We report an amphiphilic perylene diimide (1), a bimolecular analog of l-3,4-dihydroxyphenylalanine (L-DOPA), as a reversible fluorescence switching probe for the detection and sensing of cationic surfactants and Fe(3+)/Cu(2+) in an aqueous media respectively by means of host-guest interactions driven assembly and disassembly of 1. Photophysical studies of 1, going from dimethyl sulfoxide (DMSO) (State-I) to pure aqueous medium (State-II), suggested the formation of self-assembled aggregates by displaying very weak fluorescence emission along with red shifted broad absorption bands. Interestingly, the cationic surfactant cetyltrimethylammonium bromide (CTAB) could disassemble 1 in miceller conditions by restoring bright yellow fluorescence and vibronically well-defined (Franck-Condon progressions A0-0/A0-1 ≈ 1.6) absorption bands of 1 over other neutral and anionic surfactants (State-III). Owing to the metal chelating nature of L-DOPA, 1 was able to sense Fe(3+) and Cu(2+) among a pool of other metal ions by means of fluorescence switching off state, attributed to metal interaction driven assembly of 1 (State-IV). Such metallosupramolecular assemblies were found to reverse back to the fluorescence switching on state using a metal ion chelator, diethylenetriaminepentaacetic acid (DTPA, State-III), further signifying the role of metal ions toward assembly of 1. Formation of assembly and disassembly could be visualized by the diminished and increased yellow emission under green laser light. Further, the assembly-disassembly modulation of 1 has been extensively characterized using infrared (IR), mass spectrometry, microscopy and dynamic light scattering (DLS) techniques. Therefore, modulation of the molecular self-assembly of PDI derivative 1 in aqueous media (assembled state, State-II) by means of host-guest interactions provided by micellar structures of CTAB (disassembled state, State-III), metal ion (Fe(3+) and Cu(2+)) interactions (assembled state, State-IV) and metal ion sequestration using DTPA (disassembled state, State-III) is viewed as a supramolecular reversible fluorescence switching off-on probe for cationic surfactant CTAB and Fe(3+)/Cu(2+).
The synthesis and application of a neutral polymer, poly(1,4-bis-(8-imidazole-octyloxy)-benzene) (PPI), is performed by economical and simple reaction steps. The PPI polymer demonstrates exemplary activity to be used as a film on a TLC plate, or as a membrane by blending it with a desired polymer or in a solution phase to detect fluoride anions from contaminated water in the presence of competing anions at ppb levels easily and rapidly. This polymer PPI works on the simple displacement principle where fluorescence turn-on/turn-off are observed as signals. On selectively binding Cu 2+ it displays extraordinary fluorescence quenching, resulting in >97% reduction in the fluorescence intensity. This effect could be visualized in solution phase, on a TLC plate and on a blended polymer membrane. Furthermore, the fluorescence of this PPI-Cu 2+ assay showed 81% enhancement on selectively binding F À anions in contaminated water in the presence of other competing anions with higher positive free energies of hydration. Polymeric systems with such robust fluorescence dequenching activity are novel, providing a unique platform for detection and possible removal of fluoride anions. To validate this potential, two experiments were performed: (a) preparation of a film on a TLC plate and (b) preparation of a membrane by mixing 1% PPI in polystyrene and casting as a membrane film of desired shape and thickness. Our results confirm that PPI-Cu 2+ films and membranes described above have the highest specific activity to sense fluoride in a competitive environment, observed by the unique enhancements in fluorescence intensities at varying and extremely low quantities of 1 ppm, 10 ppm and 50 ppm of fluoride. The detection limit of fluoride in contaminated water for the TLC plate and membrane methods was very low and was in the range of 2.5-10.0 ppb. We have further used these methods for the detection of fluoride in natural ground water samples and ascertained the percentage of fluoride.
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