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single molecule absorption spectroscopy
James E. Kloeppel, Physical Sciences Editor
photo to enlarge
by L. Brian Stauffer
Gruebele, front, a professor of chemistry, physics
and biophysics, stands next to the device he and
Joshua Ballard, postdoctoral research associate,
and graduate student Erin Carmichael use for laser-assisted
scanning tunneling microscopy.
— A powerful new tool for probing molecular structure on surfaces
has been developed by researchers at the University of Illinois at Urbana-Champaign.
Single molecule absorption spectroscopy can enhance molecular analysis,
surface manipulation and studies of molecular energy and reactivity
at the atomic level.
“This new measurement method combines the chemical selectivity
of optical absorption spectroscopy with the atomic-scale resolution
of scanning tunneling microscopy,” said Martin Gruebele, a professor
of chemistry, physics and biophysics and corresponding author of a paper accepted for publication in the
journal Nano Letters, and posted on its Web site. “The method
literally feels how a molecule changes shape when it absorbs energy.”
Unlike single molecule fluorescence spectroscopy, which is now a commonly
used measurement technique, single molecule absorption spectroscopy
has been an elusive goal.
“Single molecules don’t absorb much light, making detection
difficult to begin with,” said Gruebele, who also is a researcher
at the university’s Beckman
Institute for Advanced Science and Technology. “An even bigger
problem, however, is that light-induced heating in the sample and in
the microscope tip can produce so much noise that the signal is lost.”
To reduce the noise, the researchers combined several special techniques
– each insufficient by itself – into a method that allows
them to detect single molecule absorption under laser illumination by
scanning tunneling microscopy.
“First, the sample molecule is placed on a transparent silicon
substrate,” said Joseph Lyding, a professor of electrical
and computer engineering and a researcher at the Beckman Institute.
“Laser light will either be absorbed by the sample or will pass
through the substrate with little or no heating effect. Second, the
tip-sample junction is illuminated through the rear face of the substrate,
significantly reducing tip heating.”
Modulating the laser light with a mechanical chopper further reduces
heating, Lyding said. A lock-in amplifier, which switches on and off
at the same rate as the laser, filters out mechanical and electronic
noise. As a result, the absorbed energy causes a change of shape in
the electron density of the sample molecule, and the scanning tunneling
microscope then measures that change of shape.
“Single molecule absorption spectroscopy is an extremely sensitive
technique for analytical chemistry, for measuring electrical properties
of molecules, and for studying energy transfer on surfaces,” Gruebele
said. “While most molecules don’t fluoresce – limiting
the usefulness of single molecule fluorescence spectroscopy –
all molecules absorb, making single molecule absorption spectroscopy
a much more general approach.”
Co-authors of the paper with Gruebele and Lyding are postdoctoral researcher
Joshua Ballard, graduate students Erin Carmichael and Dongxia Shi (now
at the Chinese Academy of Sciences in Beijing). The National Science
Foundation funded the work.
Editor’s note: To reach Martin Gruebele, call 217-333-1624; e-mail: firstname.lastname@example.org.
To reach Joseph Lyding, call 217-333-8370; e-mail: email@example.com.