Transient enhancement and spectral narrowing of the photothermal effect of plasmonic nanoparticles under pulsed excitation

Abstract

The transient 100-fold enhancement and spectral narrowing to 2 nm of the photothermal conversion by solid gold nanospheres under near-infrared excitation with a short laser pulse is reported. This non-stationary effect was observed for a wide range of optical fluences starting from 10 mJ cm(-2) for single nanospheres, their ensembles and aggregated clusters in water, in vitro and in vivo.

Figure 1

Spectra of photothermal responses of gold spheres to 70 ps laser pulse (measured through the lifetime of vapor nanobubbles). (A) Individual gold 60 nm spheres at 66 mJ cm⁻² (hollow dots) and 230 mJ cm⁻² (solid dots) in water. (B) Clusters of 20 nm (solid dots) and 60 nm (hollow squares) spheres in water (44 mJ cm⁻²). (C) Water suspensions of gold spheres with diameters of 20 nm (dotted hollow squares), 60 nm (solid dots) and 120 nm (dashed hollow triangles) (60 mJ cm⁻²). (D) Spectra of individual cancer cells in vitro targeted with C225-conjugated 60 nm gold spheres (60 mJ cm⁻²). (E) Spectra of acoustic responses of a tumor (solid dots) and intact tissue (hollow squares) in vivo in a mouse systemically treated with C225-conjugated 60 nm gold spheres (40 mJ cm⁻² ).

Figure 2

The metrics of the photothermal responses of gold spheres as a function of the laser fluence (dashed cross: 532 nm, solid circles: 780 nm, hollow circles: 776 nm). (A) The nanobubble lifetime for individual 60 nm gold spheres in water. (B) The nanobubble lifetime for clusters of 60 nm gold spheres in water. (C) The nanobubble lifetime, for a water suspension of 60 nm gold spheres and the amplitude of the thermal trace (hollow dotted triangles: 532 nm, hollow dotted squares: 780 nm). (D) The amplitude of the acoustic trace for the gold NP-C225 treated mice as a function of the fluence of a single 70 ps laser pulse (cross: 532 nm, solid dot: 780 nm, NP-treated mouse: solid dots and intact mouse: hollow squares (780 nm) and triangles (532 nm)).

Figure 3

The calculated time-course of the temperature of gold 60 nm spheres in water (solid line) during the absorption of a single laser pulse at 780 nm with a Gaussian temporal profile (dashed line) with the fluence of 66 mJ cm⁻², (A) the duration of 70 ps and (B) 400 ps. The horizontal dotted line shows a temperature threshold of NP surface melting. The solid vertical line indicates the potential onset of the transient structure and the dashed vertical line indicates the onset of the vapor nanobubble. Arrows show the temporal window for the transient nanostructure. Transmission Electron Microscopy (TEM) images of 60 nm gold spheres before (C) and after exposure to single laser pulses (70 ps, 66 mJ cm⁻²) at 532 nm (D) and 780 nm (E); (F): comparison of the nanobubble lifetime in water suspensions of 60 nm NPs under excitation at 66 mJ cm⁻² with 70 ps (solid bars) and 400 ps (hollow bars) laser pulses at resonant (532 nm) and off-resonant (780 nm) wavelengths.