As an immune response to COVID-19 infection, patients develop SARS-CoV-2-specific IgM/IgG antibodies. Here, we compare the performance of a conventional lateral flow assay (LFA) with a surface-enhanced Raman scattering (SERS)-based LFA test for the detection of SARS-CoV-2-specific IgM/IgG in sera of COVID-19 patients. Sensitive detection of IgM might enable early serological diagnosis of acute infections. Rapid detection in serum using a custom-built SERS reader is at least an order of magnitude more sensitive than the conventional LFAs with naked-eye detection. For absolute quantification and the determination of the limit of detection (LOD), a set of reference measurements using purified (total) IgM in buffer was performed. In this purified system, the sensitivity of SERS detection is even 7 orders of magnitude higher: the LOD for SERS was ca. 100 fg/mL compared to ca. 1 μg/mL for the naked-eye detection. This outlines the high potential of SERS-based LFAs in point-of-care testing once the interference of serum components with the gold conjugates and the nitrocellulose membrane is minimized.

Surface‐enhanced resonance Raman scattering (SERRS) is based on nanostructures supporting localized surface plasmon resonances as well as electronic resonances of the molecular adsorbate. Bright NIR‐SERRS nanotags require dyes with high Raman scattering cross‐sections upon NIR laser excitation in combination with functional groups for chemisorption. Here, we report on the role of isothiocyanate (ITC) and sulfonate (SO) groups in five cyanine dyes on sensitivity and performance of nanotags in SERRS‐based LFA. Dye concentration‐dependent SERRS and zeta potential measurements on bare gold nanostars (AuNS) revealed strong differences in their sensitivity and stability as a function of the number of ITC and SO groups. A positive correlation with the number of ITC groups due to stronger chemisorption and a negative correlation with the number of SO groups due to stronger desorption because of higher water solubility was observed. The application in SERRS‐based biosensing requires additional functionalization steps, for example, co‐adsorption of PEG with the NIR dye. The comparison of the Raman intensities before and after bioconjugation showed that the presence of at least one ITC group is necessary for minimizing Raman signal losses, irrespective of the additional presence of SO groups. In a SERRS‐based lateral flow assay (LFA) for sensing the pregnancy marker beta‐hCG, the two dyes NIR 4f and NIR 5e (both ITC = 2, SO = 0) showed the best performance. In contrast, we do not recommend to use IR‐783 (ITC = 0, SO = 2) for SERRS‐based biosensing. Overall, this study highlights the importance of surface‐seeking moieties such as the common ITC group for optimal performance in SERRS‐based biosensing platforms.