**4. Bio-Imaging**

Due to the magnificent properties of metal inorganic NPs, they have been directed to plenty of biomedical applications, especially in the bio-imaging field. The size of NPs plays an important role in the bio-imaging process, as NPs are preferred to other small molecules owing to their fast penetration in biological tissues and their ability to pass through the circulatory system. In addition, NPs exhibit limited renal excretion and prolonged blood circulation time, which allows repeated passing of NPs

through tumors' vessels. Unlike organic NPs, inorganic NPs exhibit ine fficient extra vasation inside tumors, which involves inorganic NPs tending to remain in the vasculatures of tumors without being in the interstitial spaces. This reduces the marking of nonspecific tumor cells within the imaging spaces [83–86].

An important approach for improving the imaging process which has been conducted by researchers has been to load one NP with di fferent contrast agents related to several imaging techniques for multimodal imaging which attempts to overcome the limitations related to single imaging techniques [87,88]. Moreover, imaging and treatment can be achieved by the same NP [89,90]. NPs such as semiconductor quantum dots (QDs) have been considered for photoluminescence and have been widely used in bio-imaging [88–90]. In addition, luminescent UCNPs and SERS nanoprobes based on gold and silver NPs can be used for bio-imaging [91–93]. Magnetic NPs with sizes 1–100 nm can display superpara-magnetism meaning they can be widely used as a contrast agen<sup>t</sup> in magnetic resonance imaging (MRI); iron oxide magnetic NPs coated with dextran can also be used in MRI [94,95]. Because of the optical properties of single-walled CNTs, such as high optical absorption and photoluminescence in the near IR region and strong resonance Raman-scattering, single-walled CNTs are widely utilized for bio-imaging [96].

## *4.1. Quantum Dots*

QDs are NPs composed of semiconductor materials or atoms from groups II-IV or III-V, includingcds, cdse, cdte, zns, znse, zno, GaAs, InAs, and InP, owing to their unique optical properties. Each semiconductor material is covered by another semiconductor material, and these have a large spectral bond-gap which allows for an increase in the photo stability and quantum yield for the emission process; QD NPs show stability against an aggregation with capping agents [97]. In addition, these materials have a high molar extinction coe fficient and high absorption from UV to near IR [88], where the changing diameter of NPs can modulate the excitation and emission peaks of QDs, with QDs showing sharp emission peaks that are considered ideal for multi-color imaging [97–99] (Figure 8).

Cai et al. [98] have reported the in vivo integrin αvβ3 imaging of RGD peptide-conjugated QDs. Their results revealed that RGD peptides when conjugated with PEG-ylated QDs demonstrate maximum emission at 705 nm when injected intravenously into mice bearing U87MG tumors. In addition, the results showed a tumor contrast 20 min after injection and reached a maximum 6 h from injection. Another in vivo study conducted by Chen et al. [87] attempted to use optical and polyethylene terephthalate (PET) imaging of VEGFR in vasculature tumors by using QDs, where, they reported that, the amine-functionalized QDs conjugated with VEGF protein and then were exposed to radiation to be radio-labelled for VEGFR-targeted NIR fluorescence and PET imaging of tumor vasculatures. Furthermore, Ostendorpet al. [93] have utilized cyclic Asn-Gly-Arg (cNGR), which was seen to conjugate with paramagnetic QDs (pQDs) as a tumor nanoprobe, where cNGR targeted (the aminopeptidase N) CD13 on the endothelium of a tumor and was used for fluorescence/MRI dual evaluation of tumor activity.

Yong et al. [89] prepared InP-ZnS QDs that were used for in vivo detection of pancreatic cancer because of the low cytotoxicity exhibited by InP-ZnS. These were considered to be biocompatible nanoprobes for diagnosis of pancreatic cancer cells. The results showed that in primary and metastatic pancreatic cells the antigen receptors for anti-claudian 4 were over-expressed and QDs conjugated to anti-claudian 4; in addition, a polyclonal antibody was conjugated to QDs and no damage to the cells was observed. However, Cai et al. [98] have explored PET imaging of radio-labelled QD-RGD, which showed predominant uptake of QDs in the spleen and liver. Another problem related to QDs could be their presence within the body, where long retention time for accumulation of QDs has been observed. There is in vivo toxicity related to the presence of II-IV groups inside the body, but there are strategies to overcome this problem where there is a new generation of QDs that has been developed to reduce toxicity, including Cd-free QDs. Repeated demands of the product development of QDs and their related fields should be considered for their side e ffects and in the long run, substantiated by prototype

modules and pilot-line fabricating, especially in light of arrangemen<sup>t</sup> producers and subsidizing organizations frequently setting their needs in relation to being dependent on these demands.

**Figure 8.** A semiconductor quantum and a quantum dot (QD) aqueous solution under UV light showing bright pink fluorescence. QDs are widely used in fluorescence imaging (adapted from [99]).
