Working to learn how biological cells adhere to each other and to develop new tools to study those cells, Iowa State University (Ames, IA) researchers have developed a way to make 3D measurements of single biological molecules with "unprecedented accuracy and precision."1 The new method, called standing wave axial nanometry (SWAN), measures height to within a nanometer without custom optics or special surfaces for the samples.
SWAN involves attaching a commercial atomic force microscope (AFM) to a single-molecule fluorescence microscope: The AFM tip is positioned over a focused laser beam, creating a standing wave pattern. When a molecule treated to emit light is placed within the standing wave, its fluorescence fluctuates in a way that corresponds to the tip's distance from the molecule's surface: That distance can be compared to a marker on the surface and measured.
The researchers, using fluorescent nanospheres and single strands of DNA to test their instrument, reported measurements of a molecule's height accurate to <1 nm, and precision of repeated measurements to 3.7 nm.
In standing wave axial nanometry (SWAN), positioning an atomic force microscope tip over a focused laser beam excites fluorescence in a particle, whose axial position can be determined with sub-nanometer accuracy and 3.7 nm precision from the phase of the emission intensity.
1. H. Li, C-F Yen, and S. Sivasankar, Nano Lett., 12, 7, 3731–3735 (2012).