Solid-State Nanopore Single-Molecule Sensing of DNAzyme Cleavage Reaction Assisted with Nucleic Acid Nanostructure

Recent advances of small molecule detection in nanopore sensing

Due to its advantages of label-free and highly sensitive, the resistive pulse sensing with a nanopore has recently become even more potent for the discrimination of analytes in single molecule level. Generally, a transient interruption of ion current originated from the captured molecule passing through a nanopore will provide the rich information on the structure, charge and translocation dynamics of the analytes. Therefore, nanopore sensors have been widely used in the fields of DNA sequencing, protein recognition, and the portable detection of varied macromolecules and particles. However, the conventional nanopore devices are still lack of sufficient selectivity and sensitivity to distinguish more metabolic molecules involving ATP, glucose, amino acids and small molecular drugs because it is hard to receive a large number of identifiable signals with the fabricated pores comparable in size to small molecules for nanopore sensing. For all this, a series of innovative strategies developed in the past decades have been summarized in this review, including host-guest recognition, engineering alteration of protein channel, the introduction of nucleic acid aptamers and various delivery carriers integrating signal amplification sections based on the biological and solid nanopore platforms, to achieve the high resolution for the small molecules sensing in micro-nano environment. These works have greatly enhanced the powerful sensing capabilities and extended the potential application of nanopore sensors.

Nanopore sensitization based on a double loop hybridization chain reaction and G-quadruplex

The widespread implementation of solid-state nanopores faces challenges such as lower resolution and increased electrical noise when compared to biological nanopores. Incorporating specific nucleic acid reactions can enhance resolution. In this study, we've developed a nucleic acid amplifier to enhance the sensitivity of solid-state nanopores, utilizing a G-rich sequence and hybridization chain reaction. This amplifier improves target concentration and volume amplification, showing promise in nanopore sensitivity tests.

Translocation of Proteins through Solid-State Nanopores Using DNA Polyhedral Carriers

The detection of biomolecules at the single molecule level has important applications in the fields of biosensing and biomedical diagnosis. The solid-state nanopore (SS nanopore) is a sensitive tool for detecting single molecules because of its unique label-free and low sample consumption properties. SS nanopore translocation of small biomolecules is typically driven by an electronic field force and is thus influenced by the charge, shape, and size of the target molecules. Therefore, it remains challenging to control the translocation of biomolecules through SS nanopores, particularly for different proteins with complex conformations and unique charges. Toward this problem, a DNA polyhedral carrier coating strategy to assist protein translocation through SS nanopores is developed, which facilitates target protein detection. The current signal-to-noise ratios are improved significantly using this DNA carrier loading strategy. The proposed method should aid the detection of proteins, which are difficult to translocate through nanopores. This coating-assisted method offers a wide range of applications for SS nanopore detection and promotes the development of single-molecule detection.

A novel highly sensitive compilation-detachment fluorescence sensing strategy based on RNA-cleavage DNAzyme for MDA-MB-231 breast cancer biomarker determination

Herein, we designed a novel and highly sensitive fluorescence multicomponent detachable platform for MDA-MB-231 breast cancer cell detection as a model. The RNA cleavage DNAzyme was used as a central operator of the multicomponent probe through which compilation and induced detachment of probe was done. During the compilation step, the dsDNA-Sybr green 1 complexes on gold nanoparticles (GNP@dsDNA@SG1) were assembled. The intercalated Sybr green in the DNA structure has been used as an amplified signal generator on one site of DNAzyme and magnetic nanoparticles (MNP) act as a biological carrier and probe collector on the opposite side. The enzyme activator co-factor (MDA-MB-231 cell cytoplasmic protein) provokes the activation of the catalytic core of enzyme sequence in the DNAzyme molecule, followed by cleavage reaction in the substrate sequence and releasing GNP@ dsDNA@SG1 into the solution. The results indicate that the Sybr green emission fluorescence (520 nm) increases with the increment of MDA-MB-231 protein concentration in the linear dynamic range of 8.10 × 10^-2 to 1.95 ng ml^-1 (0.77 × 10^-3-0.019 cell ml^-1) with a detection limit (LOD) of 1/72 × 10^-2 pg ml^-1 under optimal conditions. The proposed immunosensor has great potential in developing ultrasensitive and rapid diagnostic platforms.

The application of single molecule nanopore sensing for quantitative analysis

Nanopore-based sensors typically work by monitoring transient pulses in conductance via current-time traces as molecules translocate through the nanopore. The unique property of being able to monitor single molecules gives nanopore sensors the potential as quantitative sensors based on the counting of single molecules. This review provides an overview of the concepts and fabrication of nanopore sensors as well as nanopore sensing with a view toward using nanopore sensors for quantitative analysis. We first introduce the classification of nanopores and highlight their applications in molecular identification with some pioneering studies. The review then shifts focus to recent strategies to extend nanopore sensors to devices that can rapidly and accurately quantify the amount of an analyte of interest. Finally, future prospects are provided and briefly discussed. The aim of this review is to aid in understanding recent advances, challenges, and prospects for nanopore sensors for quantitative analysis.

Recent advances in biological nanopores for nanopore sequencing, sensing and comparison of functional variations in MspA mutants

Biological nanopores are revolutionizing human health by the great myriad of detection and diagnostic skills. Their nano-confined area and ingenious shape are suitable to investigate a diverse range of molecules that were difficult to identify with the previous techniques. Additionally, high throughput and label-free detection of target analytes instigated the exploration of new bacterial channel proteins such as Fragaceatoxin C (FraC), Cytolysin A (ClyA), Ferric hydroxamate uptake component A (FhuA) and Curli specific gene G (CsgG) along with the former ones, like α-hemolysin (αHL), Mycobacterium smegmatis porin A (MspA), aerolysin, bacteriophage phi 29 and Outer membrane porin G (OmpG). Herein, we discuss some well-known biological nanopores but emphasize on MspA and compare the effects of site-directed mutagenesis on the detection ability of its mutants in view of the surface charge distribution, voltage threshold and pore-analyte interaction. We also discuss illustrious and latest advances in biological nanopores for past 2-3 years due to limited space. Last but not the least, we elucidate our perspective for selecting a biological nanopore and propose some future directions to design a customized nanopore that would be suitable for DNA sequencing and sensing of other nontrivial molecules in question.

Structure-flexible DNA origami translocation through a solid-state nanopore

Nanopore detection is a label-free detection method designed to analyze single molecules by comparing specific translocation events with high signal-to-noise ratios. However, it is still challenging to understand the influences of structural flexibility of 100 nm DNA origami on nanopore translocations. Here, we used solid-state nanopores to characterize the translocation of “nunchaku” origami structures, the flexibility of which can be regulated by introducing specific DNA strands and streptavidin protein. The structural changes can result in significant variations in the translocation signals and distributions. It is anticipated that such a method of the flexible DNA origami translocation through a solid-state nanopore will find further applications in molecular detection as well as biosensing.

Using a solid-state nanopore to characterize the translocation of “nunchaku” origami with tunable-structures.

Single-Entity Detection With TEM-Fabricated Nanopores

Nanopore-based single-entity detection shows immense potential in sensing and sequencing technologies. Solid-state nanopores permit unprecedented detail while preserving mechanical robustness, reusability, adjustable pore size, and stability in different physical and chemical environments. The transmission electron microscope (TEM) has evolved into a powerful tool for fabricating and characterizing nanometer-sized pores within a solid-state ultrathin membrane. By detecting differences in the ionic current signals due to single-entity translocation through the nanopore, solid-state nanopores can enable gene sequencing and single molecule/nanoparticle detection with high sensitivity, improved acquisition speed, and low cost. Here we briefly discuss the recent progress in the modification and characterization of TEM-fabricated nanopores. Moreover, we highlight some key applications of these nanopores in nucleic acids, protein, and nanoparticle detection. Additionally, we discuss the future of computer simulations in DNA and protein sequencing strategies. We also attempt to identify the challenges and discuss the future development of nanopore-detection technology aiming to promote the next-generation sequencing technology.

Low-Noise Solid-State Nanopore Enhancing Direct Label-Free Analysis for Small Dimensional Assemblies Induced by Specific Molecular Binding

Solid-state nanopores show special potential as a new single-molecular characterization for nucleic acid assemblies and molecular machines. However, direct recognition of small dimensional species is still quite difficult due the lower resolution compared with biological pores. We recently reported a very efficient noise-reduction and resolution-enhancement mechanism via introducing high-dielectric additives (e.g., formamide) into conical glass nanopore (CGN) test buffer. Based on this advance, here, for the first time, we apply a bare CGN to directly recognize small dimensional assemblies induced by small molecules. Cocaine and its split aptamer (Capt assembly) are chosen as the model set. By introducing 20% formamide into CGN test buffer, high cocaine-specific distinguishing of the 113 nt Capt assembly has been realized without any covalent label or additional signaling strategies. The signal-to-background discrimination is much enhanced compared with control characterizations such as gel electrophoresis and fluorescence resonance energy transfer (FRET). As a further innovation, we verify that low-noise CGN can also enhance the resolution of small conformational/size changes happening on the side chain of large dimensional substrates. Long duplex concatamers generated from the hybridization chain reaction (HCR) are selected as the model substrates. In the presence of cocaine, low-noise CGN has sensitively captured the current changes when the 26 nt aptamer segment is assembled on the side chain of HCR duplexes. This paper proves that the introduction of the low-noise mechanism has significantly improved the resolution of the solid-state nanopore at smaller and finer scales and thus may direct extensive and deeper research in the field of CGN-based analysis at both single-molecular and statistical levels, such as molecular recognition, assembly characterization, structure identification, information storage, and target index.

Recognition of Bimolecular Logic Operation Pattern Based on a Solid-State Nanopore

Nanopores have a unique advantage for detecting biomolecules in a label-free fashion, such as DNA that can be synthesized into specific structures to perform computations. This method has been considered for the detection of diseased molecules. Here, we propose a novel marker molecule detection method based on DNA logic gate by deciphering a variable DNA tetrahedron structure using a nanopore. We designed two types of probes containing a tetrahedron and a single-strand DNA tail which paired with different parts of the target molecule. In the presence of the target, the two probes formed a double tetrahedron structure. As translocation of the single and the double tetrahedron structures under bias voltage produced different blockage signals, the events could be assigned into four different operations, i.e., (0, 0), (0, 1), (1, 0), (1, 1), according to the predefined structure by logic gate. The pattern signal produced by the AND operation is obviously different from the signal of the other three operations. This pattern recognition method has been differentiated from simple detection methods based on DNA self-assembly and nanopore technologies.

DNAzyme Sensor for the Detection of Ca2+ Using Resistive Pulse Sensing

DNAzymes are DNA oligonucleotides that can undergo a specific chemical reaction in the presence of a cofactor. Ribonucleases are a specific form of DNAzymes where a tertiary structure undergoes cleavage at a single ribonuclease site. The cleavage is highly specificity to co-factors, which makes them excellent sensor recognition elements. Monitoring the change in structure upon cleavage has given rise to many sensing strategies; here we present a simple and rapid method of following the reaction using resistive pulse sensors, RPS. To demonstrate this methodology, we present a sensor for Ca²⁺ ions in solution. A nanoparticle was functionalised with a Ca²⁺ DNAzyme, and it was possible to follow the cleavage and rearrangement of the DNA as the particles translocate the RPS. The binding of Ca²⁺ caused a conformation change in the DNAzyme, which was monitored as a change in translocation speed. A 30 min assay produced a linear response for Ca²⁺ between 1–9 μm, and extending the incubation time to 60 min allowed for a concentration as low as 0.3 μm. We demonstrate that the signal is specific to Ca²⁺ in the presence of other metal ions, and we can quantify Ca²⁺ in tap and pond water samples.

Nanopore detection of metal ions: Current status and future directions

In this review, we highlight recent research efforts that aimed at developing nanopore sensors for detection of metal ions, which play a crucial role in environmental safety and human health. Protein pores use three stochastic sensing-based strategies for metal ion detection. The first strategy is to construct engineered nanopores with metal ion binding sites, so that the interaction between the target analytes and the nanopore can slow the movement of metal ions in the nano-channel. Second, large molecules such as nucleic acids and especially peptides could be utilized as external selective molecular probes to detect metal ions based on the conformational change of the ligand molecules induced by the metal ion-ligand chelation / coordination interaction. Third, enzymatic reactions can also be used as an alternative to the molecule probe strategy in the situation that a sensitive and selective probe molecule for the target analyte is difficult to obtain. On the other hand, by taking advantage of steady-state analysis, synthetic nanopores mainly use two strategies (modification and modification-free) to detect metals. Given the advantages of high sensitivity & selectivity, and label-free detection, nanopore-based metal ion sensors should find useful application in many fields, including environmental monitoring, medical diagnosis, and so on.

DNA nanotechnology assisted nanopore-based analysis

Nanopore technology is a promising label-free detection method. However, challenges exist for its further application in sequencing, clinical diagnostics and ultra-sensitive single molecule detection. The development of DNA nanotechnology nonetheless provides possible solutions to current obstacles hindering nanopore sensing technologies. In this review, we summarize recent relevant research contributing to efforts for developing nanopore methods associated with DNA nanotechnology. For example, DNA carriers can capture specific targets at pre-designed sites and escort them from nanopores at suitable speeds, thereby greatly enhancing capability and resolution for the detection of specific target molecules. In addition, DNA origami structures can be constructed to fulfill various design specifications and one-pot assembly reactions, thus serving as functional nanopores. Moreover, based on DNA strand displacement, nanopores can also be utilized to characterize the outputs of DNA computing and to develop programmable smart diagnostic nanodevices. In summary, DNA assembly-based nanopore research can pave the way for the realization of impactful biological detection and diagnostic platforms via single-biomolecule analysis.

An investigation of solid-state nanopores on label-free metal-ion signalling via the transition of RNA-cleavage DNAzyme and the hybridization chain reaction

Recent advances have proven solid-state nanopores as a powerful analysis platform that enables label-free and separation-free single-molecule analysis. However, the relatively low resolution still limits its application because many chemicals or targets with small sizes could not be recognized in a label-free condition. In this paper, we provide a possible solution that uses solid-state nanopores for small species signaling via the transition of huge DNA assembly products. DNAzyme responding to metal ions and the hybridization chain reaction (HCR) generating nanopore-detectable dsDNA concatamers are used as the transition model set. By the two-step DNAzyme-HCR transition, Pb2+ that was too tiny to be sensed was successfully recognized by the nanopore. The whole process happened in a completely homogeneous solution without any chemical modification. During condition optimization, we also discussed one possible application challenge that may affect the HCR signal-background distinction. Solid-state nanopores provide a potential solution to this challenge due to its ability to profile product length or even 3D structure information.