Sanjeevi Sivasankar Invents Way to See Single Molecules

Sanjeevi Sivasankar has used an ingenious combination of optical and atomic force microscope technologies to perform 3-D measurements of single biological molecules with unprecedented accuracy and precision.

Unlike conventional technologies that can measure single molecules on a two-dimensional plane, Sivasankar’s technique lets researchers measure a molecule’s height to within a billionth of a meter (nanometer) without custom optics or special surfaces for samples.

“This is a completely new type of measurement that can be used to determine the z position of molecules,” said Sivasankar, an Iowa State assistant professor of physics and astronomy and an associate of the US Department of Energy’s Ames Laboratory.

Sivasankar calls his new technology standing wave axial nanometry (SWAN). His laboratory developed it as part of its mission of learning how biological cells adhere to each other and to develop new tools to study those cells.

Sivasankar attached a commercial atomic force microscope to a single molecule fluorescence microscope.

Sivasankar positioned the tip of the atomic force microscope — which is attached to a single fluorescence microscope — over a focused laser beam, creating a standing wave pattern. A molecule that has been treated to emit light is placed within the standing wave. As the tip of the atomic force microscope moves up and down, the fluorescence emitted by the molecule fluctuates in a way that corresponds to its distance from the surface. That distance can be compared to a marker on the surface and measured.

“We can detect the height of the molecule with nanometer accuracy and precision,” said Sivasankar.

Sivasankar’s team used fluorescent nanospheres and single strands of DNA to calibrate, test and prove out SWAN’s capabilities which is expected to be useful tomedical researchers who need high-resolution data from microscopes. While Sivasankar thinks the technology has commercial potential, his immediate goal is to use it for his own work in single molecule biophysics.

“In doing that, we’ll continue to invent new technologies,”

Sivasankar’s work was published along with co-authors Hui Li, an Iowa State post-doctoral research associate in physics and astronomy and an associate of the Ames Laboratory, and Chi-Fu Yen, an Iowa State doctoral student in electrical and computer engineering and a student associate of the Ames Laboratory.

Sivasankar earned a PhD from the University of Illinois at Urbana Champaign in 2001. He participated in a postdoctoral fellowship at Stanford from 2001 to 2007. He worked as an associate specialist at UC Berkeley from 2007 to 2008. From 2008 he has worked as both an asst. professor of physics and astronomy and a courtesy assistant professor of electrical and computer engineering at Iowa State University.