Scientists at ETH Zurich (Switzerland) and the Lawrence Livermore National Laboratory (LLNL, Livermore, California) have developed a sensor for surface-enhanced Raman spectroscopy (SERS) that has allowed them to detect a certain organic species (1, 2bis(4-pyridl)ethylene, or BPE) in a concentration of a few hundred femtomoles per liter. The results of the study, conducted by Hyung Gyu Park, professor of energy technology at ETH Zurich, and Tiziana Bond, capability leader at LLNL, were published in the August 27th issue of Advanced Materials.
Prior to the development of their method, the detection limit of common SERS systems was in the nanomolar range, or one billionth of a mole.
The researchers set a goal of developing a sensor that massively amplifies the signals of Raman-scattered light. Their substrate of choice of choice turned out to be vertically arranged, caespitose, densely packed carbon nanotubes that guarantee a high density of “hot spots.” The group developed techniques to grow dense forests of carbon nanotubes in a uniform and controlled manner.
In their abstract, the researchers wrote of a highly sensitive substrate for SERS formed by arrays of gold-coated metallic carbon nanotubes having a nanoinsert of high-k dielectric (hafnia) as an energy coupling barrier.
The researchers coated the sharply-curved carbon nanotube tips with gold and hafnium dioxide, a dielectric insulating material. The point of contact between the surface of the sensor and the sample reportedly resembles a plate of spaghetti topped with sauce. However, between the strands of spaghetti, there are numerous randomly arranged holes that let through scattered light, and the many points of contact — the hot spots — amplify the signals. The insulation layer increased the sensitivity of its sensor substrate by a factor of 100,000 in the molar concentration unit.