Débora M. Martino

EPR spectroscopy and exchange interaction parameters in Cu(glycine)2·H2O

Abstract EPR measurements have been performed in single crystals of Cu(glycine) 2 ·H 2 O, at 9.7 and 33 GHz and at room temperature. this orthorhombic crystal has four chemically identical but magnetically non-equivalent copper ions per unit cell. The EPR data at both microwave frequencies show two partially resolved EPR lines for some of the field directions in the ab plane, while only a single line is observed along any direction in the other two crystallographic planes. This indicates that the four copper ions are arranged in two weakly coupled and symmetry-related subsets , each involving two non-equivalent and interacting copper sites. The decoupling of the spectra leads to g 1 = 2.256(2), g 2 = 2.071(2) and g 3 = 2.061(2) for the principal values of the molecular g -factors. The EPR data, used in conjunction with the classical Andersons model and the Kubo and Tomitas theory, allow to evaluate the exchange interaction parameters between the magnetically non-equivalent copper ions. The Andersons model and the observed EPR line positions data lead to | J ′/ k | = 19(3) mK for the intersubset exchange coupling. The analysis of the angular variation of the line width in terms o the Kubo and Tomitas theory, gives | J ′/ k | = 21(2) mK and | J / k | = 57(1) mK for the intersubset and intrasubset exchange couplings, respectively. These values are discussed in terms of the crystal structure of the compound and compared with the results obtained by other authors.

Synthesis of “bioinspired” copolymers: experimental and theoretical investigation on poly(vinyl benzyl thymine-co-triethyl ammonium chloride)

Abstract “Bioinspired” copolymers based on vinylbenzyl thymine (VBT) and an ionically-charged monomer, such as vinylbenzyl triethylammonium chloride (VBA), were synthesized and theoretically investigated. These water-soluble copolymers are polystyrene- (PS) based, and their structure mimics DNA. In the presence of short-wavelength UV light, the thymine groups dimerize into non-toxic, environmentally benign, and biodegradable photo-resistant materials. Copolymerizations with different comonomer ratios were carried out at 65°C. Samples were taken along the reactions to determine monomer conversion, chemical composition, and molecular weight distribution. While average molecular weights fall along the reaction, the average composition remains almost constant and coincident with the initial comonomer ratios, thus indicating a similar reactivity of all the comonomer radicals. A mathematical model was developed that simulates the synthesis of the base biopolymer, in the sense of predicting the evolution of the global reaction variables and molecular structure of the polymer. The termination and propagation kinetic constants were adjusted to the experimental data. The resulting values are quite different to those of a normal styrene homopolymerization, thus suggesting a noticeable effect of the solvent and the comonomer pending groups.

A highly sensitive and stable glucose biosensor using thymine-based polycations into laponite hydrogel films.

A series of glucose bioelectrodes were prepared by glucose oxidase (GOx) immobilization into laponite hydrogel films containing DNA bioinspired polycations made of vinylbenzyl thymine (VBT) and vinylbenzyl triethylammonium chloride (VBA) with general formulae (VBT)m(VBA)n](n+)≈25 with m=0, 1 and n=2, 4, 8, deposited onto glassy carbon electrode. The bioelectrodes were characterized by chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. Results indicated that the electrochemical properties of the laponite hydrogel films were largely improved by the incorporation of thymine-based polycations, being proportional to the positive charge density of the polycation molecule. After incorporation of glucose oxidase, the sensitivity of the bioelectrode to glucose increased with the positive charge density of the polycation. Additionally, the presence of the vinylbenzyl thymine moiety played a role in the long-term stability and reproducibility of the bioelectrode signal. As a consequence, the [(VBT)(VBA)8](8+)≈25 was the most appropriate polycation for bioelectrode preparation and glucose sensing, with a specific sensitivity of se=176 mA mmol(-1)Lcm(-2)U(-1), almost two-order of magnitude larger than other laponite immobilized GOx bioelectrodes reported elsewhere. These features were confirmed by testing the bioelectrode for a selective determination of glucose in powder milk and blood serum samples without interference of either ascorbic or uric acids under the experimental conditions. The present study demonstrates the suitability of DNA bioinspired water-soluble polycations [(VBT)m(VBA)n](n+)≈25 for enzyme immobilization like GOx into laponite hydrogels, and the preparation of highly sensitive and stable bioelectrodes on glassy carbon surface.

The effect of light intensity, film thickness, and monomer composition in styrene-based bioinspired polymers

Abstract The effect of irradiation light intensity, film thickness, and polymer composition upon photo-irradiation of water-soluble polymers containing thymine was studied by ultraviolet (UV)-vis spectroscopy. Coatings of aqueous solution of the polymer on PET substrates were exposed to UV light at 254 nm through a standard mask. The effect of irradiation dose is similar to the typical behavior of photo-resists: the degree of crosslink increases over the time until it reaches saturation. The polymer composition effect shows an expected trend on the curing process, the more vinylbenzylthymine (VBT) content in the copolymer the higher the degree of crosslinking, a situation that agrees with the fact the VBT is the monomer playing the main role in the photo-reaction. It was observed that the effect of film thickness was as expected, the thicker the film the slower the immobilization of the polymer on the substrate. It was demonstrated that by varying these parameters one could control the crosslinking rate of the polymer.

Second order multivariate curve resolution of Fourier transform infrared spectroscopic data of the photo-induced crosslinking of thymine functionalized polymers

A meaningful characterization of the photo-induced curing process of materials based on styrene monomers functionalized with thymine and charged ionic groups was accomplished using FT-IR spectroscopy in combination with second-order multivariate calibration algorithms. The polymer composition as well as the irradiation dose effects on the photo-crosslinking of copolymer films was experimentally determined. Each FT-IR absorption spectra was decomposed into the contribution of individual species by means of chemometric algorithms. A second-order strategy involving a three-way array for each sample and analyzing all arrays simultaneously was used. Temperature and solvent frequently have a strong influence on the FT-IR peak producing shifts and trilinearity lost. A new methodology to properly pre-align the spectroscopic matrix data is used based on the decomposition of a three-way array via a suitably initialized and constrained PARAFAC model. The chemical reaction mechanism describing the underlying process in terms of identifiable steps was determined. Associated key parameters and equilibrium rate constants that characterize the interconversion and stability of diverse species were predicted. Additionally, it was possible to quantify all the species even in the presence of a non-calibrated compound.

Optimization of Environmentally Benign Polymers Based on Thymine and Polyvinyl Sulfonate Using Plackett-Burman Design and Surface Response

Traditional approaches to the development of integrated circuits involve the use and/or manufacture of toxic materials that have a potential environmental impact. An extensive research has been done to design environmentally benign synthetic polymers containing nucleic acid bases, which can be used to enhance the photoresistor technologies. Water soluble, environmentally benign photopolymers of 1-(4-vinylbenzyl) thymine (VBT) and vinylphenyl sufonate (VPS) undergo a photodimerization reaction when exposed to low levels of ultraviolet irradiation leading to an immobilization of the copolymer on a variety of substrates. Plackett-Burman design (PBD) and central composite design (CCD) were applied to identify the significant factors influencing the polymer crosslinking and dye adsorption processes, which are relevant in the fabrication of copolymer films for potential photoresist use. The PBD results assigned a maximum absorption signal of 0.67, while optimal conditions obtained in this experiment following the CCD method predictions provided a response of 0.83 ± 0.03, being a solid foundation for further use of this methodology in the production of potential photoresistors. The pH effect was relevant for low concentrations but not significant for higher concentrations. To the best of our knowledge, this was the first report applying statistical experimental designs to optimize the crosslinking of thymine-based polymers.

Thymine based copolymers: feasible sensors for the detection of persistent organic pollutants in water

Polycyclic Aromatic Hydrocarbons (PAHs) are among the main persistent organic pollutants in water, because they can cause serious diseases in living organisms. The PAHs trace levels in environmental samples makes their detection particularly difficult. The development of new fluorescence spectroscopic sensors is a realistic alternative for the quantification of PAHs at very low concentrations. Bio-inspired copolymers based on thymine and charged groups showed high affinity for benzo[a]pyrene, the nastiest contaminant of the PAHs group, and can be used to enhance their native luminescence. In the present work we rationalized the observed experimental evidence using a theoretical model that studies the plausible non-covalent interactions (polar hydrogen–π, π–π “stacking”, etc.) between these compounds, in order to make a realistic design of new thymine-based copolymers sensors.

First Steps in the Aggregation Process of Copolymers Based on Thymine Monomers: Characterization by Molecular Dynamics Simulations and Atomic Force Microscopy.

Atomistic molecular dynamic simulations were performed to study the structure of isolated VBT-VBA (vinylbenzylthymine-vinylbenzyltriethylammonium chloride) copolymer chains in water at different monomeric species ratios (1:1 and 1:4). The geometric parameters of the structure that the copolymers form in equilibrium together with the basic interactions that stabilize them were determined. Atomic force microscopy (AFM) measurements of dried diluted concentrations of the two copolymers onto highly oriented pyrolytic graphite (HOPG) substrates were carried out to study their aggregation arrangement. The experiments show that both copolymers arrange in fiber-like structures. Comparing the diameters predicted by the simulation results and those obtained by AFM, it can be concluded that individual copolymers arrange in bunches of two chains, stabilized by contra-ions-copolymer interactions for the 1:1 copolymerization ratio at the ionic strength of our samples. In contrast, for the 1:4 system the individual copolymer chains do not aggregate in bunches. These results remark the relevance of the copolymerization ratio and ionic strength of the solvent in the mesoscopic structure of these materials.

Fluorescence spectroscopy and multivariate analysis as a greener monitoring tool: characterization of the curing kinetics of bioinspired polymers

The photo-induced curing kinetics of bioinspired copolymers vinylbenzyl thymine (VBT) and vinylphenyl sulfonate was studied using fluorescence spectroscopy in combination with multivariate analysis. Fluorescence spectroscopy enables a detailed description of the curing process of VBT copolymers in real time, combining selectivity, simplicity and sensitivity, without the need of sample pre-treatments, being an advantage from the green analytical chemistry point of view. Two chemometric strategies were used to decompose the data matrix generated while monitoring the curing reaction, identifying the evolution of each species involved in the process in conjunction with the corresponding pure spectra. A comprehensive comparison between the developed approaches was made, clearly highlighting the advantages and disadvantages of both of them. The use of multivariate analysis applied to fluorescence spectroscopic data to study curing reactions have several advantages such as no sample pre-treatment, no sophisticated equipment requirement, reduced analysis time and use of non-toxic solvents, among others.

Experimental studies and mathematical modeling of the curing reaction of bioinspired copolymers

ABSTRACT Polymer curing is a complex process that significantly delineate the final properties of the synthetized material. In this work, the photo-induced crosslinking reaction of synthetic “bio-inspired” copolymers based on thymine and ionic groups was studied by gel permeation chromatography and UV absorption spectroscopy. A mathematical model for the curing process based on statistical techniques and coupled to the kinetics of crosslinking was developed. The model allows to predict both, the evolution of the crosslinking degree as a function of curing time and the gel times as a function of the molecular structure of the copolymer and the curing conditions. The theoretical values showed a very good agreement with the UV–Vis and GPC spectroscopy experimental results for all the copolymers studied. A better knowledge of the curing kinetics of thymine-based biopolymers will enable to develop materials with pre-specified properties and to improve their applications. GRAPHICAL ABSTRACT