The Marangoni contraction of sessile drops of a binary mixture of a volatile and a nonvolatile liquid has been investigated experimentally and theoretically. The origin of the contraction is the locally inhomogeneous evaporation rate of sessile drops. This leads to surface tension gradients and thus to a Marangoni flow. Simulations show that the interplay of Marangoni flow, capillary flow, diffusive transport, and evaporative losses can establish a quasistationary drop profile with an apparent nonzero contact angle even if both liquid components individually wet the substrate completely. Experiments with different solvents, initial mass fractions, and gaseous environments reveal a previously unknown universal power-law relation between the apparent contact angle and the relative undersaturation of the ambient atmosphere: θapp ∼ (RHeq – RH)1/3. This experimentally observed power law is in quantitative agreement with simulation results. The exponent can also be inferred from a scaling analysis of the hydrodynamic-evaporative evolution equations of a binary mixture of liquids with different volatilities.

Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.

The aim of this study was to develop a one-step synthesis of gold nanotriangles (NTs) in the presence of mixed phospholipid vesicles followed by a separation process to isolate purified NTs.

Anisotropic plasmonic particles such as gold nanotriangles have extraordinary structural, optical, and physicochemical properties. For many applications in different fields, it is essential to prepare them in a chemically and physically stable, structurally well-defined manner, e.g., as large and uniform coverage on a substrate. We present a direct method for the large scale close-packed monolayer formation of edge-to-edge ordered, ultrathin crystalline gold nanotriangles on Si wafers or quartz glass via the transfer of these asymmetric particles to the air-liquid interface after adding ethanol-toluene mixtures without any subsequent surface functionalization. X-ray diffraction monitoring of the close-packed, large area monolayer with a mosaicity of less than 0.1° allows for calibrating the temperature of the particles during continuous laser heating. This is important for characterizing the microscopic temperature of the metal particles in the plasmon-driven dimerization process of 4-nitrothiophenol (4-NTP) into 4,4'-dimercaptoazobenzene (DMAB), monitored in real time by surface-enhanced Raman scattering (SERS). The gold nanotriangles can act as a source of hot electrons and initiate the dimerization process.

The mechanism of nanotriangle formation in multivesicular vesicles (MMV) is investigated by using time dependent SAXS measurements in combination with UV-vis spectroscopy, light and transmission electron microscopy. In the first time period 6.5 nm sized spherical gold nanoparticles are formed inside of the vesicles, which build up soft nanoparticle aggregates. In situ SAXS experiments show a linear increase of the volume and molar mass of nanotriangles in the second time period. The volume growth rate of the triangles is 16.1 nm3/min and the growth rate in vertical direction only 0.02 nm/min. Therefore, flat nanotriangles with a thickness of 7 nm and diameters of 23 nm are formed. This process can be described by a diffusion-limited Ostwald ripening growth mechanism. TEM micrographs visualize soft coral-like structures with thin nanoplatelets at the periphery of the aggregates, which disaggregate in the third time period into nanotriangles and spherical particles. The 16 times faster growth of nanotriangles in lateral than that in vertical direction is related to the adsorption of symmetry breaking components, i.e., AOT and the polyampholyte PalPhBisCarb, on the {111} facets of the gold nanoplatelets in combination with confinement effects of the vesicular template phase.