Nanoparticle-Loaded Thermosensitive Microgel Changes Color

Daniel S. Burgess

A hybrid material whose optical properties change color with temperature may find a place in biomedical applications such as drug delivery. Developed by Haruma Kawaguchi and Daisuke Suzuki of Keio University in Yokohama, Japan, the material comprises metallic nanoparticles in a thermosensitive microgel matrix.

The microgel swells and shrinks in response to changes in temperature, altering the relative distance of metallic nanoparticles suspended in the polymer matrix, the interaction of their surface plasmons and, thus, their optical properties. The difference in color at 25 and 40 °C (left and right in each frame, respectively) is shown for samples loaded with gold nanoparticles (a), gold/silver core/shell nanoparticles (b) and gold/silver/gold multishell nanoparticles (c). Courtesy of Daisuke Suzuki. ©2006, American Chemical Society.

Balls of copolymerized N-isopropylacrylamide and glycidyl methacrylate microgel shrink and swell in response to changes in temperature. At 20 °C, the hydrodynamic diameter of the microgel in an aqueous dispersion is approximately 700 nm, and it is approximately 350 nm at 40 °C. This determines the relative distance between the metallic nanoparticles in the gel and, thus, the interaction of their surface plasmons — optical excitations coupled with oscillations of their conduction electrons — which thereby affects the optical properties of the particles.

To produce the hybrid material, the researchers prepared aminofunctionalized polymer using 2aminoethanethiol so that it would offer sites for the synthesis of 14nm-diameter gold particles that served as seeds for further nanoparticle growth. The introduction of the amino group increased the micro-gel’s hydrodynamic diameter, but it retained its thermal response, varying from approximately 1000 nm at 20 °C to approximately 500 nm at 40 °C.

Using a Hitachi U-2001 UV/VIS spectrophotometer, Kawaguchi and Suzuki recorded absorption spectra of samples with gold/gold, gold/silver or gold/silver/gold nanoparticles for 10 heating/cooling cycles between 25 and 40 °C. The color-change effect continued to be visible to the naked eye following the series of experiments, and the spectra of the core/shell dispersion at 25 °C were essentially unaffected. The spectrum of the multishell dispersion had broadened slightly in the near-infrared, indicating an irreversible aggregation of the nanoparticles.

Suzuki predicted that a potential application may be in targeted drug delivery, by exploiting the relative transparency of human skin and subdermal tissue to near-IR radiation and the switchable absorption of the nanoparticle-loaded microgel at those wavelengths. He said that the researchers next will investigate the response of the hybrid material to stimuli similar to those in a biological system.

Langmuir, online March 14, 2006, doi:10.1021/la052999f.