ABSTRACT
Plasmon-resonant gold nanorods are demonstrated as low backscattering albedo contrast agents for optical coherence tomography (OCT). We define the backscattering albedo, a', as the ratio of the backscattering to extinction coefficient. Contrast agents which modify a' within the host tissue phantoms are detected with greater sensitivity by the differential OCT measurement of both a' and extinction. Optimum sensitivity is achieved by maximizing the difference between contrast agents and tissue, |a'(ca) - a'(tiss)|. Low backscattering albedo gold nanorods (14x 44 nm; lambda(max) = 780 nm) within a high backscattering albedo tissue phantom with an uncertainty in concentration of 20% (randomized 2+/-0.4% intralipid) were readily detected at 82 ppm (by weight) in a regime where extinction alone could not discriminate nanorods. The estimated threshold of detection was 30 ppm.
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.
Subject(s)
Diagnostic Imaging/methods , Gold , Luminescent Measurements/methods , Animals , Blood Circulation , Blood Vessels , Ear/blood supply , Female , Fluorescence Polarization , Mice , Mice, Inbred BALB C , Microscopy, Confocal/methods , NanostructuresABSTRACT
The growth of gold nanorods can be arrested at intermediate stages by treatment with Na(2)S, providing greater control over their optical resonances. Nanorods prepared by the seeded reduction of AuCl(4) in aqueous cetyltrimethylammonium bromide solutions in the presence of AgNO(3) typically exhibit a gradual blueshift in longitudinal plasmon resonance, over a period of hours to days. This "optical drift" can be greatly reduced by adding millimolar concentrations of Na(2)S to quench nanorod growth, with an optimized sulfur:metal ratio of 4:1. The sulfide-treated nanorods also experience a marked redshift as a function of Na(2)S concentration to produce stable plasmon resonances well into the near-infrared. Sulfide treatment permitted a time-resolved analysis of nanorod growth by transmission electron microscopy, revealing two distinct periods: an initial growth burst (t < 15 min) that generates dumbbell-shaped nanorods with flared ends and a slower phase (t > 30 min) favoring growth around the midsection, leading to nanorods with the more familiar oblate geometry. The blueshift in plasmon resonance that accompanies the dumbbell-to-oblate shape transition correlates more strongly with changes in the length-to-midsection (L/D(1)) ratio rather than the length-to-end width (L/D(2)) ratio, based on the empirical relationship introduced by El-Sayed and co-workers.
