Surface coatings of stimuli-responsive polymer brushes that are chemically grafted to the substrate have received much attention due to numerous applications in material and life science. The surface-grafted brushes can be employed as templates for the immobilization of gold nanoparticles, thus creating composite materials with interesting optical properties that can be used for colorimetric sensing. Color changes are induced by employing stimuli-responsive polymers that undergo reversible swelling-shrinking transitions upon variations of outer stimuli (1,2). As the stimuli-induced color change of the composites depends on the particle loading and interparticle distance, it is crucial to control the assembly of the immobilized nanoparticles. Ways to control the assembly of nanoparticles in polymer brushes include variations of brush grafting density (3), brush thickness (4), polymer-particle interaction strength, particle size, and particle surface modification (1). Here we employed UV/vis spectroscopy, neutron reflectometry, scanning force microscopy and ellipsometry to study the assembly of gold nanoparticles in stimuli-responsive poly(N-isoacrylamide) brushes. We found that the particle assembly and temperature-induced color change depended on the particle surface functionalization. Whereas citrate-coated nanoparticles penetrated into the PNIPAM brushes and formed a 3D assembly, nanoparticles coated with 12-mercaptododecanoic acid molecules remained near the brush surface, thus forming a 2D assembly (1). Furthermore, we investigated the color change of the citrate-coated particles/PNIPAM composites in presence of different salts and found that the salt-induced color change depended on the type of salt.
References:
(1) Christau, S.; Möller, T.; Brose, F.; Genzer, J.; Soltwedel, O.; von Klitzing, R. Effect of gold nanoparticle hydrophobicity on thermally induced color change of PNIPAM brush/gold nanoparticle hybrids, Polymer, 2016, doi: 10.1016/j.polymer.2016.03.088
(2) Kesal, D.; Christau, S.; Krause, P.; Moeller, T.; von Klitzing, R. Uptake of pH-Sensitive Gold Nanoparticles in Strong Polyelectrolyte Brushes, Polymer, 2016, 8, 134; doi:10.3390/polym8040134
(3) Christau, S.; Möller, T., Yenice, Z.; Genzer, J.; von Klitzing, R. Brush/gold nanoparticle hybrids: Effect of grafting density on particle uptake and distribution within weak polyelectrolyte brushes. Langmuir, 2014, 30, 13033-13041.
(4) Christau, S.; Thurandt, S.; Yenice, Z.; von Klitzing, R. Stimuli-Responsive Polyelectrolyte Brushes as a Matrix for the Attachment of Gold Nanoparticles: The Effect of Brush Thickness on Particle Distribution. Polymers, 2014, 6, 1877.
