Polyacrylamide–gelatine carrier system used for invertase immobilization

Invertase was immobilized into polyacrylamide–gelatin carrier system by chemical cross-linking with chromium (III) acetate, chromium (III) sulphate, and potassium chromium (III) sulphate. The optimum conditions, namely substrate concentration, temperature, and pH were determined. The effect of polyacrylamide–gelatin ratio and cross-linker concentration on immobilized enzyme activity were analysed. Maximum immobilized enzyme activities were obtained with chromium (III) acetate (0.01 mol dm⁻³), chromium (III) sulphate (0.004 mol dm⁻³) and potassium chromium (III) sulphate (0.001 mol dm⁻³) for 0.177 (w/w) polyacrylamide–gelatin carrier ratio as 79%, 72% and 79%, respectively. The K_m values were 86 and 166 mM for free and immobilized enzyme, respectively. All immobilized samples were used 20 times over a period of 2 months without a considerable loss of activity.     

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 carbohydrates by capillary zone electrophoresis with amperometric detection at a nano-nickel oxide modified carbon paste electrode

In this paper, a nano-NiO modified carbon paste electrode in capillary zone electrophoresis with amperometric detection (CZE-AD) was firstly applied to the determination of carbohydrates. Effects of several important factors such as detection potential, the concentration of running buffer, separation voltage and injection time were investigated to acquire the optimum conditions. Under the selected optimum conditions, three carbohydrates: glucose, sucrose and fructose could be perfectly separated within 20 min. The relationships between peak current and concentration of three carbohydrates were linear over about 3 orders of magnitude with detection limits (S/N = 3) ranging from 3.0 × 10⁻⁷ to 6.0 × 10⁻⁷ mol L⁻¹. The electrode was stable, and can be used for at least one week. The proposed method has been successfully applied to monitor carbohydrates in the real samples with satisfactory assay results.     

C NMR and electrospray ionization mass spectrometric study of sucrose aqueous solutions at high pH: NMR measurement of sucrose dissociation constant

The ¹³C NMR technique is used for the measurement of the first dissociation constant of sucrose (HL) in highly alkaline solutions. In 1.0 M NaCl/NaOH medium and for 25 °C, the concentration dissociation constant (pK₁) was 13.1 ± 0.3; and, for 60 °C, pK₁ = 12.30 ± 0.05. The β-d-fructofuranosyl ring was found to be responsible for dissociation. The NMR data reveal no clear evidence of the second dissociation step below pH 14, either at 25 °C or at 60 °C. In the solutions with 4–10 mol dm⁻³ NaOH content the ¹³C NMR technique indicated the chemical shift changes, treated as the second dissociation step of sucrose and a sodium complex formation. A very rough estimation, for variable ionic strength, gives the value: pK₂ ∼ 15.8 ± 0.8. The anionic species L⁻ and NaH₋₁L⁻ have been registered by electrospray ionization time-of-flight mass spectrometry (ESI-ToF MS) for 0.01 M sucrose solutions with initial pH 13.     

Amperometric assessment of glucose electrode behaviour in mixed solvents and determination of glucose in dairy products

An amperometric biosensor based on a ruthenium(III), nickel(II) and iron(II) hexacyanometallate (HCM)-modified graphite electrode and immobilized glucose oxidase has been used for the determination of glucose in water-miscible organic solvent/aqueous buffer mixtures. Although the specific activity of biochemically active molecule such as enzyme is reduced in organic environment, it was established that the presence of water soluble organic solvents such as methanol, ethanol and acetonitrile (φ = 10%) enhance the biosensor response. Hydrogen peroxide, produced by enzyme-catalysed reduction of glucose, was measured in phosphate buffer solution (pH = 6.86) at −50 mV against a reference Hg|Hg₂Cl₂|3 M KCl electrode to determine the concentration of glucose. The influence of the addition of different volume fractions (φ = 10–60%) of methanol, ethanol, acetone, acetonitrile and isopropanol on biosensor response was investigated. The obtained amperometric signals were fast, reproducible and linearly proportional to glucose concentrations in the range of 0.1–0.8 mM, with a squared correlation coefficient of 0.9994 for buffer solution. With the addition of ethanol (φ = 10% and 40%) the plateau on I/c curve was obtained for concentrations of glucose higher than 0.8 and 1.1 mM, respectively. The biosensor proved to be stable for several months. The recoveries of added glucose (0.200 and 0.300 mM) from aqueous solution and from solution with ethanol φ = 10% ranged from 96.0% to 108.0%. The biosensor was used for the determination of glucose in some food samples of dairy industry, and the results were consistent with those obtained with the commercially available glucose enzyme photometric kit.     

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.     

Amperometric detection of ascorbic acid in honey using ascorbate oxidase immobilised on amberlite IRA-743

A differential amperometric method for the specific determination of ascorbic acid in honey was developed by association of a flow injection analysis (FIA) system and a tubular reactor containing the ascorbate oxidase enzyme immobilised. A gold electrode modified by electrochemical deposition of palladium was employed as working electrode. Ascorbic acid was quantified in seven samples of commercial honeys using a potential of +0.60 V vs. Ag/AgCl_(sat). The linear dynamic range in ascorbic acid extends from 1 to 50 μmol L⁻¹, at pH 7.0. At flow rate of 1.5 mL min⁻¹ and injecting 250 μL sample volumes, a sampling frequency of 180 determinations per hour is afforded. The detection and quantification limit of this method are 0.14 and 0.49 μmol L⁻¹, respectively. The samples analyses were compared with the volumetric method, and showed an excellent correlation between the methods.     

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.     

Effects of skin and grilling method on formation of heterocyclic amines in chicken pectoralis superficialis muscle

Mutagenic heterocyclic amines (HAs) originate in processed proteinaceous foods. The effects of the presence of skin (with vs. without) and of grilling method (two plate vs. infrared) on the content of HAs in grilled chicken pectoralis superficialis muscle (temperature, 220 °C) were investigated. HA precursors (creatine, creatinine, free amino acids and carbohydrates) and HAs of these raw and grilled breast muscles were determined. The muscles originated from 24 birds of either sex (provenance Ross; aged 40-45 days). The HA content was determined in homogenates of the upper and lower surface slices of the grilled muscles (T_i = 82 °C). A higher content of total free amino acids was seen for the muscle (27.1 mmol kg⁻¹ raw meat) than for the skin (21.7 mmol kg⁻¹ raw meat). The creatine, creatinine and carbohydrate levels in the skin were below the limits of detection. The contents of creatine (31.8-38.7 mmol kg⁻¹) and creatinine (0.24-0.33 mmol kg⁻¹) in the breast muscle were determined. Relatively high levels were seen for glucose (23 mmol kg⁻¹ raw meat) and fructose (10 mmol kg⁻¹ raw meat) in the muscle, with other sugars present at low levels (<2 mmol kg⁻¹ raw meat). For the chicken muscle grilled on a two-plate grill, the contents of total HAs (PhIP, MeIQx, DiMeIQx, Harman and Norharman) were lower with the skin in place than in the muscle grilled without the skin (3.5 μg kg⁻¹vs. 4.8 μg kg⁻¹). Also, during infrared grilling with the skin, lower amounts of HAs were formed than with grilling on the two-plate grill (2.4 μg kg⁻¹vs. 3.5 μg kg⁻¹). On average, the infrared-grilled samples with skin contained 3-fold more total HAs than similar samples without the skin (2.4 μg kg⁻¹vs. 0.8 μg kg⁻¹), with the highest levels seen for PhIP and MeIQx.     

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.     

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.     

Electrochemical biosensor based on gold coated iron nanoparticles/chitosan composite bound xanthine oxidase for detection of xanthine in fish meat

A xanthine oxidase was immobilized covalently onto chitosan bound gold coated iron nanoparticles (CHIT/Fe-NPs@Au) electrodeposited on the surface of pencil graphite electrode (PGE). A xanthine biosensor was fabricated using XOD/CHIT/Fe-NPs@Au/PGE as working, Ag/AgCl as reference and Pt as auxiliary electrode connected through potentiostat. The enzyme electrode was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and electrochemical impedance spectroscopy (EIS). The biosensor exhibited optimum current response within 3 s at pH 7.4, 35 °C and working range 0.1–300 μM, when polarized at 0.5 V vs Ag/AgCl. The sensitivity of the biosensor was 0.001169 mAμ M^–1 cm^–2 with detection limit of 0.1 μM (S/N = 3). The biosensor showed only 25% loss in its initial activity after its 100 uses over 100 days, when stored at 4 °C.     

Effect of pre- and postharvest salicylic acid treatment on ethylene production,fungal decay and overall quality of Selva strawberry fruit

The effect of salicylic acid (SA) treatment at different concentrations and growth stages of strawberry (Fragaria ananassa cv. Selva) fruit on postharvest ethylene production, fungal decay and overall quality index was studied. SA at all concentrations effectively reduced fruit ethylene production and fungal decay and retained overall quality. Treatment of plants at vegetative stage and fruit development stage followed by postharvest treatment of fruits with 1 and 2 mmol L⁻¹ was the most effective strategy, whilst with decrease in treatment time the effects of treatment decreased. Single stage treatment strategy of fruits with 2 mmol L⁻¹ SA at postharvest stage was most effective. Postharvest treatment with 4 mmol L⁻¹ SA slightly damaged the fruits and was less effective than 2 mmol L⁻¹ in retaining fruit quality.     

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.     

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.     

Quantitative determination of the phenolic antioxidants using voltammetric techniques

Synthetic antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ), show sensitivity to voltammetric waves. The waves of these antioxidants, however, are seriously overlapped and it is difficult to determine them simultaneously. The influence of different parameters (working electrode, supporting electrolyte, pH, voltammetric technique) was evaluated in a quantitative simultaneous determination of three antioxidants in alcoholic mixtures and real sample foods. Glassy carbon (GC) and platinum (Pt) working electrodes were investigated as mediators of oxireduction reactions. Two supporting electrolytes were investigated: Britton-Robinson 0.1 mol l⁻¹ buffer (pH 2.0) and HCl 0.1 mol l⁻¹ (pH 2.0) both with 2 g l⁻¹ (p/v) of methanol. In this paper, voltammetric conditions for the analysis of up to three-component mixtures of antioxidant present at levels: 2.0-100.0 mg l⁻¹ for BHA, 4.0-100.0 mg l⁻¹ for TBHQ and 2.0-20.0 mg l⁻¹ for BHT at GC in HCl 0.1 mol l⁻¹ and 8.0-120.0, 10.0-130.0 and 4.0-30.0 mg l⁻¹ for BHA, TBHQ and BHT, respectively, at Pt in the same supporting electrolyte. The results show that for real food samples, the parameters investigated were satisfactory for quantitative determination using square wave voltammetry (SWV) without chemometric approaches and without suffering overlapping problems.     

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.     

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 molybdenum by adsorptive anodic stripping voltammetry of molybdenum–alizarin violet complex at an acetylene black paste electrode

A reliable and sensitive procedure for the determination of trace levels of molybdenum by adsorptive anodic stripping voltammetry is proposed. The method is based on adsorptive accumulation of the molybdenum (Mo)–alizarin violet (AV) complex onto an acetylene black paste electrode (ABPE), followed by oxidation of the adsorbed species by voltammetric scan using a second-order derivative modulation. For voltammetric determination of molybdenum, the parameters influencing the peak current have been optimized. Under the selected conditions, the electrode demonstrated linear response over a wide range of Mo(VI) concentration (6.0 × 10⁻⁹–1.0 × 10⁻⁵ mol/L), the detection limit was 2.0 × 10⁻⁹ mol/L (S/N = 3) for 120 s accumulation. The effects of potential interfering ions were studied, and it was found that the proposed procedure was free from most interferences. The method has been applied to the determination of molybdenum in plant foodstuffs, and satisfied results were obtained.     

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.