Electrochemical aptasensor for mucin 1 based on dual signal amplification of poly(o-phenylenediamine) carrier and functionalized carbon nanotubes tracing tag

Conjugated Polymers for Aptasensing Applications

Rapid and specific assaying of molecules that report on a pathophysiological condition, environmental pollution, or drug concentration is pivotal for establishing efficient and accurate diagnostic systems. One of the main components required for the construction of these systems is the recognition element (receptor) that can identify target analytes. Oligonucleotide switching structures, or aptamers, have been widely studied as selective receptors that can precisely identify targets in different analyzed matrices with minimal interference from other components in an antibody-like recognition process. These aptasensors, especially when integrated into sensing platforms, enable a multitude of sensors that can outperform antibody-based sensors in terms of flexibility of the sensing strategy and ease of deployment to areas with limited resources. Research into compounds that efficiently enhance signal transduction and provide a suitable platform for conjugating aptamers has gained huge momentum over the past decade. The multifaceted nature of conjugated polymers (CPs), notably their versatile electrical and optical properties, endows them with a broad range of potential applications in optical, electrical, and electrochemical signal transduction. Despite the substantial body of research demonstrating the enhanced performance of sensing devices using doped or nanostructure-embedded CPs, few reviews are available that specifically describe the use of conjugated polymers in aptasensing. The purpose of this review is to bridge this gap and provide a comprehensive description of a variety of CPs, from a historical viewpoint, underpinning their specific characteristics and demonstrating the advances in biosensors associated with the use of these conjugated polymers.

Nanotechnology‐based electrochemical biosensors for monitoring breast cancer biomarkers

Breast cancer is categorized as the most widespread cancer type among women globally. On-time diagnosis can decrease the mortality rate by making the right decision in the therapy procedure. These features lead to a reduction in medication time and socioeconomic burden. The current review article provides a comprehensive assessment for breast cancer diagnosis using nanomaterials and related technologies. Growing use of the nano/biotechnology domain in terms of electrochemical nanobiosensor designing was discussed in detail. In this regard, recent advances in nanomaterial applied for amplified biosensing methodologies were assessed for breast cancer diagnosis by focusing on the advantages and disadvantages of these approaches. We also monitored designing methods, advantages, and the necessity of suitable (nano) materials from a statistical standpoint. The main objective of this review is to classify the applicable biosensors based on breast cancer biomarkers. With numerous nano-sized platforms published for breast cancer diagnosis, this review tried to collect the most suitable methodologies for detecting biomarkers and certain breast cancer cell types.

Interfacial charge storage mechanisms of composite electrodes based on poly(ortho-phenylenediamine)/carbon nanotubes via advanced electrogravimetry

To reach a deeper understanding of the charge storage mechanisms of electrode materials is one of the challenges toward improving their energy storage performance. Herein, we investigate the interfacial ion exchange of a composite electrode made of carbon nanotube/poly( ortho-phenylenediamine) (CNT/P oPD) in a 1M NaCl aqueous electrolyte via advanced electrogravimetric analyses based on electrochemical quartz crystal microbalance (EQCM). Classical EQCM at different scan rates of the potential revealed the complex electrogravimetric behavior likely due to multi-species participation at different temporal scales. Thereafter, in order to better understand the behavior of each species (ions, counter ions, and co-ions) in the charge compensation mechanism, the electrogravimetric impedance spectroscopy analysis (also called ac-electrogravimetry) was pursued. Ac-electrogravimetry revealed the role of each species where Na+ cations and Cl− anions as well as protons participate in the charge compensation mechanism of the CNT/P oPD composite with different kinetics and proportions. The water molecules with opposite flux direction with the cations are also detected, suggesting their exclusion during cationic species transfer. Having analyzed ac-electrogravimetry responses in depth, the synergistic interaction between the CNT and P oPD is highlighted, revealing the improved accessibility of species to new sites in the composite.

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.

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.

An electrochemical aptasensor for the milk allergen β-lactoglobulin detection based on a target-induced nicking site reconstruction strategy

Food allergy is an immune system reaction to a particular food, milk being the most common one. β-Lactoglobulin (β-Lg) is the main ingredient of milk protein and the main cause of infant milk allergy. On such an occasion, the determination of β-Lg is very important and the electrochemical sensors are a good alternative for this purpose since they are sensitive, selective and inexpensive. In this work, an electrochemical aptasensor was fabricated for the quantitative detection of β-Lg in hypoallergenic formula (HF) milk. A tri-functional hairpin (HP) was designed, which was composed of an aptamer sequence, a nicking site and a DNA sequence (T1). In the absence of β-Lg, the aptamer part hybridized with T1 to form a stable stem-loop structure. However, in the presence of β-Lg, the capture of the aptamer sequence towards β-Lg caused the reconstruction of HP and thus the nicking sites were exposed. Then, the nicking enzyme was activated and T1 could be released, which bound with the end of the hairpin 1-methylene blue (HP1-MB)/HP2-MB conjugation on the Au nanoparticle (AuNP) modified electrode surface. Thus, the insulating property of the electrode was enhanced and the current response of MB decreased, which built the quantitative basis for β-Lg detection. In this way, the proposed aptasensor exhibited a wide linear range of 0.01-100 ng mL^-1 and a low detection limit of 5.7 pg mL^-1. This aptasensor also displayed high selectivity, reproducibility and stability, and became a promising platform for β-Lg detection in real food samples.

A Comprehensive Study on Aptasensors For Cancer Diagnosis

Cancer is the most devastating disease in the present scenario, killing millions of people every year. Early detection, accurate diagnosis, and timely treatment are considered to be the most effective ways to control this disease. Rapid and efficient detection of cancer at their earliest stage is one of the most significant challenges in cancer detection and cure. Numerous diagnostic modules have been developed to detect cancer cells early. As nucleic acid equivalent to antibodies, aptamers emerge as a new class of molecular probes that can identify cancer-related biomarkers or circulating rare cancer/ tumor cells with very high specificity and sensitivity. The amalgamation of aptamers with the biosensing platforms gave birth to "Aptasensors." The advent of highly sensitive aptasensors has opened up many new promising point-of-care diagnostics for cancer. This comprehensive review focuses on the newly developed aptasensors for cancer diagnostics.

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.

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.

Recent advances in aptamer-based sensors for breast cancer diagnosis: special cases for nanomaterial-based VEGF, HER2, and MUC1 aptasensors

Cancer is one of the most common and important diseases with a high mortality rate. Breast cancer is among the three most common types of cancer in women, and the mortality rate has reached 0.024% in some countries. For early-stage preclinical diagnosis of breast cancer, sensitive and reliable tools are needed. Today, there are many types of biomarkers that have been identified for cancer diagnosis. A wide variety of detection strategies have also been developed for the detection of these biomarkers from serum or other body fluids at physiological concentrations. Aptamers are single-stranded DNA or RNA oligonucleotides and promising in the production of more sensitive and reliable biosensor platforms in combination with a wide range of nanomaterials. Conformational changes triggered by the target analyte have been successfully applied in fluorometric, colorimetric, plasmonic, and electrochemical-based detection strategies. This review article presents aptasensor approaches used in the detection of vascular endothelial growth factor (VEGF), human epidermal growth factor receptor 2 (HER2), and mucin-1 glycoprotein (MUC1) biomarkers, which are frequently studied in the diagnosis of breast cancer. The focus of this review article is on developments of the last decade for detecting these biomarkers using various sensitivity enhancement techniques and nanomaterials.

Strategies and perspectives to develop SARS-CoV-2 detection methods and diagnostics

To develop diagnostics and detection methods, current research is focussed on targeting the detection of coronavirus based on its RNA. Besides the RNA target, research reports are coming to develop diagnostics by targeting structure and other parts of coronavirus. PCR based detection system is widely used and various improvements in the PCR based detection system can be seen in the recent research reports. This review will discuss multiple detection methods for coronavirus for developing appropriate, reliable, and fast alternative techniques. Considering the current scenario of COVID-19 diagnostics around the world and an urgent need for the development of reliable and cheap diagnostic, various techniques based on CRISPR technology, antibody, MIP, LAMP, microarray, etc. should be discussed and tried.

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.

Controlled/“living” radical polymerization-based signal amplification strategies for biosensing

Controlled/“living” radical polymerization-based signal amplification strategies and their applications in highly sensitive biosensing of clinically relevant biomolecules are reviewed.

Impedimetric aptamer based determination of the tumor marker MUC1 by using electrospun core-shell nanofibers

An impedimetric single-shot assay is described for the determination of the proteinic breast cancer marker MUC1. The surface of a glassy carbon electrode was modified with core-shell nanofibers, multi-walled carbon nanotubes and gold nanoparticles that were covalently modified with the MUC1-binding aptamer. Detection is based on the change of the resistance of the electrode surface as measured by electrochemical impedance spectroscopy using hexacyanoferrate(II/III) as an electrochemical probe in working potential is 0.25 V. Scanning electron microscopy and cyclic voltammetry were also applied to characterize the electrode. The analytical response ranges from 5 to 115 nM of MUC1, with a detection limit of 2.7 nM. The assay was successfully applied to MUC1 determination in spiked serum samples where it gave satisfactory results. Graphical abstractAn impedimetric nanoprobe for the tumor marker MUC1 is proposed. It is based on use of electrospun honey core-shell nanofibers. The nanoprobe exhibits excellent sensitivity, good stability and a low detection limit.

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%.

Electrochemical biosensors for the detection of lung cancer biomarkers: A review

Cancer is one of the most widespread challenges and important diseases, which has the highest mortality rate. Lung cancer is the most common type of cancer, so that about 25% of all cancer deaths are related to the lung cancer. The lung cancer is classified as two different types with different treatment methodology: the small cell lung carcinoma and nonsmall cell lung carcinoma are two categories of the lung cancer. Since the lung cancer is often in the latent period in its early stages, therefore, early diagnosis of lung cancer has many challenges. Hence, there is a need for sensitive and reliable tools for preclinical diagnosis of lung cancer. Therefore, many detection methods have been employed for early detection of lung cancer. As lung cancer tumors growth in the body, the cancerous cells release numerous DNA, proteins, and metabolites as special biomarkers of the lung cancer. The levels of these biomarkers show the stages of the lung cancer. Therefore, detection of the biomarkers can be used for screening and clinical diagnosis of the lung cancer. There are numerous biomarkers for the lung cancer such as EGFR, CEA, CYFRA 21-1, ENO1, NSE, CA 19-9, CA 125 and VEGF. Nowadays, electrochemical methods are very attractive and useful in the lung cancer detections. So, in this paper, the recent advances and improvements (2010-2018) in the electrochemical detection of the lung cancer biomarkers have been reviewed.

Covalent organic framework-based electrochemical aptasensors for the ultrasensitive detection of antibiotics

We designed and synthesized a novel covalent organic framework (COF) by condensation polymerization of 1,3,6,8-tetrakis(4-formylphenyl)pyrene and melamine through imine bonds (represented by Py-M-COF). The basic characterizations revealed that the Py-M-COF not only exhibited an extended π-conjugation framework, a large specific surface area (495.5 m² g^-1), big pore cavities, and nanosheet-like structure but also possessed rich functional groups, such as C˭C, C˭N, C˭O, and NH₂. These features endowed the Py-M-COF with high charge carrier mobility, further improving the strong immobilization of DNA aptamer strands via π-π stacking interaction and electrostatic interaction. As such, the Py-M-COF-based electrochemical aptasensors are ultrasensitive in detecting different antibiotics, including enrofloxacin (ENR) and ampicillin (AMP), yielding extremely low detection limits of 6.07 and 0.04 fg mL^-1 (S/N = 3) toward ENR and AMP, respectively, along with other excellent sensing performances. This biosensing platform based on Py-M-COF has potential applications for the sensitive detection of antibiotics or other analytes by replacing the corresponding aptamers.

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.

Recent Advances in Enhancement Strategies for Electrochemical ELISA-Based Immunoassays for Cancer Biomarker Detection

Electrochemical enzyme-linked immunosorbent assay (ELISA)-based immunoassays for cancer biomarker detection have recently attracted much interest owing to their higher sensitivity, amplification of signal, ease of handling, potential for automation and combination with miniaturized analytical systems, low cost and comparative simplicity for mass production. Their developments have considerably improved the sensitivity required for detection of low concentrations of cancer biomarkers present in bodily fluids in the early stages of the disease. Recently, various attempts have been made in their development and several methods and processes have been described for their development, amplification strategies and testing. The present review mainly focuses on the development of ELISA-based electrochemical immunosensors that may be utilized for cancer diagnosis, prognosis and therapy monitoring. Various fabrication methods and signal enhancement strategies utilized during the last few years for the development of ELISA-based electrochemical immunosensors are described.

Current Advances in Aptamers for Cancer Diagnosis and Therapy

Nucleic acid aptamers are single-stranded oligonucleotides that interact with target molecules with high affinity and specificity in unique three-dimensional structures. Aptamers are generally isolated by a simple selection process called systematic evolution of ligands by exponential enrichment (SELEX) and then can be chemically synthesized and modified. Because of their high affinity and specificity, aptamers are promising agents for biomarker discovery, as well as cancer diagnosis and therapy. In this review, we present recent progress and challenges in aptamer and SELEX technology and highlight some representative applications of aptamers in cancer therapy.

Aptasensor Based on Hierarchical Core-Shell Nanocomposites of Zirconium Hexacyanoferrate Nanoparticles and Mesoporous mFe3O4@mC: Electrochemical Quantitation of Epithelial Tumor Marker Mucin-1

A novel nanostructured hierarchical core-shell nanocomposite of zirconium hexacyanoferrate (ZrHCF) and a mesoporous nanomaterial composed of Fe₃O₄ and carbon nanospheres (denoted as ZrHCF@mFe₃O₄@mC) was prepared and used as a novel platform for an aptasensor to detect the epithelial tumor marker mucin-1 (MUC1) sensitively and selectively. The prepared ZrHCF@mFe₃O₄@mC nanocomposite exhibited good chemical functionality, water stability, and high specific surface area. Therefore, large amounts of aptamer molecules resulted in high sensitivity of the developed electrochemical aptasensor toward traces of MUC1. The constructed sensor also showed a good linear relationship with the logarithm of MUC1 concentration in the broad range of 0.01 ng·mL^-1 to 1.0 μg·mL^-1, with a low detection limit of 0.90 pg·mL^-1. The fabricated ZrHCF@mFe₃O₄@mC-based aptasensor exhibited not only high selectivity because of the formation of aptamer-MUC1 complex but also good stability, acceptable reproducibility, and applicability. The proposed novel strategy based on a newly prepared hierarchical core-shell nanocomposite demonstrated outstanding biosensing performance and presents potential applications in biomedical fields.

Nano-Aptasensing in Mycotoxin Analysis: Recent Updates and Progress

Recent years have witnessed an overwhelming integration of nanomaterials in the fabrication of biosensors. Nanomaterials have been incorporated with the objective to achieve better analytical figures of merit in terms of limit of detection, linear range, assays stability, low production cost, etc. Nanomaterials can act as immobilization support, signal amplifier, mediator and artificial enzyme label in the construction of aptasensors. We aim in this work to review the recent progress in mycotoxin analysis. This review emphasizes on the function of the different nanomaterials in aptasensors architecture. We subsequently relate their features to the analytical performance of the given aptasensor towards mycotoxins monitoring. In the same context, a critically analysis and level of success for each nano-aptasensing design will be discussed. Finally, current challenges in nano-aptasensing design for mycotoxin analysis will be highlighted.

Carbon Nanomaterial Based Biosensors for Non-Invasive Detection of Cancer and Disease Biomarkers for Clinical Diagnosis

The early diagnosis of diseases, e.g., Parkinson's and Alzheimer's disease, diabetes, and various types of cancer, and monitoring the response of patients to the therapy plays a critical role in clinical treatment; therefore, there is an intensive research for the determination of many clinical analytes. In order to achieve point-of-care sensing in clinical practice, sensitive, selective, cost-effective, simple, reliable, and rapid analytical methods are required. Biosensors have become essential tools in biomarker sensing, in which electrode material and architecture play critical roles in achieving sensitive and stable detection. Carbon nanomaterials in the form of particle/dots, tube/wires, and sheets have recently become indispensable elements of biosensor platforms due to their excellent mechanical, electronic, and optical properties. This review summarizes developments in this lucrative field by presenting major biosensor types and variability of sensor platforms in biomedical applications.

Thrombin aptasensor enabled by Pt nanoparticles-functionalized Co-based metal organic frameworks assisted electrochemical signal amplification

In this work, a Pt nanoparticles-functionalized Co-based metal organic frameworks (PtNPs@Co(II)MOFs@PtNPs) was synthesized and applied in electrochemical aptasensor for thrombin (TB) detection. First, the Co(II)MOFs@PtNPs were prepared via the mixed solvothermal method, which consists of inner Pt nanoparticles (PtNPs) encapsulated by aminofunctionalized Co(II)MOFs materials. Following that, additional PtNPs were adsorbed on the surface of Co(II)MOFs@PtNPs, resulting in the formation of PtNPs@Co(II)MOFs@PtNPs nanocomposite. The PtNPs@Co(II)MOFs@PtNPs nanocomposites with a large surface area were implimented as nanocarriers to immobilize a mass of TBA II for the formation of the TBA II bioconjugates that could be captured onto the electrode surface by sandwich-type format. Moreover, the PtNPs@Co(II)MOFs@PtNPs nanocomposites could directly use as redox tags for charge-generating and electron-transporting with the electron transfer from Co(II) to Co(III). Furthermore, in the presence of H₂O₂, the PtNPs@Co(II)MOF@PtNPs could effectively catalyze H₂O₂ oxidation with improvement electron transfer of redox probe, resulting in electrochemical signal amplification. Based on the above superior advantages, TB was determined in the concentration range from 0.1pM to 50nM with a detection limit of 0.33fM. Furthermore, the excellent sensitivity and selectivity can be easily established for quantitative analysis of other analytes.

SERS- and luminescence-active Au-Au-UCNP trimers for attomolar detection of two cancer biomarkers

Alpha-fetoprotein (AFP) and mucoprotein1 (mucin-1) are two important disease biomarkers. Self-assembled gold nanoparticles (AuNPs) and upconversion nanoparticle (Au-Au-UCNP) trimers based on aptamers were developed for the ultrasensitive detection of AFP and mucin-1. The Au-Au-UCNP trimers produced ideal optical signals, with prominent Raman enhancement and fluorescence quenching effects. The surface-enhanced Raman scattering (SERS) intensity decreased in the presence of mucin-1 and the luminescence intensity increased in the presence of AFP. A limit of detection (LOD) of 4.1 aM and a wide linear range of 0.01-10 fM for the detection of mucin-1 were obtained with this SERS-encoded sensing system. Using the luminescence-encoded sensing system, a LOD of 0.059 aM and a wide linear range of 1-100 aM for the detection of AFP were obtained. These LODs are the lowest values reported so far. This approach has the advantage of detecting two disease biomarkers simultaneously.

Aptamers: versatile molecular recognition probes for cancer detection

In the past two decades, aptamers have emerged as a novel class of molecular recognition probes comprising uniquely-folded short RNA or single-stranded DNA oligonucleotides that bind to their cognate targets with high specificity and affinity. Aptamers, often referred to as "chemical antibodies", possess several highly desirable features for clinical use. They can be chemically synthesized and are easily conjugated to a wide range of reporters for different applications, and are able to rapidly penetrate tissues. These advantages significantly enhance their clinical applicability, and render them excellent alternatives to antibody-based probes in cancer diagnostics and therapeutics. Aptamer probes based on fluorescence, colorimetry, magnetism, electrochemistry, and in conjunction with nanomaterials (e.g., nanoparticles, quantum dots, single-walled carbon nanotubes, and magnetic nanoparticles) have provided novel ultrasensitive cancer diagnostic strategies and assays. Furthermore, promising aptamer targeted-multimodal tumor imaging probes have been recently developed in conjunction with fluorescence, positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). The capabilities of the aptamer-based platforms described herein underscore the great potential they hold for the future of cancer detection. In this review, we highlight the most prominent recent developments in this rapidly advancing field.

A highly sensitive metronidazole sensor based on a Pt nanospheres/polyfurfural film modified electrode

A sensitively electrochemical metronidazole sensor basing on a Pt nanospheres/polyfurfural modified glassy carbon electrode.

Electrochemical immunosensor for N6-methyladenosine detection in human cell lines based on biotin-streptavidin system and silver-SiO2 signal amplification

N6-methyladenosine (m6A), a kind of RNA methylation form and important epigenetic event, plays crucial roles in many biological progresses. Thus it is essential to quantitatively detect m6A in complicated biological samples. Herein, a simple and sensitive electrochemical method was developed for m6A detection using N6-methyladenosine-5'-triphosphate (m6ATP) as detection target molecule. In this detection strategy, anti-m6A antibody was selected as m6A recognition and capture reagent, silver nanoparticles and amine-PEG3-biotin functionalized SiO₂ nanospheres (Ag@SiO₂) was prepared and used as signal amplification label, and phos-tag-biotin played a vital role of "bridge" to link m6ATP and Ag@SiO₂ through the two forms of specific interaction between phosphate group of m6ATP and phos-tag, biotin and streptavidin, respectively. Under the optimal experimental conditions, the immunosensor presented a wide linear range from 0.2 to 500nM and a low detection limit of 0.078nM (S/N=3). The reproducibility and specificity were acceptable. Moreover, the developed method was also validated for detect m6A content in human cell lines. Importantly, this detection strategy provides a promising immunodetection platform for ribonucleotides and deoxyribonucleotides with the advantages of simplicity, low-costing, specificity and sensitivity.

One Step Assembly of Thin Films of Carbon Nanotubes on Screen Printed Interface for Electrochemical Aptasensing of Breast Cancer Biomarker

Thin films of organic moiety functionalized carbon nanotubes (CNTs) from a very well-dispersed aqueous solution were designed on a screen printed transducer surface through a single step directed assembly methodology. Very high density of CNTs was obtained on the screen printed electrode surface, with the formation of a thin and uniform layer on transducer substrate. Functionalized CNTs were characterized by X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and Brunauer–Emmett–Teller (BET) surface area analyzer methodologies, while CNT coated screen printed transducer platform was analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The proposed methodology makes use of a minimum amount of CNTs and toxic solvents, and is successfully demonstrated to form thin films over macroscopic areas of screen printed carbon transducer surface. The CNT coated screen printed transducer surface was integrated in the fabrication of electrochemical aptasensors for breast cancer biomarker analysis. This CNT coated platform can be applied to immobilize enzymes, antibodies and DNA in the construction of biosensor for a broad spectrum of applications.

Electrochemical, Electrochemiluminescence, and Photoelectrochemical Aptamer-Based Nanostructured Sensors for Biomarker Analysis

Aptamer-based sensors have been intensively investigated as potential analytical tools in clinical analysis providing the desired portability, fast response, sensitivity, and specificity, in addition to lower cost and simplicity versus conventional methods. The aim of this review, without pretending to be exhaustive, is to give the readers an overview of recent important achievements about electrochemical, electrochemiluminescence, and photoelectrochemical aptasensors for the protein biomarker determination, mainly cancer related biomarkers, by selected recent publications. Special emphasis is placed on nanostructured-based aptasensors, which show a substantial improvement of the analytical performances.

Voltammetric aptasensors for protein disease biomarkers detection: A review

An electrochemical aptasensor is a compact analytical device where the bioreceptor (aptamer) is coupled to a transducer surface to convert a biological interaction into a measurable signal (current) that can be easily processed, recorded and displayed. Since the discovery of the Systematic Evolution of Ligands by Enrichment (SELEX) methodology, the selection of aptamers and their application as bioreceptors has become a promising tool in the design of electrochemical aptasensors. Aptamers present several advantages that highlight their usefulness as bioreceptors such as chemical stability, cost effectiveness and ease of modification towards detection and immobilization at different transducer surfaces. In this review, a special emphasis is given to the potential use of electrochemical aptasensors for the detection of protein disease biomarkers using voltammetry techniques. Methods for the immobilization of aptamers onto electrode surfaces are discussed, as well as different electrochemical strategies that can be used for the design of aptasensors.

Aptamer-nanoparticle complexes as powerful diagnostic and therapeutic tools

Correct diagnosis and successful therapy are extremely important to enjoy a healthy life when suffering from a disease. To achieve these aims, various cutting-edge technologies have been designed and fabricated to diagnose and treat specific diseases. Among these technologies, aptamer-nanomaterial hybrids have received considerable attention from scientists and doctors because they have numerous advantages over other methods, such as good biocompatibility, low immunogenicity and controllable selectivity. In particular, aptamers, oligonucleic acids or peptides that bind to a specific target molecule, are regarded as outstanding biomolecules. In this review, several screening techniques for aptamers, also called systematic evolution of ligands by exponential enrichment (SELEX) methods, are introduced, and diagnostic and therapeutic aptamer applications are also presented. Furthermore, we describe diverse aptamer-nanomaterial conjugate designs and their applications for diagnosis and therapy.

Towards the electrochemical diagnosis of cancer: nanomaterial-based immunosensors and cytosensors

In this review, nanomaterial based electrochemical biosensors including electrochemical immunosensors and cytosensors towards cancer detection are covered.