Scanning near-field optical microscopy of a cell membrane in liquid

The applications of scanning near-field optical microscopy to biological specimens under physiological conditions have so far been very rare since common techniques for a probe-sample distance control are not as well suited for operation in liquid as under ambient conditions. We have shown previously that our own approach for a distance control, based on a short aperture fibre probe and a tuning fork as force sensor in a tapping mode, works well even on soft material in water. By means of an electronic self-excitation circuit, which compensates for changes of the resonance frequency due to evaporation of liquid, the stability of the force feedback has now been further improved. We present further evidence for the excellent suitability of the tapping-mode-like distance control to an operation in liquid, for example, by force-imaging of double-stranded DNA. Moreover, we demonstrate that a nuclear envelope in liquid can be imaged with a high optical resolution of approximately 70 nm without affecting its structural integrity. Thereby, single nuclear pores in the nuclear envelope with a nearest neighbour distance of approximately 120 nm have been optically resolved for the first time.