Spectral properties and dynamics of gold nanorods revealed by EMCCD-based spectral phasor method

Abstract

Gold nanorods (NRs) with tunable plasmon-resonant absorption in the near-infrared region have considerable advantages over organic fluorophores as imaging agents. However, the luminescence spectral properties of NRs have not been fully explored at the single particle level in bulk due to lack of proper analytic tools. Here we present a global spectral phasor analysis method which allows investigations of NRs' spectra at single particle level with their statistic behavior and spatial information during imaging. The wide phasor distribution obtained by the spectral phasor analysis indicates spectra of NRs are different from particle to particle. NRs with different spectra can be identified graphically in corresponding spatial images with high spectral resolution. Furthermore, spectral behaviors of NRs under different imaging conditions, e.g. different excitation powers and wavelengths, were carefully examined by our laser-scanning multiphoton microscope with spectral imaging capability. Our results prove that the spectral phasor method is an easy and efficient tool in hyper-spectral imaging analysis to unravel subtle changes of the emission spectrum. Moreover, we applied this method to study the spectral dynamics of NRs during direct optical trapping and by optothermal trapping. Interestingly, spectral shifts were observed in both trapping phenomena.

Self-assisted optothermal trapping of gold nanorods under two-photon excitation

“…However the thermal effect could be restored by the heat converted from absorbed photons via GNRs [11, 12]. Since GNRs are freely diffusing in water, a GNR can be directly trapped at the laser focus for a short period of time [35], e.g. tens of milliseconds, or simply pass over the illumination region.…”

“…As schematically shown in figure 1, the optothermal trapping of GNRs is performed on a confocal laser scanning microscope with the addition of two-photon excitation capability as described previously [35]. As shown in figure 1(A), a NIR beam from a MaiTai HP Ti:Sapphire (Newport, USA) laser is coupled into an inverted Olympus FV1000 confocal system (Olympus, USA).…”

“…The signal is reflected to the internal PMT for intensity imaging or coupled into a 200 μ m fiber for hyperspectral imaging. In hyperspectral imaging analysis, the phasor approach is used to identify components in the image based on spectral changes [35]. …”

“…We used 488 nm Ar + laser and 840 nm Ti:Sapphire for excitation with a 488 nm or a SDM690 dichroic mirror. The emission is also reported to from 400 nm to NIR range [35]. Here an emission bandpass filter of 505–540 nm was used in both trapping modes for collection of luminescence signal.…”

“…Recently we reported that GNRs can undergo thermal trapping with a femtosecond laser at LSPR wavelength [35]. Given that the water absorption of photons at 840 nm is small, GNRs effectively convert absorbed photons into heat and lead to the thermal trapping without using the water absorption window.…”