**1. Introduction**

Gold nanoparticles (AuNPs) are the most versatile material in nanotechnology, with a huge range of biological and biomedical applications, such as diagnostic, therapeutic and biosensing applications [1–7]. In particular, AuNPs have been often proposed as non-toxic carriers for drug and gene-delivery applications [8–13]. In fact, the specific properties of AuNPs, such as their high surface-to-volume ratio, peculiar optical properties, easy synthesis and versatile surface functionalization, hold pledge in the clinical field for cancer therapeutics [14,15]. Moreover, AuNPs present optical properties, which can be easily tuned to desirable wavelengths according to their shape (e.g., nanoparticles, nanoshells, nanorods, etc.), size (e.g., 1 to 100 nm) and composition (e.g., core/shell or alloy noble metals) [16–20], enabling their imaging and photothermal applications [21–26]. AuNPs can also be easily functionalized

with di fferent moieties, such as antibodies, peptides and/or DNA/RNA to specifically target di fferent cells [10,27,28], and with biocompatible molecules to prolong their in vivo circulation for drug delivery applications [29,30]. Furthermore, it is well known that passive targeting can be achieved by using AuNPs as a carrier, because of their preferential accumulation in tumor cells (enhanced permeability and retention (EPR) e ffect) [21].

In recent years, the biomedical research of new metal-based anticancer drugs alternative to Pt(II) derivatives (cisplatin, oxaliplatin and carboplatin, which are currently utilized in clinical practice) has been focused on complexes including, among other metals, gold, ruthenium, silver and copper [31–36]. The purpose of these studies is to circumvent severe toxicity in non-tumor cells as well as inherited and/or acquired resistance phenomena caused by Pt(II) drugs [37–39]. In particular, among the abovementioned metals, copper is receiving increasing attention [36]. Copper, as an essential micronutrient in mammalians, plays a pivotal role in redox-chemistry, growth and development, and is a key co-factor for the function of several enzymes involved in energy metabolism, respiration and DNA synthesis [40]. In addition, homeostatic mechanisms strictly define the concentration of copper in mammalian cells, which have also developed a physiological active transport mechanism for its internalization based on a trans-membrane protein referred to as human copper transporter 1 (hCtr1) [41,42]. Novel copper-based antitumor agents have been studied according to the view that endogenous metals may be less toxic toward normal cells with respect to cancer cells. Since the generally assessed mechanism of copper cell uptake implies the reduction from copper(II) to copper(I) followed by internalization through transmembrane transporters [43,44], our research has been mainly focused on copper(I) derivatives. The synthetic strategy utilizes ligands with soft donor atoms such as phosphorous in tertiary phosphanes or aromatic sp<sup>2</sup> hybridized nitrogen of pyrazolyl derivatives. Among these compounds, homoleptic, cationic phosphane complexes well match the ability of hCtr1 protein to internalize specifically monovalent ions, thus leading to outstanding cytotoxic e fficiency toward cancer cells in both in vitro and in vivo trials [45–49]. In addition, neutral mixed-ligand complexes containing both scorpionate-like (N-donor) and phosphane ligands showed remarkable cytotoxic activity in in vitro and in vivo tests as well [50]. Despite the promising results, open problems remain, such as the low solubility and bioavailability of some of these compounds and their uncontrolled release. In this preliminary work, hydrophilic AuNPs were synthesized and loaded with either a representative of a water-soluble, cationic complex—[Cu(PTA)4] + [BF4] − (A; PTA = 1,3,5-triaza-7-phosphadamantane)—or a lipophilic, neutral complex—[HB(pz)3Cu(PCN)] (B; HB(pz)3 = tris(pyrazolyl)borate, PCN = tris(cyanoethyl)phosphane)—aiming at the construction of a novel drug delivery system. The use of hydrophilic AuNPs as a vehicle for copper complexes is an innovative and strategic approach to improve the solubility and stability in water of the copper complexes, and consequently to increase their bioavailability. Moreover, these drug delivery systems allow the investigation of a slow and controlled release of copper complexes, opening the way for promising scenarios of in vivo and in vitro experimentation.
