Design of optical and electrochemical sensors based on porphyrin nanostructured systems.

HAMER MARIANA; IRENE N. REZZANO

Chascomus

Otro; IV Latin American Meeting on Biological Inorganic Chemistry - V WOQUIBIO; 2014

INTRODUCTION The porphyrins and metalloporphyrins are important π-conjugated compounds with specific properties that can be easily modulated through the variation in molecular symmetry, metal coordination, dipole moment and extent of conjugation of the π-systems. For this reason, they have been widely used in the design of electrochemical and optical analytical devices with a number of applications [1]. The synthetic dinuclear copper and iron systems have been decisive to the understanding of dioxygen binding and showed excellent results in catalysis attempting to design mimics of enzymes [2]. A number of dinuclear complexes have been investigated to activate small molecules but undoubtedly the synthetic Fe porphyrins as biomimetic analogs of the heme/Cu site of cytochrome c oxidase was the most intensively studied [3]. Our research has been focused on biomimetic catalytic systems based on porphyrins where the combination of two different transition metal complexes resulted in more active catalysts than their mononuclear counterparts.   EXPERIMENTAL METHODS Four different nanocomposites were prepared with the combination of iron (III) and copper (II) metalloporphyrins. Nanostructured electrocatalytic surfaces were obtained by sequential deposition of metalloporphyrins (polyFeCuPP) onto gold nanoparticle previously deposited on flat gold electrodes by SAM ("Self assembled monolayers") method. The morphology, electronic transfer and electrochemical catalysis of hydrogen peroxide of the modified electrodes were evaluated (SEM, CV, amperometry, IES). Then the amorphous porphyrin films were doped with multiple walled carbon nanotubes. The nanostructured material was studied by electrochemical impedance spectroscopy and cyclic voltammetry. Also the electrochemical responses were evaluated in presence of different ligands. Modified glass surfaces with gold nanoparticles and subsequently with metalloporphyrins films were assembled and characterized (UV-Visible, FTIR, Raman and SEM) in order to evaluate the absorption spectroscopy of porphyrin monolayer and spectral changes produced by ligands. Nanorods were prepared with FeIII and CuII metalloporphyrins and characterized by (UV-Visible, FTIR, Raman, VC and TEM). Their potential application as optical sensors was screened to assess the binding of ligands of different nature in bimetallic structures. RESULTS AND DISCUSSION The amperometric response of gold electrodes modified with bimetallic films of metalloporphyrins (polyFeCuPP) to H2O2 is dramatically higher than the corresponding to the bare electrode (up to 60 times) and far exceeding the individual polyCuPP or polyFePP. This response is attributed to the incorporation of H2O2 as ligand of both metal centers. We performed the sequential electrochemical deposition of the metalloporphyrins films onto gold nanoparticles (AuNps) surface previously immobilized on a solid electrode. SEM analysis showed a heterogeneous distribution of material aggregates (0.1 ? 1 mm) for the bulk modified surface whereas the nanostructured gold electrode, covered with the porphyrin films depicted a microporous morphology. The voltamperometric experiments of an external redox couple were consistent with a highly porous film structure enabling a net increase of redox active sites in the nanostructured electrode. The increase in surface area and nanoparticle hopping sites can compensate the hamper charge transfer between outer layers and underlying electrodes by increasing number of deposited polymer layers. The electrocatalytic reduction of H2O2 showed almost four times higher sensitivity for the bimetallic nanostructured electrode. Polymeric mono and bimetallic porphyrins doped with multiwalled carbon nanotubes were studied by EIS and voltammetry to understand the differential response. Polymers deposited by electro-co-deposition present a network of interconnected fibrils with diameters similar to the nanotubes serving as backbones of the porphyrin aggregates by SEM micrograph. The experiments conducted us to think that the presence of oxygen is crucial to obtain a highly conducting polymer with the formation of the intermediate Cu(III) specie. The metallic centers are spontaneous oriented during polymerization resulting in a bimetallic composite which is capable to accommodate hydrogen peroxide as ligand, linking both metals. The azide molecule is also incorporated as ligand facilitating the connection between the two metal centres whereas the monometallic structures are not affected by the explored ligands. The covalent linkage of the porphyrin to the gold surface was very useful to evaluate the absorption spectroscopy of the immobilized system. A significant higher peak at 412 nm was observed, which could be attributed to the coupling of the molecular π system to the gold substrate. SEM images showed a dense and uniform distribution of nanoparticles on the gold substrate. The ATR-FTIR spectra showed typical absorptions bands of porphyrin core at around 1000 cm−1 and 800 cm−1, which were only observed in the Cu(II) complex suggesting that the CuTPyP molecule is tilted with respect to the substrate surface. The presence of FeTPyP produces changes in the geometry of the adsorbed CuTPyP porphyrins, indicating a flat accommodation of the porphyrinic rings in FeTPyP?CuTPyP composite that could be the reason of the differential tendency to include H2O2 as ligand. This method allowed us to observe the effect of a strong ligand, such as nitrite anion, which irreversibly affects the bimetallic complex. The cationic Cu(II) and Fe(III) complexes of tetra(4-pyridyl)porphyrin [TPyP]4+ have been combined with the anionic [H4TPPS]2− to obtain bimetallic J-aggregate nanostructures defined by the tendency to form the FeIIIOCuII bridge along with electrostatic interactions. The Resonant Raman spectra of the bimetallic structures revealed subtle but detectable changes of two significant bands: 661 cm−1 (attributed to the out-of-plane bending of the Cu N bond) and 805 cm−1 (sensitive to the nature of the axial ligand opposite to the oxo ligand), both indicating the important role of the metallic centers in the formation of the CuFe nanorod aggregate. The sensing performance of the aggregates was explored monitoring the absorbance changes in the visible region induced by adding different analytes (H2O2, NO2−, SO32− and N3−). The bimetallic nanostructures maintain the J-aggregation structure after successive addition of H2O2 with an increasing absorption response at 435 cm−1 (monomer band); this architecture was the most sensitive arrangement confirming the synergic effect of the two metals   CONCLUSION Four different nanocomposites were prepared with the combination of iron (III) and copper (II) metalloporphyrins. A synergic effect in every system was observed forward to link H2O2 and other ligands.   REFERENCES 1. Wöhrle D. et. al., The Porphyrin Handbook, (2000),17:110. 2. Collman J.P. et. al., J. Org. Chem. (2004),69:3546?3549. 3. Selverstone V.J. et. al., Biological Inorganic Chemistry, University Science Books (2007).  ACKNOWLEDGMENTS Financial support from University of Buenos Aires (UBACyT), ANPCyT and CONICET are gratefully thanked. Mariana Hamer also thanks CONICET for a doctoral fellowship.