Ultrasensitive electrochemical detection of miRNA-21 by using an iridium(III) complex as catalyst

An electrochemical aptasensor for the detection of bisphenol A based on triple signal amplification assisted by gold nanoparticles, hemin/G-quadruplex DNAzyme, and exonuclease I

A triple signal amplified electrochemical aptasensor for the detection of bisphenol A (BPA) was developed for the first time based on gold nanoparticles (AuNPs), hemin/G-quadruplex DNAzyme, and exonuclease I (Exo I) assisted amplification strategies. The BPA aptamer (Apt) hybridized with the capture probe (CP) was fixed on the gold electrode (GE) to form the double-stranded DNA (dsDNA) structure. When BPA was present, the Apt was detached from the GE surface by specific recognition between the BPA and Apt, forming BPA-Apt complexes in solution. The complexes could be selectively digested by Exo I, releasing BPA to participate in the cycle for binding to other Apt in dsDNA. The hybridization of the CP and auxiliary DNA (aDNA) within the detect probe DNA (dpDNA)-AuNP-aDNA nanocomplex allowed the nanocomplex to connect to the GE surface. The dpDNA interacted with K+ and hemin to produce hemin/G-quadruplex DNAzyme, which catalyzed H2O2 reduction, accelerated methylene blue (MB) oxidization, and further amplified the electrochemical signal. The integration of triple signal amplification strategies with aptamer-specific recognition enabled sensitive and specific detection of BPA. Under optimized conditions, the aptasensor exhibited a linear range of 0.1 pM-10 nM, with a low detection limit of 76 fM. Moreover, the designed aptasensor was successfully applied to detect BPA in lake water, fruit juice, and honey samples.

Minimally Invasive Real-Time Monitoring for Rapid and Sensitive Diagnosis of Spinal Cord Injury

Spinal cord injury (SCI) is a serious neurological injury that is currently extremely difficult to cure clinically. SCI involves numerous pathophysiological processes, and microRNAs (miRNAs) play an important role in these processes. Meanwhile, miRNAs have received a lot of attention for their role in other diseases as well. Therefore, the detection of disease-related miRNAs is important for the study of disease development, treatment, and prognosis. With the rapid development of molecular biology, the traditional detection methods of miRNA can no longer meet the needs of experiments. Electrochemical detection methods are widely used because of their excellent detection performance. Here, we designed an electrochemical sensor prepared using borosilicate glass microneedle electrodes for real-time monitoring of miR-21-5p expression in vivo after SCI. The sensor showed a good linear relationship between the oxidation peak current value and the concentration of miR-21-5p in the concentration range 0-2 fM (Y = 12.025X + 90.396, R2 = 0.98). The limit of detection (LOD) of the sensor was 0.3667 fM. The experimental results showed that the borosilicate glass microneedle electrochemical sensor achieved fast, accurate, highly sensitive, highly specific, highly stable, and reproducible monitoring of miR-21-5p. More importantly, the electrochemical sensor has a better clinical translation prospect, which is important for the research of clinical diseases.

A pH ultra-sensitive hydrated iridium oxyhydroxide films electrochemical sensor for label-free detection of Vibrio parahaemolyticus

Vibrio parahaemolyticus (V. parahaemolyticus) is a major foodborne pathogen, which can cause serious foodborne illnesses like diarrhoea. Rapid on-site detection of foodborne pathogens is an ideal way to respond to foodborne illnesses. Herein, we provide an electrochemical sensor for rapid on-site detection. This sensor utilized a pH-sensitive metal-oxide material for the concurrent isothermal amplification and label-free detection of nucleic acids. Based on a pH-sensitive hydrated iridium oxide oxyhydroxide film (HIROF), the electrode transforms the hydrogen ion compound generated during nucleic acid amplification into potential, so as to achieve a real-time detection. The results can be transmitted to a smartphone via Bluetooth. Moreover, HIROF was applied in nucleic acid device detection, with a super-Nernst sensitivity of 77.6 mV/pH in the pH range of 6.0-8.5, and the sensitivity showed the best results so far. Detection of V. parahaemolyticus by this novel method showed a detection limit of 1.0 × 103 CFU/mL, while the time consumption was only 30 min, outperforming real-time fluorescence loop-mediated isothermal amplification (LAMP). Therefore, the characteristics of compact, portable, and fast make the sensor more widely used in on-site detection.

Electrochemiluminescence detection of miRNA-21 based on dual signal amplification strategies: Duplex-specific nuclease -mediated target recycle and nicking endonuclease-driven 3D DNA nanomachine

DNA nanomachines have shown potential application in the construction of various biosensors. Here, an electrochemiluminescence biosensor for the sensitive detection of miRNA-21 were reported based on three-dimensional (3D) DNA nanomachine and duplex-specific nuclease (DSN)-mediated target recycle amplification strategy. First, the bipedal DNA walkers were obtained by DSN-mediated digestion reaction initiated by target miRNA-21.3D DNA tracks were prepared by modifying Fe₃O₄ magnetic beads (MBs) with ferrocene-labeled DNA (Fc-DNA). The produced DNA walkers autonomously moved along 3D DNA tracks powered by nicking endonuclease. During the movement, ferrocene-labeled DNA was cleaved, resulting in large amounts of Fc-labeled DNA fragments away from the MBs surface. Finally, the liberated Fc-labeled DNA fragments were dropped on the C-g-C₃N₄ modified electrode surface, leading to the quenching of C-g-C₃N₄ electrochemiluminescence (ECL). Benefiting from the dual amplification strategy of 3D DNA nanomachine and DSN-mediated target recycling, the developed ECL biosensor exhibited an excellent performance for miRNA-21 detection with a wide linear range of 10 fM to 10 nM and a low detection limit of 1.0 fM. This work offers a new thought for the application of DNA walkers in the construction of various biosensors.

Programmable Nanostructures Based on Framework-DNA for Applications in Biosensing

DNA has been actively utilized as bricks to construct exquisite nanostructures due to their unparalleled programmability. Particularly, nanostructures based on framework DNA (F-DNA) with controllable size, tailorable functionality, and precise addressability hold excellent promise for molecular biology studies and versatile tools for biosensor applications. In this review, we provide an overview of the current development of F-DNA-enabled biosensors. Firstly, we summarize the design and working principle of F-DNA-based nanodevices. Then, recent advances in their use in different kinds of target sensing with effectiveness have been exhibited. Finally, we envision potential perspectives on the future opportunities and challenges of biosensing platforms.

Rapid electrochemical biosensor composed of DNA probe/iridium nanoparticle bilayer for Aphanizomenon flos-aquae detection in fresh water

Toxic cyanobacteria pose a serious threat to aquatic ecosystems and require adequate detection and control systems. Aphanizomenon flos-aquae is a harmful cyanobacterium that produces the toxicant saxitoxin. Therefore, it is necessary to detect the presence of A. flos-aquae in lakes and rivers. We proposed a rapid electrochemical biosensor composed of DNA primer/iridium nanoparticles (IrNP) bilyer for the detection of A. flos-aquae in freshwater. The extracted A. flos-aquae gene (rbcL-rbcX) is used as a target, and it was fixed to the electrode using a 5'-thiolated DNA primer (capture probe). Then, Avidin@IrNPs complex for amplification of electrical signals was bound to the target through a 3'-biotinylated DNA primer (detection probe). To rapidly detect the target, an alternating current electrothermal flow technique was introduced in the detection step, which could reduce the detection time to within 20 min. To confirm the biosensor fabrication, atomic force microscopy was used to investigate the surface morphology. To evaluate the biosensor performance, cyclic voltammetry and electrochemical impedance spectroscopy were used. The target gene was detected at a concentration of 9.99 pg/mL in tap water, and the detection range was 0.1 ng/mL to 10³ ng/mL with high selectivity. Based on the combined system, we employed A. flos-aquae in tap water. This rapid cyanobacteria detection system is a powerful tool for CyanoHABs in the field.

Diagnostic kit based on halloysite nanoclay-ionic liquid nanocomposite modified electrode for electrochemical determination of cancer biomarker

Nucleic acid hybridization is occurred between the selective single-stranded nucleic acid sequence and its target sequence, which is one of the essential procedure for electrochemical detection of nucleic acid. microRNA-21 (miRNA-21) is known as a biomarker in various cancers. The determination of miRNA-21 was attained through by hybridization of inosine substituted miRNA-21 specific DNA probe (P_inosine) with its target miRNA-21. In this study, the surface of pencil graphite electrode (PGE) was firstly modified with halloysite nanoclay-ionic liquid (HNT/IL) nanocomposite. The characterization of surface was performed by Scanning Electron Microscope (SEM) images and Energy Dispersive X-Ray Analysis (EDX) analysis, and the differences at surface modifications were also shown by electrochemical methods with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). For sensitive and selective determination of miRNA-21, P_inosine and target miRNA concentration, immobilization and hybridization time were optimized by using HNT/IL modified PGE in combination with differential pulse voltammetry (DPV). The detection limit was achieved as 0.17 μg/mL (equals to 23.69 nM) in the linear range of 0.25-2 μg/mL miRNA-21. The selectivity of voltammetric method based on HNT/IL-PGE developed for miRNA-21 was examined in the presence of mismatch (MM) and non-complementary (NC) sequences. Because miRNA-21 is over-expressed in cancer cells, it has been tested in total RNA samples isolated from cancer cell line (breast cancer cell line, MCF-7). In the total RNA samples obtained from MCF-7, the detection limit was calculated as 0.28 μg/mL in the linear range of 1-4 μg/mL. Besides, the healthy cell line (human embryonic kidney cell line, HEK-293) was used as a control group and the results obtained by MCF-7 total RNA samples were compared to the results using HEK-293 total RNA samples in terms of miRNA-21 level.

G-quadruplex-selective iridium(III) complex as a novel electrochemiluminescence probe for switch-on assay of double-stranded DNA

In this work, we synthesized an iridium(III) complex and studied its selective ability to interact with a specific G-quadruplex DNA sequence (GTGGGTAGGGCGGGTTGG). Results showed that the iridium(III) complex exhibits high selectivity for the G-quadruplex DNA and could be used as an efficient electrochemiluminescence (ECL) probe in a switch-on assay format for the detection of double-stranded DNA (dsDNA). To construct the assay, a hairpin-structured capture probe (CP) which was modified by thiol at its 3' end and contained the G-quadruplex sequence at its 5' end was firstly immobilized on a gold electrode. Upon the specific recognition of the dsDNA sequence with the corresponding CP, the hairpin structure of the CP was opened to free G-quadruplex sequence, forming the G-quadruplex structure with the assistance of K⁺. Then, the iridium(III) complex was able to specifically interact with the G-quadruplex to produce an obvious ECL signal that was proportional to the dsDNA concentration. Notably, this iridium(III) complex/G-quadruplex-based strategy was universal and was not limited to the analysis of DNA using specific sequences, thus opening a new avenue for the application of the G-quadruplex-selective iridium(III) complex in the field of ECL.

Interference Reduction Biosensing Strategy for Highly Sensitive microRNA Detection

MicroRNAs are potential biomarkers for human cancers and other diseases due to their roles as post-transcriptional regulators for gene expression. However, the detection of miRNAs by conventional methods such as RT-qPCR, in situ hybridization, northern blot-based platforms, and next-generation sequencing is complicated by short length, low abundance, high sequence homology, and susceptibility to degradation of miRNAs. In this study, we developed a nicking endonuclease-mediated interference reduction rolling circle amplification (NEM-IR-RCA) strategy for the ultrasensitive and highly specific detection of miRNA-21. This method exploits the advantages of the optical properties of long-lived iridium(III) probes, in conjunction with time-resolved emission spectroscopy (TRES) and exponential rolling circle amplification (E-RCA). Under the NEM-IR-RCA-based signal enhancement processes, the limit of detection of miRNA-21 was down to 0.0095 fM with a linear range from 0.05 to 100 fM, which is comparable with the conventional RT-qPCR. Unlike RT-qPCR, the strategy was performed at a lower and constant temperature without heating/cooling cycles and reverse transcription. The strategy could clearly discriminate between matched and mismatched targets, demonstrating high specificity. Moreover, the potential application of this method was demonstrated in cancer cells and mouse serum samples, showing good agreement with RT-qPCR results. Apart from miRNA-21 detection, this platform could be also adapted for detecting other miRNAs, such as let-7a and miRNA-22, indicating its excellent potential for biomedical research and clinical diagnostics.

Non-Coding RNA-Based Biosensors for Early Detection of Liver Cancer

Primary liver cancer is an aggressive, lethal malignancy that ranks as the fourth leading cause of cancer-related death worldwide. Its 5-year mortality rate is estimated to be more than 95%. This significant low survival rate is due to poor diagnosis, which can be referred to as the lack of sufficient and early-stage detection methods. Many liver cancer-associated non-coding RNAs (ncRNAs) have been extensively examined to serve as promising biomarkers for precise diagnostics, prognostics, and the evaluation of the therapeutic progress. For the simple, rapid, and selective ncRNA detection, various nanomaterial-enhanced biosensors have been developed based on electrochemical, optical, and electromechanical detection methods. This review presents ncRNAs as the potential biomarkers for the early-stage diagnosis of liver cancer. Moreover, a comprehensive overview of recent developments in nanobiosensors for liver cancer-related ncRNA detection is provided.

Label free detection of miRNA-21 with electrolyte gated organic field effect transistors (EGOFETs)

We report a dual gate/common channel organic transistor architecture designed for quantifying the concentration of one of the strands of miRNA-21 in solution. The device allows one to measure the differential response between two gate electrodes, viz. one sensing and one reference, both immersed in the electrolyte above the transistor channel. Hybridization with oligonucleotide in the picomolar regime induces a sizable reduction of the current flowing through the transistor channel. The device signal is reported at various gate voltages, showing maximum sensitivity in the sublinear regime, with a limit of detection as low as 35 pM. We describe the dose curves with an analytical function derived from a thermodynamic model of the reaction equilibria relevant in our experiment and device configuration, and we show that the apparent Hill dependence on analyte concentration, whose exponent lies between 0.5 and 1, emerges from the interplay of the different equilibria. The binding free energy characteristic of the hybridization on the device surface is found to be approximately 20% lower with respect to the reaction in solution, hinting to partially inhibiting effect of the surface and presence of competing reactions. Impedance spectroscopy and surface plasmon resonance (SPR) performed on the same oligonucleotide pair were correlated to the electronic current transduced by the EGOFET, and confirmed the selectivity of the biorecognition probe covalently bound on the gold surface.

New insight into G-quadruplexes; diagnosis application in cancer

Biochemical properties and flexibility of nitrogenous bases allow DNA to fold into higher-order structures. Among different DNA secondary structure, G-quadruplexes (tetrapelexes-G4) - which are formed in guanine rich sequences - have gained more attention because of their biological significance, therapeutic intervention, and application in molecular device and biosensor. G4-quadruplex studies categorize into three main fields, in vivo, in vitro, and in silico. The in vitro field includes G4 synthetic oligonucleotides. This review focuses on the G-quadruplex synthetic aptamers structure features and considers the applicability of G4-aptamers for cancer biomarkers detection. Various biosensing methods will be reviewed based on G-quadruplex aptamers for cancer detection.

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.

One-step and ultrasensitive ATP detection by using positively charged nano-gold@graphene oxide as a versatile nanocomposite

A versatile nanocomposite was simply prepared based upon the electrostatic adsorption of positively charged gold nanoparticles with negatively charged graphene oxide (nano-gold@GO), and utilized as a novel fluorescence quenching platform for ultrasensitive detection of adenosine triphosphate (ATP). In the designed system, DNA-stabilized Ag nanoclusters (DNA/AgNCs) were used as fluorescent probes, DNA duplex was formed in the presence of ATP, and they can electrostatically adsorb onto the surface of nano-gold@GO to quench the fluorescence signal. Upon the addition of exonuclease III (Exo III), the DNA duplex would be hydrolyzed into DNA fragments and resulted in the recovery of the fluorescence signals due to the diffusion of AgNCs away from nano-gold@GO. Based on these, sensitive detection of ATP was realized with a detection range of 5.0 pM-20 nM. Notably, a good recovery in the range of 94-104% was obtained when detecting ATP in human serum samples, indicating a promising application value in early disease diagnosis. Graphical abstract A functional positively charged nano-gold@graphene oxide was fabricated and utilized as an enhanced fluorescence quenching platform for the detection of ATP, coupled with exonuclease III-assisted signal amplification.

Paper based colorimetric detection of miRNA-21 using Ag/Pt nanoclusters

Abnormal expression of MicroRNA-21 (miRNA-21) is considered to be a reliable biomarker for the early diagnosis of cancer. In this work, a novel paper based biosensor was fabricated to detect sub-micro molar concentrations of miRNA-21 based on peroxidase mimetic activity of DNA-templated Ag/Pt nanoclusters (DNA-Ag/Pt NCs), which could catalyze the reaction of hydrogen peroxide and 3,3',5,5' tetramethylbenzidine (TMB), to produce a blue color. The Mechanism of reaction was based on the inhibition effect of miRNA-21 on peroxidase-like activity of nanosensor which resulted to quantitative determination of miRNA-21 concentration. It was found that miRNA-21 could be linearly detected in the range from 1-700 pM (A₆₅₂ = 0.16x-0.96, R² = 0.99; x = -log [miRNA-21]) with a detection limit of 0.6 pM. Moreover, a paper assay was carried out on a Y-shaped paper-based microfluidic device in order to use the distinctive features of micro-channels such as short response time, very low reagent volume, low fabrication cost, etc. After performing paper based assay, a good linear range was observed between 10-1000 pM (y = 0.06x+147.48, R² = 0.99; x = [miRNA-21]) with detection limit of 4.1 pM. The practical application of proposed method for detection of miRNA-21 in real sample was assayed in the human urine sample and indicated the colorimetric method had acceptable accuracy.

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

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

A luminescent microRNA nanoprobe based on the target-triggered release of an iridium(III)-solvent complex from mesoporous silica nanoparticles

A luminescent microRNA nanoprobe based on the target-triggered Ir(III)-solvent complex release has been fabricated. The complex is initially embedded into mesoporous silica nanoparticles (MSNs), and then is capped by single-stranded (ss) DNA. In the presence of the target microRNA, the ssDNA hybridize with the microRNA forming a rigid DNA/RNA heteroduplexes and leaving the surface of MSN. Thus, the capped Ir(III) solvent complex is released and re-coordinated with histidine (His) to form a new luminescent complex. The luminescence intensity of the nascent complex (with excitation/emission maxima at 340/570 nm) is positively correlated with the concentrations of the target microRNA in the range from 0.05 to 2 nM, and the detection limit of microRNA is estimated as 0.2 pM (S/N = 3). The ability of this nanoprobe to detect microRNA in cell extract further demonstrates its potential in practical application. Graphical abstractSchematic of a luminescent microRNA nanoprobe based on the target-triggered release of an Ir(III)-solvent complex from mesoporous silica nanoparticles.

Construction of subtype-specific prognostic gene signatures for early-stage non-small cell lung cancer using meta feature selection methods

Feature selection in the framework of meta-analyses (meta feature selection), combines meta-analysis with a feature selection process and thus allows meta-analysis feature selection across multiple datasets. In the present study, a meta feature selection procedure that fitted a multiple Cox regression model to estimate the effect size of a gene in individual studies and to identify the overall effect of the gene using a meta-analysis model was proposed. The method was used to identify prognostic gene signatures for lung adenocarcinoma and lung squamous cell carcinoma. Furthermore, redundant gene elimination (RGE) is of crucial importance during feature selection, and is also essential for a meta feature selection process. The current study demonstrated that the proposed meta feature selection procedure with RGE outperforms that without RGE in terms of predictive ability, model parsimony and biological interpretation.

Target-assisted FRET signal amplification for ultrasensitive detection of microRNA

The recycling of target miRNA and high quenching efficiency of nanogold greatly improved the sensitivity for miRNA detection.

Smart engineering of a dual-DNA machine with a high signal-to-noise ratio for one-pot robust and sensitive miRNA signaling

A target triggered dual-DNA machine composed of RCA and SDA was developed for robust and one-pot determination of miRNA.

Applications of Gold Nanoparticles in Non-Optical Biosensors

Due to their unique properties, such as good biocompatibility, excellent conductivity, effective catalysis, high density, and high surface-to-volume ratio, gold nanoparticles (AuNPs) are widely used in the field of bioassay. Mainly, AuNPs used in optical biosensors have been described in some reviews. In this review, we highlight recent advances in AuNP-based non-optical bioassays, including piezoelectric biosensor, electrochemical biosensor, and inductively coupled plasma mass spectrometry (ICP-MS) bio-detection. Some representative examples are presented to illustrate the effect of AuNPs in non-optical bioassay and the mechanisms of AuNPs in improving detection performances are described. Finally, the review summarizes the future prospects of AuNPs in non-optical biosensors.

Biosensing methods for determination of creatinine: A review

Creatinine is a metabolic product of creatine phosphate in muscles, which provides energy to muscle tissues. Creatinine has been considered as indicator of renal function specifically after dialysis, thyroid malfunction and muscle damage. The normal level of creatinine in the serum and its excretion through urine in apparently healthy individuals is 45-140 μM and 0.8-2.0 gm/day respectively. The level of creatinine reaches >1000 μM in serum during renal, thyroid and kidney dysfunction or muscle disorder. A number of conventional methods such as colorimetric, spectrophotometric and chromatographic are available for determination of creatinine. Besides the advantages of being highly sensitive and selective, these methods have some drawbacks like time-consuming, requirement of sample pre-treatment, high cost instrumental set-up and skilled persons to operate. The sensors/biosensors overcome these drawbacks, as these are fast, easy, cost effective and highly sensitive. This review article describes the classification, operating principles, merits and demerits of various creatinine sensors/biosensors, specifically nanomaterials based biosensors. Creatinine biosensors work optimally within 2-900 s, potential range 0.1-1.0 V, pH range 4.0-10.0, temperature range 25-35 °C and had linear range, 0.004-30000 µM for creatinine with the detection limit between 0.01.01 µM and 520 µM. These biosensors measured creatinine level in sera and urine samples and had storage stability between 4 and 390 days, while being stored dry at 4 °C. The future perspective for further improvement and commercialization of creatinine biosensors are discussed.

MicroRNA sensors based on gold nanoparticles

MicroRNAs (miRNAs) are small regulatory RNAs, the dysregulation of which has been associated with the progression of several human diseases, including cancer. Interestingly, these molecules can be used as biomarkers for early disease diagnosis and can be found in a variety of body fluids and tissue samples. However, their specific properties and very low concentrations make their detection rather challenging. In this regard, current detection methods are complex, cost-ineffective, and of limited application in point-of-care settings or resource-limited facilities. Recently, nanotechnology-based approaches have emerged as promising alternatives to conventional miRNA detection methods and paved the way for research towards sensitive, fast, and low-cost detection systems. In particular, due to their exceptional properties, the use of gold nanoparticles (AuNPs) has significantly improved the performance of miRNA biosensors. This review discusses the application of AuNPs in different miRNA sensor modalities, commenting on recently reported examples. A practical overview of each modality is provided, highlighting their future use in clinical diagnosis. Graphical abstract ᅟ.

A facile DNA strand displacement reaction sensing strategy of electrochemical biosensor based on N-carboxymethyl chitosan/molybdenum carbide nanocomposite for microRNA-21 detection

Herein, we report a facile enzyme-free microRNA (miRNA) target-triggered strand displacement reaction (SDR) amplification strategy with ferrocene (Fc) as a signal molecule to fabricate a two-dimensional electroactive molybdenum carbide (Mo₂C)-based biosensor. In the presence of miRNA-21, SDR was initiated and many hairpin DNA1 (HDNA1) and hairpin DNA2 (HDNA2) duplexes, which could be captured by probe DNA leading the Fc-modified HDNA2 close to the electrode surface, were produced continuously. MiRNA-21 could be detected by monitoring the redox signal of Fc. The prepared N-carboxymethyl chitosan/Mo₂C nanocomposite featured excellent conductivity, great dispersion, and multiple functional groups (amine groups). When the nanocomposite was introduced to a miRNA biosensor electrode interface to ensure its strong connection to the DNA probe, the developed miRNA-21 biosensor demonstrated a reliable linear range of 1.0 fM to 1.0 nM with a detection limit of 0.34 fM and showed good selectivity, reproducibility, and stability. The biosensor was employed to detect miRNA-21 in human serum samples, and it showed great potential in the early clinical diagnosis of various genetic diseases.

Interaction of monohydrogensulfide with a family of fluorescent pyridoxal-based Zn(ii) receptors

We studied the reactivity of HS⁻ with a family of fluorescent zinc complexes. In the case of complexes 1 and 3, we have evidence that the interaction with HS⁻ results in the displacement of the coordinated ligand from the Zn center. For complex 2, our data points to the coordination of HS⁻ to the metal center likely assisted by hydrogen bondings with the OH of the pyridoxal moiety.

Gold-loaded nanoporous ferric oxide nanocubes for electrocatalytic detection of microRNA at attomolar level

A crucial issue in microRNA (miRNA) detection is the lack of sensitive method capable of detecting the low levels of miRNA in RNA samples. Herein, we present a sensitive and specific method for the electrocatalytic detection of miR-107 using gold-loaded nanoporous superparamagnetic iron oxide nanocubes (Au-NPFe₂O₃NC). The target miRNA was directly adsorbed onto the gold surfaces of Au-NPFe₂O₃NC via gold-RNA affinity interaction. The electrocatalytic activity of Au-NPFe₂O₃NC was then used for the reduction of ruthenium hexaammine(III) chloride (RuHex, [Ru(NH₃)₆]³⁺) bound with target miRNA. The catalytic signal was further amplified by using the ferri/ferrocyanide [Fe(CN)₆]^3-/4- system. These multiple signal enhancement steps enable our assay to achieve the detection limit of 100aM which is several orders of magnitudes better than most of the conventional miRNA sensors. The method was also successfully applied to detect miR-107 from cancer cell lines and a panel of tissue samples derived from patients with oesophageal squamous cell carcinoma with excellent reproducibility (% RSD = < 5%, for n = 3) and high specificity. The analytical accuracy of the method was validated with a standard RT-qPCR method. We believe that our method has the high translational potential for screening miRNAs in clinical samples.

In situ biosensor for detection miRNA in living cells based on carbon nitride nanosheets with catalytic hairpin assembly amplification

In this study, an ultrasensitive fluorescence turn-on assay for in situ sensing of intracellular microRNA (miRNA) was developed utilizing a carbon nitride nanosheet (CNNS) and a catalytic hairpin assembly (CHA). The CHA showed favourable signal amplification for low-level biomarkers, and CNNS was an excellent candidate as a fluorescence quencher and gene vector. Moreover, the hairpin DNA of CHA could be adsorbed onto the surface of CNNS. An enzyme-free fluorescence biosensor for ultrasensitive sensing of intracellular miRNA in cells based on CHA and CNNS was designed. When faced with target miRNA, the fluorescence was recovered due to the miRNA, which could trigger cycling of CHA circuits, leading to the production of a marked enhanced fluorescence signal. Compared with traditional methods, the proposed method is convenient, with low cytotoxicity, and high specificity and ultrasensitivity. It has promising potential for detection low-level biomarkers.

Non-Invasive Breast Cancer Diagnosis through Electrochemical Biosensing at Different Molecular Levels

The rapid and accurate determination of specific circulating biomarkers at different molecular levels with non- or minimally invasive methods constitutes a major challenge to improve the breast cancer outcomes and life quality of patients. In this field, electrochemical biosensors have demonstrated to be promising alternatives against more complex conventional strategies to perform fast, accurate and on-site determination of circulating biomarkers at low concentrations in minimally treated body fluids. In this article, after discussing briefly the relevance and current challenges associated with the determination of breast cancer circulating biomarkers, an updated overview of the electrochemical affinity biosensing strategies emerged in the last 5 years for this purpose is provided highlighting the great potentiality of these methodologies. After critically discussing the most interesting features of the electrochemical strategies reported so far for the single or multiplexed determination of such biomarkers with demonstrated applicability in liquid biopsy analysis, existing challenges still to be addressed and future directions in this field will be pointed out.