Abstract
A fluorescent naphthalimide conjugate of calix[4]arene (L1 ) has been synthesized and characterized. The selective and efficient detection of trinitrophenol (TNP) by L1 among nine other different nitroaromatic compounds was demonstrated using absorption and fluorescence spectroscopy. The minimum detection limit is 29 nM, which is the lowest reported so far by any conjugate of calixarene toward TNP. The fluorescence quenching is associated with a high Stern-Volmer constant of 3.3 ± 0.4 × 105 M-1. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) data revealed a network structure with pores having a weighted average size of 0.66 ± 0.08 μm for L1 . When incubated with TNP, the pores were filled with fibril structures, as supported by both SEM and TEM data. In order to demonstrate the real time applications, the L1 has been coated onto a Whatman filter paper and the imprint of TNP contaminated thumb has been detected upon physical contact. The 1HNMR titration and the studies carried out using the control molecule support the necessity of both the naphthalimide moiety and the calixarene platform for sensing. In order to mend L1 as a reversible sensor for TNP, the same is incorporated into carrageenan beads (L1 @Cb ) and the reversible sensing has been shown for three cycles by reusing the same material upon recovery followed by washing it. The solid-state detection of TNP has also been demonstrated using the lyophilized L1 @Cb bead powder. The fluorescence intensity of L1 was quenched upon addition of solid TNP to the lyophilized bead powder of L1 @Cb as studied by fluorescence microscopy. The computational studies show that one of the arms of the calixarene takes a bent conformation, and the 1:1 TNP complex of L1 is stabilized by exhibiting differential extents of hydrogen bonding interactions with the two arms owing to their conformational difference. The result of such complexation was already felt through the shifts observed in the experimentally measured 1HNMR spectra.