Atomic force microscopy as a multifunctional molecular toolbox in nanobiotechnology.

Nanobiotechnology — the characterization, design and application of biological systems at the nanometre scale — is a rapidly evolving area at the crossroads of nanoscience, biology and engineering1,2. One fascinating challenge in nanobiotechnology is the bottom-up design and operation of nanoscale machines and motors made up of supramolecular systems3,4. The molecular components of these systems are typically cellular machineries such as haemolysin5, porin6, myosin7, kinesin8, RNA polymerase9 and ATP synthase10. These machines can be set in motion in a controlled manner to work as bio-inspired nanoscale valves, engines, motors and shuttles. Progress in nanobiotechnology strongly relies on the development of scanning probe techniques, particularly atomic force microscopy (AFM)11,12. For the first time, AFM techniques are allowing us to observe and manipulate biomolecular machinery in its native environment (Fig. 1). The principle of AFM is to raster-scan a sharp tip over the sample surface and to probe interaction forces with piconewton sensitivity. AFM can image surfaces in buffer solution with outstanding signal-to-noise ratio, offering a means of observing single biomolecules without the need for fixation or staining. Cellular functions are governed ultimately by the dynamic character of their working parts, which makes them difficult to understand solely from static structural information. Time-lapse AFM imaging directly observes biological specimens at work, thus providing a unique tool to approach the relationship between their structure and function. Moreover, using the AFM probe as a ‘lab-on-a-tip’ enables us to probe simultaneously the structure and specific biological, chemical and physical parameters of the cell’s machinery. Molecular interactions make up the basic language of all biological processes. They determine how molecules assemble into complex functional structures and switch these cellular machineries ‘on’ or ‘off ’. In addition, molecular interactions control the way that cells communicate with each other, and form higher-ordered organisms. Remarkably, AFM can measure interactions between and within single biomolecules, giving us clues on how cells direct adhesion between themselves. Such information is important for understanding tissue development, tumour metastasis, bacterial infection and an almost uncountable number of medical and biotechnological questions. AFM can also be used to determine the energy landscape of molecular recognition events and protein folding pathways. AFM is clearly emerging as a powerful, multifunctional nanoscale tool, flourishing in modern biological and medical laboratories, and opening up exciting new possibilities for nanobiotechnologists (Fig. 1).