Enhancing the signal strength of surface sensitive 2D IR spectroscopy

Spectroscopic techniques that are capable of measuring surfaces and interfaces must overcome two technical challenges: one, the low coverage of molecules at the surface, and two, discerning between signals from the bulk and surface. We present surface enhanced attenuated reflection 2D infrared (SEAR 2D IR) spectroscopy, a method that combines localized surface plasmons with a reflection pump-probe geometry to achieve monolayer sensitivity. The method is demonstrated at 6 µm with the amide I band of a model peptide, a cysteine terminated α-helical peptide tethered to a gold surface. Using SEAR 2D IR spectroscopy, the signal from this sample is enhanced 20 000-times over a monolayer on a dielectric surface. Like attenuated total reflection IR spectroscopy, SEAR 2D IR spectroscopy can be applied to strongly absorbing solvents. We demonstrated this capability by solvating a peptide monolayer with H₂O, which cannot normally be used when measuring the amide I band. SEAR 2D IR spectroscopy will be advantageous for studying chemical reactions at electrochemical surfaces, interfacial charge transfer in photovoltaics, and structural changes of transmembrane proteins in lipid membranes.