This article reports on a novel microsensor for amperometric measurement of ascorbic acid (AA) under acidic conditions (pH 2) based on a carbon fiber microelectrode (CFME) modified with nickel oxide and ruthenium hexacyanoferrate (NiO–RuHCF). This sensing layer was deposited electrochemically in a two-step procedure involving an initial galvanostatic NiO deposition followed by a potentiodynamic RuHCF deposition from solutions containing the precursor salts. Several important parameters were examined to characterize and optimize the NiO–RuHCF sensing layer with respect to its current response to AA by using cyclic voltammetry, and scanning electron microscopy–energy dispersive X-ray spectroscopy methods. With the NiO–RuHCF coated CFME, the AA oxidation potential under acidic conditions was shifted to a less positive value for about 0.2 V (Ep of ca. 0.23 V vs. Ag/AgCl) as compared to a bare CFME, which greatly improves the electrochemical selectivity. Using the hydrodynamic amperometry mode, the current vs. AA concentration in 0.01 M HCl, at a selected operating potential of 0.30 V, was found to be linear over a wide range of 10–1610 μM (n = 22, r = 0.999) with a calculated limit of detection of 1.0 μM. The measurement repeatability was satisfactory with a relative standard deviation (r.s.d.) ranging from 4% to 5% (n = 6), depending on the AA concentration, and with a sensor-to-sensor reproducibility (r.s.d.) of 6.9% at 100 μM AA. The long-term reproducibility, using the same microsensor for 112 consecutive measurements of 20 μM AA over 11 h of periodic probing sets over 4 days, was 16.1% r.s.d., thus showing very good stability at low AA levels and suitability for use over a prolonged period of time. Moreover, using the proposed microsensor, additionally coated with a protective cellulose acetate membrane, the calibration plot obtained in the extremely complex matrix of real undiluted gastric juice was linear from 10 to 520 μM (n = 14, r = 0.998). These results demonstrated the unique featuring of the proposed NiO–RuHCF microsensor under acidic conditions with enhanced sensitivity and stability and proved its promising potentiality for direct amperometric probing of AA at physiological levels in real gastric juice environments.

Novel electrochemical microsensor based on mixed iron-ruthenium hexacyanoferrate (FeRuHCF) modified carbon fiber microelectrode (CFME) is presented for voltammetric and amperometric measurement of hydrogen peroxide at physiological pH. The FeRuHCF coating was electrochemically deposited using a one step procedure onto the substrate carbon fiber microelectrode by cycling the potential between 0.0 and + 1.0 V (vs. Ag/AgCl) in a solution containing all precursor salts. The microsensor displayed good stability in neutral and alkaline media and had a nonstop working lifetime of up to 12 hours. The amperometric response time varied from 5 to 15 s depending on the hydrogen peroxide concentration level. The newly developed electrochemical microsensor exhibited a highly linear behavior in the examined concentration range from 5 to 1000 mu mol L-1 (R-2 = 0.999), an LoD (3 sigma) of 0.9 mu mol L-1, and a favorable reproducibility with a calculated RSD of 2.9% (n=6) for 100 mu mol L-1 hydrogen peroxide, thus holding great promise for its further application in real samples and its exploitation in combination with biorecognition elements in advanced microbiosensor design.

The bismuth film microelectrode (BiFME), consisting of a bismuth film ex situ preplated onto a single carbon fibre substrate microelectrode, is presented for the direct simultaneous determination of trace Co(II) and Ni(II) in selected low-volume simulated and real human body fluid samples, using the adsorptive cathodic stripping voltammetric (AdCSV) protocol with dimethylglyoxime (DMG) as complexing agent. The BiFME exhibited well-defined and undistorted current signals for the Co(II) and Ni(II) ions, with the signals for each metal being independent of each other. Factors influencing the stripping performance at BiFME, including complexing agent concentration, accumulation potential and time, were examined. The stripping performance of the BiFME for simultaneous measurement of Co(II) and Ni(II), in the presence of dissolved oxygen, is characterised by very low detection limits of 69ng/L (1.2×10−9M) for Co(II) and 56ng/L (9.5×10−10M) for Ni(II) employing a preconcentration time of only 60s, good linearity (r>0.995), and excellent reproducibility with relative standard deviations of 4.6% and 3.8% for 0.5μg/L (n=10) of Co(II) and Ni(II), respectively. The applicability of the BiFME in direct measurement of trace Co(II) and Ni(II) in some simulated (saliva, sweat) and real (aqueous humor, cerebrospinal fluid) unpretreated low-volume (100–225μL) biological fluids is demonstrated. The attractive behaviour of the new “mercury-free” BiFME microsensor augurs well for its use in the monitoring of Co(II) and Ni(II) release from customarily employed biomedical devices and routinely encountered metallic items, and for its deployment in clinical testing of biological fluids for cobalt and nickel exposure from medical implants.

The bismuth film electrode (BiFE), in combination with anodic stripping voltammetry, offers convenient measurement of low concentrations of tin. The procedure involves simultaneous in situ formation of the bismuth film electrode on a glassy carbon substrate electrode, together with electrochemical deposition of tin, in a non-deaerated model solution containing bismuth ions, catechol as complexing agent and the metal analyte, followed by an anodic stripping scan. The BiFE is characterized by an attractive electroanalytical performance, with two distinct voltammetric stripping signals corresponding to tin, accompanied with low background contributions. Several experimental parameters were optimized, such as concentration of bismuth ions and catechol, deposition potential, deposition time and pH of the model solution. In addition, a critical comparison is given with bare glassy carbon and mercury film electrodes, revealing the superior characteristics of BiFE for measurement of tin. BiFE exhibited highly linear behavior in the examined concentration range from 1 to 100 microg L(-1) of tin (R2=0.997), an LoD of 0.26 microg L(-1) tin, and good reproducibility with a calculated R.S.D. of 7.3% for 10 microg L(-1) tin (n=10). As an example, the practical applicability of BiFE was tested with the measurement of tin in a real sample of seawater.

The in vivo performance of a voltammetric microprobe based on overoxidized poly(1,2-phenylenediamine) coated carbon fiber microelectrode (OPPD/CFME), developed in our laboratory, is presented. For this purpose, an OPPD microprobe was stereotaxicaly implanted in the striatum of a deeply anesthetized Wistar rat for the simultaneous measurement of dopamine, serotonin and ascorbate. Furthermore, the post mortem levels of these physiologically important compounds were monitored after the rats were terminated with an overdose of anesthetic introduced through an indwelling jugular catheter. Using cyclic (CV) and square-wave (SWV) voltammetry, the OPPD/CFME was demonstrated to exhibit efficient separation of the voltammetric signals of dopamine, serotonin and ascorbate in the presence of biological matrix, with the SWV mode allowing more convenient detection regarding both sensitivity and selectivity. Explicit proof of in vivo dopamine detection at the OPPD/CFME was achieved via the absence of the dopamine signal in rats with unilateral lesions of nigrostriatal dopaminergic neurons, which was induced by the use of the selective dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). In stark contrast to non-treated rats, where a strong signal corresponding to 1.2 micro M dopamine was measured at ca. 90 s after the rats' death, the signal for dopamine in dopamine-depleted striatum of 6-OHDA rats was absent. A critical comparison of the in vivo performance of the OPPD/CFME to that of bare and Nafion-coated carbon fiber microelectrodes, often used in neurophysiological studies, clearly showed a significant advantage of the former microprobe. The OPPD/CFME allowed multiple and repetitive in vivo measurements, along with pre- and post-measurement external calibration, with no loss in selectivity and an acceptable loss in sensitivity, indicating that the active sensing sites were not adversely blocked by the components of the extracellular fluid, thus affirming the great practical in vivo applicability of the OPPD microprobe.

A new sol–gel precursor, based on 1-methyl-3-[3-(trimethoxy-λ4-silyl)propyl]imidazolium iodide (MTMSPI+I−), was synthesized and investigated as a potential novel quasi-solid-state ionic liquid redox electrolyte for dye-sensitized photoelectrochemical cells of the Graetzel type. MTMSPI+I− was hydrolyzed with acidified water and the reaction products of the sol–gel condensation reactions were assessed with the help of 29Si NMR and infrared spectroscopic techniques, which revealed that the aged MTMSPI+I− sols consisted of a positively charged polyhedral cube-like silsesquioxane and iodide/triiodide species. The addition of iodine increased the specific conductivity of the non-hydrolyzed and hydrolyzed MTMSPI+I−, which was attributed to the formation of triiodide ions and demonstrated from Raman spectra measurements. Steady-state voltammetric measurements at a Pt disc microelectrode, performed for the hydrolyzed MTMSPI+I− and MTMSPI+I− + I2 sols, revealed changes in the apparent diffusion coefficients (Dapp) during the course of transformation of the ionic liquid from a liquid to a quasi-solid-state.

A study on the preparation and characterisation of ex situ formed bismuth film microelectrodes (BiFMEs) is presented, focusing in particular on their stable and reliable stripping electroanalytical performance. The potentiostatic pre-plating of the bismuth film onto a single carbon fibre substrate microelectrode was investigated and optimised with the aim of achieving long-term electrochemical and mechanical film stability. Several important film preparation parameters, such as plating agent, potential and time, and composition of the plating solution were examined with respect to the current signals of 40 consecutive adsorptive cathodic stripping voltammetry (AdCSV) measurements of trace Co(II) as model analyte. A comparison, also presented, of the stripping performance between bismuth and mercury film microelectrodes revealed a distinct practical advantage of the BiFME. The resulting optimised BiFME exhibited, besides excellent long-term film functional stability, attractive stripping analytical performance. Employing AdCSV with square-wave voltammetric detection, highly linear behaviour was obtained in the examined concentration range, with limits of detection of 70 and 90ng/l and excellent reproducibility with 2.4 and 2.9% relative standard deviation at the 1μg/l level (n=10), for Co(II) and Ni(II), respectively, achieved using only 2min preconcentration time in the presence of dissolved oxygen. In addition, the performance of the proposed ex situ prepared BiFME in both anodic stripping voltammetry (ASV) of Cd(II) and Pb(II) and in AdCSV of Co(II) and Ni(II) from the same test solution is demonstrated. The ex situ prepared BiFME represents a promising non-toxic, environmentally friendly microsensor for detection at microlocations and in microvolumes, in particular where in situ bismuth film electrode preparation is inappropriate, inconvenient or impossible.

The bismuth film electrode (BiFE) is presented for use in both batch voltammetric and flow injection (FI) amperometric detection of some nitrophenols (2-nitrophenol, 2-NP; 4-nitrophenol, 4-NP; 2,4-dinitrophenol, 2,4-DNP). The bismuth film was deposited ex situ (batch measurements) and in-line (FI) onto a glassy carbon substrate electrode. Batch analysis of the nitrophenols was carried out in 0.04 M Britton Robinson (BR) buffer pH 4, while for FI measurements, a carrier/electrolyte solution composed of 0.1 M BR buffer pH 4 mixed with methanol (20 + 80, v/v%) was employed to resemble media used in preconcentration/clean-up and flow separation sample pretreatment procedures. Under batch conditions, the voltammetric behavior of the nitrophenols was examined for dependence on medium pH in the range of 2 to 10. Employing the square-wave voltammetry mode, the limits of detection were 0.4 mug L-1, 1.4 mug L-1, and 3.3 mug L-1 for 2-NP, 4-NP, and 2,4-DNP, respectively. Under flow conditions, a simple in-line electrochemical bismuth film renewal procedure was tested and shown to provide very good inter- and intra-electrode reproducibility of the current signals at low mug L-1 analyte concentrations. The limits of detection for 2-NP, 4-NP and 2,4-DNP obtained using FI and amperometric detection at -1.0 V (vs. Ag/AgCl) were 0.3 mug L-1, 0.6 mug L-1 and 0.7 mug L-1, respectively, with linear ranges extending up to 20 mug L-1. The attractive performance of the BiFE under flow analysis conditions offers great promise with respect to its detection capability and to its use for a prolonged period of time with no need for inconvenient removal of the electrode from the system for mechanical surface treatment.

A study is presented on the use of the bismuth film electrode (BiFE) operated in the anodic stripping and the cathodic adsorptive stripping voltammetry (ASV, CAdSV) modes, for the determination of two trace heavy metals (Cd and Co, respectively), in soil extract samples. Two types of BiFE were examined in this study: the in situ prepared BiFE, which was employed in ASV determination of Cd, and the ex situ prepared BiFE, which was used in CAdSV of Co with dimethylglyoxime (DMG) as complexing agent. A series of unpretreated soil extracts with varying Cd and Co concentrations were analyzed, and the results obtained compared to those determined using inductively coupled plasma–mass spectrometry (ICP–MS). The results revealed the suitability of stripping analysis at the BiFE for determination of μg l−1 levels of heavy metals in soil extracts. The promising results obtained here, coupled with the non-toxic nature of bismuth (in comparison to commonly used mercury electrodes employed in stripping analysis), offer great promise in centralized and decentralized analysis of trace heavy metals in complex environmental matrices.

Bismuth film electrode (BiFE) is presented as a promising alternative to mercury electrodes for the simultaneous determination of trace cobalt and nickel in non-deoxygenated solutions. The preplated BiFE was employed under adsorptive stripping constant current chronopotentiometric and adsorptive stripping voltammetric conditions in the presence of dimethylglyoxime complexing agent. BiFE exhibited well-defined and undistorted signals with favorable overall resolution for cobalt and nickel cations, with the signals for both metal cations being practically independent of each other. The stripping performance of BiFE is characterized by good reproducibility (RSD 1.4% for Co(II), and 4.3% for Ni(II)), low detection limits of 0.08 μg l−1 for Co(II) and 0.26 μg l−1 for Ni(II) employing a deposition time of 60 s, in addition to good linearity. The non-toxic character of bismuth imparts the possibility of tailoring disposable and one-shot electrochemical sensors for decentralized environmental, clinical and industrial monitoring of trace cobalt and nickel.

A new electrode surface design, the bismuth film electrode (BiFE), is presented as a promising alternative to mercury and other solid electrodes for direct cathodic electrochemical detection of organic compounds. The preparation of the BiFE, involving an ex situ electroplating of metallic bismuth onto a glassy carbon (GC) substrate electrode, was optimised. The useful negative potential windows of the BiFE in the pH range 1 (−0.2 to −0.8 V vs Ag/AgCl) to 10 (−0.2 to −1.5 V) were determined. The reproducibility of measuring 2-nitrophenol as a model compound (relative standard deviation, r.s.d., n=10) was found to be 0.5% at the same BiFE, and 1.0% at successive newly prepared BiFEs. No polishing or any other pre-treatment of the substrate GC surface was required prior to re-plating of a new Bi film. The BiFE showed similar or even favourable voltammetric behaviour when compared to mercury and bare GC electrodes, and was successfully tested for amperometric detection under hydrodynamic conditions. The results revealed that BiFE is an attractive new non-mercury metallic electrode particularly suitable for cathodic electrochemical detection in flow analytical systems.