Multi-Stimuli Responsive Hybrid Gel:Design Rationale,Properties, And Applications

Abstract:Stimuli responsive gels refer to a novel class of functional materials which exhibit a wild application prospect in many fields, including signal sensors, molecular motors, shape memory devices, drug delivery systems, display devices, and other applications. By combing the redox and aromatic properities of ferrocenyl moiety with the excellent self-assembly ability and biocompatibility of phenylalaine group, weuccessfully designed a new multi-stimuli responsive hydrogelator, ferrocene-phenylalanine (Fc-F), which exhibits sharp and reversible phase transitions in response to a series of disparate stimuli, including oxidation-reduction reactions (redox), pH changes, temperature and shear force. So far, Fc-F is one of the most concise multi-stimuli responsive hydrogelator ever reported and the rare example of ferrocenyl gels to form well-organized nanofibril structure.By taking the size advantage of Fc-F, we performed DFT calculations that predicted the energies and possible geometries of monomeric, dimeric, and tetrameric arrangements that Fc-F might experience during the self-assembly process, respectively. On the basis of microscopic imaging results and DFT calculations, we have proposed a tentative mechanism of oligomerization. The key component of this hypothesis is that the complementary π-π and hydrogen bonding takes place between the ferrocenyl and benzene rings of the phenylalanino moieties and between both carboxylic acid groups, respectively, during dimer formation. Of particular importance is the hydrogen bond promoted assembly of two dimers in a side by side fashion to generated tetramers and π-π bonding to form a stacked tetrameric species. According to the interaction of Fc-F and β-cyclodextrin, it is further confirmed that Fc groups play a vital role in the formation of Fc-F fibrils.These intermolecular interactions are the key effects for Fc-F gels owning excellent stability, multi-stimuli responsivities, biocompatiblity, and extremely high salt tolerance, helping Fc-F gels to find uses as controllable drug medium, enzyme matrix, template for nanoparticle preparation or as chiral catalyst. We hope that Fc-F will serve as a valuable archetypical template for future researchers when designing new, multiresponsive hydrogelators.In addition, GO and its derivatives exhibit excellent electronic, mechanical and chemical properties, due to its two dimensional carbon network structure. Researchers have attempted to introduce GO and its derivatives into supramolecular systems in order to tailor their intrinsic undesirable properties. In this paper, we successfully combined the two functional factors, Fc-F and GO, to get an excellent hybrid hydrogel. The incorporation of GO can promote the fibrillation property of Fc-F, decrease its critical gelation concentration and improve its stablity on the electrode and enhance the responsive current significantly, by involving π-π stacking triggered co-assembly between GO and Fc. The last but not the least, we constructed a hybrid aerogel containing size-controllable Fe3O4nanoparticles based on Fc-F and GO hybrid hydrogel through hydrothemal method and heat treatment without addition of any reducing and conductive crosslinking agents. This kind of aerogels own low density and large specific surface area and exhibit a desirably catalytic effect on oxygen reduction reaction (ORR) with the catalytic current density reaching up to3mA·cm-2.