Opto-Thermophoretic Attraction, Trapping, and Dynamic Manipulation of Lipid Vesicles

Lipid
vesicles are important biological assemblies, which are critical to biological
transport processes, and vesicles prepared in the lab are a workhorse
for studies of drug delivery, protein unfolding, biomolecular interactions,
compartmentalized chemistry, and stimuli-responsive sensing. The current
method of using optical tweezers for holding lipid vesicles in place
for single-vesicle studies suffers from limitations such as high optical
power, rigorous optics, and small difference in the refractive indices
of vesicles and water. Herein, we report the use of plasmonic heating
to trap vesicles in a temperature gradient, allowing long-range attraction,
parallel trapping, and dynamic manipulation. The capabilities and
limitations with respect to thermal effects on vesicle structure and
optical spectroscopy are discussed. This simple approach allows vesicle
manipulation using down to 3 orders of magnitude lower optical power
and at least an order of magnitude higher trapping stiffness per unit
power than traditional optical tweezers while using a simple optical
setup. In addition to the benefit provided by the relaxation of these
technical constraints, this technique can complement optical tweezers
to allow detailed studies on thermophoresis of optically trapped vesicles
and effects of locally generated thermal gradients on the physical
properties of lipid vesicles. Finally, the technique itself and the
large-scale collection of vesicles have huge potential for future
studies of vesicles relevant to detection of exosomes, lipid-raft
formation, and other areas relevant to the life sciences.