Application of Ionic Liquid on Biological Samples in Correlative Optical Microscopy and Scanning Electron Microscopy

Correlative optical microscopy and scanning electron microscopy (SEM) combines two techniques enabling the use both optical and electronic analysis techniques on the same sample. For example, optical techniques, such as fluorescence staining, fluorescence in situ hybridization (FISH) could in principle be used with SEM and energy dispersive spectroscopy (EDS) [1]. Optical microscopy provides efficient species identification, biomolecule-labeling and live sample imaging. On the other hand, SEM enables nanometer-spatial resolution and elemental mapping [2]. Conventional biological sample preparation for SEM, which involves fixation, dehydration and coating with gold or other heavy-metal, make subsequent optical microscopy difficult. With SEM sample preparation processes it is difficult to relocate same field of view in optical microscopy. Gold coating renders samples opaque under an optical beam. Moreover, the heavy-metal coating prevents EDS analysis. Ionic liquids (IL) are highly conductive, stable under electron beam and have extremely low vapor pressure, which renders them suitable to use in SEM at vacuum conditions around 10 Pa [3]. Using IL may eliminate some of the problems with carrying out correlative microscopy of biological samples. The IL sample preparation processes may also be less expensive and simpler than conventional methods. This paper reports the preliminary results from a feasibility study using IL in correlative microscopy.