SPR response of stimuli-sensitive microgel on sensor chip

Behavior of stimuli-sensitive microgels on SPR (surface plasmon resonance) sensor chip was studied using poly(N-isopropylacrylamide-co-acrylic acid) microgels. The microgels were prepared by aqueous free-radical precipitation copolymerization. They were monodisperse spheres with a diameter less than 200 nm. The dispersions of microgels in different buffer solutions were run over the SPR chip which was covered with a self-assembled monolayer (SAM) having amine functional groups at the top surface. The number of microgels bound to the chip was inversely proportional to the hydrodynamic size of microgel. SPR response of microgels bound to the chip was found to be controlled by two factors: the microgel-inducing change in evanescent wave amplitude and the number of microgels. The former is a function of depth-depending evanescent field strength, refractive index, and volume of swollen microgel. The simulated SPR response based on this function was consistent with experimental response.