Direct Observation of Spatiotemporal Heterogeneous Gelation by Rotational Tracking of a Single Anisotropic Nanoprobe

Polymer network gels usually exhibit spatial heterogeneity of local defects and cross-link density, which can affect their elasticity on the microscopic scale differently. The ability to evaluate the formation and distribution of these heterogeneities is important for guiding the application of gels in biology, medicine, and separation science. Previously, it has been reported that single-particle tracking based microrheology could provide local properties of gel networks with high resolution; however, the particle probes have been limited to spherical micro/nanotracers undergoing translational motions. In this work, we used single gold nanorods (AuNRs) as rotational microrheology probes to study the polyacrylamide gelation process by dual-channel polarization dark-field microscopy. The AuNRs were in Brownian motion during the initial stages of the gelation. As the reaction continues, individual AuNRs are confined locally and almost lost translational motion, but still maintained rotational motion. As the reaction proceeded further, the rotation state of the AuNRs gradually changed from free rotation in 3D to restricted rotation in 2D and eventually stopped completely. The appearance of the intermediate 2D plane indicated the existence of localized anisotropic compression of the gel during the heterogeneous gelation process. Our method can be further applied to investigate the formation of different polymer gels and a wide variety of heterogeneous biophysical and soft material systems.