ABSTRACT
Nucleic acid detection methods based on isothermal amplification techniques show great potential for point-of-care diagnostic applications. However, most current methods rely on fluorescent or lateral flow assay readout, requiring external excitation or post-amplification reaction transfer. Here, we developed a bioluminescent nucleic acid sensor (LUNAS) platform in which target dsDNA is sequence-specifically detected by a pair of dCas9-based probes mediating split NanoLuc luciferase complementation. Whereas LUNAS itself features a detection limit of [~]1 pM for dsDNA targets, the LUNAS platform is easily integrated with recombinase polymerase amplification (RPA), providing attomolar sensitivity in a single-pot assay. We designed a one-pot RT-RPA-LUNAS assay for detecting SARS-CoV-2 RNA without the need for RNA isolation and demonstrated the diagnostic performance for COVID-19 patient nasopharyngeal swab samples using a digital camera to record the ratiometric signal. Detection of SARS-CoV-2 from samples with viral RNA loads of [~]200 cp/L was achieved within [~]20 minutes, showing that RPA-LUNAS is attractive for point-of-care diagnostic applications.
ABSTRACT
Heterogeneous immunoassays such as ELISA have become indispensable in modern bioanalysis, yet translation into easy-to-use point-of-care assays is hindered by their dependence on external calibration and multiple washing and incubation steps. Here, we introduce RAPPID (Ratiometric Plug-and-Play Immunodiagnostics), a "mix-and-measure" homogeneous immunoassay platform that combines highly specific antibody-based detection with a ratiometric bioluminescent readout that can be detected using a basic digital camera. The concept entails analyte-induced complementation of split NanoLuc luciferase fragments, photoconjugated to an antibody sandwich pair via protein G adapters. We also introduce the use of a calibrator luciferase that provides a robust ratiometric signal, allowing direct in-sample calibration and quantitative measurements in complex media such as blood plasma. We developed RAPPID sensors that allow low-picomolar detection of several protein biomarkers, anti-drug antibodies, therapeutic antibodies, and both SARS-CoV-2 spike protein and anti-SARS-CoV-2 antibodies. RAPPID combines ratiometric bioluminescent detection with antibody-based target recognition into an easy-to-implement standardized workflow, and therefore represents an attractive, fast, and low-cost alternative to traditional immunoassays, both in an academic setting and in clinical laboratories for point-of-care applications.
