Aromatic-aromatic interactions enhance interfiber contacts for enzymatic formation of a spontaneously aligned supramolecular hydrogel

Abstract

Anisotropy or alignment is a critical feature of functional soft materials in living organisms, but it remains a challenge for spontaneously generating anisotropic gel materials. Here we report a molecular design that increases intermolecular aromatic-aromatic interactions of hydrogelators during enzymatic hydrogelation for spontaneously forming an anisotropic hydrogel. This process, relying on both aromatic-aromatic interactions and enzyme catalysis, results in spontaneously aligned supramolecular nanofibers as the matrices of a monodomain hydrogel that exhibits significant birefringence. This work, as the first example of monodomain hydrogels formed via an enzymatic reaction, illustrates a new biomimetic approach for generating aligned anisotropic soft materials.

Figure 1

(A) Illustration of aromatic–aromatic interactions, in a hydrogel (Gel_1b) formed by treating the solution of a precursor (1a) with an enzyme (ALP), to enhance interfiber interactions that favors alignment of nanofibers and results in an inherently anisotropic hydrogel that causes optical retardance. (B) Structures of two hydrogelators, differing only in the number of phenylalanine residues.

Figure 2

Optical images of Gel_1b and Gel_2b, formed by treating the solutions of 1a and 2a with ALP (2 U/mL) at the concentration of 0.8 wt % and pH of 7.4 overnight. The images are taken with the vials or capillaries (d = 0.3 mm) placed between cross polarizers, illuminating by ambient light. (A, B) Gel_1b in a vial and (C, D) Gel_2b in a vial. (E, F, G) Gel_1b formed in two capillaries that are placed at three different angles with the cross polarizers.

Figure 3

Polarized optical microscopy retardance images (scale bar = 100 μm) of (A) Gel_1b, (B) Gel_2b, (C) Gel_1b′, (D) Gel_2b′. The images are taken with a sample thickness of 235 μm and concentration of 0.8 wt %.

Figure 4

TEM images of the nanofiber matrices of (A) Gel_1b and (B) Gel_2b. The scale bar is 250 nm.

Figure 5

Rheological characterization of Gel_1b, Gel_2b, Gel_1b′, and Gel_2b′. (A) The strain dependence of the dynamic storage (G′) and loss storage (G″) is taken at a frequency equal to 6.28 rad/s, and (B) the frequency dependence is taken at a strain equal to 0.99%.