Stimulus-responsive Electrospun Nanofibers: Preparation, Structure Analysis and Sensing Applications

Electrospinning technique has been extensively studied because it is inexpensive, facile, and enables nanometer-scaled fibers with controllable structures. Multifunctional stimulus-responsive electrospun (ES) nanofibers could change one or more properties, such as morphology, photoluminescence and wettability upon exposure to external signals such as gas, pH, temperature or metal ions. However, most aforementioned studies were based on solutions or thin films, but not on nanofibers. ES nanofibers with the advantage of high surface-to-volume ratio could achieve high sensitivity toward different stimuli. In this thesis, we design and prepare ES nanofibers with different structures using multifunctional copolymers for various sensory applications, as described in the following. In the first part of this thesis (Chapter 2), multifunctional ES nanofibers were successfully prepared from the poly((N-isopropylacrylamide)-co-(N-hydroxymethyl acrylamide)) (poly(NIPAAm-co-NMA)) blending with 1,2-diaminoanthraquinone (DAQ). The experimental results show that non-porous and uniform P(NIPAAm:NMA) (100:20)/1.5wt% DAQ ES nanofibers exhibit the fastest NO(g) sensing characteristic due to the optimized NMA content for maintaining the fiber morphology in water. On the other hand, a significant on/off switching of the UV-vis absorption spectra on detecting the NO(g) were observed during 25 and 50℃ due to the low critical solution temperature (LCST) characteristic of the NIPAAm moiety. In the second part (Chapter 3), multifunctional ES nanofibers were prepared from random copolymers of poly{2-{2-hydroxyl-4-[5-(acryloxy)hexyloxy]phenyl} benzoxazole}-co-(N-isopropylacrylamide)-co-(stearyl acrylate)} (poly(HPBO-co- NIPAAm-co-SA)) using free-radical polymerization and followed by electrospinning. The moieties of HPBO, NIPAAm, and SA were designed to exhibit zinc ion (Zn2+) and pH sensing, thermoresponsiveness, and physical cross-linking, respectively. The ES nanofibers prepared from the copolymer (1:93:6 composition ratio for HPBO/NIPAAm/SA), showed ultrasensitivity to Zn2+ (as low as 10-8 M) because its photoluminescence emission maximum underwent a blue shift of 75 nm, and the emission intensity was enhanced 2.5-fold after detecting Zn2+ ion. Furthermore, the nanofibers exhibited a substantial volume (or hydrophilic–hydrophobic) change during the heating and cooling cycle between 10 °C and 40 °C. Such a temperature-dependent variation of the prepared nanofibers under a Zn2+ or basic condition led to a distinct on–off switching of photoluminescence. In the third part (Chapter 4), fluorescent ES nanofibers prepared from random copolymers of poly{[9,9-dihexylfluorene-2-bipyridine-7-(4-vinylphenyl)]-co- (N-isopropylacrylamide)-co-(stearylacid)} (poly(FBPY-co-NIPAAm-co-SA) were successfully prepared from the electrospinning technique with a single-capillary spinneret. The smooth nanofibers prepared from the copolymer (FBPY:NIPAAm:SA = 1:93:6) demonstrated superior sensitivity as low as 10-5 M in sensing zinc ions as compared to polymer films (10-3 M) due to the high specific surface area of nanofibers. The porous nanofibers were also manufactured to further enhance sensing performance and got the best sensitivity (10-6 M) among three states when sensing with zinc ions (sensitivity of polymer solution in THF was about 10-5 M). These nanofibers also exhibited an interesting “on/off” switch behavior with decreasing temperature from 40 to 10 ℃ due to the hydrophobic-hydrophilic transition of NIPAAm moiety. In the fourth part (Chapter 5), multifunctional electrospun porous fibers prepared from random copolymers of poly{2-{6-[5-(pyrene-3-yl)pyridine-2-yl] pyridine-3-yl-amino} ethylacrylate-co-(N-isopropylacrylamide)-co-(stearylacrylate)]} (poly(BPYP-co-NIPAAm-co-SA) were successfully prepared from the electrospinning technique with a single-capillary spinneret. The polymer porous fibers prepared from 1:93:6 composition ratios for BPYP/NIPAAm/SA showed a superior performance (10-8 M) in sensing zinc ions as compared to polymer films (10-6 M) and smooth fibers (10-7 M) due to the higher specific surface area than those of films and smooth fibers. Cadmium ion, with the higher formation constant than zinc ion with the BPYP probe, showed the best performance in 10-10 M by porous fibers. These fibers also exhibit an off/on switch behavior as the temperature decreased from 40 to 10 oC due to the hydrophobic-hydrophilic transition of NIPAAm moieties which cause the gradual swelling of fibers. The high surface-to-volume ratio of the above ES nanofibers significantly enhanced their sensitivity compared to that of thin films, which have the potentials in ultrahigh sensory device applications.