3.2. Assessment of silver ion metabolism of individualD.r. bacteria
While the dissimilar SR levels of individual D.r. are clear,
their abilities to reduce silver ions and to form less toxic Ag metal
particles are carefully characterised both at the bulk level and the
single bacterium level. First, anodic stripping voltammetry was
conducted as a function of time to a gold macrodisc electrode inserted
in a grown bacterial solution initially containing 2 mM silver ions and
the broth compositions. Figure 2a shows throughout the experiment an
oxidation peak at the potentials of 0.2~0.3 V in
contrast to the blank voltammgram obtained with the bacterial solution
without Ag+ added. This indicates that the peak
corresponds to the oxidation of the reduced silver from the bacterial
solutions in the pre-concentration step. Importantly, the charge
transferred during the voltammetric peak was observed to gradually
decrease to nearly none (<5%), suggesting an almost complete
consumption of silver ions by the bacteria in the bulk solution over the
9 hours. This inference is on the basis that since the stripping of a
monolayer deposition of silver is estimated to correspond to an
oxidative charge of 14 μC (see Supporting Information Section 1 for the
calculation), which is far more than the largest amount
(~ 3 μC) measured in our experiment, the electrode used
is thus thought to be large enough to minimise inhomogeneous surface
redox reactions,31 and the measured charge can be
quantitatively related to the silver ion concentration. Besides, it is
noted that the negative shift of the peak potential over time, despite
the inferred decrease in the silver ion concentration, is probably due
to an increased level of halide anions in the extracellular solution
that results from the microbial metabolism. Next, the resulting
bacterial mixture with the minimum Ag+ content was
briefly examined by UV-vis spectroscopy. In comparison with the bacteria
incubated without silver ions, a distinct absorption band at around 430
nm was seen in Figure 2b, which coincides with the well-known
characteristic wavelength of the plasmonic absorption of silver
nanoparticles, suggesting the product of the silver ion uptake by the
bacteria is metallic nanosilver.