Digital CRISPR/Cas-Assisted Assay for Rapid and Sensitive Detection of SARS-CoV-2

Recent advances in the CRISPR/Cas system-based visual detection method

Currently, various infectious pathogens and bacterial toxins as well as heavy metal pollution pose severe threats to global environmental health and the socio-economic infrastructure. Therefore, there is a pressing need for rapid, sensitive, and convenient visual molecular detection methods. The rapidly evolving detection approach based on clustered regularly interspaced short palindromic repeats (CRISPR)/associated nucleases (Cas) has opened a new frontier in the field of molecular diagnostics. This paper reviews the development of visual detection methods in recent years based on different Cas and analyzes their advantages and disadvantages as well as the challenges of future research. Firstly, different CRISPR/Cas effectors and their working principles in the diagnosis of various diseases are briefly reviewed. Subsequently, the article focuses on the development of visual readout signals in point-of-care testing using laboratory-based CRISPR/Cas technology, including colorimetric, fluorescence, and lateral flow analysis. Finally, the challenges and prospects of visual detection methods based on CRISPR/Cas technology are discussed.

CRISPR/Cas-mediated "one to more" lighting-up nucleic acid detection using aggregation-induced emission luminogens

CRISPR diagnostics are effective but suffer from low signal transduction efficiency, limited sensitivity, and poor stability due to their reliance on the trans-cleavage of single-stranded nucleic acid fluorescent reporters. Here, we present CrisprAIE, which integrates CRISPR/Cas reactions with "one to more" aggregation-induced emission luminogen (AIEgen) lighting-up fluorescence generated by the trans-cleavage of Cas proteins to AIEgen-incorporated double-stranded DNA labeled with single-stranded nucleic acid linkers and Black Hole Quencher groups at both ends (Q-dsDNA/AIEgens-Q). CrisprAIE demonstrates superior performance in the clinical nucleic acid detection of norovirus and SARS-CoV-2 regardless of amplification. Moreover, the diagnostic potential of CrisprAIE is further enhanced by integrating it with spherical nucleic acid-modified AIEgens (SNA/AIEgens) and a portable cellphone-based readout device. The improved CrisprAIE system, utilizing Q-dsDNA/AIEgen-Q and SNA/AIEgen reporters, exhibits approximately 80- and 270-fold improvements in sensitivity, respectively, compared to conventional CRISPR-based diagnostics. We believe CrisprAIE can be readily extended as a universal signal generation strategy to significantly enhance the detection efficiency of almost all existing CRISPR-based diagnostics.

Portable wide-field femtoliter-chamber imaging system for point-of-care digital bioanalysis

Recently, digital bioanalysis using femtoliter (fL)-chamber arrays has significantly improved the sensitivity, accuracy, and throughput of conventional nucleic acid and antigen tests, with great potential for the diagnosis of infectious diseases and underlying disorders. However, the large size of conventional platforms with costly assay consumables for digital bioanalysis complicates its use in point-of-care testing (POCT). To solve these problems, in this study, we developed a wide-field fL-chamber imaging system (COWFISH2), a portable wide-field femtoliter-chamber imaging system (footprint: 14 × 22 cm), by redesigning various electronic controls and optical systems of COWFISH, accompanied by the development of low-cost and durable consumables for digital bioanalysis. As a proof of concept, the point-of-care digital bioanalysis was successfully performed in a hospital setting, using amplification-free multiplex digital RNA detection of SARS-CoV-2, influenza A virus, and influenza B virus. Collectively, COWFISH2 will facilitate versatile and convenient digital bioanalysis in POCT, contributing to the improvement of public health, including the prevention of infectious diseases.

Enhanced CRISPR/Cas-Based Immunoassay through Magnetic Proximity Extension and Detection

Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas-associated systems have recently emerged as a focal point for developing next-generation molecular diagnosis, particularly for nucleic acid detection. However, the detection of proteins is equally critical across diverse applications in biology, medicine, and the food industry, especially for diagnosing and prognosing diseases like cancer, Alzheimer's and cardiovascular conditions. Despite recent efforts to adapt CRISPR/Cas systems for protein detection with immunoassays, these methods typically achieved sensitivity only in the femtomolar to picomolar range, underscoring the need for enhanced detection capabilities. To address this, we developed CRISPR-AMPED, an innovative CRISPR/Cas-based immunoassay enhanced by magnetic proximity extension and detection. This approach combines proximity extension assay (PEA) with magnetic beads that converts protein into DNA barcodes for quantification with effective washing steps to minimize non-specific binding and hybridization, therefore reducing background noise and increasing detection sensitivity. The resulting DNA barcodes are then detected through isothermal nucleic acid amplification testing (NAAT) using recombinase polymerase amplification (RPA) coupled with the CRISPR/Cas12a system, replacing the traditional PCR. This integration eliminates the need for thermocycling and bulky equipment, reduces amplification time, and provides simultaneous target and signal amplification, thereby significantly boosting detection sensitivity. CRISPR-AMPED achieves attomolar level sensitivity, surpassing ELISA by over three orders of magnitude and outperforming existing CRISPR/Cas-based detection systems. Additionally, our smartphone-based detection device demonstrates potential for point-of-care applications, and the digital format extends dynamic range and enhances quantitation precision. We believe CRISPR-AMPED represents a significant advancement in the field of protein detection.

Recent Advances in DNA Nanotechnology-Based Sensing Platforms for Rapid Virus Detection

Several major viral pandemics in history have significantly impacted the public health of human beings. The COVID-19 pandemic has further underscored the critical need for early detection and screening of infected individuals. However, current detection techniques are confronted with deficiencies in sensitivity and accuracy, restricting the capability of detecting trace amounts of viruses in human bodies and in the environments. The advent of DNA nanotechnology has opened up a feasible solution for rapid and sensitive virus determination. By harnessing the designability and addressability of DNA nanostructures, a range of rapid virus sensing platforms have been proposed. This review overviewed the recent progress, application, and prospect of DNA nanotechnology-based rapid virus detection platforms. Furthermore, the challenges and developmental prospects in this field were discussed.

CRISPR-Cas12a-based ultrasensitive assay for visual detection of SARS-CoV-2 RNA

The development of ultrasensitive and visual methods is of great significance for molecular diagnosis at the point-of-care. In this study, we have integrated recombinase polymerase amplification (RPA) with the CRISPR-Cas12a system to design an ultrasensitive strategy for visual nucleic acid testing. RPA is utilized to amplify the target nucleic acid, producing amplicons that activate the single-stranded DNase property of CRISPR-Cas12a. The activated CRISPR-Cas12a then degrades the single-stranded DNA on magnetic nanoparticles (MNPs), releasing immobilized GOx from the MNPs which catalyses the chromogenic substrate. The developed method exhibits remarkable sensitivity, successfully detecting as low as 10 aM (∼6 copies per μL) of the target nucleic acid by visual colour changes in solution. The instrumental limit of detection is calculated to be 2.86 aM (∼2 copies per μL), comparable to the sensitivity of polymerase chain reaction (PCR). Importantly, this approach only requires isothermal incubation operation and does not involve costly instruments. The method has been validated by visually detecting the SARS-CoV-2 RNA gene fragment within 50 minutes. With its ultrasensitivity, simplicity of operation, and potential for integration into a point-of-care detection kit, this strategy holds great promise for nucleic acid testing in various settings.

Evaluation and comparison of one-step real-time PCR and one-step RT-LAMP methods for detection of SARS-CoV-2

BACKGROUND

There is an increasing disease trend for SARS-COV-2, so need a quick and affordable diagnostic method. It should be highly accurate and save costs compared to other methods. The purpose of this research is to achieve these goals.

METHODS

This study analyzed 342 samples using TaqMan One-Step RT-qPCR and fast One-Step RT-LAMP (Reverse Transcriptase Loop-Mediated Isothermal Amplification). The One-Step LAMP assay was conducted to assess the sensitivity and specificity.

RESULTS

The research reported positive samples using two different methods. In the RT-LAMP method, saliva had 92 positive samples (26.9%) and 250 negative samples (73.09%) and nasopharynx had 94 positive samples (27.4%) and 248 negative samples (72.51%). In the RT-qPCR method, saliva had 86 positive samples (25.1%) and 256 negative samples (74.8%) and nasopharynx had 93 positive samples (27.1%) and 249 negative samples (72.8%). The agreement between the two tests in saliva and nasopharynx samples was 93% and 94% respectively, based on Cohen's kappa coefficient (κ) (P < 0.001). The rate of sensitivity in this technique was reported at a dilution of 1 × 101 and 100% specificity.

CONCLUSIONS

Based on the results of the study the One-Step LAMP assay has multiple advantages. These include simplicity, cost-effectiveness, high sensitivity, and specificity. The One-Step LAMP assay shows promise as a diagnostic tool. It can help manage disease outbreaks, ensure prompt treatment, and safeguard public health by providing rapid, easy-to-use testing.

Digitized Kinetic Analysis Enhances Genotyping Capacity of CRISPR-Based Biosensing

CRISPR/Cas systems have been widely employed for nucleic acid biosensing and have been further advanced for mutation detection by virtue of the sequence specificity of crRNA. However, existing CRISPR-based genotyping methods are limited by the mismatch tolerance of Cas effectors, necessitating a comprehensive screening of crRNAs to effectively distinguish between wild-type and point-mutated sequences. To circumvent the limitation of conventional CRISPR-based genotyping, here, we introduce Single-Molecule kinetic Analysis via a Real-Time digital CRISPR/Cas12a-assisted assay (SMART-dCRISPR). SMART-dCRISPR leverages the differential kinetics of the signal increase in CRISPR/Cas systems, which is modulated by the complementarity between crRNA and the target sequence. It employs single-molecule digital measurements to discern mutations based on kinetic profiles that could otherwise be obscured by variations in the target concentrations. We applied SMART-dCRISPR to genotype notable mutations in SARS-CoV-2, point mutation (K417N) and deletion (69/70DEL), successfully distinguishing wild-type, Omicron BA.1, and Omicron BA.2 SARS-CoV-2 strains from clinical nasopharyngeal/nasal swab samples. Additionally, we introduced a portable digital real-time sensing device to streamline SMART-dCRISPR and enhance its practicality for point-of-care settings. The combination of a rapid and sensitive isothermal CRISPR-based assay with single-molecule kinetic analysis in a portable format significantly enhances the versatility of CRISPR-based nucleic acid biosensing and genotyping.

Identifying SARS-CoV-2 Variants Using Single-Molecule Conductance Measurements

The global COVID-19 pandemic has highlighted the need for rapid, reliable, and efficient detection of biological agents and the necessity of tracking changes in genetic material as new SARS-CoV-2 variants emerge. Here, we demonstrate that RNA-based, single-molecule conductance experiments can be used to identify specific variants of SARS-CoV-2. To this end, we (i) select target sequences of interest for specific variants, (ii) utilize single-molecule break junction measurements to obtain conductance histograms for each sequence and its potential mutations, and (iii) employ the XGBoost machine learning classifier to rapidly identify the presence of target molecules in solution with a limited number of conductance traces. This approach allows high-specificity and high-sensitivity detection of RNA target sequences less than 20 base pairs in length by utilizing a complementary DNA probe capable of binding to the specific target. We use this approach to directly detect SARS-CoV-2 variants of concerns B.1.1.7 (Alpha), B.1.351 (Beta), B.1.617.2 (Delta), and B.1.1.529 (Omicron) and further demonstrate that the specific sequence conductance is sensitive to nucleotide mismatches, thus broadening the identification capabilities of the system. Thus, our experimental methodology detects specific SARS-CoV-2 variants, as well as recognizes the emergence of new variants as they arise.

Ratiometric nonfluorescent CRISPR assay utilizing Cas12a-induced plasmid supercoil relaxation

Most CRISPR-based biosensors rely on labeled reporter molecules and expensive equipment for signal readout. A recent approach quantifies analyte concentration by sizing λ DNA reporters via gel electrophoresis, providing a simple solution for label-free detection. Here, we report an alternative strategy for label-free CRISPR-Cas12a, which relies on Cas12a trans-nicking induced supercoil relaxation of dsDNA plasmid reporters to generate a robust and ratiometric readout. The ratiometric CRISPR (rCRISPR) measures the relative percentage of supercoiled plasmid DNA to the relaxed circular DNA by gel electrophoresis for more accurate target concentration quantification. This simple method is two orders of magnitude more sensitive than the typical fluorescent reporter. This self-referenced strategy solves the potential application limitations of previously demonstrated DNA sizing-based CRISPR-Dx without compromising the sensitivity. Finally, we demonstrated the applicability of rCRISPR for detecting various model DNA targets such as HPV 16 and real AAV samples, highlighting its feasibility for point-of-care CRISPR-Dx applications.

Palm-Sized Lab-In-A-Magnetofluidic Tube Platform for Rapid and Sensitive Virus Detection

Simple, sensitive, and accurate molecular diagnostics are critical for preventing rapid spread of infection and initiating early treatment of diseases. However, current molecular detection methods typically rely on extensive nucleic acid sample preparation and expensive instrumentation. Here, a simple, fully integrated, lab-in-a-magnetofluidic tube (LIAMT) platform is presented for "sample-to-result" molecular detection of virus. By leveraging magnetofluidic transport of micro/nano magnetic beads, the LIAMT device integrates viral lysis, nucleic acid extraction, isothermal amplification, and CRISPR detection within a single engineered microcentrifuge tube. To enable point-of-care molecular diagnostics, a palm-sized processor is developed for magnetofluidic separation, nucleic acid amplification, and visual fluorescence detection. The LIAMT platform is applied to detect SARS-CoV-2 and HIV viruses, achieving a detection sensitivity of 73.4 and 63.9 copies µL-1, respectively. Its clinical utility is further demonstrated by detecting SARS-CoV-2 and HIV in clinical samples. This simple, affordable, and portable LIAMT platform holds promise for rapid and sensitive molecular diagnostics of infectious diseases at the point-of-care.

Advances in Nucleic Acid Assays for Infectious Disease: The Role of Microfluidic Technology

Within the fields of infectious disease diagnostics, microfluidic-based integrated technology systems have become a vital technology in enhancing the rapidity, accuracy, and portability of pathogen detection. These systems synergize microfluidic techniques with advanced molecular biology methods, including reverse transcription polymerase chain reaction (RT-PCR), loop-mediated isothermal amplification (LAMP), and clustered regularly interspaced short palindromic repeats (CRISPR), have been successfully used to identify a diverse array of pathogens, including COVID-19, Ebola, Zika, and dengue fever. This review outlines the advances in pathogen detection, attributing them to the integration of microfluidic technology with traditional molecular biology methods and smartphone- and paper-based diagnostic assays. The cutting-edge diagnostic technologies are of critical importance for disease prevention and epidemic surveillance. Looking ahead, research is expected to focus on increasing detection sensitivity, streamlining testing processes, reducing costs, and enhancing the capability for remote data sharing. These improvements aim to achieve broader coverage and quicker response mechanisms, thereby constructing a more robust defense for global public health security.

Exploring fluoropolymers for fabrication of femtoliter chamber arrays used in digital bioanalysis

The global supply of fluoropolymers and fluorinated solvents is decreasing due to environmental concerns regarding polyfluoroalkyl substances. CYTOP has been used for decades primarily as a component of a femtoliter chamber array for digital bioanalysis; however, its supply has recently become scarce, increasing the urgency of fabricating a femtoliter chamber array using alternative materials. In this study, we investigated the feasibility of fabricating a femtoliter chamber array using four types of fluoropolymers in stable supply as candidate substitutes and verified their applicability for digital bioanalysis. Among these candidates, Fluorine Sealant emerged as a viable option for fabricating femtoliter chamber arrays using a conventional photolithography process. To validate its efficacy, we performed various digital bioanalysis using FP-A-based chamber arrays with model enzymes such as CRISPR-Cas, horseradish peroxidase, and β-galactosidase. The results demonstrated the similar performance to that of CYTOP, highlighting the broader utility of FP-A in digital bioanalysis. Our findings underscore the potential of FP-A to enhance the versatility of digital bioanalysis and foster the ongoing advancement of innovative diagnostic technologies.

On-Site and Visual Detection of the H5 Subtype Avian Influenza Virus Based on RT-RPA and CRISPR/Cas12a

Avian influenza viruses (AIVs) of the H5 subtype rank among the most serious pathogens, leading to significant economic losses in the global poultry industry and posing risks to human health. Therefore, rapid and accurate virus detection is crucial for the prevention and control of H5 AIVs. In this study, we established a novel detection method for H5 viruses by utilizing the precision of CRISPR/Cas12a and the efficiency of RT-RPA technologies. This assay facilitates the direct visualization of detection results through blue light and lateral flow strips, accurately identifying H5 viruses with high specificity and without cross-reactivity against other AIV subtypes, NDV, IBV, and IBDV. With detection thresholds of 1.9 copies/μL (blue light) and 1.9 × 103 copies/μL (lateral flow strips), our method not only competes with but also slightly surpasses RT-qPCR, demonstrating an 80.70% positive detection rate across 81 clinical samples. The RT-RPA/CRISPR-based detection method is characterized by high sensitivity, specificity, and independence from specialized equipment. The immediate field applicability of the RT-RPA/CRISPR approach underscores its importance as an effective tool for the early detection and management of outbreaks caused by the H5 subtype of AIVs.

An autocatalytic CRISPR-Cas amplification effect propelled by the LNA-modified split activators for DNA sensing

CRISPR-Cas systems with dual functions offer precise sequence-based recognition and efficient catalytic cleavage of nucleic acids, making them highly promising in biosensing and diagnostic technologies. However, current methods encounter challenges of complexity, low turnover efficiency, and the necessity for sophisticated probe design. To better integrate the dual functions of Cas proteins, we proposed a novel approach called CRISPR-Cas Autocatalysis Amplification driven by LNA-modified Split Activators (CALSA) for the highly efficient detection of single-stranded DNA (ssDNA) and genomic DNA. By introducing split ssDNA activators and the site-directed trans-cleavage mediated by LNA modifications, an autocatalysis-driven positive feedback loop of nucleic acids based on the LbCas12a system was constructed. Consequently, CALSA enabled one-pot and real-time detection of genomic DNA and cell-free DNA (cfDNA) from different tumor cell lines. Notably, CALSA achieved high sensitivity, single-base specificity, and remarkably short reaction times. Due to the high programmability of nucleic acid circuits, these results highlighted the immense potential of CALSA as a powerful tool for cascade signal amplification. Moreover, the sensitivity and specificity further emphasized the value of CALSA in biosensing and diagnostics, opening avenues for future clinical applications.

Point-Of-Care Ultra-Portable Single-Molecule Bioassays for One-Health

Screening asymptomatic organisms (humans, animals, plants) with a high-diagnostic accuracy using point-of-care-testing (POCT) technologies, though still visionary holds great potential. Convenient surveillance requires easy-to-use, cost-effective, ultra-portable but highly reliable, in-vitro-diagnostic devices that are ready for use wherever they are needed. Currently, there are not yet such devices available on the market, but there are a couple more promising technologies developed at readiness-level 5: the Clustered-Regularly-Interspaced-Short-Palindromic-Repeats (CRISPR) lateral-flow-strip tests and the Single-Molecule-with-a-large-Transistor (SiMoT) bioelectronic palmar devices. They both hold key features delineated by the World-Health-Organization for POCT systems and an occurrence of false-positive and false-negative errors <1-5% resulting in diagnostic-selectivity and sensitivity >95-99%, while limit-of-detections are of few markers. CRISPR-strip is a molecular assay that, can detect down to few copies of DNA/RNA markers in blood while SiMoT immunometric and molecular test can detect down to a single oligonucleotide, protein marker, or pathogens in 0.1mL of blood, saliva, and olive-sap. These technologies can prospectively enable the systematic and reliable surveillance of asymptomatic ones prior to worsening/proliferation of illnesses allowing for timely diagnosis and swift prognosis. This could establish a proactive healthcare ecosystem that results in effective treatments for all living organisms generating diffuse and well-being at efficient costs.

Portable, single nucleotide polymorphism-specific duplex assay for virus surveillance in wastewater

The Argonaute protein from the archaeon Pyrococcus furiosus (PfAgo) is a DNA-guided nuclease that targets DNA with any sequence. We designed a virus detection assay in which the PfAgo enzyme cleaves the reporter probe, thus generating fluorescent signals when amplicons from a reverse transcriptase loop-mediated isothermal amplification (RT-LAMP) assay contain target sequences. We confirmed that the RT-LAMP-PfAgo assay for the SARS-CoV-2 Delta variant produced significantly higher fluorescent signals (p < 0.001) when a single nucleotide polymorphism (SNP), exclusive to the Delta variant, was present, compared to the samples without the SNP. Additionally, the duplex assay for Pepper mild mottle virus (PMMOV) and SARS-CoV-2 detection produced specific fluorescent signals (FAM or ROX) only when the corresponding sequences were present. Furthermore, the RT-LAMP-PfAgo assay does not require dilution to reduce the impact of environmental inhibitors. The limit of detection of the PMMOV assay, determined with 30 wastewater samples, was 28 gc/μL, with a 95 % confidence interval of [11,103]. Finally, using a point-of-use device, the RT-LAMP-PfAgo assay successfully detected PMMOV in wastewater samples. Based on our findings, we conclude that the RT-LAMP-PfAgo assay can be used as a portable, SNP-specific duplex assay, which will significantly improve virus surveillance in wastewater.

RNase H-Driven crRNA Switch Circuits for Rapid and Sensitive Detection of Various Analytical Targets

The clustered regularly interspaced short palindromic repeats (CRISPR/Cas12a) system has exhibited great promise in the rapid and sensitive molecular diagnostics for its trans-cleavage property. However, most CRISPR/Cas system-based detection methods are designed for nucleic acids and require target preamplification to improve sensitivity and detection limits. Here, we propose a generic crRNA switch circuit-regulated CRISPR/Cas sensor for the sensitive detection of various targets. The crRNA switch is engineered and designed in a blocked state but can be activated in the presence of triggers, which are target-induced association DNA to initiate the trans-cleavage activity of Cas12a for signal reporting. Additionally, RNase H is introduced to specifically hydrolyze RNA duplexed with the DNA trigger, resulting in the regeneration of the trigger to activate more crRNA switches. Such a combination provides a generic and sensitive strategy for the effective sensing of the p53 sequence, thrombin, and adenosine triphosphate. The design is incorporated with nucleic acid nanotechnology and extensively broadens the application scope of the CRISPR technology in biosensing.

Precise Detection of Viral RNA by Programming Multiplex Rolling Circle Amplification and Strand Displacement

Detection of viral infections (e.g., SARS-CoV-2) with high precision is critical to disease control and treatment. There is an urgent need to develop point-of-care detection methods to complement the gold standard laboratory-based PCR assay with comparable sensitivity and specificity. Herein, we developed a method termed mCAD to achieve ultraspecific point-of-care detection of SARS-CoV-2 RNA while maintaining high sensitivity by programming multiplex rolling circle amplification and toehold-mediated strand displacement reactions. RCA offers sufficient amplification of RNA targets for subsequent detection. Most importantly, a multilayer of detection specificity is implemented into mCAD via sequence-specific hybridization of nucleic acids across serial steps of this protocol to fully eliminate potential false-positive detections. Using mCAD, we demonstrated a highly specific, sensitive, and convenient visual detection of SARS-CoV-2 RNA from both synthetic and clinical samples, exhibiting performance comparable to qPCR. We envision that mCAD will find its broad applications in clinical prospects for nucleic acid detections readily beyond SARS-CoV-2 RNA.

Digital Recombinase Polymerase Amplification, Digital Loop-Mediated Isothermal Amplification, and Digital CRISPR-Cas Assisted Assay: Current Status, Challenges, and Perspectives

Digital nucleic acid detection based on microfluidics technology can quantify the initial amount of nucleic acid in the sample with low equipment requirements and simple operations, which can be widely used in clinical and in vitro diagnosis. Recently, isothermal amplification technologies such as recombinase polymerase amplification (RPA), loop-mediated isothermal amplification (LAMP), and clustered regularly interspaced short palindromic repeats-CRISPR associated proteins (CRISPR-Cas) assisted technologies have become a hot spot of attention and state-of-the-art digital nucleic acid chips have provided a powerful tool for these technologies. Herein, isothermal amplification technologies including RPA, LAMP, and CRISPR-Cas assisted methods, based on digital nucleic acid microfluidics chips recently, have been reviewed. Moreover, the challenges of digital isothermal amplification and possible strategies to address them are discussed. Finally, future directions of digital isothermal amplification technology, such as microfluidic chip and device manufacturing, multiplex detection, and one-pot detection, are outlined.

Recombinase Polymerase Amplification-Based Biosensors for Rapid Zoonoses Screening

Recent, outbreaks of new emergency zoonotic diseases have prompted an urgent need to develop fast, accurate, and portable screening assays for pathogen infections. Recombinase polymerase amplification (RPA) is sensitive and specific and can be conducted at a constant low temperature with a short response time, making it especially suitable for on-site screening and making it a powerful tool for preventing or controlling the spread of zoonoses. This review summarizes the design principles of RPA-based biosensors as well as various signal output or readout technologies involved in fluorescence detection, lateral flow assays, enzymatic catalytic reactions, spectroscopic techniques, electrochemical techniques, chemiluminescence, nanopore sequencing technologies, microfluidic digital RPA, and clustered regularly interspaced short palindromic repeats/CRISPR-associated systems. The current status and prospects of the application of RPA-based biosensors in zoonoses screening are highlighted. RPA-based biosensors demonstrate the advantages of rapid response, easy-to-read result output, and easy implementation for on-site detection, enabling development toward greater portability, automation, and miniaturization. Although there are still problems such as high cost with unstable signal output, RPA-based biosensors are increasingly becoming one of the most important means of on-site pathogen screening in complex samples involving environmental, water, food, animal, and human samples for controlling the spread of zoonotic diseases.

Unmodificated stepless regulation of CRISPR/Cas12a multi-performance

As CRISPR technology is promoted to more fine-divided molecular biology applications, its inherent performance finds it increasingly difficult to cope with diverse needs in these different fields, and how to more accurately control the performance has become a key issue to develop CRISPR technology to a new stage. Herein, we propose a CRISPR/Cas12a regulation strategy based on the powerful programmability of nucleic acid nanotechnology. Unlike previous difficult and rigid regulation of core components Cas nuclease and crRNA, only a simple switch of different external RNA accessories is required to change the reaction kinetics or thermodynamics, thereby finely and almost steplessly regulating multi-performance of CRISPR/Cas12a including activity, speed, specificity, compatibility, programmability and sensitivity. In particular, the significantly improved specificity is expected to mark advance the accuracy of molecular detection and the safety of gene editing. In addition, this strategy was applied to regulate the delayed activation of Cas12a, overcoming the compatibility problem of the one-pot assay without any physical separation or external stimulation, and demonstrating great potential for fine-grained control of CRISPR. This simple but powerful CRISPR regulation strategy without any component modification has pioneering flexibility and versatility, and will unlock the potential for deeper applications of CRISPR technology in many finely divided fields.

Unidirectional trans-cleaving behavior of CRISPR-Cas12a unlocks for an ultrasensitive assay using hybrid DNA reporters containing a 3' toehold

CRISPR-Cas12a can induce nonspecific trans-cleavage of dsDNA substrate, including long and stable λ DNA. However, the mechanism behind this is still largely undetermined. In this study, we observed that while trans-activated Cas12a didn't cleave blunt-end dsDNA within a short reaction time, it could degrade dsDNA reporters with a short overhang. More interestingly, we discovered that the location of the overhang also affected the susceptibility of dsDNA substrate to trans-activated Cas12a. Cas12a trans-cleaved 3' overhang dsDNA substrates at least 3 times faster than 5' overhang substrates. We attributed this unique preference of overhang location to the directional trans-cleavage behavior of Cas12a, which may be governed by RuvC and Nuc domains. Utilizing this new finding, we designed a new hybrid DNA reporter as nonoptical substrate for the CRISPR-Cas12a detection platform, which sensitively detected ssDNA targets at sub picomolar level. This study not only unfolded new insight into the trans-cleavage behavior of Cas12a but also demonstrated a sensitive CRISPR-Cas12a assay by using a hybrid dsDNA reporter molecule.

Digital Cascade Assays for ADP- or ATP-Producing Enzymes Using a Femtoliter Reactor Array Device

Digital enzyme assays are emerging biosensing methods for highly sensitive quantitative analysis of biomolecules with single-molecule detection sensitivity. However, current digital enzyme assays require a fluorogenic substrate for detection, which limits the applicability of this method to certain enzymes. ATPases and kinases are representative enzymes for which fluorogenic substrates are not available; however, these enzymes form large domains and play a central role in biology. In this study, we implemented a fluorogenic cascade reaction in a femtoliter reactor array device to develop a digital bioassay platform for ATPases and kinases. The digital cascade assay enabled quantitative measurement of the single-molecule activity of F1-ATPase, the catalytic portion of ATP synthase. We also demonstrated a digital assay for human choline kinase α. Furthermore, we developed a digital cascade assay for ATP-synthesizing enzymes and demonstrated a digital assay for pyruvate kinase. These results show the high versatility of this assay platform. Thus, the digital cascade assay has great potential for the highly sensitive detection and accurate characterization of various ADP- and ATP-producing enzymes, such as kinases, which may serve as disease biomarkers.

Programmable Clostridium perfringens Argonaute-Based, One-Pot Assay for the Multiplex Detection of miRNAs

Assays for the molecular detection of miRNAs are typically constrained by the level of multiplexing, especially in a single tube. Here, we report a general and programmable diagnostic platform by combining mesophilic Clostridium perfringens Argonaute (CpAgo) with exponential isothermal amplification (EXPAR), which is a dual-signal amplification strategy, allowing for the rapid and sensitive detection of multiple miRNAs with single-nucleotide discrimination in one pot. The CpAgo-based One-Pot (COP) assay achieved a limit of detection of 1 zM miRNA within 30 min of turnaround time and a wide concentration range. This COP assay was applied to simultaneously detect four miRNAs in a single tube from clinical serum samples, showing superior analytical performance in distinguishing colorectal cancer patients from healthy individuals. This programmable, one-pot, multiplex, rapid, and specific strategy offers great promise in scientific research and clinical applications.

Outlook for CRISPR-based tuberculosis assays now in their infancy

Tuberculosis (TB) remains a major underdiagnosed public health threat worldwide, being responsible for more than 10 million cases and one million deaths annually. TB diagnosis has become more rapid with the development and adoption of molecular tests, but remains challenging with traditional TB diagnosis, but there has not been a critical review of this area. Here, we systematically review these approaches to assess their diagnostic potential and issues with the development and clinical evaluation of proposed CRISPR-based TB assays. Based on these observations, we propose constructive suggestions to improve sample pretreatment, method development, clinical validation, and accessibility of these assays to streamline future assay development and validation studies.

Advances in Simple, Rapid, and Contamination-Free Instantaneous Nucleic Acid Devices for Pathogen Detection

Pathogenic pathogens invade the human body through various pathways, causing damage to host cells, tissues, and their functions, ultimately leading to the development of diseases and posing a threat to human health. The rapid and accurate detection of pathogenic pathogens in humans is crucial and pressing. Nucleic acid detection offers advantages such as higher sensitivity, accuracy, and specificity compared to antibody and antigen detection methods. However, conventional nucleic acid testing is time-consuming, labor-intensive, and requires sophisticated equipment and specialized medical personnel. Therefore, this review focuses on advanced nucleic acid testing systems that aim to address the issues of testing time, portability, degree of automation, and cross-contamination. These systems include extraction-free rapid nucleic acid testing, fully automated extraction, amplification, and detection, as well as fully enclosed testing and commercial nucleic acid testing equipment. Additionally, the biochemical methods used for extraction, amplification, and detection in nucleic acid testing are briefly described. We hope that this review will inspire further research and the development of more suitable extraction-free reagents and fully automated testing devices for rapid, point-of-care diagnostics.

A triple amplification strategy using GR-5 DNAzyme as a signal medium for ultrasensitive detection of trace Pb2+ based on CRISPR/Cas12a empowered electrochemical biosensor

Lead ions (Pb²⁺) are a well-known toxic heavy metal that poses a significant threat to human health. Therefore, the development of a simple and ultrasensitive technique for detecting Pb²⁺ is essential. With their trans-cleavage properties, the newly discovered CRISPR-V effectors have become a potential high-precision biometric tool. In this regard, a CRISPR/Cas12a-based electrochemical biosensor (E-CRISPR) has been developed, which is combined with the GR-5 DNAzyme that can specifically recognize Pb²⁺. In this strategy, the GR-5 DNAzyme acts as a signal-mediated intermediary, which can convert Pb²⁺ into nucleic acid signals, thereby becoming single-stranded DNA that triggers strand displacement amplification (SDA) reaction. This is coupled with following activated CRISPR/Cas12a cleavage of the electrochemical signal probe, enabling cooperative signal amplification for ultrasensitive Pb²⁺ detection. The proposed method has a detection limit as low as 0.02 pM. Therefore, we have developed an E-CRISPR detection platform with GR-5 DNAzyme as a signal medium (called SM-E-CRISPR biosensor). This provides a method for the CRISPR system to specifically detect non-nucleic substances by converting the signal using a medium.

CRISPR Assays for Disease Diagnosis: Progress to and Barriers Remaining for Clinical Applications

Numerous groups have employed the special properties of CRISPR/Cas systems to develop platforms that have broad potential applications for sensitive and specific detection of nucleic acid (NA) targets. However, few of these approaches have progressed to commercial or clinical applications. This review summarizes the properties of known CRISPR/Cas systems and their applications, challenges associated with the development of such assays, and opportunities to improve their performance or address unmet assay needs using nano-/micro-technology platforms. These include rapid and efficient sample preparation, integrated single-tube, amplification-free, quantifiable, multiplex, and non-NA assays. Finally, this review discusses the current outlook for such assays, including remaining barriers for clinical or point-of-care applications and their commercial development.

STAMP-Based Digital CRISPR-Cas13a for Amplification-Free Quantification of HIV-1 Plasma Viral Loads

Quantification of HIV RNA in plasma is critical for identifying the disease progression and monitoring the effectiveness of antiretroviral therapy. While RT-qPCR has been the gold standard for HIV viral load quantification, digital assays could provide an alternative calibration-free absolute quantification method. Here, we reported a Self-digitization Through Automated Membrane-based Partitioning (STAMP) method to digitalize the CRISPR-Cas13 assay (dCRISPR) for amplification-free and absolute quantification of HIV-1 viral RNAs. The HIV-1 Cas13 assay was designed, validated, and optimized. We evaluated the analytical performances with synthetic RNAs. With a membrane that partitions ∼100 nL of reaction mixture (effectively containing 10 nL of input RNA sample), we showed that RNA samples spanning 4 orders of dynamic range between 1 fM (∼6 RNAs) to 10 pM (∼60k RNAs) could be quantified as fast as 30 min. We also examined the end-to-end performance from RNA extraction to STAMP-dCRISPR quantification using 140 μL of both spiked and clinical plasma samples. We demonstrated that the device has a detection limit of approximately 2000 copies/mL and can resolve a viral load change of 3571 copies/mL (equivalent to 3 RNAs in a single membrane) with 90% confidence. Finally, we evaluated the device using 140 μL of 20 patient plasma samples (10 positives and 10 negatives) and benchmarked the performance with RT-PCR. The STAMP-dCRISPR results agree very well with RT-PCR for all negative and high positive samples with Ct < 32. However, the STAMP-dCRISPR is limited in detecting low positive samples with Ct > 32 due to the subsampling errors. Our results demonstrated a digital Cas13 platform that could offer an accessible amplification-free quantification of viral RNAs. By further addressing the subsampling issue with approaches such as preconcentration, this platform could be further exploited for quantitatively determining viral load for an array of infectious diseases.

Nano-biosensor for SARS-CoV-2/COVID-19 detection: methods, mechanism and interface design

The epidemic of coronavirus disease 2019 (COVID-19) was a huge disaster to human society. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to COVID-19, has resulted in a large number of deaths. Even though the reverse transcription-polymerase chain reaction (RT-PCR) is the most efficient method for the detection of SARS-CoV-2, the disadvantages (such as long detection time, professional operators, expensive instruments, and laboratory equipment) limit its application. In this review, the different kinds of nano-biosensors based on surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), field-effect transistor (FET), fluorescence methods, and electrochemical methods are summarized, starting with a concise description of their sensing mechanism. The different bioprobes (such as ACE2, S protein-antibody, IgG antibody, IgM antibody, and SARS-CoV-2 DNA probes) with different bio-principles are introduced. The key structural components of the biosensors are briefly introduced to give readers an understanding of the principles behind the testing methods. In particular, SARS-CoV-2-related RNA mutation detection and its challenges are also briefly described. We hope that this review will encourage readers with different research backgrounds to design SARS-CoV-2 nano-biosensors with high selectivity and sensitivity.

Rapid and Portable Quantification of HIV RNA via a Smartphone-enabled Digital CRISPR Device and Deep Learning

For the 28.2 million people in the world living with HIV/AIDS and receiving antiretroviral therapy, it is crucial to monitor their HIV viral loads with ease. To this end, rapid and portable diagnostic tools that can quantify HIV RNA are critically needed. We report herein a rapid and quantitative digital CRISPR-assisted HIV RNA detection assay that has been implemented within a portable smartphone-based device as a potential solution. Specifically, we first developed a fluorescence-based reverse transcription recombinase polymerase amplification (RT-RPA)-CRISPR assay for isothermally and rapidly detecting HIV RNA at 42 °C in < 30 min. When realized within a commercial stamp-sized digital chip, this assay yields strongly fluorescent digital reaction wells corresponding to HIV RNA. The isothermal reaction condition and the strong fluorescence in the small digital chip unlock compact thermal and optical components in our device, allowing us to engineer a palm-size (70 × 115 × 80 mm) and lightweight (< 0.6 kg) device. Further leveraging the smartphone, we wrote a custom app to control the device, perform the digital assay, and acquire fluorescence images throughout the assay time. We additionally trained and verified a Deep Learning-based algorithm for analyzing fluorescence images and detecting strongly fluorescent digital reaction wells. Using our smartphone-enabled digital CRISPR device, we were able to detect 75 copies of HIV RNA in 15 min and demonstrate the potential of our device toward convenient monitoring of HIV viral loads and combating the HIV/AIDS epidemic.

Nanotechnologies for the Diagnosis and Treatment of SARS-CoV-2 and Its Variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the global coronavirus disease 2019 (COVID-19) pandemic, has caused well over 750 million infections and 6.8 million deaths. Rapid diagnosis and isolation of infected patients are the primary aims of the concerned authorities to minimize the casualties. The endeavor to mitigate the pandemic has been impeded by the emergence of newly identified genomic variants of SARS-CoV-2. Some of these variants are considered as serious threats because of their higher transmissibility and potential immune evasion, leading to reduced vaccine efficiency. Nanotechnology can play an important role in advancing both diagnosis and therapy of COVID-19. In this review, nanotechnology-based diagnostic and therapeutic strategies against SARS-CoV-2 and its variants are introduced. The biological features and functions of the virus, the mechanism of infection, and currently used approaches for diagnosis, vaccination, and therapy are discussed. Then, nanomaterial-based nucleic acid- and antigen-targeting diagnostic methods and viral activity suppression approaches that have a strong potential to advance both diagnostics and therapeutics toward control and containment of the COVID-19 pandemic are focused upon.

An Integrated Amplification-Free Digital CRISPR/Cas-Assisted Assay for Single Molecule Detection of RNA

Conventional nucleic acid detection technologies usually rely on amplification to improve sensitivity, which has drawbacks, such as amplification bias, complicated operation, high requirements for complex instruments, and aerosol pollution. To address these concerns, we developed an integrated assay for the enrichment and single molecule digital detection of nucleic acid based on a CRISPR/Cas13a and microwell array. In our design, magnetic beads capture and concentrate the target from a large volume of sample, which is 100 times larger than reported earlier. The target-induced CRISPR/Cas13a cutting reaction was then dispersed and limited to a million individual femtoliter-sized microwells, thereby enhancing the local signal intensity to achieve single-molecule detection. The limit of this assay for amplification-free detection of SARS-CoV-2 is 2 aM. The implementation of this study will establish a "sample-in-answer-out" single-RNA detection technology without amplification and improve the sensitivity and specificity while shortening the detection time. This research has broad prospects in clinical application.

Microfluidics: the propellant of CRISPR-based nucleic acid detection

Since the discovery of collateral cleavage activity, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas systems have become the new generation of nucleic acid detection tools. However, their widespread application remains limited. A pre-amplification step is required to improve the sensitivity of CRISPR systems, complicating the operating procedure and limiting quantitative precision. In addition, nonspecific collateral cleavage activity makes it difficult to realize multiplex detection in a one-pot CRISPR reaction with a single Cas protein. Microfluidics, which can transfer nucleic acid analysis process to a chip, has the advantages of miniaturization, integration, and automation. Microfluidics coupled with CRISPR systems improves the detection ability of CRISPR, enabling fast, high-throughput, integrated, multiplex, and digital detection, which results in the further popularization of CRISPR for a range of scenarios.

Adsorption-Free Self-Priming Direct Digital Dual-crRNA CRISPR/Cas12a-Assisted Chip for Ultrasensitive Detection of Pathogens

Rapid and sensitive pathogen detection methods are critical for disease diagnosis and treatment. RPA-CRISPR/Cas12 systems have displayed remarkable potential in pathogen detection. A self-priming digital PCR chip is a powerful and attractive tool for nucleic detection. However, the application of the RPA-CRISPR/Cas12 system to the self-priming chip still has great challenges due to the problems of protein adsorption and two-step detection mode of RPA-CRISPR/Cas12. In this study, an adsorption-free self-priming digital chip was developed and a direct digital dual-crRNAs (3D) assay was established based on the chip for ultrasensitive detection of pathogens. This 3D assay combined the advantages of rapid amplification of RPA, specific cleavage of Cas12a, accurate quantification of digital PCR, and point-of-care testing (POCT) of microfluidics, enabling accurate and reliable digital absolute quantification of Salmonella in POCT. Our method can provide a good linear relationship of Salmonella detection in the range from 2.58 × 10¹ to 2.58 × 10⁴ cells/mL with a limit of detection ∼0.2 cells/mL within 30 min in a digital chip by targeting the invA gene of Salmonella. Moreover, the assay could directly detect Salmonella in milk without nucleic acid extraction. Therefore, the 3D assay has the significant potential to provide accurate and rapid pathogen detection in POCT. This study provides a powerful nucleic detection platform and facilitates the application of CRISPR/Cas-assisted detection and microfluidic chips.

Recent progress in nucleic acid detection with CRISPR

Recent advances in CRISPR-based biotechnologies have greatly expanded our capabilities to repurpose CRISPR for the development of molecular diagnostic systems. The key attribute that allows CRISPR to be widely utilized is its programmable and highly specific nature. In this review, we first illustrate the principle of the class 2 CRISPR nucleases for molecular diagnostics which originates from their immunologic defence systems. Next, we present the CRISPR-based schemes in the application of diagnostics with amplification-assisted or amplification-free strategies. By highlighting some of the recent advances we interpret how general bioengineering methodologies can be integrated with CRISPR. Finally, we discuss the challenges and exciting prospects for future CRISPR-based biosensing development. We hope that this review will guide the reader to systematically learn the start-of-the-art development of CRISPR-mediated nucleic acid detection and understand how to apply the CRISPR nucleases with different design concepts to more general applications in diagnostics and beyond.

Microfluidic-based technologies for diagnosis, prevention, and treatment of COVID-19: recent advances and future directions

The COVID-19 pandemic has posed significant challenges to existing healthcare systems around the world. The urgent need for the development of diagnostic and therapeutic strategies for COVID-19 has boomed the demand for new technologies that can improve current healthcare approaches, moving towards more advanced, digitalized, personalized, and patient-oriented systems. Microfluidic-based technologies involve the miniaturization of large-scale devices and laboratory-based procedures, enabling complex chemical and biological operations that are conventionally performed at the macro-scale to be carried out on the microscale or less. The advantages microfluidic systems offer such as rapid, low-cost, accurate, and on-site solutions make these tools extremely useful and effective in the fight against COVID-19. In particular, microfluidic-assisted systems are of great interest in different COVID-19-related domains, varying from direct and indirect detection of COVID-19 infections to drug and vaccine discovery and their targeted delivery. Here, we review recent advances in the use of microfluidic platforms to diagnose, treat or prevent COVID-19. We start by summarizing recent microfluidic-based diagnostic solutions applicable to COVID-19. We then highlight the key roles microfluidics play in developing COVID-19 vaccines and testing how vaccine candidates perform, with a focus on RNA-delivery technologies and nano-carriers. Next, microfluidic-based efforts devoted to assessing the efficacy of potential COVID-19 drugs, either repurposed or new, and their targeted delivery to infected sites are summarized. We conclude by providing future perspectives and research directions that are critical to effectively prevent or respond to future pandemics.

A critical review of microfluidic systems for CRISPR assays

Reviewed are nucleic acid detection assays that incorporate clustered regularly interspaced short palindromic repeats (CRISPR)-based diagnostics and microfluidic devices and techniques. The review serves as a reference for researchers who wish to use CRISPR-Cas systems for diagnostics in microfluidic devices. The review is organized in sections reflecting a basic five-step workflow common to most CRISPR-based assays. These steps are analyte extraction, pre-amplification, target recognition, transduction, and detection. The systems described include custom microfluidic chips and custom (benchtop) chip control devices for automated assays steps. Also included are partition formats for digital assays and lateral flow biosensors as a readout modality. CRISPR-based, microfluidics-driven assays offer highly specific detection and are compatible with parallel, combinatorial implementation. They are highly reconfigurable, and assays are compatible with isothermal and even room temperature operation. A major drawback of these assays is the fact that reports of kinetic rates of these enzymes have been highly inconsistent (many demonstrably erroneous), and the low kinetic rate activity of these enzymes limits achievable sensitivity without pre-amplification. Further, the current state-of-the-art of CRISPR assays is such that nearly all systems rely on off-chip assays steps, particularly off-chip sample preparation.

Recent development of microfluidics-based platforms for respiratory virus detection

With the global outbreak of SARS-CoV-2, the inadequacies of current detection technology for respiratory viruses have been recognized. Rapid, portable, accurate, and sensitive assays are needed to expedite diagnosis and early intervention. Conventional methods for detection of respiratory viruses include cell culture-based assays, serological tests, nucleic acid detection (e.g., RT-PCR), and direct immunoassays. However, these traditional methods are often time-consuming, labor-intensive, and require laboratory facilities, which cannot meet the testing needs, especially during pandemics of respiratory diseases, such as COVID-19. Microfluidics-based techniques can overcome these demerits and provide simple, rapid, accurate, and cost-effective analysis of intact virus, viral antigen/antibody, and viral nucleic acids. This review aims to summarize the recent development of microfluidics-based techniques for detection of respiratory viruses. Recent advances in different types of microfluidic devices for respiratory virus diagnostics are highlighted, including paper-based microfluidics, continuous-flow microfluidics, and droplet-based microfluidics. Finally, the future development of microfluidic technologies for respiratory virus diagnostics is discussed.

Nanozyme-catalysed CRISPR-Cas12a system for the preamplification-free colorimetric detection of lead ion

CRISPR-based detection was often based on the target preamplification to realize the high sensitivity. Here, we prepared a CRISPR-Cas12a system for the colorimetric detection of lead ion (Pb²⁺) based on the assistance of DNAzyme and nanozyme instead of preamplification. The recognition between GR-5 DNAzyme and Pb²⁺ could trigger the CRISPR-Cas12a system. MnO₂ nanozymes connected with magnetic beads through single stranded DNA were prepared as the colorimetric signal probes and catalyst of CRISPR-Cas12a system for the strong oxidase-like activity inducing the color change of 3,3',5,5'-tetramethylbenzidine. The nanozyme-catalysed CRISPR-Cas12a system could be used to detect Pb²⁺ through the color change with high specificity and sensitivity. The linear range of this approach was 0.8 nM-2500 nM, with a limit of detection of 0.54 nM. This method was applied for the detection of the Pb²⁺ in food samples indicating good accuracy and anti-interference ability.

Elucidating the Role of CRISPR/Cas in Single-Step Isothermal Nucleic Acid Amplification Testing Assays

Developing assays that combine CRISPR/Cas and isothermal nucleic acid amplification has become a burgeoning research area due to the novelty and simplicity of CRISPR/Cas and the potential for point-of-care uses. Most current research explores various two-step assays by appending different CRISPR/Cas effectors to the end of different isothermal nucleic acid amplification methods. However, efforts in integrating both components into more ideal single-step assays are scarce, and poor-performing single-step assays have been reported. Moreover, lack of investigations into CRISPR/Cas in single-step assays results in incomplete understanding. To fill this knowledge gap, we conducted a systematic investigation by developing and comparing assays that share the identical recombinase polymerase amplification (RPA) but differ in CRISPR/Cas12a. We found that the addition of CRISPR/Cas12a indeed unlocks signal amplification but, at the same time, impedes RPA and that CRISPR/Cas12a concentration is a key parameter for attenuating RPA impediment and ensuring assay performance. Accordingly, we found that our protospacer adjacent motif (PAM)-free CRISPR/Cas12a-assisted RPA assay, which only moderately impeded RPA at its optimal CRISPR/Cas12a concentration, outperformed its counterparts in assay design, signal, sensitivity, and speed. We also discovered that a new commercial Cas12a effector could also drive our PAM-free CRISPR/Cas12a-assisted RPA assay and reduce its cost, though simultaneously lowering its signal. Our study and the new insights can be broadly applied to steer and facilitate further advances in CRISPR/Cas-based assays.

CASMART, a one-step CRISPR Cas12a-mediated isothermal amplification for rapid and high-resolution digital detection of rare mutant alleles

Convenient, ultrasensitive, and accurate detection of rare variants is essential for early cancer diagnosis and precision medicine, however, despite years of efforts, tools that have all these qualities remain elusive. Here, we developed a one-step CRISPR/Cas12a-based digital diagnostic platform for accurately quantifying mutant alleles, referred to as the CRISPR ASsoaciated Mutation Allele Rapid Test (CASMART). The platform accurately quantifies the variant allele frequency of EGFR L858R within 1 h at 42 °C and can detect mutant targets as low as 0.3 copies/μL (0.498 aM) in mock multiplex cfDNA samples. We further investigated the applicability of CASMART using human genomic samples with confirmed EGFR L858R mutations previously measured variant allele frequency by next-generation sequencing. Comparison across platforms revealed equivalent detection performance (Pearson's correlation coefficient, R² = 0.9208) and high quantification accuracy for mutation allele frequency (intraclass correlation coefficient = 0.959). Our one-step approach enables easy and accurate variant allele frequency measurement of rare mutant alleles without PCR instrumentation, while the assay time was reduced by approximately half compared to the digital PCR with the shortest turnaround. The CASMART is an alternative to conventional single nucleotide polymorphism detection methods with great potential as a next-generation biosensor for rapidly quantifying the variant allele fraction, especially in resource-limited settings.

Digital CRISPR Systems for the Next Generation of Nucleic Acid Quantification

Digital CRISPR (dCRISPR) assays are an emerging platform of molecular diagnostics. Digital platforms introduce absolute quantification and increased sensitivity to bulk CRISPR assays. With ultra-specific targeting, isothermal operation, and rapid detection, dCRISPR systems are well-prepared to lead the field of molecular diagnostics. Here we summarized the common Cas proteins used in CRISPR detection assays. The methods of digital detection and critical performance factors are examined. We formed three strategies to frame the landscape of dCRISPR systems: (1) amplification free, (2) in-partition amplification, and (3) two-stage amplification. We also compared the performance of all systems through the limit of detection (LOD), testing time, and figure of merit (FOM). This work summarizes the details of digital CRISPR platforms to guide future development. We envision that improvements to LOD and dynamic range will position dCRISPR as the leading platform for the next generation of molecular biosensing.

PddCas: A Polydisperse Droplet Digital CRISPR/Cas-Based Assay for the Rapid and Ultrasensitive Amplification-Free Detection of Viral DNA/RNA

Clustered regularly interspaced short palindromic repeats (CRISPR)-based assays have been an emerging diagnostic technology for pathogen diagnosis. In this work, we developed a polydisperse droplet digital CRISPR-Cas-based assay (PddCas) for the rapid and ultrasensitive amplification-free detection of viral DNA/RNA with minimum instruments. LbaCas12a and LbuCas13a were used for the direct detection of viral DNA and RNA, respectively. The reaction mixtures were partitioned with a common vortex mixer to generate picoliter-scale polydisperse droplets in several seconds. The limit of detection (LoD) for the target DNA and RNA is approximately 100 aM and 10 aM, respectively, which is about 3 × 10⁴-10⁵ fold more sensitive than corresponding bulk CRISPR assays. We applied the PddCas to successfully detect severe acute respiratory syndrome coronavirus (SARS-CoV-2) and human papillomavirus type 18 (HPV 18) in clinical samples. For the 23 HPV 18-suspected cervical epithelial cell samples and 32 nasopharyngeal swabs for SARS-CoV-2, 100% sensitivity and 100% specificity were demonstrated. The dual-gene virus detection with PddCas was also established and verified. Therefore, PddCas has potential for point-of-care application and is envisioned to be readily deployed for frequent testing as part of an integrated public health surveillance program.

Recent advancements in nucleic acid detection with microfluidic chip for molecular diagnostics

The coronavirus disease 2019 (COVID-19) has extensively promoted the application of nucleic acid testing technology in the field of clinical testing. The most widely used polymerase chain reaction (PCR)-based nucleic acid testing technology has problems such as complex operation, high requirements of personnel and laboratories, and contamination. The highly miniaturized microfluidic chip provides an essential tool for integrating the complex nucleic acid detection process. Various microfluidic chips have been developed for the rapid detection of nucleic acid, such as amplification-free microfluidics in combination with clustered regularly interspaced short palindromic repeats (CRISPR). In this review, we first summarized the routine process of nucleic acid testing, including sample processing and nucleic acid detection. Then the typical microfluidic chip technologies and new research advances are summarized. We also discuss the main problems of nucleic acid detection and the future developing trend of the microfluidic chip.

An ultra-sensitive and specific nanoplasmonic-enhanced isothermal amplification platform for the ultrafast point-of-care testing of SARS-CoV-2

The novel mutations attributed by the high mutagenicity of the SARS-CoV-2 makes its prevention and treatment challenging. Developing an ultra-fast, point-of-care-test (POCT) protocol is critical for responding to large-scale spread of SARS-CoV-2 in public places and in resource-poor remote areas. Here, we developed a nanoplasmonic enhanced isothermal amplification (NanoPEIA) strategy that combines a nanoplasmonic sensor with isothermal amplification. The novel strategy provides an ideal easy-to operate detection platform for obtaining accurate, ultra-fast and high-throughput (96 samples can be tested together) data. For clinical samples with viral detection at Ct value <25, the entire process (including sample preparation, virus lysis, detection, and data analysis) can be completed within six minutes. The method is also appropriate for detection of SARS-CoV-2 γ-coronavirus mutants. The NanoPEIA method was validated using clinical samples from 21 patients with SARS-CoV-2 infection and 31 healthy individuals. The detection result on the 52 clinical samples for SARS-CoV-2 showed that the NanoPEIA platform had a 100% sensitivity for N and orf1ab genes, which was higher than those obtained using RT-qPCR (88.9% and 90.0%, respectively). The specificities of 31 clinical negative samples were 92.3% and 91.7% for the N gene and the orf1ab gene, respectively. The limits of detection (LoD) of the clinical samples were 28.3 copies/mL and 23.3 copies/mL for the N gene and the orf1ab gene, respectively. The efficient NanoPEIA detection strategy facilitates real-time detection and visualization within ultrashort durations and can be applied for POCT diagnosis in resource-poor and highly populated areas.

Systematically investigating the fluorescent signal readout of CRISPR-Cas12a for highly sensitive SARS-CoV-2 detection

The CRISPR/Cas system is widely used for molecular diagnostics after the discovery of trans-cleavage activity, especially now with the COVID-19 outbreak. However, the majority of contemporary trans-cleavage activity-based CRISPR/Cas biosensors exploited standard single-strand DNA (ssDNA) reporters, which were based on the FRET principle from pioneering research. An in-depth comparison and understanding of various fluorescent readout types are essential to facilitate the outstanding analytical performance of CRISPR probes. We investigated various types of fluorescent reporters of Cas12a comprehensively. Results show that trans-cleavage of Cas12a is not limited to ssDNA and dsDNA reporters, but can be extended to molecular beacons (MB). And MB reporters can achieve superior analytical performance compared with ssDNA and ds DNA reporters at the same conditions. Accordingly, we developed a highly-sensitive SARS-CoV-2 detection with the sensitivity as low as 100 fM were successfully achieved without amplification strategy. The model target of ORF1a could robustly identify the current widespread emerging SARS-CoV-2 variants. A real coronavirus GX/P2V instead of SARS-CoV-2 were chosen for practical application validation. And a minimum of 27 copies/mL was achieved successfully. This inspiration can also be applied to other Cas proteins with trans-cleavage activity, which provides new perspectives for simple, highly-sensitive and universal molecular diagnosis in various applications.

A Sensitive and Nonoptical CRISPR Detection Mechanism by Sizing Double-Stranded λ DNA Reporter

CRISPR-based biosensors often rely on colorimetric, fluorescent, or electrochemical signaling mechanism, which involves expensive reporters and/or sophisticated equipment. Here, we demonstrated a simple, inexpensive, nonoptical, and sensitive CRISPR-Cas12a-based sensing platform to detect ssDNA targets by sizing double-stranded λ DNA as novel report molecules. In this platform, the size reduction of λ DNA was quantified by gel electrophoresis analysis. We hypothesize that the massive trans-nuclease activity of Cas12a toward λ DNA is due to the presence of single-stranded looped structures along the λ DNA sequence. In addition, we observed a strong binding affinity between Cas12a and λ DNA, which further promotes the trans-cleavage activity and helps achieve sub-picomolar detection sensitivity, ≈100 times more sensitive than the fluorescent counterpart. The concept of utilizing the physical size change of λ DNA unlocks the possibility of using a variety of dsDNA as CRISPR reporters.