Sponsored by the Department of Energy, two Ames Laboratory scientists have devised a method to detect and even track the movement of single molecules within chemical solutions.

With this method, medical scientists may be able to identify the presence of specific genes, proteins and viral agents which indicate the presence of cancer, the HIV virus or other undesirable pathogens.

Ed Yeung, Ames Lab program director for physical and biological chemistry and distinguished professor of chemistry at ISU, and Xiao-Hong Xu, a post-doctoral research associate at Ames Lab, have been working on this research for one year.

Last fall they demonstrated their technique by tracking the motion of rhodamine, which is a highly fluorescent compound, and of rhodamine-labeled DNA in water solution.

“We have demonstrated how our technique works, and the next step is to see if we can detect single viruses or not,” Yeung said.

Yeung said, “It is roughly a million times more sensitive than existing techniques; normally you would need a million copies of the virus or the gene to get this much information.”

Their technique combines the use of a blue-green Argon ion laser to induce fluorescence in individual molecules with an optical microscope and an Intensified Charge-Coupled Device (ICCD) camera to gather the light from within the solution.

The dynamics of the molecules of interest are recorded continuously in the ICCD, giving the equivalent of a video representation for the movement of the molecules.

The ICCD can record the motion of the molecules with better than millisecond resolution in time, Yeung said.

The secret to getting such impressive resolution — and in fact to tracking molecules speeding about in solution in any case — is not in the ICCD camera itself, but rather in the way it’s operated, a way of moving it around which gives a time resolution that is more than a hundred times faster than prior techniques, Yeung said.

Fundamental packets of light, or photons, from lasers or any source of light are absorbed by molecules they come into contact with if they happen to have the right amount of energy to induce the molecule to vibrate or rotate in particular ways. This means that for certain molecules, a laser can cause them to fluoresce, or in other words, to give off light of a lower energy, or different color.

“You use your chemistry and biochemistry to color the species you are interested in, (i.e., to tag it with a known fluorescent molecule), and then you use our method to detect the color,” Yeung said.

The principal benefit of using an ICCD camera to image the light gathered by the microscope is extreme sensitivity, which allows the detection of light emitted by a single molecule, Yeung said.

Yeung said their new method for collecting information about the behaviors and characteristics of individual molecules represents a significant advancement over traditional methods, which determine the properties based on statistical averages of populations of molecules.

“In the gas phase, people have been able to monitor single molecules for a long time, because in gases you have better control and fewer interfering species,” Yeung said.