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...
Folate‐conjugated gold nanorods targeted to tumor cell surfaces produced severe membrane damage upon near‐infrared irradiation. Photoinduced injury to the plasma membrane resulted in a rapid increase in intracellular calcium (shown in green) with subsequent disruption of the actin network, featured prominently by the formation of membrane blebs.
SummaryPlasmon-resonant gold nanorods, which have large absorption cross sections at near-infrared (NIR) frequencies, are excellent candidates as multifunctional agents for image-guided therapies based on localized hyperthermia. The controlled modification of the nanorods' surface chemistry is of critical importance, as issues of cell-specific targeting and nonspecific uptake must be addressed prior to clinical evaluation. Nanorods coated with CTAB (a cationic surfactant used in nanorod synthesis) are internalized within hours into KB cells by a nonspecific uptake pathway, whereas the careful removal of CTAB from nanorods functionalized with folate results in their accumulation on the cell surface over the same time interval. In either case, the nanorods render the tumor cells highly susceptible to photothermal damage when irradiated at the nanorods' longitudinal plasmon resonance, generating extensive blebbing of the cell membrane at laser fluences as low as 44 W/cm 2 .
Gold nanorods (NRs) have plasmon-resonant absorption and scattering in the near-infrared (NIR) region, making them attractive probes for in vitro and in vivo imaging. In the cellular environment, NRs can provide scattering contrast for darkfield microscopy, or emit a strong two-photon luminescence due to plasmon-enhanced two-photon absorption. NRs have also been employed in biomedical imaging modalities such as optical coherence tomography or photoacoustic tomography. Careful control over surface chemistry enhances the capacity of NRs as biological imaging agents by enabling cell-specific targeting, and by increasing their dispersion stability and circulation lifetimes. NRs can also efficiently convert optical energy into heat, and inflict localized damage to tumor cells. Laser-induced heating of NRs can disrupt cell membrane integrity and homeostasis, resulting in Ca 2+ influx and the depolymerization of the intracellular actin network. The combination of plasmonresonant optical properties, intense local photothermal effects and robust surface chemistry render gold NRs as promising theragnostic agents.
Theoretical and semiempirical studies of two-dimensional (2D) metal nanoparticle arrays under periodic boundary conditions yield quantitative estimates of their electromagnetic (EM) field factors, revealing a critical relationship between particle size and interparticle spacing. A new theory based on the RLC circuit analogy has been developed to produce analytical values for EM field enhancements within the arrays. Numerical and analytical calculations suggest that the average EM enhancements for Raman scattering (G h) can approach 2 × 10 11 for Ag nanodisks (5 × 10 10 for Au) and 2 × 10 9 for Ag nanosphere arrays (5 × 10 8 for Au). Radiative losses related to retardation or damping effects are less critical to the EM field enhancements from periodic arrays compared to that from other nanostructured metal substrates. These findings suggest a straightforward approach for engineering nanostructured arrays with direct application toward surface-enhanced Raman scattering (SERS).Nanostructured metal-dielectric interfaces often exhibit enhanced optical phenomena at visible and near-infrared (NIR) frequencies via excitation of surface plasmon modes. 1,2 The enticing possibilities of engineering such properties for applications in photonics and chemical sensing have led to a resurgence of activity in the design of plasmonic materials with subwavelength dimensions. 3 Enhanced electromagnetic (EM) field effects can be generated either in a broad spectral range, as is the case for disordered metal-dielectric composites, 2,4 or at select frequencies from periodically ordered metal nanostructures. Periodicity plays a key role in tuning the optical response of the latter, and has been documented in experimental and theoretical investigations of plasmonenhanced effects such as surface-enhanced Raman scattering (SERS), 5-7 extraordinary optical transmission, [8][9][10] and robust photonic band gaps at visible and NIR wavelengths. [11][12][13] SERS has attracted widespread attention because of its demonstrated potential for single-molecule spectroscopy and chemical sensing with high information content. [14][15][16] The rational design of optimized SERS substrates remains a challenging goal, despite extensive efforts to elucidate the physical basis of signal enhancement. Several theoretical studies have described highly localized EM fields at the junction of metal nanostructures, 7,17-19 with local EM enhancement factors G loc ) |E loc (λ)/E 0 (λ)| 4 as high as 10 11 -10 12 for a two-particle system. 20 However, less attention has been paid to the average EM enhancement factors (G h ) 〈G loc 〉), which has greater relevance for the design and optimization of SERS-based chemical sensors. In this regard, García-Vidal and Pendry have provided electrodynamics calculations on periodic nanostructured metal films with G h values on the order of 10 6 , a level of activity commonly observed in many experimental systems. 7 Here we provide numerical calculations and a simple analytical theory for calculating EM field enhancements in...
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