Amplified thrombin aptasensor based on alkaline phosphatase and hemin/G-quadruplex-catalyzed oxidation of 1-naphthol

Visual/Photoelectrochemical Off-On Sensor Based on Cu/Mn Double-Doped CeO2 and Branched Sheet Embedded Cu2O/CuO Nanocubes

An integrated dual-signal bioassay was devised to fulfil thrombin (TB) ultrasensitive detection by integrating visualization with the photoelectrochemical technique based on G-quadruplex/hemin. During the process, branched sheet embedded copper-based oxides prepared with illumination and alkaline condition play a vital role in obtaining the desirable photocurrent. The switchover of photoelectrochemical signal was realized by the adjustable distance between electron acceptor G-quadruplex/hemin and interface materials due to dissociation of the Cu/Mn double-doped cerium dioxide (CuMn@CeO₂)/DNA caused by the addition of TB. Then, CuMn@CeO₂ transferred onto visual zones triggered catalytic reactions under the existence of 3,3',5,5'-tetramethylbenzidine and hydrogen peroxide, making a variation in color recognized by the naked eye and providing visual prediction. Under optimized conditions, this bioassay protocol demonstrated wide linear ranges (0.0001-50 nM), high selectivity, stability, and reproducibility. More importantly, the proposed visual/photoelectrochemical transduction mechanism platform exhibits a lower background signal and more reliable detection results, which also offers an effective way for detecting other proteins and nucleic acids.

Target-Responsive Smart Nanomaterials via a Au-S Binding Encapsulation Strategy for Electrochemical/Colorimetric Dual-Mode Paper-Based Analytical Devices

Target-responsive nanomaterials attract growing interest in the application of drug delivery, bioimaging, and sensing due to the responsive releasing of guest molecules by the smart molecule gate. However, it remains a challenge to develop smart nanomaterials with simple assembly and low nonspecific leakage starting from encapsulation strategies, especially in the sensing field. Herein, Au nanoclusters (Au NCs) were first grown on porous carbon derived from ZIF-8 (PCZIF) to be employed as nanocarriers. By employing the Au NCs as linkers and aptamer (Apta) double-strand hybrids (target Apta and SH-complementary DNA) as capping units, we reported the novel target-responsive nanomaterials of Apta/Au NCs-PCZIF/hemin through Au-S binding encapsulation for sensing assays. The Au-S binding encapsulation strategy simplified the packaging procedure and reduced non-target responsive leakage. As a proof, ochratoxin A (OTA) as a model target participates in the double-strand hybrid competitive displacement reaction and triggered Apta conformation switches from a coil to a G-quadruplex structure accompanied by the dissociation of the gatekeeper. Simultaneously, the released hemin can initiate a self-assembly to form G-quadruplex/hemin DNAzyme. Interestingly, owing to DNAzyme providing electron transfer mediators and peroxidase-like activity, we proposed an electrochemical/colorimetric dual-mode paper-based analytical device (PAD) that provided self-verification to enhance reliability and accuracy, benefiting from independent signal conversion and transmission mechanism. As a consequence, the proposed dual-mode PAD could achieve sensitive electrochemical detection and visual prediction of OTA in the range of 1 pg/mL to 500 ng/mL and 50 pg/mL to 500 ng/mL, respectively. The electrochemical detection limit for OTA was as low as 0.347 pg/mL (S/N = 3). We believe that this work provides point-of-care testing (POCT) tools for a broad spectrum of applications.

Recent trends in nanomaterial-based signal amplification in electrochemical aptasensors

Ultrasensitive biosensors have become a necessity in the world of scientific research, and several signal enhancement strategies have been employed to attain exceptionally low detection limits. Nanotechnology turns out to be a strong contender for signal amplification, as they can be employed as platform modifiers, catalysts, carriers or labels. Here, we have described the most recent advancements in the utilization of nanomaterials as signal amplification components in aptamer-based electrochemical biosensors. We have briefly reviewed the methods that utilized nanomaterials, namely gold and carbon, as well as nanocomposites such as: graphene, carbon nanotubes, quantum dots, and metal-organic frameworks.

Femtomolar Detection of Thrombin in Serum and Cerebrospinal Fluid via Direct Electrocatalysis of Oxygen Reduction by the Covalent G4-Hemin-Aptamer Complex

Thrombin, a serine protease playing a central role in the coagulation cascade reactions and a potent neurotoxin produced by injured brain endothelial cells, is a recognized cardiac biomarker and a critical biomarker for Alzheimer's disease development. Both in vivo and in vitro, its low physiological concentrations and nonspecific binding of other components of physiological fluids complicate electroanalysis of thrombin. Here, femtomolar levels of thrombin in serum and an artificial cerebrospinal fluid (CSF) were detected by the indicator-free electrochemical methodology exploiting the O₂ reduction reaction directly, with no electron transfer mediators, electrocatalyzed by the covalent G4-hemin DNAzyme complex naturally self-assembling upon thrombin binding to the hemin-modified 29-mer DNA aptamer sequence tethered to gold via an alkanethiol linker. Coadsorbed PEG inhibited nonspecific protein binding and allowed the sought signal resolution. The proposed assay exploiting the "oxidase" activity of G4-hemin DNAzyme does not require any coreactants necessary for the traditional peroxidase activity-based assays with this DNAzyme, such as H₂O₂ and redox mediators, or solution deaeration and allows fast, overall 30 min analysis of thrombin in aerated buffer, CSF, and 1% human serum solutions. This pioneer approach exploiting the oxidase activity G4-hemin DNAzyme is simple, sensitive, and selective and represents a new tool for ultrasensitive electrocatalytic assays based on simple and efficient O₂-dependent DNAzyme labels.

A Novel Electrochemical Aptasensor for the Ultrasensitive Detection of Adenosine Triphosphate Based on DNA-Templated Copolymers

Adenosine triphosphate (ATP) is a small but significant biological molecule that plays an important role in regulating cellular metabolism processes. Accurate and sensitive analytical techniques for detecting ATP are urgently needed. Herein, a new electrochemical aptasensor was designed in light of DNA-templated copolymers that parallel to the electrode for sensitive gauging of ATP. The ATP aptamer decorated by the electronic medium ferrocene can be regarded as a ″bridge″ connecting two DNA-templated copolymers. When ATP exists, because of the extraordinary binding selectivity of DNA-templated copolymers for target ATP, the rapid electron transfer of ferrocene was beneficial to the sensitive detection of target ATP. Specifically, our parallel DNA copolymers are more stable than upright DNA copolymers and have a faster signal transmission because of the close distance to the electrode; meanwhile, the nonspecific pollution is less. Consequently, the developed novel aptasensor exhibited a wide range of linear response toward ATP that was from 0.1 to 10 000 pM and high selectivity with a detection limit down to 11.5 fM. The electrochemical aptasensor has a broad application prospect in the detection of small biological molecules.

Colorimetric aptasensor for ochratoxin A detection based on enzyme-induced gold nanoparticle aggregation

An innovative colorimetric method based on enzyme-induced gold nanoparticle aggregation was developed to detect the activity of alkaline phosphatase (ALP), and it was further applied to construct an aptasensor to monitor ochratoxin A (OTA) concentrations. In the presence of ALP, the substrate ascorbic acid 2-phosphate was hydrolyzed to generate ascorbic acid (AA). Subsequently, reduction of MnO₂ nanosheets by AA produced manganese ions, which mediated gold nanoparticle aggregation. The color of the detection solution changed from brown-red to purple to blue as the ALP concentration increased, and a detection limit of 0.05 U·L^-1 was achieved. Furthermore, this strategy was successfully utilized to devise a target-responsive aptasensor for colorimetric detection of an important mycotoxin, OTA, which causes food poisoning and has various toxic effects on humans. The proposed method offers high sensitivity with a detection limit as low as 5.0 nM together with high specificity. When applied to analyze red wine and grape juice samples, no complex sample pretreatment or bulky instruments were required. Overall, a colorimetric platform based on enzyme-induced gold nanoparticle aggregation was successfully established to improve the simplicity and sensitivity of ALP and OTA detection. This platform appears highly promising for mycotoxin-related food safety monitoring.

Highly sensitive host-guest mode homogenous electrochemical thrombin signal amplification aptasensor based on tetraferrocene label

The development of sensitive and convenient detection methods to monitor thrombin without the use of enzymes or complex nanomaterials is highly desirable for the diagnosis of cardiovascular diseases. In this article, tetraferrocene was first synthesized and then a sensitive and homogeneous electrochemical aptasensor was developed for thrombin detection based on host-guest recognition between tetraferrocene and β-cyclodextrin (β-CD). In the absence of thrombin, the double stem-loop of thrombin aptamer (TBA) prevented tetraferrocenes labeled at both ends from entering the cavity of β-CD deposited on gold electrode surface. After binding with thrombin, the stem-loop structure of TBA opened and transformed into special G-quarter structure, forcing tetraferrocene into the cavity of β-CD. As a result, thrombin allowed eight ferrocene molecules to reach the gold electrode surface, greatly amplifying the response signal. The obtained aptasensors showed dynamic detection range from 4 pM to 12.5 nM with detection limit around 1.2 pM. Overall, the results indicate that the proposed aptasensors are promising for future rapid clinical detection of thrombin and development of signal amplification strategies for detection of various proteins.

One-step sensitive thrombin detection based on a nanofibrous sensing platform

Convenient and time-saving one-step strategies for detecting ultralow concentrations of protein biomarkers play key roles in rapid disease diagnosis. In this study, we report a one-step detection method based on a nanofibrous sensing platform via the combination of proximity-induced DNA strand displacement (PiDSD), catalytic hairpin assembly (CHA) amplification and thioflavin T (ThT) binding. The interface behaviors on the nanofibrous membrane were studied to promote interface reaction kinetics and thermodynamics. Thrombin was used as a model biomarker, and the nanofibrous sensing platform achieved a limit of detection as low as 1.0 pM, a wide linear range of 50 pM to 5 nM, excellent specificity and good long-term stability. Compared with previous one-step thrombin detection methods, our one-step detection method is label-free, convenient and much more sensitive; it has potential applications for protein detection in point-to-care testing (POCT) and early diagnosis.

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

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

Combined electrochemiluminescent and electrochemical immunoassay for interleukin 6 based on the use of TiO2 mesocrystal nanoarchitectures

A dual-responsive sandwich-type immunosensor is described for the detection of interleukin 6 (IL-6) by combining electrochemiluminescent (ECL) and electrochemical (EC) detection based on the use of two kinds of TiO₂ mesocrystal nanoarchitectures. A composite was prepared from TiO₂ (anatase) mesocages (AMCs) and a carboxy-terminated ionic liquid (CTIL) and then placed on a glassy carbon electrode (GCE). In the next step, the ECL probe Ru(bpy)₃(II) and antibody against IL-6 (Ab₁) were immobilized on the GCE. Octahedral anatase TiO₂ mesocrystals (OAMs) served as the matrix for immobilizing acid phosphatase (ACP) and secondary antibody (Ab₂) labeled with horseradish peroxidase (HRP) to form a bioconjugate of type Ab₂-HRP/ACP/OAMs. It was self-assembled on the GCE by immunobinding. 1-Naphthol, which is produced in-situ on the surface of the GCE due to the hydrolysis of added 1-naphthyl phosphate by ACP, is oxidized by HRP in the presence of added H₂O₂. This results in an electrochemical signal (typically measured at 0.4 V vs. Ag/AgCl) that increases linearly in the 10 fg·mL^-1 to 90 ng·mL^-1 IL-6 concentration range with a detection limit of 0.32 fg·mL^-1. Secondly, the oxidation product of 1-naphthol quenches the ECL emission of Ru(bpy)₃²⁺. This leads to a decrease in ECL intensity which is linear in the 10 ag·mL^-1 to 90 ng·mL^-1 concentration range, with a detection limit of 3.5 ag·mL^-1. The method exhibits satisfying selectivity and good reproducibility which demonstrates its potential in clinical testing and diagnosis. Graphical abstract A dual-responsive sandwich-type immunosensor was fabricated for the detection of interleukin 6 by combining electrochemiluminescence and electrochemical detection based on the use of two kinds of TiO₂ mesocrystal nanoarchitectures.

Two-Dimensional Fluorescence Difference Spectroscopy of ZnO and Mg Composites in the Detection of Physiological Protein and RNA Interactions

Two-dimensional fluorescence difference spectroscopy (2-D FDS) was used to determine the unique spectral signatures of zinc oxide (ZnO), magnesium oxide (MgO), and 5% magnesium zinc oxide nanocomposite (5% Mg/ZnO) and was then used to demonstrate the change in spectral signature that occurs when physiologically important proteins, such as angiotensin-converting enzyme (ACE) and ribonuclease A (RNase A), interact with ZnO nanoparticles (NPs). When RNase A is bound to 5% Mg/ZnO, the intensity is quenched, while the intensity is magnified and a significant shift is seen when torula yeast RNA (TYRNA) is bound to RNase A and 5% Mg/ZnO. The intensity of 5% Mg/ZnO is quenched also when thrombin and thrombin aptamer are bound to the nanocomposite. These data indicate that RNA–protein interaction can occur unimpeded on the surface of NPs, which was confirmed by gel electrophoresis, and importantly that the change in fluorescence excitation, emission, and intensity shown by 2-D FDS may indicate specificity of biomolecular interactions.

Electrochemical aptasensor for thrombin using co-catalysis of hemin/G-quadruplex DNAzyme and octahedral Cu2O-Au nanocomposites for signal amplification

In this work, novel octahedral Cu₂O-Au nanocomposites were synthesized and first applied in an electrochemical aptasensor to detect thrombin (TB) with the aid of a DNAzyme for signal amplification. The octahedral Cu₂O-Au nanocomposites have not only simultaneously served as signal amplifying molecules but have also been utilized as an ideal loading platform to immobilize a large number of electroactive substances and recognition probes. Gold nanoparticles (AuNPs) were grown directly on the surface of the octahedral Cu₂O nanocrystals, and the Cu₂O-Au nanocomposites obtained had the advantages of large surface areas and excellent biocompatibilities. The hemin/G-quadruplex, which was formed by intercalating hemin into the amino terminated thrombin binding aptamer (NH₂-TBA), and the electroactive toluidine blue (Tb) were immobilized onto the Cu₂O-Au nanocomposite surfaces through a stable Au-N bond. AuNPs, Cu₂O and hemin/G-quadruplex co-catalyse the H₂O₂ in the working buffer to promote the electron transfer of Tb as a multiple signal amplification strategy in order to improve the performance of the electrochemical aptasensor. Under optimal conditions, the designed aptasensor exhibited sensitive detection of TB from 100 fM to 20nM with a lower detection limit of 23fM. This proposed aptasensor exhibited good sensitivity, high specificity and acceptable reproducibility and could be widely applied in bioassay analysis.

Glucose oxidase-initiated cascade catalysis for sensitive impedimetric aptasensor based on metal-organic frameworks functionalized with Pt nanoparticles and hemin/G-quadruplex as mimicking peroxidases

Based on cascade catalysis amplification driven by glucose oxidase (GOx), a sensitive electrochemical impedimetric aptasensor for protein (carcinoembryonic antigen, CEA as tested model) was proposed by using Cu-based metal-organic frameworks functionalized with Pt nanoparticles, aptamer, hemin and GOx (Pt@CuMOFs-hGq-GOx). CEA aptamer loaded onto Pt@CuMOFs was bound with hemin to form hemin@G-quadruplex (hGq) with mimicking peroxidase activity. Through sandwich-type reaction of target CEA and CEA aptamers (Apt1 and Apt2), the obtained Pt@CuMOFs-hGq-GOx as signal transduction probes (STPs) was captured to the modified electrode interface. When 3,3-diaminobenzidine (DAB) and glucose were introduced, the cascade reaction was initiated by GOx to catalyze the oxidation of glucose, in situ generating H₂O₂. Simultaneously, the decomposition of the generated H₂O₂ was greatly promoted by Pt@CuMOFs and hGq as synergistic peroxide catalysts, accompanying with the significant oxidation process of DAB and the formation of nonconductive insoluble precipitates (IPs). As a result, the electron transfer in the resultant sensing interface was effectively hindered and the electrochemical impedimetric signal (EIS) was efficiently amplified. Thus, the high sensitivity of the proposed CEA aptasensor was successfully improved with 0.023pgmL^-1, which may be promising and potential in assaying certain clinical disease related to CEA.

G-quadruplex-based aptamers against protein targets in therapy and diagnostics

Nucleic acid aptamers are single-stranded DNA or RNA molecules identified to recognize with high affinity specific targets including proteins, small molecules, ions, whole cells and even entire organisms, such as viruses or bacteria. They can be identified from combinatorial libraries of DNA or RNA oligonucleotides by SELEX technology, an in vitro iterative selection procedure consisting of binding (capture), partitioning and amplification steps. Remarkably, many of the aptamers selected against biologically relevant protein targets are G-rich sequences that can fold into stable G-quadruplex (G4) structures. Aiming at disseminating novel inspiring ideas within the scientific community in the field of G4-structures, the emphasis of this review is placed on: 1) recent advancements in SELEX technology for the efficient and rapid identification of new candidate aptamers (introduction of microfluidic systems and next generation sequencing); 2) recurrence of G4 structures in aptamers selected by SELEX against biologically relevant protein targets; 3) discovery of several G4-forming motifs in important regulatory regions of the human or viral genome bound by endogenous proteins, which per se can result into potential aptamers; 4) an updated overview of G4-based aptamers with therapeutic potential and 5) a discussion on the most attractive G4-based aptamers for diagnostic applications. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

Molybdenum disulfide sphere-based electrochemical aptasensors for protein detection

In this work, we report the development of an ultrasensitive sandwich-type electrochemical aptasensor for protein detection.

A sensitive impedimetric platform biosensing protein: Insoluble precipitates based on the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrinin in HCR-assisted dsDNA

In this study, a sensitive biosensing interface for protein was reported based on nonconductive insoluble precipitates (IPs) by the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrin (MnTMPyP), which was intercalated into formed double-strand DNA (dsDNA) scaffold triggered by hybridization chain reaction (HCR). In the proposed impedimetric aptasensor, carcinoembryonic antigen (CEA) and its aptamer were used as testing model. PtPd nanowires (PtPdNWs) with large surface area and superior conductivity were employed as nanocarriers to greatly immobilize biomolecules (e.g. CEA aptamers). Then, two DNA hairpins H1 and H2 were introduced to trigger HCR with the assistance of DNA initiator, resulting in the formation of a long dsDNA scaffold. Meanwhile, mimicking enzyme MnTMPyP molecules were embedded into the resultant dsDNA, in situ generating the complex MnTMPyP-dsDNA with peroxidase-like activity. Under the biocatalysis of MnTMPyP-dsDNA, 3,3-diaminobenzidine (DAB) was oxidized to form nonconductive IPs. As a result, the electron transfer between electrode interface and redox probe was vastly hindered, leading to the significant amplification of electrochemical impedimetric signal. So, greatly improved analytical performances of the proposed aptasensor were achieved with a detection limit as low as 0.030pgmL(-1). And the successful assay of CEA in human serum samples enabled the developed biosensing platform to have promising potential in bioanalysis.

Ultrasensitive electrochemical aptasensor for the detection of thrombin based on dual signal amplification strategy of Au@GS and DNA-CoPd NPs conjugates

In this work, an ultrasensitive electrochemical aptasensor for the detection of thrombin was developed based on Au nanoparticles decorated graphene sheet (Au@GS) and CoPd binary nanoparticles (CoPd NPs). A sulfydryl-labeled thrombin capture probe (Apt1) and a biotin-labeled thrombin reporter probe (Apt2) were designed to achieve a sandwich-type strategy. Au@GS was used as a sensing platform for the facile immobilization of Apt1 through Au-S bond, forming a sensing interface for thrombin. The specific recognition of thrombin induced the attachment of Apt2-CoPd NPs to the electrode. The labeled CoPd NPs showed good catalytic properties toward the reduction of H2O2, resulting in an amperometric signal. The amperometric response was correlated to the thrombin concentration in sample solutions. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) confirmed the successful fabrication of the aptasensor. A linear response to thrombin in the range of 0.01-2.00 ng mL(-1) with a low detection limit (5 pg mL(-1)) was achieved. The proposed aptasensor showed good selectivity, good reproducibility and acceptable stability. This proposed strategy may find many potential applications in the detection of other biomolecules.

Ultrasensitive biosensing platform based on layered vanadium disulfide–graphene composites coupling with tetrahedron-structured DNA probes and exonuclease III assisted signal amplification

An electrochemical aptasensor is developed to sensitively detect PDGF-BB based on vanadium disulfide–graphene composites and exonuclease III signal amplification.