## 3 Results  
It has been shown that Si nanomaterials are not stable in an aqueous environment [11; 20]. We verified the assumption that the dissolution reaction at the surface of silicon oxide will become more pronounced with a higher surface to volume ratio by measuring the Si release into the surrounding solution using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Figure 1b). This elemental analysis revealed that a bulk Si surface covered with silicon oxide released a small amount of Si into the surrounding solution. However, when the surface area was increased by decorating the bare silicon surface with SiNWs (Figure 1a), all of which were covered with native oxide of roughly 1.5 nm in thickness [21] (see Figure 2a), the release of Si drastically increased by up to 4.5 folds in the surrounding PBS (Figure 1b, gray triangles compared to dark gray dots). This preliminary experiment supported the notion that SiNWs degraded in a (simulated) physiological environment by a dissolution reaction at their oxide surface. To characterize the dissolution rate quantitatively and elucidate the relevant changes in SiNWs for biochemical sensor applications, we monitored the diameter of single SiNWs employing atomic force microscopy (AFM). We took the height of a SiNW as a representative measure for the diameter, as the height would be least affected by artifacts emerging from the scanning tip geometry compared to lateral measurements. Phosphate buffered saline (PBS) and a temperature of 37 \({}^{\circ}\)C were chosen to simulate typical physiological test conditions. Figure 1: (**a**) SEM image of SiNW densely grown onto a Si wafer piece. Scale bar 1 \(\upmu\)m. (**b**) Si release from a sample covered densely with SiNW (triangles) to the surrounding PBS in the course of immersion at a temperature of at 37 \({}^{\circ}\)C as measured with ICP-AES, compared to a bare silicon wafer piece serving as control (dots). For comparison, the Si content of PBS alone is plotted at Day 0 (square). Transmission electron microscopy (TEM) analysis (Figure 2a) revealed the morphology of the SiNWs before immersion in PBS. The images confirm the single-crystalline structure of the SiNW and the presence of a thin (ca. 2 nm thick) amorphous layer around the nanowire, most likely corresponding to the native oxide layer [21]. Figure 2: (**a**) TEM analysis of a SiNW covered with a native oxide layer prior to immersion in PBS. Scale bar 10 nm. (**b**) Decrease of SiNW height (corresponding to the diameter) as measured with AFM over time during immersion in PBS at 37 \({}^{\circ}\)C. The error bars denote the standard deviation and the dotted line represents a linear fit of the data. (**c–f**) Height scans of a single SiNW after immersion in PBS at 37 \({}^{\circ}\)C for: 3 days (**c**); 14 days (**d**); 35 days (**e**); and 63 days (**f**) as measured with AFM. The p–n junction encoded as a kink is marked with an arrow in sub-figure (**c**). The AFM measurements (Figure 2c-f) showed that it took about 10 weeks until SiNWs with an initial diameter of 75-95 nm were no longer discernible from the surrounding substrate roughness. The height values extracted from the AFM analysis are shown in in Figure 2b. To consider the varying initial diameters of individual SiNWs, the measured height was individually normalized to the respective Day 0 value. Whereas initially the diminutive changes in SiNW height were not discernible at the large variation of height values measured (see Figure S1 in the supporting information), the cumulative changes became apparent after around two weeks. The variation between the measured values was high even for values measured on a single SiNW. This was partly due to the intrinsic variation of the material and the measurement method but might also be attributed to the SiNWs bending upwards away from the substrate surface instead of lying flat. The clearly visible temporary side bending of the SiNW in Figure 2c-f may serve as an indication for this assumption.