Gires-Tournois Immunoassay Platform for Label-Free Colorimetric Detection of SARS-CoV-2

COVID-19 pandemic, caused by respiratory syndrome-coronavirus-2 (SARS-CoV-2), is an ongoing global challenge. Although the reverse transcription polymerase chain reaction (RT-PCR) test is effective in detecting SARS-CoV-2 virus as a gold standard clinical diagnosis, it is insufficient as a preemptive test to prevent the rapid transmission of SARS-CoV-2. To further improve the diagnostic rapidity, direct and sensitive approaches for label-free and/or amplification-free detection of SARS-CoV-2 have been presented. Representatively, electrochemical sensors based primarily on carbon/graphene electrodes have been successfully demonstrated to provide diagnostic results on clinical samples in minutes using immunological responses without an additional pre-sampling process. Even so, these electro-signaling strategies still require expertise and specialized equipment with limited scalability of the sensing area, and are less intuitive for ordinary users. In practical applications for virus detection, the most intuitive method is to easily identify optical or color changes with the human eye, as is the case with typical diagnostic techniques such as fluorescence immunoassays and enzyme-linked immunosorbent assays (ELISA). Although several optical-based diagnostic techniques for rapid detection of SARS-CoV-2 without labeling or amplification have been reported including colorimetric assays or surface plasmon methods using gold nanostructures or particles, they are still challenging in sensitivity and observability compared to methods involving amplification and labeling processes. Over the past few decades, advances in optics to make it possible to control the speed of light propagating through optically dispersive media presents many opportunities for wise use of light. Slow light promotes stronger light-matter interactions, which also have applications in biosensing by providing additional control over the spectral bandwidth of these interactions. Previously reported slow light-based biosensing platform mainly utilize nanovolume-scale structural slow light in the form of nanospheres or nanocrescents as surface-enhanced Raman scattering substrates. Meanwhile, as a simple structure for effectively controlling light, Gires-Tournois resonators using non-linear effective phase changes have been recently studied as flat metasurfaces including a dynamic phase-change absorber, achromatic surface cloaking and lensing, and colored perovskite solar cells. This strong designable modulator, which can effectively control light with various media combinations, has not yet been optimized for biosensing applications including low-index virus detections through light control such as slow light that induces strong light-matter interactions. Here, we introduce the Gires-Tournois immunoassay platform (GTIP) for amplification/label-free rapid detection of SASR-CoV-2. In slow light conditions, GTIP exhibits an intuitive colorimetric response to viral particles of SARS-CoV-2. Viral distribution and concentrations are provided through chromaticity analysis from virus cluster images obtained by microscopic scanning. For practical applications, GTIP, attached to an everyday item such as a face mask, also detects sprayed droplet transmission in infectious environments such as person-to-person conversations and coughing. Moreover, for extended applications with versatility, GTIP provides optimal structures for analytes of different refractive indices and sizes with the design parameters of the tri-layer configuration.