Thin-layer Electrochemical Flow Cell Research Articles

Online monitoring of electrocatalytic reactions of alcohols at platinum and gold electrodes in acidic, neutral and alkaline media by capillary electrophoresis with contactless conductivity detection (EC-CE-C4 D).

An EC-CE-C4 D flow system was applied to the investigation of electrocatalytic processes by monitoring carboxylic acids formed during the electro-oxidation at various potentials of primary alcohols (mixture of 1mmol/L of ethanol, n-propanol, n-butanol and n-pentanol) in acidic, neutral and alkaline media. The electro-oxidation was carried out on gold and platinum disk electrodes (3mm of diameter) in a thin-layer electrochemical flow cell. Products were sampled 50μm apart from the electrode directly into the capillary. All the generated carboxylates were determined in near real time (less than 2min) by CE-C4 D in counter-flow mode, with Tris/HCl buffer solution (pH8.6) as BGE. Long sequences of 5-min experiments were run automatically, exploring the applied potential, electrolysis time and solution composition. Electro-oxidation at 1.5V (versus Ag/AgCl quasi-reference) during 50s in acidic medium was found appropriate for both Pt and Au electrodes when the determination of alcohols after derivatization is intended. A noteworthy selectivity effect was observed on the Au electrode. The signal corresponding to pentanoate is similar on both electrodes while the signal of ethanoate (acetate) is four times larger on gold than on platinum. The carboxylate signals were lower in alkaline medium (below the determination limit on Pt) than in acidic and neutral media. On gold, the formation of carboxylates was anticipated (0.85V in alkaline medium versus 1.40V in neutral medium). The automatic online monitoring of electrochemical processes by EC-CE-C4 D holds great potential to investigate ionic/ionizable intermediates/products of new electrocatalysts and/or alternative fuels.

Online Electrochemical Monitoring of Dynamic Change of Hippocampal Ascorbate: Toward a Platform for In Vivo Evaluation of Antioxidant Neuroprotective Efficiency against Cerebral Ischemia Injury

Effective monitoring of cerebral ascorbate following intravenous antioxidant treatment is of great importance in evaluating the antioxidant efficiency for neuroprotection because ascorbate is closely related to a series of ischemia-induced neuropathological processes. This study demonstrates the validity of an online electrochemical system (OECS) for ascorbate detection as a platform for in vivo evaluation of neuroprotective efficiency of antioxidants by studying the dynamic change of hippocampal ascorbate during the acute period of cerebral ischemia and its responses to intravenous administration of antioxidants including ascorbate and glutathione (GSH). The OECS consists of a selective electrochemical detector made of a thin-layer electrochemical flow cell integrated with in vivo microdialysis. With such a system, the basal level of hippocampal ascorbate is determined to be 5.18 ± 0.60 μM (n = 20). This level is increased by 10 min of two-vessel occlusion (2-VO) ischemia treatment and reaches 11.51 ± 3.43 μM (n = 5) at the time point of 60 min after the ischemia. The 2-VO ischemia-induced hippocampal ascorbate increase is obviously attenuated by immediate intravenous administration of ascorbate (2.94 g/kg) or glutathione (5.12 g/kg) within 10 min after ischemia and the ascorbate level remains to be 3.75 ± 1.66 μM (n = 4) and 5.30 ± 0.79 μM (n = 5), respectively, at the time point of 60 min after ischemia. To confirm if the attenuated hippocampal ascorbate increase is attributed to the antioxidant-induced oxidative stress alleviation, we further study the immunoreactivity of 8-hydroxy-2-deoxyguanosine (8-OHdG) in the ischemic hippocampus and find that the 8-OHdG immunoreactivity is decreased by the administration of ascorbate or GSH as compared to the ischemic brain without antioxidant treatment. These results substantially demonstrate that the OECS for ascorbate detection could be potentially used as a platform for evaluating the efficiency of antioxidant neuroprotection in cerebral ischemia treatment.

Indoor allergen assessment quantified by a thin-layer electrochemical cell and magnetic beads

We report the electrochemical determination of mite allergen in real house dust by using a thin layer electrochemical flow cell and magnetic beads. Der p1, which is an allergen from Dermatophagoides pteronyssinus, was immunochemically sandwiched between two dispersed monoclonal antibodies; one was modified on the surface of magnetic beads and the other was modified with alkaline phosphatase. After washing the beads, a small volume of p-aminophenol phosphate (p-APP) was added to produce p-aminophenol (p-AP). And then the p-AP concentration was measured electrochemically with a homemade electrochemical cell. The Der p1 assay was completed within 30min and a low detection limit of 0.3ng/mL was achieved. This is because the diffusion distance of Der p1 and the detection antibody was reduced to 22.3μm by using dispersed magnetic beads. Only 10min was required to complete the entire immunoreaction, and 54% of the Der p1 was confirmed to have immunoreacted in only 1min of mixing. Furthermore, the p-APP volume could be reduced using the thin-layer electrochemical flow cell. This is advantageous in terms of concentrating p-AP, and provides a high signal-to-noise ratio measurement in a short time. We achieved a high correlation (r=0.967, p<0.001) between our assay and a conventional enzyme-linked immunosorbent assay (ELISA) for real house dust measurements.

From Sample Processing to Quantification: A Full Electrochemical Approach for Neutral Analyte Derivatization, Capillary Electrophoresis Separation, and Contactless Conductivity Detection

A thin-layer electrochemical flow cell coupled to capillary electrophoresis with contactless conductivity detection (EC-CE-C(4)D) was applied for the first time to the derivatization and quantification of neutral species using aliphatic alcohols as model compounds. The simultaneous electrooxidation of four alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol) to the corresponding carboxylates was carried out on a platinum working electrode in acid medium. The derivatization step required 1 min at 1.6 V vs. Ag/AgCl under stopped flow conditions, which was preceded by a 10 s activation at 0 V. The solution close to the electrode surface was then hydrodynamically injected into the capillary, and a 2.5 min electrophoretic separation was carried out. The fully automated flow system operated at a frequency of 12 analyses per hour. Simultaneous determination of the four alcohols presented detection limits of about 5 × 10(-5) mol L(-1). As a practical application with a complex matrix, ethanol concentrations were determined in diluted pale lager beer and in nonalcoholic beer. No statistically significant difference was observed between the EC-CE-C(4)D and gas chromatography with flame ionization detection (GC-FID) results for these samples. The derivatization efficiency remained constant over several hours of continuous operation with lager beer samples (n = 40).

Versatile Flow-Injection Amperometric Ion Detector Based on an Interface between Two Immiscible Electrolyte Solutions: Numerical and Experimental Characterization

The present paper describes a flexible thin layer electrochemical flow cell for ultrasensitive amperometric detection at a supported interface between immiscible electrolyte solutions. Nanomolar detection limits were demonstrated using the cell design, and 3D finite element simulations allowed a detailed characterization of the flow cell. The cell design employed in the present work allowed the sensing oil membrane and the aqueous reference electrode to be placed in close contact, thereby minimizing cell resistance. The adjustable cell volume means that the same cell design can be used for different application with different requirement for detection limits and dynamic range. A disposable membrane was employed which reduces the need for surface cleaning and prevents sample carryover between different applications. For the lowest cell volumes the detection chamber approaches a thin layer electrochemical flow cell detector with a large surface to volume ratio.

Automated two‐dimensional separation flow system with electrochemical preconcentration, stripping, capillary electrophoresis and contactless conductivity detection for trace metal ion analysis

This paper describes the automation of a fully electrochemical system for preconcentration, cleanup, separation and detection, comprising the hyphenation of a thin layer electrochemical flow cell with CE coupled with contactless conductivity detection (CE-C⁴D). Traces of heavy metal ions were extracted from the pulsed-flowing sample and accumulated on a glassy carbon working electrode by electroreduction for some minutes. Anodic stripping of the accumulated metals was synchronized with hydrodynamic injection into the capillary. The effect of the angle of the slant polished tip of the CE capillary and its orientation against the working electrode in the electrochemical preconcentration (EPC) flow cell and of the accumulation time were studied, aiming at maximum CE-C⁴D signal enhancement. After 6 min of EPC, enhancement factors close to 50 times were obtained for thallium, lead, cadmium and copper ions, and about 16 for zinc ions. Limits of detection below 25 nmol/L were estimated for all target analytes but zinc. A second separation dimension was added to the CE separation capabilities by staircase scanning of the potentiostatic deposition and/or stripping potentials of metal ions, as implemented with the EPC-CE-C⁴D flow system. A matrix exchange between the deposition and stripping steps, highly valuable for sample cleanup, can be straightforwardly programmed with the multi-pumping flow management system. The automated simultaneous determination of the traces of five accumulable heavy metals together with four non-accumulated alkaline and alkaline earth metals in a single run was demonstrated, to highlight the potentiality of the system.

Fabrication and Characterization of a Thin-Layer Electrochemical Flow Cell and Its Application for Flow Analysis

The design and fabrication of a home-made thin-layer electrochemical flow cell are reported. The performance and electrochemical characteristics of thin-layer cells are evaluated in terms of flow rate and channel thickness. The adaptability of the flow cell for amperometric and electrochemiluminescence measurements is demonstrated. Also, a flow injection method comprising electrochemical generation followed by spectrophotometric detection is proposed for determination of bromide using the fabricated cell. The influence of chemical, electrochemical, and flow parameters on the absorbance and current signals are investigated, and under optimized conditions, the analytical features of the proposed method with respect to bromide detection are evaluated.

A non-oxidative electrochemical approach to online measurements of dopamine release through laccase-catalyzed oxidation and intramolecular cyclization of dopamine

A new electrochemical approach to selective online measurements of dopamine (DA) release in the cerebral microdialysate is demonstrated with a non-oxidative mechanism based on the distinct reaction properties of DA and the excellent biocatalytic activity of laccase. To make the successful transition of the distinct sequential reaction properties of DA from a conceptual determination protocol to a practical online analytical system, laccase enzyme is immobilized onto magnetite nanoparticles and the nanoparticles are confined into a fused-silica capillary through an external magnetic field to fabricate a magnetic microreactor. The microreactor is placed in the upstream of the thin-layer electrochemical flow cell to efficiently catalyze the oxidation of DA into its quinonoid form and thereby initialize the sequential reactions including deprotonation, intramolecular cyclization, disproportionation and/or oxidation to finally give 5,6-dihydroxyindoline quinone. The electrochemical reduction of the produced 5,6-dihydroxyindoline quinone at bare glassy carbon electrode is used as the readout for the DA measurement. The laccase-immobilized microreactor is also found to catalyze the oxidation of ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) into electroinactive species and, as such, to eliminate the great interference from both species. Moreover, the successful transition of the mechanism for DA detection from the conventional oxidative electrochemical approach to the non-oxidative one substantially enables the measurements virtually interference-free from physiological levels of uric acid, 5-hydroxytryptamine, norepinephrine, and epinephrine. The current response is linear with DA concentration within a concentration range from 1 to 20 μM with a sensitivity of 3.97 nA/μM. The detection limit, based on a signal-to-noise ratio of 3, is calculated to be 0.3 μM. The high selectivity and the good linearity as well as the high stability of the online method make it very potential for continuous monitoring of cerebral DA release in physiological and pathological processes.

Online Coupling of Electrochemical Reactions with Liquid Sample Desorption Electrospray Ionization-Mass Spectrometry

The combination of electrochemistry (EC) and mass spectrometry (MS) is a powerful analytical tool to study redox reactions. This work reports the online coupling of a thin-layer electrochemical flow cell with liquid sample desorption electrospray ionization mass spectrometry (DESI-MS) and its applications in investigating various electrochemical reactions of biological molecules such as oxidative formation and reductive cleavage of disulfide bonds and online derivatization of peptides/proteins. As a result of the direct sampling nature of DESI, several useful features of such a coupling have been found, including simple instrumentation, fast response time (e.g., 3.6 s in the case of dopamine oxidation), freedom to choose a favorable ionization mode of DESI or traditional electrolysis solvent systems, and the absence of background signal possibly resulting from ionization when the cell is off (e.g., in the case of dopamine oxidation). More importantly, with the use of this new coupling apparatus, three disulfide bonds of insulin were fully cleaved by electrolytic reduction and both the A and B chains of the protein were successfully detected online by DESI-MS. In addition, online tagging of free cysteine residues of peptides/proteins employing electrogenerated dopamine o-quinone can be performed. These revealed characteristics of the coupling along with examined electrochemical reactions suggest that EC/DESI-MS has good potential in bioanalysis.

Nafion-Coated Bismuth Film and Nafion-Coated Mercury Film Electrodes for Anodic Stripping Voltammetry Combined On-Line with ICP-Mass Spectrometry

Nafion-coated bismuth film electrodes (NCBFEs) and Nafion-coated mercury film electrodes (NCMFEs) were used to electrochemically preconcentrate metal analytes for subsequent analysis by inductively coupled plasma-mass spectrometry (ICP-MS). Either type of electrodes is part of a thin-layer electrochemical flow cell that is positioned upstream of a microconcentric nebulizer for the ICP-MS. Performances of these electrodes were compared in terms of the analytical "figures of merit" (e.g., dynamic ranges, reproducibility, hydrodynamic stability, and elimination of matrix effects detrimental to ICP-MS). The coupled technique (ASV-ICP-MS) is found to possess a wide dynamic range (at least 4 to 5 orders of magnitude) and to be reproducible. Both electrodes are much more stable than the thin mercury film electrode (TMFE) traditionally used for ASV-ICP-MS, with the lifetime of the NCBFE exceeding 8 h. Adopting these electrodes for ASV-ICP-MS overcomes the problems associated with a TMFE, the erosion of which decreases the sample throughput, affects the analysis precision, and contaminates conventional glass nebulizers and spray chambers of the spectrometer. The medium exchange procedure inherent in ASV is successfully implemented with a two-valve flow injection system for the accumulation of trace Cd2+ into the electrode from a certified seawater sample, followed by stripping Cd into a solution that is compatible to the ICP-MS operation.

Sequential injection/electrochemical immunoassay for quantifying the pesticide metabolite 3,5,6-trichloro-2-pyridinol

An automated and sensitive sequential injection electrochemical immunoassay was developed to monitor a potential insecticide biomarker, 3,5,6-trichloro-2-pyridinol. The current method involved a sequential injection analysis (SIA) system equipped with a thin-layer electrochemical flow cell and permanent magnet, which was used to fix 3,5,6-trichloro-2-pyridinol (TCP) antibody coated magnetic beads (TCP-Ab-MBs) in the reaction zone. After competitive immunoreactions among TCP-Ab-MBs, TCP analyte, and horseradish peroxidase (HRP) labeled TCP, a 3,3′,5,5′-tetramethylbenzidine dihydrochloride and hydrogen peroxide (TMB-H 2O 2) substrate solution was injected to produce an electroactive enzymatic product. The activity of HRP tracers was monitored by a square wave voltammetric scanning electroactive enzymatic product in the thin-layer flow cell. The voltammetric characteristics of the substrate and the enzymatic product were investigated under batch conditions, and the parameters of the immunoassay were optimized in the SIA system. Under the optimal conditions, the system was used to measure as low as 6 ng L −1(ppt) TCP, which is around 50-fold lower than the value indicated by the manufacturer of the TCP RaPID Assay ® kit (0.25 μg/L, colorimetric detection). The performance of the developed immunoassay system was successfully evaluated on tap water and river water samples spiked with TCP. This technique could be readily used for detecting other environmental contaminants by developing specific antibodies against contaminants and is expected to open new opportunities for environmental and biological monitoring.

Anodic stripping voltammetry combined on-line with inductively coupled plasma-MS via a direct-injection high-efficiency nebulizer.

A direct-injection high-efficiency nebulizer (DIHEN) is used to couple a thin-layer electrochemical flow cell on-line with an ICP-mass spectrometer to perform anodic stripping voltammetry (ASV) at a thin mercury film followed by subsequent ICPMS measurements for the stripped metal analytes. The resultant hyphenated technique (ASV-DIHEN-ICPMS) is capable of analyzing select heavy metals present at ultratrace levels (down to low-ppt to sub-ppt levels) that are lower than the detection limits obtained by conventional ICPMS. In addition to its good analytical performance, the technique offers other attractive features such as the ability to eliminate detrimental matrix effects that can compromise ICPMS analyses and the possibility of probing electrode reactions involving trace amounts metal species with ICPMS. For conducting ASV on-line with ICPMS, the DIHEN was found to be more advantageous than the microconcentric nebulizer in terms of minimizing memory effects and potential artifacts caused by the erosion of the Hg film into the flowing solution stream. Compared to a direct injection nebulizer (DIN), the DIHEN was easier to operate. Moreover, its simpler design and the lack of back pressure from the DIHEN capillary made it more compatible with coupling to the thin-layer electrochemical cell than a DIN system.

An acetylcholinesterase/choline oxidase-based amperometric biosensors as a liquid chromatography detector for acetylcholine and choline determination in brain tissue homogenates.

A liquid chromatography (LC) detector based on a fast response and sensitive bienzyme amperometric biosensor for acetylcholine (ACh) and choline (Ch) is described. The detector fabrication consisted of glutaraldehyde co-crosslinking of acetylcholinesterase and choline oxidase with bovine serum albumin on the Pt working electrode of a conventional thin-layer electrochemical flow cell. The influence of some experimental parameters (e.g., enzyme loading, thickness of the bienzyme layer, flow rate) on the detector characteristics has been studied in order to optimize the analyte response while minimizing band-broadening and distortion. A mobile phase consisting of a phosphate buffer (I, 0.1 M; pH, 6.5) containing 5 mM sodium hexane sulfonate and 10 mM tetramethylammonium phosphate was found to give very satisfactory resolution and peak shape in ion-pair, reversed-phase LC. Linear responses were observed over at least four decades and absolute detection limits (at a signal-to-noise ratio of 3) were 12 and 27 fmol injected for Ch and ACh, respectively. After one month of intensive use in the LC system, the detector retained about 70% of its initial sensitivity. The potential of the described approach is demonstrated by the simultaneous determination of Ch and ACh in rat brain tissue homogenates.

Electrochemical Polymerization of Aniline Investigated Using On-Line Electrochemistry/Electrospray Mass Spectrometry

A thin-layer electrochemical flow cell coupled on-line with electrospray mass spectrometry (EC/ES-MS) was used to investigate the soluble products from the controlled-potential anodic polymerization of aniline in H(2)O and H(2)O/CH(3)OH (1/1 v/v) with ammonium acetate and acetic acid or ammonium hydroxide as electrolytes (pH 4, 6.5, or 9). At a working electrode (glassy carbon) potential of 1.0 V versus Ag/AgCl, singly protonated aniline oligomers containing as many as 10 aniline units (10-mer) were observed in the ES mass spectra when the polymerization in H(2)O/CH(3)OH at pH 4 was carried out. The abundance of the higher n-mers decreased at higher solution pH and in 100% H(2)O at pH 4. Most of the oligomers were observed in more than one redox state ranging from fully oxidized (all imine nitrogens) to fully reduced (all amine nitrogens). The number of different redox states observed for the n-mers increased with increasing n. The structures of the reduced (m/z 185) and oxidized (m/z 183) aniline dimer ions (head-to-tail, tail-to-tail, or head-to-head) produced from the polymerization of aniline at pH 4, 6.5, and 9 in H(2)O/CH(3)OH were revealed to vary as a function of pH by comparison of their tandem mass spectrometry product ion spectra with the product ion spectra of the dimer standards. EC/ES-MS potential scan experiments, in which the working electrode current and major n-mer ions for n = 2, 3, and 4 were monitored as a function of electrode potential, were used to probe the growth mechanism to higher aniline oligomers. Under the conditions used, the controlled-current electrolytic process inherent to the operation of the ES ion source did not significantly influence the formation or nature of the oligomers observed. Beyond the current application, the results presented here serve to demonstrate the utility of EC/ES-MS as a tool in identifying the initial products of electropolymerization and in studying the products of electrode reactions in general.

A Thin-Layer Electrochemical Flow Cell Coupled On-Line with Electrospray-Mass Spectrometry for the Study of Biological Redox Reactions

A three electrode thin-layer, flow-by electrode cell was coupled on-line with electrospray-mass spectrometry (i.e., EC/ES-MS) for the study of biological redox reactions. The cell made use of a commercially available 6.0 mm diameter, offset glassy carbon disk working electrode, a Ag/AgCl reference electrode, and a working/counter electrode spacing gasket (16 μm), along with a home-built PEEK counter electrode block with a Pt foil (7.0 mm wide) counter electrode (cell volume ca. 1.1 μL). Off-line hydrodynamic voltammetry experiments and electrolytic conversion efficiency measurements using chronocoulometry were used to characterize the performance of the cell. On-line EC/ES-MS conversion efficiency was found consistent with the off-line results and the EC/ES-MS response time was measured as 5.1 s at a flow rate of 30 μL/min and 2.4 s at 62 μL/min. The use of this hybrid system for the study of the products of biologically relevant redox reactions was demonstrated using the oxidation of dopamine in aqueous CH3OH (pH 4.0) as a test case. Tandem mass spectrometry experiments provided evidence to conclude that the structure of the two major dopamine oxidation products observed in the ES mass spectra were 5,6-dihydroxyindoline and 5,6-hydroxyindole. Reaction of oxidized dopamine with the surrogate biogenic nucleophile benzene thiol resulted in the formation of mono-, di-, and tri-benzene thiol addition products. These spectra also indicated that the reaction of oxidized dopamine with benzene thiol was significantly faster than the intermolecular cyclization reaction of this intermediate, which normally produces 5,6-dihydroxyindoline.

Optically transparent thin-layer electrochemical flow cell for liquid chromatography

Optically transparent thin-layer electrochemical flow cell for liquid chromatography