# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 1 Introduction  
Nanomedicine is an emerging branch of science dealing with the use of smart materials for medical applications. Some practical examples include new generation functional materials for physical therapy (hyperthermia, photodynamics and sonodynamics) and advanced drug delivery systems, but also for tissue engineering, contrast agents and in vivo imaging. The integration of smart materials with nanomedicine is challenging as it poses a severe limitation about the corresponding size, which must be comparable to that characterizing most of the biological systems. For this purpose, nanomaterials are the most promising candidates to meet such requirements as the average size lies at the nanometer scale. Moreover, the medical technologies resulting from the use of nanomaterials benefit from this reduced size, as they are considered to be less invasive and possibly implantable in the body. Another advantage is that the physical-chemical properties of nanomaterials can be easily tuned with various methods so that unprecedented functionalities with respect to the bare material can be provided. In this sense, nanomaterials for nanomedicine are conceived as smart, i.e., materials designed for the exploitation of specific functions, even remotelycontrolled and activated by an external stimulus [1]. This specific function can thus be exploited after the injection of the nanomaterial in the human body and only once the site of interest has been reached. Among nanomedicine applications, the efficacy of nanoparticles-based drug delivery systems has been largely demonstrated [2; 3]. With respect to the standard methods, the use of nanoparticles with a high surface area as drug carriers has highly improved the encapsulation efficiency of drugs and a more targeted delivery of the drug to the site of interest is foreseen. Moreover, drug delivery systems based on the use of nanoparticles also show improved pharmacokinetics and biodistribution of the drug [2]. Complex delivery mechanisms have been developed to achieve the controlled release of the drug, including pH-triggered release systems [4] or are driven by an external activation with light/mechanical stimulation [5; 6]. In some cases, nanoparticles have been also investigated as nanoantibiotics due to their promising antimicrobial activity [7] and high potential in fighting antibiotic resistance [8]. More recently, nanoparticles started to be studied on their own as new generation nanodrugs for anticancer therapy [9]. Actually, the conventional treatments based on chemotherapy show several limitations: for example, the lack of selectivity and solubility of the chemotherapeutic agents, which induced undesirable side effects [10]. On the other side, the successful treatment of cancer cells by using nanoparticles can benefit from the following aspects: (i) the high surface area to volume ratio and (ii) the rich surface chemistry, both of which allow the anchoring of specific functional groups and make the functionalized nanoparticles more selective toward selected tumor cells [11; 12]; (iii) the reduced size (5-100 nm), allowing their preferential accumulation at tumor sites that lack an effective lymphatic drainage system [13; 14; 15; 16]; (iv) a spontaneous biodegradation into non-toxic reaction products, thus avoiding the accumulation of toxic elements in the body once the therapeutic action has been completed [17; 18]. Nanoparticles with improved functionalities have been developed for in vivo imaging as well [19]. The reduction of the particle size down to the nanometer scale results in quantum confinement effects such as size-tunable light emission properties (quantum dots, QDs). In this sense, nanoparticles can be used as contrast agents with improved imaging properties. For example, fluorescent nanoparticles based on semiconducting QDs, or polymeric or metallic nanoparticles have been developed to achieve higher contrast imaging than using standard molecular fluorophores [20; 21].