**Bifunctionalized Silver Nanoparticles as Hg 2** + **Plasmonic Sensor in Water: Synthesis, Characterizations, and Ecosafety**

**Paolo Prosposito 1,2, Luca Burratti 1, Arianna Bellingeri 3, Giuseppe Protano 3, Claudia Faleri 4, Ilaria Corsi 3, Chiara Battocchio 5, Giovanna Iucci 5, Luca Tortora 5,6, Valeria Secchi 5, Stefano Franchi 7 and Iole Venditti 5,\***


Received: 20 August 2019; Accepted: 16 September 2019; Published: 20 September 2019

**Abstract:** In this work, hydrophilic silver nanoparticles (AgNPs), bifunctionalized with citrate (Cit) and L-cysteine (L-cys), were synthesized. The typical local surface plasmon resonance (LSPR) at λ max = 400 nm together with Dynamic Light Scattering (DLS) measurements (<2R H> = 8 ± 1 nm) and TEM studies (Ø = 5 ± 2 nm) confirmed the system nanodimension and the stability in water. Molecular and electronic structures of AgNPs were investigated by FTIR, SR-XPS, and NEXAFS techniques. We tested the system as plasmonic sensor in water with 16 di fferent metal ions, finding sensitivity to Hg<sup>2</sup>+ in the range 1–10 ppm. After this first screening, the molecular and electronic structure of the AgNPs-Hg<sup>2</sup>+ conjugated system was deeply investigated by SR-XPS. Moreover, in view of AgNPs application as sensors in real water systems, environmental safety assessment (ecosafety) was performed by using standardized ecotoxicity bioassay as algal growth inhibition tests (OECD 201, ISO 10253:2006), coupled with determination of Ag+ release from the nanoparticles in fresh and marine aqueous exposure media, by means of ICP-MS. These latest studies confirmed low toxicity and low Ag+ release. Therefore, these ecosafe AgNPs demonstrate a grea<sup>t</sup> potential in selective detection of environmental Hg<sup>2</sup>+, which may attract a grea<sup>t</sup> interest for several biological research fields.

**Keywords:** silver nanoparticles; Hg<sup>2</sup>+ sensors; heavy metal sensing; plasmonic sensors; optical sensors; ecosafety
