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.