Au Nanoshell Enhanced Fluorophores for Fluorescence Optical Tomography
R. Bardhan, N.K. Grady, A. Joshi, M. Bartels, S. Ke, N.J. Halas
Rice University, US
Keywords: optical tomography, nanoshells, ICG
Abstract:Noble metal nanoparticles exhibit remarkably intense optical properties due to excitation of plasmons by incident light, which gives rise to many interesting applications including plasmon enhanced fluorescence of molecular species. Indocyanine green (ICG), the only FDA-approved near-infrared-emitting fluorophore, is extensively used in clinical settings for biomedical imaging. However, ICG is a relatively weak fluorophore with a quantum yield of only 1.3% in aqueous media. Here we examine the fluorescence enhancement of ICG molecules as a function of distance from the surface of Au nanoshells. The distance between ICG molecules and Au-NS surface is controlled by growing silica epilayers of varying thickness (Au-NS@SiO2). A maximum fluorescence enhancement of 50 fold is achieved at a distance of ~ 7 nm from the Au-NS surface, and even for the thickest silica layer fabricated on the Au-NS (~ 42 nm) a 7 fold enhancement is still observed. This approach can not only provide a technique for the quantitative measurement of the nanoscale spatial extent of the fluorophore – nanoparticle interaction but also be utilized for biomedical imaging such as in fluorescence optical tomography (FOT), a powerful non-invasive molecular imaging modality. The Au-NS@SiO2 conjugated with ICG nanoparticle solutions filled in 3.5 µL glass bulbs were implanted in mouse carcass and FOT was employed to detect fluorescence signal at significant tissue depths. Momentous fluorescence emission was achieved even at 8 mm tissue depth which would not be possible with free ICG due to low quantum yield as well as photobleaching. This makes these nanoshell based probes well suited for early detection of tumors and metastasis.