  We performed a visualization of membrane proteins labeled with 10-nm gold
nanoparticles in cells, using an all-optical method based on photothermal
interference contrast. The high sensitivity of the method and the stability of
the signals allows 3D imaging of individual nanoparticles without the drawbacks
of photobleaching and blinking inherent to fluorescent markers. A simple
analytical model is derived to account for the measurements of the signal
amplitude and the spatial resolution. The photothermal interference contrast
method provides an efficient, reproducible, and promising way to visualize low
amounts of proteins in cells by optical means.
