Quantum dots (QDs) are considered to be pseudo-atoms that have highly tunable opto-electronic properties & used in a variety of applications, ranging from electronic devices to in vivo cellular labeling. The ionization energy of a typical solid is determined from the constituent atoms and, in general, cannot be modified. In QD solids, however, the ionization energy, as well as other beneficial opto-electronic properties, can be modified in controlled and rational ways.

Quantum dots can be very flexible chemical systems, and one can tune their ionization energy over a very large range of value.Ligands, are  are organic molecules attached to the surfaces of a QD. Surface modifications is done to render semiconductor quantum dots water soluble and biocompatiable which are suitable for biological applications such as molecular imaging and medical diagnosis.Organic molecules including oligomeric ligands, phospholipids, dendrimers, & amphiphilic polymers have all been used for surface modification.Modifying the interface between a PbS QD and a ligand produces different chemical systems.

Due to small direct band gap and large excitation Bohr radius, lead sulphide quantum dots (PBS) are important semiconductor material. The red shifted emission band, coupled with the small particle size, is facilitating clearance of imaging. Lead sulphide quantum dots bio-conjugates are promising candidates for targeted infrared molecular imaging and future infrared tissue imaging applications.

 QDs contains amino group. This amino group acts as a ligand, which attach to cells. Further modification of the amino group can be done by tagging it with ligands. This modification includes tagging with antigens, lectins, functional groups, receptor binding proteins and other molecules.