We report here on the electroreduction of p-benzoquinone (BQ) or H2O2 as a new trigger for simple, fast, uniform, and controllable electrodeposition of chitosan (CS) hydrogels and biosensing nanocomposite films of CS, multiwalled carbon nanotubes (MWCNTs), and glucose oxidase (GOD). The multiparameter electrochemical quartz crystal microbalance (EQCM) based on crystal electroacoustic impedance analysis was used to dynamically monitor the deposition processes. When the EQCM Au electrode was immersed in a weakly acidic solution (here pH 5.1 acetic buffer) containing BQ (or H2O2) and CS, the proton consumption during BQ (or H2O2) electroreduction increased the local solution pH near the electrode surface and led to the deposition of CS hydrogel on the electrode surface at local pH near and above the pKa value of CS. The concentration of BQ (or H2O2) required for CS electrodeposition was theoretically evaluated based on an electrogenerated base-to-acid titration model and supported by experiments. Co-deposition of GOD and MWCNTs with the CS hydrogel was achieved, and the resulting MWCNTs-CS-GOD nanocomposite films were demonstrated for glucose biosensing. The MWCNTs-CS-GOD enzyme electrode prepared by BQ reduction exhibited a current sensitivity of 6.7 microA mM-1 cm-2 to glucose, and the linear range for glucose detection at 0.7 V vs SCE was from 5 microM to 8 mM, with a detection limit of 2 microM and a Michaelis-Menten constant of 6.8 mM. The BQ-electroreduction protocol exhibited the best sensor performance, as compared with H2O2-reduction and previously reported water-reduction values. The present protocol via electroreduction of a deliberately added oxidant that is accompanied by a local pH change is highly recommended for wider applications in pH-dependent deposition of other films.

No abstract

An efficient and highly E-selective protocol for the synthesis of vinyl sulfones is described. This simple protocol demonstrates the first synthesis of vinyl sulfones via a silver-catalyzed C-S bond coupling reaction. In addition, the success of the reaction was found to be critically dependent on the use of TEMPO as the additive.