**4. Using optical properties in new technologies**

The optical properties of lanthanides are still number one in rare earth (RE) ions applications since the late nineteenth century. In 1964 two scientists discovered possibilities for this elements group so-called red phosphors. The red phosphors were used mainly in the TV screens production process. That technology used a mixture of two metal elements from the sub-shell "*d*" group of the periodic table as europium and yttrium ions. As a result, it became possible to obtain a TV color [29]. These excellent optical properties of RE are still used in several of new technologies such as modern lighting displays, photodynamic therapy, and biodetection. These technologies give an opportunity to develop numerous industry sectors.

The reason of the great array optical properties is an unusual electronic structure on the sub-shell "*d.*" This characteristic electronic structure is related to the existence of an important phenomenon called luminescence. That phenomena depend on three mechanisms of electron transfer: 4f-4f or 5d-4f and charge transfer (CT).

Moreover in the CT mechanism, the easily oxidizing or reducing ions like Ce3+, Pr3+, Tb3+, and Eu, Tm, Yb entail depending the transitions intensity broad bands produces from the symmetry of the surroundings of the RE ion.

RE ions due to base on their 4f-4f or 5d-4f energy transfer and thus the emission of powerful light sources in a wide range [29]. The 5d-4f energy transfer largely depends on the ligands surrounding the RE ions. Hence it becomes more reactive unluckily. However, the 4f-4f transitions are the most commonly used in the range of visible light the lifetime of the luminescence range from tens of microseconds to single milliseconds. In addition, there is the shielding effect of the filled 5s2 and 5p6 shells which contributes to the environment protection. Thus external factors have no influence on them. This makes them much better candidates suited to applications in the field of optics than organic compounds.

In the last years, a large development of nanotechnology in optics is observed. Among the interesting new materials, there are also nanomaterials containing RE ions. Those modern nanomaterials are called witching others: fluorescent nanoparticles, quantum dots, or nanocolloidal metallic nanoparticles. The nanoluminophores and nanoplatforms demonstrated desirable properties and were devised functionally in the last years [30]. The most commonly used RE elements in nanotechnology are: Eu3+, Tb3+, and co-doped phosphors Yb3+-Tm3+, Yb3+-Er3+, or Yb3+-Ho3+.The emission of RE ions covers the entire range from UV (Gd3+) by visible light (Tb3+, Tm3+, Sm3+,) to the NIR range (Yb3+, Er3+, Pr3+ Nd3+). However, two RE ions: La3+ and Lu3+ are unable to give 4f-4f transfer and do not emit light. A lifetime of emission light depends on the size of the crystallites. It was noted that the light emission in phosphors is growing with the reduction increasing of the phosphors crystalline [31]. This could apply e.g. nanomarkers doped with RE. These nanomaterials can actively influence on the cells. The environment of cells can be heating locally and formation of free-radicals at once. That phenomena have a significant role in photodynamic therapy (PDT) and hyperthermia. The most important is using NIR range light to excite the phosphors which allows to less invasive and deep penetrations of cancer cells.

Currently, however, trials are underway to extend the therapeutic indications for extracorporeal analysis as screening test genetic diseases, detection of infections caused by different microorganisms, and the presence of impurities of bacteria in water and food products [32]. Therefore, nanophosphors becoming increasingly fashionable in various ranges of applications. Their unique physical and chemical properties allow for quick and easy applications in a new range of applications notably in medicine range.

Nevertheless, one of the most developed directions is biological applications such as biodetection and bioimaging [33]. The extraordinary spectroscopic properties of RE as stable luminescence, up-conversion ability, and a narrow band of light emission resulted in an increased interest in them. Recent decades have seen the development of research and an increased interest in RE in nanoform in various biological applications. A pie chart **Figure 6** shows all characteristic biological application RE ions which are the most popular in the last years.

Among the widespread type of biodetection and bioimaging are phosphorus markers. These makers give opportunities for a wide range of applications in medicine as drags transfers, photodynamic therapy, in vivo imaging of biochemical reactions in real-time, cancer therapy, or imaging DNA mutations. From a medical viewpoint, these opportunities give wide and new perspectives.

The use of RE in multiplexing also plays an important role. The multiplexing with RE being capable to for the analysis of multiple analytes in one sample. This possibility allows to apply RE in genetic tests or new studies on cancer drugs.

#### **Figure 6.**

*New way of biological applications of RE elements. \*PET, positron emission tomography; \*PDT, photodynamic therapy.*

Interesting results are silica-based or polymeric-based hybrid phosphors. The silica in SiO form and polymeric in PEG form plays a role of ligands. The matrix where RE ions are embedded can be various. The type of matrix usually used in phosphors for biological applications were shown in **Table 2**. As can be observed, all of these list medical applications are based on phosphors or nanophosphors.

Diagnostics and photodynamic therapy in the use of lanthanides are one of the most modern methods of imaging and cancer therapy of PDT. The PDT method is based on the up-convergence mechanism. This process generally depends on the absorption of successive photons (**Figure 7**). For example, the β-NaYF4 matrix doped with Er3+ and Yb3+ is characterized by about 105 times greater efficiency than organic compounds, e.g. rhodamine 6G [39].


#### **Table 2.**

*A selected list of the luminescence materials with the most popular RE ions used in biological experiments [34–38].*

#### **Figure 7.**

*Scheme of PDT mechanism based on nanoluminophores doped with RE–the matrix with nanoparticles doped RE ions is excited by infrared light NIR and the core is emitting visible light VIS. The VIS light exciting photosensitizers molecules and generate reactive oxygen form 1 O2. The cancer cells undergo apoptosis [30].*

On the other hand, biological imaging is one of the most important applications of RE elements in a science experiments, various electronic devices, and medicine. NaYF4:Er3+, Tm3+, Ho3+ nanocrystallite was used for cancer cells imaging [40]. The up-conversion mechanism of RE ions in special matrix was also used. The deep penetration excitation infrared light of cells and absence of photobleaching problem of nanoparticles especially. In this case doping of several RE ions of matrix NaYbF4 gives the opportunity to obtain wavelength ranges with different colors.

However, biological or medical applications are not the only ones where RE phosphors are used. The up-conversion is using also to various sensors production with nanoluminophores doped with RE ions as pH sensors, oxygen sensors, ammonia sensors, or carbon dioxide sensors [41–43]. The most frequently the ytterbium, thulium, or europium ions are used for this purpose. The first designed oxygen sensors NIR light excited was NaYF4:Yb3+, Tm3+.

Silicate phosphor KBaScSi2O7:Ce3+ (KBSS:Ce3+) can emit cyan light with an emission peak at 509 nm under n-UV light excitation (300–400 nm). Those optical properties indicate that KBSS:Ce3+ phosphor is favorable in LED and field emission display applications [44].
