## 4 Discussion  
We confirmed that SiNWs degrade in a simulated physiological environment (PBS) and release Si into the surrounding solution. The degradation of single SiNWs can be monitored by using AFM measurements to estimate the etch rate. We extrapolated that, under the present conditions, SiNWs lose about 1 nm in diameter per day. This rate is well within the range of dissolution rates given for silicon oxide layers that were investigated at different temperatures, including non-physiological ones, demonstrating the temperature dependence of the dissolution process [25; 26]. Literature on the dissolution of SiNW under similar conditions [11; 27] seem to show comparable dissolution rates, albeit under presumably static conditions, i.e. without a regular exchange of PBS, and with the use of other methods to determine the SiNW diameter. The SiNW devices were functional for up to seven weeks in PBS at the physiological temperature of 37 \({}^{\circ}\)C. The magnitude of current that was flowing through the device, however, varied by orders of magnitude during degradation. This indicates a variability in conductivity that is detrimental to the reproducibility of silicon nanowire transducer signal interpretation, if SiNWs are used for long-term analytical applications. Electrical characterization of SiNWs with encoded p-n junctions in parallel with the AFM measurements support the overall diameter decrease. However, the changes in electrical properties do not correlate simply with decreasing diameter. In particular within the first 48 h, immersion in PBS had a drastic effect on the SiNW surface band structure as determined by CPD and APS measurements. During this period, the Fermi level shifted towards the conduction band and was finally situated inside the conduction band itself. This implies a significant accumulation of majority charge carriers, here electrons, at the nanowire surface and thus a significant increase in SiNW conductivity, which is fully supported by the increased forward current of the measured p-n junctions. The exact reason for this Fermi level shift remains to be found, but three aspects are discussed here. First, it is well known that SiO\({}_{2}\) can passivate surface states which are present at the silicon surface, thus reducing charge carrier trapping [28]. The effectiveness of this passivation is directly linked to the oxide quality and the preparation method. The hydrolytic dissolution of the SiNWs is accompanied by the dissolution of the native oxide, followed by a re-oxidation of the silicon [10]. Considering the estimated etch rate, it is reasonable to assume that the initial native oxide grown in air is exchanged within the first 48 h by an oxide grown in the liquid. Thus, with the re-arrangement of the oxide layer, the passivation of surface states may change. A second aspect that can be discussed is the evolution of a fixed positive interface charge in SiO\({}_{2}\)[22; 28] that would lead to an electron accumulation as observed. The oxide-recovering oxidation reaction may be the source of such a charge imbalance and may lead to the observed increased Fermi level [29]. The third point is the PBS electrolyte itself and the ions that should in principle balance the overall charge situation. The electrolyte, however, potentially also affects the formation of surface states. Alkali metal cations can replace hydrogen on the silanolhydroxyl groups and the deposition of cations such as sodium and potassium may lead to a strong accumulation of electrons at the surface. As a side effect, this replacement can promote an increasing bond angle and therefore facilitate the access of water molecules to the surface, which in turn increase the dissolution rate [30]. However, further studies are required to determine the origin for the observed initial change in surface band structure to fully explain the observed behavior including the significant shift of the Fermi level well above the conduction band edge. After a week of immersion, the band structure seemed to stabilize.