Air-saturated Media Research Articles

A preliminary voltammetric investigation of silver ions in food simulants

In this paper, the voltammetric behavior of the Ag+/Ag0 system was investigated in milli-Q water, 3% v/v acetic acid/milli-Q water, 10% v/v, 20% v/v and 50% v/v ethanol/milli-Q water media, which, according to the European Commission recommendations, can be employed as simulants of liquid foods for tests regarding the release of contaminants from plastic food packaging. Cyclic voltammetric measurements were performed using AgNO3, as source of Ag+ ions, and the various media without and with KNO3 or CH3COONa (for the aqueous acetic acid solution) as supporting electrolyte. Media with no deliberately added supporting electrolytes were investigated to better adhering to the experimental conditions used in conventional tests for establishing the amounts of Ag+ released from food packaging, loaded with silver nanoparticles (AgNPs), in the simulant media. To avoid strong distortions of the voltammetric responses, especially in the solutions without supporting electrolyte, the measurements were performed using a microdisk electrode. Diffusion coefficient values of Ag+, evaluated by the quasi steady-state limiting currents obtained from the Ag+ reduction process, depended mainly on viscosity of the specific food simulant investigated. The ionic strength of the medium affected diffusivity parameters to a lower extent. However, current values in the media without supporting electrolyte were larger than those recorded with excess electrolyte due to migration, which adds to diffusion in the mass transport. This contribution was observed when the Ag+ concentration levels were larger than a few tens of micromolar. Preliminary anodic stripping voltammetric measurements performed in milli-Q and 3% HAc, containing commercially available AgNPs (30–90nm diameter), incubated for about 6h in ambient conditions (i.e., temperature 24°C and air saturated media), provided Ag+ ion concentrations of 1.1×10−7molL−1 and 2.4×10−7molL−1, respectively.

Light‐Driven Horseradish Peroxidase Cycle by Using Photo‐activated Methylene Blue as the Reducing Agent

In this work, the regeneration of native horseradish peroxidase (HRP), following the consecutive reduction of oxo-ferryl pi-cation (compound I) and oxo-ferryl (compound II) forms, was observed by UV-visible spectrometry and electron paramagnetic resonance (EPR) in the presence of methylene (MB+), in the dark and under irradiation. In the dark, MB+ did not affect the rate of HRP compound I and II reduction, compatible with hydrogen peroxide as the solely reducing species. Under irradiation, the dye promoted a significant increase in the native HRP regeneration rate in a pH-dependent manner. Flash photolysis measurements revealed significant redshift of the MB+ triplet absorbance spectrum in the presence of native HRP. This result is compatible with the dye binding on the enzyme structure leading to the increase in the photogenerated MB* yield. In the presence of HRP compound II, the lifetime of the dye at 520 nm decreased approximately 75% relative to the presence of native HRP that suggests MB* as the heme iron photochemical reducing agent. In argon and in air-saturated media, photoactivated MB+ led to native HRP regeneration in a time- and concentration-dependent manner. The apparent rate constant for photoactivated MB+-promoted native HRP regeneration, in argon and in air-saturated medium and measured as a function of MB+ concentration, exhibited saturation that is suggestive of dye binding on the HRP structure. The dissociation constant (KMB) observed for the binding of dye to HRP was 5.4+/-0.6 microM and 0.57+/-0.05 microM in argon and air-saturated media, respectively. In argon-saturated medium, the rate of the conversion of HRP compound II to native HRP was significantly higher, k2argon=(2.1+/-0.1)x10(-2) s(-1), than that obtained in air-equilibrated medium, k2air=(0.73+/-0.02)x10(-2) s(-1). Under these conditions the efficiency of photoactivated MB(+)-promoted native HRP regeneration was determined in argon and air-equilibrated media as being, respectively: k2/KMB=3.9x10(3) and 12.8x10(3) M(-1) s(-1).