Quantum Optical Immunoassay: Upconversion Nanoparticle-based Neutralizing Assay for COVID-19

TL;DR

This paper introduces a highly sensitive quantum optical immunoassay using upconversion nanoparticles to measure neutralizing antibodies against COVID-19, outperforming existing commercial kits in sensitivity.

Contribution

It presents a novel upconversion nanoparticle-based assay for detecting SARS-CoV-2 neutralizing antibodies with improved sensitivity and dynamic range.

Findings

Sensitivity of 4.0 ng/ml, better than 19 ng/ml of commercial kits

Dynamic range from 1.0 ng/ml to 3.2 μg/ml

Prototype system successfully quantifies antibody neutralization

Abstract

In a viral pandemic, a few important tests are required for successful containment of the virus and reduction in severity of the infection. Among those tests, a test for the neutralizing ability of an antibody is crucial for assessment of population immunity gained through vaccination, and to test therapeutic value of antibodies made to counter the infections. Here, we report a sensitive technique to detect the relative neutralizing strength of various antibodies against the SARS-CoV-2 virus. We used bright, photostable, background-free, fluorescent upconversion nanoparticles conjugated with SARS-CoV-2 receptor binding domain as a phantom virion. A glass bottom plate coated with angiotensin-converting enzyme 2 (ACE-2) protein imitates the target cells. When no neutralizing IgG antibody was present in the sample, the particles would bind to the ACE-2 with high affinity. In contrast, a neutralizing antibody can prevent particle attachment to the ACE-2-coated substrate. A prototype system consisting of a custom-made confocal microscope was used to quantify particle attachment to the substrate. The sensitivity of this assay can reach 4.0 ng/ml and the dynamic range is from 1.0 ng/ml to 3.2 {\mu}g/ml. This is to be compared to 19 ng/ml sensitivity of commercially available kits.