1/7/2024 0 Comments Date a live origami![]() The released Ag(I) in turn suppresses urease activity, leading to lower pH conditions and a yellow readout. Therefore, to detect viral RNA, the authors designed the DNA probe so that Ag(I) is released on binding to a SARS-CoV-2 sequence (Fig. At the same time, the urease enzyme, which hydrolyses urea to release the weak base ammonia, is strongly inhibited by free Ag(I) ions 11. Ag(I) ions, as with many metal cations, interact strongly with negatively charged DNA, forming C–Ag–C complexes when exposed to C:C mismatches within a DNA double helix 10. To couple a change in pH to the presence of viral RNA, the authors made clever use of the interactions of silver (Ag) ions with two components: a DNA-based probe, and the enzyme urease. The assay technology, which they named MARVE (short for multiplexed, nucleic-acid-amplification-free, single-nucleotide-resolved viral evolution), involves phenol red - a common pH indicator - that undergoes a dramatic change in colour (from red to yellow) as pH decreases. To create such a sensitive, fast and simple assay, Li and co-authors pursued a strategy that does not require nucleic acid amplification. The assay can detect as little as 400 copies per microlitre of viral RNA, and leverages highly specific nucleic acid interactions to identify SARS-CoV-2 variants of concern. Writing in Nature Biomedical Engineering, Jinghong Li, Weimin Li, Ruijie Deng and colleagues now describe a paper-based nucleic acid assay for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that is fast (30 min from sample in to result out) and cheap (under one US dollar per test), and that can be read out by the naked eye 9. However, developing diagnostics that match the convenience of lateral-flow tests and the accuracy of RT–qPCR has been challenging. ![]() A variety of recently developed alternative nucleic acid tests 2, 3, 4, 5, 6, 7, 8 are easier to perform and are less costly than RT–qPCR, and offer better sensitivity than lateral-flow assays. But RT–qPCR nucleic acid assays require trained personnel and expensive equipment to amplify the genetic material from the pathogen, and hence the tests are performed by centralized facilities and have slower turnaround times. However, antigen tests have lower sensitivity and specificity - and hence produce more false positives - than the gold-standard test based on quantitative PCR with reverse transcription (RT–qPCR) 1. Antigen tests in a lateral-flow format are being widely used during the coronavirus disease 2019 (COVID-19) pandemic because they provide results fast, and are simple to use.
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