A carbon paste electrode modified with a metal-organic framework of type MIL-101(Fe) for voltammetric determination of citric acid.

A carbon paste electrode (CPE) modified with a metal-organic framework composite of type MIL-101(Fe) is described for determination of citric acid (CA). The electrochemical activity of the modified CPE was studied by cyclic voltammetry and differential pulse voltammetry. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms and X-ray powder diffraction were used for characterization of the MIL-101(Fe). Under optimized electrochemical conditions, the anodic peak current, best measured at working potentials around 0.02 V (vs. Ag/AgCl); decreases linearly in the 5.0 to 100 µM CA concentration range, and the detection limit is 4.0 µM (at S/N = 3). The electrode exhibits good selectivity for CA, with no significant interference in the wide pH range of 3.0 to 9.0. The electrochemical sensitivity of the MIL-CPE is -0.67 µA·µM-1·cm-2. The method was successfully applied to the determination of CA in some commercial beverages. The good recoveries (98-102%) and the agreement of data with those obtained by HPLC indicate the applicability of the method. Graphical abstract Schematic presentation of a new modified carbon paste electrode based on the metal-organic framework of type MIL-101(Fe) for the simple and sensitive determination of citric acid. The results show the MIL-101(Fe)-modified electrode to have good selectivity for citric acid and to enable real sample analysis.

Ascorbic Acid Determination Based on Electrocatalytic Behavior of Metal-Organic Framework MIL-101-(Cr) at Modified Carbon-Paste Electrode.

Background: Metal-organic frameworks (MOFs) are classified as microporous materials. They have large surface areas, low framework densities, accessible cages, and tunnels with modifiable pores. Objective: The MOF MIL-101(Cr) with a large pore volume was synthesized hydrothermally and used in the electrochemical reactions as an electrocatalyst for the determination of ascorbic acid (AA). The synthesized MOF was characterized by scanning electron microscopy and X-ray powder diffraction. Methods: The electrocatalytic behavior of a carbon-paste electrode modified with MIL-101(Cr) was studied through electro-oxidation of AA by cyclic voltammetry and square wave voltammetry. The conventional three-electrode cell system, consisting of Ag/AgCl (3 M KCl) as the reference, platinum wire as the counter electrode, and modified carbon paste as the working electrode, was used in the experiment. Results: Under optimized experimental conditions, the electrode revealed a linear relationship between the oxidation peak current and concentration of AA over a wide range from 0.01 to 10 mM with the LOD of 0.006 mM (3 Sb/m). The results showed that 100-fold of Na+, K+, Mg²+, Ca2+, Cl-, SO42-, sorbitol, sucrose, fructose, citric acid, 40-fold of NO3-, glucose, sucrose, urea, and 10-fold of uric acid had no significant interference. The method was adapted for the determination of the concentration of AA present in two real samples (vitamin C tablet and vitamin C effervescent tablet) with recovery of 97.0 and 96.0%, respectively. Conclusions: A simple, sensitive and reliable modified electrode has been established and applied for the determination of AA. Highlights: The modified electrode represented a good performance in the analysis of the real sample.

Synthesis of highly stable and biocompatible gold nanoparticles for use as a new X-ray contrast agent.

This work reports a novel reduction procedure for the synthesis of Gum Arabic (GA) capped-gold nanoparticles (AuNPs) in glucosammonium formate as a new ionic liquid. The GA coated AuNPs show good stability in physiological media. The synthesized AuNPs were characterized by UV-Vis spectroscopy, transmission electron microscopy, dynamic light scattering and X-ray diffraction analysis. These stable AuNPs are introduced as a new contrast agent for X-ray Computed Tomography (X-ray CT). These nanoparticles have higher contrasting properties than the commercial contrast agent, Visipaque. The precursors used (Gum Arabic and glucose based-ionic liquid) for synthesis of AuNPs are biocompatible and non-toxic.

Aptamer-conjugated silver nanoparticles for electrochemical dual-aptamer-based sandwich detection of staphylococcus aureus.

Staphylococcus aureus (S. aureus) is one of the most important human pathogens and causes numerous illnesses. In this study, we report a sensitive and highly selective dual-aptamer-based sandwich immunosensor for the detection of S. aureus. In this bioassay system, a biotinylated primary anti-S.aureus aptamer was immobilized on streptavidin coated magnetic beads (MB), which serves as a capture probe. A secondary anti-S.aureus aptamer was conjugated to silver nanoparticles (Apt-AgNP) that sensitively reports the detection of the target. In the presence of target bacterium, an Apt/S.aureus/apt-AgNP sandwich complex is formed on the MB surface and the electrochemical signal of AgNPs followed through anodic stripping voltammetry. The proposed sandwich assay benefits from advantageous of a sandwich assay for increased specificity, MB as carriers of affinity ligands for solution-phase recognition and fast magnetic separation, AgNPs for signal amplification, and an electrochemical stripping voltammetry read-out as a simple and sensitive detection. The electrochemical immunosensor shows an extended dynamic range from 10 to 1×10(6) cfu/mL with a low detection limit of 1.0 cfu/mL (S/N=3). Furthermore, the possible interference of other analog bacteria was studied. To assess the general applicability of this sensor, we investigated the quantification of S. aureus in real water samples. The results were compared to the experimental results obtained from a plate counting method, which demonstrated an acceptable consistency.

A highly selective voltammetric sensor for nanomolar detection of mercury ions using a carbon ionic liquid paste electrode impregnated with novel ion imprinted polymeric nanobeads.

This work reports the preparation of a voltammetric sensor for selective recognition and sensitive determination of mercury ions using a carbon ionic liquid paste electrode (CILE) impregnated with novel Hg(2+)-ion imprinted polymeric nanobeads (IIP) based on dithizone, as a suitable ligand for complex formation with Hg(2+) ions. The differential pulse anodic stripping voltammetric technique was employed to investigate the performance of the prepared IIP-CILE for determination of hazardous mercury ions. The designed modified electrode revealed linear responses in the ranges of 0.5nM-10nM and 0.08µM-2µM with a limit of detection of 0.1nM (S/N=3). It was found that the peak currents of the modified electrode for Hg(2+) ions were at a maximum value in phosphate buffer of pH4.5. The optimized preconcentration potential and accumulation time were to be -0.9V and 35s, respectively. The applicability of the proposed sensor to mercury determination in waste water samples is reported.

Electrochemical detection of individual single nucleotide polymorphisms using monobase-modified apoferritin-encapsulated nanoparticles.

An electrochemical approach for detection of individual single nucleotide polymorphisms (SNPs) based on nucleobase-conjugated apoferritin probe loaded with metal phosphate nanoparticles is reported. Coupling of the nucleotide-modified nanoparticle probe to the mutant sites of duplex DNA was induced by DNA polymerase I (Klenow fragment) to preserve Watson-Crick base-pairing rules. After sequential liquid hybridization of biotinylated DNA probes with mutant DNA and complementary DNA, the resulting duplex DNA helixes were captured to the surface of magnetic beads through a well known and specific biotin-streptavidin affinity binding. For signaling each of eight possible Single-nucleotide polymorphisms (SNPs), Pb, Cu, Cd and Zn phosphate-loaded apoferritin nanoparticle probes were linked to adenosine (A), cytidine (C), guanosine (G), and thymidine (T) mononucleotides, respectively. Monobase-conjugated apoferritin probes were coupled to the mutant sites of the formed duplex DNA in the presence of DNA polymerase. Electrochemical stripping analyses of the metals loaded in apoferritin nanoparticle probes provide a means for detection and quantification of mutant DNA. Each mutation captures different nucleotide-conjugated apoferritin probe and provide a distinct four-potential voltammogram, whose peak potentials reflect the identity of the mismatch. The method is sensitive enough to accurately determine AG mutation, as the most thermodynamically stable mismatch to detect, in the range of 50-600 pM. The proposed protocol provides a simple, fast, cost-effective, accurate and sensitive method for detection of SNPs.

A cyclodextrin host-guest recognition approach to a label-free electrochemical DNA hybridization biosensor.

A novel label-free electrochemical DNA hybridization biosensor using a ß-cyclodextrin/poly(N-acetylaniline)/carbon nanotube composite modified screen printed electrode (CD/PNAANI/CNT/SPE) has been developed. The proposed DNA hybridization biosensor relies on the intrinsic oxidation signals of guanine (G) and adenine (A) from single-stranded DNA entered into the cyclodextrin (CD) cavity. Due to the binding of G and A bases to complementary cytosine and thymine bases in dsDNA, the signals obtained for ssDNA were much higher than that of dsDNA. The synergistic effect of the multi-walled carbon nanotubes provides a significantly enhanced voltammetric signal, and the CD encapsulation effect makes anodic peaks of G and A shift to less positive potentials than that at the bare SPE. The peak heights of G and A signals are dependent on both the number of the respective bases in oligonucleotides and the concentration of the target DNA sequences. Hybridization of complementary strands was monitored through the measurements of oxidation signal of purine bases, which enabled the detection of target sequences from 0.01 to 1.02 nmol µl(-1) with the detection limit of target DNA as low as 5.0 pmol µl(-1) (S/N = 3). Implementation of label-free and homogeneous electrochemical hybridization detection constitutes an important step toward low-cost, simple, highly sensitive and accurate DNA assay. Discrimination between complementary, noncomplementary, and two-base mismatch targets was easily accomplished using the proposed electrode.

A cyclodextrin host-guest recognition approach to an electrochemical sensor for simultaneous quantification of serotonin and dopamine.

An electrochemical sensor for simultaneous quantification of serotonin (5-hydroxytryptamine, 5-HT) and dopamine (DA) using a ß-cyclodextrin/poly(N-acetylaniline)/carbon nanotube composite modified carbon paste electrode has been developed. Synergistic effect of multi-walled carbon nanotube (MWCNT) in addition to the pre-concentrating effect of ß-cyclodextrin (ß-CD) as well as its different inclusion complex stability with 5-HT and DA was used to construct an electrochemical sensor for quantification of these important neurotransmitters. The overlapping anodic peaks of 5-HT and DA at 428 mV on bare electrode resolved in two well-defined voltammetric peaks at 202 and 363 mV vs. Ag/AgCl respectively. The oxidation mechanism of 5-HT and DA on the surface of the electrode was investigated by cyclic voltammetry and it was found that the electrode processes are pH dependent and electrochemical oxidation of 5-HT is totally irreversible while the electrode gave a more reversible process to DA. Under optimized conditions, linear calibration curves were obtained in the range of about 4-200 µM with a detection limits down to sub-µM levels (S/N=3) after 20-s accumulation, for both. The proposed sensor was shown to be remarkably selective for 5-HT and DA in matrices containing different species including ascorbic acid and uric acid. The suitability of the developed method was tested for the determination of 5-HT and DA in the Randox Synthetic Plasma samples and acceptable recoveries were obtained for a set of spiked samples.

Method for preparation of a sol-gel-derived carbon ceramic electrode using microwave irradiation.

A fast and new method for preparation of a sol-gel carbon ceramic electrode (CCE) by microwave (MW) irradiation is introduced. In comparison to previous preparation methods which require a very long time (mostly 48 h for ceramic completion and drying in air), this method requires only a few minutes. Furthermore, before MW irradiation an ultrasonic wave was applied to influence the gelation time and dispersion of particles in the sol-gel. The composition of the proposed carbon ceramic was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy, which was very similar to the air-dried sol-gel carbon ceramics. The proposed electrode was used for determination of dopamine (DA); the results show that this method has a wider linear range (LR) and lower detection limit (DL) than the air-dried CCE and exhibits a greater sensitivity for determination of DA compared to a recently reported CCE.

A simple and cost-effective method, as an appropriate alternative for visible spectrophotometry: development of a dopamine biosensor.

In this study, a new, simple, fast and inexpensive method as an alternative to visible spectrophotometry is developed. In this method the cells containing the sample solution were scanned with a scanner, then the color of each cell was analyzed with software written in visual basic (VB 6) media to red, green and blue values. The cells were built by creating holes in the Plexiglas sheet. The dimensions of identical cells were examined by Cr (III) solution with known concentrations. The validity of this new method was studied by determination of dopamine (DA) without using any other reagent. The parameters which affect the system were optimized. The comparison between the current and traditional UV-Vis spectrophotometry methods was studied and the results revealed similar trends in both methods. The developed method was successfully applied to the determination of dopamine in serum and urine without using any pretreatment. Finally comparing the results obtained in the developed method showed that microwave irradiation of the solution can decrease the experimental time, increase sensitivity and improve the limit of detection.

Electrochemical studies on the oxidation of guanine and adenine at cyclodextrin modified electrodes.

An electrochemical sensor for guanine and adenine using cyclodextrin-modified poly(N-acetylaniline) (PNAANI) on a carbon paste electrode has been developed. The oxidation mechanism of guanine and adenine on the surface of the electrode was investigated by cyclic voltammetry. It was found that the electrode processes are irreversible, pH dependent, and involve several reaction products. The electron transfer process occurs in consecutive steps with the formation of a strongly adsorbed intermediate on the electrode surface. Also, a new method for estimating the apparent formation constants of guanine and adenine with the immobilized cyclodextrins, through the change of surface coverage of studied analytes has been reported. Both guanine and adenine showed linear concentrations in the range of 0.1-10 microM by using differential pulse voltammetry, with an experimental limit of detection down to 0.05 microM. Linear concentration ranges of 2-150 microM for guanine and 6-104 microM for adenine have been found when cyclic voltammetry was used for determination of both analytes.

A simple and selective sensor for the determination of ascorbic acid in vitamin C tablets based on paptode.

A method for the determination of ascorbic acid in vitamin C tablets based on a very simple paptode design on TLC strips is described. This procedure is based on the reduction of iron(III) with ascorbic acid and the formation of a colorful red complex with immobilized 2,2'-dipyridyl (dipy) on TLC strips. The linear range of the system was 20-200 ppm with a detection limit of 1 ppm and a relative standard deviation of 1.5% (n = 28). The parameters, such as pH, concentration of iron(III), concentration of dipy and the volume of dipy per 1 cm(2) of TLC strips, were optimized. The proposed sensor was successfully applied for the determination of ascorbic acid in vitamin C tablets.

Spectrophotometric determination of acidity constants by two-rank annihilation factor analysis.

Annihilation of the contribution of one chemical component from the original data matrix is a general method in rank annihilation factor analysis (RAFA). However, RAFA is not applicable for studying the protonation equilibria of multiprotic acids. In this work, a two-rank annihilation factor analysis (TRAFA) method was proposed for determination of the acidity constants of diprotic acids. After recording the electronic absorbance spectra of the acids at different pH, the contributions of both H(2)A and A(2-) were annihilated from the absorbance data, which made feasible the determination of two successive acidity constants. The method was validated by analysis of simulated data and its application to the determination of the acidity constants of calmagite, as a reference compound. A close agreement was obtained between the resulted values by TRAFA and the declared values. Indeed, the method was used for determination of the acidity constants of two new chromenone derivatives in binary solvents mixtures of methanol and water. The effects of changing solvent composition on acidity constant data were explained by linear solvation free energy relationships (LSFER) utilizing solvatochromic parameters.

Electrochemical monitoring of piroxicam in different pharmaceutical forms with multi-walled carbon nanotubes paste electrode.

The electrochemical behavior of piroxicam on a multi-walled carbon nanotubes electrode for the first time was investigated. A highly sensitive and fast responding sensor for determination of piroxicam was simply and conveniently fabricated. The constructed electrode exhibits efficiently catalytic activity for the electrooxidation of piroxicam at a reduced over potential with high sensitivity, stability, and long lifetime in the wide concentration rang of piroxicam. The oxidation process was found to be dependent on the pH of the supporting electrolyte. The behavior is further exploited as a sensitive detection method for piroxicam determination by differential pulse voltammetry. Under the optimized conditions the calibration plots are linear in the concentration range of 0.15-5 microg ml(-1). Application of the method for the determination of the drug in the dosage form (Feledene capsules and tablets and also piroxicam gel), without any interference, from the excipients, resulted in acceptable deviation from the stated concentrations. Recoveries were obtained in the range 96.35-104.16%. The detection limit of 0.1 microg ml(-1) was obtained for piroxicam determination.

Carbon nanotube composite coated platinum electrode for detection of Cr(III) in real samples.

This paper demonstrates the application of composite multi-walled carbon nanotube (MWNT) polyvinylchloride (MWNT-PVC) based on 1,5-diphenylcarbazide as chromium ionophore in potentiometric measurement. The sensor shows a good Nernstian slope of 19.52+/-0.40mV/decade in a wide linear range concentration of 6.3x10(-8) to 1.0x10(-2)M for Cr(NO(3))(3). The detection limit of this electrode was found to be 3.2x10(-8)M of Cr(NO(3))(3) and is applicable in a pH range of 3.0-6.8. It has a short response time of about 10s. This chromium electrode has a good selectivity over 16 various metal ions. The practical analytical utility of this electrode was demonstrated by measurement of Cr(III) in drinking water and mineral water samples without any serious preliminary pre-treatment and chromium in multivitamin.

Platinum electrode coated with a bentonite-carbon composite as an environmental sensor for detection of lead.

A Pt wire coated with a bentonite-carbon composite in a poly(vinyl chloride) membrane was used for detection of lead. The sensor has a Nernstian slope of 29.42+/-0.50 mV per decade over a wide range of concentration, 1.0 x 10(-7) to 1.0 x 10(-3) mol L(-1) Pb(NO(3))(2). The detection limit is 5.0 x 10(-8) mol L(-1) Pb(NO(3))(2) and the electrode is applicable in the pH range 3.0-6.7. It has a response time of approximately 10 s and can be used at least for three months. The electrode has good selectivity relative to nineteen other metal ions. The practical analytical utility of the electrode is demonstrated by measurement of Pb(II) in industrial waste and river water samples.

Partial least squares-based multivariate spectral calibration method for simultaneous determination of beta-carboline derivatives in Peganum harmala seed extracts.

The partial least squares regression method has been applied for simultaneous spectrophotometric determination of harmine, harmane, harmalol and harmaline in Peganum harmala L. (Zygophyllaceae) seeds. The effect of pH was optimized employing multivariate definition of selectivity and sensitivity and best results were obtained in basic media (pH>9). The calibration models were optimized for number of latent variables by the cross-validation procedure. Determinations were made over the concentration range of 0.15-10 microg mL(-1). The proposed method was validated by applying it to the analysis of the beta-carbolines in synthetic quaternary mixtures of media at pH 9 and 11. The relative standard errors of prediction were less than 4% in most cases. Analysis of P. harmala seeds by the proposed models for contents of the beta-carboline derivatives resulted in 1.84%, 0.16%, 0.25% and 3.90% for harmine, harmane, harmaline and harmalol, respectively. The results were validated against an existing HPLC method and it no significant differences were observed between the results of two methods.

Electrocatalytic activity of Ce(III)-EDTA complex toward the oxidation of nitrite ion.

The electrochemical behavior of Ce(III) ion in the presence of EDTA and their application for electrocatalytic oxidation of nitrite ion is described in this manuscript. The electrochemical properties of Ce(III)-EDTA complex as well as the two-electron oxidation of nitrite ion were investigated using cyclic voltammetry and hydrodynamic voltammetry methods. Kinetic parameters such as transfer coefficient, homogeneous rate constant for electrocatalytic oxidation of nitrite ion at the experimental conditions, were obtained. Also the possible mechanism for oxidation of nitrite ion using Ce(III)-EDTA complex is proposed. The detection limit of 4.8x10(-6)M and dynamic linear range 1.0x10(-5) to 1.0x10(-2)M were obtained for determination of nitrite ion using electrocatalytic oxidation of this ion by Ce(III)-EDTA complex.

Electrocatalytic oxidation of guanine and DNA on a carbon paste electrode modified by cobalt hexacyanoferrate films.

The electrochemical behavior of cobalt hexacyanoferrate complex adsorbed on a carbon paste electrode (CPE) and its application to the electrocatalytic oxidation of guanine and single-strand DNA (ss-DNA) in aqueous solution are investigated in this report. The modification of CPE by the adsorption of this complex results in excellent amplification of the guanine oxidation response of ss-DNA. The effects of paste composition, scan rate, DNA, and guanine concentration were studied. The detection limits of 52 and 920 ng mL(-)(1) were obtained for guanine and ss-DNA, respectively.

Simultaneous determination of zirconium and molybdenum by first-derivative spectrophotometry.

A first-derivative spectrophotometric method for the simultaneous determination of Zr and Mo with Alizarin Red S is described. Measurements were made at the zero-crossing wavelengths at 490.5 nm for Zr and 446.0 nm for Mo. The calibration graphs were linear at 0.5-20 and 0.5-13.0 microg ml(-1) for Zr and Mo, respectively. The possible interfering effects of various ions were studied. Only iron was interfered in this system. The validity of the method was examined by using some mixtures of Mo and Zr. The method was applied in different matrix in both presence and absence of some foreign metal ions.