A label-free and double recognition-amplification novel strategy for sensitive and accurate carcinoembryonic antigen assay

Advancing polarity-transcendent design: Development of a photoelectrochemical sensor with extended detection range

In photoelectrochemical (PEC) sensors, traditional detection modes such as "signal-on", "signal-off", and "polarity-switchable" limit target signals to a single polarity range, necessitating novel design strategies to enhance the operational scope. To overcome this limitation, we propose, for the first time, a "polarity-transcendent" design concept that enables a continuous response across the polarity spectrum, significantly broadening the sensor's concentration detection range. This concept is exemplified in our new "background-enhanced signal-off polarity-switchable" (BESOPS) mode, where the model analyte let-7a activates a cascade shearing reaction of a DNAzyme walker in conjunction with CRISPR/Cas12a, quantitatively peeling off Cu2O-H2 strands at the Cu2O/TiO2 electrode interface to expose the TiO2 surface. This exposure generates an anodic photocurrent at the expense of the cathodic photocurrent from Cu2O/TiO2, facilitating a seamless transition of the target signal from cathodic to anodic. Through systematic experiments and comparative analyses, the BESOPS sensor demonstrates highly sensitive and precise quantification of let-7a, with a detection limit of 2.5 aM and a broad operating range of 10 aM to 10 nM. Its performance exceeds most reported sensor platforms, highlighting the significant potential of our polarity-transcendent design in expanding the operational range of PEC sensors. This innovative approach paves the way for developing next-generation PEC sensors with enhanced applicability and heightened sensitivity in various critical fields.

Triblock polyadenine-based electrochemical aptasensor for ultra-sensitive detection of carcinoembryonic antigen via exonuclease III-assisted target recycling and hybridization chain reaction

Carcinoembryonic antigen (CEA), a key colon biomarker, demands a precise detection method for cancer diagnosis and prognosis. This study introduces a novel electrochemical aptasensor using a triblock polyadenine probe for ultra-sensitive detection of CEA. The method leverages Exonuclease III (Exo III)-assisted target recycling and hybridization chain reaction. The triblock polyadenine probe self-assembles on the bare gold electrode through the strong affinity between adenine and gold electrode, blocking CEA diffusion and providing a large immobilization surface. CEA binding to hairpin probe 1 (HP1), followed by the hybridization between HP1 and hairpin probe 2 (HP2), triggers DNA cleavage by Exo III, amplifying the signal via a hybridization chain reaction and producing numerous dsDNA walkers that generates a dramatic electrochemical impedance signal. Under optimized conditions, the aptasensor achieved two ultra-low detection limits: 0.39 ag∙mL-1 within the concentration range of 5 ag∙mL-1 to 5 × 106 ag∙mL-1, and 1.5 ag∙mL-1 within the concentration range of 5 × 106 ag∙mL-1 to 1 × 1010 ag∙mL-1. Its performance in human serum samples meets the practical standards, offering a promising new tool for ultrasensitive tumor marker detection, potentially revolutionizing early cancer diagnosis.

Relay-type sensing mode: A strategy to push the limit on nanomechanical sensor sensitivity based on the magneto lever

Ultrasensitive molecular detection and quantization are crucial for many applications including clinical diagnostics, functional proteomics, and drug discovery; however, conventional biochemical sensors cannot satisfy the stringent requirements, and this has resulted in a long-standing dilemma regarding sensitivity improvement. To this end, we have developed an ultrasensitive relay-type nanomechanical sensor based on a magneto lever. By establishing the link between very weak molecular interaction and five orders of magnitude larger magnetic force, analytes at ultratrace level can produce a clearly observable mechanical response. Initially, proof-of-concept studies showed an improved detection limit up to five orders of magnitude when employing the magneto lever, as compared with direct detection using probe alone. In this study, we subsequently demonstrated that the relay-type sensing mode was universal in application ranging from micromolecule to macromolecule detection, which can be easily extended to detect enzymes, DNA, proteins, cells, viruses, bacteria, chemicals, etc. Importantly, we found that, sensitivity was no longer subject to probe affinity when the magneto lever was sufficiently high, theoretically, even reaching single-molecule resolution.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (experimental section) is available in the online version of this article at 10.1007/s12274-022-5049-0.

Nucleic acid isothermal amplification-based soft nanoarchitectonics as an emerging electrochemical biosensing platform

The emergence of nucleic acid isothermal amplification strategies based on soft nanoarchitectonics offers a new dimension to the traditional electrochemical technique, particularly because of its flexibility, high efficiency, and increased sensitivity for analytical applications. Various DNA/RNA isothermal amplification strategies have been developed for the design and fabrication of new electrochemical biosensors for efficient and important biomolecular detection. Herein, we provide an overview of recent efforts in this research field and the strategies for signal-amplified sensing systems, with their biological applications, current challenges and prospects in this promising new area.

An electrochemical aptasensor for highly sensitive detection of CEA based on exonuclease III and hybrid chain reaction dual signal amplification

At present, carcinoembryonic antigen (CEA) is considered a broad-spectrum cancer biomarker, and its accurate analysis in clinical samples can assist early cancer diagnosis and treatment. Herein, a novel electrochemical aptasensor has been proposed for CEA detection based on exonuclease III and hybrid chain reaction. The target CEA specifically binds to the aptamer region in hairpin probe 1 (defined as H1) by strong attraction, which leads the rest of the H1 triggering catalytic hairpin assembly to form a high quantity of H1 and hairpin probe 2 (defined as H2) double chain complex (denoted as H1@H2). Subsequently, the exonuclease III digests the complex of H1@H2 and liberates H1 to induce the first signal amplification. Simultaneously, a large number of generated trigger chains initiate a hybrid chain reaction and produce a second signal amplification. This proposed sensor exhibited excellent analytical performance for the detection of CEA, with wide linear range from 10 pg.mL-1 to 100 ng.mL-1 and low limit of detection of 0.84 pg.mL-1. Additionally, the biosensing strategy was successfully verified for direct measurement of CEA in human serum. Therefore, this elaborated sensor provides a new simple method for detecting CEA and exhibits great promise in the early screening of cancer.

DNA Tetrahedron-Based MNAzyme for Sensitive Detection of microRNA with Elemental Tagging

Heterogeneous immunoassay based on magnetic separation is commonly used in inductively coupled plasma-mass spectrometry (ICP-MS)-based biomedical analysis with elemental labeling. However, the functionalized magnetic beads (MBs) often suffer from non-specific adsorption and random distribution of the functional probes. To overcome these problems, DNA tetrahedron (DT)-functionalized MBs were designed and further conjugated with substrate modified Au NPs (Sub-AuNP). Based on the prepared MB-DT-AuNP probes, an MB-DT based multicomponent nucleic acid enzyme (MNAzyme) system involving Au NPs as the elemental tags was proposed for highly sensitive quantification of miRNA-155 by ICP-MS. Target miRNA would trigger the assembly of MNAzyme, and Sub-AuNP would be cleaved from the MB-DT-AuNP probe, resulting in a cyclic amplification. Single-stranded DNA-functionalized MB (MB-ssDNA)-AuNP probes were prepared as well. Comparatively, the amount of Au NPs grafted onto MB-ssDNA-AuNP probes was higher than that grafted onto MB-DT-AuNP probes. Meanwhile, a higher signal-to-noise ratio was obtained by using MB-DT-AuNP probes over MB-ssDNA-AuNP probes in the MNAzyme system. Under the optimal experimental conditions, the limit of detection for target miRNA obtained by using MB-DT-AuNP probes was 1.15 pmol L^-1, improved by 23 times over that obtained by the use of MB-ssDNA-AuNP probes. The proposed MB-DT-MNAzyme-ICP-MS method was applied to the analysis of miRNA-155 in serum samples, and recoveries of 86.7-94.6% were obtained. This method is featured with high sensitivity, good specificity, and simple operation, showing a great application potential in biomedical analysis.

Electrochemistry/Photoelectrochemistry-Based Immunosensing and Aptasensing of Carcinoembryonic Antigen

Recently, electrochemistry- and photoelectrochemistry-based biosensors have been regarded as powerful tools for trace monitoring of carcinoembryonic antigen (CEA) due to the fact of their intrinsic advantages (e.g., high sensitivity, excellent selectivity, small background, and low cost), which play an important role in early cancer screening and diagnosis and benefit people's increasing demands for medical and health services. Thus, this mini-review will introduce the current trends in electrochemical and photoelectrochemical biosensors for CEA assay and classify them into two main categories according to the interactions between target and biorecognition elements: immunosensors and aptasensors. Some recent illustrative examples are summarized for interested readers, accompanied by simple descriptions of the related signaling strategies, advanced materials, and detection modes. Finally, the development prospects and challenges of future electrochemical and photoelectrochemical biosensors are considered.

The TDs/aptamer cTnI biosensors based on HCR and Au/Ti3C2-MXene amplification for screening serious patient in COVID-19 pandemic

The accurate assay of cardiac troponin I (cTnI) is very important for acute myocardial infarction (AMI), it also can be employed as an effective index for screening serious patients in COVID-19 pandemic before fatal heart injury to reduce the mortality. A ratiometric sensing strategy was proposed based on electrochemiluminescent (ECL) signal of doxorubicin (Dox)-luminol or the electrochemical (EC) signal of methylene blue (MB) vs. referable EC signal of Dox. The bio-recognitive Tro4-aptamer ensures the high specificity of the sensor by affinity binding to catch cTnI, and the tetrahedral DNA (TDs) on Au/Ti₃C₂-MXene built an excellent sensing matrix. An in situ hybrid chain reaction (HCR) amplification greatly improved the sensitivity. The ratiometric sensing responses ECL_Dox-luminol/Current_Dox or Current_MB/Current_Dox linearly regressed to cTnI concentration in the range of 0.1 fM-1 pM or 0.1 fM-500 fM with the limit of detection (LOD) as 0.04 fM or 0.1 fM, respectively. Served as the reference signal, Current_Dox reflected the variation of sensor, it is very effective to ensure the accuracy of detection to obviate the false results. The proposed biosensors show good specificity, sensitivity, reproducibility and stability, have been applied to determine cTnI in real samples with satisfactory results. They are worth looking forward to be used for screening serious patient of COVID-19 to reduce the mortality, especially in mobile cabin hospital.

The importance of precision medicine in modern molecular oncology

With the rapid development of modern medical technology, information data modeling has been gradually applied to clinical diagnosis and treatment. Precision medicine is an important approach that focuses on individual patients in terms of their own characteristics, genomic information, proteomics and even social environments. Genome-wide high-throughput technologies, including DNA-seq, RNA-seq, exosome-seq…, contribute enormous amounts of molecular data to aid in diagnosis and analysis. Here, we review the developmental history of different next-generation sequencing platforms, introduce their applications in different tumor diagnosis and therapy, and further discuss the remaining challenges in precision medicine.

Novel electrochemical biosensor based on Exo III-assisted digestion of dsDNA polymer from hybridization chain reaction in homogeneous solution for CYFRA 21-1 DNA assay

A novel electrochemical biosensing strategy was proposed to detect cytokeratin fragment antigen 21-1 (CYFRA 21-1) DNA based on Exo III-assisted digestion of dsDNA polymer (EADDP) from hybridization chain reaction (HCR). Primarily, the presence of target can drive a catalytic hairpin assembly (CHA) reaction, which was aimed to achieve target recognition and circulation. Then the HCR can be triggered for further signal amplification and generate long dsDNA polymer with signal tags. Subsequently, the introduction of Exo III can digest the long dsDNA polymer to produce large amounts of double signal fragments (DSFs). The above experiments were all carried out in homogeneous solution. Finally, the released DSF can be captured onto the electrode directly by capture probe (CP) and a highly amplified electrochemical signal can be detected. The EADDP in homogeneous solution circumvented complex solid-liquid interface reaction and tedious operation steps on electrode. Besides, one target can be converted into abundant DSFs, which greatly improved the sensitivity. This biosensor exhibited a low detection limit (0.0348 fM) and wide linear range (5 fM ∼ 50 nM) for CYFRA 21-1 DNA biosensing with reliable specificity and stability.

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.

Label free, electric field mediated ultrasensitive electrochemical point-of-care device for CEA detection

Developing point-of-care (PoC) diagnostic platforms for carcinoembryonic antigen detection is essential. However, thefew implementations of transferring the signal amplification strategies in electrochemical sensing on paper-based platforms are not satisfactory in terms of detection limit (LOD). In the quest for pushing down LOD, majority of the research has been targeted towards development of improved nanostructured substrates for entrapping more analyte molecules and augmenting the electron transfer rate to the working electrode. But, such approaches have reached saturation. This paper focuses on enhancing the mass transport of the analyte towards the sensor surface through the application of an electric field, in graphene-ZnO nanorods heterostructure. These hybrid nanostructures have been deposited on flexible polyethylene terephthalate substrates with screen printed electrodes for PoC application. The ZnO nanorods have been functionalized with aptamers and the working sensor has been integrated with smartphone interfaced indigenously developed low cost potentiostat. The performance of the system, requiring only 50 µl analyte has been evaluated using electrochemical impedance spectroscopy and validated against commercially available ELISA kit. Limit of detection of 1 fg/ml in human serum with 6.5% coefficient of variation has been demonstrated, which is more than three orders of magnitude lower than the existing attempts on PoC device.

Immunoassay-aptasensor for the determination of tumor-derived exosomes based on the combination of magnetic nanoparticles and hybridization chain reaction

The detection of tumor-related exosomes is of great significance. In this work, a fluorescence aptasensor was designed for the determination of tumor-related exosomes based on the capture of magnetic nanoparticles (MNPs) and specific recognition of an aptamer. MNPs were used as substrates to capture the exosomes by modifying the CD63 antibody on the MNP surface. Probe 1 consists of PDL-1 aptamer sequence and a section of other sequences. PDL-1 expression was observed on the surface of exosomes; the aptamer of PDL-1 could combine with PDL-1 with high affinity. Thus, the immunoassay-type compounds of "MNPs-exosomes-probe 1" were formed. The other section of probe 1 triggered the HCR with probe 2 and probe 3 and formed the super-long dsDNA. The addition of GelRed resulted in the generation of an amplified fluorescence signal. The proposed design demonstrated a good linearity with the exosome concentration ranging from 300 to 107 particles per mL and with a low detection limit of 100 particles per mL. This aptasensor also exhibited high specificity for tumor-related exosomes, and was successfully applied in biological samples.

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.

Grafting homogenous electrochemical biosensing strategy based on reverse proximity ligation and Exo III assisted target circulation for multiplexed communicable disease DNA assay

Rapid and effective diagnosis of communicable disease is one of the critical issues of the modern society, especially for detecting different targets at the same time. In this work, a grafting homogenous electrochemical biosensing strategy is proposed by integrating of reverse proximity ligation and exonuclease III (Exo III) assisted target circulation to analyze hepatitis B (HBV) and human immunodeficiency (HIV). Specially, a two-wing nanodevice (TWD) with two detection paths is elaborately designed based on analogous proximity ligation assay. The reverse proximity ligation process provides a new way of signal conversion and amplification, what accomplished by demolishing the TWD in the presence of targets. Meanwhile, a vast number of signal probes are released via Exo III assisted target circulation. Then the signal probes are grafted on the universal sensing interface, which is decorated with graftable tetrahedron DNA (GTD). These lead to a highly amplified electrochemical signal. Compared with the conventional strategies, the grafting homogenous electrochemical biosensing strategy not only achieves convenient sensitive detection of multiple communicable diseases DNA simultaneously, but also performs well in the detection of sole target. This strategy effectively decreases the background, homogenizes the distribution of probes, and avoids the complex and time-consuming modification process of the working electrode, which holds great potential application in early diagnosis for communicable disease in the future.

Highly sensitive detection of carcinoembryonic antigen using copper-free click chemistry on the surface of azide cofunctionalized graphene oxide

In this study, we reported a highly sensitive method for detecting carcinoembryonic antigen (CEA) based on an azide cofunctionalized graphene oxide (GO-N₃) and carbon dot (CDs) biosensor system. Carbon dots-labeled DNA (CDs-DNA) combined with GO-N₃ using copper-free click chemistry (CFCC), which quenched the fluorescence of the CDs via fluorescence resonance energy transfer (FRET). Upon the addition of CEA, fluorescence was recovered due to the combination of CEA and aptamer. Under optimal conditions, the relative fluorescence intensity was linear with CEA concentration in the range of 0.01-1 ng/mL (R² = 0.9788), and the limit of detection (LOD) was 7.32 pg/mL (S/N = 3). This biosensor had a high sensitivity and good selectivity for CEA detection in serum samples, indicating that the novel sensor platform holds a great potential for CEA and other biomarkers in practical applications.

Dually enhanced homogenous synthesis of molybdophosphate by hybridization chain reaction and enzyme nanotags for the electrochemical bioassay of carcinoembryonic antigen

A magnetic bead (MB)-based sandwich biorecognition reactions is combined with a gold nanoprobe-induced homogenous synthesis of molybdophosphate to develop a novel bioassay method for the electrochemical detection of the tumor biomarker of carcinoembryonic antigen (CEA). The nanoprobe is prepared through the specific loading of numerous alkaline phosphatase (ALP)-functionalized gold nanoparticles (Au NPs) on a double-stranded DNA (dsDNA) produced by the CEA aptamer-triggered hybridization chain reaction (HCR). Both the large amounts of PO₄^3- produced by the ALP catalytic hydrolysis of pyrophosphate and the phosphate backbones of dsDNA can react with the added MoO₄^2- to generate electroactive molybdophosphates. So, the gold nanoprobe was used for signal tracing of the sandwich bioassay of CEA at a constructed antibody-functionalized MB platform. The sensitive electrochemical measurement of molybdophosphate produced from the quantitatively captured nanoprobes at a carbon nanotube-modified electrode (measured at about 0.12 V vs. Ag/AgCl, 3 M KCl) enabled the convenient signal transduction of the method. Due to the dually enhanced synthesis of molybdophosphate by the HCR and multi-enzyme Au NP nanotags, this method shows a wide linear range from 0.05 pg mL^-1 to 10 ng mL^-1 along with a low detection limit of 0.027 pg mL^-1. In addition, the MB-based biorecognition reaction and the homogeneous synthesis of molybdophosphate are much convenient in manipulations. These excellent performances decide the extensive application potentials of the method. Graphical abstract A magnetic bead-based bioassay method was simply developed for the electrochemical detection of carcinoembryonic antigen. The dually enhanced homogenous synthesis of molybdophosphate by hybridization chain reaction (HCR) and enzyme nanotags and the sensitive electrochemical measurement of molybdophosphate at a carbon nanotube (CNT)-electrode enable ultrasensitive signal transduction of the method.

Electrochemical aptasensors for cancer diagnosis in biological fluids - A review

The tunability of SELEX procedure is an essential feature to supply bioaffinity receptors (aptamers) almost on demand for analytical and therapeutic purposes. This longstanding ambition is, however, not straightforward. Non-invasive cancer diagnosis, so called liquid biopsy, requires collection of body fluids with minimal or no sample pretreatment. In those raw matrices, aptamers must recognize minute amounts of biomarkers that are not unique entities but large sets of variants evolving with the disease stage. The susceptibility of aptasensors to assay conditions has driven the selection of aptamers to natural environments to ensure their optimum performance in clinical samples. We present herein a compilation of the SELEX procedures in natural milieus. By revising the electrochemical aptasensors applied to clinical samples for cancer diagnosis and tracing back to the original SELEX we analyze whether aptamers raised using these SELEX strategies are being incorporated to the diagnostic devices and how aptasensors are finding their way to a market dominated by antibody-based assays.

Sensitive detection of carcinoembryonic antigen (CEA) by a sandwich-type electrochemical immunosensor using MOF-Ce@HA/Ag-HRP-Ab2 as a nanoprobe

Sandwich-type electrochemical immunosensor was one of the main methods for detecting carcinoembryonic antigen (CEA). In this work, using Ce-MoF as the skeleton precursor, hyaluronic acid (HA) was coated on the surface of Ce-metal organic framework (Ce-MoF), which loaded with silver nanoparticles (Ag NPs) and horseradish peroxidase (HRP) to catalyze H₂O₂ and double amplified the current signal. Thus, a sensitive sandwich-type electrochemical immunosensor (Ce-MoF@ HA/Ag-HRP) was designed to detect carcinoembryonic antigen (CEA). The designed immunosensor used Au NPs to enhance the ability of attach more the first antibody (Ab₁). This was due to Au NPs had good electrical conductivity and biocompatibility to accelerate electron transfer on the surface of the electrode. HA was riched in -COOH, -OH and had excellent biocompatibility, which can carry more Ag NPs to catalyze H₂O₂. Finally, the prepared sandwich-type electrochemical immunosensor had excellent biocompatibility and great catalytic performance. The immunosensor can be tested within 30 min and the logarithm of the current signal and CEA concentration showed a broad linear response range of 1 pg ml^-1-80 ng ml^-1, and the detection limit of CEA was 0.2 pg ml^-1. More importantly, the proposed immunosensor had good reproducibility, selectivity, stability and without matrix effect. This confirmed that the proposed immunosensor had broad prospects in early clinical trials.

The Translational Potential of Electrochemical DNA-Based Liquid Biopsy

Latest technological advancement has tremendously expanded the knowledge on the composition of body fluids and the cancer-associated changes, which has fueled the replacement of invasive biopsies with liquid biopsies by using appropriate specific receptors. DNA emerges as a versatile analytical reagent in electrochemical devices for hybridization-based or aptamer-based recognition of all kind of biomarkers. In this mini review, we briefly introduce the current affordable targets (tumor-derived nucleic acids, circulating tumor cells and exosomes) in body fluids, and then we provide an overview of selected electrochemical methods already applied in clinical samples by dividing them into three large categories according to sample type: red (blood), yellow (urine), and white (saliva and sweat) diagnostics. This review focuses on the hurdles of the complex matrices rather than a comprehensive and detailed revision of the format schemes of DNA-based electrochemical sensing. This diverse perspective compiles some challenges that are often forgotten and critically underlines real sample analysis or clinical validation assays. Finally, the needs and trends to reach the market are briefly outlined.

Construction of electrochemical aptasensor of carcinoembryonic antigen based on toehold-aided DNA recycling signal amplification

Carcinoembryonic antigen (CEA), serves as a broad-spectrum tumor marker, and plays an important role in reflecting the existence, therapeutic evaluation, development, monitoring and prognosis of many types of cancer. An electrochemical aptasensor was designed for CEA detection based on toehold-aided DNA recycling. A partially hybridized Probe-4 (i.e. P2/P3/P4) was self-assembled on the surface of a gold electrode serving as the sensing platform. For CEA detection, CEA can bind with aptamer and free probe-1 (P1) can hybridize with P4, triggering toehold-aided DNA recycling. This enables the hybridization of more probe-5 (P5) (labeled with methylene blue (MB)) with P4, causing more methylene blue (MB) to be brought close to the electrode surface. An amplified current signal was thus generated due to more MB in the electrode surface. The proposed design showed good linearity between current response and log CEA concentration ranging from 0.1 to 50 ng·mL^-1, with a detection limit of 20 pg mL^-1. This aptasensor also showed high specificity for CEA detection, and was successfully used in spiked biological samples.

Sandwich-type electrochemical immunosensor for carcinoembryonic antigen detection based on the cooperation of a gold-vertical graphene electrode and gold@silica-methylene blue

In this study, a sandwich-type electrochemical (EC) immunosensor was proposed to detect a carcinoembryonic antigen (CEA) based on Au-graphene and Au@SiO₂-methylene blue (MB). The Au nanoparticles (NPs)-vertical graphene (VG) electrode efficiently amplifies the response signal by immobilizing a large amount of the coating antibody (Ab) and is characterized by excellent electrocatalytic activity. The MB nanodot-loaded Au@SiO₂ carriers with core-shell nanostructure and detection Ab were used to construct the Ab-Au@SiO₂-MB label, which improved the sensitivity due to the high EC signal of MB nanodots and the high labeling effect between the detection Ab and MB probe. A novel double-Ab sandwich strategy was developed to further improve the sensitivity and stability based on the same specificity of the coating and detection Abs for the recognition of CEA. Under optimal conditions, the developed EC sensor exhibited a wide linear range from 1 fg mL^-1 to 100 ng mL^-1, with an ultralow detection limit of 0.8 fg mL^-1 (S/N = 3). The feasibility in the clinical application of the EC sensor was verified by the in vitro detection of CEA in human serum.

A "signal-on" electrochemical biosensor based on DNAzyme-driven bipedal DNA walkers and TdT-mediated cascade signal amplification strategy

In this work, a dual amplified signal enhancement approach based on coupling deoxyribozyme (DNAzyme)-driven bipedal DNA walkers (BDW) and terminal deoxynucleotidyl transferase (TdT)-mediated DNA elongation signal amplifications has been developed for highly sensitive and label-free electrochemical detection of thrombin in human serums. In presence of thrombin, the BDW complex, which is comprised from the target thrombin and two DNAzyme-containing probes, can exhibit autonomous cleavage behavior on the surface of the substrate DNA (SD) modified electrode, and remove the cleaved DNA fragment from the electrode surface. Subsequently, the TdT can catalyze the elongation of the SD with free 3'-OH termini and formation of many G-quadruplex sequence replicates with the presence of 2'-deoxyaguanosine-5'-triphosphate (dGTP) and adenosine 5'-triphosphate (dATP) at a molar ratio of 6:4. These G-quadruplex sequences bind hemin and generate drastically amplified current response for sensitive detection of thrombin in a "signal-on" and completely label-free fashion. Under optimized conditions, the response peak current was linear with the concentration of thrombin in the range from 0.5 pM to 100000 pM with detection limit of 0.31 pM. This research provides us a sustainable idea for the hyphenated multiple amplification strategies and a stable and effective method for the detection of protein biomarkers.

Highly ordered 3D electrochemical DNA biosensor based on dual orientation controlled rolling motor and graftable tetrahedron DNA

Herein, a robust and highly ordered three-dimensional electrochemical DNA (3D E-DNA) biosensor was proposed, and its orientation was controlled from top down by poly adenine oligonucleotides (polyA-ODNs)-mediated rolling motor (PRM) and graftable tetrahedron DNA (GTD). The GTD with a grafting domain was immobilized on the electrode surface to construct a well-organized sensing interface and controlled the orientation and distribution of the whole system at the "bottom" of this biosensor. The polyA-ODNs regulated the direction and density of the leg DNA attached on PRM at the "top" of the biosensor. The motion was achieved through the target induced cyclic cleaving, which triggered the motor rolling rather than walk. Impressively, the duplex strand DNA (dsDNA) formed after grafting, as a girder, provided a stable support to the soft long single strand (ssDNA), which facilitated the formation of the catalytic center, elevated the efficiency of the rolling cleavage. Under the optimal conditions, the designed biosensor exhibited a lower limit of 0.17 nM and wide linear range from 0.5 nM to 1.5 μM for adenosine rapid detection. Unique dual orientation regulated characteristics of the system increased the probability hybridization enormously and improved the motion efficiency significantly, which offered new avenue of DNA nanomachines development in biosensor platform.

A label-free IFN-γ aptasensor based on target-triggered allosteric switching of aptamer beacon and streptavidin-inorganic hybrid composites

A label-free electrochemical aptasensor was developed for the sensitive detection of interferon-gamma (IFN-γ). To do this, a diblock dual-aptamer allosteric hairpin (DDAH) was designed, followed by conjugation with gold nanoparticles (DDAH&AuNP). The presence of target destroyed the stable hairpin structure, and then the catalytic cleavage of DNAzymes removed the IFN-γ-binding molecules, triggering the allosteric switching from inactive hairpin to active streptavidin aptamer (A-DDAH&AuNP) in homogeneous system. Moreover, streptavidin-inorganic hybrid nanoflowers decorated with graphene composites (SFG) were synthesized and used as substrates to modify glassy carbon electrodes (SFG/GCE). SFG specifically bind to the A-DDAH&AuNP to realize high-efficient readout of signals. Under the optimal conditions and by using differential pulse stripping voltammetry (DPSV), the response peak currents increases linearly with the logarithm of the IFN-γ concentration in the range between 0.1 pg mL^-1 and 500 ng/mL. The detection limit is as low as 19 fg mL^-1. The aptasensor also has excellent electrochemical performances, which exhibits broad application prospects in biometric analysis.

An enzyme-free and label-free signal-on aptasensor based on DNAzyme-driven DNA walker strategy

Herein, a signal-on electrochemical aptasensor for highly sensitive detection of thrombin (TB) was constructed based on the DNAzyme-driven DNA walker strategy. We developed a new dual functional hairpin DNA (HP) containing a substrate sequence of the Mg²⁺-dependent DNAzyme (in the loop region) and the G-quadruplex forming segment (in the stem region). The DNA walker (TBA₂-DWs), containing a TB aptamer and an enzymatic sequence, was introduced onto gold electrode (GE) by aptamers-target specific recognition, and thus initiated the enzymatic sequences to hybridize with the substrate sequence. Then, the DNA walker could repeatedly bind and cleave HP in the assistance of Mg²⁺, unlocking many active G-quadruplex forming sequences. Finally, hemin can further bind the G-quadruplex to form G-quadruplex/hemin complexes and generate enhanced current output. The aptasensor for TB assay showed a linear detection range from 1 pM to 60000 pM with a lower detection limit of 0.58 pM. And more, the proposed detection strategy was enzyme-free and label-free.

Recent developments of photoelectrochemical biosensors for food analysis

Recent developments of photoelectrochemical biosensors for food analysis are summarized and the future prospects in this field are discussed.