## 3 Results and Discussion
The same happens to all other components of the samples that are in the liquid phase. Through repetition of the washing steps, not only is the selection of the virus species of interest achieved, but also the complexity of the sample is significantly reduced. In the final step, all three sample types essentially consist of the washing buffer, the magnetic beads and, in the case of the SARS-CoV-2 sample, the virus particles. To enable the Raman detection on a dried sample, a further washing step with dH\({}_{2}\)O is implemented to remove the buffer salts. For this study, inactivated SARS-CoV-2 viruses were used. Further information is provided in Section 1 (and associated Figure S1) of the Supporting Information. Figure 2: Detailed display of the magnetic bead-based sample preparation scheme. To the sample 1  
Figure 3a shows the Raman-based identification scheme, for which two data sets have been investigated, each consisting of four replicates per sample type (SARS-CoV-2, Influenza A virus, negative control). The replicates were processed and measured on independent days. It is noteworthy that the two data sets have been investigated several months apart to ensure the reproducibility of the study under different environmental conditions (ex. season change creating variating humidity index, slight lab temperature variation, new batches of magnetic beads, etc.). Two significant differences have been introduced voluntarily: (1) For data set 1, the ACE2 functionalized beads were prepared in one batch, while for data set 2, a fresh batch of ACE2 beads has been prepared for each replicate; (2) As shown in Figure 3a, the order in which the samples were measured within an experimental day has been varied for data set 2, with the objective of ruling out the potential impact of any minor optic changes that can occur in the course of an experimental day. During the Raman measurements, we also observed that the SARS-CoV-2 samples often looked microscopically different from the other sample types. As shown in the microscopic images of Figure S4, the magnetic beads form large agglomerates for the SARS-CoV-2 samples, while for the other sample types, this effect is much less pronounced, if it occurs at all. This formation of large clusters can be ascribed to the multiple binding  
Figure 3: (**a**) Structure of the data sets used for the Raman spectroscopic investigation. For each data set four independent replicates (R1–R4) were considered. (**b**) Raman mean spectra of SARS-CoV-2 (S), Influenza A virus (A) and the negative control (N) as well as the difference spectra. The spectra were shifted vertically for clarity. sites that are present both on the spherical virus particles and on the ACE2 decorated magnetic beads. In principle, this effect alone has great potential for detecting the presence of SARS-CoV-2 using a simple assay. However, we found that the extent of this cluster formation with the current workflow can differ significantly and is therefore not a reliable criterion for identifying SARS-CoV-2 by itself. The Raman measurements were conducted with an incident irradiation of 808 nm, which remains significantly under the typical size of a single bead (2.8 um in diameter). The agglomeration of the beads (domain typically composed of a few tens to several hundreds of beads) does not appear to create additional scattering that could modify the background of the spectra (vide infra). However, to ensure the absence of interferences in the data due to this phenomenon, a minimum of 49 spectra were acquired at different locations on the sample and where the morphology of the bead arrangement differed.