell CdSe/CdS quantum dot‐in‐rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light‐emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot‐in‐rods. Films made from water‐soluble dot‐in‐rods show amplified spontaneous emission (ASE) with a similar threshold (130 μJ/cm²) as the pristine material (115 μJ/cm²). To demonstrate feasibility for lasing applications, self‐assembled microlasers are fabricated via the “coffee‐ring effect” that display single‐mode operation and a very low threshold of ～10 μJ/cm². The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot‐in‐rods.     

Geordnete Nanoinseln für schnellere Transistoren

The spontaneous formation of self‐assembled nanostructures in strained‐layer epitaxy offers the possibility to fabricate perfectly passivated nanostructures on planar substrate surfaces in a very simple way. However, as plentiful investigations are on these nano‐islands, as limited is the control over their exact position. Since fabrication is based on a self‐assembling process the nanostructures nucleate more or less randomly on the surface. Hence, one of the biggest challenges in this research field is the controlled ordering of self‐assembled nanostructures on a planar substrate surface. Taking the SiGe material system it is shown that pre‐patterning in combination with self‐assembly leads to long‐range ordered lines of self‐assembled nanoislands. The ordering opens the way to a new concept for faster Si‐based field effect transistors, which exploits the specific electronic properties of self‐assembled Ge/Si islands. The transistor is called 'DotFET'.     

Thermoreversible Self‐Assembly of Perfluorinated Core‐Coronas Cellulose‐Nanoparticles in Dry State

Self‐assembly of nanoparticles (NPs) forming unique structures has been investigated extensively over the past few years. However, many self‐assembled structures by NPs are irreversible, because they are generally constructed using their suspensions. It is still challenging for NPs to reversibly self‐assemble in dry state, let alone of polymeric NPs with general sizes of hundreds of nm. Herein, this study reports a new reversible self‐assembly phenomenon of NPs in dry state, forming thermoreversible strip‐like supermolecular structures. These novel NPs of around 150 nm are perfluorinated surface‐undecenoated cellulose nanoparticles (FSU‐CNPs) with a core‐coronas structure. The thermoreversible self‐assembled structure is formed after drying in the air at the interface between FSU‐CNP films and Teflon substrates. Remarkably, the formation and dissociation of this assembled structure are accompanied by a reversible conversion of the surface hydrophobicity, film transparency, and anisotropic properties. These findings show novel feasibility of reversible self‐assembly of NPs in dry state, and thereby expand our knowledge of self‐assembly phenomenon.     

Multi‐Step Crystallization of Self‐Organized Spiral Eutectics

A method for the solidification of metallic alloys involving spiral self‐organization is presented as a new strategy for producing large‐area chiral patterns with emergent structural and optical properties, with attention to the underlying mechanism and dynamics. This study reports the discovery of a new growth mode for metastable, two‐phase spiral patterns from a liquid metal. Crystallization proceeds via a non‐classical, two‐step pathway consisting of the initial formation of a polytetrahedral seed crystal, followed by ordering of two solid phases that nucleate heterogeneously on the seed and grow in a strongly coupled fashion. Crystallographic defects within the seed provide a template for spiral self‐organization. These observations demonstrate the ubiquity of defect‐mediated growth in multi‐phase materials and establish a pathway toward bottom‐up synthesis of chiral materials with an inter‐phase spacing comparable to the wavelength of infrared light. Given that liquids often possess polytetrahedral short‐range order, our results are applicable to many systems undergoing multi‐step crystallization.     

Selective Absorption of Hydrophobic Cations in Nanostructured Porous Materials from Crosslinked Hydrogen‐Bonded Columnar Liquid Crystals

A nanostructured porous material is obtained by crosslinking of a self‐assembled system consisting of columnar liquid crystals with polyamines and removal of the template. For this purpose, a columnar liquid crystal with liquid crystalline properties at room temperature is synthesized and fully characterized. The orthogonal self‐assembly of the columnar liquid crystal with polyamines (i.e., PPI dendrimers) results in the formation of nanosegregated structures. When crosslinked by photopolymerization a nanostructured crosslinked material is obtained. Partial removal of the polyamine template leads to a nanostructured porous material, which is characterized and the absorbent properties are investigated. The polarity of the porous material is probed and the porous material is used for the selective absorption of cationic dye molecules.     

Mixed Fluorinated/Hydrogenated Self‐Assembled Monolayer‐Protected Gold Nanoparticles: In Silico and In Vitro Behavior

Gold nanoparticles (AuNPs) covered with mixtures of immiscible ligands present potentially anisotropic surfaces that can modulate their interactions at complex nano–bio interfaces. Mixed, self‐assembled, monolayer (SAM)‐protected AuNPs, prepared with incompatible hydrocarbon and fluorocarbon amphiphilic ligands, are used here to probe the molecular basis of surface phase separation and disclose the role of fluorinated ligands on the interaction with lipid model membranes and cells, by integrating in silico and experimental approaches. These results indicate that the presence of fluorinated amphiphilic ligands enhances the membrane binding ability and cellular uptake of gold nanoparticles with respect to those coated only with hydrogenated amphiphilic ligands. For mixed monolayers, computational results suggest that ligand phase separation occurs on the gold surface, and the resulting anisotropy affects the number of contacts and adhesion energies with a membrane bilayer. This reflects in a diverse membrane interaction for NPs with different surface morphologies, as determined by surface plasmon resonance, as well as differential effects on cells, as observed by flow cytometry and confocal microscopy. Overall, limited changes in monolayer features can significantly affect NP surface interfacial properties, which, in turn, affect the interaction of SAM‐AuNPs with cellular membranes and subsequent effects on cells.     

Hierarchical Structured Multifunctional Self‐Cleaning Material with Durable Superhydrophobicity and Photocatalytic Functionalities

Self‐cleaning materials, which are inspired and derived from natural phenomena, have gained significant scientific and commercial interest in the past decades as they are energy‐ and labor‐saving and environmentally friendly. Several technologies are developed to obtain self‐cleaning materials. The combination of superhydrophobic and photocatalytic properties enables the efficient removal of solid particles and organic contaminations, which could reduce or damage the superhydrophobicity. However, the fragility of the nanoscale roughness of the superhydrophobic surface limits its practical application. Here, a hierarchical structure approach combining micro‐ and nanoscale architectures is created to protect the nanoscale surface roughness from mechanical damage. Glass beads of 75 µm are partially embedded into a low‐density polyethylene film. This composite surface is coated with silicone nanofilaments (SNFs) via the droplet‐assisted growth and shaping approach, providing the nanoscale surface roughness as well as the support for the photocatalyst with enlarged surface area. TiO₂ nanoparticles, which serve as the photocatalyst, are synthesized in situ on SNFs through a hydrothermal reaction. The self‐cleaning effect is proved using wettability measurements for various liquids, degradation of organic contamination under UV light, and antibacterial tests. The enhanced mechanical durability of the hierarchical structure of the composite material is verified with an abrasion test.     

Confined Assembly of Asymmetric Block‐Copolymer Nanofibers via Multiaxial Jet Electrospinning

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene‐b‐isoprene), PS‐b‐PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post‐fabrication annealing of the fibers to obtain equilibrium self‐assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self‐assembled structures. Various confined assemblies such as co‐existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self‐assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self‐assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block‐copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS‐b‐PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS‐b‐PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self‐assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high‐end applications such as drug delivery.     

Increasing the Morphological Stability of DNA‐Templated Nanostructures with Surface Hydrophobicity

DNA has been extensively used as a versatile template to assemble inorganic nanoparticles into complex architectures; thanks to its programmability, stability, and long persistence length. But the geometry of self‐assembled nanostructures depends on a complex combination of attractive and repulsive forces that can override the shape of a molecular scaffold. In this report, an approach to increase the morphological stability of DNA‐templated gold nanoparticle (AuNP) groupings against electrostatic interactions is demonstrated by introducing hydrophobicity on the particle surface. Using single nanostructure spectroscopy, the nanometer‐scale distortions of 40 nm diameter AuNP dimers are compared with different hydrophilic, amphiphilic, neutral, and negatively charged surface chemistries, when modifying the local ionic strength. It is observed that, with most ligands, a majority of studied nanostructures deform freely from a stretched geometry to touching particles when increasing the salt concentration while hydrophobicity strongly limits the dimer distortions. Furthermore, an amphiphilic surface chemistry provides DNA‐linked AuNP dimers with a high long‐term stability against internal aggregation.     

Photoreversible Fluorescent Modulation of Nanoparticles via One‐Step Miniemulsion Polymerization

A nitrobenzoxadiazolyl(NBD)‐based fluorescent dye and a photochromic spiropyran derivative are incorporated into polymeric nanoparticles via a one‐step miniemulsion polymerization. The diameter of the nanoparticles can be varied from approximately 40 nm to 80 nm by adjusting the polymerization conditions. The prepared nanoparticles exhibit the spectral properties of both NBD dye and spiropyran, indicating that the two chromophores are incorporated into the nanoparticles. The determined amount of NBD and spiropyran in the nanoparticles are about ≈85–90% of the feed amount, while the determined weight ratios of spiropyran to NBD in nanoparticles are very close to that of feed ratios, suggesting the miniemulsion polymerization is a suitable approach for incorporating multiple chromophores into individual nanoparticles with controlled amounts (content) and ratio. UV and visible light can be applied to modulate the fluorescence emission of NBD dye in nanoparticles. Upon UV irradiation, the spiropyran moieties in nanoparticles are converted to the open‐ring (McH form) structure and upon visible‐light irradiation they return to the closed‐ring (SP form) structure; as a result, the fluorescence of NBD can be reversibly “switched off” and “switched on”. Fluorescence resonance energy transfer from the excited NBD dye molecules to the McH form of the spiropyran moieties is the drives the fluorescence modulation. The nanoparticles display fairly good photoreversibility, photostability, and relatively fast photoresponsivity upon alternate UV/Vis irradiation. This class of photoresponsive nanoparticles may find applications in biological fields, such as labeling and imaging, as well as in optical fields, for example, individually light‐addressable nanoscale devices.     

Dual pH‐Induced Reversible Self‐Assembly of Gold Nanoparticles by Surface Functionalization with Zwitterionic Ligands

The dual pH‐induced reversible self‐assembly (PIRSA) of Au‐nanoparticles (Au NPs) is reported, based on their decoration with the self‐complementary guanidiniocarbonyl pyrrole carboxylate zwitterion (GCPZ). The assembly of such functionalized Au NPs is found at neutral pH, based on supramolecular pairing of the GCPZ groups. The resulting self‐assembled system can be switched back to the disassembled state by addition of base or acid. Two predominant effects that contribute to the dual‐PIRSA of Au NPs are identified, namely the ionic hydrogen bonding between the GCPZ groups, but also a strong hydrophobic effect. The contribution of each interaction is depending on the concentration of GCPZ on NPs, which allows to control the self‐assembly state over a wide range of different water/solvent ratios.     

A 3D Self‐Shaping Strategy for Nanoresolution Multicomponent Architectures

3D printing or fabrication pursues the essential surface behavior manipulation of droplets or a liquid for rapidly and precisely constructing 3D multimaterial architectures. Further development of 3D fabrication desires a self‐shaping strategy that can heterogeneously integrate functional materials with disparate electrical or optical properties. Here, a 3D liquid self‐shaping strategy is reported for rapidly patterning materials over a series of compositions and accurately achieving micro‐ and nanoscale structures. The predesigned template selectively pins the droplet, and the surface energy minimization drives the self‐shaping processing. The as‐prepared 3D circuits assembled by silver nanoparticles carry a current of 208–448 µA at 0.01 V impressed voltage, while the 3D architectures achieved by two different quantum dots show noninterfering optical properties with feature resolution below 3 µm. This strategy can facilely fabricate micro‐nanogeometric patterns without a modeling program, which will be of great significance for the development of 3D functional devices.     

Endoplasmic Reticulum–Targeted Fluorescent Nanodot with Large Stokes Shift for Vesicular Transport Monitoring and Long‐Term Bioimaging

Herein, a highly stable aggregation‐induced emission (AIE) fluorescent nanodot assembled by an amphiphilic quinoxalinone derivative‐peptide conjugate, namely Quino‐1‐Fmoc‐RACR (also termed as Q1‐PEP), which exhibits large Stokes shift and an endoplasmic reticulum (ER)‐targeting capacity for bioimaging is reported. It is found that the resulting nanodot can effectively enter the ER with high fluorescent emission. As the ER is mainly involved in the transport of synthesized proteins in vesicles to the Golgi or lysosomes, the Q1‐PEP nanodot with ER‐targeting capacity can be used to monitor vesicular transport inside the cells. Compared to conventional fluorescent dyes with small Stokes shifts, the self‐assembled fluorescent nanodot shows superior resistance to photobleaching and aggregation‐induced fluorescence quenching, and elimination of the spectra overlap with autofluorescence of biosubstrate owning to their AIE‐active and red fluorescence emission characteristics. All these optical properties make the fluorescent nanodot suitable for noninvasive and long‐term imaging both in vitro and in vivo.     

Investigating the Optical Properties of a Laser Induced 3D Self‐Assembled Carbon–Metal Hybrid Structure

Carbon‐based and carbon–metal hybrid materials hold great potential for applications in optics and electronics. Here, a novel material made of carbon and gold–silver nanoparticles is discussed, fabricated using a laser‐induced self‐assembly process. This self‐assembled metamaterial manifests itself in the form of cuboids with lateral dimensions on the order of several micrometers and a height of tens to hundreds of nanometers. The carbon atoms are arranged following an orthorhombic unit cell, with alloy nanoparticles intercalated in the crystalline carbon matrix. The optical properties of this metamaterial are analyzed experimentally using a microscopic Müller matrix measurement approach and reveal a high linear birefringence across the visible spectral range. Theoretical modeling based on local‐field theory applied to the carbon matrix links the birefringence to the orthorhombic unit cell, while finite‐difference time‐domain simulations of the metamaterial relates the observed optical response to the distribution of the alloy nanoparticles and the optical density of the carbon matrix.     

Transparent Nanofibrous Mesh Self‐Assembled from Molecular LEGOs for High Efficiency Air Filtration with New Functionalities

Alarming levels of particulate matter pollution in air pose a serious health threat in several countries, therefore intriguing a strong need for an economic and a viable technology of air filtration. Current air purification technology is rather expensive with certain types even having the risk of emitting hazardous by‐products. The authors have developed a multifunctional air filter inspired from the nasal hairs possessing an ability to specifically trap/exhale the foreign particles and allergens while still letting the air flow. This design is achieved by introducing different functionalities at different dimensional scale employing a bottom‐up approach starting with an organic molecule which is further self‐organized to form nanoparticles and ultimately to a nanofibrous mesh. While the molecular building block inherently possesses the property of shielding Ultraviolet (UV) rays, the nanofibrous mesh built up from it aids in trapping the particulate matter while maintaining good air flow. By controlling the concentration of the organic molecule, the formation of fibers has been enabled in the nanoscale regime to obtain high particle‐capture possibilities. The self‐assembled nanofibrous filter thus designed has achieved a high filtration efficiency of ≈90% for the PM 2.5 particle in congruence with the ability to block the harmful UV radiations.     

Cholesteric Liquid Crystal Shells as Enabling Material for Information‐Rich Design and Architecture

The responsive and dynamic character of liquid crystals (LCs), arising from their ability to self‐organize into long‐range ordered structures while maintaining fluidity, has given them a role as key enabling materials in the information technology that surrounds us today. Ongoing research hints at future LC‐based technologies of entirely different types, for instance by taking advantage of the peculiar behavior of cholesteric liquid crystals (CLCs) subject to curvature. Spherical shells of CLC reflect light omnidirectionally with specific polarization and wavelength, tunable from the UV to the infrared (IR) range, with complex patterns arising when many of them are brought together. Here, these properties are analyzed and explained, and future application opportunities from an interdisciplinary standpoint are discussed. By incorporating arrangements of CLC shells in smart facades or vehicle coatings, or in objects of high value subject to counterfeiting, game‐changing future uses might arise in fields spanning information security, design, and architecture. The focus here is on the challenges of a digitized and information‐rich future society where humans increasingly rely on technology and share their space with autonomous vehicles, drones, and robots.     

3D Orientational Control in Self‐Assembled Thin Films with Sub‐5 nm Features by Light

While self‐assembled molecular building blocks could lead to many next‐generation functional organic nanomaterials, control over the thin‐film morphologies to yield monolithic sub‐5 nm patterns with 3D orientational control at macroscopic length scales remains a grand challenge. A series of photoresponsive hybrid oligo(dimethylsiloxane) liquid crystals that form periodic cylindrical nanostructures with periodicities between 3.8 and 5.1 nm is studied. The liquid crystals can be aligned in‐plane by exposure to actinic linearly polarized light and out‐of‐plane by exposure to actinic unpolarized light. The photoalignment is most efficient when performed just under the clearing point of the liquid crystal, at which the cylindrical nanostructures are reoriented within minutes. These results allow the generation of highly ordered sub‐5 nm patterns in thin films at macroscopic length scales, with control over the orientation in a noncontact fashion.     

Interparticle Forces Underlying Nanoparticle Self‐Assemblies

Studies on the self‐assembly of nanoparticles have been a hot topic in nanotechnology for decades and still remain relevant for the present and future due to their tunable collective properties as well as their remarkable applications to a wide range of fields. The novel properties of nanoparticle assemblies arise from their internal interactions and assemblies with the desired architecture key to constructing novel nanodevices. Therefore, a comprehensive understanding of the interparticle forces of nanoparticle self‐assemblies is a pre‐requisite to the design and control of the assembly processes, so as to fabricate the ideal nanomaterial and nanoproducts. Here, different categories of interparticle forces are classified and discussed according to their origins, behaviors and functions during the assembly processes, and the induced collective properties of the corresponding nanoparticle assemblies. Common interparticle forces, such as van der Waals forces, electrostatic interactions, electromagnetic dipole‐dipole interactions, hydrogen bonds, solvophonic interactions, and depletion interactions are discussed in detail. In addition, new categories of assembly principles are summarized and introduced. These are termed template‐mediated interactions and shape‐complementary interactions. A deep understanding of the interactions inside self‐assembled nanoparticles, and a broader perspective for the future synthesis and fabrication of these promising nanomaterials is provided.