## 3 Results and Discussion
### Sensing Measurements  
In a recent work [60], we tested the response of colloidal aqueous solution of AgNCs-PMAA in the presence of several heavy metal ions. Briefly, we have found an enhancement of the PL emission with respect to the reference solution for Zn(II), Cd(II), and Pb(II) with a fixed concentration of 50 \(\upmu\)M, while for the same concentration of many other heavy metal ions, the fluorescence signal was the same as the reference one. The best response in terms of change of fluorescence intensity was found for Pb(II). The same experiment was carried out also in the case of the hybrid matrix containing AgNCs. The results are shown in Figure S5 of Supplementary Materials, highlighting that the selectivity was unchanged with respect to the trials in the liquid phase.  
We focused our investigation to Pb(II) ions since they presented the highest optical response. The sensitivity is related to the AgNCs content; the study of different AgNCs-PMAA contents in the polymeric matrix has shown that the lower the amount of NCs, the higher the optical response in presence of Pb(II). Figure S6 shows the ratio F/F\({}_{0}\) as a function of Pb(II) concentration (0-100 \(\upmu\)M) for different AgNCs densities, where F is the fluorescence intensity of the polluted sample and F\({}_{0}\) is the intensity of the reference sample. In this graph, the linear fittings for each case are also presented. The limit of detection (LOD, 3\(\upsigma\)) for each sample was estimated and reported in the Table S7. The shape of the AgNCs emission for the lowest concentrated sample (0.75 mL) was too weak to be used for sensing tests. Therefore, the sample labelled with \"1 mL\" represents the best compromise among sensitivity and intensity of AgNCs emission. Figure 3a reports the PL spectra of the samples with different Pb(II) concentrations and the emission spectrum of bare AgNCs-PMAA colloidal solution as reference is included in the graph (dotted grey line). The PL intensity of the bare AgNCs-PMAA is arbitrary and not quantitatively comparable with the fluorescence of the hydrogel matrix. Figure 3b shows the F/F\({}_{0}\) ratio as a function of Pb(II) concentration. A linear behavior in the range 0-100 \(\upmu\)M can be observed, while for higher contaminant concentrations, the F/F\({}_{0}\) ratio reaches a plateau where the value is almost constant. This is a clear indication that the centers responsible for the interaction with lead ions, namely the carboxyl groups of PMAA chains, are completely saturated by the polluting cations and an additional amount of lead ions does not produce a change in the fluorescence intensity of the nanoclusters. The limit of detection of doped hydrogel matrix was estimated. The value was 8 \(\upmu\)M that compared to the AgNCs-PMAA colloidal solution was quite high. In that case, we found a low LOD of the order of 60 nM [60]. This behavior can be understood considering that fluorescence signal is a sum of two different fluorescence components. The first one was related to the fluorescence of the nanoclusters that increases with the increasing concentration of the lead ions. The second one was related to the PEGDA matrix itself. The sum of the two signals, producing an inseparable overlap, reduces the sensitivity of the system consistently. Nevertheless, the strategy to insert the sensitive elements in a solid matrix offers many advantages due to a greater stability in terms of handling, portability, and storage and the possibility of in situmeasurements. Moreover, the sensitivity of the solid system to Cd(II) and Zn(II) in addition to Pb(II) make this system potentially suitable to be used as a water filter for the removal of dangerous heavy metal ions. These preliminary results are very encouraging and open a new path towards solid-state optical sensors for Pb(II) ions based on fluorescent AgNCs.