Celso P. de Melo

Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Effect of electron correlation on the static longitudinal hyperpolarizability of polyacetylene

Abstract We continue our development of the finite oligomer method for computing nonlinear optical properties of conjugated polymers. Using polyacetylene as an example it is shown how the effect of electron correlation on the static longitudinal. hyperpolarizability can be accurately determined from calculations on small oligomers.

Hartree–Fock static longitudinal (hyper)polarizability of polyyne

The evolution in the static longitudinal polarizability and second hyperpolarizability of the C4H2 through C54H2 oligomers of polyyne is studied at the Hartree–Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit.

Hierarchical Composite Polyaniline–(Electrospun Polystyrene) Fibers Applied to Heavy Metal Remediation

We describe the in situ preparation of a multipurpose hierarchical polyaniline-polystyrene (PANI-PS) composite based in the chemical polymerization of PANI on nonwoven (NW) electrospun PS mats. We performed a detailed study of the properties of these materials to select the best strategies to incorporate PANI chains into pristine NW PS mats without compromising the original porosity and mechanical flexibility of the matrices. The resulting composites presented nanostructured PANI chains highly dispersed in the interior of the NW PS mat and showed good electrical properties and surface-wetting characteristics that could be easily controlled. In particular, we show that these NW PANI-PS mats exhibit interesting properties in their interaction with heavy metal ions. For instance, their high adsorption capacities toward dispersed Hg(II), Cd(II), Pb(II), Cr(VI), and Cu(II) ions make them promising materials for water remediation, by providing a simple manner of collecting and removing these metals from aqueous systems. In fact, the NW electrospun mats here presented do not suffer from the usual limitations found in materials commonly employed as adsorbents, such as a tendency to agglomerate or accumulate in the environment because of difficulties of properly recovering them after use. To better understand the nature of each pairwise metal-PANI interaction, we performed a thorough investigation of the optical and electrical changes induced by the metal adsorption in the NW PANI-PS mats. As a consequence of their interaction with the metal ions, the visual aspect of the mats change, a fact more evident in the case of Cr(VI) removal, when the matrices vary their color from green to purple. These changes are related to the variation of the oxidation state of the PANI chains: as the ion metals are progressively adsorbed into the mat, they promote the conversion in varying degrees of the PANI chains from salt emeraldine to the pernigraniline form, and the mats become more resistive. We implemented an electrical impedance investigation of the charge transport characteristics of NW PANI-PS mat, and the results indicate that they are sensitive to the type of metal ion adsorbed and that the amount of ions adsorbed in each case is mostly related to the standard electrode potential of the metal considered.

A COMPARATIVE STUDY OF THE EFFECT OF ELECTRON CORRELATION IN THE HYPERPOLARIZABILITY OF POLYYNE, POLYACETYLENE AND POLYPYRROLE OLIGOMERS

Abstract The effect of electron correlation on the static longitudinal second-order hyperpolarizability of polyyne, polyacetylene and polypyrrole oligomers is compared using second-order Moller-Plesset perturbation theory (MP2). In general, the MP2 hyperpolarizability is found to be significantly greater than the Hartree-Fock (HF) estimate. The bulk of the difference is traced to a correlation-induced reduction in the bond length alternation of the polymer backbone. A more efficient electron correlation scaling procedure for hyperpolarizabilities of π-conjugated polymers based on HF values found using the MP2 infinite polymer geometry is proposed.

An impedimetric biosensor for detection of dengue serotype at picomolar concentration based on gold nanoparticles-polyaniline hybrid composites.

In this work, we describe the preparation and characterization of a novel gold nanoparticles-polyaniline hybrid composite (AuNpPANI) with SH-terminal groups that, due to its ability of immobilizing dengue serotype-specific primers 1, 2 and 3 (ST1, ST2 and ST3), can be used for the development of biosensors. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed. CV and EIS results demonstrated that the AuNpPANI can immobilize ST1, ST2 and ST3, forming AuNpPANI-ST complexes. Well-defined cyclic voltammograms characteristic of a diffusion-limited redox process were observed both for the bare gold electrode and after these electrodes have been modified by the adsorption of AuNpPANI or AuNpPANI-ST. The AuNpPANI-ST(1-3) systems were able to recognize the dengue serotype of different patients at picomolar concentrations. Even when small volumes and low concentrations of the analyte were used, the CV and EIS results showed unequivocal evidence of an existing interaction between dengue serotype-specific primers and their complementary genomic DNA targets.

Abinitio polarizability study of polypyrrole

The longitudinal polarizability, αzz, and second order hyperpolarizability, γzzzz, of the (C4H3N)RH2 oligopyrroles, where R=1−9, are determined at the Hartree–Fock (HF) level of theory using a 6‐31G basis set. The effect of electron correlation is taken into account using Moller–Plesset perturbation theory through second order for the R=1−5 oligopyrroles. Methods for extrapolation to the infinite polymer values are explored. The convergence behavior of the correlated results as a ratio with the corresponding HF values may indicate that a scaled HF procedure could be appropriate for finding correlated estimates to the infinite polymer property values. We also propose a generalized finite field recipe which can easily be used in conjunction with popular black‐box electronic structure programs for determining molecular polarizabilities through second order. The αzz and γzzzz values obtained from using this procedure on a variety of polyacetylene and polypyrrole oligomers are in excellent agreement with those...

Ab initio polarizabilities of polyenic chains with conformational defects

Abstract We present an ab initio Hartree-Fock study of the electronic polarizabilities of the C 2 n +1 H 2 n +3 + (2 n 2 n H 2 n +2 ++ (2 n β xxy of the soliton chains was obtained analytically. We confirm that the polarizabilities of these polyenic oligomers are quite sensitive to the nature of the conformational defect present.

Impedimetric sensor of bacterial toxins based on mixed (Concanavalin A)/polyaniline films

In this paper, we report the use of Concanavalin A (ConA) and electrosynthesized polyaniline (PANI) thin films for the development of a new electrochemical sensor that allows the specific detection of two bacterial toxins: lipopolysaccharide (LPS) from Escherichia coli and lipoteichoic acid from Staphylococcus aureus. The impedimetric sensor is fabricated by using glutaraldehyde to self-assemble ConA lectin on PANI-modified steel electrodes through covalent binding. ConA acts as a recognition element for bacterial toxins. Electrical impedance spectroscopy (EIS) and scanning electron microscope (SEM) were applied to characterize the assembly process on the modified electrode. The EIS measurements revealed that the resistance charge transfer (RCT) of the electrode/electrolyte interface increases considerably after the ConA lectin interacts with specific carbohydrate moieties present in the molecule of the bacterial toxin. Our results showed that the ConA lectin retained its activity after immobilization on the PANI surface and also the existence of electrochemical impedance response of the bioelectrode which is linear to the extent of the lectin-toxin interaction, with maximum values of RCT for E. coli (14.40 kΩ), and S. aureus (17.80 kΩ). We have observed that electrosynthesized PANI is an excellent support layer for the covalent binding of lectins on the electrode surface. Thus, the recognition system provides an appropriate biomimetic interface for detection of specific constituents of gram-positive and gram-negative bacteria.

Time‐resolved picosecond optical nonlinearity and all‐optical Kerr gate in poly (3‐hexadecylthiophene)

A time‐resolved anisotropic configuration exploiting the Z‐scan technique in a one color pump‐and‐probe arrangement was implemented to measure the time response and the value of the nondiagonal component of the third‐order nonlinear susceptibility of a chloroform solution of poly (3‐hexadecylthiophene). In addition, an all‐optical Kerr gate with picosecond response using the polymeric solution was implemented.

Use of magnetic polyaniline/maghemite nanocomposite for DNA retrieval from aqueous solutions.

We demonstrated that the magnetic polyaniline/maghemite nanocomposite (Pani/γ-Fe2O3 MNC) is an efficient agent for retrieval of pure double stranded deoxyribonucleic acid (dsDNA) chains from aqueous solutions. The dsDNA chains used in the retrieval experiments were of sodium salt of Salmon Sperm DNA. Based on λ=260 nm absorption measurements, we have employed UV-Vis spectroscopy to estimate the concentration of DNA present in solutions, before and after the interaction with the MNC. The best results corresponded to a maximum amount of 75.2 mg of DNA absorbed per gram of MNC reached within only 10 min of joint exposure into the aqueous solution. After magnetic separation of the fully DNA-loaded Pani/γ-Fe2O3 MNC, we achieved essentially complete DNA desorption by appropriate changes in the pH of the solution. We have shown that it is possible to recycle the use of these MNC in several adsorption-desorption cycles. By comparing the present results to those of other DNA retrieval systems reported in the literature, we argued that the Pani/γ-Fe2O3 MNC here described represent a promising low-cost material for use as a fast, simple and efficient method of DNA separation and concentration.