Electrochemical impedance spectroscopy detection of lysozyme based on electrodeposited gold nanoparticles

Development of Nanomaterial-Modified Impedimetric Aptasensor—A Single-Step Strategy for 3,4-Methylenedioxymethylamphetamine Detection

Developing rapid, sensitive detection methods for 3,4-Methylenedioxymethylamphetamine (MDMA) is crucial to reduce its current misuse in the world population. With that aim, we developed an aptamer-modified tin nanoparticle (SnNP)-based nanoarchitecture as an electrochemical sensor in this study. This platform exhibited a high electron transfer rate with enhanced conductivity arising from its large surface area in comparison to the bare electrode. This observation was explained by the 40-fold higher electroactive surface area of SnNPs@Au, which provided a large space for 1.0 μM Apt_MDMA (0.68 ± 0.36 × 10¹² molecule/cm²) immobilization and yielded a significant electrochemical response in the presence of MDMA. Furthermore, the Apt_MDMA-modified SnNPs@Au sensing platform proved to be a simple yet ultrasensitive analytical device for MDMA detection in spiked biological and water samples. This novel electrochemical aptasensor showed good linearity in the range of 0.01–1.0 nM for MDMA (R² = 0.97) with a limit of detection of 0.33 nM and a sensitivity of 0.54 ohm/nM. In addition, the device showed high accuracy and stability along with signal recoveries in the range of 92–96.7% (Relative Standard Deviation, RSD, 1.1–2.18%). In conclusion, the proposed aptasensor developed here is the first to combine SnNPs and aptamers for illicit compound detection, and it offers a reliable platform for recreational drug detection.

Molecular dynamics study on the effects of charged amino acid distribution under low pH condition to the unfolding of hen egg white lysozyme and formation of beta strands

Aggregation of unfolded or misfolded proteins into amyloid fibrils can cause various diseases in humans. However, the fibrils synthesized in vitro can be developed toward useful biomaterials under some physicochemical conditions. In this study, atomistic molecular dynamics simulations were performed to address the mechanism of beta-sheet formation of the unfolded hen egg-white lysozyme (HEWL) under a high temperature and low pH. Simulations of the protonated HEWL at pH 2 and the non-protonated HEWL at pH 7 were performed at the highly elevated temperature of 450 K to accelerate the unfolding, followed by the 333 K temperature to emulate some previous in vitro studies. The simulations showed that HEWL unfolded faster, and higher beta-strand contents were observed at pH 2. In addition, one of the simulation replicas at pH 2 showed that the beta-strand forming sequence was consistent with the 'K-peptide', proposed as the core region for amyloidosis in previous experimental studies. Beta-strand formation mechanisms at the earlier stage of amyloidosis were explained in terms of the radial distribution of the amino acids. The separation between groups of positively charged sidechains from the hydrophobic core corresponded to the clustering of the hydrophobic residues and beta-strand formation.

2D Materials-Based Aptamer Biosensors: Present Status and Way Forward

Current advances in constructing functional nanomaterials and elegantly designed nanostructures have opened up new possibilities for the fabrication of viable field biosensors. Two-dimensional materials (2DMs) have fascinated much attention due to their chemical, optical, physicochemical, and electronic properties. They are ultrathin nanomaterials with unique properties such as high surface-to-volume ratio, surface charge, shape, high anisotropy, and adjustable chemical functionality. 2DMs such as graphene-based 2D materials, Silicate clays, layered double hydroxides (LDHs), MXenes, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs) offer intensified physicochemical and biological functionality and have proven to be very promising candidates for biological applications and technologies. 2DMs have a multivalent structure that can easily bind to single-stranded DNA/RNA (aptamers) through covalent, non-covalent, hydrogen bond, and π-stacking interactions, whereas aptamers have a small size, excellent chemical stability, and low immunogenicity with high affinity and specificity. This review discussed the potential of various 2D material-based aptasensor for diagnostic applications, e.g., protein detection, environmental monitoring, pathogens detection, etc.

Impedimetric Aptamer-Based Biosensors: Applications

Impedimetric aptamer-based biosensors show high potential for handheld devices and point-of-care tests. In this review, we report on recent advances in aptamer-based impedimetric biosensors for applications in biotechnology. We detail on analytes relevant in medical and environmental biotechnology as well as food control, for which aptamer-based impedimetric biosensors were developed. The reviewed biosensors are examined for their performance, including sensitivity, selectivity, response time, and real sample validation. Additionally, the benefits and challenges of impedimetric aptasensors are summarized.

HER-2 aptamer-targeted Ecoflex® nanoparticles loaded with docetaxel promote breast cancer cells apoptosis and anti-metastatic effect

Breast cancer is a major cause of cancer mortality. Regarding the advantages of polymeric nanoparticles as drug delivery systems with targeting potential, in this study the antitumor mechanism of targeted docetaxel polymeric nanoparticles of Ecoflex^® was exploited. Since the overexpression of HER-2 receptor in breast cancer cases is associated with poor prognosis and more aggressive disease, the proposed nanoparticles were conjugated to HER-2 specific aptamer molecules. In vitro cytotoxicity was evaluated by MTT assay. Flow-cytometry analysis was performed to evaluate the cellular uptake of nanoparticles loaded with a fluorescent probe. Anti-migration effects of samples were studied. Annexin IV-FITC and propidium iodide were implemented to investigate apoptosis induction and cell cycle analysis. Enhanced cytotoxicity compared with free docetaxel was explained considering improved cellular uptake of the nanoparticles and induced apoptosis in a larger portion of cells. Lower relative migration demonstrated enhanced anti-migration effect of nanoparticles, and cell cycle was arrested in G2/M phase using both formulations so the anti-microtubule mechanism of the drug was not altered. Therefore, this system could offer a potential substitute for the currently marketed docetaxel formulations, which may reduce adverse effects of the drug, while further in vivo and clinical investigations are required.

An electrochemical immunosensor for the corona virus associated with the Middle East respiratory syndrome using an array of gold nanoparticle-modified carbon electrodes

The Middle East respiratory syndrome corona virus (MERS-CoV) is highly pathogenic. An immunosensor for the determination of MERS-CoV is described here. It is based on a competitive assay carried out on an array of carbon electrodes (DEP) modified with gold nanoparticles. Recombinant spike protein S1 was used as a biomarker for MERS CoV. The electrode array enables multiplexed detection of different CoVs. The biosensor is based on indirect competition between free virus in the sample and immobilized MERS-CoV protein for a fixed concentration of antibody added to the sample. Voltammetric response is detected by monitoring the change in the peak current (typically acquired at a working potential of −0.05 V vs. Ag/AgCl) after addition of different concentrations of antigen against MERS-CoV. Electrochemical measurements using ferrocyanide/ferricyanide as a probe were recorded using square wave voltammetry (SWV). Good linear response between the sensor response and the concentrations from 0.001 to 100 ng.mL⁻¹ and 0.01 to 10,000 ng.mL⁻¹ were observed for MERS-CoV and HCoV, respectively. The assay was performed in 20 min with detection limit as low as 0.4 and 1.0 pg.mL⁻¹ for HCoV and MERS-CoV, respectively. The method is highly selective over non-specific proteins such as Influenza A and B. The method is single-step, sensitive and accurate. It was successfully applied to spiked nasal samples.

Graphical abstract

An electrochemical immunosensor for the corona virus associated with the Middle East respiratory syndrome using an array of gold nanoparticle-modified carbon electrodes

The Middle East respiratory syndrome corona virus (MERS-CoV) is highly pathogenic. An immunosensor for the determination of MERS-CoV is described here. It is based on a competitive assay carried out on an array of carbon electrodes (DEP) modified with gold nanoparticles. Recombinant spike protein S1 was used as a biomarker for MERS CoV. The electrode array enables multiplexed detection of different CoVs. The biosensor is based on indirect competition between free virus in the sample and immobilized MERS-CoV protein for a fixed concentration of antibody added to the sample. Voltammetric response is detected by monitoring the change in the peak current (typically acquired at a working potential of -0.05 V vs. Ag/AgCl) after addition of different concentrations of antigen against MERS-CoV. Electrochemical measurements using ferrocyanide/ferricyanide as a probe were recorded using square wave voltammetry (SWV). Good linear response between the sensor response and the concentrations from 0.001 to 100 ng.mL^-1 and 0.01 to 10,000 ng.mL^-1 were observed for MERS-CoV and HCoV, respectively. The assay was performed in 20 min with detection limit as low as 0.4 and 1.0 pg.mL^-1 for HCoV and MERS-CoV, respectively. The method is highly selective over non-specific proteins such as Influenza A and B. The method is single-step, sensitive and accurate. It was successfully applied to spiked nasal samples. Graphical abstract An electrochemical immunoassay is described for the Middle East Respiratory Syndrome Corona Virus (MERS-CoV). The method is based on a competitive assay carried out on a carbon array electrodes (DEP) nanostructured with gold nanoparticles. The array electrodes enable the multiplexed detection of different types of Corona Virus.

An impedimetric biosensor for DNA damage detection and study of the protective effect of deferoxamine against DNA damage

The detection and inhibition of DNA damage are of great importance in the prevention and treatment of diseases. Developing a simple and sensitive tool for this purpose would be a chance to monitor the DNA damage and could be helpful in introducing some drugs which can prevent this phenomenon. Here, we report a novel and sensitive electrochemical biosensor based on DNA/Au nanoparticles (AuNPs) modified screen printed gold electrode (DNA/AuNPs/SPGE) to investigate the DNA damage process and also to study the protective behavior of deferoxamine (DFO). The proposed biosensor was fabricated by electrodeposition of AuNPs onto SPGE, followed by chemical immobilisation of thiol-terminated DNA. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) have been used to characterise this biosensor. Hydroxyl radical (OH), which is generated during the Fenton reaction, is responsible for the induced damage to DNA. EIS technique was applied to monitor the DNA damage, and the increase in charge transfer resistance (R_ct) following the DNA damage, was considered as an indicator. Furthermore, the ability of the electrochemical screening system was proved by the investigation of the antioxidant effect of DFO in prohibiting the DNA damage.

Pharmacokinetics and in vitro/in vivo antitumor efficacy of aptamer-targeted Ecoflex® nanoparticles for docetaxel delivery in ovarian cancer

Purpose

Epithelixal ovarian cancer is the fourth cause of cancer death in developed countries with 77% of ovarian cancer cases diagnosed with regional or distant metastasis, with poor survival rates. Docetaxel (DTX) is a well-known anticancer agent, with clinically proven efficacy in several malignancies, including ovarian cancer. However, the adverse effects caused by the active ingredient or currently marketed formulations could even deprive the patient of the advantages of treatment. Therefore, in the current study, polymeric nanoparticles (NPs) equipped with aptamer molecules as targeting agents were proposed to minimize the adverse effects and enhance the antitumor efficacy through directing the drug cargo toward its site of action.

Materials and methods

Electrospraying technique was implemented to fabricate poly (butylene adipate-co-butylene terephthalate) (Ecoflex^®) NPs loaded with DTX (DTX-NPs). Afterward, aptamer molecules were added to the DTX-NPs, which bound via covalent bonds (Apt-DTX-NPs). The particle size, size distribution, zeta potential, entrapment efficiency, and release profile of the NPs were characterized. Using MTT assay and flow-cytometry analysis, the in vitro cytotoxicity and cellular uptake of the NPs were compared to those of the free drug. Following intravenous administration of Taxotere^®, DTX-NPs, and Apt-DTX-NPs (at an equivalent dose of 5 mg/kg of DTX), pharmacokinetic parameters and antitumor efficacy were compared in female Balb/c and HER-2-overexpressing tumor-bearing B6 athymic mice, respectively.

Results

The obtained results demonstrated significantly enhanced in vitro cytotoxicity and cellular uptake of Apt-DTX-NPs in a HER-2-overexpressing cell line, comparing to DTX-NPs and the free drug. The results of in vivo studies indicated significant increment in pharmacokinetic parameters including the area under the plasma concentration–time curve, mean residence time, and elimination half-life. Significant increment in antitumor efficacy was also observed, probably due to the targeted delivery of DTX to the tumor site and enhanced cellular uptake as evaluated in the aforementioned tests.

Conclusion

Hence, the proposed drug delivery system could be considered as an appropriate potential substitute for currently marketed DTX formulations.

A Low-Cost Inkjet-Printed Aptamer-Based Electrochemical Biosensor for the Selective Detection of Lysozyme

Recently, inkjet-printing has gained increased popularity in applications such as flexible electronics and disposable sensors, as well as in wearable sensors because of its multifarious advantages. This work presents a novel, low-cost immobilization technique using inkjet-printing for the development of an aptamer-based biosensor for the detection of lysozyme, an important biomarker in various disease diagnosis. The strong affinity between the carbon nanotube (CNT) and the single-stranded DNA is exploited to immobilize the aptamers onto the working electrode by printing the ink containing the dispersion of CNT-aptamer complex. The inkjet-printing method enables aptamer density control, as well as high resolution patternability. Our developed sensor shows a detection limit of 90 ng/mL with high target selectivity against other proteins. The sensor also demonstrates a shelf-life for a reasonable period. This technology has potential for applications in developing low-cost point-of-care diagnostic testing kits for home healthcare.

Ensuring food safety using aptamer based assays: Electroanalytical approach

Aptamers, are being increasingly employed as favorable receptors for constructing highly sensitive biosensors, for their remarkable affinities towards certain targets including a wide scope of biological or chemical substances, and their superiority over other biologic receptors. The selectivity and affinity of the aptamers have been integrated with the wise design of the assay, applying suitable modifications, such as nanomaterials on the electrode surface, employing oligonucleotide-specific amplification strategies or, their combinations. After successful performance of the electrochemical aptasensors for biomedical applications, the food sector with its direct implication for human health, which demands rapid and sensitive and economic analytical solutions for determination of health threatening contaminants in all stages of production process, is the next field of research for developing efficient electrochemical aptasensors. The aim of this review is to categorize and introduce food hazards and summarize the recent electrochemical aptasensors that have been developed to address these contaminants.

Highly sensitive amperometric detection of cardiac troponin I using sandwich aptamers and screen-printed carbon electrodes

In this study, we developed a sandwich aptamer-based screen-printed carbon electrode (SPCE) using chronoamperometry for the detection of cardiac troponin I (cTnI), one of the promising biomarkers for acute myocardial infarction (AMI). Disposable three-electrode SPCEs were manufactured using a screen printer, and various modifications such as electrodeposition of gold nanoparticles and electropolymerization of conductive polymers were performed. From the bare electrode to the aptamer-immobilized SPCE, all processes were monitored and analyzed via various techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The quantification of cTnI was conducted based on amperometric signals from the catalytic reaction between hydrazine and H₂O₂. The fabricated aptasensor in a buffer, as well as in a serum-added solution, exhibited great analytical performance with a dynamic range of 1-100 pM (0.024-2.4ng/mL) and a detection limit of 1.0 pM (24pg/mL), which is lower than the existing cutoff values (40-700pg/mL). Furthermore, the developed sensor showed high sensitivity to cTnI over other proteins. It is anticipated that this potable SPCE aptasensor for cTnI will become an innovative diagnostic tool for AMI.

Electrochemical and in vitro neuronal recording characteristics of multi-electrode arrays surface-modified with electro-co-deposited gold-platinum nanoparticles

In order to complement the high impedance electrical property of gold nanoparticles (Au NPs) we have performed electro-co-deposition of gold-platinum nanoparticles (Au-Pt NPs) onto the Au multi-electrode array (MEA) and modified the Au-Pt NPs surface with cell adhesive poly-D-lysine via thiol chemistry based covalent binding. The Au-Pt NPs were analyzed to have bimetallic nature not the mixture of Au NPs and Pt NPs by X-ray diffraction analysis and to have impedance value (4.0 × 10(4) Ω (at 1 kHz)) comparable to that of Pt NPs. The performance of Au-Pt NP-modified MEAs was also checked in relation to neuronal signal recording. The noise level in Au-Pt NP-modified MEAs was lower than in that of Au NP-modified MEA.

Dual-Target Electrochemical Biosensing Based on DNA Structural Switching on Gold Nanoparticle-Decorated MoS2 Nanosheets

A MoS2-based electrochemical aptasensor has been developed for the simultaneous detection of thrombin and adenosine triphosphate (ATP) based on gold nanoparticles-decorated MoS2 (AuNPs-MoS2) nanocomposites. Two different aptamer probes labeled with redox tags were simultaneously immobilized on an AuNPs-MoS2 film modified electrode via Au-S bonds. The aptamers presented structural switches with the addition of target molecules (thrombin and ATP), resulting in methylene blue (MB) far from or ferrocene (Fc) close to the electrode surface. Therefore, a dual signaling detection strategy was developed, which featured both "signal-on" and "signal-off" elements in the detection system because of the target-induced structure switching. This proposed aptasensor could simultaneously determine ATP and thrombin as low as 0.74 nM ATP and 0.0012 nM thrombin with high selectivity, respectively. In addition, thrombin and ATP could act as inputs to activate an AND logic gate.

A highly sensitive and selective impedimetric aptasensor for interleukin-17 receptor A

Interleukin-17 receptor A (IL-17RA) has been recognized as a valuable biomarker for diverse diseases, including autoimmune diseases. In this work, an electrochemical biosensor with great sensitivity and selectivity toward IL-17RA was fabricated using an IL-17RA aptamer (Kd=14.00nM) for the first time. The aptasensor was manufactured using electrodeposition of gold nanoparticles, and then quantitative detection of IL-17RA was performed based on impedimetry. The developed sensor exhibited a superior analytical performance for IL-17RA with a wide dynamic range of 10-10,000pg/mL in buffer and a detection limit of 2.13pg/mL, which is lower than that of commercially available ELISA kits. In addition, we validated the high specificity of the designed aptasensor to only IL-17RA, which showed good sensitivity even in human serum solution. Furthermore, the detection of the differentiated HL-60 cells expressing IL-17RA was successfully performed. Clinical applicability of the sensor was also demonstrated utilizing neutrophils separated from asthma patients. It is expected that the fabricated aptasensor will become an excellent diagnostic platform for IL-17RA-mediated diseases.

Feasible electrochemical biosensor based on plasma polymerization-assisted composite of polyacrylic acid and hollow TiO2 spheres for sensitively detecting lysozyme

A composite made of polyacrylic acid and hollow TiO2 spheres (TiO2@PPAA) was prepared by the plasma polymerization method and subsequently used as an electrode material for detecting lysozyme. The chemical structure, surface morphology, and electrochemical performance of the TiO2@PPAA composite were mainly affected by the plasma input power used during plasma polymerization. After optimizing plasma conditions, aptamer strands exhibited high adsorption affinity toward the surface of TiO2@PPAA composite via synergistic effects between TiO2 and PPAA. Electrochemical impedance spectroscopy results showed that the developed TiO2@PPAA aptasensor presents highly sensitive detection ability toward lysozyme; the limit of detection of the proposed aptasensor is 0.015 ng mL(-1) (1.04 pM) within the range of 0.05-100 ng mL(-1) in terms of 3σ value. The film further showed excellent selectivity toward lysozyme in the presence of interfering proteins, such as thrombin, bovine serum albumin, and immunoglobulin E. Thus, this aptasensing strategy might broaden the applications of plasma polymerized nanomaterials in the field of biomedical research and early clinical diagnosis.

A label-free and self-assembled electrochemical biosensor for highly sensitive detection of cyclic diguanylate monophosphate (c-di-GMP) based on RNA riboswitch

Cyclic diguanylate monophosphate (c-di-GMP) is an important second messenger that regulates a variety of complex physiological processes involved in motility, virulence, biofilm formation and cell cycle progression in several bacteria. Herein we report a simple label-free and self-assembled RNA riboswitch-based biosensor for sensitive and selective detection of c-di-GMP. The detectable concentration range of c-di-GMP is from 50 nM to 1 μM with a detection limit of 50 nM.

RNA aptamer based electrochemical biosensor for sensitive and selective detection of cAMP

Cyclic adenosine monophosphate (cAMP) is an important small biological molecule associated with the healthy state of living organism. In order to realize highly sensitive and specific detection of cAMP, here an RNA aptamer and electrochemical impedance spectroscopy (EIS) based biosensor enhanced by gold nanoparticles electrodeposited on the surface of gold electrode is designed. The designed aptasensor has a wide effective measuring range from 50pM to 250pM with a detection limit of 50pM in PBS buffer, and an effective measuring range from 50nM to 1μM with a detection limit of 50nM in serum. The designed biosensor is also able to detect cAMP with high sensitivity, specificity, and stability. Since the biosensor can be easily fabricated with low cost and repeatedly used for at least two times, it owns great potential in wide application fields such as clinical test and food inspection, etc.

A new and highly sensitive resonance Rayleigh scattering assay for lysozyme using aptamer-nanogold as a probe

Gold nanoparticles (GN), 10 nm in size, were modified by using lysozyme aptamer (Apt) to obtain a stable Apt–GN probe in pH 8.05 Tris/HCl buffer solutions containing 0.04 mol/L NaCl. Upon addition of lysozyme (LYS), it reacted with the Apt of the probe to form a very stable Apt–LYS complex and to release GNs, which aggregated to form large clusters with a resonance Rayleigh scattering (RRS) peak at 368 nm. The enhanced peak intensity, ΔI, was linear to the LYS concentration in the range 0.2–5.2 nmol/L, with a detection limit of 0.05 nmol/L. The influence of foreign substance was tested, and the results showed that this RRS method has high selectivity. This Apt–GN RRS method was applied to the analysis of LYS in a real sample, with satisfactory results.

Developing trends in aptamer-based biosensor devices and their applications

Aptamers are, in general, easier to produce, easier to store and are able to bind to a wider variety of targets than antibodies. For these reasons, aptamers are gaining increasing popularity in environmental monitoring as well as disease detection and disease management applications. This review article examines the research and design of RNA and DNA aptamer based biosensor systems and applications as well as their potential for integration in effective biosensor devices. As single stranded DNA or RNA molecules that can bind to specific targets, aptamers are well suited for biomolecular recognition and sensing applications. Beyond being able to be designed for a near endless number of specific targets, aptamers can also be made which change their conformation in a predictable and consistent way upon binding. This can lead to many unique and effective detection methods using a variety of optical and electrochemical means.

Chitosan-graphene oxide based aptasensor for the impedimetric detection of lysozyme

An impedimetric detection of lysozyme (LYS) was performed for the first time in this study at the surface of chitosan-graphene oxide (CHIT-GO) modified sensor based on the specific interaction process between DNA aptamer and its cognate protein, LYS. The amino linked DNA aptamer (APT) was covalently immobilized without using any chemical agents onto the surface of pencil graphite electrode (PGE). These PGEs are inexpensive and simple to use, and thus, they can be furtherly developed for a single-use application in a portable protein chip device. The electrochemical impedance spectroscopy (EIS) technique was used herein to analyze (i) the surface characterization of unmodified PGE and CHIT-GO modified PGE, and also (ii) the interaction between APT and LYS. The limit of detection (DL) was found as 0.38 μg/mL (equals to 28.53 nM). This impedimetric LYS aptasensor exhibited a higher selectivity toward thrombin and bovine serum albumin, even in the mixture samples.

Novel homogeneous label-free electrochemical aptasensor based on functional DNA hairpin for target detection

We first developed a label-free and immobilization-free homogeneous electrochemical aptasensor, which combined a smart functional DNA hairpin and a designed miniaturized electrochemical device. Cocaine was chosen as a model target. The anticocaine aptamer and peroxidase-mimicking DNAzyme were integrated into one single-stranded DNA hairpin. Both aptamer and G-quadruplex were elaborately blocked by the stem region. The conformation switching induced by the affinity interaction between aptamer and cocaine released G-quadruplex part and turned on DNAzyme activity. The designed electrochemical device, constructed by a disposable micropipet tip and a reproducible carbon fiber ultramicroelectrode, was applied to the detection of homogeneous DNAzyme catalytic activity at the microliter level. The aptasensor realized the quantification of cocaine ranging from 1 to 500 μM with high specificity. The clever combination of the functional DNA hairpin and the novel device achieved an absolutely label-free electrochemical aptasensor, which showed excellent performance like low cost, easy operation, rapid detection, and high repeatability.

Aptamer biosensor for label-free impedance spectroscopy detection of potassium ion based on DNA G-quadruplex conformation

Herein, a label-free and highly sensitive electrochemical impedance spectroscopy (EIS) aptasensor for the detection of potassium ion (K⁺) was developed based on a conformational change in which a K⁺-stabilized single stranded DNA (ssDNA) with G-rich sequence was used as the recognition element. In the measurement of K⁺ ions, the change in interfacial electron transfer resistance (R(ct)) of the sensor using a redox couple of [Fe(CN)₆]³⁻/⁴⁻ as the probe was monitored. In the presence of K⁺, the G-rich DNA folded into the G-quadruplex structure, and then K⁺ can bind to the G-quadruplex structure, leading to an increase in the R(ct). The Rct increased with K⁺ concentration, and the plot of R(ct) against the logarithm of K⁺ concentration is linear over the range from 0.1 nM to 1 mM with a detection limit of 0.1 nM. Other metal ions, such as Ca²⁺, Mg²⁺, Na⁺, Li⁺, Al³⁺, Zn²⁺, Cu²⁺, and Ni²⁺ caused no notable interference on the detection of K⁺. The scheme reported herein is applicable to the detection of other kinds of G-rich aptamer-binding chemicals and biomolecules.

A highly selective electrochemical impedance spectroscopy-based aptasensor for sensitive detection of acetamiprid

A simple aptasensor for sensitive and selective detection of acetamiprid has been developed based on electrochemical impedance spectroscopy (EIS). To improve sensitivity of the aptasensor, gold nanoparticles (AuNPs) were electrodeposited on the bare gold electrode surface by cycle voltammetry (CV), which was employed as a platform for aptamer immobilization. With the addition of acetamiprid, the formation of acetamiprid-aptamer complex on the AuNPs-deposited electrode surface resulted in an increase of electron transfer resistance (Ret). The change of Ret strongly depends on acetamiprid concentration, which is applied for acetamiprid quantification. A wide linear range was obtained from 5 to 600nM with a low detection limit of 1nM. The control experiments performed by employing the pesticides that may coexist or have similar structure with acetamiprid demonstrate that the aptasensor has only specific recognition to acetamiprid, resulting in high selectivity of the aptasensor. The dissociation constant, Kd of 23.41nM for acetamiprid-aptamer complex has been determined from the differential capacitance (Cd) by assuming a Langmuir isotherm, which indicates strong interaction between acetamiprid and aptamer, further proving high selectivity of the aptasensor. Besides, the applicability of the developed aptasensor has been successfully evaluated by determining acetamiprid in the real samples, wastewater and tomatoes.

Signal-on architecture for electrochemical aptasensors based on multiple ion channels

In this work, we described a signal-on architecture for electrochemical aptasensors that is applicable to a wide range of aptamers. Herein, we use thrombin as the model sensing target. The signal-on aptasensor is composed of multiple ion channels embedded within a polymeric membrane, with the antithrombin aptamers chemically modified onto the inner walls of each ion channel working as the sensing element. As the thrombin concentration increased, [Ru(NH(3))(6)](3+), which was electrostatically absorbed onto the negatively charged phosphate backbones of aptamers beforehand, was displaced and pushed into the ion channels from the inner walls, leading to an increase in the current of redox cations at the working electrode surface. Compared with the traditional two-electrode design using a single ion channel sensing system, our ion channel sensing system is applied multiple times within an ordinary three-electrode system, providing such advantages as a high signal-to-noise ratio and suitability for a wide variety of redox species. The results indicate that multiple ion channel sensing provides improvements of orders of magnitude in signal sensitivity. In particular, this signal-on architecture avoids the problems of limited signal gain and "false positives". Moreover, the proposed aptasensor is simple, highly selective, stable, and applicable to real samples.

Label-free impedimetric immunosensor for sensitive detection of 2,4-dichlorophenoxybutyric acid (2,4-DB) in soybean

Electrochemical impedance immunosensor, with its high sensitivity from electrochemical impedance analysis and ideal specificity from the immunoassay, is increasingly used in the detection of a kind of phenoxy acid herbicides which is 2,4-Dichlorophenoxybutyric acid (2,4-DB). In this experiment, synthetic 2,4-DB antibodies were immobilized on the electrode by the crosslinking of L-Cysteine/glutaraldehyde, and 2,4-DB were measured by the increase of electron-transfer resistance when the immune reaction occurred, with Fe(CN)(6)(3-)/Fe(CN)(6)(4-) as the probe. Under optimal conditions, the change of resistance is in a linear relationship with the logarithm of the concentration in the range of 1.0×10(-7)-1.0×10(-3) g/L (R=0.994) with the detection limit of 1.0×10(-7) g/L (0.1 ppb). This method bears such merits as simplicity in operation, high sensitivity, wide linear range, specificity, reproducibility and good stability. The actual soybean samples were analyzed with the recovery of 82.8%-102.3%.