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.

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.

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.

Simultaneous measurement of ochratoxin A and aflatoxin B1 using a duplexed-electrochemical aptasensor based on carbon nanodots decorated with gold nanoparticles and two redox probes hemin@HKUST-1 and ferrocene@HKUST-1

An electrochemical aptasensor was developed for the simultaneous determination of ochratoxin A (OTA) and aflatoxin B1 (AFB1). Two compounds hemin@HKUST-1 and ferrocene@HKUST-1 were synthesized by encapsulating hemin and ferrocene inside the MOF made by copper nodes and trimesic acid, known as HKUST-1 MOF, and then bind to complementary DNA sequences of OTA aptamer (cDNA1) and AFB1 aptamer (cDNA2), respectively. Then, the hemin@HKUST-1/cDNA1 and ferrocene@HKUST-1/cDNA2 bioconjugates were deposited on the glassy carbon electrode (GCE) modified with carbon nanodots decorated with gold nanoparticles (AuNPs-CNDs). In the absence of the two mycotoxins, the current response related to the electroactive labels of hemin and ferrocene in the discussed bioconjugates have been measured at two separate potentials simultaneously by the differential pulse voltammetry technique (DPV). Using the described aptasensor, it is possible to measure both OTA and AFB1 mycotoxins in the linear concentration range of 1.0 × 10-2 to 100.0 ng mL-1. Also, the detection limits of OTA and AFB1 were 4.3 × 10-3 ng mL-1 and 5.2 × 10-3 ng mL-1, respectively. Finally, the ability of the proposed duplex aptasensor for the simultaneous measurement of OTA and AFB1 in a corn flour sample was investigated and completely satisfactory results were achieved.

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.

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.

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.

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.

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.

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.

Structural effect of different carbon nanomaterials on the corrosion protection of Ni–W alloy coatings in saline media

The equivalent circuit models used for the fitting of the EIS data of the Ni–W alloy and Ni–W carbon nanomaterial nanocomposite coatings in a corrosive solution of 3.5% NaCl.

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.

Electrooxidation of homogentisic acid in aqueous and mixed solvent solutions: experimental and theoretical studies

Electrochemical behavior of homogentisic acid (HGA) has been studied in both aqueous and mixed solvent solution of water-acetonitrile. Physicochemical parameters of the electrochemical reaction of HGA in these solutions are obtained experimentally by cyclic voltammetry method and are also calculated theoretically using accurate ab initio calculations (G3MP2//B3LYP). Solvation energies are calculated using the available solvation model of CPCM. The pH dependence of the redox activity of HGA in aqueous and the mixture solutions at different temperatures was used for the experimental determination of the standard reduction potential and changes of entropy, enthalpy, and Gibbs free energy for the studied reaction. The experimental standard redox potential of the compound in aqueous solution was obtained to be 0.636 V versus the standard hydrogen electrode. There is a good agreement between the theoretical and experimental values (0.702 and 0.636 V) for the standard electrode potential of HGA. The changes of thermodynamic functions of solvation are also calculated from the differences between the solution-phase experimental values and the gas-phase theoretical values. Finally, using the value of solvation energy of HGA in water and acetonitrile solvents which calculated by the CPCM model of energy, we proposed an equation for calculating the standard redox potential of HGA in mixture solution of water and acetonitrile. A good agreement between the result of electrode potential calculated by the proposed equation and the experimental value confirms the validity of the theoretical models used here and the accuracy of experimental methods.

Thermodynamic parameters of electrochemical oxidation of L-DOPA: experimental and theoretical studies

Electrode potential and thermodynamic parameters of the electrochemical reaction of L-DOPA in aqueous solution are obtained experimentally by cyclic voltammetry method and also calculated theoretically using accurate ab initio calculations (G3MP2//B3LYP) along with the available solvation model of CPCM. The pH dependence of the redox activity of L-DOPA in aqueous solution at temperatures in the range of 10-30 °C was used for the experimental determination of the standard reduction potential, changes of entropy, enthalpy, and Gibbs free energy for the studied reaction. The experimental formal redox potential of the two-proton-two-electron reduction process was obtained to be 0.745 V versus standard hydrogen electrode (SHE). The theoretical and experimental values (0.728 and 0.745 V) for the standard electrode potential of L-DOPA are in agreement with each other. The difference between the peak potential of the L-DOPA and the products, which are produced by chemical reactions, has been measured experimentally and also calculated theoretically. There is also an agreement between experimental and theoretical potential difference. Also in this work, the changes of thermodynamic functions of solvation are calculated from the differences between the solution-phase experimental values and the gas-phase theoretical values.

Theoretical Study of the Oxidation Mechanism of Hematoxylin in Aqueous Solution

The oxidation of the two catechol rings A and B in the chemical structure of hematoxylin in aqueous solution has been studied theoretically in order to identify the mechanism of oxidation. In a recent experimental study, the oxidation mechanism of hematoxylin was designated an ErCiEr process in which an irreversible chemical reaction (Ci) followed the reversible chemical electrochemical oxidation (Er) of the catechol unit connected to the six-membered ring of the molecule (ring A). The theoretical results presented herein indicate that the electrochemical oxidation of ring B is actually slightly more favoured than ring A, although the potential separation is so small that they were unable to be distinguished in the experimental study. We therefore suggest that the most likely mechanism is ErErCiEr, in which two reversible electrochemical oxidation reactions (Er) occur preceding the irreversible chemical reaction (Ci), though we cannot rule out a contribution from ErCiEr. The calculated oxidation potential (0.719 V v. standard hydrogen electrode) is in close accord with the experimental value (0.759 V v. standard hydrogen electrode). The deprotonation of five hydroxyl groups of hematoxylin in aqueous solution is also studied and the order of acidic strength of these groups has been identified.

Electrocatalytic oxidation and differential pulse voltammetric determination of using a carbon nanotubes modified electrode

The electrocatalytic oxidation of hydroxylamine was studied at the surface of a carbon paste electrode (CPE) spiked with multi-wall carbon nanotubes (MWCNT) and 4-hydroxy-2-(triphenylphosphonio)phenolate (HTP). The modified electrode (HTP-MWCNT-CPE) exhibits an excellent electrochemical catalytic activity toward hydroxylamine oxidation. The results show that there is a dramatic enhancement on the anodic peak current of hydroxylamine oxidation at the HTP-MWCNT-CPE in comparison the value obtained at HTP-CPE and MWCNT-CPE. The kinetic parameters of the electron transfer coefficient, α, the heterogeneous electron transfer rate constant, k', and the exchange current density, j0, for oxidation of hydroxylamine at the HTP-MWCNT-CPE were determined using cyclic voltammetry. Differential pulse voltammetry exhibits three dynamic linear ranges of 2.0-10.0 μM, 10.0-1000.0 μM and 1000.0-8000.0 μM, and a lower detection limit of 0.16 μM for hydroxylamine. Finally, the modified electrode activity was studied for hydroxylamine determination in two water samples and satisfactory results were obtained.

Introduction of hematoxylin as an electroactive label for DNA biosensors and its employment in detection of target DNA sequence and single-base mismatch in human papilloma virus corresponding to oligonucleotide

For the detection of DNA hybridization, a new electrochemical biosensor was developed on the basis of the interaction of hematoxylin with 20-mer deoxyoligonucleotides (from human papilloma virus, HPV). The study was performed based on the interaction of hematoxylin with an alkanethiol DNA probe self-assembled gold electrode (ss-DNA/AuE) and its hybridization form (ds-DNA/AuE). The optimum conditions were found for the immobilization of HPV probe on the gold electrode (AuE) surface and its hybridization with the target DNA. Electrochemical detection of the self-assembled DNA and the hybridization process were performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) over the potential range where the accumulated hematoxylin at the modified electrode was electroactive. Observing a remarkable difference between the voltammetric signals of the hematoxylin obtained from different hybridization samples (non-complementary, mismatch and complementary DNAs), we confirmed the potential of the developed biosensor in detecting and discriminating the target complementary DNA from non-complementary and mismatch oligonucleotides. Under optimum conditions, the electrochemical signal had a linear relationship with the concentration of the target DNA ranging from 12.5 nM to 350.0 nM, and the detection limit was 3.8 nM.