Success stories
Pablo Etchegoin:
Raman Scattering - Portal into a Nanoscopic World
Where's Wally?
Its hard enough to find Wally here so imagine the sort of sensitivity you'd need to locate him in a population three times that of the Earth. The methods Pablo Etchegoin is developing for amplifying Raman signals does this with molecules, making it possible to detect a single target molecule in a beaker of solvent. These revolutionary techniques are having a major impact in medical research.
What is Raman Spectroscopy?
Basically Raman Spectroscopy is inelastic light scattering. It is a non-destructive technique for identifying specific chemical species in a sample. A laser is shone at a sample and the inelastically scattered photons are used to identify species in the sample. Laser photons interact with the target molecule, changing its vibrational state. The molecule either emits or absorbs a photon. Each chemical species has characteristic quantised vibrational frequencies. Energy shifted photons are produced on each side of the laser frequency (Stokes and anti-Stokes branches of the Raman Spectrum).
Unfortunately Raman scattered photons make up a very small proportion of the total intensity, so without amplification the Raman peaks are very weak. This is where Surface Enhanced Raman Scattering (SERS) comes into the picture.
Pablo with the Raman spectrometer
SERS (Surface Enhanced Raman Scattering)
This is an area of particular interest for Pablo. The molecule to be detected is placed in solution with metal colloids (tiny insoluble pieces of metal~20-50nm diameter). Electrons within these colloids are much lighter than the ionic structure they move within, so in the presence of an oscillating electromagnetic field they oscillate in a sort of 'Electron jelly' within the colloid. This vibration is called the plasmon frequency. When the target molecule sits on the surface of a colloid (or between two colloids) and the photon frequency is in resonance with the plasmon the Raman signal is greatly amplified.
An analogy will help to describe how this works. Imagine the molecule on the colloid as a very tiny creature hiding in a large room. When you shout at the creature it lets out a tiny distinctive yelp (the Raman signal) and if the wavelength of that tiny yelp is proportional to the dimensions of the room (the plasmon resonance) the tiny yelp will resonate and become a very loud yelp. The colloids act in the same sort of way as the room to amplify the Raman signals. They do not change the wavelength of the signal, they just amplify it so it can be easily detected.
The resonance of one colloid has a single value but when the number of colloids is large, resonance conditions split above and below this value and become complicated. Pablo has found unusually intense hot spots in the Raman signal occurring at frequencies just below the individual plasmon resonance at points located in between two close together colloids.
a: A random configuration of colloids in 2d; B: Electric field intensity in the space surrounding th colloids - red represents the largest field and green the smallest; c: Enlargement of region containing a 'hot spot'.
No one knows the reason for the extraordinary intensity of these hotspots. A new field of 'lasmonics' is opening up exploring this sort of phenomena. In particular when light is incident on a sheet of metal with a regular array of holes, it is found that more light comes through than is incident on the holes. It is suspected that the answer lies in Quantum Mechanics.
The Effect Of Surface Topology
Surface Topology also effects the amplification of the signal. It has been observed that the more complicated the metal surface the greater the amplification. This could be explained by 奏he lightening bolt effect ie. the photon energy is focused and intensified by features of the metal surface so the signal is amplified.
Single Particle Detection
SERS can be used to detect single molecules . A laser shines photons into a small region of the solution. Every now and then one of the molecules floats through this region and its Raman signal is amplified by the colloids. A large peak appears on the spectrum. There might be a peak every 15 minutes or so when the molecule floats through the laser beam.
Experimental setup for single particle detection
How could these new methods be used in Medical Research?
Pablo confesses he is a biologist at heart, inspired by the biological and medical applications of Raman spectroscopy, particularly early cancer detection. Problems in biology, he says are far more complicated than inorganic ones. There can be thousands of proteins produced by the same cell differing only very slightly so the Raman spectra become very complicated. Pablo has developed a new method for early detection of cancer. A tiny amount of an antibody (developed especially to stick to the anomalous cancer protein) is attached to dye molecules and injected into the blood stream. The antibodies attach to any anomalous proteins in the body. After some time has passed a micro-volume of flesh from the area to be tested is taken. The dye molecule can be detected using Raman Spectroscopy and by using the SERS technique described above, single anomalous proteins can be detected. Pablo is working on such cancer detection methods with the Mallaghan Institute for Medical Research, situated in the Central Services Building at VUW.
Other areas of Research
Raman Spectroscopy has such a wide range of uses that Pablo's expertise is highly sort after. Raman Spectroscopy will need to be used to detect species in drug synthesis using the microstructures embedded with active nano-structure that Jeff Tallon and his team are working on at IRL. Almost every researcher you talk to has some use for Raman Spectroscopy but there still remain a lot of unanswered questions, especially in the theoretical aspects. Pablo hopes to throw further light on the subject during his time at Victoria.
