Homogeneous electrochemical aptasensor for mucin 1 detection based on exonuclease I-assisted target recycling amplification strategy

Portable electrochemical aptasensor for highly sensitive detection of 3,3',4,4'-tetrachlorobiphenyl

Aptamer-based electrochemical sensors are frequently used as independent, surface-functionalized, passive electrodes. However, their sensitivity and detection limits become limited, particularly when the electrode area is reduced to facilitate miniaturization. A mobile phone-based microfluidic electrochemical aptamer sensing platform for 3,3',4,4'-tetrachlorobiphenyl (PCB77) detection was developed in this work. This aptamer sensor utilized Exonuclease I (Exo I) and DNA/AuNPs/horseradish peroxidase (DNA/AuNPs/HRP) nanoprobes as a merged signal amplification method, which resulted in an increase in the electrochemical sensing performance. Sensitive detection of PCB77 was accomplished by functionalizing the hierarchically structured Au@MoS2/CNTs/GO modified working/sensing electrode with the specific aptamer. The aptamer sensor was tested with different concentrations of PCB77 within the microfluidic platform. Afterward, the differential pulse voltammograms were recorded using a wireless integrated circuit device. Subsequently, the collected data was transmitted to a smartphone using Bluetooth communication. A detection limit of 0.0085 ng/L was obtained for PCB77 detection, with a detection range from 0.1 to 1000 ng/L. In addition, the detection of PCB77 in spiked water samples validated the possibility of using this aptamer sensor in a real environment, and the aptamer sensor demonstrated high selectivity in distinguishing PCB77 from other potential interfering species. The merging of electrochemical aptamer sensors with purposefully engineered microfluidic and integrated devices in this study is a novel and promising method that provides a dependable platform for on-site applications.

An electrochemical method based on CRISPR-Cas12a and enzymatic reaction for the highly sensitive detection of tumor marker MUC1 mucin

Anti-cancer therapy is crucial in cancer prevention and anti-cancer, and thus, highly sensitive methods for detecting cancer biomarkers are essential for cancer early diagnosis. Herein, an electrochemical aptamer biosensor based on the CRISPR-Cas12a system was constructed for the detection of cancer tumor biomarker MUC1 mucin. The sensitivity was significantly prompted by enzyme-catalyzed signal amplification, and the selectivity was improved by the dual recognition of the aptamer to MUC1 and crRNA-Cas12a system to the aptamer. Glucose oxidase (GOD) was loaded on the surface of magnetic Fe3O4@Au (MGNP) via probe single-stranded DNA (pDNA) with the terminal modification of mercapto (-SH) to form GOD-pDNA/MGNP. The corresponding aptamer of MUC1 (MUC1 Apt) binds to its complementary ssDNA (cDNA) to form the activator Apt/cDNA, which is specifically recognized by crRNA-Cas12a and excites the trans-cleavage function of Cas12a, thus in turn trans-cleaves pDNA and detaches GOD from the magnetic particles. The magnetic beads were separated and transferred into a glucose solution, and the oxidation current of H2O2 produced by the catalytic reaction of GOD was measured on a Pt-modified magnetically-controlled glassy carbon electrode, resulting in an indirect determination of MUC1. The current change was linear with the logarithm of MUC1 concentration in the range from 1.0 × 10-17 g mL-1 to 1.0 × 10-10 g mL-1. The detection limit was as low as 7.01 × 10-18 g mL-1. The method was applied for the detection of MUC1 in medical samples.

A Magnetically Driven Tandem Chip Enables Rapid Isolation and Multiplexed Profiling of Extracellular Vesicles

The protein phenotypes of extracellular vesicles (EVs) have emerged as promising biomarkers for cancer diagnosis and treatment monitoring. However, the technical challenges in rapid isolation and multiplexed molecular detection of EVs have limited their clinical practice. Herein, we developed a magnetically driven tandem chip to achieve streamlined rapid isolation and multiplexed profiling of surface protein biomarkers of EVs. Driven by magnetic force, the magnetic nanomixers not only act as tiny stir bars to promote mass transfer and enhance reaction efficiency of EVs, but also transport on communicating vessels of the tandem chip continuously and expedite the assay workflow. We designed cyclic surface enhancement of Raman scattering (SERS) tags to bind with target EVs and then release them by exonuclease I, eliminating steric hindrance and amplifying the SERS signal of multiple protein biomarkers on EVs. Due to the excellent assay performance, six breast cancer biomarkers were detected simultaneously on EVs using only 10 μL plasma within 1.5 h. The unweighted SUM signature offers great accuracy in discriminating breast cancer patients from healthy donors. Overall, the dynamic magnetic driving tandem chip offers a new avenue to advance the clinical application of EV-based liquid biopsy.

Base-Stacking-Driven Catalytic Hairpin Assembly: A Nucleic Acid Amplification Reaction Using Electrode Interface as a "Booster" for SARS-CoV-2 Point-of-Care Testing

Electrochemical DNA (E-DNA) biosensors based on interface-mediated hybridization reactions are promising for point-of-care testing (POCT). However, the low efficiency of target recycle amplification and the steric hindrance at the electrode interface limit their sensing performance. Herein, we propose a base-stacking-driven catalytic hairpin assembly (BDCHA), a nucleic acid amplification reaction strategy, for POCT. The introduction of the base-stacking effect in this strategy increases the thermodynamic stability of the product, thereby effectively improving the recycling efficiency. Also, it enables the interface-mediated hybridization to maintain stability with even fewer bases in the reaction-binding domain, hence minimizing DNA secondary structure formation or intertwining at the electrode surface and ameliorating the steric hindrance limitation. The introduced base-stacking effect makes the electrode serve as a "booster" by integrating the advantages of homogeneous and heterogeneous reactions, giving BDCHA an increased reaction rate of about 20-fold, compared to the conventional catalytic hairpin assembly. As a proof of concept, our BDCHA was applied in constructing a portable E-DNA biosensor for the detection of a SARS-CoV-2 N gene sequence fragment. A simple 30 min one-pot incubation is required, and the results can be readily read on a smartphone, making it portable and user-friendly for POCT.

Immobilization-free electrochemical homogeneous aptasensor for highly sensitive detection of carcinoembryonic antigen by dual amplification strategy

Electrochemical aptasensor has been widely studied, while its practical application is limited by the unavoidable variations of aptamer loading densities and low signal amplification efficiency. To overcome these restrictions, an immobilization-free and label-free electrochemical homogeneous aptasensor was constructed for carcinoembryonic antigen (CEA) assay by combining RecJ_f exonuclease-mediated target cycling strategy and rolling circle amplification technology. In this system, the pre-immobilization of aptamers or other relevant signal elements on the electrode substrate is no longer necessary, thus the electrochemical homogeneous aptasensor shows good versatility on different transducers. Moreover, the whole recognition and signal amplification process are activated instantaneously by a non-professional operation of the solution mixture. This strategy can not only increase the stability (95.1% after 30 days of storage) and reproducibility (2.12% among five independent electrodes), but also further improve the sensitivity (detection limit of fg mL^-1 level) due to the free target recognition and dual signal amplification in the homogeneous solution phase. The proposed immobilization-free electrochemical homogeneous aptasensors on different electrode substrates both achieve satisfactory results in actual sample tests, which has the potential for commercial applications and the establishment of other target platforms in the future.

Fiber SPR biosensor sensitized by MOFs for MUC1 protein detection

The concentration of tumor markers is low, which needs a highly sensitive, stable and fast detection method. In this paper, we proposed and demonstrated a U-shape fiber SPR biosensor sensitized by MOFs materials. The surface of the U-shape SPR sensor was modified with MOFs materials to enhance the sensitivity, and the nucleic acid aptamer was immobilized on the sensor surface because of the biocompatibility of MOFs materials. By the high specificity of the nucleic acid aptamer, the MUC1 protein was recognized and detected. The testing results indicate that the sensor has a logarithmic linear response in the MUC1 protein concentration detection range of 1 pg/ml-100 μg/ml, its sensitivity and detection limit are 5.33 nm/log(μg/ml) and 0.16 pg/ml respectively. After being sensitized by MOFs, the detection sensitivity of the sensor can be increased by 1.62 times，the LOD can be decreased by 0.75 times. The sensor has high sensitivity and specificity, which has broad application prospects in clinical detection of tumor markers.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

A novel detection of MicroRNA based on homogeneous electrochemical sensor with enzyme-assisted signal amplification

Rapid and sensitive detection of microRNAs is of great importance in biological researches and cancer diagnosis. Herein, we proposed a novel homogeneous electrochemical sensor to detect microRNA-21 (miRNA-21) using functionalized magnetic nanoparticles combined with enzyme-assisted signal amplification. The biotinylated capture probe (CP) labeled magnetic nanoparticles can capture miRNA-21 and introduce streptavidin-conjugated hydroxyapatite (HAP) nanoparticles. In the presence of miRNA-21, hybridization between RNA and DNA results in the formation of RNA/DNA duplexes, and then duplex-specific nuclease (DSN) cleave the duplexes to digest the capture chain and release the miRNA-21 in a loop. Meanwhile, the HAP nanoparticles strip from the magnetic nanoparticles and electrochemical signal by the reaction of HAP with molybdate is changed. The current variation before and after incubation with miRNA-21 is linearly correlated with the miRNA-21 concentration between 1 aM and 1 pM with a low detection limit (LOD) of 0.27 aM. Remarkably, the expression of miRNA-21 in human serum and different cell lysate was successfully performed, which fully demonstrates the great practical potentials in biomedical diagnostics and clinical therapeutics.

Triple Amplification Ratiometric Electrochemical Aptasensor for CA125 Based on H-Gr/SH-β-CD@PdPtNFs

A triple-amplified and ratiometric electrochemical aptasensor for CA125 was designed based on hemin-graphene/SH-β-cyclodextrin@PdPt nanoflower (H-Gr/SH-β-CD@PdPtNF) composites and an exonuclease I (Exo I)-assisted strategy. In the nanocomposite, hemin acts as an internal reference signal owing to the reversible hemin_ox/hemin_red pair. PdPtNFs can significantly improve the electron transfer rate. SH-β-CD can efficiently enrich quercetin probes through host-guest recognition and increase the second indicator signal. In the presence of CA125, due to the specific binding between the aptamer and CA125, the conformational change of dsDNA (designed by the CA125 aptamer and its complementary DNA) results in the release of quercetin embedded in dsDNA. Subsequently, the free quercetin and DNA fragments are enriched on the H-Gr/SH-β-CD@PdPtNF-modified electrode. Thus, an enhanced oxidation peak from quercetin (I_Q) and a reduced peak from hemin (I_hemin) can indicate the same biological identification event. In addition, the recycling amplification of CA125 by Exo I can effectively assist the increase of the quercetin signal. The value of I_Q/I_hemin is linear with the concentration of CA125 in the range from 6.0 × 10^-4 to 1.0 × 10³ ng/mL, and the limit of detection is 1.4 × 10^-4 ng/mL. The recovery of CA125 in human blood serum samples was from 99.2 to 104.4%. The proposed sensor is sensitive and reliable, which provides an avenue for the development of triple amplification and ratiometric signal strategies for detecting tumor markers in clinical diagnostics.

Highly sensitive detection of MUC1 by microchip electrophoresis combining with target recycling amplification and strand displacement amplification

Mucin 1 (MUC1) is usually overexpressed in a variety of malignant tumors, and quantitative analysis of MUC1 plays an important role in the early diagnosis of cancer. In this work, a highly sensitive MUC1 assay was developed by integrating microchip electrophoresis (MCE) with target recycling amplification (TRA) and strand displacement amplification (SDA). Specifically, the presence of MUC1 can trigger the exposure of the designed hairpin probe (HP) to initiate SDA and an amplified amount of ssDNA is produced finally. The amount of these ssDNA can be detected by MCE, then the concentration of MUC1 can be obtained through the correlation between MUC1 concentration and ssDNA concentration. The experimental results show that the MCE signal had a good linear relationship with MUC1 concentration in the range of 1.0 pg/mL - 1.0 × 10³ pg/mL with a low limit of detection of 0.23 pg/mL under the optimal conditions (S/N = 3). Additionally, the assay had been successfully applied to detect MUC1 in biological samples with satisfactory results, providing an alternative assay for the detection of other tumor markers owing to the high sensitivity, high selectivity, simple operation and low sample consumption.

Boronate-Affinity Cross-Linking-Based Ratiometric Electrochemical Detection of Glycoconjugates

Despite the widespread application of the boronate-affinity cross-linking (BAC) in the separation, enrichment, and sensing of glycoconjugates, it remains a huge challenge to integrate the BAC into the selective electrochemical detection of glycoconjugates due to the poor selectivity of the BAC. Herein, we demonstrate a BAC-based ratiometric electrochemical method for the simple, low-cost, and highly sensitive and selective detection of glycoconjugates. Briefly, the methylene blue (MB)-tagged nucleic acid aptamer is exploited as the recognition element to selectively capture target glycoconjugate, to which a large number of ferrocene (Fc) tags are subsequently labeled via the BAC between the phenylboronic acid (PBA) group and the cis-diol site of the oligosaccharide chains on the captured targets. Using the MB tag as the internal reference and the Fc tag as the reporter of the target capture, the dual-signal output enables the ratiometric detection. Due to the presence of a high density of the cis-diol sites on a glycoconjugate, sufficiently high sensitivity can be obtained even without using any amplification strategies. Using glycoprotein mucin 1 (MUC1) as the model target, the signal ratio (I_Fc/I_MB) exhibits good linearity over the range from 0.05 to 50 U/mL, with a detection limit of 0.021 U/mL. In addition to the high sensitivity and selectivity, the results of the analysis of MUC1 in serum samples are acceptable. By virtue of its simplicity, cost-effectiveness, and high robustness and reproducibility, this BAC-based ratiometric electrochemical method holds great promise in the highly sensitive and selective detection of glycoconjugates.

Flow-homogeneous electrochemical sensing system based on 2D metal-organic framework nanozyme for successive microRNA assay

Considering DNA-based homogeneous electrochemical assay allows identification of targets to be carried out in a homogeneous solution, it would be of significance to develop the successive homogeneous assay system in dynamic solution for rapid disease diagnosis and high-throughput bioanalysis. In homogeneous assay, the work electrodes generally have capability of DNA capture but lack signal amplification, restricting its sensitivity. Here, a flow-homogeneous sensing system was proposed to realize the successive assay of microRNA, a model biomarker. Ultrathin 2D metal-organic framework (MOF) nanozymes with thickness of about 1 nm were facilely prepared by ultrasonic approach. Due to the excellent enzyme-like activity and adsorption capacity towards single-strand DNA (ssDNA), MOF nanozymes adsorbed on electrode simultaneously played two roles of ssDNA collector and signal-amplifier. To adapt the recoverable electrode to on-line monitoring, duplex-specific nuclease-assisted circle reaction was conducted to produce the turn-on amplified signal. Flow injection device was employed to realize the recycling of electrodes and the successive microRNA assay. The assay strategy showed low limit of detection (0.12 pM, S/N = 3) for microRNA, excellent renewability and acceptable reliability for real sample assay. The established system exerts the advantages of DNA-based homogeneous electrochemical sensing strategy. This work would not only expand homogeneous electrochemical assay to successive bioassay, but also provide the possibility for practical application of homogeneous sensing strategy.

MOF-derivated MnO@C nanocomposite with bidirectional electrocatalytic ability as signal amplification for dual-signal electrochemical sensing of cancer biomarker

Dual-signal strategy has great potential in improving the accuracy and sensitivity of cancer biomarker determination. However, most sensors based on nanomaterials as signal amplification usually output single detectable signal. It is still a challenge to achieve dual-signal sensing of biomarkers with nanomaterials as signal amplification. Herein, MnO@C nanocomposite was prepared with Mn-MOF-74 as precursor by pyrolysis. It possesses bidirectional electrocatalytic ability toward both oxidation and reduction of H₂O₂ for its fully exposed crystal facets. After loading AuNPs, MnO@C@AuNPs can connect aptamer (Apt) via Au-S and then as a signal amplification for the construction of sandwich-type aptasensor for dual-signal electrochemical sensing of cancer biomarker. Thus, taking mucin 1 (MUC1) as a model system. The aptasensor has the parallel output of differential pulse voltammetry (DPV) and chronoamperometry responses based on oxidation and reduction of H₂O₂, respectively, which implemented sensitive and accurate measurements to avoid false results. The linear response ranges of 0.001 nM-100 nM (detection limit of 0.31 pM) for DPV technique and 0.001 nM-10 nM (detection limit of 0.25 pM) for chronoamperometry technique were obtained. It opens up a new way to design elegant dual-signal aptasensors with potential applications in early disease diagnosis.

Ultra-sensitive and high efficiency detection of multiple non-small cell lung cancer-related miRNAs on a single test line in catalytic hairpin assembly-based SERS-LFA strip

Accurate quantification of multiple miRNAs biomarkers in body fluid is still a challenge for early screening of cancer. Herein, by catalytic hairpin assembly as a signal amplification strategy, we designed a novel surface-enhanced Raman scattering (SERS)-lateral flow assay (LFA) strip for ultrasensitive detection of miR-21 and miR-196a-5p in non-small cell lung cancer (NSCLC) urine on a single test (T) line. 4-mercaptobenzoic acid or 5,5'-dithiobis-2-nitrobenzoic acid as Raman molecules was labeled and two hairpin DNA sequence was modified gold nanocages (GNCs) were designed as two SERS tags. Through target miRNA-triggered catalytic hairpin assembly (CHA), the double-stranded DNAs (H1-H2 complex) formed by SERS tags and the related hairpin-structured DNA sequence 2 (H2) were immobilized on a single T line of SERS-LFA strip. This generated abundant "hot spots" because of the formation of numerous H1-H2 complex thus facilitated the SERS measurement. Through this method, two kinds of miRNAs were analyzed, resulting in limits of detection of 2.08 pM and 3.31 pM for miR-21 in PBS buffer and human urine, 1.77 pM and 2.18 pM for miR-196a-5p in PBS buffer and human urine. Significantly, the SERS-LFA strip exhibited high specificity and good repeatability toward miRNAs. The whole detection time was only 30 min, which means that the high detection efficiency of the strip. The clinical feasibility of the proposed method was also evaluated by detecting the levels of miR-21 and miR-196a-5p in urine samples from NSCLC patients and healthy subjects. The developed SERS-LFA strip has wide application prospect in biomedical research, drug development and early clinical diagnosis.

Aptamer-based sensing of breast cancer biomarkers: a comprehensive review of analytical figures of merit

INTRODUCTION

Accurate determination of the aberrantly expressed biomarkers such as human epidermal growth factor receptor 2 (HER2), carcinoembryonic antigen (CEA), platelet-derived growth factor (PDGF), mucin 1 (MUC1), and vascular endothelial growth factor VEGF₁₆₅ have played an essential role in the clinical management of the breast cancer. Assessment of these cancer-specific biomarkers has conventionally relied on time-taking methods like the enzyme-linked immunosorbent assay and immunohistochemistry. However, recent development in the aptamer-based diagnostics has allowed developing tools that may substitute the conventional means of biomarker assessment in breast cancer. Adopting the aptamer-based diagnostic tools (aptasensors) to clinical practices will depend on their analytical performance on clinical samples.

AREAS COVERED

In this review, we provide an overview of the analytical merits of HER2, CEA, PDGF, MUC1, and VEGF₁₆₅ aptasensors. Scopus and Pubmed databases were searched for studies reporting aptasensor development for the listed breast cancer biomarkers in the past one decade. Linearity, detection limit, and response time are emphasized.

EXPERT OPINION

In our opinion, aptasensors have proven to be on a par with the antibody-based methods for detection of various breast cancer biomarkers. Though robust validation of the aptasensors on significant sample size is required, their ability to detect pathophysiological range of biomarkers suggest the possibility of future clinical adoption.

Multifunctional nanoparticles as optical biosensing probe for breast cancer detection: A review

Optical biosensors show attractive performance in medical sensing in the event of using different nanoparticles in their design. Owing to their unique optical characteristics and biological compatibility, gold nanoparticles (GNPs), silver nanoparticles (AgNPs), bimetallic nanoparticles and magnetic nanoparticles have been broadly implemented in making sensing tools. The functionalization of these nanoparticles with different components provides an excellent opportunity to assemble selective and sensitive sensing materials to detect various biological molecules related to breast cancer. This review summarizes the recent application of optical biosensing devices based on nanomaterials and discusses their pros and cons to improve breast cancer detection in real samples. In particular, the main constituent elements of these optical biosensors including recognition and transducer elements, types of applied nanostructures, analytical sensing procedures, sensor detection ranges and limit of detection (LOD), are expressed in detail.

DNA branch migration amplification cascades for enzyme-free and non-label aptamer sensing of mucin 1

The sensitive and quantitative analysis of mucin 1 (MUC1) is very important for the prevention and early diagnosis of cancers. In the present work, based on the mechanism of the four-way DNA branch migration cascades, we constructed a simple and effective signal amplification strategy for aptamer-based sensitive detection of MUC1. The specific binding of MUC1 to the aptamer sequence in the hairpin probe unfolds and switches its structure, triggering the formation of the DNA Holliday junction structure for cascaded branch migrations with the assistance of two fuel DNA duplexes. Importantly, a target analogue DNA complex can be generated in such processes for recycling the branch migration reactions for the production of substantial amounts of G-quadruplexes, which can bind the thioflavin T dye to show significantly intensified fluorescence for detecting MUC1 with a low detection limit of 2.8 nM without the involvement of any labels or enzymes. In addition, this detection strategy could be successfully applied to monitor the target MUC1 in diluted human serums with a high selectivity and acceptable accuracy to demonstrate its potential application for real samples with the advantages of simplicity and signal amplification capability.

The synchronization of multiple signal amplifications for label-free and sensitive aptamer-based sensing of a protein biomarker

The abnormal variation of the mucin 1 (MUC1) protein level is associated with the development of multiple cancers, and the monitoring of trace MUC1 can be useful for early disease diagnosis. Here, on the basis of the synchronization of DNA-fueled sequence recycling and dual rolling circle amplification (RCA), the establishment of a non-label and highly sensitive fluorescent aptamer-based detection strategy for the MUC1 protein biomarker is described. The target MUC1 binds the aptamer hairpin probe and causes its structure switching to release an ssDNA tail to trigger the recycling of the complex via two toehold-mediated strand displacement reactions under assistance of a fuel DNA. Such a recycling amplification leads to the formation of a partial dsDNA duplex with two primers at both ends, which cooperatively bind the circular DNA ring template to start the dual RCA to produce many G-quadruplex sequences. The protoporphyrin IX dye further associates with the G-quadruplex structures to show a dramatically elevated fluorescent signal for sensitively detecting MUC1 with a low detection limit of 0.5 pM. The established aptamer-based detecting strategy is also highly selective and can realize assay of MUC1 in diluted human serums, highlighting its potential for the detection of different protein biomarkers at low contents.

An overview of nanomaterial based biosensors for detection of Aflatoxin B1 toxicity in foods

Aflatoxin B1 (AFB1) is one of the most potent mycotoxin contaminating several foods and feeds. It suppresses immunity and consequently increases mutagenicity, carcinogenicity, teratogenicity, hepatotoxicity, embryonic toxicity and increasing morbidity and mortality. Continuous exposure of AFB1 causes liver damage and thus increases the prevalence of cirrhosis and hepatic cancer. This article was planned to provide understanding of AFB1 toxicity and provides future directions for fabrication of cost effective and user-friendly nanomaterials based analytical devices. In the present article various conventional (chromatographic & spectroscopic), modern (PCR & immunoassays) and nanomaterials based biosensing techniques (electrochemical, optical, piezoelectrical and microfluidic) are discussed alongwith their merits and demerits. Nanomaterials based amperometric biosensors are found to be more stable, selective and cost-effective analytical devices in comparison to other biosensors. But many unresolved issues about their stability, toxicity and metabolic fate needs further studies. In-depth studies are needed for development of advanced nanomaterials integrated biosensors for specific, sensitive and fast monitoring of AFB1 toxicity in foods. Integration of biosensing system with micro array technology for simultaneous and automated detection of multiple AFs in real samples is also needed. Concerted efforts are also required to reduce their possible hazardous consequences of nanomaterials based biosensors.

Recent Developments in Aptasensors for Diagnostic Applications

Aptamers are exciting smart molecular probes for specific recognition of disease biomarkers. A number of strategies have been developed to convert target-aptamer binding into physically detectable signals. Since the aptamer sequence was first discovered, a large variety of aptamer-based biosensors have been developed, with considerable attention paid to their potential applications in clinical diagnostics. So far, a variety of techniques in combination with a wide range of functional nanomaterials have been used for the design of aptasensors to further improve the sensitivity and detection limit of target determination. In this paper, the advantages of aptamers over traditional antibodies as the molecular recognition components in biosensors for high-throughput screening target molecules are highlighted. Aptamer-target pairing configurations are predominantly single- or dual-site binding; the design of recognition modes of each aptamer-target pairing configuration is described. Furthermore, signal transduction strategies including optical, electrical, mechanical, and mass-sensitive modes are clearly explained together with examples. Finally, we summarize the recent progress in the development of aptamer-based biosensors for clinical diagnosis, including detection of cancer and disease biomarkers and in vivo molecular imaging. We then conclude with a discussion on the advanced development and challenges of aptasensors.

Design Strategies for Electrochemical Aptasensors for Cancer Diagnostic Devices

Improved outcomes for many types of cancer achieved during recent years is due, among other factors, to the earlier detection of tumours and the greater availability of screening tests. With this, non-invasive, fast and accurate diagnostic devices for cancer diagnosis strongly improve the quality of healthcare by delivering screening results in the most cost-effective and safe way. Biosensors for cancer diagnostics exploiting aptamers offer several important advantages over traditional antibodies-based assays, such as the in-vitro aptamer production, their inexpensive and easy chemical synthesis and modification, and excellent thermal stability. On the other hand, electrochemical biosensing approaches allow sensitive, accurate and inexpensive way of sensing, due to the rapid detection with lower costs, smaller equipment size and lower power requirements. This review presents an up-to-date assessment of the recent design strategies and analytical performance of the electrochemical aptamer-based biosensors for cancer diagnosis and their future perspectives in cancer diagnostics.

A novel dual signal and label-free electrochemical aptasensor for mucin 1 based on hemin/graphene@PdPtNPs

A dual signal and label-free electrochemical aptasensor for mucin 1 was constructed based on hemin/graphene@PdPtNPs nanocomposite (H-Gr@PdPtNPs). Hemin attached on the graphene surface not only improves the solubility of graphene and acts as an in-situ electrochemical probe but also exhibits excellent peroxidase-like properties to electrocatalyze the reduction of H2O2. PdPtNPs also show outstanding catalytic capacity to the reduction of H2O2 and provide numerous binding sites for loading dDNA (mucin 1 aptamer and cDNA) to form the sensing interface. In the presence of mucin 1, due to the specific affinity between aptamer and mucin 1, double helix would be induced dissociation and the aptamer would be pulled off from the electrode. As a result, the electrochemical signals of hemin and H2O2 were recovered. Based on these properties, the label-free and sensitive dual signal electrochemical biosensor for mucin 1 detection has been developed. The one is differential pulse voltammetry (DPV) signal of hemin and the other is chronoamperometry signal arisen from the catalytic reduction of H2O2. The linear ranges for mucin 1 were 8.0 pg mL-1 to 80 ng mL-1 and 0.8 pg mL-1 to 80 ng mL-1 with the limit of detection 2.5 pg mL-1 and 0.25 pg mL-1 by DPV and chronoamperometry, respectively. The recovery of mucin 1 in human blood serum samples was from 95.0% to 104.2%. The detection platform does not need signal labeling which greatly reduced the sophisticated and expensive procedures. The aptasensor provide a promising strategy for the determination of mucin 1 in clinical diagnostics.

Structure-switching fluorescence aptasensor for sensitive detection of chloramphenicol

The performance of chloramphenicol aptamer, including binding thermodynamics, structure switching, and binding domain, was investigated by isothermal titration calorimetry, circular dichroism, and molecular docking. Then, a new fluorescence aptasensor was developed with signal amplification mediated by exonuclease I-catalyzed reaction and hybridization chain reaction (HCR) for chloramphenicol detection. In this system, the aptamer-binding domain is blocked by the initiator of HCR, the aptamer undergoes structure switching in the presence of chloramphenicol, and DNA dissociation occurs. The released aptamer is subsequently recognized and cleaved by Exo I to set free chloramphenicol. With the Exo I-assisted chloramphenicol recycling, an increasing number of initiators were exposed from the digestion of the initiator-aptamer complex. Then, the chain-like assembly of FAM labeled H1 and H2 through HCR was triggered by the initiator, generating a long DNA polymer. Under optimum conditions, the aptasensor exhibited a log-linear range from 0.001 to 100 nM of chloramphenicol and a detection limit of 0.3 pM. Additionally, the designed biosensing platform was applied to determine chloramphenicol in milk and lake water with high accuracy. The current approach provides a new avenue to develop sensitive aptasensors with the assistance of binding mechanism between aptamer and target compounds. Graphical abstract.

Silver nanoclusters-based fluorescent biosensing strategy for determination of mucin 1: Combination of exonuclease I-assisted target recycling and graphene oxide-assisted hybridization chain reaction

A novel label-free fluorescent biosensing strategy was described for the sensitive detection of mucin 1 (MUC1). It consisted of an M-shaped aptamer probe for exonuclease I (Exo I)-assisted target recycling (EATR) amplification, and two AgNCs-hairpin probes for graphene oxide (GO)-assisted hybridization chain reaction (HCR) amplification. Based on the specificity of aptamer-target recognition, the addition of MUC1 caused a conformational change in the M-shaped aptamer probe, which was split into a MUC1-P3 complex and a P1-P2 duplex. Exo I then catalyzed the cleavage of aptamer sequence P3 from the MUC1-P3 complex and released the target MUC1. The released target MUC1 was free to bind with a new M-shaped probe to perform EATR amplification. Furthermore, the P1-P2 duplex with three single-stranded arms can act as a primer to initiate HCR between hairpin probes AgNCs-H1 and AgNCs-H2. In the process of HCR, two AgNCs-hairpins were autonomously cross-opened, generating long linear double-stranded nanowires containing large numbers of AgNCs. These nanowires cannot be quenched by GO due to the weak affinity between the long double-stranded DNA and GO, thereby retaining a strong fluorescent signal indicative of the concentration of MUC1. With these designs, in addition to an extremely low detection limit of 0.36 fg mL^-1, the method exhibited an acceptable linear response to detect MUC1 from 1 fg mL^-1 to 1 ng mL^-1. Additionally, this method could be exerted with a high degree of success to detect MUC1 in diluted human serum with satisfactory results.

Dual-output toehold-mediated strand displacement amplification for sensitive homogeneous electrochemical detection of specie-specific DNA sequences for species identification

The determination of specie-specific DNA sequences is a key factor for identification of animal species and detection of meat adulteration. Herein, a simple homogeneous electrochemical biosensor was developed for sensitive detection of specie-specific DNA sequences from meat products based on high efficient and specific dual-output toehold-mediated strand displacement (TMSD). After incubation with target DNA, large amount of methylene blue (electrochemical signal molecule) labeled probes (MB-P) were released from preformed DNA duplex structures by the process of dual-output TMSD amplification. The free MB-P could be further digested by Exonuclease I, and the enzymatic products contain little negative charge could diffuse to the surface of indium tin oxide electrode, generating significantly electrochemical signal. As a result, the designed biosensor showed a broad dynamic range from 0.01 pM to 100 pM, with a low detection limit of 8.2 fM, and ideal selectivity and reproducibility. Meanwhile, the approach exhibited acceptable accuracy for the detection of specie-specific DNA sequences, and possessed the potential application for the identification of animal species from meat products.

Cascaded multiple recycling amplifications for aptamer-based ultrasensitive fluorescence detection of protein biomarkers

Highly sensitive detection of molecular biomarkers plays a significant role in diagnosing various types of diseases at the early stage. We demonstrated in this paper an ultrasensitive aptamer-based fluorescence method for detecting mucin 1 (MUC1) in human serum via a cascaded multiple recycling signal amplification strategy. The MUC1 target molecules present in the samples cause structure switching of the hairpin aptamer probes, which initiates three cascaded recycling cycles for the cleavage of the fluorescently quenched signal probes to recover significant fluorescence for highly sensitive detection of MUC1. The developed method has a linear range from 100 fM to 1 nM for MUC1 detection. Besides, owing to the substantial signal amplification by the integrated and cascaded recycling cycles, a low detection limit of 35 fM is achieved with high selectivity. Moreover, the monitoring of trace MUC1 in human serum can also be realized with such a method, indicating its great potential for highly sensitive detection of different disease biomarkers.

Aptamer-based approaches for in vitro molecular detection of cancer

Cancer is typically associated with abnormal production of various tumor-specific molecules known as tumor markers. Probing these markers by utilizing efficient approaches could be beneficial for cancer diagnosis. The current widely-used biorecognition probes, antibodies, suffer from some undeniable shortcomings. Fortunately, novel oligonucleotide-based molecular probes named aptamers are being emerged as alternative detection tools with distinctive advantages compared to antibodies. All of the existing strategies in cancer diagnostics, including those of in vitro detection, can potentially implement aptamers as the detecting moiety. Several studies have been performed in the field of in vitro cancer detection over the last decade. In order to direct future studies, it is necessary to comprehensively summarize and review the current status of the field. Most previous studies involve only a few cancer diagnostic strategies. Here, we thoroughly review recent significant advances on the applications of aptamer in various in vitro detection strategies. Furthermore, we will discuss the status of diagnostic aptamers in clinical trials.

A review: Recent advances in ultrasensitive and highly specific recognition aptasensors with various detection strategies

One of the most studied topics in analytical chemistry and physics is to develop bio-sensors. Aptamers are small single-stranded RNA or DNA oligonucleotides (5-25 kDa), which have advantages in comparison to their antibodies such as physicochemical stability and high binding specificity. They are able to integrate with proteins or small molecules, including intact viral particles, plant lectins, gene-regulation factor, growth factors, antibodies and enzymes. The aptamers have reportedly shown some unique characteristics, including long shelf-life, simple modification to provide covalent bonds to material surfaces, minor batch variation, cost-effectiveness and slight denaturation susceptibility. These features led important efforts toward the development of aptamer-based sensors, known as apta-sensors classified into optical, electrical and mass-sensitive based on the signal transduction mode. This review provided a number of current advancements in selecting, development criteria, and aptamers application with the focus on the effect of apta-sensors, specifically for disease-associated analyses. The review concentrated on the current reports of apta-sensors that are used for evaluating different food and environmental pollutants.

A silver nanoparticle-assisted signal amplification electrochemiluminescence biosensor for highly sensitive detection of mucin 1

A novel electrochemiluminescence (ECL) biosensor was developed in this study, which was based on the Ag-NP modified tetrahedral DNA nanostructure.

Determination of soluble CD44 in serum by using a label-free aptamer based electrochemical impedance biosensor

CD44 is a promising biomarker in the diagnosis and prognosis of malignancies. The serum CD44 level is closely related to disease progression and metastasis of malignancies. It is of great clinical significance for the detection of serum soluble CD44. In this study, a facile, label-free aptamer based electrochemical impedance sensor for serum CD44 has been proposed. The aptamer showing high affinity to CD44 was immobilized on the gold electrodes through Au-S interaction. The interaction between target CD44 and the immobilized aptamer will cause a complex structure change of the aptamer, which makes the diffusion of [Fe(CN)₆]^3-/4- toward the electrode surface easy, thus resulting in the decrease of the impedance of the system. The decreased degree of the impedance had a good linear relationship with the logarithm of the CD44 concentration in the range of 0.1-1000 ng mL^-1 with a detection limit of 0.087 ng mL^-1 (S/N = 3). The developed biosensor has been applied to detect CD44 in serum samples with satisfactory results.

An electrochemical aptasensor for analysis of MUC1 using gold platinum bimetallic nanoparticles deposited carboxylated graphene oxide

A simple electrochemical strategy has been designed for the analysis of MUC1 using electrodeposited gold platinum bimetallic nanoparticles (Au-PtBNPs) on the surface of carboxylated graphene oxide (CGO)/FTO electrode as a signal amplification platform. The carboxylic groups of CGO were activated with EDS-NHS linker and subsequently immobilized with streptavidin for further deposition of biotin labelled aptamer. All the modification steps were characterized by FE-SEM, EDS mapping, FT-IR, contact angle measurements and electrochemical methods. After incubating with target protein MUC1, the aptaelectrode produced some concentration dependent responses which were measured electrochemically by DPV assay. The prepared aptasensor exhibits wide linear range from 1 fM-100 nM with detection limit of 0.79 fM under optimal experimental conditions. The performance of this aptaelectrode was also evaluated showing good selectivity, storage stability (15 days), reproducibility and reusability (up to 3 times). Furthermore, the applicability of the aptasensor for spiked serum samples showed recovery range from 92% to 97%.

Competitive electrochemical aptasensor based on a cDNA-ferrocene/MXene probe for detection of breast cancer marker Mucin1

A competitive electrochemical aptasensor based on a cDNA-ferrocene/MXene probe is used to detect the breast cancer marker Mucin1 (MUC1). MXene (Ti₃C₂) nanosheets with excellent electrical conductivity and large specific surface area are selected as carriers for aptamer probes. The ferrocene-labeled complementary DNA (cDNA-Fc) is first bound on the surface of MXene to form a cDNA-Fc/MXene probe. Then, the MUC1 aptamer is fixed to the electrode by Au-S bonds. The sensing electrode is named Apt/Au/GCE. After the probe is complementary to the aptamer, a cDNA-Fc/MXene/Apt/Au/GCE aptasensor is fabricated. When the aptasensor is used for detection of MUC1, a competitive process happens between the cDNA-ferrocene/MXene probe and MUC1, which makes cDNA-Fc/MXene probe detach from the sensing electrode, resulting in a decrease in electrical signal. The difference in the corresponding peak current before and after the competition can be used to indicate the quantitative change in bound MUC1. The proposed competitive electrochemical aptasensor gives a wide linear range of 1.0 pM-10 μM and a low detection limit of 0.33 pM (S/N = 3), which is promising for clinical diagnosis.

Ultrasensitive electrochemiluminescence aptasensor for 8-hydroxy-2'-deoxyguanosine detection based on target-induced multi-DNA release and nicking enzyme amplification strategy

8-Hydroxy-2'-deoxyguanosine (8-OH-dG) is a principal stable marker of DNA oxidative damage. Sensitive and specific detection of 8-OH-dG is of great importance for early disease diagnosis. In this paper, we developed an electrochemiluminescence aptasensor for 8-OH-dG detection based on target induced multi-DNA release and nicking enzyme signaling amplification strategy. First, three kinds of short DNAs were aligned on the aptamers immobilized on the magnetic beads. In the presence of 8-OH-dG, the aptamer recognized and specifically bound with 8-OH-dG, leading to the release of three kinds of short DNAs and three-fold signal amplification. Then the released short DNAs hybridized with ferrocence (Fc) labeled hairpin DNA (Fc-HP) immobilized on the gold electrode to form a double strand DNA. Subsequently, nicking endonuclease (Nt.AlwI) recognized the asymmetric sequence in the dsDNA and cleaved the substrate strand (Fc-HP) into two parts, one fragments containing Fc would leave the surface of electrode. Based on the quenching effect of Fc on the electrochemiluminescence (ECL) of Ru(bpy)₃²⁺/TPA, a signal-on ECL aptasensor was developed. At the same time, three kinds of short DNAs were released again and reused to initiate the repeated cycles of hybridization-cleavage. Under double signal amplification, this aptasensor achieved a low detection of 25 fM and a wide linear range from 100 fM to 10 nM for 8-OH-dG. Besides, the amount of 8-OH-dG in urine samples derived from different people were determined with satisfactory results.

Exonuclease I-assisted fluorescent method for ochratoxin A detection using iron-doped porous carbon, nitrogen-doped graphene quantum dots, and double magnetic separation

In this paper, a fluorescent method was developed for ochratoxin A (OTA) detection that uses iron-doped porous carbon (MPC) and aptamer-functionalized nitrogen-doped graphene quantum dots (NGQDs-Apt) as probes. In this method, the adsorbance of the NGQDs-Apt on the MPC due to a π-π interaction between the aptamer and the MPC results in the quenching of the fluorescence of the NGQDs-Apt. However, since OTA interacts strongly with the aptamer, the presence of OTA leads to the detachment of the NGQDs-Apt from the MPC, resulting in the resumption of fluorescence from the NGQDs-Apt. When exonuclease I (Exo I) is also added to the solution, this exonuclease specifically digests the aptamer, leading to the release of the OTA back into the solution. This free OTA then interacts with another MPC-NGQDs-Apt system, inducing the release of more NGQDs into the solution, which enhances the fluorescent intensity compared to that of the system with no Exo I. Utilizing this behavior of OTA in the presence of NGQDs-Apt, it was possible to detect concentrations of OTA ranging from 10 to 5000 nM, with a limit of detection of 2.28 nM. Our method was tested by applying it to the detection of OTA in wheat and corn samples. This method has four advantages: (1) the magnetic porous carbon is easy to prepare, its porosity enhances its loading capacity for NGQDs, it highly efficiently quenches the fluorescence of the NGQDs, and its magnetic properties facilitate the separation of the MPC from other species in solution; (2) applying double magnetic separation decreases the background signal; (3) Exo I digests the free aptamer effectively, which allows the resulting free OTA to induce the release of more NGQDs-Apt, ultimately enhancing the fluorescent signal; and (4) the proposed method presented high sensitivity and a wide linear detection range. This method may prove helpful in food safety analysis and new biosensor development (achieved by using different aptamer sequences to that used in the present work). Graphical abstract Exonuclease I (Exo I)-assisted fluorescent method for ochratoxin A (OTA) detection using magnetic porous carbon (MPC), nitrogen-doped graphene quantum dots (NGQDs), and double magnetic separation.

A sensitive electrochemical aptasensor for Mucin 1 detection based on catalytic hairpin assembly coupled with PtPdNPs peroxidase-like activity

In this work, an ultrasensitive aptasensor for the detection of Mucin 1 (MUC1) was presented based on the target-induced catalytic hairpin assembly combined with excellent mimic peroxidase performance of PtPd bimetallic nanoparticles (PtPdNPs). Traditionally, the cyclic reuse of target protein was achieved by protein conversion with enzyme cleavage or polymerization, which is costly and complex. However, in this work, it can be performed by simple strand displacement. In addition, PtPdNPs, a mimic peroxidase, was used a probe to catalyze the oxidation of tetramethylbenzidine (TMB) by H₂O₂, leading to the electrochemical signal amplification. With this ingenious design, the prepared aptasensor for MUC1 detection showed a favorable linear response from 100 fg mL^-1 to 1 ng mL^-1 and a relatively low detection limit of 16 fg mL^-1. The proposed biosensor possessed acceptable stability, selectivity and reproducibility for MUC1 assay. Additionally, the fabricated aptasensor has been successfully applied to detect MUC1 in serum samples with satisfactory results. This new strategy supplied one efficient approach to improve signal amplification, which also open an avenue for sensitivity enhancement in targets detection.

Aptasensors as a new sensing technology developed for the detection of MUC1 mucin: A review

Mucin 1 protein (MUC1) is a membrane-associated glycoprotein overexpressed in the majority of human malignancies and considered as a predominant protein biomarker in cancers. Owing to the crucial role of MUC1 in cancer dissemination and metastasis, detection and quantification of this biomarker is of great importance in clinical diagnostics. Today, there exist a wide variety of strategies for the determination of various types of disease biomarkers, especially MUC1. In this regard, aptamers, as artificial single-stranded DNA or RNA oligonucleotides with catalytic and receptor properties, have drawn lots of attention for the development of biosensing platforms. So far, various sensitivity-enhancement techniques in combination with a broad range of smart nanomaterials have integrated into the design of novel aptamer-based biosensors (aptasensors) to improve detection limit and sensitivity of analyte determination. This review article provides a brief classification and description of the research progresses of aptamer-based biosensors and nanobiosensors for the detection and quantitative determination of MUC1 based on optical and electrochemical platforms.