Intrinsic properties of a polyethylene glycol nanofiber are utilized for the first time to dynamically modify the spontaneous emission rate of encapsulated boradiazaindacene dye molecules with fluorescence lifetime imaging. Nanofibers, fabricated by the electrospinning technique, are exposed to relative humidity up to 80%. The spontaneous emission rate of the confined boradiazaindacene is observed to inhibit and enhance the nanofibers' swelling. The Purcell factor is determined to demonstrate an oscillatory behavior resulting from the distinct characteristics of the mode volume, and the step-like increase of the quality factor of the nanofiber cavity due to the changes in the refractive index and fiber diameter. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement