# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 1 Introduction
Magnetic nanoparticles have also been combined with the magnetic resonance imaging (MRI) technique and used as contrast agents for tumor visualization [22]. Metallic nanoparticles have been successfully tested as contrast agents for optical or X-ray imaging and allowed for the detection of atherosclerotic plaques, intravascular thrombus, or fibrotic tissue [23]. More recently, theranostic nanoparticles have captured considerable attention as well [24]. Theranostics deals with the development of multifunctional platforms able to perform diagnostics and therapy at the same time, also with the aid of nanoparticles and nanotechnologies (nanotheranostics). Some materials are characterized by the coexistence of specific physical and chemical properties, such as the optical, magnetic or catalytic properties, so that both the imaging and therapeutic effects are combined together within the same nanoparticles. Within this scenario, it is thus possible to prepare nanosized materials able to be injected in the human body, reach the specific target organ and release the therapeutic agent, but also be directly monitored and visualized in the target site. As an example, Au nanoparticles (NPs) have been demonstrated to serve as extremely versatile theranostic nanovectors since they act as an imaging probe and as the therapeutic agent for hyperthermia at the same time [25; 26; 27]. Another successful approach involves the use of magnetic nanoparticles (MNPs) [28; 29; 30] or the combination of MNPs (acting as contrast agents) with mesoporous silica nanoparticles with high surface areas to maximize the drug loading efficiency of the overall theranostic system and allow for a stimuli-responsive release of the therapeutic payload [31; 32]. Other techniques currently being investigated involve photothermal, photodynamic or sonodynamic therapy in combination with other imaging tools such as optical imaging, magnetic resonance imaging and ultrasound imaging [33; 34; 35; 36]. Nanomaterials have also been successfully applied in tissue engineering (TE) as tools able to promote the repair of damaged tissues and organs [37]. Similar to the other nanomedicine applicationspreviously mentioned, the advantage of using nanoparticles in TE derives from their small size and large surface to volume ratio. Among others, these features allow for an easy surface functionalization of the particles with peptides, ligands or proteins, their easy diffusion across membranes and facilitated cellular uptake. Moreover, nanoparticles can show superior biocompatible properties in some cases, while the nanoparticles' size resembles the nanometric scale of components present in the extracellular matrix of tissues. Carbon-based nanomaterials, as well as metal oxide- and silica-based nanoparticles, have been used either alone or else in combination with other polymeric materials for the fabrication of smart composite scaffolds with improved functionalities in terms of their biological, electrical and mechanical properties [38; 39; 40; 41]. The use of metal/metal oxide nanoparticles also led to very promising antibacterial activity, thus regulating bacterial infections often present after surgery to remove/replace organs and tissues [42]. In other cases, the use of ceramic and metal oxide nanoparticles allowed for the induction of the remotely-controlled mechanotransduction of cells, with evident effects on cells proliferation and tissue regeneration rates [43; 44]. The use of magnetic nanoparticles for remote cell patterning was also successfully demonstrated [45]. As expected, a plethora of nanomaterials, both of organic and inorganic derivation, have been successfully explored as smart functional materials in nanomedicine.