A one-pot synthetic approach to prepare palladium nanoparticles embedded hierarchically porous TiO2 hollow spheres for hydrogen peroxide sensing

A simple one-step method to fabricate hierarchically porous TiO₂/Pd composite hollow spheres without any template was developed by using solvothermal treatment. Pd nanoparticles (2-5 nm) were well dispersed in the mesopores of the TiO₂ hollow spheres via in-situ reduction. In our experiment, polyvinylpyrrolidone played an important role in the synthetic process as the reducing agent and the connective material between TiO₂ and Pd nanoparticles. HF species generated from solvothermal reaction leaded to the formation of TiO₂ hollow spheres and Ostwald ripening was another main factor that affected the size and structure of the hollow spheres. The as-prepared TiO₂/Pd composite hollow spheres exhibited high electrocatalytic activity towards the reduction of H₂O₂. The sensitivity was about 226.72 μA mM⁻¹ cm⁻² with a detection limit of 3.81 μM at a signal-to-noise ratio of 3. These results made the hierarchically porous TiO₂/Pd composite a promising platform for fabricating new nonenzymic biosensors.     

Fabrication of Pt/polypyrrole hybrid hollow microspheres and their application in electrochemical biosensing towards hydrogen peroxide

Pt/polypyrrole (PPy) hybrid hollow microspheres were successfully prepared by wet chemical method via Fe₃O₄ template and evaluated as electrocatalysts for the reduction of hydrogen peroxide. The as-synthesized products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), inductive coupled plasma emission spectrum (ICP) and Fourier-transform infrared spectra (FTIR) measurements. The results exhibited that ultra-high-density Pt nanoparticles (NPs) were well deposited on the PPy shell with the mean diameters of around 4.1 nm. Cyclic voltammetry (CV) results demonstrated that Pt/PPy hybrid hollow microspheres, as enzyme-less catalysts, exhibited good electrocatalytic activity towards the reduction of hydrogen peroxide in 0.1 M phosphate buffer solution (pH = 7.0). The composite had a fast response of less than 2 s with linear range of 1.0-8.0 mM and a relatively low detection limit of 1.2 μM (S/N = 3). The sensitivity of the sensor for H₂O₂ was 80.4 mA M⁻¹ cm⁻².     

Enzyme mimics of spinel-type CoxNi1−xFe2O4 magnetic nanomaterial for eletroctrocatalytic oxidation of hydrogen peroxide

A series of spinel-type Co_xNi_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) magnetic nanomaterials were solvothermally synthesized as enzyme mimics for the eletroctrocatalytic oxidation of H₂O₂. X-ray diffraction and scanning electron microscope were employed to characterize the composition, structure and morphology of the material. The electrochemical properties of spinel-type Co_xNi_1−xFe₂O₄ with different (Co/Ni) molar ratio toward H₂O₂ oxidation were investigated, and the results demonstrated that Co_0.5Ni_0.5Fe₂O₄ modified carbon paste electrode (Co_0.5Ni_0.5Fe₂O₄/CPE) possessed the best electrocatalytic activity for H₂O₂ oxidation. Under optimum conditions, the calibration curve for H₂O₂ determination on Co_0.5Ni_0.5Fe₂O₄/CPE was linear in a wide range of 1.0 × 10⁻⁸–1.0 × 10⁻³ M with low detection limit of 3.0 × 10⁻⁹ M (S/N = 3). The proposed Co_0.5Ni_0.5Fe₂O₄/CPE was also applied to the determination of H₂O₂ in commercial toothpastes with satisfactory results, indicating that Co_xNi_1−xFe₂O₄ is a promising hydrogen peroxidase mimics for the detection of H₂O₂.     

A simple and an efficient strategy to synthesize multi-component nanocomposites for biosensor applications

We demonstrate that core–shell multi-component nanocomposites can be grown in situ at room temperature by a novel one-step approach without adding any reductant and stabilizer. We have presented a one-step method for the synthesis of multi-component nanocomposites in water solution, the multi-component nanocomposites could be produced directly and quickly in an in situ wet-chemical reaction. Here, Au–polypyrrole (PPy)/Prussian blue (PB) nanocomposites have been synthesized successfully under the same circumstance. With the addition of pyrrole monomers into mixture solutions, the autopolymerization of pyrrole into PPy and AuCl₄⁻ was reduced to elemental Au instantaneously as well as simultaneously. At the same time, PB produced along with elemental Au serving as a catalyst. Furthermore, we investigated the performance of Au–PPy/PB nanocomposites as amperometric sensor toward the reduction of H₂O₂, which displayed high sensitivity, fast response and good stability. The peak current of H₂O₂ increased linearly with the concentration of H₂O₂ in the range from 2.5 × 10⁻⁹ to 1.2 × 10⁻⁶ M, and the low detection limit of 8.3 × 10⁻¹⁰ M (S/N = 3) was obtained. Therefore, this work provides a new pathway to design and fabricate novel multi-component nanocomposites, which have unique characteristics and hold great applications in the fields of sensors, electrocatalysis and others.     

Photocatalytic cleavage of C-F bond in pentafluorobenzoic acid with titanium dioxide-P25

The photocatalytic C-F bond cleavage in pentafluorobenzoic acid (PFBA) with TiO₂-P25 using UV-C light has been investigated under different conditions. Complete cleavage of C-F is observed with TiO₂-P25 under UV-C light irradiation. Oxidants such as IO₄⁻, BrO₃⁻, S₂O₈²⁻, H₂O₂ and ClO₃⁻ ions enhance the defluoridation of PFBA. The order of their activities is IO₄⁻ > H₂O₂ > S₂O₈²⁻ ≈ BrO₃⁻ > ClO₃⁻. C-F cleavage is also influenced by the addition of inorganic anions and metal ions. The defluoridation intermediates were analyzed by GC-MS technique.     

Fluorometric method for the determination of hydrogen peroxide and glucose with Fe3O4 as catalyst

In this paper, we utilized the instinct peroxidase-like property of Fe₃O₄ magnetic nanoparticles (MNPs) to establish a new fluorometric method for determination of hydrogen peroxide and glucose. In the presence of Fe₃O₄ MNPs as peroxidase mimetic catalyst, H₂O₂ was decomposed into radical that could quench the fluorescence of CdTe QDs more efficiently and rapidly. Then the oxidization of glucose by glucose oxidase was coupled with the fluorescence quenching of CdTe QDs by H₂O₂ producer with Fe₃O₄ MNPs catalyst, which can be used to detect glucose. Under the optimal reaction conditions, a linear correlation was established between fluorescence intensity ratio I₀/I and concentration of H₂O₂ from 1.8 × 10⁻⁷ to 9 × 10⁻⁴ mol/L with a detection limit of 1.8 × 10⁻⁸ mol/L. And a linear correlation was established between fluorescence intensity ratio I₀/I and concentration of glucose from 1.6 × 10⁻⁶ to 1.6 × 10⁻⁴ mol/L with a detection limit of 1.0 × 10⁻⁶ mol/L. The proposed method was applied to the determination of glucose in human serum samples with satisfactory results.     

Macroporous ordered titanium dioxide (TiO2) inverse opal as a new label-free immunosensor

Photonic crystal sensing materials have been validated that they are very sensitive to refractive index changes. Herein, three-dimensionally ordered macroporous (3DOM) (>50 nm) TiO₂ inverse opal film has been fabricated by the self-assembly technique. Based on the TiO₂ inverse opal film, the optical spectrometer was established for label-free immunosensor. The sensing performance of the 3DOM TiO₂ was investigated using human IgG/goat anti-human IgG couple, which showed that the sensitivity of 3DOM TiO₂ inverse opal film could reach to 1 μg mL⁻¹ (equivalent to 1.5 pg mm⁻²) of protein concentration detection limit. The 3DOM TiO₂ inverse opal has a large internal surface area, low fluorescence background and unique optical properties. These characteristics indicated the feasibility of 3DOM TiO₂ inverse opal in label-free immunoassay.     

Sensing hydrogen peroxide involving intramolecular charge transfer pathway: A boronate-functioned styryl dye as a highly selective and sensitive naked-eye sensor

The synthesis, properties and applications of a novel boronate-functioned styryl dye, BSD, as a colorimetric sensor for hydrogen peroxide is presented. The dye displayed remarkable color change from colorless (λ_max = 391 nm) to deep red (λ_max = 522 nm) in the presence of H₂O₂ and the behavior could be rationalized by the chemoselective H₂O₂-mediated transformation of arylboronate to phenolate, resulting in the release of the merocyanine dye which featured with strong intramolecular charge transfer (ICT) absorption band. The absorption increment of merocyanine at λ_max = 522 nm (? = 87000 L mol⁻¹ cm⁻¹) is linear with the concentration of H₂O₂ in the range of 1.0 × 10⁻⁷-2.5 × 10⁻⁵ mol L⁻¹ with the detection limit of 6.8 × 10⁻⁸ mol L⁻¹ under optimum conditions. There is almost no interference by other species that commonly exist due to the specific deprotection of H₂O₂ towards arylboronate group on BSD. The chromogenic sensor has been applied to the detection of trace amounts of hydrogen peroxide in rain water.     

The effects of carbon nanotubes on the electrocatalysis of hydrogen peroxide by metallo-phthalocyanines

The pre-grafted screen-printed gold electrode modified with phenyl-amino monolayer was investigated for covalent immobilization of phenyl-amine functionalized single-walled carbon nanotubes (PA-SWCNT) and metal tetra-amino phthalocyanine (MTAPc) using Schiff-base reactions with benzene-1,4-dicarbaldehyde (BDCA) as cross-linker. The PA-SWCNT and MTAPc modified electrodes were applied as hybrids for electrochemical sensing of H₂O₂. The step-by-step fabrication of the electrode was followed using electrochemistry, impedance spectroscopy, scanning electron microscopy and Raman spectroscopy and all these techniques confirmed the fabrication and the immobilization of PA-SWCNT, MnTAPc and CoTAPc onto gold surfaces. The apparent electron transfer constant (k_app) showed that the carbon nanotubes and metallo-phthalocyanines hybrids possess good electron transfer properties compared to the bare, pre-grafted and the MTAPc modified gold electrode surfaces without PA-SWCNT. The electrochemical sensing of hydrogen peroxide was successful with PA-SWCNT-MTAPc hybrid systems showing higher electrocatalytic currents compared to the other electrodes. The analytical parameters obtained using chronoamperometry gave good linearity at H₂O₂ concentrations ranging from 1.0 to 30.0 μmol L⁻¹. The values for the limit of detection (LoD) were found to be of the orders of 10⁻⁷ M using the 3δ for all the electrodes. The PA-SWCNT-MTAPc modified SPAuEs were much more sensitive compared to PA-MTAPc modified SPAuEs.     

Synthesis, structure and magnetic properties of porous magnetic composite, based on MCM-41 molecular sieve with Fe3O4 nanoparticles

Porous magnetic composites were prepared by the synthesis of molecular sieve MCM-41 in the presence of Fe₃O₄ nanoparticles with average diameter of 15 nm. Nanoparticles were captured by porous silica matrix MCM-41, which resulted in their incorporation, as it was confirmed by TEM, SEM and X-ray diffraction. The materials possessed high surface area (392-666 m² g⁻¹), high pore volume (0.39-0.73 cm³ g⁻¹) along with high magnetic response (M_S up to 28.4 emu g⁻¹ at 300 K). Calcination of samples resulted in partial oxidation of Fe₃O₄ to α-Fe₂O₃. The influence of nanoparticles content on sorption and magnetic properties of the composites was shown. No hysteresis was found for the samples at 300 K; at 5 K, H_C was in the range 370-385 G for non-calcinated samples and 350-356 G for calcinated ones.     

Electrogenerated chemiluminescence from CdS nanotubes and its sensing application in aqueous solution

Electrogenerated chemiluminescence (ECL) of CdS nanotubes in aqueous solution and its sensing application were studied by entrapping the CdS nanotubes in carbon paste electrode. Two ECL peaks were observed at −0.9 V (ECL-1) and −1.2 V (ECL-2), respectively, when the potential was cycled between 0 and −1.6 V. The electrochemically reduced nanocrystal species of CdS nanotubes could collide with the oxidized species in an annihilation process to produce the peak of ECL-1. The electron-transfer reaction between the reduced CdS nanocrystal species and oxidant coreactants such as S₂O₈²⁻, H₂O₂, and reduced dissolved oxygen led to the appearance of the ECL-2 peak. Based on the enhancing effect of H₂O₂ on ECL-2 intensity, a novel CdS ECL sensor was developed for H₂O₂ detection. The sensor exhibited a detection limit of 0.1 μM and a linear range from 0.5 μM to 0.01 mM. The relative standard deviations of five replicate determinations of 5 μM H₂O₂ was 2.6%. In addition, the ECL spectrum in aqueous solution also exhibited two peaks at 500 and 640 nm, respectively.     

Hydrogen production from water decomposition by redox of Fe2O3 modified with single- or double-metal additives

Iron oxide modified with single- or double-metal additives (Cr, Ni, Zr, Ag, Mo, Mo-Cr, Mo-Ni, Mo-Zr and Mo-Ag), which can store and supply pure hydrogen by reduction of iron oxide with hydrogen and subsequent oxidation of reduced iron oxide with steam (Fe₃O₄ (initial Fe₂O₃)+4H₂↔3Fe+4H₂O), were prepared by impregnation. Effects of various metal additives in the samples on hydrogen production were investigated by the above-repeated redox. All the samples with Mo additive exhibited a better redox performance than those without Mo, and the Mo-Zr additive in iron oxide was the best effective one enhancing hydrogen production from water decomposition. For Fe₂O₃-Mo-Zr, the average H₂ production temperature could be significantly decreased to 276 °C, the average H₂ formation rate could be increased to 360.9-461.1 μmol min⁻¹ Fe-g⁻¹ at operating temperature of 300 °C and the average storage capacity was up to 4.73 wt% in four cycles, an amount close to the IEA target.     

Ultrasensitive fluorometric determination of hydrogen peroxide and glucose by using multiferroic BiFeO3 nanoparticles as a catalyst

BiFeO₃ magnetic nanoparticles (BFO MNPs) are used as a catalyst to develop an ultrasensitive method for the determination of H₂O₂. It is found that BFO MNPs can catalyze the decomposition of H₂O₂ to produce OH radicals, which in turn oxidize the weakly fluorescent benzoic acid to a strongly fluorescent hydroxylated product with a maximum emission at 405 nm. This makes it possible to sensitively quantify traces of H₂O₂. Under optimized conditions, the fluorescence intensity is observed to be well linearly correlated with H₂O₂ concentration from 2.0 × 10⁻⁸ to 2.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 4.5 × 10⁻⁹ mol L⁻¹ (S/N = 3). In addition, a selective method for glucose determination is developed by using both glucose oxidase and BFO MNPs, which has a linear range for glucose concentration from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁷ mol L⁻¹. These new methods have been successfully applied for the determination of H₂O₂ in rainwater and glucose in human serum samples.     

Synthesis and photocatalytic applications of Ag/TiO2-nanotubes

TiO₂- and Ag/TiO₂-nanotubes (NTs) were synthesized by hydrothermal methods and microwave-assisted preparation, respectively. Scanning electron microscopy, high resolution transmission electron microscopy, Brunauer-Emmett-Teller particle surface area measurement and X-ray diffraction were used to characterize the nanotubes. Rutile TiO₂-NTs with Na₂Ti₅O₁₁ crystallinity had a length range of 200-400 nm and diameters of 10-20 nm. TiO₂- and Ag/TiO₂-NTs with a 0.4% deposition of Ag had high surface areas of 270 and 169 m² g⁻¹, respectively. The evaluation of photocatalytic activity showed that Ag/TiO₂-NTs displayed higher photocatalytic activity than pure TiO₂-NTs and a 60.91% degradation of Rhodamine-B with 0.8% deposition of Ag species. Also 60% of Rhodamine-6G was physisorbed and 40% chemisorbed on the surface of TiO₂-NTs. In addition, the photocatalytic degradations of organochlorine pesticides taking α-hexachlorobenzene (BHC) and dicofol as typical examples, were compared using Ag/TiO₂-NTs, and found that their degradations rates were all higher than those obtained from commercial TiO₂.     

Application of Fenton reaction for nanomolar determination of hydrogen peroxide in seawater

A simple and sensitive method for the determination of nanomolar levels of hydrogen peroxide (H₂O₂) in seawater has been developed and validated. This method is based on the reduction of H₂O₂ by ferrous iron in acid solution to yield hydroxyl radical (OH) which reacts with benzene to produce phenol. Phenol is separated from the reaction mixture by reversed phase high performance liquid chromatography and its fluorescence intensity signals were measured at excitation and emission of 270 and 298 nm, respectively. Under optimum conditions, the calibration curve exhibited linearity in the range of (0-50) × 10³ nmol L⁻¹ H₂O₂. The relative standard deviations for five replicate measurements of 500 and 50 nmol L⁻¹ H₂O₂ are 1.9 and 2.4%, respectively. The detection limit for H₂O₂, defined as three times the standard deviation of the lowest standard solution (5 nmol L⁻¹ H₂O₂) in seawater is 4 nmol L⁻¹. Interference of nitrite ion (NO₂⁻) on the fluorescence intensity of phenol was also investigated. The result indicated that the addition of 10 μmol L⁻¹ NO₂⁻ to seawater samples showed no significant interference, although, the addition of 50 μmol L⁻¹ NO₂⁻ to the seawater samples decreases the fluorescence intensity signals of phenol by almost 40%. Intercomparison of this method with well-accepted (p-hydroxyphenyl) acetic acid (POHPAA)-FIA method shows excellent agreement. The proposed method has been applied on-board analysis of H₂O₂ in Seto Inland seawater samples.     

Electrogenerated chemiluminescence from thiol-capped CdTe quantum dots and its sensing application in aqueous solution

In this paper, the electrogenerated chemiluminescence (ECL) from thiol-capped CdTe quantum dots (QDs) was reported. The ECL emission was occurred at −1.1 V and reached a maximum value at −2.4 V when the potential was cycled between 0.0 and −2.5 V. The reduced species of CdTe QDs could react with the coreactants to produce the ECL emission. The CdTe QD concentration (6.64 × 10⁻⁷ mol L⁻¹) of ECL is lower than that (1.0 × 10⁻³ mol L⁻¹) of chemiluminescence (CL). Based on the enhancement of light emission from thiol-capped CdTe QDs by H₂O₂ in the negative electrode potential, a novel method for the determination of H₂O₂ was developed. The light intensity was linearly proportional to the concentration of H₂O₂ between 2.0 × 10⁻⁷ and 1.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 6.0 × 10⁻⁸ mol L⁻¹. Compared with most of previous reports, the proposed method has higher sensitivity for the determination of H₂O₂. In addition, the ECL spectrum of thiol-capped CdTe QDs exhibited a peak at around 620 nm, which was substantially red shifted from the photoluminescence (PL) spectrum, suggesting the surface states play an important role in this ECL process.     

Polyethyleneimine-capped silver nanoclusters as a fluorescence probe for sensitive detection of hydrogen peroxide and glucose

In this work, we utilized polyethyleneimine-capped silver nanoclusters (PEI-Ag nanoclusters) to develop a new fluorometric method for the determination of hydrogen peroxide and glucose with high sensitivity. The PEI-Ag nanoclusters have an average size of 2 nm and show a blue emission at 455 nm. The photostable properties of the PEI-Ag nanoclusters were examined. The fluorescence of the PEI-Ag nanoclusters could be particularly quenched by H₂O₂. The oxidization of glucose by glucose oxidase coupled with the fluorescence quenching of PEI-Ag nanoclusters by H₂O₂ can be used to detect glucose. Under optimum conditions, the fluorescence intensity quenched linearly in the range of 500 nM–100 μM with high sensitivity. The detection limit for H₂O₂ was 400 nM. And a linear correlation was established between fluorescence intensity (F₀ − F) and concentration of glucose in the range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ M and 1.0 × 10⁻⁵ to 1.0 × 10⁻³ M with a detection limit of 8.0 × 10⁻⁷ M. The method was used for the detection of glucose in human serum samples with satisfactory results. Furthermore, the mechanism of sensitive fluorescence quenching response of Ag nanoclusters to glucose and H₂O₂ has been discussed.     

A facile one-pot interfacial synthesis of Pt nanoparticles/polypyrrole composites and their application for non-enzymatic sensing hydrogen peroxide

Pt nanoparticles (PtNPs)/polypyrole (PPy) composites were successfully prepared through a facile one-pot interfacial polymerization of pyrrole by using H₂PtCl₆ as the oxidant for the first time. The as-prepared PPy was granular particles with particle size within a few hundred nanometers, on which PtNPs (1.7–3.5) nm were homogeneously dispersed. The PtNPs/PPy composites displayed excellent electrocatalytic activity toward redox of H₂O₂. The non-enzyme sensor constructed with PtNPs/PPy composites displayed good sensing ability toward H₂O₂ at ?0.1 V with a significantly high sensitivity of 6056 μAmM^?1cm^?2 and a low detection limit of 1.8 μM (S/N = 3).     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized in a magnetic nanoparticles-chitosan film

Magnetic nanoparticles (Fe₃O₄) were synthesized by a chemical coprecipitation method. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to confirm the crystallite structure and the particle's radius. The Fe₃O₄ nanoparticles and chitosan (CS) were mixed to form a matrix in which haemoglobin (Hb) can be immobilized for the fabrication of H₂O₂ biosensor. The Fe₃O₄-CS-Hb film exhibited a pair of well-defined and quasi-reversible cyclic voltammetric peaks due to the redox of Hb-heme Fe (III)/Fe (II) in a pH 7.0 phosphate buffer. The formal potential of Hb-heme Fe(III)/Fe(II) couple varied linearly with the increase of pH in the range of 4.0-10.0 with a slope of 46.5 mV pH⁻¹, indicating that electron transfer was accompanied with single proton transportation in the electrochemical reaction. The surface coverage of Hb immobilized on Fe₃O₄-CS film glassy carbon electrode was about 1.13 × 10⁻¹⁰ mol cm⁻². The heterogeneous electron transfer rate constant (k_s) was 1.04 s⁻¹, indicating great facilitation of the electron transfer between Hb and magnetic nanoparticles-chitosan modified electrode. The modified electrode showed excellent electrocatalytic activity toward oxygen and hydrogen peroxide reduction. The apparent Michaelis-Menten constant for H₂O₂ was estimated to be 38.1 μmol L⁻¹.     

Synthesis and thermochemistry of SrB2O4·4H2O and SrB2O4

Two pure strontium borates SrB₂O₄·4H₂O and SrB₂O₄ have been synthesized and characterized by means of chemical analysis and XRD, FT-IR, DTA-TG techniques. The molar enthalpies of solution of SrB₂O₄·4H₂O and SrB₂O₄ in 1 mol dm⁻³ HCl(aq) were measured to be −(9.92 ± 0.20) kJ mol⁻¹ and −(81.27 ± 0.30) kJ mol⁻¹, respectively. The molar enthalpy of solution of Sr(OH)₂·8H₂O in (HCl + H₃BO₃)(aq) were determined to be −(51.69 ± 0.15) kJ mol⁻¹. With the use of the enthalpy of solution of H₃BO₃ in 1 mol dm⁻³ HCl(aq), and the standard molar enthalpies of formation for Sr(OH)₂·8H₂O(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpies of formation of −(3253.1 ± 1.7) kJ mol⁻¹ for SrB₂O₄·4H₂O, and of −(2038.4 ± 1.7) kJ mol⁻¹ for SrB₂O₄ were obtained.