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In bioanalysis, especially in single cell analysis, label-free detections and recognitions for molecular analysis are highly demanded. IR spectroscopy is one of label-free methods and it gives us chemical specificity and molecular information, yet its application in bioanalysis is limited due to its low sensitivity. Recently, plasmonic nanostructures were intensively studied to improve the sensitivity of IR spectroscopy by several orders of magnitude, however positioning analytes exactly at the hot-spots is still challenging. We propose a device that utilizes nanofluidics to manipulate analytes into the hot-spots of metamaterials, consequently an ultra-high sensitivity of IR absorption detection can be achieved.
The structure consists of metal square-disks array and metal mirror separated by a nano fluidic channel. The interaction between top square nanostructure and bottom mirror forms the quadrupole resonance, and it suppresses the light reflection from the device. When the molecule whose absorption is overlapped with this mode is introduced, strong interaction between molecules and metamterials is excited and it creates the reflection light within the absorption band of the metamaterial. The sensitivity was achieved at molecule density of ~10^-4 molecules/Å^2, which is improved by 2 orders compared to reported plasmonic induced IR detection methods. We also succeeded in the quantitative determination of absolute number of molecules by precise fluidic operation.
Moreover, the device allows the confinement of both molecules and plasmonic energy inside the nanocavities. We confirmed the presence of a strong H-bond network and the scaling behavior of water confined in 10-100 nm regime. Our method can provide the capability for in-situ probing molecules and chemical reactions under nanoconfinement.
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