Just like humans have unique fingerprints, molecules do too.

There is a fingerprint region that lies in the IR spectrum, which corresponds to molecules’ unique IR absorption spectrum.

From detecting food additives to skin health conditions, or even to identifying real gems from fake ones, IR spectroscopy is one quick non-invasive method to identify substances.

Infrared spectroscopy is a measurement technique used to study and identify functional groups in different kinds of matter. By utilizing the interaction between mid-infrared spectrum and highly specific covalent bonds in chemical structures, IR spectroscopy can be used to verify molecular fingerprint of a particular substance. Because the resonant vibrational frequencies in functional groups and molecular skeletons lie in the range of ~2.5 to 20 microns, this spectrum range is also called the fingerprint region. However, because light wavelengths within this range are significantly longer than the size of chemical bonds, this mismatch leads to a limitation in measurement sensitivity due to weak interaction between light and the molecules.

Plasmons as Means to Enhance IR spectroscopy

To increase the peak for IR absorption in molecules, surface-enhanced infrared absorption (SEIRA) based on plasmon resonances is commonly used. By utilizing the strong coupling effect between free electrons in metals and electric fields, plasmon structures can concentrate the light field in the subwavelength region, which in turn significantly intensify the local electric field. However, due to the narrow range of plasmon structure (within the submicron and nanometer range), high-resolution nanolithography techniques, such as electron-beam lithography, are needed. Such technologies require state-of-the-art equipment that cost million dollars. Such high cost subsequently limits the scale and affordability ofproducing nanostructures, and therefore creates a challenge for more widespread, practical usage of SEIRA.