Historically, high-resolution optical microscopy has played a vital role in biomedical research. The resolution of conventional optical microscopy is limited by the diffraction effect to about 200 nm and 500-900 nm in the radial and axial directions, respectively. This modest resolution is a major limitation in using optical microscopy for demanding applications. New techniques such as near field optical microscopy, scanning tunneling microscopy, and atomic force microscopy have been developed to circumvent this problem and have achieved atomic resolution in solid state specimens. Unfortunately, the applications of these new techniques have been difficult in biological systems. First, scanning probe techniques require mechanical interaction with the specimens. The mechanical forces exerted by the probe tip may cause sample deformation. This interaction further degrades the achievable image resolution. Resolutions in the range of 50 to 100 nm are often reported for near field optical systems. Second, scanning probe technique has a relatively low frame rate since the required high precision position control imposes severe bandwidth limitations. Scanning probe techniques are more suitable for studying static sample but often have difficulties in addressing many inherently dynamical phenomena in biology. Third, these are surface imaging techniques and are inherently limited in their ability to investigate the 3-D structures of biological specimens.
Effect of the gold crystallinity on the enhanced luminescence signal of scanning probe tips in apertureless near-field optical microscopy
