# Thermoresponsive Behavior of Magnetic Nanoparticle Complexed pNIPAm-_co_-AAc Microgels
## 3 Results and Discussions  
The responsive hydrogel nanoparticles are prepared by aqueous free-radical precipitation copolymerization of \(N\)-isopropylacrylamide (NIPAm) and acrylic acid (AAc) with the use of the  
Figure 1: TEM image of Zn\({}_{0.4}\)Fe\({}_{2.6}\)O\({}_{4}\) nanoparticles (**left panel**) and silica coated Zn\({}_{0.4}\)Fe\({}_{2.6}\)O\({}_{4}\) nanoparticles (**right panel**). The average sizes of the nanoparticles are 13 nm \(\pm\) 1.1 nm and 21 nm \(\pm\) 1.5 nm, respectively. cross-linker, \(N\),\(N^{\prime}\)-Methylenebisacrylamide (BIS). Microgels are hydrogel particles that are in a range of sub-micron size and with high water content in their polymeric networks. PNIPAm-_co_-AAc microgels are responsive to external stimuli such as temperature and pH, resulting in the decrease or increase of the hydrodynamic radius of microgels, as shown in Figure 2a,b. In a previous study, we report that pNIPAm-_co_-AAc microgels are complexed with poly(allylaminehydrochloride) (PAH) via ion pairing, which shows a decrease or increase of the microgel's hydrodynamic size mainly due to the crosslinking and charge shielding effects [37]. Thus, we hypothesize that pNIPAm-_co_-AAc microgels are able to form the complex structure with amine-functionalized magnetic nanoparticles (aMNPs) in similar mechanisms, which provides an easy and practical method for preparation of hybrid MNP-microgels, as shown in Figure 2c,d, while retaining the thermoresponsivity. Since TEM imaging has been widely used to visualize magnetic nanoparticles, we utilize TEM analysis which displays the image of aMNP incorporated pNIPAm-_co_-AAc microgels, as shown in Figure 3. In addition, zeta potential and DLS values are measured to confirm the aMNP-microgel complexation, which shows the increase of the surface charge and the decrease of hydrodynamic diameter upon addition of cationic aMNP into anionic microgel solution, as shown in Figure 4.  
Figure 3: TEM images of aMNP complexed pNIPAm-_co_-AAc microgel. Low magnification (**left**) and high magnification (**right**). Note that dark circles represent aMNP incorporated to microgels. Figure 2: pNIPAm-_co_-AAc microgel (**a**) and its thermoresponsive behavior (**b**). Amine-functionalized magnetic nanoparticle (aMNP) enhanced microgel via electrostatic coupling (**c**) and its temperature dependent volume change (**d**). To characterize how the pNIPAm-_co_-AAc microgels respond to aMNPs, we measure the hydrodynamic diameter (Dh) of the microgels upon the addition of aMNPs in aqueous solutions, as shown in Figure 5. The use of dynamic light scattering (DLS) allows the determination of the average Dh, which shows the decrease of each Dh for various microgels, regardless of the AAc and BIS content in the microgels. In the case of different AAc content, the microgel size is decreased with positive correlation upon the addition of aMNPs. For instance, the diameter of microgels decreases from 630, 470, and 410 nm to 250, 250, and 300 nm for 20%, 10%, and 5% of AAc, respectively, as shown in Figure 5a. It is reasonable to consider the fact that higher content of AAc yields the larger Dh of microgels mainly due to Coulombic swelling providing room for higher deswelling in which the microgel charge is compensated by positive charge of aMNPs. In contrast, the higher content of BIS shows the smaller Dh of the microgels as measured by 760, 470, and 430 nm, respectively, while the acrylic acid ratio is constant at 10%, as shown in Figure 5b. Their response to addition of aMNPs is inversely correlated with BIS content in the microgels. It is interesting to note that the microgels with 0% BIS dramatically change in the Dh (760 to 350 nm) after initial titration with aMNPs and then keep the size with no further response to aMNPs. This is partially explained by the fact that crosslink free microgels are highly deformable by eternal stimuli [38], resulting in such abrupt change in the hydrodynamic diameter for 0% BIS microgels.