Direct Imaging of Transmembrane Dynamics of Single
Nanoparticles with Darkfield Microscopy: Improved Orientation Tracking
at Cell Sidewall
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Abstract
Investigation
of the cellular internalization processes of individual
nanoparticles (NPs) is of great scientific interest with implications
to drug delivery and NP biosafety. Herein, by using dual-channel polarization
darkfield microcopy (DFM) and single gold nanorods (AuNRs) as orientation
probes, we developed a method that is capable of monitoring AuNR orientation
dynamics during its transmembrane process. With annular oblique illumination
and a birefringent prism to split AuNR plasmonic scattering into two
channels of orthogonal polarizations, the AuNR azimuth and polar angles
are obtained from their intensity difference and intensity sum. By
placing the focal plane of the microscope objective at the elevated
cell sidewall rather than at the flat cell top, interference from
cellular background is reduced and the signal-to-noise ratio of AuNR
orientation sensing is improved significantly, especially for AuNRs
inserting into the membrane at a large out-of-plane angle. As a result,
we were able to capture the complete membrane-crossing dynamics of
single AuNRs. This powerful method could be utilized to obtain valuable
insights on NP endocytosis mechanisms of various cells