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The purpose of this paper is to investigate the dynamics of the fluorescence mechanism of boradiazaindacene (BODIPY) dye molecules, which are covalently bound to a polyethylene glycol based hydrogel structure with different concentrations, using a picosecond time-resolved spectroscopic technique. Since the hydrogel structure is capable of absorbing a large amount of water, without dissolving and without losing its shape, upon swelling, the distance between the BODIPY azide dyes is controllably changed; it is observed that the intensity weighted fluorescence lifetime for the highly concentrated donor dye molecules embedded in the hydrogel cluster network changes from 2.03 to 7.14 ns. Calculations based on our experimental results suggest that the fluorescence dynamics of the BODIPY azide dye molecules confined within the hydrogel network obeys the Forster resonance energy transfer (FRET) rather than self (or contact) quenching. If the hydrogel is dry, in which the distance between donors and acceptors is minimum, the energy transfer efficiency is found to be about 72%, and the distance between the two dye molecules is calculated to be 4.59 nm. Such a close placement causes a significant reduction in the fluorescence intensity due to a strong dipole-dipole interaction of the dye molecules. As the separation increases upon hydrogel swelling, the FRET efficiency reduces to 2%, which corresponds to a separation of 10 nm between two BODIPY dyes and hence a considerable increase in the level of fluorescence intensity. For the dilute hydrogel samples, the distance between the dye molecules is larger than the critical Forster distance. Therefore, the energy transfer efficiency for this type of dilute samples is found to be much lower.

Humidity induced change in the refractive index and thickness of the polyethylene glycol (PEG) coatings are in situ investigated for a range from 10 to 95%, using an optical waveguide spectroscopic technique. It is experimentally demonstrated that, upon humidity change, the optical and swelling characteristics of the PEG coatings can be employed to build a plastic fibre optic humidity sensor. The sensing mechanism is based on the humidity induced change in the refractive index of the PEG film, which is directly coated onto a polished segment of a plastic optical fibre with dipcoating method. It is observed that PEG, which is a highly hydrophilic material, shows no monotonic linear response to humidity but gives different characteristics for various ranges of humidity levels both in index of refraction and in thickness. It undergoes a physical phase change from a semi-crystalline structure to a gel one at around 80% relative humidity. At this phase change point, a drastic decrease occurs in the index of refraction as well as a drastic increase in the swelling of the PEG film. In addition, PEG coatings are hydrogenated in a vacuum chamber. It is observed that the hydrogen has a preventing effect on the humidity induced phase change in PEG coatings. Finally, the possibility of using PEG coatings in construction of a real plastic fibre optic humidity sensor is discussed.

Green fluorescent protein (GFP) molecules are attached to titanium dioxide and cadmium oxide nanoparticles via sol-gel method and fluorescence dynamics of such a protein-metal oxide assembly is investigated with a conventional time correlated single photon counting technique. As compared to free fluorescent protein molecules, time-resolved experiments show that the fluorescence lifetime of GFP molecules bound to these metal oxide nanoparticles gets shortened dramatically. Such a decrease in the lifetime is measured to be 22 and 43 percent for cadmium oxide and titanium dioxide respectively, which is due to photoinduced electron transfer mechanism caused by the interaction of GFP molecules (donor) and metal oxide nanoparticles (acceptor). Our results yield electron transferrates of 3.139 x 108 sand 1.182 x 108 s_ 1 from the GFP molecules to titanium dioxide and cadmium oxide nanoparticles, respectively. The electron transfer rates show a marked decrease with increasing driving force energy. This effect represents a clear example of the Marcus inverted region electron transfer process

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