Inmaculada Prieto

Aggregate formation in mixed monolayers at the air–water interface of metal-complex tetracationic water-soluble porphyrins attached to a phospholipid matrix

The conformational behaviour of mixtures of the metallated Ni-TMPyP/DMPA and Co-TMPyP/DMPA, with molar ratio 1∶4, has been investigated at the air–water interface. In order to characterize the interactions of the metallated TMPyP derivatives with DMPA, different surface methods have been used. Thus, the surface pressure–surface area, π–A, and surface potential–surface area, ΔV–A, isotherms were measured and valuable information about the presence of the metallated porphyrin molecules underneath the lipid matrix of DMPA, as well as the stability and the hysteresis phenomena of these monolayers was obtained. To reveal the molecular organization of the metallated porphyrins in the mixed monolayers, the reflection spectra of the mixed monolayers at the air–water interface were measured. Additional information on the organization of Ni-TMPyP and Co-TMPyP was obtained by using Brewster angle microscopy (BAM). An analysis of the molecular organization of Ni-TMPyP and Co-TMPyP in a mixed monolayer containing DMPA as lipid anchor, by using the oscillator strength and in accord with the extended dipole approximation, leads us to propose different organization models of these metal-complex porphyrins at the air–water interface depending upon the coordination number of the metal ion.

Investigation of the reduction mechanism of the NAD+ model compound N'-methyl nicotinamide on mercury electrodes in strongly acidic solutions

The electroreduction of the NAD+ model compound N′-methyl nicotinamide has been studied in acidic media (5 M H2SO4 to pH 5) by dc and DP polarography and linear-sweep cyclic voltammetry. Tafel slopes and reaction orders were obtained from I-E curves traced at potentials corresponding to the foot of the polarographic wave. The protonation pK of the heterocyclic nitrogen was obtained from spectroscopic (UV) and potentiometric measurements, having a value of 3.50 ± 0.15. Controlled-potential electrolysis indicated that at pH < 2.5 two electrons are transferred per reactant molecule and that the carbonyl group is reduced to hydroxyl. The reduction pathway consists of the reversible transfer of an electron and an H+ ion followed by an irreversible one-electron transfer, which is the rate-determining step, in agreement with what occurs in the reduction of NAD+ coenzyme at these pH values.

Langmuir-Blodgett films containing water-soluble molecules: the methylene blue-dimyristoyl phosphatidic acid system

Abstract In this work, isotherms for mixtures of methylene blue (MB+) and dimyristoyl phosphatidic acid (DMPA) were studied. Co-spreading techniques were used to spread a mixed solution of MB+ and DMPA in 3:1 CHCl 3 CH 3 OH at the air-water interface. This system exhibits markedly expanded isotherms at low π values. By using the Langmuir-Blodgett (LB) technique, a 1:1 DMPA MB + monolayer was transferred to an indium tin oxide electrode at π = 25 mN m−1. The presence of MB+ in LB films was detected by visible spectroscopy and electrochemical methods.

Numerical determination of extended semi integrals and semi differentials by using spline cubic functions. Applications to an EE reversible mechanism in cyclic voltammetry

In this work, we propose a new general method based on the use of cubic spline functions, suitable for the numerical determination of extended semi integrals and semi differentials. The procedure combines these interpolation functions with different numerical integration and differentiation standard methods, including commercial software. The method has been applied to the electrochemical reduction of the meso-tetra(4-N-methylpyridyl)porphyrin in acidic aqueous solution, where this compound exhibits a two-electron wave in cyclic voltammetry. In this work, we determine the semi differential of the current and demonstrate that the signals obtained correspond to an EE process, where both electrochemical steps are reversible and their formal potentials are very close.

Determination of porphyrin dimer in a mixed monolayer of porphyrin/ phospholipid transferred on ITO electrodes

Abstract The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ , transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ m =5.19×10 −11 mol cm −2 , and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ d =8.65×10 −11 mol cm −2 , and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m −1 . Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, Δ T . This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.

Study of non-faradaic 2D phase transitions by use of cyclic voltammetry and capacitance-potential curves

Abstract Non-faradaic peaks produced by the two-dimensional (2D) condensation of organic molecules on an electrode were studied theoretically by using cyclic voltammetry. Under limiting conditions of low scan rate and overpotential, the classical and atomistic theories of nucleation were found to predict the same behaviour for this type of process. The model developed from the previous study was also used to predict hysteresis in capacitance-potential curves for this type of process. With slight nomenclature changes, the ensuing mathematical expressions are also applicable to 2D phase transitions involving the simultaneous exchange of electrons between deposited molecules and the electrode.

ED mechanisms: electrodimerization of N′-methyl nicotinamide on mercury electrodes in aqueous solutions

Abstract The electrodimerization of the NAD + model compound N ′-methyl nicotinamide has been studied in aqueous media in the pH range 4–11 by direct current and differential pulse polarography and linear-sweep cyclic voltammetry. Tafel slopes and reaction orders were obtained from I - E curves traced at potentials corresponding to the foot of the polarographic wave. Controlled-potential electrolysis indicated that at pH 5 and 8 one electron is transferred per reactant molecule and a dimer must be obtained. The reduction pathways consisted of the reversible transfer of one electron, preceded by the transfer of either two (first wave) or one (second wave) H + ions, followed by an irreversible electrodimerization which was the rate-determining step, in agreement with that which occurs in the reduction of other model compounds and the NAD + coenzyme itself at these pH values. The dimerization reaction is a radical-radical coupling taking place in the reaction layer.

Chiral and achiral 1D copper(II) coordination polymers based on glycolato and chelating aromatic diamine ligands

Achiral glycolic acid and chelating aromatic diamine ligands were employed at room temperature to prepare five 1D copper(II) coordination polymers which were structurally characterized. The polymers ∞1{[Cu(HG)(dipyam)](HG)} (1) (dipyam = 2,2′-dipyridylamine) and ∞1[Cu(HG)(ClO4)(NN)] (NN = 2,2′-bipyridine (2), NN = 1,10-phenanthroline (3)) adopt homochiral helical structures in the crystal state and undergo spontaneous chiral resolution in the solid state. On the other hand, the polymers containing bidepronated glycolato ligand G, ∞1{[Cu(G)(dipyam)]·H2O} (1a), obtained from a solution of 1 in MeOH/iPrOH and ∞1{[Cu2(μ-G)(ClO4)(H2O)(dipyam)2](ClO4)(H2O)2} (4b) are achirals. Crystals of 4b have been obtained together with crystals of the molecular complex [Cu(HG)(H2O)(dipyam)](ClO4) (4a). Analysis by Hirshfeld surface generation and two-dimensional fingerprinting was carried out to study the nature of the interactions and their contributions towards the crystal packing.

Molecular organization and electrochemical reduction of a Ni(II)Porphyrin complex in LB films

Abstract In this work, mixed films of a tetra-cationic porphyrin, Ni(II)TMPyP, and an anionic phospholipid, DMPA, in molar ratio of 1:4, were formed at the air–water interface and transferred onto glass and optically transparent indium tin oxide (ITO) electrodes. Transmission spectroscopy (on glass and ITO) and cyclic voltammetry (on ITO) were used to infer the molecular organization and the electrochemical reduction of these LB films. Likewise, we compare the electrochemical reduction of the Ni(II)TMPyP in water solution with that in LB films. The porphyrin molecules in water solution show three two-electron reduction waves, which are related to the two-electron reduction of the central ring of the porphyrin and to the one-electron reductions of the four methyl–pyridyl groups of the molecule, respectively, while only two reversible one-electron reduction waves are observed in LB films corresponding to the reduction of the central ring of the porphyrin and to the Ni(II) to Ni(I) reduction, respectively.

New Applications and Properties of Langmuir-Blodgett Films

The advance in nanotechnology is mainly determined by the development of novel functionalized materials and the use of new principles of device functioning. The biomimetic approach may provide new insights in the design, development, assembly and functionalization of new intelligent materials and systems at the molecular level. The physical principles which drive the interactions leading to formation of self-assembled and self-organized molecular and nanoscopic systems have been widely investigated during the last years in order to establish a relation between chemical composition, structure and organization of these materials and systems with desired physical properties and functions. In this context, the Langmuir-Blodgett (LB) method is considered one of the most versatile techniques for fabricating organic thin films with well-controlled composition, structure and thickness. Due to these features, the LB technique has been widely used to handle and assemble different functional molecules in a predefined manner with the aim of, e.g., fabricating molecular devices, or investigating physicochemical processes occurring in the monolayer at the gas-substrate interface. The present chapter tries to describe the latest developments in the use of this technique to fabricate functional nanoassemblies. Typical Langmuir films consist of amphiphilic molecules with long alkyl chains that form organized assemblies at the air-water interface (or gas-liquid interface in general). When amphiphilic molecules are spread on a trough containing an aqueous subphase, they form a monolayer where the polar head group is immersed into the water and the hydrocarbon chain oriented toward the air, as depicted in Figure 1. The trough has a moving barrier controlled mechanically over the air-water interface that serves to reduce the surface area. As the monolayer is compressed, it will undergo several phase transitions that can be identified by monitoring the surface pressure as a function of the area occupied by the film (A). A typical isotherm can be observed in Figure 1.