We demonstrate that optical trapping of multiple silver nanoparticles is strongly influenced by plasmonic coupling of the nanoparticles. Employing dark-field Rayleigh scattering imaging and spectroscopy on multiple silver nanoparticles optically trapped in three dimensions, we experimentally investigate the time-evolution of the coupled plasmon resonance and its influence on the trapping stability. With time the coupling strengthens, which is observed as a gradual red shift of the coupled plasmon scattering. When the coupled plasmon becomes resonant with the trapping laser wavelength, the trap is destabilized and nanoparticles are released from the trap. Modeling of the trapping potential and its comparison to the plasmonic heating efficiency at various nanoparticle separation distances suggests a thermal mechanism of the trap destabilization. Our findings provide insight into the specificity of three-dimensional optical manipulation of plasmonic nanostructures suitable for field enhancement, for example for surface-enhanced Raman scattering.

We present a simple method for the fabrication of highly luminescent gold-containing films by thermal decomposition of gold(I) dodecylthiolate synthesized by treating an ethanol solution of gold tetrachloroauric acid with an ethanol solution of 1-dodecanethiol. During the heat treatment of gold(I) dodecylthiolate complexes homogeneously distributed in the polystyrene matrix, extinction spectra of the films change from the well resolved spectrum of a gold(I) thiolate complex peaked at 390 nm to the broad plasmon peak of metallic gold centered around 600 nm. All samples, both before and after heat treatment, show a strong red emission, with a maximum in the spectral range of 620-640 nm. The room temperature emission quantum yield strongly increases from the nonheated films to the briefly heated films reaching a value of ∼8%, and decreases for the samples where larger gold particles are formed after prolonged heating. Transmission electron microscopy shows the occurrence of monodisperse gold nanoparticles with 1.8 nm mean diameter in the polymer matrix. Extended heat treatment leads to the partial coalescence of these nanoparticles into larger particles of irregular shapes, with a size in the range of 10-200 nm. We explain these findings by the formation from the original luminescent Au(I)-thiolate complexes of an emissive species with even stronger luminescence. This species is consumed in the growth of larger Au particles upon further heating. From the spectroscopic data, the strongly luminescent species could be few-atom thiolated Au clusters or polynuclear Au(I)-thiolate complexes with strong aurophilic interactions.

We explore a new application of optical tweezers for ultrasensitive detection of sound waves in liquid media. Position tracking of a single gold nanoparticle confined in a three-dimensional optical trap is used to readout acoustic vibrations at a sound power level down to -60  dB, causing a ∼90  μeV increase in kinetic energy of the nanoparticle. The unprecedented sensitivity of such a nanoear is achieved by processing the nanoparticle's motion in the frequency domain. The concept developed here will enable us to access the interior of biological microorganisms and micromechanical machines not accessible by other microscopy types.

Microfluidic jetting is a promising method to produce giant unilamellar phospholipid vesicles for mimicking living cells in biomedical studies. We have investigated the chemical composition of membranes of vesicles prepared using this approach by means of Raman scattering spectroscopy. The membranes of all jetted vesicles are found to contain residuals of the organic solvent decane used in the preparation of the initial planar membrane. The decane inclusions are randomly distributed over the vesicle surface area and vary in thickness from a few to several tens of nanometers. Our findings point out that the membrane properties of jetted vesicles may differ considerably from those of vesicles prepared by other methods and from those of living cells.

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