Submicromolar Detection Limits Research Articles

A novel l-leucine modified Sol-Gel-Carbon electrode for simultaneous electrochemical detection of homovanillic acid, dopamine and uric acid in neuroblastoma diagnosis

Neuroblastoma is a pediatric neuroblastic tumor arising in the sympathetic nervous crest cells. A high grade of Neuroblastoma is characterized by a high urinary excretion of homovanillic acid and dopamine. In this work l-leucine modified Sol-Gel-Carbon electrode was used for a sensitive voltammetric determination of homovanillic acid and dopamine in urine. The electrochemical response characteristics were investigated by cyclic and differential pulse voltammetry; the modified electrode has shown an increase in the effective area of up to 40%, a well-separated oxidation peaks and an excellent electrocatalytic activity. High sensitivity and selectivity in the linear range of 0,4–100μML−1 of homovanillic acid and 10–120μML−1 of dopamine were also obtained. Moreover, a sub-micromolar limit of detection of 0.1μM for homovanillic acid and 1.0μM for the dopamine was achieved. Indeed, high reproducibility with simple preparation and regeneration of the electrode surface made this electrode very suitable for the determination of homovanillic acid and dopamine in pharmaceutical and clinical preparations. The mechanism of homovanillic acid and the electrochemical oxidation at l-leucine modified Sol-Gel-Carbon electrode is described out the B3P86/6-31+G(d,p) level of theory as implemented in Gaussian software.

Circumventing Traditional Conditioning Protocols in Polymer Membrane-Based Ion-Selective Electrodes.

Preparation of ion-selective electrodes (ISEs) often requires long and complicated conditioning protocols limiting their application as tools for in-field measurements. Herein, we eliminated the need for electrode conditioning by loading the membrane cocktail directly with primary ion solution. This proof of concept experiment was performed with iodide, silver, and sodium selective electrodes. The proposed methodology significantly shortened the preparation time of ISEs, yielding functional electrodes with submicromolar detection limits. Moreover, it is anticipated that this approach may form the basis for the development of miniaturized all-solid-state ion-selective electrodes for in situ measurements.

A robust method for iodine status determination in epidemiological studies by capillary electrophoresis.

Iodine deficiency is the most common preventable cause of intellectual disabilities in children. Global health initiatives to ensure optimum nutrition thus require continuous monitoring of population-wide iodine intake as determined by urinary excretion of iodide. Current methods to analyze urinary iodide are limited by complicated sample pretreatment, costly infrastructure, and/or poor selectivity, posing restrictions to large-scale epidemiological studies. We describe a simple yet selective method to analyze iodide in volume-restricted human urine specimens stored in biorepositories by capillary electrophoresis (CE) with UV detection. Excellent selectivity is achieved when using an acidic background electrolyte in conjunction with dynamic complexation via α-cyclodextrin in an unmodified fused-silica capillary under reversed polarity. Sample self-stacking is developed as a novel online sample preconcentration method to boost sensitivity with submicromolar detection limits for iodide (S/N ≈ 3, 0.06 μM) directly in urine. This assay also allows for simultaneous analysis of environmental iodide uptake inhibitors, including thiocyanate and nitrate. Rigorous method validation confirmed good linearity (R(2) = 0.9998), dynamic range (0.20 to 4.0 μM), accuracy (average recovery of 93% at three concentration levels) and precision for reliable iodide determination in pooled urine specimens over 29 days of analysis (RSD = 11%, n = 87).

A quinoline derivative as an efficient sensor to detect selectively Al³⁺ ion.

A quinoline-based Schiff base 1 has been utilized as a fluorescence chemosensor for the selective detection of Al(3+). The receptor 1 exhibited a high association constant (3.67 × 10(5) M(-1)) with submicromolar detection limit (0.18 ppm) towards Al(3+) in CH3CN solution.

Colorimetric and fluorometric probe for the highly selective and sensitive detection of cyanide based on coumarinyloxime

We report a coumarinyloxime-based fluorescent probe (1) for cyanide. The probe has shown a highly selective and sensitive response to the cyanide anion over other various anions with a submicromolar limit of fluorimetric detection (LOD=0.49μM). When CN− was added to 1, probe 1 showed dramatic fluorescence and color changes, which were easily observable with the naked eye in aqueous solvent.

Use of potentiometric detection in (ultra) high performance liquid chromatography and modelling with adsorption/desorption binding kinetics

Observation of a potentiometric sensor's response behaviour after injection in flow injection analysis at different concentrations allowed studying “on” and “off” kinetics of the analyte's adsorption/diffusion behaviour. The alkaloid metergoline was mostly used as an example. kon and koff rate constant values were measured, and the association constant Kass, and ΔG values of the analyte–surface interaction were calculated with an adsorption-based model which proved to be fully applicable. kon increased by decreasing the sensor dimensions, while koff was unaffected by miniaturization. Increasing acetonitrile concentrations in the running buffer increased koff, while kon was unaffected. The experimentally determined ΔG values of the analyte–surface interaction showed a linear relation to the response of the sensor, in mV. This knowledge was applied to optimize the potentiometric detection of plant alkaloids in (U)HPLC. Sub-micromolar detection limits were obtained with the potentiometric detector/(U)HPLC combination. This is the first time that the response rates and the response itself can be modelled accurately for coated wire potentiometric sensors, and it is the first application of a potentiometric detector in UPLC.

Simultaneous determination of Sunset yellow and Tartrazine in soft drinks using gold nanoparticles carbon paste electrode

The monitoring of synthetic dyes in foods is very important due to their potential harmfulness to human beings. Herein, a carbon-paste electrode (CPE) that is chemically modified with gold nanoparticles (nAu) was fabricated and used for the determination of Sunset yellow (SY) and Tartrazine (Tz). Cyclic and differential pulse voltammetry (CV and DPV) results revealed two well-resolved anodic peaks for SY and Tz with remarkably increase in oxidation signals of these colourants. Based on this, a novel electrochemical method was developed for the simultaneous determination of SY and Tz. High sensitivity and selectivity, sub-micromolar detection limit, high reproducibility and regeneration of the electrode surface by simple polishing make the nAu-CPE electrode very suitable for the determination of SY and Tz in commercially available soft drinks. The detection limits was 3.0×10−8 and 2.0×10−9moll−1 for SY and Tz, respectively, which are remarkably lower than those reported previously for SY and Tz using other modified electrodes.

Hydroxy-Iron/β-cyclodextrin-Film Amperometric Sensor for the Endocrine Disruptor Substance Bisphenol-A in an Aqueous Medium with Reduced Fouling Effects

Amperometric sensors for the detection of bisphenol A (BPA) were developed by drop coating an amorphous hydroxy iron product (FeOx) and its composites with β-cyclodextrin (BCD) on the surface of glassy carbon electrodes (GCE). The composite materials were prepared in an aqueous medium by adding a stoichiometric mixture of iron (II) and iron (III) ions into a strongly alkaline solution of BCD under reflux at 90°C. The electrochemical oxidation behavior and detection of BPA at the modified electrode was investigated by using cyclic voltammetry. The potentials (vs. Ag/AgCl) for BPA-oxidation at the modified electrodes (727–771 mV) were significantly lower than observed for the bare GCE (834 mV) in aq. KCl. The lowest oxidation potential was observed for the iron oxide film without BCD. However, the analytical figures of merit as an amperometric sensor based on cyclic voltammetric peak measurements were the best in the case of the iron oxide which was synthesized in the presence of BCD. Its two orders of magnitude of linearity, sub-micromolar detection limit, and reproducibility within experimental error make this sensor potentially exploitable for real sample analysis. The significantly shorter linearity range of the bare GCE indicated that the iron oxide product and its composites with BCD were less prone to passivation by BPA oxidation products. Application of the sensor in a real sample matrix was also demonstrated successfully using “Stoney Ginger Beer samples” spiked with a saturated aq. solution of BPA. The recovery rate was 100%.

Simultaneous electrochemical determination of dopamine and ascorbic acid using poly ( l-serine) modified carbon paste electrode

A polymerized film of l-serine was prepared on the surface of a carbon paste electrode by electropolymerization using cyclic voltammetry. The polymer film-modified electrode was successfully used for the simultaneous determination of dopamine and ascorbic acid. It enhanced the redox peak currents of dopamine and ascorbic acid conspicuously and could determine them sensitively and separately at their low concentrations in 0.1 M acetate buffer solution (pH 5.5). The negative electrocatalytic activity of the poly- l-serine on the electrochemical oxidation of ascorbic acid reduced its oxidation potential to a more negative potential. It results in the separation of the overlapping voltammogram of dopamine and ascorbic acid at the carbon paste electrode in a mixture. The separation of anodic peak potentials of dopamine and ascorbic acid reached 180 mV by cyclic voltammetry. Using differential pulse voltammetry the calibration curves of dopamine and ascorbic acid were obtained in the range of 50 × 10 −6–3 × 10 −4 M and 0.1–0.3 × 10 −4 M respectively. The detection limit of dopamine was 5 × 10 −8 M in the presence of 1 mM ascorbic acid. The effect of pH and scan rate on the electrode process was investigated. High sensitivity and selectivity, sub-micro molar detection limit, high reproducibility together with ease of preparation and regeneration of the electrode surface make the electrode very suitable for the determination of dopamine in the presence of ascorbic acid in pharmaceutical and clinical preparations.

Ion-Selective Electrodes Based on a Pyridyl-Containing Triazolophane: Altering Halide Selectivity by Combining Dipole-Promoted Cooperativity with Hydrogen Bonding

Triazolophanes are ionophores, with preorganized cyclic cavities that have tunable selectivities for halides. The interaction with halides is based on hydrogen bonding between the eight CH hydrogen atoms of the cavity and the halide anion. The rigidity of the cavity in tetraphenylene triazolophane along with the hydrogen bonding favors planar 1:1 complexation of "snugly" encapsulated chloride and bromide. Manipulating the triazolophane's structure by introducing two pyridyl moieties into the cavity alters the receptor's binding mode. This change adds a dipole-promoted driving force that combines with hydrogen bonding to favor the formation of 2:1 sandwich complexes around halides. The potentiometric response of electrodes based on this new ionophore was evaluated for optimal halide selectivity. The new triazolophane-based electrode showed an anti-Hofmeister selectivity toward iodide with a submicromolar detection limit. The stoichiometry of complexation and the stability constants with different halides were evaluated using a segmented sandwich membranes method. The pyridyl-triazolophane demonstrated a response consistent with a 2:1 sandwich-type complex with iodide, in polyvinyl chloride (PVC) membranes.

Biocompatible hybrid film of β-cyclodextrin and ionic liquids: A novel platform for electrochemical biosensing

A novel electrochemical biosensing platform was developed based on the modification of biocompatible hybrid film of β-cyclodextrin (β-CD) and ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4) onto glass carbon electrode (GCE). Then an unmediated biosensor was successfully prepared by immobilizing the horseradish peroxidase (HRP) into the β-CD/ILs composite film. β-CD could provide a biocompatible microenvironment for HRP, and ILs could accelerate the electron transfer between HRP and the electrode. Results showed that the HRP entrapped in the β-CD/ILs film could maintain its native structure and the direct electrochemistry of HRP were facilely achieved. A couple of well-defined redox peaks of HRP were observed at about −0.32 V (vs. SCE), corresponding to the protein heme Fe(III)/Fe(II) redox couples. The electrocatalysis of this biosensor to both quercetin and hydrogen peroxide was characterized. The biosensor exhibited a low operating potential (−0.05 V vs. SCE), fast amperometric response, high sensitivity, good selectivity and sub-micromolar limit of detection.

Selective Determination of Epinephrine in the Presence of Ascorbic Acid and Dopamine Using a Glassy Carbon Electrode Modified with Valine

A glassy carbon electrode (GCE) modified with valine was used for the sensitive volt metric determination ofepinephrine (EP). The electrochemical response characteristics of the modified electrode toward EP, ascorbic acid (AA)and dopamine (DA) were investigated by cyclic voltammetry (CV). The results show an efficient catalytic activity ofthe electrode for the electro-oxidation of EP, which leads to an improvement to the reversibility of the electroderesponse. The effect of pH and potential sweep rate on the mechanism of the electrode process was investigated. Themodified electrode exhibits an efficient electron mediating behavior along with well-separated oxidation peaks for EP,AA, and DA. Under the optimum pH of 7.0 in phosphate buffer solution(PBS), the CV anodic peak current showed alinear relation versus EP concentration in the range of 4.5×10-6~1.0×10-5mol·L-1 and 1.0×10-5~1.4×10-4 mol·L-1, withcorrelation coefficients of 0.9997 and 0.9942. The detection limit is 7.6×10-7 mol·L-1. High sensitivity and selectivity,submicromolar detection limit, high reproducibility, along with ease of preparation and regeneration of the electrodesurface by simple polishing make this method suitable for the determination of EP in pharmaceutical and clinicalpreparations.

A digital microfluidic platform for the automation of quantitative biomolecular assays

A digital microfluidic platform for the automation of quantitative, multi-step biomolecular assays is developed and optimized. The platform consists of a 2-dimensional array of microvalves that can be programmed to perform reagent routing, mixing, rinsing, serial dilution, and many other operations using nanolitre scale volumes of sample. Discrete transfer of fluid between microvalves is characterized using gravimetric flow analysis and optimized to achieve maximum efficiency. Protocols for on-chip reagent mixing and serial dilution are optimized to achieve linearity over a 1000-fold dilution range. These optimized programs are used to develop a rapid, quantitative assay for hydrogen peroxide, a biomarker of oxidative stress. A sub-micromolar limit of detection is demonstrated with an 8.5 min program runtime, thus establishing this platform as an effective tool for the automation of multi-step bioassays. The programmability of this system enables rapid development of diverse assay protocols on a common chip format.

Application of carbon-paste electrode modified with iron phthalocyanine for voltammetric determination of epinephrine in the presence of ascorbic acid and uric acid

A carbon-paste electrode (CPE) modified with iron(II) phthalocyanine was used for the sensitive voltammetric determination of epinephrine (EN). The electrochemical response characteristics of the modified electrode toward EN, ascorbic acid (AA) and uric acid (UA) were investigated by cyclic and differential pulse voltammetry (CV and DPV). The results show an efficient catalytic activity of the electrode for the electro-oxidation of EN, which leads to improvement of reversibility of the electrode response and lowering its overpotential by more than 100 mV. The effect of pH and potential sweep rate on the mechanism of the electrode process was investigated. The modified electrode exhibits an efficient electron mediating behavior together with well-separated oxidation peaks for EN, AA and UA. Under the optimum pH of 4.0 in 0.1 M acetate buffer solution, the DPV anodic peak current showed a linear relation versus EN concentration in the range of 1–300 μM, with a correlation coefficient of 0.998 and a detection limit of 0.5 μM. High sensitivity and selectivity, sub-micromolar detection limit, high reproducibility, together with ease of preparation and regeneration of the electrode surface by simple polishing, make the electrode very suitable for the determination of EN in pharmaceutical and clinical preparations.

Carbon film resistor electrode for amperometric determination of acetaminophen in pharmaceutical formulations

Flow injection analysis (FIA) with amperometric detection was employed for acetaminophen quantification in pharmaceutical formulations using a carbon film resistor electrode. This sensor exhibited sharp and reproducible current peaks for acetaminophen without chemical modification of its surface. A wide linear working range (8.0 × 10 −7 to 5.0 × 10 −4 mol L −1) in phosphate buffer solution as well as high sensitivity (0.143 A mol −1 L cm −2) and low submicromolar detection limit (1.36 × 10 −7 mol L −1) were achieved. The repeatability (R.S.D. for 10 successive injections of 5.0 × 10 −6 and 5.0 × 10 −5 mol L −1 acetaminophen solutions) was 3.1 and 1.3%, respectively, without any memory effect between injections. The new procedure was applied to the analyses of commercial pharmaceutical products and the results were in good agreement with those obtained utilizing a spectrophotometric method. Consequently, this amperometric method has been shown to be very suitable for quality control analyses and other applications with similar requirements.

Determination of cationic neurotransmitters and metabolites in brain homogenates by microchip electrophoresis and carbon nanotube-modified amperometry

An electrophoretic method for simultaneous determination of catecholamines and their O-methoxylated metabolites on the microchip as well as in the capillary is presented. A complex separation system employing sodium dodecyl sulfate (SDS) micelles, dendrimers forming a second pseudostationary phase and borate complexation is needed for the satisfactory separation of the selected compounds on the short migration length. A carbon nanotube-modified working electrode has been applied for the sensitive amperometric detection with submicromolar detection limits. The applicability of this new method for the analytics of real samples is demonstrated by analysis of mouse brain homogenate on the microchip and human urine by capillary electrophoresis.

The potentiometric behavior of polymer-supported metallophthalocyanines used as anion-selective electrodes

Liquid polymer membrane electrodes based on nickel and manganese phthalocyanines were examined for use as anion-selective electrodes. The electrodes were prepared by incorporating the ionophores into plasticized poly(vinyl chloride) membranes, which were directly coated onto the surfaces of graphite electrodes. The resulting electrodes demonstrate near-Nernstian responses over a wide linear range of perchlorate anion (5 x 10(-7) to 1 x 10(-1) M). The electrodes have a fast response time, submicromolar detection limits (5 x 10(-7) M perchlorate), and could be used over a wide pH range of 3.5-10. The influences of lipophilic cationic and anionic additives on the response properties of the electrodes were investigated. The proposed sensors revealed high selectivity for perchlorate over a number of common inorganic and organic anions. The highest selectivity was observed for the electrode based on manganese phthalocyanine in the presence of the lipophilic anionic additive sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. Application of the electrodes to determine perchlorate in tap water and human urine is also reported.

Carbon paste electrode incorporating multi-walled carbon nanotube/cobalt salophen for sensitive voltammetric determination of tryptophan

A modified carbon paste electrode is prepared by incorporating multi-walled carbon nanotube (MWCNT) and cobalt salophen (CoSal). A mixture of fine graphite powder with 10 wt% of MWCNT is applied for the preparation of carbon paste (by dispersing in Nujol) and finally modified with CoSal. The electrocatalytic oxidation of tryptophan (Trp) is investigated on the surface of the MWCNT/CoSal-modified electrode using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Using the modified electrode, the kinetics of the electro-oxidation of Trp is considerably enhanced, by lowering the anodic overpotential through a catalytic fashion. The mechanism of electrochemical behavior of Trp at the surface of modified electrode is analyzed by investigating the effect of sweep rate and pH on the responses of CV method. The prepared modified electrode showed voltammetric responses with high sensitivity and selectivity for Trp, which makes it very suitable for sub-micromolar detection of Trp. The electrode is able to completely resolve the voltammetric response of Trp from potentially interfering species, e.g. ascorbic acid (AA) and cysteine, which can be oxidized effectively on the surface of the modified electrodes with transition metal complexes. The voltammetric studies using the modified electrode showed three well-resolved anodic peaks for AA, cysteine and Trp, revealing the applicability of the electrode for simultaneous electrochemical detections of these compounds in mixtures. A linear dynamic range of 5 × 10 −5 to 5.0 × 10 −7 M for Trp is obtained in buffered solutions with pH 4.0. The linear range of the calibration curve remains constant in binary mixture solutions of Trp and cysteine or AA. By use of modified electrode, the voltammetric detection limit for Trp in DPV measurements was 1.0 × 10 −7 M. The reproducibility of the electrode response, based on 6 measurements during 1 month, was 2.2% for the slope of the calibration curve. High sensitivity and selectivity together with sub-micromolar detection limit of the electrode response make it very suitable for the determination of trace amounts of Trp in pharmaceutical and clinical preparations.

Single-step enantioselective amino acid flux analysis by capillary electrophoresis using on-line sample preconcentration with chemical derivatization

Capillary electrophoresis (CE) represents a versatile platform for integrating sample pretreatment with chemical analysis because of its ability to tune analyte electromigration and band dispersion properties in discontinuous electrolyte systems. In this article, a single-step method that combines on-line sample preconcentration with in-capillary chemical derivatization is developed for rapid, sensitive, and enantioselective analysis of micromolar levels of amino acids that lack intrinsic chromophores by CE with UV detection. Time-resolved electrophoretic studies revealed two distinct stages of amino acid band narrowing within the original long sample injection plug occurring both prior to and after in-capillary labeling via zone passing by ortho-phthalaldehyde/ N-acetyl l-cysteine (OPA/NAC). This technique enabled direct analysis of d-amino acids in a 95% enantiomeric excess mixture with sub-micromolar detection limits and minimal sample handling, where the capillary functions as a preconcentrator, microreactor, and chiral selector. On-line sample preconcentration with chemical derivatization CE (SPCD–CE) was applied to study the enantioselective amino acid flux in Escherichia coli bacteria cultures, which demonstrated a unique l-Ala efflux into the extracellular medium. New strategies for high-throughput analyses of low-abundance metabolites are important for understanding fundamental physiological processes in bacteria required for screening the efficacy of new classes of antibiotics as well as altered metabolism in genetically modified mutant strains.

Coated Wire Potentiometric Detection for Capillary Electrophoresis Studied Using Organic Amines, Drugs, and Biogenic Amines

Capillary electrophoresis was coupled successfully and reliably to potentiometric sensors, which are based on an ionically conductive rubber phase coating, applied on a 250 microm diameter metal substrate. The membrane components included potassium tetrakis(p-chlorophenyl)borate (TCPB), bis(2-ethylhexyl)sebacate (DOS), and high molecular mass poly(vinyl chloride) (PVC). Potentiometry reveals a very sensitive CE detection mode, with sub-micromolar detection limits for amines and the randomly chosen drugs quinine, clozapine, cocaine, heroine, noscapine, papaverine, and ritodrine. The lowest detection limit, 1 x 10(-8) M injected concentration, was obtained for the quaternary ammonium compound tetrahexylammonium chloride. The more polar lower aliphatic amines and the biogenic amines dopamine, adrenaline, and cadaverine have much higher detection limits. The detection limits are log P dependent. Addition of a commercially available calixarene molecule or a synthetic macrocyclic amphiphilic receptor molecule to the electrode coatings enhanced the sensitivity respectively for the lower aliphatic amines and for the biogenic amines. A transpose of the Nikolskii-Eisenman-type function was suggested and used to convert the signal of the detector to a concentration-dependent signal.