Plasmonic gold nanorod instability and reshaping behavior below melting points are important for many future applications but are yet to be fully understood, with existing nanoparticle melting theories unable to explain the observations. Here, we have systematically studied the photothermal reshaping behavior of gold nanorods irradiated with femtosecond laser pulses to report that the instability is driven by curvature-induced surface diffusion rather than a threshold melting process, and that the stability dramatically decreases with increasing aspect ratio. We successfully utilized the surface diffusion model to explain the observations and found that the activation energy for surface diffusion was dependent on the aspect ratio of the rods, from 0.6 eV for aspect ratio of 5 to 1.5 eV for aspect ratio less than 3. This result indicates that the surface atoms are much easier to diffuse around in larger aspect ratio rods than in shorter rods and can induce reshaping at any given temperature. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement will therefore need to consider surface diffusion driven shape change even at low temperatures.

Recent advances in two-photon excited photoluminescence of plasmonic gold nanorods have greatly expanded their application. Shape-controlling of nanorods can enhance a particular photophysical process to tailor needs, but its application to two-photon luminescence is yet to be fully developed despite its importance in biolabeling. Here, we report direct, independent measurements of two-photon action cross sections (TPACS) of single gold bipyramids, hemispherically capped cylindrical nanorods, and dumbbells. The effect of radius of curvature at the tip on TPACS are measured and compared per same aspect ratio and volume. Bipyramids have shown 33% more TPACS per volume for the aspect ratio range 3.8–4.2, with 0.28 GM/nm3 compared to 0.21 and 0.20 GM/nm3 for nanorods and dumbbells, respectively, at 27% measurement error. This value is not as high as the field strength at the tip, which can be 3–4 times higher for sharp bipyramids. Such moderate increase is attributed to the small surface area of the tip that the field is subjected to which competes with the increase in field. We have used the z-scan technique to measure nonlinear absorption on a single crystalline gold nanosheet as a bulk gold and an analytical field enhancement theory for prolate spheroids to successfully confirm the trend of TPACS/volume with respect to the aspect ratio. It was found that the prolate spheroids do provide a good approximation of TPACS values for these rods. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement and subsequent two-photon luminescence will need to consider the tip surface area that limits the overall efficiency of the process.

Cholesterol is believed to induce the formation of membrane domains, “rafts”, which are implicated in a range of natural and pathologic membrane processes. Therefore, it is important to understand the role that cholesterol plays in the formation of these structures. Here, we use label-free spectroscopic imaging to investigate cholesterol fractioning in supported bilayer membranes at nanoscale. Scattering-type scanning near-field optical microscopy (s-SNOM) was used to visualize the formation of cholesterol-induced domains in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes. Our results revealed the coexistence of phase separated domains in DMPC lipids with 10 mol % cholesterol content, whereas a mostly homogeneous bilayer was found at low (5 mol %) and high (15 mol %) cholesterol content. Near-field nano-FTIR spectroscopy was used to identify the cholesterol-rich domains based on their qualitative chemical compositions. It was determined that cholesterol binds to phosphodiester and alkyl glycerol ester moieties, likely via hydrogen bonding of the alcohol to either of the ester oxygens. The results also confirm the existence of an ideal cholesterol-lipid mixture ratio (∼15:85) with a geometrically defined packing. At lower cholesterol content there is phase separation between liquid ordered and almost neat DMPC domains. Thus, the liquid ordered phase exists at an energy minimum at a given lipid–cholesterol ratio.