3.2.1. AgNP coated with alginate
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).narrowing in presence of NaCl indicated that the surface species (possibly
ALG coating or Ag species) are being replaced by chloride ions
(Espinoza et al., 2012). Nevertheless, at [NaCl] N 31mM,Wh/2 increased
with time (up to 180 nm) until a constant value was reached at the end
of the experiment. At enhanced NaCl concentrations, the AgNP aggregation
could contribute to the widening of the SPRB due to light absorption
of the aggregates and/or non-spherical nanoparticle shapes at
wavelengths higher than 500–550 nm.