*3.3. Evaluation of Signal Uniformity and Stability*

Apart from sensitivity, signal uniformity is another important aspect for assessing the performance of SERS substrates. In this experiment, 100 μg L−<sup>1</sup> R6G was still used as the testing solution. As shown in Figure 3a, by measuring 16 randomly distributed points on the optimized substrate, signal intensity at 1358 cm−<sup>1</sup> was analyzed, and the result showed the prepared substrate provided uniform enhancement of the Raman signal. The RSD was calculated as 5.33% (see Figure 3b), which is comparable with previous studies. In practical application, a SERS substrate with longer time stability has extra advantage. To evaluate the stability of our AgNPs-PDMS substrate, 100 μg L−<sup>1</sup> R6G solution was dropped and dried. After recording the SERS spectrum, it was stored in a nitrogen environment and remeasured every 5 days. From Figure 3c,d, we can tell that the substrate showed relatively good stability, with 80% SERS activity preserved in the period of one month. Even though a decrease of SERS activity was observed, considering the easy to oxidize nature of Ag, the stability of this substrate is still acceptable [54]. In addition, benefitting from the easy to fabricate procedure and low cost, it is easy to resupply the substrate in large quantities.

**Figure 3.** SERS spectra (**a**) and distribution of signal intensity (**b**) at 1358 cm−<sup>1</sup> of R6G (100 μg L<sup>−</sup>1) collected from 16 random points on optimized AgNPs−PDMS substrate. The solid line in b represents the average SERS intensity, and the dotted lines indicate the RSD. SERS spectra (**c**) and the ratio of intensities (**d**) at 1358 cm−<sup>1</sup> of R6G (100 μg L−1) acquired on optimized AgNPs-PDMS sub-strate stored in nitrogen atmosphere between 0 and 30 days.
