A electrochemiluminescence aptasensor for detection of thrombin incorporating the capture aptamer labeled with gold nanoparticles immobilized onto the thio-silanized ITO electrode

Sandwich-type aptamer-based biosensors for thrombin detection

Thrombin, a proteolytic enzyme, plays an essential role in catalyzing many blood clotting reactions. Thrombin can act as a marker for some blood-related diseases, such as leukemia, thrombosis, Alzheimer's disease and liver disease. Therefore, its diagnosis is of great importance in the fields of biological and medical research. Biosensors containing sandwich-type structures have attracted much consideration owing to their superior features such as reproducible and stable responses with easy improvement in the sensitivity of detection. Sandwich-type platforms can be designed using a pair of receptors that are able to bind to diverse locations of the same target. Herein, we investigate recent advances in the progress and applications of thrombin aptasensors containing a sandwich-type structure, in which two thrombin-binding aptamers (TBAs) identify different parts of the thrombin molecule, leading to the formation of a sandwich structure and ultimately signal detection. We also discuss the pros and cons of these approaches and outline the most logical approach in each section.

Tris(2,2'-bipyridyl)ruthenium (II) complex as a universal reagent for the fabrication of heterogeneous electrochemiluminescence platforms and its recent analytical applications

In recent years, electrochemiluminescence (ECL) has received enormous attention and has emerged as one of the most successful tools in the field of analytical science. Compared with homogeneous ECL, the heterogeneous (or solid-state) ECL has enhanced the rate of the electron transfer kinetics and offers rapid response time, which is highly beneficial in point-of-care and clinical applications. In ECL, the luminophore is the key element, which dictates the overall performance of the ECL-based sensors in various analytical applications. Tris(2,2'-bipyridyl)ruthenium (II) complex, Ru(bpy)32+, is a coordination compound, which is the gold-standard luminophore in ECL. It has played a key role in translating ECL from a "laboratory curiosity" to a commercial analytical instrument for diagnosis. The aim of the present review is to provide the principles of ECL and classical reaction mechanisms-particularly involving the heterogeneous Ru(bpy)32+/co-reactant ECL systems, as well as the fabrication methods and its importance over solution-phase Ru(bpy)32+ ECL. Then, we discussed the emerging technology in solid-state Ru(bpy)32+ ECL-sensing platforms and their recent potential analytical applications such as in immunoassay sensors, DNA sensors, aptasensors, bio-imaging, latent fingerprint detection, point-of-care testing, and detection of non-biomolecules. Finally, we also briefly cover the recent advances in solid-state Ru(bpy)32+ ECL coupled with the hyphenated techniques.

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

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

Electrochemiluminescence methods using CdS quantum dots in aptamer-based thrombin biosensors: a comparative study

The detection of thrombin by using CdS nanocrystals (CdS NCs), gold nanoparticles (AuNPs) and luminol is investigated in this work. Thrombin is detected by three methods. One is called the quenching method. It is based on the quenching effect of AuNPs on the yellow fluorescence of CdS NCs (with excitation/emission wavelengths of 355/550 nm) when placed adjacent to CdS NCs. The second method (called amplification method) is based on an amplification mechanism in which the plasmonics on the AuNPs enhance the emission of CdS NCs through distance related Förster resonance energy transfer (FRET). The third method is ratiometric and based on the emission by two luminophores, viz. CdS NCs and luminol. In this method, by increasing the concentration of thrombin, the intensity of CdS NCs decreases, while that of luminol increases. The results showed that ratiometric method was most sensitive (with an LOD of 500 fg.mL^-1), followed by the amplification method (6.5 pg.mL^-1) and the quenching method (92 pg.mL^-1). Hence, the latter is less useful. Graphical abstract Schematic representation of three different methods (quenching, amplification and ratiometric) were applied for detection of thrombin via aptasensor. The CdS nanocrystals, streptavidin (Str) coated AuNPs and also Str-luminol coated AuNPs were used for the construction steps of the electrochemiluminescence (ECL)-based biosensor.

An electrochemical chlorpyrifos aptasensor based on the use of a glassy carbon electrode modified with an electropolymerized aptamer-imprinted polymer and gold nanorods

A highly selective and sensitive aptasensor is described for voltammetric determination of the pesticide chlorpyrifos (CPS). The sensor was constructed by modifying a glassy carbon electrode (GCE) with gold nanorods and a polymer that was molecularly imprinted with an aptamer against CPS. This results in double specific recognition. Under optimal conditions and a working potential as low as 0.22 V (vs. Ag/AgCl), the nanotools has a dynamic range that covers the 1.0 fM - 0.4 pM CPS concentration range, and the detection limit is 0.35 fM. This is lower than any of the previously reported methods. This MIP-aptasensor is selective over structural analogs, stable, and adequately reproducible. It was successfully applied to the determination of CPS in spiked food samples. Graphical abstract Impedimetric detection of Chlorpyrifos by using a Fe(CN)₆^3-/4- probe based on double recognition of aptamer-molecular imprinted polymer onto a glassy carbon electrode modified with gold nanorod nanocomposite. The incubation with Chlorpyrifos lead to an increase of electron transfer resistance.

Ultrasensitive and reusable electrochemical aptasensor for detection of tryptophan using of [Fe(bpy)3](p-CH3C6H4SO2)2 as an electroactive indicator

In this paper, we report the application of a reusable electrochemical aptasensor for detection of tryptophan by using [Fe(bpy)₃](p-CH₃C₆H₄SO₂)₂ as an electroactive indicator and based on the target-compelled aptamer displacement. The aptasensor fabricated by self-assembling the thiolated probe on the surface of graphite screen-printed electrode modified with gold nanoparticles/multiwalled carbon nanotubes and chitosan nanocomposite (AuNPs/MWCNTs-Chit/SPE). Afterward, Trp aptamer (Apt) immobilized on the modified electrode surface through hybridization. In the absence of Trp, a sharp peak of [Fe(bpy)₃](p-CH₃C₆H₄ SO₂)₂ can be observed in differential pulse voltammetry (DPV) study. The introduction of Trp led to the formation of aptamer-Trp complex and dissociation of the aptamer from the DNA-Apt duplex on the electrode surface into the solution and decreases the peak current intensity of electroactive indicator. This is because, [Fe(bpy)₃](p-CH₃C₆H₄SO₂)₂ tends to bind to the two strands DNA. Therefore, the peak current of [Fe(bpy)₃](p-CH₃C₆H₄ SO₂)₂ linearly decreased with increasing the concentration of Trp over a range of 3.0 nM- 100 μM. The detection limit (3 σ) was 1.0 nM. In addition, we examined the selectivity of the constructed biosensor for tyrosine, histidine, arginine, lysine, valine and methionine that belonged to the amino acid family. The obtained results showed that the fabricated sensor had a good selectivity for Trp against the other examined amino acids. Also, the potential applicability of the aptasensor was investigated by detecting the Trp in a complex media such as human blood plasma spiked with Trp.

Recent trends in electrochemiluminescence aptasensors and their applications

Aptamers are single stranded DNA or RNA ligands which can be selected for different targets from proteins to small organic dyes. In the past few years great progress has been accomplished in the development of aptamer based bioanalytical assays with different detection techniques. Among them, electrochemiluminescence (ECL) aptasensors are very promising because they have the advantages of both electrochemical and chemiluminescence biosensors, such as high sensitivity, low background, cost effectiveness, and ease of control. In this review, we summarize the recent efforts to construct novel and improved ECL aptasensors and their application.

A colorimetric aptamer biosensor based on cationic polythiophene derivative as peroxidase mimetics for the ultrasensitive detection of thrombin

A colorimetric assay for the ultrasensitive determination of thrombin was presented, in which the cationic polythiophene derivative was used as catalyst of the 3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ reaction and the thrombin-binding aptamer (TBA) was used as inducing polymer's different conformation elements. It was found the cationic polythiophene derivative, poly[3-(3'-N,N,N-triethylamino-1'-propyloxy)-4-methyl-2,5-t-hiophene hydrochloride] (PMNT), can catalyze the oxidation reaction of TMB in the presence of H₂O₂ to produce a blue color solution. The catalytic activity of PMNT on the TMB-H₂O₂ reaction was closely relevant to the conformation of PMNT. The absorbance of TMB-H₂O₂ was distinctly increased in the presence of TBA. With the addition of thrombin, TBA interacted with thrombin to form a G-quadruplex structure. The conformational change weakened the catalytic activity of PMNT and resulted in a decrease in the absorbance. The colorimetric sensor could detect thrombin down to 4pM with high selectivity against other interfering proteins. This work is not only of importance for a better understanding of the unique properties of cationic polythiophenes derivative but also have great potential for medical diagnostics and therapy for human health.

Selection and Biosensor Application of Aptamers for Small Molecules

Small molecules play a major role in the human body and as drugs, toxins, and chemicals. Tools to detect and quantify them are therefore in high demand. This review will give an overview about aptamers interacting with small molecules and their selection. We discuss the current state of the field, including advantages as well as problems associated with their use and possible solutions to tackle these. We then discuss different kinds of small molecule aptamer-based sensors described in literature and their applications, ranging from detecting drinking water contaminations to RNA imaging.

A simple and rapid chemiluminescence aptasensor for acetamiprid in contaminated samples: Sensitivity, selectivity and mechanism

Ultralow concentration and selective detection of pesticide residue is important to evaluate the environmental and biological pollution and the threat to human health which single component pesticide can bring. Herein, we report an amplified chemiluminescence (CL) sensing platform for ultrasensitive and selective acetamiprid (widely used pesticide) detection. It is based on aptamer's high binding affinity to target and the relevance between AuNPs' morphology and its catalytic effect to stimulate the generation of CL in the presence of H2O2 and luminol. Moreover, AuNPs morphological slight change induced by aptamers' conformation during targets binding could lead to the significant change of catalytic properties. Therefore, the proposed sensing platform for pesticide residue exhibited a high sensitivity toward acetamiprid with a detection limit of 62pM, which was about 100-fold lower than that of other aptamer-based sensor for acetamiprid detection. Because of the intrinsic specificity of aptamer's recognization, this sensing platform has high selectivity. So, this sensing platform provides a label-free and cost-effective approach for sensitive and selective detection of single component pesticide residue. More importantly, this CL method was successfully used to determine acetamiprid in real contaminated samples.

Nanozymes in bionanotechnology: from sensing to therapeutics and beyond

Nanozymes are nanomaterials with enzyme-like characteristics, which have found broad applications in various areas including bionanotechnology and beyond.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Electrochemical aptasensor for the detection of adenosine by using PdCu@MWCNTs-supported bienzymes as labels

A highly sensitive electrochemical adenosine aptasensor was fabricated by covalently immobilizing 3'-NH2-terminated capture probe (SSDNA1) and thionine (TH) on Au-GS modified glassy carbon electrode. 3'-SH-terminated adenosine aptamer (SSDNA2) was adsorbed onto palladium/copper alloyed supported on MWCNTs (PdCu@MWCNTs)-conjugated multiple bienzymes, glucose oxidase (GOx), and horseradish peroxidase (HRP) (SSDNA2/PdCu@MWCNTs/HRP/GOx). Then, it was immobilized onto the electrode surface through the hybridization between the adenosine aptamer and the capture probe. The signal was amplified based on the gradual electrocatalytic reduction of GOx-generated hydrogen peroxide by the multiple HRP through the mediating ability of the loaded multiple TH. However, the peak current of TH decreased in the presence of adenosine because the interaction between adenosine and its aptamer made SSDNA2/PdCu@MWCNTs/HRP/GOx release from the modified electrode. Various experimental parameters have been optimized for the detection of adenosine and tests for selectivity, reproducibility and stability have also been performed. Under the optimal condition, the proposed aptasensor displayed a wide linear range (10-400 nM) with the low detection limit (2.5 nM), which has been applied in human serum samples with satisfactory results. Thus, the combination of Au-GS as a sensor platform and PdCu@MWCNTs/HRP/GOx as labels can be a promising amplification strategy for highly sensitive adenosine detection.

Facile preparation of an immobilized surfactant-free palladium nanocatalyst for metal hydride trapping: a novel sensing platform for TXRF analysis

In this work, a simple route for the synthesis of surfactant-free immobilized palladium nanoparticles (Pd NPs) and their use as effective nanocatalysts for metal hydride decomposition is described. A mixture of ethanol : water was used as the reducing agent. Ethanol was added in a large excess to reduce the ionic Pd and stabilize the obtained Pd NPs. Ethanol is adsorbed on the surface of Pd allowing steric stabilization. Freshly prepared Pd NPs were immobilized onto quartz substrates modified with 3-mercaptopropyltrimethoxysilane. Pd interacts with the thiol group of the alkoxysilane that is adsorbed on the surface of NPs without the dissociation of the S-H bond. Different parameters affecting the synthesis of Pd NPs and their immobilization onto quartz substrates were evaluated. A comprehensive characterization of the synthesized Pd NPs was carried out by transmission electron microscopy (TEM), whereas total reflection X-ray fluorescence (TXRF) spectrometry was applied in order to evaluate their catalytic activity for solid-gas reactions. Immobilized Pd NPs were applied as nanocatalysts for the dissociative chemisorption of arsine at room temperature, yielding the formation of As-Pd bonds. Quartz substrates coated with nanosized Pd could be used as novel sensing platforms for total reflection X-ray fluorescence analysis. Arsenic can be detected in situ in natural water with a limit of detection of 0.08 μg L(-1).

A facile label-free colorimetric aptasensor for acetamiprid based on the peroxidase-like activity of hemin-functionalized reduced graphene oxide

A facile aptasensor has been developed for the colorimetric detection of acetamiprid by using the hemin-functionalized reduced graphene oxide (hemin-rGO) composites. The as-prepared hemin-rGO composites possessed both the ability of rGO to physically adsorb the aptamers and the peroxidase-like activity of hemin that could catalyse 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2, to produce a solution with blue color. The well-dispersed hemin-rGO composites coagulated completely at the proper salt concentration; however, the coagulation of hemin-rGO was vanished when abundant aptamers were adsorbed on its surface because the attached negatively charged DNA backbone increased individual hemin-rGO electrostatic repulsion. In the detection scheme, acetamiprid with different concentrations was firstly incubated with the same amount of aptamer. The more acetamiprid in the tested solution, the less free aptamers were absorbed on the hemin-rGO surface, making the composites coagulate to a higher degree in the presence of the optimum NaCl concentration. As a consequence, the content of hemin-rGO in the supernatant was decreased after centrifugation, which catalysed oxidation of TMB in the presence of H2O2 to produce light blue color with a low absorbance. The color variation relavant to the acetamiprid concentration can be judged by the naked eyes and easily monitored by the inexpensive UV-vis spectrometer. Such designed aptasensor displayed a linear response for acetamiprid in the range from 100nM to 10μM with a detection limit of 40nM (S/N=3). This colorimetric aptasensing platform offers great advantages including the simple operation process, low-cost portable instrument, and user-friendly applications.

Ruthenium polypyridine complexes combined with oligonucleotides for bioanalysis: a review

Ruthenium complexes are among the most interesting coordination complexes and they have attracted great attention over the past decades due to their appealing biological, catalytic, electronic and optical properties. Ruthenium complexes have found a unique niche in bioanalysis, as demonstrated by the substantial progress made in the field. In this review, the applications of ruthenium complexes coordinated with polypyridine ligands (and analogues) in bioanalysis are discussed. Three main detection methods based on electrochemistry, electrochemiluminescence, and photoluminscence are covered. The important targets, including DNA and other biologically important targets, are detected by specific biorecognition with the corresponding oligonucleotides as the biorecognition elements (i.e., DNA is probed by its complementary strand and other targets are detected by functional nucleic acids, respectively). Selected examples are provided and thoroughly discussed to highlight the substantial progress made so far. Finally, a brief summary with perspectives is included.

Electrogenerated chemiluminescence aptasensor for ultrasensitive detection of thrombin incorporating an auxiliary probe

A novel electrogenerated chemiluminescence (ECL) aptasensor for ultrasensitive detection of thrombin incorporating an auxiliary probe was designed by employing specific anti-thrombin aptamer as a capture probe and a ruthenium(II) complex-tagged reporter probe as an ECL probe and an auxiliary probe to assist the ECL probe close to the surface of the electrode. The ECL aptasensor was fabricated by self-assembling a thiolated capture probe on the surface of gold electrode and then hybridizing the ECL probe with the capture probe, and further self-assembling the auxiliary probe. When analyte thrombin was bound with the capture probe, the part of the dehybridized ECL probe was hybridized with the neighboring auxiliary probe, led to the tagged ruthenium(II) complex close to the electrode surface, resulted in great increase in the ECL intensity. The results showed that the increased ECL intensity was directly related to the logarithm of thrombin concentrations in the range from 5.0 × 10(-15)M to 5.0 × 10(-12)M with a detection limit of 2.0 × 10(-15)M. This work demonstrates that employing an auxiliary probe which exists nearby the capture probe can enhance the sensitivity of the ECL aptasensor. This promising strategy will be extended to the design of other biosensors for detection of other proteins and genes.

A novel label-free electrochemiluminescence aptasensor based on layered flowerlike molybdenum sulfide-graphene nanocomposites as matrix

A label-free and ultrasensitive electrochemiluminescence (ECL) aptasensor was constructed for the detection of thrombin. Molybdenum sulfide-graphene nanocomposites with good conductivity and large surface area were immobilized on glassy carbon electrode (GCE), and then Nafion was fixed to chemosorb the Ru(bpy)3(2+) used as luminescence agent. Subsequently, gold nanoparticles (AuNPs) were modified on the electrode to immobilize the thiol-modified thrombin aptamer for fabrication of the thrombin aptasensor. The proposed ECL aptasensor produced the ultrasensitive detection of thrombin with a low detection limit of 3.6×10(-15)M (S/N=3) and over a wide target concentration range from 1.0×10(-14) to 5.0×10(-9)M. The aptasensor has been successfully applied in the determination of thrombin in human plasma samples of both traumatic and non-traumatic injury patients, indicating its promise in biochemical analysis. The recoveries of thrombin in human plasma samples are between 88.6% and 105.0%, and the RSD values are no more than 3.7%. The results demonstrate that this aptasensor has excellent sensitivity, selectivity and stability.

A colorimetric sandwich-type assay for sensitive thrombin detection based on enzyme-linked aptamer assay

A colorimetric sandwich-type assay based on enzyme-linked aptamer assay has been developed for the fast and sensitive detection of as low as 25 fM of thrombin with high linearity. Aptamer-immobilized glass was used to capture the target analyte, whereas a second aptamer, functionalized with horseradish peroxidase (HRP), was employed for the conventional 3,5,3',5'-tetramethylbenzidine (TMB)-based colorimetric detection. Without the troublesome antibody requirement of the conventional enzyme-linked immunosorbent assay (ELISA), as low as 25 fM of thrombin could be rapidly and reproducibly detected. This assay has superior, or at least equal, recovery and accuracy to that of conventional antibody-based ELISA.

An off-on-off electrochemiluminescence approach for ultrasensitive detection of thrombin

This work demonstrates an aptasensor for ultrasensitive electrochemiluminescence (ECL) detection of thrombin based on an "off-on-off" approach. The system is composed of an Eu(3+)-doped CdS nanocrystals (CdS:Eu NCs) film on glassy carbon electrode (GCE) as ECL emitter. Then gold nanoparticles (AuNPs) labeled hairpin-DNA probe (ssDNA1) containing thrombin-binding aptamer (TBA) sequence was linked on the NCs film, which led to ECL quenching (off) as a result of Förster-resonance energy transfer (FRET) between the CdS:Eu NC film and the proximal AuNPs. Upon the occurrence of hybridization with its complementary DNA (ssDNA2), an ECL enhancement (on) occurred owing to the interactions of the excited CdS:Eu NCs with ECL-induced surface plasmon resonance (SPR) in AuNPs at large separation. Thrombin could induce ssDNA1 forming a G-quadruplex and cause the AuNPs to be close to CdS:Eu NCs film again, which resulted in an enhanced ECL quenching (off). This "off-on-off" system showed a maximum 7.4-fold change of ECL intensity due to the configuration transformation of ssDNA1 and provides great sensitivity for detection of thrombin in a wide detection range from 50 aM to 1 pM.

Electrochemiluminescence aptasensor for adenosine triphosphate detection using host-guest recognition between metallocyclodextrin complex and aptamer

A sensitive and label-free electrochemiluminescence (ECL) aptasensor for the detection of adenosine triphosphate (ATP) was successfully designed using host-guest recognition between a metallocyclodextrin complex, i.e., tris(bipyridine)ruthenium(II)-β-cyclodextrin [tris(bpyRu)-β-CD], and an ATP-binding aptamer. In the protocol, the NH2-terminated aptamer was immobilized on a glassy carbon electrode (GCE) by a coupling interaction. After host-guest recognition between tris(bpyRu)-β-CD and aptamer, the tris(bpyRu)-β-CD/aptamer/GCE produced a strong ECL signal as a result of the photoactive properties of tris(bpyRu)-β-CD. However, in the presence of ATP, the ATP/aptamer complex was formed preferentially, which restricted host-guest recognition, and therefore less tris(bpyRu)-β-CD was attached to the GCE surface, resulting in an obvious decrease in the ECL intensity. Under optimal determination conditions, an excellent logarithmic linear relationship between the ECL decrease and ATP concentration was obtained in the range 10.0-0.05 nM, with a detection limit of 0.01 nM at the S/N ratio of 3. The proposed ECL-based ATP aptasensor exhibited high sensitivity and selectivity, without time-consuming signal-labeling procedures, and is considered to be a promising model for detection of aptamer-specific targets.

Protein-directed approaches to functional nanomaterials: a case study of lysozyme

Using lysozyme as a model, protein-directed approaches to functional nanomaterials were reviewed, making rational materials design possible in the future.

Highly selective and sensitive adenosine aptasensor based on platinum nanoparticles as catalytical label for amplified detection of biorecognition events through H2O2 reduction

Based on a combination of aptamer and platinum nanoparticles a new sensitive and selective adenosine aptasensor was fabricated. Functionalized thiol-terminated adenosine aptamer (5'-AGAGAACCTGGGGGAGTATTGCGGAGGAAGGT-SH-3') with Pt Nanoparticles (Pt-NPs) was employed as highly catalytic label for electrochemical detection of adenosine based on electrocatalytic activity of Pt-NPs toward H2O2 reduction. Multiwalled carbon nanotubes/ionic liquid/chitosan (MWCNTs/IL/CHIT) nanocomposite was applied as the interface for covalent attachment of 3'-amine-terminated capture probe (3'-NH2-(CH2)6-TCTCTTGGACCC-5'). The presence of Pt nanoparticles improvement the conductivity and performance characteristics of the biosensor as well as incensement in the loading amount of the aptamer DNA sequence. The interaction of adenosine with the aptamer causes the releasing of aptamer with PtNPs into solution which resulted in a decreasing of hydrogen peroxide reduction peak current. Sensitive quantitative detection of adenosine was achieved by monitoring the decrease of voltammetric responses of H2O2 peak current. The peak current of H2O2 decreased with increase in the concentration of adenosine over a range of 1-750 nM with detection limit 1 nM. In addition the proposed aptasensor showed excellent selectivity toward adenosine in compared to some other nucleosides such as guanosine, cytidine and uridine. The proposed aptasensor was successfully used to detect adenosine in human serum samples. The elimination of enzymes or antibodies for the amplified detection of adenosine and the use of platinum nanoparticles as inorganic catalytic label, are the advantage of the proposed aptasensor.

Sensitive electrochemical aptasensor by coupling "signal-on'' and "signal-off'' strategies

A simple electrochemical aptasensor for sensitive and selective determination of adenosine triphosphate (ATP) has been developed on the basis of a new dual-signaling amplification strategy. This aptasensor features both ''signal-on'' and ''signal-off'' elements. The ferrocene (Fc)-labeled aptamer probe (Fc-P) is designed to hybridize with the thiolated methylene blue (MB)-modified DNA probe (MB-P) on gold electrode to form rigid duplex DNA. In the presence of ATP, the interaction between ATP and the aptamer leads to the dissociation of the duplex DNA structure and thereby the release of the Fc-P from the sensing interface. The single-stranded MB-P could thus tend to form a hairpin structure through the hybridization of the complementary sequences at both its ends. Such conformational changes result in the oxidation peak current of Fc decreases and that of MB increases, and the changes of dual signals are linear with the concentration of ATP. When "ΔI = ΔI(MB) + |ΔI(Fc)|" (ΔI(MB) and ΔI(Fc) are the change values of the oxidation peak currents of MB and Fc, respectively.) is used as the response signal for quantitative determination of ATP, the detection limit (1.9 nM) is much lower than that by using either MB-P or Fc-P alone. This new dual-signaling aptasensor is readily regenerated and shows good response toward the target. It will have important applications in the sensitive and selective electrochemical determination of other small molecules and proteins.

An aptamer-gated silica mesoporous material for thrombin detection

An aptamer-capped mesoporous material for the selective and sensitive detection of α-thrombin in human plasma and serum has been prepared and characterised.

Fabrication of a highly sensitive adenosine aptasensor based on covalent attachment of aptamer onto chitosan-carbon nanotubes-ionic liquid nanocomposite

The present study describes the fabrication of a novel electrochemical aptasensor for the label-free determination of adenosine. The immobilization surface is prepared by the modification of a glassy carbon (GC) electrode with a robust nanocomposite containing multiwalled carbon nanotubes, ionic liquid and chitosan(MWCNTs-IL-CHIT). Amine-terminated 12-mer capture probe(ssDNA1) is covalently attached onto the nanocomposite using glutaraldehyde (GA) as the linking agent, a 32-mer adenosine-specific aptamer (ssDNA2) immobilized onto the electrode surface through hybridization with the ssDNA1 and methylene blue (MB) used as the redox probe. The peak current of MB decreased linearly with increasing adenosine concentration due to the formation of aptamer-adenosine complex and displacement of the aptamer from the modified electrode surface. The aptasensor showed a low detection limit of 150 pM and high sensitivity of 0.67 μAnM⁻¹ at a concentration range of up to 0.4 μM. Through the control experiments performed by using some other nucleosides such as guanosine, cytidine and uridine, the excellent specificity of this sensor toward adenosine detection is demonstrated. The potential applicability of the aptasensor is successfully applied for measuring adenosine concentration in blood serum and drug formulation samples.The herein described methodology may hold great promise for fabrication of other aptasensors and immunosensors.

Electrochemiluminescence of blue-luminescent graphene quantum dots and its application in ultrasensitive aptasensor for adenosine triphosphate detection

A simple approach based on exfoliating and disintegrating treatments for graphite oxide, followed by hydrothermal synthesis, was developed to prepare water-soluble graphene quantum dots (GQDs). The as-prepared GQDs exhibited bright blue emission under ultraviolet irradiation (∼365nm), and showed an excitation-independent photoluminescence feature. More importantly, a newly anodic electrochemiluminescence (ECL) was observed from the water-soluble GQDs with H2O2 as coreactant for the first time, and the ECL induced a strong light emission at a low potential (ca. 0.4V vs. Ag/AgCl). The ECL mechanism is investigated in detail. Employing SiO2 nanospheres as signal carrier, a novel SiO2/GQDs ECL signal amplification labels were synthesized based on which a ultrasensitive ECL aptamer sensor was proposed. Under the optimized experimental conditions, the proposed ECL aptamer sensor exhibited excellent analytical performance for adenosine triphosphate (ATP) determination, ranging from 5.0×10(-12) to 5.0×10(-9)molL(-1) with the detection limit of 1.5×10(-12)molL(-1). Due to the low cytotoxicity and excellent biocompatibility, GQDs are demonstrated to be an eco-friendly material as well as excellent ECL labeling agents for biosensor.

Aptamer-modified nanoparticles as biosensors

Aptamers are short oligonucleotides that are capable of selectively binding to their corresponding target. Therefore, they can be thought of as a nucleic acid-based alternative to antibodies and can substitute for their amino acid-based counterparts in analytical applications, including as receptors in biosensors. Here they offer several advantages because their nucleic acid nature and their binding via an induced fit mechanism enable novel sensing strategies. In this article, the utilization of aptamers as novel bio-receptors in combination with nanoparticles as transducer elements is reviewed. In addition to these analytical applications, the medical relevance of aptamer-modified nanoparticles is described.

Development and Optimization of a Thrombin Sandwich Aptamer Microarray

A sandwich microarray employing two distinct aptamers for human thrombin has been optimized for the detection of subnanomolar concentrations of the protein. The aptamer microarray demonstrates high specificity for thrombin, proving that a two-site binding assay with the TBA1 aptamer as capture layer and the TBA2 aptamer as detection layer can ensure great specificity at times and conditions compatible with standard routine analysis of biological samples. Aptamer microarray sensitivity was evaluated directly by fluorescent analysis employing Cy5-labeled TBA2 and indirectly by the use of TBA2-biotin followed by detection with fluorescent streptavidin. Sub-nanomolar LODs were reached in all cases and in the presence of serum, demonstrating that the optimized aptamer microarray can identify thrombin by a low-cost, sensitive and specific method.

Nucleic acid and peptide aptamers: fundamentals and bioanalytical aspects

In recent years new nucleic acid and protein-based combinatorial molecules have attracted the attention of researchers working in various areas of science, ranging from medicine to analytical chemistry. These molecules, called aptamers, have been proposed as alternatives to antibodies in many different applications. The aim of this Review is to illustrate the peculiarities of these combinatorial molecules which have initially been explored for their importance in molecular medicine, but have enormous potential in other biotechnological fields historically dominated by antibodies, such as bioassays. A description of these molecules is given, and the methods for their selection and production are also summarized. Moreover, critical aspects related to these molecules are discussed.

Applications of tris(2,2'-bipyridyl)ruthenium(II) in electrochemiluminescence

Electrochemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(II) [Ru(bpy)(3)(2+)] has received considerable interest over broad applications due to its remarkably high sensitivity and extremely wide dynamic range. After a brief introduction of the ECL of Ru(bpy)(3)(2+), an overview of our recent research on enhanced ECL, fabrication of solid-state ECL sensors, analytical application of an effective bioassay, and alignment of ECL with capillary electrophoresis (CE) and microchip CE is discussed in detail. Finally, we conclude with a look at the future challenges and prospects of the development of ECL.

In vitro selection and characterization of DNA aptamers recognizing chloramphenicol

Chloramphenicol (Cam), although an effective antibiotic, has lost favour due to some fatal side effects. Thus there is an urgent need for rapid and sensitive methods to detect residues in food, feed and environment. We engineered DNA aptamers that recognize Cam as their target, by conducting in vitro selections. Aptamers are nucleic acid recognition elements that are highly specific and sensitive towards their targets and can be synthetically produced in an animal-friendly manner, making them ethical innovative alternatives to antibodies. None of the isolated aptamers in this study shared sequence homology or structural similarities with each other, indicating that specific Cam recognition could be achieved by various DNA sequences under the selection conditions used. Analyzing the binding affinities of the sequences, demonstrated that dissociation constants (K(d)) in the extremely low micromolar range, which were lower than those previously reported for Cam-specific RNA aptamers, were achieved. The two best aptamers had G rich (>35%) nucleotide regions, an attribute distinguishing them from the rest and apparently responsible for their high selectivity and affinity (K(d)∼0.8 and 1μM respectively). These aptamers open up possibilities to allow easy detection of Cam via aptamer-based biosensors.

Signal-on electrochemiluminescent biosensor for ATP based on the recombination of aptamer chip

A novel signal-on electrochemiluminescent (ECL) biosensor based on adenosine triphosphate (ATP)-induced recombination of split aptamer chips is presented.