Nanoplasmonics and surface enhanced spectroscopy

Laser-driven phase transformations in PNIPAM copolymer / Au nanoparticles hybrid nanosystem: the role of plasmon heating 
and attractive optical forces
Yeshchenko O.A.1, Naumenko A.P. 1, Kutsevol N.V.2, Harahuts I.I.2
1 Physics Department and  2 Chemistry Department, Taras Shevchenko National University of Kyiv, 64/13 Volodymyrs'ka str., 01601 Kyiv, Ukraine. 
E-mail: yes@univ.kiev.ua

A laser induced structural transformations in dextran grafted-poly (N-isopropylacrylamide) copolymer / Au nanoparticles (D-g-PNIPAM/AuNPs) hybrid nanosystem in water have been observed. The laser induced local plasmonic heating of Au NPs leads to Lower Critical Solution Temperature (LCST) phase transition in D-g-PNIPAM/AuNPs macromolecules accompanied by their shrinking and aggregation. The hysteresis non-reversible character of the structural transformation in D-g-PNIPAM/AuNPs system has been observed at the decrease of laser intensity, i.e. the aggregates remains in solution after the turn-off the laser illumination. Such hehavior has been rationalized as the result of action of attractive optical forces arising due to the excitation of surface plasmons in Au NPs. The attractive plasmonic forces facilitate the formation of the aggregates and counteract their destruction. The laser induced structural transformations have been found to be very sensitive to matching condition of the resonance of the laser light with surface plasmon resonance proving the plasmonic nature of observed phenomena. 

Fig. 1. Extinction spectra of Au NPs in D-g-PNIPAM/AuNPs nanosystem after laser illumination with various wavelengths (laser intensity 425 W/cm2) and without illumination.