Research on fluorescent semiconductor nanocrystals (also known as quantum dots or qdots) has evolved over the past two decades from electronic materials science to biological applications. We review current approaches to the synthesis, solubilization, and functionalization of qdots and their applications to cell and animal biology. Recent examples of their experimental use include the observation of diffusion of individual glycine receptors in living neurons and the identification of lymph nodes in live animals by near-infrared emission during surgery. The new generations of qdots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics.

Background-The current method of analyzing myocardial cell transplantation relies on postmortem histology. We sought to demonstrate the feasibility of monitoring transplanted cell survival in living animals using molecular imaging techniques. Methods and Results-For optical bioluminescence charged-coupled device imaging, rats (nϭ20) underwent intramyocardial injection of embryonic rat H9c2 cardiomyoblasts (3ϫ10 6 to 5ϫ10 5 ) expressing firefly luciferase (Fluc) reporter gene. Cardiac bioluminescence signals were present for more than 2 weeks with 3ϫ10 6 cells: day 1 (627 000Ϯ15%), day 2 (346 100Ϯ21%), day 4 (112 800Ϯ20%), day 8 (78 860Ϯ24%), day 12 (67 780Ϯ12%), and day 16 (62 200Ϯ5% p · s Ϫ1 · cm 2Ϫ1 · sr

The ability to monitor reporter gene expression noninvasively offers significant advantages over current techniques such as postmortem tissue staining or enzyme activity assays. Here we demonstrate a novel method of repetitively tracking in vivo gene expression of firefly luciferase (FL) in skeletal muscles of mice using a cooled charged coupled device (CCD) camera. We first show that the cooled CCD camera provides consistent and reproducible results within +/-8% standard deviation from mean values, and a detection sensitivity (range tested: 1 x 10(4) - 1 x 10(9) plaque form-ing units (pfu)) of 1 x 10(6) pfu of E1-deleted adenovirus expressing FL driven by a cytomegalovirus promoter (Ad-CMV-FL). The duration and magnitude of adenoviral mediated (1 x 10(9) pfu) FL gene expression were then followed over time. FL gene expression in immunocompetent Swiss Webster mice peaks within the first 48 hours, falls by 98% after 20 days, and persists for >150 days. In contrast, FL activity in nude mice remains elevated for >110 days. Finally, transduced Swiss Webster and nude mice were sacrificed to show that the in vivo CCD signals correlate well with in vitro luciferase enzyme assays (r(2)=0.91 and 0.96, respectively). Our findings demonstrate the ability of the cooled CCD camera to sensitively and noninvasively track the location, magnitude, and persistence of FL gene expression. Monitoring of gene therapy studies in small animals may be aided considerably with further extensions of this technique.