A shrinkable dendritic glutamic acid based supromolecular gel: From microscopic volume phase transition to functional applications

Abstract

Supramolecular gel is an important class of soft materials formed through the self-assembly of low molecular weight molecules in water or organic solvents via various non-covalent interactions to immobilize the solvents. Supramolecular gels have attracted widespread attentions due to their structural and functional diversity. One of the important features of the gel materials is their responsiveness, which include the reversible gel-to-sol transition, macroscopic volume phase transition (shrinkage-swelling) and so on depending upon the external stimulus such as temperature, light, metal ions, electrical or magnetic field. Among these, volume phse transition is important since it can be used to develop various functions in drug release, biosensing, controlled microvalves, self-healing and so on. Although there are large amount of reports on the volume phase transition in polymer gels, there is fewer reproted on the supramolecualr gels due to the the inherent weak character of non-covalent interactions. Supramolecular hydrogels suffer the reality that the brittle 3D structures have difficulty in maintaining physical change such as folding and curling of the supramolecular chains as in polymer hydrogels to accordingly exhibit phase transition in volume (shrinkage-swelling) and shape (deformation- recovery). Among several shrinkable supramolecular gels, the dendritic glutamic acid amphiphile OGAc containing multi-carboxylic acid as the head group and a long alkyl chain appeared to be one of the good examples of such gels. This minireview focuses on the formation of the supramolecular hydrogel OGAc, the shrinkage-swelling transition, the stimuli-responsive together with such volume phase transition and the potential applications by utilizing the shrinkage/ swelling properties. The paper firstly introduces the unique reversible shrinking and swelling supramolecular gel triggered by divalent metal ions. Then the function of the various metal ions as the inducer for the gel shrinkage was discussed and a mechanism was proposed. Based on these discussions, the design concept for the shrinkable supramolecular gels was suggested. Secondly, the pH responsiveness of the OGAc-Mg2+ is discussed. Thirdly, in order to functionalize the supramolecular gels, charged organic compounds were introduced into the shrinkable gels. It was found that OGAc-AZOC2Py complex system also exhibited reversible shrinkage/swelling transition. Simultaneously, the gel showed a change in supramolecular chirality. Fourthly, the potential applications of the supramolecular assemblies by using the shrinkable gel are discussed, which mainly highlights the application in the efficient separation of the mixtures composed of positively and negatively charged dyes, visualized recognition of chiral amines based on OGAc- Mg2+-PIC system, the construction of dual reversible thermal and light-responsive chiroptical switches for OGAc-AZOC2Py hydrogel and controlled step-wise release of drug or biological substances in OGAc-Mg2+ gels. Finally, the future development and application of supramolecular shrinkable gel is outlooked. It is anticipated to design more sophisticated multi-functional materials based on reversible macroscopic volume phase transition and also to combine the advantage of the shrinkable polymer gels to develop new soft materials. It is expected that the shrinkable gels will find more applications in biological system and biotechnology.