Metallic nanoparticles with diameters from 10 nm to 250 nm can be optically trapped and manipulated in 3D using
a single tightly focused near infrared laser beam. This will result in a significant heating of the particle and its vicinity,
with temperature increases easily reaching hundreds degrees Celsius. If such a hot metallic nanoparticle is brought
into the contact zone between two cells or vesicles, this local temperature increase can cause a total fusion of the
selected cells or vesicles. Upon fusion, both the membrane and the cargos become completely mixed and we also
show that the cells remain viable after fusion. The presented method has potential for single-cell targeted drug
delivery and for the creation of hybrid cells.
Optically trapped plasmonic nano-heaters are used to mediate efficient and controlled fusion of biological membranes. The fusion method is demonstrated by optically trapping plasmonic nanoparticles located in between vesicle membranes leading to rapid lipid and content mixing. As an interesting application we show how direct control over fusion can be used for studying diffusion of peripheral membrane proteins and their interactions with membranes and for studying protein reactions. Membrane proteins encapsulated in an inert vesicle can be transferred to a vesicle composed of negative lipids by optically induced fusion. Mixing of the two membranes results in a fused vesicle with a high affinity for the protein and we observe immediate membrane tubulation due to the activity of the protein. Fusion of distinct membrane compartments also has applications in small scale chemistry for realizing pico-liter reactions and offers many exciting applications within biology which are discussed here.
When irradiated at its resonance frequency, a metallic nanoparticle efficiently converts the absorbed energy into heat which is locally dissipated. This effect can be used in photothermal treatments, e.g., of cancer cells. However, to fully exploit the functionality of metallic nanoparticles as nanoscopic heat transducers, it is essential to know how the photothermal efficiency depends on parameters like size and shape. Here we present the measurements of the temperature profile around single irradiated gold nanorods and nanospheres placed on a biologically relevant matrix, a lipid bilayer.  We developed a novel assay based on molecular partitioning between two coexisting phases, the gel and fluid phase, within the bilayer. [2, 3] This assay allows for a direct measurement of local temperature gradients, an assay which does not necessitate any pre-assumptions about this system and is generally applicable to any irradiated nanoparticle system. The nanorods are irradiated with a tightly focused laser beam at a wavelength of 1064 nm where biological matter exhibits a minimum in absorption. By controlling the polarization of the laser light we show that the absorption of light by the nanorod and the corresponding dissipated heat strongly depends on the orientation of the nanorod with respect to the polarization. Finally, by comparing to spherical gold nanoparticles, we demonstrate how a change in shape, from spherical to rod like, leads to a dramatic enhancement of heating when using near infrared light.
Optically trapped metallic nanoparticles hold great promise as heat transducers in photothermal applications such as drug delivery assays or photothermal therapy. We use the heat dissipated from an optically trapped gold nanosphere to perform a controlled release of a fluorescently labeled vesicle lumen. In the assay, the ambient
temperature is kept below the phase transition temperature of the vesicle. When the temperature reaches the phase transition temperature of the lipid, the vesicle becomes leaky and the fluorescently marked lumen diffuses out. We used gel phase vesicles as sensors to quantify the temperature profile around a nanoparticle optically trapped in three dimensions in a similar way as presented in Ref.1 Trapping of 200 nm gold particles resulted in lower than expected heating, which may be accredited to the displacement of the particle from the optical focus due to high scattering forces experienced by the particle.