Multiplexed two-photon excitation spectroscopy of single gold nanorods.

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 30 min, demonstrating the absence of thermal reshaping. Spectra featured a signal-to-noise ratio of >10 and a plasmon peak width of typically 30 nm. Changes in the refractive index of the medium of less than 0.04, corresponding to a change in surface plasmon resonance of 8 nm, 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.