Multilayered iron oxide/reduced graphene oxide nanocomposite electrode for voltammetric sensing of bisphenol-A in lake water and thermal paper samples.

A multilayered iron oxide/reduced graphene oxide (ION-RGO) nanocomposite electrode is reported for the voltammetric sensing of bisphenol-A (BPA). Structural characterizations reveal the nanocomposite features RGO sheets decorated with nanometric spherical ION in a mixture of maghemite and magnetite phases. ITO substrate modified with the ION-RGO multilayered film exhibits strong electrocatalytic effect toward BPA oxidation, which is made possible by Fe(III) catalysts generated at the ION's surface after scanning the electrode potential from below 0 V (vs Ag/AgCl) and followed by the RGO phase conducting the transferred electrons. Under optimized differential pulse voltammetry conditions, the proposed sensor shows three linear working ranges 0.09-1.17 (r2 = 0.999), 1.17-3.81 (r2 = 0.995) and 3.81-8.20 (r2 = 0.998), with the highest sensitivity equaling 7.76 µA cm-2/µmol L-1 and the lowest limit of detection of 15 nmol L-1. A single electrode can be used for at least twenty consecutive runs loosing less than 15% of sensitivity, whereas electrodes fabricated in different bacthes exhibit almost identical perfomances. Determination of BPA in a thermal paper sample shows no difference (at 95% confidence level) between the proposed sensor and HPLC/UV. The sensor is neither influenced by the matrix composition nor by other emerging contaminants.

Starch-Mediated Immobilization, Photochemical Reduction, and Gas Sensitivity of Graphene Oxide Films.

This work unveils the roles played by potato starch (ST) in the immobilization, photochemical reduction, and gas sensitivity of graphene oxide (GO) films. The ST/GO films are assembled layer by layer (LbL) onto quartz substrates by establishing mutual hydrogen bonds that drive a stepwise film growth, with equal amounts of materials being adsorbed in each deposition cycle. Afterward, the films are photochemically reduced with UV irradiation (254 nm), following a first-order kinetics that proceeds much faster when GO is assembled along with ST instead of a nonoxygenated polyelectrolyte, namely, poly(diallyl dimethylammonium) hydrochloride (PDAC). Finally, the gas-sensing performance of ST/reduced graphene oxide (RGO) and PDAC/RGO sensors fabricated via LbL atop of gold interdigitated microelectrodes is evaluated at different relative humidity levels and in different concentrations of ammonia, ethanol, and acetone. In comparison to the PDAC/RGO sensor, the ones containing ST are much more sensitive, especially when operating in a high-relative-humidity environment. An array comprising these chemical sensors provides unique electrical fingerprints for each of the investigated analytes and is capable of discriminating and quantifying them in a wide range of concentrations, from 10 to 1000 ppm.

Phase Transition in all-trans-ß-Carotene Crystal: Temperature-Dependent Raman Spectra.

In this study, we studied the stability of an all-trans-ß-carotene single crystal using Raman spectroscopy with line excitation at 632.8 nm, in the temperature range 20­300 K. The Raman spectra exhibit clear modifications in the spectral range of the lattice and internal vibrational modes. The temperature dependence of the most intense vibrational modes ν1 (1511 cm(­1)) and ν2 (1156 cm(­1)) that are related to the C═C and C­C stretching vibrations of the polyene chain, respectively, shows an upward shift on the Raman modes. This behavior is similar to that stated in the theoretical work of Wei-Long Liu et al. We conclude that the all-trans-ß-carotene crystal undergoes a temperature-induced phase transition at approximately 219 K. This transition is interpreted as a rotation experienced by ß-ring groups at each end of the all-trans-ß-carotene molecule around the dihedral angle. At low temperatures, the new molecular configuration affects the sliding plane of the space group C2h(5)(P2(1)/n), and the phase transition leads to an unchanged monoclinic structure; however, the original space group is possibly lowered to the space group C2. In the temperature range 200­220 K, the spectral ratio (S) of the integrated intensities of the spectral modes around the symmetric and asymmetric stretching wavenumbers of the methyl group (CH3) changes as a function of temperature in agreement with the phase transition. Furthermore, according to phase transition undergone by the all-trans-ß-carotene, the thermal results obtained by differential scanning calorimetry show an exothermic process that occurs near the transition temperature assigned by the Raman spectra.

Thermal and electrical properties of starch-graphene oxide nanocomposites improved by photochemical treatment.

Bionanocomposite films have been prepared by casting an aqueous suspension of acetylated starch (ST) and poly(vinyl alcohol) (PVA) loaded with graphene oxide (GO). A photochemical and reagentless method has been successfully performed to convert the GO phase into reduced graphene oxide (RGO). The nanocomposites have displayed improved thermal and electrical properties when the amount of the GO phase is increased and properly converted to RGO. The molecular-level interactions between components are mainly hydrogen-bonding type according to attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and Raman spectroscopies, as well as thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has confirmed the effective mixing between the GO and the ST-PVA matrix. The thermal diffusivity and electrical resistivity of ST-GO nanocomposites have increased one order and decreased two orders of magnitude, respectively, after the photochemical treatment. These findings have confirmed the effectiveness of the proposed approach to produce starch-based nanocomposites with improved thermal and electrical properties.