Fabrication of Patterned Fluorescence Images in Electrospun Microfibers
Bulletin of the Korean Chemical Society (Print)
Recently, generation of patterned fluorescence images based on the "precursor approach" has gained much attention.            The concept of the 'precursor approach' is to use different electronic properties between the protected and unprotected forms. For example, a dye molecule is nonfluorescent when the key functional group of the dye molecule is protected with a protecting group. If the protecting group is removed under photoinduced chemical
... ical transformation, the fluorescence can be regenerated, allowing patterned fluorescent images in the polymer film by selective removal of the protecting group in the exposed areas. The "precursor approach" allows rapid and costeffective generation of patterned images in one step, without the need for additional wet developing processes. We have previously reported that the fluorescent quinizarin molecule can be readily converted to a nonfluorescent protected molecule t-BocQ by introduction of acid-labile tert-butoxycarbonyl (t-Boc) groups to the quinizarin hydroxyl moieties 1 (Scheme 1). 8 When the t-Boc groups were selectively removed by photogenerated acids, yellow fluorescent patterns were appeared in the UV exposed areas. Thus, the nonfluorescent t-BocQ precursor molecule was transformed to unprotected quinizarin by the photoinduced chemical transformation. The vast majorities of the fluorescence patterns generated based on the "precursor approach" have been fabricated in polymer film. During the course toward the development of fiber-based chemosensor systems, 12 we have discovered a new strategy for the fabrication of patterned fluorescence images in microfibers. The key strategy and procedure employed for the fluorescence patterning is schematically presented in Figure 1 . A viscous chloroform solution containing the precursor molecule t-BocQ (1.3 wt%), poly-(methyl methacrylate) (PMMA) (19.6 wt%), and a photoacid generator (PAG), triphenylsulfonium triflate (0.6 wt%) is placed in a syringe. A high voltage (18 kV) is then applied to the metal syringe needle causing ejection of a charged polymer jet from the polymer solution. 13 The microfibers are collected on the surface of a grounded aluminum plate (15 cm from the needle). A scanning electron microscope (SEM) image confirms the formation of microfibers ( Figure 1 ). Photomasked UV irradiation of the electrospun fiber should generate strong acids from the PAG in the exposed areas. The photochemically produced strong acids should promote catalytic deprotection of the t-Boc groups from the precursor molecules, allowing patterned fluorescence images in the polymer fibers. In order to investigate the feasibility of fluorescence patterning, the polymer fiber mats obtained as described above were irradiated with 254 nm UV light through a photomask for 2 min. The UV-treated fiber mats were then placed on a hot plate (100 o C) for 1 min for a post-exposure Scheme 1. Structure of quinizarin and its t-Boc protected form, t-BocQ. Figure 1. Schematic representation of the fabrication of fluorescence patterns in electrospun microfibers (top). SEM image of the electrospun microfibers (bottom).