Gold-coated substrates of gallium nitride with specifically formed surface, developed by researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences and the Institute of High Pressure Physics of the PAS display worldwide unique properties. Thanks to these new substrates, an extremely sensitive SERS analytical technique, capable of detecting even single molecules, after decades of waiting in specialized laboratories has finally got a chance to widespread and revolutionize medical diagnostics.
Laboratory on a microchip, capable to detect most types of antibodies in patient's blood in a single test would be a revolutionary breakthrough in medical diagnostics. The physicians would gain rapid access to examined patient's personal health records with the data on both past and present diseases, as well as those that will develop only in future. This futuristic vision gets closer to reality due to innovative, gold-coated gallium nitride substrates developed by researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Institute of High Pressure Physics of the PAS (IHPP PAS). Worldwide unique properties of the substrates open for the first time the way to long expected dissemination of surface enhanced Raman spectroscopy (SERS). Using this unusually sensitive research tool one can detect bacteria, viruses, and even single molecules. „Lack of substrates with appropriate nanostructures has, for last three decades, prevented SERS application in medicine. We managed finally to overcome the obstacle" - says Prof. Robert Hołyst (IPC PAS).
Substrates with roughness features in nanometer scale play a key role in Raman spectroscopy. The Raman effect is the inelastic scattering of photons on molecules. Usually a light scattering molecule absorbs a photon and immediately emits another one with the same energy - the phenomenon is termed by physicists elastic or Rayleigh scattering. Sometimes it happens, however, that a part of photon energy is transferred, e.g., into vibrations or rotations of a molecule. Then, the photon emitted by the molecule will have slightly lower energy. An opposite situation can also happen: the molecule will give some energy to the emitted photon. In both cases the scattering is known as Raman scattering, after the name of its discoverer, Indian physicist and Nobel Prize winner Chandrasekhara Venkata Raman. The scattering is an extremely rare phenomenon: only one photon per dozens of millions is scattered in that way, which means it is very hard to detect.
In 1974, British chemist Martin Fleischmann with co-workers observed an unusually strong Raman scattering signal from molecules placed on a roughened silver substrate. Further studies revealed that the enhancement effect is real and occurs also on substrates made of gold, platinum and copper. In each case, the necessary condition for enhancement to occur was an appropriate surface morphology. „If the surface is appropriately roughened, the electromagnetic field intensity on sharp edges of surface unevennesses is significantly increased. For a similar reason, all spire structures attract thunderbolts" - explains Prof. Hołyst. The molecules deposited on so prepared surface are in a very strong electromagnetic field and much more frequently scatter photons inelastically via Raman scattering. Surface Enhanced Raman Spectroscopy (SERS) is an exceptionally attractive analytical tool yielding powerful enhancement of the original signal, usually from million to billion times. So high method sensitivity allows detecting of even single molecules.
A good SERS substrate should have rather regular, periodic surface structure assuring that the signals recorded by the measurement setup are reproducible. Its design is based on gallium nitride (GaN) substrate, developed in the Institute of High Pressure Physics of the PAS. „The manufacturing starts with fabrication of a GaN substrate covered evenly with structural defects of nanometer size" - says D.Sc. Janusz Weyher from IHPP PAS. The surface is then photoetched, the process takes place everywhere except for the defects and results in formation of vertical pillars with approximately the same diameters. Due to tension forces, free terminals of adjacent nanopillars attach to each other, forming numerous structures resembling stacks in the field. At that stage, the substrates are transferred to the Institute of Physical Chemistry of the PAS, where a thick gold layer is evaporated on to them. The process yields a surface coated with an even thicket of nanometer size golden cones.
SERS substrates designed by Polish researchers have unique properties. „The substrates commercially available so far had to be handled with extreme caution. They had to be stored under nitrogen, could not be touched and even though they were loosing their enhancement ability within hours only. Our substrates can be put into an ordinary drawer for a couple of months and they will be still usable" - stresses Ph.D. Agnieszka Michota-Kamińska from the IPS PAS. For economic reasons, it is important that Polish substrates are suitable for multiple use as the only ones worldwide. Their surface structure is so stable that the researchers from the IPC PAS were able to develop efficient cleaning procedures to assure that a high level of SERS signal enhancement is maintained.
The substrates for SERS studies have been developed under the project "Quantum semiconductor nanostructures for applications in biology and medicine". One of the project's long term goals is to develop a sensor for detection of antibodies in blood. The core element of the device will be a semiconductor with a surface network of thousands of micrometer size wells. Each well will contain a specific carefully selected sequence of aminoacids to lure specific antibodies. A suitable microfluidic system will provide blood, and the antibodies contained in blood will attach only to matching polypeptides in wells. The data will be read out using SERS technique. The over 73 million PLN project is funded in 85% from the European Regional Development Fund under the Innovative Economy Operational Programme 2007-2013. The main participants to the project are the Institute of Physics of the PAS (coordinating body), the Institute of Physical Chemistry of the PAS and the Institute of High Pressure Physics of the PAS.