Metamaterials-Enhanced Infrared Spectroscopic Study of Nanoconfined Molecules by Plasmonics-Nanofluidics Hydrid Device

The behavior of molecules under nanoconfinement is crucial for understanding the chemical processes in biological and nanomaterial systems. We demonstrated here an infrared spectroscopic method to characterize the molecular structures of molecules confined in several tens of nanometer cavities by employing the plasmonics-nanofluidics hybrid device. This device consists of an array of metal nanostructures and a metal mirror separated by a nanofluidic cavity. Its configuration enables the confinement of both molecules and light energy as localized surface plasmons inside the physicochemically well-defined nanocavity. Exploiting the plasmons-molecular coupling, the vibrational modes of the nanoconfined molecules are selectively detected with a prominent sensitivity. Applying water as a proof-of-concept sample, we have successfully measured the infrared absorption characteristic and elucidated the molecular structures of water confined in a 10 nm cavity. They unveiled the presence of a strong H-bond network with respect to bulk water. Our method was also able to distinguish the subtle differences in the molecular structures, revealing the scaling behavior of confined water in the several tens of nanometer size regime. This effect is also found not being driven by the interaction with the interfaces; yet the constrained geometry itself promotes the intermolecular interactions of water and results in the modification of the H-bond network. This study has offered an ultrasensitive platform for in situ probing of the nanoconfined molecules and chemical reactions in their intact condition, and thus gives us a fundamental insight into the nanoconfinement effects.