Self-assembling gold nanoparticles on thiol-functionalized poly(styrene-co-acrylic acid) nanospheres for fabrication of a mediatorless biosensor

A novel strategy to construct a sensitive mediatorless sensor of H₂O₂ was described. At first, a cleaned gold electrode was immersed in thiol-functionalized poly(styrene-co-acrylic acid) (St-co-AA) nanosphere latex prepared by emulsifier-free emulsion polymerization St with AA and function with dithioglycol to assemble the nanospheres, then gold nanoparticles were chemisorbed onto the thiol groups and formed monolayers on the surface of poly(St-co-AA) nanospheres. Finally, horseradish peroxidase (HRP) was immobilized on the surface of the gold nanoparticles. The sensor displayed an excellent electrocatalytical response to reduction of H₂O₂ without the aid of an electron mediator. The biosensor showed a linear range of 8.0 μmol L⁻¹–7.0 mmol L⁻¹ with a detection limit of 4.0 μmol L⁻¹. The biosensor retained more than 97.8% of its original activity after 60 days’ storage. Moreover, the studied biosensor exhibited good current reproducibility and good fabrication reproducibility.     

Multi-elemental analysis of non-food packaging materials by inductively coupled plasma-time of flight-mass spectrometry

Commercial non-food packaging materials of four different matrices (paper, low density polyethylene (LDPE), polyethylene-polypropylene (PE-PP) and high density polyethylene (HDPE)) were examined for the content of Cr, Ni, Cu, Zn, As, Mo, Cd, Sb, Ba, Hg, Tl, Pb and U. The examined samples (0.17–0.35 g) were digested in HNO₃ and H₂O₂ (papers, LDPE and PE-PP) and in HNO₃, H₂SO₄ and H₂O₂ (HDPE) using microwave assisted high pressure system. The inductively coupled plasma-time of flight-mass spectrometry (ICP-TOFMS) has been employed as the detection technique. All measurements were carried out using internal standardization. Yttrium and rhodium (50 ng g⁻¹) were used as internal standards. The detection and quantification limits obtained were in the range of 0.005 ng g⁻¹ (⁵²Cr) to 0.51 ng g⁻¹ (⁶⁶Zn) and 0.015 μg g⁻¹ (⁵²Cr) to 2.02 μg g⁻¹ (⁶⁶Zn) of dry mass, respectively. The evaluated contents (mg kg⁻¹) of particular elements in the examined materials were as follows: 0.22–219; <1.05–9.03; 1.25–112; <2.02–449; <0.98–<1.30; <0.36–2.06; <0.29–113; <0.22–44.1; <0.06–57.4; <0.66–<0.88; <0.08–0.24; <0.13–1222 and <0.08–0.44 for Cr, Ni, Cu, Zn, As, Mo, Cd, Sb, Ba, Hg, Tl, Pb and U, respectively.     

Determination of organochlorine pesticides in ground water samples using solid-phase microextraction by gas chromatography-electron capture detection

A direct immersion solid-phase microextraction coupled with gas chromatography-electron capture detection (SPME-GC-ECD) method was optimized and validated for the quantitative determination of 18 organochlorine pesticides in ground water. Ionic strength, stirring speed, adsorption and desorption time and pH were some of the parameters investigated in order to select the optimum conditions for SPME with a 50/30 DVB/CAR/PDMS fiber coating. The SPME-GC/ECD method showed good linear response below 10 ng L⁻¹ with R² values in the range of 0.9950–0.9997. The repeatability of the measurements were lower than 10%. Values of relative recoveries located within the acceptable range (80–120%). Limits of quantification (LOQ) from 4.5 × 10⁻³ to 1.5 ng L⁻¹ were obtained. On average 8 organochlorines were found per sample, even so all the 18 organochlorines were quantified among them. Substances such as endrin ketone, γ-BHC and β-BHC were the pesticides determined in larger concentration (0.06–305 ng L⁻¹), while methoxychlor and aldrin in smaller amounts (0.151–1.55 ng L⁻¹). Measured levels of organochlorine pesticides were above the limits established by Brazilian regulations.     

Peroxidase immobilized on Amberlite IRA-743 resin for on-line spectrophotometric detection of hydrogen peroxide in rainwater

The importance of atmospheric hydrogen peroxide (H₂O₂) in the oxidation of SO₂ and other compounds has been well established. A spectrophotometric method for the determination of hydrogen peroxide in rainwater is proposed. This method is based on selective oxidation of hydrogen peroxide using an on-line tubular reactor containing peroxidase immobilized on Amberlite IRA-743 resin. The hydrogen peroxide in the presence of phenol, 4-aminoantipyrine and peroxidase, produces a red compound (λ = 505 nm). Beer's law is obeyed in a concentration range of 1–100 μmol l⁻¹ hydrogen peroxide with an excellent correlation coefficient (r = 0.9991), at pH 7.0, with a relative standard deviation (R.S.D.) <2%. The detection limit of the method is 0.7 μmol l⁻¹ (4.8 ng of H₂O₂ in a 200 μl sample). Measurements of hydrogen peroxide in rain samples were carried out over the period from November 2003 to January 2005, in the central area of the Juiz de Fora city, Brazil. The concentration of H₂O₂ varied from values lower than the detection limit to 92.5 μmol l⁻¹. The effects of the presence of nonseasalt (NSS) SO₄²⁻, NO₃⁻ and H⁺ in the concentration of hydrogen peroxide in the rainwater had been evaluated. The average concentrations of H₂O₂, NO₃⁻, NSS SO₄²⁻ and SO₄²⁻ are 23.4, 18.9, 7.9 and 10.3 μmol l⁻¹, respectively. The pH values for 82% of the collected samples are greater than 5.0. The spectrophotometeric method developed in this work that uses enzyme immobilized on the resin ion-exchange compared with the amperometric method did not present any significant difference in the results.     

Detection of TMV with ODA-H(2)O(2)-HRP voltammetric enzyme-linked immunoassay system

o-Dianisidine (ODA)-H₂O₂-horseradish peroxidase (HRP) voltammetric enzyme-linked immunoassay system has firstly been used for the detection of tobacco mosaic virus (TMV). HRP catalyzes strongly the oxidation reaction of ODA by H₂O₂, the product of which produces a sensitive second order derivative linear sweep voltammetric peak at potential of −0.56 V (versus SCE) in Britton–Robinson (BR) buffer. HRP activity has been measured with this voltammetric peak and TMV detected through immunoreaction. The detection limit for HRP is 9.25×10^-7 mU l⁻¹ and the linear range is 2.5×10⁻⁶–5.0×10⁻⁴ mU l⁻¹. The detection limit for the clarified TMV is 0.25 ng ml⁻¹ and the highest dilution ratio detected for the infected leaf sap is 1:8×10⁵. The sensitivity for TMV detection with this method is higher than that with the enzyme-linked immunosorbent spectrophotometric assay (ELISA) using ODA-H₂O₂-HRP system. The processes of the enzyme-catalyzed reaction and the electro-reduction of the product of the enzyme-catalyzed reaction have been described.     

Diagnosis of copper ions in vascular tracts using a fluorine-doped carbon nanotube sensor

The voltammetric assay of Cu(II) was investigated using a carbon nanotube electrode (CNE) and fluorine immobilized onto a carbon nanotube electrode (FCNE) in cyclic voltammetry (CV), square-wave (SW) stripping voltammetry, and chronoamperometry. Optimum SW conditions were attained at working ranges of 0.01–0.11 ng L⁻¹ Cu(II) (11 points), and a relative standard deviation of 1.68% (RSD, n = 15) was observed at 10.0 μg L⁻¹ Cu(II). Within a 200 s accumulation time, detection limit of 0.006 μg L⁻¹ was attained. The life span of each electrode was more than 1 month. The sensor was applied to tap water, blood, and rat tail vascular (in vivo). It was found that the sensor could be used with an interface system in the assay of live cells and non-treated blood.     

Thermal decomposition study on mixed ligand thymine complexes of divalent nickel(II) with dianions of some dicarboxylic acids

Thermal decomposition of mixed ligand thymine (2,4-dihydroxy-5-methylpyrimidine) complexes of divalent Ni(II) with aspartate, glutamate and ADA (N-2-acetamido)iminodiacetate dianions was monitored by TG, DTG and DTA analysis in static atmosphere of air. The decomposition course and steps of complexes [Ni(C₅H₆N₂O₂)(C₄H₅NO₄)²⁻(H₂O)₂]·H₂O, [Ni(C₅H₆N₂O₂)(C₅H₇NO₄)²⁻(H₂O)₂]·H₂O and [Ni(C₅H₆N₂O₂)(C₆H₈N₂O₅)²⁻(H₂O)₂]·1.5H₂O were analyzed. The final decomposition products are found to be the corresponding metal oxides. The kinetic parameters namely, activation energy (E*), enthalpy (ΔH*), entropy (ΔS*) and free energy change of decomposition (ΔG*) are calculated from the TG curves using Coats–Redfern and Horowitz–Metzger equations. The stability order found for these complexes follows the trend aspartate > ADA > glutamate.     

Use of CdSe/ZnS core-shell quantum dots as energy transfer donors in sensing glucose

In the present work, CdSe/ZnS core-shell quantum dots were synthesized and conjugated with enzymes, glucose oxidase (GOD) and horseradish peroxidase (HRP). The complex of enzyme-conjugated QDs was used as QD-FRET-based probes to sense glucose. The QDs were used as an electron donor, whereas GOD and HRP were used as acceptors for the oxidation/reduction reactions involved in oxidizing glucose to gluconic acid. Electron transfer between the redox enzymes and the electrochemical reduction of H₂O₂ (or O₂) occurred rapidly, resulting in an increase of the turnover rate of the electron exchange between the substrates (e.g. glucose, H₂O₂ and O₂) and the enzymes (GOD, HRP), as well as between the QDs and the enzymes. The transfer of non-radiative energy from the QDs to the enzymes resulted in the fluorescence quenching of the QDs, corresponding to the increase in the concentration of glucose. The linear detection ranges of glucose concentrations were 0–5.0 g/l (R = 0.992) for the volume ratios of 10/5/5, 0.2–5.0 g/l (R = 0.985) for the volume ratios of 10/5/3 and 1.0–5.0 g/l (R = 0.982) for the volume ratios of 10/5/0. Temperature (29–37 °C), pH (6–10) and some ions (NH₄⁺, NO₃⁻, Na⁺, Cl⁻) had no interference effect on the glucose measurement.     

Voltammetric determination of glucose based on reduction of copper(II)–glucose complex at lanthanum hydroxide nanowire modified carbon paste electrodes

A novel voltammetric method for the determination of β-d-glucose (GO) is proposed based on the reduction of Cu(II) ion in Cu(II)(NH₃)₄²⁺–GO complex at lanthanum(III) hydroxide nanowires (LNWs) modified carbon paste electrode (LNWs/CPE). In 0.1 mol L⁻¹ NH₃·H₂O–NH₄Cl (pH 9.8) buffer containing 5.0 × 10⁻⁵ mol L⁻¹ Cu(II) ion, the sensitive reduction peak of Cu(II)(NH₃)₄²⁺–GO complex was observed at −0.17 V (versus, SCE), which was mainly ascribed to both the increase of efficient electrode surface and the selective coordination of La(III) in LNW to GO. The increment of peak current obtained by deducting the reduction peak current of the Cu(II) ion from that of the Cu(II)(NH₃)₄²⁺–GO complex was rectilinear with GO concentration in the range of 8.0 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹, with a detection limit of 3.5 × 10⁻⁷ mol L⁻¹. A 500-fold of sucrose and amylam, 100-fold of ascorbic acid, 120-fold of uric acid as well as gluconic acid did not interfere with 1.0 × 10⁻⁵ mol L⁻¹ GO determination.     

Immobilization enzyme fluorescence capillary analysis for determination of lactic acid

A new method for the determination of lactic acid based on the immobilization enzyme fluorescence capillary analysis (IE-FCA) was proposed. Lactic dehydrogenase (LDH) was immobilized on inner surface of a capillary with glutaraldehyde, and an immobilized enzyme lactate capillary bioreactor (IE-LCBR) was formed for the determination of lactic acid. After nicotinamide adenine dinucleotide (NAD⁺) is mixed with lactic acid solution, it was sucked into the IE-LCBR and was detected at λ_ex 353 nm/λ_em 466 nm. Optimized conditions are as follows: the temperature is 38 °C; the reaction time is 15 min; the concentrations of Tris buffer (pH 8.8) and NAD⁺ are 0.1 mol L⁻¹ and 4 mmol L⁻¹, respectively; the concentration of LDH used for immobilization is 15 kU L⁻¹. The concentration of lactic acid is directly proportional to the fluorescence intensity measured from 0.50 to 2.0 mmol L⁻¹; and the analytical recovery of added lactic acid was 99–105%. The minimum detection limit of the method is 0.40 mmol L⁻¹ and sensitivity of the IE-CBR is 4.6 F mmol⁻¹ L⁻¹ lactate. Its relative standard deviation (R.S.D.) is ≤2.0%. This IE-FCA method was employed for determination of lactate in milk drink.