Story 6 - 17/7/2008
The Single Molecule
Who would you hire to localize a single molecule? A new detective is now available! A nano-lightning rod can do the job. It can act as a single-molecule fingerprint detective with an unprecedented spatial resolution up to 15 nanometers.
A single-molecule fingerprint. TERS image of a single molecule about 10 nm wide.
When Sherlock Holmes arrives on the scene of a crime, he immediately looks for potential culprits' fingerprints using a magnifying glass. In the same way, scientists can use sharp
metallic nanotips to look for the Raman fingerprints of molecules.
Recently, Jens Steidtner and Bruno Pettinger at the German
Fritz-Haber-Institut of the Max-Planck-Gesellschaft in Berlin have
proven that a metallic nanotip can be employed to localize single
molecules with an unprecedented spatial resolution up to 15
When light impinges upon a molecule, different parts of the
molecule start vibrating at specific frequencies. Just like we all
have our characteristic fingerprints, each molecule has its own
distinctive set of characteristic frequencies, known as the Raman
spectrum of the molecule. These vibrational frequencies have been
widely used by scientists as a tool to detect and discriminate
molecules, and they are at the core of techniques such as infrared
spectroscopy or Raman spectroscopy.
Raman spectroscopy, in particular, has specific advantages that
make it a good molecular detective: samples can be prepared easily
and it can be used in various environments. However, this
detective has some drawbacks that have prevented it from detecting
single molecules: poor signal efficiency and a spatial resolution
essentially limited to a few hundred nanometers.
Luckily, these disadvantages are overcome by using a more powerful
Raman detective, Surface Enhanced Raman Spectroscopy (SERS). In
this technique the Raman signal efficiency is greatly enhanced by
metal nanostructures brought either in close vicinity to or in
contact with the molecule . If the metal nanostructure is a
tiny, sharp, and nanosized tip, like a lightning rod at the
nanoscale, then the technique is called Tip Enhanced Raman
Spectroscopy (TERS). Compared to SERS, TERS is a more efficient
molecular detective with spatial resolution at the nanoscale.
"TERS is a very young approach," Pettinger explains, "that
combines a raster scanning probe device with a Raman spectrograph.
The core of this unit is a tip of suitable material and size that,
when illuminated, greatly enhances Raman scattering, making single
molecule Raman spectroscopy possible with a spatial resolution
around 10-20 nanometers. In a sense, the tip is working as an
optical antenna for the incident and radiated light, thereby
providing the huge signal enhancement in its close vicinity."
Before Steidtner and Pettinger's work, TERS had only shown
indirect evidence for single molecule detection sensitivity. Now,
Steidtner and Pettinger have come up with substantial and direct
evidence by performing TERS experiments in ultra-high vacuum
rather than in air. "One advantage to our approach," Pettinger
reveals, "is that the state of the sample surface can be
controlled and characterized much better in ultra-high vacuum than
in air. In this respect we presented one particularly interesting
example: the substantially reduced photobleaching in ultra-high
vacuum." Indeed, it is only thanks to this increased stability of
the molecule that it has been possible to image single molecules
by employing TERS.
This development has significantly improved the spatial resolution
of the molecular detective up to 15 nanometers. Renato Zenobi, of
the Swiss Federal Institute of Technology (ETH) in Zürich,
whose group actively works on TERS technique , notes that "the
work reveals high sensitivity and localization of TERS at fast
signal accumulation times both of which are excellent results."
Experiments like this usually require expertise from different
fields, ranging from Raman spectroscopy to scanning probe
microscopy, a fact which makes them all the more challenging.
According to Pettinger, "the most challenging part was the
precise optical alignment in ultra-high vacuum to produce a sharp
focus and to get the tip in its center."
When compared to other molecular detectives, "TERS is superior
with respect to identification of unknown molecules," Pettinger
explains. "It also works for molecules which do not show
fluorescence, although the sensitivity drops by about two orders
of magnitude." Furthermore, there is more room for improvement in
TERS resolution. "The research on TERS started in the year 2000
and," Pettinger remarks, "the quality of the tip is of crucial
importance. We routinely get tips with a radius of the apex of
about 20 nanometers. Since the achievable resolution depends
strongly on this radius, we expect that sharper tips will provide
a higher resolution."
"TERS has a wide range of applications," Pettinger concludes,
"for example, in surface science, heterogeneous catalysis, single
molecule spectroscopy, and biology. So far, these fields have only
been briefly explored." Zenobi clarifies that "TERS has immense
implications in biology, especially in imaging sub-cellular
components such as lipid rafts, membrane protein and others.
However, since this work uses ultra-high vacuum, the imaging of
bio-entities in this way would be difficult." Although biological
entities, like living cells, cannot sustain ultra-high vacuum,
this method shows plenty of promise for high resolution chemical
imaging which can be further harnessed.
 K. Kneipp et al.
, Surface enhanced Raman scattering: Physics and application, Springer-Verlag, Berlin (2006).
 W. Zhang et al.
, Nanoscale Roughness on Metal Surfaces Can Increase Tip-Enhanced Raman Scattering by an Order of Magnitude
, Nano Lett 7
, 1401-1405 (2007).
G. V. Pavan Kumar
2008 © Optics & Photonics Focus
PK is currently working as a researcher at ICFO - The Institute of Photonic Sciences, Barcelona (Spain)
J. Steidtner and B. Pettinger, Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with 15 nm resolution, Phys Rev Lett (2008) 100, 236101 (link).