Advances in Soft Functional Materials Research

Soft matter plays an increasingly important role in our daily life. Soft materials together with metals and ceramics are listed among the top three most important materials. The research efforts on soft materials are targeting three main aspects, namely high performance, multi-functionality and smartness. In this regard, the current major attention from the community is focused on the identification and fabrication of new materials with high performance for use in devices, environmentally friendly, intelligent building materials and innovations in biomedicine-related fields. Since the soft matter research was started more than one hundred years ago, people have identified many systems of various types. All these materials share some common characteristics, such as notable thermal fluctuations, multiple metastable states, mesoscopic multiscale self-assembled structures, entropy-driven order-disorder transitions, macro flexibility. Briefly, these are the systems having “small stimulus, big response” and displaying strong nonlinearities. These characteristics are not so much related to their microstructures (at atomic or molecular levels) but more to their mesoscopic self-assembled structures. In the last few decades, soft materials have been widely examined in molecular machines, bio-sensors, filters, nano-reactors, which are widely applied to chemo-catalysis, drug delivery, gene delivery, biological imaging and tissue engineering. The research results have been generating great impact correspondingly. In particular, wearable, implantable, bio-degradable/absorbable and injectable flexible devices will exert a huge impact on human health and life. This special issue contains 7 feature articles and 14 full papers, covering a broad spectrum of the most important aspects in the research of soft materials, ranging from the approaches for self-assembly and characterization of soft materials, to their diverse applications in biological systems and optoelectronic devices. Thanks to the recent exciting advances of supramolecular chemistry, self-assembly by means of covalent and/or noncovalent molecular interactions with a synergistical effect has become one of the key techniques for constructing well-defined architectures for multiple functionalities of soft materials. The feature article by Zhang et al. (adfm.201602998 (8920)) highlights the approaches to functional assemblies based on a kind of newly emerging soft materials, namely the supra-amphiphiles. They are fabricated by various noncovalent interactions and/or dynamic covalent bonds, such as host-guest interaction, electrostatic interaction, charge-transfer interaction, hydrogen bond, coordination bond, imine bond, boronic ester bond, etc. The design strategies of supra-amphiphiles have been categorized into noncovalent synthesis and dynamic covalent synthesis, accordingly. The engineering of their molecular and supramolecular architectures leads to broad applications of these novel soft materials in the fields of surfactants, chemical analysis, nanocarriers, photodynamic therapies, surface modification, and catalysis, as summarized by Zhang and his colleagues. The dynamic nature of the covalent and/or noncovalent interactions, which is the fundamental of the responsiveness, reversibility and adaptivity, promises the integration of structural motifs with various functions. Colloids are a special type of soft matter which might be regarded as “big atoms”. In their feature article, Han et al. (adfm.201603077 (8903)) provide a comprehensive survey on the recent experimental and simulated research progresses on the superheating, melting, and premelting of colloidal crystals. Colloids in this context serve as an ideal model system for studying the melting of crystals, a process that is extensively studied but the mechanism of which remains a mystery. The melting processes of colloids as well as crystals can be either homogeneous or heterogeneous. The homogeneous melting of 3D crystals typically undergoes stages of incubation, nucleus growth, and coalescence. The heterogeneous melting of 3D crystals, however, starts usually from the surfaces or defects, and a premelting process is often involved. As for the melting of 2D crystals, two major theories have been introduced, namely the the Kosterlitz–Thouless–Halperin– Nelson–Young (KTHNY) scenario of two continuous transitions from solid → hexatic phase → liquid, and Chui's grain-boundary-mediated single-step first-order melting scenario. The direct observation of the dynamic melting of colloidal crystals provides deep insights in the melting kinetics of bulk crystals, which will improve the future design and development of materials based on colloids such as photonic crystals. Apart from self-assembly of synthetic building blocks, another way of obtaining novel soft materials is by the functionalization of natural materials. The feature article of Liu and colleagues (adfm.201603826 (8885)) introduces multiple functionalizations of silk fibroin materials, abundantly spun from silkworms or spiders. The silk fibers by itself have unique properties such as mechanical strength, biocompatibility and optical transparency, which can serve as ideal platforms for further functionalization for multiple applications. The authors introduce a “mesoscopic material assembly (MMA)” approach, involving in vivo or in vitro assemblies, implemented by three different paths: molecular recognition, surface functionalization of nanomaterials, or foreign molecule mediation. The as-functionalized silk fibers find applications in optical components such as bio-wave guides, laser damage protection, and white-light emitters, as well as for bio-imaging applications. The knowledge of functionalization of silk fibers by MMA provides guidelines of rational design of mesoscopic soft materials in a controllable manner. Soft materials such as carbon materials, polymers/macromolecules and biomaterials find diverse applications because of their rich surface chemistry and versatile functionalities. However, most of the soft materials commonly lack high durability and mechanical rigidity. The applications of these materials would be greatly extended if they can be endowed with similar stable, rigid scaffolds and hierarchically porous structures as those of hard materials. In the feature articles by Li and colleagues (adfm.201602697 (8824)), the strategies for building hard structures based on soft materials and their applications in the fields of electrochemical energy conversion and storage as well as in in-situ electrochemical molecular biosensing are extensively reviewed. Composite hard structures such as low dimensional 0D nanoparticles or 1D nanowire arrays, 2D graphene based layered structures, hierarchical and/or porous structures can be properly constructed based on soft materials by means of direct binding, layered or core/shell structures. Another approach to build such composite structures is to use the soft materials as template for the fabrication of hard structures. The resulting composite materials take advantage of both soft and hard materials with synergistic effects, resulting excellent physico- and electro-chemical properties for advanced applications as electrode materials in solar cells, fuel cells, electrochemical water splitting, lithium-ion batteries, lithium-sulfur batteries and/or supercapacitors and in-situ molecular biosensing. The understanding of the mechanisms of the improved performances of these composite materials is discussed and is expected to offer fundamental guidance for the further development of the performance of soft materials. Biomaterials, one of the most important categories of soft materials, can also serve as mediator for the growth of one of the most important hard materials in living creatures, namely bones. In their feature article, Wang and co-workers (adfm.201603197 (8810)) highlight the applications of biomaterials as enhancers of growth factors in bone regeneration. The regeneration of bone defects has been a serious clinical challenge for orthopedic surgeons in modern society with its increasing life expectancy and aging population. In this context, the integration of biomaterials and growth becomes a powerful strategy for bone regenerative therapy. Soft materials have the capacity of harnessing the interactions between the growth factor and the specific cells, and therefore promoting bone formation. The reinforcing mechanisms involve mainly temporal regulation and efficacy manipulation. The superior communications among growth factors, cells, and biomaterials in the micro environment are expected to provide new concepts of designing medical devices for bone regeneration. The development of soft materials also provides opportunities for energy devices, especially for flexible and wearable energy converters. Guo et al. (adfm.201603378 (8855)) provide an overview on the recent developments of transparent thin film solar cells, with special emphasis on flexible transparent conducting electrodes (TCEs) without conducting oxides. The flexible indium tin oxide (ITO)-free thin film solar cells (SCs), including organic solar cells, dye-sensitized solar cells and perovskite solar cells, have been categorized into three groups according to their configurations: (i) frontside illuminated planar configuration; (ii) back-side illuminated planar configuration, and (iii) fiber-shaped solar cells. In the near future, ITO-free TCEs are expected to occupy more and more market share in flexible thin film SCs due to the technical breakthroughs in mass production of ITO-alternative TCEs and development of new functional materials and SCs. The development of advanced analyzing techniques provides opportunities for the characterization of novel soft materials. In his feature article, Elemans (adfm.201603145 (8932)) reviewed the development of one particular tool, scanning tunneling microscopy (STM), to characterize and manipulate functional soft surfaces. Its versatility to operate in a wide range of environments, varying from ultrahigh vacuum (UHV) to ultralow temperatures to high pressures and liquids and in ambient atmospheres, and its superior spatial resolution in topography allows for the direct visualization at the molecular level. More importantly, the tunneling current of an STM is highly sensitive to electronic changes in the probed surface and thus can provide quantitative information about the local electronic density of states of adsorbed molecules and, by carefully choosing the tunneling conditions (bias voltage Vbias and tunneling current iset), various parts of a molecule can be identified, even if they are topographically obscured by additional layers of molecules. The feature article highlights the capacities of STM to characterize functional 2D self-assembly at the solid-liquid interface and the possibilities to change the organization of 2D assemblies after their formation by externally applied manipulation. This ability allows for a molecular level understanding of the interplay between surface, liquid and molecular assemblies formed between the two phases, which opens new avenues for creating functional surfaces in these chemically and biologically relevant and realistic environments. In organizing this special issue, we received invaluable support from Dr. Jörn Ritterbusch, Editor-in-Chief, and Dr. Till von Graberg, Editor of Advanced Functional Materials. Our appreciation also extends to the whole editorial team of Advanced Functional Materials for their professionalism throughout the whole editing process. We are grateful for the supports from the National Nature Science Foundation of China (NSFC), the Netherlands Organization for Scientific Research (NWO), and the Royal Netherlands Academy of Arts and Sciences (KNAW). Finally, we are greatly indebted to all contributing authors of this special issue for their efforts and enthusiastic supports. Although this special issue covers only the recent progress in some areas of soft matter research, it does highlight the major advances in some emerging key research topics. We hope that this special issue brings readers a timely overview on the latest breakthroughs in the related areas. Finally, it is our sincere wish that this will further stimulate exciting multilateral collaborations among the international scientific communities. Jialiang Xu obtained his PhD in July 2010 from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), under the supervision of Prof. Yuliang Li. Afterwards he joined as a Marie-Curie Fellow in Prof. Alan E. Rowan's group at Radboud University, Nijmegen. In July 2013, he started to develop his own research line at the interface of Chemistry and Physics, to study the coupling between light and (supra-)molecular materials, hosted by Prof. Theo Rasing in the Institute for Molecules and Materials (IMM), Radboud University. He is now an Associate Professor at the School of Chemical Engineering and Technology, Tianjin University. Xiang Yang Liu is a tenured full Professor in the National University of Singapore, and also the State Distinguished Professor of the 1000 Talent Program, Chair Professor of China MOE Changjiang Scholars Program in Xiamen University. Prof. Liu serves for many national and international boards/Societies, i.e. the 5th President of the Asia Association for Crystal Growth and Crystal Technology, councilor of the International Organization for Crystal Growth, etc. Prof. Liu's research interests range from biomimetics, crystallization, soft functional materials and flexible devices, etc. He has received numerous awards. i.e. outstanding researcher's award, in the last decade. Alan E. Rowan studied at the University of Liverpool, England, where he obtained his PhD in Physical Organic Chemistry in 1992. In 1994 he moved to Radboud University, Nijmegen, the Netherlands as a junior lecturer in the area of hierarchal materials, where he remained and became Full Professor and head of and the Department of Molecular Materials in 2005. He is now the Director of Australian Institute for Bioengineering and Nanotechnology (AIBN), at The University of Queensland. His interests are ‘how things work' and in particular the relationship between molecular architecture and function, in self-assembling (bio)-mimetic materials. Theo Rasing is full professor of experimental physics and Director of the Institute for Molecules and Materials at Radboud University Nijmegen, elected member of the Royal Dutch Academy of Arts and Sciences (KNAW) and of the Academia Europaea and member of the Executive Board of the Netherlands organization for Fundamental Research FOM. After his PhD (Nijmegen) and postdoctoral stay at UC Berkeley he became staff scientist at the Lawrence Berkeley National Laboratory, where he developed nonlinear optical techniques for surface and interface studies. His interests are the interaction and manipulation of light with matter and vice-versa, in particular in the areas of ultrafast magnetism and molecular materials.