Unambiguous Discrimination of Multiple Protein Biomarkers by Nanopore Sensing with Double-Stranded DNA-Based Probes

Beta-Barrel Nanopores as Diagnostic Sensors: An Engineering Perspective

Biological nanopores are ultrasensitive and highly attractive platforms for disease diagnostics, including the sequencing of viral and microbial genes and the detection of biomarkers and pathogens. To utilize biological nanopores as diagnostic sensors, they have been engineered through various methods resulting in the accurate and highly sensitive detection of biomarkers and disease-related biomolecules. Among diverse biological nanopores, the β-barrel-containing nanopores have advantages in nanopore engineering because of their robust structure, making them well-suited for modifications. In this review, we highlight the engineering approaches for β-barrel-containing nanopores used in single-molecule sensing for applications in early diagnosis and prognosis. In the highlighted studies, β-barrel nanopores can be modified by genetic mutation to change the structure; alter charge distributions; or add enzymes, aptamers, and protein probes to enhance sensitivity and accuracy. Furthermore, this review discusses challenges and future perspectives for advancing nanopore-based diagnostic sensors.

Identification of Conformational Variants for Bradykinin Biomarker Peptides from a Biofluid Using a Nanopore and Machine Learning

There is a current need to develop methods for the sensitive detection of peptide biomarkers in complex mixtures of molecules, such as biofluids, to enable early disease detection. Moreover, to our knowledge, there is currently no detection method capable of identifying the different conformations of a peptide biomarker differing by a single amino acid. Single-molecule nanopore sensing promises to provide this level of resolution. In order to be able to identify these differences in a biofluid such as serum, it is necessary to carefully characterize electrical parameters to obtain specific signatures of each biomarker population observed. We are interested here in a family of peptide biomarkers, kinins such as bradykinin and des-Arg9 bradykinin, that are involved in many disabling pathologies (allergy, asthma, angioedema, sepsis, or cancer). We show the proof of concept for direct identification of these biomarkers in serum at the single-molecule level using a protein nanopore. Each peptide exhibits two unique electrical signatures attributed to specific conformations in bulk. The same signatures are found in serum, allowing their discrimination and identification in a complex mixture such as biofluid. To extend the utility of our experimental results, we developed a principal component analysis approach to define the most relevant electrical parameters for their identification. Finally, we used semisupervised classification to assign each event type to a specific biomarker at physiological serum concentration. In the future, single-molecule scale analysis of peptide biomarkers using a powerful nanopore coupled with machine learning will facilitate the identification and quantification of other clinically relevant biomarkers from biofluids.

An aptamer-assisted nanopore strategy with a salt gradient for direct protein sensing

Nanopore sensing is at the forefront of the technological revolution of the protein research field and has been widely used in molecular diagnosis and molecular dynamics, as well as for various sequencing applications. However, direct protein sensing with biological nanopores is still challenging owing to the large molecular size. Here, we propose an aptamer-assisted nanopore strategy for direct protein sensing and demonstrate its proof-of-concept utilities by experiments with SARS-Cov-2 nucleocapsid protein (NP), the most abundantly expressed viral protein, that is widely used in clinical diagnosis for COVID-19. NP binds with an oligonucleotide-tailed aptamer to form a protein-DNA complex which induces a discriminative two-level pattern of current blockades. We reveal the potential molecular interaction mechanism for the characteristic blockades and identify the salt gradient condition as the dominant factor of the phenomenon. Furthermore, we achieve a high sensitivity of 10 pM for NP detection within one hour and make a preliminary exploration on clinical diagnosis. This work promises a new platform for rapid and label-free protein detection.

Advances in miniaturized nanosensing platforms for analysis of pathogenic bacteria and viruses

Pathogenic bacteria and viruses are the main causes of infectious diseases all over the world. Early diagnosis of such infectious diseases is a critical step in management of their spread and treatment of the infection in its early stages. Therefore, the innovation of smart sensing platforms for point-of-care diagnosis of life-threatening infectious diseases such as COVID-19 is a prerequisite to isolate the patients and provide them with suitable treatment strategies. The developed diagnostic sensors should be highly sensitive, specific, ultrafast, portable, cheap, label-free, and selective. In recent years, different nanosensors have been developed for the detection of bacterial and viral pathogens. We focus here on label-free miniaturized nanosensing platforms that were efficiently applied for pathogenic detection in biological matrices. Such devices include nanopore sensors and nanostructure-integrated lab-on-a-chip sensors that are characterized by portability, simplicity, cost-effectiveness, and ultrafast analysis because they avoid the time-consuming sample preparation steps. Furthermore, nanopore-based sensors could afford single-molecule counting of viruses in biological specimens, yielding high-sensitivity and high-accuracy detection. Moreover, non-invasive nanosensors that are capable of detecting volatile organic compounds emitted from the diseased organ to the skin, urine, or exhaled breath were also reviewed. The merits and applications of all these nanosensors for analysis of pathogenic bacteria and viruses in biological matrices will be discussed in detail, emphasizing the importance of artificial intelligence in advancing specific nanosensors.

Nanopore discrimination and sensitive plasma detection of multiple natriuretic peptides: The representative biomarker of human heart failure

Natriuretic peptides can relieve cardiovascular stress and closely related to heart failure. Besides, these peptides also have preferable interactions of binding to cellular protein receptors, and subsequently mediate various physiology actions. Hence, detection of these circulating biomarkers could be evaluated as a predictor ("Gold standard") for rapid, early diagnosis and risk stratification in heart failure. Herein, we proposed a measurement to discriminate multiple natriuretic peptides via the peptide-protein nanopore interaction. The nanopore single-molecular kinetics revealed that the strength of peptide-protein interactions was in the order of ANP > CNP > BNP, which was demonstrated by the simulated peptide structures using SWISS-MODEL. More importantly, the peptide-protein interaction analyzing also allowed us to measure the peptide linear analogs and structure damage in peptide by single-chemical bond breakup. Finally, we presented an ultra-sensitive detection of plasma natriuretic peptide using asymmetric electrolyte assay, obtaining a detection limit of ∼770 fM for BNP. At approximately, it is 1597 times lower than that of using symmetric assay (∼1.23 nM), 8 times lower than normal human level (∼6 pM), and 13 times lower than the diagnostic values (∼10.09 pM) complied in the guideline of European Society of Cardiology. That said, the designed nanopore sensor is benefit for natriuretic peptides measurement at single molecule level and demonstrates its potential for heart failure diagnosis.

Nanopore Single-molecule Analysis of Biomarkers: Providing Possible Clues to Disease Diagnosis

Biomarker detection has attracted increasing interest in recent years due to the minimally or non-invasive sampling process. Single entity analysis of biomarkers is expected to provide real-time and accurate biological information for early disease diagnosis and prognosis, which is critical to the effective disease treatment and is also important in personalized medicine. As an innovative single entity analysis method, nanopore sensing is a pioneering single-molecule detection technique that is widely used in analytical bioanalytical fields. In this review, we overview the recent progress of nanopore biomarker detection as new approaches to disease diagnosis. In highlighted studies, nanopore was focusing on detecting biomarkers of different categories of communicable and noncommunicable diseases, such as pandemic Covid-19, AIDS, cancers, neurologic diseases, etc. Various sensitive and selective nanopore detecting strategies for different types of biomarkers are summarized. In addition, the challenges, opportunities, and direction for future development of nanopore-based biomarker sensors are also discussed.

Recognition Receptor for Methylated Arginine at the Single Molecular Level

Among the various types of post-translational modifications (PTMs), methylation is the simple functionalized one that regulates the functions of proteins and affects interactions of protein-protein and protein-DNA/RNA, which will further influence diverse cellular processes. The methylation modification has only a slight effect on the size and hydrophobicity of proteins or peptides, and it cannot change their net charges at all, so the methods for recognizing methylated protein are still limited. Here, we designed a recognition receptor consisting of a α-hemolysin (α-HL) nanopore and polyamine decorated γ-cyclodextrin (am₈γ-CD) to differentiate the methylation of peptide derived from a heterogeneous nuclear ribonucleoprotein at the single molecule level. The results indicate that the modification of a methyl group enhances the interaction between the peptide and the recognition receptor. The results of molecular simulations were consistent with the experiments; the methylated peptide interacts with the receptor strongly due to the more formation of hydrogen bonds. This proposed strategy also can be used to detect PTM in real biological samples and possesses the advantages of low-cost and high sensitivity and is label-free. Furthermore, the success in the construction of this recognition receptor will greatly facilitate the investigation of pathogenesis related to methylated arginine.

Protein discrimination using erythrosin B-based GUMBOS in combination with UV-Vis spectroscopy and chemometrics

GUMBOS (Group of Uniform Materials Based on Organic Salts) have recently emerged as interesting materials for protein analysis due to their unique features and high tunability. In this regard, four novel erythrosin B (EB)-based GUMBOS were synthesized and their potential to discriminate among proteins with distinct properties (e.g., size, charge, and hydrophobicity) was assessed. These solid-phase materials were prepared using a single-step metathesis reaction between EB and various phosphonium and ammonium cations, namely tetrabutylphosphonium (P₄₄₄₄⁺), tributylhexadecylphosphonium (P₄₄₄₁₆⁺), tetrabutylammonium (N₄₄₄₄⁺), and benzyldimethylhexadecylammonium (BDHA⁺). Subsequently, the effect of pH (3.0, 4.5, and 6.0) and reaction time (5, 10, and 15 min) on the discriminatory power of synthesized GUMBOS was evaluated. Absorption spectra resulting from the interaction between EB-based GUMBOS and proteins were analyzed using partial least squares discriminant analysis (PLSDA). Unlike time, the pH value was determined to have influence over GUMBOS discrimination potential. Correct protein assignments varied from 86.5% to 100.0%, and the best discriminatory results were observed for [P₄₄₄₄]₂[EB] and [N₄₄₄₄]₂[EB] at pH 6.0. Additionally, these two GUMBOS allowed discrimination of protein mixtures containing different ratios of albumin and myoglobin, which appeared as individualized clusters in the PLSDA scores plots. Overall, this study showcases EB-based GUMBOS as simple synthetic targets to provide a label-free, cost-effective, rapid, and successful approach for discrimination of single proteins and their mixtures.

Solid-state and polymer nanopores for protein sensing: A review

In two decades, the solid state and polymer nanopores became attractive method for the protein sensing with high specificity and sensitivity. They also allow the characterization of conformational changes, unfolding, assembly and aggregation as well the following of enzymatic reaction. This review aims to provide an overview of the protein sensing regarding the technique of detection: the resistive pulse and ionic diodes. For each strategy, we report the most significant achievement regarding the detection of peptides and protein as well as the conformational change, protein-protein assembly and aggregation process. We discuss the limitations and the recent strategies to improve the nanopore resolution and accuracy. A focus is done about concomitant problematic such as protein adsorption and nanopore lifetime.

Nanopore-Based Single-Molecule Investigation of DNA Sequences with Potential to Form i-Motif Structures

i-Motifs are DNA secondary structures present in cytosine-rich sequences. These structures are formed in regulatory regions of the human genome and play key regulatory roles. The investigation of sequences capable of forming i-motif structures at the single-molecule level is highly important. In this study, we used α-hemolysin nanopores to systematically study a series of DNA sequences at the nanometer scale by providing structure-dependent signature current signals to gain in-sights into the i-motif DNA sequence and structural stability. Increasing the length of the cytosine tract in a range of 3-10 nucleobases resulted in a longer translocation time through the pore, indicating improved stability. Changing the loop sequence and length in the sequences did not affect the formation of the i-motif structure but changed its stability. Importantly, the application of all-atom molecular dynamics simulations revealed the structural morphology of all sequences. Based on these results, we postulated a folding rule for i-motif formation, suggesting that thousands of cytosine-rich sequences in the human genome might fold into i-motif structures. Many of these were found in locations where structure formation is likely to play regulatory roles. These findings provide insights into the application of nanopores as a powerful tool for discovering potential i-motif-forming sequences and lay a foundation for future studies exploring the biological roles of i-motifs.

MASH-FRET: A Simplified Approach for Single-Molecule Multiplexing Using FRET

Multiplexed detection has been a big motivation in biomarker analysis as it not only saves cost and labor but also improves the reliability of diagnosis. Among the many approaches for multiplexed detection, fluorescence resonance energy transfer (FRET)-based multiplexing is gaining popularity particularly due to its low background and quantitative nature. Although several FRET-based approaches have been developed for multiplexing, they require either multiple FRET pairs in combination with multiple excitation sources or complicated algorithms to accurately assign signals for individual FRET pairs. Therefore, the need for multiple FRET pairs and multiple excitation sources not only complicates the experimental design but also increases the cost and labor. In this regard, multiplexed sensing by tuning the interdye distance of a single FRET pair could be an ideal solution if identification of multiple FRET efficiencies in a single imaging is possible. Here, implementing a program called MASH-FRET, we evaluated the rigor and capability of this program in identifying seemingly overlapped FRET populations obtained from a multiplexed detection experiment using a single FRET pair. Through MASH-FRET-enabled bootstrap-based analysis of FRET data (also called BOBA-FRET), we demonstrated that the resolution and statistical confidence of the poorly resolved or even unresolved FRET populations can be readily determined. Using simulated FRET data, we further demonstrated that the program can easily identify FRET populations separated by ∼0.1 in mean FRET values, indicating an upper limit of ∼9-fold multiplexing without the need for complicated labeling schemes and multiexcitation sources. Therefore, this paper presents a data analysis approach on an existing platform that has a great potential to simplify the technological needs for multiplexing and to broaden the scope of FRET-based single-molecule analyses.

Nanodiagnostics: A review of the medical capabilities of nanopores

Modern diagnostics strive to be accurate, fast, and inexpensive in addition to properly identifying the presence of a disease, infection, or illness. Early diagnosis is key; catching a disease in its early stages can be the difference between fatality and treatment. The challenge with many diseases is that detectability of the disease scales with disease progression. Since single molecule sensors, e.g., nanopores, can sense biomolecules at low concentrations, they have the potential to become clinically relevant in many of today's medical settings. With nanopore-based sensing, lower volumes and concentrations are required for detection, enabling it to be clinically beneficial. Other advantages to using nanopores include that they are tunable to an enormous variety of molecules and boast low costs, and fabrication is scalable for manufacturing. We discuss previous reports and the potential for incorporating nanopores into the medical field for early diagnostics, therapeutic monitoring, and identifying relapse/recurrence.

Analysis with biological nanopore: On-pore, off-pore strategies and application in biological fluids

Inspired from ion channels in biology, nanopores have been developed as promising analytical tools. In principle, nanopores provide crucial information from the observation and analysis of ionic current modulations caused by the interaction between target analytes and fluidic pores. In this respect, the biological, chemical and physical parameters of the nanopore regime need to be well-understood and regulated for intermolecular interaction. Because of well-defined molecular structures, biological nanopores consequently are of a focal point, allowing precise interaction analysis at single-molecule level. In this overview, two analytical strategies are summarized and discussed accordingly, upon the challenges arising in case-dependent analysis using biological nanopores. One kind of strategies relies on modification, functionalization and engineering on nanopore confined interface to improve molecular recognition sites (on-pore strategies); The other kind of highlighted strategies concerns to measurement of various chemistry/biochemistry based interactions triggered by employed molecular agents or probes (off-pore strategies). In particularly, a few recent paradigms using these strategies for practical application of accurate analysis of biomarkers in biological fluids are illustrated. To end, the challenging and future outlook of using analytical tools by means of biological nanopores are depicted.

Sensing with Nanopores and Aptamers: A Way Forward

In the 90s, the development of a novel single molecule technique based on nanopore sensing emerged. Preliminary improvements were based on the molecular or biological engineering of protein nanopores along with the use of nanotechnologies developed in the context of microelectronics. Since the last decade, the convergence between those two worlds has allowed for biomimetic approaches. In this respect, the combination of nanopores with aptamers, single-stranded oligonucleotides specifically selected towards molecular or cellular targets from an in vitro method, gained a lot of interest with potential applications for the single molecule detection and recognition in various domains like health, environment or security. The recent developments performed by combining nanopores and aptamers are highlighted in this review and some perspectives are drawn.