Fluorescence Quenching Kinetics of Py Excimer in PS Films
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Fluorescence Quenching Kinetics of Py Excimer in PS Films Hyun-Sook Jang1, Yu Lei2, Mu-Ping Nieh1, 2 Institute of Materials Science, Polymer program1, Chemical and Biomolecular engineering2, University of Connecticut
ABSTRACT Excimer fluorescence of two-component thin films made of pyrene (Py) and polystyrene (PS) can be quenched by the vapor of nitro-aromatic and nitro-ester explosives with a high selectivity and sensitivity. Normally, an electrospun film can be quenched in minutes by the vapor of the explosives. In order to understand the origin of the mechanism, we have investigated the fluorescence quenching rate of the binary thin films as functions of the molecular weights (MW) of the polystyrene (from 2,500 to 900,000 g/mol) and film thicknesses (110nm and 610 nm) in presence of the vapor of 2,4-dinitrotoluene (2,4-DNT, a type of nitro-explosives). The diffusion coefficients of 2, 4-DNT in the solid films are found nearly independent of MW but have strong dependence on the film thicknesses.
INTRODUCTION Fluorescent conjugated polymers of poly(phenylenethynylene) (PPE) have been employed as a sensor for detecting explosives presumably through its efficient exciton migration along the polymer chains known as “molecular wire theory”1. This method was able to detect the trace amount of 10–100 femtograms level of trinitrotoluene (TNT). It has also been reported that high porous electrospun films composed of Py/PS/TBAPF6 can detect nitro-explosive at the ppb level, indicative of reasonably high sensitivity. This material is much affordable and readily available, thus providing the potential platform for cost-effective sensors2. The function of TBAPF6 in this 3-component electrospun film is to increase the conductivity of the solution and generate nanofibers with good morphology. In fact, it has reported that Py/ PS solution only generates electrospun microfibers with poor morphology2. A possible “sandwich-like” conformation which can facilitate π – π stacking between PS and Py similar to molecular wire theory was proposed for the sensing mechanism of this 3-component system.2 The different structures of polymer were introduced to demonstrate an interaction between Py and PS with similar fluorescent films of nanofibers. Only Py/PS/TBAPF6 film shows a high quenching efficiency in presence of 1
explosives compared to other polymers.2 However, a solid evidence to support this enhanced π – π stacking between Py and PS in the 3-component film is lacking2 and no clear π – π stacking peak was observed in the XRD in our previous study as well.3 Therefore, the validity of molecular wire theory in this 3-component system is controversial. To elucidate this issue, our initial attempt is to eliminate unnecessary factors such as TBAPF6, high-electric field, and high porous morphology from an electrospinning in order to examine an effect of polymer in explosives detections. Previously, we have found that the excimer fluorescence of threecomponent films can be greatly enhanced by annealing in high solvent vapor pressure.3 In t
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