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.     

Determination of oxalic acid in spinach with carbon nanotubes-modified electrode

This work investigated the electrocatalytic oxidation of oxalic acid on multiwall carbon nanotubes (MWNTs) modified glassy carbon (GC) electrode. The result indicated that the oxidation of oxalic acid was greatly improved at the MWNTs-modified GC (MWNTs/GC) electrode as compared with the bare GC electrode. The effects of the MWNTs suspension amount, pH and scan rate on the electrooxidation response of oxalic acid were studied by voltammetry. The surface concentration and diffusion coefficient (D_R) of oxalic acid at the MWNTs/GC electrode were determined by chronocoulometry. This MWNTs/GC electrode presented a wide linear response ranging from 5.0 × 10⁻⁵ to 1.5 × 10⁻² M for oxalic acid. The detection limit (S/N = 3) was estimated to be 1.20 × 10⁻⁵ M. Based on this MWNTs/GC electrode, the content of oxalic acid in spinach was successfully determined.     

Amperometric determination of bisphenol A in milk using PAMAM–Fe3O4 modified glassy carbon electrode

A simply and sensitively electroanalytical method for determination of bisphenol A (BPA) using poly(amidoamine) (PAMAM) and Fe₃O₄ magnetic nanoparticles modified glassy carbon electrode (GCE) was presented. Compared with bare electrode, PAMAM–Fe₃O₄ modified electrode not only significantly enhanced the oxidation peak current of BPA, but also lowered the oxidation overpotential, suggesting that the modified electrode can remarkably improve the determining sensitivity of BPA. Factors influencing the detection processes were optimised and kinetic parameters were calculated. Under the optimal conditions, the oxidation current increased linearly with increasing the concentration of BPA in the range of 1 × 10⁻⁸–3.07 × 10⁻⁶ M with the correlation coefficient of 0.9996 and the detection limit of 5 × 10⁻⁹ M. The current reached 95% of the steady-state current within about 6 s. The proposed method was successfully applied to determine BPA in milk samples and satisfactory results were obtained.     

Voltammetric method using a lead film electrode for the determination of caffeic acid in a plant material

This paper deals with development of an adsorptive stripping voltammetric method for determination of caffeic acid (CA) at a lead film electrode (PbFE). The working electrode was prepared in situ on a glassy carbon substrate as a “mercury-free” electrochemical sensor. The method is based on the accumulation by adsorption of caffeic acid on PbFE and then the oxidation of CA during the stripping step. In a acetate buffer based supporting medium the oxidation signal for caffeic acid was found to be proportional to the CA concentration in the range from 1 × 10⁻⁸ to 5 × 10⁻⁷ mol/L with the limit of detection equal to 4 × 10⁻⁹ mol/L (with preconcentration for 30 s). The method, operated in the square-wave voltammetric mode, was successfully applied to the determination of CA in a plant material (herbs of Plantago lanceolata). The content of caffeic acid received by the proposed method was in close agreement with that obtained by high performance thin-layer chromatography (HPTLC) combined with densitometry. This appears to be the first application of a lead film electrode to the determination of an organic compound in such complicated matrix.     

Selective determination of chloramphenicol at trace level in milk samples by the electrode modified with molecularly imprinted polymer

A novel voltammetric sensor for determination of chloramphenicol (CAP) in milk samples was introduced. The CAP selective molecularly imprinted polymer (MIP) and non imprinted polymer (NIP) were synthesized and then added to the carbon paste (CP) electrode composition in order to prepare MIP-CP electrode. The MIP, embedded in the carbon paste electrode, acted as the selective recognition element and pre-concentrator agent for CAP. CAP was extracted in the electrode for a definite time and then it was analyzed by differential pulse voltametry, found to be an effective determination method. The MIP-CP showed very high CAP recognition ability, compared to NIP-CP. The electrode washing, after CAP extraction, led to an enhanced selectivity. Various factors, known to affect the response behavior of the electrode, were investigated and optimized. This sensor showed a linear response range of 8.0 × 10⁻⁹−1.0 × 10⁻⁶ M and lower detection limit of 2.0 × 10⁻⁹ M (S/N = 3). The sensor was successfully applied to the determination of CAP in milk samples.     

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.     

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.     

Electrocatalysis and sensitive determination of Sudan I at the single-walled carbon nanotubes and iron(III)-porphyrin modified glassy carbon electrodes

Iron-porphyrin (5,10,15,20-tetraphenyl-21H, 23H-porphine iron(III) chloride) was used in combination with single-wall carbon nanotubes (SWNTs) to modify a glassy carbon electrode (GCE). The electrochemical behavior of Sudan I on the iron-porphyrin-SWNT-DMF (N,N-dimethylformamide) modified GCE was studied by the cyclic voltammetry and square wave voltammetry. In pH 7.0 Tris–HCl buffers, Sudan I has a sensitive catalytic reduction peak at −0.08 V on the iron-porphyrin-SWNT-DMF modified GCE. Using square wave voltammetry, the linear relationship of Sudan I is 5.03 × 10⁻⁸ mol L⁻¹–2.01 × 10⁻⁶ mol L ⁻¹ with the tropics equation: Δ_iP = 3.40C + 3.43 × 10⁻⁶, and the detection limit is 1 × 10⁻⁸ mol L⁻¹. And the iron-porphyrin-SWNT-DMF modified GCE was applied successfully in the determination of Sudan I in real samples.     

New voltammetric procedure for determination of thiamine in commercially available juices and pharmaceutical formulation using a lead film electrode

A simple, reliable and reproducible method, based on adsorptive stripping voltammetry (AdSV), for determination of vitamin B₁ (thiamine) in pharmaceutical preparation and food is described in this paper. The in situ plated lead film electrode was used as a working electrode. The lead film was formed and thiamine was accumulated at −1.25 V (vs. Ag/AgCl) on a glassy carbon electrode. Then, the preconcentrated thiamine was reduced by scanning the potential of the electrode from −1.25 to −1.55 V using a square-wave technique. The linear range was from 0.0133 to 0.265 mg L⁻¹ for vitamin B₁, with the regression coefficient of 0.999. The detection limit for vitamin B₁ was 0.0053 mg L⁻¹ for the accumulation time 120 s. The method developed was applied to the determination of thiamine in certified reference material (BCR-485), pharmaceutical formulation and commercially available juices, and the assay results were satisfactory.     

Analysis of some selected catechins and caffeine in green tea by high performance liquid chromatography

Green tea seems to have a positive impact on health due to the catechins-found as flavanols. Thus, the present study was aimed to develop a low cost reversed phase high performance liquid chromatographic (HPLC) method for simultaneous determination of flavanol contents, namely catechin (C), epicatechin (EC), epigallocatechin (EGC), epicatechin 3-gallate (ECG) and epigallocatechin 3-gallate (EGCG) and caffeine in 29 commercial green tea samples available in a Saudi Arabian local market. A C-18 reversed-phase column, acetonitrile–trifluoroacetic acid as a mobile phase, coupled with UV detector at 205 nm, was successfully used for precise analysis of the tested analytes in boiled water of digested tea leaves. The average values of N (No. of theoretical plates), HETP (height equivalent of theoretical plates) and R_s (separation factor) (at 10 μg ml⁻¹ of the catechins EC, EGC, EGCG and ECG) were 2.6 × 10³ ± 1.2 × 10³, 1.7 × 10⁻³ ± 4.7 × 10⁻⁴ cm and 1.7 ± 5.53 × 10⁻², respectively. The developed HPLC method demonstrated excellent performance, with low limits of detection (LOD) and quantification (LOQ) of the tested catechins of 0.004–0.05 μg ml⁻¹ and 0.01–0.17 μg ml⁻¹, respectively, and recovery percentages of 96–101%. The influence of infusion time (5–30 min) and temperature on the content of the flavanols was investigated by HPLC. After a 5 min infusion of the tea leaves, the average concentrations of caffeine, catechin, EC, EGC, ECG and EGCG were found to be in the ranges 0.086–2.23, 0.113–2.94, 0.58–10.22, 0.19–24.9, 0.22–13.9 and 1.01–43.3 mg g⁻¹, respectively. The contents of caffeine and catechins followed the sequence: EGCG > EGC > ECG > EC > C > caffeine. The method was applied satisfactorily for the analysis of (+)-catechin, even at trace and ultra trace concentrations of catechins. The method was rapid, accurate, reproducible and ideal for routine analysis.     

Development of novel potentiometric sensors for determination of tartrazine dye concentration in foodstuff products

Tartrazine dye Na₃TZ in foodstuff products was determined by a new modified carbon paste electrode, encoded sensor A, and a coated silver wire electrode, encoded sensor B, based on tartrazine TZ- cetryltrimethyl ammoniumbromide CTAB as a chemical modifier TZ-CTA. The electrodes exhibit the following characteristics listed respectively: a Nernstian slope of 17.9 ± 0.5 and 19.4 ± 0.2 mV/decade for tartrazine ion over a wide concentration range from 4.3 × 10⁻⁷ to 1.0 × 10⁻² and 1.1 × 10⁻⁷ to 1.0 × 10⁻² M. The lower detection limits: 3.2 × 10⁻⁷ and 5.5 × 10⁻⁸ M. Short response time (5–8 s) over the pH range 3.8–7.7 and 4.2–8.1. The proposed sensors display significantly high selectivity for TZ ion over a wide variety of sugars, some anions, common organic, inorganic compounds and additives. The developed electrodes were applied to the potentiometric determination of tartrazine ion in different kinds of foodstuffs: solid jelly (strawberry and custard) powder samples and soft drink (orange) samples with satisfactory results.     

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.     

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.     

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.     

Measurement of caffeine in coffee beans with UV/vis spectrometer

In this research work using UV/vis spectrophotometer the molar decadic absorption coefficients and transitional dipole moment of pure caffeine in water and dichloromethane were obtained at 272 and 274.7 nm. The molar decadic absorption coefficients of caffeine in water and dichloromethane at these wavelengths are 1115 and 1010 m² mol⁻¹, respectively. The calculated values for the transitional dipole moment of caffeine in water and in dichloromethane are 10.40 × 10⁻³⁰ and 10.80 × 10⁻³⁰ C m, respectively. After characterizing caffeine in water and dichloromethane, fast and simple methods were developed that enable to quantify the content of caffeine in coffee beans. The methods helped in extracting caffeine from coffee dissolved in water by dichloromethane, and Gaussian fit was applied to eliminate the possible interference with the caffeine spectra.     

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.     

Decaffeination of fresh green tea leaf (Camellia sinensis) by hot water treatment

Hot water treatment was used to decaffeinate fresh tea leaf in the present study. Water temperature, extraction time and ratio of leaf to water had a statistically significant effect on the decaffeination. When fresh tea leaf was decaffeinated with a ratio of tea leaf to water of 1:20 (w/v) at 100 °C for 3 min, caffeine concentration was decreased from 23.7 to 4.0 mg g⁻¹, while total tea catechins decreased from 134.5 to 127.6 mg g⁻¹; 83% of caffeine was removed and 95% of total catechins was retained in the decaffeinated leaf. It is considered that the hot water treatment is a safe and inexpensive method for decaffeinating green tea. However, a large percentage of tea catechins was lost if rolled leaf and dry tea were decaffeinated by the hot water treatment and so the process is not suitable for processing black tea.     

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.     

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.