2.2.5. Gold Nanoparticles

Gold nanoparticles (GNPs) are defined as a colloid of nanometer-sized particles with better properties than bulk gold. They are produced in different shapes such as spheres [314], rods [315], star-like [316], and cages [317]. GNPs possess unique characteristics, such as easy-to-control, nanoscale size, easy preparation, high surface area, easy functionalization, and excellent biocompatibility, that make them highly suited for many tissue engineering and more in general for biotechnology applications [318,319]. GNPs are definitely superior over other types of nanoparticles in terms of low toxicity and colloidal stability. Furthermore, they present an outstanding physicochemical behavior, which is related to local plasmon resonance phenomena.

Gold nanoparticles were utilized for biosensing [320], bioimaging [321–323], cancer therapy [324], gene delivery to enhance osteogenic differentiation [325], and photo-thermal therapy [229,314,316,323]. Gold nanoparticles were also combined with other materials such as silica (to produce core and shell nanoparticles) [318,323] as well as natural polymers (to improve mechanical properties) and synthetic polymers (to enhance biocompatibility) [318].

Due to their excellent biocompatibility and chemical inertness, gold nanoparticles became the ideal choice for the preparation of scaffolds in many cases [318]. The mission of GNPs in tissue engineering and regenerative medicine is to act as a multimodal tool in order to improve scaffold properties, cell differentiation and intracellular growth factor delivery (Figure 17), while monitoring cellular events in real-time [326].

**Figure 17.** Scheme representing the importance of introducing GNPs in tissue engineering and the regenerative medicine (TERM) realm [326]. (Adapted with permission from Elsevier © 2017).
