DNA immobilization on a polypyrrole nanofiber modified electrode and its interaction with salicylic acid/aspirin

A double-stranded calf thymus DNA (dsDNA) was physisorbed onto a polypyrrole (PPy) nanofiber film that had been electrochemically deposited onto a Pt electrode. The surface morphology of the polymeric film was characterized using scanning electron microscopy (SEM). The electrochemical characteristics of the PPy film and the DNA deposited onto the PPy modified electrode were investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Then the interaction of DNA with salicylic acid (SA) and acetylsalicylic acid (ASA), or aspirin, was studied on the electrode surface with DPV. An increase in the DPV current was observed due to the oxidation of guanine, which decreased with the increasing concentrations of the ligands. The interactions of SA and ASA with the DNA follow the saturation isotherm behavior. The binding constants of these interactions were 1.15 × 10⁴ M for SA and 7.46 × 10⁵ M for ASA. The numbers of binding sites of SA and ASA on DNA were approximately 0.8 and 0.6, respectively. The linear dynamic ranges of the sensors were 0.1–2 μM (r² = 0.996) and 0.05–1 mM (r² = 0.996) with limits of detection of 8.62 × 10⁻¹ and 5.24 × 10⁻⁶ μM for SA and ASA, respectively.     

Ultrasensitive DNA hybridization biosensor based on polyaniline

Ultrasensitive DNA hybridization biosensor based on polyaniline (PANI) electrochemically deposited onto Pt disc electrode has been fabricated using biotin-avidin as indirect coupling agent to immobilize single-stranded 5'-biotin end-labeled polydeoxycytidine (BdC) probes and 5'-biotin end-labeled 35 base-long oligonucleotide probe (BdE) to detect complementary target, using both direct electrochemical oxidation of guanine and redox electroactive indicator methylene blue (MB), respectively. These polyaniline-based disc electrodes have been characterized using differential pulse voltammetry (DPV), Fourier transform infrared spectroscopy (FT-IR), impedance measurements and scanning electron microscopy (SEM) techniques, respectively. Compared to direct electrochemical oxidation of guanine, hybridization detection using MB results in the enhanced detection limit by about 100 times. These DNA immobilized PANI electrodes have hybridization response time of about 60 s.     

Ultrasensitive electrochemical immunosensor for HE4 based on rolling circle amplification

A novel electrochemical immunoassay system for the detection of human epididymis-specific protein 4 (HE4) was developed. A chitosan-titanium carbide (TiC) nanocomposition film was first electrodeposited onto a tin-doped indium oxide (ITO) electrode at a constant potential. Gold (Au) nanoparticles were then electrodeposited on the surface of the chitosan-TiC film by cyclic voltammetry (CV). The capture antibody (anti-HE4) was adsorbed onto the Au and TiC nanoparticles. After a specific sandwich immunoreaction among the capture antibody, HE4, and biotinylated secondary antibody, biotinylated primer DNA was immobilized on the secondary antibody by biotin-streptavidin system. Appropriate amounts of circular template DNA and biotinylated primer DNA were used for rolling circle amplification (RCA) under optimal conditions. The RCA products provided a large number of sites to link DNA detection probes. Doxorubicin hydrochloride intercalated the CG-GC steps between the RCA products and the DNA detection probes, which was monitored by differential pulse voltammetry (DPV) based on the current signal of doxorubicin hydrochloride. With the above-mentioned amplification factors, the current responded to HE4 linearly in the concentration range of 3-300 pM under optimal detection conditions, with a detection limit of 0.06 pM. Stepwise changes in the microscopic features of the surfaces and electrochemical properties upon the formation of each layer were confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS). This system was successfully employed for the detection of HE4 with good accuracy and renewable ability.     

Detection of protein-DNA interaction with a DNA probe: distinction between single-strand and double-strand DNA-protein interaction

A simple, direct method for the detection of DNA-protein interaction was developed with electrochemical methods. Single-stranded DNA (ss-DNA) probes were prepared through the chemical bonding of an oligonucleotide to a polymer film bearing carboxylic acid groups, and double-stranded DNA (ds-DNA) probes were prepared through hybridization of the complementary sequence DNA on the ss-DNA probe. Impedance spectroscopy and differential pulse voltammetry (DPV) distinguished the interaction between the DNA probes with mouse Purbeta (mPurbeta), an ss-DNA binding protein, and with Escherichia coli MutH, a ds-DNA binding protein. Impedance spectra obtained before and after the interaction of DNA probes with these proteins clearly showed the sequence-specific ss-DNA preference of mPurbeta and the sequence-specific ds-DNA preference of MutH. The concentration dependence of proteins on the response of the DNA probes was also investigated, and the detection limits of MutH and mPurbeta were 25 and 3 microg/ml, respectively. To confirm the impedance results, the variation of the current oxidation peak of adenine of the DNA probe was monitored with DPV. The formation constants of the complexes formed between the probe DNA and the proteins were estimated based on the DPV results.     

Studies on sildenafil citrate (Viagra) interaction with DNA using electrochemical DNA biosensor

The interaction of sildenafil citrate (Viagra) with DNA was studied by using an electrochemical DNA biosensor. The binding mechanism of sildenafil citrate was elucidated by using constant current potentiometry and differential pulse voltammetry at DNA-modified glassy carbon electrode. The decrease in the guanine oxidation peak area or peak current was used as an indicator for the interaction in 0.2M acetate buffer (pH 5). The binding constant (K) values obtained were 2.01+/-0.05 x 10(5) and 1.97+/-0.01 x 10(5)M(-1) with constant current potentiometry and differential pulse voltammetry, respectively. A linear dependence of the guanine peak area or peak current was observed within the range of 1-40 microM sildenafil citrate with slope=-2.74 x 10(-4)s/microM, r=0.989 and slope=-2.78 x 10(-3)microA/microM, r=0.995 by using constant current potentiometry and differential pulse voltammetry, respectively. Additionally, binding constant values for sildenafil citrate-DNA interaction were determined for the pH range of 4-8 and in biological fluids (serum and urine) at pH 5. The influence of sodium and calcium ions was also studied to elucidate the mechanism of sildenafil citrate-DNA interaction under different solution conditions. The present study may prove to be helpful in extending our understanding of the anticancer activity of sildenafil citrate from cellular to DNA level.     

Molecularly imprinted polyaniline film for ascorbic acid detection

Molecularly imprinted polyaniline (PANI) film (～ 100 nm thick) has been electrochemically fabricated onto indium-tin-oxide (ITO) coated glass plate using ascorbic acid (AA) as template molecule. Fourier transform infra-red spectroscopy, scanning electron microscopy, cyclic voltammetry and differential pulse voltammetry (DPV) studies indicate the presence of AA in PANI matrix, which also acts as a dopant for PANI. Further, the AA selective molecularly imprinted PANI electrode (AA-MI-PANI/ITO) has been developed via over-oxidation of AA doped PANI electrode which leads to the removal of AA moieties from PANI film. The response studies using DPV technique have revealed that this molecularly imprinted AA-MI-PANI/ITO electrode can detect AA in the range of 0.05-0.4 mM with detection limit of 0.018 mM and sensitivity of 1.2 × 10(-5) AmM(-1). Interestingly, this AA-MI-PANI/ITO electrode shows excellent reusability, selectivity and stability.     

Omp85 genosensor for detection of human brain bacterial meningitis

The 5′-thiolated DNA probe based on specific virulence gene, Omp85, was immobilized onto a screen-printed gold electrode followed by hybridization with 6–100 ng/6 μl (5.9 × 10⁵–9.3 × 10⁶ c.f.u.) of Neisseria meningitidis single stranded genomic DNA (ssG-DNA) for 10 min at 25 °C from the cerebrospinal fluid (CSF) of a meningitis patient. The Omp85 genosensor can detect as little as 6 ng ssG-DNA in 6 μl CSF of a human brain meningitis patient in 30 min including a response time of 1 min by cyclic voltammetry, differential pulse voltammetry (DPV) and electrochemical impedance. The sensitivity of the genosensor electrode was 2.6(μA/cm²)/ng using DPV with regression coefficient (R²) 0.954. The genosensor was characterized using Fourier transform infrared spectroscopy and atomic force microscopy. Omp85 genosensor was stable for 12 months at 4 °C with 12 % loss in DPV current.     

Electrochemical DNA biosensor for the study of ciprofloxacin-DNA interaction

The interaction of ciprofloxacin with DNA was studied by using an electrochemical DNA biosensor. The binding mechanism of ciprofloxacin was elucidated by using constant current potentiometry and differential pulse voltammetry at DNA-modified glassy carbon electrode. The decrease in the guanine oxidation peak area or peak current at +0.9 V was used as an indicator for the interaction mechanism in 0.2M acetate buffer (pH 5). The binding constant (K) values obtained were 1.33+/-0.02 x 10(4) and 1.32+/-0.08 x 10(4) M(-1) with constant current potentiometry and differential pulse voltammetry, respectively. A linear dependence of the guanine peak area or peak currents was observed in the range of 40-80 microM ciprofloxacin, with a detection limit of 24 microM with r=0.995 and 9 microM with r=0.999 by using constant current potentiometry and differential pulse voltammetry, respectively. Moreover, the influence of sodium and calcium ions was also studied to elucidate the mechanism of ciprofloxacin-DNA interaction at different solution conditions, and this proved to be helpful in understanding the ciprofloxacin-DNA interaction.     

Voltammetric investigation of DNA damage induced by nitrofurazone and short-lived nitro-radicals with the use of an electrochemical DNA biosensor

Electrochemical behavior of nitrofurazone (NFZ) was investigated with the use of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods. The pH-dependence of NFZ was studied at a glassy carbon electrode (GCE) in ethanol/Britton-Robinson buffer (30:70), and short-lived nitro-radicals were generated by the reduction of NFZ at high pHs (>7.0). In the presence of DNA, the DPV peak current of NFZ decreased and the peak potential shifted negatively, which indicated that there was an electrostatic interaction between NFZ and DNA. An electrochemical dsDNA/GCE biosensor was prepared to study the DNA damage produced in the presence NFZ; this process was followed with the use of the Co(phen)(3)(2+) electroactive probe. Also, the oxidation peaks of guanosine (750mV) and adenosine (980mV) indicated that DNA damage was related directly to the nitro-radicals. Experiments demonstrated that DNA damage occurred via two different steps while NFZ was metabolized and nitro-radicals were produced. Novel work with AFM on the NFZ/DNA interaction supported the suggestion that in vivo, the nitro-radicals were more cytotoxic than the NFZ molecules. A linear DPV calibration plot was obtained for NFZ analysis at a modified dsDNA/GCE (concentration range: 2.50×10(-6)-3.75×10(-5)molL(-1); limit of detection: 8.0×10(-7)molL(-1)), and NFZ was determined successfully in pharmaceutical samples.     

Synthesis and bioelectrochemical behavior of aromatic amines

Four aromatic amines 1-amino-4-phenoxybenzene (A₁), 4-(4-aminophenyloxy) biphenyl (A₂), 1-(4-aminophenoxy) naphthalene (A₃) and 2-(4-aminophenoxy) naphthalene (A₄) were synthesized and characterized by elemental, spectroscopic (FTIR, NMR), mass spectrometric and single crystal X-ray diffraction methods. The compounds crystallized in monoclinic crystal system with space group P2₁. Intermolecular hydrogen bonds were observed between the amine group and amine/ether acceptors of neighboring molecules. Electrochemical investigations were done using cyclic voltammetry (CV), square wave voltammetry (SWV) and differential pulse voltammetry (DPV). CV studies showed that oxidation of aromatic amines takes place at about 0.9 V (vs. Ag/AgCl) and the electron transfer (ET) process has irreversible nature. After first scan reactive intermediate were generated electrochemically and some other cathodic and anodic peaks also appeared in the succeeding scans. DPV study revealed that ET process is accompanied by one electron. DNA binding study of aromatic amines was performed by CV and UV–visible spectroscopy. These investigations revealed groove binding mode of interaction of aromatic amines with DNA.     

Detection of protein–DNA interaction with a DNA probe: distinction between single-strand and double-strand DNA–protein interaction

A simple, direct method for the detection of DNA–protein interaction was developed with electrochemical methods. Single-stranded DNA (ss-DNA) probes were prepared through the chemical bonding of an oligonucleotide to a polymer film bearing carboxylic acid groups, and double-stranded DNA (ds-DNA) probes were prepared through hybridization of the complementary sequence DNA on the ss-DNA probe. Impedance spectroscopy and differential pulse voltammetry (DPV) distinguished the interaction between the DNA probes with mouse Purβ (mPurβ), an ss-DNA binding protein, and with Escherichia coli MutH, a ds-DNA binding protein. Impedance spectra obtained before and after the interaction of DNA probes with these proteins clearly showed the sequence-specific ss-DNA preference of mPurβ and the sequence-specific ds-DNA preference of MutH. The concentration dependence of proteins on the response of the DNA probes was also investigated, and the detection limits of MutH and mPurβ were 25 and 3 μg/ml, respectively. To confirm the impedance results, the variation of the current oxidation peak of adenine of the DNA probe was monitored with DPV. The formation constants of the complexes formed between the probe DNA and the proteins were estimated based on the DPV results.     

Investigation of DNA damage treated with perfluorooctane sulfonate (PFOS) on ZrO2/DDAB active nano-order film

The interactions between DNA and small molecules with planar heterocyclic structure were indicated in previous researches. This study investigated the interactions between PFOS with linear chain structure and DNA. A new phenomenon of DNA damage due to PFOS using electrochemistry technique was proved. The data was obtained on a modified glassy carbon electrode, on which didodecyldimethylammonium bromide (DDAB), ZrO(2) and calf thymus DNA were immobilized layer-by-layer. Electrochemical response of DNA damage caused by PFOS was detected by differential pulse voltammetry (DPV) using methylene blue (MB) as electro-active indicator. The current of MB attenuated obviously after DNA/ZrO(2)/DDAB/GCE were incubated in PFOS. The shift of MB reduction peak potential indicates that PFOS is bound with DNA in groove probably by the first step of hydrophobic interaction and then the second step of intercalation into the base of DNA. X-ray photoelectron spectroscopic (XPS) was used to elucidate in detail the intercalation of PFOS into DNA and the formation of hydrogen bond between PFOS and DNA base. Electrochemical quartz crystal microbalance (EQCM) proved the formation of adducts of DNA and PFOS. Moreover, electrochemical impedance spectroscopy (EIS) indicates that the PFOS influence DNA structure and attenuate DNA charge transport. These results demonstrate that PFOS intercalated into DNA do induce DNA base damage.     

Application of electrochemically prepared polypyrrole-polyvinyl sulphonate films to DNA biosensor

Double stranded calf thymus deoxyribonucleic acid (DNA) was physisorbed onto polypyrrole-polyvinyl sulphonate (PPY-PVS) films electrochemically deposited onto indium-tin-oxide (ITO) coated glass plates. These DNA immobilized PPY-PVS films optimized for various conditions, such as polymerization potential, pH of buffer, DNA concentration and scan rate were characterized using Fourier-transform infrared (FT-IR) spectroscopy, atomic force microscopy (AFM) and cyclic voltammetry (CV) techniques, respectively. The amperometric response studies of these DNA/PPY-PVS electrodes were carried out as a function of 2-aminoantharcene (2-AA, 0.01-20 ppm) and o-chlorophenol (OCP, 0.1-30 ppm) concentration, respectively at 25 degrees C. The observed amperometric current arising due to oxidation of guanine in the DNA/PPY-PVS films decreased linearly with the increase in the concentration of 2-AA and OCP. It has been revealed that 10 ppm of 2-AA is sufficient to reduce the observed guanine oxidation peak current by approximately -95+/-10% as compared to the reported values. A 25 ppm of OCP was capable enough to reduce the guanine oxidation current to zero. These DNA/PPY-PVS electrodes were found to have a shelf life of about 4 months when stored at 25 degrees C.     

Cadmium sulfide-modified GCE for direct signal-amplified sensing of DNA hybridization

A novel DNA hybridization sensor based on nanoparticle CdS modified glass carbon electrode (GCE) was constructed and characterized coupled with Cyclic Voltammogram (CV) and Differential Pulse Voltammogram (DPV) techniques. The mercapto group-linked probe DNA was covalently immobilized onto the CdS layer and exposed to oligonucleotide (ODN) target for hybridization. The structure of DNA sensor was characterized by X-ray diffraction (XRD), field-emission microscopy (FESEM) and X-ray photoelectron spectra (XPS). Sensitive electrical readout achieved by CV and DPV techniques shown that when the target DNA hybridized with probe CdS-ODN conjugates and the double helix formed on the modified electrode, a significant increased response was observed comparing with the bare electrodes. The selectivity of the sensor was tested using a series of matched and certain-point mismatched sequences with concentration grads ranging from 10(-6) microM to 10(1) microM. The signal was in good linear with the minus logarithm of target oligonucleotide concentration with detection limit <1 pM and the optimized target DNA concentration was 10(-6) microM for the signal amplification. Due to great surface properties, the additional negative charges and space resistance of as-prepared CdS nanoparticles, the sensor was able to robustly discriminate the DNA hybridization responses with good sensitivity and stability.     

Thermodynamic and kinetic analysis of the interaction between hepatitis B surface antibody and antigen on a gold electrode modified with cysteamine and colloidal gold via electrochemistry

Hepatitis B surface antibody (HBsAb) was immobilized to the surface of a gold electrode modified with cysteamine and colloidal gold as matrices to detect hepatitis B surface antigen (HBsAg). Differential pulse voltammetry (DPV) method was used for the investigation of the specific interaction between the immobilized HBsAb and HBsAg in solution, which was followed as a change of peak current in DPV with time. With the modified gold electrode, the differences in affinity of HBsAb with HBsAg at the temperatures of 37 and 40 °C were easily distinguished and the kinetic rate constants (k_ass and k_diss) and kinetic affinity constant K were determined from the curves of current versus time. In addition, the thermodynamic constants, ΔG, ΔH and ΔS, of the interaction at 37 °C were calculated, which were −56.65, −64.54 and −25.45 kJ mol⁻¹, respectively.     

Voltammetric behavior of complexation of salbutamol with calf thymus DNA and its analytical application

The interaction of salbutamol (Sal), an animal growth promoter, with DNA was investigated by differential pulse voltammetry (DPV), cyclic voltammetry (CV), and fluorescence spectroscopy. An irreversible reduction was observed from the cyclic voltammograms, and the reaction mechanism involved a one-electron change irreversible oxidation. In the presence of DNA, the DPV peak current decreased and the Sal peak shifted to higher potentials, indicating that Sal interacted with DNA to form an intercalation Sal–DNA complex. In addition, reaction binding parameters were extracted from the DPV data with the use of the multivariate curve resolution–alternating least squares (MCR–ALS) method; the binding constant and ratio were found to be (2.0 ± 0.5) × 10⁵ M⁻¹ and 1:1, respectively. Quantitative voltammetric analysis of Sal was performed in the concentration range of 3.02 × 10⁻⁶ to 1.23 × 10⁻⁴ mol L⁻¹, and it was found that the detection limit was 5.11 × 10⁻⁷ mol L⁻¹ in the presence of 1.00 × 10⁻⁶ mol L⁻¹ DNA. The method was applied for the determination of Sal in spiked urine and human serum samples, and the calibration was successfully verified.     

Hybridization signal enhancement via long-stranded probe for detection of DNA using aquadichloro (benzimidazole)-copper(II) as hybridization indicator

A simple and sensitive method for electrochemical detection of DNA was designed. This DNA sensor was based on a "sandwich" detection strategy, which involved a long capture probe DNA immobilized on glassy carbon electrodes that flanked both the reference DNA and target DNA. Electrochemical signals were measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using aquadichloro(benzimidazole)-copper(II), Cu(bzim)(H(2)O)Cl(2), as an electroactive indicator. An improving amount of Cu(bzim)(H(2)O)Cl(2) was interacted with the hybrid DNA via the incorporation of a long-probe DNA and a reference DNA in this sensor. As a result of this effect, this sensor design significantly enhanced the sensitivity. With 48-mer probe DNA and 27-mer reference DNA, the proposed method could be used for detection of 21-mer ssDNA ranging from 1.32 x 10(-7) to 2.52 x 10(-6) M with a detection limit of 2.94 x 10(-8) M. Electrochemical DNA biosensors were also developed using the same long-probe sequence as the target sequence with the novel hybridization indicator, Cu(bzim) (H(2)O)Cl(2). The detection limits for the complementary 21-mer target and 27-mer target were 9.52 x 10(-8) M and 5.81 x 10(-8) M, respectively. The results showed that the sensor with long-probe DNA and reference DNA is far more sensitive than that with nonswitch assay.     

Voltammetric studies of the interaction of transition-metal complexes with DNA

The interaction of the two new synthesized transition-metal complexes, ML(2) (M=Co, Cu, L=1,8-dihydroxyethyl-1, 3,8,10,13-hexa-azacyclotetradecane) with calf thymus DNA was probed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Adding deoxyribonucleic acid (DNA) into [CoL](2+) and [CuL](2+) solution, the i(p) value of all the peaks of [CoL](2+) and [CuL](2+) significantly decreased in proportion to concentration of DNA. Glassy carbon electrodes (GCEs) were modified with DNA by adsorption, and it was electrochemically characterized with transition-metal complexes, [ML](2+). The DNA modification layer on the GCE is unstable to alkali and to heat, but stable to acid solutions and very stable in long stock in a dry state. It could be seen that peak potential shifted positively and the peak current increased significantly. The electrochemical parameters, binding constant (k(n+)) and binding sites(s) were calculated by a nonlinear regression method.     

Electrochemical behaviors of diethylstilbestrol and its application in pharmacokinetics

The electrochemical behaviors of diethylstilbestrol (DES) at a glassy carbon electrode were investigated by cyclic voltammetry, linear sweep voltammetry, and differential pulse voltammetry (DPV). Based on these, a sensitive and selective DPV method was developed for determination of DES. The linear response range of DES is 1.0 x 10(-4)-2.0 x 10(-6) mol/L and the detection limit was 8.0 x 10(-8) mol/L. The developed method has been used for the pharmacokinetics of DES in rabbit blood plasma.