The invention and development of the scanning tunnel-Three-dimensional imaging, atomic resolution, and operations ing microscope (STM) have opened new and original approaches underwater have prompted the application of STM for imaging for atomic-and nanometer-scale studies of surfaces. However, its biomolecules. A variety of organic and biological molecules have application for imaging biomolecules has to overcome the poor elecbeen examined by STM [3]. DNA bases [4], DNA [5], DNAtrical conductivity of biological samples. This article describes an protein complexes [6], globular and elongated proteins [7-9], operation mode of the STM that allows high-resolution imaging of several protein membranes [10-12], and microtubules [13] are hydrated purple membranes and their selective modification. The imsome examples of biomolecules studied by STM. However, those aging requires very low currents (below 1 pA) and applied voltages
experiments have to deal with the poor electrical conductivity of above 5 V. This mode also allows performance of nanometer-scale biomolecules that makes it difficult to establish a constant current.
modifications of the membranes. These modifications are generated by removing the proteins and lipids from a selected region of the They also pose the problem of the contrast mechanism that allows membrane. The removal takes place by establishing tip-membrane their imaging by STM.
mechanical contact. This happens when the operating current is Several models and calculations based on the coupling beabove 2 pA. These experiments pose the problem of electron transtween substrate and molecule orbitals [14], resonant tunneling port through 5-10-nm-thick insulating materials. We propose a model , or the modulation of the effective barrier by polarizable in which the contrast mechanism is controlled by two factors: the molecular adsorbates [16] give some reasonable explanations of electric field at the interface and the transmission through empty the observed contrast for very thin organic films (below 1 nm).
states in the membrane. We also compare these results with STM
The situation is more complicated when biomolecules are inexperiments imaging DNA molecules deposited on insulating substrates. There, the contrast is based on the lateral conductivity of volved, and direct tunneling through them gives a negligible conwater films.