Extended research on molecular gels: From the perspective of development of three dimensional fluorescent sensing films and low-density porous materials

Abstract

Low molecular-mass gelators (LMMGs) were first reported in the 1930s, but interest in the field was dwindled for several decades until the mid-1990s when Shinkai, Weiss and their colleagues re-started the research in the field based on newly developed supramolecular principles. Since then, remarkable progress has been achieved on LMMGs-based molecular gel research. However, most studies till now have been focused on the design of new LMMGs, discovery of new gels, and reveal of new gelation mechanisms. As pointed out by Weiss in one of his recent reviews ( J Am Chem Soc , 2014, 136: 7519–7530), few applications of molecular gels have been realized as of yet. The reasons behind partially are their limited lifetimes and mechanical strengths at ambient conditions. Therefore, in future studies, the way to make molecular gels work into their advantages and promote their practical applications will be highly concerned. With consideration of the following characteristics of molecular gels, fluorescent films for sensing, and gel-emulsions based on LMMGs: (1) presence of three dimensional (3D) networked structures is a pre-requirement for molecular gel formation, (2) increasing porosity is beneficial for enhancing the sensing performance of fluorescent films, and (3) the internal structures of gel-emulsions would change from liquid-liquid to liquid-solid (liquid-gel) or even solid-solid (gel-gel) after gelation of one or both of the continuous and dispersed phases of the systems which may be induced by introduction of LMMGs, we introduced molecular gel strategy in fabrication of smart fluorescent sensing films and creation of new kind of gel-emulsions as well as the relevant low-density porous materials. It is known that the structure of a gel could be tuned by variation of the structure and concentration of the gelator used in the system. The gel structure is also dependent upon the nature of the solvent or solvent mixtures. These facts suggest that molecular gel structures could be largely tuned via different ways. Moreover, the molecular gel structures, which is in fact the self-assembled structures of the gelators within the gels, are generally ordered, at least partially ordered, due to competition of crystallization and dissolution during the gelation process, laying foundation for specific applications. Generally, the amount of gelator in a molecular gel accounts for only 2% (w/v) around, which means the main component of a gel is solvent. Therefore, evaporation of solvent produces networked porous xerogels, implying that porous and highly, at least partially, ordered films could be fabricated via a physical coating technique with the molecular gels as coating materials, which is the so called molecular gel strategy. The key point in the fabrication of high-performance fluorescence sensing films via the molecular gel strategy is design of fluorescent LMMGs. Through comprehensive study of the photophysical properties, the gel formation behaviors of the fluorescent gelators, fluorescent films with preferable adlayer structure could be obtained. In this way, a variety of fluorescent sensing films with 3D porous and networked structures have been developed in the authors’ laboratory during the last few years. The analytes studied include mainly aniline, ammonia, hydrogen chloride, nitro-aromatics, some organophosphorus pesticides, nerve agents, formaldehyde, and some illegal drugs, etc. As for gel-emulsions, stabilizer is the most important component in the systems. For conventional gel-emulsions, the stabilizers used are generally surfactants and micro/nano particles. Nearly ten years ago, we introduced LMMGs as a new kind of stabilizers. Unlike surfactants and micro-/nano-particles, the amount of LMMGs used in gel-emulsions is much less, which normally accounts less than 4% (w/v) of the continuous phase. The LMMGs-based gel-emulsions have better stability and several sticky problems encountered in conventional gel-emulsions have been avoided. In addition, the volume fraction of dispersed phase in the LMMGs-based gel- emulsions is not restricted by 74%, which is the minimum volume fraction of dispersed phase for surfactants or micro-/nano-particles based gel-emulsions. In other words, with introduction of LMMGs as stabilizers, the internal structures of gel-emulsions could be tuned in a much wider range, which is favorable for template preparation of porous polymeric materials. Importantly, less stabilizer consumption is favorable for cost saving and makes separation and purification of the final materials much easier. Based upon these considerations, LMMGs-based gel-emulsions have been employed for preparation of low density porous monoliths. The materials as obtained show remarkable adsorption of organic liquids both in condensed and gaseous phases. Moreover, the materials can also be used for absorption of oil (organic liquids) from immiscible oil-water mixtures. In this review paper, we elaborate the application of molecular gel strategy in developing functional surface and interface materials based on the work conducted in our group. We will summarize the challenges in further research related to molecular gels and make prospect on the future study on this important area of science.