Near-field fluorescence imaging of genetic material: toward the molecular limit

Chromosomes, DNA, and single fluorescent molecules are studied using an aperture-type near-field scanning optical microscope with tuning fork shear force feedback. Fluorescence in situ hybridization labels on repetitive and single copy probes on human metaphase chromosomes are imaged with a width of 80 nm, allowing their localisation with nanometer accuracy, in direct correlation with the simultaneously obtained topography. Single fluorophores, both in polymer and covalently attached to amino-silanized glass, are imaged using two-channel fluorescence polarization detection. The molecules are selectively excited according to their dipole orientation. The orientation of the dipole moment of all molecules in one image could be directly determined. Rotational dynamics on a 10-ms to 100-s timescale is observed. Finally, shear force imaging of double-stranded DNA with a vertical sensitivity of 0.2 nm is presented. A DNA height of 1.4 nm is measured, which indicates the nondisturbing character of the shear force mechanism.