## 3 Applications of Multimodal Fluorescent-Magnetic Nanoparticles
### Multimodal Bioimaging  
Molecular imaging refers to the study of biological processes at the cellular and/or molecular level. There are different biological imaging techniques such as optical bioluminescence, optical fluorescence, ultrasound imaging, MRI, single-photon-emission computed tomography (SPECT), and positron emission tomography (PET). The important feature in the use of multimodal magnetic nanoparticles is that they can act as probes for imaging applications and also can be loaded with drugs and used as drug carriers for therapeutic applications. MRI is among one of the main in vivo imaging techniques while fluorescence and confocal microscopies are widely used for in vitro studies, which makes these imaging techniques complementary. Figure 11: Synthetic procedure for the preparation of fluorescent CoFe\({}_{2}\)O\({}_{4}\) magnetic nanoparticles. Adapted from ref. [76]. Therefore, fluorescent-magnetic nanomaterials offer a unique opportunity as dual contrast agents that can be used both in fluorescent microscopy and in MRI--thus opening new prospects in medical diagnostics. MRI involves the imaging of protons present in the soft tissues in the human body composed of proteins, water and lipids. Proton spins align under the presence of an applied magnetic field, B0. These protons are perturbed from B0 when a transverse radiofrequency pulse is applied. Subsequently, protons return to their original state through a process of relaxation. These processes are known as T1 (longitudinal relaxation) and T2 (transverse relaxation time) and generate a magnetic resonance image. Difference in relaxations is related to the density or the chemical and physical nature of the tissues under study [83]. The acting mechanism of T2 contrast agents is as follows: both relaxation time, T1 and T2, can be shortened by the use of a magnetic contrast agent. The coating of the nanoparticles can cause a delay in the shortening of the relaxation time but the distance between the protons and the contrast agent has to be minimized. Generally, T1 uses paramagnetic complexes, such as gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) as contrast agents [84; 85]. Magnetic field gradients arise when T2 is shortened due to the difference in the susceptibility between the magnetic material and the medium around them. This gradient causes the diffusion of the protons that leads to a decrease in the relaxation time of the protons. In this case, superparamagnetic nanoparticles are useful as negative contrast agents (T2) [86]. Optical imaging is a complementary tool to magnetic resonance imaging since it provides a better performance in sensibility and spatial resolution for in vitro imaging. However, tissue penetration is limited to few millimetres, and so MRI provides better in vivo images and also provides excellent deep tissue contrast and spatial resolution. Fluorescent magnetic nanoparticles can be used as multimodal agents combining contrast agents and magnetic manipulation. Interesting advances have been achieved in recent years in the development of these types of materials. For example, [PERSON] et al. designed iron oxide nanoparticles of about 10 nm coated with fluorescent mesoporous silica for neutral progenitor cell MRI. The material demonstrated high magnetic resonance sensitivity and excellent cell labelling efficiency (Figure 12) [87]. Iron oxide nanoparticles were engineered with dopamine to modify the surface of the material that was subsequently encapsulated into human serum albumin matrices. This nanosystem was labelled with \({}^{64}\)Cu-DOTA and Cy5.5 and tested in a subcutaneous U87 MG xenograft mouse model. In vivo positron emission tomography (PET)/NIR/magnetic resonance imaging (MRI) tri-modal imaging, and ex vivo analyses and histological examinations were conducted to investigate the in vivo behavior of the nanostructures [88]. Monodisperse P(PEGMA)-grafted magnetic nanoparticles functionalized with the organic dye FITC showed optical activity and good biocompatibility.