Carbon Nanotubes Modified Graphite Electrodes for Monitoring of Biointeraction Between 6‐Thioguanine and DNA

In this present study, single‐walled carbon nanotubes (SWCNT) modified disposable pencil graphite electrodes (SWCNT‐PGEs) were developed for the electrochemical monitoring of anticancer drug, and its interaction with double stranded DNA (dsDNA). Under this aim, SWCNT‐PGEs were applied for the first time in the literature to analyse of 6‐Thioguanine (6‐TG), and also to investigate its interaction with DNA by voltammetric and impedimetric methods. The surface morphologies of PGE and SWCNT‐PGE were explored using scanning electron microscopy (SEM) and electrochemical characterization of unmodified/modified electrodes was performed by cyclic voltammetry (CV). Experimental parameters; such as, the concentration of 6‐TG and its interaction time with dsDNA were optimized by using differential pulse voltammetry (DPV). Additionally, the interaction of 6‐TG with dsDNA was studied in case of different interaction times by electrochemical impedance spectroscopy (EIS) in contrast to voltammetric results. The detection limit of 6‐TG was found to be 0.25 μM by SWCNT‐PGE.     

Graphene Oxide Modified Chemically Activated Graphite Electrodes for Detection of microRNA

In our study, graphene oxide (GO) modified graphite electrodes were used for sensitive and selective impedimetric detection of miRNA. After chemical activation of pencil graphite electrode (PGE) surface using covalent agents (CA), GO modification was performed at the surface of chemically activated PGE. Then, CA‐GO‐PGEs were applied for impedimetric miRNA detection. The microscopic and electrochemical characterization of CA‐GO‐PGEs was performed by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The optimization of experimental conditions; such as GO concentration, DNA probe concentration and miRNA target concentration was performed by using EIS technique. After the hybridization occurred between miRNA‐34a RNA target and its complementary DNA probe, the hybrid was immobilized onto the surface of CA‐GO‐PGEs. Then, the impedimetric detection of miRNA‐DNA hybridization was performed by EIS. The selectivity of our assay was also tested under the optimum experimental conditions.     

Direct DNA Hybridization on the Single‐Walled Carbon Nanotubes Modified Sensors Detected by Voltammetry and Electrochemical Impedance Spectroscopy

Carboxylic acid functionalized single‐walled carbon nanotubes modified graphite sensors (SWCNT‐PGEs) were developed for electrochemical monitoring of direct DNA hybridization related to specific sequence of Hepatitis B virus, which substantially enhance the electrochemical transduction resulting from guanine oxidation signal comparison to bare PGEs. The performance characteristics of DNA hybridization on disposable CNT‐PGE were explored measuring the guanine signal in terms of optimum analytical conditions; probe and target concentration, hybridization time, and selectivity. The voltammetric results were also complemented with electrochemical impedance spectroscopy (EIS), that was used to characterize the successful construction of carbon nanotubes modification onto the surface of PGEs.     

Electrochemical Analysis of Acrylamide Using Screen‐Printed Carboxylated Single‐Walled Carbon Nanotube Electrodes

Acrylamide (AA) was electrochemically detected and quantified by means of its voltammetric response on carboxylic modified Single‐Walled Carbon Nanotube Screen Printed Electrodes (COOH‐SWCNT‐SPEs). The electroreduction signal of AA was proportional to AA concentration at low values (below 300 µM) and the observed sensitivity was explained in terms of AA adsorption on the COOH‐SWCNT‐SPEs that was demonstrated using the electrochemical response of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? and Raman spectroscopy experiments. In order to test the suggested analytic approach (LOD of 0.03 µM, LOQ of 0.04 µM), detection and quantification of AA in fried potatoes was carried out using the proposed electrochemical method and HPLC. Both techniques showed similar contents of AA.     

Stable and Sensitive Amperometric Determination of Endocrine Disruptor Bisphenol A at Residual Metal Impurities Within SWCNT

A simple and attentive method was attempted for the determination of endocrine disruptor molecule, Bisphenol A (BPA) using residual metal impurity act as a reactant present in as received SWCNT. The electrochemical behavior of BPA oxidation and its reaction mechanism was investigated by cyclic voltammetry using “as received SWCNT modified on glassy carbon electrode” (SWCNT/GCE) and the obtained results were compared with bare GCE. The SWCNT/GCE showed high electrocatalytic activity, sensitivity, stability and more importantly not much surface fouling compared with bare GCE. This is because of the formation of electroactive quinone and catechol as byproducts on SWCNT/GCE during electro‐oxidation of BPA. More interestingly, the electrochemically acid treated SWCNT/GCE not showed any characteristic oxidation and reduction peaks during electro‐oxidation of BPA which indicates that the presence of a residual metal impurity in SWCNT plays a vital role in electro‐oxidation of BPA. The amperometric detection of BPA oxidation on SWCNT/GCE showed excellent stability and good linear response from the wide range of concentration of 10–100 μM. The limit of detection and sensitivity of BPA electro‐oxidation on SWCNT/GCE is to be 7.3 μM (S/N=3) and 0.6494 μA/μM cm² respectively. Finally, the fabricated sensor using SWCNT/GCE was successfully applied for the detection of BPA in plastic water bottles with excellent recovery range from 98–102 %.     

Genomagnetic electrochemical assays of DNA hybridization

An electrochemical genomagnetic hybridization assay has been developed to take advantage of a new and efficient magnetic separation/mixing process, the amplification feature of enzyme labels, and single-use thick-film carbon transducers operated in the pulse-voltammetric mode. It represents the first example of coupling a magnetic isolation with electrochemical detection of DNA hybridization. The new protocol employs an enzyme-linked sandwich solution hybridization, with a magnetic-particle labeled probe hybridizing to a biotinylated DNA target that captures a streptavidin-alkaline phosphatase (AP). The alpha-naphthol product of the enzymatic reaction is quantitated through its well-defined, low-potential (+0.1 V vs. Ag/AgCl) differential pulse-voltammetric peak at the disposable screen-printed electrode. The efficient magnetic isolation is particularly attractive for electrical detection of DNA hybridization which is commonly affected by the presence of non-hybridized nucleic acid adsorbates. The new biomagnetic processing combines such magnetic separation with a low-volume magnetic mixing, and allows simultaneous handling of 12 samples. The attractive bioanalytical behavior of the new enzyme-linked genomagnetic electrical assay is illustrated for the detection of DNA segments related to the breast-cancer BRCA1 gene.     

Electrochemical Detection of Activated Protein C Using an Aptasensor Based on PAMAM Dendrimer Modified Pencil Graphite Electrodes

Electrochemical aptasensing of APC was carried out using PAMAM dendrimer modified pencil graphite electrodes (PGEs) for the first time herein. Poly(amidoamine) dendrimer having 16 succinamic acid surface groups (generation 2, G2‐PS) modified PGEs were developed, and then were utilized for APC monitoring using differential pulse voltammetry, electrochemical impedance spectroscopy and cyclic voltammetry. The selectivity of single‐use aptasensor was tested against to other proteins; BSA and THR as well as to the affinity of APC binding to different DNA aptamer, or oligonucleotide. Voltammetric APC detection was also explored in a diluted fetal bovine serum resulting with a detection limit DL as 1.5 µg/mL.     

Interaction of Mitomycin C with DNA Immobilized onto Single‐walled Carbon Nanotube/Polymer Modified Pencil Graphite Electrode

The electrochemical investigation of the interaction between the anticancer drug mitomycin C (MC) and DNA was described using a single‐walled carbon nanotube (SWCNT)/poly(vinylferrocenium) (PVF⁺) modified pencil graphite electrode (PGE). The electrochemical oxidation signals of guanine were monitored before and after the interaction between MC and DNA by using differential pulse voltammetry. The effects of DNA and MC concentration and MC interaction time were examined based on the electrode response. Cyclic voltammetry and electrochemical impedance spectroscopy were used for the characterization of SWCNT/PVF⁺ modified and PVF⁺ modified PGEs. The detection limit corresponded to 625 ng/mL for MC using calf thymus double‐stranded DNA immobilized SWCNT/PVF⁺ modified PGE.     

Simultaneous Detection of Dopamine and Uric Acid on Indium Tin Oxides Modified with Cost‐effective Gas‐phase Synthesized Single Walled Carbon Nanotubes

Cost‐effective gas‐phase synthesized single walled carbon nanotubes (SWCNTs) were first employed for the surface modification of indium tin oxides (ITO) via electrostatic coating of poly‐l‐lysine (PLL). Compared with control substrates of bare ITO and SWCNT‐PLL‐slide glasses, SWCNT‐PLL‐ITO, with high catalytic properties associated with large surface areas, showed significant improvement of electro catalytic activity toward the oxidation of dopamine (DA) and uric acid (UA). The cyclic voltammetric (CV) peak separation of oxidation for both DA and UA was estimated to be ∼ 0.13 V, which renders them capable of simultaneously detecting DA and UA in a mixed solution. The differential pulse voltammetry (DPV) technique was further performed to obtain a linear detection range of DA from 1.0 μM to 100 μM, and a detection limit of 1.0 μM was confirmed in the mixed solution. Furthermore, the practical analytical ability of the method was finally confirmed by selective detection of both DA and UA in human urine samples without any preliminary treatment, implying the feasibility of applying bio‐sensors in a low‐cost manner.     

A Novel DNA Probe Based on Molecularly Imprinted Polymer Modified Electrode for the Electrochemical Monitoring of DNA

A DNA probe that was based on methylene blue (MB) imprinted polyvinyl pyridine polymer (MIP) modified carbon paste electrodes were developed for the first time for electrochemical monitoring of DNA. Probes were built up by adsorbing MB onto modified electrodes prior to DNA immobilization. It was shown that DNA strongly immobilizes on MIP modified electrodes when MB was adsorbed in advance of DNA immobilization. The performance of the MB imprinted polymer modified carbon paste electrodes (MIP‐CPE) to rebind the template molecule (MB) were compared to those of control polymer modified (non‐imprinted polymer NIP‐CPE) and bare (CPE) electrodes. Electrochemical signal resulting from the oxidation of guanine moiety of the immobilized probe DNA was high enough on the constructed platform, implicating that probes of this kind could be favorably used for DNA analysis. These probes exhibited high selectivity for its complementary DNA sequences (target). HBV‐DNA hybridization was studied to evaluate the selectivity of the probes for complementary, non‐complementary and mismatch sequences. The detection limit of the probe for the target DNA was 8.72 µg/mL (1.38 µM), which was better than those attained by some earlier DNA sensor studies.     

Electrochemical Determination of Pyrogallol at Conducting Poly(3,4‐ethylenedioxythiophene) Film‐Modified Screen‐Printed Carbon Electrodes

The electrochemical oxidation of pyrogallol at electrogenerated poly(3,4‐ethylenedioxythiophene) (PEDOT) film‐modified screen‐printed carbon electrodes (SPCE) was investigated. The voltammetric peak for the oxidation of pyrogallol in a pH 7 buffer solution at the modified electrode occurred at 0.13 V, much lower than the bare SPCE and preanodized SPCE. The experimental parameters, including electropolymerization conditions, solution pH values and applied potentials were optimized to improve the voltammetric responses. A linear calibration plot, based on flow‐injection amperometry, was obtained for 1–1000 µM pyrogallol, and a slope of 0.030 µA/µM was obtained. The detection limit (S/N=3) was 0.63 µM.     

Flow Injection Photoamperometric Investigation of Ascorbic Acid Using Methylene Blue Immobilized on Titanium Phosphate

Abstract

Methylene blue (MB) was incorporated into titanium phosphate (TiP) after pretreatment of TiP with the gas, n‐butyl amine. The dye is strongly retained and not easily leached from the layered host matrix. The adsorbed MB on TiP was used to prepare modified carbon paste electrodes (MCPE), which were studied voltammetrically and in amperometric flow injection (FI) mode for the electrocatalytic oxidation of ascorbic acid (AA). The electrochemical behavior of the immobilized dye was investigated with cyclic voltammetry, at a pH 7.0 phosphate buffer containing 0.5 M KCl, at different potential scan rates. The MB immobilized on the support underwent a quasi‐reversible electrochemical redox reaction. A homemade flow‐through electrochemical cell with a suitable transparent window for irradiation of the electrode surface was constructed and used for amperometric FI studies. The photoamperometric‐FI conditions were optimized for sensitivity and reproducibility at a flow rate of 1.5 mL/min, a transmission tubing length of 25 cm, a sample injection volume of 100 µL, and a constant applied potential of +100 mV vs. SCE. The calibration curve for AA was linear over the concentration range from 1.0×10^?6 to 2.5×10^?5 mol l^?1 for both amperometric and photoamperometric studies. But the slope of the photoelectrocatalytic FIA procedure was improved about 52% compared with those obtained without irradiation. The results obtained for AA determination in some pharmaceutical products are in good agreement with those obtained using the procedure involving the reaction between triiodide and AA.     

A low-cost,fast, disposable and sensitive biosensor study: flow injection analysis of glucose at poly-methylene blue-modified pencil graphite electrode

In this work, firstly methylene blue (MB) was electropolymerized onto pencil graphite electrode (PGE) surface for the electrocatalytic oxidation of NADH. Cyclic voltammograms show that oxidation potential of NADH at Poly-MB/PGE shifted to negative direction about 300 mV compared with bare PGE. These results indicate that Poly-MB/PGE exhibits a good electrocatalytic activity toward NADH oxidation. Then, a glucose biosensor study was performed based on the determination of enzymatically generated NADH by glucose dehydrogenase (GDH) which immobilized onto Poly-MB/PGE using glutaraldehyde cross-linking procedure. The biosensing of glucose in flow injection analysis (FIA) system was performed at GDH/Poly-MB/PGE for the first time. The electrocatalytic oxidation currents of enzymatically produced NADH obtained from FI amperometric current–time curves recorded at + 200 mV and in phosphate buffer solution at pH 7.0 containing 1.0 M KCl were linearly related to the concentration of glucose. Linear calibration plots are obtained in the concentration range from 0.01 to 1.0 mM. The limit of detection (LOD) was found to be 4.0 µM. A fast, sensitive, low-cost and disposable glucose biosensor was constructed in FIA system using GDH/Poly-MB/PGE; therefore, it might provide a new perspective for the fabrication of biosensor of other compounds such as glutamate, lactate and alcohol.     

Electroanalysis of Norepinephrine at Bare Gold Electrode Pure and Modified with Gold Nanoparticles and S‐Functionalized Self‐Assembled Layers in Aqueous Solution. Part II

Gold electrodes modified with S‐containing compounds and gold were used for determination of norepinephrine (NEP) in aqueous solution. A linear relationship between norepinephrine concentration and current response was obtained in the range of 0.1 µM to 600 µM with the detection limit ≤0.090 µM for the electrodes modified at 2D template and in the range of 0.1 µM to 700 µM with the detection limit ≤0.075 µM for the electrodes modified at 3D template. The results have shown that modified electrodes could clearly resolve the oxidation peaks of norepinephrine, ascorbic (AA) and uric acid (UA) with peak‐to‐peak separation enabling determination of NEP, AA and UA in the presence of each other.     

Development of a Carbon Paste Electrode Modified with Reduced Graphene Oxide and an Imidazole Derivative for Simultaneous Determination of Biological Species of N‐acetyl‐L‐cysteine,Uric Acid and Dopamine

In this work, the modified carbon paste electrode (CPE) with an imidazole derivative 2‐(2,3 dihydroxy phenyl) 4‐methyl benzimidazole (DHPMB) and reduced graphene oxide (RGO) was used as an electrochemical sensor for electrocatalytic oxidation of N‐acetyl‐L‐cysteine (NAC). The electrocatalytic oxidation of N‐acetyl‐L‐cysteine on the modified electrode surface was then investigated, indicating a reduction in oxidative over voltage and an intensive increase in the current of analyte. The scan rate potential, the percentages of DHPMB and RGO, and the pH solution were optimized. Under the optimum conditions, some parameters such as the electron transfer coefficient (α) between electrode and modifier, and the electron transfer rate constant) k_s) in a 0.1 M phosphate buffer solution (pH=7.0) were obtained by cyclic voltammetry method. The diffusion coefficient of species (D) 3.96×10^?5 cm² s^?1 was calculated by chronoamperometeric technique and the Tafel plot was used to calculate α (0.46) for N‐ acetyl‐L‐cysteine. Also, by using differential pulse voltammetric (DPV) technique, two linear dynamic ranges of 2–18 µM and 18–1000 µM with the detection limit of 61.0 nM for N‐acetyl‐L‐cysteine (NAC) were achieved. In the co‐existence system of N‐acetyl‐L‐cysteine (NAC), uric acid (UA) and dopamine (DA), the linear response ranges for NAC, UA, and DA are 6.0–400.0 µM, 5.0–50.0 µM and 2.0–20.0 µM, respectively and the detection limits based on (C=3s_b/m) are 0.067 µM, 0.246 µM and 0.136 µM, respectively. The obtained results indicated that DHPMB/RGO/CPE is applicable to separate NAC, uric acid (UA) and dopamine (DA) oxidative peaks, simultaneously. For analytic performance, the mentioned modified electrode was used for determination of NAC in the drug samples with acceptable results, and the simultaneous determination of NAC, UA and DA oxidative peaks was investigated in the serum solutions, too.     

Multi‐responses optimization of simultaneous adsorption of methylene blue and malachite green dyes in binary aqueous system onto Ni:FeO(OH)‐NWs‐AC using experimental design: derivative spectrophotometry method

In this research, response surface methodology (RSM) approach using Central Composite Design (CCD) coupled by derivative spectrophotometry method was applied to develop mathematical model and optimize process parameters for simultaneous adsorption of methylene blue (MB) and malachite green (MG) from aqueous solution using Ni:FeO(OH) ‐ NWs‐AC. The optimal conditions to adsorption of MB and MG in binary mixture solution from aqueous solution were found at pH 8.0, MB concentration 20 mg L^‐1, MG concentration 20 mg L^‐1, adsorbent dosage 0.033 g and contact time 40 min. At these conditions, high adsorption efficiency (99.39% and 100.0% for MB and MG, respectively) was achieved. Among experimental equilibrium, Langmuir isotherm model fitted well with maximum monolayer adsorption capacity of 28.6 and 29.8 mg g^‐1 for MB and MG, respectively. The adsorption kinetic data followed pseudo second‐order kinetics for MB and MG dyes.     

Surface‐Confined Electropolymerization of Methylene Blue on Gold Electrodes

We report on the electrochemical behaviour and electropolymerization of self‐assembled monolayers (SAMs) of methylene blue (MB) on gold electrodes. The SAMs of MB on gold electrodes were prepared by immersing the substrates into a solution of 1.0 mM MB in absolute ethanol for different times at room temperature. Cyclic voltammetry experiments exhibited that reductive desorption of MB monolayer takes place at three different potentials on polycrystalline gold electrodes, while reductive desorption of MB monolayer consists of only one peak on single crystal Au(111) substrates. Calculated charge densities for different immersion times indicated that optimal immersion time for self‐assembly of MB is 96 h. Electropolymerization of SAMs of MB on gold electrode was achieved by applying 0.95 V for 1 s in 0.1 M borate buffer solution (pH: 9.0). It was observed that poly(MB) monolayers are highly stable in acidic media. ATR‐FTIR and UV‐vis spectra exhibited differences between monomer and polymer monolayers, which are attributed to surface‐confined electropolymerization. STM image of poly(MB) monolayer on Au(111) substrate revealed a surface that is covered by well‐ordered, collateral nanowires with an average size of 3 nm.     

G-quadruplex functionalized nano mesoporous silica for assay of the DNA methyltransferase activity

The abnormal level of DNA methyltransferase (MTase) may cause the aberrant DNA methylation, which has been found being associated with a growing number of human diseases, so it is necessary to create a sensitive and selective method to detect DNA MTase activity. In this paper, a new type of DNA functionalized nano mesoporous silica (MSNs) was creatively introduced to the detection of DNA MTase activity with G-quadruplex as a lock for signal molecule to release. The method was carried out by designing a particular DNA which could fold into G-quadruplex and complement with probe DNA. Next, MSNs was prepared before blocking methylene blue (MB) by G-quadruplex. Probe DNA was then fixed on gold nanoparticles modified glass carbon electrode, and the material was able to be transferred to the surface of electrode by DNA hybridization. After methylation of DNA MTase and the cutting of restriction endonuclease, the electrode was transferred to phosphate buffer solution (pH 9.0) for the releasing of MB. The response of differential pulse voltammetry was obtained from the release of MB. Consequently, the difference of signals with or without methylation could prove the assay of M. SssI MTase activity. The results showed that the responses from MB increased linearly with the increasing of the M. SssI MTase concentrations from 0.28 to 50 U mL⁻¹. The limit of detection was 0.28 U mL⁻¹. In addition, Zebularine, a nucleoside analog of cytidine, was utilized for studying the inhibition activity of M. SssI MTase.     

4‐Aminothiophenol Self‐Assembled Monolayer for the Development of a DNA Biosensor Aiming the Detection of Cylindrospermopsin Producing Cyanobacteria

The development of a DNA biosensor for the detection of cylindrospermopsin, based on self‐assembled monolayers (SAMs) of 4‐aminothiophenol, is investigated. SAMs were characterized by electrochemical reductive desorption. Detection of probe immobilization and hybridization has been achieved by cyclic and square‐wave voltammetry (SWV), using methylene blue (MB) as electroactive indicator. The SWV data obtained in phosphate buffer, with and without NaCl, after MB accumulation, revealed an increase of the redox indicator current peaks after the hybridization step. This behavior is consistent with MB intercalation into DNA, for high ionic strength media and attributed to electrostatic interactions in the absence of salt. Evidence for surface modification is also provided by atomic force microscopy and ellipsometry.     

DNA‐Directed Attachment of Carbon Nanotubes for Enhanced Label‐Free Electrochemical Detection of DNA Hybridization

Multi‐walled carbon nanotubes (MWNTs) were used as nanowires, which combined DNA molecules to a carbon paste electrode (CPE). The attachment of MWNT on the electrode surface was controlled by a hybridization assay between adenine and thymine containing oligonucleotides. The appearance of guanine oxidation signal after hybridization with target DNA greatly simplified the specific sequence DNA detection mechanism. Combination of sidewall‐ and end‐functionalization of MWNT provided a significant enhancement in the voltammetric signal of guanine oxidation in comparison with the signals obtained from only end‐oxidized MWNT modified CPE and a bare CPE. A control experiment involving adenine containing polynucleotide (poly(A)) instead of adenine probe modified MWNT was performed. The effect of target and noncomplementary DNA concentration on the guanine signal was also monitored. Discrimination against single‐base mismatch and noncomplementary DNA was achieved by surfactant containing washing solution. The promising conductivity of carbon nanotubes, and the creation of a larger surface area for DNA immobilization by sidewall‐ and end‐oxidation of MWNT provided a detection limit down to 10 pg/mL, which is compatible with the demand of the genetic tests.