Our group is investigating the use of ZnS-capped CdSe quantum dot (QD) bioconjugates combined with fluorescence
endoscopy for improved early cancer detection in the esophagus, colon and lung. A major challenge in using fluorescent
contrast agents in vivo is to extract the relevant signal from the tissue autofluorescence (AF). Our studies are aimed at
maximizing the QD signal to AF background ratio (SBR) to facilitate detection. This work quantitatively evaluates the
effect of the excitation wavelength on the SBR, using both experimental measurements and mathematical modeling.
Experimental SBR measurements were done by imaging QD solutions placed onto (surface) or embedded in (sub-surface)
ex vivo murine tissue samples (brain, kidney, liver, lung), using a polymethylmethacrylate (PMMA)
microchannel phantom. The results suggest that the maximum contrast is reached when the excitation wavelength is set
at 400±20 &mgr;m for the surface configuration. For the sub-surface configuration, the optimal excitation wavelength varies
with the tissue type and QD emission wavelengths. Our mathematical model, based on an approximation to the
diffusion equation, successfully predicts the optimal excitation wavelength for the surface configuration, but needs
further modifications to be accurate in the sub-surface configuration.