1. excellent contrast agents for imaging and labels for bioassays.
Quantum dots
Quantum dots are colloidal fluorescent semiconductor nanocrystals (2– 10 nm).
The central core of quantum dots consists of combinations of elements from groups II–VI of the periodic system(CdSe, CdTe, CdS, PbSe, ZnS and ZnSe) or III–V (GaAs, GaN, InP and InAs), which are ‘overcoated’ with a layer of ZnS. Quantum dots are photostable.
They show size- and composition-tuneable emission spectra and high quantum yield.
They are resistant to photo bleaching and show exceptional resistance to photo and chemical degradation.
All these characteristics make quantum dots
excellent contrast agents for imaging and labels for bioassays.

3. Many nanoparticles have been investigated for biomedical applications targeting cancer

4. Examples of nanoparticles (a) Organic nanoparticles, from left to right: liposomes, dendrimers and carbon nanotubes. (b) Inorganic nanoparticles. From left to right: quantum dots, magnetic nanoparticles and gold nanoparticles.

5. Metallic nanoparticles
Metallic MNPs, made of iron, cobalt, or nickel, are often overlooked for biological applications due to their chemical instability.
Readily forming oxides in the presence of water and oxygen, these metallic MNPs are typically protected by coatings, such as gold or silica, to form a core-shell structure.
Despite complex synthesis processes, research continues on these metallic nanoparticles due to the unique advantages some of these MNPs can offer. For example, iron nanoparticles possess relatively high magnetization and are able to maintain superparamagnetism at larger particle sizes compared to their oxide counterparts

6. Gold Nanoparticles
Gold nanoparticles are one type of metallic nanoparticle; others are Ni, and TiO2 nanoparticles.
It has advantages over other metal nanoparticles due to their biocompatibility and non-cytotoxicity.
Nanoparticles are nanometers in size. These are 100 to times smaller than human cells.
The size of gold nanoparticles can be controlled during their synthesis and functionalization with different groups. Gold nanoparticles accumulate in the tumour cells and show optical scattering.