Anodic stripping voltammetry at in situ bismuth-plated carbon and gold microdisc electrodes in variable electrolyte content unstirred solutions

Carbon and gold microdisc electrodes (30 and 10 μm, respectively) have been tested as substrates for in situ bismuth film plating from unstirred solutions of variable acetate buffer content and were subsequently used in the anodic stripping voltammetry determination of Pb(II) and Cd(II) ions. The effects of Bi(III) concentration, analyte accumulation time, stirring as well as supporting electrolyte content have been studied. Under optimal conditions good voltammetric responses were obtained by means of square wave anodic stripping voltammetry in unstirred analyte solutions of 5 × 10⁻⁸ to 10⁻⁶ M, even in the absence of added buffer. In an indicative application, Pb(II) ion levels were determined in tap water using bismuth-plated carbon microdisc electrodes.     

Sensitive and stable monitoring of lead and cadmium in seawater using screen-printed electrode and electrochemical stripping analysis

Sensitive and stable monitoring of heavy metals in seawater using screen-printed electrodes (SPE) is presented. The analytical performance of SPE coupled with square wave anodic stripping voltammetry (SWASV) for the simultaneous determination of Pb and Cd in seawater samples, in the low μg L⁻¹ range, is evaluated. The stripping response for the heavy metals following 2 min deposition was linear over the concentration range examined (10-2000 μg L⁻¹) with detection limits of 1.8 and 2.9 μg L⁻¹ for Pb and Cd, respectively. The accuracy of the method was validated by analyzing metal contents in different spiked seawater samples and comparing these results to those obtained with the well-established anodic stripping voltammetry using the hanging mercury drop electrode. Moreover, a certified reference material was also used and the results obtained were satisfactory.     

Determination of trace heavy metals in herbs by sequential injection analysis-anodic stripping voltammetry using screen-printed carbon nanotubes electrodes

A method for the simultaneous determination of Pb(II), Cd(II), and Zn(II) at low μg L⁻¹ concentration levels by sequential injection analysis-anodic stripping voltammetry (SIA-ASV) using screen-printed carbon nanotubes electrodes (SPCNTE) was developed. A bismuth film was prepared by in situ plating of bismuth on the screen-printed carbon nanotubes electrode. Operational parameters such as ratio of carbon nanotubes to carbon ink, bismuth concentration, deposition time and flow rate during preconcentration step were optimized. Under the optimal conditions, the linear ranges were found to be 2-100 μg L⁻¹ for Pb(II) and Cd(II), and 12-100 μg L⁻¹ for Zn(II). The limits of detection (S_bl/S = 3) were 0.2 μg L⁻¹ for Pb(II), 0.8 μg L⁻¹ for Cd(II) and 11 μg L⁻¹ for Zn(II). The measurement frequency was found to be 10-15 stripping cycle h⁻¹. The present method offers high sensitivity and high throughput for on-line monitoring of trace heavy metals. The practical utility of our method was also demonstrated with the determination of Pb(II), Cd(II), and Zn(II) by spiking procedure in herb samples. Our methodology produced results that were correlated with ICP-AES data. Therefore, we propose a method that can be used for the automatic and sensitive evaluation of heavy metals contaminated in herb items.     

Disposable screen-printed sensors modified with bismuth precursor compounds for the rapid voltammetric screening of trace Pb(II) and Cd(II)

In this article, a study of novel screen-printed electrodes bulk-modified with five potential bismuth precursor compounds (bismuth citrate, bismuth titanate, bismuth oxide, bismuth aluminate and bismuth zirconate) is presented for the determination of Cd(II) and Pb(II) by anodic stripping voltammetry. During the electrolytic deposition step, the precursor was reduced and served as the source of bismuth. Different key parameters were investigated in detail such as the nature of the bismuth precursor compound, the precursor content in the carbon ink, the polarisation range of the sensors, the supporting electrolyte, the stripping waveform, the deposition time, the deposition potential and the long-term stability of the sensors under continuous use. Using bismuth citrate as the precursor, the limit of detection was 0.9 μg L⁻¹ for Pb(II) and 1.1 μg L⁻¹ for Cd(II). The reproducibility on the same sensor (expressed as % relative standard deviation, (n = 8)) was 5.4% for Pb(II) and 7.2% for Cd(II) at the 20 μg L⁻¹ level. Finally, the sensors were applied to the determination of Cd(II) and Pb(II) in water samples.     

Disposable Nafion-modified micro-fabricated bismuth-film sensors for voltammetric stripping analysis of trace metals in the presence of surfactants

This work is a study of the analytical utility of Nafion-modified microfabricated bismuth film electrodes (BiFEs) for the determination of Pb(II) and Cd(II) by anodic stripping voltammetry (ASV) in the presence of surfactants. Micro-fabricated BiFEs were prepared by depositing a thin film of bismuth on the surface of a silicon substrate by sputtering while the two-dimensional geometry of the final sensors was defined by photolithography. The BiFEs were further drop-coated with a Nafion film. These devices were applied to the determination of Pb(II) and Cd(II) by square wave ASV (SWASV) in the presence of Triton X-100 (a non-ionic surfactant), cetyltrimethylammonium bromide (CTAB) (a cationic surfactant) and sodium dodecyl sulphate (SDS) (an anionic surfactant). It was found that the presence of Nafion afforded an increase in sensitivity and the tolerance against surfactants but these properties were severely influenced by both the thickness of the Nafion film and the nature of the interfering surfactant. Using a Nafion of 0.4 μm thickness and 120 s of preconcentration, the repeatability (expressed as the % relative standard deviation on the same sensor (n = 8)) at the 20 μg l⁻¹ level was 3.8% for Pb(II) and 3.1% for Cd(II) and the limits of detection were 0.5 μg l⁻¹ for Cd(II) and Pb(II). The sensors were applied to Cd(II) and Pb(II) determination in a certified lake-water sample.     

Simultaneous trace-levels determination of Hg(II) and Pb(II) ions in various samples using a modified carbon paste electrode based on multi-walled carbon nanotubes and a new synthesized Schiff base

A modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTs) and 3-(4-methoxybenzylideneamino)-2-thioxothiazolodin-4-one as a new synthesized Schiff base was constructed for the simultaneous determination of trace amounts of Hg(II) and Pb(II) by square wave anodic stripping voltammetry. The modified electrode showed an excellent selectivity and stability for Hg(II) and Pb(II) determinations and for accelerated electron transfer between the electrode and the analytes. The electrochemical properties and applications of the modified electrode were studied. Operational parameters such as pH, deposition potential and deposition time were optimized for the purpose of determination of traces of metal ions at pH 3.0. Under optimal conditions the limits of detection, based on three times the background noise, were 9.0 × 10⁻⁴ and 6.0 × 10⁻⁴ μmol L⁻¹ for Hg(II) and Pb(II) with a 90 s preconcentration, respectively. In addition, the modified electrode displayed a good reproducibility and selectivity, making it suitable for the simultaneous determination of Hg(II) and Pb(II) in real samples such as sea water, waste water, tobacco, marine and human teeth samples.     

Construction and performance characteristics of new ion selective electrodes based on carbon nanotubes for determination of meclofenoxate hydrochloride

This work offers construction and comparative evaluation the performance characteristics of conventional polymer (I), carbon paste (II) and carbon nanotubes chemically modified carbon paste ion selective electrodes (III) for meclofenoxate hydrochloride are described. These electrodes depend mainly on the incorporation of the ion pair of meclofenoxate hydrochloride with phosphomolybdic acid (PMA) or phosphotungestic acid (PTA). They showed near Nernestian responses over usable concentration range 1.0 × 10⁻⁵ to 1.0 × 10⁻² M with slopes in the range 55.15–59.74 mV (concentration decade)⁻¹. These developed electrodes were fully characterized in terms of their composition, response time, working concentration range, life span, usable pH and temperature range. The electrodes showed a very good selectivity for Meclo with respect to a large number of inorganic cations, sugars and in the presence of the degradation product of the drug (p-chloro phenoxy acetic acid). The standard additions method was applied to the determination of MecloCl in pure solution, pharmaceutical preparations and biological samples. Dissolution testing was also applied using the proposed sensors.     

Antimony film screen-printed carbon electrode for stripping analysis of Cd(II), Pb(II), and Cu(II) in natural samples

An in-situ antimony film screen-printed carbon electrode (in-situ SbSPCE) was successfully used for the determination of Cu(II) simultaneously with Cd(II) and Pb(II) ions, by means of differential pulse anodic stripping voltammetry (DPASV), in a certified reference groundwater sample with a very high reproducibility and good trueness. This electrode is proposed as a valuable alternative to in-situ bismuth film electrodes, since no competition between the electrodeposited copper and antimony for surface sites was noticed. In-situ SbSPCE was microscopically characterized and experimental parameters such as deposition potential, accumulation time and pH were optimized. The best voltammetric response for the simultaneous determination of Cd(II), Pb(II) and Cu(II) ions was achieved when deposition potential was −1.2 V, accumulation time 120 s and pH 4.5. The detection and quantification limits at levels of μg L⁻¹ suggest that the in-situ SbSPCE could be fully suitable for the determination of Cd(II), Pb(II) and Cu(II) ions in natural samples.     

A study of bismuth-film electrodes for the detection of trace metals by anodic stripping voltammetry and their application to the determination of Pb and Zn in tapwater and human hair

This work reports the simultaneous determination of Cd(II), Pb(II) and Zn(II) at the low μg l⁻¹ concentration levels by square wave anodic stripping voltammetry (SWASV) on a bismuth-film electrode (BFE) plated in situ. The metal ions and bismuth were simultaneously deposited by reduction at −1.4 V on a rotating glassy carbon disk electrode. Then, the preconcentrated metals were oxidised by scanning the potential of the electrode from −1.4 to 0 V using a square-wave waveform. The stripping current arising from the oxidation of each metal was related to the concentration of each metal in the sample. The parameters for the simultaneous determination of the three metals were investigated with the view to apply this type of voltammetric sensor to real samples containing low concentrations of metals. Using the selected conditions, the limits of detection were 0.2 μg l⁻¹ for Cd and for Pb and 0.7 μg l⁻¹ for Zn at a preconcentration time of 10 min. Finally, BFE's were successfully applied to the determination of Pb and Zn in tapwater and human hair and the results were in satisfactory statistical agreement with atomic absorption spectroscopy (AAS).     

Stripping voltammetric determination of silymarin in formulations and human blood utilizing bare and modified carbon paste electrodes

Silymarin is one of the most powerful natural substances that have the ability to protect and rebuild the liver cells damaged by alcohol and other toxic substances. Silymarin showed two irreversible anodic peaks in buffered solutions (pH 2.5-8.0) at either the bare carbon paste electrode or the montmorillonite-Ca modified carbon paste one. These two peaks have been attributed to oxidation of the two phenolic OH groups at positions C-20 and C-7 of silymarin molecule. A square-wave adsorptive anodic stripping voltammetry method was optimized for determination of silymarin utilizing the bare and the modified carbon paste electrodes. The method was fully validated and successfully applied for the determination of silymarin in commercial formulations and human serum without prior extraction utilizing both carbon paste electrodes. Limits of quantitation of 1 × 10⁻⁷ and 7 × 10⁻⁹ mol L⁻¹ silymarin have been achieved in bulk form or in formulations while 2 × 10⁻⁷ and 8 × 10⁻⁹ mol L⁻¹ silymarin were achieved in spiked human serum utilizing the bare carbon paste electrode and the modified one, respectively. The two electrodes exhibited excellent selectivity towards silymarin even in the presence of 10²to 10³-fold excess of its co-formulated drugs, common excipients, and common metal ions. The pharmacokinetic parameters of silymarin in plasma of healthy human volunteers were estimated following the administration of a single oral dose of 120 mg silymarin utilizing the modified carbon paste electrode. The estimated pharmacokinetic parameters were favorably compared with those reported in literature.     

On-line stripping voltammetry of trace metals at a flow-through bismuth-film electrode by means of a hybrid flow-injection/sequential-injection system

In this work, we describe an automated stripping analyzer operating on a hybrid flow-injection/sequential-injection (FIA/SIA) mode and utilizing a bismuth-film electrode (BiFE) as a flow-through sensor for on-line stripping voltammetry of trace metals. The instrument combines the advantages of FIA and SIA and is characterised by simplicity, low-cost, rapidity, versatility and low consumption of solutions. The proposed analytical flow methodology was applied to the determination of Cd(II) and Pb(II) by anodic stripping voltammetry (ASV) and of Ni(II) and Co(II) by adsorptive stripping voltammetry (AdSV). The steps of the rather complex experimental sequence (i.e. the bismuth-film formation, the analyte accumulation, the voltammetric stripping and the electrode cleaning/regeneration) were conducted on-line and the critical parameters related to the respective analytical procedures were investigated. In ASV, for a accumulation time of 180 s the limits of detection for Cd(II) and Pb(II) were 2 and 1 μg l⁻¹, respectively (S/N = 3) and the relative standard deviations were 5.3% and 4.7%, respectively (n = 8). In AdSV, for a total sample volume of 1000 μl, the limits of detection for Ni(II) and Co(II) were 1 μg l⁻¹ (S/N = 3) and the relative standard deviations were 5.5% and 6.2%, respectively (n = 8). The measurement frequency ranged between 15 and 20 stripping cycles h⁻¹. The results indicate that the BiFE is well suited as a flow-through detector for on-line stripping analysis and, by virtue of its low toxicity, can serve as a viable alternative to mercury-based flow-through electrodes.     

High selective SiO2-Al2O3 mixed-oxide modified carbon paste electrode for anodic stripping voltammetric determination of Pb(II)

The main purpose of this study is to develop an inexpensive, simple, selective and especially highly selective modified mixed-oxide carbon paste electrode (CPE) for voltammetric determination of Pb(II). For the preliminary screening purpose, the catalyst was prepared by modification of SiO₂-Al₂O₃ mixed-oxide and characterized by TG, CHN elemental analysis and FTIR spectroscopy. Using cyclic voltammetry the electroanalytical characteristics of the catalyst have been determined, and consequently the modified mixed-oxide carbon paste electrode was constructed and applied for determination of Pb(II). The electroanalytical procedure for determination of the Pb(II) comprises two steps: the chemical accumulation of the analyte under open-circuit conditions followed by the electrochemical detection of the preconcentrated species using differential pulse anodic stripping voltammetry. During the preconcentration step, Pb(II) was accumulated on the surface of the modifier by the formation of a complex with the nitrogen atoms of the pyridyl groups in the modifier. The peak currents increases linearly with Pb(II) concentration over the range of 2.0 × 10⁻⁹ to 5.2 × 10⁻⁵ mol L⁻¹ (r² = 0.9995).The detection limit (three times signal-to-noise) was found to be 1.07 × 10⁻⁹ mol L⁻¹ Pb(II). The chemical and instrumental parameters have been optimized and the effect of the interferences has been determined. The Proposed method was used for determination of lead ion in the real samples.     

Potentiometric determination of cetylpyridinium chloride using a new type of screen-printed ion selective electrodes

A new type of screen-printed ion-selective electrode for the determination of cetylpyridinium chloride (CPC) is presented. These new electrodes involve in situ, modified and unmodified screen-printed ion-selective electrodes for the determination of CPC. The screen-printed electrodes (SPEs) show a stable, near-Nernstian response for 1 × 10⁻² to 1 × 10⁻⁶ M CPC at 25 °C over the pH range 2-8 with cationic slope 60.66 ± 1.10. The lower detection limit is found to be 8 × 10⁻⁷ M and response time of about 3 s and exhibit adequate shelf-life (6 months). The fabricated electrodes can be also successfully used in the potentiometric titration of CPC with sodium tetraphenylborate (NaTPB). The analytical performances of the SPEs are compared with those for carbon paste electrode (CPE) and polyvinyl chloride (PVC) electrodes. The method is applied for pharmaceutical preparations with a percentage recovery of 99.60% and R.S.D. = 0.53. The frequently used CPC of analytical and technical grade as well as different water samples has been successfully titrated and the results obtained agreed with those obtained with commercial electrode and standard two-phase titration method. The sensitivity of the proposed method is comparable with the official method and ability of field measurements.     

Diamond paste based electrodes for the determination of Pb(II) at trace concentration levels

Three types of monocrystalline diamond: natural diamond 1 μm, synthetic diamond 50 μm (synthetic-1), and synthetic diamond 1 μm (synthetic-2) were used for electrodes’ construction. The linear concentration ranges recorded for Pb(II), when natural diamond, synthetic-1 and synthetic-2 based electrodes were used were between 10⁻⁹ and 10⁻⁶; 10⁻¹⁰ and 10⁻⁷; and between 10⁻¹⁰ and 10⁻⁸ mol l⁻¹, respectively. Low detection limits which lie between 10 and 100 pmol l⁻¹ proves the sensitivity of the electrodes. It was found that Pb(II) yielded a peak at about +0.3±0.02 V (versus Ag/AgCl) for all the electrodes. Lead was determine with high reliability from water and tea samples at trace concentration levels using the proposed diamond paste based electrodes.     

Individual and simultaneous determination of lead, cadmium, and zinc by anodic stripping voltammetry at a bismuth bulk electrode

A bismuth bulk electrode (BiBE) has been investigated as an alternative electrode for the anodic stripping voltammetric (ASV) analysis of Pb(II), Cd(II), and Zn(II). The BiBE, which is fabricated in-house, shows results comparable to those of similar analyses at other Bi-based electrodes. Metal accumulation is achieved by holding the electrode potential at −1.4 V (vs. Ag/AgCl) for 180 s followed by a square wave voltammetric stripping scan from −1.4 to −0.35 V. Calibration plots are obtained for all three metals, individually and simultaneously, in the10-100 μg L⁻¹ range, with a detection limit of 93, 54, and 396 ng L⁻¹ for Pb(II), Cd(II), Zn(II), respectively. A slight reduction in slope is observed for Cd(II) and Pb(II) when the three metals are calibrated simultaneously vs. individually. Comparing the sensitivities of the metals when calibrated individually vs. in a mixture reveals that Zn(II) is not affected by stripping in a mixture. However, Pb(II) and Cd(II) have decreasing sensitivities in a mixture. The optimized method has been successfully used to test contaminated river water by standard addition. The results demonstrate the ability of the BiBE as an alternative electrode material in heavy metal analysis.     

Manganese detection in marine sediments: anodic vs. cathodic stripping voltammetry

Three different electroanalytical techniques for the detection of manganese in marine sediments are evaluated. The anodic stripping voltammetry of manganese at an in situ bismuth-film-modified boron-doped diamond electrode and cathodic stripping voltammetry at a carbon paste electrode are shown to lack the required sensitivity and reproducibility whereas cathodic stripping voltammetry at a bare boron-doped diamond electrode is shown to be reliable and selective with a limit of detection, from applying a 60 s accumulation period of 7.4 × 10⁻⁷ M and a sensitivity of 0.24 A M⁻¹. The method was used to evaluate the manganese content of marine sediments taken from Šibenik, Croatia.     

Ionic liquid-based miniature electrochemical sensors for the voltammetric determination of catecholamines

Screen-printed electrodes modified with carbon paste that consisted of graphite powder dispersed in ionic liquids (IL) were used for the electrochemical determination of dopamine, adrenaline and dobutamine in aqueous solutions by means of cyclic voltammetry. The IL plays a dual role in modifying compositions, acting both as a binder and chemical modifier (ion-exchanger); ion-exchange analyte pre-concentration increases analytical signal and improves the sensitivity. Calibration graphs are linear in concentration range 3.9 × 10⁻⁶ to 1.0 × 10⁻⁴ M (dopamine), 2.9 × 10⁻⁷ to 1.0 × 10⁻⁴ M (adrenaline) and 1.7 × 10⁻⁷ to 1.0 × 10⁻⁴ M (dobutamine); detection limits are (1.2 ± 0.1) × 10⁻⁶, (1.3 ± 0.1) × 10⁻⁷ and (5.3 ± 0.1) × 10⁻⁸ M, respectively. Using an additive of Co (III) tetrakis-(tert-butyl)-phthalocyanine leads to the increase of signal and lowering detection limit. Some practical advises concerning both the sensor design and selectivity of catecholamine determination are provided.     

Construction of novel simple phosphate screen-printed and carbon paste ion-selective electrodes

The construction and performance characteristics of different phosphate ion-selective electrodes are described. Three types of electrodes are demonstrated, namely screen-printed, carbon paste and the conventional PVC membrane electrodes. The cited electrodes are based on bisthiourea ionophores and show a considerable selectivity towards hydrogenphosphate with Nernstian slopes depending on the type of the electrode and the ionophore used. Matrix compositions of each electrode are optimised on the basis of effects of type and concentration of the ionophore as well as influence of the selected plasticizers. The screen-printed electrodes work satisfactorily in the concentration range 10⁻⁵ to 10⁻² mol L⁻¹ with anionic Nernstian compliance (32.8 mV/decade activity) and detection limit 4.0 × 10⁻⁶ mol L⁻¹. The screen-printed electrodes show fast response time of about 2.2 s and exhibit adequate shelf-life (4 months). The fabricated electrodes can be also successfully used in the potentiometric titration of HPO₄²⁻ with Ba²⁺.     

Colloidal-gold cysteamine-modified carbon paste electrodes as suitable electrode materials for the electrochemical determination of sulphur-containing compounds Application to the determination of methionine

The suitability of colloidal-gold cysteamine-modified carbon paste electrodes (nAu-Cyst-CPE) for the electrochemical determination of sulphur-containing compounds is illustrated in this work by determining the amino acid methionine in real samples, as well as a methionine-based peptide. Voltammograms from methionine solutions at nAu-Cyst-CPE exhibited improved electroanalytical characteristics when compared with colloidal-gold cysteamine-modified Au disk electrodes (nAu-Cyst-AuE). Differential pulse voltammetry in phosphate buffer of pH 7.0 was used for the determination of methionine, with a range of linearity of (1.0-100) × 10⁻⁶ mol l⁻¹ and a detection limit of 5.9 × 10⁻⁷ mol l⁻¹. This detection limit is remarkably lower than those reported previously using other modified electrodes or amperometric detection. Methionine peptides also exhibited anodic peaks suitable to detect this kind of molecules. Methionine was determined with good results in a pharmaceutical product containing several vitamins, amino acids and other compounds, and in spiked meat peptone, a complex sample containing enzimatically digested protein.     

Recent Advances in Anodic Stripping Voltammetry with Bismuth‐Modified Carbon Paste Electrodes

In this article, the results of some recent investigations on two types of bismuth‐modified carbon paste electrodes are presented. In the first study, the bismuth‐film carbon paste electrode (BiF‐CPE) operated in situ and employed in anodic stripping voltammetry of Cd(II) and Pb(II) at the low μg L^?1 level was of interest in view of choosing the proper Bi(III)‐to‐Me(II) concentration ratios (where Me: Pb or Cd). Such optimization has resulted in significant improvement of detection limits down to 1.0 μg L^?1 Cd and 0.8 μg L^?1 for Pb, which allowed us to apply the BiF‐CPE for analysis of selected real samples of tap and sea water. The BiF‐CPE was also further investigated for its application in highly alkaline media. In this case, attention was focused on the complex‐forming capabilities of the OH ^– ions and their effect on the anodic stripping characteristics of some heavy metals (i.e. Cd, Pb, Tl) as well as upon the formation of the bismuth film itself. The last example deals with the continuing characterization of the recently introduced carbon paste electrodes modified with bismuth powder (Bi‐CPEs) which combine the advantageous properties of carbon paste material with the favorable electrochemical properties of bismuth. Three series of electrodes, differing either in the content of metallic bismuth (from 8 to 50% w/w) or in the type of the carbon powder used (two spectroscopic types of graphite and powdered glassy carbon), were compared and the respective relations to the optimal carbon paste composition evaluated. Attractive electroanalytical performance of the Bi‐CPE in anodic stripping voltammetry is demonstrated for selected model mixtures of heavy metals (Mn, Zn, Cd, Pb, Tl, and In).