– UV-Visible spectrophotometry.The

– UV-Visible spectrophotometry.
The UV-Visible spectra were recorded using a deuterium tungsten
halogen light source (DT-Mini 2GS, Ocean Optics), a quartz cuvette
and a miniature CCD array spectrophotometer (USB-4000, Ocean
Optics), connected through two optical fibres (600 μm fibre, P600-
025-SR). Using this set-up we were able to obtain a complete scan
(from 250 to 800 nm) in b1 s. A control solution without AgNPs
was used to blank correct the instrument before the start of themeasurements.
The solutions at fixed pH (8.0 ± 0.2), with a range of
AgNP, total silver concentrations (44 and 90 μM), different NaCl concentrations
(from 0 to 997 mM) and also natural fjord waters, were
analysed in terms of SPRB evolution (A, Wh/2, SPRB height and position
of the absorbance maximum) over time (Videos S1–S8). The
same integration time (100 ms), number of averaged scans (3) and
boxcar smoothing (10) were used during the experiments. At least
two replicates were measured for each experiment. In order to
avoid possible contributions to A of AgNP aggregates, that would
present absorption at higher wavelengths (N500 nm), we obtained
A, Wh/2, SPRB height and position of the absorbance maximum
from a straight line traced from the base of the SPRB between 325
and 500 nm. An in-house Matlab script was used to record a full
spectrum (from 250 to 800 nm) every second at the beginning of
the aggregation process when the changes of the SPRB were fast,
and increasing this time step as the experiment proceeded. The
experiments were conducted until the SPRB disappeared in the
case of fast aggregation processes, and for at least 1500 s in the
case of slow processes.