Nanoscale Spatial Control over the Self-Assembly of Small Molecule Hydrogelators

Abstract

Spatial control over molecular self-assembly at the nano scale offers great potential for many high-tech applications, yet remains a challenging task. Here, we report a polymer brush-mediated strategy to confine the self-assembly of hydrazone-based hydrogelators exclusively at nanoparticle surfaces. The surfaces of these nanoparticles are grafted with negatively charged polyacrylic acid, which enrich protons that can catalyze the in situ formation and self-assembly of hydrazone-based gelators. We found that, with respect to the polymer lengths, the concentration of the nanoparticles presents more significant effects on the self-assembly process and the properties of the resultant hydrogels, including gelation time, stiffness, and network morphology. More interestingly, the hydrogel fibers are found to be formed specifically around the nanoparticles, demonstrating the directed nanoscale molecular self-assembly. This work demonstrates that triggering molecular self-assembly using catalysis can serve as an effective way to realize directed molecular self-assembly at the nano scale, which may serve as a powerful approach to improve many material properties, such as the mechanical properties of supramolecular materials as we found in this work.

Keywords: directed self-assembly; gelation; gels; small-molecular-weight-hydrogelators; supramolecular chemistry.

Figure 1

Concept of directed molecular self-assembly at the surfaces of nanoparticles: (a) scheme showing the catalytic formation and self-assembly of the hydrogelator HA₃; (b) the formation and self-assembly of HA₃ around the catalytic nanoparticles; (c) TEM image showing the formation of hydrogel fibers around the nanoparticles.

Figure 2

DLS results showing the dynamic hydration diameter of the nanoparticles grafted with negatively charged polymer chains of different lengths. All the nanoparticles grafted with PAA chains (Sample 1–3) are prepared using the same batch of core nanoparticles with a diameter of 96 nm.

Figure 3

Rheological tests on the hydrogel formation in the presence of different nanoparticles. Effects of (a) nanoparticle concentration and (b) brush length on the hydrogelation process and the stiffness of the resultant hydrogels. All the samples: [H] = 20 mM, [A] = 80, at pH 7.0.

Figure 4

CLSM images showing the hydrogel networks formed in the presence of different concentrations of nanoparticles. Samples: [H] = 20 mM, [A] = 80, [FITC] = 30 µM at pH 7.0.

Figure 5

SEM pictures showing the hydrogel network morphology formed in the presence of different nanoparticle concentrations, (a–c) is 0, 0.125, and 0.5 wt%, respectively, insets are the magnified images. Samples: [H] = 20 mM, [A] = 80 at pH 7.0; and (d) TEM pictures demonstrating the localized formation of the hydrogel fibers in the vicinity of the nanoparticles. The sample: [H] = 5 mM, [A] = 20 at pH 7.0.