Poly (N-Isopropylacrylamide) Microgel-Based Etalons for Optical Sensing

Poly (N-isopropylacrylamide) (pNIPAm) is one of the most completely studied “smart” polymers due to its unique reversible thermoresponsivity.That is, when pNIPAm in water is heated > ~31°C, it transits from a random coil to a globule conformation; this transition is reversed when T < ~31°C. This conformational change is accompanied by water exchange process. When pNIPAm undergoes the coil to globule transition, water is expelled, while water is “sorbed” when the polymer undergoes the opposite process. Researchers have synthesized various structures from pNIPAm, e.g., block [1], random [2], and star [3] polymers, as well as crosslinked hydrogels, and ultimately colloidally stable particles (microgels [4], and nanogels [5]). Of these, microgels are generally defined as cross-linked networks of water soluble polymers. The special network structure leads microgels to swell with water. Since stimuli responsive microgels were discovered, they have emerged as important building blocks for sensing, [6,7] chemical separation, [8,9] and drug delivery. [10,11] Compared with hydrogels, the size of microgels is in the range of tens of nanometers to several micrometers that gives rise to a rapid response to external stimuli. [12] Additionally, pNIPAm microgels can be functionalized with other comonomers, including acrylic acid (AAc), [13-15] 2-hydroxyethyl methacrylate, [16] N-hydroxymethyl acrylamide, [17] and so on. Of these, AAc is commonly used to functionalize pNIPAm microgels to render them pH responsive. Since AAc has a pKa of ~ 4.25, pNIPAm-co-AAc microgels are negatively charged at pH> 4.25, and “neutral” (initiator affords the microgel with a slight negative charge) at pH< 4.25. As a result, pNIPAm-co-AAc microgels are not completely thermoresponsive at pH> 4.25 due to Coulombic repulsions between the deprotonated AAc in the microgel structure. At pH< 4.25 the microgels are fully thermoresponsive. AAc also allows for facile functionalization of the microgels via reaction of amine containing molecules with the AAc on the microgel via carbodiimide coupling [18,19].