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. 2021 Nov 11;1:46. doi: 10.1038/s43856-021-00045-9

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — a Schematic of working principle and workflow of cellulose pull-down VNT (cpVNT). (i) Un-diluted plasma is mixed with receptor binding domain (RBD) tagged to cellulose binding domain (RBD–CBD) and angiotensin-converting enzyme 2 receptor (ACE2) tagged to biotin and streptavidin horse-radish peroxidase (ACE2-BA-SA-HRP). (ii) The reaction is incubated for 5 min to allow neutralizing antibodies (Nabs)/RBD–CBD or ACE2-BA-SA-HRP/RBD–CBD complex formations. (iii) The mixture is applied on the cellulose-based vertical-flow device. (iv) Washing solution and ready-to-use 3,3′,5,5′-tetramethylbenzidine (TMB) substrate solution are sequentially applied to the device. Colorimetric reaction is allowed to develop for 3 min. Signals were captured using camera. b Images of cpVNT test results at different concentrations of mouse anti-SARS-CoV-2 neutralizing antibodies and human anti-RBD non-neutralizing antibodies. c Plots of inhibitory percentages derived from cyan intensity signals. Limit of detections (LOD) were determined using mean + 3 standard deviation (SD) formula and represented as the dotted lines. All data were represented as mean ± SD. Each data points were performed in triplicates. 4 Parameters logistic model was used to draw the fitted curve with R² value of 0.9534.