A signal amplification electrochemical aptasensor for the detection of breast cancer cell via free-running DNA walker

Precision in cancer diagnostics: ultra-sensitive detection of MCF-7 breast cancer cells by gold nanostructure-enhanced electrochemical biosensing

Timely identification of cancers is pivotal in optimizing treatment efficacy and reducing their widespread impact. This study introduces a novel biosensor for the sensitive electrochemical detection of cancer cells overexpressing mucin 1 (MUC1), a well-established model for breast cancer. The sensor substrate comprises gold columnar nanostructures obtained through glancing angle deposition (GLAD) of copper nanostructures, subsequently replaced by gold via a facile galvanic replacement process. Functionalizing these gold nanostructures with aptamers targeting the MUC1 glycoproteins, a prominent cancer biomarker, enables specific recognition of MCF-7 breast cancer cells. The proposed electrochemical sensing platform offers several advantages, including high selectivity, a wide linear range of detection, a low detection limit of 30 cells per mL, and long-term stability, rendering this sensor highly desirable for definitive breast cancer diagnosis.

A dual-core 3D DNA nanomachine based on DNAzyme positive feedback loop for highly sensitive MicroRNA imaging in living cells

A double 3D DNA walker nanomachine by DNAzyme self-driven positive feedback loop amplification for the detection of miRNA was constructed. This method uses two gold nanoparticles as the reaction core, and because of the spatial confinement effect the local concentration of the reactants increase the collision efficiency was greatly improved. Meanwhile, the introduction of positive feedback loop promotes the conversion efficiency. In presence of miRNA-21, a large amount of DNAzyme was released and hydrolyze the reporter probe, resulting the recovery of fluorescence signal. The linear range for miRNA-21 is 0.5-60 pmol/L, and the detection limit is 0.41 pmol/L (S/N = 3). This nanomachine has been successfully used for accurate detection of miRNA-21 expression levels in cell lysates. At the same time, it can enter cells for intracellular miRNA-21 fluorescence imaging, distinguishing tumor cells from normal cells. This combination of in vitro detection and imaging analysis of living cells can achieve the goal of jointly detecting cancer markers through multiple pathways, providing new ideas for early diagnosis and screening of diseases.

Host-Guest Recognition-Mediated Supramolecular Aggregation-Induced Electrochemiluminescence of Iridium(III) Complexes for Nucleic Acid Bioassay

Currently reported aggregation-induced electroluminescence (AIECL) is usually based on the electrostatic integration of luminous monomers, and its application is still limited by the low ECL efficiency and poor structural stability of electrostatic integration-based AIECL emitters. Herein, host-guest recognition-mediated supramolecular AIECL was creatively developed to overcome the defects of electrostatic-integration-based AIECL. Cucurbit[8]uril (CB[8]) as the host recognized tris (2-phenylpyridine) iridium(III) [Ir(ppy)3] as the guest to form a novel supramolecular complex Ir-CB[8]. CB[8] can not only provide a large hydrophobic cavity to efficiently load Ir(ppy)3 and enrich coreactant tripropylamine but also utilize its carbonyl-laced portals to form intramolecular hydrogen bonds to stabilize the supramolecular structure, so Ir-CB[8] revealed excellent AIECL performance. The AIECL emitter Ir-CB[8] coupled the efficient DNA walker to construct a sensing system for miRNA-16 detection. Au nanoparticles@norepinephrine (AuNPs@NE) trapped by single-strand S1 was developed to significantly quench the ECL emission of Ir-CB[8]. When the target microRNA-16 (miRNA-16) existed, H1 was opened and the sequential assembly from H2 to H7 was triggered, forming "windmill"-like DNA walker with six Pb2+-dependent leg DNA. The assembled DNA walker, which was centered on DNA structure, had high efficiency and biocompatibility and can cut S1 to keep the DNA fragment-carrying quencher AuNPs@NE away from the electrode surface, thus restoring the ECL emission of Ir-CB[8] and realizing ultrasensitive detection of miRNA-16. Supramolecular AIECL mediated by host-guest recognition provides a new way for constructing AIECL emitters with excellent structural stability and AIECL efficiency, and an Ir-CB[8] coupling "windmill"-like DNA walker builds a promising ECL-sensing system for bioassay.

Construction and Application of DNAzyme-based Nanodevices

The development of stimuli-responsive nanodevices with high efficiency and specificity is very important in biosensing, drug delivery, and so on. DNAzymes are a class of DNA molecules with the specific catalytic activity. Owing to their unique catalytic activity and easy design and synthesis, the construction and application of DNAzymes-based nanodevices have attracted much attention in recent years. In this review, the classification and properties of DNAzyme are first introduced. The construction of several common kinds of DNAzyme-based nanodevices, such as DNA motors, signal amplifiers, and logic gates, is then systematically summarized. We also introduce the application of DNAzyme-based nanodevices in sensing and therapeutic fields. In addition, current limitations and future directions are discussed.

Cancer Diagnostics and Early Detection Using Electrochemical Aptasensors

The detection of early-stage cancer offers patients the best chance of treatment and could help reduce cancer mortality rates. However, cancer cells or biomarkers are present in extremely small amounts in the early stages of cancer, requiring high-precision quantitative approaches with high sensitivity for accurate detection. With the advantages of simplicity, rapid response, reusability, and a low cost, aptamer-based electrochemical biosensors have received considerable attention as a promising approach for the clinical diagnosis of early-stage cancer. Various methods for developing highly sensitive aptasensors for the early detection of cancers in clinical samples are in progress. In this article, we discuss recent advances in the development of electrochemical aptasensors for the early detection of different cancer biomarkers and cells based on different detection strategies. Clinical applications of the aptasensors and future perspectives are also discussed.

Integration of exonuclease III-powered three-dimensional DNA walker with single-molecule detection for multiple initiator caspases assay

Initiator caspases are important components of cellular apoptotic signaling and they can activate effector caspases in extrinsic and intrinsic apoptotic pathways. The simultaneous detection of multiple initiator caspases is essential for apoptosis mechanism studies and disease therapy. Herein, we develop a sensitive nanosensor based on the integration of exonuclease III (Exo III)-powered three-dimensional (3D) DNA walker with single-molecule detection for the simultaneous measurement of initiator caspase-8 and caspase-9. This assay involves two peptide-DNA detection probe-conjugated magnetic beads and two signal probe-conjugated gold nanoparticles (signal probes@AuNPs). The presence of caspase-8 and caspase-9 can induce the cleavage of peptides in two peptide-DNA detection probes, releasing two trigger DNAs from the magnetic beads, respectively. The two trigger DNAs can serve as the walker DNA to walk on the surface of the signal probes@AuNPs powered by Exo III digestion, liberating numerous Cy5 and Texas Red fluorophores which can be quantified by single-molecule detection, with Cy5 indicating caspase-8 and Texas Red indicating caspase-9. Notably, the introduction of the AuNP-based 3D DNA walker greatly reduces the background signal and amplifies the output signals, and the introduction of single-molecule detection further improves the detection sensitivity. This nanosensor is very sensitive with a detection limit of 2.08 × 10-6 U μL-1 for caspase-8 and 1.71 × 10-6 U μL-1 for caspase-9, and it can be used for the simultaneous screening of caspase inhibitors and the measurement of endogenous caspase activity in various cell lines at the single-cell level. Moreover, this nanosensor can be extended to detect various proteases by simply changing the peptide sequences of the detection probes.

Nucleic acid-based electrochemical biosensors for rapid clinical diagnosis: Advances, challenges, and opportunities

Clinical diagnostic tests should be quick, reliable, simple to perform, and affordable for diagnosis and treatment of diseases. In this regard, owing to their novel properties, biosensors have attracted the attention of scientists as well as end-users. They are efficient, stable, and relatively cheap. Biosensors have broad applications in medical diagnosis, including point-of-care (POC) monitoring, forensics, and biomedical research. The electrochemical nucleic acid (NA) biosensor, the latest invention in this field, combines the sensitivity of electroanalytical methods with the inherent bioselectivity of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The NA biosensor exploits the affinity of single-stranded DNA/RNA for its complementary strand and is used to detect complementary sequences of NA based on hybridization. After the NA component in the sensor detects the analyte, a catalytic reaction or binding event that generates an electrical signal in the transducer ensues. Since 2000, much progress has been made in this field, but there are still numerous challenges. This critical review describes the advances, challenges, and prospects of NA-based electrochemical biosensors for clinical diagnosis. It includes the basic principles, classification, sensing enhancement strategies, and applications of biosensors as well as their advantages, limitations, and future prospects, and thus it should be useful to academics as well as industry in the improvement and application of EC NA biosensors.

Aptamer-based biosensors and application in tumor theranostics

An aptamer is a short oligonucleotide chain that can specifically recognize targeting analytes. Due to its high specificity, low cost, and good biocompatibility, aptamers as the targeting elements of biosensors have been applied widely in non-invasive tumor imaging and treatment in situ to replace traditional methods. In this review, we will summarize recent advances in using aptamer-based biosensors in tumor diagnosis. After a brief introduction of the advantage of aptamers compared with enzyme sensors and immune sensors, the different sensing designs and mechanisms based on 3 signal transduction modes will be reviewed to cover different kinds of analytical methods, including: electrochemistry analysis, colorimetry analysis, and fluorescence analysis. Finally, the prospective advantages of aptamer-based biosensors in tumor theranostics and post-treatment monitoring are also evaluated in this review.

Aptamer Embedded Arch-Cruciform DNA Assemblies on 2-D VS2 Scaffolds for Sensitive Detection of Breast Cancer Cells

Arch-cruciform DNA are self-assembled on AuNPs/VS₂ scaffold as a highly sensitive and selective electrochemical biosensor for michigan cancer foundation-7 (MCF-7) breast cancer cells. In the construction, arch DNA is formed using two single-strand DNA sequences embedded with the aptamer for MCF-7 cells. In the absence of MCF-7 cells, a cruciform DNA labeled with three terminal biotin is bound to the top of arch DNA, which further combines with streptavidin-labeled horseradish peroxidase (HRP) to catalyze the hydroquinone-H₂O₂ reaction on the electrode surface. The presence of MCF-7 cells can release the cruciform DNA and reduce the amount of immobilized HRP, thus effectively inhibiting enzyme-mediated electrocatalysis. The electrochemical response of the sensor is negatively correlated with the concentration of MCF-7 cells, with a linear range of 10~1 × 10⁵ cells/mL, and a limit of detection as low as 5 cells/mL (S/N = 3). Through two-dimensional materials and enzyme-based dual signal amplification, this biosensor may pave new ways for the highly sensitive detection of tumor cells in real samples.

Recent Development of Nanomaterials-Based Cytosensors for the Detection of Circulating Tumor Cells

The accurate analysis of circulating tumor cells (CTCs) holds great promise in early diagnosis and prognosis of cancers. However, the extremely low abundance of CTCs in peripheral blood samples limits the practical utility of the traditional methods for CTCs detection. Thus, novel and powerful strategies have been proposed for sensitive detection of CTCs. In particular, nanomaterials with exceptional physical and chemical properties have been used to fabricate cytosensors for amplifying the signal and enhancing the sensitivity. In this review, we summarize the recent development of nanomaterials-based optical and electrochemical analytical techniques for CTCs detection, including fluorescence, colorimetry, surface-enhanced Raman scattering, chemiluminescence, electrochemistry, electrochemiluminescence, photoelectrochemistry and so on.

DNA Nanotechnology-Based Biosensors and Therapeutics

Over the past few decades, DNA nanotechnology engenders a vast variety of programmable nanostructures utilizing Watson-Crick base pairing. Due to their precise engineering, unprecedented programmability, and intrinsic biocompatibility, DNA nanostructures cannot only interact with small molecules, nucleic acids, proteins, viruses, and cancer cells, but also can serve as nanocarriers to deliver different therapeutic agents. Such addressability innate to DNA nanostructures enables their use in various fields of biomedical applications such as biosensors and cancer therapy. This review is begun with a brief introduction of the development of DNA nanotechnology, followed by a summary of recent applications of DNA nanostructures in biosensors and therapeutics. Finally, challenges and opportunities for practical applications of DNA nanotechnology are discussed.

Novel cytosensor for accurate detection of circulating tumor cells based on a dual-recognition strategy and BSA@Ag@Ir metallic-organic nanoclusters

Herein, based on a dual-recognition strategy and BSA@Ag@Ir metallic-organic nanoclusters (BSA@Ag@Ir MONs), a highly specific and sensitive cytosensor was developed for detecting circulating tumor cells (CTCs). To amplify current signal, novel BSA@Ag@Ir MONs with outstanding catalytic activity and huge specific surface area were synthesized, and conjugated with hairpin DNA strands as signal probes. Orion carbon black 40 (Ocb40)//AuNPs were firstly used to modify electrode to increase its conductivity and surface area. Moreover, the dual recognition strategy based on DNA proximity effect was designed to improve the specificity of cytosensor. When two capture probes respectively bound to two adjacent membrane markers of target cells, the probes could form the associative toehold through the proximity effect to capture the signal probes. Only CTCs simultaneously expressing two membrane markers could be captured and generate current responses. The developed cytosensor could detect CTCs in the range of 3 - 3 × 10⁶ cells mL^-1 with a detection limit of 1 cell mL^-1. Notably, the cytosensor could accurately identify CTCs even in whole blood. Therefore, this cytosensor has great potential for application in biological science, biomedical engineering and personalized medicine.

Electrochemical sensing technology for liquid biopsy of circulating tumor cells-a review

In recent years, a lot of new detection techniques for circulating tumor cells (CTCs) have been developed. Among them, electrochemical sensing technology has gradually developed because of its advantages of good selectivity, high sensitivity, low cost and rapid detection. Especially in the latest decade, the field of electrochemical biosensing has witnessed great progress, thanks to the merging of biosensing research area with nanotechnology, immunotechnology, nucleic acid technology, and microfluidic technology. In this review, the recent progress for the detection of CTCs according to the principle of detection was summarized and how they can contribute to the enhanced performance of such biosensors was explained. The latest electrode construction strategies such as rolling circle amplification reaction, DNA walker and microfluidic technology and their advantages were also introduced emphatically. Moreover, the main reasonswhy the existing biosensors have not been widely used clinically and the next research points were clearly put forward.

A label-free electrochemical aptasensor for breast cancer cell detection based on a reduced graphene oxide-chitosan-gold nanoparticle composite

Regarding the cancer fatal consequences, early detection and progression monitoring are the most vital issues in patients' treatment and mortality reduction. Therefore, there is a great demand for fast, inexpensive, and selective detection methods. Herein, a graphene-based aptasensor was designed for sensitive human breast cancer cell detection. A reduced graphene oxide-chitosan-gold nanoparticles composite was used as a biocompatible substrate for the receptor stabilization. The significant function of the aptamer on this composite is due to the synergistic effects of the components in improving the properties of the composite, including increasing the electrical conductivity and effective surface area. After the aptasensor incubation in MCF-7 cancer cells, the cell membrane proteins interacted specifically with the three dimensional-structure of the AS1411 aptamer, resulting in the cell capture on the aptasensor. The aptasensor fabrication steps were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The higher cell concentrations concluded to the higher captured cells on the aptasensor which blocked the Ferro/Ferricyanide access to the sensor, causing increases in the charge transfer resistances. This aptasensor shows a linear relationship with the cell concentration logarithm, high selectivity, a wide linear range of 1 × 10¹-1 × 10⁶ cells/mL, and a low detection limit of 4 cells/mL.

Efficient strand displacement amplification via stepwise movement of a bipedal DNA walker on an electrode surface for ultrasensitive detection of antibiotics

DNA walkers, one of the artificial molecular machines which are constructed via smart synthetic DNA, have attracted rapidly growing attention from researchers in the biosensing field. In this work, we design an Exonuclease III (Exo III)-aided target-aptamer binding recycling (ETBR) activated bipedal DNA machine for highly sensitive electrochemical detection of antibiotics. To the best of our knowledge, this is the first time that a bipedal DNA machine has been applied in electrochemical sensing for antibiotics. On the one hand, the bipedal DNA walker exceeds the conventional single swing arm DNA walker in terms of walking efficiency and stability. On the other hand, the ETBR strategy, along with efficient strand displacement amplification via stepwise movement of a bipedal DNA walker significantly promotes the signal amplification efficiency. Under optimal conditions, this bipedal DNA machine possesses a detection limit of 7.1 fM within a linear detection range from 10 fM to 100 pM. Moreover, this electrochemical biosensor is expected to detect a wide variety of analytes using the corresponding target recognition probes. Thus, our proposed strategy offers a highly efficient, stable and practical platform for small molecule analysis.

Electrochemical Analysis of Target-Induced Hairpin-Mediated Aptamer Sensors

The state of probe DNA at the biosensing interface greatly affects the detection performance of electrochemical DNA biosensors. Herein, we constructed a target-induced hairpin-mediated biosensing interface to study the effect of probe DNA on the analytical performance of adenosine triphosphate aptamer (ATPA) and adenosine triphosphate (ATP) detection. Moreover, we also explored the electrochemical contribution of the coexisting hairpin and double-stranded DNA (dsDNA) to this sensing interface. Experimental results suggested that the molecular recognition ability and detection performance of the biosensing interface were majorly dependent on the surface density of methylene blue (MB)-labeled probe hairpin DNA and partly affected by the spatial state of the formed dsDNA. When the surface density of hairpin DNA was moderate (5.72 pmol cm^-2), this sensing interface determined as low as 0.74 fM ATPA and 5.04 pM ATP with high selectivity and excellent regeneration, respectively. Furthermore, we calculated that the formed dsDNA had a 31.87% contribution in the total electrochemical signal for 10 pM ATPA detection. Based on the above results, we designed an XOR logic gate based on the biosensing interface for ATPA and ATP detection.

Critical role of biosensing on the efficient monitoring of cancer proteins/biomarkers using label-free aptamer based bioassay

Cancer is the second most extended disease during the world with an improved death rate over the past several time. Due to the restrictions of cancer analysis methods, the patient's real survival rate is unknown. Therefore early stage diagnosis of cancer is crucial for its strong detection. Bio-analysis based on biomarkers may help to overcome this problem. Aptamers can be employed as high-affinity tools for cancer detection. The utilization of aptamer-based strategy in cancer investigation and strategy shows new opportunities in biotechnology. The label-free system is an important method to study biomolecules in different sizes, such as biomarkers in real-time because of their greatest sensitivity, selectivity, and multi examination. In this review (with 75 references), excellent features of the label-free aptasensors on the sensitive and accurate monitoring of cancer biomarkers were discussed. Also, the role of advanced of nanomaterials on the construction of label-free aptasensors were investigated. In addition, application of different detection methods such as electrochemical, optical, electronic, and photoelectrochemical (PEC), electrochemiluminescence (ECL) were surveyed. Finally, advantages and limitation of different strategies on the early stage diagnosis of cancer biomarkers were discussed. This article has been updated until July 2020.

Aptamers in Drug Design: An Emerging Weapon to Fight a Losing Battle

Implementation of novel and biocompatible polymers in drug design is an emerging and rapidly growing area of research. Even though we have a large number of polymer materials for various applications, the biocompatibility of these materials remains as a herculean task for researchers. Aptamers provide a vital and efficient solution to this problem. They are usually small (ranging from 20 to 60 nucleotides, single-stranded DNA or RNA oligonucleotides which are capable of binding to molecules possessing high affinity and other properties like specificity. This review focuses on different aspects of Aptamers in drug discovery, starting from its preparation methods and covering the recent scenario reported in the literature regarding their use in drug discovery. We address the limitations of Aptamers and provide valuable insights into their future potential in the areas regarding drug discovery research. Finally, we explained the major role of Aptamers like medical imaging techniques, application as synthetic antibodies, and the most recent application, which is in combination with nanomedicines.

Detection of Circulating Tumor Cells in Breast Cancer Patients by Nanopore Sensing with Aptamer-Mediated Amplification

Circulating tumor cells (CTCs) have been utilized in the diagnosis and prognosis of tumor. However, the CTC concentration is extremely low to be detected in peripheral blood. Many existing methods suffer from either expensive labeling or complex operation. In this study, we constructed a label- and enzyme-free and sensitive method to detect the breast cancer CTCs. First of all, a probe containing a breast cancer cell-specific aptamer and a complementary single-stranded DNA (trigger DNA P1) were designed. When the target cells are present, the aptamer binds to the CTCs and releases P1 which triggers the strand displacement amplification. This process generates three-way junction structure DNA, the specific translocation signals of which are identified by nanopore assay. The detection limit of tumor cells is 5 in the current experimental setup and can be further reduced. Furthermore, the method is demonstrated in a clinical sample test with high recovery rate and accuracy. Our results suggest that this method could be applied to early diagnosis of metastatic recurrence and prognosis determination.

Ultrasensitive aptasensor for isolation and detection of circulating tumor cells based on CeO2@Ir nanorods and DNA walker

Herein, based on dual signal amplification by CeO₂@Ir nanorods (Ce@IrNRs) and enzyme-free DNA walker, a novel electrochemical aptasensor was developed for simultaneous isolation and detection of circulating tumor cells (CTCs). A membrane protein MUC1-targeting aptamer was used to specifically recognize and capture MCF-7 cells. Uracil DNA glycosylase could hydrolyze deoxyuracils of the aptamer to isolate the captured cells. Novel Ce@IrNRs with large surface area and high peroxidase activity were synthesized to amplify the signal, and the enzyme-free DNA walker was applied to release more signal probes combined with Ce@IrNRs. Furthermore, to reduce steric hindrance by cells, the signal probes rather than the target cells, were directly combined with the electrode. The aptasensor could detect CTCs in the range of 2 to 2 × 10⁶ cells mL^-1 with a limit of detection 1 cell mL^-1. The developed aptasensor, which can simultaneously isolate and detect CTCs, has great application potential in the early monitoring of tumor metastasis and in individualized treatment.

Recent advances on functional nucleic acid-based biosensors for detection of food contaminants

It has seen increasing development of reliable, robust, and flexible biosensors for rapid food-safety analysis in the past few decades. Recently, functional nucleic acid-based biosensors have attracted attention because of their programmability, bottom-up characteristics, and structural switches. However, few systematic reviews devoted to categorizing the potential of DNA nanostructures and devices were found for detecting food contaminants. Hence, the applications of functional nucleic acid-based biosensors were reviewed for analyzing food contaminants, including foodborne pathogen bacteria, biotoxins, heavy metals, and et al. In addition to categorizing the various biosensors, multiple signal readout strategies, such as optical, electrochemical, and mass-based signals were also examined. Finally, the future changes and potential opportunities, as well as practical applications of functional nucleic acid-based biosensors were discussed.

Dynamic DNA Assemblies in Biomedical Applications

Deoxyribonucleic acid (DNA) has been widely used to construct homogeneous structures with increasing complexity for biological and biomedical applications due to their powerful functionalities. Especially, dynamic DNA assemblies (DDAs) have demonstrated the ability to simulate molecular motions and fluctuations in bionic systems. DDAs, including DNA robots, DNA probes, DNA nanochannels, DNA templates, etc., can perform structural transformations or predictable behaviors in response to corresponding stimuli and show potential in the fields of single molecule sensing, drug delivery, molecular assembly, etc. A wave of exploration of the principles in designing and usage of DDAs has occurred, however, knowledge on these concepts is still limited. Although some previous reviews have been reported, systematic and detailed reviews are rare. To achieve a better understanding of the mechanisms in DDAs, herein, the recent progress on the fundamental principles regarding DDAs and their applications are summarized. The relative assembly principles and computer-aided software for their designing are introduced. The advantages and disadvantages of each software are discussed. The motional mechanisms of the DDAs are classified into exogenous and endogenous stimuli-triggered responses. The special dynamic behaviors of DDAs in biomedical applications are also summarized. Moreover, the current challenges and future directions of DDAs are proposed.

Functionalized reduced graphene oxide with aptamer macroarray for cancer cell capture and fluorescence detection

An integrated aptamer macroarray functionalized with reduced graphene oxide (rGO) to specifically capture and sensitively detect cancer cells is reported. The capture for cancer cells is based on effective recognition of the modified rGO surface through the aptamer against epithelial cell adhesion molecule (EpCAM). The rough structure of rGO enhances morphologic interactions between rGO film interface and the cancer cells, while super-hydrophilicity of modified rGO hinders nonspecific cell capture. The synergistic interactions offer the aptamer macroarray high efficiency of cancer cell capture. By means of a turn-on fluorescence strategy based on the conformation change of the aptamer induced by the target recognition, the enriched cancer cells can be directly read out at excitation/emission wavelengths of 550/680 nm without washing, separation, and dying steps. The working range is 1 × 10² to 2 × 10⁴ cells per mL with a detection limit of 22 cells per mL. The results indicate that the aptamer macroarray has a considerable foreground for early diagnosis, therapy, and monitoring of cancer. Graphical abstract.

Effect of polyvalency on tethered molecular walkers on independent one-dimensional tracks

We study the motion of random walkers with residence time bias between first and subsequent visits to a site, as a model for synthetic molecular walkers composed of coupled DNAzyme legs known as molecular spiders. The mechanism of the transient superdiffusion has been explained via the emergence of a boundary between the new and the previously visited sites, and the tendency of the multilegged walker to cling to this boundary, provided residence time for a first visit to a site is longer than for subsequent visits. Using both kinetic Monte Carlo simulation and an analytical approach, we model a system that consists of unipedal walkers, each on its own one-dimensional track, connected by a tether, i.e., a kinematic constraint that no two walkers can be more than a certain distance apart. Even though a single unipedal walker does not at all exhibit directional, superdiffusive motion, we find that a team of unipedal walkers on parallel tracks, connected by a flexible tether, does enjoy a superdiffusive transient. Furthermore, unipedal walker teams exhibit a greater expected number of steps per boundary period and are able to diffuse more quickly than bipedal walker teams, which leads to longer periods of superdiffusion.

Advances in aptasensor technology

Aptasensors form a class of biosensors that function on the basis of a biological recognition. An aptasensor is advantageous because it incorporates a unique biologic recognition element, i.e., an aptamer, coupled to a transducer to convert a biological interaction to readable signals that can be easily processed and reported. In such biosensors, the specificity of aptamers is comparable to and sometimes even better than that of antibodies. Using the SELEX technique, aptamers with high specificity and affinity to various targets can be isolated from large pools of different oligonucleotides. Nowadays, new modifications of the SELEX technique and, as a result, easy generation and synthesis of aptamers have led to the wide application of these materials as biological receptors in biosensors. In this regard, aptamers promise a bright future. In the present research a brief account is initially provided of the recent developments in aptasensors for various targets. Then, immobilization methods, design strategies, current limitations and future directions are discussed for aptasensors.

An anchored monopodial DNA walker triggered by proximity hybridization for amplified amperometric biosensing of nucleic acid and protein

This work designed an anchored monopodial DNA walker to amplify amperometric biosensing signal for sensitive detection of nucleic acid and protein. The biosensing surface was constructed by self-assembling hairpin DNA1 (H1) and small amount of P1-W (probe DNA1 hybridized with walking DNA) on a gold electrode. In the presence of target molecule, the walker could be triggered by the surface proximity hybridization product of P1, target and P2 to induce the cyclic hybridization of H1 with ferrocene modified hairpin DNA2 (H2-Fc), which took electroactive Fc to the electrode surface for amplified amperometric detection of the target. By linking P1 and P2 with dual specific DNA strands, aptamers or antibodies to recognize the target for proximity hybridization of P1 and P2, the walker amplified amperometric strategy could be used for highly sensitive biosensing of different targets. Using DNA and thrombin as the target models, the proposed biosensing methods achieved the linear range from 0.2 pM to 2 nM with a detection limit of 0.11 pM and 1.0 pM to 10 nM with a detection limit of 0.61 pM, respectively. The specific recognition process endowed the strategy with high selectivity and potential applications.

Enhanced biosensing strategies using electrogenerated chemiluminescence: recent progress and future prospects

An overview of enhancement strategies for highly sensitive ECL-based sensing of bioanalytes enabling early detection of cancer.

Electrochemical cytosensors for detection of breast cancer cells

Breast cancer is one of lethal cancers among women with its metastasis leading to cancer-related morbidity and mortality. Circulating tumor cells (CTCs) derived from a primary tumor can be detected in the venous blood of cancer patients. Monitoring CTCs in blood samples has increased exponentially over the past decades and holds great promise in the diagnosis and treatment of metastatic breast cancer. Electrochemical cytosensors, classified as a class of electrochemical biosensors for sensitive detection and enumeration of targeted cells with minimally invasive methods, have the advantages of electrochemical biosensors, such as simplicity, low cost, and low limit of detection. Here, we review recent progress in the detection of CTCs from breast cancer with a focus on electrochemical cytosensors. This review describes platforms benefiting from these cytosensors to identify cancerous breast cells. Furthermore, strategies for signal amplification and also generation of reusable electrochemical cytosensors are introduced. In addition, breast cancer markers and biorecognition elements for cell capturing are reviewed.

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.

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%.

An electrochemical signal-on apta-cyto-sensor for quantitation of circulating human MDA-MB-231 breast cancer cells by transduction of electro-deposited non-spherical nanoparticles of gold

A highly simple, sensitive, specific and low-cost electrochemical apta-cyto-sensor for determination of circulating human MDA-MB-231 breast cancer cells was fabricated. Non-spherical nanoparticles of gold were electro-deposited in the presence of ethosuximide as a shape directing and size controlling agent. The nanoparticles had dimensions ranging 50-150 nm, and covered the underlying surface with a roughness factor of 8.03. The Non-spherical nanoparticles were then employed as the apta-cyto-sensor transducer. A 83-mer DNA aptamer that is specific to capturing the cell surface proteins was immobilized on the transducer surface, and binding with the cells was followed using the ferro/ferricyanide redox marker. The aptamer was immobilized within ∼200 min on the transducer surface. The cells were quantified with an equation of regression of ΔIp(μA) = (1.028 ± 0.027) log (C (cell mL^-1)) + (0.2199 ± 0.0944), a sensitivity of 1.028 μA (log (concentration / cell mL^-1))^-1 and a quantitation limit of 2 cell mL^-1, in a concentration range of 5 to 2 × 10⁶ cell mL^-1. The apta-cyto-sensor selectivity was also evaluated toward AsPC-1, Calu-6, HeLa, MCF-7 and melanoma B16/F10 cell lines. The apta-cyto-sensor had a fabrication reproducibility of 4.2%, regeneration capability of 5.1%, a stability of 35 days, and a potential application for the detection of MDA-MB-231 cells in the spiked blood serum samples with a sensitivity of 0.8975 μA (log (concentration / cell mL^-1))^-1 and a quantitation limit of 5 cell mL^-1, in a concentration range of 10 to 1 × 10³ cell mL^-1. The apta-cyto-sensor would be applicable for breast cancer diagnosis at early stage.

A sensitive electrochemical strategy via multiple amplification reactions for the detection of E. coli O157: H7

The sensitive and efficient strategy remains a central challenge for early diagnosis of pathogenic bacteria. Herein, an ultrasensitive electrochemical biosensor was proposed based on the multiple amplification strategy via the 3D DNA walker, rolling circle amplification (RCA) and hybridization chain reaction (HCR) for the accurate detection of Escherichiacoli O157:H7 (E. coli O157:H7). Firstly, the target sequence extracted from E. coli O157:H7 was transformed and amplified by the DNA walker firstly. Subsequently, a large number of transformed nucleic acid sequences were amplified by the RCA reaction. And then, the progress of HCR was triggered by every fragment in RCA products to form a long double-stranded DNA sequence to immobilize electrochemical indicators, generating a significantly enhanced electrochemical signal. As expected, a high sensitivity with a detection limit of 7 CFU/mL was achieved based on the proposed multiple amplification strategy, which is superior to most current methods for E. coli O157: H7 assay. The multiple amplification strategy could be readily expanded for the detection of various pathogenic bacteria, providing a new approach for early diagnosis of pathogenic microorganisms or other diseases.

Aptamer-based electrochemical cytosensors for tumor cell detection in cancer diagnosis: A review

Circulating tumor cells, a type of viable cancer cell circulating from primary or metastatic tumors in the blood stream, can lead to the parallel development of primary tumors and metastatic lesions. Highly selective and sensitive detection of tumor cells has become a hot research topic and can provide a basis for early diagnosis of cancers and anticancer drug evaluation to develop the best treatment plan. Aptamers are single-stranded oligonucleotides that can bind to target tumor cells in unique three-dimensional structures with high specificity and affinity. Aptamer-based methods or signal amplification methods using aptamers show great potential in improving the selectivity and sensitivity of electrochemical (EC) cytosensors for tumor cell detection. This review covers the remarkable developments in aptamer-based EC cytosensors for the identification of cell type, cell counting and detection of crucial proteins on the cell surface. Various EC techniques have been developed for cancer cell detection, including common voltammetry or impedance, electrochemiluminescence and photoelectrochemistry in a direct approach (aptamer-target cell), sandwich approach (capture probe-target cell-signaling probe) or other approach. The current challenges and promising opportunities in the establishment of EC aptamer cytosensors for tumor cell detection are also discussed.

Perspective of Molecular Diagnosis in Healthcare: From Barcode to Pattern Recognition

Barcode technology has a broad spectrum of applications including healthcare, food security, and environmental monitoring, due to its ability to encode large amounts of information. With the rapid development of modern molecular research, barcodes are utilized as a reporter with different molecular combinations to label many biomolecular targets, including genomic and metabolic elements, even with multiplex targeting. Along with the advancements in barcoded bioassay, the improvements of various designs of barcode components, encoding and decoding strategies, and their portable adoption are indispensable in satisfying multiple purposes, such as medical confirmation and point-of-care (POC) testing. This perspective briefly discusses the current direction and progress of barcodes development and provides a hypothesis for barcoded bioassay in the near future.

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.

A rolling circle amplification-assisted DNA walker triggered by multiple DNAzyme cores for highly sensitive electrochemical biosensing

A DNA walker triggered by multiple DNAzyme cores was constructed with the assistance of rolling circle amplification for electrochemical biosensing.