Quantum Dots for Molecular Imaging and Cancer Medicine

he past few decades have witnessed technicaladvances that have introduced cell biologistsand physicians to a new, dynamic, subcellularworld where genes and gene products can be visualizedto interact in space and time and in health and disease.The accelerating field of molecular imaging has beencritically dependent on indicator probes which showwhen and where genetically or biochemically definedmolecules, signals or processes appear, interact and dis-appear, with high spatial and temporal resolution in liv-ing cells and whole organisms.For example, the use of radionuclide tracers combinedwith 3-dimensional (3-D) imaging systems such asPositron Emission Tomography (PET) and SinglePhoton Emission Computed Tomography (SPECT) arenow helping clinicians to characterize the molecularstatus of tumors deep within patients.Other types of imaging probes rely on the biolumines-cence and fluorescence of genetically encoded proteins(originally found in fireflies and jellyfish, respectively)or entirely synthetic fluorochromes, or a combination ofboth.New powerful biological fluorescence microscopesprovide the ability to study single molecules within sin-gle cells. Multiphoton confocal microscopy has beendeveloped to allow for the capturing of high-resolution,3-D images of living tissues that have been tagged withhighly specific fluorophores.Each of these indicator molecules is designed to maxi-mize sensitivity and specificity and each approach hasits own pros and cons.While effective, the use of radioactive tracers is accom-panied by limitations inherent to their complex fabrica-tion, short half-life due to radioactive decay and issuesrelated to patient/provider safety and radioactive wastedisposal. Furthermore the radioactivity-based methodsare limited by their inability to visualize simultaneoussignals (of more than one marker) and the lack of detec-tion sensitivity towards very small tumor burdens thusconfining its use to macro-scale imaging. Typically,millions of tumor cells are needed to provide a suffi-