Interfacial Nanoarchitectonics: Lateral and Vertical, Static and Dynamic


Interfacial Nanoarchitectonics: Lateral and Vertical, Static and Dynamicpubs.acs.org/doi/pdf/10.1021/la4006423Similarby...

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Invited Feature Article pubs.acs.org/Langmuir

Interfacial Nanoarchitectonics: Lateral and Vertical, Static and Dynamic Katsuhiko Ariga,* Taizo Mori, and Jonathan P. Hill World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and JST, CREST, Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ABSTRACT: 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.



INTRODUCTION Advances in human social activity are supported by various functional systems that have been developed over time but whose functions are being replaced by so-called nanotechnologies.1,2 Additionally, materials with specialized properties including electronic, photonic, magnetic, ecological, sensing, catalytic, biocompatible, and medical applications have been investigated.3−5 However, the exploration of nanostructures and nanomaterials is essential for the further development of useful advanced functionalities. Advanced functions may be exhibited by nanoscale structural units through their mutual interactions in appropriate combinations. For these innovations, nanoarchitectonics has been proposed as a novel paradigm in nanotechnology aimed at assembling nanoscale structural units into predesigned configurations. This terminology was first proposed by Dr. Masakazu Aono at the first International Symposium on Nanoarchitectonics Using Suprainteractions (NASI-1) at Tsukuba in 2000. Researchers in nanoarchitectonics construct materials through advanced methodologies for the manipulation of atoms, molecules, and their assemblies.6−8 This concept should be developed by the synergistic combination of technical innovations in various fields including atomic/molecular-level control, chemical nanofabrication, self-organization, and fieldcontrolled organization. The combined development of these © XXXX American Chemical Society

technologies will result in innovation in materials and systems regardless of their dimensions or chemical identity. Investigating what to synthesize for nanoarchitectonics and how to prepare nanoarchitectures is not sufficient to create useful systems. The locations at which nanoarchitectures are prepared could be another crucial factor. We can synthesize molecular complexes or nanoclusters in solution and these may be of scientific interest, but most of them cannot be accessed in order to exploit their properties. It is necessary to immobilize functional nanoarchitectures at known locations in order to address them. As media for the immobilization of nanoarchitectures, various kinds of interfaces including solid device surfaces, soft matter surfaces, and even highly flexible fluid interfaces should be considered. Therefore, the initial concept of nanoarchitectonics should be further developed to the more advanced concept of interfacial nanoarchitectonics for useful future technologies. In this Feature Article, we introduce several recent examples of interfacial nanoarchitectonics. These examples generally have two featuresstructure development in lateral directions and vertical growthand are constructed on solid static surfaces or operate on flexible dynamic interfaces. Here, dynamic interface means an interface that permits a variation of the conformation Received: February 18, 2013 Revised: April 1, 2013

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Figure 1. Formation of the Kagomé structure through the hydrogen-bonding network of (5,10,15,20-tetrakis(3,5-dimethyl-4-hydroxyphenyl)porphyrin) on the Cu(111) surface.

or structure of the component molecules as can be seen at the air−water interface. According to these features, this Feature Article is divided into three main categories: (i) lateral nanoarchitectonics at a static interface (molecular organization as bottom-up nanoarchitectonics and device integration as topdown nanoarchitectonics), (ii) lateral nanoarchitectonics at dynamic interfaces (pattern formation by 2D molecular patterning and molecular machine operation by hand-operating nanotechnology), and (iii) vertical nanoarchitectonics from interfaces (layer-by-layer assembly with novel materials, methods, and emerging applications). Recent research examples of nanostructure formation and functions including the formation of precise molecular arrangement, organization of living cells, operation of atomic switches, and control of molecular machines in manual modes are included. Finally, anticipated developments of interfacial nanoarchitectonics are described.

Porphyrin molecules are expected to possess roughly planar conformations on a solid surface, and their available chemical modifications are well established. In addition, arrays of porphyrin derivatives are thought to be important models of lightharvesting systems. In a typical example, Yokoyama and coworkers formed controlled molecular arrays of porphyrin derivatives through hydrogen bonding between cyano groups on a Au(111) surface.11 cis- or trans-dicyanophenyl-substituted tetraphenylporphyrins exhibited the formation of either isolated clusters or nanowire chains of single molecules depending on hydrogen-bonding motifs. Hill and co-workers observed the hydrogen-bonded network organization of (5,10,15,20-tetrakis(3,5-dimethyl-4-hydroxyphenyl)porphyrin) on the Cu(111) surface (Figure 1).12 Hydrogen bonds are formed between the meso substituents, and the molecules tend to form a trimer unit through interactions at the meso substituents at submonolayer coverage. Both macrocyclic and ring-open forms of the trimer are commonly observed. The trimeric units formed vertices of an extended hexagonal structure. The simplest case for a hexagonal structure built from the trimer having C3 symmetry forms a porous honeycomb-type structure that was observed as a Kagomé lattice containing void areas. In the formation of a 2D cavity structure, Barth and coworkers used 1,4-dicarboxylic benzoic acid (terephthalic acid), 1,2,4-tricarboxylic benzoic acid (trimellitic acid), and 4,1′,4′,1″terphenyl-1,4″-dicarboxylic acid as building blocks for cavity formation through coordination with Fe atoms.13 The space between molecules was functionalized to capture C60 molecules. The formation of unoccupied space leads to a novel concept for the preparation of 2D nanospaces with molecular-level structural precision. Lin, Barth, and co-workers used 1,3,5tricarboxylic benzoic acid (trimesic acid) molecules that can chelate Fe atoms to form a pattern.14 In selected cases, chiral molecular spaces composed of 16 molecules of trimesic acid and 9 iron atoms were formed. Recently, Linderoth, Gothelf, and co-workers prepared chiral molecular spaces by applying a welldesigned molecular system.15 They studied the surface selfassembly of a class of linear compounds based on an oligo(phenylene ethynylene) backbone to form achiral windmill motifs. However, the replacement of peripheral tert-butyl substituents by



LATERAL NANOARCHITECTONICS AT STATIC INTERFACES Because instrumental techniques and expertise for atomic/ molecular image observation have been rapidly developed, the demand for the control of atomic/molecular arrangements with extremely high precision has concurrently increased. These efforts have resulted in both a fundamental understanding of molecular interactions within two dimensions and practical developments of the fabrication of ultrasmall devices. Most of these studies have been conducted at solid static interfaces because of the ease of observation of atomic/molecular images. Therefore, the nanoarchitectonics of these systems has been mostly performed in lateral directions to develop regular structures and functional arrays in two dimensions. In the following few sections, several recent typical examples of lateral nanoarchitectonics at static interfaces are briefly summarized. Molecular Organization: Bottom-Up Nanoarchitectonics. The control of molecular organization on a solid surface based on molecular self-assembly is one of the most advanced and promising bottom-up approaches in nanofabrication. Although various molecules have been targeted to form 2D molecular patterns on solid surfaces,9 the use of porphyrin derivatives has been widespread for regular pattern formation.10 B

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sec-butyl groups containing an S chiral center leads to a strong preference for windmill motifs with one chiral orientation. This example demonstrates that the control of the chiral organization of the molecular spaces can be accomplished through rational molecular design. All of these examples indicate that 2D nanoarchitectonics leading to designed nanospaces on the molecular level can be achieved only by the selection of appropriately designed molecular units. Therefore, we can expect to create various regular structures and controlled nanospaces by applying molecular design principles based on concepts and methodologies of supramolecular science, crystal lattice formation, and coordination chemistry. Another aspect of lateral nanoarchitectonics at a solid interface is its contribution to basic science because properties of the molecules such as molecular motion and conformational changes can be directly observed. These characteristics are inconvenient to assess using conventional solution techniques. As shown below, Hill and co-workers demonstrated an advantage of lateral nanoarchitectonics in two phenomena: (i) the phase transition of a 2D array and (ii) the conformational adaptation at a phase boundary. Two-dimensional molecular arrays of the oxoporphyrinogen (tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin) were first investigated. At submonolayer coverage on Cu(111), surfacemobile hexagonally packed domain islands interspersed in a 2D gas phase were observed at lower temperature.16 Increasing the temperature to room temperature induced a phase transition of the hexagonally packed phase to a new phase, including a squarepacked grid motif (Figure 2). This transition was observed by

square phase is highly cooperative, and a dominolike effect in this transition structure took place. Device Integration: Top-Down Nanoarchitectonics. In the previous examples, 2D nanoarchitectures are formed through self-assembling processes of the molecules. This can be regarded as a bottom-up approach to nanofabarication. As a counterpart of the bottom-up approaches, nanoarchitectonics including topdown features has been accomplished by Aono and co-workers and is introduced in this section. Okawa et al. developed electrical-stimuli-driven molecular wire formation based on bottom-up self-assembly and top-down microtip technology (Figure 3).17 A self-assembled monolayer of a diacetylene compound was first prepared on a solid surface. When an electrical bias was applied using the probe tip of an STM situated appropriately in the vicinity of the molecular row of the diacetylene compound, a conductive polydiacetylene nanowire forms by the chain polymerization of the diacetylene groups in adjacent molecules. This process can be used to create nanowires on the molecular scale at predesignated positions. In addition, the front edge of chain polymerization is chemically active and can form covalent bonds with an appropriately situated molecule. It was demonstrated that two conductive polymer nanowires can be connected to a single phthalocyanine molecule. This methodology could be called chemical soldering. If this strategy were to be combined with the molecular patterning described above, the writing of electrical circuits with molecular elements would become possible. The proposed method is useful for wiring each component of single-molecule devices and could be a key method in lateral nanoarchitectonics for future single-molecule circuits. Advances in microfabrication techniques allow us to draw electric circuits on a solid surface within incredibly small areas. One of the ultimate goals of the closely related approaches is the preparation of devices that can be reversibly switched on and off by migrations of single (or a few) atoms. Such devices have been actually realized in the concept of the atomic switch reported by Aono and co-workers.18,19 An atomic switch is a nanoionic device operated by controlling the diffusion of metal ions/atoms and their reduction/oxidation processes in a switching operation that forms/annihilates a conductive path. On the basis of this fundamental concept, various devices have been created using this atomic switch concept. They are highly successful examples of top-down-type nanoarchitectonics on static surfaces. Tsuruoka and co-workers reported the preparation of resistive switching memory based on a Ag+-conductive solid polymer electrolyte (Figure 4).20 Polyethylene oxide−silver perchlorate complexes exhibit bipolar resistive switching under bias voltage sweeping. The observed switching behavior is driven by the formation and dissolution of a silver metal filament inside a solid polymer electrolyte stimulated by electrochemical reactions. The application of positive bias to the Ag electrode causes the transportation of Ag+ and ClO4− toward the counter Pt and Ag electrodes, respectively. The transported Ag+ ions are reduced to Ag atoms at the electrolyte−Pt interface. Ag nucleates and grows preferentially toward the opposite electrode, and eventually a metal filament is formed between the two electrodes (switch-on process). When a negative bias voltage is applied, the oxidation reaction of Ag to Ag+ occurs at surfaces of the metal filament, resulting in the dissolution of the thinnest part (switch-off process). The fabricated devices exhibited on/off resistance ratios greater than 105, programming speeds higher than 1 μs, and retention times longer than 1 week. The solid polymer

Figure 2. Phase transition of 2D molecular arrays of the oxoporphyrinogen (tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin) at submonolayer coverage on Cu(111) between the hexagonally packed phase and the square-packed grid phase.

scanning tunneling microscopy (STM) on a time scale of seconds. The disclike shape of the planar conformations of these molecular units favors a hexagonal arrangement for the optimization of intermolecular van der Waals contacts. However, nonplanar conformations optimize the van der Waals contacts between tert-butyl groups by the formation of the square grid structure. Square packing is preferable from the viewpoint of the conformational energy of an individual molecule. However, van der Waals contacts among neighboring molecules and the surface are enhanced in hexagonal packing domains. The coexistence of the hexagonal and square domains indicates that both phases are in fact metastable and the energy barrier between these domains is relatively small. The transition from the hexagonal phase to the C

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Figure 3. Electrical-stimuli-driven molecular wire formation by the tip of a scanning tunneling microscope (STM) to the molecular row of the diacetylene compound and connection to a single phthalocyanine.

Figure 4. Resistive switching memory based on a silver-ion-conductive solid polymer electrolyte where the application of positive bias to the Ag electrode causes the formation of a metal filament between the two electrodes (switch-on process).

electrolyte-based electrochemical device is suitable for flexible switch and memory applications. Applying atomic switch mechanisms to new three-terminal operations can be used to create atom transistors.21 The fabricated atom transistor shows volatile/nonvolatile selective operations with high on/off resistance ratios (106−108) and very small power consumption. The nonvolatile operation is suitable for achieving nonvolatile logic, where a very small write/erase current of pA is advantageous for other devices. Selective volatile and nonvolatile operations might enable new types of logic operations/architectures. With the use of atomic switch mechanisms, two types of synaptic plasticity, short-term plasticity and longterm potentiation, have been mimicked.22 Ag2S inorganic synapse models in an atomic switch realized both memory modes. The operation of the atomic switch at critical voltages stores information with short-term plasticity and a spontaneous decay of the conductance level in response to intermittent input stimuli. However, frequent stimulation results in a transition to long-term potentiation. These functional structures would be appropriate for the design of neural systems including hardwareimplemented artificial neural networks. Apart from these

examples, various modes of memory and switching devices based on atom-level operations have been prepared.23−26 These could be useful in the near future for realistic practical applications of 2D nanoarchitectonics.



LATERAL NANOARCHITECTONICS AT DYNAMIC INTERFACES Because the precise observation of molecular images at the surfaces of liquids and/or soft matter is difficult, an evaluation of molecular and nanolevel structures at dynamic interfaces requires indirect methods. Therefore, the investigation of nanostructures at dynamic interfaces is somewhat less advanced than those at static solid interfaces. However, the dynamic interface is a powerful medium for creating sophisticated functions. This can be understood if we consider the many complex functions that occur at cell membranes in biological systems. Photosynthesis, signal transduction, and energy conversions are conducted in highly flexible cell membrane media. Functional molecules including proteins, dyes, and carbohydrates are arranged precisely at fluidic lipid membranes. Therefore, we expect that nanoarchitectonics in lateral dimensions at dynamic interfaces D

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Figure 5. Two-dimensional molecular pattern formation through the binding of flavine adenine dinucleotide (FAD) to two guanidinium molecules at phosphate groups and one orotate molecule at adenine sites. The height difference of bound molecules results in 2D patterns with submolecular precision.

molecule at adenine sites. Monoalkyl guanidinium and dialkyl orotate were mixed at the air−water interface and interacted with an aqueous FAD molecule. The monolayer transferred onto a mica surface was observed by AFM, revealing regular height differences in the angstrom range. This pattern can be reproduced in molecular models (Figure 5). Because the guanidiniums and orotate bind to FAD through hydrogen bonding, the two kinds of amphiphilic molecules locate their polar heads close to the FAD molecule, so the positions of terminal methyl groups of the amphiphiles could be differentiated. The formation of regular mesoscopic patterns has also been investigated. Liu and co-workers demonstrated several systems involving the formation of chiral structures from achiral components at the air−water interface.31 For example, the amphiphilic imidazole monolayer forms chiral spiral structures as a result of intermolecular interactions at the air−water interface.32 Interestingly, the chirality of the aggregates is determined by chance, and the chirality of the starting aggregate is communicated over a wider area through cooperative interaction. Oishi and co-workers demonstrated the formation of a nanoscopic domain structure in a mixed monolayer consisting of alkyl guanidinium and a fluorocarbon carboxylic acid.33 The balance between the ionic and/or hydrogen-bonding interactions at the two polar head groups and the surface free-energy difference at the hydrophobic parts (hydrocarbon and fluorocarbon chains) determines the domain size. Control of the domain size and shape from the nanometer to micrometer scales was achieved by varying the values of accessible external parameters such as the pressure, composition, temperature, pH, and ionic strength of the subphase. Mori et al. reported the formation of monodisperse regularly sized disks of several nanometers thickness and with diameters of less than 100 nm (Figure 6) by combining relatively weak intermolecular interactions at the air−water interface and 2D quick dewetting.34 An amphiphilic triimide, tri-n-dodecylmellitic triimide with three alkyl chains, was spread as a monolayer on an aqueous phase containing 1,4,7,10-tetraazacyclododecane (cyclen). The imide moieties act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. Monolayer transfer onto freshly

should have great potential for the development of functional systems. In the following sections, we briefly describe lateral nanoarchitectonics at a dynamic interface from two viewpoints, molecular pattern formation and molecular machine operation Pattern Formation (Two-Dimensional Molecular Patterning). It has been proven that the air−water interface provides an effective medium for promoting molecular recognition between interfacial film components and aqueous guest molecules.27 This attractive feature of supramolecular chemistry at the air−water interface allows us to design molecular-level regular structures through molecular interactions between monolayer components and aqueous template molecules. This concept is referred to as 2D molecular patterning at a dynamic interface. Several examples are briefly introduced below. Because guanidinium groups can selectively bind certain kinds of functional groups such as carboxylate and phosphate, the molecular ordering of amphiphilc guanidinium can be regulated through the binding of carboxylate compounds as templates.28 If bifunctional carboxylates were to be used as templates, then the crystallinity of the guanidinium amphiphiles could be adjusted. Templates with shorter spacers between two carboxylates, such as malonate [(CH2) spacer] and succinate [(CH2)2 spacer] in the subphase, induced a crystalline state of the guanidinium monolayer. However, an increase in the spacer length [glutarate with (CH2)3 and adipate with (CH2)4] results only in the formation of an amorphous ordering of alkyl chains. Such ordering of alkyl chains can be induced through the formation of a continuous hydrogen bonding array as seen in extended arrays with alternating hydrogen-bonding pairs between melamine and barbituric acid or related molecules.29 An atomic force microscope (AFM) image of the didodecylmelamine monolayer reveals that perfectly aligned molecular patterns are obtained only in the presence of barbituric acid or thiobarbituric acid in the subphase. Bifacial complementary hydrogen-bonding-pair formation between melamine units and barbituric (or thiobarbituric) acid groups results in a regular molecular pattern. If a template has different binding sites within one molecule, then binding with different kinds of amphiphile molecules leads to more sophisticated complexes and patterns.30 A single flavine adenine dinucleotide (FAD) can potentially bind two guanidinium molecules at phosphate groups and one orotate E

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Figure 6. Formation of monodisperse regularly sized disks of several nanometers thickness and with diameters of less than 100 nm by combining relatively weak intermolecular interactions within at the air−water interface and 2D quick dewetting using an amphiphilic triimide, tri-n-dodecylmellitic triimide with three alkyl chains and 1,4,7,10-tetraazacyclododecane.

Figure 7. Bending of a larger object (cantilever) through the accumulation of motions of immobilized molecular shuttles on the surface of an AFM cantilever.

only by spectroscopic observations. For practical applications of molecular machines, their immobilization onto a solid surface should be performed and has been investigated mostly during the last 10 years. For example, Stoddart and co-workers immobilized molecular shuttles at the surface of an AFM cantilever (Figure 7).40 Shifting the position of the shuttle unit along the molecular axis resulted in molecular-level torsions. Surprisingly, the accumulated tension at the solid interface caused the bending of the much larger cantilever corresponding to the cantilever deformation of 35 to 50 nm through the collection of a mean molecular force of 14 to 21 pN. To ensure the free motion of molecular machines, the use of a dynamic interface could have advantages over a solid static interface. Interfacial media provide a boundary between two media with different dielectric constants, where differences in chemical potential might act as driving forces for machine operation. A pioneering example can be seen in the molecular rotor operations at the gas−liquid interface reported by Tabe and Yokoyama.41 A coherent molecular precession driven by the transmembrane transfer of water molecules using a chiral liquidcrystalline monolayer spread on an air−glycerol interface was demonstrated. The rotational direction could be controlled

cleaved mica by a surface touching (i.e., Langmuir−Schaefer) method led to a 2D quick dewetting process. An AFM investigation revealed the formation of well-dispersed nanodisks, and the diameters of these objects on mica were narrowly distributed at 46 and 73 nm for samples prepared at 15 and 30 mN m−1, respectively, with well-defined thicknesses of 2.6 and 2.9 nm again at 15 and 30 mN m−1, respectively. The disk diameter was tunable through the mechanical motion of the monolayer whereas the height remained constant at the molecular level. The surface touching method yields bilayer structures through layer overturning, and the presence of cyclen probably strongly enhances the association of the head groups of the triimide amphiphiles. This methodology should lead to unique approaches for economical and energy-efficient nanofabrication. Recently, Chi, Fuchs, and co-workers also reported elegant interfacial processes to create organized stripe patterns.35−37 In addition, various methods for preparing molecular nanostructures are summarized in several review articles.38,39 Molecular Machine Operation (Hand-Operated Nanotechnology). One of the ultimate goals in the miniaturization of functional systems is the operation of molecular machines. In the initial stages of molecular machine science, molecules were studied in solution where their operation could be confirmed F

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Figure 8. Capture and release of aqueous guest molecule by steroid cyclophane upon application of macroscopic mechanical forces.

recognition through mechanical tuning by applying an external force, which could be an alternative methodology to the more traditional molecular design strategies. A mechanically controlled indicator displacement assay (MCIDA) was demonstrated by fluorescent signaling between a host and an indicator that can be switched by surface compression.46 An amphiphilic dilysine peptide host (Figure 9) was designed to contain a phenylboronic acid, a cholesterol moiety, and a carboxyfluorescein indicator at the N-terminus of the peptide. Carboxyfluorescein was chosen as a fluorescent probe because it is known to serve as a fluorescence resonance energy transfer (FRET) acceptor for coumarin-based indicators. The distance between these moieties dictates the extent of the FRET signal, and the donor and acceptor units will have a closer arrangement when lateral mechanical force was applied. As a result, an optimal fluorescence switching/sensing scheme could be obtained under optimized conditions. The addition of a small amount of D-glucose to the water subphase led to a decrease in the magnitude of the green fluorescence observed for the monolayer. This sensing approach allowed for the quantitative sensing of D-glucose through clear fluorescence switching. The method of using the host-indicator complex in a specifically defined interfacial region may be further applicable to biological and environmental analyses.

through the selection of either the molecular chirality or the transfer direction of water molecules. At a dynamic interface, the harmonized motion of molecular machines within an organized array can be coupled with macroscopic force.42 Dynamic deformation including compression, expansion, and bending of the interface could be connected to the structural alteration of individual molecular machines contained in the array. These macroscopic motions in a lateral direction can induce nanoscopic and/or molecular-level changes in molecular machines within such a film. For example, 1,6,20,25tetraaza[6.1.6.1]-paracyclophane connected to four cholic acid moieties through a flexible L-lysine spacer (so-called steroid cyclophane) was used as a molecular machine operated at the air−water interface (Figure 8).43 The compression of the monolayer induced the shrinkage of the steroid cyclophane molecule, resulting in a cavity conformation. The capture of aqueous guest molecules upon cavity formation of the steroid cyclophane was demonstrated. Repeated compression and expansion cycles of the steroid cyclophane monolayer by external mechanical force induced periodic guest capture and release. Because these stimuli were performed manually, this process could be conceptually referred to as hand-operating nanotechnology. The twisting motion of N-substituted cyclen containing a 1,4,7,10-tetraazacyclododecane core with four cholesteric side arms has similarly been used as a molecular machine.44 Packing control of the N-substituted cyclen receptor through the application of lateral mechanical force was shown to control the modification of enatioselectivity in the binding of aqueous amino acid guests. Chiral discrimination was caused by the pairwise packing of adjacent chiral centers of this molecular machine. An inversion of the magnitude of the binding constant between L and D enantiomers was observed in the case of the valine guest. The same concept was also applied to one of the most challenging biomolecular recognition problemsthat of discriminating thymine from uracil. The monolayer of a triazacyclononane host selectively recognized uracil over thymine (ca. 64 times) under optimized conditions ([LiCl] = 10 mM at a surface pressure of 35 mN m−1).45 These two examples suggest a novel methodology in the host−guest chemistry of the optimization of host structures for the best



VERTICAL NANOARCHITECTONICS AT INTERFACES Buildings in the macroscopic world are often constructed with only a single floor (i.e., the ground floor). However, modern buildings have advanced architectures with multiple floors or stories. In the nanoworld, vertical nanoarchitectonics on interfaces has also been pursued in order to prepare advanced functional systems. As methods for the construction of multilayered structures, the Langmuir−Blodgett (LB) technique and layer-by-layer (LbL) assembly are widely known. The LB method provides unit-layer structure with well-controlled packing, ordering, orientation, and arrangement, which can be assembled into multilayer structures. Similarly, multilayer films can be prepared by using LbL assembly. Although molecular ordering and packing are not as good as LB films, the LbL technique can be applied to a wider range of materials by a G

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Figure 9. Mechanically controlled indicator displacement assay (MC-IDA).

relatively simple and inexpensive procedure. Therefore, the use of the LbL method has been transferred to many areas of science and technology. In the following sections, recent developments of vertical nanoarchitectonics from interfaces are briefly summarized. Layer-by-Layer (LbL) Assembly: Novel Materials and Methods. The assembly of nanoscale objects such as colloidal particles through electrostatic attraction in a layer-by-layer manner was first proposed by Iler,47 followed by a pioneering realization of the idea by Decher.48,49 Various materials can be assembled into multilayer films with a predesigned thickness and sequence through very simple procedures, with the minimum requirements of the necessary apparatus being beakers and tweezers. Therefore, many scientists from various research specialties have begun using this method for the preparation of regular assemblies.50 Materials useful for LbL techniques have been widely researched, and the exploration of LbL assemblies remains intensive. For example, graphene and graphene oxides have been recently investigated as emerging nanomaterials and have already been assembled in LbL structures.51 The example shown in Figure 10 shows LbL assemblies between graphene and ionic liquids.52 A graphene oxide sheet was reduced to a graphene sheet in the presence of ionic liquids in water, resulting in charged composites of a graphene sheet/ionic liquid. These charge-decorated nanosheets were assembled alternately with poly(sodium styrenesulfonate) (PSS) by LbL adsorption on appropriate solid supports such as quartz crystal microbalance (QCM) sensors. Exposure of the composite LbL films to various saturated vapors caused an in situ decrease in the frequency of QCM as a result of gas adsorption. Because these LbL films provide a well-defined π-electron-rich nanospace, the sensing films showed significantly higher selectivity (more than 10 times) for benzene vapor over cyclohexane despite the similarities in their molecular sizes, molecular weights, and vapor pressures. The application of LbL techniques to functional nanomaterials is not limited to graphene nanosheets. Various kinds of nanosheets

Figure 10. Gas sensor by LbL assemblies between graphene and ionic liquids.

can be assembled to give functional multilayer structures, which have been amply described in other review articles.53,54 In one surprising example, the preparation of an all-metal LbL film has been recently reported.55 Multilayered mesoporous bimetallic (Pt/Pd) films were assembled through electrochemical deposition. Each layer thickness of mesoporous Pt and Pd could be altered by simply controlling the applied deposition time as a result of the linearity of the relationship between the deposition time and deposition thickness of Pt and Pd layers. Driving forces and mechanisms of LbL assemblies are still an active subject of vertical nanoarchitectonics. For example, Shi and co-workers proposed LbL assembly based on gravity control.56 Recently, Li et al. demonstrated a novel mode of LbL assembly by so-called electrochemical coupling layer-bylayer (ECC-LbL), where the electrochemically driven dimerization of N-alkylcarbazole was used (Figure 11).57 The ECC-LbL H

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materials combined with procedural simplicity, scientists in a wide range of research fields have considered its use. Recent trends in LbL research indicate that LbL assembly can be used for diverse purposes, including sensing, drug delivery, and biomedical and device technology.59,60 Wide material applicability of the LbL method enables us to assemble functional nanostructured materials into film assemblies. For example, mesoporous materials with well-defined nanopores can adsorb guest molecules with high selectivity and high efficiency. Mesoporous materials themselves have been used only in conventional applications such as filtration and the removal of specific materials.61 In contrast, assembling them into LbL films on a device surface leads to modern and advanced sensor applications. For example, the LbL assembly of oxidized mesoporous carbon CMK-3 with cationic polyelectrolyte on a QCM plate exhibited a sensing performance in aqueous solution (Figure 12).62 Frequency shifts upon the adsorption of tannic acid greatly exceed those for catechin and caffeine. The superior adsorption capacity for tannic acid likely originates in its molecular structure (i.e., multiple phenyl rings of the tannic acid molecule can interact with the carbon surface through π−π interactions and hydrophobic effects). In addition, the size fitting of tannic acid to the structure may be important. Adsorption quantities of tannic acid on the LbL film at equilibrium exhibited a sigmoidal profile at low concentrations. Highly cooperative behavior might result from confinement effects during adsorption. Similarly, LbL assemblies of mesoporous carbon capsules have been assembled with counterionic polyelectrolyte.63 These LbL films were tested for their detection of specific molecules in the gas phase. Aromatic hydrocarbons such as benzene and toluene are better detected in this sensing system than aliphatic hydrocarbons such as cyclohexane. The selectivity could be easily tuned by impregnation with additional recognition components, which can be introduced after film preparation. The carbon capsule film impregnated with lauric acid showed the greatest affinity for nonaromatic amines, and the impregnation of dodecylamine in the carbon capsule films resulted in a strong preference for acetic acid. LbL-assembled films of a mesoporous silica capsule can be used for unusual material delivery.64 The frequency shifts upon water evaporation from the mesoporous capsule films possess a stepwise profile even though no external stimulus was applied (Figure 13). The observed stepwise release is assumed to originate from the combination of two processes,

Figure 11. Electrochemical coupling layer-by-layer (ECC-LbL) upon the electrochemically driven dimerization of N-alkylcarbazole to immobilize functional units such as porphyrin, fluorene, and fullerene.

assembly of various carbazole derivatives carrying distinctive donor (porphyrin and fluorene) and acceptor (fullerene) moieties was demonstrated. For example, ECC-LbL assembly with alternating layers of porphyrin and fullerene showed switching behavior in absorbance at 437 nm for the porphyrin Soret band during film assembly, indicating the electronic interlayer communications between porphyrin and fullerene. Thin-film photoelectronic performances of ECC-LbL films were investigated using a prototype p/n heterojunction device with the structure ITO/porphyrin (donor)/fullerene (acceptor)/Al. Regular photocurrent switching was realized. An appropriate variation of the film structure and composition for broad optical absorption, low contact resistance, and trapping density would improve the device performance. One of the prominent differences between this ECC-LbL method and the other LbL techniques is that assembling processes can be commanded from the solid supports. Area-selective assembly in patterning mode can be performed by varying the spatial application of voltage. In fact, ECC-LbL in area-selective patterned mode was conducted on an ITO electrode to prepare an area-selective display.58 Commanded assembly could be coupled with the techniques of top-down nanotechnology such as highly miniaturized integrated circuit formation. Layer-by-Layer (LbL) Assembly: Emerging Applications. Because LbL assembly has wide applicability for applicable

Figure 12. LbL assembly of mesoporous carbon CMK-3 with polyelectrolyte on a QCM plate for sensing applications in aqueous solution. I

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Figure 13. LbL assemled films of the mesoporous silica capsule for unusual material delivery and energyless, clean stimulus-free controlled drug release applications.

water evaporation from the pores and capillary penetration into the pores. Initially, water entrapped in mesopore channels evaporates to the exterior, which is observed as the first step of water release. This release profile was also used in a demonstration of controlled release of various fluid drugs such as fragrance molecules. This feature could be useful for controlled-release drug delivery and is also of great utility for the development of energyless, clean stimulus-free controlled drug release applications. The LbL assembly procedure is conducted under mild ambient conditions, which is highly advantageous for the immobilization of biological materials. Therefore, applications of LbL films to biological and biomedical fields are becoming increasingly popular.65 For example, according to a report by Komatsu et al., a virus trap nanotube was prepared by the LbL assembly of human serum albumin, poly(L-arginine), poly(L-glutamic acid), and antiHBsAb (hepatitis B surface antigen)-antibody within a nanoporous polycarbonate membrane.66 The removal of the porous polycarbonate template resulted in multilayered protein nanotubes having an anti-HBsAg antibody layer as an internal wall. They anticipated that this kind of structure would be used as a novel type of virus detection and removal apparatus. LbL applications do not stop at biomolecular immobilization. This method can be used even for cell modification. Akashi and coworkers used the LbL technique for designing artificial cell organization (Figure 14).67 In their method, a single cell was first coated with LbL films of fibronectin and gelatin. The fibronectin−gelatin films can interact with integrin receptors of the cell membrane, resulting in the cell−cell adhesion of all seeded cells in three dimensions. This method enables us to form cell architectures with a large diversity of the layer number, cell type, and location, which could be a promising methodology for the in vitro construction of tissue or organ models. Device applications are also a promising target as uses of LbL assemblies. The mildness and simple nature of this technique are also advantageous in using device structures as substrates of the LbL assembly. For example, Cho and co-workers prepared a biomemory device using the LbL assembly of anionic ferritin and cationic polyelectrolyte on Pt-coated substrates.68 Reversible resistance changes based on ferritin nanoparticles were derived by the charge trap/release of Fe3+/Fe2+ redox couples upon applying an external voltage. One ferritin nanoparticle can be operated as a nanoscale-memory device. In another surprising example, Sun and co-workers demonstrated an actuating device through LbL assembly (Figure 15).69 They prepared

Figure 14. Designing artificial cell organization with an LbL-coated cell.

Figure 15. Autowalking device decorated by LbL assembly.

actuators through attaching LbL bilayers of poly(acrylic acid)/ poly(allylamine hydrochloride) to UV-cured Norland Optical Adhesive 63 substrate. Because of the moisture sensitivity of this actuator, the modulation of relative humidity induced unidirectional walking of the device on a ratchet substrate. The swelling and contracting of polyelectrolyte LbL films depending on the surrounding moisture caused stretching and bending of the walking device forward in the designated direction. This design could be useful for miniaturized devices with rapid responses to a wide variety of stimuli.



FUTURE OF NANOARCHITECTONICS In this Feature Article, we reviewed several recent research works on interfacial nanoarchitectonics in three major categories: (i) J

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lateral nanoarchitectonics at a static interface, (ii) lateral nanoarchitectonics at dynamic interfaces, and (iii) vertical nanoarchitectonics from interfaces. The examples presented indicate the rapid development of the field of formation of molecular-level organization at an interface. It should be noted that these strategies can also be applied to surfaces of nonplanar nanoobjects such as nanoparticles and nanotubes.70−72 In addition, micro- and nanofabrication have already enabled us to prepare device structures that can be operated using atomicscale phenomena. We can expect the integration of functional molecules within ultrasmall device structures in a defined way. This will lead to the realization of our long-term dream: the operation of molecular machines by external inputs. The dynamic nature of an interface is advantageous for the connection of macroscopic dynamic motions with nano/ molecular phenomena. Such features may be coupled with biological functions because biological functions are usually operated by dynamic interactions within assemblies of wellarranged components. As several of the examples shown here have illustrated, biofunctional elements can be easily immobilized within thin films using lateral or vertical nanoarchitectonics. Therefore, the construction of artificial arrangements of various biomolecules such as proteins, carbohydrates, and DNA have become possible, yielding high-quality biomimetic functional systems such as light-harvesting, signal transduction, and immunological responses in artificial systems.73,74 Because device fabrication techniques are developing, device structures integrated with biomolecules can be constructed through interfacial nanoarchitectonics. It is said that the human body is an assembly of well-designed biodevices and biomachines. That system is the ultimate specimen of nanotechnology. Features of interfacial nanoarchitectonics capable of fusion with both device fabrication and biomolecular organization would significantly contribute to the ultimate goals of nanotechnology.



of the American Chemical Society since 1991 and currently serves as an editorial advisory board member for Langmuir, Chemisty of Materials, and ACS Applied Materials & Interfaces.

Taizo Mori received his Ph.D. from the department of polymer chemistry at Kyoto University in 2009. He is currently a postdoctoral researcher in the supermolecules group at the National Institute for Materials Science (NIMS).

AUTHOR INFORMATION Jonathan P. Hill received his Ph.D. degree from Brunel University, U.K, in 1995. He is currently subgroup leader of the supermolecules group at the National Institute for Materials Science. Current research interests include the synthesis and properties of tetrapyrroles and their supramolecular manifolds as well as unusual methods for preparing organic nanomaterials.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies



ACKNOWLEDGMENTS This work was partially supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, and the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST), Japan. We thank Dr. Yuji Okawa and Dr. Qingmin Ji for providing illustrations.



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