**4. Conclusions**

In this work AgNPs functionalized with hydrophilic capping agents, i.e., Cit and L-cys, were synthesized and characterized by means of UV-Vis, FT-IR, SR-XPS and NEXAFS spectroscopies, confirming the surface functionalization. Their nanodimensions were studied by DLS and TEM analysis, showing diameter less than 10 nm. AgNPs showed high selectivity and sensitivity for Hg<sup>2</sup>+ in water (concentration range:1–10 ppm) respect to 16 different metal ions investigated. The AgNPs-Hg<sup>2</sup>+ system was deeply investigated by means of DLS, ICP-MS, TEM and SR-XPS. Measurements of Ag concentration in fresh and marine aqueous media showed low Ag+ ions release, probably due to the good Cit/L-cys covering, also confirmed by SR-XPS data. Moreover, AgNPs ecosafety was confirmed by ecotoxicity tests which showed no effects on algal growth of both freshwater *R. subcapitata* and

marine *P. tricornutum* algae in the range of tested concentrations (10–500 mg/L). Our results further support the hypothesis that the Cit/L-cys coating of AgNPs prevent dissolution of Ag+ ions in both fresh and saltwater. These results open new ways for AgNPs sensing applications in environmental tests on more complex biological systems, up to tests on real environmental aquatic scenarios.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/10/1353/s1, Figure S1. TEM images of AgNPs; Figure S2. L-Cys FTIR spectrum; Figure S3. Uv-Vis spectra of of AgNPs in presence of 2.5 ppm of metal ions (As<sup>3</sup>+, As5+, Ca2+, Co2+, Mg<sup>2</sup>+, Nd3+, Ni2+, Zn2+, Pb2+) in water; all references curves (AgNPs alone) are in black line, while red curves represent the interaction between AgNPs and metal ions; Table S1. Table XPS. BE, FWHM, Atomic Ratio values and proposed assignments for all measured core-level signals title.

**Author Contributions:** I.V. designed and made the chemical experiments and synthesis; P.P. and L.B. performed sensing tests; I.C., G.P., C.F., and A.B. carried out the AgNPs biological eValuation and TEM, ICP-MS and DLS measurements; G.I. and L.T. performed measurements and the data analysis by ATR-FTIR; G.I. and V.S. performed measurements and the data analysis, by NEXAFS; C.B. performed measurements and the data analysis, by SR-XPS; S.F. provided technical support in SR-XPS measurements. All authors contributed to the paper writing.

**Funding:** This research received no external funding.

**Acknowledgments:** The Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314–337 LEGGE 232/2016), is gratefully acknowledged by authors of Roma Tre University. This research was partially funded by Regione Lazio, through Progetto di ricerca 85-2017-15125, according to L.R.13/08. We gratefully acknowledge the assistance of our colleagues at Elettra for providing good quality synchrotron light. CERIC-ERIC consortium, Grant Agency of Charles University (GAUK project No. 1054217) and Czech Ministry of Education (LM2015057) are acknowledged for the access to experimental facility and financial support.

**Conflicts of Interest:** The authors report no conflict of interest in this work.
