Abstract
Plasmonic metallic nanoparticles are commonly used in (bio-)sensing applications because their localized surface plasmon resonance is highly sensitive to changes in the environment. Although optical detection of scattered light from single particles provides a straightforward means of detection, the two-photon luminescence (TPL) of single gold nanorods (GNRs) has the potential to increase the sensitivity due to the large anti-Stokes shift and the non-linear excitation mechanism. However, two-photon microscopy and spectroscopy are restricted in bandwidth and have been limited by the thermal stability of GNRs. Here, we used a scanning multi-focal microscope to simultaneously measure the two-photon excitation spectra of hundreds of individual GNRs with sub-nanometer accuracy. By keeping the excitation power under the melting threshold, we show that GNRs were stable in intensity and spectrum for more than 30min, demonstrating the absence of thermal reshaping. Spectra featured a signal-to-noise ratio of >10 and a plasmon peak width of typically 30nm. Changes in the refractive index of the medium of less than 0.04, corresponding to a change in surface plasmon resonance of 8nm, could be readily measured and over longer periods. We used this enhanced spectral sensitivity to measure the presence of neutravidin, exploring the potential of TPL spectroscopy of single GNRs for enhanced plasmonic sensing.
Highlights
The unique optical properties Gold nanorods (GNRs) have found multiple applications in research and industry
To identify and characterize the Two-photon luminescence (TPL) signal of single GNRs, we first compared TPL imaging with scanning electron microscopy (SEM) images
Differences between the orientation as obtained by TPL and from the SEM images were within 10 degrees and may originate from optical aberrations in the excitation and/or imaging path
Summary
The unique optical properties Gold nanorods (GNRs) have found multiple applications in research and industry. Mohamed et al later observed that the luminescence of gold could be enhanced by >106 when exciting rod-shaped gold nanoparticles in resonance with their surface plasmon wavelength, typically in the near-infrared (NIR) part of the spectrum, increasing the quantum yield to ~10-4 2. This field enhancement yields a signal intensity similar to that of quantum dots[3], being bright enough for straightforward detection of individual particles. In combination with the reduced absorbance and scattering of NIR light in vivo, GNRs form attractive labels for in vivo imaging[3,4], but GNRs are used in cancer therapy, fluorescence enhancement and bio-sensing[5,6,7,8,9,10,11,12,13]
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