MicroRNA detection at femtomolar concentrations with isothermal amplification and a biological nanopore

Simultaneous Recognition of Over- and Under-Expressed MicroRNAs Using Nanopore Decoding

This paper describes a strategy for simultaneous recognition of over- and under-expressed microRNAs (miRNAs) using the method of signal classification-based nanopore decoding. MiRNA has attracted attention as a promising biomarker for cancer diagnosis owing to its cancer-type-specific expression patterns. While nanopore technology has emerged as a simple and label-free method to detect miRNAs and their expression patterns, recognizing patterns involving simultaneous over/under-expression is still challenging due to the inherent working principles. Here, inspired by the sequence design for DNA computation with nanopore decoding, we designed diagnostic DNA probes targeting two individual over/under-expressed miRNAs in the serum of oral squamous cell carcinoma. Through nanopore measurements, our designed probes exhibited characteristic current signals depending on the hybridized miRNA species, which were plotted on the scatter plot of duration versus current blocking ratio. The classified signals reflected the relative abundance of target miRNAs, thereby enabling successful pattern recognition of over/under-expressed miRNAs, even when using clinical samples. We believe that our method paves the way for miRNA-targeting simple diagnosis as a liquid biopsy.

Nanopore Filter: A Method for Counting and Extracting Single DNA Molecules Using a Biological Nanopore

This paper describes a method for the real-time counting and extraction of DNA molecules at the single-molecule level by nanopore technology. As a powerful tool for electrochemical single-molecule detection, nanopore technology eliminates the need for labeling or partitioning sample solutions at the femtoliter level. Here, we attempt to develop a DNA filtering system utilizing an α-hemolysin (αHL) nanopore. This system comprises two droplets, one filling with and one emptying DNA molecules, separated by a planar lipid bilayer containing αHL nanopores. The translocation of DNA through the nanopores is observed by measuring the channel current, and the number of translocated molecules can also be verified by quantitative polymerase chain reaction (qPCR). However, we found that the issue of contamination seems to be an almost insolvable problem in single-molecule counting. To tackle this problem, we tried to optimize the experimental environment, reduce the volume of solution containing the target molecule, and use the PCR clamp method. Although further efforts are still needed to achieve a single-molecule filter with electrical counting, our proposed method shows a linear relationship between the electrical counting and qPCR estimation of the number of DNA molecules.

Detection of Biological Molecules Using Nanopore Sensing Techniques

Modern biomedical sensing techniques have significantly increased in precision and accuracy due to new technologies that enable speed and that can be tailored to be highly specific for markers of a particular disease. Diagnosing early-stage conditions is paramount to treating serious diseases. Usually, in the early stages of the disease, the number of specific biomarkers is very low and sometimes difficult to detect using classical diagnostic methods. Among detection methods, biosensors are currently attracting significant interest in medicine, for advantages such as easy operation, speed, and portability, with additional benefits of low costs and repeated reliable results. Single-molecule sensors such as nanopores that can detect biomolecules at low concentrations have the potential to become clinically relevant. As such, several applications have been introduced in this field for the detection of blood markers, nucleic acids, or proteins. The use of nanopores has yet to reach maturity for standardization as diagnostic techniques, however, they promise enormous potential, as progress is made into stabilizing nanopore structures, enhancing chemistries, and improving data collection and bioinformatic analysis. This review offers a new perspective on current biomolecule sensing techniques, based on various types of nanopores, challenges, and approaches toward implementation in clinical settings.

Carbon-Based Fluorescent Nano-Biosensors for the Detection of Cell-Free Circulating MicroRNAs

Currently, non-communicable diseases (NCDs) have emerged as potential risks for humans due to adopting a sedentary lifestyle and inaccurate diagnoses. The early detection of NCDs using point-of-care technologies significantly decreases the burden and will be poised to transform clinical intervention and healthcare provision. An imbalance in the levels of circulating cell-free microRNAs (ccf-miRNA) has manifested in NCDs, which are passively released into the bloodstream or actively produced from cells, improving the efficacy of disease screening and providing enormous sensing potential. The effective sensing of ccf-miRNA continues to be a significant technical challenge, even though sophisticated equipment is needed to analyze readouts and expression patterns. Nanomaterials have come to light as a potential solution as they provide significant advantages over other widely used diagnostic techniques to measure miRNAs. Particularly, CNDs-based fluorescence nano-biosensors are of great interest. Owing to the excellent fluorescence characteristics of CNDs, developing such sensors for ccf-microRNAs has been much more accessible. Here, we have critically examined recent advancements in fluorescence-based CNDs biosensors, including tools and techniques used for manufacturing these biosensors. Green synthesis methods for scaling up high-quality, fluorescent CNDs from a natural source are discussed. The various surface modifications that help attach biomolecules to CNDs utilizing covalent conjugation techniques for multiple applications, including self-assembly, sensing, and imaging, are analyzed. The current review will be of particular interest to researchers interested in fluorescence-based biosensors, materials chemistry, nanomedicine, and related fields, as we focus on CNDs-based nano-biosensors for ccf-miRNAs detection applications in the medical field.

MicroRNA Detection at Femtomolar Concentrations with Isothermal Amplification and a Biological Nanopore

Nanopore sensing is a powerful tool for the rapid and label-free detection of oligonucleotides, including microRNA. When moving towards actual diagnostic applications, detection of microRNA at low concentrations is one of the significant issues to be addressed. We here describe a method to detect ultra-low concentrations of microRNA using isothermal amplification and nanopore technology. Using this method, the amplified DNA from 1 fM of target microRNA can be measured by a nanopore measurement.

Pattern Recognition of microRNA Expression in Body Fluids Using Nanopore Decoding at Subfemtomolar Concentrations

This paper describes a method for detecting microRNA (miRNA) expression patterns using the nanopore-based DNA computing technology. miRNAs have shown promise as markers for cancer diagnosis due to their cancer type specificity, and therefore simple strategies for miRNA pattern recognition are required. We propose a system for pattern recognition of five types of miRNAs overexpressed in bile duct cancer (BDC). The information of miRNAs from BDC is encoded in diagnostic DNAs (dgDNAs) and decoded electrically by nanopore analysis. With this system, we succeeded in the label-free detection of miRNA expression patterns from the plasma of BDC patients. Moreover, our dgDNA-miRNA complexes can be detected at subfemtomolar concentrations, which is a significant improvement compared to previously reported limits of detection (∼10^-12 M) for similar analytical platforms. Nanopore decoding of dgDNA-encoded information represents a promising tool for simple and early cancer diagnosis.

Application of Nanomaterials in Isothermal Nucleic Acid Amplification

Because of high sensitivity and specificity, isothermal nucleic acid amplification are widely applied in many fields. To facilitate and improve their performance, various nanomaterials, like nanoparticles, nanowires, nanosheets, nanotubes, and nanoporous films are introduced in isothermal nucleic acid amplification. However, the specific application, roles, and prospect of nanomaterials in isothermal nucleic acid amplification have not been comprehensively reviewed. Here, the application of different nanomaterials (0D, 1D, 2D, and 3D) in isothermal nucleic acid amplification is comprehensively discussed and recent progress in the field is summarized. The nanomaterials are mainly used for reaction enhancer, signal generation/amplification, or surface loading carriers. In addition, 3D nanomaterials can be also functioned as isolated chambers for digital nucleic acid amplification and the tools for DNA sequencing of amplified products. Challenges and future recommendations are also proposed to be better used for recent covid-19 detection, point-of-care diagnostic, food safety, and other fields.

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.

Single polypeptide detection using a translocon EXP2 nanopore

DNA sequencing using nanopores has already been achieved and commercialized; the next step in advancing nanopore technology is towards protein sequencing. Although trials have been reported for discriminating the 20 amino acids using biological nanopores and short peptide carriers, it remains challenging. The size compatibility between nanopores and peptides is one of the issues to be addressed. Therefore, exploring biological nanopores that are suitable for peptide sensing is key in achieving amino acid sequence determination. Here, we focus on EXP2, the transmembrane protein of a translocon from malaria parasites, and describe its pore-forming properties in the lipid bilayer. EXP2 mainly formed a nanopore with a diameter of 2.5 nm assembled from 7 monomers. Using the EXP2 nanopore allowed us to detect poly-L-lysine (PLL) at a single-molecule level. Furthermore, the EXP2 nanopore has sufficient resolution to distinguish the difference in molecular weight between two individual PLL, long PLL (Mw: 30,000-70,000) and short PLL (Mw: 10,000). Our results contribute to the accumulation of information for peptide-detectable nanopores.

Nanopore decoding for a Hamiltonian path problem

DNA computing has attracted attention as a tool for solving mathematical problems due to the potential for massive parallelism with low energy consumption. However, decoding the output information to a human-recognizable signal is generally time-consuming owing to the requirement for multiple steps of biological operations. Here, we describe simple and rapid decoding of the DNA-computed output for a directed Hamiltonian path problem (HPP) using nanopore technology. In this approach, the output DNA duplex undergoes unzipping whilst passing through an α-hemolysin nanopore, with information electrically decoded as the unzipping time of the hybridized strands. As a proof of concept, we demonstrate nanopore decoding of the HPP of a small graph encoded in DNA. Our results show the feasibility of nanopore measurement as a rapid and label-free decoding method for mathematical DNA computation using parallel self-assembly.

Concentration effects on capture rate and translocation configuration of nanopore-based DNA detection

Nanopore is a kind of powerful tool to detect single molecules and investigate fundamental biological processes. In biological cells or real detection systems, concentration of DNA molecules is various. Here, we report an experimental study of the effects of DNA concentration on capture rate and translocation configuration with different sized nanopores and applied voltages. Three classes of DNA translocation configurations have been observed including linear translocation, folded translocation, and cotranslocation. In the case of relatively large sized nanopore or high applied voltage, considerable cotranslocation events have been detected. The percentage of cotranslocation events also increases with DNA concentration, which leads to the relationship between capture rate and DNA concentration deviates from linearity. Therefore, in order to reflect the number of translocation molecules accurately, the capture rate should be corrected by double-counting cotranslocation events. These results will provide a valuable reference for the design of nanopore sensors.

Early Monitoring Drug Resistant Mutation T790M with a Two-Dimensional Simultaneous Discrimination Nanopore Strategy

With the aim of detecting low frequency of drug resistant mutation T790M against wild-type sequences, we reported a two-dimensional signal analysis strategy by combining a three locked nucleic acids (LNAs)-modified probe (LP15-3t) and an α-HL nanopore. The specific hybridization of the LP15-3t probe with the T790M generated unique long two-level signals, including characteristic blocking current and characteristic dwell time. Due to the significant dwell time difference (114.2-fold) and the blocking current difference ranging from 81% to 96%, this two-dimensional signal analysis strategy can simultaneously distinguish T790M sequences with a sensitivity of 0.0001% against wild-type sequences. The LOD of T790M was 0.1 pM. This high discrimination capability would have great potential in the detection of rare mutation sequences and the early monitoring of clinical outcome of NSCLC patients with TKI drug resistance.

Detection of microRNA using enzyme-assisted amplifying and DNA-templated silver nanoclusters signal-off fluorescence bioassay

A Simple and fast analysis strategy of fluorescence quenching based on DNA-templated silver nanoclusters was developed for detection of miR-122 related to diseases such as human liver. We used Exo III to cleave the silver cluster template and assist in the DNA-RNA complex cycle. When the target is absent, the silver cluster template remains intact, and DNA-AgNCs are generated under the action of AgNO₃/NaBH₄, producing a strong background fluorescence signal. Once the target is added, the site of the Exo III occurs after a series of hybridization cycles, the Exo III acts, the template DNA is continuously hydrolyzed, and the fluorescence intensity of the system is significantly reduced. By comparing the changes in the fluorescence signal, we found that this strategy has good sensitivity and the detection limit is as low as 84.0 pM. The strategy also has excellent discriminating ability and good selectivity, it can provide a persuasive reference for the early diagnosis of liver cancer and hepatitis.

Membrane protein-based biosensors

This review highlights recent development of biosensors that use the functions of membrane proteins. Membrane proteins are essential components of biological membranes and have a central role in detection of various environmental stimuli such as olfaction and gustation. A number of studies have attempted for development of biosensors using the sensing property of these membrane proteins. Their specificity to target molecules is particularly attractive as it is significantly superior to that of traditional human-made sensors. In this review, we classified the membrane protein-based biosensors into two platforms: the lipid bilayer-based platform and the cell-based platform. On lipid bilayer platforms, the membrane proteins are embedded in a lipid bilayer that bridges between the protein and a sensor device. On cell-based platforms, the membrane proteins are expressed in a cultured cell, which is then integrated in a sensor device. For both platforms we introduce the fundamental information and the recent progress in the development of the biosensors, and remark on the outlook for practical biosensing applications.

Nanopore Decoding of Oligonucleotides in DNA Computing

In conventional DNA-computation methods involving logic gate operations, the output molecules are detected and decoded mainly by gel electrophoresis or fluorescence measurements. To employ rapid and label-free decoding, nanopore technology, an emerging methodology for single-molecule detection or DNA sequencing, is proposed as a candidate for electrical and simple decoding of DNA computations. This review describes recent approaches to decoding DNA computation using label-free and electrical nanopore measurements. Several attempts have been successful in DNA decoding with the nanopore either through enzymatic reactions or in water-in-oil droplets. Additionally, DNA computing combined with nanopore decoding has clinical applications, including microRNA detection for early diagnosis of cancers. Because this decoding methodology is still in development and not yet widely accepted, this review aims to inform the scientific community regarding usefulness.

Channel current analysis estimates the pore-formation and the penetration of transmembrane peptides

We measured the current signal of the transmembrane model peptides using the barrel-stave, toroidal pore, and penetration models in order to establish a precise assignment of the channel signals.

Rapid and label-free fluorescence bioassay for microRNA based on exonuclease III-assisted cycle amplification

A new assay for the rapid and simple detection of microRNA based on G-quadruplex and Exonuclease III (ExoIII) dual signal amplification was constructed.

Synthetic Ion Channels and DNA Logic Gates as Components of Molecular Robots

A molecular robot is a next-generation biochemical machine that imitates the actions of microorganisms. It is made of biomaterials such as DNA, proteins, and lipids. Three prerequisites have been proposed for the construction of such a robot: sensors, intelligence, and actuators. This Minireview focuses on recent research on synthetic ion channels and DNA computing technologies, which are viewed as potential candidate components of molecular robots. Synthetic ion channels, which are embedded in artificial cell membranes (lipid bilayers), sense ambient ions or chemicals and import them. These artificial sensors are useful components for molecular robots with bodies consisting of a lipid bilayer because they enable the interface between the inside and outside of the molecular robot to function as gates. After the signal molecules arrive inside the molecular robot, they can operate DNA logic gates, which perform computations. These functions will be integrated into the intelligence and sensor sections of molecular robots. Soon, these molecular machines will be able to be assembled to operate as a mass microrobot and play an active role in environmental monitoring and in vivo diagnosis or therapy.