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.

▲ Figure 1 The process of preparing SEIRA substrate and the spectroscopy in detecting octadecanethiol (ODT).

To address this bottleneck issue, ZJUI’s researcher, Assist Prof. HU Huan, together with Prof. XU Yang (from School of Micro-Nano Electronics) and Prof. MA Yungui (from College of Optical Science and Engineering) led a research team to develop a large-scale and low-cost manufacturing method for IR spectroscopy by using a more efficient vertical-coupled nano-structure. Their research work has recently been published on Sensors and Actuators B: Chemical, a well-known international journal in the field of chemical transducers. The research article is published with ZJUI’s postgraduate WU Shaoxiong as the first author and Assist Prof. HU Huan as the sole corresponding author.

The team’s research work introduced a complimentary vertically coupled plasmonic structure that is stable and highly scalable. Through nanosphere lithography, the fabrication approach can be implemented on a 4-inch wafer scale with a much lower cost than conventional methods. By adjusting the size of the nanosphere, the resonance frequencies for plasmonic structures can be tuned, which then makes the SEIRA substrate spectrum capable of covering the range of molecules’ functional group region and fingerprint region. Through further refining the depth of plasmonic-structures coupling, local light field can be intensified up to almost 800 times. Their experimental results also show that by enhancing the nano-cavity of plasmonic structures (an enhancement factor of 2100 times), the team managed to detect monolayer octadecanethiol.

▲ Figure 2 Simulation results showing the presence of metallic nanoparticles intensify localized electric field.

Self-Assembled Metallic Nanoparticles to Strengthen Electric Fields

Another innovative part of the research lies in the self-assembled metallic nanoparticles on the sidewall of the nanostructures.. Simulation results show that these metallic nanoparticles deposition on the sidewall of nanostructures can further improve the enhancement efficiency of local electric field for better IR absorption. Every nanoparticle represents a new hotspot, and these densely packed hotspots significantly increase the average electric field in the nanostructure, bringing the absorption strength of octadecanethiol up to 1.45%.

▲ Figure 3 Comparison between SEIRA and flat gold substrate in detecting monolayer octadecanethiol.

By exploiting the advantage of capable of producing SEIRA substrate in large scale, the team managed to increase theoptical field up to 1 mm x 1mm. As a result, even small amounts of CH3 bond can be detected within this area. The results show that SEIRA substrate detection limit can reach a sensitivity of 10 nM, four times better than using flat gold substrates. Moving forward, this research strive to apply appropriate surface chemistry protocols to anchor ligands on surface, to achieve detection of biological compounds such as viruses, biomarker proteins and others possible. With promising future applications, this research and its technology is currently in the process of filing for the national invention patent. 

Article link: https://www.sciencedirect.com/science/article/pii/S0925400523002757