# Thermoresponsive Behavior of Magnetic Nanoparticle Complexed pNIPAm-_co_-AAc Microgels
## 3 Results and Discussions
The microgels with 2% and 5% BIS also decrease in the size upon addition of aMNPs and their reactivity is lower than that of 0% BIS microgels. Figure 4: The zeta potential value and the hydrodynamic size for 10% AAc (acrylic acid), 2% BIS (\(N\),\(N^{\prime}\)-Methylenebisacrylamide) microgels upon 0.6, 1.2, 1.8, and 2.4 \(\upmu\)g of aMNP additions. Note that zeta potential values of aMNP and microgels were measured as 19 mV and \(-\)18 mV, respectively. Figure 5: Hydrodynamic diameter of 5% (\(\square\)), 10% (\(\Circle\)), and 20% (\(\triangle\)) AAc (**a**) or 0% (\(\square\)), 2% (\(\Circle\)), and 5% (\(\triangle\)) BIS microgels (**b**) upon the addition of 0.6 \(\upmu\)g of aMNP aliquots. The hydrodynamic diameter of microgels measured by dynamic light scattering (DLS) at 25 \({}^{\circ}\)C.  
Thermoresponsivity of the microgels is evaluated by measuring the Dh at 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C five times, repetitively. The microgels with various AAc content (5%, 10%, 20%) show the decreased Dh at 50 \({}^{\circ}\)C, the temperature above the LCST, and their size is recovered by cooling at 25 \({}^{\circ}\)C. Such thermoresponsivity is consistently observed for the remaining cycles for all microgels, as shown in Figure 6a, and the Dh changes increase with the AAc content of microgels. We then test the effect of aMNP complexation with microgels on their thermoresponsivity, which illustrates repeated Dh changes in the cycles of temperature, as shown in Figure 6b. It is worthwhile to note that the thermoresponsivity of aMNP-microgels remains for the all cycles, while their initial size is smaller than those of the uncomplexed microgels. This finding is informative when the stimuli-responsive microgels are tailored for future applications. For instance, to design more sensitive microgels for nanoparticle binding, the higher AAc composition would be beneficial. This also suggests that electrostatic complexation of microgels with aMNP is an efficient route to prepare thermoresponsive MNP-microgels for various applications. Similarly, thermoresponsivity of the microgels with various BIS content (0%, 2%, 5%) is evaluated in the same temperature cycle mode, as shown in Figure 6c. The higher BIS content in microgels, the smaller the Dh at 25 \({}^{\circ}\)C, resulting in the lesser change in the Dh at 50 \({}^{\circ}\)C. The effects of BIS on the microgel thermoresponsivity are mimicking those of AAc, but in an inversed manner. Such trends are also measured in the case of aMNP-microgels, as shown in Figure 6d. Again, this finding suggests that the microgels with higher responsivity to nanoparticles are prepared with the lower BIS content. Thus, we confirm that the nanoparticle-microgel complexation is suitable to form advanced nanomaterials with the characteristic thermoresponsivity in a repeatable manner. To characterize the thermoresponse behavior of microgels, we also measure the Dh of microgels at 1 \({}^{\circ}\)C intervals from 25 to 55 \({}^{\circ}\)C, which allows for the determination of the volume phase transition temperature (VPTT), as shown in Figure 7. Although the microgels with various AAc begin to undergo transition in their size at \(\sim\)31 \({}^{\circ}\)C, the ending temperature of the phase transition differs by 10 \({}^{\circ}\)C. As a result, the VPTTs of microgels are increased to 36, 37, and 41 \({}^{\circ}\)C for 5%, 10%, and 20% of AAc, respectively, as shown in Figure 7a and Table 1. The complexation of aMNPs with microgels slightly alters their thermoresponsivity by increasing VPTT and beginning/ending transition temperature with 2\(\sim\)3 \({}^{\circ}\)C, respectively, with the exception of the 5% AAc, as shown in Figure 7b. It is interesting to note that the initial size of aMNP-microgels is smaller than that of microgels alone but their deswelling yields similar size to the microgels.