Aptamer-based piezoelectric quartz crystal microbalance biosensor array for the quantification of IgE

Optical Label-Free Aptasensor Based on Weak Value Amplification for Real-Time and Ultrasensitive Detection of IgE

Allergy is a prevalent disease, and the potential allergic population is expanding with industrialization and changes in people's living standards. Serum immunoglobulin E (IgE) level is one of the critical indicators for determining allergy. Here, we proposed a simple, real-time monitoring, low chip cost, label-free aptamer biosensing strategy based on weak value amplification (WVA) for the quantitative detection of IgE in serum samples, enabling early and accurate diagnosis of allergic or hypersensitive patients. The aptasensor combined an imaging weak measurement system with the high specificity of the aptamer for the marker IgE. By modifying the amino group at the 3-terminal end, the anti-IgE aptamers can attach to a dopamine-modified prism's surface and selectively recognize IgE in human serum. In the presence of IgE, a specific binding reaction occurred, resulting in a change in the refractive index of the reactive region's surface, manifested as a change in the light intensity of the camera acquired experimental images. As the concentration of IgE increased, the relative light intensity advanced sequentially. The WVA-aptasensing strategy achieved a wide detection range of 0.01 ng/mL to 2 μg/mL in phosphate buffered saline buffer, with the resolution as low as 4.3 pg/mL. IgE testing experiments in human serum have proved the feasibility of our methods in detecting complex samples. In addition, the method specifically recognized IgE without interference from other proteins. We believe that our proposed sensing strategy opens up new possibilities for ultrahigh sensitivity screening of IgE and can be expanded to detecting other biomolecules.

New Insights into Aptamers: An Alternative to Antibodies in the Detection of Molecular Biomarkers

Aptamers are short oligonucleotides with single-stranded regions or peptides that recently started to transform the field of diagnostics. Their unique ability to bind to specific target molecules with high affinity and specificity is at least comparable to many traditional biorecognition elements. Aptamers are synthetically produced, with a compact size that facilitates deeper tissue penetration and improved cellular targeting. Furthermore, they can be easily modified with various labels or functional groups, tailoring them for diverse applications. Even more uniquely, aptamers can be regenerated after use, making aptasensors a cost-effective and sustainable alternative compared to disposable biosensors. This review delves into the inherent properties of aptamers that make them advantageous in established diagnostic methods. Furthermore, we will examine some of the limitations of aptamers, such as the need to engage in bioinformatics procedures in order to understand the relationship between the structure of the aptamer and its binding abilities. The objective is to develop a targeted design for specific targets. We analyse the process of aptamer selection and design by exploring the current landscape of aptamer utilisation across various industries. Here, we illuminate the potential advantages and applications of aptamers in a range of diagnostic techniques, with a specific focus on quartz crystal microbalance (QCM) aptasensors and their integration into the well-established ELISA method. This review serves as a comprehensive resource, summarising the latest knowledge and applications of aptamers, particularly highlighting their potential to revolutionise diagnostic approaches.

A high-frequency QCM biosensing platform for label-free detection of the SARS-CoV-2 spike receptor-binding domain: an aptasensor and an immunosensor

Herein, high-frequency quartz crystal microbalance biosensing platforms were constructed using an aptamer and antibody as bioreceptors for fast and label-free detection of the SARS-CoV-2 RBD.

Quartz Crystal Microbalance-Based Aptasensors for Medical Diagnosis

Aptamers are important materials for the specific determination of different disease-related biomarkers. Several methods have been enhanced to transform selected target molecule-specific aptamer bindings into measurable signals. A number of specific aptamer-based biosensors have been designed for potential applications in clinical diagnostics. Various methods in combination with a wide variety of nano-scale materials have been employed to develop aptamer-based biosensors to further increase sensitivity and detection limit for related target molecules. In this critical review, we highlight the advantages of aptamers as biorecognition elements in biosensors for target biomolecules. In recent years, it has been demonstrated that electrode material plays an important role in obtaining quick, label-free, simple, stable, and sensitive detection in biological analysis using piezoelectric devices. For this reason, we review the recent progress in growth of aptamer-based QCM biosensors for medical diagnoses, including virus, bacteria, cell, protein, and disease biomarker detection.

Recent Developments in Aptasensors for Diagnostic Applications

Aptamers are exciting smart molecular probes for specific recognition of disease biomarkers. A number of strategies have been developed to convert target-aptamer binding into physically detectable signals. Since the aptamer sequence was first discovered, a large variety of aptamer-based biosensors have been developed, with considerable attention paid to their potential applications in clinical diagnostics. So far, a variety of techniques in combination with a wide range of functional nanomaterials have been used for the design of aptasensors to further improve the sensitivity and detection limit of target determination. In this paper, the advantages of aptamers over traditional antibodies as the molecular recognition components in biosensors for high-throughput screening target molecules are highlighted. Aptamer-target pairing configurations are predominantly single- or dual-site binding; the design of recognition modes of each aptamer-target pairing configuration is described. Furthermore, signal transduction strategies including optical, electrical, mechanical, and mass-sensitive modes are clearly explained together with examples. Finally, we summarize the recent progress in the development of aptamer-based biosensors for clinical diagnosis, including detection of cancer and disease biomarkers and in vivo molecular imaging. We then conclude with a discussion on the advanced development and challenges of aptasensors.

Aptamer-based approaches for the detection of waterborne pathogens

Waterborne ailments pose a serious threat to public health and are a huge economic burden. Lack of hygiene in drinking and recreational water is the chief source of microbial pathogens in developing countries. Poor water quality and sanitation account for more than 3.4 million deaths a year worldwide. This has urged authorities and researchers to explore different avenues of pathogen detection. There is a growing demand for rapid and reliable sensor technologies, in particular those that can detect in situ and perform in harsh conditions. Some of the major waterborne pathogens include Vibrio cholerae, Leptospira interrogans, Campylobacter jejuni, Shigella spp., enterotoxigenic Escherichia coli, Clostridium difficile, Cryptosporidium parvum, Entamoeba histolytica, and Hepatitis A virus. While conventional methods of pathogen detection like serodiagnosis and microbiological methods have been superseded by nucleic acid amplification methods, there is still potential for improvement. This review provides an insight into aptamers and their utility in the form of aptasensors. It discusses how aptamer-based approaches have emerged as a novel strategy and its advantages over more resource-intensive and complex biochemical approaches.

Paper-based upconversion fluorescence aptasensor for the quantitative detection of immunoglobulin E in human serum

Immunoglobulin E (IgE), a biomarker of allergic diseases, plays a critical role in allergic mechanism. Because of its low abundance in serum, the demand of developing sensitive, selective and simple methods for IgE detection is still very urgent. Paper-based analytical devices using upconversion nanoparticles (UCNPs) as the label can be promising point-of-care test (POCT) methods in rapid diagnosis, owing to their NIR-excitation and visible light emission nature, which can avoid the interference of autofluorescence and scattering light from biological samples and paper substrates. In this work, we proposed a paper-based analytical device for the sensitive, selective and accurate detection of total immunoglobulin E (IgE) in human serum. The assay was based on resonance energy transfer between UCNPs and organic dye tetramethylrhodamine (TAMRA), and IgE aptamer with stem-loop structure was used as the recognizing probe. The existence of IgE change the conformation of IgE aptamer, enlarge the distance between donor and acceptor, and block the energy transfer process. Thus, the luminescence of UCNPs recovered with an IgE concentration independent manner. A linear calibration was obtained in the range of 0.5-50 IU/mL, with a detection limit of 0.13 IU/mL. The results of our method were well correlated with that of commercial ELISA kit (20 human serum samples). This work suggests promising prospect of the paper-based UC-LRET analytical devices in real samples and may promote the application of paper-based analytical devices in clinical diagnosis.

Aptamer-Based Sandwich Assay for Measurement of Thymidine Kinase 1 in Serum of Cancerous Patients

Thymidine kinase 1 (TK1) is traditionally a serum biomarker that is elevated in the early stages of malignancies. The diagnostic and prognostic role of TK1 for screening and monitoring human malignancies has recently been investigated. Anti-human TK1 aptamers were selected through 12 iterative rounds of systematic evolution of ligands by exponential enrichment from a DNA library. The aptamer pool of round 12 was amplified, and the polymerase chain reaction product was cloned on the TA vector. Of the 85 colonies obtained, 52 were identified as positive clones. These aptamers were screened for TK1 with surface plasmon resonance, where apta37 and apta69 showed the highest affinity for TK1. The TK1_apta37 and TK1_apta69 aptamers were used in a sandwich assay platform and successfully detected TK1 in the concentration range of 54-3500 pg mL^-1. Clinical samples from 60 cancerous patients were also tested with this assay system and compared using the conventional antibody-based enzyme-linked immunosorbent assay kit. The aptamer sandwich assay demonstrated a dynamic range for TK1 at clinically relevant serum levels, covering subpicogram per milliliter concentrations. The new approach offers a simple and robust method for detecting serum biomarkers that have low and moderate abundance. The results of this study demonstrate the screening capability of the aptamer sandwich assay platform and its potential applicability to the point-of-care testing system.

A graphene aptasensor for biomarker detection in human serum

This paper presents an affinity graphene nanosensor for detection of biomarkers in undiluted and non-desalted human serum. The affinity nanosensor is a field-effect transistor in which graphene serves as the conducting channel. The graphene surface is sequentially functionalized with a nanolayer of the polymer polyethylene glycol (PEG) and a biomarker-specific aptamer. The aptamer is able to specifically bind with and capture unlabeled biomarkers in serum. A captured biomarker induces a change in the electric conductivity of the graphene, which is measured in a buffer of optimally chosen ionic strength to determine the biomarker concentration. The PEG layer effectively rejects nonspecific adsorption of background molecules in serum while still allowing the aptamer to be readily accessible to serum-borne biomarkers and increases the effective Debye screening length on the graphene surface. Thus, the aptamer-biomarker binding sensitively changes the graphene conductivity, thereby achieving specific and label-free detection of biomarkers with high sensitivity and without the need to dilute or desalt the serum. Experimental results demonstrate that the graphene nanosensor is capable of specifically capturing human immunoglobulin E (IgE), used as a representative biomarker, in human serum in the concentration range of 50 pM-250 nM, with a resolution of 14.5 pM and a limit of detection of 47 pM.

MAFG is a potential therapeutic target to restore chemosensitivity in cisplatin-resistant cancer cells by increasing reactive oxygen species

Adjuvant chemotherapy for solid tumors based on platinum-derived compounds such as cisplatin is the treatment of choice in most cases. Cisplatin triggers signaling pathways that lead to cell death, but it also induces changes in tumor cells that modify the therapeutic response, thereby leading to cisplatin resistance. We have recently reported that microRNA-7 is silenced by DNA methylation and is involved in the resistance to platinum in cancer cells through the action of the musculoaponeurotic fibrosarcoma oncogene family, protein G (MAFG). In the present study, we first confirm the miR-7 epigenetic regulation of MAFG in 44 normal- and/or tumor-paired samples in non-small-cell lung cancer (NSCLC). We also provide translational evidence of the role of MAFG and the clinical outcome in NSCLC by the interrogation of two extensive in silico databases of 2019 patients. Moreover, we propose that MAFG-mediated resistance could be conferred due to lower reactive oxygen species production after cisplatin exposure. We developed specifically selected aptamers against MAFG, with high sensitivity to detect the protein at a nuclear level probed by aptacytochemistry and histochemistry analyses. The inhibition of MAFG activity through the action of the specific aptamer apMAFG6F increased the levels of reactive oxygen species production and the sensitivity to cisplatin. We report first the specific nuclear identification of MAFG as a novel detection method for diagnosis in NSCLC, and then we report that MAFG modulates the redox response and confers cell protection against free radicals generated after platinum administration, thus also being a promising therapeutic target.

A Proof-of-Concept of Label-Free Biosensing System for Food Allergy Diagnostics in Biophotonic Sensing Cells: Performance Comparison with ImmunoCAP

Food allergy is a common disease worldwide with over 6% of the population (200⁻250 million people) suffering from any food allergy nowadays. The most dramatic increase seems to be happening in children and young people. Therefore, improvements in the diagnosis efficiency of these diseases are needed. Immunoglobulin type E (IgE) biomarker determination in human serum is a typical in vitro test for allergy identification. In this work, we used a novel biosensor based on label-free photonic transducers called BICELLs (Biophotonic Sensing Cells) for IgE detection. These BICELLs have a thin film of nitrocellulose over the sensing surface, they can be vertical optically interrogated, and are suitable for being integrated on a chip. The BICELLs sensing surface sizes used were 100 and 800 µm in diameter. We obtained calibration curves with IgE standards by immobilizating anti-IgE antibodies and identified with standard IgE calibrators in minute sample amounts (3 µL). The results, in similar assay format, were compared with commercially available ImmunoCAP^®. The versatility of the interferometric nitrocellulose-based sensing surface was demonstrated since the limit of detections for BICELLs and ImmunoCAP^® were 0.7 and 0.35 kU/L, respectively.

An aptamer-based biosensor for detection of doxorubicin by electrochemical impedance spectroscopy

An aptamer-based biosensor was developed for the detection of doxorubicin using electrochemical impedance spectroscopy. Doxorubicin and its 14-dehydroxylated version daunorubicin are anthracyclines often used in cancer treatment. Due to their mutagenic and cardiotoxic effects, detection in groundwater is desirable. We developed a biosensor using the daunorubicin-binding aptamer as biological recognition element. The aptamer was successfully co-immobilized with mercaptohexanol on gold and a density of 1.3*10¹³ ± 2.4*10¹² aptamer molecules per cm² was achieved. The binding of doxorubicin to the immobilized aptamer was detected by electrochemical impedance spectroscopy. The principle is based on the inhibition of electron transfer between electrode and ferro-/ferricyanide in solution caused by the binding of doxorubicin to the immobilized aptamer. A linear relationship between the charge transfer resistance (R _ct ) and the doxorubicin concentration was obtained over the range of 31 nM to 125 nM doxorubicin, with an apparent binding constant of 64 nM and a detection limit of 28 nM. With the advantages of high sensitivity, selectivity, and simple sensor construction, this method shows a high potential of impedimetric aptasensors in environmental monitoring. Graphical abstract Measurement chamber and immobilization principle for the detection of doxorubicin by electrochemical impedance spectroscopy.

Development of Novel Piezoelectric Biosensor Using PZT Ceramic Resonator for Detection of Cancer Markers

A novel biosensor based on piezoelectric ceramic resonator was developed for direct detection of cancer markers in the study. For the first time, a commercially available PZT ceramic resonator with high resonance frequency was utilized as transducer for a piezoelectric biosensor. A dual ceramic resonators scheme was designed wherein two ceramic resonators were connected in parallel: one resonator was used as the sensing unit and the other as the control unit. This arrangement minimizes environmental influences including temperature fluctuation, while achieving the required frequency stability for biosensing applications. The detection of the cancer markers Prostate Specific Antigen (PSA) and α-Fetoprotein (AFP) was carried out through frequency change measurement. The device showed high sensitivity (0.25 ng/ml) and fast detection (within 30 min) with small samples (1 μl), which is compatible with the requirements of clinical measurements. The results also showed that the ceramic resonator-based piezoelectric biosensor platform could be utilized with different chemical interfaces, and had the potential to be further developed into biosensor arrays with different specificities for simultaneous detection of multiple analytes.

Carbon-based nanocomposites with aptamer-templated silver nanoclusters for the highly sensitive and selective detection of platelet-derived growth factor

We synthesized two kinds of carbon-based nanocomposites of silver nanoclusters (AgNCs). An aptamer for targeted platelet-derived growth factor-BB (PDGF-BB) detection was used as the organic phase to produce AgNCs@Apt, three dimensional reduced graphene oxide@AgNCs@Aptamer (3D-rGO@AgNCs@Apt), and graphene quantum dots@AgNCs@Aptamer (GQD@AgNCs@Apt) nanocomposites. The formation mechanism of the developed nanocomposites was described by detailed characterizations of their chemical and crystal structures. Subsequently, the as-synthesized nanoclusters containing aptamer strands were applied as the sensitive layers to fabricate a novel electrochemical aptasensor for the detection of PDGF-BB, which may be directly used to determine the target protein. Electrochemical impedance spectra showed that the developed 3D-rGO@AgNCs@Apt-based biosensor exhibited the highest sensitivity for PDGF-BB detection among three kinds of fabricated aptasensors, with an extremely low detection limit of 0.82pgmL^-1. In addition, the 3D-rGO@AgNCs@Apt-based biosensor showed high selectivity, stability, and applicability for the detection of PDGF-BB. This finding indicated that the AgNC-based nanocomposites prepared by a one-step method could be used as an electrochemical biosensor for various detection procedures in the biomedical field.

Aptamers Against Immunologic Targets: Diagnostic and Therapeutic Prospects

The concept of in vitro selection of nucleic acid aptamers emerged 25 years ago, and since then tremendous progress has been achieved in the development of different aptamers and their applications for various bioanalytical and therapeutic purposes. Among other protein targets of aptamers, immune system proteins are of particular interest both as diagnostic markers and therapeutic targets. The present review summarizes up-to-date articles concerning the selection and design of DNA and RNA aptamers against immunologic targets such as antibodies, cytokines, and T-cell and B-cell receptors. We also discuss the prospects of employing aptamers as recognizing modules of diagnostic aptasensors, potential therapeutic candidates for the treatment of autoimmune diseases and cancer, and specific tools for functional studies of immune system proteins.

Biosensor Regeneration: A Review of Common Techniques and Outcomes

Biosensors are ideally portable, low-cost tools for the rapid detection of pathogens, proteins, and other analytes. The global biosensor market is currently worth over 10 billion dollars annually and is a burgeoning field of interdisciplinary research that is hailed as a potential revolution in consumer, healthcare, and industrial testing. A key barrier to the widespread adoption of biosensors, however, is their cost. Although many systems have been validated in the laboratory setting and biosensors for a range of analytes are proven at the concept level, many have yet to make a strong commercial case for their acceptance. Though it is true with the development of cheaper electrodes, circuits, and components that there is a downward pressure on costs, there is also an emerging trend toward the development of multianalyte biosensors that is pushing in the other direction. One way to reduce the cost that is suitable for certain systems is to enable their reuse, thus reducing the cost per test. Regenerating biosensors is a technique that can often be used in conjunction with existing systems in order to reduce costs and accelerate the commercialization process. This article discusses the merits and drawbacks of regeneration schemes that have been proven in various biosensor systems and indicates parameters for successful regeneration based on a systematic review of the literature. It also outlines some of the difficulties encountered when considering the role of regeneration at the point of use. A brief meta-analysis has been included in this review to develop a working definition for biosensor regeneration, and using this analysis only ∼60% of the reported studies analyzed were deemed a success. This highlights the variation within the field and the need to normalize regeneration as a standard process across the field by establishing a consensus term.

[DNA aptamer based sorbents for binding human IgE]

DNA aptamer based sorbents are synthesized for binding human IgE. Sorbents effectively removed IgE from human blood plasma. The experimental values of IgE desorption constants were from 11 x 10(-l0) to 1.7 x 10(-10) M depending on the orientation of the aptamer, an insoluble matrix. The sorbents were stable during multiple use. Conditions for sorbent regeneration were picked up. These chromatographic materials can be used for medical and biotechnological applications.

Biosensors for hepatitis B virus detection

A biosensor is an analytical device used for the detection of analytes, which combines a biological component with a physicochemical detector. Recently, an increasing number of biosensors have been used in clinical research, for example, the blood glucose biosensor. This review focuses on the current state of biosensor research with respect to efficient, specific and rapid detection of hepatitis B virus (HBV). The biosensors developed based on different techniques, including optical methods (e.g., surface plasmon resonance), acoustic wave technologies (e.g., quartz crystal microbalance), electrochemistry (amperometry, voltammetry and impedance) and novel nanotechnology, are also discussed.

Highly sensitive electrochemical aptasensor for immunoglobulin E detection based on sandwich assay using enzyme-linked aptamer

Here, we describe the fabrication of an electrochemical immunoglobulin E (IgE) aptasensor using enzyme-linked aptamer in the sandwich assay method and thionine as redox probe. In this protocol, 5'-amine-terminated IgE aptamer and thionine were covalently attached on glassy carbon electrode modified with carbon nanotubes/ionic liquid/chitosan nanocomposite. Furthermore, another IgE aptamer was modified with biotin and enzyme horseradish peroxidase (HRP), which attached to the aptamer via biotin-streptavidin interaction. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry were performed at each stage of the chemical modification process to confirm the resulting surface changes. The presence of IgE induces the formation of a double aptamer sandwich structure on the electrode, and the electrocatalytic reduction current of thionine in the presence of hydrogen peroxide was measured as the sensor response. Under optimized conditions and using differential pulse voltammetry as the measuring technique, the proposed aptasensor showed a low detection limit (6 pM) and high sensitivity (1.88 μA nM(-1)). This aptasensor also exhibited good stability and high selectivity for IgE detection without an interfering effect of some other proteins such as bovine serum albumin (BSA) and lysozyme. The application of the aptasensor for IgE detection in human serum sample was also investigated. The proposed protocol is quite promising as an alternative sandwich approach for various protein assays.

Voltammetric aptasensor combined with magnetic beads assay developed for detection of human activated protein C

A sensitive and selective label free voltammetric aptasensor based on magnetic beads assay was performed for the first time in our study for monitoring of human activated protein C (APC), which is a serine protease (i.e., key enzyme of the protein C pathway). An amino modified DNA aptamer (DNA APT) was covalently immobilized onto the surface of carboxylated magnetic beads (MBs), and then, the specific interaction between DNA APT and its cognate protein, APC, was performed at the surface of MBs. Similarly a biotinylated DNA APT was immobilized onto the surface of streptavidin coated MBs. Before and after interaction process, the oxidation signal of guanine was measured at disposable pencil graphite electrode (PGE) surface in combination with differential pulse voltammetry (DPV) technique and accordingly, the decrease at the guanine signal was evaluated. The biomolecular recognition of APC was successfully achieved with a low detection limit found as 2.35 µg mL(-1) by using MB-COOH based assay. Moreover, the selectivity of this aptasensor assay was tested in the presence of numerous proteins and other biomolecules: protein C (PC), thrombin (THR), bovine serum albumin (BSA), factor Va (FVa) and chromogenic substrate (KS).

A new system for the amplification of biological signals: RecA and complimentary single strand DNA probes on a leaky surface acoustic wave biosensor

This research describes a new amplification signals system of the leaky surface acoustic wave (LSAW) bis-peptide nucleic acid (bis-PNA) biosensor for the simple, sensitive and rapid detection of the target double-stranded DNA (dsDNA). The system consists of a RecA protein-coated complementary single-stranded DNA (cssDNA) probe complex that amplifies the biological signal to improve the sensitivity of the biosensor. The bis-PNA probe for detecting HPV was first immobilized on a gold surface membrane of the detection channel. After the probe was completely hybridized with the corresponding target DNA, different concentrations of the "RecA protein-complementary single strand DNA probe" were added to react with the bis-PNA/dsDNA complex. The phase shift of the LSAW biosensors, which was measured and found to be most significant when the RecA protein was 45 μg/mL and the ATPγS was 2.5 mmol/L. Compared with other concentrations (P<0.01) of RecA and ATPγS, the value of the phase shift was (11.74 ± 1.03) degrees and the ratio of the phase shift and hybridization time clearly outperformed that of the other concentrations. Compared to the direct hybridization of the bis-PNA probe and the target DNA sequence, the sensitivity was effectively improved and the detection time was significantly shortened. PNA binding adjacent to the area of the target sequence homologous to the probe significantly increased the yield of the hybridization reaction between the PNA/dsDNA complex and the RecA protein-coated cssDNA probe. In this condition, the phase shift was significantly obvious and the detection time was significantly shortened. In conclusion, the combination of the RecA protein-coated cssDNA probe and the LSAW bis-PNA biosensor provides sensitivity and simple and rapid detection of clinical trace pathogenic microorganisms.

Development of an aptamer-based impedimetric bioassay using microfluidic system and magnetic separation for protein detection

An aptamer-based impedimetric bioassay using the microfluidic system and magnetic separation was developed for the sensitive and rapid detection of protein. The microfluidic impedance device was fabricated through integrating the gold interdigitated array microelectrode into a flow cell made of polydimethylsiloxane (PDMS). Aptamer modified magnetic beads were used to capture and separate the target protein, and concentrated into a suitable volume. Then the complexes were injected into the microfluidic flow cell for impedance measurement. To demonstrate the high performance of this novel detection system, thrombin was employed as the target protein. The results showed that the impedance signals at the frequency of 90 kHz have a good linearity with the concentrations of thrombin in a range from 0.1 nM to 10nM and the detection limit is 0.01 nM. Compared with the reported impedimetric aptasensors for thrombin detection, this method possesses several advantages, such as the increasing sensitivity, improving reproducibility, reducing sample volume and assay time. All these demonstrate the proposed detection system is an alternative way to enable sensitive, rapid and specific detection of protein.

Aptamer-based biosensor for detection of phenylalanine at physiological pH

A simple, sensitive aptamer-based biosensor for the detection of phenylalanine is developed using the electrochemical transduction method. For this proposed aptasensor, a 5-thiol-terminated aptamer is covalently attached onto a gold electrode. At the first time, the electrode was evaluated as an electrochemical aptasensor for determination of phenylalanine in aqueous solutions. This sensor was tested in a Tris-HCl buffer with physiological pH = 7.4 by cyclic voltammetry and differential pulse voltammetry. The detection limit and sensitivity of the modified electrode toward phenylalanine were estimated to be 1 nM (S/N = 3) and 0.367 μA nM(-1), respectively. The linear range of the signal was observed between 1 and 10 nM of phenylalanine with 0.9914 correlation factor. The herein-described approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies.

Label-free electrochemical IgE aptasensor based on covalent attachment of aptamer onto multiwalled carbon nanotubes/ionic liquid/chitosan nanocomposite modified electrode

A simple, sensitive and label-free aptamer-based biosensor for the detection of human immunoglobulin E (IgE) is developed using the electrochemical transduction method. A special immobilization interface consisting of multiwalled carbon nanotubes/ionic liquid/chitosan nanocomposite (MWCNTs/IL/Chit) is utilized to improve the conductivity and performance characteristics of the biosensor as well as to increase the loading amount of aptamer DNA sequence. A 5'-amino-terminated aptamer is covalently attached onto MWCNTs/IL/Chit modified glassy carbon (GC) electrode via a linker of glutaraldehyde (GA). Methylene blue (MB) is used as an electrochemical indicator which is intercalated into the aptamer through the specific interaction with its guanine bases. In the absence of IgE, MB bound to the aptamer produces a strong differential pulse voltammetric (DPV) signal. But when IgE exists, the intercalated MB releases from the aptamer, resulting an obviously decreased DPV signal. This phenomenon can be applied for human IgE detection. The peak current of MB linearly decreases with the concentration of IgE over a range of 0.5-30 nM with a detection limit of 37 pM. By using Bovine serum albumin (BSA) and lysozyme, the excellent specificity of this sensing system for the detection of IgE is also demonstrated. Finally, the proposed aptasensor is successfully used to IgE analysis in human serum sample. The obtained result is well agreed with the value obtained by the standard ELISA method. The herein described approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies.

Challenges and opportunities for small molecule aptamer development

Aptamers are single-stranded oligonucleotides that bind to targets with high affinity and selectivity. Their use as molecular recognition elements has emerged as a viable approach for biosensing, diagnostics, and therapeutics. Despite this potential, relatively few aptamers exist that bind to small molecules. Small molecules are important targets for investigation due to their diverse biological functions as well as their clinical and commercial uses. Novel, effective molecular recognition probes for these compounds are therefore of great interest. This paper will highlight the technical challenges of aptamer development for small molecule targets, as well as the opportunities that exist for their application in biosensing and chemical biology.

Challenges and opportunities for small molecule aptamer development

Aptamers are single-stranded oligonucleotides that bind to targets with high affinity and selectivity. Their use as molecular recognition elements has emerged as a viable approach for biosensing, diagnostics, and therapeutics. Despite this potential, relatively few aptamers exist that bind to small molecules. Small molecules are important targets for investigation due to their diverse biological functions as well as their clinical and commercial uses. Novel, effective molecular recognition probes for these compounds are therefore of great interest. This paper will highlight the technical challenges of aptamer development for small molecule targets, as well as the opportunities that exist for their application in biosensing and chemical biology.

An electronic channel switching-based aptasensor for ultrasensitive protein detection

Due to the ubiquity and essential of the proteins in all living organisms, the identification and quantification of disease-specific proteins are particularly important. Because the conformational change of aptamer upon its target or probe/target/probe sandwich often is the primary prerequisite for the design of an electrochemical aptameric assay system, it is extremely difficult to construct the electrochemical aptasensor for protein assay because the corresponding aptamers cannot often meet the requirement. To circumvent the obstacles mentioned, an electronic channel switching-based (ECS) aptasensor for ultrasensitive protein detection is developed. The essential achievement made is that an innovative sensing concept is proposed: the hairpin structure of aptamer is designed to pull electroactive species toward electrode surface and makes the surface-immobilized IgE serve as a barrier that separates enzyme from its substrate. It seemingly ensures that the ECS aptasensor exhibits most excellent assay features, such as, a detection limit of 4.44×10(-6)μg mL(-1) (22.7fM, 220zmol in 10-μL sample) (demonstrating a 5 orders of magnitude improvement in detection sensitivity compared with classical electronic aptasensors) and dynamic response range from 4.44×10(-6) to 4.44×10(-1)μg mL(-1). We believe that the described sensing concept here might open a new avenue for the detection of proteins and other biomacromolecules.

Highly sensitive optical biosensor for thrombin based on structure switching aptamer-luminescent silica nanoparticles

We describe here the construction of a sensitive and selective optical sensor system for the detection of human α-thrombin. The surface functionalized luminescent [Ru(dpsphen)(3)](4-) (dpsphen-4,7-diphenyl-1,10-phenanthroline disulfonate) ion doped silica nanoparticles (SiNPs) with a size ~70 nm have been prepared. The DABCYL (2-(4-dimethylaminophenyl)diazenyl-benzoic acid) quencher labeled thrombin binding aptamer is conjugated to the surface of SiNPs using BS(3) (bis(sulfosuccinimidyl) suberate) as a cross-linker, resulting in the conformational change of aptamer to form G-quadruplex structure upon the addition of thrombin. The binding event is translated into a change in the luminescence intensity of Ru(II) complex via FRET mechanism, due to the close proximity of DABCYL quencher with SiNPs. The selective detection of thrombin using the SiNPs-aptamer system up to 4 nM is confirmed by comparing its sensitivity towards other proteins. This work demonstrates the application of simple aptamer-SiNPs conjugate as a highly sensitive system for the detection of thrombin and also it is highly sensitive towards thrombin in the presence of other proteins and complex medium such as BSA.

Rapid and sensitive detection of Nampt (PBEF/visfatin) in human serum using an ssDNA aptamer-based capacitive biosensor

A single-stranded DNA (ssDNA) aptamer was successfully developed to specifically bind to nicotinamide phosphoribosyl transferase (Nampt) through systematic evolution of ligands by exponential enrichment (SELEX) and successfully implemented in a gold-interdigitated (GID) capacitor-based biosensor. Surface plasmon resonance (SPR) analysis of the aptamer revealed high specificity and affinity (K(d)=72.52 nM). Changes in surface capacitance/charge distribution or dielectric properties in the response of the GID capacitor surface covalently coupled to the aptamers in response to changes in applied AC frequency were measured as a sensing signal based on a specific interaction between the aptamers and Nampt. The limit of detection for Nampt was 1 ng/ml with a dynamic serum detection range of up to 50 ng/ml; this range includes the clinical requirement for both normal Nampt level, which is 15.8 ng/ml, and Nampt level in type 2 diabetes mellitus (T2DM) patients, which is 31.9 ng/ml. Additionally, the binding kinetics of aptamer-Nampt interactions on the capacitor surface showed that strong binding occurred with increasing frequency (range, 700 MHz-1 GHz) and that the dissociation constant of the aptamer under the applied frequency was improved 120-240 times (K(d)=0.3-0.6 nM) independent on frequency. This assay system is an alternative approach for clinical detection of Nampt with improved specificity and affinity.

Applications of aptasensors in clinical diagnostics

Aptamers are artificial oligonucleotides (DNA or RNA) selected in vitro that bind a broad range of targets with high affinity and specificity; a sensitive yet simple method to utilize aptamers as recognition elements for the development of biosensors (aptasensors) is to transduce the signal electrochemically. So far, aptasensors have been applied to clinical diagnostics and several technologies are in development. Aptasensors will extend the limits of current clinical diagnostics. Although the potential diagnostic applications are unlimited, the most current applications are foreseen in the areas of biomarker detection, cancer clinical testing, detection of infectious microorganisms and viruses. This review attempts to list examples of the research progresses of aptamers in biosensor platforms that have been published in recent years; in particular, we display cases of aptasensors that are already incorporated in clinical diagnostics or have potential applications in clinical diagnostics.

Design and construction of a label free aptasensor for electrochemical detection of sodium diclofenac

The present manuscript describes a label free electrochemical aptasensor for the detection of sodium diclofenac (DCF). In order to construct the biosensor, the amino-functionalized diclofenac binding aptamer (DBA) was covalently immobilized on the surface of the glassy carbon electrode (GCE). The conformation of the DBAs on the surface of the electrode is changed when this is exposed to different concentrations of DCF. The introduction of DCF induces an alteration in the conformation of the surface immobilized DBA and causes a decrease in the charge transfer resistance of the aptasensor. However, the charge transfer resistance is increased by incubation of GCE/DBA/DCF in the secondary DBA. The changes in the charge transfer resistance have been monitored using the voltammetric and electrochemical impedance spectroscopic (EIS) techniques. The aptasensor shows two different linear dynamic ranges over 0-5.0 μM and 10 μM to 1mM, and the sensitivity of 15.7 kΩ μM(-1) and detection limit of 2.7 × 10(-7)M were obtained. The validity of the method and applicability of the aptasensor were successfully evaluated by detection of DCF in a blood serum sample without interference from the sample matrix. Furthermore, the aptasensor has shown good stability.

Fluorescence enhancement of silver nanoparticle hybrid probes and ultrasensitive detection of IgE

An ultrasensitive protein assay method was developed based on silver nanoparticle (AgNP) hybrid probes and metal-enhanced fluorescence. Two aptamer based silver nanoparticles, Aptamer/Oligomer-A/Cy3-modified AgNPs (Tag-A) and Aptamer/Oligomer-B/Cy3-modified AgNPs (Tag-B) were hybridized to form a silver nanoparticle aggregate that produced a red shift and broadening of the Localized Surface Plasmon Resonance (LSPR) peak. The enhanced fluorescence resulted from the increased content of Cy3 molecules and their emission resonance coupled to the broadened localized surface plasmon (LSP) of AgNP aggregate. The separation distance between Cy3 and AgNPs was 8 nm which was the most optimal for metal enhanced fluorescence and the separation distance between adjacent AgNPs was about 16 nm and this was controlled by the lengths of oligomer-A and oligomer-B. The protein array was prepared by covalently immobilizing capture antibodies on aldehyde-coated slide. After addition of protein IgE sample, two kinds of aptamer-modified AgNPs (Tag-A and Tag-B) were employed to specifically recognize IgE and form the AgNP aggregate on the arrays based on their hybridization. The detection property of the aptamer-modified AgNP aggregate was compared to two other modified aptamer-based probes, aptamer-modified Cy3 and Tag-A. The modified AgNP hybrid probe (Tag-A and Tag-B) showed remarkable superiority in both sensitivity and detection limit due to the formed AgNP aggregate. The new hybrid probe also produced a wider linear range from 0.49 to 1000 ng/mL with the detection limit reduced to 40 pg/mL (211 fM). The presented method showed that the newly designed strategy of combining aptamer-based nanomaterials to form aggregates results in a highly sensitive optical detection method based on localized surface plasmon.

Aptameric system for the highly selective and ultrasensitive detection of protein in human serum based on non-stripping gold nanoparticles

A novel approach is proposed in this study for the development of an aptameric assay system for protein based on non-stripping gold nanoparticles (NPs)-triggered chemiluminescence (CL) upon target binding. The strategy chiefly depends on the formation of a sandwich-type immunocomplex among the capture antibody immobilized on the polystyrene microwells, target protein and aptamer-functionalized gold NPs. Introduction of target protein into the assay system leads to the attachment of gold NPs onto the surface of the microwells and thus the assembled gold NPs could trigger the reaction between luminol and AgNO(3) with a CL emission. Further signal amplification was achieved by a simple gold metal catalytic deposition onto the gold NPs. Such an amplified CL transduction allowed for the detection of model target IgE down to the 50 fM, which is better than most existing aptameric methods for IgE detection. This new protocol also provided a good capability in discriminating IgE from nontarget proteins such as IgG, IgA, IgM and interferon. The practical application of the proposed gold NPs-based immunoassay was successfully carried out for the determination of IgE in 35 human serum samples. Overall, the proposed assay system exhibits excellent analytical characteristics (e.g., a detection limit on the attomolar scale and a linear dynamic range of 4 orders of magnitude), and it is also straightforward to adapt this strategy to detect a spectrum of other proteins by using different aptamers. This new CL strategy might create a novel technology for developing simple biosensors in the sensitive and selective detection of target protein in a variety of clinical, environmental and biodefense applications.

Rapid real-time electrical detection of proteins using single conducting polymer nanowire-based microfluidic aptasensor

Single polypyrrole (PPy) nanowire-based microfluidic aptasensors were fabricated using a one-step electrochemical deposition method. The successful incorporation of the aptamers into the PPy nanowire was confirmed by fluorescence microscopy image. The microfluidic aptasensor showed responses to IgE protein solutions in the range from 0.01 nM to 100 nM, and demonstrated excellent specificity and sensitivity with faster response and rapid stabilization times (~20 s). At the lowest examined IgE concentration of 0.01 nM, the microfluidic aptasensor still exhibited ~0.32% change in the conductance. The functionality of this aptasensor was able to be regenerated using an acid treatment with no major change in sensitivity. In addition, the detection of cancer biomarker MUC1 was performed using another microfluidic aptasensor, which showed a very low detection limit of 2.66 nM MUC1 compared to commercially available MUC1 diagnosis assay (800 nM).

Surface plasmon resonance investigation of RNA aptamer-RNA ligand interactions

Instruments based on the surface plasmon resonance (SPR) principle allow label-free detection of interactions between targets immobilized at a solid-liquid interface and partners in solution. This method is well suited to determine the kinetic parameters, the equilibrium constant and the stoichiometry of a reaction. Aptamers are ligands identified from random libraries of RNA, DNA or chemically modified oligonucleotides by in vitro selection (SELEX). Aptamers can be raised against a great variety of targets (small molecules, proteins, nucleic acids, cells, viruses, bacteria). SPR is routinely used in our laboratory for the analysis of RNA aptamer-RNA target complexes. To illustrate SPR investigation of such complexes, we describe here methods that were successfully used to monitor the interaction between the trans-activating responsive element of HIV-1 and an RNA aptamer.

Nanocrystalline diamond impedimetric aptasensor for the label-free detection of human IgE

Like antibodies, aptamers are highly valuable as bioreceptor molecules for protein biomarkers because of their excellent selectivity, specificity and stability. The integration of aptamers with semiconducting materials offers great potential for the development of reliable aptasensors. In this paper we present an aptamer-based impedimetric biosensor using a nanocrystalline diamond (NCD) film as a working electrode for the direct and label-free detection of human immunoglobulin E (IgE). Amino (NH(2))-terminated IgE aptamers were covalently attached to carboxyl (COOH)-modified NCD surfaces using carbodiimide chemistry. Electrochemical impedance spectroscopy (EIS) was applied to measure the changes in interfacial electrical properties that arise when the aptamer-functionalized diamond surface was exposed to IgE solutions. During incubation, the formation of aptamer-IgE complexes caused a significant change in the capacitance of the double-layer, in good correspondence with the IgE concentration. The linear dynamic range of IgE detection was from 0.03 μg/mL to 42.8 μg/mL. The detection limit of the aptasensor reached physiologically relevant concentrations (0.03 μg/mL). The NCD-based aptasensor was demonstrated to be highly selective even in the presence of a large excess of IgG. In addition, the aptasensor provided reproducible signals during six regeneration cycles. The impedimetric aptasensor was successfully tested on human serum samples, which opens up the potential of using EIS for direct and label-free detection of IgE levels in blood serum.

Aptamer-barcode based immunoassay for the instantaneous derivatization chemiluminescence detection of IgE coupled to magnetic beads

We report on a highly sensitive aptameric assay system for the determination of IgE, where a special chemiluminescence (CL) reagent, 3,4,5-trimethoxylphenylglyoxal (TMPG), acts as the signaling molecule and polystyrene beads as the amplification platform. Briefly, a "sandwich-type" detection strategy is employed in our design, where magnetic beads functionalized with a capture antibody were reacted with the target protein IgE, and then sandwiched with the aptamer-barcodes which were prepared by assembling polystyrene beads with IgE aptamer. The target immunoreaction event could be sensitively detected via an instantaneous derivatization reaction between TMPG and the guanine (G) nucleotides within the aptamer-barcodes to form an unstable CL intermediate for the generation of light. Further signal amplification is achieved by extending the G nucleotide-rich domain on the aptamer backbone for second amplification. Such simple amplified CL transduction allows the detection of IgE down to the 4.6 pM level, which is better than most previous aptameric methods for IgE detection. This new protocol also provides a good capability in discriminating IgE from nontarget proteins such as IgG, IgA, IgM, interferon and thrombin. The practical application of the proposed aptamer-barcode based immunoassay was successfully carried out for the determination of IgE in 20 human serum samples. It is straightforward to adapt this strategy to detect a spectrum of other proteins by using different aptamers, thus this method may offer a new direction in designing high-performance CL aptasensors for early diagnoses of diseases.