Multiplexed droplet loop-mediated isothermal amplification with scorpion-shaped probes and fluorescence microscopic counting for digital quantification of virus RNAs

Pathogen detection via inductively coupled plasma mass spectrometry analysis with nanoparticles

Infections caused by viruses and bacteria pose a significant threat to global public health, emphasizing the critical importance of timely and precise detection methods. Inductively coupled plasma mass spectrometry (ICP-MS), a contemporary approach for pathogen detection, offers distinct advantages such as high sensitivity, a wide linear range, and multi-index capabilities. This review elucidates the underexplored application of ICP-MS in conjunction with functional nanoparticles (NPs) for the identification of viruses and bacteria. The review commences with an elucidation of the underlying principles, procedures, target pathogens, and NP requirements for this innovative approach. Subsequently, a thorough analysis of the advantages and limitations associated with these techniques is provided. Furthermore, the review delves into a comprehensive examination of the challenges encountered when utilizing NPs and ICP-MS for pathogen detection, culminating in a forward-looking assessment of the potential pathways for advancement in this domain. Thus, this review contributes novel perspectives to the field of pathogen detection in biomedicine by showcasing the promising synergy of ICP-MS and NPs.

Rapid Detection of Uropathogens Using an Integrated Multiplex Digital Nucleic Acid Detection Assay Powered by a Digital-to-Droplet Microfluidic Device

The digital nucleic acid detection assay features the capability of absolute quantitation without the need for calibration, thereby facilitating the rapid identification of pathogens. Although several integrated digital nucleic acid detection techniques have been developed, there are still constraints in terms of automation and analysis throughput. To tackle these challenges, this study presents a digital-to-droplet microfluidic device comprising a digital microfluidics (DMF) module at the bottom and a droplet microfluidics module at the top. Following sample introduction, the extraction of nucleic acid and the dispensation of nucleic acid elution for mixing with the multiple amplification reagents are carried out in the DMF module. Subsequently, the reaction droplets are transported to the sample inlet of the droplet microfluidic module via a liquid outlet, and then droplet generation in four parallel units within the droplet microfluidics module is actuated by negative pressure generated by a syringe vacuum. The digital-to-droplet microfluidic device was employed to execute an integrated multiplex digital droplet nucleic acid detection assay (imDDNA) incorporating loop-mediated isothermal amplification (LAMP). This assay was specifically designed to enable simultaneous detection of four uropathogens, namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterococcus faecalis. The entire process of the imDDNA is completed within 75 min, with a detection range spanning 5 orders of magnitude (9.43 × 10-2.86 × 104 copies μL-1). The imDDNA was employed for the detection of batched clinical specimens, showing a consistency of 91.1% when compared with that of the conventional method. The imDDNA exhibits simplicity in operation and accuracy in quantification, thus offering potential advantages in achieving rapid pathogen detection.

Nucleic Acid Target Sensing Using a Vibrating Sharp-Tip Capillary and Digital Droplet Loop-Mediated Isothermal Amplification (ddLAMP)

Nucleic acid tests are key tools for the detection and diagnosis of many diseases. In many cases, the amplification of the nucleic acids is required to reach a detectable level. To make nucleic acid amplification tests more accessible to a point-of-care (POC) setting, isothermal amplification can be performed with a simple heating source. Although these tests are being performed in bulk reactions, the quantification is not as accurate as it would be with digital amplification. Here, we introduce the use of the vibrating sharp-tip capillary for a simple and portable system for tunable on-demand droplet generation. Because of the large range of droplet sizes possible and the tunability of the vibrating sharp-tip capillary, a high dynamic range (~2 to 6000 copies/µL) digital droplet loop-mediated isothermal amplification (ddLAMP) system has been developed. It was also noted that by changing the type of capillary on the vibrating sharp-tip capillary, the same mechanism can be used for simple and portable DNA fragmentation. With the incorporation of these elements, the present work paves the way for achieving digital nucleic acid tests in a POC setting with limited resources.

Droplet digital molecular beacon-LAMP assay via pico-injection for ultrasensitive detection of pathogens

A pico-injection-aided digital droplet detection platform is presented that integrates loop-mediated isothermal amplification (LAMP) with molecular beacons (MBs) for the ultrasensitive and quantitative identification of pathogens, leveraging the sequence-specific detection capabilities of MBs. The microfluidic device contained three distinct functional units including droplet generation, pico-injection, and droplet counting. Utilizing a pico-injector, MBs are introduced into each droplet to specifically identify LAMP amplification products, thereby overcoming issues related to temperature incompatibility. Our methodology has been validated through the quantitative detection of Escherichia coli, achieving a detection limit as low as 9 copies/μL in a model plasmid containing the malB gene and 3 CFU/μL in a spiked milk sample. The total analysis time was less than 1.5 h. The sensitivity and robustness of this platform further demonstrated the potential for rapid pathogen detection and diagnosis, particularly when integrated with cutting-edge microfluidic technologies.

Multiplexing LAMP Assays: A Methodological Review and Diagnostic Application

The loop-mediated isothermal amplification (LAMP) technique is a great alternative to PCR-based methods, as it is fast, easy to use and works with high sensitivity and specificity without the need for expensive instruments. However, one of the limitations of LAMP is difficulty in achieving the simultaneous detection of several targets in a single tube, as the methodologies that allow this rely on fluorogenic probes containing specific target sequences, complicating their adaptation and the optimization of assays. Here, we summarize different methods for the development of multiplex LAMP assays based on sequence-specific detection, illustrated with a schematic representation of the technique, and evaluate their practical application based on the real-time detection and quantification of results, the possibility to visualize the results at a glance, the prior stabilization of reaction components, promoting the point-of-care use, the maximum number of specific targets amplified, and the validation of the technique in clinical samples. The various LAMP multiplexing methodologies differ in their operating conditions and mechanism. Each methodology has its advantages and disadvantages, and the choice among them will depend on specific application interests.

Digital microfluidics methods for nucleic acid detection: A mini review

Many serious infectious diseases have occurred throughout human history. Rapid and accurate detection as well as the isolation of infected individuals, through nucleic acid testing, are effective means of containing the spread of these viruses. However, traditional nucleic acid testing methods rely on complex machines and specialized personnel, making it difficult to achieve large-scale, high-throughput, and rapid detection. In recent years, digital microfluidics has emerged as a promising technology that integrates various fields, including electrokinetics, acoustics, optics, magnetism, and mechanics. By leveraging the advantages of these different technologies, digital microfluidic chips offer several benefits, such as high detection throughput, integration of multiple functions, low reagent consumption, and portability. This rapid and efficient testing is crucial in the timely detection and isolation of infected individuals to prevent the virus spread. Another advantage is the low reagent consumption of digital microfluidic chips. Compared to traditional methods, these chips require smaller volumes of reagents, resulting in cost savings and reduced waste. Furthermore, digital microfluidic chips are portable and can be easily integrated into point-of-care testing devices. This enables testing to be conducted in remote or resource-limited areas, where access to complex laboratory equipment may be limited. Onsite testing reduces the time and cost associated with sample transportation. In conclusion, bioassay technologies based on digital microfluidic principles have the potential to significantly improve infectious disease detection and control. By enabling rapid, high-throughput, and portable testing, these technologies enhance our ability to contain the spread of infectious diseases and effectively manage public health outbreaks.

Microfluidic systems for infectious disease diagnostics

Microorganisms, encompassing both uni- and multicellular entities, exhibit remarkable diversity as omnipresent life forms in nature. They play a pivotal role by supplying essential components for sustaining biological processes across diverse ecosystems, including higher host organisms. The complex interactions within the human gut microbiota are crucial for metabolic functions, immune responses, and biochemical signalling, particularly through the gut-brain axis. Viruses also play important roles in biological processes, for example by increasing genetic diversity through horizontal gene transfer when replicating inside living cells. On the other hand, infection of the human body by microbiological agents may lead to severe physiological disorders and diseases. Infectious diseases pose a significant burden on global healthcare systems, characterized by substantial variations in the epidemiological landscape. Fast spreading antibiotic resistance or uncontrolled outbreaks of communicable diseases are major challenges at present. Furthermore, delivering field-proven point-of-care diagnostic tools to the most severely affected populations in low-resource settings is particularly important and challenging. New paradigms and technological approaches enabling rapid and informed disease management need to be implemented. In this respect, infectious disease diagnostics taking advantage of microfluidic systems combined with integrated biosensor-based pathogen detection offers a host of innovative and promising solutions. In this review, we aim to outline recent activities and progress in the development of microfluidic diagnostic tools. Our literature research mainly covers the last 5 years. We will follow a classification scheme based on the human body systems primarily involved at the clinical level or on specific pathogen transmission modes. Important diseases, such as tuberculosis and malaria, will be addressed more extensively.

The development of droplet-based microfluidic virus detection technology for human infectious diseases

Virus-based human infectious diseases have a significant negative impact on people's health and social development. The need for quick, accurate, and early viral infection detection in preventive medicine is expanding. A microfluidic control is particularly suitable for point-of-care-testing virus diagnosis due to its advantages of low sample consumption, quick detection speed, simple operation, multi-functional integration, small size, and easy portability. It is also thought to have significant development potential and a wide range of application prospects in the research on virus detection technology. In an effort to aid researchers in creating novel microfluidic tools for virus detection, this review highlights recent developments of droplet-based microfluidics in virus detection research and also discusses the challenges and opportunities for rapid virus detection.

StratoLAMP: Label-free, multiplex digital loop-mediated isothermal amplification based on visual stratification of precipitate

Multiplex, digital nucleic acid detections have important biomedical applications, but the multiplexity of existing methods is predominantly achieved using fluorescent dyes or probes, making the detection complicated and costly. Here, we present the StratoLAMP for label-free, multiplex digital loop-mediated isothermal amplification based on visual stratification of the precipitate byproduct. The StratoLAMP designates two sets of primers with different concentrations to achieve different precipitate yields when amplifying different nucleic acid targets. In the detection, deep learning image analysis is used to stratify the precipitate within each droplet and determine the encapsulated targets for nucleic acid quantification. We investigated the effect of the amplification reagents and process on the precipitate generation and optimized the assay conditions. We then implemented a deep-learning image analysis pipeline for droplet detection, achieving an overall accuracy of 94.3%. In the application, the StratoLAMP successfully achieved the simultaneous quantification of two nucleic acid targets with high accuracy. By eliminating the need for fluorescence, StratoLAMP represents a unique concept toward label-free, multiplex nucleic acid assays and an analytical tool with great cost-effectiveness.

Advancing Multiple Detection in RT-LAMP with a Specific Probe Assembled from Plural Three-Way-Junction Structures

The timely detection of diseases and the accurate identification of pathogens require the development of efficient and reliable diagnostic methods. In this study, we have developed a novel specific multivariate probe termed MRTFP (multivariate real-time fluorescent probe) by assembling strand exchange three-way-junction (3WJ) structures. The 3WJ structures were incorporated into a four-angle probe (FP) and a hexagonal probe (HP), to target the multivariate genes of Salmonella. The FP and HP enable single-step and multiplexed detection in RT-LAMP (real-time loop-mediated isothermal amplification) with exceptional sensitivity and specificity. Encouragingly, real food samples contaminated with Salmonella (Salmonella enteritidis and Salmonella typhimurium) can be readily identified and distinguished with a minimum detectable concentration (MDC) of 103 CFU/mL without the need for further culture. The introduction of MRTFP allows for simultaneous detection of dual or three targets in a single tube for LAMP, thereby improving detection efficiency. The MRTFP simplifies the design of robust multivariate probes, exhibits excellent stability, and avoids interference from multiple probe units, offering significant potential for the development of specific probes for efficient and accurate disease detection and pathogen identification.

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.

A Smartphone-Enabled Continuous Flow Digital Droplet LAMP Platform for High Throughput and Inexpensive Quantitative Detection of Nucleic Acid Targets

Molecular tests for infectious diseases and genetic anomalies, which account for significant global morbidity and mortality, are central to nucleic acid analysis. In this study, we present a digital droplet LAMP (ddLAMP) platform that offers a cost-effective and portable solution for such assays. Our approach integrates disposable 3D-printed droplet generator chips with a consumer smartphone equipped with a custom image analysis application for conducting ddLAMP assays, thereby eliminating the necessity for expensive and complicated photolithographic techniques, optical microscopes, or flow cytometers. Our 3D printing technique for microfluidic chips facilitates rapid chip fabrication in under 2 h, without the complications of photolithography or chip bonding. The platform's heating mechanism incorporates low-powered miniature heating blocks with dual resistive cartridges, ensuring rapid and accurate temperature modulation in a compact form. Instrumentation is further simplified by integrating miniaturized magnification and fluorescence optics with a smartphone camera. The fluorescence quantification benefits from our previously established RGB to CIE-xyY transformation, enhancing signal dynamic range. Performance assessment of our ddLAMP system revealed a limit of detection at 10 copies/μL, spanning a dynamic range up to 104 copies/μL. Notably, experimentally determined values of the fraction of positive droplets for varying DNA concentrations aligned with the anticipated exponential trend per Poisson statistics. Our holistic ddLAMP platform, inclusive of chip production, heating, and smartphone-based droplet evaluation, provides a refined method compatible with standard laboratory environments, alleviating the challenges of traditional photolithographic methods and intricate droplet microfluidics expertise.

Design of a Digital LAMP Detection Platform Based on Droplet Microfluidic Technology

Loop-mediated isothermal amplification (LAMP) is a rapid and high-yield amplification technology for specific DNA or RNA molecules. In this study, we designed a digital loop-mediated isothermal amplification (digital-LAMP)-functioning microfluidic chip to achieve higher sensitivity for detection of nucleic acids. The chip could generate droplets and collect them, based on which we could perform Digital-LAMP. The reaction only took 40 min at a constant temperature of 63 °C. The chip enabled highly accurate quantitative detection, with the limit of detection (LOD) down to 10² copies μL^-1. For better performance while reducing the investment of money and time in chip structure iterations, we used COMSOL Multiphysics to simulate different droplet generation ways by including flow-focusing structure and T-junction structure. Moreover, the linear structure, serpentine structure, and spiral structure in the microfluidic chip were compared to study the fluid velocity and pressure distribution. The simulations provided a basis for chip structure design while facilitating chip structure optimization. The digital-LAMP-functioning chip proposed in the work provides a universal platform for analysis of viruses.

Multiplex Detection of Infectious Diseases on Microfluidic Platforms

Infectious diseases contribute significantly to the global disease burden. Sensitive and accurate screening methods are some of the most effective means of identifying sources of infection and controlling infectivity. Conventional detecting strategies such as quantitative polymerase chain reaction (qPCR), DNA sequencing, and mass spectrometry typically require bulky equipment and well-trained personnel. Therefore, mass screening of a large population using conventional strategies during pandemic periods often requires additional manpower, resources, and time, which cannot be guaranteed in resource-limited settings. Recently, emerging microfluidic technologies have shown the potential to replace conventional methods in performing point-of-care detection because they are automated, miniaturized, and integrated. By exploiting the spatial separation of detection sites, microfluidic platforms can enable the multiplex detection of infectious diseases to reduce the possibility of misdiagnosis and incomplete diagnosis of infectious diseases with similar symptoms. This review presents the recent advances in microfluidic platforms used for multiplex detection of infectious diseases, including microfluidic immunosensors and microfluidic nucleic acid sensors. As representative microfluidic platforms, lateral flow immunoassay (LFIA) platforms, polymer-based chips, paper-based devices, and droplet-based devices will be discussed in detail. In addition, the current challenges, commercialization, and prospects are proposed to promote the application of microfluidic platforms in infectious disease detection.

Isothermal nucleic acid amplification technology in HIV detection

Nucleic acid testing for HIV plays an important role in the early diagnosis and monitoring of antiretroviral therapy outcomes in HIV patients and HIV-infected infants. Currently, the main molecular diagnostic methods employed are complex, time-consuming, and expensive to operate in resource-limited areas. Isothermal nucleic acid amplification technology overcomes some of the shortcomings of traditional assays and makes it possible to use point-of-care tests for molecular HIV detection. Here, we summarize and discuss the latest technological advances in isothermal nucleic acid amplification for HIV detection, with the intent of providing guidance for the development of subsequent HIV assays with high sensitivity and specificity.

Challenges and perspectives of multi-virus biosensing techniques: A review

In the context of globalization, individuals have an increased chance of being infected by multiple viruses simultaneously, thereby highlighting the importance of developing multiplexed devices. In addition to sufficient sensitivity and rapid response, multi-virus sensing techniques are expected to offer additional advantages including high throughput, one-time sampling for parallel analysis, and full automation with data visualization. In this paper, we review the optical, electrochemical, and mechanical platforms that enable multi-virus biosensing. The working mechanisms of each platform, including the detection principle, transducer configuration, bio-interface design, and detected signals, are reviewed. The advantages and limitations, as well as the challenges in implementing various detection strategies in real-life scenarios, were evaluated. Future perspectives on multiplexed biosensing techniques are critically discussed. Earlier access to multi-virus biosensors will efficiently serve for immediate pandemic control, such as in emerging SARS-CoV-2 and monkeypox cases.

Droplet Encoding-Pairing Enabled Multiplexed Digital Loop-Mediated Isothermal Amplification for Simultaneous Quantitative Detection of Multiple Pathogens

Despite the advantages of digital nucleic acid analysis (DNAA) in terms of sensitivity, precision, and resolution, current DNAA methods commonly suffer a limitation in multiplexing capacity. To address this issue, a droplet encoding-pairing enabled DNAA multiplexing strategy is developed, wherein unique tricolor combinations are deployed to index individual primer droplets. The template droplets and primer droplets are sequentially introduced into a microfluidic chip with a calabash-shaped microwell array and are pairwise trapped and merged in the microwells. Pre-merging and post-amplification image analysis with a machine learning algorithm is used to identify, enumerate, and address the droplets. By incorporating the amplification signals with droplet encoding information, simultaneous quantitative detection of multiple targets is achieved. This strategy allows for the establishment of flexible multiplexed DNAA by simply adjusting the primer droplet library. Its flexibility is demonstrated by establishing two multiplexed (8-plex) droplet digital loop-mediated isothermal amplification (mddLAMP) assays for individually detecting lower respiratory tract infection and urinary tract infection causative pathogens. Clinical sample analysis shows that the microbial detection outcomes of the mddLAMP assays are consistent with those of the conventional assay. This DNAA multiplexing strategy can achieve flexible high-order multiplexing on demand, making it a desirable tool for high-content pathogen detection.

Web hybrid chain reaction enhanced fluorescent magnetic bead array for digital nucleic acid detection

The detection of biomarkers at low concentrations is important in clinical diagnostic analyses and has attracted continuous research. In this work, absolute quantification of hepatitis B virus (HBV) DNA was achieved using magnetic beads with isothermal, enzyme-free DNA nanostructure for fluorescence amplification. Firstly, the DNA-functionalized bead captured the target nucleic acid in the form of sandwich hybridization, and the individual target lighted up the entire bead by isothermal web hybridization chain reaction (wHCR). After the microarray scanning, the target nucleic acids can be digitally quantified based on the Poisson statistics. Therefore, the fluorescent bead assay enabled precise detection of HBV DNA down to 5 fM level without external calibration curves. Moreover, this method not only specifically distinguished single-base mismatched sequences, but also obtained the quantitative detection of HBV DNA in serum samples. Unlike routine digital detection usually combined with complex compartment partitioning operations, the amplification structure immobilized on beads can be conducted in microcentrifuge tubes with a volume of microliter scale. This work expands the application of magnetic beads in the digital quantitative detection via enzyme-free and isothermal method.

Cell-Free Production Systems in Droplet Microfluidics

The use of cell-free production systems in droplet microfluidic devices has gained significant interest during the last decade. Encapsulating DNA replication, RNA transcription, and protein expression systems in water-in-oil drops allows for the interrogation of unique molecules and high-throughput screening of libraries of industrial and biomedical interest. Furthermore, the use of such systems in closed compartments enables the evaluation of various properties of novel synthetic or minimal cells. In this chapter, we review the latest advances in the usage of the cell-free macromolecule production toolbox in droplets, with a special emphasis on new on-chip technologies for the amplification, transcription, expression, screening, and directed evolution of biomolecules.

Recent Advances in Digital Biosensing Technology

Digital biosensing assays demonstrate remarkable advantages over conventional biosensing systems because of their ability to achieve single-molecule detection and absolute quantification. Unlike traditional low-abundance biomarking screening, digital-based biosensing systems reduce sample volumes significantly to the fL-nL level, which vastly reduces overall reagent consumption, improves reaction time and throughput, and enables high sensitivity and single target detection. This review presents the current technology for compartmentalizing reactions and their applications in detecting proteins and nucleic acids. We also analyze existing challenges and future opportunities associated with digital biosensing and research opportunities for developing integrated digital biosensing systems.

Digital Droplet Loop-Mediated Isothermal Amplification Featuring a Molecular Beacon Assay, 3D Printed Droplet Generation, and Smartphone Imaging for Sequence-Specific DNA Detection

Nucleic acid detection is widely used in the amplification and quantitation of nucleic acids from biological samples. While polymerase chain reaction (PCR) enjoys great popularity, expensive thermal cyclers are required for precise temperature control. Loop-mediated isothermal amplification (LAMP) enables highly sensitive, rapid, and low-cost amplification of nucleic acids at constant temperatures. LAMP detection often relies on double-stranded DNA-binding dyes or metal indicators that lack sequence selectivity. Molecular beacons (MBs) are hairpin-shaped oligonucleotide probes whose sequence specificity in LAMP provides the capability of differentiating between single-nucleotide polymorphisms (SNPs). Digital droplet LAMP (ddLAMP) enables a large number of independent LAMP reactions to be performed and provides quantification of target DNA sequences. However, a major challenge with ddLAMP is the requirement of expensive droplet generators to form homogeneous microdroplets. In this study, we demonstrate for the first time that a three-dimensional (3D) printed droplet generation platform can be coupled to a LAMP assay featuring MBs as sequence-specific probes. The low-cost 3D printed droplet generator system was designed, and its customizability was demonstrated in the formation of monodisperse ddLAMP assay-in-oil microdroplets. Additionally, a smartphone-based imaging system is demonstrated to increase accessibility for point-of-care applications. The MB-ddLAMP assay is shown to discriminate between two SNPs at various amplification temperatures to afford a useful platform for sequence-specific, sensitive, and accurate DNA quantification. This work expands the utility of MBs to ddLAMP for quantitative studies in the detection of SNPs and exploits the customizability of 3D printing technologies to optimize the homogeneity, size, and volume of oil-in-water microdroplets.

Droplet microfluidic-based loop-mediated isothermal amplification (dLAMP) for simultaneous quantification of multiple targets

The quantification of trace nucleic acids in biological samples is a frequent requirement in experimental and clinical diagnostics. Here, we present a protocol for the digital quantification of multiple nucleic acid targets with droplet microfluidics-based loop-mediated isothermal amplification (dLAMP). Our protocol provides a fundamental platform for the absolute quantification of multiple nucleic acid targets with high specificity, allowing readily adaption in various in vitro diagnostic settings.

For complete details on the use and execution of this protocol, please refer to Tan et al. (2021a, 2021b).

•

Protocol for droplet microfluidic-based loop-mediated isothermal amplification (dLAMP)

•

Fluorescence-activating scorpion-shaped probes-based dLAMP for fluorescence generation

•

Fast and accurate fluorescence microscopy-based droplets counting

•

Can be applied for the absolute quantification of multiple nucleic acid targets

Agarose-Droplet-Based Digital LAMP Assay for Counting Virus DNA in Single-Particle ICP-MS

Inductively coupled plasma mass spectrometry (ICP-MS) has emerged as a promising analytical platform for the quantification of biomolecules using elemental tags; however, absolute quantification at extremely low concentrations by ICP-MS without a calibration curve remains challenging. Here, we developed a digital loop-mediated isothermal amplification (LAMP) assay for counting hepatitis B virus (HBV) DNA using single-particle (sp) ICP-MS. The sample and LAMP reagents were mixed and encapsulated in agarose droplets, which were generated by homemade centrifugal droplet generators. The agarose droplets were incubated at 65 °C for amplifying the virus DNA with LAMP primers and then cooled to 4 °C for generating "gel" particles during the temperature-dependent "sol-gel" transition. The LAMP amplicons were intercalated into the agarose particles using polyacrylamide-modified LAMP primers, enabling the labeling of dsDNA with [Ru(bpy)₂dppz]²⁺ and the removal of excess reagents. Only those agarose particles, containing virus DNA, could be labeled with ¹⁰¹Ru and detected in spICP-MS. We also embedded the ¹⁵³Eu-containing polystyrene microspheres into agarose droplets as the internal standard for counting the total number of agarose droplets. The copy number of virus DNA could be counted from the ¹⁰¹Ru/¹⁵³Eu pulse numbers in spICP-MS. We achieved the lowest quantification of 25 copy μL^-1 virus DNA in one analysis without the need for a calibration curve. The developed assay can be easily tuned for counting multiple types of nucleic acid targets and extended for new possibilities of the spICP-MS-based digital assay.