**4. Conclusions**

Strongly hydrophilic gold nanoparticles, AuNPs, were prepared to be conjugated with copper(I) complexes. In particular, loading and release studies were performed using two di fferent copper(I) antitumor complexes, namely [Cu(PTA)4] + [BF4] − (A; PTA = 1,3,5-triaza-7-phosphadamantane) and [HB(pz)3Cu(PCN)] (B; HB(pz)3 = tris(pyrazolyl)borate, PCN = tris(cyanoethyl)phosphane). In the water-soluble compound A, the metal is tetrahedrally arranged in a cationic moiety, while compound B is a mixed-ligand (scorpionate/phosphane), neutral complex insoluble in water. Loading protocols and

efficiency are also related to these structural aspects and were optimized to obtain η = 90 ± 4% and η = 65 ± 10%, respectively, for AuNPs-A and AuNPs-B. Structural differences of A and B induced different behaviours regarding the interactions with the gold surface, as showed by the HR-XPS studies. In fact, for compound A, nitrogen partially transfers electrons to the surface of the metal nanoparticles, creating an interaction that causes a slow release in water, less than 10% in 4 days. On the other hand, in B compound the N ≡ C − R groups hook onto the surface of the gold, producing a strong interaction that makes the release not appreciable in the same time interval (up to 4 days). Therefore, both AuNPs-A and AuNPs-B represent promising examples of water-soluble gold nanocarriers suitable to improve the bioavailability of synthetic drugs, especially considering the EPR effect of AuNPs. In particular AuNPs-A, which achieved a slow release, opens the way for biological in vitro studies to explore the synergic activity of copper complexes and gold nanoparticles.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/5/772/s1, Figure S1: Uv–Vis spectra and calibration curves for complex A (green) and complex B (blue); Figure S2: SEM-EDX analysis on AuNPs-A; Figure S3: ATR data of conjugates systems: (a) AuNPs-A (PTA); (b) AuNPs-B (PCN); Table S1. C1s and P2p spectra data analysis BE, FWHM values and assignments for pristine Cu(I) complexes and AuNPs carriers; Figure S4: XPS P2p spectra confirming the molecular structure stability of A and B complexes; Figure S5: (a) XPS Au4f spectrum of AuNP-A; (b) Cu2p spectra of complex A and AuNP-A (rough data, confirming the stability of Cu(I) complex).

**Author Contributions:** I.F. and I.V. designed and made the chemical experimental synthesis with AuNPs and conjugate systems, and performed the DLS measurements and evaluations with S.A. M.P. (Marina Porchia), F.T. and M.P. (Maura Pellei), C.S. performed synthesis and characterizations of Cu(I) complexes and the relative data analysis. L.C. and C.B. performed the H-XPS measurements and the relative data analysis. S.N. and E.M. provided technical support in SR-XPS measurements. All authors contributed to the paper writing.

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

**Acknowledgments:** Sapienza authors acknowledged Ateneo Sapienza 2017 project. XPS measurements at the ELETTRA facility were supported within project N. 20160181. The Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314–337 LEGGE 232/2016) is gratefully acknowledged by the authors of Roma Tre University. We are grateful to CIRCMSB (Consorzio Interuniversitario di Ricerca in Chimica dei Metalli nei Sistemi Biologici). Images in Figures 2b and 3 were generated with BioRender.com

**Conflicts of Interest:** The authors declare no conflict of interest.
