Coupling of hydrophobic graphene quantum dots with photochromic molecule for fabrication of transparent photo-responsive polymeric films manifesting FRET functioning

Abstract

In this study, a novel sensor on the basis of polymeric film comprising hydrophobic graphene quantum dots (GQDs-Amine) as fluorescent donor and spiropyran (SP) as photochromic acceptor was developed. The polymeric fluorescence sensor was synthesized through a four-step approach, beginning with the synthesize of photochromic component (SP) having hydroxyl functional group. Then, the amine-functionalized GQDs denoted as GQDs-Amine were synthesized through direct carbonization of citric acid in the presence of benzyl amine. GQDs-Amine was then decorated with SP to achieve GQDs-Amine-SP. In the final step, by using either polylactic acid (PLA) or polymethyl meta acrylate (PMMA) as polymeric precursors and via a simple solvent casting methodology, the related polymeric films comprising photoactive elements were prepared. All synthesized compounds were characterized using various methods such as nuclearmagnetic resonance (NMR) spectroscopy, elemental analysis (CHN), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, and field-emission scanning electron microscopy (FE-SEM). According to the related experiments, 0.582 mmol SP was successfully immobilized per gram of GQDs-Amine-SP. Also, after grafting of SP onto the surface of GQDs-Amine, the contact angle remarkably increased from 19° to 64° due to the difference hydrophobic nature between SP and its merocyanine (MC) form. The fluorescence resonance energy transfer (FRET) phenomenon occurred via exposure of GQDs-Amine-SP to alternating cycles of UV–vis irradiations. To confirm the FRET phenomenon and existing a good matching between the UV–vis absorption region of SP and the fluorescence emission spectra of GQDs-Amine, the photoluminescence activity of material was also performed in different mediums as well as in solid state. These solutions were exposed to UV light(365 nm) at various time intervals and their fluorescence properties were investigated under excitation at 400 nm. With increasing the time of UV light exposure up to 220 s, the fluorescence intensity increased at 620 nm, and inversely, the fluorescence intensity decreased at 471 nm. A similar pattern but with higher intensity were also observed for the polymeric films fabricated using phase inversion method. According to the UV–vis and fluorometric analyses, the GQDs-Amine-SP and the corresponding films showed excellent photoreversibility, photostability, and prominentfatigue resistance.