Surface-enhanced near-infrared absorption (SENIRA) of C-H and N-H groups with gold nanoarray

Excitation of localized surface plasmon resonance in noble metal nanoparticles leads to enhancement and localization of electromagnetic fields in the immediate vicinity of nanoparticles. These properties may be employed to amplify the lightmatter interaction in the near-infrared range where the overtone molecular vibrations are situated. Since the overtone vibration bands are much weaker than the fundamental bands, the amplification is essential. Here we explored SENIRA in the framework of molecular overtones sensing, particularly, those overtones that correspond to the C-H (1676 nm) and NH (1494 nm) stretching modes overtones. The gold nanorods (GNRs) are placed on the dielectric substrate and embedded into a thin layer of organic probe molecules (N-Methylaniline). The dispersion characteristics of N-Methylaniline, namely, its wavelength-dependent absorption and refractive indices in the spectral vicinity of the overtone transitions were fully taken into account. To find out the enhancement of overtone transitions provided by the GNR, we numerically calculated the differential transmission (DT) as a function of the gold nanoantenna's size and grating periods. The computational results evidence that in sparse arrays of GNRs when the near fields of the neighbor GNRs do not overlap with each other, the differential transmission of stretching overtone modes shows the resonance at the right spectral position which is around 8.8 times larger as compared to the absorption of the bare molecular film of the same thickness. Thus, the obtained results substantiate a new sensing spectroscopy concept for identification of versatile "fingerprints" in the near-IR range based on plasmon-overtones interactions.