Two-photon Fluorescence Measurement of a Targeted Nanodevice Using a Sensitive Double Clad Optical Fiber

Abstract

[[abstract]]Fluorescence quantification in tissues using conventional techniques is problematic owing to the absorption and scattering of light in the tissues. Whole body fluorescence imaging techniques do not provide accurate quantitative information on the distribution of a fluorescently tagged molecule in tissues. Owing to the limited tissue penetration of light, these methods also lack sensitivity for detection of low concentrations of tissue fluorescence. Previously we have developed a two-photon optical fiber fluorescence (TPOFF) probe as a minimally invasive technique for quantifying fluorescence in solid tumors in live mice in a real-time basis (Thomas et al, Proceedings of the SPIE, 2006, Vol 6095). In those studies we have used a single mode optical fiber (SMF) through which femtosecond laser pulses were delivered into the tumor, which enabled us to measure low micromolar concentrations of targeted fluorescent nanoparticles. It is essential that a more sensitive TPOFF device is developed for quantification of lower levels of a targeted fluorescent agent. Here we demonstrate the biological application of a double-clad optical fiber (DCF) that can keep high excitation rate by propagating ultrashort laser pulses down an inner single mode core, while improving the collection efficiency by using a high-NA multimode outer core confined with a second clad. The DCF does not have a hole which prevents the capillary suction of biological fluids which is a problem for the biological application of a previously described Double Clad Photonic Crystal Fiber (DCPCF, Ye, et al, Proceedings of the SPIE, 2005. 5700: p. 23-27). The solid DCF used has a numeric aperture of 0.46, which is smaller than that of a DCPCF. Although it does not achieve as high a collection efficiency as the DCPCF, it provides significant improvement over traditional single-clad fibers. We have compared the two-photon fluorescence detection efficiency of using the DCF vs. SMF with standard solutions of the dye 6-TAMRA (6T), and the generation 5 dendrimer (G5) nanoparticles G5-6T and G5-6T-Folic acid (G5-6T-FA). We have observed about 6-fold increase in the detection efficiency of these fluorescent agents. We then compared the targeting of G5-6T-FA in FA receptor (FAR)-expressing cells in vitro, and finally demonstrated the applicability of the DCF fiber to quantify the in vivo targeted uptake of G5-6T-FA in mice tumor expressing FAR. In summary, the DCF-TPOFF probe is an appropriate tool to quantify nanomolar levels of a targeted nanoparticle in deep tissue in vivo