**2. Types of nanoparticles**

Modern and advanced synthesis techniques have led to the preparation of a great variety of nanoparticles with different shapes and sizes, together with the use of a great variety of materials. The classification of nanoparticles can be based on different physical and/or chemical parameters. This is a brief summary of the most important characteristics and functions of different types of nanoparticles used in biomedicine, classified based on the materials used in their synthesis.

#### **2.1 Metal nanoparticles**

Metallic nanoparticles have attracted great interest for use in medicine as anticancer agents [18], imaging contrast agents [19], and drug carriers [20]. One of the most exploited properties of these nanoparticles is the increase in molar absorptivity that colloidal dispersions present due to the intensity of their surface plasmon resonance [21], classic examples of this being nanoparticles of metals such as gold, silver or copper. Plasmon resonance can radiate light (Mie scattering), a process that finds great utility in the optical and imaging fields, or it can be rapidly converted to heat (absorption). The latter mechanism can be used to convert metallic nanoparticles into light-activated heat sources for use in medicine in selective laser photothermolysis of cancer cells [22–24]. The properties of the resonance plasmon can be tuned by modifying the size, morphology and nature of the metals used for the synthesis of nanoparticles, thus being able to serve different purposes [18]. Their optical properties and high capacity to catalyze reactions and electron transfer also give them applications as biosensors [21] which, through ingenious modifications, are capable of significantly amplifying signals [25].

Metallic nanoparticles are also of interest as vehicles for the administration of drugs and other active principles due to their high surface-volume ratio, stability, functionality through chemical modifications of their surface and relative harmlessness. For example, Libutti et al. [26] functionalized the surface of 27 nm gold nanoparticles with tumor necrosis factor-α and polyethylene glycol. The nanoparticles managed to passively accumulate in the cancerous tissues avoiding healthy tissues. This allowed the researchers to administer doses of tumor necrosis factor-α that were previously considered toxic. Iron oxide nanoparticles have been approved by the United States Food and Drug Administration (FDA) for the treatment of anemia [27]. Recently, molecular docking studies propose the reuse of these nanoparticles to combat the current global pandemic of SARS-CoV-2 [28]. The studies revealed that both Fe2O3 and Fe3O4 nanoparticles interact effectively with the different proteins and glycoproteins of the virus. These interactions associated with conformational changes in proteins are expected to result in the inactivation of the virus.

However, despite the great boom in metallic nanoparticles due to their long history and simplicity in terms of their synthesis, they present toxicity problems in prolonged use as they cannot be biodegraded [29–31]. In addition, different authors have already expressed their concerns regarding the neurotoxicity of these particles as they are capable of crossing the blood–brain barrier [32, 33].

#### **2.2 Liposomes**

Liposomes are spherical vesicles composed of one or more concentric membranes of lipid bilayers with an internal compartment that normally contains water. Liposomes have the ability to encapsulate both lipophilic molecules in their membrane and hydrophilic in their internal cavity. The size of these vesicles can vary from a few nanometers to several microns. However, liposomes applied for medical use range between 50 and 450 nm [34]. Liposomes were discovered in the 1960s [35] and claim to be the first nanoparticles to be used for the delivery of nanomedicines after Doxil® was approved by the FDA in 1995 [14]. At present, there have been technological advances that have managed to use various natural or synthetic lipids, as well as surfactants to modify the physicochemical properties of liposomes, giving rise to the second and third generation of them [36]. Changes in the physicochemical properties of liposomes influence their interaction with cells, their half-life in circulation, their ability to penetrate tissues, and their final fate in vivo [36]. For example, through the exchange of a phospholipid bilayer in liquid phase for a bilayer in solid phase in liposomes, by incorporating cholesterol (bilayer tightening effect) or sphingomyelin, the retention of the drug loaded in the liposomes increases, delaying the release.

Despite all the hopes for conventional liposomes, they have presented various problems and pharmacological implications over the years. A major drawback of

*Nanoparticles as Drug Delivery Systems DOI: http://dx.doi.org/10.5772/intechopen.100253*

conventional liposomes is their rapid capture by the reticuloendothelial system [37]. Liposomes accumulate mainly in the liver and spleen, due to their abundant blood supply and the abundance of phagocytic cells resident in these tissues [38]. The marked increase in the retention and accumulation of liposomal drugs in these organs may delay the clearance of lipophilic anticancer drugs from the circulation [39]. Furthermore, during chemotherapy, it can lead to partial depletion of macrophages and interfere with important host defense functions in these cell types [40].
