The key to getting good Raman data - separating "hot" nanoparticles for signal enhancement
Prof. Mazur has invented a new method that allows one to separate out highly-active SERS metal nanoparticles, either from solution or adhered to a surface. Metal nanoparticles, with adsorbed Raman-active molecules on their surface, are suspended within a photo-polymerizable resin (or coated with the resin on a substrate). A laser setup is used to excite the surface-attached molecules and cause them to radiate a specific frequency of light due to Raman scattering. However, only the molecules at the surface of “hot” nanoparticles (i.e. those able to enhance their radiation) produce an intense enough Raman signal to polymerize the surrounding resin. These active nanoparticles thus become coated with a cross-linked polymer, and can be selected out either by their increased mass in solution, or by their adherence to a surface.
Metallic carbon nanotubes can also be separated from semiconducting ones based on their electrical conductivity using this technique. The ability to efficiently isolate metallic carbon nanotubes at a rapid rate has posed a significant challenge in the materials industry. With this sorting method, carbon nanotubes can be easily separated in solution or on a surface and be used for a variety of applications, including molecular electronic devices. The photo-polymerizable resin can even provide an electrically insulating coating for the selected particles adhered to substrates.
This method allows for the separation of a large quantity of nanoparticles or nanotubes in a short period of time, at a rate on the order of one liter per second in solution. The laser setup can be tuned to accommodate the different resonances of the metallic nanoparticles and vibrational modes of the Raman-active molecules. The ability to greatly enhance Raman signals can lead to a wider array of samples than can be analyzed with Raman microscopy for applications in biology and pharmaceuticals. This method can even make single-molecule Raman detection possible.
Intellectual Property Status: Patent(s) Pending
Raman microscopy can give one a tremendous wealth of chemical and structural information about a material with virtually no sample preparation or labeling. Unlike IR, it can be used with wet or packaged samples and has at least 10x greater resolution. However, Raman microscopy suffers from low-intensity signals. A discovery in the 1970s that Raman signals can be significantly enhanced at the surface of metal nanoparticles gained considerable interest (Surface Enhanced Raman Spectroscopy, SERS). However, this method has not been widely adapted due to the fact that not all metallic particles are made equal, having widely varying enhancement factors. Thus, there is a serious need in the field of Raman to be able to separate SERS-active (“hot”) nanoparticles from inactive ones.