Patchy gold coated Fe3O4 nanospheres with enhanced catalytic activity applied for paper-based bipolar electrode-electrochemiluminescence aptasensors

Optical Image Sensors for Smart Analytical Chemiluminescence Biosensors

Optical biosensors have emerged as a powerful tool in analytical biochemistry, offering high sensitivity and specificity in the detection of various biomolecules. This article explores the advancements in the integration of optical biosensors with microfluidic technologies, creating lab-on-a-chip (LOC) platforms that enable rapid, efficient, and miniaturized analysis at the point of need. These LOC platforms leverage optical phenomena such as chemiluminescence and electrochemiluminescence to achieve real-time detection and quantification of analytes, making them ideal for applications in medical diagnostics, environmental monitoring, and food safety. Various optical detectors used for detecting chemiluminescence are reviewed, including single-point detectors such as photomultiplier tubes (PMT) and avalanche photodiodes (APD), and pixelated detectors such as charge-coupled devices (CCD) and complementary metal-oxide-semiconductor (CMOS) sensors. A significant advancement discussed in this review is the integration of optical biosensors with pixelated image sensors, particularly CMOS image sensors. These sensors provide numerous advantages over traditional single-point detectors, including high-resolution imaging, spatially resolved measurements, and the ability to simultaneously detect multiple analytes. Their compact size, low power consumption, and cost-effectiveness further enhance their suitability for portable and point-of-care diagnostic devices. In the future, the integration of machine learning algorithms with these technologies promises to enhance data analysis and interpretation, driving the development of more sophisticated, efficient, and accessible diagnostic tools for diverse applications.

Progress in quantum dot-based biosensors for microRNA assay: A review

MicroRNAs (miRNAs) are responsible for post-transcriptional gene regulation, and may function as valuable biomarkers for diseases diagnosis. Accurate and sensitive analysis of miRNAs is in great demand. Quantum dots (QDs) are semiconductor nanomaterials with superior optoelectronic features, such as high quantum yield and brightness, broad absorption and narrow emission, long fluorescence lifetime, and good photostability. Herein, we give a comprehensive review about QD-based biosensors for miRNA assay. Different QD-based biosensors for miRNA assay are classified by the signal types including fluorescent, electrochemical, electrochemiluminescent, and photoelectrochemical outputs. We highlight the features, principles, and performances of the emerging miRNA biosensors, and emphasize the challenges and perspectives in this field.

Paper-Based Aptasensors: Working Principles, Detection Modes, and Applications

Aptamers are short oligonucleotides designed to possess high binding affinity towards specific target compounds (ions, molecules, or cells). Due to their function and unique advantages, aptamers are considered viable alternatives to antibodies as biorecognition elements in bioassays and biosensors. On the other hand, paper-based devices (PADs) have emerged as a promising and powerful technology for the fabrication of low-cost analytical tools, mainly intended for on-site and point-of-care applications. The present work aims to provide a comprehensive overview of paper-based aptasensors. The review describes the fabrication methods and working principles of paper-based devices, the properties of aptamers as bioreceptors, the different modes of detection used in conjunction with aptasensing PADs, and representative applications for the detection of ions, small molecules, proteins, and cells. The future challenges and prospects of these devices are also discussed.

Paper-based bipolar electrode electrochemiluminescence sensors for point-of-care testing

In the past few years, point-of-care testing (POCT) technology has crossed the boundaries of laboratory determination and entered the stage of practical applications. Herein, the latest advances and principal issues in the design and fabrication of paper-based bipolar electrode electrochemiluminescence (BPE-ECL) sensors, which are widely used in the POCT field, are highlighted. After introducing the attractive physical and chemical properties of cellulose paper, various approaches aimed at enhancing the functions of the paper, and their underlying principles are described. The materials typically employed for fabricating paper-based BPE are also discussed in detail. Subsequently, the universal method of enhancing BPE-ECL signal and improving detection accuracy is put forward, and the ECL detector widely used is introduced. Furthermore, the application of paper-based BPE-ECL sensors in biomedical, food, environmental and other fields are displayed. Finally, future opportunities and the remaining challenges are analyzed. It is expected that more design concepts and working principles for paper-based BPE-ECL sensors will be developed in the near future, paving the way for the development and application of paper-based BPE-ECL sensors in the POCT field and providing certain guarantee for the development of human health.

Novel electrochemiluminescence biosensor of fumonisin B1 detection using MWCNTs-PDMS flexible bipolar electrode

A novel portable and disposable bipolar electrode (BPE)-electrochemiluminescence (ECL) device was fabricated for fumonisin B₁ (FB₁) detection. BPE was fabricated by using MWCNTs and polydimethylsiloxane (PDMS) due to their excellent electrical conductivity and good mechanical stiffness. After the deposition of Au NPs on the cathode of BPE, the ECL signal could be improved 89-fold. Then a specific aptamer-based sensing strategy was constructed by grafting capture DNA on Au surface, followed by hybridizing with aptamer. Meanwhile, an excellent catalyst, Ag NPs was labeled on aptamer to activate oxygen reduction reaction, leading to a 13.8-fold enhancement in ECL signal at the anode of BPE. Under the optimal conditions, the biosensor exhibited a wide linear range of 0.10 pg/mL to 10 ng/mL for FB₁ detection. Meanwhile, it demonstrated satisfactory recoveries for real sample detection with good selectivity, making it to be a convenient and sensitive device for mycotoxin assay.

Recent progress in assembly strategies of nanomaterials-based ultrasensitive electrochemiluminescence biosensors for food safety and disease diagnosis

The Electrochemiluminescence (ECL)-based biosensors have received considerable attention in food contaminants and disease diagnosis, due to their fascinating advantages such as low cost, fast analysis speed, wide linear range, high sensitivity, and excellent anti-interference ability. Meanwhile, with the vigorous development and improvement of nanotechnology, biosensor assembly strategies tend to diversify and be multifunctional. This review focuses on the representative ECL biosensors in food safety and disease diagnosis reported by our research group and other research groups based on nanomaterials assembly strategies in recent years. According to the different roles of nanomaterials played in the constitution of ECL biosensors, nanomaterials would be divided into the following two categories to be summarized: (1) Nanomaterials for signal amplification. (2) Nanomaterials as ECL emitters. Finally, this review prospects the perspectives on the future development direction of ECL biosensor in food safety and disease diagnosis.

Insights into the Mechanism of Bipolar Electrodeposition of Au Films and Its Application in Visual Detection of Prostate Specific Antigens

Au particles are commonly used for deposition on the surface of a bipolar electrode (BPE) in order to amplify electrochemical and electrochemiluminescence (ECL) signal because of their excellent conductivity, biocompatibility, and large surface area. In this work, a closed BPE device was fabricated and Au particles were deposited on the two poles of a BPE via bipolar deposition. Results indicated that the electrochemical stability of Au film on the anode part of the BPE and the reduction of AuCl₄^- to Au on the cathode part of the BPE depended on the conductivity of the solution. The prepared Au-Au BPE exhibited a remarkable amplification effect on the ECL signal. Then, a specific sensing interface was constructed on one pole of the BPE for the visual detection of prostate-specific antigens (PSA) based on sandwich-type immunoreactions between primary PSA antibodies (Ab₁) on the electrode surface, PSA, and SiO₂ nanoparticles labeled secondary PSA antibodies (SiO₂-Ab₂). The designed biosensor exhibited a good linear relationship for the ECL detection of PSA in the range of 1 × 10^-6 to 1 × 10^-10 g/mL with a correlation coefficient of 0.9866; the limit of detection (LOD) was 1.5 × 10^-11 g/mL. Additionally, the biosensor can realize the electrochemical imaging of PSA by regulating the electrochemical oxidation of the Au anode with the immunoreactions on the cathode part of BPE. Therefore, the small, portable and highly sensitive biosensors have great potential for on-site detection.

Enhancement of the Detection Performance of Paper-Based Analytical Devices by Nanomaterials

Paper-based analytical devices (PADs), including lateral flow assays (LFAs), dipstick assays and microfluidic PADs (μPADs), have a great impact on the healthcare realm and environmental monitoring. This is especially evident in developing countries because PADs-based point-of-care testing (POCT) enables to rapidly determine various (bio)chemical analytes in a miniaturized, cost-effective and user-friendly manner. Low sensitivity and poor specificity are the main bottlenecks associated with PADs, which limit the entry of PADs into the real-life applications. The application of nanomaterials in PADs is showing great improvement in their detection performance in terms of sensitivity, selectivity and accuracy since the nanomaterials have unique physicochemical properties. In this review, the research progress on the nanomaterial-based PADs is summarized by highlighting representative recent publications. We mainly focus on the detection principles, the sensing mechanisms of how they work and applications in disease diagnosis, environmental monitoring and food safety management. In addition, the limitations and challenges associated with the development of nanomaterial-based PADs are discussed, and further directions in this research field are proposed.

Bipolar (Bio)electroanalysis

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this "wireless" electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

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.

Chlorin e6 and polydopamine modified gold nanoflowers for combined photothermal and photodynamic therapy

Combinational photo-based approaches with enhanced efficacy for cancer therapy have garnered increasing attention in recent years. In this work, a multifunctional system for synergistic photothermal and photodynamic cancer therapy was successfully prepared. The system consists of gold nanoflowers (AuNFs) conjugated with Chlorin e6 (Ce6), and then coated with a polydopamine (PDA) layer. AuNFs with diameters around 80 nm and a broad absorbance in the visible-near infrared (Vis-NIR) range of 500 to 800 nm, were successfully synthesized by a two-step process at 0 °C, using HAuCl4, ascorbic acid (AA), and hydroxylamine hydrochloride (NH2OH·HCl) as the reactants. Glutathione (GSH) molecules chemically anchored to the gold surfaces were used to provide addressable sites for Ce6 conjugated to GSH-AuNFs through an amidation reaction. A PDA layer was then wrapped outside the Ce6-GSH-AuNFs via self-polymerization of dopamine, which provided additional chemical modification and functionalization. Finally, the multifunctional PDA-Ce6-GSH-AuNFs were obtained. The content of Ce6 incorporated into the AuNFs was 14.0 wt%, and the singlet oxygen yield of PDA-Ce6-GSH-AuNFs was approximately 91.0% of that of free Ce6. PDA-Ce6-GSH-AuNFs showed better photothermal conversion efficiency (η = 23.6%), lower cytotoxicity, and faster cell internalization. Both in vitro and in vivo investigation of the combined treatment with a near-infrared (NIR) laser (660 nm for photodynamic therapy, and 808 nm for photothermal therapy) demonstrated that PDA-Ce6-GSH-AuNFs had excellent phototoxicity and synergistic effects of killing cancer cells. Hence, PDA-Ce6-GSH-AuNFs are a dual phototherapeutic agent that exhibits photodynamic and photothermal therapeutic effects and has potential application in enhanced cancer therapy.

Biosensing platform on ferrite magnetic nanoparticles: Synthesis, functionalization, mechanism and applications

Ferrite magnetic nanoparticles (FMNPs) are gaining popularity to design biosensors for high-performance clinical diagnosis. The fusion of information shows that FMNPs based biosensors require well-tuned FMNPs as detection probes to produce large and specific biological signals with minimal non-specific binding. Nevertheless, there is a noticeable lacuna of information to solve the issues related to suitable synthesis route, particle size reduction, functionalization, sensitivity towards targeted intercellular biological tiny particles, and lower signal-to-noise ratio. Therefore it allows exploring unique characteristics of FMNPs to design a suitable sensing device for intracellular measurements and diseases detection. This review focuses on the extensively used synthesis routes, their advantages and limitations, crystalline structure, functionalization, along with recent applications of FMNPs in biosensors, taking into consideration their analytical figures of merit and range of linearity. This work also addresses the current progress, key factors for sensitivity, selectivity and productivity improvement along with the challenges, future trends and perspectives of FMNPs based biosensors.

Ultrasensitive detection of cyclin D1 by a self-enhanced ECL immunosensor based on Bi2S3 quantum dots

Bismuth sulfide quantum dots (Bi₂S₃ QDs), which have excellent optical and thermoelectric properties, represent a green and non-toxic semiconductor material that has been widely used in catalysis and photoelectric conversion devices. At present, research on this material has gradually expanded into the biological field. Herein, the biomineralization method mediated by bovine serum albumin (BSA) was utilized to synthesize Bi₂S₃ QDs with monodispersity, excellent colloidal stability, and good biocompatibility. This is the first study on the electrochemiluminescence (ECL) characteristics of Bi₂S₃ QDs and related ECL mechanisms in detail. In addition, on the basis of Bi₂S₃ QDs, an ECL immunosensor was used for the ultrasensitive measurement of cyclin D1 (CCND1). The composite material, namely Au@Cu-Bi₂S₃ QDs was used as a high-sensitivity ECL probe, in which AuNPs were connected with Bi₂S₃ QDs through a copper(ii) ion bridge. PDA-AgNPs made of dopamine (DA) and silver nanoparticles (AgNPs) were utilized as a carrier for fixing the primary antibody (Ab₁), ultimately presenting a relatively wide detection range of 10 fg mL^-1-1 μg mL^-1. Moreover, quite a low detection limit (6.34 fg mL^-1) was also obtained for an assay of CCND1. Results indicated that the immunosensor can provide a potential platform with fine stability and creditable reproducibility for clinical diagnosis.

Strategies for the detection of target analytes using microfluidic paper-based analytical devices

Microfluidic paper-based analytical devices (μPADs) have developed rapidly in recent years, because of their advantages, such as small sample volume, rapid detection rates, low cost, and portability. Due to these characteristics, they can be used for in vitro diagnostics in the laboratory, or in the field, for a variety of applications, including food evaluation, disease screening, environmental monitoring, and drug testing. This review will present various detection methods employed by μPADs and their respective applications for the detection of target analytes. These include colorimetry, electrochemistry, chemiluminescence (CL), electrochemiluminescence (ECL), and fluorescence-based methodologies. At the same time, the choice of labeling material and the design of microfluidic channels are also important for detection results. The construction of novel nanocomponents and different smart structures of paper-based devices have improved the performance of μPADs and we will also highlight some of these in this manuscript. Additionally, some key challenges and future prospects for the use of μPADs are briefly discussed.

Paper-Based Bipolar Electrode Electrochemiluminescence Platform for Detection of Multiple miRNAs

This paper introduces a novel potential-resolved paper-based biosensor for simultaneous detection of multiple microRNAs (miRNAs) (taking miRNA-155 and miRNA-126 as examples) based on the bipolar electrode (BPE) electrochemiluminescence (ECL) strategy. The proposed multiple-channel paper-based sensing microfluidic platform was prepared by wax-printing technology, screen-printing method, and in situ Au nanoparticles (AuNPs) growth to form hydrophilic areas, hydrophobic boundaries, waterproof electronic bridge, driving electrode regions, and parallel bipolar electrode regions. CdTe quantum dots (QDs)-H2 and Au@g-C₃N₄ nanosheets (NSs)-DNA1 were used as dual electrochemiluminescence signal probes, and carboxylated Fe₃O₄ magnetic nanoparticles existed as carriers. CdTe QDs-H2/S₂O₈^2- and Au@g-C₃N₄ NSs-DNA1/S₂O₈^2- could exhibit two strong and stable ECL emissions at a drive voltage of 9 and 12 V, respectively, which can be used as effective potential-resolved signal tags. In addition, the proposed three-dimensional (3D) DNA nanomachine model and the target miRNA cycle strategy were used to achieve double amplification of electrochemiluminescence intensity. More importantly, the combination of the bipolar electrode system and the potential-resolved multitarget electrochemiluminescence method can greatly reduce the spatial interference between substances. The prepared ECL biosensor showed a favorable linear response for the detection of miRNA-155 and miRNA-126 with relatively low detection limits of 5.7 and 4.2 fM, respectively. With excellent sensitivity, the strategy may provide an efficient method for clinical application, especially in detection of trace multiple targets.

Application of magnetic nanomaterials in electroanalytical methods: A review

Magnetic nanomaterials (MNMs) have gained high attention in different fields of studies due to their ferromagnetic/superparamagnetic properties and their low toxicity and high biocompatibility. MNMs contain magnetic elements such as iron and nickel in metallic, bimetallic, metal oxide, and mixed metal oxide. In electroanalytical methods, MNMs have been applied as sorbents for sample preparation before the electrochemical detection (sorbent role), as the electrode modifier (catalytic role), and the integration of the above two roles (as both sorbent and catalytic agent). In this paper, the application of MNMs in electroanalytical methods have been classified based on the main role of the nanomaterial and discussed separately. Furthermore, catalytic activities of MNMs in electroanalytical methods such as redox electrocatalytic, nanozymes catalytic (peroxidase, catalase activity, oxidase activity, superoxide dismutase activity), catalyst gate, and nanocontainer have been discussed.

Paper-based microfluidic aptasensors

For in-situ disease markers detection, point-of-care (POC) diagnosis has great advantages in speed and cost compared with traditional techniques. The rapid diagnosis, prognosis, and surveillance of diseases can significantly reduce disease-related mortality and trauma. Therefore, increasing attention has been paid to the POC diagnosis devices due to their excellent diagnosis speed and portability. Over the past ten years, paper-based microfluidic aptasensors have emerged as a class of critical POC diagnosis devices and various aptasensors have been proposed to detect various disease markers. However, most aptasensors need further improvement before they can actually enter the market and be widely used. There is thus an urgent need to sort out the key points of preparing the aptasensors and the direction that needs to be invested in. This review summarizes the representative articles in the development of paper-based microfluidic aptasensors. These works can be divided into paper-based optical aptasensors and paper-based electrochemical aptasensors according to their output signals. Significant focus is applied to these works according to the following three parts: (1) The ingenious design of device structure; (2) Application and synthesis of nanomaterial; (3) The detection principle of the proposed aptasensor. This is a detailed and comprehensive review of paper-based microfluidic aptasensors. The accomplishments and shortcomings of the current aptasensors are outlined, the development direction and the future prospective are given. It is hoped that the research in this review can provide a reference for further development of more advanced, more effective paper-based microfluidic aptasensors for POC disease markers diagnosis.

A Novel NiFe2O4/Paper-Based Magnetoelastic Biosensor to Detect Human Serum Albumin

For the first time, a novel NiFe2O4/paper-based magnetoelastic (ME) biosensor was developed for rapid, sensitive, and portable detection of human serum albumin (HSA). Due to the uniquely magnetoelastic effect of NiFe2O4 nanoparticles and the excellent mechanical properties of the paper, the paper-based ME biosensor transforms the surface stress signal induced by the specific binding of HSA and antibody modified on the paper into the electromagnetic signal. The accumulated binding complex generates a compressive stress on the biosensor surface, resulting in a decrease in the biosensor's static magnetic permeability, which correlates to the HSA concentrations. To improve the sensitivity of the biosensor, the concentration of NiFe2O4 nanofluid and the impregnated numbers of the NiFe2O4 nanofluid-impregnated papers were optimized. The experimental results demonstrated that the biosensor exhibited a linear response to HSA concentrations ranging from 10 μg∙mL-1 to 200 μg∙mL-1, with a detection limit of 0.43 μg∙mL-1, which is significantly lower than the minimal diagnosis limit of microalbuminuria. The NiFe2O4/paper-based ME biosensor is easy to fabricate, and allows the rapid, highly-sensitive, and selective detection of HSA, providing a valuable analytical device for early monitoring and clinical diagnosis of microalbuminuria and nephropathy. This study shows the successful integration of the paper-based biosensor and the ME sensing analytical method will be a highly-sensitive, easy-to-use, disposable, and portable alternative for point-of-care monitoring.

Electrochemical aptasensors based on the gold nanostructures

Electrochemical aptasensors as novel diagnostic tools have attracted sufficient research interest in biomedical sciences. In this review, recent leading trends about gold (Au) nanostructures based electrochemical aptasensors have been collected, reviewed, and compared. Here, the considered electrochemical aptasensors were categorized based on the analytes and diagnostic techniques. Pharmaceutical analytes and biomolecules were reviewed in a separate section consisting of a variety of antibiotics, analgesics, and other biomolecules. Various aptasensors have also measured toxins, ions, and hazardous chemicals, and the findings of them have also been reviewed. Many aptasensors have been designed to detect different disease biomarkers that will play an essential role in the future of early diagnosis of diseases. Pathogen microorganisms have been considered as the analyte in several designed electrochemical aptasensors in recent researches, and their results have been reviewed and discussed as another section. Important aspects considered in the review of the mentioned aptasensors were the type of analyte, features of the aptamer as the biorecognition element, type of Au nanostructures, diagnostic technique, diagnostic mechanism, detection range and the limit of detection (LOD). In the last section, an in-depth analysis has been provided based on the crucial features of all included aptasensors.

Resonance Raman scattering-infrared absorption dual-mode immunosensing for carcinoembryonic antigen based on ZnO@SiO2 nanocomposites

Detection of cancer biomarkers is crucial for the diagnosis and monitoring of malignant tumors. However, the accuracy and sensitivity still require sufficient improvement for practically clinical application. In this work, a reliable and sensitive dual-mode immunosensing method is described for carcinoembryonic antigen (CEA) detection using a biofunctional ZnO@SiO₂ nanocomposite as a resonance Raman scattering (RRS)-infrared (IR) absorption nanoprobe. The multiphonon RRS signal originating from the ZnO and the characteristic IR fingerprint signal of the transverse optical and longitudinal optical phonon modes of the asymmetric stretching of Si-O-Si bonds showed no interference with each other. A CEA antibodies-immobilized substrate was fabricated to capture the analyte/nanoprobe complexes. The RRS intensity at 569 cm^‒1 and the IR absorption at 1061 cm^‒1 were used for quantitative analysis. Accurate CEA detection was performed as a result of the strong resistance of the dual-mode nanoprobe to surrounding interference. The limit of detection was 98.0 fg mL^‒1. The detection range was 500 ng mL^‒1 - 50 fg mL^‒1, which is wider than those of single-mode RRS or IR absorption immunosensings. High reproducibility, selectivity and specificity were achieved. The assay performance of human serum samples demonstrated the practicability of the method in clinical cancer diagnosis.

Aptamer-based biosensor for detecting carcinoembryonic antigen

Carcinoembryonic antigen (CEA), as one of the common tumor markers, is a human glycoprotein involved in cell adhesion and is expressed during human fetal development. Since the birth of human, CEA expression is largely inhibited, with only low levels in the plasma of healthy adults. Generally, CEA will overexpressed in many cancers, including gastric, breast, ovarian, lung, and pancreatic cancers, especially colorectal cancer. As one of the important tumor markers, the detection of CEA has great significance in differential diagnosis, condition monitoring and therapeutic evaluation of diseases. Conventional CEA testing typically uses immunoassay methods. However, immunoassay methods require complex and expensive instruments and professional personnel to operate. Moreover, radioactive element may cause certain damage to the human body, which limits their wide application. In the past few years, biosensors, especially aptamer-based biosensors, have attracted extensive attention due to their high sensitivity, good selectivity, high accuracy, fast response and low cost. This review briefly classifies and describes the advance in optical and electrochemical aptamer biosensors for CEA detection, also explains and compares their advantages and disadvantages.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook.

Advances in functional nucleic acid based paper sensors

This article provides an extensive review of paper-based sensors that utilize functional nucleic acids, particularly DNA aptamers and DNAzymes, as recognition elements.

A new Bi2MoO6 nano-tremella-based electrochemical immunosensor for the sensitive detection of a carcinoembryonic antigen

Novel Bi₂MoO₆ nanohybrids with a tremella-like structure modified with gold nanoparticles were used to fabricate an electrochemical immunosensing platform of CEA.

Enhanced synergistic effects from multiple iron oxide nanoparticles encapsulated within nitrogen-doped carbon nanocages for simple and label-free visual detection of blood glucose

Hollow-structured carbon materials play a crucial role in research of biosensors, energy storage and nanomedicine as a kind of material with advantages like high surface area, tunable pore volume, excellent mechanical properties, and good biocompatibility. Herein, we developed a simple, facile and controllable method for synthesis of Fe₃O₄ nanoparticles encapsulated in hollow carbon nanocages (FNHCs) with SiO₂ nanospheres as a sacrificial template. Owing to the unique structure of multiple Fe₃O₄ nanoparticles cores integrated with N-doped carbon nanocages, the as-synthesized FNHCs exhibited greatly enhanced peroxidase mimicking activity with extremely high signal-to-noise ratio of ∼91 fold. Also, it was found that the FNHCs possessed a higher peroxidase-like activity than that of other similar-structured Fe₃O₄ architectures (e.g. Fe₃O₄@C NPs). The resulting steady-state kinetic curve demonstrated the enzymatic activity of FNHCs with classic Michaelis-Menton kinetics following a ping-pong mechanism. On the basis of the superior enzymatic activity, the FNHCs performed as a high-efficiency peroxidase mimic, realizing facile, label-free, highly sensitive/selective colorimetric detection of H₂O₂ and glucose. Furthermore, the colorimetric sensor successfully determined glucose in patients' serum samples with high accuracy and precision, suggesting great potential for real applications.

Ultrasensitive electrochemical genosensor for detection of CaMV35S gene with Fe3O4-Au@Ag nanoprobe

We report an attomolar sensitive electrochemical genosensor for the detection of cauliflower mosaic virus 35S (CaMV35S) gene. The sandwich-type genosensor uses gold-silver core-shell (Au@Ag)-loaded iron oxide (Fe₃O₄) nanocomposite (Fe₃O₄-Au@Ag) as label of signal DNA probe (sDNA). Electrochemical sensing is accomplished at interface of electrodeposited AuNPs and carboxylated multiwalled carbon nanotubes-modified glassy carbon electrode through the specific interaction between the capture probe and target CaMV35S (tDNA), and tDNA and the labeled sDNA. The detection sensitivity was improved by the amplified reduction signal of hydrogen peroxide (H₂O₂), which takes advantage of the enhanced electrocatalytic activity of Fe₃O₄-Au@Ag. Under the optimal experimental conditions, an ultralow limit of detection was calculated to be 1.26 × 10^-17 M (S/N = 3), and the blank value subtracted reduction signal of H₂O₂ of the sensor increased linearly with the logarithm of CaMV35S concentration over a wide range (1 × 10^-16 M to 1 × 10^-10 M). This genosensor displayed excellent stability, selectivity and reproducibility, and was successful in detecting the target CaMV35S in genetically modified tomato samples.

Bipolar electrode-electrochemiluminescence (ECL) biosensor based on a hybridization chain reaction

A novel electrochemiluminescence (ECL) closed bipolar electrode (BPE) chip was designed based on a hybridization chain reaction (HCR)-induced ECL amplification strategy for the detection of both DNA and H₂O₂. Without the utilization of a patterned ITO bipolar electrode (BPE), this chip platform consisted of an ITO glass coated with two layers of PDMS slices. The ITO cathode was modified with Au nanoparticles for further functionalization of biomolecules, which could also amplify the ECL signal at the anode of the BPE. Based on the specific hybridization and hybridization chain reaction (HCR), DNA sequences were greatly extended, leading to a significant increase in the resistance of the cathode. The reduction of H₂O₂ was inhibited on the cathode of the BPE, resulting in a quenching effect on the ECL intensity on the anode of the BPE. The designed biosensor displayed a satisfactory linear relationship for the detection of both DNA and H₂O₂. Therefore, the biosensor could not only be employed for DNA assays but also used in enzyme reactions based on the generation of H₂O₂.

Photoelectrochemical biosensor for CEA detection based on SnS2-GR with multiple quenching effects of Au@CuS-GR

A novel signal on-off type photoelectrochemical (PEC) biosensing system was designed for sensitive detection of carcinoembryonic antigen (CEA) based on tin disulfide nanosheets loaded on reduced graphene cxide (SnS₂-GR) as the photoactive material and gold nanoparticles coated on reduced graphene oxide-functionalized copper sulfide (Au@CuS-GR) for signal amplification. It's the first time for SnS₂-GR was exploited as a sensing matrix. Here, the photocurrent signals of SnS₂ were amplified attributed to the sensitization effect of graphene. As signal amplifier, Au@CuS-GR could quench the photocurrents of SnS₂-GR not only through the p-n type semiconductor quenching effect as well as the steric hindrance effect, but also as peroxidase mimetics to catalyze the oxidation of 4-Chloro-1-naphthol (4-CN) to produce insoluble product on the electrode surface. Based on the multiple signal amplification ability of Au@CuS-GR, CEA was detected sensitively with a linear range from 0.1 pg mL^-1 to 10 ng mL^-1 and limit of detection down to 59.9 fg mL^-1 (S/N = 3). Meanwhile, the PEC biosensor displayed excellent performance in the assay of human serum sample, showing good application prospects for various target analysis.

Dual-signal-amplified electrochemiluminescence biosensor for microRNA detection by coupling cyclic enzyme with CdTe QDs aggregate as luminophor

In this work, a dual-signal-amplified electrochemiluminescence (ECL) biosensor was proposed for the first time to detect microRNAs (miRNAs) based on cyclic enzyme and seeded-watermelon-like mesoporous nanospheres (mSiO₂@CdTe@SiO₂, mSQS NSs). mSQS NSs were successfully fabricated by inlaying the CdTe quantum dots (QDs) into the mesoporous silica (mSiO₂) and future coating the surface with the silica layer. The obtained mSQS NSs contained tens of QDs and exhibited much stronger ECL signal than single QDs. The ECL biosensor achieved firstly signal amplification by using mSQS NSs to label the functional oligonucleotide probe (DNA-F) as enhanced ECL signal probes. Well-dispersed Fe₃O₄@Au nanoparticles were prepared as immobilization matrices to load hairpin-structured DNA probe (DNA-P). When the target miRNAs were present, hairpin DNA undertook conformation changes. Meanwhile, RNA/DNA duplexes was formed which cleaved by duplex-specific nuclease (DSN) to release miRNAs. Target miRNAs were cycled to hybridize with hairpin DNA, which achieved secondly signal amplification of the ECL biosensor. Thereafter, the complementarily parts between DNA-F and the rest DNA-P generated conjugates. The obtained conjugates would be collected on the surface of the electrode by effecting of magnet. Under the optimal conditions, the developed biosensor showed a wide linear range from 0.1 pM to 100 pM with a low detection limit of 33 fM (S/N = 3). The results of detection for the stability, specificity and reproducibility of ECL biosensor were outstanding. Simultaneously, the potential application of ECL biosensor was verified by using biosensor in serum sample.

Rapid Detection of Severe Fever with Thrombocytopenia Syndrome Virus via Colloidal Gold Immunochromatography Assay

To develop the point-of-care testing method to facilitate the clinical detection of severe fever with thrombocytopenia syndrome virus (SFTSV), colloidal gold paper-based lateral flow immunochromatography test strips (LFITSs) have been fabricated for the rapid detection for the first time. The pH value and the amount of monoclonal antibody to prepare colloidal gold nanoparticle-labeled monoclonal antibody bioconjugates were optimized. In addition, 0.4% bovine serum albumin was considered to be the best concentration for blocking nitrocellulose membranes. Under optimal conditions, the limit of detection for SFTSV was as low as 1 ng/mL depending on a visual line. Meanwhile, the entire detection process required no more than 10 min with a volume of only 50 μL of the analyte solution. Moreover, paper-based LFITSs were evaluated in real samples of human serum of patients with satisfactory results. In addition, all strips were of high stability and specificity. In the light of advantages such as simple, portable, rapid, and low cost, the developed LFITSs will extensively come into service, especially in remote areas.