# BODIPY-Based Nanomaterials--Sensing and Biomedical Applications
## 1 Introduction
### Metallic Nanoparticles in Medicine  
Among numerous types of nanomaterials, metallic nanoparticles are one of the most interesting ones. The widely spread method of obtaining metal-based nanoparticles is the chemical reduction of the metal compound (e.g., chloroauric acid or silver nitrate) via a proper reducing agent (e.g., sodium citrate or sodium borohydride), in some cases also used as a stabilizer allowing the prevention of unwelcome agglomeration [13]. The diameter of metallic nanoparticles can be easily tuned as reaction parameters change, including time of reaction, temperature, amount of precursor, and reducing agent. Nowadays, the synthesis of nanoparticles also involves green chemistry methods to reduce the usage of environmentally harmful reagents [13; 14]. In this way, some reducing agents such as green sources are used, e.g., lactic acid, citrus fruits, coffee seeds, etc. In addition, sonochemical, microwave-assisted, and electrochemical methods are also involved in the production of metallic nanoparticles [15].  
The unique properties of metallic nanoparticles enable their use for various purposes in nanomedicine. One of the most important fields is tumor imaging, where metallic nanoparticles form a class of new contrast and tracking agents [16]. Due to their structures, they are suitable for detection, diagnosis, and treatment of various tumor lesions. Metallic nanoparticles can also be a part of the theranostic nanoconcept as a combination of both diagnosis and treatment [16]. Utilized as drug delivery carriers, metallic nanoparticles are solid colloidal particles with diameters from 10 to 1000 nm [17]. There are several possibilities to bind a therapeutic agent with its nanocar, such as (i) encapsulation within a polymer shell or a structure, (ii) dispersion in a polymer carrier matrix, and (iii) covalent attachment or adsorption to the particle surface [18]. Due to site-specificity, prevention of multidrug resistance, and effective delivery of therapeutic agents (based on targeting ligands), metallic nanoparticles are able to increase the therapeutic index of specified drugs and are now being investigated for cancer treatment. The increased surface-area-to-volume ratio of some metallic nanoparticles allows chemical modification of their surface, which results in an increased cellular uptake, protection of the therapeutic agents, and in some cases even an improvement of the bioavailability of the anticancer agents [19]. Nanocarriers, both alone and with modified surfaces, have many interesting features. They reveal protective effects towards various drug molecules in different compartments, which can increase their stability and minimize eradication. Nanometer size range carriers can also participate in the enhanced permeability and retention (EPR) effect, which is important for medical applications. What is more, they can also be involved in multiple drug deliveries and various combined therapies, e.g., by utilizing chemotherapeutic and photothermal effects (Figure 3) [20].  
Figure 3: Features of nanocarriers for free drug administration [20].