Diana M. Fernandes

Novel electrochemical sensor based on N-doped carbon nanotubes and Fe3O4 nanoparticles: simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid.

A new modified electrode based on N-doped carbon nanotubes functionalized with Fe3O4 nanoparticles (Fe3O4@CNT-N) has been prepared and applied on the simultaneous electrochemical determination of small biomolecules such as dopamine (DA), uric acid (UA) and ascorbic acid (AA) using voltammetric methods. The unique properties of CNT-N and Fe3O4 nanoparticles individually and the synergetic effect between them led to an improved electrocatalytic activity toward the oxidation of AA, DA and UA. The overlapping anodic peaks of these three biomolecules could be resolved from each other due to their lower oxidation potentials and enhanced oxidation currents when using the Fe3O4@CNT-N modified electrode. The linear response ranges for the square wave voltammetric determination of AA, DA and UA were 5-235, 2.5-65 and 2.5-85μmoldm(-3) with detection limit (S/N=3) of 0.24, 0.050 and 0.047μmoldm(-3), respectively. These results show that Fe3O4@CNT-N nanocomposite is a promising candidate of cutting-edge electrode materials for electrocatalytic applications.

Novel Composite Material Polyoxovanadate@MIL-101(Cr): A Highly Efficient Electrocatalyst for Ascorbic Acid Oxidation

A novel hybrid composite material, PMo10V2@MIL-101 was prepared by the encapsulation of the tetra-butylammonium (TBA) salt of the vanadium-substituted phosphomolybdate [PMo10V2O40](5-) (PMo10V2) into the porous metal-organic framework (MOF) MIL-101(Cr). The materials characterization by powder X-ray diffraction, Fourier transform infrared spectra and scanning electron microscopy confirmed the preparation of the composite material without disruption of the MOF porous structure. Pyrolytic graphite electrodes modified with the original components (MIL-101(Cr), PMo10V2), and the composite material PMo10V2@MIL-101 were prepared and their electrochemical responses were studied by cyclic voltammetry. Surface confined redox processes were observed for all the immobilized materials. MIL-101(Cr) showed one-electron reduction process due to chromium centers (Cr(III) → Cr(II)), while PMo10V2 presented five reduction processes: the peak at more positive potentials is attributed to two superimposed 1-electron vanadium reduction processes (V(V) → V(IV)) and the other four peaks to Mo-centred two-electron reduction processes (Mo(VI) → Mo(V)). The electrochemical behavior of the composite material PMo10V2@MIL-101 showed both MIL-101(Cr) and PMo10V2 redox features, although with the splitting of the two vanadium processes and the shift of the Mo- and Cr- centered processes to more negative potentials. Finally, PMo10V2@MIL-101 modified electrode showed outstanding enhanced vanadium-based electrocatalytic properties towards ascorbic acid oxidation, in comparison with the free PMo10V2, as a result of its immobilization into the porous structure of the MOF. Furthermore, PMo10V2@MIL-101 modified electrode showed successful simultaneous detection of ascorbic acid and dopamine.

Multielectrocatalysis by layer-by-layer films based on pararosaniline and vanadium-substituted phosphomolybdate.

Hybrid multilayer films based on the two molecular species pararosaniline (PR) and Keggin-type polyoxometalate K5[PMo11VO40)] (PMo11V) were prepared on different substrates using the electrostatic layer-by-layer (LbL) self-assembly method. The film buildup, monitored by electronic spectroscopy, showed a regular stepwise growth, and X-ray photoelectron spectroscopy data confirmed the presence of both molecular components within the LbL films. Scanning electron microscopy images revealed a completely covered surface with a nonuniform distribution of film components, and atomic force microscopy images confirmed a rough surface. The film electrochemical responses and permeability were studied by cyclic voltammetry. Films revealed three Mo-based redox processes (Mo(VI) → Mo(V)) and one V-based redox process (V(V) → V(IV)) in the potential range between 0.8 and -0.4 V vs Ag/AgCl. Studies with the redox probes [Fe(CN)6](3-/4-) and [Ru(NH3)6](3+/2+) showed that the films maintain the permeability even after six bilayers. Furthermore, the {PR/PMo11V}n multilayer films exhibit excellent Mo-based electrocatalytic activity toward reduction of iodate and V-based electrocatalytic activity toward ascorbic acid oxidation, thus acting as a versatile multielectrocatalyst.

Europium phosphomolybdate and osmium metallopolymer multi-functional LbL films: Redox and electrocatalytic properties

Hybrid multilayer films composed by osmium metallopolymer [Os(bpy)2(PVP)10Cl]Cl (Os-poly) and europium phosphomolybdate, K₁₁[Eu(III)(PMo₁₁O₃₉)₂] (Eu(PMo11)2), were prepared using the electrostatic layer-by-layer (LbL) self-assembly method. The film build-up, monitored by electronic spectroscopy, showed a regular stepwise growth indicating a strong interaction between layers. The XPS measurements corroborated the successful fabrication of the hybrid films with the Os-poly/Eu(PMo11)2 composition. SEM images revealed a completely covered surface with a highly roughened texture. Electrochemical characterisation of films by cyclic voltammetry revealed three Mo-based reduction processes (Mo(VI)→Mo(V)) in the potential range between -0.4 and 0.1 V and one Os reduction process (Os(III)→Os(II)) at ≈0.270 V. The cyclic voltammograms of two electroactive probes, [Fe(CN)₆](3-/4-) and [Ru(NH₃)₆](3+/2+) on {Os-poly/Eu(PMo11)2}n modified electrodes revealed redox mediation between film and the probes. Furthermore, the {Os-poly/Eu(PMo11)2}n multilayer films also showed excellent Mo-based electrocatalytic activity towards reduction of nitrite and iodate, confirming the multi-functional properties of the hybrid europium phosphomolybdate - osmium metallopolymer LbL films.

Reduction of 4-nitrophenol to 4-aminophenol using a novel Pd@NixB–SiO2/RGO nanocomposite: enhanced hydrogen spillover and high catalytic performance

A nanocomposite catalyst containing palladium–nickel boride–silica and reduced graphene oxide (Pd@NixB–SiO2/RGO, abbreviated as Pd@NSG) was successfully fabricated and its enhanced hydrogen spillover mechanism and high catalytic performance towards reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) is discussed. The structure, composition and morphology of the Pd@NSG nanocomposite were characterized by various techniques. The H2 adsorption experiment directly reveals the spillover effect on the Pd@NSG nanocomposite and its enhanced H2 uptake capacity (0.7 wt%) compared to SiO2/RGO (0.05 wt%) under 50 bar pressure at RT. 4-NP reduction reaction shows remarkably high activity (120 s) of Pd@NSG compared to NixB–SiO2/RGO (7200 s) with excellent stability up to 5 cycles. Both the experiments showed the facile H2 dissociation on the Pd (active sites) activator and subsequent transportation of hydrogen atoms on receptor sites.

Synthesis, structural characterization, cytotoxic properties and DNA binding of a dinuclear copper(II) complex.

In this study a novel dinuclear copper(II) complex with adenine and phenanthroline has been synthesized and its structure determined by single crystal X-ray diffraction. In the dinuclear complex [Cu₂(μ-adenine)₂(phen)₂(H2O)2](NO3)4·0.5H2O (phen=1,10-phenanthroline) (1) the two Cu(II) centres exhibit a distorted square pyramidal coordination geometry linked by two nitrogen donors from adenine bridges leading to a Cu-Cu distance of 3.242(3)Å. Intramolecular and intermolecular π⋯π interactions as well as an H-bonding network were observed. The antitumor capacity of the complex has been tested in vitro against human cancer cell lines, cervical carcinoma (HeLa) and colorectal adenocarcinoma (Caco-2), by metabolic tests, using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide as reagent. The complex 1 has remarkable low IC50 values of 0.87±0.06μM (HeLa) and 0.44±0.06μM (Caco-2), when compared with values for cisplatin against the same cell lines. The interaction of complex 1 with calf thymus DNA (CT DNA) was further investigated by absorption and fluorescence spectroscopic methods. A binding constant of 5.09×10(5)M(-1) was obtained from UV-vis absorption studies.

Redox behaviour, electrochromic properties and photoluminescence of potassium lanthano phosphomolybdate sandwich-type compounds

Sandwich-type phosphomolybdates with general formula Kn[LnIII(PMo11O39)2], where LnIII = Sm, Eu, Gd, Tb and Dy, were prepared and characterized by several techniques. The crystal structure of Sm(PMo11)2 and Gd(PMo11)2 were studied and showed that they crystallise in a P21/c space group. All Ln(PMo11)2 revealed four Mo-based electrochemical reduction processes with very similar E1/2 values (≈0.5, ≈0.3, ≈0.02 and ≈−0.1 V) for all the LnIII atoms. The two more positive reduction processes correspond to pH independent one-electron reduction processes, whereas the two more negative processes correspond to pH dependent two-electron reduction processes. Electrolysis at two different potentials, 0.1 V – related to the two one-electron reduction processes – and −0.3 V – related to the two more negative two-electron reduction processes, confirmed the electrochromic properties of the Ln(PMo11)2 species: their original yellow coloured solutions turned blue, corresponding to the appearance of four new electronic bands in the near UV-Vis-near IR region. These electronic bands were tentatively assigned based on their molar absorption coefficients (e) and absorbance (Abs) variation as a function of electrolysis time: bands A (λmax ≈ 855–870 nm) and B (λmax ≈ 670–695 nm) were assigned to MoV → MoVI intervalence charge transfer transitions, band C (λmax ≈ 525 nm) to d–d transition due to d1 configuration of the reduced addenda atom (MoV) and band D (λ ≈ 310–315 nm) to an O → MoV charge transfer transition (CTT). The emission features of the Eu(PMo11)2 and Tb(PMo11)2 samples reveal broad emission in the UV/vis spectral region resulting from d–d transition transitions. The Eu(PMo11)2 also display the typical Eu3+ 5D0 → 7F0–4 transitions, when excited through the O → EuIII and O → MoVI CCTs.

Sucrose-derived activated carbons: electron transfer properties and application as oxygen reduction electrocatalysts

The development of carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) is one of the most attractive topics in fuel cell field. Herein, we report the application of two sustainable sucrose-based activated carbons (ACs), denominated SC800 and SH800, as ORR electrocatalysts. In alkaline medium the ACs showed similar onset potentials at Eonset ≈ −0.20 V vs. Ag/AgCl (0.76 V vs. ERHE), which are 0.06 V more negative than that observed for 20 wt% Pt/C used as a reference. Higher diffusion-limiting current densities (jL(−1.0 V, 1600 rpm) = −3.44 mA cm−2) were obtained for the SH800 electrocatalyst, in contrast to SC800 (jL(−1.0 V, 1600 rpm) = −3.04 mA cm−2). These differences can be related with their different textural properties. The SH800 electrocatalyst revealed a higher specific surface area (ABET ≈ 2500 m2 g−1), larger micropores (widths between 0.7 and 2 nm) and sponge-like morphology. Conversely, SC800 showed a spherical shape, ABET ≈ 1400 m2 g−1 and narrow micropores with pore width <0.7 nm. Both ACs were neither selective to 2- or 4-electron ORR processes, opposing Pt/C which showed selectivity towards direct O2 reduction to water. SH800 and SC800 showed very similar Tafel plots, but with SH800 showing in both low and high current density regions, the lowest slopes values 53/171 mV dec−1 vs. 68/217 mV dec−1. Furthermore, the ACs presented excellent tolerance to methanol, with the SH800 electrocatalyst also showing greater long-term electrochemical stability than the Pt/C electrocatalyst which are very important advantages. The ACs-based electrocatalysts also showed ORR catalytic activity in acidic media, which makes them promising candidates for applications with acidic electrolytes (e.g. proton exchange fuel cells). In this case, Eonset = 0.06 V vs. Ag/AgCl (0.41 V vs. ERHE) for SC800 and Eonset = −0.01 V vs. Ag/AgCl (0.34 V vs. ERHE) for SH800, and the diffusion-limiting current densities are very similar for both ACs (jL = −2.59/−2.76 mA cm−2 at −1.3 V vs. Ag/AgCl, at 1600 rpm). SH800 and SC800 Tafel plots also showed two different slopes, but with higher values in both low and high current density regions, when compared with those obtained in an alkaline medium; still SH800 continues to show the lowest slopes.

Modified electrodes with Keggin-type silicotungstates and poly(brilliant cresyl blue)

Electrosynthesis of poly(brilliant cresyl blue) in aqueous solution in the presence of Keggin-type polyoxotungstates, [SiW11Fe(H2O)O39]5− or [SiW11Co(H2O)O39]6−, was used to prepare modified glassy carbon electrodes. The deposited hybrid organic/inorganic films were studied and characterised by cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammetry showed that the electrochemical features of the polyoxoanions were maintained after immobilisation, with the first tungsten reduction peak involving the uptake of protons from the solution. The chemically modified electrodes were stable, and their preparation was easy to perform. The results provide valuable information for exploring future applications of these films in electrochemical sensors or electrocatalysis.

POM & MOF-based Electrocatalysts for Energy-related Reactions

Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies and the development of highly efficient and cost‐effective materials is one of the current major challenges. This results from the current global energy crisis, reflected in the depletion of fossil fuels and growth of the environmental pollution, which has stimulated the development of novel renewable energy storage and conversion technologies. Currently, several electrocatalysts have been proposed and among them are the polyoxometalates (POMs), the metal‐organic frameworks (MOFs) and their based composites.