Highly sensitive and label-free electrochemical detection of microRNAs based on triple signal amplification of multifunctional gold nanoparticles, enzymes and redox-cycling reaction

Overview on the Development of Electrochemical Immunosensors by the Signal Amplification of Enzyme- or Nanozyme-Based Catalysis Plus Redox Cycling

Enzyme-linked electrochemical immunosensors have attracted considerable attention for the sensitive and selective detection of various targets in clinical diagnosis, food quality control, and environmental analysis. In order to improve the performances of conventional immunoassays, significant efforts have been made to couple enzyme-linked or nanozyme-based catalysis and redox cycling for signal amplification. The current review summarizes the recent advances in the development of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling for signal amplification. The special features of redox cycling reactions and their synergistic functions in signal amplification are discussed. Additionally, the current challenges and future directions of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling are addressed.

Fenton-like reaction triggered chemical redox-cycling signal amplification for ultrasensitive fluorometric detection of H2O2 and glucose

An ultrasensitive fluorescent biosensor is reported for glucose detection based on a Fenton-like reaction triggered chemical redox-cycling signal amplification strategy. In this amplified strategy, Cu2+ oxidizes chemically o-phenylenediamine (OPD) to generate photosensitive 2,3-diaminophenazine (DAP) and Cu+/Cu0. On the one hand, the generated Cu0 catalyzes the oxidation of OPD. On the other hand, H2O2 reacts with Cu+ to produce hydroxyl radicals (˙OH) and Cu2+ through a Cu+-mediated Fenton-like reaction. The generated ˙OH and recycled Cu2+ ions take turns oxidizing OPD to produce more photoactive DAP, triggering a self-sustaining chemical redox-cycling reaction and a remarkable fluorescent enhancement. It is worth mentioning that the cascade reaction did not stop until OPD molecules were completely consumed. Benefiting from H2O2-triggered chemical redox-cycling signal amplification, the strategy was exploited for the development of an ultrasensitive fluorescent biosensor for glucose determination. Glucose content monitoring was realized with a linear range from 1 nM to 1 μM and a limit of detection of 0.3 nM. This study validates the practicability of the chemical redox-cycling signal amplification on the fluorescent bioanalysis of glucose in human serum samples. It is expected that the method offers new opportunities to develop ultrasensitive fluorescent analysis strategy.

Cascade signal amplification strategy by coupling chemical redox-cycling and Fenton-like reaction: Toward an ultrasensitive split-type fluorescent immunoassay

An ultrasensitive split-type fluorescent immunobiosensor has been reported based on a cascade signal amplification strategy by coupling chemical redox-cycling and Fenton-like reaction. In this strategy, Cu2+ could oxidize chemically o-phenylenediamine (OPD) to generate photosensitive 2, 3-diaminophenazine (DAP) and Cu+/Cu0. On one hand, the generated Cu0 in turn catalyzed the oxidation of OPD. On the other hand, the introduced H2O2 reacted with Cu + ion to produce hydroxyl radicals (·OH) and Cu2+ ion through a Cu + -mediated Fenton-like reaction. The produced ·OH and recycled Cu2+ ion could take turns oxidizing OPD to generate more photoactive DAP, which triggering a self-sustaining chemical redox-cycling reaction and leading to a remarkable fluorescent improvement. It was worth mentioning that the cascade reaction did not stop until OPD molecules were completely consumed. Based on the H2O2-triggered cascade signal amplification, the strategy was exploited for the construction of split-type fluorescent immunoassay by taking interleukin-6 (IL-6) as the model target. It was realized for the ultrasensitive determination of IL-6 in a linear ranging from 20 fg/mL to 10 pg/mL with a limit of detection of 5 fg/mL. The study validated the practicability of the cascade signal amplification on the fluorescent bioanalysis and the superior performance in fluorescent immunoassay. It is expected that the strategy would offer new opportunities to develop ultrasensitive fluorescent methods for biosensor and bioanalysis.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Biosensors with Boronic Acid-Based Materials as the Recognition Elements and Signal Labels

It is of great importance to have sensitive and accurate detection of cis-diol-containing biologically related substances because of their important functions in the research fields of metabolomics, glycomics, and proteomics. Boronic acids can specifically and reversibly interact with 1,2- or 1,3-diols to form five or six cyclic esters. Based on this unique property, boronic acid-based materials have been used as synthetic receptors for the specific recognition and detection of cis-diol-containing species. This review critically summarizes the recent advances with boronic acid-based materials as recognition elements and signal labels for the detection of cis-diol-containing biological species, including ribonucleic acids, glycans, glycoproteins, bacteria, exosomes, and tumor cells. We also address the challenges and future perspectives for developing versatile boronic acid-based materials with various promising applications.

Streptavidin-Conjugated DNA for the Boronate Affinity-Based Detection of Poly(ADP-Ribose) Polymerase-1 with Improved Sensitivity

This work reports the development of a fluorescence method for the detection of poly(ADP-ribose) polymerase-1 (PARP1), in which a phenylboronic acid-modified fluorescein isothiocyanate dye (FITC-PBA) was used to recognize the formed poly(ADP-ribose) (PAR) polymer. The detection system was designed by conjugating recombinant streptavidin (rSA) with PARP1-specific double-stranded DNA (dsDNA) through streptavidin-biotin interaction. Capture of PARP1 via rSA-biotin-dsDNA allowed for the poly-ADP-ribosylation (PARylation) of both rSA and PARP1 in a homogeneous solution. The resulting rSA-biotin-dsDNA/PAR conjugates were then captured and separated via the commercialized nitrilotriacetic acid-nickel ion-modified magnetic bead (MB-NTA-Ni) through the interaction between NTA-Ni on MB surface and oligohistidine (His₆) tag in rSA. The PAR polymer could capture the dye of FITC-PBA through the borate ester interaction between the boronic acid moiety in PBA and the cis-diol group in ribose, thus causing a decrease in fluorescence signal. The PARylation of streptavidin and the influence of steric hindrance on PARylation efficiency were confirmed using reasonable detection strategies. The method showed a wide linear range (0.01~20 U) and a low detection limit (0.01 U). This work should be valuable for the development of novel biosensors for the detection of poly(ADP-ribose) polymerases and diol-containing species.

3-Aminophenylboronic Acid Conjugation on Responsive Polymer and Gold Nanoparticles for Qualitative Bacterial Detection

Background

Because of their sensitive and selective responses to a wide variety of analytes, colorimetric sensors have gained widespread acceptance in recent years. Gold nanoparticles (AuNPs) are widely employed in visual sensor strategies due to their high stability and ease of use. Combining AuNPs with a responsive polymer can result in distinct surface plasmon resonance (SPR) changes that can be utilized as colorimetric biosensors.

Objectives

The purpose of this research is to develop a colorimetric-based sensor through the utilization of the optical properties of gold nanoparticles (AuNPs) crosslinked with pH-responsive polymers poly (acrylic acid) (PAA) conjugated to 3-aminophenyl boronic acid (APBA).

Methods

The polymer (PAA) was synthesized via RAFT polymerization. The inversed Turkevic method was used to produce AuNPs, which were subsequently used in a self-assembly process using poly (acrylic acid)-aminophenyl boronic acid (PAA-APBA) to create the self-assembled AuNPs-APBA-PAA. The particle size, zeta potential, and reversibility of the polymer-modified gold nanoparticles were determined using a transmission electron microscope (TEM), a particle size analyzer (PSA), and an Ultraviolet-Visible spectrophotometer (UV-Vis spectrophotometer). Visual, UV-Vis spectrophotometer and TEM observations confirmed the system's ability to identify bacteria. Statistical analysis was performed using a one-way analysis of variance using Excel software.

Results

Using UV-Vis spectrophotometry, the particle size of AuNPs was determined to be 25.7 nm, and the maximum absorbance occurred at 530 nm. AuNPs PAA APBA colloid exhibited an absorbance maximum of 532 nm, a zeta potential of -41.53, and a pH transition point between 4 and 5. At E. coli concentrations of 4.5 x 107 CFU/mL, the color of the system sensors changed from red to blue after 15 hours of incubation, whereas at S. aureus concentrations of 1.2 x 109 CFU/mL, the color changed to purple immediately after mixing. The TEM confirmed that the detection mechanism is based on the boronate-polyol bonding of saccharides on the outer membranes of Escherichia coli and Staphylococcus aureus.

Conclusions

The use of APBA in conjunction with pH-responsive PAA polymers containing AuNPs to detect E. coli and S. aureus bacteria induces a maximum wavelength transition, followed by a color change from red to blue. By the process of de-swelling of the responsive polymer, which induces the aggregation of the AuNPs, the established sensor system is able to alter the color. The conjugated polymer and gold nanoparticle-based sensor system demonstrated a promising method for bacterial detection.

Gold Nanoparticles as a Biosensor for Cancer Biomarker Determination

Molecular biology applications based on gold nanotechnology have revolutionary impacts, especially in diagnosing and treating molecular and cellular levels. The combination of plasmonic resonance, biochemistry, and optoelectronic engineering has increased the detection of molecules and the possibility of atoms. These advantages have brought medical research to the cellular level for application potential. Many research groups are working towards this. The superior analytical properties of gold nanoparticles can not only be used as an effective drug screening instrument for gene sequencing in new drug development but also as an essential tool for detecting physiological functions, such as blood glucose, antigen-antibody analysis, etc. The review introduces the principles of biomedical sensing systems, the principles of nanomaterial analysis applied to biomedicine at home and abroad, and the chemical surface modification of various gold nanoparticles.

On-chip miRNA extraction platforms: recent technological advances and implications for next generation point-of-care nucleic acid tests

Circulating microRNAs (or miRNAs) in bodily fluids, are increasingly being highlighted as promising diagnostic and predictive biomarkers for a broad range of pathologies. Although nucleic acid sensors have been developed that can detect minute concentrations of biomarkers with high sensitivity and sequence specificity, their robustness is often compromised by sample collection and processing prior to analysis. Such steps either (i) involve complex, multi-step procedures and toxic chemicals unsuitable for incorporation into portable devices or (ii) are inefficient and non-standardised therefore affecting the reliability/reproducibility of the test. The development of point-of-care nucleic acid tests based on the detection of miRNAs is therefore highly dependent on the development of an automated, on-chip, sample processing platform that would enable extraction or pre-purification of the biological specimen prior to reaching the sensing platform. In this review we categorise and critically discuss the most promising technologies that have been developed to facilitate the transition of nucleic acid tests based on miRNA detection from bench to bedside.

Recent Progresses in Electrochemical DNA Biosensors for MicroRNA Detection

MicroRNAs (miRNAs), as the small, non-coding, evolutionary conserved, and post-transcriptional gene regulators of the genome, have been highly associated with various diseases such as cancers, viral infections, and cardiovascular diseases. Several techniques have been established to detect miRNAs, including northern blotting, real-time polymerase chain reaction (RT-PCR), and fluorescent microarray platform. However, it remains a significant challenge to develop sensitive, accurate, rapid, and cost-effective methods to detect miRNAs due to their short size, high similarity, and low abundance. The electrochemical biosensors exhibit tremendous potential in miRNA detection because they satisfy feature integration, portability, mass production, short response time, and minimal sample consumption. This article reviewed the working principles and signal amplification strategies of electrochemical DNA biosensors summarized the recent improvements. With the development of DNA nanotechnology, nanomaterials and biotechnology, electrochemical DNA biosensors of high sensitivity and specificity for microRNA detection will shortly be commercially accessible.

In Situ Single-Cell Stimulation and Real-Time Electrochemical Detection of Lactate Response Using a Microfluidic Probe

Metabolism of a single cell, even within the same organization, differs from other cells by orders of magnitude. Single-cell analysis provides key information for early diagnosis of cancer as well as drug screening. Any slight change in the microenvironment may affect the state of a single cell. Timely and effective cell monitoring is conducive to better understand the behavior of single cells. The immediate response of a single cell described in this study is a liquid transfer-based approach for real-time electrochemical detection. The cell was in situ stimulated by continuous flow with glucose, and lactate secreted from the cell would diffuse into the microflow. The microflow was aspirated into the detection channel where lactate was then decomposed by coupled enzyme reactions and detected by an electrode. This work provides a novel approach for detecting lactate response from a single cell by noninvasive measurements, and the position resolution of the microfluidic probe reaches the level of a single cell and permits individual heterogeneity in cells to be explored in the diagnosis and treatment of cancer as well as in many other situations.

Multimodal/Multifunctional Nanomaterials in (Bio)electrochemistry: Now and in the Coming Decade

Multifunctional nanomaterials, defined as those able to achieve a combined effect or more than one function through their multiple functionalization or combination with other materials, are gaining increasing attention in the last years in many relevant fields, including cargo targeted delivery, tissue engineering, in vitro and/or in vivo diseases imaging and therapy, as well as in the development of electrochemical (bio)sensors and (bio)sensing strategies with improved performance. This review article aims to provide an updated overview of the important advances and future opportunities exhibited by electrochemical biosensing in connection to multifunctional nanomaterials. Accordingly, representative aspects of recent approaches involving metal, carbon, and silica-based multifunctional nanomaterials are selected and critically discussed, as they are the most widely used multifunctional nanomaterials imparting unique capabilities in (bio)electroanalysis. A brief overview of the main remaining challenges and future perspectives in the field is also provided.

From bench to bedside: Biosensing strategies to evaluate endocrine disrupting compounds based on epigenetic events and their potential use in medicine

The relationship between endocrine system disorders and health risks due to chemical environmental compounds has become a growing concern in recent years. Involuntary exposure to endocrine disruptors (EDCs) is associated with the worldwide increase of diseases such as cancer, obesity, diabetes, and neurocortical disorders. EDCs are compounds that target the nuclear hormonereceptors (NHR) leading to epigenetic changes. Consequently, the use of biosensing strategies based on epigenetic events have a great potential to provide outstanding information about the exposition of EDCs and their evaluation in human health. This review addresses the novel trends in biosensing EDCs evaluation based on DNA methylation assays associated with different human diseases.

Electrochemiluminescence Biosensors Using Screen-Printed Electrodes

Electrogenerated chemiluminescence (also called electrochemiluminescence (ECL)) has become a great focus of attention in different fields of analysis, mainly as a consequence of the potential remarkably high sensitivity and wide dynamic range. In the particular case of sensing applications, ECL biosensor unites the benefits of the high selectivity of biological recognition elements and the high sensitivity of ECL analysis methods. Hence, it is a powerful analytical device for sensitive detection of different analytes of interest in medical prognosis and diagnosis, food control and environment. These wide range of applications are increased by the introduction of screen-printed electrodes (SPEs). Disposable SPE-based biosensors cover the need to perform in-situ measurements with portable devices quickly and accurately. In this review, we sum up the latest biosensing applications and current progress on ECL bioanalysis combined with disposable SPEs in the field of bio affinity ECL sensors including immunosensors, DNA analysis and catalytic ECL sensors. Furthermore, the integration of nanomaterials with particular physical and chemical properties in the ECL biosensing systems has improved tremendously their sensitivity and overall performance, being one of the most appropriates research fields for the development of highly sensitive ECL biosensor devices.

Ultrasensitive analysis of microRNAs with gold nanoparticle-decorated molybdenum disulfide nanohybrid-based multilayer nanoprobes

The nanoprobe-based signal amplification strategy is a powerful way to ultrasensitively detect biomolecules. Herein, a gold nanoparticle-decorated molybdenum disulfide (MoS₂-AuNP)-based multilayer nanoprobe (MLNP) was designed for ultrasensitive analysis of microRNA-21 (miRNA-21). The MLNP-amplified electrochemical biosensor exhibited an ultrawide dynamic range (10 aM-1 μM) and an ultralow detection limit (38 aM) for target miRNA-21 analysis. Furthermore, this biosensor can determine miRNA-21 expression in cell lysates of 100 human cervical cancer (HeLa) cells. Our results demonstrate that MoS₂-AuNP nanocomposites have great potential in constructing biosensors for target molecule analysis.

Sensitive tear screening of diabetic retinopathy with dual biomarkers enabled using a rapid electrokinetic patterning platform

Bead-based immunosensors have intrigued the scientific community over the past decades due to their rapid and multiplexed capabilities in the detection of various biological targets. Nevertheless, their use in the detection of low-abundance analytes remains a continuing challenge because of their limited number of active enrichment approaches. To this end, our research presents a delicate microbead enrichment technique using an optoelectrokinetic platform, followed by the detection of dual biomarkers for the sensitive screening of an eye disease termed diabetic retinopathy (DR). In this study, microbeads turned fluorescent as their surfaces formed sandwiched immunocomplexes in the presence of target antigens. The tiny fluorescent dots were then concentrated using the optoelectrokinetic platform for the enhancement of their signals. The signal rapidly escalated in 10 s, and the optimal limit of detection was nearly 100 pg mL^-1. For practical DR screening, two biomarkers, lipocalin 1 (LCN1) and vascular endothelial growth factor (VEGF), were used. Approximately 20 μL of analytes were collected from the tear samples of the tested patients. The concentrations of both biomarkers showed escalating trends with the severity of DR. Two concentration thresholds of LCN1 and VEGF that indicate proliferative DR were determined out of 24 clinical samples based on the receiver operating characteristic curves. For verification, a single-blind test was conducted with additional clinical tear samples from five random subjects. The final outcome of this evaluation showed an accuracy of >80%. This non-invasive screening provides a potential means for the early diagnosis of DR and may increase the screening rate among the high-risk diabetic population in the future.

Rolling circle amplification based colorimetric determination of Staphylococcus aureus

A colorimetric microplate assay for determination of Staphylococcus aureus DNA is described. Linear padlock probes were designed to recognize target sequences. After DNA binding, the linear padlock probes were circularized by ligation and then hybridize with biotin-labeled capture probes. Biotin-labeled capture probes act as primers to initiate the RCA. The biotin-labeled RCA products hybridize with digoxin-labeled signal probes fixed on streptavidin-functionalized wells of a 96-well plate. To enhance sensitivity, an AuNP-anti-digoxigenin-POx-HRP conjugate was added to the wells and then bound to digoxin-labeled signalling probes. The oxidation of tetramethylbenzidine (TMB) by H₂O₂ produces a color change from colorless to blue via HRP catalysis. After the reaction was terminated, absorbance is measured at 450 nm. For target sequences of Staphylococcus aureus, the detection limit is 1.2 pM. For genomic DNA, the detection limit is 7.4 pg.μL^-1. The potential application of the method was verified by analyzing spiked food samples. Graphical abstractSchematic representation of rolling circle amplification and functionalized AuNP-based colorimetric determination of Staphylococcus aureus. The method uses streptavidin-functionalized 96-well plates and RCA as a molecular tool and AuNP-anti-digoxigenin-POx-HRP as signal transduction markers to increase sensitivity.

Electrochemical nano-genosensor for highly sensitive detection of miR-21 biomarker based on SWCNT-grafted dendritic Au nanostructure for early detection of prostate cancer

MicroRNAs (miRNAs) appear as a novel reliable candidate in biomarkers for early diagnosis of cancer. Due to their roles in various types of cancer, their potential as a diagnostic biomarker is getting more attention. Here, a novel electrochemical biosensor for detection of miR-21 was demonstrated, through combining the advantages of electrochemical methods and nanomaterials with the selectivity of oligonucleotides, based on thiolated receptor probe-functionalized dendritic gold nanostructures (den-Au) via the self-assembly monolayer (SAM) process which grafted on the single-wall carbon nanotubes (SWCNTs) platform on the surface of the fluorine-doped tin oxide (FTO) electrode. Cadmium ions (Cd²⁺) were used as signal units and also signal amplification substance which labeled before on miR-21 target. The oxidation signal of Cd²⁺ as a signal unit was measured by differential pulse voltammetry (DPV) technique that had a very wide linear relationship with the concentration of miR-21 target (0.01 fmol L^-1 to 1 μmol L^-1) and low experimental detection limit of 0.01 fmol L^-1. Furthermore, fabricated biosensor showed acceptable performance in human serum samples and also good selectivity indiscriminate between the complementary target and non-complementary one, so this nano-genosensor can clinically be used for prostate cancer diagnosis through the detection of miR-21 in human serum samples.

Wireless bipotentiostat circuit for glucose and H2O2 interrogation

Here we present a cost-effective point-of-use wireless platform for the electrochemical detection of low concentrations of glucose and hydrogen peroxide (H₂O₂), simultaneously. The electrochemical system utilizes a dual sensor integrated with a portable bipotentiostat. The bipotentiostat hardware implements a basic designed that reduces the cost of construction and increase the affordability of the instrument, while providing similar functionality as the more expensive bench-top potentiostats. The bipotentiostat utilizes inexpensive components and common Ag/AgCl reference and platinum counter electrodes and two working electrodes, and it is designed to detect currents within the range of 20 uA - 7 mA. Additionally, the bipotentiostat is integrate with wireless module ESP8266 that interfaces with a smartphone to enable real-time monitoring and visualization of the analyte concentration levels. The results show that the selfdesigned bipotentiostat is capable of performing chronoamperometry and demonstrate an electrochemical detection system that is a portable alternative system for laboratory and point-of-use testing.

One Step Preparation of Peptide-Coated Gold Nanoparticles with Tunable Size

Gold nanoparticles (AuNPs) made from self-assembling peptides have been used in many research fields and attracted a great deal of attention due to their high stability, biocompatibility and functionality. However, existing preparation methods for peptide-coated AuNPs are post-synthesis processes, which are complicated and time consuming. Therefore, a one-step preparation method for peptide-coated AuNPs is proposed here. The AuNPs obtained by this method exhibit good stability. Importantly, peptide-coated AuNPs with precise different sizes can be prepared by this method through pH control of reducing reagent tyrosine in range of 10.0~12.7. Thus, the one-step preparation method proposed here provides a significant tool for the research in different fields concerning NP size, stability and biocompatibility.

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.

Recent advances in nanomaterial-based electrochemical and optical sensing platforms for microRNA assays

This review describes recent efforts in the application of nanomaterials as sensing elements in electrochemical and optical miRNAs assays.

Molecular methods in electrochemical microRNA detection

High-throughput profiling/sensing of nucleic acids has recently emerged as a highly promising strategy for the early diagnosis and improved prognosis of a broad range of pathologies, most notably cancer. Among the potential biomarker candidates, microRNAs (miRNAs), a class of non-coding RNAs of 19-25 nucleotides in length, are of particular interest due to their role in the post-transcriptional regulation of gene expression. Developing miRNA sensing technologies that are quantitative, ultrasensitive and highly specific has proven very challenging because of their small size, low natural abundance and the high degree of sequence similarity among family members. When compared to optical based methods, electrochemical sensors offer many advantages in terms of sensitivity and scalability. This non-comprehensive review aims to break-down and highlight some of the most promising strategies for electrochemical sensing of microRNA biomarkers.

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.

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.

Magnetic Beads-Based Sensor with Tailored Sensitivity for Rapid and Single-Step Amperometric Determination of miRNAs

This work describes a sensitive amperometric magneto-biosensor for single-step and rapid determination of microRNAs (miRNAs). The developed strategy involves the use of direct hybridization of the target miRNA (miRNA-21) with a specific biotinylated DNA probe immobilized on streptavidin-modified magnetic beads (MBs), and labeling of the resulting heteroduplexes with a specific DNA–RNA antibody and the bacterial protein A (ProtA) conjugated with an horseradish peroxidase (HRP) homopolymer (Poly-HRP40) as an enzymatic label for signal amplification. Amperometric detection is performed upon magnetic capture of the modified MBs onto the working electrode surface of disposable screen-printed carbon electrodes (SPCEs) using the H₂O₂/hydroquinone (HQ) system. The magnitude of the cathodic signal obtained at −0.20 V (vs. the Ag pseudo-reference electrode) demonstrated linear dependence with the concentration of the synthetic target miRNA over the 1.0 to 100 pM range. The method provided a detection limit (LOD) of 10 attomoles (in a 25 μL sample) without any target miRNA amplification in just 30 min (once the DNA capture probe-MBs were prepared). This approach shows improved sensitivity compared with that of biosensors constructed with the same anti-DNA–RNA Ab as capture instead of a detector antibody and further labeling with a Strep-HRP conjugate instead of the Poly-HRP40 homopolymer. The developed strategy involves a single step working protocol, as well as the possibility to tailor the sensitivity by enlarging the length of the DNA/miRNA heteroduplexes using additional probes and/or performing the labelling with ProtA conjugated with homopolymers prepared with different numbers of HRP molecules. The practical usefulness was demonstrated by determination of the endogenous levels of the mature target miRNA in 250 ng raw total RNA (RNA_t) extracted from human mammary epithelial normal (MCF-10A) and cancer (MCF-7) cells and tumor tissues.

Pyrrolidinyl PNA polypyrrole/silver nanofoam electrode as a novel label-free electrochemical miRNA-21 biosensor

A label-free electrochemical miRNA biosensor was developed based on a pyrrolidinyl peptide nucleic acid (acpcPNA)/polypyrrole (PPy)/silver nanofoam (AgNF) modified electrode. The AgNF was electrodeposited as redox indicator on a gold electrode, which was then functionalized with an electropolymerized layer of PPy, a conducting polymer, to immobilize the PNA probes. The fabrication process was investigated by electrochemical impedance spectroscopy. The biosensor was used to detect miRNA-21, a biomarker abnormally expressed in most cancers. The signal was monitored by the change in current of the AgNF redox reaction before and after hybridization using cyclic voltammetry. Two PNA probe lengths were investigated and the longer probe exhibited a better performance. Nucleotide overhangs on the electrode side affected the signal more than overhangs on the solution side due to the greater insulation of the sensing surface. Under optimal conditions, the electrochemical signal was proportional to miRNA-21 concentrations between 0.20fM and 1.0nM, with a very low detection limit of 0.20fM. The biosensor showed a high specificity which could discriminate between complementary, single-, doubled-base mismatched, and non-complementary targets. Three out of the seven tested plasma samples provided detectable concentrations (63 ± 4, 111 ± 4 and 164 ± 7fM). The sensor also showed good recoveries (81-119%). The results indicated the possibilities of this biosensor for analysis without RNA extraction and/or amplification, making the sensor potentially useful for both the prognosis and diagnosis of cancer in clinical application.