Gold nanoparticles doped conducting polymer nanorod electrodes: ferrocene catalyzed aptamer-based thrombin immunosensor

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Nucleic acid-based electrochemical biosensors for rapid clinical diagnosis: advances, challenges, and opportunities

Clinical diagnostic tests should be quick, reliable, simple to perform, and affordable for diagnosis and treatment of diseases. In this regard, owing to their novel properties, biosensors have attracted the attention of scientists as well as end-users. They are efficient, stable, and relatively cheap. Biosensors have broad applications in medical diagnosis, including point-of-care (POC) monitoring, forensics, and biomedical research. The electrochemical nucleic acid (NA) biosensor, the latest invention in this field, combines the sensitivity of electroanalytical methods with the inherent bioselectivity of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The NA biosensor exploits the affinity of single-stranded DNA/RNA for its complementary strand and is used to detect complementary sequences of NA based on hybridization. After the NA component in the sensor detects the analyte, a catalytic reaction or binding event that generates an electrical signal in the transducer ensues. Since 2000, much progress has been made in this field, but there are still numerous challenges. This critical review describes the advances, challenges, and prospects of NA-based electrochemical biosensors for clinical diagnosis. It includes the basic principles, classification, sensing enhancement strategies, and applications of biosensors as well as their advantages, limitations, and future prospects, and thus it should be useful to academics as well as industry in the improvement and application of EC NA biosensors.

A membraneless self-powered photoelectrochemical biosensor based on Bi2S3/BiPO4 heterojunction photoanode coupling with redox cycling signal amplification strategy

The photoelectrochemical (PEC) self-powered system has attracted great attention in disease detection. The determination of a simple and efficient approach for disease-related biomarkers is highly interesting and appealing. Herein, an ingenious visible light-induced membraneless self-powered PEC biosensing platform was constructed, integrating a signal amplification strategy for ultrasensitive split-type PEC bioanalysis. The system was comprised of a Bi₂S₃/BiPO₄ heterojunction photoanode and a platinum (Pt) cathode in a one compartment chamber. An alkaline phosphatase (ALP)-loaded sandwich immunoassay was used to generate the signal reporter ascorbic acid (AA) in a 96-well plate, and myoglobin (Myo) was used as a model protein. In the presence of AA, ferrocene (Fc), and Tris (2-carboxyethyl) phosphine (TCEP), the chemical-chemical redox cycling scheme was operated upon the initial oxidation of Fc by the holes in the Bi₂S₃/BiPO₄ photoelectrode, and Fc was regenerated from Fc⁺ by AA. Subsequently, AA was regenerated by TCEP after its oxidation, and cycling was triggered. As a result, the proposed self-powered PEC sensing exhibited excellent performance with a wide linear range from 5.0 × 10^-13 to 1.0 × 10^-7 g/mL, and a low detection limit of 2.0 × 10^-13 g/mL for Myo. This work provided a new design of a redox cycling strategy in the self-powered PEC biosensor, and showed an effective approach for the clinical diagnosis.

Nucleic acid-based electrochemical biosensors for rapid clinical diagnosis: Advances, challenges, and opportunities

Clinical diagnostic tests should be quick, reliable, simple to perform, and affordable for diagnosis and treatment of diseases. In this regard, owing to their novel properties, biosensors have attracted the attention of scientists as well as end-users. They are efficient, stable, and relatively cheap. Biosensors have broad applications in medical diagnosis, including point-of-care (POC) monitoring, forensics, and biomedical research. The electrochemical nucleic acid (NA) biosensor, the latest invention in this field, combines the sensitivity of electroanalytical methods with the inherent bioselectivity of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The NA biosensor exploits the affinity of single-stranded DNA/RNA for its complementary strand and is used to detect complementary sequences of NA based on hybridization. After the NA component in the sensor detects the analyte, a catalytic reaction or binding event that generates an electrical signal in the transducer ensues. Since 2000, much progress has been made in this field, but there are still numerous challenges. This critical review describes the advances, challenges, and prospects of NA-based electrochemical biosensors for clinical diagnosis. It includes the basic principles, classification, sensing enhancement strategies, and applications of biosensors as well as their advantages, limitations, and future prospects, and thus it should be useful to academics as well as industry in the improvement and application of EC NA biosensors.

A Sensitive Thrombin Aptasensor Based on Target Circulation Strategy

A convenient homogeneous electrochemical thrombin sensor based on potential-assisted Au-S deposition and a dual signal amplification strategy was established in this study. Potential-assisted Au-S deposition does not require the modification of the gold electrode, thus eliminating the tedious pre-modification of the electrode. To better amplify the output signal, both ends of the signal hairpin probes were modified with a new electroactive substance, tetraferrocene, which was synthesized by the authors. Thrombin was immediately hybridized with a thiol-modified probe to open the stem-loop structure. After chain hybridization, thrombin was replaced and participated in the next round of the reaction; thus, the cascade amplification of the signal was realized. The hybrid chain formed an Au-S deposition under potential assistance, and the electrochemical signal of tetraferrocene could then be measured through differential pulse voltammetry (DPV) and consequently used for the quantitative detection of target thrombin. In addition, the detection limit of thrombin was as low as 0.06 pmol/L, and the detection of common interfering proteins was highly specific.

Fast Aptamer Generation Method Based on the Electrodynamic Microfluidic Channel and Evaluation of Aptamer Sensor Performance

We demonstrate for the first time a fast aptamer generation method based on the screen-printed electrodynamic microfluidic channel device, where a specific aptamer selectively binds to a target protein on channel walls, following recovery and separation. A malaria protein as a model target, Plasmodium vivax lactate dehydrogenase (PvLDH) was covalently bonded to the conductive polymer layer formed on the carbon channel walls to react with the DNA library in a fluid. Then, the AC electric field was symmetrically applied on the channel walls for inducing the specific binding of the target protein to DNA library molecules. In this case, the partitioning efficiency between PvLDH and DNA library in the channel was attained to be 1.67 × 10⁷ with the background of 5.56 × 10^-6, which was confirmed using the quantitative polymerase chain reaction (qPCR). The selectively captured DNAs were isolated from the protein and separated in situ to give five aptamers with different sequences by one round cycle. The dissociation constants (K_d) of the selected aptamers were determined employing both electrochemical impedance spectroscopy (EIS) and the fluorescence method. The sensing performance of each aptamer was evaluated for the PvLDH detection after individual immobilization on the screen-printed array electrodes. The most sensitive aptamer revealed a detection limit of 7.8 ± 0.4 fM. The sensor reliability was evaluated by comparing it with other malaria sensors.

The application of antibody-aptamer hybrid biosensors in clinical diagnostics and environmental analysis

The growing number of various diseases and the increase of environmental contamination are the causes for the development of novel methods for their detection. The possibility of the application of affinity-based biosensors for such purposes seems particularly promising as they provide high selectivity and low detection limits. Recently, the usage of hybrid antibody-aptamer sandwich constructs was shown to be more advantageous in terms of working parameters in comparison to aptamer-based and immune-based biosensors. This review is focused on the usage of hybrid antibody-aptamer receptor layers for the determination of clinically and environmentally important target molecules. In this work, antibodies and aptamer molecules are characterized and the methods of their immobilization as well as analytical signal generation are shown. This is followed by the critical presentation of examples of hybrid sandwich biosensors that have been elaborated in the past 12 years.

Multiplexed aptasensor based on metal ions labels for simultaneous detection of multiple antibiotic residues in milk

A dual-target electrochemical aptasensor was developed for the simultaneous detection of multiple antibiotics based on metal ions as signal tracers and nanocomposites as signal amplification strategy. Metal ions such as Cd²⁺ and Pb²⁺ could generate distinct differential pulse voltammetry (DPV) peaks. When targets were present, kanamycin (KAN) and streptomycin (STR) as models, the KAN aptamer (KAP) and STR aptamer (STP) were released from their complementary strands, with more change of Cd²⁺ and Pb²⁺ corresponding to peak currents. At the same time, complementary strand of KAP (cKAP) and STP (cSTP) were linked with the poly (A) structure (cSTP-PolyA-cKAP) to increase their conformational freedom. Graphitized multi-walled carbon nanotubes (MWCNT_Gr) and carbon nanofibers-gold nanoparticles (CNFs-AuNPs) as a biosensor platform enhanced the surface area to capture a large amount of cSTP-PolyA-cKAP, thus amplifying the detection response. Under the optimal conditions, the aptasensor could detect KAN and STR as low as 74.50 pM and 36.45 pM respectively with the range from 0.1 to 100 nM and exhibited excellent selectively. Moreover, this aptasensor showed promising applications for the detection of other analytes by changing corresponding aptamers.

Detection of Thrombin Based on Fluorescence Energy Transfer between Semiconducting Polymer Dots and BHQ-Labelled Aptamers

Carboxyl-functionalized semiconducting polymer dots (Pdots) were synthesized as an energy donor by the nanoprecipitation method. A black hole quenching dye (BHQ-labelled thrombin aptamers) was used as the energy acceptor, and fluorescence resonance energy transfer between the aptamers and Pdots was used for fluorescence quenching of the Pdots. The addition of thrombin restored the fluorescence intensity. Under the optimized experimental conditions, the fluorescence of the system was restored to the maximum when the concentration of thrombin reached 130 nM, with a linear range of 0-50 nM (R² = 0.990) and a detection limit of 0.33 nM. This sensor was less disturbed by impurities, showing good specificity and signal response to thrombin, with good application in actual samples. The detection of human serum showed good linearity in the range of 0-30 nM (R² = 0.997), with a detection limit of 0.56 nM and a recovery rate of 96.2-104.1%, indicating that this fluorescence sensor can be used for the detection of thrombin content in human serum.

Electrochemical and AFM Characterization of G-Quadruplex Electrochemical Biosensors and Applications

Guanine-rich DNA sequences are able to form G-quadruplexes, being involved in important biological processes and representing smart self-assembling nanomaterials that are increasingly used in DNA nanotechnology and biosensor technology. G-quadruplex electrochemical biosensors have received particular attention, since the electrochemical response is particularly sensitive to the DNA structural changes from single-stranded, double-stranded, or hairpin into a G-quadruplex configuration. Furthermore, the development of an increased number of G-quadruplex aptamers that combine the G-quadruplex stiffness and self-assembling versatility with the aptamer high specificity of binding to a variety of molecular targets allowed the construction of biosensors with increased selectivity and sensitivity. This review discusses the recent advances on the electrochemical characterization, design, and applications of G-quadruplex electrochemical biosensors in the evaluation of metal ions, G-quadruplex ligands, and other small organic molecules, proteins, and cells. The electrochemical and atomic force microscopy characterization of G-quadruplexes is presented. The incubation time and cations concentration dependence in controlling the G-quadruplex folding, stability, and nanostructures formation at carbon electrodes are discussed. Different G-quadruplex electrochemical biosensors design strategies, based on the DNA folding into a G-quadruplex, the use of G-quadruplex aptamers, or the use of hemin/G-quadruplex DNAzymes, are revisited.

Aptamer-based surface-enhanced Raman scattering (SERS) sensor for thrombin based on supramolecular recognition, oriented assembly, and local field coupling

A supramolecular recognition and oriented assembly system was developed on chip for the highly selective surface-enhanced Raman scattering (SERS) detection of thrombin by means of the aptamer-based SERS tag method. A 15-base thrombin-binding aptamer (TBA₁₅) with a thiol end was first immobilized on an Ag nanoprism array by the S-Ag bond. This aptamer has high binding affinity with thrombin when it folds into a G-quadruplex structure. After the recognition between the aptamer and thrombin, a bridge is built between the SERS tag (4-mercaptobenzoic acid marked Ag nanoparticle) and the fixed thrombin based on the activation of the carboxylic group of 4-mercaptobenzoic acid. Thus, the quantitative detection of thrombin can be achieved based on the SERS intensity of the immobilized SERS tags. The obvious advantages of this sensing method are as follows: (1) remarkable SERS enhancement due to the high electric field coupling effect via the gap structure formation, which improves the sensitivity of the SERS detection and the limit of detection of this method arrives in 1.6 × 10^-11 M, (2) high selectivity based on the specific aptamer recognition toward thrombin, which can be extended to other enzymes easily by changing a proper sequence, (3) high repeatability of SERS signals according to a highly ordered structure, and (4) highly efficient oriented assembly of a sandwich structure over an Ag nanoprism array. The proposed method is expected to be a practical implement in medical diagnosis. Graphical Abstract Illustration of the aptamer-based SERS sensor for thrombin detection.

Novel multifunctionalized peryleneteracarboxylic/amine supramolecules for electrochemical assay

In this work, a series of novel multifunctionalized peryleneteracarboxylic supramolecules were synthesized based on hydrogen bonding interactions between 3,4,9,10-perylenetetracarboxylic acid (PTCA) and amines, which possess large specific surface area, good membrane-forming properties and high stability. Importantly, an interesting phenomenon was found in that these series of supramolecules could conciliate disorderly redox peaks of PTCA and result in a pair of well-defined redox peaks, which were able to act as redox carriers for charge-generation and electron-transportion. And the probable mechanism for this phenomenon was discussed for the first time in detail through the integration of theoretical with practical research. To further reveal the advantages of these novel multifunctionalized supramolecule nanomaterials, PTCA/triethylamine (PTCA/TEA) was chosen as the best candidate for a redox carrier to participate in a "signal-on" aptasensor for thrombin (TB) detection by employing Fe₃O₄ magnetic beads (MBs) as a good enzyme mimic to catalyze the PTCA/TEA for signal amplification. As a result, a wide linear detection range of 0.0001-50 nM is acquired with a relatively low detection limit of 0.05 pM. And the proposed aptasensor exhibited good specificity and acceptable reproducibility and stability. After all, the explorations between PTCA and amines would set up a meaningful basis in seeking multifunctionalized supramolecule nanomaterials based on PTCA for extending the application of PTCA in a wider range of fields, and exploring the essential reason for the referred peculiar phenomenon for PTCA.

Highly sensitive chemiluminescence biosensor for protein detection based on the functionalized magnetic microparticles and the hybridization chain reaction

An ultrasensitive chemiluminescence (CL) biosensor for the detection of protein is developed in this study based on the functionalized magnetic microparticles (MMPs) and the hybridization chain reaction (HCR). First, the primer hybridized with the thrombin aptamer conjugated on the surface of MMPs. Then the HCR was triggered by part of the primer and its products were assembled on the surface of the MMPs. Through the interaction between streptavidin and biotin, the streptavidin-horseradish peroxidase (SA-HRP) was coupled with the HCR products. In the presence of thrombin, the HCR products conjugating with SA-HRP were released from the surface of MMPs after the aptamer recognized and bound to its target molecule. So the released SA-HRP in the supernatant produced a significant chemiluminescence imaging signal after the addition of H₂O₂-luminol. The detection limit of thrombin with this method could be as low as 9.7fM. Besides, the sensing strategy was modified by changing the adding order of reagents that was then successfully applied in the detection of thrombin in complex sample. What's more, the DNA detection also could be carried out with this method, which demonstrated the universality of the proposed sensing strategy.

An electrochemical aptasensor electrocatalyst for detection of thrombin

This work reports a novel signal amplification strategy based on three-dimensional ordered macroporous C60-poly(3,4-ethylenedioxythiophene)-1-butyl-3-methylimidazolium hexafluorophosphate (3DOM C60-PEDOT-[BMIm][BF6]) for ultrasensitive detection of thrombin by cascade catalysis of Au-PEDOT@SiO2 microspheres and alcohol dehydrogenase (ADH). Au-PEDOT@SiO2 microspheres were constructed not only as nanocarriers to anchor the large amounts of secondary thrombin aptamers but also as nanocatalysts to catalyze the oxidation of ethanol efficiently. Significantly, the electrochemical signal was greatly enhanced based on cascade catalysis: First, ADH catalyzed the oxidation of ethanol to acetaldehyde with the concomitant generation of NADH in the presence of β-nicotinamide adenine dinucleotide hydrate (NAD(+)). Then, gold nanoparticles (AuNPs) as nanocatalysts could effectively catalyze NADH to produce NAD(+) with the help of PEDOT as redox probe. Under the optimal conditions, the proposed aptasensor exhibits a linear range of 2 × 10(-13) to 2 × 10(-8) M with a low detection limit of 2 × 10(-14) M for thrombin detection and shows high sensitivity and good specificity.

One-step immobilization antibodies using ferrocene-containing thiol aromatic aldehyde for the fabrication of a label-free electrochemical immunosensor

This work focuses on a facile method for antibody immobilization to fabricate a label-free electrochemical immunosensor using ferrocene-containing thiol aromatic aldehyde (FcSA) synthesized by us.

Group 6 metal pentacarbonyl complexes of air-stable primary, secondary, and tertiary ferrocenylethylphosphines

The synthesis and characterization of a series of Group 6 metal pentacarbonyl complexes of air stable primary, secondary, and tertiary phosphines containing ferrocenylethyl substituents are reported [M(CO)5L: M = Cr, Mo, W; L = PH2(CH2CH2Fc), PH(CH2CH2Fc)2, P(CH2CH2Fc)3]. The structure and composition of the complexes were confirmed by multinuclear NMR spectroscopy, IR and UV-Vis absorption spectroscopy, mass spectrometry, X-ray crystallography, and elemental analysis. The solid-state structural data reported revealed trends in M-C and M-P bond lengths that mirrored those of the atomic radii of the Group 6 metals involved. UV-Vis absorption spectroscopy and cyclic voltammetry highlighted characteristics consistent with electronically isolated ferrocene units including wavelengths of maximum absorption between 435 and 441 nm and reversible one-electron (per ferrocene unit) oxidation waves between 10 and -5 mV relative to the ferrocene/ferrocenium redox couple. IR spectroscopy confirmed that the σ donating ability of the phosphines increased as ferrocenylethyl substituents were introduced and that the tertiary phosphine ligand described is a stronger σ donor than PPh3 and a weaker σ donor than PEt3, respectively.

Sensitive electrochemical immunosensor based on three-dimensional nanostructure gold electrode

A sensitive electrochemical immunosensor was developed for detection of alpha-fetoprotein (AFP) based on a three-dimensional nanostructure gold electrode using a facile, rapid, “green” square-wave oxidation-reduction cycle technique. The resulting three-dimensional gold nanocomposites were characterized by scanning electron microscopy and cyclic voltammetry. A “sandwich-type” detection strategy using an electrochemical immunosensor was employed. Under optimal conditions, a good linear relationship between the current response signal and the AFP concentrations was observed in the range of 10–50 ng/mL with a detection limit of 3 pg/mL. This new immunosensor showed a fast amperometric response and high sensitivity and selectivity. It was successfully used to determine AFP in a human serum sample with a relative standard deviation of <5% (n=5). The proposed immunosensor represents a significant step toward practical application in clinical diagnosis and monitoring of prognosis.

An amplified electrochemical aptasensor for thrombin detection based on pseudobienzymic Fe3O4-Au nanocomposites and electroactive hemin/G-quadruplex as signal enhancers

A sensitive and selective electrochemical aptasensor for thrombin detection was constructed based on hemin/G-quadruplex as the signal label and Fe3O4-Au nanocomposites with glucose oxidase (GOx-) and peroxide-mimicking enzyme activity as the signal enhancers. Due to their large surface area and good biocompatibility, Fe3O4-Au nanocomposites were employed to immobilize electroactive hemin/G-quadruplex, which was formed by the conjugation between a single-stranded guanine-rich nucleic acid and hemin. Based on the GOx-mimicking enzyme activity, Au nanoparticles on the surface of the Fe3O4-Au nanocomposites effectively catalyzed the oxidization of glucose in the presence of dissolved O2, accompanied by the production of H2O2. Both the Fe3O4 cores of Fe3O4-Au nanocomposites and hemin/G-quadruplex with H2O2-mimicking enzyme activity could catalyze the reduction of the generated H2O2, which promoted the electron transfer of hemin and amplified the electrochemical signal. The proposed electrochemical aptasensor had a wide dynamic linear range of 0.1 pM to 20 nM with a lower detection limit of 0.013 pM, which provided a promising method for a sensitive assay for the detection of proteins in electrochemical aptasensors.

Dynamics of roughening and growth kinetics of CdS–polyaniline thin films synthesized by the Langmuir–Blodgett technique

Layer variation dynamic scaling properties of CdS–polyaniline Langmuir–Blodgett thin films.

Noncovalent assembly of carbon nanoparticles and aptamer for sensitive detection of ATP

Here base on competitive interaction of electrostatic repulsion and π–π stacking, noncovalent assembly of carbon nanoparticles (cCNPs) with aptamer that allows sensitive and selective detection of ATP is reported. The sensor exhibits minimal background fluorescence and rapid kinetics response depending on the spherical structure of cCNPs.

Scanning electrochemical microscopy for study of aptamer-thrombin interfacial interactions on gold disk microelectrodes

A feasibility for the determination of thrombin on gold disk microelectrodes (GDMs) using scanning electrochemical microscopy (SECM) is reported. The assembly process step-by-step of thrombin aptasensor on GDMs is monitored by SECM. SECM analysis reveals the immobilization of thrombin aptamers on GDMs. The interaction between thrombin aptamers and thrombin on GDMs is imaged by SECM with feedback mode using ferrocenemethanol as an electrochemical mediator. The formation of thrombin/thrombin aptamer complex on GDMs results in a decrease in the tip peak current on spatial SECM images. This method is able to linearly and selectively detect thrombin over a linear range from 10(-12) to 10(-5)M with a detection limit of 6.07 fM.

Assembly of single-stranded polydeoxyadenylic acid and β-glucan probed by the sensing platform of graphene oxide based on the fluorescence resonance energy transfer and fluorescence anisotropy

Based on the fluorescence resonance energy transfer (FRET) and fluorescence anisotropy (FA), the present study reported proof-of-principle for a highly sensitive and rapid detection technique that can be precisely utilized for investigating the self-assembly of polydeoxyadenylic acid (poly(dA)) and β-glucan, and the interactions of the poly(dA)-β-glucan complex on the surface of graphene oxide (GO). Due to the noncovalent assembly of fluorescein amidite (FAM)-labeled poly(dA) and GO via π-π stacking, the fluorescence of (FAM)-labeled poly(dA) as a molecular aptamer beacon (MAB) was completely quenched by GO. Conversely, the addition of single-stranded lentinan (s-LNT) resulted in the significant restoration of fluorescence due to the formation of poly(dA)-s-LNT complexes with a stiff rod-like structure, which had a weak affinity to GO and kept the dyes away from GO. However, relatively weak fluorescence restoration was observed by adding another single-stranded curdlan (s-CUR) for positive control, indicative of complex formation with higher binding ability to GO. The fluorescence anisotropy (FA) was also combined to confirm the occurrence with different increments of anisotropy relative to the free poly(dA), which could be conveniently extended for detecting the assembly of other biomolecules with higher sensitivity.

A pseudo triple-enzyme cascade amplified aptasensor for thrombin detection based on hemin/G-quadruplex as signal label

In this work, a pseudo triple-enzyme cascade amplified electrochemical aptasensor based on hemin/G-quadruplex as signal label for thrombin (TB) was constructed and the amplified electrochemical signal was achieved by the corporate catalysis of alcohol dehydrogenase-graphene sheets (ADH-GSs) bionanocomposite and hemin/G-quadruplex, which simultaneously acted as NADH oxidase and HRP-mimicking DNAzyme. Through "sandwich" reaction, hemin/G-quadruplex labeled gold nanoparticles-ADH-GSs bionanocomposite (AuNPs-ADH-GSs) was captured on electrode surface and thus obtained the electrochemical signal. After the addition of ethanol into the electrolytic cell, ADH availably catalyzed the oxidation of ethanol with the reduction of NAD(+) to NADH. Then, hemin/G-quadruplex as NADH oxidase catalyzed the oxidization of NADH, accompanying with the production of H2O2. Simultaneously, hemin/G-quadruplex as HRP-mimicking DNAzyme catalyzed the reduction of the generated H2O2. Such a catalysis strategy greatly promoted the electron transfer of hemin and resulted in the specific enhancement of electrochemical signal. The proposed TB aptasensor achieved a linear range of 1 pM-50 nM with a detection limit of 0.3 pM (defined as S/N=3). In addition, it showed satisfying stability and reproducibility, good specificity and sensitivity, indicating promising application for the detection of various proteins in clinical analysis.

Bead assembly magnetorotation as a signal transduction method for protein detection

This paper demonstrates a proof-of-principle for a new signal transduction method for protein detection called Bead Assembly Magnetorotation (BAM). BAM is based on using the target protein to mediate the formation of aptamer-coated 1 μm magnetic beads into a bead assembly, formed at the bottom of a 1 μL hanging droplet. The size, shape and fractal dimension of this bead assembly all depend on the protein concentration. The protein concentration can be measured in two ways: by magnetorotation, in which the rotational period of the assembly correlates with the protein concentration, or by fractal analysis. Additionally, a microscope-free magnetorotation detection method is introduced, based on a simple laser apparatus built from standard laboratory components. In this paper, we chose to focus on the protein thrombin, a popular choice for proof-of-principle work in this field.

A chronocoulometric aptasensor based on gold nanoparticles as a signal amplification strategy for detection of thrombin

A sensitive chronocoulometric aptasensor for the detection of thrombin has been developed based on gold nanoparticle amplification. The functional gold nanoparticles, loaded with link DNA (LDNA) and report DNA (RDNA), were immobilized on an electrode by thrombin aptamers performing as a recognition element and capture probe. LDNA was complementary to the thrombin aptamers and RDNA was noncomplementary, but could combine with [Ru(NH₃)₆]³⁺ (RuHex) cations. Electrochemical signals obtained by RuHex that bound quantitatively to the negatively charged phosphate backbone of DNA via electrostatic interactions were measured by chronocoulometry. In the presence of thrombin, the combination of thrombin and thrombin aptamers and the release of the functional gold nanoparticles could induce a significant decrease in chronocoulometric signal. The incorporation of gold nanoparticles in the chronocoulometric aptasensor significantly enhanced the sensitivity. The performance of the aptasensor was further increased by the optimization of the surface density of aptamers. Under optimum conditions, the chronocoulometric aptasensor exhibited a wide linear response range of 0.1-18.5 nM with a detection limit of 30 pM. The results demonstrated that this nanoparticle-based amplification strategy offers a simple and effective approach to detect thrombin.

Coffee ring aptasensor for rapid protein detection

We introduce a new biosensing platform for rapid protein detection that combines one of the simplest methods for biomolecular concentration, coffee ring formation, with a sensitive aptamer-based optical detection scheme. In this approach, aptamer beacons are utilized for signal transduction where a fluorescence signal is emitted in the presence of the target molecule. Signal amplification is achieved by concentrating aptamer-target complexes within liquid droplets, resulting in the formation of coffee ring "spots". Surfaces with various chemical coatings were utilized to investigate the correlation among surface hydrophobicity, concentration efficiency, and signal amplification. On the basis of our results, we found that the increase in the coffee ring diameter with larger droplet volumes is independent of surface hydrophobicity. Furthermore, we show that highly hydrophobic surfaces produce enhanced particle concentration via coffee ring formation, resulting in signal intensities 6-fold greater than those on hydrophilic surfaces. To validate this biosensing platform for the detection of clinical samples, we detected α-thrombin in human serum and 4-fold-diluted whole blood. Coffee ring spots from serum and blood produced detection signals up to 40 times larger than those from samples in liquid droplets. Additionally, this biosensor exhibits a lower limit of detection of 2 ng/mL (54 pM) in serum, and 4 ng/mL (105 pM) in blood. On the basis of its simplicity and high performance, this platform demonstrates immense potential as an inexpensive diagnostic tool for the detection of disease biomarkers, particularly for use in developing countries that lack the resources and facilities required for conventional biodetection practices.

A novel electrochemical aptasensor for thrombin detection based on the hybridization chain reaction with hemin/G-quadruplex DNAzyme-signal amplification

In this work, a novel signal amplification electrochemical aptasensor for the sensitive and selective detection of thrombin was successfully fabricated. The amplification method was based on the hybridization chain reaction (HCR) and a pseudobienzyme electrocatalytic system. HCR-based double-stranded DNA (dsDNA) polymers not only constructed an effective carrier for anchoring larger amounts of electron mediator methylene blue (MB) into the DNA duplexes to produce a strong differential pulse voltammetry (DPV) signal, but also resulted in the formation of hemin/G-quadruplex DNAzymes nanowires by intercalating hemin into two induced single-stranded DNA (ssDNA). With the addition of NADH into the electrolytic cell, the hemin/G-quadruplex acting as an NADH oxidase and HRP-mimicking DNAzyme for the pseudobienzyme amplifying system could in situ biocatalyze the formation of H₂O₂ with local concentrations and low transfer loss resulting in dramatic signal enhancements. The binding event can be detected by a decrease in the integrated charge of MB which electrostatically absorbed onto dsDNA polymers. In the presence of thrombin, the dsDNA polymers associated with MB and hemin/G-quadruplex structures were removed from the electrode surface, leading to a significant decrease of redox current. DPV signals of MB provided quantitative measures of the concentrations of thrombin, with a linear calibration range of 0.01-50 nM and a detection limit of 2 pM. Moreover, the resulting aptasensor also exhibited good specificity, acceptable reproducibility and stability, indicating that the present strategy was promising for broad potential application in clinic assay and various protein analyses.