sensitive detection of photons in vivo. NIR-QD also may be a superior alternative because they can circumvent the problem of auto-ﬂuorescence which is often associated with optical imaging of naturally-occurring compounds in animal tissue. Thus, these QDs have tremendous potential for in vivo imaging, and have already been used in various in vivo studies, including lymphatic mapping in animal models.34,35 Dendrimers Dendrimers are a unique class of repeatedly branched poly- meric macromolecules with numerous arms extending from a center, resulting in a nearly-perfect three-dimensional geo- metric pattern. Dendrimers can be synthesized via two major strategies: a) divergent methods and b) convergent methods, which differ in their direction of synthesis; either outward from the core or inwardly toward the core, respectively. Divergent methods were ﬁrst introduced by Tomalia36 in the 1980’s, where his group synthesized three-dimensional polyamindoamine (PAMAM) dendrimers by the growth of branches extending radially from a core site to the periph- ery.37 Furthermore, PAMAM dendrimers contain tertiary amines and amide linkages which allow for the binding of numerous targeting and guest molecules. Hawker and Frechet are well known for having established the convergent strategies for the synthesis of dendrimers.38 In convergent methods, dendrimer surfaces are ﬁrst synthesized by gradually linking surface unit monomers together. When the growing surface wedges are large enough, several are attached to a suitable core to give a complete dendrimer. Polypropylenemine (PPI) and polyaryl ethers dendrimers are synthesized by using this convergent method. Dendrimers have three components: an initiator core, branches, and terminal functional groups. The initiator core Core molecule Surface molecule Nanoparticles and cancer therapy is in the heart of the molecule, and branches extend outward from it. The monomers attached to the core (G0), are called ﬁrst generation monomers (G1) and two second generation monomers (G2) are attached to the each ﬁrst generation mono- mers. Successive generations will form in this same manner, being two monomers attached to the monomer from the previous generation (Figure 1). The molecular weight of the dendrimer nearly doubles with each additional generation.39 Furthermore, terminal groups can be modiﬁed to obtain both a charged, and hydrophilic or lipophilic function for the desired biological and drug delivery application.40 The advantage of dendrimers is that they can be synthe- sized and designed for speciﬁc applications. They are ideal drug delivery systems due to their feasible topology, func- tionality and dimensions; and also, their size is very close to various important biological polymers and assemblies such as DNA and proteins which are physiologically ideal.41 Many of the properties of dendrimers include:42 1. Nanoscale sizes that have similar dimensions to important bio-building blocks, eg, proteins, DNA. 2. Numbers of terminal surface groups (Z) suitable for bio- conjugation of drugs, signaling groups, targeting moieties or biocompatibility groups. 3. Surfaces that may be designed with functional groups to augment or resist trans-cellular, epithelial or vascular biopermeability. 4. An interior void space may be used to encapsulate small molecule drugs, metals, or imaging moieties. Encapsu- lating in that void space reduces the drug toxicity and facilitates controlled release. 5. Positive biocompatibility patterns that are associated with lower generation anionic or neutral polar terminal surface groups as compared to higher generation neutral apolar and cationic surface groups. Generation: Diameter: G0 1.4 nm G1 1.9 nm G2 2.6 nm G3 3.6 nm Figure 1 Graphical representation of dendrimers from core to generation G = 4, showing the linear increase in diameter. International Journal of Nanomedicine 2009:4 G4 4.4 nm 3