Jinmyoung Joo^†, Xiangyou Liu^‡, Venkata Ramana Kotamraju^‡, Erkki Ruoslahti ^‡§, Yoonkey Nam^∥, and Michael J. Sailor ^*†

^† Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, mc 0358, La Jolla, California 92093, United States

^‡ Cancer Research Center, Sanford-Burnham Medical Research Institute, La Jolla, California 92037, United States

^§ Center for Nanomedicine and Department of Cell, Molecular and Developmental Biology, University of California, Santa Barbara, Santa Barbara, California 93106-9610, United States

^∥ Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea

^*Correspondence to Michael J. Sailor

Abstract

The luminescence lifetime of nanocrystalline silicon is typically on the order of microseconds, significantly longer than the nanosecond lifetimes exhibited by fluorescent molecules naturally present in cells and tissues. Time-gated imaging, where the image is acquired at a time after termination of an excitation pulse, allows discrimination of a silicon nanoparticle probe from these endogenous signals. Because of the microsecond time scale for silicon emission, time-gated imaging is relatively simple to implement for this biocompatible and nontoxic probe. Here a time-gated system with ∼10 ns resolution is described, using an intensified CCD camera and pulsed LED or laser excitation sources. The method is demonstrated by tracking the fate of mesoporous silicon nanoparticles containing the tumor-targeting peptide iRGD, administered by retro-orbital injection into live mice. Imaging of such systemically administered nanoparticles in vivo is particularly challenging because of the low concentration of probe in the targeted tissues and relatively high background signals from tissue autofluorescence. Contrast improvements of >100-fold (relative to steady-state imaging) is demonstrated in the targeted tissues.

Keywords:  time-gated luminescence imaging; bioimaging; intravital imaging; targeting peptides; tumor; cancer; in vivo imaging; iRGD; porous silicon