Adding IAV to the supernatant solution (Figure ), we monitored SEIRA spectra taking the spectrum of the tBLM as background (Figure ; black to purple spectra). We observe the appearance of several spectral features, in particular, the bands at 1659 and 1548 cm -1 assigned to protein amide I and II modes, respectively, which indicate a favorable interaction of the IAV particles with the tBLM. Comparing the shape of the amide bands in the SEIRA spectra with a reference spectrum of IAV particles recorded using ATR-IR spectroscopy (purple spectrum below SEIRA spectra in Figure ), we note considerable differences, for instance, in the relative intensities of the amide I and amide II: while in the SEIRA spectra both bands are of similar intensity (amide I/amide II ratio of ~1.2), the ATR-IR spectrum shows a considerably larger amide I. As the ATR-IR spectrum represents a sum of all constituents of the virus (lipids, envelope proteins, capsid, and genetic material), the spectral differences in the amide bands indicate that SEIRA specifically detects the portion of the HA proteins of the IAV particles that interact with the GD1a headgroups at the membrane interface. This is supported by a set of the following observations. The CHn stretching region (3000 -2800 cm -1 ) of the ATR-IR reference spectra of IAV, GD1a and POPC show dominating peaks at 2924 and 2852 cm -1 , which are assigned to CH2 stretches typical for long alkyl chains of lipids. Instead, in the SEIRA spectra the CH3 stretch at 2962 cm -1 is strongest as typically observed in protein spectra (grey reference spectrum); the SEIRA spectrum is therefore indicative of the branched alkyl sidechains in HA. In addition, we observe changes to the tBLM. In particular in the initial SEIRA spectra, we note a broad band at ~1600 cm -1 , which coincides with the GD1a absorption band (orange ATR-IR spectrum in Figure ). Furthermore, in later spectra, a difference peak at 1745(+)/1728(-) cm -1 occurs matching the C=O stretch band of the lipid ester groups (blue ATR-IR spectrum in Figure ). The lipid C=O band itself is composed of two components, i.e., non-H-bonded and H-bonded C=O groups at roughly 1740 and 1730 cm -1 , respectively. As such, the SEIRA difference feature indicates a change in the H-bonding network and/or reduction of lipid hydration upon IAV-GD1a interaction. This is further supported by negative water bands (i.e. removal of water from the membrane interface) shown in Figure .