  Correlative light electron microscopy (CLEM) requires the availability of
robust probes which are visible both in light and electron microscopy. Here we
demonstrate a CLEM approach using small gold nanoparticles as a single probe.
Individual gold nanoparticles bound to the epidermal growth factor protein were
located with nanometric precision background-free in human cancer cells by
light microscopy using resonant four-wave-mixing (FWM), and were correlatively
mapped with high accuracy to the corresponding transmission electron microscopy
images. We used nanoparticles of 10 nm and 5 nm radius, and show a correlation
accuracy below 60 nm over an area larger than 10 um size, without the need for
additional fiducial markers. Correlation accuracy was improved to below 40 nm
by reducing systematic errors, while the localisation precision is below 10 nm.
Polarisation-resolved FWM correlates with nanoparticle shapes, promising for
multiplexing by shape recognition in future applications. Owing to the
photostability of gold nanoparticles and the applicability of FWM microscopy to
living cells, FWM-CLEM opens up a powerful alternative to fluorescence-based
methods.
