The area under the SPRB curve in the frequency spectrumis proportional
to the quantity of surface electrons in the AgNPs and consequently
to the concentration of silver atoms in the solution (each silver atom
donates one electron to the metal). Therefore, the increase in peak area
confirmed the formation of new small NPs from Ag+ reduction in the
presence of low amounts of Cl− ions and/or silver-chloride species.
The SPRB evolution over time at different NaCl concentrations is
shown in Videos S1–S2. For NaCl concentrations ≤ 42 mM there was
an increase in A between 5–25 s and finally a constant value was
achieved (inset Fig. 1a). Small varying trendswere also observed at longer
times for the lowest salt concentrations tested. Nevertheless, at
[NaCl] N 61 mM a rather continuous reduction of A was observed
(Figs. 1b, S5a–b).