Multimodal Detection of Low Water Contents in Ethanol Using a Plasmon-Berreman-Enhanced Metasurface Infrared Absorber.

The molecular vibrational peaks of a target material can be enhanced by designing an infrared (IR) absorber in its fingerprint region. Here, we report a periodic metasurface absorber (MSA) with two resonance modes in the near-IR and long-wavelength IR regions with 80 and 99.5% absorbance, respectively, due to the coupling of incident photons with plasmons and phonons. This dual-mode MSA is used to detect water impurity in ethanol, as high-purity ethanol is essential for biofuel and pharmaceutical industries. The plasmonic mode enhances the -OH bending and stretching bands of water, while the Berreman (BE) mode, induced by phonons, enhances the C-O stretching peak of ethanol. The plasmon and BE-enhanced IR absorption (SEIRA and BEIRA) address the limitations of conventional IR spectroscopy by enhancing weak molecular vibrational signals for ultra-low quantities of sensing materials. Additionally, a unique plasmonic shift is observed with varying water concentrations in ethanol, enabling quantification of low water content (up to 1%), with a limit of detection (LOD) of 1.14 (% v/v), and a high sensitivity of 14.64 nm/(% v/v). Further, the BEIRA at grazing angles produces concentration-dependent thermal contrast across the full range of ethanol-water mixtures with 0.8 °C/(% v/v) of water and an LOD of 0.31 (% v/v). Selectivity and anti-interference performance were evaluated using ethanol solutions mixed with other impurities such as methanol, acetone, and acetaldehyde. This highly selective, versatile multimodal sensing strategy, based on a single metasurface chip, enables label-free visual quantification for chemical analysis, process monitoring, and quality assurance.