Engineering Gold Nanorods for Cancer Treatment: Biological Profile, Protein Interactions and Drug Delivery.

Rod-shaped gold nanoparticles, namely gold nanorods (GNRs) have recently attracted a widespread attention due to their unique optical properties and facile synthesis. In fact, GNRs exhibit two distinct localized surface plasmon resonances (SPRs): the transversal mode in the visible region (TSPR) and the longitudinal one in the upper visible or nearinfrared part of the spectrum (LSPR) that correspond to the oscillations perpendicular or parallel to the rod length direction, respectively. In particular, the LSPR can be tailored to a particular wavelength: there is a linear relationship between the absorption maximum of the LSPR band and the mean aspect ratio (AR, ratio between the length and the width of the rod) of GNRs, which can be tuned during the synthesis. Other factors of impact on the frequencies of both TSPR and LSPR bands are the refractive index of the environment and the degree of particle aggregation. In addition, the surface of GNRs can be functionalized with a variety of molecules, providing stability and biocompatibility. All these features make GNRs ideal platforms for several applications and, in particular, for biomedical applications such as sensing, photo-acoustic imaging, photothermal treatment of cancer and drug delivery. However, several problems are connected to the synthesis and purification of GNRs, as well as to their use for biomedical applications. All these issues are addressed within the present PhD thesis, with special attention for GNRs which are well suitable for protein sensing, the photothermal treatment of cancer, and the realization of nanocapsules as drug delivery systems. Specifically, in the Introduction, a brief history of gold nanoparticles and gold nanorods, together with the synthetic procedure with an insight into the growth mechanism of GNRs, their optical properties and their biomedical applications, are addressed. In Chapter 1 the issues connected to the most common synthesis of GNRs (e.g. the difficulty to obtain relatively small GNRs and the precise control of the LSPR) are illustrated. Therefore a simple and reproducible method for the synthesis of small-sized GNRs with a good control over the LSPR is reported. A modified method for the purification of small GNRs from reaction by-products (i.e. spherical and cubic nanoparticles or aggregates) is accurately described in Chapter 2 with particular attention to the quantification of the separation. Chapter 3 is focused on the realization and characterization of functionalized GNRs for therapeutic applications. Although the available literature is inconclusive, particle size may modulate critical parameters such as: the cellular penetration, the intracellular localization, the biodistribution, features that depend on specific surface area, including the rate of interaction with proteins, residual toxicity of contaminants, the ratio between absorption and scattering of the particles and the efficiency and stability of photothermal conversion. Specifically, this chapter offers an extensive survey on the size related biological effects of functionalized GNRs (i.e. PEGylated GNRs), with special attention for the cytotoxicity and cellular uptake of GNRs on a panel of cellular models. In Chapter 4, the possibility to tune the interactions between GNRs with proteins is illustrated by direct monitoring the LSPR of GNRs, which is highly sensitive to changes in the refractive index. In particular, the effect of the insertion of charged groups in PEGylated GNRs on their protein interactions is reported, providing useful hints for protein sensing applications. Finally, in Chapter 5 is focused on the generation of nanosized capsules through self assembling of GNRs to be used for the delivery of hydrophobic anticancer drugs. These GNR-stabilized nanocapsules could provide highly localized release of anticancer therapeutics, maximizing the therapeutic efficacy and minimizing off-target effects. These studies could provide the basis for future pre-clinical animal studies that could lead to an important new therapeutic strategy for tumors.