Synthesis of Gold Nanoparticles Coated with Polystyrene-block-poly(N-isopropylacrylamide) and Their Thermoresponsive Ultraviolet−Visible Absorbance

Abstract

Poly(styrene-b-N-isopropylacrylamide) (PSt-b-PNIPAM) with a narrow molecular weight distribution is prepared by reversible addition−fragmentation transfer radical polymerization. The dithioester group at the chain end of PSt-b-PNIPAM is converted into thiol terminal group by LiB(C2H5)3H. Gold nanoparticles and the PSt-b-PNIPAM interact via the terminal thiol group (-SH). The size of the gold nanoparticles coated with copolymers can be easily manipulated by adjusting the molar ratio of HAuCl4/PSt-b-PNIPAM. Interestingly, the thermosensitive gold nanoparticles exhibit a sharp and reversible transparent−opaque transition between 25 and 40 °C. Moreover, the transition change is sensitive to the size of the gold nanoparticles and the macromolecular weight of the copolymer. The maximum wavelength of the surface plasmon band and the hydrodynamic size of the PSt-b-PNIPAM-Au micelles are sensitive to temperature, pH, and salt concentration. A considerable red shift from 524 to 534 nm in the plasmon band is observed in 0.9% NaCl aqueous solution, but no appreciable change in the band is observed in pure water when the temperature increases from 25 to 40 °C. With a decrease of the pH of the solution, the maximum wavelength of the surface plasmon band exhibits a red shift (from 520 to 534 nm). In addition, dynamic light scatting (DLS) reveals that the hydrodynamic size of coated gold nanoparticles exhibits a small change under alkaline and neutral (pH = 7) conditions, but it gives a pronounced change in the acidic condition (from 350 to 280 nm) when the temperature increases from 25 to 40 °C. Furthermore, the UV−vis absorption spectrum clearly shows that the red shift of the thermosensitive gold nanoparticles is reversible.