**1. Introduction**

Nanosized inorganic particles, in simple or composite formulation, have unique physical and chemical properties and represent an increasingly important material in the development of novel nanodevices that can be used in numerous fields such as catalysis, energy, optoelectronics, biomedicine, and sensors [1–11]. Several recent achievements show the opportunity of producing new types of nanostructured materials with planned surface and desired physico-chemical properties [12–16]. Among other materials, silver nanoparticles (AgNPs) are deeply studied for their optical and antibacterial properties, easy functionalizations and cheap preparations [17,18].

Nowadays, for the AgNPs use in sensing field, the main goal is the preparation of uniform nanosized particles with precise requirements in terms of size, shape, surface functionalities, and structural features [19–22]. In fact, many studies involved AgNPs as optical sensor, using the localized surface plasmon resonance (LSPR) that is specific feature of the colloidal silver nanoparticle solutions: the energy of the absorption maximum and the shape of the peak are strongly related with the size, shape and interparticle distance, but also with the surrounding environment, which can influence the degradation or aggregation of the particles [23–25]. These optical properties of NPs have allowed researchers to mature new diagnostic methods that are useful for optical and colorimetric measurements [26–28].

In particular, AgNPs have drawn grea<sup>t</sup> attention for mercury sensors development, especially due to their high sensitivity to Hg<sup>2</sup>+ micro-level concentration changes [29,30]. This behavior has been related, both with redox chemistry of Hg<sup>2</sup>+ and silver surface (Ag◦), and both with the soft-soft chemistry between metals and sulfur-containing capping agents used for AgNPs [31]. In both cases, the mercury presence produce important changes in the absorbance intensity and peak position of AgNPs. Surface functionalization has the main role both in stabilization and in reactivity of AgNPs. Therefore, the ligand surface chemistry is used to tune their sensing properties. For example, many ligands, such as *N*-cholyl-Lcysteine [32], citrate [33–35], 6-thioguanine [36], cytosine triphosphate [37], have been used for the surface modifications of AgNPs and AuNPs, and the di fferent functionalizations change the inter and intra-ligand interactions. Obviously, all this produces di fferent properties and capacities of the nanoparticles, that can aggregate or disaggregate and interact with elements and ions present in their surroundings, and it a ffects sensing features towards metals ions, such as Hg<sup>2</sup>+, modifying the selectivity and sensitivity. So far there are several reports available for the colorimetric detection of Hg<sup>2</sup>+ ions using green synthesized unmodified AgNPs in aqueous medium [38]. Frequently used, citrus extracts from lemons and sweet orange fruits have been investigated for green synthesis of AuNPs and AgNPs used as Hg<sup>2</sup>+ colorimetric sensors [39]. There are many bio extracts reported from di fferent plants, fruits, leafs, etc. which have been used for photoinduced green synthesis of AgNPs where a bio extract acts as reducing as well as stabilizing agen<sup>t</sup> and applied in Hg<sup>2</sup>+ detection [38–40].

Nevertheless, the use of NPs for heavy metals removal known as nanoremediation is a matter of concern due to the potential risks associated with the scarcity of information regarding their behavior, fate and impact on the environment and human health [41]. In order to overcome such limitations, an ecosafe approach is developed with the aim to test the ecotoxicity of NPs to selected biota having an important ecological role in aquatic ecosystems, as for instance primary producers such as freshwater and marine microalgae [42,43].

AgNPs are recognized to exert toxic e ffects to biota, both from fresh- and marine waters, from bacteria [44] and microalgae [45–48] to crustacean [49], mussels [50], fishes [51] and mammalian cells [52,53]. Among these studies, those reporting Ag+ release in exposure media [44–48,52,54], report that AgNPs toxicity is often closely linked to the dissolution of the particles and consequent release of Ag ions during exposure. Therefore, based on such eVidence, the ecosafety assessment of AgNPs is mandatory in order to avoid any potential toxicological risks associated with their application in environmental remediation.

In this work AgNPs functionalized with hydrophilic capping agents, i.e., citric acid (Cit) and L-Cysteine (L-cys) were synthesized and characterized by means of UV-Vis, Fourier Transform Infrared (FTIR), Synchrotron radiation-X-ray photoelectron (SR-XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopies. Their nanodimensions were confirmed by Dynamic Light Scattering (DLS) and Transmission Electron Microscope (TEM) analysis. AgNPs were tested as plasmonic sensor for heavy metal detection in water, showing selectivity and sensitivity for Hg<sup>2</sup>+ ions. The AgNPs-Hg<sup>2</sup>+ system was deeply studied by means of UV-Vis, and, SR-XPS spectroscopies, DLS, Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and TEM studies. Moreover, in view of AgNPs application in real water systems, their ecosafety was investigated.
