Study of water adsorption in Poly(N-isopropylacrylamide) thin films using fluorescence emission of 3-hydroxyflavone probes

Published Citation: Study of water adsorption in Poly(N-isopropylacrylamide) thin films using fluorescence emission of 3-hydroxyflavone probes. Note: This is the final accepted version that includes corrections but may not include all the proofing corrections. The final definitive version of the manuscript is available on the Macromolecules website. Abstract: The non-contact, measurement of water uptake in micro-scale (1-100 µm), thermoresponsive Poly(N-isopropylacrylamide) thin films is challenging. We assessed the efficacy of three different 3-Hydroxyflavone (3-HF) based fluorophores; to monitor water uptake in pNIPAM thin films close to the Lower Critical Solution Temperature (LCST) at 25 and 37 ºC. These 3-HF fluorophores undergo excited state intramolecular proton transfer, yielding emission from normal (N*) and tautomeric (T*) excited-state forms. The emission intensity ratio, log(I N* /I T*) and N* band position are environmentally sensitive. Water adsorption in pNIPAM thin films follows a non-linear, two phase process: at low relative humidity, the adsorbed water disrupts polymer-fluorophore hydrogen bonding, yielding small changes in log(I N* /I T*) ratios, and overall intensity, at higher relative humidity, these intensity parameters (but not fluorescence lifetime) change dramatically, indicating a larger change in local polarity. These probes are thus sensitive indicators of water uptake in pNIPAM.