Heparin-stabilized gold nanoparticles embedded in graphene for the electrochemical determination of esculetin.

A conductive nanocomposite consisting of heparin-stabilized gold nanoparticles embedded in graphene was prepared and characterized to develop an electrochemical sensor for the determination of esculetin in tea and jam samples. The gold nanoparticles were characterized by spectroscopic and microscopic techniques. The different proportions of graphene in the nanocomposite were evaluated and characterized by electrochemical practices. The heterostructure material on the glassy carbon electrode with esculetin showed π-π stacking interactions with an adsorption-controlled process. The voltammetric profile of esculetin using the proposed nanomaterial presented oxidation and reduction peaks at +0.61 and +0.58 V vs. Ag/AgCl, respectively, facilitating the electron transfer with esculetin through the transfer of two moles of protons and two moles of electrons per mole of esculetin. Using optimized conditions and square wave voltammetry, the calibration curve was obtained with two linear ranges, from 0.1 to 20.5 µmol L-1, with a detection limit of 43.0 nmol L-1. The electrochemical sensor showed satisfactory results for repeatability and stability, although interferences were observed in the presence of high concentrations of ascorbic acid or quercetin. The sensor was successfully applied in the determination of esculetin in samples of mulberry jam, white mulberry leaf tea, and white mulberry powder tea, presenting adequate recovery ranges. This directive provides valuable insights for the development of novel electrochemical sensors using heparin-based conductive nanomaterials with improved sensitivity and sensibility.

Anticorrosive properties of chitosan-derivatives coatings on Mg AZ31 alloy in Hank's Balanced Salt Solution.

This study describes the preparation of chitosan-derivatives coatings on AZ31 Mg alloy for corrosion protection in Hank's Balanced Salt Solution (HBSS). The derivatives were prepared by reacting chitosan with natural aldehydes (vanillin, benzaldehyde and cinnamaldehyde) and the coatings were characterized by means of water contact angle, scanning electron microscopy and swelling essays. The corrosion behavior of the samples was investigated using potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution essays. All derivatives present superior corrosion protection than neat chitosan and the best performance is observed for the vanillin derivative with the highest modification degree, which present hydrogen evolution rate of 0.05 mL cm-2 day-1, below the tolerance limit for biomedical application, and |Z|max in the order of 104.6 Ω cm2 even after 14 days of exposure to the corrosive solution.

Nitrogen and sulfur co-doped graphene quantum dot-modified electrode for monitoring of multivitamins in energy drinks.

A glassy carbon electrode was modified with nitrogen and sulfur co-doped graphene quantum dots immobilized in chitosan for the monitoring of multivitamins. The graphene quantum dots were synthesized using a simple citric acid/l-cysteine pyrolysis procedure. The co-doping with nitrogen and sulfur in the graphene matrix was confirmed by spectroscopic techniques. Electron microscopy results showed that the synthesized quantum dots had a diameter of 3.4 ± 1.4 nm. Electrochemical techniques showed excellent current responses to vitamin oxidation provided by the modified electrode compared to the bare electrode. The parameters of square wave voltammetry were optimized in order to obtain the best current responses and to study the electrochemical oxidation of vitamins. The calibration plots for vitamins B2, B6 and B12 were constructed in 0.1 mol L-1 sodium acetate buffer (pH 5.0) with limits of detection of 0.30, 30.1 and 0.32 nmol L-1, respectively. Lastly, the modified electrode was effectively implemented in the quantification of vitamins in classic and fruit-based energy drink samples.

New relations between modification degree, swelling and impedance in anticorrosion chitosan-derivative coatings on magnesium alloy AZ31.

Environmentally friendly coatings of chitosan-derivatives with natural aldehydes were investigated for corrosion protection of magnesium AZ31 alloy. The derivatives were characterized using FTIR and UV-Vis, and the swelling degree of their films was determined in 3.5 wt% NaCl solution. The coated samples were characterized using scanning electron microscopy, potentiodynamic polarization and electrochemical impedance spectroscopy. The results indicated that, as the modification degree increases, the corrosion current density and the swelling degree decrease, whereas the impedance modulus increases. For the first time it was proposed relations between |Z| and swelling with the modification degree.

A non-mercury electrode for the voltammetric determination of butralin in foods.

A modified electrode was developed with Co-Ag bimetallic nanoparticles stabilized in poly(vinylpyrrolidone) for butralin monitoring. This is the first non-mercury electrode proposed for the quantification of butralin. The bimetallic nanoparticles were characterized by spectroscopic and microscopic techniques, which showed that they are composed of a cobalt core partially covered with silver. The modified electrode was characterized by field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy and electrochemical impedance spectroscopy. The experimental parameters (pH, supporting electrolyte, accumulation step, pulse technique) were optimized. The calibration plot for butralin obtained by square wave voltammetry was linear in the range of 0.1-1.0 µmol L-1 with limits of detection and quantification of 32 and 106 nmol L-1, respectively. Lastly, the modified electrode was effectively implemented in the quantification of butralin in honey and apple jam samples. The results were in agreement with those furnished by UV-vis spectrometry and endorsed by statistical tests.

A carbon paste electrode improved with poly(ethylene glycol) for tannic acid surveillance in beer samples.

Tannic acid is often used as additive in beers and is an important parameter to be evaluated in quality control of beverages. Thus, this paper describes the improvement of a carbon paste electrode through its modification with poly(ethylene glycol) for determination of tannic acid in beers. Microscopic and electrochemical techniques were used to characterize the modified surface. The electrochemical behavior of tannic acid and the optimization of experimental parameters (pH, supporting electrolyte and accumulation step) on the modified surface were performed by cyclic voltammetry. The calibration plot for tannic acid by differential pulse voltammetry was linear in the range of 0.08-2.10 µmol L-1 with limits of detection and quantification of 72.6 and 220 nmol L-1, respectively. Lastly, the carbon paste electrode improved with poly(ethylene glycol) was effectively implemented in the quantification of tannic acid in beer samples. The results were similar to those furnished by the Folin-Ciocalteu method.

Electrode modified with graphene quantum dots supported in chitosan for electrochemical methods and non-linear deconvolution of spectra for spectrometric methods: approaches for simultaneous determination of triclosan and methylparaben.

Two analytical methods were developed using electrochemical and spectrometric techniques for the simultaneous determination of endocrine disruptors triclosan and methylparaben in the monitoring of personal care products. For the electroanalytical analyses, a sensitive electrode based on graphene quantum dots supported in chitosan was employed. Under optimized conditions and a working potential of typically + 0.60 V for triclosan and + 0.81 V (vs. Ag/AgCl) for methylparaben, the calibration plots obtained by differential pulse voltammetry were linear in the range 0.10 to 10.0 µmol L-1. The detection limits were 0.03 and 0.04 µmol L-1 for triclosan and methylparaben, respectively. For the spectrometric method, UV/VIS spectrometry was used with a mathematical processing of non-linear deconvolution. This processing was used to solve the problem of overlapping absorption bands of triclosan (282 nm) and methylparaben (257 nm), which enabled simultaneous determination. The calibration plots by UV/VIS spectrometry were linear in the range 1.0 to 14.0 µmol L-1 with detection limits of 0.42 and 0.37 µmol L-1, respectively, for triclosan and methylparaben. Similar results obtained from the calibration plots of individual analytes suggest that the methods can be applied for individual or simultaneous determination of these species. Both methods were employed in the analysis of five samples of personal care products: toothpaste, antiseptic soap, antiseptic deodorant, shampoo, and a bath kit (soap and shampoo). The statistical tests indicated that there were no significant differences regarding the accuracy and precision of the data provided by the two methods described herein. Graphical abstract Schematic representation for simultaneous determination of triclosan and methylparaben: electrochemical method employing an electrode modified with graphene quantum dots supported in chitosan and spectrometric method applying a non-linear deconvolution of spectrum.

Ionic liquid-supported magnetite nanoparticles as electrode modifier materials for estrogens sensing.

This paper reports the application of a carbon paste electrode modified with magnetite nanoparticles and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate in the electroanalytical determination of 17ß-estradiol and estriol. These estrogens are potential endocrine disruptors and thus it is relevant the development of devices for their monitoring. Transmission electron microscopy, scanning electron microscopy and zeta potential techniques were applied to characterization of the modifier materials. In cyclic voltammetry experiments, irreversible oxidation peaks were observed for 17ß-estradiol and estriol at +0.320 V and +0.400 V, respectively. The anodic currents obtained were approximately three times greater than those provided by the unmodified electrode due to the presence of magnetic nanoparticles and the ionic liquid, which improved the sensitivity of modified electrode. For the analysis, the parameters of the square-wave voltammetry (scan increment, amplitude and frequency) were optimized by Box-Behnken factorial design for each estrogen. For 17ß-estradiol in B-R buffer pH 12.0, the calibration plot was linear from 0.10 to 1.0 µmol L-1, with a detection limit of 50.0 nmol L-1. For estriol in B-R buffer pH 11.0, the linear range was 1.0 to 10.0 µmol L-1, with a detection limit of 300.0 nmol L-1. The modified electrode was applied in the determination of 17ß-estradiol and estriol in pharmaceutical formulations and the results were comparable to those obtained using UV/VIS spectrometry. Statistical tests were applied to evaluate the results and it was concluded that there was no significant difference regarding the precision and accuracy of the data provided by the two methods.

Chitosan coatings crosslinked with genipin for corrosion protection of AZ31 magnesium alloy sheets.

In this study, coatings of chitosan crosslinked with genipin were prepared on sheets of AZ31 magnesium alloy and their corrosion protection properties were characterized by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The coatings were also characterized by means of FTIR and XPS. It was observed that the crosslinking process decreases the corrosion current and shifts the corrosion potential of the alloy to less negative values. The EIS analysis demonstrated that the crosslinking process increases the maximum impedance after short and long exposure times. The superior performance of the crosslinked coatings is related to a lower degree of swelling, as observed in the swelling tests carried out on free-standing films.

Magnetite-platinum nanoparticles-modified glassy carbon electrode as electrochemical detector for nitrophenol isomers.

A glassy carbon electrode was modified with magnetite and platinum nanoparticles stabilized with 3-n-propyl-4-picoline silsesquioxane chloride. This chemically-modified electrode is proposed for the first time for the individual or simultaneous electrochemical detection of nitrophenol isomers. Nanoparticles act as catalysts and also increase the surface area. The polymer stabilizes the particles and provides the electrochemical separation of isomers. Under optimized conditions, the reduction peak currents, obtained by differential-pulse voltammetry, of 2-, 3-, and 4-nitrophenol increased linearly with increases in their concentration in the range of 0.1-1.5µmolL-1. In individual analysis, the detection limits were 33.7nmolL-1, 45.3nmolL-1 and 48.2nmolL-1, respectively. Also, simultaneous analysis was possible for 2-, and 4-nitrophenol. In this case, the separation of the peak potentials was 0.138V and the detection limits were 69.6nmolL-1 and 58.0nmolL-1, respectively. These analytical figures of merit evidence the outstanding performance of the modified electrode, which was also successfully applied to the individual determination of isomers in environmental and biological samples. The magnetite and platinum nanoparticles modified glassy carbon electrode was able to detect nitrophenol isomers at the ppm level in rain water and human urine samples.

Silver nanoparticle-modified electrode for the determination of nitro compound-containing pesticides.

This paper reports the electroanalytical determination of pendimethalin and ethyl parathion by square-wave adsorptive stripping voltammetry using a material comprised of chitosan-stabilized silver nanoparticles to modify a glassy carbon electrode. Under optimized experimental conditions, the peak current was found to vary linearly with the concentration of pendimethalin in the range of 70 to 2000 nmol L(-1) and with concentration of ethyl parathion in the range of 40 to 8000 nmol L(-1). Detection limits of 36 and 40 nmol L(-1) were obtained for pendimethalin and ethyl parathion, respectively. The silver - nanoparticle-modified electrode was successfully employed for the analysis of pesticides in tap and mineral water (pendimethalin) and in lettuce and honey (ethyl parathion) samples. Pendimethalin recovery was between 94 and 100 %, and ethyl parathion recovery was between 97 and 101 %, indicating no significant matrix interference effects on the analytical results. The accuracy of the electroanalytical methodology using the proposed modified electrode was also compared to that of the UV-vis spectrophotometric method.

Environmentally-friendly in situ plated bismuth-film electrode for the quantification of the endocrine disruptor parathion in skimmed milk.

An in situ bismuth-film electrode (BiFE) together with square-wave cathodic voltammetry (SWCV) was used to determine the concentration of the endocrine disruptor parathion in skimmed milk. The experimental conditions (deposition time, deposition potential and Bi (III) concentration) were optimized for the preparation of the BiFE. A glassy carbon electrode was used as the substrate. The selection of the chemical composition of the supporting electrolyte and the solution pH was aimed at improving the reduction of parathion at the BiFE surface. In addition, the parameters of the square-wave cathodic voltammetry were adjusted to improve the sensor performance. A cathodic current identified at -0.523 V increased linearly with the parathion concentration in the range of 0.2-2.0 µmol L(-1) (R=0.999). The sensitivity of the calibration curve obtained was 4.09 µA L µmol(-1), and the limits of detection (LOD) and quantification (LOQ) were 55.7 nmol L(-1) and 169.0 nmol L(-1), respectively. The performance of the sensor was tested using a sample of skimmed milk with parathion added. The same determination was carried out by UV-vis spectroscopy and the results obtained were used for the statistical evaluation of the data obtained.

A label-free electrochemical immunosensor based on an ionic organic molecule and chitosan-stabilized gold nanoparticles for the detection of cardiac troponin T.

A label-free electrochemical immunosensor based on an ionic organic molecule ((E)-4-[(4-decyloxyphenyl)diazenyl]-1-methylpyridinium iodide) and chitosan-stabilized gold nanoparticles (CTS-AuNPs) was developed for the detection of cardiac troponin T (cTnT). The new ionic organic molecule was strategically employed as a redox probe, and CTS-AuNPs were applied as a "green" platform for the immobilization of the monoclonal anti-cTnT antibody, for the construction of the immunosensor. The characterization of the proposed immunosensor was carried out by employing cyclic and square-wave voltammetry and electron microscopy. The film of ionic organic molecules acts as a redox probe and from its electrochemical response the presence of cTnT antigens, which interact specifically with the anti-cTnT antibody immobilized on the surface of the immunosensor, can be detected. This interaction results in a decrease in the analytical signal, which is proportional to the amount of cTnT antigens present in the sample analyzed. Under optimized conditions, using square-wave voltammetry (a frequency of 100 Hz, an amplitude of 100 mV and an increment of 8 mV) and an incubation time of 10 min, the proposed immunosensor showed linearity in the range of 0.20 to 1.00 ng mL(-1) cTnT, with a calculated limit of detection of 0.10 ng mL(-1). The proposed immunosensor shows some advantages when compared to other sensors reported in the literature, especially with regard to the detection limit and the time of incubation. A study of the interday precision (n = 8) showed a coefficient of variation of 3.33%. The potential interference of some compounds (glucose, ascorbic acid, albumin, uric acid, creatine, and creatinine) on the response of the immunosensor was evaluated and the inhibition of the immunosensor response was found to be less than 8.0%. The immunosensor was successfully used for the determination of cTnT in samples of simulated blood serum with a relative error of <13.0%. Furthermore, the proposed methodology provides a working range that allows the detection of cTnT antigens at levels below the cutoff value used for the diagnosis of acute myocardial infarction and was also found to be faster than the conventional methods.

Troponin T immunosensor based on liquid crystal and silsesquioxane-supported gold nanoparticles.

A nanostructured immunosensor based on the liquid crystal (E)-1-decyl-4-[(4-decyloxyphenyl)diazenyl]pyridinium bromide (Br-Py) and gold nanoparticles supported by the water-soluble hybrid material 3-n-propyl-4-picolinium silsesquioxane chloride (AuNP-Si4Pic(+)Cl(-)) was built for the detection of troponin T (cTnT), a cardiac marker for acute myocardial infarction (AMI). The functionalized nanostructured surface was used to bind anti-cTnT monoclonal antibodies through electrostatic interaction. The immunosensor (ab-cTnT/AuNP-Si4Pic(+)Cl(-)/Br-Py/GCE) surface was characterized by microscopy techniques. The electrochemical behavior of the immunosensor was studied by cyclic voltammetry and electrochemical impedance spectroscopy. A calibration curve was obtained by square-wave voltammetry. The immnunosensor provided a limit of detection of 0.076 ng mL(-1) and a linear range between 0.1 and 0.9 ng mL(-1) (appropriate for AMI diagnosis).

Gold nanoparticles hosted in a water-soluble silsesquioxane polymer applied as a catalytic material onto an electrochemical sensor for detection of nitrophenol isomers.

The water-soluble 3-n-propyl-4-picolinium silsesquioxane chloride (Si4Pic(+)Cl(-)) polymer was prepared, characterized and used as a stabilizing agent for the synthesis of gold nanoparticles (nAu). The ability of Si4Pic(+)Cl(-) to adsorb anionic metal complexes such as AuCl4(-) ions allowed well-dispersed nAu to be obtained with an average particle size of 4.5nm. The liquid suspension of nAu-Si4Pic(+)Cl(-) was deposited by the drop coating method onto a glassy carbon electrode (GCE) surface to build a sensor (nAu-Si4Pic(+)Cl(-)/GCE) which was used for the detection of o-nitrophenol (o-NP) and p-nitrophenol (p-NP). Under optimized experimental conditions the reduction peak current increased with increasing concentrations of both nitrophenol isomers in the range of 0.1-1.5µmolL(-1). The detection limits were 46nmolL(-1) and 55nmolL(-1) for o-NP and p-NP, respectively. These findings indicate that the nAu-Si4Pic(+)Cl(-) material is a very promising candidate to assemble electrochemical sensors for practical applications in the field of analytical chemistry.

Electroanalytical determination of estriol hormone using a boron-doped diamond electrode.

A boron-doped diamond (BDD) electrode was used for the electroanalytical determination of estriol hormone in a pharmaceutical product and a urine sample taken during pregnancy by square-wave voltammetry. The optimized experimental conditions were: (1) a supporting electrolyte solution of NaOH at a pH of 12.0, and (2) a frequency of 20 Hz, a pulse height of 30 mV and a scan increment of 2 mV (for the square-wave parameters). The analytical curve was linear in the concentration range of 2.0 x 10(-7) to 2.0 x 10(-5) mol L(-1) (r=0.9994), with a detection limit of 1.7 x 10(-7) mol L(-1) and quantification limit of 8.5 x 10(-7) mol L(-1). Recoveries of estriol were in the range of 98.6-101.0%, for the pharmaceutical sample, and 100.2-103.4% for the urine sample, indicating no significant matrix interference effects on the analytical results. The accuracy of the electroanalytical methodology proposed was compared to that of the radioimmunoassay method. The values for the relative error between the proposed and standard methods were -7.29% for the determination of estriol in the commercial product and -4.98% in a urine sample taken during pregnancy. The results obtained suggest a reliable and interesting alternative method for electroanalytical determination of estriol in pharmaceutical products and urine samples taken during pregnancy using a boron-doped diamond electrode.

In vivo human electrochemical properties of a NiTi-based alloy (Nitinol) used for minimally invasive implants.

The development of a homemade device for in vivo human determination of the open circuit potential (OCP) of Nitinol is described. Pseudo-reference electrodes (316L stainless steel and Pt) were initially tested and validated in vitro using simulated body fluids. As judged from the excellent electrochemical responses in terms of both accuracy and precision, the most ideal system comprised the combination of sterilized Pt (pseudo-reference) and Nitinol (working) needle-shaped electrodes. The average in vivo human OCP determined from independent measurements on six human patients with indication of direct arterial surgery was -0.334 +/- 0.030 V/SCE. This value was in good agreement with data recorded in vitro using simulated body fluids (-0.313 +/- 0.003 V/SCE in AFNOR S90-701 artificial saliva; -0.334 +/- 0.001 V/SCE in artificial urine; -0.239 +/- 0.007 V/SCE in Ringer's solution). The thin surface film protecting the bulk NiTi alloy is therefore not susceptible to active dissolution at rest as long as the break down potentials (>>0.0 V/SCE) so far reported are well above OCP measured in this study. These results highlight the importance of evaluating the corrosion resistance of Nitinol under realistic conditions (mechanical loads, wear and fatigue) in order to establish multifaceted mechanisms that might lead to accelerated dissolution and failure of implanted stents.

Biosensor based on laccase and an ionic liquid for determination of rosmarinic acid in plant extracts.

Novel biosensors based on laccase from Aspergillus oryzae and the ionic liquids (ILs) 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIPF(6)) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF(4)) were constructed for determination of rosmarinic acid by square-wave voltammetry. The laccase catalyzes the oxidation of rosmarinic acid to the corresponding o-quinone, which is electrochemically reduced back to rosmarinic acid at +0.2V vs. Ag/AgCl. The biosensor based on BMIPF(6) showed a better performance than that based on BMIBF(4). The best performance was obtained with 50:20:15:15% (w/w/w/w) of the graphite powder:laccase:Nujol:BMIPF(6) composition in 0.1mol L(-1) acetate buffer solution (pH 5.0). The rosmarinic acid concentration was linear in the range of 9.99 x 10(-7) to 6.54 x 10(-5)mol L(-1) (r=0.9996) with a detection limit of 1.88 x 10(-7)mol L(-1). The recovery study for rosmarinic acid in plant extract samples gave values from 96.1 to 105.0% and the concentrations determined were in agreement with those obtained using capillary electrophoresis at the 95% confidence level. The BMIPF(6)-biosensor demonstrated long-term stability (300 days; 920 determinations) and reproducibility, with a relative standard deviation of 0.56%.

Rutin determination in pharmaceutical formulations using a carbon paste electrode modified with poly(vinylpyrrolidone).

A carbon paste electrode modified with poly(vinylpyrrolidone) (PVP) was developed for the determination of rutin. PVP enhances the adsorption of rutin on the electrode surface due to the presence of hydrogen bonding between the imide group in PVP and the hydroxyl group in rutin. The current responses in cyclic and linear sweep voltammetric experiments were investigated and an oxidation peak was observed at +0.87 V vs. Ag/AgCl. Several parameters were investigated to evaluate the performance of the modified electrode. The best analytical response was obtained employing (75:15:10%, w/w/w) graphite powder:Nujol:PVP, with an accumulation time of 10 min, scan rate of 100 mV s(-1) and 0.1M phosphate buffer solution (pH 6.0) as supporting electrolyte. The analytical curve was linear for rutin concentrations of 3.9 x 10(-7) to 1.3 x 10(-5)M (r=0.9991) and the detection limit was 1.5 x 10(-7)M. The recovery of rutin from pharmaceutical samples ranged from 98.3 to 101.7% and the relative standard deviation was 3.3% for a solution containing 7.7 x 10(-6)M rutin (n=5). This electrode was successfully applied to the determination of rutin in pharmaceutical formulations. The results obtained using the proposed modified carbon paste electrode and those obtained by the standard method are in agreement at the 95% confidence level.

Antioxidant activity of phenolic and related compounds: a density functional theory study on the O-H bond dissociation enthalpy.

We report here on calculations at the hybrid DFT/HF (B3-LYP/6-31G(d, p)) level of the O-H bond dissociation enthalpy (O-H BDE) of phenylpropenoic acids (caffeic, ferulic, p-coumaric and cinnamic) and phenolic acids and related compounds (gallic, methylgallate, vanillic and gentisic) in order to gain insight into the understanding of structure-antioxidant activity relationships. The results were correlated and discussed mainly on the basis of experimental data in a companion work (Galato D, Giacomelli C, Ckless K, Susin MF, Vale RMR, Spinelli A. Antioxidant capacity of phenolic and related compounds: correlation among electrochemical, visible spectroscopy methods and structure-antioxidant activity. Redox Report 2001; 6: 243-250). The O-H BDE values showed remarkable dependence on the hydroxyl position in the benzene ring and the existence of additional interaction due to hydrogen bonding. For parent molecules, the experimental antioxidant activity (AA) order was properly obeyed only when intramolecular hydrogen bonding was present in the radicalized structures of o-dihydroxyl moieties. In structurally related compounds, excellent correlation with experimental data was in general observed (0.64 < rho < 0.99). However, it is shown that excellent correlation can also be obtained for this series of compounds considering p-radicalized structures which were not stabilized by intramolecular hydrogen bonding, but this had no physical meaning. These findings suggested that the antioxidant activity evaluation of phenolic and related compounds must take into consideration the characteristics of each particular compound.