Laser Flash Photolysis of Au-PNIPAM Core–Shell Nanoparticles: Dynamics of\nthe Shell Response

Abstract

Hydrophobic forces play a key role\nin the processes of collapse\nand reswelling of thermoresponsive polymers. However, little is known\nabout the dynamics of these processes. Here, thermoresponsive poly­(<i>N</i>-isopropylacrylamide)-encapsulated gold nanoparticles (Au-PNIPAM)\nare heated via nanosecond laser flash photolysis. Photothermal heating\nvia excitation of the localized surface plasmon resonance of the Au\nnanoparticle cores results in rapid PNIPAM shell collapse within the\n10 ns pulse width of the laser. Remarkably, reswelling of the polymer\nshell takes place in less than 100 ns. A clear pump fluence threshold\nfor the collapse of the PNIPAM shell is demonstrated, below which\ncollapse is not observed. Reswelling takes longer at higher laser\nintensities.