Construction of chemical-responsive supramolecular hydrogels from guest-modified cyclodextrins

A methodology for preparing supramolecular hydrogels from guest-modified cyclodextrins (CDs) based on the host-guest and hydrogen-bonding interactions of CDs is presented. Four types of modified CDs were synthesized to understand better the gelation mechanism. The 2D ROESY NMR spectrum of beta-CD-AmTNB (Am=amino, TNB=trinitrobenzene) reveals that the TNB group was included in the beta-CD cavity. Pulsed field gradient NMR (PFG NMR) spectroscopy and AFM show that beta-CD-AmTNB formed a supramolecular polymer in aqueous solution through head-to-tail stacking. Although beta-CD-AmTNB did not produce a hydrogel due to insufficient growth of supramolecular polymers, beta-CD-CiAmTNB (Ci=cinnamoyl) formed supramolecular fibrils through host-guest interactions. Hydrogen bonds between the cross-linked fibrils resulted in the hydrogel, which displayed excellent chemical-responsive properties. Gel-to-sol transitions occurred by adding 1-adamantane carboxylic acid (AdCA) or urea. (1)H NMR and induced circular dichroism (ICD) spectra reveal that AdCA released the guest parts from the CD cavity and that urea acts as a denaturing agent to break the hydrogen bonds between CDs. The hydrogel was also destroyed by adding beta-CD, which acts as the competitive host to reduce the fibrils. Furthermore, the gel changed to a sol by adding methyl orange (MO) as a guest compound, but the gel reappeared upon addition of alpha-CD, which is a stronger host for MO.