## 3 Results
Due to our use of height values as a measure for the diameter, the diameter in the present data might have been overestimated. However, as this is a systematic error, a dissolution rate derived from the relative comparison of the different data points should nevertheless represent the true dissolution rate. Such a linear fit of our data yielded a dissolution or etching rate of (1 \(\pm\) 0.1) nm/day for SiNWs in PBS at 37 \({}^{\circ}\)C.  
The most relevant properties of SiNWs in terms of sensing applications are electrical properties. These were monitored in parallel to the AFM measurements using the same SiNWs by recording _I-V_ characteristics at 14 time points between 1 and 70 days of immersion. In the simplest scenario, the conductance of a homogeneously doped SiNW should decrease, if the diameter and consequently the cross-section shrinks due to dissolution. Besides the conductance, the transconductance, commonly utilized as output value for SiNW sensors in FET configuration, should also be affected. However, changes in the leakage current, e.g., due to salt deposition and surface contamination, would also affect the measurements but the measurement of their contribution is non-trivial. To overcome this, we used SiNWs with an encoded p-n junction. As we grew SiNWs in a bottom-up manner, the implementation of p-n junctions was facile and the synthesis did not differ in principle from the one of SiNWs used as FET sensors. To enable localization of the p-n junctions within the SiNWs, we introduced the aforementioned crystal kink at the p-n junction interface (marked with an arrow in Figure 2c) [12]. For a schematic of the measurement set-up, please refer to Figure 3a. _I-V_ curves are shown in Figure 3c. For improved visibility, only selected but representative curves for a single p-n junction SiNW are depicted (all measured curves for this SiNW can be found in Figure S2). Based on a continuous decrease of the diameter of about 1 nm/day, one would expect in a first assumption to see a decrease of forward current due to an increase of the SiNW resistance. However, on the contrary, we observed an initial increase in forward current after the first days of immersion in PBS. The most pronounced increase by almost one order of magnitude was measured after seven days (circle in Figure 3c, green diamonds compared to black squares of Day 0; see Figure S3 for a plot of relative changes in forward currents). After this increase in forward current, the expected decrease occurred, which was accordingly also visible in the reverse current. After 56 days, the current at both forward and reverse bias was lower than 10\({}^{-11}\) A. At this point, with a symmetric shape of the curve, the typical rectifying diode characteristics of the p-n junction was no longer present and the measured current most likely a result of leakage current. The counterintuitive behavior in conductivity may be rooted in a change in silicon surface states present at the interface between silicon and its oxide. These surface states lead to the well-known Fermi level pinning, typically in the mid-bandgap region [22], and consequently lead to charge carrier depletion or space charge region formation at the silicon surface. As the initial native oxide layer formed in air is dissolved and rebuilt in an aqueous environment, silicon surface states might be better passivated. This would lower the degree of surface charge carrier depletion and could hence enhance the conductivity despite the diameter decrease [23; 24]. To test this hypothesis and examine the electronic surface band structure during dissolution more closely, we performed CPD measurements and APS (Figure 3b). The silicon growth substrate densely covered with n-type SiNWs was measured prior to immersion in PBS at 37 \({}^{\circ}\)C and then repeatedly at different time intervals.