The Fascinating World of the Functional Hybrid and Biohybrid Materials

The world of materials of organic and inorganic origin converges in a wide family of so-called organic-inorganic hybrid materials that show the characteristic aspects of both types of components and in some cases additional functionalities inherent to the new hybrid compound. These materials represent a fast-growing subject dealing with very important implications from the Academia and from the Industry point of view, covering areas of interest as important as biomedicine, pharmacy, and health care, environmental science and technology, engineering and building materials, energy conversion and storage, among other crucial fields. Surprisingly, this topic is not new at all. In fact, since the dawn of humanity, hybrid materials have been developed, such as those used in prehistorical mural paintings. Several centuries ago, clay-organic materials were already applied in cosmetics, medicine, and stabilized organic-dyes excellently represented by the Maya Blue pigments prepared more than five hundred years ago using nanotech approaches. Modern hybrids make reference to advanced nanostructured materials showing specific functionalities conferred by the deliberate inclusion of active organic groups, diverse metals or metal-organic compounds, inorganic nanoparticles, carbonaceous solids, or bioactive species. In this latter class of materials, which are known as “biohybrids”, the organic moiety stems from biological origin that is of great interest since it can present an evolved functional activity. In the context of the present Themed Issue various contributions deal with advanced biohybrid systems and materials. The selected subject can be considered as an emerging topic receiving increasing attention from diverse research groups around the world. According to the ISI WOS (consulted in May 2018) approximately 175,000 articles dealing with “hybrid materials” were published, about 70,000 in the last 5 years. In particular, ca. 1,250 articles dealing with “Functional organic-inorganic hybrids” were published in the past 5 years with a total of 18,500 citations (average citations per year: 1,850), showing a marked tendency to exponential growth with time which is symptomatic of a topic of evolving behavior. This Themed Issue is devoted to the latest developments of organic-inorganic hybrid materials emphasizing on its facility to introduce functional and multifunctional properties counting on the valuable contribution of outstanding authors as consolidated experts in the matter. This Special Issue contains a total of 22 articles with a balanced number of 11 Feature Articles and 11 Full Papers from eminent research groups. The main topics dealt with in this special issue can be grouped around clay-based hybrids, biohybrids, nanocellulose hybrids, and opto/energy applications. An excellent overview of the field is provided by Faustini et al. (article number 1704158) who share with us their historical perception of hybrid materials science spanning prebiotic, organomineral hybrid chemistry; silicon chemistries; and soft chemistry sol-gel developments until modern day bioinspired hybrids including an outlook towards future “autonomous” materials. Along these lines Ariga et al. (article number 1702905) review recent concepts within the framework of nanoarchitectonics to build functional hybrid materials including nanomachines inspired by biological self-assembly mechanism. Clay-based hybrids and nanocomposites have a long-standing history being employed by mankind since millennia as also reviewed in the paper by Faustini et al.. However, only more recently within the last 1–2 decades these hybrids entered the realm of functional materials, while they initially came into focus as structural, mechanically reinforced materials. This Special Issue counts with five contributions touching different aspects of functional clay-organic hybrids. For instance, a review on traditional and state-of-the-art clay-carbon hybrids by Darder et al. (article number 1704323), a review on clay-ionic liquid nanoarchitectures with applications in electro-chemical sensors by Dedzo et al. (article number 1703845), while Micó-Vicent et al. (article number 1703553) summarize recent developments in bleaching resistant, colored materials based on pigments stabilized on platy and tubule nanoclays. On the other hand, the Grunlan group contribute with a full article on environmentally benign flame retardant polymer nanocomposites foams brought about by a layer-by-layer deposition involving halloysite clay nanotubes (Smith et al., article number 1703289). An exciting development in the field is the emergence of biohybrid materials that are composed of inorganic nanoparticles and biological molecules comprising lipids, proteins, biosurfactants, or enzymes, etc. This development is also reflected in this Special Issue with contributions from renowned research groups like the ones of Prof. Mezzenga (article number 1703609), Prof. Terrones (article number 1704990), Prof. Kaplan (article number 1702570), and Prof. Choy (article number 1704470). Specifically, Nyström et al. (article number 1703609) highlight the potential of amyloid protein fibrils to construct organic-inorganic hybrid aerogels with noble metal nanoparticles. On the other hand, Demirel et al. (article number 1704990) present a detailed survey on the controlled assembly of 2D crystals like MXenes, transition metal dichalcogenides, and graphene with biomolecules to form complex hybrids with programmable properties. Regarding biomineralization at the organic-inorganic interface Martín-Moldes et al. (article number 1702570) demonstrate a link between silica-based biomaterials and key intracellular pathways by using recombinant silk–silica fusion chimeras, which is important for the understanding of osteogenesis and bone regeneration. Finally, Choi et al. (article number 1704470) present an attempt to improve boron neutron capture therapy in tumor treatment by using layered double hydroxides as efficient endocytic nanocarriers of boron metal-organic clusters. Nanocellulose has witnessed an impressive surge of attention over the last decade owing to its outstanding mechanical, optical, and chemical properties, which are ever more exploited in hybrid and nanocomposite materials. In this context Liu et al. (article number 1703277) and González del Campo et al. (article number 1703048) explore functional nanocomposite films based on clay-nanocellulose. While in the first contribution the mechanical and optical aspects of the nacre-like microstructures are the focal point of the investigation, the latter contribution introduces additional functionalities to the nanocomposite films by incorporating catalytic, conducting and/or magnetic nanoparticles. Functional hybrid films are also presented by Wicklein et al. (article number 1704274), who studied the dual self-assembly of nanocellulose and V2O5 nanofibers rendering conducting, electrochromic films with potential interest in flexible optoelectronic devices. The formation of organic-inorganic hybrid materials is in many cases governed by the controlled self-assembly of the individual nanoscopic building blocks, which is the basis for the exceptional structural and functional properties of these materials. Herein, one of the important interactions between building blocks are hydrogen bonds, which are reviewed by Nonappa and Ikkala (article number 1704328). They describe how H-bonding can direct colloids into self-assembled 2D mesocrystals, capsids, and supracolloidal assemblies for optical and medical applications. Self-assembly mechanisms also play a crucial role in the formation of porous hybrid materials reviewed by Park and Ha (article number 1703814) and Shirman et al. (article number 1704559), revisiting hollow mesoporous hybrids and organic-inorganic raspberry particles, respectively. In the latter case a modular design approach is presented, which allows combinatorial variations of the material building blocks and their organization, resulting in the design of efficient and robust systems for a wide range of catalytic and photocatalytic reactions. Organic-inorganic hybrid materials have always been of great interest for sensing and energy related applications thanks to their molecularly exact design and functional diversity. In this Special Issue Gil-Herrera et al. (article number 1703885) report on hierarchically porous carbon photonic structures by self-assembly of heterogeneous core-shell and hollow carbon spheres showing a highly sensitive response to humidity. Another optical humidity sensor is introduced by Castellón et al. (article number 1704717) using deliquescent compounds embedded in a hybrid silica-titania matrix. The energy field is touched by Ambrosi and Pumera (article number 1700655), who describe a 3D metal and polymer printing process to fabricate a water electrolyzer. On the other hand, sodium and potassium ion batteries have become interesting and viable alternatives to the Li ion batteries partially owing to the recognition of the Na and K containing coordination compound Prussian blue (PB). In this context, Morant-Giner et al. (article number 1706125) present an approach for the preparation of PB/MoS2-based nanocomposites that exhibit excellent performance as cathode in Na+ and K+ ion batteries. We are greatly indebted to all the contributing authors for their significant efforts and enthusiastic support of this Special Issue, and we sincerely hope that the readers of Advanced Functional Materials will enjoy this issue receiving a timely overview of the most recent breakthroughs in the related areas of hybrid materials. We further hope that this Special Issue will continue to stimulate exciting discussions and interdisciplinary collaborations among the international scientific communities. Finally, we would like to acknowledge the great support and cooperation from Dr. Till von Graberg, Editor of Advanced Functional Materials, and thank him for his kind assistance in organizing this special issue. Our gratitude also goes to the whole editorial office of Advanced Functional Materials. Eduardo Ruiz-Hitzky & Bernd Wicklein Guest Editors Eduardo Ruiz-Hitzky is currently an Ad Honorem Research Professor at the National Research Council of Spain (CSIC), Head of the Hybrid, Biohybrid and Porous Nanostructured Materials Group at the Materials Science Institute of Madrid (ICMM). Graduate and PhD in Chemistry (Universidad Complutense, Madrid) and Docteur ès Sciences, (Université Catholique de Louvain, Belgium), since 1987 has been Founder and First Director of several Departments at the CSIC including the “New Architectures in Materials Chemistry Department” created at the ICMM (2010). His current research focuses on three main topics: i) nanostructured inorganic heterostructures, ii) graphene-based functional composites and, iii) bio-hybrids and bio-nanocomposite materials. Bernd Wicklein graduated from Stuttgart University in Materials Science. After he obtained his PhD from the Autonomous University of Madrid in 2011 he moved to Stockholm University for a postdoctoral position (2012–2014). Currently he is a research fellow at the Materials Science Institute of Madrid (ICMM). His main research interests are nanocellulose-based hybrids, biomass valorization, carbon materials for energy applications, and self-assembly phenomena.