Comparison of the ACE2 receptor and monoclonal antibodies immobilisation strategies for the sensitive detection of SARS-CoV-2 variants of concern.

Investigation of antibody or receptor immobilisation and binding to the target analyte is essential for the development of effective immunoassays. In our research, we applied the combination of two surface-sensitive methods: spectroscopic ellipsometry and quartz crystal microbalance with dissipation. It enabled quantitative investigation of optical and mechanical properties of formed biomolecule layers consisting of monoclonal antibodies (mAb) or angiotensin-converting enzyme 2 (ACE2) receptors coupled with the Fc fragment, in complex with severe acute respiratory syndrome coronavirus 2 spike Omicron variant (SCoV2-oS). Random and site-directed immobilisation of ACE2 receptor gave 1.8 and 2.4 times higher dry surface mass density compared to random and site-direct mAbs immobilisation, respectively. Therefore, ACE2 had better potential for more sensitive detection of the target analyte SCoV2-oS. However, the binding of SCoV2-oS to site-directed ACE2 resulted in a low 80 ng/cm2 surface mass compared to other samples. Moreover, ΔD/ΔF data revealed two-step binding of SCoV2-oS to ACE2 and mAbs. Furthermore, calculated affinity constants (KD) showed that both ACE2 and mAb have high affinity to SCoV2-oS (in the range of 10-10 to 10-11 M), and their orientation on the surface had only a minor impact on KD values. Our findings in this investigation indicated that ACE2 coupled with the Fc fragment is as effective in the recognition of SARS-CoV-2 as mAbs and it can be successfully applied for the development of immunoassays. Considering SARS-CoV-2 mutates for a better S protein binding to the ACE2 receptor, using ACE2 as a biorecognition element is useful.