AgNP-ALG and 109AgNP-ALG behaved according to the DLVO aggregation
theory. Fig. 1 shows the characteristic fast and slow aggregation
kinetic regimes obtained by UV-Visible spectrophotometry.
At pH 8 the main acid groups present in alginate (−COOH) are
deprotonated (Rey-Castro et al., 2004) providing the AgNP with a net
negative charge (see ζ-potential values in Table 1). In this case, the alginate
coating stabilizes the AgNP suspension by electro-steric repulsion,
and consequently in low ionic strength solutions aggregation is
prevented. Nevertheless, at enhanced NaCl concentrations a plot of
1/A vs t (Eq. (2)) yielded straight lineswith high regression coefficients,
confirming the aggregation process (Video S2).
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).
The percentage increase in A that occurred during the first 25 s is
presented in Table S2. At NaCl concentrations higher than 58 mM for
AgNP-ALG and 50 mM for 109AgNP-ALG, there was no observable
increase in A, so contributions from the formation of new AgNPs from
the silver ions are not expected. Usually, the time required for the
collection of a full spectrum using a standard bench-top laboratory
spectrophotometer is ≥25 s, and therefore important information,
such as the possible formation of new NPs from the silver ions present
in solution, is missed. In this study, we obtained a full spectrum from
the first second of a NP aggregation process, which facilitated our interpretation
of AgNP behaviour at the early stages of aggregation.
The CCC values obtained for AgNP-ALG and 109AgNP-ALG, following
the evolution of A in the RCA and DCA regimes,were 80±9.3 and 82±
4.2 mM (Fig. 2).