Plasmonic Gold Nanorods as Probes of Soft Matter and Biological Systems

Doctoral thesis, 2026

Motion and structure at the nanoscale encode information about local forces, mechanical properties, and dynamic interactions that are difficult to access using conventional measurement techniques. This challenge is particularly pronounced in biological and soft-matter systems, where fluctuations and structural changes demand probes combining minimal perturbation with high spatial and temporal sensitivity. Plasmonic gold nanorods provide such a platform. Their localized surface plasmon resonance gives rise to strong light scattering for sensitive optical readout, while optical forces and torques enable controlled trapping, translation and rotation. This thesis explores gold nanorods as nanoscale probes in complex biological and physical

environments. Single nanorods are optically trapped and driven to rotate by circularly polarized light, transducing local mechanical activity into rotational-frequency fluctuations. Applied to living endothelial cells, this light-driven nanomotor platform resolves heterogeneous nanoscale motions across the nucleus, perinuclear region, and cell periphery, and uncovers transient oscillations that reveal short-lived mechanical events typically inaccessible to conventional methods. The same rotational control, applied beyond biology, enables low-contact profilometry through lateral scanning of a trapped nanorod over nanoscale surface features. In another approach, nanorods serve as intracellular tracers of vesicular motion. Dark-field microscopy combined with deep-learning-based particle tracking and state segmentation enables long-duration, high-throughput analysis of intracellular transport in prostate cancer cells, resolving confined, diffusive, and directed motion across large trajectory ensembles. This reveals distinct transport phenotypes, with diffusion dominating in both cell lines and directed transport occurring as short-lived bursts. Nanorod surface chemistry further shapes this behavior, modulating the coupling between vesicle loading and active transport. Together, these studies establish plasmonic gold nanorods as a versatile platform for probing nanoscale dynamics with minimal perturbation across biological and physical systems. The results highlight the pronounced heterogeneity of such environments and demonstrate how a single nanoscale probe, viewed through light, can resolve motion and structure across diverse complex systems.