Gold Nanoparticles Grafted with PLL--PNIPAM: Interplay on Thermal/pH Dual-Response and Optical Properties.

Narrowly distributed poly(l-lysine---isopropylacrylamide) (PLL--PNIPAM) was prepared through ring-opening polymerization of ε-benzyloxycarbonyl-l-lysine -carboxy-α-amino anhydride and atom transfer radical polymerization of NIPAM, followed with the removal of ε-benzyloxycarbonyl group. Then gold nanoparticles (AuNPs) grafted with PLL--PNIPAM (PNIPAM-PLL-AuNPs) were obtained by the reduction of chloroauric acid with sodium citrate in the presence of PLL--PNIPAM. PNIPAM-PLL-AuNPs and its precursors were thoroughly characterized by proton magnetic resonance spectroscope, Fourier transform infrared spectroscope, UV-vis spectroscope, transmission electron microscopy, dynamic light scattering, thermogravimetric analysis, and circular dichroism. The obtained PNIPAM-PLL-AuNPs exhibited high colloid stability even at strong alkaline (pH = 12) and acidic (pH = 2) conditions. The thermal and pH dual-responsive behaviors of the grafting PLL--PNIPAM chains was observed to be affected by AuNPs, while not for the secondary structure of PLL chains. Correspondingly, the surface plasmon resonance (SPR) of AuNPs was found to be sensitive to both pH value and temperature. A blue shift in the SPR happened both with increasing pH value and increasing temperature. The stimuli-response was reversible in heating-cooling cycles. The gold nanoparticles with both pH and temperature response may have potential applications in biomedical areas and biosensors.