分子凝胶的拓展研究:有序三维荧光传感薄膜和低密度多孔材料的创新制备

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

经过几十年的发展,分子凝胶研究取得了巨大的进步.然而,这些研究大多还停留在开发新的胶凝剂、发现新的凝胶体系和揭示新的胶凝机理.在未来的研究中,如何更好地发挥分子凝胶的优势,推进分子凝胶的现实应用已经成为该领域研究人员的共同期盼.考虑到:(1)有序或部分有序小分子胶凝剂三维网络结构的形成是分子凝胶赖以存在的基础,(2)内相多孔网状结构的存在有助于荧光敏感薄膜材料获得良好传感性能,(3)凝胶乳液的液-液两相特点有可能因凝胶作用的发生而转化为固-固(凝胶-凝胶)或固-液(凝胶-液)两相结构等因素,在过去的10年里,作者实验室将分子凝胶策略引入到新型荧光敏感薄膜材料、凝胶乳液和以凝胶乳液为模板的低密度多孔材料制备中,由此,获得了一系列性能优异的敏感薄膜材料和油水分离材料,拓展了分子凝胶研究.本文结合本课题组工作实际,阐述分子凝胶策略在功能表界面材料制备中的应用,提出分子凝胶拓展研究面临的主要挑战,展望分子凝胶拓展研究的前景和发展趋势.

Low molecular-mass gelators（LMMGs） were first reported in the 1930 s, but interest in the field was dwindled for several decades until the mid-1990 s 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.