Adsorption of lanthanide double-decker phthalocyanines on single-walled carbon nanotubes: structural changes and electronic properties as studied by density functional theory

CONTEXT

Molecular modeling of carbon nanotubes and lanthanide double-decker phthalocyanines hybrids is challenging due to the presence of 4f-electrons. In this paper, we analyzed the trends in structural changes and electronic properties when a lanthanide (La, Gd, and Lu) bisphthalocyanine molecule is adsorbed on the surface of two single-walled carbon nanotubes (SWCNTs) models: armchair and zigzag. The density functional theory (DFT) computations showed that the height of bisphthalocyanines complexes (LnPc₂) when adsorbed on a nanotube (LnPc₂+SWCNT) is the structural feature which is most affected by the nanotube model. The formation energy of the LnPc₂+SWCNT hybrid depends on the metal atom and the nanotube chirality. LaPc₂ and LuPc₂ bind stronger to the zigzag nanotube, while for GdPc₂, bonding to the armchair nanotube is the stronger one. The HOMO-LUMO gap energy (Egap) shows a correlation between the nature of lanthanide and the nanotube chirality. In the case of adsorption on armchair nanotube, E_gap tends to match the gap of isolated LnPc₂, whereas for adsorption on the zigzag nanotube, it is closer to the value for the isolated nanotube model. The spin density is localized on the phthalocyanines ligands (plus on Gd in the case of GdPc₂), when the bisphthalocyanine is adsorbed on the surface of the armchair nanotube. For bonding to zigzag nanotube (ZNT), it extends over both components, except for LaPc₂+ZNT, where spin density is found on the nanotube only.

METHOD

All DFT calculations were carried out using the DMol³ module of Material Studio 8.0 software package from Accelrys Inc. The computational technique chosen was the general gradient approximation functional PBE in combination with a long-range dispersion correction developed by Grimme (PBE-D2), the double numerical basis set DN, and the DFT semi-core pseudopotentials.

Highly soluble tetrasubstituted lanthanide bis-phthalocyanines; synthesis, characterization, electrical properties and aggregation studies

We report the synthesis, characterization and electrical properties of lanthanide metal based bis-phthalocyanines, {M[Pc-(β-HHT)4]2} (HHT:1-hydroxyhexane-3-ylthio){ M [Formula: see text] Lu[Formula: see text] (2), Eu[Formula: see text] (3), and Yb[Formula: see text] (4)}. The interaction of bis-phthalocyanines with Ag[Formula: see text] and Pd[Formula: see text] metal ions were investigated by UV-vis spectroscopy and atomic force microscopy. Thin films of bis-phthalocyanine molecules were prepared by spin-coating method. The surface morphology of thin films were performed with Atomic Force Microscopy as further investigate. The electrical transport properties of ITO/Pc/Al devices were investigated. At low voltages all the films showed an ohmic conduction, whereas at higher voltage levels the conduction is dominated by space charged limited conduction with exponential distribution of trapping levels. Measurements of current density as a function of inverse temperature at constant applied voltage yielded a hole mobility values, μp[Formula: see text], [Formula: see text] and [Formula: see text] m2.sn[Formula: see text].V[Formula: see text] for lanthanium phthalocyanines. From the analysis of frequency, ω, dependence of electrical conductivity, σac. it was found that the [Formula: see text] obeys the power law given by σac = Aωs, in which the frequency exponent “s” decreases with temperature.

Multisensor systems based on phthalocyanines for monitoring the quality of grapes

Arrays of phthalocyanine-based sensors with complementary activity have been used to develop voltammetric electronic tongues. Such systems have demonstrated to be useful in enology for the evaluation of quality of wines in different production stages, from grapes to bottles. In this paper, the state of the art of multisensor systems based on phthalocyanines dedicated to the analysis of musts (juices obtained from crushed grapes) is described. Such multisensor systems cover different types of sensors from simple Carbon Paste Electrodes, to sophiticated nanostructured sensors, including Langmuir–Blodgett or Layer by Layer thin films and biomimetic biosensors where phthalocyanines play a crucial role as electron mediator between enzymes and electrodes. In all cases, multisensor systems based on phthalocyanines have been able to discriminate musts prepared from different varieties of grapes. The performance of these systems can be improved by combining non-specific sensors with biosensors containing enzymes selective to phenols. In this case, excellent relationships have been found between the responses provided by the array and the content in phenols and acids provided by traditional chemical analysis.

Improvement of electrocatalytic effect in voltammetric sensors based on phthalocyanines

Voltammetric sensors based on phthalocyanines have been used to detect a variety of compounds. In this paper, the state of the art of sensors prepared using classical techniques will be revised. Then, new strategies to improve the performance of the sensors will be described using as example sensors chemically modified with lutetium bisphthalocyanine (LuPc[Formula: see text] dedicated to the detection of phenols of interest in the food industry. Classical LuPc2 carbon paste electrodes can detect phenols such as catechol, caffeic acid or pyrogallol with limits of detection in the range of 10[Formula: see text]–10[Formula: see text] M. The performance can be improved by using nanostructured Langmuir–Blodgett (LB) or Layer by Layer (LbL) films. The enhanced surface to volume ratio produce an increase in the sensitivity of the sensors. Limits of detection of 10[Formula: see text]–10[Formula: see text] M are attained, which are one order of magnitude lower than those obtained using conventional carbon paste electrodes. Moreover, these techniques can be used to co-immobilize two electrocatalytic materials in the same device. The limits of detection obtained in LB sensors combining LuPc2/AuNPs or LuPc2/CNT are further improved. Finally, the LB technique has been used to prepare biosensors where a phenol oxydase (such as tyrosinase or lacasse) is immobilized in a biomimetic environment that preserves the enzymatic activity. Moreover, LuPc2 can be co-immobilized with the enzyme in a lipidic film formed by arachidic acid (AA). LuPc2 can act as an electron mediator facilitating the electron transfer. These biomimetic sensors formed by LuPc2/AA/enzyme show Limits of detection of 10[Formula: see text] M and an enhanced selectivity.

Electrochemical characterization of dilithium phthalocyanine carbonaceous electrodes

Carbonaceous electrodes of dilithium phthalocyanine were prepared using graphite, carbon microspheres and multiwall carbon nanotubes. The electrochemical behavior of the dilithium bisphthalocyanine electrodes was found to be dependent on the nature of the carbonaceous material and on the nature of the electrolytic solution. The electrocatalytic properties of the dilithium phthalocyanine electrodes for oxidation of ascorbic acid were evidenced by the enhancement of the oxidation peak current, (~10 fold compared to the bare carbon electrodes) and the decrease of the oxidation potential at which oxidation of ascorbic acid takes place. The combined use of multiwall carbon nanotubes and dilithium phthalocyanine produces a synergistic effect that improves the electrocatalytic effect towards ascorbic acid.