MIR plasmonic liquid sensing in nano-metric space driven by capillary force

Abstract

© 2019 IOP Publishing Ltd. Optical spectroscopy in the mid-infrared region (MIR) has been an important tool for non-destructive analysis of molecules through identifying the vibrational modes of chemical bonds. The use of plasmonic antenna array in the MIR region enables high selectivity and sensitivity without the labeling process. In addition, the formation of nanogap between plasmonic nano-antenna and reflective mirror allows for near-field enhancement which can be exploited for sensing applications. However, it is still challenging to realize a sensing platform which effectively delivers biochemical molecules onto the plasmonic nanogap hot-spot. Here, we experimentally demonstrate a capillary driven sensing platform confining chemical samples into a nano-metric space with a plasmonic antenna array. The quadrupole mode resonance of the plasmonic antenna combined with vertical nanogap hot-spot significantly enhanced the confined electrical field. The nano-metric spaces formed by hydrophilic silicon dioxide draw liquids into the sensing chamber by capillary effect, and hence no external liquid driving power source was required. Enhancement of both molecular vibration and plasmonic resonance were observed. Furthermore, label-free quantitative detection was achieved through coupling the plasmonic antenna resonance and the water molecular vibration. The proposed sensing platform has the potential in realizing non-destructive and label-free sensing in the MIR spectral region without an external liquid driving power source, and the enhanced signal paves the way to sense analyte of ultralow concentration.