Abstract
We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.
Highlights
Plasmonic metamaterial sensors have been a subject of increasing interest in recent years because they offer the capability to precisely engineer the interaction between electromagnetic radiation and analytes to perform highly sensitive measurements of absorption or refractive index changes [1,2,3]
We have provided a framework for efficient numerical optimization of plasmonic substrates for surface enhanced infrared absorption (SEIRA) enhancement
For the first time, our framework explicitly compensates for the perturbative effects of the analyte, with respect to spectral distortions, thickness variation and adsorption geometry
Summary
Plasmonic metamaterial sensors have been a subject of increasing interest in recent years because they offer the capability to precisely engineer the interaction between electromagnetic radiation and analytes to perform highly sensitive measurements of absorption or refractive index changes [1,2,3]. Comparison of fibroin absorbance, Aref, for the optimized array to the bare Au layer reveals remarkable signal enhancements of >103, especially for low-coverage regime (See Fig. 8, dashed lines), which is consistent with our previous work [1]. In order to better understand the difference in thickness dependence of the absorbance signals for CaF2 and Si substrates, we computed local electric fields for several field-monitoring cut planes, parallel to the substrate, at distances corresponding to the thickness values of Fig. 9(B) For these calculations, an infinitely thick transparent reference overlayer covers the plasmonic nanoarrays to simulate the index environment.
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