2D Nanoarchitectonics: Soft Interfacial Media as Playgrounds for Microobjects, Molecular Machines, and Living Cells

Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Coatable 2D Conjugated Polymers Containing Bulky Macromolecular Guests for Thermal Imaging

Dynamic properties are derived from the structural flexibility of 2D polymers. Softening layered structures has the potential for tuning and enhancing the dynamic properties. In the present work, the flexibility of layered polydiacetylene (PDA) is tuned by the interlayer polymeric guests with different branching structures. PDA shows thermoresponsive color-change properties through shortening the effective conjugation length with molecular motion. Whereas the blue-to-red color transition is observed at certain threshold temperatures for the layered PDA without the interlayer guest, the intercalation of the bulky polymer guests lowers the starting temperature and widens the temperature range for the thermoresponsive color changes. The resultant layered composite of PDA and bulky polymer affords the homogeneous coating on substrates on the centimeter scale. The thermoresponsive color-change coating is applied to temperature-distribution imaging. The specific heat of liquids is colorimetrically estimated using the coating on the bottle. The coating on a silk cloth visualizes the temperature distribution on a simulated tissue during surgical operation using an ultrasonic coagulation cutting device. The coating can be applied to thermal imaging in a variety of fields. Moreover, the softening strategy contributes to explore dynamic properties of soft 2D materials.

Mechano-Nanoarchitectonics: Design and Function

Mechanical stimuli have rather ambiguous and less-specific features among various physical stimuli, but most materials exhibit a certain level of responses upon mechanical inputs. Unexplored sciences remain in mechanical responding systems as one of the frontiers of materials science. Nanoarchitectonics approaches for mechanically responding materials are discussed as mechano-nanoarchitectonics in this review article. Recent approaches on molecular and materials systems with mechanical response capabilities are first exemplified with two viewpoints: i) mechanical control of supramolecular assemblies and materials and ii) mechanical control and evaluation of atom/molecular level structures. In the following sections, special attentions on interfacial environments for mechano-nanoarchitectonics are emphasized. The section entitled iii) Mechanical Control of Molecular System at Dynamic Interface describes coupling of macroscopic mechanical forces and molecular-level phenomena. Delicate mechanical forces can be applied to functional molecules embedded at the air-water interface where operation of molecular machines and tuning of molecular receptors upon macroscopic mechanical actions are discussed. Finally, the important role of the interfacial media are further extended to the control of living cells as described in the section entitled iv) Mechanical Control of Biosystems. Pioneering approaches on cell fate regulations at liquid-liquid interfaces are discussed in addition to well-known mechanobiology.

Bio-interactive nanoarchitectonics with two-dimensional materials and environments

Like the proposal of nanotechnology by Richard Feynman, the nanoarchitectonics concept was initially proposed by Masakazu Aono. The nanoarchitectonics strategy conceptually fuses nanotechnology with other research fields including organic chemistry, supramolecular chemistry, micro/nanofabrication, materials science, and bio-related sciences, and aims to produce functional materials from nanoscale components. In this review article, bio-interactive nanoarchitectonics and two-dimensional materials and environments are discussed as a selected topic. The account gives general examples of nanoarchitectonics of two-dimensional materials for energy storage, catalysis, and biomedical applications, followed by explanations of bio-related applications with two-dimensional materials such as two-dimensional biomimetic nanosheets, fullerene nanosheets, and two-dimensional assemblies of one-dimensional fullerene nanowhiskers (FNWs). The discussion on bio-interactive nanoarchitectonics in two-dimensional environments further extends to liquid-liquid interfaces such as fluorocarbon-medium interfaces and viscous liquid interfaces as new frontiers of two-dimensional environments for bio-related applications. Controlling differentiation of stem cells at fluidic liquid interfaces is also discussed. Finally, a conclusive section briefly summarizes features of bio-interactive nanoarchitectonics with two-dimensional materials and environments and discusses possible future perspectives.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

2D Active Nanobots Based on Soft Nanoarchitectonics Powered by an Ultralow Fuel Concentration

Enzyme catalysis to power micro/nanomotors has received tremendous attention because of the vast potential in applications ranging from biomedicine to environmental remediation. However, the current design is mainly based on a complex three-dimensional (3D) architecture, with limited accessible surface areas for the catalytic sites, and thus require a higher fuel concentration to achieve active motion. Herein we report for the first time an enzyme-powered 2D nanobot, which was designed by a facile strategy based on soft nanoarchitectonics for active motion at an ultralow fuel concentration (0.003% H 2 O 2 ). The 2D nanobot exhibited efficient positive chemotactic behavior and the ability to swim against gravity by virtue of solutal buoyancy. As a proof-of-concept, the 2D nanobots showed an excellent capability for "on-the-fly" removal of methylene blue (MB) dye with an efficiency of 85%.

Fast-Moving Self-Propelled Droplets of a Nanocatalyzed Belousov-Zhabotinsky Reaction

Self-sustained locomotion by virtue of an internalized chemical reaction is a characteristic feature of living systems and has inspired researchers to develop such self-moving biomimetic systems. Here, we harness a self-oscillating Belousov-Zhabotinsky (BZ) reaction, a well-known chemical oscillator, with enhanced kinetics by virtue of our graphene-based catalytic mats, to elucidate the spontaneous locomotion of BZ reaction droplets. Specifically, our nanocatalysts comprise ruthenium nanoparticle decorations on graphene oxide, reduced graphene oxide, and graphene nanosheets, thereby creating 0D-2D heterostructures. We demonstrate that when these nanocatalyzed droplets of the BZ reaction are placed in an oil-surfactant medium, they exhibit a macroscopic translatory motion at the velocities of few millimeters per second. This motion is brought about by the combination of enhanced kinetics of the BZ reaction and the Marangoni effect. Our investigations reveal that the velocity of locomotion increases with the electrical conductivity of our nanocomposites. Moreover, we also show that the positive feedback generated by the reaction-diffusion phenomena results in an asymmetric distribution of surface tension that, in turn, facilitates the self-propelled motion of the BZ droplets. Finally, we explore a system of multiple nanocatalyzed BZ droplets and reveal a variety of motions caused by their mutual interactions. Our findings suggest that through the use of 0D-2D hybrid nanomaterials, it is possible to design fast-moving self-propelled synthetic objects for a variety of biomimetic applications.

Nanoarchitectonics on living cells

In this review article, the recent examples of nanoarchitectonics on living cells are briefly explained. Not limited to conventional polymers, functional polymers, biomaterials, nanotubes, nanoparticles (conventional and magnetic ones), various inorganic substances, metal-organic frameworks (MOFs), and other advanced materials have been used as components for nanoarchitectonic decorations for living cells. Despite these artificial processes, the cells can remain active or remain in hibernation without being killed. In most cases, basic functions of the cells are preserved and their resistances against external assaults are much enhanced. The possibilities of nanoarchitectonics on living cells would be high, equal to functional modifications with conventional materials. Living cells can be regarded as highly functionalized objects and have indispensable contributions to future materials nanoarchitectonics.

Nanoarchitectonics: what's coming next after nanotechnology?

In science and technology today, the crucial importance of the regulation of nanoscale objects and structures is well recognized. The production of functional material systems using nanoscale units can be achieved via the fusion of nanotechnology with the other research disciplines. This task is a part of the emerging concept of nanoarchitectonics, which is a concept moving beyond the area of nanotechnology. The concept of nanoarchitectonics is supposed to involve the architecting of functional materials using nanoscale units based on the principles of nanotechnology. In this focus article, the essences of nanotechnology and nanoarchitectonics are first explained, together with their historical backgrounds. Then, several examples of material production based on the concept of nanoarchitectonics are introduced via several approaches: (i) from atomic switches to neuromorphic networks; (ii) from atomic nanostructure control to environmental and energy applications; (iii) from interfacial processes to devices; and (iv) from biomolecular assemblies to life science. Finally, perspectives relating to the final goals of the nanoarchitectonics approach are discussed.

Progress in Molecular Nanoarchitectonics and Materials Nanoarchitectonics

Although various synthetic methodologies including organic synthesis, polymer chemistry, and materials science are the main contributors to the production of functional materials, the importance of regulation of nanoscale structures for better performance has become clear with recent science and technology developments. Therefore, a new research paradigm to produce functional material systems from nanoscale units has to be created as an advancement of nanoscale science. This task is assigned to an emerging concept, nanoarchitectonics, which aims to produce functional materials and functional structures from nanoscale unit components. This can be done through combining nanotechnology with the other research fields such as organic chemistry, supramolecular chemistry, materials science, and bio-related science. In this review article, the basic-level of nanoarchitectonics is first presented with atom/molecular-level structure formations and conversions from molecular units to functional materials. Then, two typical application-oriented nanoarchitectonics efforts in energy-oriented applications and bio-related applications are discussed. Finally, future directions of the molecular and materials nanoarchitectonics concepts for advancement of functional nanomaterials are briefly discussed.

Intercalation and flexibility chemistries of soft layered materials

Layered materials, alternate stackings of two or more components, are found in a wide range of scales. Chemists can design and synthesize layered structures containing functional units. The soft-type layered materials exhibit characteristic dynamic functions originating from two-dimensional (2D) anisotropy and structure flexibility. This feature article focuses on "intercalation" and "flexibility" as two new perspectives for designing soft layered materials. Intercalation of guests is a characteristic approach for design of layered structures. Flexibility is an important factor to control the dynamic functions of the layered structures. As a model case, the intercalation-induced tunable stimuli-responsive color-change properties of layered polydiacetylene (PDA) are introduced to study the impact of the intercalation and flexibility on the dynamic functions. Recently, layered materials have drastically expanded the research area from conventional rigid inorganic compounds to new self-assembled nanostructures consisting of organic components, such as polymers, metal-organic frameworks, and covalent-organic frameworks. These new layered architectures have potentials for exhibiting dynamic functions originating from the structure flexibility beyond the static properties originating from classical intercalation and host-guest chemistries. Therefore, intercalation and flexibility chemistries of soft layered materials are regarded as new perspectives for design of advanced dynamic functional materials.

Molecular recognition at the air-water interface: nanoarchitectonic design and physicochemical understanding

Although molecular recognition at the air-water interface has been researched for over 30 years, investigations on its fundamental aspects are still active research targets in current science. In this perspective article, developments and future possibilities of molecular recognition at the air-water interface from pioneering research efforts to current examples are overviewed especially from the physico-chemical viewpoints. Significant enhancements of binding constants for molecular recognition are actually observed at the air-water interface although molecular interactions such as hydrogen bonding are usually suppressed in aqueous media. Recent advanced analytical strategies for direct characterization of interfacial molecules also confirmed the promoted formation of hydrogen bonding at the air-water interfaces. Traditional quantum chemical approaches indicate that modulation of electronic distributions through effects from low-dielectric phases would be the origin of enhanced molecular interactions at the air-water interface. Further theoretical considerations suggest that unusual potential changes for enhanced molecular interactions are available only within a limited range from the interface. These results would be related with molecular recognition in biomolecular systems that is similarly supported by promoted molecular interactions in interfacial environments such as cell membranes, surfaces of protein interiors, and macromolecular interfaces.

Helicity Manipulation of a Double-Paddled Binaphthyl in a Two-Dimensional Matrix Field at the Air-Water Interface

Molecular behavior and functionality are affected by their prevailing immediate environment. Molecular machines function according to conformational variations and have been studied largely in solution states. In order to access more highly complex functional molecular machines, it is necessary to analyze and control them in various environments. We have designed and synthesized a bisbinaphthyldurene (BBD) molecule that has two binaphthyl groups connected through a central durene moiety, allowing for the formation of several conformers. In density functional theory (DFT) calculations, BBD has five major conformers, denoted anti-1/anti-2/syn-1/syn-2/flat. It has been demonstrated that BBD exhibits different conformations in solution (anti-1 and syn-1) than on a gold surface (syn dimer and flat). In this work, the ratio of BBD conformations has been controlled in mixed monolayers with several different lipids at an air-water interface in order to compare conformational activity under different conditions. The conformations of BBD in transferred films obtained by using Langmuir-Blodgett techniques were estimated from circular dichroism spectra and DFT calculations. It has been found that the conformation of BBD in the mixed monolayer depends on its aggregated state, which has been controlled here by the mechanical properties and miscibility. In mixed monolayers with "hard" lipids having less miscibility with BBD as well as in cast film, BBD is self-aggregated and mostly forms stable anti-1 and syn-1 conformations, while unstable anti-2 and syn-2 conformers dominated in the more dispersed states involving "soft" lipids, which show good miscibility with BBD. Conformational changes in BBD are due to the formation of different aggregated states in each mixed monolayer according to the miscibility. Overall, BBD molecular conformations (and the resulting spectra) could be tuned by controlling the environment whether in solution, on a solid substrate, or in an admixture with lipids at the air-water interface.

The evolution of molecular machines through interfacial nanoarchitectonics: from toys to tools

Molecular machines are often regarded as molecular artworks and sometimes as fancy molecular toys. However, many researchers strive to operate molecular machines as useful tools for realistic practical applications. In this perspective article, shifting the working environment of molecular machines from solution to interfacial media is discussed from the viewpoint of their evolution from scientific toys to useful tools. Following a short description of traditional research into molecular machines in solution and their nanotechnological manipulation on clean solid surfaces, pioneering research into molecular machine operation at dynamic interfaces, such as liquid surfaces, is discussed, along with cutting-edge research into molecular machine functions in living cells and their models. Biomolecular machines within organisms are the products of evolution over billions of years. We may nanoarchitect such sophisticated functional systems with artificial molecular machines within much shorter periods.

Don't Forget Langmuir-Blodgett Films 2020: Interfacial Nanoarchitectonics with Molecules, Materials, and Living Objects

Designing interfacial structures with nanoscale (or molecular) components is one of the important tasks in the nanoarchitectonics concept. In particular, the Langmuir-Blodgett (LB) method can become a promising and powerful strategy in interfacial nanoarchitectonics. From this viewpoint, the status of LB films in 2020 will be discussed in this feature article. After one section on the basics of interfacial nanoarchitectonics with the LB technique, various recent research examples of LB films are introduced according to classifications of (i) growing research, (ii) emerging research, and (iii) future research. In recent LB research, various materials other than traditional lipids and typical amphiphiles can be used as film components of the LB techniques. Two-dimensional materials, supramolecular structures such as metal organic frameworks, and biomaterials such as DNA origami pieces are capable of working as functional components in the LB assemblies. Possible working areas of the LB methods would cover emerging demands, including energy, environmental, and biomedical applications with a wide range of functional materials. In addition, forefront research such as molecular manipulation and cell fate control is conducted in LB-related interfacial science. The LB technique is a traditional and well-develop methodology for molecular films with a ca. 100 year history. However, there is plenty of room at the interfaces, as shown in LB research examples described in this feature article. It is hoped that the continuous development of the science and technology of the LB method make this technique an unforgettable methodology.