Electrochemical determination of Sudan I using montmorillonite calcium modified carbon paste electrode

In the current work, a sensitive, rapid and convenient electrochemical method was developed for the determination of Sudan I utilizing the excellent properties of montmorillonite calcium (MMT-Ca). Compared with the unmodified carbon paste electrode (CPE), MMT-Ca modified CPE not only significantly enhances the oxidation peak current of Sudan I but also lowers the oxidation overpotential, suggesting that the MMT-Ca modified CPE can remarkably improve the determining sensitivity of Sudan I. The experimental conditions such as determining medium, the content of MMT-Ca and accumulation time were optimized for the determination of Sudan I. The oxidation peak current is proportional to the concentration of Sudan I over the range from 0.05 mg L⁻¹ (2.01 × 10⁻⁷ mol L⁻¹) to 1.0 mg L⁻¹ (4.03 × 10⁻⁶ mol L⁻¹), and the limit of detection is 0.02 mg L⁻¹ (8.06 × 10⁻⁸ mol L⁻¹) for 2-min accumulation. Finally this newly-proposed sensing method was successfully employed to detect Sudan I in practical samples and good recovery was achieved.     

Square wave voltammetry for analytical determination of paraquat at carbon paste electrode modified with fluoroapatite

This paper describes the construction of a carbon paste electrode (CPE) impregnated with fluoroapatite (FAP). The new electrode (FAP–CPE) was revealed an interesting determination of paraquat. The latter was accumulated on the surface of the modified electrode by adsorbing onto fluoroapatite and reduced in 0.1 mol L⁻¹ K₂SO₄ electrolyte at −0.70 and −1.0 V for peaks 1 and 2, respectively. Experimental conditions were optimized by varying the accumulation time, FAP loading and measuring solution pH. Under the optimized working conditions, calibration graphs were linear in the concentration ranging from 5 × 10⁻⁸ to 7 × 10⁻⁵ mol L⁻¹ with detection limits (DL, 3σ) of 3.5 × 10⁻⁹ and 7.4 × 10⁻⁹ mol L⁻¹, respectively, for peaks 1 and 2.     

Determination of Ge(IV) in rice in a mercury-coated glassy carbon electrode in the presence of catechol

Electrochemical adsorptive cathodic stripping voltammetry determination of Ge(IV) using the catechol complex on a mercury-coated (MC) electrode was prepared using a glassy carbon electrode (GCE), the peak potential of which was −0.5 V vs. Ag/AgCl on MCGCE. The various parameters of the catechol concentration, its pH, and others were optimized. The linear working ranges were obtained in the concentration of 2–700 μg L⁻¹ Ge(IV). The relative standard deviation at the Ge(IV) concentration of 50 μg L⁻¹ was 1.37% (n = 15) using the optimum condition, and the detection limit was found to be 0.6 μg L⁻¹ (8.26 × 10⁻⁹ M) (S/N = 3), with an adsorption time of 180 s. The Ge(IV) response was highly linear. This developed method was applied to Ge(IV) to determine the presence of rice grains.     

Sensitive voltammetric determination of Sudan I in food samples by using gemini surfactant–ionic liquid–multiwalled carbon nanotube composite film modified glassy carbon electrodes

A novel modified electrode was fabricated, which comprised of hydrophobic ionic liquid (i.e. trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, [P_6,6,6,14][NTf₂]), multiwalled carbon nanotubes (MWNTs) and cationic genimi surfactant (i.e. C₁₂H₂₅N(CH₃)₂–C₄H₈–N(CH₃)₂C₁₂H₂₅Br₂, C₁₂–C₄–C₁₂). Cyclic voltammetry and linear sweep voltammetry were used to investigate the electrochemical behaviour of Sudan ?. The components showed good synergic interaction in sensing Sudan ?, thus the modified electrode presented higher sensitivity. After optimising the experimental conditions, the anodic peak current of Sudan I was linear to its concentration in the range of 0.05–2 μmol l⁻¹, and the detection limit was 0.03 μmol l⁻¹ in pH 4.5 potassium biphthalate buffer with acetonitrile. The modified electrode had good stability and repeatability. It was applied to the detection of Sudan ? in hot chilli powder and ketchup samples, and the recovery was acceptable.     

Sensor-containing microspheres of chitosan crosslinked with 8-hydroxyquinoline-5-sulphonic acid for determination of Cu(II) in instant coffee

A novel carbon paste electrode containing chitosan microspheres for the determination of Cu(II) in instant coffee by anodic stripping voltammetry was developed. Chitosan was crosslinked with the chelating agent 8-hydroxyquinoline-5-sulphonic acid and glutaraldehyde. The microspheres of the crosslinked chitosan biopolymer were obtained by the spray drying technique and employed in the construction of the sensor. In acetate buffer solution (0.1 mol L⁻¹, pH 6.0), the calibration curve obtained was linear for concentrations of 5.0 × 10⁻⁷ to 1.4 × 10⁻⁵ mol L⁻¹ (r = 0.9990); the detection limit was 5.5 × 10⁻⁸ mol L⁻¹. The relative standard deviation (n = 8) was lower than 3.0% for solutions containing 6.0 × 10⁻⁶, 5.0 × 10⁻⁵ and 1.5 × 10⁻⁴ mol L⁻¹ of Cu(II). The method was successfully employed for determination of Cu(II) in instant coffee and the results obtained showed good agreement when compared with those using electrothermal atomic absorption spectrometry.     

Pico molar assay of riboflavin in human urine using voltammetry

Using a new type of DNA and carbon nano tube (CNT) mixed paste electrode using cyclic and square wave anodic stripping voltammetric (SWASV) methods, this study presents an assay of riboflavin (RF) under optimum conditions. Results of the experiment yielded a low working concentration range of nanograms with 1–10 and 10–170 ng L⁻¹ and 5–105 μg L⁻¹, at an accumulation time of 80 s in a 0.1 M H₃PO₄ electrolyte solution. A relative standard deviation of 30 μg L⁻¹ was observed at an accuracy level of 0.1164% (n = 15) under optimum conditions. The detection limit (S/N) was pegged at 0.2 ng L⁻¹ (5.31 × 10⁻¹³ mol L⁻¹ RF). The proposed method was successfully applied to an actual human urine and drug sample, and can be applied to assays of other biological samples.     

Electrochemical behaviour of Sudan I at Fe3O4 nanoparticles modified glassy carbon electrode and its determination in food samples

In this work, a simple and sensitive electrochemical method was developed to determine Sudan I based on magnetic Fe₃O₄ nanoparticles modified glassy carbon electrode using cyclic voltammetry and differential pulse voltammetry. The sensor exhibited an obviously electrocatalytic activity towards the oxidation of Sudan I, which can be confirmed by the increased oxidation peak current and the decreased oxidation peak potential when compared with the bare GCE. The determination conditions, such as pH, modifier amount, accumulation time and accumulation potential, were optimised. And some kinetic parameters were calculated. Under the optimum experimental conditions, the oxidation current of Sudan I was proportional to its concentration from 0.01 to 1 μM and 1 to 20 μM. The detection limit was estimated to be 0.001 μM (S/N = 3). The developed method was successfully applied to determine Sudan I content in food samples with satisfactory results.     

Sensitive electrochemical determination of luteolin in peanut hulls using multi-walled carbon nanotubes modified electrode

A simple and highly sensitive electrochemical method was developed for the determination of trace-level luteolin, based on the multi-walled carbon nanotubes (MWNTs) modified glassy carbon electrode (GCE). The electrochemical behaviours of luteolin indicate that MWNTs modified glassy carbon electrode (MWNTs/GCE) can greatly enhance the electrocatalytic activity towards the redox of luteolin. It leads to a considerable improvement of the redox peak current for luteolin and allows the development of a highly sensitive voltammetric sensor for the determination of luteolin. A series of experimental parameters including the pH of supporting electrolyte, accumulation potential and time were optimised. The results showed that the oxidative peak currents increased linearly with the concentration of luteolin in the range of 2.0 × 10⁻¹⁰ to 3.0 × 10⁻⁹ M, with a detection limit of 6.0 × 10⁻¹¹ M (S/N = 3). The analytical performance of this sensor has been evaluated for detection of luteolin in peanut hulls as a real sample.     

The reduction of Cu(II)/neocuproine complexes by some polyphenols: Total polyphenols determination in wine samples

A new method is presented for spectrophotometric determination of total polyphenols content in wine. The procedure is a modified CUPRAC method based on the reduction of Cu(II), in hydroethanolic medium (pH 7.0) in the presence of neocuproine (2,9-dimethyl-1,10-phenanthroline), by polyphenols, yielding a Cu(I) complexes with maximum absorption peak at 450 nm. The absorbance values are linear (r = 0.998, n = 6) with tannic acid concentrations from 0.4 to 3.6 μmol L⁻¹. The limit of detection obtained was 0.41 μmol L⁻¹ and relative standard deviation 1.2% (1 μmol L⁻¹; n = 8). Recoveries between 80% and 110% (mean value of 95%) were calculated for total polyphenols determination in 14 commercials and 2 synthetic wine samples (with and without sulphite). The proposed procedure is about 1.5 more sensitive than the official Folin–Ciocalteu method. The sensitivities of both methods were compared by the analytical responses of several polyphenols tested in each method.     

Determination of cyclamate in artificial sweeteners and beverages using headspace single-drop microextraction and gas chromatography flame-ionisation detection

A new method for determination of cyclamate was developed using headspace single-drop microextraction (HS-SDME) and gas chromatography (GC). The method is based on the reaction of cyclamate and nitrite in the acidic media and microextraction of cyclohexene formed for subsequent determination with GC. Conditions for both derivatisation and HS-SDME have been optimised. The calibration curve for cyclamate determination showed good linearity in the range of 30–1000 μmol L⁻¹ (R² = 0.9992) and the limit of detection (S/N = 3) was estimated to be 5 μmol L⁻¹. The repeatability and reproducibility for a 200 μmol L⁻¹ of cyclamate were 4% and 4.8% (N = 5), respectively. The purposed method was successfully applied for determination of cyclamate in beverages and sweetener tablets. The average recovery of spiked samples was 97%. The results demonstrated that the developed method is simple, rapid, inexpensive, accurate and remarkably free from interference effects.     

Measurement of integrated absorption cross-section,oscillator strength and number density of caffeine in coffee beans by integrated absorption coefficient technique

Integrated absorption cross-section and oscillator strength of caffeine in water and dichloromethane were reported by UV–Vis spectrometer in the wave number regions of 20,000–39,062 cm⁻¹ at room temperature. The integrated absorption cross-section of caffeine in water and dichloromethane in these wave number regions were (4.44 ± 0.18) × 10⁷ and (4.32 ± 0.11) × 10⁷ L mol⁻¹ cm⁻², respectively. The corresponding calculated value for oscillator strength of caffeine in water and dichloromethane were 0.19 ± 0.01 and 0.18 ± 0.01, respectively. In addition number densities of caffeine in different coffee beans were reported using integrating absorption coefficient technique. The calculated number density of caffeine for different coffee varieties were varying from (3.53–4.76) × 10⁻⁵ mol L⁻¹. The new applied technique has an advantage over other techniques in that it provides additional information about the nature of the absorbing molecules and establishes accurate evaluation of the UV–Vis absorption intensity.     

Synthesis and application of a new functionalized resin for use in an on-line,solid phase extraction system for the determination of trace elements in waters and reference cereal materials by flame atomic absorption spectrometry

The synthesis and characterization of the resin Amberlite XAD-4 functionalized with 2,6-pyridinedicarboxaldehyde and its application in an on-line system for the preconcentration of cadmium, cobalt, copper, lead and manganese prior to determination using flame atomic absorption spectrometry (FAAS) is proposed. Metal ions retained on the modified resin were eluted using 1.0 mol L⁻¹ HNO₃ solution and aspirated directly to the nebulizer–burner system of a FAAS instrument using a flow injection system. Detection limits (3σ) were determined to be 0.13 μg L⁻¹ for Cd, 0.29 μg L⁻¹ for Cu, 0.23 μg L⁻¹ for Mn, 0.58 μg L⁻¹ for Co and 2.19 μg L⁻¹ for Pb using a 10 mL of water sample loading volume. The limits of detection would be 100 times higher with units of μg kg⁻¹ for the solid samples in which their dilution ratios as (volume/weight) were 100. Enrichment factors ranged from 23.6 to 28.9 (for Co and Mn, respectively). The proposed method was successfully applied to determination of the analytes in natural water samples and certified reference materials.     

Determination of Sudan I in hot chili powder by using an activated glassy carbon electrode

A novel electroanalytical method is proposed for the determination of Sudan I in hot chili powder. Sudan I was firstly pre-concentrated by adsorption and then electroreduced at the electrochemically activated glassy carbon electrode (AGCE). A linear relationship between the reduction current and concentration of Sudan I was obtained over the range from 2.4 × 10⁻⁶ mol/L to 1.8 × 10⁻⁵ mol/L with a correlation coefficient of 0.9981. The detection limit was estimated to be about 7.1 × 10⁻⁷ mol/L.     

Potentiometric sensor for sorbic acid determination in food products

The construction, evaluation and application of a new potentiometric sensor, namely, Pt|Hg|Hg₂(SOB)₂ |Graphite, where SOB stands for sorbate ion, are described. This electrode has a wide linear dynamic range between 5.0 × 10⁻⁷ and 1.0 × 10⁻² mol L⁻¹ with a near-Nernstian slope of (−58.6 ± 1.3) mV decade⁻¹ and a detection limit of 4.3 × 10⁻⁷ mol L⁻¹. The potentiometric response is independent of the pH of the solution in the pH range 6.0–9.0. The electrode is easily constructed at a relatively low-cost with fast response time (within 15–30 s) and can be used for a period of 4 months without significant change in its performance characteristics. The proposed sensor displayed good selectivities over a variety of other anions (carboxylates and inorganic anions). The potentiometric sensor was successfully applied to the determination of sorbic acid in real food samples, that is, soft drinks, skim yogurts, jams and sauces.     

Determination of nitrite in food samples by anodic voltammetry using a modified electrode

A glassy carbon (GC) electrode modified with alternated layers of iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) and copper tetrasulfonated phthalocyanine (CuTSPc) was employed for nitrite determination by differential pulse voltammetry (DPV). This modified electrode showed excellent catalytic activity for the nitrite oxidation. After optimizing the operational conditions, a linear response range from 0.5 to 7.5 μmol l⁻¹ with a low detection limit of 0.1 μmol l⁻¹ was obtained. The proposed sensor was stable with a sensitivity of 20.0 μA, 1 μmol⁻¹ and good repeatability, evaluated in terms of relative standard deviation (R.S.D. = 1.3%) for n = 10. Possible interferences from several common ions were evaluated. This sensor was applied for the voltammetric determination of nitrite in some food samples. The results were consistent with those obtained with the standard spectrophotometric procedure.     

Microfluidic chip based chemiluminescence detection of L-phenylalanine in pharmaceutical and soft drinks

A sensitive chemiluminescence (CL) method on chip coupled with microfluidic system has been reported for the determination of L-phenylalanine (L-PA). A microfluidic chip device with the detection chamber capable of fast sensing light emitted from the luminol and hydrogen peroxide CL reaction catalyzed by copper sulphate was fabricated for the determination of L-PA. The microfluidic chip was fabricated by a soft-lithographic procedure using polydimethyl siloxane (PDMS). The fabricated PDMS microfluidic chip had four inlet microchannels for introducing the sample, chemiluminescent reagent, Cu(II), and oxidant, and a 500 μm wide, 250 μm deep, and 82 mm long microchannel. The detection was based on the enhancement effect of L-PA on the CL signals of luminol–H₂O₂–Cu²⁺ system in an alkaline medium. The CL intensity of the system was enhanced linearly with the concentration of L-PA in the range of 1.5 × 10⁻⁹–1.2 × 10⁻⁷ mol L⁻¹. The limit of detection was found to be 2.4 × 10⁻¹⁰ mol L⁻¹ with the relative standard deviation of 1.8%. The presented method offers a simple, rapid and easy to handle analytical technique in terms of sensitivity, dynamic range and low detection limit for the determination of L-PA in diet soft drinks and pharmaceutical injection samples.     

Determination of resveratrol in grains,hulls and leaves of common and tartary buckwheat by HPLC with electrochemical detection at carbon paste electrode

A reverse-phase high-performance liquid chromatographic method for the determination of trans-resveratrol with spectrophotometric detection (306 nm) and amperometric detection at carbon paste electrode (E = + 1,2 V) was developed and tested on real samples of grains, hulls and leaves of six varieties of common buckwheat (Fagopyrum esculentum Möench) and two varieties of tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.). Optimal conditions for the determination of trans-resveratrol were as follows: column Kromasil C-18 (7 μm), 125 × 4 mm; mobile phase acetonitrile: diluted BR buffer pH 7 (50:50, 30:70 for grains and hulls and 20:80 for leaves); flow rate 1 ml min⁻¹. Under these conditions, the limit of detection of trans-resveratrol (L_D) was 3.5 × 10⁻⁸ mol l⁻¹ (R² = 0.9986) for electrochemical detection and 3.2 × 10⁻⁸ mol l⁻¹ (R² = 0.9993) for spectrophotometric detection.     

Sensitive and rapid determination of catechol in tea samples using mesoporous Al-doped silica modified electrode

A mesoporous Al-doped silica (Al/SiO₂) was synthesised according to the published method, and then used to modify the carbon paste electrode (CPE). The electrochemical behaviour of catechol was investigated. Compared with the unmodified CPE, the resulting mesoporous Al/SiO₂ modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the mesoporous Al/SiO₂ exhibits catalytic activity to catechol and significantly improves the determining sensitivity. Based on this, a sensitive, rapid and convenient electrochemical method was proposed for the determination of catechol. The linear range is between 5.0 × 10⁻⁷ and 5.0 × 10⁻⁵ mol L⁻¹ with a correlation coefficient of 0.998. The limit of detection is as low as 1.0 × 10⁻⁷ mol L⁻¹. Finally, this novel method was employed to determine catechol in tea samples, which testified by high-performance liquid chromatography.     

Voltammetric behaviour and square-wave voltammetric determination of the potent antioxidant and anticarcinogenic agent ellagic acid in foodstuffs

The voltammetric behaviour of ellagic acid (EA) is investigated by cyclic, differential pulse and square-wave voltammetry (CV, DPV and SWV, respectively). Based on the anodic oxidation peak at approximately 0.42 V in acetic/acetate buffer (pH 5.5) a robust and a highly reliable square-wave voltammetric method is presented for the determination of EA. The oxidation peak current was linearly dependent on the concentration of EA in the range of 1.0 × 10⁻⁷–1.5 × 10⁻⁶ mol/L (r = 0.9997), with a detection limit of 1.0 × 10⁻⁸ mol/L (S/N = 3) and a quantification limit of 3.4 × 10⁻⁸ mol/L (S/N = 10), good reproducibility and a satisfactory level of selectivity towards others polyphenols. The proposed method was applied to the determination of free and total EA in fruits, nuts and juices with good analytical results being obtained.     

Manganese(II) hexacyanoferrate based renewable amperometric sensor for the determination of butylated hydroxyanisole in food products

A novel amperometric sensor based on chemically modified electrode for the determination of butylated hydroxyanisole (BHA) in food materials is described. The sensor was constructed by graphite-wax composite method with manganese(II) hexacyanoferrate (MnHCF) as electrocatalyst. Cyclic voltammetry and differential pulse voltammetry were employed to characterize the electrochemical behavior of the modified electrode. The MnHCF modified graphite-wax composite electrode exhibits an excellent electrocatalytic activity for the oxidation of BHA‘ at a reduced overpotential of 250 mV in comparison to the unmodified graphite-wax composite electrode. Influence of parameters such as scan rate, pH and applied potential on the electrocatalytic response was also studied. The modified electrode showed a wide linear dynamic range of 4.998 × 10⁻⁷ M–1.429 × 10⁻³ M with a detection limit of 5 × 10⁻⁸ M and a correlation coefficient of 0.9996 based on S/N = 3. The MnHCF modified electrode has been successfully applied for the assay of BHA in spiked commercial samples of dry potato chips. The sensor was found to exhibit a high current sensitivity, a short response time, a good reproducibility (relative standard deviation (RSD) < 2%) and stable response for the determination of BHA and has the merit of surface renewability. Hence the method can be employed for routine analysis of BHA in food samples.