Mechanical control of nanomaterials and nanosystems

Joining forces: integrating the mechanical and optical single molecule toolkits

Combining single molecule force measurements with fluorescence detection opens up exciting new possibilities for the characterization of mechanoresponsive molecules in Biology and Materials Science.

Tetratopic pyrimidine–hydrazone ligands modified with terminal hydroxymethyl and acryloyl arms and their Pb(ii), Zn(ii), Cu(ii) and Ag(i) complexes

Tetratopic pym–hyz strands with terminal hydroxymethyl (L1) and acryloyl groups (L2) were synthesised, characterised and reacted with Pb(ii), Zn(ii), Cu(ii) and Ag(i) ions resulting in new linear, horse-shoe shaped and double helical complexes.

25th anniversary article: what can be done with the Langmuir-Blodgett method? Recent developments and its critical role in materials science

The Langmuir-Blodgett (LB) technique is known as an elegant method for fabrication of well-defined layered structures with molecular level precision. Since its discovery the LB method has made an indispensable contribution to surface science, physical chemistry, materials chemistry and nanotechnology. However, recent trends in research might suggest the decline of the LB method as alternate methods for film fabrication such as layer-by-layer (LbL) assembly have emerged. Is LB film technology obsolete? This review is presented in order to challenge this preposterous question. In this review, we summarize recent research on LB and related methods including (i) advanced design for LB films, (ii) LB film as a medium for supramolecular chemistry, (iii) LB technique for nanofabrication and (iv) LB involving advanced nanomaterials. Finally, a comparison between LB and LbL techniques is made. The latter reveals the crucial role played by LB techniques in basic surface science, current advanced material sciences and nanotechnologies.

25th anniversary article: reversible and adaptive functional supramolecular materials: "noncovalent interaction" matters

Supramolecular materials held together by noncovalent interactions, such as hydrogen bonding, host-guest interactions, and electrostatic interactions, have great potential in material science. The unique reversibility and adaptivity of noncovalent intreractions have brought about fascinating new functions that are not available by their covalent counterparts and have greatly enriched the realm of functional materials. This review article aims to highlight the very recent and important progresses in the area of functional supramoleuclar materials, focusing on adaptive mechanical materials, smart sensors with enhanced selectivity, soft luminescent and electronic nanomaterials, and biomimetic and biomedical materials with tailored structures and functions. We cannot write a complete account of all the interesting work in this area in one article, but we hope that it can in a way reflect the current situation and future trends in this prosperously developing area of functional supramolecular materials.

Interfacial nanoarchitectonics: lateral and vertical, static and dynamic

The exploration of nanostructures and nanomaterials is essential to the development of advanced functions. For such innovations, nanoarchitectonics has been proposed as a novel paradigm of nanotechnology aimed at assembling nanoscale structural units into predesigned configurations or arrangements. In this Feature Article, we provide an overview of several recent research works from the viewpoint of interfacial nanoarchitectonics with features developed in lateral directions or grown in vertical directions with construction on solid, static, or flexible dynamic surfaces. Lateral nanoarchitectonics at a static interface provides molecular organization by bottom-up nanoarchitectonics and can also be used to realize device integration by top-down nanoarchitectonics. In particular, in the latter case, the fabrication of novel devices, so-called atomic switches, are introduced as a demonstration of atomic-level electronics. Lateral nanoarchitectonics at dynamic interfaces is exemplified by 2D molecular patterning and molecular machine operation induced by macroscopic motion. The dynamic nature of interfaces enables us to operate molecular-sized machines by macroscopic mechanical stimuli such as our hand motion, which we refer to as hand-operated nanotechnology. Vertical nanoarchitectonics is mainly discussed in relation to layer-by-layer (LbL) assembly. By using this technique, we can assemble a variety of functional materials in ultrathin film structures of defined thickness and layer sequence. The organization of biomolecules (or even living cells) within thin films and their integration with device structures is exemplified. Finally, the anticipated research directions of interfacial nanoarchitectonics are described.

Langmuir nanoarchitectonics: one-touch fabrication of regularly sized nanodisks at the air-water interface

In this article, we propose a novel methodology for the formation of monodisperse regularly sized disks of several nanometer thickness and with diameters of less than 100 nm using Langmuir monolayers as fabrication media. An amphiphilic triimide, tri-n-dodecylmellitic triimide (1), was spread as a monolayer at the air-water interface with a water-soluble macrocyclic oligoamine, 1,4,7,10-tetraazacyclododecane (cyclen), in the subphase. The imide moieties of 1 act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. The monolayer behavior of 1 was investigated through π-A isotherm measurements and Brewster angle microscopy (BAM). The presence of cyclen in the subphase significantly shifted isotherms and induced the formation of starfish-like microstructures. Transferred monolayers on solid supports were analyzed by reflection absorption FT-IR (FT-IR-RAS) spectroscopy and atomic force microscopy (AFM). The Langmuir monolayer transferred onto freshly cleaved mica by a surface touching (i.e., Langmuir-Schaefer) method contained disk-shaped objects with a defined height of ca. 3 nm and tunable diameter in the tens of nanometers range. Several structural parameters such as the disk height, molecular aggregation numbers in disk units, and 2D disk density per unit surface area are further discussed on the basis of AFM observations together with aggregate structure estimation and thermodynamic calculations. It should be emphasized that these well-defined structures are produced through simple routine procedures such as solution spreading, mechanical compression, and touching a substrate at the surface. The controlled formation of defined nanostructures through easy macroscopic processes should lead to unique approaches for economical, energy-efficient nanofabrication.

Mechanical exfoliation and characterization of single- and few-layer nanosheets of WSe₂ , TaS₂ , and TaSe₂

Single- and few-layer transition-metal dichalcogenide nanosheets, such as WSe₂ , TaS₂, and TaSe₂, are prepared by mechanical exfoliation. A Raman microscope is employed to characterize the single-layer (1L) to quinary-layer (5L) WSe₂ nanosheets and WSe₂ single crystals with a laser excitation power ranging from 20 μW to 5.1 mW. Typical first-order together with some second-order and combinational Raman modes are observed. A new peak at around 308 cm⁻¹ is observed in WSe₂ except for the 1L WSe₂, which might arise from interlayer interactions. Red shifting of the A(1g) mode and the Raman peak around 308 cm⁻¹ is observed from 1L to 5L WSe₂. Interestingly, hexagonal- and monoclinic-structured WO₃ thin films are obtained during the local oxidation of thinner (1L-3L) and thicker (4L and 5L) WSe₂ nanosheets, while laser-burned holes are found during the local oxidation of the WSe₂ single crystal. In addition, the characterization of TaS₂ and TaSe₂ thin layers is also conducted.

Investigation of Nanomechanical Properties of β-Si3N4 Thin Layers in a Prismatic Plane under Tension: A Molecular Dynamics Study

We report molecular dynamics simulations of the nanomechanical properties and fracture mechanisms of β-Si3N4 thin layers in a prismatic plane under uniaxial tension. It is found that the thin layers in the y loading direction display a linear stress-strain relationship at ε < 0.021, and afterward, the stress increases nonlinearly with the strain until fracture occurs. However, for the z direction, the linear response is located at ε < 0.051. The calculated fracture stresses and strains of the thin layers increase with strain rates both in both directions. The thin layers exhibit the higher Young's modulus of 0.345 TPa in the z direction, higher than that in the y direction. The origins of crack derive from N(2c-1)-Si and N(6h-1)-Si bonds for the y and z loading directions, respectively.

Synergy between twisted conformation and effective intermolecular interactions: strategy for efficient mechanochromic luminogens with high contrast

A strategy towards efficient mechanochromic luminogens with high contrast is developed. The twisted propeller-like conformations and effective intermolecular interactions not only endow the luminogens with AIE characteristics and high efficiency in the crystalline state, but also render them to undergo conformational planarization and disruption in intermolecular interactions upon mechanical stimuli, resulting in remarkable changes in emission wavelength and efficiency.

"Flex-activated" mechanophores: using polymer mechanochemistry to direct bond bending activation

We describe studies in mechanochemical transduction that probe the activation of bonds orthogonal to an elongated polymer main chain. Compression of mechanophore-cross-linked materials resulted in the release of small molecules via cleavage of covalent bonds that were not integral components of the elongated polymer segments. The reactivity is proposed to arise from the distribution of force through the cross-linking units of the polymer network and subsequent bond bending motions that are consistent with the geometric changes in the overall reaction. This departure from contemporary polymer mechanochemistry, in which activation is achieved primarily by force-induced bond elongation, is a first step toward mechanophores capable of releasing side-chain functionalities without inherently compromising the overall macromolecular architecture.

Fullerene nanoarchitectonics: from zero to higher dimensions

The strategic design of nanostructured materials, the properties of which could be controlled across different length scales and which, at the same time, could be used as building blocks for the construction of devices and functional systems into new technological platforms that are based on sustainable processes, is an important issue in bottom-up nanotechnology.Such strategic design has enabled the fabrication of materials by using convergent bottom-up and top-down strategies. Recent developments in the assembly of functional fullerene (C60) molecules, either in bulk or at interfaces, have allowed the production of shape-controlled nano-to-microsized objects that possess excellent optoelectronic properties, thus enabling the fabrication of optoelectronic devices. Because fullerene molecules can be regarded as an ideal zero-dimensional (0D) building units with attractive functions, the construction of higher-dimensional objects, that is, 1D, 2D, and 3D nanomaterials may realize important aspects of nanoarchitectonics. This Focus Review summarizes the recent developments in the production of nanostructured fullerenes and techniques for the elaboration of fullerene nanomaterials into hierarchic structures.

Hybrid mechanoresponsive polymer wires under force activation

Force activation is triggered in a stretched polymer wire with color changes produced as a consequence of the molecules undergoing structural and conformational changes. A markedly increased efficiency of force activation is achieved by decreasing the diameter of the wires. The hybrid mechanosensitive polymer wire can function as micro- and nanoscale force sensor.

Polymeric Thin Films for Organic Electronics: Properties and Adaptive Structures

This review deals with the correlation between morphology, structure and performance of organic electronic devices including thin film transistors and solar cells. In particular, we report on solution processed devices going into the role of the 3D supramolecular organization in determining their electronic properties. A selection of case studies from recent literature are reviewed, relying on solution methods for organic thin-film deposition which allow fine control of the supramolecular aggregation of polymers confined at surfaces in nanoscopic layers. A special focus is given to issues exploiting morphological structures stemming from the intrinsic polymeric dynamic adaptation under non-equilibrium conditions.

Round-tip dielectrophoresis-based tweezers for single micro-object manipulation

In this paper, we present an efficient methodology to manipulate a single micro-object using round-tip positive dielectrophoresis-based tweezers. The tweezers consist of a glass needle with a round-tip and a pair of thin gold-film electrodes. The round-tip, which has a radius of 3µm, is formed by melting a finely pulled glass needle and concentrates the electric field at the tip of the tweezers, which allows the individual manipulation of single micro-objects. The tweezers successfully captured, conveyed, and positioned single cell-sized liposomes with diameters of 5-23µm, which are difficult to manipulate with conventional manipulation methodologies, such as optical tweezers or glass micropipettes, due to the similarities between their optical properties and those of the media, as well as the ease with which they are deformed or broken. We used Stokes' drag theory to experimentally evaluate the positive dielectrophoresis (pDEP) force generated by the tweezers as a function of the liposome size, the content of the surrounding media, and the applied AC voltage and frequency. The results agreed with the theoretically deduced pDEP force. Finally, we demonstrated the separation of labeled single cells from non-labeled cells with the tweezers. This device can be used as an efficient tool for precisely and individually manipulating biological micro-objects that are typically transparent and flexible.

Stimuli-responsive folding and unfolding of a polymer bearing multiple cerium(IV) bis(porphyrinate) joints: mechano-imitation of the action of a folding ruler

A pivotal guest role: a new porphyrin polymer, poly(PorZn⋅DD) (pink/purple), composed of a porphyrinatozinc and a porphyrin double-decker complex as a repeating unit was synthesized. In poly(PorZn⋅DD), porphyrinatozinc complexes recognize a divalent amine (tan/red) to induce an intramolecular pivoting motion through the rotation of porphyrin double-decker complexes and the polymer undergoes shortening and compaction.

Unique piezochromic fluorescence behavior of dicyanodistyrylbenzene based donor-acceptor-donor triad: mechanically controlled photo-induced electron transfer (eT) in molecular assemblies

A novel donor-acceptor-donor triad that exhibits fluorescence on-off switching with high contrast ratio (ca. 10(3) ) in response to a mechanical stimulus in the solid state is reported. This system provides a very unique example of high-contrast fluorescence switching that is driven by a mechanical-force-controlled photo-induced electron transfer (eT) in molecular assemblies.

Mechanical properties of atomic layer deposition-reinforced nanoparticle thin films

Nanoparticle thin films (NTFs) exhibit multifunctionality, making them useful for numerous advanced applications including energy storage and conversion, biosensing and photonics. Poor mechanical reliability and durability of NTFs, however, limit their industrial and commercial applications. Atomic layer deposition (ALD) represents a unique opportunity to enhance the mechanical properties of NTFs at a relatively low temperature without drastically changing their original structure and functionality. In this work, we study how ALD of different materials, Al(2)O(3), TiO(2), and SiO(2), affects the mechanical properties of TiO(2) and SiO(2) NTFs. Our results demonstrate that the mechanical properties of ALD-reinforced NTFs are dominantly influenced by the mechanical properties of the ALD materials rather than by the compositional matching between ALD and nanoparticle materials. Among the three ALD materials, Al(2)O(3) ALD provides the best enhancement in the modulus and hardness of the NTFs. Interestingly, Al(2)O(3) ALD is able to enhance not only the modulus and hardness but also the toughness of NTFs. Our study presents an additional benefit of depositing nanometer scale ALD layers in NTFs; that is, we find that the hardness and modulus of ultrathin ALD layers (<5 nm) can be estimated from the mechanical properties of ALD-reinforced NTFs using a simple mixing rule. This investigation also provides insight into the use of nanoindentation for testing the mechanical properties of ultrathin ALD-reinforced NTFs.

Paradigm shift from self-assembly to commanded assembly of functional materials: recent examples in porphyrin/fullerene supramolecular systems

Current nanotechnology based on top-down nanofabrication may encounter a variety of drawbacks in the near future so that development of alternative methods, including the so-called bottom-up approach, has attracted considerable attention. However, the bottom-up strategy, which often relies on spontaneous self-assembly, might be inefficient in the development of the requisite functional materials and systems. Therefore, assembly processes controlled by external stimuli might be a plausible strategy for the development of bottom-up nanotechnology. In this review, we demonstrate a paradigm shift from self-assembly to commanded assembly by describing several examples of assemblies of typical functional molecules, i.e. porphyrins and fullerenes. In the first section, we describe recent progress in the design and study of self-assembled and co-assembled supramolecular architectures of porphyrins and fullerenes. Then, we show examples of assembly induced by external stimuli. We emphasize the paradigm shift from self-assembly to commanded assembly by describing the recently developed electrochemical-coupling layer-by-layer (ECC-LbL) methodology.

Permeability and micromechanical properties of silk ionomer microcapsules

We studied the pH-responsive behavior of layer-by-layer (LbL) microcapsules fabricated from silk fibroin chemically modified with different poly amino acid side chains: cationic (silk-poly L-lysine, SF-PL) or anionic (silk-poly-L-glutamic acid, SF-PG). We observed that stable ultrathin shell microcapsules can be assembled with a dramatic increase in swelling, thickness, and microroughness at extremely acidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeable disintegration. These changes are accompanied by dramatic changes in shell permeability with a 2 orders of magnitude increase in the diffusion coefficient. Moreover, the silk ionomer shells undergo remarkable softening with a drop in Young's modulus by more than 1 order of magnitude due to the swelling, stretching, and increase in material porosity. The ability to control permeability and mechanical properties over a wide range for the silk-based microcapsules, with distinguishing stability under harsh environmental conditions, provides an important system for controlled loading and release and applications in bioengineering.

Flake-shell capsules: adjustable inorganic structures

Structure-adjustable capsules are fabricated from inorganic components by using a self-template dissolution-regrowth mechanism to give flake-shell silica microcapsules. The capsules shrink under thermal stimulus and their structures can be adjusted by treatment at different pH values. Tuning of shell pore diameters leads to tailored drug release over prolonged periods.

Selecting speed-dependent pathways for a programmable nanoscale texture by wet interfaces

The realization of well-defined and ordered structures on the nanoscale is a main issue in nanoscience and nanotechnology, biotechnology and other related fields like plastic or organic electronics. Among the bottom-up approaches, to date, self-assembly (equilibrium aggregates) received a major attention. In spite of this, far from equilibrium conditions allow for the generation of a wider landscape of organized systems depending on the set of control parameters employed. Under an adaptation vision of the structures, here we report some case studies showing how it is possible to programme and control the nanoscale features of ordered super- or supra-aggregates at wet interfaces by modulating the dynamic parameters. In particular, speed is foreseen as a threshold factor for changing the aggregation mechanism along with the shape and degree of order of the structures as well as, within a specific aggregation path, their size and defectivity.

Layer-by-Layer Enzymatic Platform for Stretched-Induced Reactive Release

An original "all-in-one" platform combining polymers, enzymes, and enzymatic substrates in a unique film is designed. A polymeric barrier stratum prevents any contact between enzymes adsorbed on top of the film and substrates loaded in an underlying reservoir. Upon stretching of the film, a continuous diffusion of substrates through the barrier is triggered, followed by a catalytic reaction. This leads to the formation of products that are released from the film. This new platform acts as a stretch-induced reactive release system and emerges as an innovative concept in mechano-responsive materials.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

Force-reactivity property of a single monomer is sufficient to predict the micromechanical behavior of its polymer

We demonstrate an accurate prediction of the micromechanical behavior of a single chain of cyclopropanated polybutadiene, which is governed by rapid isomerization of the cyclopropane moieties at ~1.2 nN, from the force-rate correlation of this reaction measured in a small series of increasingly strained macrocycles. The data demonstrate that a single physical quantity, force, uniquely defines the dynamics across length scales from >100 to <1 nm and that strain imposed through molecular design and that imposed by micromanipulation techniques have equivalent effects on the kinetics of a chemical reaction. This represents a new method of screening potential monomers for applications in stress-responsive materials that could also facilitate atomistic interpretations of single-molecule force experiments.