Silver content was measured by ICP-MS after dilution and digestion
of the sampleswith 0.3MHNO3 (70%; Optima, Fisher Scientific). Oxidation
experiments were undertaken to verify the AgNP stability in
terms of solubilisation in the original batches and during the experiments.
Silver ion (Ag+) concentrations in solution produced during
the dissolution of the AgNPs were determined using different approaches
depending on the sample matrix. At negligible Cl− concentrations
an Ag Ion Selective Electrode (Ag-ISE) from Thermo
Scientific (9616BNWP) was used. For saline matrices, separation of
Ag+ (free silver ion and its complexes) and AgNP was necessary.
We carried out the separation by ultrafiltration using centrifugal
ultrafilters (3 kDa, Amicon Millipore) and a centrifuge (5430R,
Eppendorf). Subsequent quantification of Ag+ and total silver in
the filtrate and retentate was carried out by ICP-MS.