Visualizing the Lower Critical Solution Temperature Phase Transition of Individual Poly(Nipam)-Based Hydrogel Particles Using Near-Infrared Multispectral Imaging Microscopy

This manuscript reports the use of near-infrared multispectral imaging (NIR-MSI) microscopy to provide the first direct observation and spectral measurement of individual poly(n-isopropylacrylamide-co-acrylic acid) (NIPAM-co-AAc) hydrogel particles. The high sensitivity and high spatial resolution (similar to 0.9 mu m/pixel) of the NIR-MSI microscope, coupled with its ability to measure images and spectra directly and simultaneously, allows the unprecedented in situ monitoring of the size, morphology, and spectroscopic properties of individual hydrogel particles, which respond strongly to external stimuli (e.g., changes hi temperature and/or pH). Importantly, this novel technique allows, for the first time, the direct measurement of the lower critical solution temperature (LCST) phase transition of individual hydrogel particles rather than that of a collection of hydrogel particles. Furthermore, NIR-MSI measurements reveal that the LCST value is unique for each individual hydrogel particle, depending strongly on particle size, with larger particles exhibiting higher LCST values.