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.     

Electrochemical behavior of catechol, resorcinol and hydroquinone at graphene–chitosan composite film modified glassy carbon electrode and their simultaneous determination in water samples

Graphene–chitosan composite film modified glassy carbon electrode was prepared and characterized. The fabricated electrode showed excellent electrochemical catalytic activities towards the oxidation of catechol (CT), resorcinol (RS) and hydroquinone (HQ). The oxidation overpotentials of CT, RS and HQ decreased significantly and the corresponding oxidation currents increased remarkably compared with those obtained at the bare GCE and chitosan modified GCE. Some kinetic parameters, such as the electron transfer number (n), proton transfer number (m), charge transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s), were calculated. Differential pulse voltammetry was used for the simultaneous determination of CT, RS and HQ in their ternary mixture. The peak-to-peak potential separations between CT and RS, RS and HQ, and HQ and CT were 0.388, 0.484 and 0.096 V, respectively. The calibration curves for CT, RS and HQ were obtained in the range of 1 × 10⁻⁶ to 4 × 10⁻⁴, 1 × 10⁻⁶ to 5.5 × 10⁻⁴ and 1 × 10⁻⁶ to 3 × 10⁻⁴ mol L⁻¹, respectively. The detection limits were 7.5 × 10⁻⁷ mol L⁻¹ (S/N = 3).     

In situ immobilization of cobalt phthalocyanine on the mesoporous carbon ceramic SiO2/C prepared by the sol–gel process. Evaluation as an electrochemical sensor for oxalic acid

The mesoporous carbon ceramics SiO₂/20 wt% C (S_BET = 160 m² g⁻¹) and SiO₂/50 wt% C (S_BET = 170 m² g⁻¹), where C is graphite, were prepared by the sol–gel method. Scanning electron microscopy images and the respective element mapping showed that, within the magnification used, no phase segregation was detectable. The materials containing 20 and 50 wt% of C presented electric conductivities of 9.2 × 10⁻⁵ and 0.49 S cm⁻¹, respectively. These materials were used as matrices to support cobalt phthalocyanine (CoPc), prepared in situ on their surfaces, to assure homogeneous dispersion of the electroactive complex in the pores of both matrices. The surface densities of cobalt phthalocyanine on both matrix surfaces were 0.014 mol cm⁻² and 0.015 mol cm⁻² for materials containing 20 and 50 wt% of C, respectively. Pressed disk electrodes made with SiO₂/50 wt% C/CoPc and SiO₂/20 wt% C/CoPc were tested as sensors for oxalic acid. The electrode was chemically very stable and presented very high sensitivity for this analyte, with a limit of detection, LOD = 5.8 × 10⁻⁷ mol L⁻¹.     

A DNA electrochemical sensor with poly-l-lysine/single-walled carbon nanotubes films and its application for the highly sensitive EIS detection of PAT gene fragment and PCR amplification of NOS gene

A sensitive and novel DNA electrochemical biosensor for the detection of the transgenic plants gene fragment by electrochemical impedance spectroscopy (EIS) was presented. The well-dispersed carboxylic group-functionalized single-walled carbon nanotubes (SWNTs) were dripped onto the carbon paste electrode (CPE) surface firstly, and poly-l-lysine films (pLys) were subsequently electropolymerized by cyclic voltammetry (CV) to prepare pLys/SWNTs/CPE. The morphology of pLys/SWNTs films was examined using a field emission scanning electron microscope (SEM). The pLys/SWNTs films modified electrode exhibited very good conductivity. DNA probes were easily immobilized on the poly-l-lysine films via electrostatic adsorption. The hybridization events were monitored with electrochemical impedance spectroscopy using [Fe(CN)₆]^3−/4− as indicator. The PAT gene fragment from phosphinothricin acetyltransferase gene was detected by this DNA electrochemical sensor. The dynamic detection range of this sensor to the PAT gene fragment was from 1.0 × 10⁻¹² to 1.0 × 10⁻⁷ mol/L. A detection limit of 3.1 × 10⁻¹³ mol/L could be estimated. The PCR amplification of NOS gene from the sample of a kind of transgenic modified bean was also detected satisfactorily by EIS.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

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.     

An improved DNA biosensor built by layer-by-layer covalent attachment of multi-walled carbon nanotubes and gold nanoparticles

In this work, we synthesized a type of compound, MWCNTs-CONH-(CH₂)₂-SH, via carboxylation, and investigated a thickness-tunable multilayer films DNA biosensor built by layer-by-layer (LBL) covalent attachment of gold nanoparticles (GNPs) and multi-walled carbon nanotubes (MWCNTs) on an Au electrode. Fourier transform infrared (FT-IR) spectra were used to identify the products formed in the step of carboxylation; scanning electron microscopy (SEM) and cyclic voltammetry (CV) were used to study the film assembly process. The hybridization events were monitored through measurement the signal of intercalated doxorubicin by differential pulse voltammetry (DPV), and the oxidation peak currents show a good linear relationship with the logarithm of the concentration of target DNA from 5.0 × 10⁻¹⁰ to 1.0 × 10⁻¹¹ M with a detection limit of 6.2 pM. The improved DNA biosensor has a good stability and reproducibility.     

Layer by layer assembly of glucose oxidase and thiourea onto glassy carbon electrode: Fabrication of glucose biosensor

For the first time a novel, simple and facile approach is described to construct highly stable glucose oxidase (GOx) multilayer onto glassy carbon (GC) electrode using thiourea (TU) as a covalent attachment cross-linker. The layer by layer (LBL) attachment process was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy and Fourier transform infrared reflection spectroscopy (FT-IR-RS) techniques. Immobilized GOx shows excellent electrocatalytic activity toward glucose oxidation using ferrocenemethanol as artificial electron transfer mediator and biosensor response was directly correlated to the number of bilayers. The surface coverage of active GOx per bilayer, heterogeneous electron transfer rate constant (k_s) and Michaelis–Menten constant (K_M), of immobilized GOx were 1.50 × 10⁻¹² mol cm⁻², 9.2 ± 0.5 s⁻¹ and 3.42(±0.2) mM, respectively. The biosensor constructed with four-bilayers of TU/GOx showed good stability, high reproducibility, long life-time, fast amperometric response (5 s) with the high sensitivity of 5.73 μA mM⁻¹ cm⁻² and low detection limit of 6 μM at concentration range up to 5.5 mM.     

Binary CuO/Co3O4 nanofibers for ultrafast and amplified electrochemical sensing of fructose

Cobalt oxide-doped copper oxide composite nanofibers (CCNFs) were successfully achieved via electrospinning followed by thermal treatment processes and then exploited as active electrode material for direct enzyme-free fructose detection. The morphology and the structure of as-prepared samples were investigated by X-ray diffraction spectrum (XRD) and scanning electron microscopy (SEM). The electrocatalytic activity of CCNFs films towards fructose oxidation and sensing performances were evaluated by conventional electrochemical techniques. Cyclic voltammetry (CV) and chronoamperometry (I−t) revealed the distinctly enhanced sensing properties towards fructose compared to pure copper oxide nanofibers (CNFs), i.e., showing significantly lowered overpotential of 0.30 V, ultrafast (1 s) and ultrasensitive (18.988 μA mM⁻¹) current response in a wide linear range of 1.0 × 10⁻⁵ M to 6.0 × 10⁻³ M with satisfied reproducibility and stability, which could be ascribed to the synergic catalytic effect of the binary CuO/Co₃O₄ composite nanofibers and the highly porous three-dimensional network films structure of the CCNFs. In addition, a good selectivity for fructose detection was achieved. Results in this work demonstrated that CCNFs is one of the promising catalytic electrode materials for enzymeless fructose sensor fabrication.     

Sensitive DNA impedance biosensor for detection of cancer, chronic lymphocytic leukemia, based on gold nanoparticles/gold modified electrode

A simple and sensitive DNA impedance sensor was prepared for the detection of chronic lymphocytic leukemia. The DNA electrochemical biosensor is worked based on the electrochemical impedance spectroscopic (EIS) detection of the sequence-specific DNA related to chronic lymphocytic leukemia. The ssDNA probe was immobilized on the surface of the gold nanoparticles. Compared to the bare gold electrode, the gold nanoparticles-modified electrode could improve the density of the probe DNA attachment and hence the sensitivity of the DNA sensor greatly. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy were performed in a solution containing 1.0 mmol L⁻¹ K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 50 mmol L⁻¹ phosphate buffer saline pH 6.87 plus 50 mmol L⁻¹ KCl. In the CV studied, the potential was cycled from 0.0 to +0.65 V with a scan rate of 50 mV s⁻¹. Using EIS, the difference of the electron transfer resistance (ΔR_et) was linear with the logarithm of the complementary oligonucleotides sequence concentrations in the range of 7.0 × 10⁻¹²–2.0 × 10⁻⁷ mol L⁻¹, with a detection limit of 1.0 × 10⁻¹² mol L⁻¹. In addition, the DNA sensor showed a good reproducibility and stability during repeated regeneration and hybridization cycles.     

SiO2/C/Cu(II)phthalocyanine as a biomimetic catalyst for dopamine monooxygenase in the development of an amperometric sensor

A mesoporous carbon ceramic SiO₂/50 wt% C (S_BET = 170 m² g⁻¹), where C is graphite, was prepared by the sol–gel method. This material was used as matrix to support copper phthalocyanine (CuPc), prepared in situ on their surface, to assure homogeneous dispersion of the electrocatalyst complex in the pores of the matrix. Pressed disk electrodes made with SiO₂/C/CuPc was tested as amperometric sensors for dopamine. Under optimized conditions, at −20 mV vs SCE in 0.08 mol dm⁻³ Britton–Robinson buffer (BRB) solution (pH = 6.0) containing 100 μmol dm⁻³ of H₂O₂, a linear response range for dopamine from 10 up to 140 μmol dm⁻³ was obtained with a sensitivity of 0.63 (±0.006) nA dm³ μmol⁻¹ cm⁻² and the limit of detection LOD was 0.6 μmol dm⁻³. The sensors presented stable response during successive determinations. The repeatability, evaluated in terms of relative standard deviation of 1.37% for n = 10 and 10 μmol dm⁻³ dopamine. The response time was 1 s and lifetime 9 months. Finally, the sensor was tested to determine dopamine in the sample, and gives very good results for its determination. The presence of other phenols like catechol and resorcinol did not show any interference in the detection of dopamine on this electrode, even in the same concentration with the dopamine.     

Direct electrochemistry of horseradish peroxidase on graphene-modified electrode for electrocatalytic reduction towards H2O2

Graphene was synthesized by a chemical method to reduce graphite oxide and well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectra. Horseradish peroxidase (HRP) immobilized on a graphene film glassy carbon electrode was found to undergo direct electron transfer and exhibited a fast electron transfer rate constant of 4.63 s⁻¹. The HRP-immobilized electrode was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP–Fe(III) and HRP–Fe(II). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The new H₂O₂ sensor shows a linear range of 0.33–14.0 μM (R² = 0.9987) with a calculated detection limit of 0.11 μM (S/N = 3). Furthermore, the biosensor exhibits both good operational storage and storage stability.     

Synthesis, characterization and electroanalytical application of a new SiO2/SnO2 carbon ceramic electrode

A new SiO₂/SnO₂ carbon ceramic composite was prepared by the sol-gel method, and its potential application in electrochemistry as a novel electrode material has been studied. The prepared xerogel was structurally and electrochemically characterized by scanning electron microscopy coupled to energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and cyclic voltammetry. The composite was pressed in a rigid disk-shape and used as a conductive substrate to immobilize a water-soluble organic-inorganic hybrid polymer, 3-n-propyl-4-picolinium chloride silsesquioxane. The oxidation of nitrite was studied on this polymer film coated electrode in aqueous solution using cyclic voltammetry and differential pulse voltammetry. This modified electrode exhibited a better defined voltammetric peak shifted negatively about 60 mV. The linear detection limit found for nitrite was from 1.3 × 10⁻⁵ to 1.3 × 10⁻³ mol l⁻¹ and the detection limit was 3.3 × 10⁻⁶ mol l⁻¹.     

Conversion enhancement of flexible dye-sensitized solar cells based on TiO2 nanotube arrays with TiO2 nanoparticles by electrophoretic deposition

We prepared highly ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F⁻ containing electrolyte. The thickness and dye loading amount of TNAs were 26 μm and 1.06 × 10⁻⁷ mol cm⁻², respectively. TiO₂ nanoparticles (TNPs) were electrophoretically deposited on the inner wall of nanotube to produce coated nanotube arrays (TNAP). The dye loading was increased to 1.56 × 10⁻⁷ mol cm⁻², and the electron transport rate improved. TNAs and TNAP were sensitized with ruthenium dye N3 to yield dye-sensitized TiO₂ nanotube solar cells. The power conversion efficiency of TNA-based dye-sensitized solar cells (DSSCs) was 4.28%, whereas the efficiency of TNAP-based DSSCs increased to 6.28% when illuminated from the counter electrode. The increase of power conversion efficiency of TNAP-based DSSCs is ascribed to the increased surface area of TNAs and the faster electron transport rate.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.     

Electrochemical impedance study of self-assembled layer-by-layer iron–silicotungstate/poly(ethylenimine) modified electrodes

Electrochemical impedance spectroscopy (EIS) has been used to study multilayer films containing anionic iron-substituted silicotungstate [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe) and positively charged poly(ethylenimine) self-assembled by the layer-by-layer method on glassy carbon and indium tin oxide electrodes. The effect of the charge of the outermost layer of the multilayer assembly on the electron transfer of soluble species was studied using the redox probes [Fe(CN)₆]³⁻ and [Ru(NH₃)₆]³⁺; cyclic voltammetry indicating that the surface charge has a significant effect on the process. EIS demonstrated that the electrostatic attraction or repulsion between the surface and the redox probes plays a significant role. Analysis of the impedance spectra showed that the charge transfer resistance increases with an increasing number of bilayers for both redox probes and that the porosity of the multilayer film, which varies with the electrode substrate, also has a significant effect on the electrochemical response.     

Film thickness dependence of the dielectric properties of SrTiO3/BaTiO3 multilayer thin films deposited by double target RF magnetron sputtering

Four-layer SrTiO₃/BaTiO₃ thin films ((ST/BT)₄) with various thicknesses deposited on Pt/Ti/SiO₂/Si substrates at 500 °C by double target RF magnetron sputtering have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), profilometry, capacitance-voltage and current-voltage measurements. The XRD patterns reveal the frame formation of the sputter deposited (ST/BT)₄ with controlled modulation. The adhesion between the Pt bottom electrode layer and the BT layer is excellent. The dielectric constant of the (ST/BT)₄ multilayer thin film increases with increasing film thickness. The effects of temperature, frequency, and bias voltage on the dielectric constant of the (ST/BT)₄ multilayer thin films are discussed in detail. The leakage current density of the (ST/BT)₄ multilayer with a thickness of 450.0 nm is lower than 1.0 × 10⁻⁸ A/cm² for the applied voltage of less than 5 V, showing that the multilayer thin films with such a characteristic could be applied for use in dynamic random access memory (DRAMs) capacitors.     

The effect of co-doping with MgO and Yb2O3 on the structure and electrical conductivity of the Sm2Zr2O7 pyrochlore

SmYb_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.15) ceramics are pressureless-sintered at 1973 K for 10 h in air. The structure and electrical conductivity of SmYb_1−xMg_xZr₂O_7−x/2 ceramics are investigated by the X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb_1−xMg_xZr₂O_7−x/2 ceramics exhibit a defect fluorite-type structure. The measured electrical conductivities of SmYb_1−xMg_xZr₂O_7−x/2 ceramics obey the Arrhenius relation, and electrical conductivity of each composition increases with increasing temperature from 673 to 1173 K. At identical temperature levels, the electrical conductivity of SmYb_1−xMg_xZr₂O_7−x/2 ceramics gradually increases with increasing magnesia content. SmYb_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The electrical conductivity obtained in SmYb_1−xMg_xZr₂O_7−x/2 ceramics reaches the highest value of 2.72 × 10⁻³ S cm⁻¹ at 1173 K for the SmYb_0.85Mg_0.15Zr₂O_6.925 ceramic.     

Enhanced solid-state electrogenerated chemiluminescence of Au/CdS nanocomposite and its sensing to H2O2

Au nanoparticles (AuNPs) are good quenchers once they closely contact with luminophore. Here we reported a simple approach to obtain enhanced electrogenerated chemiluminescence (ECL) behavior based on Au/CdS nanocomposite films by adjusting the amount of AuNPs in the nanocomposite. The maximum enhancement factor of about 4 was obtained at an indium tin oxide (ITO) electrode in the presence of co-reactant H₂O₂. The mechanism of this enhancement was discussed in detail. The strong ECL emission from Au/CdS nanocomposites film was exploited to determine H₂O₂. The resulting ECL biosensors showed a linear response to the concentration of H₂O₂ ranging from 1.0 × 10⁻⁸ to 6.6 × 10⁻⁴ mol L⁻¹ with a detection limit of 5 nmol L⁻¹ (S/N = 3) and good stability and reproducibility.     

Hexacyanoferrate ion adsorbed on propylpyridiniumsilsesquioxane polymer film-coated SiO2/Al2O3: use in an electrochemical oxidation study of cysteine

Hexacyanoferrate ion, [Fe(CN)₆]⁴⁻, was immobilized by an ion-exchange reaction on the propylpyridiniumsilsesquioxane chloride polymer thin-film-coated SiO₂/Al₂O₃ surface. The amount of [Fe(CN)₆]⁴⁻ immobilized was 0.22 mmol g⁻¹ with a surface coverage of 9.6×10⁻⁶ mmol cm⁻². A carbon paste electrode made with this material was prepared and its electrochemical properties studied. The electrode presented two well-defined redox peaks with midpoint potentials, E_m, of 0.152 V vs SCE. This potential was not significantly affected by pH changes between 2 and 9.5. The electrode showed much reproducible responses and was successfully used to study the electrochemical oxidation of cysteine.