Infrared spectroscopy is the benchmark method for detecting and analyzing organic compounds. However, that requires complicated procedures and large, expensive instruments, making device miniaturization challenging and hindering its use for some industrial and medical applications. It is also difficult to use it for data collection in the field, such as measuring pollutant concentrations. Furthermore, since it is fundamentally limited by low sensitivity, it requires large samples.
However, scientists at EPFL's School of Engineering and at Australian National University (ANU) have developed a compact and sensitive nanophotonic system that can identify a molecule's absorption characteristics without using conventional spectrometry. The scientists have already used their system to detect polymers, pesticides, and organic compounds. What's more, it is compatible with CMOS technology.
Their system consists of an engineered surface covered with hundreds of tiny sensors called metapixels, which can generate a distinct bar code for every molecule that the surface comes into contact with. These bar codes can be massively analyzed and classified using advanced pattern recognition and sorting technologies, such as artificial neural networks. This research, which sits at the crossroads of physics, nanotechnology and big data, has been published in Science.
The chemical bonds in organic molecules each have a specific orientation and vibrational mode. This influences the way molecules absorb light, giving each one a unique “signature.” Infrared spectroscopy detects whether a given molecule is present in a sample by seeing if the sample absorbs light rays at the molecule's signature frequencies. However, such analyses require lab instruments with a hefty size and price tag.
The pioneering system developed by the EPFL scientists is both highly sensitive and capable of being miniaturized; the nanostructures that trap light on the nanoscale thereby provide very high detection levels for samples on the surface. The system's nanostructures are grouped into metapixels so that each one resonates at a different frequency. When a molecule comes into contact with the surface, the way the molecule absorbs light changes the behavior of all the metapixels it touches. The metapixels are arranged in such a way that different vibrational frequencies are mapped to different areas on the surface. This creates a pixelated map of light absorption that can be translated into a molecular bar code — all without using a spectrometer. The bar codes can be generated even with broadband light sources and detectors.
There are a number of potential applications for this new system. For instance, it could be used to make portable medical testing devices that generate bar codes for each of the bio-markers found in a blood sample.
Artificial intelligence could be used in conjunction with the new technology to create and process a whole library of molecular bar codes for compounds ranging from protein and DNA to pesticides and polymers. That would give researchers a new tool for quickly and accurately spotting miniscule amounts of compounds present in complex samples.