The present invention relates to an infrared absorption spectroscopy apparatus including an infrared transparent substrate comprising a first and second surface, an array of plasmonic nano-antennas arranged on the first surface of the infrared transparent substrate, a flow cell for holding a liquid to allow spectroscopy measurements in a liquid environment, the array of plasmonic nano-antennas being located inside the flow cell, an optical source providing an incident light probe signal incident on at least a part of the array of plasmonic nano-antennas via the second surface of the infrared transparent substrate, and an optical element to collect reflected light signal reflected by said part of the array of plasmonic nano-antennas.