A preliminary voltammetric investigation of silver ions in food simulants
Graphical abstract
Introduction
The antimicrobial activity of silver has been noted for centuries [1]. Recently, the understanding of the antimicrobial action of silver species, or their toxicity towards human cells and aquatic organisms have received a renewed interest, maily because of the commercial introduction of novel silver-based materials in a large varity of goods [[2], [3], [4], [5], [6], [7]]. In particular, investigations have been concerned with silver micro- and nano-particles (Ag-Ps), which are very attractive antimicrobial agents towards both Gram-positive and Gram-negative bacteria [[8], [9], [10]], including multiresistant strains [[8], [9], [10], [11], [12]]. The antibacterial efficiency of silver-based products is mainly attributed to ionic silver forms [[9], [10], [11], [12]].
The development of food contact materials containing Ag-Ps has led to an improvement of the antimicrobial properties of packaging materials [[13], [14], [15], [16], [17], [18], [19], [20]]. However, from such products, silver species, especially silver ions, could be released into food matrices and this could pose risks to human health or environment [2,15]. Because of the specific toxicity of soluble Ag (I) species, studies concerning their migration from food packaging to the external environment have become necessary. Investigation on silver ion release from a solid matrix, typically, involves contacting the test materials with fluids that simulate the chemical or physical properties of food [21], and then detect the amount of silver species released. According to the European Commission, for aqueous, acidic, alcoholic and semi-fatty foods, suitable fluids as food simulants for plastic food-contact packaging are water solutions, eventually acidified with acetic acid, and water/ethanol mixtures [22]. Food-simulating liquids are used in place of real foods, which are often heterogeneous, to generalize the results obtained for a giving food contact material, while avoiding interferences, in the analytical protocols, due to specific substances present in foods [23].
Quantification of silver species released in food simulants are in general performed by atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) or other spectroscopic techniques [14,21,[23], [24], [25], [26], [27]], which, apart from the high cost of the instrumentation, are often employed for total silver detection. X-ray photoelectron spectroscopy (XPS) could in principle be able to provide information on the oxidation state of Ag in Ag-Ps-composites [28,29]. However, XPS is suitable for examining solid matrices in ultra-vacuum conditions [29].
To distinguish between ionic silver from other forms (i.e., silver solid particles) in liquid media, electrochemical approaches based on voltammetric methods are more suited [14,[30], [31], [32]]. To perform such kinds of measurements, mass transport characteristics of the species of interest in the test media are of considerable importance, as they are fundamental for the definition of analytical quantification protocols. However, to the best of our knowledge, no report exists on the latter aspects for Ag+ ions in food simulants. Accordingly, in this paper we investigate on the voltammetric behavior of Ag+ in the food simulants milli-Q water (MW), 3% v/v acetic acid aqueous solution (3% HAc), 10% v/v, 20% v/v and 50% v/v ethanol/m-Q water (%E/W) mixtures, to acquire information on matrix effect on potentials and diffusion parameters. Measurements are taken with a microdisk working electrode [33], which enables obtaining almost distortion-free responses even in the absence of deliberately addition of supporting electrolyte [[34], [35], [36], [37]]. The latter conditions are typically employed in conventional tests to quantify the Ag+ release in the above food simulants [22]. However, for comparison, voltammetric responses are also recorded in the media containing 0.1 M KNO3 or 0.1 M CH3COONa (the latter for the 3% HAc solution) as supporting electrolyte. The microelectrode employed was a platinum disk of 12.5 μm nominal radius, as it revealed to be suitable for detecting trace amounts of silver ions in aqueous media [31,32].
Section snippets
Chemicals and solutions
Hexaammine ruthenium (III) chloride (Ru(NH3)6Cl3), potassium chloride (KCl), potassium nitrate (KNO3), absolute ethanol were from Sigma-Aldrich (Milan, Italy). Glacial acetic acid (CH3COOH) was from Rudipont (Eurobase SpA, S. Giuliano Milanese, Italy). Sodium acetate (CH3COONa) and silver nitrate (AgNO3) 0.1 M volumetric standard solution were from Vetrotecnica S.r.l. (Padua, Italy). All reagents employed were of analytical-reagent grade and were used as received without any further
Results and discussion
Typical cyclic voltammograms (CVs) obtained at 5 mV s−1 for Ag+ reduction in the food simulants without and with 0.1 M KNO3 (or 0.1 M CH3COONa for 3% HAc) as supporting electrolyte are shown in Fig. 1. In all experiments, the potential scan commenced from 0.6 V in the negative direction until the chosen switching potential of −0.3 V was reached, and then the potential was turned back to 0.6 V. As is evident in Fig. 1, in all cases, the features of the CVs are those expected for reduction of metal ions
Conclusions
In this paper, the voltammetric behavior of Ag+ ions in the food simulants milli-Q water, 3% v/v acetic acid aqueous solution, 10% v/v, 20% v/v and 50% v/v ethanol/milli-Q water mixtures was investigated to extract diffusion parameters and to establish suitable conditions for their evaluation at trace levels, when Ag+ ions are released from silver particles in the food simulants. In particular, the study has allowed verifying that viscosity of the media, as expected, affected sensibly diffusion
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