Since the tBLM contains patches of free-floating lipid bilayers and ChEO3SH-supported lipid monolayers (Figure and 2A), we aimed to test to which of these areas virus binding occurred preferentially. Towards this goal, we constructed tBLMs on a pure self-assembled monolayer of ChEO3SH with the same lipid mixture of POPC and 1 mol% GD1a (Figure ). Adding IAV, we again observe growing amide bands at 1659 and 1548 cm -1 , which now closely resemble the reference ATR-IR spectrum of IAV. Interestingly, very similar SEIRA spectra are detected on tBLMs without GD1a, i.e. without IAV's binding receptor (Figure ). Therefore, the spectra in Figure (and Figure ) display IAV that unspecifically sedimented onto the tBLM, contrasting markedly different SEIRA spectra in Figure due to specific IAV-GD1a interactions. One can understand the observed difference between SEIRA spectra of specific and unspecific binding (i.e. Figure vs. 2E) based on the SEIRA distance dependence. The SEIRA signal is due to an electromagnetic nearfield, which drops with a distance dependence of d -6 from the SEIRA Au film (dashed line in Figure ). As such, most of the SEIRA signal originates from the first 10 nm (red area), where a polarized SEIRA spectrum of oriented HA proteins is detected (square in Figure and). In fact, analyzing the shape of the amide I band, we obtain accurate relative contributions of secondary structure elements of HA at the membrane interface. The highlighted region in Figure , showing the structure of HA's segments interacting with the tBLM, is composed of 46% turn/random, 22% α-helices and 32% β-sheets. Fitting the amide I band observed using SEIRA in Figure 2B