An enzyme-free sensor based on La-doped CoFe-layered double hydroxide decorated on reduced graphene oxide for sensitive electrochemical detection of urea.

The successful synthesis of La-doped CoFe LDH@rGO nanocomposite is reported combining the advantages of LDH and rGO and shows promising performances in electrochemical sensors. The structure of the obtained nanocomposite was investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction pattern (XRD), and field emission scanning electron microscope images (FE-SEM). Then, it was directly utilized to construct a carbon paste electrode (CPE) for urea detection. The electrochemical performance of the sensor was evaluated by various electrochemical methods. The La-CoFe LDH@rGO electrode exhibited excellent electrocatalytic properties, including a wide linear working range of 0.001-23.5 mM, very high sensitivity of 1.07 ± 0.023 µA µM-1 cm-2, a low detection limit of 0.33 ± 0.11 µM, and rapid response time of 5 s towards urea detection at the working potential of 0.4 V. Furthermore, the sensor displayed a high selectivity in different matrices, appropriate reproducibility, and long shelf life without activity loss during 3 months of storage under ambient conditions. Further tests were performed on serum and milk samples to confirm the capability of the proposed sensor for practical applications, demonstrating a reasonable recovery of 94.8 to 102% with an RSD value below 3%. Consequently, the synergistic effect of each component led to the good electrocatalytic activity of the modified electrode towards urea.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound.

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn4Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N3)Cl]2 (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN4OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377 mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media.

Water splitting is a promising way to alleviate the energy crisis. In nature, water oxidation is done by a tetranuclear manganese cluster in photosystem II. Therefore, the study of water oxidation by Mn complexes is attractive in water splitting systems. In this report, a new mononuclear Mn(II) complex, MnL2 (HL = (E)-3-hydroxy-N'-(pyridin-2-ylmethylene)-2-naphthohydrazide) was prepared and characterized by spectroscopic techniques and single-crystal X-ray diffraction. Crystallographic analysis indicated that the geometry around the Mn(II) ion is distorted octahedral. The MnN4O2 coordination moiety is achieved by bounding of oxygen and two nitrogen donor atoms of two hydrazone ligands. The synthesized complex was also investigated for electrochemical water oxidation using electrochemical techniques, scanning electron microscopy, energy dispersive spectrometry, and PXRD analysis. Linear sweep voltammetry experiment showed that the modified carbon paste electrode by the complex displays high activity for water oxidation reaction with an overpotential of 565 mV at a current density of 10 mA cm-2 and Tafel slope of 105 mV dec-1 in an alkaline solution. It was found that the complex structure finally changes during the reaction and converts to Mn oxide nanoparticles which act as active catalytic species and oxidize the water.

CeO2/CuO/NiO hybrid nanostructures loaded on N-doped reduced graphene oxide nanosheets as an efficient electrocatalyst for water oxidation and non-enzymatic glucose detection.

In this work, the three-component heterostructure of CeO2/CuO/NiO was synthesized by a co-precipitation procedure and heating at a temperature of 750 °C. Then, CeO2/CuO/NiO nanoparticles were successfully supported on N-doped reduced graphene oxide (N-rGO) by a hydrothermal method. The obtained nanomaterials were used as effective electrocatalysts for the oxygen evolution reaction and glucose sensing in an alkaline medium. The results indicated that when CeO2/CuO/NiO is anchored on N-rGO nanosheets, active catalytic sites increase. On the other hand, N-doped rGO enhances electrical conductivity and electron transfer for water or glucose oxidation. CeO2/CuO/NiO@N-rGO has a large electrochemically active surface area and more active catalytic positions, and thus exhibits high activity for the OER with a low overpotential of 290 mV, a suitable Tafel slope of 110 mV dec-1, and superior stability and durability for at least 10 hours. CeO2/CuO/NiO@N-rGO can also detect glucose with a high sensitivity of 912.7 µA mM-1 cm-2, a low detection limit of 0.053 µM, a wide linear range between 0.001 and 24 mM, and a short response time of about 2.9 s. Moreover, the high selectivity and stability of this electrode for glucose sensing show its potential for clinical applications.

A new hexanuclear Fe(III) nanocluster: synthesis, structure, magnetic properties, and efficient activity as a precatalyst in water oxidation.

The oxo-bridged hexanuclear iron cluster formulated [FeIII6(µ4-O)2(edteH)2(piv)4(SCN)4]·2MeCN·2H2O (1) (where edteH = N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine; piv = pivalic acid) has been synthesized by the reaction of FeCl2·4H2O with edteH4 and piv in the presence of KSCN in CH2Cl2/MeCN. The single crystal X-ray measurements indicated that the cluster is centrosymmetric in structure. The magnetic study demonstrated the presence of very strong antiferromagnetic coupling between the iron centers and the Brillouin fitting showed the best fit with S = 5/2 and g = 1.87. In addition, the water oxidation activity of the cluster has been studied by electrochemical techniques. Electrochemical experiments revealed that the electrode modified by 1 has high efficiency for the oxidation of water and needs an overpotential of 484 mV under a constant current density of 15 mA cm-2 with a Tafel slope of 114 mV dec-1 in neutral media. Experiments indicated that in the presence of 1, a yellow solid film was formed on the electrode surface under the applied electrochemical conditions. This yellow material is likely a compound of iron and oxygen and has a crystalline nature. Our findings revealed that along with the cluster, this compound is active in water oxidation reactions.

Electrocatalytic water oxidation by a Ni(ii) salophen-type complex.

A new mononuclear Ni(ii) complex, NiL (1), was synthesized from the reaction of Ni(OAc)2·4H2O and salophen-type N2O2-donor ligand, H2L (where H2L = 2,2'-((1E,1'E)-((4-chloro-5-methyl-1,2-phenylene)bis(azanylylidene))bis(methanylylidene))diphenol), in ethanol. The obtained complex was characterized by elemental analysis, spectroscopic techniques and single crystal X-ray analysis. The complex was studied as a water oxidizing catalyst and its electrocatalytic activity in the water oxidation reaction was tested in 0.5 M of borate buffer at pH = 3, 7 and 11 in a typical three-electrode setup with a carbon paste electrode modified by complex 1 as a working electrode. The linear sweep voltammetry (LSV) curves indicated that complex 1 has a much superior activity and only needs 21 mV vs. Ag/AgCl overvoltage to reach a geometrical catalytic current density of 2.0 mA cm-2 at pH = 11. The onset potential decreased from 1.15 V to 0.67 V vs. Ag/AgCl with an increase of pH from 3 to 13 under a constant current density of 1.0 mA cm-2. Then, to determine the true catalyst for the water oxidation reaction in the presence of complex 1 at pH = 3, 7 and 11, cyclic voltammetry was also performed. The continuous CVs for complex 1 at neutral and alkaline solutions showed significant progress for the water oxidation reaction. In addition, the amperometry tests exhibited excellent stability and high constant current density for water oxidation by CPE-complex 1 under electrochemical conditions at pH = 11 and 7. Although X-ray powder diffraction analysis did not show a pure and crystalline structure for NiO x , the scanning electron microscopy images showed that nickel oxide at pH = 11 and nickel oxide or other Ni-based compounds at pH = 7 are true water oxidizing catalysts on the surface of a CPE electrode. Moreover at pH = 3, no clear water oxidation or NiO x formation was observed.

Optical Response of Two Azo Ligands Containing Salicyaldimine-based Ligand as Side Chains Towards Some Divalent Metal Ions and Their Antioxidant Behavior.

According to applicability of azo-azomethine compounds in chemical sensors and biological activities, two receptors: 1,2-[1-(3-imino-4-hydroxophenylazobenzene)]-4-nitrobenzene (1) and 1,2-[1-(3-imino-4-hydroxophenylazo-4-nitrobenzene)]-4-nitrobenzene (2) are investigated for detection of nickel, cobalt, copper, lead, mercury, zinc and cadmium divalent metal ions by UV-vis spectroscopy. With the addition of all metal ions to the DMSO solution of ligands, the peaks at 558 and 549 nm increase in intensity with hypsochromic or bathochromic shifts except Zn2+ ions and 2, while the peaks at 388 and 391 nm dramatically decrease in intensity. In both cases, the largest shift is observed after addition of copper ions. In solution, both receptors produce a cation blue shift from 558 and 549 nm to 503 and 497 nm with the sensible color change of solutions from purple-red to orange. Therefore, both compounds can highly recognize copper ions in DMSO solution. In the next step, Benesi-Hildebrand plot and Job's method are used for determination of binding constant (Ka) and stoichiometry of formed complexes, respectively. Also, the investigation of solvent effect in the UV-vis spectra of ligands shows that the generation of hydrazine and enaminone tautomers increases in highly polar solvents such as DMF and DMSO. Finally, the antioxidant activity of ligands is studied by DPPH method. The results show that NO2 withdrawing groups in 1,2-[1-(3-imino-4-hydroxophenylazo-4-nitrobenzene)]-4-nitrobenzene probably affect keto∆enol equilibrium. As a result, this ligand reduces free radicals to non-reactive species by donating hydrogen.

Spectroscopic properties of some new azo-azomethine ligands in the presence of Cu(2+), Pb(2+), Hg(2+), Co(2+), Ni(2+), Cd(2+) and Zn(2+) and their antioxidant activity.

Due to their potential applicability as selective receptors in optical sensors, two novel azo Schiff-base derivatives I and II are synthesized and characterized with FT-IR, (1)H NMR and elemental analysis techniques. The optical response of azo groups of I and II towards Ni(2+), Co(2+), Cu(2+), Pb(2+), Hg(2+), Zn(2+) and Cd(2+) metal ions is studied in DMSO by UV-vis spectroscopy. The absorption spectra of both compounds with cations show marked changes. In solution, azo Schiff-base I produces a cation induced 95nm blue shift for Cu(2+) ion from 555nm to 460nm with remarkable color change from red to yellow. Whereas no significant color change is observed upon addition of studied metal cations to the solution of ligand II or other metal ions to the solution of ligand I. Furthermore, Job's plot indicate 1:1 binding-stoichiometry for I with Cu(2+) ion and Benesi-Hildebrand plot is used for the determination of its association constant. Therefore receptor I is highly specific for copper ions in DMSO solution. Finally, the study of antioxidant properties of I and II with DPPH method reveals high and significant activities.

Optical spectroscopy studies of the complexation of bis(azophenol)calix[4]arene possessing chromogenic donors with Ni2+, Co2+, Cu2+, Pb2+ and Hg2+.

Due to their potential applicability as selective receptors in electrochemical or optical sensors, a bis(azophenol)calix[4]arene derivative H(2)L has been investigated. The complexation properties of this molecule towards Ni(2+) and Co(2+) metal ions has been studied. It is revealed that this ligand exhibits tetradentate with N(2)O(2) core when bound to Ni (II) or Co (II) metal ion. The optical response of azo groups of H(2)L towards Ni(2+), Co(2+), Cu(2+), Pb(2+) and Hg(2+) metal ions has been investigated in DMSO by UV-vis spectroscopy. The absorption spectra of calix[4]arene with cations show marked changes, especially for Co(2+) ion. Furthermore, Job's plot indicate 1:1 binding-stiochiometry for calix[4]arene with Co(2+) ion and Benson-Hilderbrand plot is used for the determination of its association constant. The investigation of UV-vis spectra of chromogenic calix[4]arene in different solvents shows that cis-trans isomerization of azo groups probably depends on kind of solvent. Also the different between the polarity and viscosity of organic solvents used is likely responsible for the changes of the band shape of the spectra.