Green fluorescent protein chromophore-based covalent organic polymers (GFPC-COPs): sensing of nitroaromatic organic pollutants and explosives

Abstract

Green fluorescent protein (GFP) is extremely fluorescent in nature, and the chromophore that is responsible for the remarkable fluorescence has been identified as the benzylidene imidazolinone moiety. Emulation of high fluorescence, such as that observed in GFP in synthetic models, is thwarted by rapid bond rotation about CC and C–C bonds in the singlet-excited state. We have rationally designed and developed four covalent organic polymers (COPs) based on two di- and trihydroxy-substituted GFP chromophores, that is, (Z)-4-(4-hydroxybenzylidene)-1-(4-hydroxyphenyl)-2-phenyl-5-imidazolinone (DHI) and (Z)-4-(4-hydroxybenzylidene)-1,2-bis(4-hydroxyphenyl)-5-imidazolinone (THI), to emulate the fluorescence properties of GFP. Of the four polymers, two polymers based on DHI and THI were found to exhibit the highest solid-state fluorescence quantum yields of 83 and 73%, respectively. They are porous and their BET surface areas based on CO₂ sorption studies at 195 K were found to be 107.9 and 164.8 m² g⁻¹. AFM analyses of the two polymeric materials revealed that the polymer based on DHI is comprised of nanosheets, while that based on THI exhibits a nanosphere morphology. It is shown that both polymers can be employed as sensory materials for the selective sensing of nitroaromatic compounds (NACs) containing acidic hydrogens, that is, nitrophenols, nitrothiophenols, and nitroanilines. The quenching efficiencies are found to be more than 80% for these NACs. For example, 4-nitroaniline (4-NA) and picric acid (TNP) are sensed at sub-ppm levels by the suspensions of the GFPC-COPs. Furthermore, the COP based on THI is demonstrated to serve as a mop for rapid adsorption of TNP; the latter is shown to be adsorbed by the COP by more than 50% from its solution in methanol within 10 min. The results exemplify the potential of innovative biomimetic design principles as applied to materials development and sensing technology.