Studying the fluorescence emission properties of 1,2,3 triazine based fluorophores in polymer thin films

Abstract

The application of thin polymeric films for medical purposes is growing very fast. Two of the most important uses of thin polymer films are drug coatings for controlled release and tissue engineering. Thermoresponsive polymers are extensively used in this field [1-3]. These polymer thin films are complex in geometry and water uptake into thin films (during manufacturing or aging) may change their physicochemical properties limiting their application. One challenge for the use of soft thermoresponsive polymers as thin films in regulated medical devices and drug delivery systems is their accurate characterisation on surfaces. A non-contact, non-destructive, method is required for the in situ analysis of polymer thin films. Spectroscopic techniques, both vibrational (i.e. IR, Raman) and electronic excitation (UV-Visible, Fluorescence spectroscopy) have received much attention for the analysis of polymer thin films. Among them fluorescence spectroscopy has received considerable interest due to its high sensitivity and nondestructive nature [4]. The use of fluorescence spectroscopy as a potential tool for the rapid, in situ, and routine analysis of polymer film characterization was investigated in this thesis. In order to justify the use of 1,2,3- triazine (Tr) fluorophores for the development of fluorescence based methods for thickness measurements (of thermoresponsive polymer thin films), we then investigated the photophysical properties of Tr doped poly N- (isopropyl acrylamide), PNIPAm (Tr-PNIPAm) thin films. Three derivatives such as 5- methoxycarbonyl-5-(N-phenylformimidoyl)-2,4,6-triphenyl-2,5-dihydro-1,2,3-triazine (pTr), hydrolysed form of pTr (hTr), and keto form of triazine (kpTr) were used for this study. These Tr fluorophores are hydrophobic and their overall emission is complex and composed of three emitting conformers. They have a large Stokes shift, and their absorption and emission properties are unaffected by changing the nature of the solvents [5-7]. We incorporated these fluorophores into a thermoresponsive polymer (e.g. PNIPAm) and investigated the effect of humidity on emission properties. Here we controlled the polymer environment using a controlled humidity chamber and analysed the emission properties of the fluorophores using both steady-state (S-S) and time-resolved fluorescence (TRF) measurements. The hydrophobic nature of Tr fluorophores governs its photophysical properties. The pTr doped PNIPAm (pTr-PNIPAm) thin film shows excellent stability and was insensitive to changes of microenvironment (i.e. humidity, temperature). The S-S emission properties showed only minor changes and the variation in the lifetime measurement was due to refractive index (RI) changes of the polymer matrix. Most ii significantly, pTr-PNIPAm thin film also showed unique photostability compared to the other triazine fluorophores studies. In addition, a series of Tr fluorophores doped polyvinyl alcohol, PVA (Tr-PVA) thin films were also fabricated and studied to explore the photophysical properties in another hydrophilic polymer. Unfortunately, the presence of anionic impurities in PVA (traces amount of sodium/methyl acetate) effects anion-induced quenching leading to a large decrease in emission intensity.