In vitro and in vivo two-photon luminescence imaging of single gold nanorods

Abstract

Gold nanorods excited at 830 nm on a far-field laser-scanning microscope produced strong two-photon luminescence (TPL) intensities, with a cos 4 dependence on the incident polarization. The TPL excitation spectrum can be superimposed onto the longitudinal plasmon band, indicating a plasmon-enhanced two-photon absorption cross section. The TPL signal from a single nanorod is 58 times that of the two-photon fluorescence signal from a single rhodamine molecule. The application of gold nanorods as TPL imaging agents is demonstrated by in vivo imaging of single nanorods flowing in mouse ear blood vessels.in vivo imaging ͉ plasmon resonance ͉ multiphoton ͉ nonlinear optics P hotoluminescence from noble metals was first reported in 1969 by Mooradian (1) and later observed as a broad background in surface-enhanced Raman scattering (2). Singlephoton luminescence from metals has been described as a three-step process as follows: (i) excitation of electrons from the d-to the sp-band to generate electron-hole pairs, (ii) scattering of electrons and holes on the picosecond timescale with partial energy transfer to the phonon lattice, and (iii) electron-hole recombination resulting in photon emission (1). Two-photon luminescence (TPL) was characterized by Boyd et al. (3) and is considered to be produced by a similar mechanism as singlephoton luminescence, but the relatively weak TPL signal can be amplified by several orders of magnitude when produced from roughened metal substrates. This amplification is due to a resonant coupling with localized surface plasmons, which are well known to enhance a variety of linear and nonlinear optical properties (4-9).Metal nanoparticles are also capable of photoluminescence, which has been shown to correlate strongly with their welldefined plasmon resonances (10-16). For example, Mohamed et al. (11) have observed that the quantum efficiency of singlephoton luminescence from gold nanorods is enhanced by a factor of Ͼ1 million under plasmon-resonant conditions. Plasmonresonant TPL is attractive for nonlinear optical imaging of biological samples with 3D spatial resolution (17). Gold nanorods are particularly appealing as TPL substrates because their longitudinal plasmon modes are resonant at near-infrared, where the absorption of water and biological molecules are minimized. Moreover, nanorods have larger local field enhancement factors than nanoparticles due to reduced plasmon damping (18). A scanning near-field optical microscopy study of TPL from single nanorods (diameter Ϸ 40 nm) has recently been reported by Imura et al. (16), who observed that the luminescence is greatest at their tips. However, further characterization of TPL from single gold nanorods is needed: the polarization dependence of TPL excitation and emission from nanorods has yet to be defined, as well as the relationship between TPL enhancement and the longitudinal and transverse plasmon modes. These studies can provide a deeper understanding of single-particle TPL and its potential application in nonlinear optical imag...

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