## 3 Gold Nanosystems  
Medical research on gold nanoparticles has been directed towards the study of drug delivery systems, chemotherapeutic agents, detection and diagnostics [33; 34]. Gold nanosystems turned out to be attractive due to their unique properties, mainly dependent on their size and shape [35]. The resulting physical properties of nanoparticles strongly depend on the particle size, density, nature of the protective organic shell and their shape [36]. The quantum size effect occurs when the de Broglie wavelength of the valence electrons has the same dimensional order as the particle itself. Thus, the particles electronically behave like zero-dimensional quantum boxes. Therefore, the electrons are free to move inside these metal boxes with a collective oscillation frequency characteristic of plasma resonance, called the plasmon resonance band (PRB) observed near 530 nm in the 5-20 nm diameter range. The plasmon resonance of nanoparticles is closely related to the size, shape and dielectric properties of the medium surrounding the nanoparticles [37]. By varying the shape of metal nanoparticles, such as nanospheres, nanotubes, nanoprisms or core-shell nanoparticles, their optical properties vary in a quantitative and dependent manner [38]. Gold nanoparticles are used as sensors for the early diagnosis of many diseases [39]. Alzheimer's disease and breast cancer have been targeted for essential early diagnosis [40]. With early diagnosis, current drugs have the ability to postpone the onset of symptoms typical of the disease [41] and are therefore essential for greater treatment efficacy and a higher survival rate [42]. Gold nanoshells have been used as theranostics for the diagnosis and photothermal therapy of breast cancer cells in vitro [40]. Gold nanoshells induce an important photothermal response under illumination of near infrared radiation, showing good potential for cancer therapy, with 100% efficacy in tumor remission [42; 43]. The surfaces of gold nanoshells can link targeting, diagnostic and therapeutic functionalities, forming a multifunctional nanocomplex. This system has also been used in vivo, enriching the near infrared fluorescence and, at the same time, the magnetic resonance imaging capability [44]. A gold nanoparticle delivery system conjugated with gemcitabine and cetuximab as a target agent has been tested in vitro and in vivo for the treatment of pancreatic cancer cells [45]. The results of these tests showed a greater inhibition of tumor growth through a targeted system. The targeted release of multifunctional nanoparticles [46], obtained by conjugating three different peptides, was investigated: an epidermal growth factor receptor (EGFR)-recognizing peptide, an aminoterminal peptide that recognizes the urokinase plasminogen activator receptor and a peptide cyclic that recognizes the integrin receptor, to study the accumulation of gold in tumor models. These experiments did not demonstrate a substantial improvement in tumor uptake compared to control particles in vivo. Instead, gold nanoparticles with a thiolate derivative of _cis_platin have been produced and tested against ovarian cancer cells [47]. The results showed that the gold conjugate with _cis_platin had comparable efficacy to _cis_platin alone, but toxicity to  
Figure 1: Schematic illustration of targeted strategies for cancer therapy by functional Gold Nanoparticles (Au-NPs, NPs = NanoParticles). Malignant tissue is distributed unevenly concerning healthy cells, hence the gold nanoparticles that cross the empty spaces due to the increase in permeability and the retention effect of passive drug release. This figure also illustrates active targeting with direct binding of gold nanoparticles to receptors with specific ligands. healthy cells was almost nothing, unlike the high toxicity of _cis_platin used alone [48]. The decrease in toxicity towards healthy cells is one of the many reasons why therapies with gold nanoparticles can prove superior to the use of drugs alone [49].