Liquid crystal polymer networks (LCNs) have shown great promise as responsive materials due to their ability to undergo predictable shape changes by virtue of their intrinsic anisotropy. To complement order-disrupting triggers such as heat, we are developing LCN materials that employ enzymes as molecular triggers to actuate liquid crystal polymers upon exposure to chemical signals, which has the potential to enhance the selectivity and specificity of their chemical responsiveness. This approach was investigated by immobilizing the enzyme urease in hydrogen-bonded LCN films in a way that it retains activity. Since urease converts urea to ammonia, which readily reacts with sensitive moieties in the polymer network, incorporation of this enzyme has the potential to create LC materials that respond to the chemical signal urea.<br/>First, the carboxylic acids forming the supramolecular network were converted to acyl fluorides, which were subsequently reacted with lysine residues on the surface of the enzyme. Activity measurements and confocal fluorescence imaging confirmed retention of activity after attachment and widespread distribution of enzyme on the LCN surface. Interestingly, immobilized urease showed higher tolerance to challenging heat and pH conditions than free urease. After LCN<sub>Urease </sub>films were exposed to urea, enzymatic activity triggered a response in the material whereby newly-generated ammonia reacted with the remaining free acyl fluorides to form hydrogen-bonded benzamide dimers, which self-assembled in a non-covalent supramolecular 3D scaffold as confirmed by FT-IR. Initially flat twisted nematic films actuated into a helix after exposure to ammonia, proving that aminolysis generated a substantial transformation in the inner structure of the material. Furthermore, dynamic mechanical analysis revealed that fluorination reduced the elastic modulus of the material by an order of magnitude, while aminolysis stiffened the film to close to original levels due to the differences in non-covalent crosslinking density. These findings demonstrate the untapped potential of biochemical mechanisms to trigger LC polymer actuation and opens the door to the exploitation of LCNs in applications where chemical responsiveness and selectivity are crucial.

Harnessing the power of enzymes to actuate liquid crystal polymers

ACS Fall 2021

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