Advances in optical counting and imaging of micro/nano single-entity reactors for biomolecular analysis

Ultrasensitive detection of biomarkers is of paramount importance in various fields. Superior to the conventional ensemble measurement-based assays, single-entity assays, especially single-entity detection-based digital assays, not only can reach ultrahigh sensitivity, but also possess the potential to examine the heterogeneities among the individual target molecules within a population. In this review, we summarized the current biomolecular analysis methods that based on optical counting and imaging of the micro/nano-sized single entities that act as the individual reactors (e.g., micro-/nanoparticles, microemulsions, and microwells). We categorize the corresponding techniques as analog and digital single-entity assays and provide detailed information such as the design principles, the analytical performance, and their implementation in biomarker analysis in this work. We have also set critical comments on each technique from these aspects. At last, we reflect on the advantages and limitations of the optical single-entity counting and imaging methods for biomolecular assay and highlight future opportunities in this field.

Microchamber-Free Digital Flow Cytometric Analysis of T4 Polynucleotide Kinase Phosphatase Based on Single-Enzyme-to-Single-Bead Space-Confined Reaction

Digital bioassays have attracted extensive attention in biomedical applications due to their ultrahigh sensitivity. However, traditional digital bioassays require numerous microchambers such as droplets or microwells, which restricts their application scope. Herein, we propose a microchamber-free flow cytometric method for the digital quantification of T4 polynucleotide kinase phosphatase (T4 PNKP) based on an unprecedented phenomenon that each T4 PNKP molecule-catalyzed reaction can be spatially self-confined on a single microbead, which ultimately enables the one-target-to-one-fluorescence-positive microbead digital signal transduction. The digital signal-readout mode can clearly detect T4 PNKP concentrations as low as 1.28 × 10^-10 U/μL, making it most sensitive method to date. Significantly, T4 PNKP can be specifically distinguished from other phosphatases and nucleases in complex samples by digitally counting the fluorescence-positive microbeads, which cannot be realized by traditional bulk measurement-based methods. Taking advantage of the novel space-confined enzymatic feature of T4 PNKP, this digital mechanism can use T4 PNKP as the enzyme label to fabricate digital sensing systems toward various biomolecules such as digital enzyme-linked immunosorbent assay (ELISA). Therefore, this work not only enlarges the toolbox for high-sensitivity biomolecule detection but also opens new gates to fabricate next-generation digital assays.

Single microbead-based fluorescent aptasensor (SMFA) for direct isolation and in situ quantification of exosomes from plasma

Exosomes are cell-derived membrane-enclosed biological nanoparticles that carry lots of parental molecular information, and are recognized as an ideal biomarker for non-invasive diagnosis. However, due to the low abundance of exosomes in plasma samples and the interferences from complex biological matrices, the sensitive and direct detection of exosomes still remains a challenge. Here, by combining the direct magnetic isolation with in situ fluorescence imaging, we developed a Single Microbead-based Fluorescent Aptasensor (SMFA) for specific enrichment and sensitive quantification of exosomes from plasma. In the SMFA, a single aptamer-modified microbead (MB) served as the reaction carrier so that the specific exosomes inserted with a fluorescent anchor will be highly enriched on the single MB. By in situ fluorescence imaging to monitor the fluorescence signals on the single MB, sensitive detection of exosomes can be realized without the requirement of any signal amplification routes, and as low as 4.9 × 10⁴ particles per μL of exosomes could be simply detected. More importantly, the SMFA could be applied for direct detection of the exosomes from small amounts of clinical plasma samples without prior purification procedures, indicating its great potential applications in clinical diagnostics.

Trends of Bead Counting-Based Technologies Toward the Detection of Disease-Related Biomarkers

Nowadays, the biomolecular assay platforms built-up based on bead counting technologies have emerged to be powerful tools for the sensitive and high-throughput detection of disease biomarkers. In this mini-review, we classified the bead counting technologies into statistical counting platforms and digital counting platforms. The design principles, the readout strategies, as well as the pros and cons of these platforms are introduced in detail. Finally, we point out that the digital bead counting technologies will lead the future trend for the absolute quantification of critical biomarkers, and the integration of new signal amplification approaches and routine optical/clinical instruments may provide new opportunities in building-up easily accessible digital assay platforms.

The Recent Development of Hybridization Chain Reaction Strategies in Biosensors

With the continuous development of biosensors, researchers have focused increasing attention on various signal amplification strategies to pursue superior performance for more applications. In comparison with other signal amplification strategies, hybridization chain reaction (HCR) as a powerful signal amplification technique shows its certain charm owing to nonenzymatic and isothermal features. Recently, on the basis of conventional HCR, this technique has been developed and improved rapidly, and a variety of HCR-based biosensors with excellent performance have been reported. Herein, we present a systematic and critical review on the research progress of HCR in biosensors in the last five years, including the newly developed HCR strategies such as multibranched HCR, migration HCR, localized HCR, in situ HCR, netlike HCR, and so on, as well as the combination strategies of HCR with isothermal signal amplification techniques, nanomaterials, and functional DNA molecules. By illustrating some representative works, we also summarize the advantage and challenge of HCR in biosensors, and offer a deep discussion of the latest progress and future development trends of HCR in biosensors.

Critical Review: digital resolution biomolecular sensing for diagnostics and life science research

One of the frontiers in the field of biosensors is the ability to quantify specific target molecules with enough precision to count individual units in a test sample, and to observe the characteristics of individual biomolecular interactions. Technologies that enable observation of molecules with "digital precision" have applications for in vitro diagnostics with ultra-sensitive limits of detection, characterization of biomolecular binding kinetics with a greater degree of precision, and gaining deeper insights into biological processes through quantification of molecules in complex specimens that would otherwise be unobservable. In this review, we seek to capture the current state-of-the-art in the field of digital resolution biosensing. We describe the capabilities of commercially available technology platforms, as well as capabilities that have been described in published literature. We highlight approaches that utilize enzymatic amplification, nanoparticle tags, chemical tags, as well as label-free biosensing methods.

On-bead enzyme-catalyzed signal amplification for the high-sensitive detection of disease biomarkers

The high-sensitive and rapid detection of critical biomarkers, e.g., disease-related nucleic acids and proteins, is always desired. Compared with the routine homogenous detection strategies, the on-bead flow cytometry (FCM)-based assays have drawn a lot of interests owing to their unique advantages. On one hand, microbeads (MBs) are employed for the enrichment of fluorescent signals, allowing the size encoding for multiplexed detection of biomarkers. On the other hand, FCM enables the fast read-out of the total fluorescent signals enriched on the MBs and the decoding of MBs' size information. For an improved sensitivity and versatile application scenarios, the signal amplification on MBs is required. However, the enzyme-catalyzed on-bead reactions remain challenging owing to the critical reaction conditions on the MBs/solution interface. Toward the high-sensitive detection of target biomolecules in real-samples, a series of on-bead enzyme-catalyzed signal amplification strategies have been developed. After careful optimization of the reaction conditions, the proposed sensors are proven to have ultra-high sensitivities to fulfill the requirement of real-sample detection.

Isothermal cross-boosting extension–nicking reaction mediated exponential signal amplification for ultrasensitive detection of polynucleotide kinase

A novel nucleic acid-based isothermal signal amplification strategy, named cross-boosting extension–nicking reaction (CBENR) is developed and successfully used for rapid and ultrasensitive detection of polynucleotide kinase (PNK) activity.

Enzyme-free amplified DNA assay: five orders of linearity provided by metal stable isotope detection

For the quantification of mutated genes at greatly varied concentrations in body fluids, nucleic acid amplification strategies are often challenged by limited dynamic linear ranges. The proposed metal stable isotope detection strategy demonstrates five orders of linear range and a lower attomol detection limit, showing promising potential in clinical diagnosis.

A modified exponential amplification reaction (EXPAR) with an improved signal-to-noise ratio for ultrasensitive detection of polynucleotide kinase

We reported a modified exponential amplification reaction strategy and applied it to design an ultrasensitive biosensor for the detection of endogenous polynucleotide kinase activity at single-cell level.

Dual enzyme-assisted one-step isothermal real-time amplification assay for ultrasensitive detection of polynucleotide kinase activity

A novel, simple, one-step and one-tube detection method for polynucleotide kinase (PNK) activity based on isothermal real-time amplification assay was proposed.

Detection of T4 Polynucleotide Kinase via Allosteric Aptamer Probe Platform

As a vital enzyme in DNA phosphorylation and restoration, T4 polynucleotide kinase (T4 PNK) has aroused great interest in recent years. Therefore, numerous strategies have been established for highly sensitive detection of T4 PNK based on diverse signal amplification techniques. However, they often need sophisticated design, a variety of auxiliary reagents and enzymes, or cumbersome manipulations. We have designed a new kind of allosteric aptamer probe (AAP) consisting of streptavidin (SA) aptamer and the complementary DNA (cDNA) for simple detection of T4 PNK without signal amplification and with minimized interference in complex biological samples. When the 5'-terminus of the cDNA is phosphorylated by T4 PNK, the cDNA is degraded by lambda exonuclease to release the fluorescein amidite (FAM)-labeled SA aptamer, which subsequently binds to streptavidin beads. The enhancement of the fluorescence signal on SA beads can be detected precisely and easily by a microscope or flow cytometer. Our method performs well in complex biological samples as a result of the enrichment of the signaling molecules on beads, as well as simple manipulations to discard the background interference and nonbinding molecules. Without signal amplification techniques, our AAP method not only avoids complicated manipulations but also decreases the time required. With the advantages of ease of operation, reliability, and robustness for T4 PNK detection in buffer as well as real biological samples, the AAP has great potential for clinical diagnostics, inhibitor screening, and drug discovery.

Ultrasensitive and Accurate Assay of Human Methyltransferase Activity at the Single-Cell Level Based on a Single Integrated Magnetic Microprobe

Human DNA methyltransferase (MTase) activity expression patterns and inhibition response are linked to related cancer initiation, progression, and therapeutic responses. Sensitive and accurate human MTase activity assay in cancer cells, especially at the single-cell level, is essential for biological study, clinical diagnosis, and therapy. Here, we developed an ultrasensitive and accurate DNA (cytosine-5)-methyltransferase 1 (Dnmt1) activity assay at the single-cell level based on a single integrated magnetic microprobe of functionalized double-stranded DNA (dsDNA) anchored to a single magnetic microbead surface. Functionalized dsDNA is designed with a hemimethylated DNA site for Dnmt1 recognition and a single-stranded tail to trigger in situ rolling circle amplification (RCA). Under the action of Dnmt1, hemimethylated dsDNA could be recognized and catalyzed to fully methylated dsDNA, which would protect them from the cleavage of BssHII. However, the dsDNA without full methylation would be cut by BssHII, making single-stranded tail separated from the single integrated microprobe. Subsequently, full methylation-protected in situ RCA could be performed, and multiple signal probes were hybridized to the single integrated microprobe for amplified signal accumulation. Finally, Dnmt1 activity could be evaluated by reading the fluorescence of the single integrated microprobe. Meanwhile, to minimize matrix interferences, magnetic separation was performed in the process. In this strategy, the single integrated magnetic microprobe was provided with integrated capacities of target recognition, signal amplification, signal accumulation, and matrix isolation. Therefore, an ultralow detection limit of 0.007 U/mL Dnmt1 was obtained, and accurate Dnmt1 activity assays in multiple cell lysates at the single-cell level were achieved. Furthermore, the inhibition effect of RG108 was evaluated conveniently. These results indicate that the single integrated magnetic microprobe-based strategy is an excellent candidate for sensitive monitoring of Dnmt1 activity and screening of anticancer drugs.

Hybridization chain reaction: a versatile molecular tool for biosensing, bioimaging, and biomedicine

This review provides a comprehensive overview of the fundamental principles, analysis techniques, and application fields of hybridization chain reaction and its development status.

Rational design of hybridization chain reaction monomers for robust signal amplification

We established four-point guidelines for the sequence design of hairpin monomers in hybridization chain reaction (HCR). This enabled greater flexibility to customize specific hairpin sequences for use with the readout platform of interest. Using shorter hairpin stem length, a one-pot signal amplification system was demonstrated by incorporating distance-sensitive Förster resonance energy transfer (FRET) readout.

Isothermal Amplification of Nucleic Acids

Isothermal amplification of nucleic acids is a simple process that rapidly and efficiently accumulates nucleic acid sequences at constant temperature. Since the early 1990s, various isothermal amplification techniques have been developed as alternatives to polymerase chain reaction (PCR). These isothermal amplification methods have been used for biosensing targets such as DNA, RNA, cells, proteins, small molecules, and ions. The applications of these techniques for in situ or intracellular bioimaging and sequencing have been amply demonstrated. Amplicons produced by isothermal amplification methods have also been utilized to construct versatile nucleic acid nanomaterials for promising applications in biomedicine, bioimaging, and biosensing. The integration of isothermal amplification into microsystems or portable devices improves nucleic acid-based on-site assays and confers high sensitivity. Single-cell and single-molecule analyses have also been implemented based on integrated microfluidic systems. In this review, we provide a comprehensive overview of the isothermal amplification of nucleic acids encompassing work published in the past two decades. First, different isothermal amplification techniques are classified into three types based on reaction kinetics. Then, we summarize the applications of isothermal amplification in bioanalysis, diagnostics, nanotechnology, materials science, and device integration. Finally, several challenges and perspectives in the field are discussed.

Label-free fluorescence light-up detection of T4 polynucleotide kinase activity using the split-to-intact G-quadruplex strategy by ligation-triggered and toehold-mediated strand displacement release

A label-free, fluorescence light-up detection method for T4 polynucleotide kinase activity has been developed using the split-to-intact G-quadruplex strategy.