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. 2017 Feb 1;8(2):1535-1546.
doi: 10.1039/c6sc04367d. Epub 2016 Oct 25.

A supramolecular Tröger's base derived coordination zinc polymer for fluorescent sensing of phenolic-nitroaromatic explosives in water

Affiliations

A supramolecular Tröger's base derived coordination zinc polymer for fluorescent sensing of phenolic-nitroaromatic explosives in water

Sankarasekaran Shanmugaraju et al. Chem Sci. .

Abstract

A V-Shaped 4-amino-1,8-napthalimide derived tetracarboxylic acid linker (L; bis-[N-(1,3-benzenedicarboxylic acid)]-9,18-methano-1,8-naphthalimide-[b,f][1,5]diazocine) comprising the Tröger's base (TB) structural motif was rationally designed and synthesised to access a nitrogen-rich fluorescent supramolecular coordination polymer. By adopting the straight forward precipitation method, a new luminescent nanoscale Zn(ii) coordination polymer (TB-Zn-CP) was synthesized in quantitative yield using Zn(OAc)2·2H2O and tetraacid linker L (1 : 0.5) in DMF at room temperature. The phase-purity of as-synthesised TB-Zn-CP was confirmed by X-ray powder diffraction analysis, infra-red spectroscopy, and elemental analysis. Thermogravimetric analysis suggests that TB-Zn-CP is thermally stable up to 330 °C and the morphological features of TB-Zn-CP was analysed by SEM and AFM techniques. The N2 adsorption isotherm of thermally activated TB-Zn-CP at 77 K revealed a type-II reversible adsorption isotherm and the calculated Brunauer-Emmett-Teller (BET) surface area was found to be 72 m2 g-1. Furthermore, TB-Zn-CP displayed an excellent CO2 uptake capacity of 76 mg g-1 at 273 K and good adsorption selectivity for CO2 over N2 and H2. The aqueous suspension of as-synthesized TB-Zn-CP showed strong green fluorescence (λmax = 520 nm) characteristics due to the internal-charge transfer (ICT) transition and was used as a fluorescent sensor for the discriminative sensing of nitroaromatic explosives. The aqueous suspension of TB-Zn-CP showed the largest quenching responses with high selectivity for phenolic-nitroaromatics (4-NP, 2,4-DNP and PA) even in the concurrent presence of other potentially competing nitroaromatic analytes. The fluorescence titration studies also provide evidence that TB-Zn-CP detects picric acid as low as the parts per billion (26.3 ppb) range. Furthermore, the observed fluorescence quenching responses of TB-Zn-CP towards picric acid were highly reversible. The highly selective fluorescence quenching responses including the reversible detection efficiency make the nanoscale coordination polymer TB-Zn-CP a potential material for the discriminative fluorescent sensing of nitroaromatic explosives.

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Figures

Scheme 1
Scheme 1. Synthesis of the bis-naphthalimide derived Tröger’s base tetracarboxylic acid linker (L) from commercially available 4-nitro-1,8-naphthalic anhydride, that was then used to generate the Zn(ii) coordination polymer (TB-Zn-CP).
Fig. 1
Fig. 1. (A) Scanning electron microscopy and atomic force microscopy images (B) of as-synthesized TB-Zn-CP. (C) Dynamic light scattering (DLS) measurements of TB-Zn-CP dispersed in water.
Fig. 2
Fig. 2. The N2 adsorption–desorption isotherm measured at 77 K (inset: corresponding pore-size distribution curve) of TB-Zn-CP (left). The uptake capacitates of TB-Zn-CP for CO2, N2 and H2 at 273 K (right).
Fig. 3
Fig. 3. Fluorescence emission spectra of TB-Zn-CP (λ max = 520 nm) and L (λ max = 532 nm) dispersed in water (inset: visual color of TB-Zn-CP taken under UV lamp).
Fig. 4
Fig. 4. Observed fluorescence quenching (left) of TB-Zn-CP upon addition of PA (μM) in water and its corresponding Stern–Volmer plot (right).
Fig. 5
Fig. 5. Extent of fluorescence quenching of TB-Zn-CP observed upon the addition of various analytes. Inset: the colour changes observed.
Fig. 6
Fig. 6. Competitive selective binding affinity of TB-Zn-CP towards different NACs in the presence of PA in aqueous medium.
Fig. 7
Fig. 7. Fluorescence decay profile (left) of the aqueous suspension of TB-Zn-CP upon the addition of PA in different concentrations (0–90.9 μM) (left) and extent of fluorescence quenching (right) observed after the addition of PA (47.6 μM) at different temperatures (25–45 °C).
Fig. 8
Fig. 8. Plot of quenching efficiency as a function of exposure time (0 → 8 min) monitored at different concentrations of PA (47.6 → 130.4 μM).
Fig. 9
Fig. 9. Change in fluorescence intensity of TB-Zn-CP at different concentrations of PA.
Fig. 10
Fig. 10. Reproducibility of the quenching ability of TB-Zn-CP for PA dispersed in water. The material was recovered by centrifugation after each experiment and washed several times with water.

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