Radiochemical transformation of mucilage extracted from Opuntia ficus-índica using gamma radiation.

Gamma irradiation is a powerful tool for modifying the physicochemical properties of polymers. Gamma radiation has been widely used to carry out reactions such as crosslinking, depolymerization, and graft copolymerization, with a wide variety of synthetic and natural polymers. Although gamma irradiation proved to be useful to modify the chemical properties of several polysaccharides, this method has not been applied to mucilage. However, irradiation could bring beneficial changes to mucilage for its use in different applications. Thus, the aim of this work was to study the radiochemical transformation of mucilage extracted from Opuntia ficus-indica using gamma radiation. Results obtained from FTIR-ATR, pH and electrical conductivity in aqueous solution showed that decarboxylation of mucilage occurs applying a radiation dose from 2 to 12 kGy. Further, it was observed that thermal stability of mucilage was enhanced with a radiation dose of 2 and 6 kGy. The decarboxylation degree was proportional to the radiation dose in the interval of 0-12 kGy. Further, mucilage was irradiated in the presence of acrylic acid forming mucilage/PAA hydrogels with high swelling capacity. The best swelling capacity (734%) was observed for the hydrogel obtained applying 2 kGy of gamma radiation. Hydrogels prepared with higher radiation doses (4-20 kGy) showed lower swelling capacity and higher thermal stability, which suggest a higher crosslinking degree. The experiments performed in this work were performed using 100 mg of mucilage but gamma irradiation allows a high scalability of the process. The usefulness of gamma irradiation relays on the development of a method for preparing hydrogels by an easy and scalable method.

Experimental Study of the Validity of Entangled Two-Photon Absorption Measurements in Organic Compounds.

Entangled two-photon absorption (ETPA) has recently become a topic of lively debate, mainly due to the apparent inconsistencies in the experimentally reported ETPA cross sections of organic molecules obtained by a number of groups. In this work, we provide a thorough experimental study of ETPA in the organic molecules Rhodamine B (RhB) and zinc tetraphenylporphirin (ZnTPP). Our contribution is 3-fold: first, we reproduce previous results from other groups; second, we on the one hand determine the effects of different temporal correlations─introduced as a controllable temporal delay between the signal and idler photons to be absorbed─on the strength of the ETPA signal, and on the other hand, we introduce two concurrent and equivalent detection systems with and without the sample in place as a useful experimental check; third, we introduce, and apply to our data, a novel method to quantify the ETPA rate based on taking into account the full photon-pair behavior rather than focusing on singles or coincidence counts independently. Through this experimental setup we find that, surprisingly, the purported ETPA signal is not suppressed for a temporal delay much greater than the characteristic photon-pair temporal correlation time. While our results reproduce the previous findings from other authors, our full analysis indicates that the signal observed is not actually due to ETPA but simply to linear losses. Interestingly, for higher RhB concentrations, we find a two-photon signal that, contrary to expectations, likewise does not correspond to ETPA.

Graphenic substrates as modifiers of the emission and vibrational responses of interacting molecules: The case of BODIPY dyes.

Graphenic substrates (GS), such as reduced graphene oxide (rGO) and graphene oxide (GO), are 2D materials known for their unique physicochemical properties such as their ability to enhance vibrational spectroscopic signals and quench the fluorescence of adsorbed molecules. These properties provide an opportunity to develop nanostructured GS-based systems for detecting and identifying different analytes with high sensitivity and reliability through molecular spectroscopic techniques. This work evaluated the capacities of different GS to interact with a highly fluorescent compound, thereby changing its optical emission response (fluorescence quenching) and amplifying its vibrational signal, which is the base of graphene-enhanced Raman scattering (GERS). To test these properties, we used a derivative of highly fluorescent BODIPY (BP) compounds, which cover a wide range of applications from solar energy conversion to photodynamic cancer therapy. GS prepared by using the Langmuir-Blodgett (LB) technique allowed us to quench the fluorescence emission of BP and improve its Raman spectroscopy detection limit due to the GERS effect. These results were interpreted in light of the π-π interactions taking place between the Csp2 domains of GS and the aromatic core of the BP fluorophore.

Preparation and Characterization of a Novel Organic Semiconductor Indacenedithiophene Derivative and the Corresponding Langmuir-Blodgett Thin Films.

The synthesis of a novel indacenedithiophene derivative (IDT-DPA) is described, which exhibits semiconducting behavior. Its properties were measured by means of UV-visible and fluorescence spectroscopies using toluene as solvent. An extinction molar coefficient of 2.05×104 M-1 cm-1 and a Stokes shift of 50 nm were obtained. A theoretical study was performed using the density functional theory, from which HOMO-LUMO band gap of 1.711 eV was calculated. IDT-DPA was deposited on the water-air interface to form Langmuir monolayers. π-A curves and hysteresis were measured showing reversibility behavior. The monolayers were transferred to glass substrates as Langmuir-Blodgett thin films. Their morphological properties were characterized by using scanning electron and atomic force microscopy, which showed that the films tend to form clusters with a homogeneous distribution. Absorption and emission spectra of the films were measured, from which the optical band gap and Stocks shift were derived. Based on the electronic properties and light emission spectra of IDT-DPA, this compound can be proposed for the applications in organic lightemitting diodes and other organic semiconductor devices.