Potential Use of Chitosan-TiO₂ Nanocomposites for the Electroanalytical Detection of Imidacloprid

The detection of toxic insecticides is a major scientific and technological challenge. In this regard, imidacloprid is a neonicotinoid that is a systemic insecticide that can accumulate in agricultural products and affect human health. This work aims to study the properties of chitosan–TiO2 nanocomposites in which nanoparticles with high surface area serve as molecular recognition sites for electroanalytical imidacloprid detection. We show that the best sensitivity to imidacloprid was obtained using a modified electrode with a chitosan–TiO2 nanocomposite with a 40 wt.% of TiO2 nanoparticles. By using a three-phase effective permittivity model which includes chitosan, TiO2, an interface layer between nanoparticles and a matrix, we showed that nanocomposites with 40 wt.% of TiO2 the interface volume fraction reaches a maximum. At higher nanoparticle concentration, the sensitivity of the sensor decreases due to the decreasing of the interface volume fraction, agglomeration of nanoparticles and a decrease in their effective surface area. The methodology presented can be helpful in the design and optimization of polymer-based nanocomposites for a variety of applications

Chitosan-Covered Pd@Pt Core–Shell Nanocubes for Direct Electron Transfer in Electrochemical Enzymatic Glucose Biosensor

Development of biosensors with high sensitivity, high spatial resolution, and low cost has received significant attention for their applications in medical diagnosis, diabetes management, and environment-monitoring. However, achieving a direct electrical contact between redox enzymes and electrode surfaces and enhancing the operational stability still remain as challenges. Inorganic metal nanocrystals (NCs) with precisely controlled shape and surface structure engineered with an appropriate organic coating can help overcome the challenges associated with their stability and aggregation for practical biosensor applications. Herein, we describe a facile, room-temperature, seed-mediated solution-phase route to synthesize monodisperse Pd@Pt core–shell nanocubes with subnanometer-thick platinum (Pt) shells. The enzyme electrode consisting of Pd@Pt core–shell NCs was first covered with a chitosan (CS) polymer and then glucose oxidase (GOx) immobilized by a covalent linkage to the CS. This polymer permits covalent immobilization through active amino (−NH) side groups to improve the stability and preserve the biocatalytic functions while the Pd@Pt NCs facilitate enhanced direct electron transfer (DET) in the biosensor. The resultant biosensor promotes DET and exhibits excellent performance for the detection of glucose, with a sensitivity of 6.82 μA cm^–2 mM^–1 and a wide linear range of 1–6 mM. Our results demonstrate that sensitive electrochemical glucose detection based on Pd@Pt core–shell NCs provides remarkable opportunities for designing low-cost and sensitive biosensors

Molecular Dynamics Analysis of PVA- AgnP

The molecular dynamics of PVA/AgnP composites were studied by dielectric spectroscopy (DS) in the 20–300°C temperature range. Improper water elimination leads to misinterpretation of thermal relaxations in PVA composites in agreement with the previous report for pristine PVA. The evaporation of water and its plasticizing effect are more evident in pure PVA confirming the existence of strong interaction between OH groups of PVA chains and AgnP. Dry films show a single nonlinear VFT dependence (from 45°C until melting) associated to the α-relaxation and, therefore, to the glass transition phenomenon and from dielectric measurements, the Tg of composites vary from 88°C for pristine PVA to 125°C for PVA/AgnP (5 wt%). Below 45°C, dry films exhibit a single Arrhenius behavior showing a 3D hopping conductivity as explained based on the variable range hopping model. PVA/AgnP composites have higher conductivity compared to pristine PVA, and it increases as AgnP weight percent increases. Finally, DMA measurements support the statement that a secondary relaxation was erroneously assigned as the glass transition of PVA and composites in previous reports

Novel high molecular weight aromatic fluorinated polymers from one-pot, metal-free step polymerizations

A novel series of linear, high-molecular-weight polymers and copolymers was synthesized by one-pot, metal-free superacid-catalyzed reaction of trifluoromethylalkyl (1a-1c) and trifluoromethylaryl (1d-1h) ketones with the linear, nonactivated, multiring aromatic hydrocarbons biphenyl (A), p-terphenyl (B), and p-quaterphenyl (C).The polymerizations were performed at room temperature in the Brønsted superacid trifluoromethanesulfonic acid (CF3SO3H, TFSA) and in a mixture of TFSA with methylene chloride. Polymerizations of trifluoromethyl ketones (1c, 1f-1h) bearing functional groups gave polymers with reactive lateral groups such as bromomethyl, 4-(N,N-dimethylamino)phenyl-, 3-sulfophenyl-, and 2,3,4,5,6-pentafluorophenyl. The polymers obtained were soluble in most common organic solvents and flexible transparent films could be cast from the solutions. 1H and 13C NMR analyses of the polymers synthesized revealed their linear structure with para-substitution in the phenylene fragments of the main chain. The molecular weights (Mw) of the polymers based on trifluoromethylalkyl ketones and aromatic are very high and reach 1 000 000, while the molecular weights of the polymers based on trifluoromethylaryl ketones and aromatic ranged from 30 000 to 300 000 g/mol. The polydispersity of the polymers is generally less than 2. The polymers also possess high thermostability. Mechanistic aspects of polymerization mechanism have been discussed, and a new approach for monomer design has been proposed. © 2013 American Chemical Society.Financial support from CONACYT Mexico (Grant 151842) and from DGAPA-UNAM (PAPIIT IN 104211)Peer Reviewe