Electrochemical aptamer-based nanosensor fabricated on single Au nanowire electrodes for adenosine triphosphate assay

New nanosensor fabricated on single nanopore electrode filled with prussian blue and graphene quantum dots coated by polypyrrole for hydrogen peroxide sensing

Hydrogen peroxide (H2O2) is a common oxidant that plays an important role in many biological processes and is also an important medium analysis in various fields. In this work, a new electrochemical nanosensor capable of detecting and quantifying hydrogen peroxide was introduced. This nanosensor was fabricated by electrodepositing prussian blue (PB)/graphene quantum dots (GQDs)/polypyrrole (PPy) on single nanopore electrode etched from single gold nanoelectrode. This prepapred nanosensor exhibits good electrochemical response to hydrogen peroxide with high sensitivity and stability, with a linear response in the 2.0 and 80 μM by using amperometric method and differential pulse voltammetry (DPV) method. The limit of detections are 0.33 μM (S/N = 3) for amperometric method and 0.67 μM (S/N = 3) for differential pulse voltammetry (DPV) method, respectively. This nanosensor can be used for the determination of hydrogen peroxide in human urine, and can serve as a new electrochemical platform to monitor H2O2 release from single living cells due to its small overal dimension and high sensitivity.

Single gold nanowire-based nanosensor for adenosine triphosphate sensing by using in-situ surface-enhanced Raman scattering technique

Development of new hot spots for surface enhanced Raman scattering (SERS) technique is of great significance recently. Herein, we developed a single Au nanowire (NW)-based nanosensor for adenosine triphosphate (ATP) sensing by using in-situ SERS technique. Single Au NWs, fabricated by laser-assisted pulling method and hydrofluoric acid (HF) etching process, were linked with single-stranded HS-terminated DNA. After that, gold-silver bimetallic nanoparticles (Au/Ag NPs), attached with thiol-containing Raman dyes and ATP aptamer, were immobilized on DNA-modified single AuNW due to the designed affinity between ATP aptamer and single-stranded DNA. This single AuNW-based device exhibited strong SERS signals. In the presence of adenosine triphosphate (ATP), due to the strong specific affinity between the aptamer and the target, the Au/Ag NPs will be separated from the AuNW, resulting in the obvious decrease of the Raman signals, which can be used for ATP sensing with high sensitivity, selectivity and stability. This nanosensor can be used as an ideal platform for real applications, especially at some confined-space samples, such as trace detection, single cell and in vivo analysis.

Electrochemical enzyme-based blood ATP and lactate sensor for a rapid and straightforward evaluation of illness severity

This study aimed to develop an electrochemical system for measuring blood ATP and lactate levels in a single format. The ratio of lactate to ATP levels was previously reported to provide an alternative illness severity score. Although severity evaluation is crucial to treat patients with acute disease admitted to intensive care units, no sensors are currently available to simply and rapidly measure ATP and lactate levels using the same detection method. Therefore, we constructed an integrated sensing system for ATP and lactate using enzymatic reactions and two sets of electrodes integrated into a chip connected to a single potentiostat operated by a microcontroller. The enzymatic system involves adenylate kinase, pyruvate kinase, and pyruvate oxidase for ATP, and lactate oxidase for lactate, both of which produce hydrogen peroxide. Multiplex enzyme-based reactions were designed to minimize the corresponding operations significantly without enzyme immobilization onto the electrodes. The system was robust in the presence of potentially interfering blood components, such as ascorbate, pyruvate, ADP, urate, and potassium ions. The ATP and lactate levels in the blood were successfully measured using the new sensor with good recoveries. The analytical results of blood samples obtained using our sensor were in good agreement with those using conventional methods. Integrating electrode-based analysis and a microcontroller-based system saved further operations, enabling the straightforward measurement of ATP and lactate levels within 5 min. The proposed sensor may serve as a useful tool in the management of serious infectious diseases.

A CRISPR/Cas12a-Mediated Dual-Mode Electrochemical Biosensor for Polymerase Chain Reaction-Free Detection of Genetically Modified Soybean

A clustered regularly interspaced short palindromic repeats (CRISPR)/Cas12a-mediated dual-mode electrochemical biosensor without polymerase chain reaction (PCR) amplification was designed for sensitive and reliable detection of genetically modified soybean SHZD32-1. A functionalized composite bionanomaterial Fe3O4@AuNPs/DNA-Fc&Ru was synthesized as the signal unit, while a characteristic gene fragment of SHZD32-1 was chosen as the target DNA (tDNA). When Cas12a, crRNA, and tDNA were present simultaneously, a ternary complex Cas12a-crRNA-tDNA was formed, and the nonspecific cleavage ability of the CRISPR/Cas12a system toward single-stranded DNA was activated. Thus, the single-stranded DNA-Fc in the signal unit was cleaved, resulting in the decrease in the fast scan voltammetric (FSV) signal from ferrocene (Fc) and the increase in the electrochemiluminescence (ECL) signal from ruthenium complex (Ru) inhibited by Fc. The linear range was 1-107 fmol/L for ECL and 10-108 fmol/L for FSV, and the limit of detection (LOD) was 0.3 fmol/L for ECL and 3 fmol/L for FSV. Accuracy, precision, stability, selectivity, and reliability were all satisfied. In addition, PCR-free detection could be completed in an hour at room temperature without requiring complicated operation and sample processing, showing great potential in the field detection of genetically modified crops.

In Situ SERS Monitoring of the Plasmon-Driven Catalytic Reaction by Using Single Ag@Au Nanowires as Substrates

Single nanowires (NWs), as a kind of new surface-enhanced Raman scattering (SERS) substrates, have received extensive concern owing to their distinctive properties and distinct advantages. In this contribution, single Ag nanowires (AgNWs) and single Au-coated AgNWs (Ag@AuNWs) were fabricated by the laser-assisted pulling method and the galvanic replacement reaction, respectively. The prepared single Ag@AuNWs show both high SERS activity and catalytic activity through in situ monitoring and assessing the plasmon-driven surface-catalytic reaction of 4-nitrothiophenol (4-NTP) dimerizing to 4,4'-dimercaptoazobenzene and the reduction reaction of 4-NTP to para-aminothiophenol, respectively. It was found that the intensity of the Raman peak was affected greatly by the laser power, and the Raman peak could still be observed at 0.05% power under mild conditions (633 nm wavelength) in this single nanowire system. From the Raman spectrum, the SERS enhancement factor (EF) of 5.4 × 10⁴ can be obtained, and the EF value of 1.3 × 10⁹ can be reached at optimal conditions. Results have shown that single Ag@AuNWs can be utilized as a good platform for SERS applications with high sensitivity, stability, and reproducibility, which will benefit SERS-based research at the single entity level.

Dual-signaling amplification strategy for glutathione sensing by using single gold nanoelectrodes

A new nanosensor for glutathione (GSH) detection by use of single nanoelectrodes has been developed through a dual-signaling ratiometric amplification strategy. Ag nanoparticles (Ag NPs) metalized DNA1 was modified on an Au nanoelectrode surface. Due to the strong affinity between Ag NP and GSH, Ag NPs could be removed by the addition of GSH. The remaining metalized DNA1 could hardly form a double strand, while the de-metalized DNA1 could hybrid with DNA2 and DNA3 to form a complex structure to adsorb methylene blue (MB), and then the electrochemical signal of differential pulse voltammetry (DPV) from MB oxidation could be observed. With the addition of GSH, the peak current of MB oxidation at about -0.27 V (I_MB) increases, while the signal of Ag oxidation at about 0.1 V (I_Ag) decreases. It was found that there had a linear relationship between the ratio of dual-signal (I_MB/I_Ag) and the GSH concentrations, which could be used to detect GSH. The ratiometric nanosensor is label-free, easy to operate, and can eliminate inherent system errors. Considering the advantages of nanoelectrodes, such as low IR drop, fast response, and small overall dimension, this developed nanosensor can be used for GSH detection living systems (e.g., cell lysate).

Recombinant Antibody Engineering Enables Reversible Binding for Continuous Protein Biosensing

Engineering antibodies to improve target specificity, reduce detection limits, or introduce novel functionality is an important research area for biosensor development. While various affinity biosensors have been developed to generate an output signal upon varying analyte concentrations, reversible and continuous protein monitoring in complex biological samples remains challenging. Herein, we explore the concept of directed evolution to modulate dissociation kinetics of a high affinity anti-epidermal growth factor receptor (EGFR) single-chain variable antibody fragment (scFv) to enable continuous protein sensing in a label-free binding assay. A mutant scFv library was generated from the wild type (WT) fragment via targeted permutation of four residues in the antibody-antigen-binding interface. A single round of phage display biopanning complemented with high-throughput screening methods then permitted isolation of a specific binder with fast reaction kinetics. We were able to obtain ∼30 times faster dissociation rates when compared to the WT without appreciably affecting overall affinity and specificity by targeting a single paratope that is known to contribute to the binding interaction. Suitability of a resulting mutant fragment to sense varying antigen concentrations in continuous mode was demonstrated in a modified label-free binding assay, achieving low nanomolar detection limits (K_D = 8.39 nM). We also confirmed these results using an independent detection mechanism developed previously by our group, incorporating a polarity-dependent fluorescent dye into the scFv and reading out EGFR binding based on fluorescence wavelength shifts. In future, this generic approach could be employed to generate improved or novel binders for proteins of interest, ready for deployment in a broad range of assay platforms.

ATP-responsive laccase@ZIF-90 as a signal amplification platform to achieve indirect highly sensitive online detection of ATP in rat brain

A novel electrochemical online system for indirect, highly sensitive and selective online monitoring of ATP in the cerebral microdialysate is presented based on the particular reaction of ATP with zeolitic imidazole framework-90 (ZIF-90) encapsulated laccase microcrystals (laccase@ZIF-90) and the natural catalytic activity of laccase.

Single gold nanoclusters: Formation and sensing application for isonicotinic acid hydrazide detection

Nano-sized electrodes have their special advantages for sensing applications, such as small overall dimension, fast response and low background current. In this work, single gold nanoclusters (AuNCs) were controllably prepared on single Pt nanoelectrode surface by electrodeposition method. The AuNCs covered Pt nanoelectrode (AuNCs/PtNE) had steady-state voltammetric response in redox species solution, which was similar to micro-/nano-sized electrodes. It was interesting to find isonicotinic acid hydrazide (INH, also known as isoniazid) showed good electrochemical response on AuNCs/PtNE surface, which had investigated carefully by square wave voltammetry (SWV) and chronoamperometry. Moreover, the prepared single AuNCs/PtNEs showed the capability for INH sensing with good sensitivity, reproducibility and selectivity, which was demonstrated for INH detection in human urine samples.

A novel portable biosensor based on aptamer functionalized gold nanoparticles for adenosine detection

Adenosine has received great attentions acting as a potential biomarker for monitoring lung cancer. Most of the reported studies for adenosine detection require large instruments and complicated procedures. Herein, a sensitive, rapid and in-situ colorimetric aptasensor was developed for adenosine detection. Moreover, a homemade biomimetic electronic-eye (E-eye) was established and utilized as a portable in-time detection equipment. The entire measurement can be completed within 20 min, including the combination of aptamer with adenosine or AuNPs and the detection of adenosine. Four different kinds of aptamer were compared and the results showed that the AuNPs-aptamer-biotin system was the most stable and with the widest detection range of 5.0 μM-60.0 μM and the lowest LOD of 0.17 μM. Moreover, the artificial urine samples were also tested with a linear range from 5.0 to 50.0 μM and a LOD of 0.48 μM. The results validated that the aptasensor together with the E-eye can be a promising platform for adenosine detection.

A label-free aptasensor for highly sensitive detection of homocysteine based on gold nanoparticles

In the present study, an original electrode fabrication approach was devised to create a label free sensitive electrochemical aptasensor for the detection of Homocysteine (Hcy) (Homocysteine signal was used for detection). To bind certain targets, synthetic oligonucleotides used as aptamers (APs) were specifically selected. Aptamers are substitutes for antibodies for analytical devices because of their sensitivity and high affinity. In this study, Hcy-Binding-Aptamer (HBA) was grafted onto the surface of Au nanoparticles/Glassy Carbon Electrode (Au/GCE) in order to create an aptasensor. The effects of buffer concentration, buffer type, interaction time, and aptamer concentration were investigated and optimized. In addition, Differential Pulse Voltammetry (DPV) was implemented to identify homocysteine. Favorable performance was achieved at a detection limit of 0.01 μM (S/N = 3) and linear range 0.05-20.0 μM. Furthermore, the fabricated aptasensor displayed desirable stability and reproducibility. The developed electrochemical aptasensor was found to have reasonable selectivity for the detection of homocysteine in the presence of cysteine and methionine. Analysis of real samples showed good ability of the proposed homocysteine biosensor to provide sensitive, quick, easy, and cost effective measurement of homocysteine in human blood serum and urine samples.

Electrochemical biosensors based on nucleic acid aptamers

During recent decades, nucleic acid aptamers have emerged as powerful biological recognition elements for electrochemical affinity biosensors. These bioreceptors emulate or improve on antibody-based biosensors because of their excellent characteristics as bioreceptors, including limitless selection capacity for a large variety of analytes, easy and cost-effective production, high stability and reproducibility, simple chemical modification, stable and oriented immobilization on electrode surfaces, enhanced target affinity and selectivity, and possibility to design them in target-sensitive 3D folded structures. This review provides an overview of the state of the art of electrochemical aptasensor technology, focusing on novel aptamer-based electroanalytical assay configurations and providing examples to illustrate the different possibilities. Future prospects for this technology are also discussed. Graphical abstract.

A portable electrochemical immunosensor for highly sensitive point-of-care testing of genetically modified crops

The wide cultivation of genetically modified (GM) crops has raised concerns on the risks to humans and the environment. 5-enolpyruvylshikimate-3-phosphate synthase isolated from Agrobacterium species strain CP4 (CP4-EPSPS) protein is most widely present in these crops. Therefore the measurement of CP4-EPSPS sensitively in a point-of-care testing (POCT) manner for the screening of transgenic plants is demanded. To date the development of quantitative POCT system has not yet been reported. In presented study, an electrochemical immunosensor towards CP4-EPSPS has been fabricated by integrating a portable bioanalytical device with a disposable screen-printed carbon electrode (SPCE) for POCT of GM crops. The dual-functionalized AuNPs were used as nanoprobes and prepared by simultaneously tagging horseradish peroxidase (HRP) and antibody on AuNPs with an exceptionally simple protocol. The sensitivity of the developed nanoprobe-based immunosensor was 62.5-fold higher than that using HRP-labeled antibody. As a result, the proposed immunosensor using SPCE could detect CP4-EPSPS down to 0.050 ng mL^-1 with the linear range of 0.10-10 ng mL^-1 within 65 min. In addition, the developed method has been validated with genuine GM crops and the results show a good correlation coefficient of 0.9909 compared with those of a commercial ELISA kit. Therefore, this portable electrochemical immunosensor is suitable for rapid and sensitive detection and provides a convenient and reliable platform for POCT assay.

A visible light photoelectrochemical sandwich aptasensor for adenosine triphosphate based on MgIn2S4-TiO2 nanoarray heterojunction

This work designed a MgIn₂S₄-TiONA heterojunction by growing MgIn₂S₄ nanoplates on TiO₂ nanowire array (TiONA) for preparation of visible light photoelectrochemical (PEC) sensing platform. The heterojunction exhibited strong absorption of visible light, large surface area and high loading of biomolecules, leading to high sensing sensitivity. Using adenosine triphosphate (ATP), a marker of cell vitality, as the target model, a PEC sandwich aptasensor was constructed by immobilizing capture DNA₁ on MgIn₂S₄ surface. In the presence of ATP and signal DNA₂ with terminal ferrocene as the electron donor, a sandwiched DNA₁-ATP-DNA₂ complex could be formed on the PEC aptasensor. The aptasensor showed excellent performance with a wide linear range from 50 fM to 100 nM and a detection limit of 20 fM. The sensing performance including specificity, reproducibility, stability and practical use were also evaluated, showing promising application of the MgIn₂S₄-TiONA heterojunction in PEC biosensing.

Amperometric sensing of hydrazine by using single gold nanopore electrodes filled with Prussian Blue and coated with polypyrrole and carbon dots

A nanoprobe for hydrazine sensing is described that is making use of a single gold nanopore electrode (SAuNPEs) that was modified by electro-deposition of Prussian Blue (PB) and then coated with a thin membrane of polypyrrole and carbon dots in order to enhance stability and catalytic activity. Best operated at a low potential of 0.3 V vs. Ag/AgCl, the nanosensor display good electrocatalytic activity towards the oxidation of hydrazine, with a linear response in the 0.5-80 μM hydrazine concentration range and a 0.18 μM detection limit (at S/N = 3). The method was applied to the determination of hydrazine in human urine. Graphical abstract Schematic presentation of the electrocatalytic oxidation of hydrazine using a single gold nanopore electrode that was modified by electro-deposition of Prussian Blue and then coated with a thin membrane of polypyrrole and carbon dots.

Stepping gating of ion channels on nanoelectrode via DNA hybridization for label-free DNA detection

Natural ion channels on cell membrane can gate the transport of ions and molecules by the conformational alteration of transmembrane proteins to regulate the normal physiological activities of cells. Inspired by the similarity of the conformation change under specific stimuli, here we introduce an ion channel gating model on a single nanoelectrode by anchoring DNA-gated switches on the very nanotip of gold nanoelectrode to mimic the response-to-stimulus behaviors of ion channels on bio-membranes. The surface-tethered DNA ion channels can be switched on by the Watson-Crick base pairing, which can alter the conformation of the tethered DNA from lying state to upright state. And these conformational alterations of the anchored DNA switches can effectively gate the transport of potassium ferricyanide onto the electrode interface. By continuously initiating the gates with DNA of different concentrations, we achieved the stepping gating of ion channels on a single nanoelectrode. Further, we demonstrated that the ion gating system on nanoelectrode showed excellent sensing performance. For example, the response kinetic was very fast with the signal saturation time of ~1 min, the reproducibility of the OFF/ON switch was robust enough to sustain for two cycles, and simultaneously, the specificity was high enough to distinguish complementary DNA and noncomplementary DNA. When used for label-free DNA detection, the limit of detection can be as low as 10 pM. This study provides a promising avenue to achieve label free and real-time detection of multiple biomolecules.

A signal amplification strategy and sensing application using single gold nanoelectrodes

In this work, a label-free electrochemical apta-nanosensor was fabricated on a single gold nanodisk electrode (AuNDE) for thrombin sensing with high sensitivity via a novel signal amplification strategy. This recognition platform was fabricated via self-assembly of helper DNA (HP-DNA), thrombin-binding aptamer (TBA) and gold nanoparticle (AuNP)-DNA complexes to form a sandwich structure on the AuNDE surface. A novel signal amplification strategy via designed AuNP-DNA complexes was introduced using Ru(NH3)63+ as the signal reporter based on the electrostatic interaction. In the presence of thrombin, the strong interaction between the TBA and target led to the dissociation of sandwich DNA complexes from the AuNDE, which resulted in the reduction current of Ru(NH3)63+. This proposed sensing platform showed a wide detection range of 0.1 pM-5 nM and a low detection limit of 0.02 pM. Considering the small overall dimensions and high sensitivity, this nanosensor can be potentially applied for bioanalysis in living biosystems.

Advanced Selection Methodologies for DNAzymes in Sensing and Healthcare Applications

DNAzymes have been widely explored owing to their excellent catalytic activity in a broad range of applications, notably in sensing and biomedical devices. These newly discovered applications have built high hopes for designing novel catalytic DNAzymes. However, the selection of efficient DNAzymes is a challenging process but one that is of crucial importance. Initially, systemic evolution of ligands by exponential enrichment (SELEX) was a labor-intensive and time-consuming process, but recent advances have accelerated the automated generation of DNAzyme molecules. This review summarizes recent advances in SELEX that improve the affinity and specificity of DNAzymes. The thriving generation of new DNAzymes is expected to open the door to several healthcare applications. Therefore, a significant portion of this review is dedicated to various biological applications of DNAzymes, such as sensing, therapeutics, and nanodevices. In addition, discussion is further extended to the barriers encountered for the real-life application of these DNAzymes to provide a foundation for future research.

New detection method for nucleoside triphosphates based on carbon dots: The distance-dependent singlet oxygen trapping

The distance-dependent based sensing mechanism, such as fluorescence resonance energy transfer (FRET) and surface plasmon resonance (SPR) absorption of gold nanoparticles, has been used widely in visual detection. In this work, we report another distance-dependent detection method for nucleoside triphosphates (NTPs) based on carbon dots (CDs) (¹O₂ donor) and 9, 10-diphenylanthracene-2-boronic acid (DABA, ¹O₂ acceptor). The CDs can generate singlet oxygen (¹O₂) which allows diffusion within 200 nm. Thus, the distance between CDs and DABA decreased through binding of NTPs (<200 nm), leading to absorption changes of DABA under light irradiation due to ¹O₂ trapping. This sensing system (CDs@DABA) has high selectivity for the detection of NTPs due to the double molecular recognition and a linear response in the 0-80 μM concentration range was accomplished with the detection limit as low as 4.35 μM.

Autonomous DNA nanomachine based on cascade amplification of strand displacement and DNA walker for detection of multiple DNAs

DNA can be modified to function as a scaffold for the construction of a DNA nanomachine, which can then be used in analytical applications if the DNA nanomachine can be triggered by the presence of a diagnostic DNA or some other analyte. We herein propose a novel and powerful DNA nanomachine that can detect DNA via combining the tandem strand displacement reactions and a DNA walker. Three different DNA sensing platforms are described, where the whole DNA machine was constructed on a gold electrode (GE). This cascade multiple amplification strategy exhibited an excellent sensitivity. Under optimal conditions, the electrochemical sensor could achieve a detection limit of 36 fM with a linear range from 50 to 500 fM. In particular, the electrochemical sensor could easily distinguish the base mutations. More interestingly, the DNA nanomachine could be used to construct analog AND and OR logic gates. We demonstrate that electrochemical signals generated from the different input combinations can be used to distinguish multiple target DNAs. The practical applicability of the present biosensor is demonstrated by the detection of target DNA in human serum with satisfactory results, which holds great potential for a future application in clinical diagnosis.

Amplified Detection of the Aptamer-Vanillin Complex with the Use of Bsm DNA Polymerase

The electrochemical detection of interactions between aptamers and low-molecular-weight targets often lacks sensitivity. Signal amplification improves the detection of the aptamer-analyte complex; Bsm DNA polymerase was used to amplify the signal from the interaction of vanillin and its aptamer named Van_74 on an ion-sensitive field-effect transistor (ISFET)-based biosensor. The aptamer was immobilized on the ISFET sensitive surface. A short DNA probe was hybridized with the aptamer and dissociated from it upon vanillin addition. A free probe interacted with a special DNA molecular beacon initiated the Bsm DNA polymerase reaction that was detected by ISFET. A buffer solution suitable for both aptamer action and Bsm DNA polymerase activity was determined. The ISFET was shown to detect the Bsm DNA polymerase reaction under the selected conditions. Vanillin at different concentrations (1 × 10-6-1 × 10-8 M) was detected using the biosensor with signal amplification. The developed detection system allowed for the determination of vanillin, starting at a 10-8 M concentration. Application of the Bsm DNA polymerase resulted in a 15.5 times lower LoD when compared to the biosensor without signal amplification (10.1007/s00604-017-2586-4).