Detection of vascular endothelial growth factor based on rolling circle amplification as a means of signal enhancement in surface plasmon resonance

DNA-modified Prussian blue nanozymes for enhanced electrochemical biosensing

Prussian blue nanoparticles exhibit the potential to be employed in bioanalytical applications due to their robust stability, peroxidase-like catalytic functionality, straightforward synthesis, and biocompatibility. An efficient approach is presented for the synthesis of nucleic acid-modified Prussian blue nanoparticles (DNA-PBNPs), utilizing nanoparticle porosity to adsorb nucleic acids (polyT). This strategic adsorption leads to the exposure of nucleic acid sequences on the particle surface while retaining catalytic activity. DNA-PBNPs further couple with functional nucleic acid sequences and aptamers through complementary base pairing to act as transducers in biosensors and amplify signal acquisition. Subsequently, we integrated a copper ion-dependent DNAzyme (Cu2+-DNAzyme) and a vascular endothelial growth factor aptamer (VEGF aptamer) onto screen-printed electrodes to serve as recognition elements for analytes. Significantly, our approach leverages DNA-PBNPs as a superior alternative to traditional enzyme-linked antibodies in electrochemical biosensors, thereby enhancing both the efficiency and adaptability of these devices. Our study conclusively demonstrates the application of DNA-PBNPs in two different biosensing paradigms: the sensitive detection of copper ions and vascular endothelial growth factor (VEGF). These results indicate the promising potential of DNA-modified Prussian blue nanoparticles in advancing bioanalytical sensing technologies.

Recent trends in aptamer-based nanobiosensors for detection of vascular endothelial growth factors (VEGFs) biomarker: A review

Vascular endothelial growth factor (VEGF) is a remarkable cytokine that plays an important role in regulating vascular formation during the angiogenesis process. Therefore, real-time detection and quantification of VEGF is essential for clinical diagnosis and treatment due to its overexpression in various tumors. Among various sensing strategies, the aptamer-based sensors in combination with biological molecules improve the detection ability VEGFs. Aptamers are suitable biological recognition agents for the preparation of sensitive and reproducible aptasensors (Apt-sensors) due to their low immunogenicity, simple and straightforward chemical modification, and high resistance to denaturation. Here, a summary of the strategies for immobilization of aptamers (e.g., direct or self-assembled monolayer (SAM) attachment, etc.) on different types of electrodes was provided. Moreover, we discussed nanoparticle deposition techniques and surface modification methods used for signal amplification in the detection of VEGF. Furthermore, we are investigating various types of optical and electrochemical Apt-sensors used to improve sensor characterization in the detection of VEGF biomarkers.

VEGF Detection via Impedance Spectroscopy on Surface Functionalized Interdigitated Biosensor

Vascular endothelial growth factor (VEGF), a clinically important biomarker, often plays a key role in angiogenesis, would healing, tumor growth, lung development, and in retinal diseases. Hence, detecting and quantifying VEGF is deemed medically important in clinical diagnosis for many diseases. In this report, a simple yet highly cost-effective platform was proposed for VEGF protein detection using commercially available interdigitated sensors that are surface modified to present DNA optimally for VEGF capture. The dielectric characteristics between the fingers of the sensor were modulated by the negatively charged aptamer-VEGF capture, and the impedance was estimated using an impedance analyzer. Impedance-spectra tests were compared among pristine unmodified surfaces, functionalized monolayer surfaces, and aptamer-grafted surfaces in order to evaluate the efficacy of VEGF detection. From our results, the sensitivity experiments as conducted showed the ability of the interdigitated sensor to detect VEGF at a low concentration of 5 pM (200 pg/mL). The specificity of the functionalized sensor in detecting VEGF was further examined by comparing the impedance to platelet-derived growth factor, and the results confirm the specificity of the sensor. Finally, the Nyquist plot of impedance spectra was also presented to help data visualization and the overall performance of the device was found to be a highly suitable template for a smart biosensor for the detection of VEGF.

One Single Tube Reaction of Aptasensor-Based Magnetic Sensing System for Selective Fluorescent Detection of VEGF in Plasma

In this study, a simple, easy and convenient fluorescent sensing system for the detection of the vascular endothelial growth factor (VEGF) based on VEGF aptamers, aptamer-complementary fluorescence-labeled probe and streptavidin magnetic beads was developed in one single tube. The VEGF is the most important biomarker in cancer, and it is investigated that the serum VEGF level varied according to the different types and courses of cancers. Hence, efficient quantification of VEGF is able to improve the accuracy of cancer diagnoses and the precision of disease surveillance. In this research, the VEGF aptamer was designed to be able to bind with the VEGF by forming G-quadruplex secondary structures; then, the magnetic beads would capture the non-binding aptamers due to non-steric interference; and finally, the fluorescence-labeled probes were hybridized with the aptamers captured by the magnetic beads. Therefore, the fluorescent intensity in the supernatant would specifically reflect the present VEGF. After an overall optimization, the optimal conditions for the detection of VEGF were as followed, KCl, 50 μM; pH 7.0; aptamer, 0.1 μM; and magnetic beads, 10 μL (4 μg/μL). The VEGF could be well quantified within a range of 0.2-2.0 ng/mL in plasma, and the calibration curve possessed a good linearity (y = 1.0391x + 0.5471, r = 0.998). The detection limit (LOD) was calculated to be 0.0445 ng/mL according to the formula (LOD = 3.3 × σ/S). The specificity of this method was also investigated under the appearance of many other serum proteins, and the data showed good specificity in this aptasensor-based magnetic sensing system. This strategy provided a simple, sensitive and selective biosensing platform for the detection of serum VEGF. Finally, it was expected that this detection technique can be used to promote more clinical applications.

Integration of CuS/ZnIn2S4 flower-like heterojunctions and (MnCo)Fe2O4 nanozyme for signal amplification and their application to ultrasensitive PEC aptasensing of cancer biomarker

Vascular endothelial growth factor 165 (VEGF₁₆₅) is a crucial regulator of angiogenesis and works as a major protein biomarker of cancer metastasis. Therefore, its quantitative detection is pivotal in clinic. In this work, CuS/ZnIn₂S₄ flower-like heterojunctions had strong and stable photocurrents, which behaved as photoactive material to construct a photoelectrochemical (PEC) aptasensor for detecting VEGF₁₆₅, combined by home-prepared (MnCo)Fe₂O₄ nanozyme-mediated signal amplification. The interfacial photo-induced electron transfer mechanism was chiefly discussed by UV-vis diffuse reflectance spectroscopy in details. Specifically, the (MnCo)Fe₂O₄ modified VEGF₁₆₅ aptamer was released from the PEC aptasensing platform for its highly specific affinity to target VEGF₁₆₅, which terminated the color precipitation reaction, ultimately recovering the PEC signals. The developed sensor displayed a wider linear range from 1 × 10^-2 to 1 × 10⁴ pg mL^-1 with a smaller limit of detection (LOD) of 0.1 fg mL^-1. This study provides some valuable insights for building other ultrasensitive aptasensors for clinical assays of cancer biomarkers in practice.

Signal amplification strategies in biosensing of extracellular vesicles (EVs)

Extracellular vesicles (EVs) are membrane-enclosed vesicles secreted from mammalian cells. EVs act as multicomponent delivery vehicles to carry a wide variety of biological molecular information and participate in intercellular communications. Since elevated levels of EVs are associated with some pathological states such as inflammatory diseases and cancers, probing circulating EVs holds a great potential for early diagnostics. To this end, several detection methods have been developed in which biosensors have attracted great attentions in identification of EVs due to their simple instrumentation, versatile design and portability for point-of-care applications. The concentrations of EVs in bodily fluids are extremely low (i.e. 1-100 per μl) at early stages of a disease, which necessitates the use of signal amplification strategies for EVs detection. In this way, this review presents and discusses various amplification strategies for EVs biosensors based on detection modalities including surface plasmon resonance (SPR), calorimetry, fluorescence, electrochemical and electrochemiluminescence (ECL). In addition, microfluidic systems employed for signal amplification are reviewed and discussed in terms of their design and integration with the detection methods.

Recent advances in optical aptasensors for biomarkers in early diagnosis and prognosis monitoring of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) accounts for approximately 90% of all primary liver cancers and is one of the main malignant tumor types globally. It is essential to develop rapid, ultrasensitive, and accurate strategies for the diagnosis and surveillance of HCC. In recent years, aptasensors have attracted particular attention owing to their high sensitivity, excellent selectivity, and low production costs. Optical analysis, as a potential analytical tool, offers the advantages of a wide range of targets, rapid response, and simple instrumentation. In this review, recent progress in several types of optical aptasensors for biomarkers in early diagnosis and prognosis monitoring of HCC is summarized. Furthermore, we evaluate the strengths and limitations of these sensors and discuss the challenges and future perspectives for their use in HCC diagnosis and surveillance.

Rolling Circle Amplification Tailored for Plasmonic Biosensors: From Ensemble to Single-Molecule Detection

We report on the tailoring of rolling circle amplification (RCA) for affinity biosensors relying on the optical probing of their surface with confined surface plasmon field. Affinity capture of the target analyte at the metallic sensor surface (e.g., by using immunoassays) is followed by the RCA step for subsequent readout based on increased refractive index (surface plasmon resonance, SPR) or RCA-incorporated high number of fluorophores (in surface plasmon-enhanced fluorescence, PEF). By combining SPR and PEF methods, this work investigates the impact of the conformation of long RCA-generated single-stranded DNA (ssDNA) chains to the plasmonic sensor response enhancement. In order to confine the RCA reaction within the evanescent surface plasmon field and hence maximize the sensor response, an interface carrying analyte-capturing molecules and additional guiding ssDNA strands (complementary to the repeating segments of RCA-generated chains) is developed. When using the circular padlock probe as a model target analyte, the PEF readout shows that the reported RCA implementation improves the limit of detection (LOD) from 13 pM to high femtomolar concentration when compared to direct labeling. The respective enhancement factor is of about 2 orders of magnitude, which agrees with the maximum number of fluorophore emitters attached to the RCA chain that is folded in the evanescent surface plasmon field by the developed biointerface. Moreover, the RCA allows facile visualizing of individual binding events by fluorescence microscopy, which enables direct counting of captured molecules. This approach offers a versatile route toward a fast digital readout format of single-molecule detection with further reduced LOD.

Functional nucleic acid biosensors utilizing rolling circle amplification

Functional nucleic acids regulate rolling circle amplification to produce multiple detection outputs suitable for the development of point-of-care diagnostic devices.

A Fluorescent Aptasensor Based on Assembled G-Quadruplex and Thioflavin T for the Detection of Biomarker VEGF165

VEGF165, a regulator of angiogenesis, has been widely used as a serum biomarker for a number of human diseases, including cancer, rheumatoid arthritis, bronchial asthma, and diabetic eye disease. The rapid, accurate, and convenient detection of VEGF165 is a crucial step in effective healthcare monitoring, disease diagnosis, and prognosis assessment. In this study, a fluorescent aptasensor based on an assembled G-quadruplex and the signal molecule ThT was developed for VEGF165 detection. First, G-rich DNA fragments were assembled at both ends of the anti-VEGF165 aptamer, and the B-DNA form was converted into a G-quadruplex structure aptamer (G4-Apt). Then, ThT was introduced, and the G-quadruplex significantly enhanced the fluorescence intensity of the bound ThT. When VEGF165 was present, the higher affinity of the aptamer to the target protein allowed the G4-Apt/VEGF165 complex to form and release ThT, which emitted only weak fluorescence in the free state. Therefore, the aptasensor exhibited a good linear detection window from 1.56 to 25 nM VEGF165, with a limit of detection of 0.138 nM. In addition, the aptasensor was applied to detect VEGF165 in clinical serum samples, showing good accuracy, reproducibility, and stability. These results indicate that our developed fluorescent aptasensor can potentially be a reliable, convenient, and cost-effective approach for the sensitive, specific, and rapid detection of the VEGF165 biomarker.

Recent Advancements in Aptamer-Based Surface Plasmon Resonance Biosensing Strategies

Surface plasmon resonance (SPR) can track molecular interactions in real time, and is a powerful as well as widely used biological and chemical sensing technique. Among the different SPR-based sensing applications, aptamer-based SPR biosensors have attracted significant attention because of their simplicity, feasibility, and low cost for target detection. Continuous developments in SPR aptasensing research have led to the emergence of abundant technical and design concepts. To understand the recent advances in SPR for biosensing, this paper reviews SPR-based research from the last seven years based on different sensing-type strategies and sub-directions. The characteristics of various SPR-based applications are introduced. We hope that this review will guide the development of SPR aptamer sensors for healthcare.

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.

Protein detection based on rolling circle amplification sensors

Rolling circle amplification (RCA) is an isothermal process under the action of DNA polymerases. Large-scale DNA templates have been generated using RCA for target detection. Some signal amplification strategies including optical sensors and electrochemical sensors based on RCA have been applied to achieve sensitive detection. Sensors based on RCA have attracted increasing interest. Advances in RCA-based sensors for protein detection are reviewed in this paper. The advantages and detection mechanisms of sensors based on RCA are revealed and discussed. Finally, possible challenges and future perspectives are also outlined.

Porphyrin-based covalent organic framework as bioplatfrom for detection of vascular endothelial growth factor 165 through fluorescence resonance energy transfer

A fluorescent aptasensor based on porphyrin-based covalent organic framework (p-COF) and carbon dots (CDs) was constructed for detecting vascular endothelial growth factor 165 (VEGF₁₆₅) and for imaging of the breast cancer cell line Michigan cancer foundation-7 (MCF-7). CDs synthesized with strong photoluminescence at λ∼380 nm were used as donors to label the VEGF₁₆₅-targeted aptamers (Apt_VEGF/CDs). Additionally, the p-COF nanostructure comprised rich functional groups of CN on the surface and π-stacking planar nanostructure, resulting in the CDs adsorption via weakly π-π stacking, hydrogen bond and the Van der Waals force. Thereby, the fluorescence resonance energy transfer (FRET) occurred due to the close distance between the p-COF network and CDs, leading to the quenching of the fluorescence feature of CDs and p-COF. In the presence of VEGF₁₆₅, the G-quadruplex was formed via the specific binding between VEGF₁₆₅ and aptamer. It impelled that the release of partial VEGF₁₆₅-Apt_VEGF/CDs complex, affording the fluorescence recovery of the sensing system to some extent. Consequently, the proposed Apt_VEGF/CDs/p-COF fluorescence biosensor offered excellent analytical performances for the VEGF₁₆₅ detection, displaying a detection limit of 20.9 fg mL^-1 within a wide linear range of the VEGF₁₆₅ concentration of 1.0 pg mL^-1-100 ng mL^-1. The developed fluorescence biosensor was also used to determine VEGF₁₆₅-overexpressed in MCF-7 cancer cells. Thereby, the present work can greatly widen the application of COFs in the development of aptasensors and cancer diagnosis.

Simultaneous Detection of VEGF and CEA by Time-Resolved Chemiluminescence Enzyme-Linked Aptamer Assay

BACKGROUND

As two important tumor markers, vascular endothelial growth factor (VEGF) and carcinoembryonic antigen (CEA) have a great value for clinical application in the early diagnosis of cancer. Due to the complex composition of biological samples, the results from combined detection of CEA and VEGF are often taken as a comprehensive indicator in order to make an accurate judgment on a disease. However, most of the current methods can only be used to detect the content of one biomarker. Therefore, it is necessary to explore a simple, rapid, low-cost, and highly sensitive method for the simultaneous detection of CEA and VEGF.

METHODS

Based on specific aptamers and magnetic separation, a time-resolved chemiluminescence enzyme-linked aptamer assay was developed for the simultaneous detections of CEA and VEGF in serum samples.

RESULTS

Under the optimal conditions, the linear range of the calibration curve for VEGF was from 0.5 to 80 ng mL-1, and the limit of detection was 0.1 ng mL-1. The linear range of the calibration curve for CEA was 0.5 to 160 ng mL-1, and the limit of detection was 0.1 ng mL-1. The established method was applied to detect VEGF and CEA in serum samples. The results were consistent with those of commercial kits.

CONCLUSION

The method has high sensitivity and can quickly obtain accurate results, which could greatly improve the measurement efficiency, reduce the cost, and also reduce the volume of sample consumed. It can be seen that the method established in this study has important application value and broad application prospect in clinical diagnosis.

A sensitive spectrophotometric ellipsometry based Aptasensor for the vascular endothelial growth factor detection

A sensitive and selective, aptamer and spectroscopic ellipsometry based sensor is reported here for the early diagnosis of breast cancer, which is a common type of cancer following lung cancer. It was aimed to develop a single-step and label-free assay for the sensitive and selective detection of VEGF₁₆₅. To this end, two different aptamers and spectroscopic ellipsometry were used. In the presented study, by determining the appropriate aptamer immobilization conditions, the spectroscopic ellipsometry technique was successfully applied for the detection of VEGF₁₆₅ at the range of 1 pM-1000 pM in the buffer. Aptasensors have a detection limit of 5.81 pM and 4.29 pM, respectively.

Base amount-dependent fluorescence enhancement for the assay of vascular endothelial growth factor 165 in human serum using hairpin DNA-silver nanoclusters and oxidized carbon nanoparticles

A base amount-dependent fluorescence enhancement-based strategy is put forward to determine vascular endothelial growth factor 165 (VEGF₁₆₅) in human serum by the use of hairpin DNA-silver nanoclusters (hDNA-AgNCs) and oxidized carbon nanoparticles (CNPs). The hDNA-AgNCs aptasensing probe consists of AgNCs-contained hairpin loop (that generates a fluorescence signal), hairpin stem (that makes the structure stable), and the terminal aptamer 1 (that recognizes the target together with aptamer 2). It has been demonstrated that the fluorescence intensity of hDNA-AgNCs is ~ 3-fold stronger than that of single-stranded DNA-AgNCs (ssDNA-AgNCs), and hDNA-AgNCs have a strong dependence of fluorescence enhancement on the base amount in hairpin stem and loop. Upon the addition of oxidized CNPs, the terminal aptamer 1 of hDNA-AgNCs can adsorb onto the surface of oxidized CNPs via π-π stacking, and the fluorescence of hDNA-AgNCs (with excitation/emission maxima at 490/567 nm) is quenched via fluorescence resonance energy transfer (FRET). When aptamer 2 and VEGF₁₆₅ are subsequently added, aptamer 1, VEGF₁₆₅, and aptamer 2 reassemble into an intact tertiary structure, and the fluorescence is recovered because hDNA-AgNCs are far away from the surface of oxidized CNPs and the FRET efficiency decreases. Under the optimized conditions, the aptasensing probe can selectively assay VEGF₁₆₅ with a detection limit of 14 pM. The results provide a label-free and sensitive method to monitor VEGF₁₆₅ in human serum. Schematic representation of the strong dependence of fluorescence enhancement on base amount in stem and loop of hairpin DNA-silver nanoclusters. The probe can be used to assay vascular endothelial growth factor 165 (VEGF₁₆₅) and give a judgment whether human serum VEGF₁₆₅ is at a normal or abnormal level for clinical diagnosis.

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.

A highly sensitive aptasensor for vascular endothelial growth factor based on fluorescence resonance energy transfer from upconversion nanoparticles to MoS2 nanosheets

A novel sensitive aptasensor for vascular endothelial growth factor-165 (VEGF₁₆₅) was constructed based on fluorescence resonance energy transfer (FRET) by employing upconversion nanoparticles (UCNPs) and MoS₂ nanosheets as the energy donor and acceptor, respectively. The upconversion fluorescence resonance energy transfer (UC-FRET) was triggered by the physical adsorption interaction between the aptamer and MoS₂ nanosheets, leading to a remarkable quenching of UCNP fluorescence up to 95%. Upon addition of VEGF₁₆₅ to the UCNP-aptamer system before MoS₂ nanosheets were added, the aptamer preferentially bound to VEGF₁₆₅ with the change of spatial conformation, which weakened the van der Waals' force between the MoS₂ nanosheets and the aptamer, thus leading to the separation of the donor and the acceptor. Consequently, the FRET phenomenon was inhibited and the luminescence of UCNPs was regained, which was linearly related to the concentration of VEGF₁₆₅ in the range of 0.1 ng mL^-1 to 16 ng mL^-1. By taking advantage of the extreme fluorescence quenching ability of MoS₂ nanosheets and the optical merits of UCNPs, the aptasensor based on UC-FRET exhibited favorable performance for the homogeneous assay of VEGF₁₆₅ in human serum, which is of great value for clinical diagnosis of tumors and related biological studies.

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.

A review on nanostructure-based mercury (II) detection and monitoring focusing on aptamer and oligonucleotide biosensors

Heavy metal ion pollution is a severe problem in environmental protection and especially in human health due to their bioaccumulation in organisms. Mercury (II) (Hg²⁺), even at low concentrations, can lead to DNA damage and give permanent harm to the central nervous system by easily passing through biological membranes. Therefore, sensitive detection and monitoring of Hg²⁺ is of particular interest with significant specificity. In this review, aptamer-based strategies in combination with nanostructures as well as several other strategies to solve addressed problems in sensor development for Hg²⁺ are discussed in detail. In particular, the analytical performance of different aptamer and oligonucleotide-based strategies using different signal improvement approaches based on nanoparticles were compared within each strategy and in between. Although quite a number of the suggested methodologies analyzed in this review fulfills the standard requirements, further development is still needed on real sample analysis and analytical performance parameters.

Application of DNA nanodevices for biosensing

Deoxyribonucleic acid (DNA), the carrier of genetic information in living life, is an essential biomacromolecule in almost all living systems. DNA has advantages including, programmability, predictability, high rigidity, and stability. Through self-assembly or combination with other nanomaterials (such as gold nanoparticles, graphene oxides, quantum dots, and polymers), DNA can be applied to construct specific, stable, biocompatible, and functional nanodevices. DNA nanodevices have made greater contributions in a plethora of fields. In this review, we discuss the recent progress of DNA nanodevices in molecular detection and analysis. Meanwhile, we prospect the development of various DNA devices in biological analysis, clinical diagnosis and biomedical research.

Pyridinium porphyrins and AuNPs mediated bionetworks as SPR signal amplification tags for the ultrasensitive assay of brain natriuretic peptide

Extension of the self-assembled bionanonetworks into surface plasmon resonance (SPR) assay investigation provides an effective signal amplification approach. We fabricated a bionetwork by nucleic acids, organic compounds, and supramolecular gold nanoparticles for ultrasensitive SPR detection of B-type natriuretic peptide (BNP). The SPR method was developed by a sandwich-type format of aptamer-target-antibody, and the aptamer-modified bionanonetworks induced localized SPR and large refractive index for different concentrations of the target BNP. The linear concentration range and limit of detection were 1-10,000 pg/mL (R² = 0.9852) and 0.3 pg/mL respectively. The detection recovery was in the range 92.13 to 108.69%. The approach embraces the following main advantages: (1) Cooperative double recognition was realized by calix[4]arenes for amino aptamers and pyridinium porphyrins. (2) The approach provided the specificity for supramolecular-based nanomaterials and a simple synthesis process via the ordered self-assembly under mild conditions. (3) The bionanonetworks endowed the SPR assay with signal amplification and stable determination for trace proteins. Therefore, it is expected that this study may offer a new SPR signal-amplified platform of organic-inorganic bionanonetworks to achieve sensitive, stable, and real-time determination. Graphical abstract Schematic of bionanonetwork based on porphyrin-mediated functionalized gold nanoparticles for SPR signal amplification to quantitatively detect BNP.

Aptamer-based approaches for in vitro molecular detection of cancer

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

A highly sensitive colorimetric aptasensor for the detection of the vascular endothelial growth factor in human serum

Early detection of cancer is of great significance for disease prevention and diagnosis. However, the levels of most cancer markers are quite low in the early stages of disease, so it is urgent to develop a highly sensitive detection method. In this study, a label-free and highly sensitive colorimetric strategy was developed for the detection of the vascular endothelial growth factor₁₆₅ (VEGF₁₆₅) in human serum. First, a convenient biosensor was constructed by immobilizing VEGF₁₆₅ on a microplate, where aptamers bound with VEGF₁₆₅ to form a complex. Then, streptavidin labeled-horseradish peroxidase (HRP-SA) combined with the complex via the interaction between streptavidin and biotin, thus catalyzing the 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂ system to produce colored products. In the presence of target, immobilized VEGF₁₆₅ and target competitively bound with the aptamers, resulting in a reduction of the colorimetric signal. Moreover, the optical density (OD) signal decreased with the increase of target concentration. The strategy showed a broad linear range (0.1-100 ng/mL) and a rather low detection limit of 10 pg/mL with good precision and selectivity. Further, the proposed method was successfully applied in detecting VEGF₁₆₅ in human serum. The detection results of serum samples showed that the proposed assay had a high correlation with CLEIA kits (r = 0.971, P = 0.001). It has potential for application in clinical research and 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.

Biosensors for Detection of Human Placental Pathologies: A Review of Emerging Technologies and Current Trends

Substantial growth in the biosensor research has enabled novel, sensitive and point-of-care diagnosis of human diseases in the last decade. This paper presents an overview of the research in the field of biosensors that can potentially predict and diagnosis of common placental pathologies. A survey of biomarkers in maternal circulation and their characterization methods is presented, including markers of oxidative stress, angiogenic factors, placental debris, and inflammatory biomarkers that are associated with various pathophysiological processes in the context of pregnancy complications. Novel biosensors enabled by microfluidics technology and nanomaterials is then reviewed. Representative designs of plasmonic and electrochemical biosensors for highly sensitive and multiplexed detection of biomarkers, as well as on-chip sample preparation and sensing for automatic biomarker detection are illustrated. New trends in organ-on-a-chip based placental disease models are highlighted to illustrate the capability of these in vitro disease models in better understanding the complex pathophysiological processes, including mass transfer across the placental barrier, oxidative stress, inflammation, and malaria infection. Biosensor technologies that can be potentially embedded in the placental models for real time, label-free monitoring of these processes and events are suggested. Merger of cell culture in microfluidics and biosensing can provide significant potential for new developments in advanced placental models, and tools for diagnosis, drug screening and efficacy testing.

Facile, Rapid, and Low-Cost Electrophoresis Titration of Thrombin by Aptamer-Linked Magnetic Nanoparticles and a Redox Boundary Chip

An aptamer-linked assay of a target biomarker (e.g., thrombin) is facing the challenges of long-term run, complex performance, and expensive instrument, unfitting clinical diagnosis in resource-limited areas. Herein, a facile chip electrophoresis titration (ET) model was proposed for rapid, portable, and low-cost assay of thrombin via aptamer-linked magnetic nanoparticles (MNPs), redox boundary (RB), and horseradish peroxidase (HRP). In the electrophoresis titration-redox boundary (ET-RB) model, thrombin was chosen as a model biomarker, which could be captured within 15 min by MNP-aptamer 1 and HRP-aptamer 2, forming a sandwich complex of (MNP-aptamer 1)-thrombin-(HRP-aptamer 2). After MNP separation and chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) within 10 min, an ET-RB run could be completed within 5 min based on the reaction between a 3,3',5,5'-tetramethylbenzidine radical cation (TMB^•+) and l-ascorbic acid in the ET channel. The systemic experiments based on the ET-RB method revealed that the sandwich complex could be formed and the thrombin content could be assayed via an ET-RB chip, demonstrating the developed model and method. In particular, the ET-RB method had the evident merits of simplicity, rapidity (less than 30 min), and low cost as well as portability and visuality, in contrast to the currently used thrombin assay. In addition, the developed method had high selectivity, sensitivity (limit of detection of 0.04 nM), and stability (intraday: 3.26%, interday: 6.07%) as well as good recovery (urine: 97-102%, serum: 94-103%). The developed model and method have potential to the development of a point-of-care testing assay in resource-constrained conditions.

High-sensitive immunosensing of protein biomarker based on interfacial recognition-induced homogeneous exponential transcription

A novel and versatile immunosensing strategy was developed for ultrasensitive and specific detection of proteins by organically integrating interfacial specific target recognition and homogeneous transcription amplification. In principle, classic antigen-antibody sandwich structure on the microplate could realize the specific identification of target protein. Biotinylated DNA probe was subsequently introduced by streptavidin-biotin system as a bridge linking interfacial and homogeneous reaction. The biotinylated DNA initiated exponential transcription amplification in the solution, which converted per target recognition event on the interface to numerous single-stranded RNA products in solution for highly sensitive fluorescence immunosensing. The proposed immunoassay based on interfacial recognition-induced homogeneous exponential transcription (IR-HET) for vascular endothelial growth factor (VEGF) detection showed a good linear range from 0.01 to 1000 pg/mL and the limit of detection as low as 1 fg/mL, which was 3 orders lower than traditional ELISA method. The established strategy was also successfully applied to directly detect VEGF from culture supernatants of tumor cells and clinical body fluid samples, proving very high sensitivity, selectivity and low matrix effect. Therefore, IR-HET-based immunosensing strategy might become a potential powerful tool be applied in ultrasensitive detection of low abundance protein biomarker for clinical early diagnosis, treatment and prognosis.

Advances in nanoplasmonic biosensors for clinical applications

Plasmonic biosensors can be conveniently used as portable diagnostic devices for attaining timely and cost-effective clinical outcomes. Nanoplasmonics technology opens the way for sensor miniaturization, multiplexing and point of care testing.

A chemiluminescent dual-aptasensor capable of simultaneously quantifying prostate specific antigen and vascular endothelial growth factor

An aptasensor with guanine chemiluminescence detection was developed for the early diagnosis of prostate cancer with the simultaneous quantifications of prostate specific antigen (PSA) and vascular endothelial growth factor (VEGF). A PSA DNA aptamer conjugated with Texas Red emits red light in guanine chemiluminescence reaction, whereas a VEGF DNA aptamer conjugated with 6-FAM emits green light. The PSA and VEGF aptamers immobilized on the surface of paramagnetic beads mixed with a sample were incubated for 30 min. After the incubation, PSA and VEGF bound with PSA and VEGF aptamers were simultaneously quantified for 20 s using guanine chemiluminescence detections operated with two photomultiplier tubes and two optical filters capable of selectively collecting green or red light. With the increase of PSA concentration, the strength of the red light emitted from Texas Red dropped exponentially, whereas with the increase of VEGF concentration, the green light emitted from 6-FAM was enhanced. The limits of detection were as low as 0.6 ng/ml for PSA and 0.4 ng/ml for VEGF. We confirmed that the dual-aptasensor can be applied as an advanced and new medical device capable of simultaneously quantifying PSA and VEGF to early diagnose prostate cancer with good accuracy, precision, and reproducibility.

SELEX methods on the road to protein targeting with nucleic acid aptamers

Systematic evolution of ligand by exponential enrichment (SELEX) is an efficient method used to isolate high-affinity single stranded oligonucleotides from a large random sequence pool. These SELEX-derived oligonucleotides named aptamer, can be selected against a broad spectrum of target molecules including proteins, cells, microorganisms and chemical compounds. Like antibodies, aptamers have a great potential in interacting with and binding to their targets through structural recognition and are therefore called "chemical antibodies". However, aptamers offer advantages over antibodies including smaller size, better tissue penetration, higher thermal stability, lower immunogenicity, easier production, lower cost of synthesis and facilitated conjugation or modification with different functional moieties. Thus, aptamers represent an attractive substitution for protein antibodies in the fields of biomarker discovery, diagnosis, imaging and targeted therapy. Enormous interest in aptamer technology triggered the development of SELEX that has underwent numerous modifications since its introduction in 1990. This review will discuss the recent advances in SELEX methods and their advantages and limitations. Aptamer applications are also briefly outlined in this review.

A new coccolith modified electrode-based biosensor using a cognate pair of aptamers with sandwich-type binding

In this study, we report a cognate pair of the aptamer-based sandwich-type electrochemical biosensor for type 2 diabetes biomarker (Vaspin) using coccolith modified electrodeposited on the screen-printed gold electrode (CME-SPGE). The coccolith derived from E. huxleyi used in this study were known to be highly-structured microparticles with many nano-sized pores. The CME-SPGE was successfully fabricated by drop-casting coccoliths, followed by Au sputtering and electrodeposition of Au. On this CME-SPGE electrode, the sandwich-type electrochemical aptasensor was fabricated by using a cognate pair of aptamers. The morphological, electrochemical characteristics and the performances of both the CME-SPGE and the completely fabricated sandwich-type aptasensor were investigated by SEM, EDAX, cyclic voltammetry, and chronoamperometry. Due to the synergic effect of a cognate pair of aptamers on CME-SPGE, this newly developed sandwich-type electrochemical biosensor for Vaspin showed high specificity, and good sensitivity with a limit of detection (LOD) of 298 pM, along with more widen the linear range. To the best of our knowledge, this is the first report about the use of a coccolith modified electrode with a cognate pair aptamer resulting in sandwich-type binding in an electrochemical biosensor. With the advantages of using highly-structured biomineral microparticles and a cognate pair of aptamers, this new study may pave the innovative way to design a novel sandwich-type electrochemical aptasensor platform.

Aptamer-based biosensors and nanosensors for the detection of vascular endothelial growth factor (VEGF): A review

Vascular endothelial growth factor (VEGF) is a key regulator of vascular formation and a predominant protein biomarker in cancer angiogenesis. Owing to its crucial roles in the cancer metastasis, VEGF detection and quantification is of great importance in clinical diagnostics. Today, there exist a wide variety of detection strategies for identifying many types of disease biomarkers, especially for VEGF. As artificial single-stranded DNA or RNA oligonucleotides with catalytic and receptor properties, aptamers have drawn lots of attention to be applied in biosensing platforms due to their target-induced conformational changes as well as high stability and target versatility. So far, various sensitivity-enhancement techniques in combination with a broad range of smart nanomaterials have integrated into the design of novel aptasensors to improve detection limit and sensitivity of analyte detection. 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 VEGF based on optical and electrochemical platforms.

A near-infrared BSA coated DNA-AgNCs for cellular imaging

Near-infrared silver nanoclusters, have potential applications in the field of biosensing and biological imaging. However, less stability of most DNA-AgNCs limits their application. To obtain stable near-infrared fluorescence DNA-AgNCs for biological imaging, a new kind of near-infrared fluorescent DNA-Ag nanoclusters was constructed using the C3A rich aptamer as a synthesis template, GAG as the enhancer. In particular, a new DNA-AgNCs-Trp@BSA was obtained based on the self-assembly of bovine serum albumin (BSA) and tryptophan loaded DNA-AgNCs by hydrophobic interaction. This self-assembly method can be used to stabilize DNAn-Ag (n = 1-3) nanoclusters. Hence, the near-infrared fluorescence DNA-AgNCs-Trp@BSA was applied in cellular imaging of HepG-2 cells.

Cucurbituril and Azide Cofunctionalized Graphene Oxide for Ultrasensitive Electro-Click Biosensing

To achieve high selectivity and sensitivity simultaneously in an electrochemical biosensing platform, cucurbituril and azide cofunctionalized graphene oxide, a new functional nanomaterial that acts as a go-between to connect the recognition element with amplified signal architecture, is developed in this work. The cucurbituril and azide cofunctionalized graphene oxide features a high specific surface area with abundant levels of the two types of functional groups. Specifically, it emerges as a powerful tool to link recognition elements with simplicity, high yield, rapidity, and highly selective reactivity through azide-alkynyl click chemistry. Moreover, it possesses many host molecules to interact with guest molecules (also signal molecules)-grafted branched ethylene imine polymer, through which the detection sensitivity can be greatly improved. Together with electro-click technology, a highly controllable, selective, and sensitive biosensing platform can be easily created. For VEGF₁₆₅ protein detection, the electro-click assay has high selectivity and sensitivity; a dynamic detection range from 10 fg mL^-1 to 1 ng mL^-1 with a detection limit of 8 fg mL^-1 was achieved. The electro-click biosensing strategy based on cucurbituril and azide cofunctionalized graphene oxide would have great promise for other target analytes with a broad range of applications.

Highly sensitive antibody-aptamer sensor for vascular endothelial growth factor based on hybridization chain reaction and pH meter/indicator

Vascular endothelial growth factor (VEGF) is a crucial signaling protein for the tumor growth and metastasis, which is also acted as the biomarkers for various diseases. In this research, we fabricate an aptamer-antibody sensor for point-of-care test of VEGF. Firstly, target VEGF is captured by antibody immobilized on the microplate, and then binds with aptamer to form the sandwich structure. Next, with the assist of glucose oxidase (GOx)-functionalized ssDNAs, hybridization chain reaction occurs using the aptamer as the primer. Thus, GOx are greatly gathered on the microplate, which catalyzes the oxidization of glucose, leading to the pH change. As a result, the detect limit at a signal-to-noise was estimated to be 0.5pg/mL of target by pH meter, and 1.6pg/mL of VEGF was able to be distinguished by naked eyes. Meanwhile, this method has been used assay VEGF in the serum with the satisfactory results.

Quantitative determination of VEGF165 in cell culture medium by aptamer sandwich based chemiluminescence assay

In this work, we have developed a sensitive and selective chemiluminescence (CL) assay for vascular endothelial growth factor (VEGF165) quantitative detection based on two specific VEGF165 binding aptamers (Apt). VEGF is a predominant biomarker in cancer angiogenesis, and sensitive detection method of VEGF are highly demanded for both academic study and clinical diagnosis of multiple cancers. In our experiment, VEGF165 was captured in a sandwich structure assembled by two binding aptamers, one capture aptamer was immobilized on streptavidin-coated magnetic beads (MBs) and another VEGF-binding aptamer was labeled by biotin for further phosphatase conjunction. After Apt-VEGF-Apt sandwich was formed on MBs surface, alkaline phosphatase (ALP) was modified to the second aptamer to catalyze CL reaction. By applying 4-methoxy-4-(3-phosphatephenyl)-spiro-(1,2-dioxetane-3,2-adamantane) (AMPPD) as CL substrate, strong signal intensity was achieved. VEGF165 content as low as 1ng/mL was detected in standard spiked samples by our assay, and linear range of working curve was confirmed from 1 to 20ng/mL. Then our method was successfully applied for cell culture medium analysis and on-chip hypoxic HepG2-HUVEC co-culture model study with excellent accuracy equal to ELISA Kit. Our developed assay demonstrated an outstanding performance in VEGF165 quantification and may be further extended to clinical testing of important biomarkers as well as probing microchip cell culture model.

Aptamer-based sandwich-type biosensors

Sandwich-type biosensor platforms have drawn lots of attentions due to its superior features, compared to other platforms, in terms of its stable and reproducible responses and easy enhancement in the detection sensitivity. The sandwich-type assays can be developed by utilizing a pair of receptors, which bind to the different sites of the same target. In this mini-review paper, the sandwich-type biosensors using either pairs of aptamers or aptamer-antibody pairs are reviewed in terms of its targets and platforms, the schematic designs, and their analytical performance.