Measurement of microRNA-106b as a gastric cancer biomarker based on Zn-BTC MOF label-free genosensor.

Due to the late detection of stomach cancer, this cancer usually causes high mortality. The development of an electrochemical genosensor to measure microRNA 106b (miR-106b), as a gastric cancer biomarker, is the aim of this effort. In this regard, first, 1,3,5-benzenetricarboxylate (BTC) metal-organic frameworks (Zn-BTC MOF) were self-assembled on the glassy carbon electrode and then the probe (ssDNA) was immobilized on it. The morphology Zn-BTC MOF was characterized by SEM, FT-IR, Raman and X-Ray techniques. Zn-BTC MOF as a biosensor substrate has strong interaction with ssDNA. Quantitative measurement of miR-106b was performed by electrochemical impedance spectroscopy (EIS). To perform this measurement, the difference of the charge transfer resistances (ΔRct) of Nyquist plots of the ssDNA probe modified electrode before and after hybridization with miR-106b was obtained and used as an analytical signal. Using the suggested genosensor, it is possible to measure miR-106b in the concentration range of 1.0 fM to 1.0 µM with a detection limit of 0.65 fM under optimal conditions. Moreover, at the genosensor surface, miR-106b can be detected from a non-complementary and a single base mismatch sequence. Also, the genosensor was used to assess miR-106b in a human serum sample and obtained satisfactory results.

Development of an electrochemical sensitive aptasensor based on a zeolite imidazolate framework-8 and gold nanoparticles for the determination of Staphylococcus aureus bacteria.

Staphylococcus aureus (S. aureus) is one of the most important pathogens that cause illness and food poisoning. In this research, using a glassy carbon electrode (GCE) modified with zeolite imidazolate framework-8 (ZIF 8) and gold nanoparticles (AuNPs), a sensitive electrochemical aptasensor has been made for the detection of the S. aureus bacteria. The morphology of the prepared AuNPs-ZIF 8 nanocomposite has been carefully characterized by means of transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDS). In the manufacturing process, the S. aureus aptamer is immobilized on the AuNPs-ZIF 8 surface. Electrochemical impedance spectroscopy (EIS) method has been used for quantitative determination of S. aureus bacteria. The changes in the charge transfer resistance (Rct) of the aptamer due to the change in the concentration of bacteria are considered as the analytical signals. The proposed aptasensor has linear response in the concentration range of 1.5 × 101 to 1.5 × 107 CFU mL-1 of S. aureus bacteria. The detection limit of the method is 3.4 CFU mL-1. Using the developed aptasensor, it is possible to determine S. aureus bacteria in water and milk samples.

Development of an electrochemical aptasensor based on Au nanoparticles decorated on metal-organic framework nanosheets and p-biphenol electroactive label for the measurement of aflatoxin B1 in a rice flour sample.

This study purposes designing a new aptasensor to detect aflatoxin B1 (AFB1). The AFB1 aptasensor was developed by growing gold nanoparticles on the surface of nickel-based metal-organic framework nanosheets (AuNPs/Ni-MOF) and an electroactive indicator (p-biphenol, PBP). The AFB1 aptamer was immobilized on the AuNPs/Ni-MOF and then hybridized with the complementary DNA (cDNA). PBP was intercalated within the double helix of the cDNA-aptamer. The difference between electrochemical responses of intercalated PBP before and after incubation of AFB1 with the immobilized aptamer was considered as an analytical response. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to monitor the construction processes of the aptasensor. By recording the differential pulse voltammograms of PBP in phosphate buffer (pH 7.0, 0.1 M), the linear range and the detection limit of AFB1 were found to be 5.0 × 10-3-150.0 ng mL-1 and 1.0 × 10-3 ng mL-1 (S/N = 3), respectively. Finally, the designed aptasensor has been successfully used to measure AFB1 in a rice flour sample with satisfying results. Schematic illustrated the different steps of constructing the electrochemical aptasensor based on Au nanoparticles decorated on Ni-metal-organic framework nanosheets and p-biphenol electroactive label for measuring aflatoxin B1 (AFB1).

Determination of lead ions in fish and oyster samples using a nano-adsorbent of functionalized magnetic graphene oxide nanosheets-humic acid and the flame atomic absorption technique.

This study aims at the functionalization of magnetic graphene oxide nanosheets and the binding of humic acid as a lead complex ligand. Graphene oxide nanosheets possess a large surface area and various carboxylic acid groups which can be activated easily by activating agents. Therefore, they are suitable to be used for the extraction of heavy metals. To have a better process of extracting lead ions, magnetic graphene oxide was used in this research. Humic acid, as a lead metal complex agent, has an amine functional group which can be bound to modified graphene oxide from one side. The process of constructing the nano-adsorbent proposed for the preconcentration of lead ions as well as its characterization was studied by infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV-visible), field emission scanning electron microscopy (FESEM), and vibrating sample magnetometry (VSM). The designed nano-adsorbent was tested to measure lead ions in simulated and real samples of sea water, fish, and oysters. The detection limit obtained in the simulated samples was 0.07 µg/L, and the linear range was 0.2-12 µg/L. The apparatus used to measure the ions was a flame atomic absorption device. In the analysis of the real samples, the values obtained through flame atomic absorption were compared with those obtained through furnace atomic absorption. The proposed technique is advantageous due to being cheap, precise, and sensitive for the trace measurement of lead ions.

Electrochemical sandwich aptasensor for the carcinoembryonic antigen using graphene quantum dots, gold nanoparticles and nitrogen doped graphene modified electrode and exploiting the peroxidase-mimicking activity of a G-quadruplex DNAzyme.

A sandwich-type electrochemical aptasensor has been constructed and applied for sensitive and selective detection of the carcinoembryonic antigen (CEA). The surface of a glassy carbon electrode (GCE) was first modified with nitrogen-doped graphene and then gold nanoparticles and graphene quantum dots electrodeposited on it to obtain an architecture of type GQD/AuNP/NG/GCE. In the next step, the CEA-binding aptamer was immobilized on the modified GCE. Hemin intercalates in the amino-modified hemin aptamer to form a hemin-G-quadruplex (hemin-G4) DNAzyme. The amino modified CEA aptamer II is connected to hemin-G4 by glutaraldehyde (GA) as a linker to produce CEAaptamerII/GA/hemin-G4 (=ApII/GA/DNAzyme). Through a sandwich mode, the ApII/GA/DNAzyme bioconjugates are captured on the modified GCE. Subsequently, the hemin-G4 acts as peroxidase-mimicking DNAzyme and rapidly catalyzes the electroreduction of hydrogen peroxide. The quantitative determination of CEA was achieved by differential pulse voltammetry, best at a working potential of around -0.27 V vs. Ag/AgCl. Under optimized conditions, the assay has a linear response in the 10.0 fg mL-1 to 200.0 ng mL-1 CEA concentration range and a lower detection limit of 3.2 fg mL-1. Graphical abstract Schematic presentation of a sandwich-type electrochemical aptasensor based on nitrogen doped graphene (NG), gold nanoparticles (AuNPs) and graphene quantum dots (GQDs) modified glassy carbon electrode, and the hemin-G4 DNAzyme for femtomolar detection of the carcinoembryonic antigen.

A voltammetric assay for microRNA-25 based on the use of amino-functionalized graphene quantum dots and ss- and ds-DNAs as gene probes.

The authors describe a DNA based voltammetric assay for the cancer biomarker microRNA-25. A glassy carbon electrode (GCE) was modified with amino-functionalized graphene quantum dots and used as an amplifier of electrochemical signals. p-Biphenol is introduced as a new electroactive probe with a fairly low working potential of 0.3 V (vs. Ag/AgCl). The stages of fabricating the electrode were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. ss-Probe DNA was immobilized on the modified GCE and then exposed to a sample containing microRNA-25. The results indicated that the electrode can distinguish complementary microRNA-25 from a single-base mismatch. The increase in the electrochemical response of PBP and the positive shift in the potential peak indicate that PBP is intercalated between two strands. Under optimized experimental conditions, the current of the electrode increases linearly with the logarithm of the microRNA-25 concentration in the range from 0.3 nM to 1.0 µM, and the detection limit is 95.0 pM. The assay was successfully employed to the determination of microRNA-25 in spiked human plasma. Graphical abstract A novel electrochemical nanogenosensor is introduced for simple and sensitive determination of microRNA-25, as a biomarker, based on amino-functionalized graphene quantum dots (as a surface modifier) and p-biphenol (as an electroactive label).

Single Palladium Nanoparticle Collisions Detection through Chronopotentiometric Method: Introducing a New Approach to Improve the Analytical Signals.

In the present research, the chronopotentiometric method and hydrazine, as a suitable probe, were used to detect single Pd nanoparticle (Pd-NP) collisions to the surface of a carbon fiber ultramicroelectrode (CFUME). The change in the potential, which is due to the electrocatalytic oxidation of hydrazine exactly at the time of Pd-NP collision to the CFUME surface, was used to detect each collide. It was shown that the amplitude and the frequency of the potential steps, produced through the nanoparticles collisions at the CFUME surface, are respectively proportional to their radius and concentration in an analytical solution. For the first time, a new approach is introduced for extraction of current-time plots (chronoamperograms) from experimental potential-time plots (chronopotentiograms). It is demonstrated that the signal-to-noise ratio (S/N) increases significantly based on the proposed method. Also, by using the chronoamperograms that resulted from the experimental chronopotentiograms, a higher number of collisions is achievable and, thus, the collision frequency, f, increases and the limit of detection decreases. Interestingly, the collision frequency resulted from the chronoamperograms, that has been derived from chronopotentiograms, is closer to the collision frequency calculated by using the theoretical model.

An ultrasensitive aptasensor for hemin and hemoglobin based on signal amplification via electrocatalytic oxygen reduction.

The present study aims at the fabrication of a novel electrochemical aptasensor, Ap-GA-AMSN-GCE, for the label-free determination of hemin and hemoglobin (Hb). Basically, the electrochemical reduction current of hemin or Hb incubated on Ap-GA-AMSN-GCE in the presence of oxygen serves as an excellent signal for quantitative determination of these analytes. By differential pulse voltammetry, the calibration plot was linear in the concentration range of 1.0 × 10-19-1.0 × 10-6 M of hemin and Hb. Also, the detection limits, DL, of hemin and Hb were found to be 7.5 × 10-20 M and 6.5 × 10-20 M respectively. According to the experimental results, using the proposed aptasensor in the absence of any oxygen molecule in the analytical solution, the DL value of hemin was 1.0 × 10-12 M. The very low DL obtained in the presence of oxygen is due to the excellent electrocatalytic activity of hemin and Hb incubated on the aptasensor for oxygen reduction. This electrocatalytic activity has a key role in bringing about excellent low detection limits, DL, and wide linear concentration ranges of analytes. Finally, this aptasensor was satisfactorily used for the determination of Hb in human blood samples.

A label-free aptasensor based on electrodeposition of gold nanoparticles on silver-based metal-organic frameworks for measuring ochratoxin A in black and red pepper.

Since ochratoxin A (OTA) is immunotoxic, teratogenic and carcinogenic, it is very important to monitor this compound in food samples. In the present work, the development and fabrication of a label-free electrochemical aptasensor based on the gold nanoparticles/silver-based metal-organic framework (AuNPs/Ag-MOF) for the determination of ochratoxin A (OTA) is introduced. The aptasensor was fabricated by electrodeposition of AuNPs on a glassy carbon electrode modified with Ag-MOF. The characteristics of the synthesized Ag-MOF were determined by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and UV-Visible spectroscopy. The aptamer was immobilized on the modified electrode and then OTA was incubated on it. The process of different stages of the aptasensor construction has been confirmed by two methods of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) and using [Fe(CN)6]3-/4- as a redox probe. The EIS method has also been used for the OTA quantitative determination. The difference in charge transfer resistance (Rct) before and after the interaction of OTA with the immobilized aptamer was considered as the analytical response of the aptasensor. Using the developed aptasensor, it is possible to measure OTA in the concentration range of 1.0 × 10-3 to 200.0 ng mL-1 with a detection limit of 2.2 × 10-4 ng mL-1. Finally, the ability of the aptasensor to measure OTA in red and black pepper was investigated and completely satisfactory results were obtained.

Electrodeposited acetaminophen as a bifunctional electrocatalyst for simultaneous determination of ascorbic acid, glutathione, adrenaline and tryptophan.

In this study, an acetaminophen-modified glassy carbon electrode (ACMGCE) was fabricated for the purpose of investigating its electrochemical behavior by cyclic voltammetry (CV). ACMGCE serves as an excellent bifunctional electrocatalyst for the oxidation of ascorbic acid (AA) and glutathione (GSH) in a phosphate buffer solution (pH 7.0). The kinetic parameters such as the electron transfer coefficient, α, and the heterogeneous electron transfer rate constant, k', for the oxidation of AA and GSH at the ACMGCE surface were determined by CV. In addition, detection limits of 3.89 µM for AA and 0.37 µM for GSH were obtained at the ACMGCE using a differential pulse voltammetric (DPV) method. In DPV, the bifunctional modified electrode could separate the oxidation peak potentials of AA, GSH, adrenaline (AD) and tryptophan (Trp) present in the same solution, though the peak potentials were indistinguishable at a bare GCE. Finally, the modified electrode was successfully applied to the determination of AA in a pharmaceutical preparation and GSH in a human plasma sample.

Femtomolar determination of an ovarian cancer biomarker (miR-200a) in blood plasma using a label free electrochemical biosensor based on L-cysteine functionalized ZnS quantum dots.

In the present study, a label-free electrochemical genosensor was designed based on ZnS quantum dots functionalized with l-cysteine (Cys-ZnS-QDs) to detect miR-200a, as a special ovarian cancer biomarker. The Cys-ZnS-QD genosensor was characterized by transmission electron microscopy (TEM), UV-Vis absorption and fluorescence methods. Cys-ZnS-QDs are electrodeposited on the glassy carbon electrode surface and act as a suitable substrate for immobilization of the DNA probe. The effective parameters in the preparation of the genosensor are optimized to improve its analytical performance. The analytical performance of the genosensor has been investigated using electrochemical impedance spectroscopy. Under optimal conditions, the linear range and the detection limit of miR-200a were found to be 1.0 × 10-14 to 1.0 × 10-6 M and 8.4 fM. In addition, the genosensor is used to detect the target complementary miRNA strand from a single-base mismatch miRNA strand. Finally, this label-free electrochemical biosensor was used to detect miR-200a in human plasma without using any amplification method.

Decoration of titanium dioxide nanotubes with silver nanoparticles using the photochemical deposition method and their application as an electrocatalyst to determine tinidazole.

In this study, titanium nanotube electrodes were decorated with silver nanoparticles (AgNPs/TiO2NTs) and used as an electrocatalyst for the reduction of tinidazole. AgNPs/TiO2NTs are constructed by anodization of titanium sheet metal and photochemical deposition of AgNPs on TiO2NTs. The structural and elemental analysis characteristics of the AgNPs/TiO2NT electrode have been studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) methods. Based on the cyclic voltammetric data, it has been confirmed that the AgNPs/TiO2NT electrode has good electrocatalytic activity to reduce tinidazole. Two liner concentration ranges of 0.2-55.0 µM and 55.0-111.2 µM were obtained by amperometric method. A detection limit of 60.9 nM was obtained for measuring tinidazole at the AgNPs/TiO2NT electrode surface. In addition, the designed sensor has been successfully used for quantitative measurement of tinidazole in pharmaceutical and human urine samples.

Measurement of aflatoxin M1 in powder and pasteurized milk samples by using a label-free electrochemical aptasensor based on platinum nanoparticles loaded on Fe-based metal-organic frameworks.

In the present study, a sensitive label-free electrochemical aptasensor is introduced to measure aflatoxin M1 (AFM1) by using platinum nanoparticles (PtNPs) decorated on a glassy carbon electrode (GCE) modified with Fe-based metal-organic frameworks, MIL-101(Fe). The MIL-101(Fe) and the PtNP/MIL-101(Fe) are synthesized and characterized by Fourier transform infrared spectroscopy, X-ray diffraction, UV-Visible spectroscopy, and field-emission scanning electron microscopy. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) are done to monitor the fabrication processes of the aptasensor. In optimum conditions, the linear calibration range of 1.0 × 10-2 to 80.0 ng mL-1 and the detection limit of 2.0 × 10-3 ng mL-1 are obtained to measure AFM1 concentration using the EIS method. Finally, the fabricated aptasensor is successfully applied to measure AFM1 concentration in powder and pasteurized milk samples.

Experimental and theoretical studies of redox reactions of o-chloranil in aqueous solution.

The electrochemical reduction of o-chloranil (OCA) in aqueous solution has been studied experimentally and theoretically. The effects of temperature and pH on various thermodynamic parameters were studied by means of cyclic voltammetry. The pH dependence of the redox activity of OCA in aqueous solution at temperatures in the range 25-40 degrees C was used for the experimental determination of the standard reduction potentials of both the one-proton-two-electron (0.67 V) and two-proton-two-electron (0.79 V) reduction processes. The temperature dependency of the equilibrium constants of studied reactions has been used in order to determine enthalpy, entropy, and Gibbs energy changes of the reactions. It is found that the two studied redox reactions of OCA are strongly affected by solvation effects and controlled by entropy contributions to the Gibbs free energies. High-level ab initio calculations (G3 and CBS-QB3 using the CPCM solvation model) have been employed to calculate the reduction potentials of OCA in aqueous solution for the one-proton-two-electron (0.65 and 0.69 V at the respective levels of theory) and two-proton-two-electron (0.81 and 0.83 V) reactions, which are in excellent agreement with the corresponding experimental values. The acidic strength of the reduced form of OCA in aqueous solution, pK(a), has been also calculated (5.2) and is also in excellent agreement with the experimental value (5.0). The agreement mutually verifies the accuracy of experimental method and the validity of the mathematical model.

Modeling of an electrochemical nanobiosensor in COMSOL Multiphysics to determine phenol in the presence of horseradish peroxidase enzyme.

Horseradish peroxidase enzyme selectively oxidizes phenol to o-quinone that can be reduced electrochemically to catechol and generating a current response which is directly proportional to phenol concentration. In order to investigate the o-quinone enzymatic production and its electrochemical behavior, a 2-D model was developed for a nanochip biosensor in COMSOL Multiphysics. The oxidation rate of phenol to o-quinone was predicted by the developed model based on Michaelis-Menten equation. The diffusion coefficient of o-quinone was obtained 2.17 × 10-6 cm2 s-1 based on experimental chronoamperograms. The cathodic and anodic peak potentials for o-quinone/catechol redox couple are obtained experimentally 255 and 310 mV, respectively. The obtained results from simulation were compared with the experimental results to verify the validity of the model. By comparing the cyclic voltammograms from the simulation and experimental results, the heterogeneous rate constant, k°, and the transfer coefficient, α, were calculated 0.02 cm s-1 and 0.5, respectively. Then, using simulation results, chronoamperograms were drawn for the nanochip biosensors with different heights. Also, o-quinone concentration gradients were determined at the electrode surface, which can be used to estimate the thickness of the diffusion layer. Finally, a calibration plot was obtained based on the simulation results of the proposed nanochip as phenol biosensor with the following equation I (nA) = 0.1497 C (µM)-0.3521 and a linear range of 20.0-150.0 µM.

Highly selective sensing and measurement of microRNA-541 based on its sequence-specific digestion by the restriction enzyme Hinf1.

In this study, a genosensor is introduced to detect microRNA-541 through an enzymatic digestion method and using a restriction enzyme (RE). Hinf1 is a type of RE which can cut the double helix DNA at specific sequences. The hybridization event and the corresponding enzymatic reactions are studied through guanine signal tracing on a pencil graphite electrode modified with graphene quantum dots (GQDs/PGE). The stages of fabricating the electrode are monitored by atomic force microscopy, and its electrochemical behavior is studied by cyclic voltammetry. The results indicate that the guanine current response of a 25-mer oligonucleotide of 7-guanine immobilized on the electrode surface decreases after hybridization despite an increase in the number of the guanine bases. Also, after enzyme treatment, the current decreases further due to the separation of a number of guanine bases from ds-DNA. A comparison of the analytical parameters of the proposed method with those of the conventional guanine oxidation method indicates that the linear concentration range in the proposed method, i.e. 1.0 fM to 1.0 nM, is lower than that in the conventional method, i.e. 10.0 pM-1.0 µM. On the basis of these findings, it is concluded that the use of Hinf1 enzyme makes it possible to measure microRNA at a femtomolar level. The selectivity of the designed biosensor has been proved using a non-complementary sequence with a one-base mismatch in the recognition site, rather than a complementary sequence. Finally, the proposed genosensor can be satisfactorily applied to measure microRNA-541 in human plasma samples.

Deciphering the inhibition effect of thymoquinone on xanthine oxidase activity using differential pulse voltammetry in combination with theoretical studies.

Xanthine oxidase (XO) catalyzes the oxidation of xanthine to uric acid. Over-production of uric acid is a risk factor for hyperuricemia and other diseases. Although allopurinol decreases uric acid levels, it causes severe adverse effects. Therefore, more effort is needed in finding novel XO inhibitors with fewer side effects. In this study, differential pulse voltammetry was used to investigate the inhibitory effect of thymoquinone (TQ) on the XO activity while the major problem was the overlap of the obtained signals. Thus, Parallel Factor Analysis (PARAFAC) was applied to extract the useful information. Also, docking was used to investigate how TQ and the active site of XO fit together. PARAFAC results based on the voltammetry studies revealed that TQ blocks the catalytic centers of XO, which leads to a decrease in the electrochemical signal of Mo center in XO. The results also indicated the dose-dependent inhibition of XO with TQ. Molecular docking studies were shown TQ surrounds the active sites of XO and reduces the oxidation of xanthine to uric acid. Therefore, the electrochemical response of Mo decreases in the presence of TQ. This finding is in good agreement with the results obtained from molecular docking studies.

Determination of the absolute redox potential of Rutin: experimental and theoretical studies.

The conditional formal potential, E degrees', of Rutin has been studied by cyclic voltammetry using a Rutin film deposited at the multi-wall carbon nanotubes modified glassy carbon electrode (GCE) as the working electrode in different pH phosphate buffered solutions. The experimental standard redox potential, E degrees, of Rutin is obtained to be 0.88 V versus SHE (Standard Hydrogen Electrode). High-level ab initio calculations have been also performed on a chemical model of Rutin and the absolute reduction potential has been calculated. The theoretical standard reduction potential relative to SHE (0.83 V) is in relatively good agreement with experiment.

Sulfate-selective electrode based on a complex of copper.

A new sulfate-selective electrode based on the complex N,N'-bis(2-amino-1-oxo-phenelenyl)phenylenediamine copper(II) (CuL) as the membrane carrier was developed. The electrode exhibited a good Nernstian slope of -29.5 +/- 0.5 mV/decade and a linear range of 1.0 x 10(-7) - 1.0 x 10(-1) M for sulfate. The limit of detection was 1.0 x 10(-8) M. It has a fast response time of 10 s and can be used for more than three months. The selective coefficients were determined by the fixed interference method (FIM). The electrode could be used in the pH range 3.5 - 8.0. It was employed as an indicator electrode for direct determination of sulfate in pharmacy and cement samples.

Computational electrode potential of a coumestan derivative: theoretical and experimental studies.

Electrode potential of a coumestan derivative, an important biological molecule, in aqueous solution is computed theoretically using Self-Consistent Field (SCF) theory at the level of Hartree--Fock and employing 6-31G(d) basis set and also obtained experimentally by employing electrochemical technique of cyclic voltammetry (CV). Frequency calculations have been carried out and thermal corrections and entropies have been taken into account. Polarizable continuum model is used to describe the solvent. The theoretical and experimental values for the standard electrode potential of the studied coumestan are in excellent agreement with each other and there is only 0.001 V discrepancy between experiment and theory. The agreement mutually verifies the accuracy of experimental method and the validity of mathematical model.