Alizarin Red S as an electrochemical indicator for saccharide recognition

In addition to the well-established spectroscopic Alizarin Red S (ARS) assay for the determination of binding constants between arylboronic acids and different saccharides, we report the use of ARS as a reporter in an electrochemical set-up. The electrochemical properties of ARS, the binding to phenyl boronic acid (PBA) and the competition with fructose in phosphate buffer at pH 7.4 were investigated by cyclic voltammetry (CV). By choosing a negative scan direction (starting at +0.2 V), a quasi-reversible process was detected at E⁰′ = −0.59V with ΔE_p = 0.1V. An irreversible oxidation peak at +0.42 V could also be detected. These peaks are characterised both as a 2-proton-2-electron transfer and corresponds to the oxidation and reduction of the anthraquinone or the ortho-quinone moiety. After addition of phenylboronic acid a new oxidation peaks occurred at −0.42 V which correlates with the ARS–PBA interaction. The peak current increased with increasing phenylboronic acid concentration according to the release of BA and formation of the ARS–PBA ester. After addition of fructose the peak current decreases again, in proportion to the fructose concentration, enabling the use of ARS as an electrochemical reporter for fructose detection up to 50 mM. Also the interaction with other cis-diol containing compounds such as sorbitol, mannitol, glucose and mannose was investigated and a dependence based on already published binding constants to phenylboronic acid could be shown.     

A boronic acid-based fluorescent hydrogel for monosaccharide detection

A boronic acid-based anthracene fluorescent probe was functionalised with an acrylamide unit to incorporate into a hydrogel system for monosaccharide detection. In solution, the fluorescent probe displayed a strong fluorescence turn-on response upon exposure to fructose, and an expected trend in apparent binding constants, as judged by a fluorescence response where D-fructose>D-galactose>D-mannose>D-glucose. The hydrogel incorporating the boronic acid monomer demonstrated the ability to detect monosaccharides by fluorescence with the same overall trend as the monomer in solution with the addition of D-fructose resulting in a 10-fold enhancement (≤0.25 mol/L).     

Electrocatalytic properties of Pd clusters on Au nanoparticles in formic acid electro-oxidation

Pd clusters were formed on highly dispersed Au nanoparticles (∼3.5 nm in diameter) using a seed-mediated growth process. The structural information and electrocatalytic activities of these Pd clusters on Au nanoparticles were confirmed by high-resolution-transmission-electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The resulting nanoparticles, which had a uniform size (<5 nm in diameter), were highly dispersed on carbon particles, and Pd clusters (<0.44 nm in size, <2 atomic layers) were formed selectively on Au nanoparticles. XPS results show that the Pd 3d_5/2 peak shifted to lower binding energies and that the amount of surface oxide decreased as the Pd content was decreased on the Au nanoparticles. In formic acid electro-oxidation, these Pd clusters exhibit enhanced electrocatalytic activity relative to that of carbon-supported Pd nanoparticles. These results may be due to the modified electronic and geometric structure of the Pd clusters on the Au nanoparticle substrate.     

Direct voltammetric determination of gold nanoparticles using graphite-epoxy composite electrode

A new voltammetric method for a direct determination of gold nanoparticles, based on adsorption and electrochemical detection of colloidal gold, is described. In this protocol, the absorption of gold nanoparticles onto the rough surface of graphite-epoxy composite electrode is followed by their electrochemical oxidation in 0.1 M HCl medium at a potential of +1.25 V. The resulting tetrachloroaurate ions generated near the electrode surface are detected by differential pulse voltammetry (DPV). The DPV response is linear in the range from 4.7 × 10⁸ to 4.7 × 10¹¹ nanoparticles cm⁻³ with a limit of detection of 1.8 × 10⁸ gold nanoparticles cm⁻³. The surface characteristics of the composite electrode are investigated and the parameters that affect the complete analytical detection process of gold nanoparticles are optimized.     

Development of an amperometric immunosensor based on TiO2 nanoparticles and gold nanoparticles

A highly hydrophilic, non-toxic and conductive TiO₂ nanoparticles/gold nanoparticles bilayer films as immobilization matrix via self-assembly (SA) and deposition method was prepared on a gold electrode. Subsequently, positively charged horseradish peroxidase (HRP) was assembled onto the bilayer films, which provided an interface to assemble gold nanoparticles for immobilization of carcinoembryonic antibody (anti-CEA). Finally, HRP was used to block sites against non-specific binding. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were applied to characterize the electrochemical properties of the SA process. The CVs reduction current of the immunosensor decreases linearly in two concentrations ranges of CEA from 0.3 to 10 ng/ml and from 10 to 80 ng/ml with a detection limit of 0.2 ng/ml in presence of 0.7 mM H₂O₂ in analyte solution. Effects of deposition time, pH of working buffer, temperature and incubation time were also explored for optimum analytical performance by using the amperometric method. Moreover, the proposed immunosensor exhibited good accuracy, high sensitivity, long-term stability and made it to determine CEA in serum samples with satisfactory results.     

Electrocatalytic performance of carbon nanotube-supported palladium particles in the oxidation of formic acid and the reduction of oxygen

We describe a simple approach for the synthesis of nanosized Pd particles supported on multiwall carbon nanotubes (MWCNTs) and their electrocatalytic performance in the oxidation of formic acid and the reduction of oxygen. The metal precursors are pre-organized on poly(diallyldimethylammonium) chloride-wrapped MWCNTs by electrostatic interaction and chemically reduced to obtain Pd nanoparticles. The MWCNT-supported nanoparticles are characterized by UV-visible spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy and electrochemical measurements. MWCNTs are uniformly decorated with closely packed array of nanoparticles. The nanoparticles on the MWCNTs have spherical and rod-like shapes with size ranging from 5 to 10 nm. XRD and selected area electron diffraction measurements of the nanoparticles show (1 1 1), (2 0 0) and (2 2 0) reflections of the Pd lattice. The electrocatalytic activity of the nanoparticles towards oxidation of formic acid and reduction of oxygen is examined in acidic solution. The MWCNT-supported particles exhibit excellent electrocatalytic activity. The electrocatalytic reduction of oxygen follows the peroxide pathway. Surface morphology and coverage of particles on the nanotubes control the electrocatalytic activity. The large surface area and high catalytic activity of the MWCNT-supported nanoparticles facilitate the electrocatalytic reactions at a favorable potential.     

Fabrication of CeO2 nanoparticles modified glassy carbon electrode and its application for electrochemical determination of UA and AA simultaneously

A glassy carbon electrode modified with CeO₂ nanoparticles was constructed and was characterized by electrochemical impedance spectrum (EIS) and cyclic voltammetry (CV). The resulting CeO₂ nanoparticles modified glassy carbon electrode (CeO₂ NP/GC electrode) was used to detect uric acid (UA) and ascorbic acid (AA) simultaneously in mixture. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards UA and AA with activation overpotential. For UA and AA in mixture, one can well separate from the other with a potential difference of 273 mV, which was large enough to allow the determination of one in presence of the other. The DPV peak currents obtained in mixture increased linearly on the UA and AA in the range of 5.0 × 10⁻⁶ to 1.0 × 10⁻³ mol/L and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol/L, with the detection limit (signal-to-noise ratio was 3) for UA and AA were 2.0 × 10⁻⁷ and 5.0 × 10⁻⁶ mol/L, respectively. The proposed method showed excellent selectivity and stability, and the determination of UA and AA simultaneously in serum was satisfactory.     

Optical responses, permeability and diol-specific reactivity of thin polyacrylamide gels containing immobilized phenylboronic acid

Thin semitransparent gels were prepared by radical copolymerization of N-acryloyl-m-aminophenylboronic acid (NAAPBA) and acrylamide (AAm) taken in molar ratios from 8:92 to 16:84, respectively, in water. The gels were characterized by the content of immobilized NAAPBA and monomer conversion. Scanning electron microscopy revealed the micrometer size pores in the dried gels. The wet gels displayed a linear optical response to sugars with sensitivity decreasing in the series: d-fructose, d-galactose, d-glucose, d-mannose, N-acetyl-d-glucosamine in the sugar concentration range from 1 to 40-60 mM at pH 7.3. Cross-linking of the gels with N,N-methylene-bis-acrylamide decreased the strength of optical response. Specific binding capacity of a diol-containing dye Alizarin Red S in the gels at pH 7.0 coincided with the content of immobilized NAAPBA indicating the 1:1 stoichiometry of the reaction and, therefore, good accessibility of the boronic acid ligands for water-soluble diols. Permeability of the gels was studied with a non-interacting dye Ethyl Orange exhibiting the pore diffusion coefficient of 1.4 × 10⁻⁷ cm²/s. The rate of optical response of the gels to glucose was found to be determined by diffusion of sugar into the relatively thick gels (l = 0.35-1 mm) with effective diffusion coefficients of 2 × 10⁻⁷ cm²/s. In the thinner gels (l = 0.1 mm) the input of other kinetic processes, such as affinity binding or structural rearrangements of the gel, was noticeable.     

Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode

A new gold nanoparticles-modified electrode (GNP/LC/GCE) was fabricated by self-assembling gold nanoparticles to the surface of the l-cysteine-modified glassy carbon electrode. The modified electrode showed an excellent character for electrocatalytic oxidization of uric acid (UA) and ascorbic acid (AA) with a 0.306 V separation of both peaks, while the bare GC electrode only gave an overlapped and broad oxidation peak. The anodic currents of UA and AA on the modified electrode were 6- and 2.5-fold to that of the bare GCE, respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of UA and AA has been explored at the modified electrode. DPV peak currents of UA and AA increased linearly with their concentration at the range of 6.0 × 10⁻⁷ to 8.5 × 10⁻⁴ mol L⁻¹ and 8.0 × 10⁻⁶ to 5.5 × 10⁻³ mol L⁻¹, respectively. The proposed method was applied for the detection of UA and AA in human urine with satisfactory result.     

Highly dispersed platinum nanoparticles on carbon nanocoils and their electrocatalytic performance for fuel cell reactions

Highly graphitic carbon nanocoils (GCNC) were synthesized through the catalytic graphitization of carbon microspheres obtained by the hydrothermal carbonization of different saccharides (sucrose, glucose and starch) and were used as a support for Pt nanoparticles. The Pt nanoparticles were deposited by means of a polymer mediated-polyol method. The Pt catalysts were characterized both physically (XRD, TEM, HRTEM and XPS) and electrochemically (electrooxidation of methanol in an acid medium). The electrocatalysts thus prepared show a high dispersion of Pt nanoparticles, with diameters in the 3.0-3.3 nm range and a very narrow particle size distribution. These catalytic systems possess high electroactive Pt surface areas (up to 85 m² g⁻¹ Pt) and they exhibit large catalytic activities towards methanol electrooxidation (up to 201 A g⁻¹ Pt). Moreover, they have a high resistance against oxidation, which is considerably greater than that of the Pt/Vulcan system.     

A novel label-free voltammetric immunosensor for the detection of α-fetoprotein using functional titanium dioxide nanoparticles

A highly sensitive label-free voltammetric immunosensor was developed based on the functional titanium dioxide nanoparticles (PV-NTiP), which was prepared by capping 1,1′-bis-(2-phosphonoethyl)-4,4′-bipyridinium dibromide (PV) on the surface of the titanium dioxide nanoparticles (NTiP) with covalent attachment. The PV-NTiP has prominent biocompatibility, good electron transfer ability, primarily excellent adsorption, large specific surface area and positively charged environment. As a result, the negatively charged gold nanoparticles (NGP) could be adsorbed on the PV-NTiP modified electrode surface by electrostatic adsorption, and then to immobilize α-1-fetoprotein antibody (anti-AFP) for the assay of α-1-fetoprotein (AFP). The fabricated procedures and electrochemical behaviors of the immunosensor were characterized by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and cyclic voltammetry (CV). The anti-AFP/NGP/PV-NTiP modified electrode was sensitive to AFP in linear relation between 1.25 and 200 ng/mL with the correlation coefficient of 0.9982, and the detection limit (S/N = 3) is 0.6 ng/mL under the optimal conditions. In addition, the proposed immunosensor exhibits good sensitivity, selectivity, stability and long-term maintenance of bioactivity and it may be used to immobilize other biomoleculars to develop biosensor for the detection of other antigens or biocompounds.     

Continuous synthesis of surface-modified zinc oxide nanoparticles in supercritical methanol

Continuous synthesis of surface-modified zinc oxide (ZnO) nanoparticles was examined using surface modifiers (oleic acid and decanoic acid) in supercritical methanol at 400 °C, 30 MPa and a residence time of ∼40 s. Wide angle X-ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles retained ZnO crystalline structure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the surface modifiers changed drastically the size and morphology of the ZnO nanoparticles. When the molar ratio of oleic acid to Zn precursor ratio was 30, 10 nm size particles with low degree of aggregation were produced. The surface-modified ZnO nanoparticles had higher BET surface areas (29-36 m²/g) compared to unmodified ZnO particles synthesized in supercritical water (0.7 m²/g). Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) indicated that aliphatic, carboxylate and hydroxyl groups were chemically attached on the surface of ZnO nanoparticles. Long-term (80 days) dispersion test using ultraviolet transmittance showed that the surface-modified ZnO particles had enhanced dispersion stability in ethylene glycol.     

In situ electrochemical generation of gold nanostructured screen-printed carbon electrodes. Application to the detection of lead underpotential deposition

In the present work, an electrochemical method for the reproducible and stable generation of gold nanostructures on the surface of screen-printed carbon electrodes was developed. This technique is based on the application of a constant current over an appropriate time interval. Gold nanostructured screen-printed carbon electrodes were characterized using both SEM and electrochemical methods. The mean diameter and the dispersion of gold nanoparticles that were generated electrochemically depended on the gold concentration, the time deposition and the current intensity. Smaller diameters and better distribution of nanoparticles were obtained when a shift of potential to −0.70 V occurred during the gold electrodeposition process. Moreover, the underpotential deposition (UPD) of lead on these nanostructured surfaces was studied, as was their behavior as array electrodes. The best results, using UPD combined with square wave stripping voltammetry, were obtained for gold nanostructured surfaces with a mean diameter of 78 ± 24 nm and a density of 4.4 × 10⁷ nanoparticles/mm². These gold nanostructured screen-printed carbon electrodes were obtained by applying a current intensity of −100 μA for 300 s using a gold concentration of 0.5 mM. The reproducibility and limit of detection obtained using these nanostructured electrodic surfaces were 2.4% (in terms of RSD) and 0.8 ng/mL, respectively.     

Electrochemical tolazoline sensor based on gold nanoparticles and imprinted poly-o-aminothiophenol film

Amperometric detection of tolazoline (TL) was carried out on a gold nanoparticles (AuNPs)/poly-o-aminothiophenol (PoAT)-modified electrode by a molecular imprinting technique and electropolymerization method. The modification procedure was characterized via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The recognition between the imprinted sensor and target molecule was observed by measuring the variation of amperometric response of the oxidation-reduction probe, K₃Fe(CN)₆ on electrode. Under the optimal experimental conditions, the peak currents were proportional to the concentrations of tolazoline in two ranges of 0.05-5.0 μg mL⁻¹ and 5.0-240 μg mL⁻¹ with the detection limit of 0.016 μg mL⁻¹. Meanwhile the prepared sensor showed sensitive and selective binding sites for tolazoline. The enhancement of sensitivity was attributed to the presence of AuNPs which decreased the electron-transfer impedance.     

A study on direct glucose and fructose alkaline fuel cell

Direct glucose fuel cell (DGFC) has huge potential as a power source in low power long term portable devices. Electro-oxidation of glucose and fructose on PtRu/C catalyst are studied using cyclic voltammetry in alkaline medium to study the reason for deactivation of glucose fuel cell. A simple direct glucose fuel cell with PtRu/C as anode and activated charcoal as cathode was constructed and operated to study the effect of different temperature and concentration of glucose and KOH. An open-circuit voltage (OCV) of 0.91 V is obtained using 0.3 M glucose in 1 M KOH solution. OCV increased with the increase in glucose concentration. The maximum peak power density of 1.38 mW cm⁻² is obtained using 0.2 M glucose in 1 M KOH at 30 °C and it decreases with further increase in glucose concentration and temperature. In order to determine the reason for decrease in performance of glucose fuel cell due to conversion of glucose to fructose, the fuel cell was operated using 0.2 M fructose in 1 M KOH. The peak power density delivered is 0.57 mW cm⁻². The DGFC is continuously operated for 260 h at constant load of 500 Ω produces final constant voltage of 0.21 V.     

Controlled multilayer films of sulfonate-capped gold nanoparticles/thionine used for construction of a reagentless bienzymatic glucose biosensor

A novel reagentless bienzymatic sensor for the determination of glucose in the low working potentials without interference is proposed. The bienzymatic sensor was fabricated by covalently attachment of periodate-oxidized glucose oxidase (IO₄⁻-GOx) and horseradish peroxidase (HRP) on controlled multilayer films of sulfonate-capped gold nanoparticles/thionine (SCGNPs/TH). Using the layer-by-layer method (LBL), SCGNPs and TH were deposited alternately on the gold electrode through the electrostatic and covalent interactions. SCGNPs could greatly enhance the amount of immobilized TH and ensure the good conductivity of the whole structure. UV-vis absorption spectroscopy and electrochemical methods showed that the resulting multilayer films were tridimensional conductive and porous, and TH incorporated in LBL configuration had well electroactive performance. Such superstructures can thus provide an ideal matrix for the construction of bienzymatic sensor, where TH molecules acted as a mediator for electron transfer. After IO₄⁻-GOx and HRP were covalently attached to the multilayer precursor film, the resulting biosensor exhibited good electrocatalytical response toward glucose and that the electrocatalytical response increased with the number of TH layers. This suggested that the analytical performance such as sensitivity and detection limit of the bienzymatic sensors could be tuned to the desired level by adjusting the number of deposited SCGNPs/TH bilayers. Furthermore, because of the low working potentials, the interference from other electro-oxidizable compounds (such as uric acid, ascorbic acid and acetaminophen) was avoided, which improved the selectivity of the biosensors. The biosensor constructed with six bilayers of SCGNPs/TH showed a good performance of glucose detection with a fast response less than 20 s, acceptable sensitivity of 3.8 μA mM⁻¹ cm⁻² and the detection limit of 3.5 × 10⁻⁵ M.     

Microwave heated polyol synthesis of Pt3Te/C catalysts

Pt₃Te/C nanoparticles supported on Vulcan XC-72 carbon were prepared within a few minutes under different reaction conditions by using a microwave-polyol method. Their physical and electrochemical characterization were carried out by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectrometer (XPS), selected-area electron diffraction (SAED), H₂ adsorption-desorption, ethanol oxidation and CO stripping. TEM shows that the Pt₃Te/C (pH 3) catalyst has uniform nanoparticles and is well dispersed with average particle size of about 2.8 nm. Electrochemical results show that the electrochemical activity of Pt₃Te/C catalysts synthesized at different pH values are in the order of pH 3 > pH 7 > pH 9 > pH 13. The Pt₃Te/C catalyst (pH 3) is also better than the Pt₃Te/C catalyst synthesized by formic acid as reductant. From a practical point of view, the microwave-polyol method at the pH value of 3 could be an appropriate method for synthesizing nanocatalysts.     

A new amperometric glucose biosensor based on platinum nanoparticles/polymerized ionic liquid-carbon nanotubes nanocomposites

A new amperometric glucose biosensor has been developed based on platinum (Pt) nanoparticles/polymerized ionic liquid-carbon nanotubes (CNTs) nanocomposites (PtNPs/PIL-CNTs). The CNTs was functionalized with polymerized ionic liquid (PIL) through directly polymerization of the ionic liquid, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM]BF₄), on carbon nanotubes and then used as the support for the highly dispersed Pt nanoparticles. The electrochemical performance of the PtNPs/PIL-CNTs modified glassy carbon (PtNPs/PIL-CNTs/GC) electrode has been investigated by typical electrochemical methods. The PtNPs/PIL-CNTs/GC electrode shows high electrocatalytic activity towards the oxidation of hydrogen peroxide. Taking glucose oxidase (GOD) as the model, the resulting amperometric glucose biosensor shows good analytical characteristics, such as a high sensitivity (28.28 μA mM⁻¹ cm⁻²), wide linear range (up to 12 mM) and low detection limit (10 μM).     

Preparation of high solids stable translucent nanolatexes of MMA/BA copolymers and MMA/BA/Veova-10 terpolymers with low MFFT using green industrial surfactants

In this study, high solids (∼40 wt%) stable translucent nanolatexes of MMA/BA copolymers and MMA/BA/Veova-10 terpolymers with MFFT ≤ 0 °C were synthesized by a modified semi-continuous microemulsion copolymerization. Ammonium persulfate was used as initiator and a redox system for post-polymerization to reduce the free monomers and achieve over 99.9% conversion. Methacrylic acid or acrylic acid at 1 wt% (calculated on monomers) and a combination of various “green” industrial anionic and non-ionic surfactants free of alkylphenol ethoxylates (APEO-free) at low concentrations (up to 10 wt% on monomers) were used. The resulting latexes were composed of polymer nanoparticles with average diameter size 50–80 nm and low polydispersity index. These friendly to the environment latexes can be potentially used as impregnation primer instead of classic solvent acrylic primers in architectural coatings.     

On the preferential crystallographic orientation of Au nanoparticles: Effect of electrodeposition time

The crystallographic orientation of Au nanoparticles electrodeposited at glassy carbon (nano-Au/GC) electrodes (prepared by potential step electrolysis) is markedly influenced by the width of the potential step. The oxygen reduction reaction (ORR) and the reductive desorption of cysteine have been studied on nano-Au/GC electrodes. Furthermore, electron backscatter diffraction (EBSD) technique has been used to probe the crystallographic orientation of the electrodeposited Au nanoparticles. That is, Au nanoparticles prepared in short time (5-60 s) have been found rich in the Au(1 1 1) facet orientation and are characterized by a relatively small particle size (ca. 10-50 nm) as well as high particle density (number of particles per unit area) as revealed by SEM images. Whereas Au nanoparticles prepared by longer electrolysis time (>60 s) are found to be much enriched in the Au(1 0 0) and Au(1 1 0) facets and are characterized by a relatively large particle size (>100 nm). EBSD patterns provided definitive information about the crystal orientations mapping of Au nanoparticles prepared at various deposition times.