# Molecular Imaging with \({}^{68}\)Ga Radio-Nanomaterials: Shedding Light on Nanoparticles
## 4 Biomedical Applications of \({}^{68}\)Ga-Labeled Nanoparticles
### Nanoparticles for Optical Imaging
#### 4.3.2 Upconverting Nanoparticles  
Rare-earth upconverting nanoparticles (UCNPs) constitute another probe widely used for OI [136; 137]. Most of the conventional fluorescent probes for bioimaging, including QDs, are based on single-photon excitation, emitting low energy fluorescence when excited by high energy light. Consequently, images produced by this type of probes possess low SNR caused by significant auto-fluorescence from biological tissues. Furthermore, due to the high energy excitation light, the penetration depth in biological tissues is poor. UCNPs emerged as an alternative aiming to cast out these limitations. Upconversion luminescence (UCL) is a unique process where low-energy light, usually NIR light, is converted into higher energy light through the sequential absorption of multiple photons or energy transfers [138]. Upon a continuous wave (CW) excitation at 980 nm UCNPs, in particular lanthanide (Ln)-doped UCNPs, exhibit unique UCL properties, such as long lifetimes, high photostability and sharp emission lines that can be multiple when doped with several lanthanides (Figure 8) [139].  
The unique properties provided by UCL makes UCNPs truly advantageous for in vivo luminescence imaging. Firstly, due to the low harm that infrared excitation produces in tissues. Secondly, due to the lack of autofluorescence from biological samples, since they do not produce UCL under CW excitation at 980 nm. Finally, due to the increase in SNR, which produces high imaging contrast. For these reasons, research focusing on in vivo OI with UCNPs has been growing throughout the past years [140]. Furthermore, these UCNPs can be used in photoacoustic imaging, expanding their application field [141].  
In addition to their use in OI, several examples of multimodal UCNPs have been developed. Gd\({}^{3+}\) doped UCNPs have been used for T\({}_{1}\)-MRI [142; 143; 144]. Moreover, the high X-ray attenuation of Lanthanide elements has been exploited for the use of UCNPs as CT contrast agents [145; 146]. Finally, \({}^{18}\)F-labeled UCNPs were synthesized for PET imaging [147].  
Concerning the use of these type of nanoparticles with \({}^{68}\)Ga, [PERSON] et al. [148] labeled UCNPs via DOTA chelator with this isotope and conjugated them to RGD peptide to accomplish targeted visualization of tumor integrin \(\alpha_{\mathrm{v}}\beta_{3}\) expression in M21 tumor bearing mice using RGD peptide as targeting moiety. Amines in the surface of the UCNPs reacted with DOTA-NHS to attach DOTA to UCNPs. RGD functionalization took place using SPDP heterobifunctional crosslinker. Different radiolabeling pH conditions were tried. Highest radiolabeling percentage of UCNPs was obtained at pH 6 (87.5%), however they selected pH 5 as, above pH 6.3, Ga\({}^{3+}\) forms Ga(OH)\({}_{3}\) which is insoluble in water. Radiolabeling yield at pH 5 was 15.2% for the targeted UCNPs. Accumulation at tumor site of \({}^{68}\)Ga-labeled UCNPs was confirmed by PET imaging and ex vivo quantification.