Surface-enhanced infrared absorption spectroscopy and spectral analysis. SEIRA spectra were recorded using a Bruker Vertex 80v FTIR spectrometer equipped with a liquid N2-cooled MCT detector, in a Kretschmann-ATR geometry at an angle of incident of 60°, and at a spectral resolution of 4 cm -1 . Nanostructured SEIRA Au films were obtained according to Miyake et al. by electroless deposition from a NaAuCl4 solution. Spectra obtained in H2O buffer were analyzed using a global fitting procedure written in Python (Global Peak and 2 nd Derivative Fitting; GP2DF.py; see SI Section 1 for further details). A fit with Gaussian line shapes is performed simultaneously to spectra and their 2 nd derivatives (a measure for peak curvature enhancing narrow features), modelling sharp peptide bands with higher accuracy than broad overlapping water bands; furthermore, as low-curvature background drifts disappear in the 2 nd derivative, this fitting strategy can be used for convenient automatized background correction. 2 nd derivatives of spectra are obtained after Savitzky-Golay smoothing with a window of 21 data points and are multiplied with a factor of 100 (to appear with similar intensities as spectra). An initial set of Gaussian bands is set up with HWHM of < 23.6 cm -1 (σ < 20 cm -1 ; peptide contributions) or > 41.2 cm -1 (σ < 35 cm -1 ; water contributions) and a 4 th -order polynomial baseline. Each Gaussian band is fit with a maintained shape throughout the entire set of SEIRA spectra, and peak intensity changes describe the spectral evolution with time. After a fitting has converged, the polynomial baseline and broad Gaussian components are removed, to reveal the narrow bands due to Aβ. Fits have been repeated with several starting sets of Gaussians and with data recorded in D2O ensuring that similar results are obtained.