(e) Detection curve obtained from the confocal images. quantitatively measure the glycoproteins and antibodies more efficiently compared with the previous methods reported to date. Furthermore, the intrinsic limit of detection in the metasurface FL sensors was examined via confocal microscopy and found to be less than 0.64 pg/mL for glycoprotein peptides. Moreover, the sensors had a dynamic range more than five orders that of the target concentrations, indicating extremely high sensitivity. These two-way functions of the metasurface FL sensors can be helpful in reducing daily loads in clinics and in providing quantitative test values for proper diagnosis and cures. Keywords: COVID-19, SARS-CoV-2, spike glycoprotein, peptide, antibody, all-dielectric metasurface, fluorescence sensor, sandwich assay 1. Introduction For more than two years, the COVID-19 (SARS-CoV-2) pandemic has substantially reduced social and economic activities. To overcome such damages, efficient and quantitative techniques for evaluating the status of human bodies are required. Particularly, quantification of the concentrations of COVID-19 and neutralizing antibodies (Abs) are highly preferred for proper therapeutic approaches. Currently, polymerase chain reaction (PCR) [1] serves as a high sensitivity method for COVID-19 detection; however, it is widely known that PCR is costly as a daily practice and has high demand with regard Rabbit Polyclonal to B4GALNT1 to human resources. As handy chips, antigen test kits relying on paper-based immunochromatography are used; they are known to be far less sensitive than PCR and qualitative platforms for supplementary tests. Numerous proposals for detecting COVID-19 have been reported in the last two years, and are summarized in several review papers [2,3,4,5,6,7,8]. However, to the best of our knowledge, there is no newly established technique that meets the aforementioned criterion, suggesting practical difficulties that have not been noted in previous reports. Thus, there is room to explore a practical and quantitative technique to detect the antigens and Abs. Here, we address proof-of-concept experiments for the quantitative detection of COVID-19 spike proteins and Abs. When implementing sandwich assays to detect AbCproteinCAb complexes, it becomes possible to set the proteins or Abs to be a target in the assay, meaning that two-way detection ONC212 can be realized in one platform through the employment of all-dielectric ONC212 metasurface fluorescence (FL) sensors [9,10,11]. The configurations of the two-way assay are schematically illustrated in Figure 1a,b. The optical features of the metasurfaces are described in Section 3.1. Section 3.2 shows that efficient detection of the target glycoprotein can be realized in a practical setup which is superior to the conventional enzyme-linked immunosorbent assays (ELISAs), and that the intrinsic detection capability of metasurface FL sensors reaches sub-pg/mL range in low-background confocal measurements. Additionally, we obtained linear quantitative detection down to 10 picomolars (pM) when setting the Abs as a target; this is elaborated in Section 3.3. Importantly, the all-dielectric metasurface FL sensors were applied to the detection of immunoglobulin G (IgG) Abs in human serums, and it was substantiated that they can retain their high sensitivity even in the presence of abundant other biomolecules, such as albumin, lipids, and other Abs [9]. One of the key factors is binding molecules that are immobilized on the surface of Si nanorods, which specifically capture biomolecules. In this study, we used the same binding molecule used in previous reports [9,10,11]. The metasurfaces were arrayed on a substrate in an assembled manner, as shown in Figure 1c. By combining the metasurface substrate with a microfluidic (MF) chip made of transparent polydimethylsiloxane (PDMS), a metasurface FL sensor was ONC212 formed (Figure 1d), which is illuminated by the green LED light. The outlook of the experimental setup, including the ONC212 MF.