Protein‐Functionalized Indium‐Tin Oxide Nanoantenna Arrays for Selective Infrared Biosensing

Plasmonic nanoantennas afford significantly enhanced near‐fields that can be exploited for ultrasensitive molecular sensing in plasmon‐enhanced vibrational spectroscopy. While elemental gold is usually used in plasmonic biosensing due to its stability and well‐established surface functionalization, compound plasmonic materials with versatile surface functionalization, low‐cost, and lower optical loss especially in the infrared range are receiving considerable attention. This study demonstrates the application of protein‐functionalized single‐crystalline indium‐tin oxide (ITO) nanorod arrays for biosensing in infrared spectroscopy. The ITO nanorod arrays show angle‐dependent plasmon resonances in the mid‐infrared range, whose position and intensity can be readily tuned by varying the incident angle, thereby providing an easy means for selective enhancement of targeted molecular vibrations. The use of phosphonic‐acid‐derived biotin molecules is examined to chemically functionalize the antennas, and the specific adsorption of streptavidin onto them is subsequently demonstrated. The successful surface functionalization of the ITO nanorods demonstrated here facilitates the detection of functional biomolecules and proteins with high specificity.