Visualizing the effect of gold nanocages on absorption, imaging, and lower critical solution temperature phase transition of individual poly(NiPAM)-based hydrogel particles by near infrared multispectral imaging microscopy.

We have successfully utilized the near-infrared multispectral imaging (NIR-MSI) microscope to observe and measure directly images and spectra of individual hydrogel particles alone or with added gold nanocages (GNs). The NIR-MSI is suited for this task because it can simultaneously record spectral and spatial information of a sample with high sensitivity (single pixel resolution) and high spatial resolution (∼0.9 μm/pixel). Because both images and spectra of the individual particles can be directly and simultaneously measured by the microscope, it is possible to detect any changes in the spectroscopic properties and/or nature (size, volume) of individual hydrogel particles induced by external factors (e.g., temperature and/or pH). These features make it possible to determine lower critical solution temperature (LCST) values based on monitoring either changes in the NIR spectra or the volume of the hydrogel particle in response to variations in temperature. More importantly, the measured volume transition temperature or LCST value is not of a collection of many hydrogel particles, but rather of individual hydrogel particles. GNs were found to significantly affect not only absorption but also images and properties of individual hydrogel particles. Specifically, GNs were found to enhance absorption of individual hydrogel particles, particularly the C-H band at 1716 nm, by about 25%. Of particular interest is the fact that not all individual hydrogel particles were enhanced by GNs; only about 50% of total number of particles were enhanced by GNs. GNs were also found to make it difficult to observe individual hydrogel particles, i.e., it seems that GNs defocused images of hydrogel particles. The defocusing effect by GNs might be due to photothermal generation of heat and vapor bubbles by the GNs. Of particular interest is the effect of GNs on the volume transition temperature of individual hydrogel particles. It seems that individual hydrogel particles lose their LCST in the presence of GNs, i.e., when heated, they undergo a gradual decrease in the volume but do not exhibit any clear and observable discontinued phase transition temperature.