Sensitive electrochemical biosensor for MicroRNAs based on duplex-specific nuclease-assisted target recycling followed with gold nanoparticles and enzymatic signal amplification

Nanomaterial-based electrochemical biosensors as tools for detecting the tumor biomarker miR-21

MicroRNAs (miRNAs) are noncoding RNA transcripts with myriad physiologically important regulatory roles in the human body. These miRNAs have also recently emerged as promising biomarkers for the diagnosis of particular cancers. Conventional miRNA detection strategies, however, are characterized by many limitations. As electrochemical biosensors offer advantages including low costs, high levels of sensitivity, and amenability to miniaturization, they hold great promise as an alternative approach to miRNA detection. Nanomaterials are commonly used in the context of electrochemical sensor production, and this review provides an overview of the use of various carbon nanomaterials, metallic nanomaterials, metal-organic frameworks, magnetic nanomaterials, and conductive polymer nanocomposites to modify electrochemical biosensors in order to facilitate the detection of miRNA-21. A range of materials and detection methods for particular cancer types are discussed herein highlighting the superior sensitivity and specificity of these analytical strategies., which allow for the stable and reproducible detection of miRNAs in clinical samples. Ultimately, this review demonstrates the promising clinical prospects of these modified electrochemical biosensors as tools for early cancer diagnosis and the prognostic evaluation of affected patients.

Enzyme-Assisted Electrochemical Point-of-Care Test for miRNA Detection in Liquid Biopsy

In the personalized medicine era, affordable and portable devices for quicker cancer monitoring, even in remote areas, are crucial. To address this need, we have developed an enzyme-assisted electrochemical point-of-care (POC) test for application toward liquid biopsy. In particular, miR-200a-5p has been taken into account as the model target due to its correlation for triple negative breast cancer (TNBC) prognosis. The proposed platform has been conceived as signal-ON, and the detection architecture is based on the presence of a duplex-specific nuclease (DSN) that is selective for DNA-RNA heteroduplexes. When the miRNA is recognized by an ad-hoc designed DNA probe, the DSN enzyme enables for the isothermal target recycling and signal enhancement, which is essential for detecting the miRNA trace in biofluids. Introducing a methylene blue (MB) modification on the DNA probe, a single miRNA strand is capable of triggering multiple DSN cleavage circles, increasing the free MB and thus the electrochemical signal recorded. All the optimization studies have been carried out using a screen-printed strip, resulting in a dynamic range comprised between 0.1 pM and 100 nM and a detection limit down to the fM level. A satisfactory selectivity was highlighted by interrogating the system toward random miRNA target mixtures, and the platform was also tested in spiked commercial serum samples.

Nanomaterials promote the fast development of electrochemical MiRNA biosensors

Cancer has become the leading cause of death worldwide. In recent years, molecular diagnosis has demonstrated great potential in the prediction and diagnosis of cancer. MicroRNAs (miRNAs) are short oligonucleotides that regulate gene expression and cell function and are considered ideal biomarkers for cancer detection, diagnosis, and patient prognosis. Therefore, the specific and sensitive detection of ultra-low quantities of miRNA is of great significance. MiRNA biosensors based on electrochemical technology have advantages of high sensitivity, low cost and fast response. Nanomaterials show great potential in miRNA electrochemical detection and promote the rapid development of electrochemical miRNA biosensors. Some methods and signal amplification strategies for miRNA detection in recent years are reviewed herein, followed by a discussion of the latest progress in electrochemical miRNA detection based on different types of nanomaterial. Future perspectives and challenges are also proposed for further exploration of nanomaterials to bring breakthroughs in electrochemical miRNA detection.

Nanomaterials in electrochemical nanobiosensors of miRNAs

Nanomaterial-based biosensors have received significant attention owing to their unique properties, especially enhanced sensitivity. Recent advancements in biomedical diagnosis have highlighted the role of microRNAs (miRNAs) as sensitive prognostic and diagnostic biomarkers for various diseases. Current diagnostics methods, however, need further improvements with regards to their sensitivity, mainly due to the low concentration levels of miRNAs in the body. The low limit of detection of nanomaterial-based biosensors has turned them into powerful tools for detecting and quantifying these biomarkers. Herein, we assemble an overview of recent developments in the application of different nanomaterials and nanostructures as miRNA electrochemical biosensing platforms, along with their pros and cons. The techniques are categorized based on the nanomaterial used.

A plasmonic MNAzyme signal amplification strategy for quantification of miRNA-4739 breast cancer biomarker

A major challenge in the 21st century is the development of point-of-care diagnostic tools capable to detect and quantify disease biomarkers in a straightforward, affordable, sensitive, and specific manner. The remarkable plasmonic properties of gold nanoparticles (AuNPs) have promoted their use for development of simple methodologies for nucleic acid detection in combination with a variety of oligonucleotides amplification techniques. Here, assemblies of AuNPs with Multicomponent Nucleic Acid enzymes (MNAzymes) has been successfully used in the design of a highly sensitive and simple bioassay for rapid spectroscopic detection and quantification of miRNA-4739 in blood samples. The miRNA selected is a doxorubicin chemoresistant biomarker in breast cancer which overexpression promotes the proliferation, progression, and survival of cancer cells. In this work, two alternatives experimental designs, based on use of MNAzymes and AuNPs, have been optimized and applied for sensitive miRNA-4739 quantification: one based on a traditional direct measurement of wavelength shift and a second non-conventional simple approach based on isolation and measurement of free nanoparticles absorbance. Improvement in sensitivity and, higher measurement accuracy and precision were achieved with the second approach. The developed bioassay provides a detection limit as low as 7 pmolL-1 for miRNA-4739 quantification and performed satisfactory selectivity and well practical applicability by analysis of the miRNA-4739 in blood, demonstrating that the proposed strategy is a promising and suitable spectroscopic method for breast cancer diagnosis thought liquid biopsy of circulating tumoral cells.

Recent advancements in biosensor designs toward the detection of intestine cancer miRNA biomarkers

Cancer diagnosis and treatment have been of broad interest among scientists in the last decades due to the high death rate, widespread occurrence, and recurrence after treatment. The survival rate of cancer patients depends greatly on early detection and appropriate treatments. Therefore developing new technologies applicable to sensitive and specific methods of cancer detection is an inevitable task for cancer researchers. Abnormal miRNA expression is contributed to severe diseases such as cancers and since their expression level and type differ strictly during carcinogenesis and later metastasis and treatments, the improved detection accuracy of these miRNAs would undoubtedly lead to early diagnosis, prognosis, and targeted therapy. Biosensors are accurate and straightforward analytical devices that have had practical applications especially in the last decade. Their domain is still growing through a combination of attractive nanomaterials and amplification methods, leading to innovative biosensing platforms for the efficient detection of miRNAs as diagnostic and prognostic biomarkers. In this review, we will provide the recent developments in biosensors to detect intestine cancer miRNA biomarkers and also discuss the challenges and outcomings of this field.

Hybridization Chain Reaction-Based Electrochemical Biosensors by Integrating the Advantages of Homogeneous Reaction and Heterogeneous Detection

The conventional hybridization chain reaction (HCR)-based electrochemical biosensors usually require the immobilization of probes on the electrode surface. This will limit the applications of biosensors due to the shortcomings of complex immobilization processes and low HCR efficiency. In this work, we proposed astrategy for the design of HCR-based electrochemical biosensors by integrating the advantages of homogeneous reaction and heterogeneous detection. Specifically, the targets triggered the autonomous cross-opening and hybridization oftwobiotin-labeled hairpin probes to form long-nicked dsDNA polymers. The HCR products with many biotin tags were then captured by a streptavidin-covered electrode, thus allowing for the attachment of streptavidin-conjugated signal reporters through streptavidin-biotin interactions. By employing DNA and microRNA-21 as the model targets and glucose oxidase as the signal reporter, the analytical performances of the HCR-based electrochemical biosensors were investigated. The detection limits of this method were found to be 0.6 fM and 1 fM for DNA and microRNA-21, respectively. The proposed strategy exhibited good reliability for target analysis in serum and cellular lysates. The strategy can be used to develop various HCR-based biosensors for a wide range of applications because sequence-specific oligonucleotides exhibit high binding affinity to a series of targets. In light of the high stability and commercial availability of streptavidin-modified materials, the strategy can be used for the design of different biosensors by changing the signal reporter and/or the sequence of hairpin probes.

Electrochemical Detection of a Few Copies of Unamplified SARS-CoV-2 Nucleic Acids by a Self-Actuated Molecular System

The existing electrochemical biosensors lack controllable and intelligent merit to modulate the sensing process upon external stimulus, leading to challenges in analyzing a few copies of biomarkers in unamplified samples. Here, we present a self-actuated molecular-electrochemical system that consists of a tentacle and a trunk modification on a graphene microelectrode. The tentacle that contains a probe and an electrochemical label keeps an upright orientation, which increases recognition efficiency while decreasing the pseudosignal. Once the nucleic acids are recognized, the tentacles nearby along with the labels are spontaneously actuated downward, generating electrochemical responses under square wave voltammetry. Thus, it detects unamplified SARS-CoV-2 RNAs within 1 min down to 4 copies in 80 μL, 2-6 orders of magnitude lower than those of other electrochemical assays. Double-blind testing and 10-in-1 pooled testing of nasopharyngeal samples yield high overall agreement with reverse transcription-polymerase chain reaction results. We fabricate a portable prototype based on this system, showing great potential for future applications.

Electrochemical Signal Amplification Strategies and Their Use in Olfactory and Taste Evaluation

Biosensors are powerful analytical tools used to identify and detect target molecules. Electrochemical biosensors, which combine biosensing with electrochemical analysis techniques, are efficient analytical instruments that translate concentration signals into electrical signals, enabling the quantitative and qualitative analysis of target molecules. Electrochemical biosensors have been widely used in various fields of detection and analysis due to their high sensitivity, superior selectivity, quick reaction time, and inexpensive cost. However, the signal changes caused by interactions between a biological probe and a target molecule are very weak and difficult to capture directly by using detection instruments. Therefore, various signal amplification strategies have been proposed and developed to increase the accuracy and sensitivity of detection systems. This review serves as a reference for biosensor and detector research, as it introduces the research progress of electrochemical signal amplification strategies in olfactory and taste evaluation. It also discusses the latest signal amplification strategies currently being employed in electrochemical biosensors for nanomaterial development, enzyme labeling, and nucleic acid amplification techniques, and highlights the most recent work in using cell tissues as biosensitive elements.

Hybrid Nanobioengineered Nanomaterial-Based Electrochemical Biosensors

Nanoengineering biosensors have become more precise and sophisticated, raising the demand for highly sensitive architectures to monitor target analytes at extremely low concentrations often required, for example, for biomedical applications. We review recent advances in functional nanomaterials, mainly based on novel organic-inorganic hybrids with enhanced electro-physicochemical properties toward fulfilling this need. In this context, this review classifies some recently engineered organic-inorganic metallic-, silicon-, carbonaceous-, and polymeric-nanomaterials and describes their structural properties and features when incorporated into biosensing systems. It further shows the latest advances in ultrasensitive electrochemical biosensors engineered from such innovative nanomaterials highlighting their advantages concerning the concomitant constituents acting alone, fulfilling the gap from other reviews in the literature. Finally, it mentioned the limitations and opportunities of hybrid nanomaterials from the point of view of current nanotechnology and future considerations for advancing their use in enhanced electrochemical platforms.

Ultrasensitive Detection of Ochratoxin A With a Zeolite Imidazolate Frameworks Composite–Based Electrochemical Aptasensor

Ochratoxin A (OTA) is a harmful mycotoxin, which is mainly secreted by Penicillium and Aspergillus species. In this work, an electrochemical aptasensor is presented for OTA detection based on Au nanoparticles (AuNPs) modified zeolite imidazolate frameworks (ZIFs) ZIF-8 platform and duplex-specific nuclease (DSN) triggered hybridization chain reaction (HCR) signal amplification. G-quadruplex-hemin assembled HCR nanowire acted as a nicotinamide adenine dinucleotide (NADH) oxidase and an HRP-mimicking DNAzyme. Besides, thionine (Thi) was enriched as a redox probe for signal amplification in this pseudobienzyme electrocatalytic system. Under the optimal conditions, the analytical response ranged from 1 to 10⁷ fg ml⁻¹ with a detection limit of 0.247 fg ml⁻¹. Furthermore, the aptasensor was proven to be applied in real wheat samples with a recovery between 96.8 and 104.2%, illustrating the potential prospects in practical detection.

Noninvasive Prognosis of Postmyocardial Infarction Using Urinary miRNA Ultratrace Detection Based on Single-Target DNA-Functionalized AuNPs

Urine is the most appropriate body fluid for analysis because it is easily and less-invasively obtained than blood; thus, urinary miRNAs can better represent the local stage of the disease and might grow up to be a new class of noninvasive biomarkers of postmyocardial infarction (MI). Monofunctionalized Au nanoparticles (AuNPs) with only one selective DNA at a specific location are more promising in nanotechnology. This study developed a urinary miRNA ultratrace detection strategy based on single-target DNA-functionalized AuNPs for the noninvasive prognosis of post-MI. The AuNPs were designed with only single-stranded biotinylated DNA complementary to the target miRNA through a ratio-optimized stoichiometric method for the first time. Combined with the duplex specific nuclease-assisted target recycling amplification, the single-target DNA-functionalized AuNPs for the first time were used in inductively coupled plasma-mass spectrometry for the determination of urinary miRNA with high sensitivity. After optimizing the reaction conditions, a linear detection range between 1 fM and 10 pM for miR-155 and a detection limit of 0.47 fM were obtained. Finally, the target miR-155 in urine samples collected from MI rats was quantified and the level of miR-155 in MI groups was 30 times higher than in the control groups. The results suggest that urinary miR-155 could be a novel biomarker for the noninvasive diagnosis of MI.

Highly Sensitive Fluorescence Assay for miRNA Detection: Investigation of the DNA Spacer Effect on the DSN Enzyme Activity toward Magnetic-Bead-Tethered Probes

Researchers have recently designed various biosensors combining magnetic beads (MBs) and duplex-specific nuclease (DSN) enzyme to detect miRNAs. Yet, the interfacial mechanisms for surface-based hybridization and DSN-assisted target recycling are relatively not well understood. Thus, herein, we developed a highly sensitive and selective fluorescent biosensor to study the phenomenon that occurs on the local microenvironment surrounding the MB-tethered DNA probe via detecting microRNA-21 as a model. Using the above strategy, we investigated the influence of different DNA spacers, base-pair orientations, and surface densities on DSN-assisted target recycling. As a result, we were able to detect as low as 170 aM of miR-21 under the optimized conditions. Moreover, this approach exhibits a high selectivity in a fully matched target compared to a single-base mismatch, allowing the detection of miRNAs in serum with improved recovery. These results are attributed to the synergetic effect between the DSN enzyme activity and the neutral DNA spacer (triethylene glycol: TEG) to improve the miRNA detection's sensitivity. Finally, our strategy could create new paths for detecting microRNAs since it obliterates the enzyme-mediated cascade reaction used in previous studies, which is more expensive, more time-consuming, less sensitive, and requires double catalytic reactions.

Recent Progress in Nanomaterials Modified Electrochemical Biosensors for the Detection of MicroRNA

MicroRNAs (miRNAs) are important non-coding, single-stranded RNAs possessing crucial regulating roles in human body. Therefore, miRNAs have received extensive attention from various disciplines as the aberrant expression of miRNAs are tightly related to different types of diseases. Furthermore, the exceptional stability of miRNAs has presented them as biomarker with high specificity and sensitivity. However, small size, high sequence similarity, low abundance of miRNAs impose difficulty in their detection. Hence, it is of utmost importance to develop accurate and sensitive method for miRNA biosensing. Electrochemical biosensors have been demonstrated as promising solution for miRNA detection as they are highly sensitive, facile, and low-cost with ease of miniaturization. The incorporation of nanomaterials to electrochemical biosensor offers excellent prospects for converting biological recognition events to electronic signal for the development of biosensing platform with desired sensing properties due to their unique properties. This review introduces the signal amplification strategies employed in miRNA electrochemical biosensor and presents the feasibility of different strategies. The recent advances in nanomaterial-based electrochemical biosensor for the detection of miRNA were also discussed and summarized based on different types of miRNAs, opening new approaches in biological analysis and early disease diagnosis. Lastly, the challenges and future prospects are discussed.

Nucleic acid-based electrochemical biosensor: Recent advances in probe immobilization and signal amplification strategies

With the increasing importance of accurate and early disease diagnosis and the development of personalized medicine, DNA-based electrochemical biosensor has attracted broad scientific and clinical interests in the past decades due to its unique hybridization specificity, fast response time, and potential for miniaturization. In order to achieve high detection sensitivity, the design of DNA electrochemical biosensors depends critically on the improvement of the accessibility of target molecules and the enhancement of signal readout. Here, we summarize the recent advances in DNA probe immobilization and signal amplification strategies with a special focus on DNA nanostructure-supported DNA probe immobilization method, which provides the opportunity to rationally control the distance between probes and keep them in upright confirmation, as well as the contribution of functional nanomaterials in enhancing the signal amplification. The next challenge of biosensors will be the fabrication of point-of-care devices for clinical testing. The advancement of multidisciplinary areas, including nanofabrication, material science, and biochemistry, has exhibited profound promise in achieving such portable sensing devices. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures.

A dual-signaling electrochemical ratiometric strategy combining "signal-off" and "signal-on" approaches for detection of MicroRNAs

A dual-signaling electrochemical ratio metric strategy was developed for detection microRNA-18a based on the duplex-specific nuclease-assisted target recycling and electrochemical atom transfer radical polymerization signal amplification. In the presence of target microRNA, RNA/DNA duplexes are formed, which become the substrate of the duplex-specific nuclease-assisted target recycling. Hence only the DNA strand is cleaved by duplex-specific nuclease enzyme, resulting in the throw away of methylene blue (MB) from the electrode (signal off) accompanied by releasing of target microRNA, which can be recycled in the next hybridization. The remaining piece of capture DNAs on the electrode surface hybridize with the Azide labeled-signal DNAs. "Click reactions" were carried out between 3-Butynyl-2-bromoisobutyrate and Azide to initiate the electrochemical atom transfer radical polymerization reaction. This process could bring a great number of ferrocenylmethyl methacrylate (FMMA) on the surface of electrode (signal on). The I_FMMA/I_MB value was proportionate to the microRNA-18a concentration and measured by square wave voltammetry. The promising potential of the proposed biosensor in clinical analyses was exhibited by its remarkable features such as strong performance, high specificity, agreeable storage stability, and notable selectivity in real sample evaluation with no pretreatment or amplification. Finally, our biosensing method offers such an application to be used for the early clinical diagnosis of Pancreatic Cancer.

Simultaneous and sensitive detection of two pathogenic genes of thrombotic diseases using SPRi sensor with one-step fixation probe by a poly-adenine oligonucleotide approach

Single-base mutations of Factor V Leiden G1691A and Prothrombin gene G20210A are the main genetic risk factors for inherited thrombotic tendency. The establishment for rapid and efficient detection method is of great significance to the prevention of venous thrombosis. In this work, a multiplexed, highly sensitive and regenerable surface plasmon resonance imaging (SPRi) sensor is described to identify and detect the two pathogenic genes by fixing probes in one-step. The probes are fixed by ployA, which is a simpler, faster and lower cost modification method compared with traditional thiol (-SH). PolyA-DNA-AuNPs is used to amplify the signal to improve sensitivity. The detection limit of the sensor is 8 pM, and it has a wide dynamic range between 8 pM and 100 nM and a good linear relationship between 8 pM to 50 pM. The equilibrium dissociation constant (KD) of 3.0 (± 0.3) pM indicates a high binding capacity. Based on the advantages of high-throughput detection, the SPRi chip can simultaneously identify and detect two genes related to thrombotic Diseases. In addition, more than 90% signal intensity can still be obtained on the surface of the chip after being regenerated of 25 times, indicating that this SPRi sensor has good stability and reproducibility. The established SPRi sensor has the advantages of high-throughput, high-sensitivity, label-free and no need for amplification, which is expected to become an effective technical means for real-time online detection of gene point mutations, and can be extended to detect and quantify a wider range of DNA mutation diseases.

An ultrasensitive and specific electrochemical biosensor for DNA detection based on T7 exonuclease-assisted regulatory strand displacement amplification

An electrochemical biosensor for determination of DNA is developed based on T7 exonuclease-assisted regulatory strand displacement dual recycling signal amplification strategy. First, the hairpin probe recognized and bound the target DNA to form a double strand nucleotide structure, and then the T7 exonuclease was introduced. After be digested by T7 exonuclease, the target DNA was released and entered the next cycle of T7 exonuclease-assisted recycle amplification, while accompanied by a large number of mimic targets (output DNAs) into another cycle. Second, the mimic target reacted with double-chain substrated DNA (CP) by a regulated toehold exchange mechanism, yielding the product complex of detection probes with the help of assisted DNA (S). Finally, after many cycles, a large number of detection probes were produced for binding numerous streptavidin-alkaline phosphatases. The electrochemical biosensor showed very high sensitivity and selectivity with a dynamic response ranged from 0.1 fM to 10 pM with a detection limit of 31.6 aM. Furthermore, this proposed biosensor was successfully applied to the detection of target DNA in 20% diluted serum. The developed strategy has been demonstrated to have the potential for application in molecular diagnostics.

A highly sensitive electrochemical biosensor for microRNA122 detection based on a target-induced DNA nanostructure

Specific and sensitive biomarker detection is significant for the early diagnosis of cancers. Herein, a highly sensitive electrochemical biosensor employing a tetrahedral DNA nanostructure (TDN) probe and multiple signal amplification strategies has been constructed, and successfully applied to microRNA-122 (miR-122) detection. The platform consisted of a TDN probe anchoring on a gold nanoparticle-coated gold electrode and multiple signal amplification procedures combining the electrodeposition of gold nanoparticles, hybridization chain reaction (HCR), and horseradish peroxidase enzymatic catalysis (HPEC). In the presence of the target, the hairpin structure of the helper probe could be opened and trigger the HCR through the hybridization of H1 and H2 probes, and then avidin-HRP was attached on the surface of the gold electrode that can produce an electro-catalytic signal. We used TDN probe as the scaffold to increase the reactivity and multiple signal amplification greatly improve the sensitivity of this biosensor. This biosensor offers an excellent sensitivity (a limit of detection of 0.74 aM) and differentiation ability for single and multiple mismatches. This multiplexing biosensor for trace microRNA detection shows promising applications in the early diagnosis of cancer.

Nanotechnology in emerging liquid biopsy applications

Liquid biopsy is considered as the most attractive alternative to traditional tissue biopsies. The major advantages of this approach lie in the non-invasive procedure, the rapidness of sample collection and the potential for early cancer diagnosis and real-time monitoring of the disease and the treatment response. Nanotechnology has dynamically emerged in a wide range of applications in the field of liquid biopsy. The benefits of using nanomaterials for biosensing include high sensitivity and detectability, simplicity in many cases, rapid analysis, the low cost of the analysis and the potential for portability and personalized medicine. The present paper reports on the nanomaterial-based methods and biosensors that have been developed for liquid biopsy applications. Most of the nanomaterials used exhibit great analytical performance; moreover, extremely low limits of detection have been achieved for all studied targets. This review will provide scientists with a comprehensive overview of all the nanomaterials and techniques that have been developed for liquid biopsy applications. A comparison of the developed methods in terms of detectability, dynamic range, time-length of the analysis and multiplicity, is also provided.

Label-free exonuclease I-assisted signal amplification colorimetric sensor for highly sensitive detection of kanamycin

A label-free colorimetric method based on exonuclease I (Exo I)-assisted signal amplification with protamine as a medium was developed for analysis of kanamycin. In this study, a double-stranded DNA (dsDNA) probe was tailored by manipulating an aptamer and its complementary DNA (cDNA) ensuring detection of target with high selectivity and excellent sensitivity. Herein, protamine could not only combine with negatively charged gold nanoparticles but also interaction with polyanion DNA. Upon addition of target kanamycin, the target-aptamer complex was formed and the cDNA was released. Thus, both aptamer and cDNA could be digested by Exo I, and the captured kanamycin was liberated for triggering target recycling and signal amplification. Under optimized conditions, the proposed colorimetric method realized a low detection limit of 2.8 × 10^-14 M along with a wide linear range plus excellent selectivity. Our strategy exhibited enormous potentials for fabricate various kinds of biosensors based on target-induced aptamer configuration changes.

A light-up "G-quadruplex nanostring" for label-free and selective detection of miRNA via duplex-specific nuclease mediated tandem rolling circle amplification

MicroRNA (miRNA) has emerged as a promising genetic marker for cancer diagnosis and therapy because its expression level is closely related to the progression of malignant diseases. Herein, a label-free and selective fluorescence platform was proposed for miRNA based on light-up "G-quadruplex nanostring" via duplex-specific nuclease (DSN) mediated tandem rolling circle amplification (RCA). First, a long DNA generated from upstream RCA was designed with the antisense sequences for miR-21 and downstream RCA primer. Upon recognizing miR-21, the resulting DNA-RNA permitted DSN digestion and triggered downstream two-way RCA, and generation of abundant "G-quadruplex nanostring" binding with ZnPPIX for label-free fluorescent responses. In our strategy, the strong preference of DSN for perfectly matched DNA/RNA ensures its excellent selectivity. The developed method generated wide linear response with LOD of 1.019 fM. Additionally, the miR-21 levels in cell extracts have been evaluated, revealing the utility of this tool for biomedical research and clinical diagnosis.

Electrochemical Biosensors for Detection of MicroRNA as a Cancer Biomarker: Pros and Cons

Cancer is the second most fatal disease in the world and an early diagnosis is important for a successful treatment. Thus, it is necessary to develop fast, sensitive, simple, and inexpensive analytical tools for cancer biomarker detection. MicroRNA (miRNA) is an RNA cancer biomarker where the expression level in body fluid is strongly correlated to cancer. Various biosensors involving the detection of miRNA for cancer diagnosis were developed. The present review offers a comprehensive overview of the recent developments in electrochemical biosensor for miRNA cancer marker detection from 2015 to 2020. The review focuses on the approaches to direct miRNA detection based on the electrochemical signal. It includes a RedOx-labeled probe with different designs, RedOx DNA-intercalating agents, various kinds of RedOx catalysts used to produce a signal response, and finally a free RedOx indicator. Furthermore, the advantages and drawbacks of these approaches are highlighted.

Electrochemical immunosensor based on binary nanoparticles decorated rGO-TEPA as magnetic capture and Au@PtNPs as probe for CEA detection

Using gold and magnetic nanoparticles co-decorated reduced graphene oxide-tetraethylenepentamine (rGO-TEPA/Au-MNPs) as the magnetic platform for capturing the primary antibody (Ab₁), separation and preconcentration of immunocomplex, a novel homogeneous electrochemical immunosensor was successfully developed. The newly prepared magnetic rGO-TEPA/Au-MNPs, compared with MNPs, exhibited better stability and enhanced electrical conductivity attributed to rGO-TEPA, and showed higher biorecognition efficiency due to AuNPs. In addition, Au@PtNPs were prepared and modified with secondary antibody (Ab₂) as an efficient signal probe for signal readout. Using carcinoembryonic antigen (CEA) as a model analyte, the prepared immunosensor demonstrated satisfactory properties like high stability, good repeatability and selectivity, wide linear range (5.0 pg mL^-1~200.0 ng mL^-1) as well as low detection limit (1.42 pg mL^-1). The homogenous electrochemical immunosensor was applied to the detection of CEA in human serum and was found to exhibit good correlation with the reference method. Thus, the proposed rGO-TEPA/Au-MNPs-based homogenous immunoassay platform might open up a new way for biomarker diagnosis. Graphical Abstract.

Electrochemical biosensor for miRNA-21 based on gold-platinum bimetallic nanoparticles coated 3-aminopropyltriethoxy silane

We report an electrochemical biosensor based on gold platinum bimetallic nanoparticles (AuPtBNPs)/3-aminopropyltriethoxy silane (APTS) nanocomposite coated fluorine-doped tin oxide (FTO) as a biosensing platform for hybridization-based detection of miRNA-21. Field Emission-Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and electrochemical measurements were carried out to ensure the successful construction of the biosensor. The amount of cDNA immobilized on electrode surface and hybridization time required for the miRNA-21 sensing were optimized. The biosensing platform showed detection limit of 0.63 fM with wide linear range i.e. 1 fM-100 nM for miRNA-21 detection. The biosensing strategy demonstrates a good recovery yield from 90.18% to 94.6% in serum samples. It offers good selectivity for its complementary miRNA compared to the non-complementary miRNAs. Other analytical features of the biosensor such as stability, reusability and reproducibility were also tested, providing appropriate results.

Nanobioconjugates for Signal Amplification in Electrochemical Biosensing

Nanobioconjugates are hybrid materials that result from the coalescence of biomolecules and nanomaterials. They have emerged as a strategy to amplify the signal response in the biosensor field with the potential to enhance the sensitivity and detection limits of analytical assays. This critical review collects a myriad of strategies for the development of nanobioconjugates based on the conjugation of proteins, antibodies, carbohydrates, and DNA/RNA with noble metals, quantum dots, carbon- and magnetic-based nanomaterials, polymers, and complexes. It first discusses nanobioconjugates assembly and characterization to focus on the strategies to amplify a biorecognition event in biosensing, including molecular-, enzymatic-, and electroactive complex-based approaches. It provides some examples, current challenges, and future perspectives of nanobioconjugates for the amplification of signals in electrochemical biosensing.

New trends in the development of electrochemical biosensors for the quantification of microRNAs

MicroRNAs (miRNAs) are non-coding regulatory RNAs that play an important role in RNA silencing and post-transcriptional gene expression regulation. Since their dysregulation has been associated with Alzheimer disease, cardiovascular diseases and different types of cancer, among others, miRNAs can be used as biomarkers for early diagnosis and prognosis of these diseases. The methods commonly used to quantify miRNAs are, in general, complex, costly, with limited application for point-of-care devices or resource-limited facilities. Electrochemical biosensors, mainly those based on nanomaterials, have emerged as a promising alternative to the conventional miRNA detection methods and have paved the way to the development of sensitive, fast, and low-cost detection systems. This review is focused on the most relevant contributions performed in the field of electrochemical miRNAs biosensors between 2017 and the beginning of 2020. The main contribution of this article is the critical discussion of the different amplification strategies and the comparative analysis between amplified and non-amplified miRNA electrochemical biosensing and between the different amplification schemes. Particular emphasis was given to the importance of the nanostructures, enzymes, labelling molecules, and special sequences of nucleic acids or analogues on the organization of the different bioanalytical platforms, the transduction of the hybridization event and the generation the analytical signal.

Sensing of circulating cancer biomarkers with metal nanoparticles

The analysis of circulating cancer biomarkers, including cell-free and circulating tumor DNA, circulating tumor cells, microRNA and exosomes, holds promise in revolutionizing cancer diagnosis and prognosis using body fluid analysis, also known as liquid biopsy. To enable clinical application of these biomarkers, new analytical tools capable of detecting them in very low concentrations in complex sample matrixes are needed. Metal nanoparticles have emerged as extraordinary analytical scaffolds because of their unique optoelectronic properties and ease of functionalization. Hence, multiple analytical techniques have been developed based on these nanoparticles and their plasmonic properties. The aim of this review is to summarize and discuss the present development on the use of metal nanoparticles for the analysis of circulating cancer biomarkers. We examine how metal nanoparticles can be used as (1) analytical transducers in various sensing principles, such as aggregation induced colorimetric assays, plasmon resonance energy transfer, surface enhanced Raman spectroscopy, and refractive index sensing, and (2) signal amplification elements in surface plasmon resonance spectroscopy and electrochemical detection. We critically discuss the clinical relevance of each category of circulating biomarkers, followed by a thorough analysis of how these nanoparticle-based designs have overcome some of the main challenges that gold standard analytical techniques currently face, and what new directions the field may take in the future.

An exonuclease-assisted triple-amplified electrochemical aptasensor for mucin 1 detection based on strand displacement reaction and enzyme catalytic strategy

The development of some sensitive methods for MUC1 is critical for preclinical diagnosis of tumors. In this experiment, we built a triple-amplified electrochemical aptasensor to achieve sensitive detection of MUC1, which was based on exonuclease III (Exo III)-assisted with strand displacement reaction and enzyme catalytic strategy. Firstly, with the help of Exo III, MUC1 and aptamer could be recycled during the cycle I, the single stranded DNA-1 (S-1) was produced during the process and was introduced to the hybride reaction on the electrode. Secondly, during the cycle II, strand displacement reaction was triggered on the electrode with the adding of hairpin DNA-2 (H-2). Thirdly, after the gold nanoparticles (AuNPs)-DNA-enzyme conjugates hybrided with the H-2 on the electrode, the AuNPs-DNA-enzyme conjugates could act as signal probe to produce electrochemical catalytic signal. We used the fabricated triple-amplified electrochemical aptasensor that could detect MUC1 from 0.1 pg mL^-1 to 10 ng mL^-1 with the detection limit of 0.04 pg mL^-1 under the optimized experimental conditions. The constructed triple-amplified electrochemical aptasensor could be applied in real samples determination. Besides, the strategy can be applied to detect other proteins for health monitoring.