(411f) Surface Plasmon Resonance Enhanced Infrared Reflection Absorption Spectroscopy Using Tunable Compressive Gratings

Surface Plasmon Resonance Enhanced Infrared Reflection Absorption Spectroscopy Using Tunable Compressive Gratings

Zhiqi Yao and Andrew C. Hillier*

Department of Chemical and Biological Engineering, Iowa State University

Surface Plasmon Resonance (SPR) sensing has attracted attention as it is a versatile and label-free method for monitoring the binding of analytes to surfaces and in the characterization of thin films. SPR combined with infrared reflection spectroscopy is an attractive analytical technique that can be used to simultaneously measure the chemistry and structure of thin films. Combining SPR with infrared measurements also allows for an enhanced spectroscopy known as surfaced enhanced infrared absorption (SEIRA) spectroscopy in which infrared signals can be enhanced due to the large surface electric field associated with the surface plasmons. Although nanoparticles are frequently used in SEIRA, nanostructured gratings have also found application and provide several advantages. In particular, nanostructured gratings provide a flexible and controllable platform for exciting SPR, as the surface plasmon resonance location can be tuned by adjusting the grating pitch and incident angle.

In this work, we demonstrate a means of tuning the response of a grating substrate for SPR applications by exploiting a compliant or flexible substrate. We have created gratings on elastomeric substrates, whereby compression or tension can be used to actively change the grating pitch in order to tune the plasmon response. We demonstrate that grating pitch values can be modified by up to nearly 15% from their original size, which allows SPR response locations to be actively moved by several hundred cm^-1 in in the infrared spectrum. The grating pitches can be decreased by compressing the substrate. The SP response locations are also strongly linear-fit with the compression percentages, which makes it possible to predict the adjustment of the surface plasmon peak.

In summary, we demonstrate that this novel pitch tunable grating substrate could be used for high performance optical sensing. Since many vibrational modes of organic molecules exist in infrared regimes, we are able to enhance several signal associated with vibrational spectroscopy on the same grating substrate by controlling the grating pitch. Also we can compress or stretch the substrate to study the adjustment of dielectric film thickness to overlay the signal with the plasmon resonance.