Surface Of Hanging Mercury Drop Electrode Research Articles

Adsorption, 2D-condensation, and redox reactions of bile acids on the hanging mercury drop electrode

Bile acids (BAs) are natural surfactants thanks to the facial amphiphilic character of the steroid skeleton. For the first time, the electrochemical and adsorption behaviour of selected bile acids differing in the number of hydroxyl groups on the steroid skeleton – lithocholic acid (3α-OH), deoxycholic acid (3α- and 12α-OH), and cholic acid (3α-, 7α- and 12α-OH) – was investigated on hanging mercury drop electrode (HMDE) in mixed Britton – Robinson buffer: methanol (9:1) media. In cyclic voltammetry, all BAs yielded a negative signal at the potential of ca −1.4 V with intensity depending on pH of the solution. Further, sharp tensammetric signals confirming adsorption/desorption or re-organization processes of the adsorbed molecules were observed. AC voltammetric measurements revealed two-dimensional (2D) condensation of lithocholic acid in the pH range 12.0–7.0 visualized by the formation of a capacitance pit. Adsorption of the other studied BAs was indicated by the AC current density decrease. Measurements of differential capacitance dependences - potential (C-E curves) revealed this behaviour to be concentration and temperature dependent. A study on effect of halides (Cl−, Br−, I−) on the adsorption of bile acids showed involvement of primarily hydrophobic interactions in the BA interaction with the HMDE surface.

Interfacial behaviour of oligodeoxynucleotides prone to G-quadruplex formation on negatively charged electrode surface monitored by electrochemical probes

Development of analytical tools enabling discrimination amongst various structures of nucleic acids attracts interest due to diverse biological functions and prospective biotechnological applications of the latter. In this work, two electrochemically active compounds, copper porphyrin complex Cu-TMPy2PP and base-modified deoxyguanosine triphosphate conjugate bearing 3-nitrophenyl moiety (dGNPTP), were used as probes to study electrode potential effects on oligodeoxynucleotides (ODNs) of various sequences and abilities to form secondary structures (including unstructured single strands, duplexes, guanine quadruplexes, i-motifs) at the surface of hanging mercury drop electrode (HMDE). Our experiments revealed formation of less dense adsorbed layers by G-rich ODNs with the propensity to fold into G-quadruplexes (G4), compared to layers formed by other studied sequences. Strikingly, the G4-forming ODNs were significantly more susceptible to desorption from the HMDE surface during its polarization to highly negative potentials compared to other ODNs. Negative potential-induced desorption of the ODNs from HMDE was further confirmed by gel electrophoresis. Application of the electrochemical probes offers a useful method for further research of DNA conformations and transitions taking place in solution and at electrically charged surfaces.

Voltammetric studies of the interaction of genotoxic 2-nitrofluorene with DNA

The interaction of genotoxic environmental pollutant 2-nitrofluorene (2-NF) with double-stranded DNA has been studied using a hanging mercury drop electrode (HMDE) as an electrochemical sensor. Two types of DNA damage were investigated and electrochemically detected using cyclic voltammetry and differential pulse voltammetry: (i) DNA damage caused by the direct interaction with 2-NF and (ii) DNA damage caused by short-lived radicals generated by the electrochemical reduction of 2-NF. For the study of the direct interaction, the HMDE was modified by DNA and the interaction of DNA with 2-NF was studied after their mutual interaction right at the HMDE surface, or DNA was preincubated with 2-NF in solution and, subsequently, the interaction was studied voltammetrically. Using both detection techniques, the formation of DNA–2-NF complex was observed and the mutual interaction was interpreted as an intercalation between DNA base pairs. On the basis of obtained results, we suppose that expected formation of 8-oxoguanosine leads to guanosine–cytidine base pair interruption and DNA double-strand break formation. The binding constants (K) of the DNA–2-NF complex formed in solution and on the HMDE surface (DNA/HMDE) were determined from the changes in the voltammetric peaks of the studied analyte.

Label‐free Voltammetric Detection of Products of Terminal Deoxynucleotidyl Transferase Tailing Reaction

AbstractA label‐free approach that takes advantage of intrinsic electrochemical activity of nucleobases has been applied to study the products of terminal deoxynucleotidyl transferase (TdT) tailing reaction. DNA homooligonucleotides A30, C30 and T30 were used as primers for the tailing reaction to which a dNTP – or a mixture of dNTPs – and TdT were added to form the tails. Electrochemical detection enabled study of the tailing reaction products created by various combinations of primers and dNTPs, with pyrolytic graphite electrode (PGE) being suitable for remarkably precise analysis of the length of tailing reaction products. Furthermore, the hanging mercury drop electrode (HMDE) was able to reveal formation of various DNA structures, such as DNA hairpins and G‐quadruplexes, which influence the behavior of DNA molecules at the negatively charged surface of HMDE. Thus, the described approach proves to be an excellent tool for studying the TdT tailing reactions and for exploring how various DNA structures affect both the tailing reactions and electrochemical behavior of DNA oligonucleotides at electrode surfaces.

Voltammetry of Se(IV) Part I: Cyclic Voltammetric Study of Se(IV) on HMDE in HNO3

The electrochemical behavior of HSeO3− and H2SeO3 in HNO3 has been investigated on hanging mercury drop electrode (HMDE) by the use of cyclic voltammetry. Electro-reduction of HSeO3− is chemically irreversible and gives a shoulder at positive potential side of H2SeO3 peak at +200mV. When scan rate is increased, the peak of HSeO3− is separated from the peak of H2SeO3. The product of the first peak which appears at a more positive potential effect the reaction which may take place upon the electrode surface because this product changes the characteristic properties of it. The reduction product or products passivate HMDE surface. Therefore, formation of Se(0) is observed by chemical reaction between H2Se and Se(IV) at the passivated electrode surface. It is found that the number of reduction peaks depends on sweep rate in positive side of potential scale. The chemical reaction between Se and Hg is slower than that of the reaction between H2Se and Hg. It is also observed that HgxSey is formed at electrode surface and reduced at more positive potential than that of corresponding to the reduction of Se(IV) to H2Se. At a very low scan rate, a broad peak at around −950mV is observed for the reduction of Se(IV) to Se(0) at the electrode passivated by selenium compounds. Number of sweeps affects the passivity of HMDE without application of deposition potential for Se(IV).

Electrochemical behavior of 7-deazaguanine- and 7-deazaadenine-modified DNA at the hanging mercury drop electrode

DNA modification with synthetic analogs of natural nucleotides and/or their conjugates with external redox active groups is applied in the development of electrochemical DNA sensors or assay for DNA hybridization, SNP typing, DNA damage and so forth. 7-Deazapurines (Pu*) are analogs of natural purine bases in which N7 atom is replaced by CH group. The Pu* bases retain Watson–Crick base pairing of their parent purines (and the ability to form duplex DNA) but are incapable of Hoogsteen pairing (and thus cannot be involved in triplex or quadruples DNA structures). Previously, we studied electrochemical oxidation of Pu* residues in DNA fragments (prepared by PCR in the presence of Pu* deoxynucleoside triphosphates) at a carbon electrode and reported on significantly lower potentials of oxidation of both 7-deazaguanine (G*) and 7-deazaadenine (A*), compared to natural guanine (G) and adenine (A), respectively. In this work, we studied faradaic and tensammetric responses of G*- or A*-modified DNA on the hanging mercury drop electrode (HMDE). While A* was reduced at the HMDE, giving rise to a similar irreversible cathodic peak as the natural A, G* did not yield any peak analogous to the peak G due to guanine, in agreement with a loss of corresponding redox site in G*. Responses of DNA modified with A* were relatively similar to those of unmodified DNA (albeit we observed certain differences in tensammetric peak currents). Effects of G substitution by G* were more pronounced, being reflected in diminution of peak due to guanine, decrease of the peak CA (due to cytosine and adenine reduction) and in significantly changed shape of tensammetric DNA signals, indicating altered adsorption/desorption processes. While substitution of A by A* resulted in certain destabilization of the DNA duplex at the negatively charged HMDE surface (in qualitative agreement with significantly decreased melting temperature of the same DNA duplexes in solution), G*-modified duplex DNA displayed apparently lower susceptibility to surface denaturation.

Adsorption of cationic surfactants on covered hanging mercury drop electrode surface of variable area

Cetyldimethylbenzylammonium chloride (CDBACl) or cetyltrimethylammonium bromide (CTAB) is preadsorbed on mercury and used as substrate. The adsorptive stripping voltammetry with the two-step procedure is used. The mercury droplet with the preadsorbed surfactant is expanded in aqueous solutions of KCl, KBr, CTAB, CDBACl, or cetylethyldimethylammonium bromide (CEDAB). The surface area was increased from 0.0022cm2 up to 0.0571cm2. The surfactant molecules are maintained close to each other and in the vicinity of the electrode by the applied electric field. The expanding of the droplets resulted in a reorientation of the adsorbed molecules depending on the surfactant surface concentration. In some cases, condensed films were observed. Differences were noticed in the adsorption and desorption potential region. A linear increase in the capacitance current with the surface area was found in all cases up to a maximum increase in the surface area. Partly disorganized films were also observed. In some cases, defects were noticed during expansion. In one case, fractal structure was observed.

Electrochemical interfacial adsorption mechanism of polyphenolic molecules onto Hanging Mercury Drop Electrode surface (HMDE)

The electrochemical interfacial adsorption of a series of polyphenolic molecules (i.e. polyhydroxybenzoic acids) at the Hanging Mercury Drop Electrode (HMDE)–electrolyte interfaces were investigated using Square Wave-Adsorption Cathodic Stripping Voltammetry (SW-AdCSV) at pH 7.5. Polyhydroxybenzoic acids bearing one 4-hydroxybenzoic acid, two 3,4-dihydroxybenzoic acid (protocatechuic) or three 3,4,5-trihydroxybenzoic acid (gallic) OH-groups at positions C3, C4 and C5 on the benzene ring were studied. The complex interfacial electrochemical behaviour of these molecules has been deconvoluted to (i) adsorption evens and (ii) redox formations at the HMDE interface. The approach presented is based on the comparative analysis of the SW-AdCSV signals in conjunction with the molecular structure of the molecules. Accordingly, theoretical calculations, involving nonlinear-fit, resulted in the identification of the interfacial reaction mechanism of adsorbed gallic acid on the HMDE.

Protein‐based electrochemical biosensor for detection of silver(I) ions

Silver(I) ions are extremely toxic to aquatic animals. Hence, monitoring of these ions in the environment is needed. The aim of the present study was to suggest a simple biosensor for silver(I) ions detection. The suggested biosensor is based on the modification of a hanging mercury drop electrode (HMDE) by the heavy metal binding protein metallothionein (MT) for silver(I) ions detection. Metallothionein accumulated for 120 s onto the HMDE surface. After rinsing the electrode, the biosensor (MT modified HMDE) was prepared prior to detection of silver(I) ions. The biosensor was immersed in a solution containing silver(I) ions. These ions were bound to the MT structure. Furthermore, the electrode was rinsed and transferred to a pure supporting electrolyte solution, in which no interference was present. Under these experimental conditions, other signals relating to heavy metals naturally occurring in MT were not detected. This phenomenon confirms the strong affinity of silver(I) ions for MT. The suggested biosensor responded well to higher silver(I) ion concentrations. The relative standard deviation for measurements of concentrations higher than 50 microM was approximately 2% (n = 8). In the case of concentrations lower than 10 microM, the relative standard deviation increased to 10% (n = 8). The detection limit (3 signal/noise) for silver(I) ions was estimated as 500 nM.

Electrochemistry of riboflavin-binding protein and its interaction with riboflavin

Riboflavin-binding protein (RBP, a carrier of riboflavin) plays an essential role in embryo development. Electrochemical studies of the riboflavin–RBP interactions have been so far limited to changes in polarographic and voltammetric responses of riboflavin because of lack of methods capable to detect electrochemical changes in the RBP responses. Here we used constant current chronopotentiometric stripping analysis (CPSA) with the hanging mercury drop electrode (HMDE) and square wave voltammetry (SWV) with carbon paste electrode (CPE) to investigate RBP. We found that CPSA of RBP produces electrocatalytic peak H, capable to discriminate between apoprotein and holoprotein forms of RBP. This peak is suitable for studies of RBP–riboflavin interaction at nanomolar concentrations. We observed no sign of a release of riboflavin from holoprotein adsorbed at the HMDE surface. SWV at CPE required higher concentrations of RBP and displayed almost identical oxidation peaks of apoprotein and holoprotein.

Preconcentration of the lipid-lowering drug lovastatin at a hanging mercury drop electrode surface

Adsorption and reduction of lovastatin were investigated by cyclic and square-wave voltammetry on a hanging mercury drop electrode in aqueous solutions over a wide pH range (4–9). The electroreduction of lovastatin proceeds via a surface EC mechanism in the whole pH range investigated. Using adsorptive stripping voltammetry, the drug yielded a well-defined voltammetric response in Britton-Robinson buffer, pH 6 at −1.49 V which can be used to determine trace amount of lovastatin. The linear concentration range of application was 1.0 × 10−8–1.0 × 10−7 M by using an accumulation potential of −0.5 V and a 90 s pre-concentration time. The method has been successfully applied for the determination of lovastatin in a spiked human serum sample.

In situ CCD video and voltammetric studies on enhanced cathodic peak observed at a hanging mercury drop electrode during consecutive two one-electron redox reactions in aprotic solutions

We have extensively studied the enhancement of the second reduction peak current I p c 2 of a stepwise one-electron redox reaction at a hanging mercury drop electrode (HMDE) in aprotic solution on the basis of cyclic voltammetric and in situ CCD video measurements. The cyclic voltammograms of the redox reactions of molecular oxygen, 1,4-dinitrobenzene, benzoquinone, 2,3,5,6-tetramethyl benzoquinone, azobenzene, methylviologen dichloride and 9,10-diphenylanthracene (DPA) at HMDE and glassy carbon (GC) electrode in dimethyl sulfoxide solutions containing 0.1 M tetraethylammonium perchlorate (TEAP) were compared. Generally, the I p c 2 was found to be greater than the first reduction peak current I p c 1 (i.e., I p c 2 / I p c 1 > 1 ) at HMDE, while the opposite result (i.e., I p c 2 / I p c 1 < 1 ) was obtained at GC electrode. Changing the state (and shape) of the mercury electrode from the liquid (and spherical) (i.e., HMDE) to semi-solid (and planar) (e.g., mercury film-coated gold electrode), increasing the concentration of TEAP and the use of poly(vinylchloride) as a surfactant were found to effectively allow the value of I p c 2 / I p c 1 to be closer to 1, indicating that the so-called streaming phenomenon of HMDE results in I p c 2 / I p c 1 > 1 . From the CCD video of HMDE captured during the electrochromic redox reaction of DPA, it was clarified that the upward streaming of the HMDE surface causes a continuous supply of the redox species from the bulk of the solution to the electrode surface, resulting in the enhanced I p c 2 .

Creatinine sensor based on a molecularly imprinted polymer-modified hanging mercury drop electrode

Molecularly imprinted polymers (MIP) have been elucidated to work as artificial receptors. In our present study, a MIP was applied as a molecular recognition element to a chemical sensor. We have constructed a creatinine sensor based on a MIP layer selective for creatinine and its differential pulse, cathodic stripping voltammetric detection (DPCSV) on a hanging mercury drop electrode (HMDE). The creatinine sensor was fabricated by the drop coating of dimethylformamide (DMF) solution of a creatinine-imprinted polymer onto the surface of HMDE. The modified-HMDE, preanodised in neutral medium at +0.4 V versus Ag/AgCl for 120 s, exhibited a marked enhancement in DPCSV current in comparison to the less anodised (≤+0.3 V) HMDE. The creatinine was preconcentrated and instantaneously oxidised in MIP layer giving DPCSV response in the concentration range of 0.0025–84.0 μg mL −1 [detection limit (3 σ) 1.49 ng mL −1]. The sensor was found to be highly selective for creatinine without any response of interferents viz., NaCl, urea, creatine, glucose, phenylalanine, tyrosine, histidine and cytosine. The non-imprinted polymer-modified electrode did not show linear response to creatinine. The imprinting factor as high as 9.4 implies that the imprinted polymer exclusively acts as a recognition element of creatinine sensor. The proposed procedure can be used to determine creatinine in human blood serum without any preliminary treatment of the sample in an accurate, rapid and simple way.

A catalytic activity of a mercury electrode towards dioxygen reduction in room-temperature ionic liquids

Oxygen (O 2) reduction reaction (ORR) at a hanging mercury drop electrode (HMDE) and gold (Au) electrode in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4) was studied. Cyclic voltammogram (CV) with a couple of well-defined cathodic and anodic peaks corresponding to the redox reaction of the O 2/superoxide ion ( O 2 - ) couple was obtained at Au electrode. On the other hand, three cathodic peaks (at ca. −0.37, −0.53 (main peak) and −0.68 V) and one anodic peak (at ca. −0.32 V) were found on the CV for the ORR at HMDE. The main cathodic peak at HMDE was observed at more positive potential by ca. 0.23 V than that observed at Au electrode, and the ratios of the cathodic peak current and of the integrated amount of the cathodic current obtained at HMDE to those at Au electrode were found to be ca. 1.4 and 2, respectively. The obtained results suggest that a catalytic two-electron quasi-reversible ORR mainly takes place at HMDE in EMIBF 4 and that the adsorption of 1-ethyl-3-methylimidazolium ion on the surface of HMDE plays a crucial role in the ORR.

Current oscillatory phenomena based on electrogenerated superoxide ion at the HMDE in dimethylsulfoxide

A study on the current oscillation based on the electrogenerated superoxide ion ( O 2 - ) at a hanging mercury drop electrode (HMDE) in dimethylsulfoxide solution containing tetra- n-alkylammonium perchlorate as the electrolyte was carried out. Cyclic voltammetric, potential step chronoamperometric, normal and reverse pulse voltammetric techniques were employed in this study. A bare glassy carbon electrode, a liquid hemispherical mercury (Hg) drop-coated gold (Au) electrode and a Hg film-coated Au electrode were also used as working electrodes to understand the mechanism of the current oscillation. The experimental conditions were optimized for a simple, regular and reproducible current oscillation. The specific adsorption phenomenon of iodide ion and the adsorption of the alkyl chain of the supporting electrolyte on the HMDE surface were also considered to clarify the experimental factors governing the current oscillation phenomenon. It has been concluded that the oscillation is mainly due to the promoted oxidation (dissolution) of the HMDE itself by the adsorption of O 2 - , resulting in the formation–destruction of a passive film such as Hg 2 ( O 2 - ) 2 on the electrode surface. A probable mechanism for the observed current oscillation is discussed.

In situ color video observation of polarographic streaming phenomenon of an HMDE using electrochromic reaction

In the present study, visualization of a characteristic upward streaming phenomenon of hanging mercury drop electrode (HMDE) on the cathodic branch of electrocapillary curve was successfully accomplished. The images of HMDE serving as a cathode for the electrochromic reaction of the BTD (2,1,3-benzothiadiazole)/BTD − (a one-electron reduced form: a radical anion of BTD) couple in dimethylsulfoxide solution were captured by a home-assembled microscopic video camera combined with an electrochemical measurement system. The electrogenerated BTD − of pink color was found to be gradually accumulated around the neck of HMDE as the potential was scanned to the negative direction. This inhomogeneous accumulation of BTD − was considered to be due to the well-known upward streaming phenomenon of HMDE. Expansion and shrinking phenomena of Hg drop were also noticed during the reduction process and were ascribed to the gradual increase of surface tension along the surface of HMDE from the bottom to the neck of it. These direct and visual observations evidence the existence of upward streaming phenomenon of HMDE for the first time since the streaming phenomena have been reported in 1966.

In situ color video observation of polarographic streaming phenomenon of an HMDE using electrochromic reaction

In the present study, visualization of a characteristic upward streaming phenomenon of hanging mercury drop electrode (HMDE) on the cathodic branch of electrocapillary curve was successfully accomplished. The images of HMDE serving as a cathode for the electrochromic reaction of the BTD (2,1,3-benzothiadiazole)/BTD − (a one-electron reduced form: a radical anion of BTD) couple in dimethylsulfoxide solution were captured by a home-assembled microscopic video camera combined with an electrochemical measurement system. The electrogenerated BTD − of pink color was found to be gradually accumulated around the neck of HMDE as the potential was scanned to the negative direction. This inhomogeneous accumulation of BTD − was considered to be due to the well-known upward streaming phenomenon of HMDE. Expansion and shrinking phenomena of Hg drop were also noticed during the reduction process and were ascribed to the gradual increase of surface tension along the surface of HMDE from the bottom to the neck of it. These direct and visual observations evidence the existence of upward streaming phenomenon of HMDE for the first time since the streaming phenomena have been reported in 1966.

Determination of cefonicid in human urine by adsorptive square-wave stripping voltammetry

The adsorption behavior of cefonicid on the hanging mercury drop electrode (HMDE) has been examined using cyclic voltammetry and square-wave voltammetry techniques in Britton–Robinson (B–R) buffers in the pH range of 2.0–11.0. The effect of different parameters on the accumulation behavior of the adsorbed species has been evaluated. Sensitive measurements can be achieved after controlled adsorption on the surface of HMDE followed by square-wave voltammetry. Under optimal conditions, a detection limit of 4.0×10 −8 M and a linear calibration graph in the range 1.0×10 −7–1.0×10 −6 M were obtained. Direct simple determination of cefonicid in urine was established with no manipulation of urine sample other than dilution and subsequent adsorptive stripping voltammetric determination. The detection limit of the method was 1.0 μg ml −1 of cefonicid in urine.

Differential-pulse voltammetric determination of low μg l−1 cyanide levels using EDTA, Cu(II) and a hanging mercury drop electrode

The proposed method for cyanide determination at the ultratrace level by differential pulse voltammetry is based in the sensitivity enhancement obtained when both Cu(II) and EDTA are present in the background electrolyte. Comparison of the detection limits and linear dynamic ranges using the conventional borate (pH 9.75), and the proposed borate-EDTA–Cu(II) background electrolytes was carried out. Best results have been obtained with the addition of 0.5mmoll−1 EDTA and 0.02mmoll−1 of Cu(II), which allow a detection limit of 1.7μgl−1 CN− (65nmoll−1 — absolute detection limit 34ng) with a precision better than ±2% for a 40μgl−1 level. Calibration range extended from detection limit up to 100μgl−1. Cyclic voltammetry indicates that the measured cyanide peak is obtained when the electrogenerated CuCN adsorbed onto the hanging mercury drop electrode surface, is oxidised at positive going potential scan. The method has been successfully applied to various industrial waste waters such as metal-finishing waste waters, water/sand mixtures from cleaning processes of coke production, leachates from wastes obtained from electrolytic cells of aluminium production, and liquors from gold extraction industry. Results obtained by the proposed method showed good agreement with those obtained by the standard methods (ion-selective potentiometry and the spectrophotometric pyridine method).

Determination of secnidazole in urine by adsorptive stripping voltammetry.

Cyclic voltammetry was used to explore the adsorption behavior of secnidazole on a hanging mercury drop electrode (HMDE). The effects of various operational parameters on the accumulation behavior of the adsorbed species were tested. Thus, a sensitive stripping voltammetry procedure for the determination of secnidazole with an adsorptive accumulation on the surface of HMDE has been developed. Measurements were taken by differential-pulse voltammetry after determination of the optimum conditions. The linear concentration range was 1 x 10(-8)-1 x 10(-7) s when using a 120 s preconcentration at -0.1 V vs. Ag/AgCl in acetate buffer of pH 4.0. The detection limit of secnidazole was 5 x 10(-9) M. The precision, expressed by the coefficient of variation, was 2.5% (n = 10) at a concentration of 1 x 10(-7) m. The method was successfully applied to the analysis of secnidazole in urine.