Study of Water Adsorption in Poly(N-isopropylacrylamide) Thin Films Using Fluorescence Emission of 3-Hydroxyflavone Probes

• Published in: Macromolecules Vol. 43; no. 22; pp. 9488 - 9494
• Main Authors: Morris, Cheryl, Szczupak, Boguslaw, Klymchenko, Andrey S, Ryder, Alan G
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 23.11.2010
• Subjects: Applied sciences; Biochemistry, Molecular Biology; Cellular Biology; Exact sciences and technology; Life Sciences; Miscellaneous; Organic polymers; Physicochemistry of polymers; Properties and characterization; Emissive probe; Investigation method; Moisture regain; Flavone derivatives; Experimental study; Acrylamide derivative polymer; Fluorescence spectrometry; Thin film
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma102152j

The noncontact measurement of water uptake in microscale (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 °C. 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 nonlinear, 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.