Structurally, nanoparticles also have large surface areas for the attachment of multiple diagnostic (e.g., optical, radioisotopic, or magnetic) and therapeutic (e.g., anticancer) agents. When conjugated with biomolecular affinity ligands, such as antibodies, peptides or small molecules, these nanoparticles can be used to target malignant tumors with high specificity.

The basic rationale is that nanometer-sized particles have functional and structural properties that are not available from either discrete molecules or bulk materials. The development of biocompatible nanoparticles for molecular imaging and targeted therapy is an area of considerable current interest.

We also examine key issues for in vivo imaging and therapy, such as nanoparticle biodistribution, pharmacokinetics, and toxicology. Here we discuss the synthesis and development of state-of-the-art QD probes and their use for molecular and cellular imaging.

When linked with targeting ligands such as antibodies, peptides or small molecules, QDs can be used to target tumor biomarkers as well as tumor vasculatures with high affinity and specificity. QDs also provide a versatile nanoscale scaffold for designing multifunctional nanoparticles with both imaging and therapeutic functions. In comparison with organic dyes and fluorescent proteins, they have unique optical and electronic properties, with size-tunable light emission, superior signal brightness, resistance to photobleaching, and broad absorption spectra for simultaneous excitation of multiple fluorescence colors. Semiconductor quantum dots (QDs) are tiny light-emitting particles on the nanometer scale, and are emerging as a new class of fluorescent labels for biology and medicine.

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