Chemically modified electrodes in liquid chromatography detection: A review

Recent trends in the use of chemically modified electrodes (CMEs) in electrochemical detection systems used in flow-injection analysis or high-performance liquid chromatography are reviewed, with the objective of indicating the most promising approaches for practical CME applications. Four specific application areas of CMEs are identified: (1) those using perm-selective coatings to enhance selectivity and inhibit surface fouling, (2) those using immobilized electron-transfer mediators to catalyse slow electrode reactions, (3) those using enzymes as modifiers to provide biological activity on the electrode surface and (4) those using ion-exchange coatings for the electrochemical detection of non-electroactive anions and cations. By virtue of these approaches, the judicious use of CMEs has already had a significant impact on the performance and scope of electrochemical detection in flow systems. The most important improvements have been in CME selectivity, which produces simpler assay procedures with less need for sample treatment, and increased usefulness for complex samples, and in CME reactivity, which expands the range of analytes amenable to electrochemical monitoring.     

Chemically modified electrodes for the determination of sulphydryl compounds

Under neutral conditions, the electrocatalytic oxidation of sulphydryl compounds at rotating graphite electrodes modified with tetracyano-p-quinodimethane, tetrathiafulvalene and 1,1′-dimethylferrocene proceeds at potentials higher than ?0.1 V vs. SCE. The sensitivity of the electrodes depends on the nature of both the modifier and the sulphydryl compounds, and in 0.1 M potassium phosphate buffer solution (pH 7) it reaches 0.16 A 1 mol^?1 cm^?2. The electrode sensitivity is higher in a weakly alkaline medium. The sensitivity is only slightly dependent on the electrode rotation speed.     

Electrocatalysis and amperometric detection of organic peroxides at modified carbon-paste electrodes

Cobalt-phthalocyanine modified carbon-paste electrodes are shown to catalyze the electro-oxidation of organic peroxides. Cyclic voltammetry offers useful insights into the catalytic behavior. Such behavior is exploited for developing an effective amperometric detection scheme for butanone peroxide, cumene hydroperoxide and tert-butyl hydroperoxide with optimum response at a potential of +0.70 V (vs. Ag/AgCl). Highly sensitive and stable flow injection measurements, with detection limits of 2.4-8.3 ng and relative standard deviations of 1.7-1.8% (n = 30), are reported. Applicability to measurements in drinking water is illustrated.     

Novel amperometric sensors for nitrite detection using electrodes modified with PEDOT prepared in ionic liquids

Journal of Solid State Electrochemistry - Novel amperometric sensors based on poly-3,4-ethylenedioxythiophene (PEDOT) were prepared and characterized for nitrite detection. An experimental matrix...     

Quinoprotein glucose dehydrogenase modified thick-film electrodes for the amperometric detection of phenolic compounds in flow injection analysis

The use of thick-film electrodes as basic transducers for highly sensitive amperometric biosensors using PQQ (pyrroloquinoline quinone) dependent glucose dehydrogenase (GDH) with short response times is described. The enzyme is embedded in a polyurethane matrix on top of a platinum based thick film electrode and its ability to reduce oxidized phenolic compounds is exploited. The electrochemical amplification is based on the oxidation of the analyte on the surface of the electrode followed by its enzymatic reduction. Different parameters of the glucose dehydrogenase electrode system using dopamine as a model analyte were optimized, e.g., membrane thickness, pH value, buffer system, flow rate and storage conditions. Using optimized parameters the sensitivity and detection limits for various phenolic compounds were evaluated. The comparison of electrodes from the identical as well as from different batches shows the ability to produce a number of well reproducible sensors showing remarkably small differences with respect to parameters as sensitivity, response times, and measuring range. Received: 15 August 2000 / Revised: 17 October 2000 / Accepted: 24 October 2000     

Chemically modified pencil electrodes for application in reagentless chronopotentiometric antioxidant activity measurement

Recently a novel chronopotentiometric method has been introduced that was found useful for assessing antioxidant activity. In our experiments, the performance of chemically modified working electrodes made of different pencil materials were tested in the novel method. Redox mediator film of Meldola Blue was formed on electrode surfaces. The stability of the film, the number of molecules structuring it has been studied. In antioxidant measurements, controlled potential oxidation of the mediator was made followed by recording the open cell potential in time. The initial slope of this was used as analytical signal. Results proved applicability of the pencil probes.     

Quinoprotein glucose dehydrogenase modified thick-film electrodes for the amperometric detection of phenolic compounds in flow injection analysis

The use of thick-film electrodes as basic transducers for highly sensitive amperometric biosensors using PQQ (pyrroloquinoline quinone) dependent glucose dehydrogenase (GDH) with short response times is described. The enzyme is embedded in a polyurethane matrix on top of a platinum based thick film electrode and its ability to reduce oxidized phenolic compounds is exploited. The electrochemical amplification is based on the oxidation of the analyte on the surface of the electrode followed by its enzymatic reduction. Different parameters of the glucose dehydrogenase electrode system using dopamine as a model analyte were optimized, e.g., membrane thickness, pH value, buffer system, flow rate and storage conditions. Using optimized parameters the sensitivity and detection limits for various phenolic compounds were evaluated. The comparison of electrodes from the identical as well as from different batches shows the ability to produce a number of well reproducible sensors showing remarkably small differences with respect to parameters as sensitivity, response times and measuring range.     

Reaction, diffusion and migration in conducting polymer electrodes: analysis of the steady-state amperometric response

The processes of reaction, diffusion and electromigration of a charged substrate within an electronically conducting polymer film deposited on an inert supporting electrode are examined in terms of a quantitative analysis and solution of the pertinent differential equations. An analytical expression for the concentration profiles of substrate within the polymer layer is derived and a theoretical expression for the corresponding steady-state amperometric current response is presented. The transport and kinetics of the substrate are discussed in terms of a dimensionless reaction/diffusion parameter γ and a migration/diffusion parameter β. Received: 13 March 1998 / Accepted: 17 July 1998     

The amperometric response of tubular electrodes applied to flow-injection determinations

The classical treatment by Taylor for the dispersion of mass injected into a fluid stream in a linear, tubular channel is applied for amperometric detection in flow-injection systems with a tubular, flowthrough electrode. Peak current is predicted to be dependent upon the $\text{l}\text{3}$rd root of flow rate for low dispersion and the —$\text{1}\text{6}$th root of flow rate for high dispersion. Agreement between experimental results and theory is excellent for flow rates of 0.5 ml min^-1 or less in a linear, tubular channel with an inner radius of 0.0483 cm. As examples, experimental and predicted currents agree to better than 0.1 μA throughout a peak with a maximum value near 2 μA, and effects of retention volume, sample volume, and flow rate on the ratio of peak current to steady-state current are predicted with errors of 9% or less.     

Electrocatalysis of metalloporphyrins: Part 12. Chemically modified electrodes with porphyrin-incorporated Nafion coatings

The electrocatalytic behaviour of cobalt(III) tetrakis(4-trimethylammoniumphenyl)porphyrin (CoTMAP) and the electroreduction mechanisms of dioxygen and dithiodipropionic acid (PSSP) at glassy carbon electrodes (GCEs) modified with Nafion coatings incorporating CoTMAP and Ru(NH₃)₆³⁺ have been studied. The experimental results indicate that the catalytic electroreduction of dioxygen using CoTMAP as a catalyst and Ru(NH ₃)₆³⁺ as a mediator could proceed at the Nafion coating—solution interface regardless of the state of the electrode surface. The catalytic electroreduction of dithiodipropionic acid could not proceed at the Nafion coating—solution interface but only at the GCE—solution interface, suggesting the importance of the adsorption of Co(I)TMAP on the GCE to the catalytic electroreduction of PSSP.     

Voltammetry and amperometric detection of tetracyclines at multi-wall carbon nanotube modified electrodes

The voltammetric behaviour and amperometric detection of tetracycline (TC) antibiotics at multi-wall carbon nanotube modified glassy carbon electrodes (MWCNT-GCE) are reported. Cyclic voltammograms of TCs showed enhanced oxidation responses at the MWCNT-GCE with respect to the bare GCE, attributable to the increased active electrode surface area. Hydrodynamic voltammograms obtained by flow-injection with amperometric detection at the MWCNT-GCE led us to select a potential value E _det = +1.20 V. The repeatability of the amperometric responses was much better than that achieved with bare GCE (RSD ranged from 7 to 12%), with RSD values for i _p of around 3%, thus demonstrating the antifouling capability of MWCNT modified electrodes. An HPLC method with amperometric electrochemical detection (ED) at the MWCNT-GCE was developed for tetracycline, oxytetracycline (OTC), chlortetracycline and doxycycline (DC). A mobile phase consisting of 18:82 acetonitrile/0.05 mol L⁻¹ phosphate buffer of pH 2.5 was selected. The limits of detection ranged from 0.09 μmol L⁻¹ for OTC to 0.44 μmol L⁻¹ for DC. The possibility to carry out multiresidue analysis is demonstrated. The HPLC-ED/MWCNT-GCE method was applied to the analysis of fish farm pool water and underground well water samples spiked with the four TCs at 2.0 × 10⁻⁷ mol L⁻¹. Solid-phase extraction was accomplished for the preconcentration of the analytes and clean-up of the samples. Recoveries ranged from 87 ± 6 to 99 ± 3%. Under preconcentration conditions, limits of detection in the water samples were between 0.50 and 3.10 ng mL⁻¹.     

Effects of competitive binding on the amperometric determination of copper with electrodes modified with chromotrope 2B

Electrodes modified with Chromotrope 2B incorporated by ion-exchange into a polycationic film of electropolymerized [Ru(v-bpy)(3)](2+) (v-bpy = 4-vinyl-4'-methyl-2,2'-bipyridyl) have been employed in the amperometric determination of copper in solution and exhibit very high sensitivity as well as linear calibration curves in the concentration range 7 x 10(-8)-1 x 10(-4)M. The effects of competing ligands, including chloride, bromide, oxalate, ammonia, acetate, citrate, borate, humic and fulvic acids, or the presence of competing metal ions such as cobalt or nickel on the uptake of copper by the modified electrodes have also been studied. The presence of competing ligands or metal ions decreases the analytical signal due to copper incorporation. The magnitude of this effect is dependent on the relative strength of coordination of the competing ligands for copper ions or of Chromotrope 2B for the competing metals, and also on the concentration of the interferents. The relevance of this work to speciation studies is discussed.     

Gold electrodes from compact discs modified with platinum for amperometric determination of ascorbic acid in pharmaceutical formulations

A simple, rapid and precise amperometric method has been developed for quantification of ascorbic acid (AA) in pharmaceutical formulations using flow-injection analysis (FIA). A slice of recordable compact disc (CD) modified by electrodeposition of platinum was employed as the working electrode. The proposed flow system allows determinations in the 1 mumol l(-1) of the analyte and enables 90 determinations per h, employing only 150-mul sample. The method permits the direct quantification of ascorbic acid in many pharmaceutical products, avoiding cumbersome processes as previous separations, solvent extraction or sample filtration. This new procedure was applied to commercial pharmaceutical tablets and the results obtained were identical than the ones obtained by the classical iodometric method. The calibration plots for freshly prepared ascorbic acid standards were highly linear in the concentration range of 1-10 mumol l(-1) with a relative standard deviation (R.S.D.) <1%. For all real samples studied, the deviations were situated between 0.5 and 8.7%.     

Potentiometric response of graphite electrodes coated with modified polymer films

Graphite electrodes coated with chemically-modified polymer films are described. Several different polymers were used, including poly(acrylic acid), poly[triethyl(vinylbenzyl)ammonium chloride], poly[trihexyl(vinylbenzyl)ammonium chloride], and poly[trihexyl(vinylbenzyl)ammonium thiocyanate]. A cation-responsive electrode can be prepared from poly(acrylic acid)-coated graphite. Anion-responsive electrodes can be prepared from graphite coated with polymeric quaternary amines. In these electrodes, the ion-sensing species is irreversibly attached to the polymer (rather than physically entrapped within a polymer matrix); this factor eliminates leaching of the active component, and the addition of a plasticizer is unnecessary. A selective sensor for thiocyanate is described; it yields a Nernstian response over the concentration range 1 × 10^?1–1 × 10^?5 M sodium thiocyanate.     

Determination of cyanides by continuous distillation and flow analysis with cylindrical amperometric electrodes

A continuous system for the determination of free and complex cyanides has been developed. Hydrogen cyanide is released in an acidic solution in a counter-current system operated by a peristaltic pump, absorbed in dilute sodium hydroxide and then fed into the amperometric detector with a cylindrical silver flow-through electrode. The parameters affecting the release and absorption of cyanide, as well as the electrode response and sensitivity, are described. Differentiation between total cyanide and strongly bound metal cyanide complexes is achieved by u.v. decomposition of the complexes.     

Mathematical treatment of concentration profiles and anodic current of amperometric enzyme electrodes with chemically-amplified response

Formulae are presented for explicit solutions of partial differential equations describing the transient and stationary behaviour of concentration profiles and the anodic current in amperometric chemically-amplified enzyme electrodes. The mathematical treatment is based on reaction/diffusion models with irreversible first-order catalytic reactions. Numerical results for an L-lactate-sensing electrode are used to demonstrate the application of the derived formulae.     

Mediated amperometric biosensors for lactic acid based on carbon paste electrodes modified with baker's yeast Saccharomyces cerevisiae

Carbon paste electrodes modified with baker's yeast Saccharomyces cerevisiae (a source of flavocytochrome b(2)) were investigated as amperometric biosensors for lactic acid. Phenazine methosulphate was used as a mediator. The optimal operational conditions of the electrodes were: an operating potential 0.0 V, solution pH 7.2, concentration of phenazine methosulphate in solution 0.2 mM. A linear range in the dependence of the current responses on the concentration of lactic acid was up to 1 mM. The suitability of the electrodes for determination of lactic acid in milk and dairy products such as kefir and yoghurt was tested. The yeast cells in the paste remained viable at least for 1 month.