Highly Sensitive Serum Protein Analysis Using Magnetic Bead-Based Proximity Extension Assay.

Many protein biomarkers are present in biofluids at a very low level but may play critical roles in important biological processes. The fact that these low-abundance proteins remain largely unexplored underscores the importance of developing new tools for highly sensitive protein detection. Although digital enzyme-linked immunosorbent assay (ELISA) has demonstrated ultrahigh sensitivity compared with conventional ELISA, the requirement of specialized instruments limits the accessibility and prevents the widespread implementation. On the other hand, proximity ligation assays (PLA) and proximity extension assays (PEA) show sensitive and specific protein detection using regular laboratory setups, but their sensitivity needs to be further improved to match digital ELISA. To achieve highly sensitive protein detection with minimal accessibility limitation, we develop a magnetic bead-based PEA (magPEA), which posts triple epitope recognition requirement and enables extensive washing for improved sensitivity and enhanced specificity. We demonstrate that the incorporation of magnetic beads into PEA workflow facilitates orders of magnitude sensitivity improvement compared with conventional ELISA, homogeneous PEA, and solid-phase PLA and achieves limits of detection close to that of digital ELISA when using IL-6, IL-8, and GM-CSF as validation. Our magPEA provides a simple approach for highly sensitive protein detection that can be readily implemented to other laboratories and will thus ultimately accelerate the study of the low abundance protein biomarkers in the future.

Magnetofluidic immuno-PCR for point-of-care COVID-19 serological testing.

Serological tests play an important role in the fight against Coronavirus Disease 2019 (COVID-19), including monitoring the dynamic immune response after vaccination, identifying past infection and determining community infection rate. Conventional methods for serological testing, such as enzyme-linked immunosorbent assays and chemiluminescence immunoassays, provide reliable and sensitive antibody detection but require sophisticated laboratory infrastructure and/or lengthy assay time. Conversely, lateral flow immunoassays are suitable for rapid point-of-care tests but have limited sensitivity. Here, we describe the development of a rapid and sensitive magnetofluidic immuno-PCR platform that can address the current gap in point-of-care serological testing for COVID-19. Our magnetofluidic immuno-PCR platform automates a magnetic bead-based, single-binding, and one-wash immuno-PCR assay in a palm-sized magnetofluidic device and delivers results in ∼30 min. In the device, a programmable magnetic arm attracts and transports magnetically-captured antibodies through assay reagents pre-loaded in a companion plastic cartridge, and a miniaturized thermocycler and a fluorescence detector perform immuno-PCR to detect the antibodies. We evaluated our magnetofluidic immuno-PCR with 108 clinical serum/plasma samples and achieved 93.8% (45/48) sensitivity and 98.3% (59/60) specificity, demonstrating its potential as a rapid and sensitive point-of-care serological test for COVID-19.

A vacuum-assisted, highly parallelized microfluidic array for performing multi-step digital assays.

There remains an unmet need for a simple microfluidic platform that can perform multi-step and multi-reagent biochemical assays in parallel for high-throughput detection and analysis of single molecules and single cells. In response, we report herein a PDMS-based vacuum-driven microfluidic array that is capable of multi-step sample loading and digitalization. The array features multi-level bifurcation microchannels connecting to 4096 dead-end microchambers for partitioning liquid reagents/samples. To realize multi-step repetitive liquid sample loading, we attach an external vacuum onto the chip to create internal negative pressure for a continuous liquid driving force. We demonstrated a high uniformity of our device for three sequential liquid loadings. To further improve its utility, we developed a thermosetting-oil covering method to prevent evaporation for assays that require high temperatures. We successfully performed digital PCR assays on our device, demonstrating the efficient multi-step reagent handling and the effective anti-evaporation design for thermal cycling. Furthermore, we performed a digital PCR detection for single-cell methicillin-resistant Staphylococcus aureus using a three-step loading approach and achieved accurate single-cell quantification. Taken together, we have demonstrated that our vacuum-driven microfluidic array is capable of multi-step sample digitalization at high throughput for single-molecule and single-cell analyses.

A fast nucleic acid extraction system for point-of-care and integration of digital PCR.

Digital PCR is a powerful amplification method for absolute quantification of nucleic acids. The systems that integrated the nucleic acid extraction and amplification can reduce detection time, improve accuracy, and reduce labor costs. However, current nucleic acid extraction systems cannot be integrated well with integrated fluidic circuit (IFC) dPCR or droplet digital PCR chips perfectly and limit the application of digital PCR. In this study, a polytetrafluoroethylene (PTFE)-based nucleic acid extraction (PNE) system, which was able to achieve fully closed extraction for micro samples and was able to be integrated with IFC dPCR or droplet digital dPCR (ddPCR) chips perfectly is proposed. For this system, PTFE tubing with an inner diameter of 1 mm was used to load the reagents and superparamagnetic particles (PMPs) were used to extract nucleic acids. The system can extract nucleic acids from cells and blood in 5 minutes. Meanwhile, when nucleic acid extraction was completed, PNE was able to be directly combined with IFC dPCR or ddPCR chips without any intermediate steps. Therefore, the PNE system can realize sample-in-digital-answer-out. It will be highly useful in point-of-care (POC) and promote the development and application of dPCR.

Proximity ligation assays for precise quantification of femtomolar proteins in single cells using self-priming microfluidic dPCR chip.

The quantification of low concentration proteins can facilitate the discovery of some significant biomarkers, and provide us a more profound understanding of cell heterogeneity when applied to single cell analysis. However, most state-of- art single cell protein detection platforms are bulky, expensive and complicated. Here we report a simple and low cost microfluidic dPCR (digital polymerase chain reaction) chip-based proximity ligation assay (PLA) for the quantification of low concentration proteins. First, standard hCSTB (human cystatin B) protein was used to optimize the related experimental conditions. Comparing to ordinary PLA tests, the results showed that our method achieved femtomolar limit of detection (LOD) with a linear dynamic range over three to four orders of magnitude. Then human CD147 protein, a reported biomarker for hepatoma carcinoma, was detected in single HepG2 and L02 cells. The results showed that there were wide disparities in single cell CD147 abundance for both of the two cell lines. And the average CD147 protein content in single HepG2 cells displayed 2-fold increase in comparison to that in single L02 cells. Comparing to the research findings obtained at bulk level, our method can provide more useful information for diagnosis and targeted therapy of tumors.

Power-free polydimethylsiloxane femtoliter-sized arrays for bead-based digital immunoassays.

A novel microfluidic chip employing power-free polydimethylsiloxane (PDMS) femtoliter-sized arrays was developed for the detection of low concentrations of protein biomakers by isolating individual paramagnetic beads in single wells. Arrays of femtoliter-sized wells were fabricated with PDMS using well-developed molding techniques. Paramagnetic beads were functionalized with specific antibodies to capture the antigens. These antigens were labeled with enzymes via conventional multistep immunosandwich approach. After suspending in aqueous solutions of enzyme substrate, the solutions were delivered to the arrays using a conventional micropipette. The aqueous solutions were introduced into the microwells by capillarity and the beads were loaded into microwells by gravity. A fluorocarbon oil was then flowed into the chip to remove excess beads from the surface of the array and meanwhile isolated the femtoliter-sized wells. All processes were achieved by conventional micropipette, without external pumping systems and valves. Finally, the arrays were imaged using standard fluorescence imaging after incubation 30 min for digital counting enzyme molecules. It was demonstrated that the chip platform possessed the performance of digital counting with a linear dynamic range from 1 aM to 1 fM for the detection of biotinylated ß-galactosidase (BßG), achieving a limit of detection (LOD) of 930 zM. Using this chip, a digital immunoassay to detect Tumor Necrosis Factor α (TNF- α) was developed. Since the chip fabrication is low-cost and circumvents the surface modification, we expect it can become a new chip-based digital immunoassay platform for ultrasensitive diagnostic of biomarkers.

A new method using machine learning for automated image analysis applied to chip-based digital assays.

Chip-based digital assays such as the digital polymerase chain reaction (digital PCR), digital loop-mediated amplification (digital LAMP), digital enzyme-linked immunosorbent assay (digital ELISA) and digital proximity ligation assay (digital PLA) need high-throughput quantification of the captured fluorescence image data. However, traditional methods that are mainly based on image segmentation using either a fixed threshold or an automated hard threshold failed to extract valid signals over a broad range of image characteristics. In this study, we introduce a new method for automated image analysis to extract signals applied to chip-based digital assays. This approach precisely locates each micro-compartment based on the structure design of the chip, thereby eliminating the interference of non-signal noise in the image. Utilizing the principle that the human eyes can distinguish between the positive micro-compartments and the negative micro-compartments, we take the parameters of each micro-compartment together with its surrounding micro-compartments as the training dataset of the Random Forest classifier to classify the micro-compartments and extract valid signals, thus solving the problem caused by the differences among images. Furthermore, we adopted the iteration methodology that adds the output of a model's prediction to the input of the next model's training dataset, until the output of a model's prediction reaches the accuracy we expected, which improves the work efficiency during data training greatly. We demonstrate the method on the dPCR dataset and it performs well without any manual adjustment of settings. The results show that our proposed method can recognize the positive signals from the fluorescence images with an accuracy of 97.78%. With minor modification, bio-instrument companies or researchers can integrate this method into their digital assay devices' software conveniently.

A highly integrated real-time digital PCR device for accurate DNA quantitative analysis.

Misclassification of positive partitions in microfluidic digital polymerase chain reaction (dPCR) can cause the false positives and false negatives, which significantly alter the resulting estimate of target DNA molecules. To address this issue, establishing real-time fluorescence interrogation of each partition in microfluidic arrays is an effective way in which false positive and false negative partitions can be eliminated. However, currently available devices for real-time fluorescence interrogation are either not competent for microfluidic digital array, or they are bulky, expensive and entail peripheral equipment due to low integration. Therefore, in this study, a Raspberry Pi based, low-cost and highly integrated device is presented to achieve real-time fluorescence detection for microfluidic digital array, termed real-time dPCR device. In the device, uniform thermocycler, streamlined real-time fluorescence imaging setup, and compact data processing system are all integrated to undergo on-chip dPCR amplification, real-time fluorescence detection, and data analysis. Using this real-time dPCR device, the accuracy of DNA absolute quantification by dPCR is improved, since the misclassification of positive partitions is efficiently reduced based on the characteristic real-time fluorescence curves of positive partitions in a self-priming microfluidic chip. Compared with end-point dPCR on our device and commercialized QuantStudio™ 3D dPCR system, the real-time dPCR on our device exhibits a higher accuracy for DNA quantification. In addition, this real-time dPCR device is much smaller and cheaper than the commercialized Digital PCR system, but not sacrificing the capability of error correction for absolute quantitation analysis. Conclusively, this highly integrated real-time dPCR device is very beneficial for DNA quantitative analysis where the determination accuracy is pivotal.

Development of a self-priming PDMS/paper hybrid microfluidic chip using mixed-dye-loaded loop-mediated isothermal amplification assay for multiplex foodborne pathogens detection.

Foodborne pathogen is the primary cause of foodborne disease outbreak. Given its great damage, a sensitive, simple and rapid detection method is demanded. Herein, we described a self-priming polydimethylsiloxane (PDMS)/paper hybrid microfluidic chip, termed SPH chip, with mixed-dye-loaded loop-mediated isothermal amplification (LAMP) for multiplex foodborne pathogens detection. Staphylococcus aureus (SA) and Vibrio parahaemolyticus (VP) were chosen to verify the novel method. Compared to other similar detection devices, the SPH chip required easier fabrication process, less operation steps and lower cost. Additionally, the reaction result, especially for the weak-positive reaction, could be judged more accurately and conveniently due to the use of mixed-dye. Without pre-enrichment of bacteria in the food contaminated sample, the limit of detection (LOD) reached down to 1000 CFU mL-1 with high specific. Additionally, for fully exploiting the potential of SPH chip, a conceptual eight-channel detection chip was also developed. Overall, the reliable and excellent result demonstrated that the novel method had great potential to be applied in the wider range of pathogens detection or disease diagnose, especially in some resource-limited area.

High throughput single cell separation and identification using a self-priming isometric and Equant screw valve-based (SIES) microfluidic chip.

The emergence of various single cell separation and identification platforms has greatly promoted the development of single cell research. Among these platforms, microfluidic chip-based strategies occupy a significant position in single cell separation and identification. Here, we proposed a self-priming isometric and Equant screw valve-based microfluidic chip (SIES chip) for high throughput single cell isolation and identification. With several special designs, such as a peripheral water tank to balance negative pressure distribution in a marginal area of the chip, a screw valve to preserve the suction power during the step-by-step sample loading, and multistage branching "T" shape channels to separate cells evenly into the chambers, up to 2000 single cells can be well dispersed and analyzed at the same time using this chip. We applied this chip for the isolation and identification of single A549 cells targeting the activated leukocyte cell adhesion molecule (ALCAM) gene. The results showed that only a small proportion (approximately 5.1%) of A549 cells expressed ALCAM, which can potentially provide a reference for A549 cell reclassification. Besides being inexpensive, user-friendly and portable, our chip can be used in some resource-limited settings and may have a great potential in POC (Point-of-Care) applications.

Smartphone-based mobile digital PCR device for DNA quantitative analysis with high accuracy.

Digital polymerase chain reaction (dPCR) circumventing the external calibration and potentially providing absolute quantification of nucleic acids has become an increasingly popular manifestation of PCR in biological researches. However, currently reported or commercial dPCR devices are not suitable for applications in laboratories or zones with limited infrastructures, due to low function integration, cost-inefficiency, or weak mobility. Herein, in order to enable accurate DNA quantitative analysis in such situations, we have developed a smartphone-based mobile dPCR device integrated with thermal cycling control, on-chip dPCR, data acquisition, and result analysis. All the function units are automatically controlled using a customized Android software. The device is approximately 90 mm × 90 mm × 100 mm in size and about 500 g in weight, only costing about 320 dollars except the smartphone. Coupled with the self-priming dPCR chip previously developed by our lab, the device is able to accurately quantify down to 10 copies of the human 18 S ribosomal DNA fragment inserted in a plasmid. Comparing to the commercial QuantStudio™ 3D dPCR platform, our device achieves a comparable analytical accuracy. Besides, our device is capable of detecting single molecule of cancer biomarker gene CD147 in a low number of HepG2 cells. Therefore, our dPCR device as a low-cost, potable, and robust tool for highly accurate DNA quantitative analysis has a great potential in Point-of-care (POC) applications.

Co-detection of ALDH1A1, ABCG2, ALCAM and CD133 in three A549 subpopulations at the single cell level by one-step digital RT-PCR.

Cancer stem-like cells (CSCs) displaying the properties of normal stem cells have become the main culprit associated with cancer transportation and recurrence. As of now, various CSC functions and marker genes have been identified due to the heterogeneity of cancer, such as aldehyde dehydrogenase (ALDH), the second member of the ABC transporter G-subfamily (ABCG2), activated leukocyte cell adhesion molecule (ALCAM) and CD133. To investigate these markers, most conventional approaches are bulk-based strategies, which may veil the disparity of single cells' gene expression. In this study, one-step digital RT-PCR at the single cell level was developed to co-determine the expression of ALDH1A1, ABCG2, ALCAM and CD133 genes in A549 cancer stem cells that perform high ALDH activities (ALDH+ A549 cells), as well as in ALDH- A549 cells and A549 cells, with 36, 20 and 20 cell samples each. The results demonstrated that, when compared to single ALDH- or A549 cells, the majority of single ALDH+ A549 cells displayed a 1.5- and 2.0-fold increase in the gene expression of ALDH1A1 and ALCAM (P < 0.001), respectively. However, for ABCG2 and CD133, there was no significant difference (P > 0.05), which means that they are not appropriate as co-indicated markers to identify ALDH+ A549 cells. Conclusively, as a single cell level approach, one-step digital RT-PCR has potential in exploring efficient co-detection markers for the classification and identification of CSCs.

Rapid and Quantitative Detection of Vibrio parahemolyticus by the Mixed-Dye-Based Loop-Mediated Isothermal Amplification Assay on a Self-Priming Compartmentalization Microfluidic Chip.

Vibrio parahemolyticus (VP) mostly isolated from aquatic products is one of the major causes of bacterial food-poisoning events worldwide, which could be reduced using a promising on-site detection method. Herein, a rapid and quantitative method for VP detection was developed by applying a mixed-dye-loaded loop-mediated isothermal amplification (LAMP) assay on a self-priming compartmentalization (SPC) microfluidic chip, termed on-chip mixed-dye-based LAMP (CMD-LAMP). In comparison to conventional approaches, CMD-LAMP was advantageous on the limit of detection, which reached down to 1 × 103 CFU/mL in food-contaminated samples without the pre-enrichment of bacteria. Additionally, as a result of the use of a mixed dye and SPC chip, the quantitative result could be easily acquired, avoiding the requirement of sophisticated instruments and tedious operation. Also, CMD-LAMP was rapid and cost-effective. Conclusively, CMD-LAMP has great potential in realizing the on-site quantitative analysis of VP for food safety.

Unusual isothermal multimerization and amplification by the strand-displacing DNA polymerases with reverse transcription activities.

ABSTARCT: Existing isothermal nucleic acid amplification (INAA) relying on the strand displacement activity of DNA polymerase usually requires at least two primers. However, in this paper, we report an unusual isothermal multimerization and amplification (UIMA) which only needs one primer and is efficiently initiated by the strand-displacing DNA polymerases with reverse transcription activities. On electrophoresis, the products of UIMA present a cascade-shape band and they are confirmed to be multimeric DNAs with repeated target sequences. In contrast to current methods, UIMA is simple to product multimeric DNA, due to the independent of multiple primers and rolling circle structures. Through assaying the synthesized single-stranded DNA targets, UIMA performs high sensitivity and specificity, as well as the universality. In addition, a plausible mechanism of UIMA is proposed, involving short DNA bending, mismatch extension, and template slippage. UIMA is a good explanation for why nonspecific amplification easily happens in existing INAAs. As the simplest INAA till now, UIMA provides a new insight for deeply understanding INAA and opens a new avenue for thoroughly addressing nonspecific amplification.