Fast detection of nitroaromatics using phosphonate pyrene motifs as dual chemosensors

Article abstract of DOI:10.1039/c4cc03980g

A new class of dual fluorescent chemosensors for nitroaromatic compounds (NACs) based on phosphonated pyrene derivatives is reported, showing high selectivity towards trinitrotoluene (TNT). The strong intermolecular interactions (π-π stacking and hydrogen bonding) allow high fluorescence quenching with visual detection in short response times. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03980g

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Fast detection of nitroaromatics using
phosphonate pyrene motifs as dual
chemosensors†
Cite this: Chem. Commun., 2014,
50, 9683
Received 24th May 2014,
Accepted 30th June 2014
N. Venkatramaiah,ab Ana D. G. Firmino,ab Filipe A. Almeida Paz*b and
Joao P. C. Tome*ac
˜
´
DOI: 10.1039/c4cc03980g
www.rsc.org/chemcomm
A new class of dual fluorescent chemosensors for nitroaromatic
compounds (NACs) based on phosphonated pyrene derivatives is
reported, showing high selectivity towards trinitrotoluene (TNT).
The strong intermolecular interactions (p–p stacking and hydrogen
bonding) allow high fluorescence quenching with visual detection
in short response times.
Scheme 1 Synthetic route for the preparation of phosphonated pyrene
derivatives.
Rapid screening of explosive nitroaromatic compounds (NACs)
in pre and post-blast debris is of high importance for both
environmental and security reasons.1 Current detection meth-
ods are cumbersome, require pre-treatment of the samples, are the guest molecules. In this communication, we report the
susceptible to interference from other compounds, and suffer design and simple (and fast) preparation of two pyrene deriva-
from low sensitivity.2 Fluorescence studies have received great tives covalently functionalized with phosphonate groups
attention in this field due to their intrinsic high sensitivity (protected and hydrolyzed, PE and PO respectively; Scheme 1).
(even at the atto-gram level) and rapid detection by photo We have investigated their potential utility as chemosensors in
induced electron transfer from the excited state of the donor the detection of NACs (Fig. S1, ESI†), in solution and solid state
to NACs (acceptors).3,4 A considerable number of fluorescent methods. These molecules were designed based on the principle
probes have been designed for this purpose including a wide that the PQO functional group reduces the lowest unoccupied
variety of functional groups.5 It remains, nevertheless, a great molecular orbital (LUMO) energy level of the resulting molecule
challenge to find new materials that could improve the selec- and, consequently, facilitates the electron inoculation to electron
tivity and sensitivity towards the detection of NACs.6
deficient NACs. While the phosphoryl groups can limit molecular
Polycyclic p-conjugated systems, such as pyrene, have an aggregation (due to steric hindrance), the unprotected hydroxyl
intrinsic high fluorescent quantum yield.7 The presence of a groups further act as donors/receptors in strong intermolecular
large conjugated backbone is additionally highly beneficial to hydrogen bonding interactions with electron-deficient NACs.
bind to aromatic rings through p–p interactions.8 Covalent
PE was easily prepared in quantitative yields (ca. 95%) by a
attachment of pyrene derivatives with polymer networks and palladium-catalysed cross-coupling reaction under microwave
physical encapsulation with nanoparticles were used for the irradiation at 230 1C for 45 min. Acid hydrolysis was carried out
detection of NACs based on the behaviour of monomer to using a 6 M HCl solution at 80 1C, which yields the pyrene
excimer emission.9 The occurrence of stable and pure excimer tetraphosphonic acid (PO) in quantitative yields. The molecular
emission is a challenging task because of its high sensitivity to structures of the final compounds were investigated in solution
using a myriad of techniques: 1H, 31P, 13C NMR and MALDI-
1
a Department of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal.
E-mail: jtome@ua.pt; Fax: +351 234370084; Tel: +351 234370342
TOF MS (Fig. S2–S12, ESI†). H NMR of PE and PO reveals the
presence of a set of singlet peaks corresponding to the four
protons in the 4,5,9,10 positions of pyrene (d 9.19 and 8.53 ppm
respectively), while the protons at the 2,7 positions appear as
triplets due to long range phosphorous–hydrogen coupling (for
PE: d 9.23–9.33 ppm with 5JHP = 30 Hz; for PO: d 8.58–8.60 ppm
b Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro,
Portugal. E-mail: filipe.paz@ua.pt; Fax: +351 234401470; Tel: +351 234 401418
c Department of Organic Chemistry, Ghent University, B-9000 Gent, Belgium
† Electronic supplementary information (ESI) available: Synthesis, UV-Vis and
fluorescence quenching studies with various studied analytes, both in solution
and in the solid state, NMR and DFT studies. See DOI: 10.1039/c4cc03980g
5
with JHP = 6 Hz). In the aliphatic region of PE the resonances
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is attributed to strong p–p interactions between the quencher
and PO through a photo-induced electron transfer (PET).
In addition to p–p interactions, PO should also exhibit
strong intermolecular hydrogen bonding interactions between
the hydroxyl groups of the phosphonic acids and nitro groups
of the quencher, resulting in an enhanced quenching efficiency
in comparison to that observed for parent PE (Fig. S16, ESI†).
Non-aromatic analytes such as nitromethane (NM) showed a
lower quenching response mainly because of its inability to
interact via p–p interactions. We further emphasize that addi-
tion of other NACs and volatile organic compounds did not
produce any sensible changes in the fluorescence intensity. It
further evidences that both PE and PO exhibit high selectivity
towards TNT and TNP in methanol (Fig. S15–S16, ESI†).
The visual modification of the strong blue fluorescence
emission of PE and PO was monitored by adding different
NACs. Both PE and PO show turn-off fluorescence behaviour in
Fig. 1 (a) Absorption, (b) emission spectra and (c) fluorescence lifetime
decay of PE and PO in methanol and in thin films. The inset in Fig. 1b shows
the emission behaviour of PE and PO in solution and in the solid state
under UV light at 365 nm. (d) Emission spectra of PO upon gradual addition
of different concentrations of TNT at lex = 330 nm. The inset shows the the presence of TNT and TNP (Fig. S17, ESI†). Fig. 2a shows the
trend in the quenching efficiency of PO at different concentrations of TNT.
comparison of the quenching efficiency of PE and PO with
different nitroaromatics: TNP has a moderate quenching effi-
ciency while TNT shows significantly enhanced selectivity for PO
clearly appear corresponding to the etoxy groups at d 1.38 rather than for PE (Fig. S18, ESI†). The fluorescence attenuation
(–CH3) and 4.27 (–CH2) ppm, while in PO, these signals of PO against NACs at different concentrations was quantitatively
disappear confirming the full deprotection of their hydroxyl evaluated using Stern–Volmer (SV) plots (Fig. 2b). The SV plots
groups. 31P NMR spectra show one set of triplet peaks centred indicate high binding constants for TNT (Ksv = 8.4 Â 104 MÀ1
)
at d 17 ppm (5JHP = 12.1 Hz) and 13.5 ppm (5JHP = 14.5 Hz in and TNP (Ksv = 3.6 Â 104
M
À1; Table S2, ESI†). They further
D2O) for PE and PO, respectively. The MALDI-TOF MS results evidence linear response behaviour upon addition of TNT at low
confirm the proposed structures. The absorption spectra of PE concentrations, with a positive upward deviation indicating the
and PO in methanol exhibit two major bands centred at formation of static and dynamic quenching effects. PE shows,
ca. 283 nm and 375 nm attributed to the electronic transitions however, Ksv = 6.6 Â 104 MÀ1 for TNT and Ksv = 2.9 Â 104 MÀ1 for
between the S0 - S2 and S0 - S1 energy levels (Fig. 1a). The TNP and exhibits linear SV plots upon addition of different NACs.
emission spectra are typical of the monomeric form of pyrene This is ascribed due to static quenching phenomena (Fig. S19,
with the emission maxima centred at ca. 387 nm (Fig. 1b), with ESI†). We note that the rate constants of these phosphonate
lifetimes of 3.74 ns (PE) and 3.42 ns (PO) (Fig. 1c), and optical pyrene derivatives were found to be higher than those of related
band gaps of 3.10 and 3.01 eV, respectively. The absorption and derivatives employed for detection of NACs.10 The obvious mod-
emission spectra in different solvents are shown in Fig. S13 and ifications in the UV-Vis spectra of PO upon addition of TNT
S14, ESI.† In contrast to the absorption spectra in solution, confirmed the formation of complexes in the ground state
the emission of thin films shows broad emission bands at (Fig. S20, ESI†). Furthermore, time-resolved fluorescence emis-
ca. 505 nm (PE) and 515 nm (PO) with a Stokes shift of 132 nm sion (Fig. S21, ESI†) reveals a change in the lifetime of PO upon
and 140 nm indicating the formation of an excimer in the solid addition of different concentrations of TNT, ultimately confirm-
state. The excimer emission of PE and PO was found to be ing the quenching of the excited state (Table S3, ESI†).
B45 Æ 5 nm higher than that of pyrene.9d The inset in Fig. 1b
The DFT optimized structures of PE and PO and their adducts
shows comparative emission behaviour of PE and PO in with TNT and TNP were obtained using the B3LYP/6-31g* basis
solution and in thin films under UV light (lex = 365 nm). While
the fluorescence quantum yields (FF) of PE and PO in solution
are ca. 36% and 22%, a notable decrease to ca. 3.2% and 1.7%
was observed in the solid state. The photophysical properties of
PE and PO, both in solution and the solid state, are summar-
ized in Table S1, ESI.† Fig. 1d depicts the spectrofluorimetric
addition of PO with different concentrations of TNT in metha-
nol while exciting at 330 nm. As expected the fluorescence
intensity gradually depletes, showing ca. 82% and 91% quench-
ing effect for PE and PO with a limit of detection (LOD) in the
range of 2–14 ppb; for TNP the analogous results for PE and PO
are 51% and 74%, respectively (Fig. S15, ESI†). The high
quenching efficiency of PO emission with the addition of TNT Volmer (SV) plot of PO treated with different NACs at different concentrations.
Fig. 2 (a) Quenching efficiency of PE and PO for various NACs. (b) Stern–
9684 | Chem. Commun., 2014, 50, 9683--9686
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set in the Gaussian 03 package (Fig. S22–S24, ESI†). The phos- was achieved within 180 s of exposure time, reaching the
phonate groups of PO and PE reveal a decrease in the LUMO equilibrium for elapsed time of exposure. The inset in Fig. 3a
energy level in comparison with that of pyrene (Fig. S25, ESI†). depicts the change in the emission intensity as a function of
The electron density of the HOMO orbitals of PO-TNT and time: PE shows a 24% and 86% quenching effect in 30 and
PE-TNT adducts is localized on the pyrene backbone; the LUMO 360 s of exposure time, respectively. Fig. 3b shows the variation
orbitals localized on TNT are strong evidence for an efficient in the quenching efficiency of PE and PO films to saturated
charge transfer process occurring between the higher energy vapours of different NACs. These results show that both PE and
state of the pyrene phosphonates and the lower-energy state of PO films are highly efficient and selective towards sensing of
NACs (Fig. S26, ESI†). The PO-TNT adduct is energetically more TNT. The other tested NACs exhibit, however, a much less
stable than PE-TNT by ca. 3.23 kcal moleÀ1. Cyclic-voltammetric quenching efficiency: for example, for TNP only a ca. 56%
studies with a Fc/Fc+ couple showed one reversible reduction quenching effect in 270 s of exposure was observed. The
half wave potential at (E1/2) À1.05 V, À1.08 V for PE and PO variation in the quenching efficiency arises from a combination
(Fig. S27, ESI†). The LUMO energy of PO (À3.43 eV) is close to of: (i) the variation in the vapour pressure of NACs; (ii) exergo-
the LUMO energy of TNT (À3.7 eV), favouring electron transfer. nicity (DG1) of electron transfer between pyrene motifs and TNT
To gain more insight into the mechanism of sensing, we with an effective binding strength; (iii) easy diffusion on the
have performed 1H NMR titration with TNT (Fig. S28–S32, surface. This study was extended to the vapours of other
ESI†). Upon gradual addition of TNT at different mole ratios molecules such as toluene, DDQ and amine derivatives. We
to PE, the singlet and triplet peaks are shifted towards the have observed no significant variation in the emission pattern,
upfield region. The degree of the shift was, however, greater for further evidencing the high selectivity of PO towards TNT
the singlet peak (Dd = À0.07 ppm) which is an indication of p–p (Fig. S35, ESI†). Fig. 3d depicts the SEM images of the pyrene
stacking (Fig. S29, ESI†). Similar results were obtained for PO derivatives on the glass surface, showing that PE leads to a
treated with different concentrations of TNT (Fig. S30–S32, layer-like film while PO promotes the self-assembly of fibres.
ESI†). The –OH protons of PO at d = 4.24 ppm broaden with
Aiming towards practical application of the herein devel-
small up-field shift upon addition of TNT, evidencing the oped materials, the reversibility of the sensing process was
formation of hydrogen bonds between the –OH and –NO2 further examined by exposing the films to the saturated
groups (Fig. S32, ESI†). A possible mode of interaction between vapours of TNT for 120 s. The emission spectrum of the film
PO and TNT is illustrated in Fig. S33, ESI.† The sensing was recorded before and after exposure to the TNT vapours. The
performance of thin films of PE and PO was tested using recovery of excimer emission intensity was significant indicat-
saturated vapours of the herein investigated NACs. Fig. 3a ing high photo-stability of the films after 8 cycles (Fig. S36,
shows the time-dependent fluorescence emission spectra of ESI†). Furthermore, portable paper-based test strips allow a
PO exposed to saturated vapours of TNT (ca. 10 ppb) with simple and low cost protocol for on-site instant detection of
excitation at lex = 370 nm.1b The fluorescence intensity of the TNT (Fig. S37, ESI†).
PO film decreases significantly, showing a 30% quenching at
In summary, we have developed novel sensor compounds
517 nm in 30 s of exposure. A fluorescence quenching of 93% based on phosphonate groups (protected and deprotected)
appended to a pyrene core. This system acts as a dual chemo-
sensor for the fast detection of TNT: as a monomer in solution
and as a stable excimer in the solid state. The phosphonate
groups attached to the pyrene decrease the LUMO energy, with
the PO exhibiting simultaneously p–p stacking and strong
hydrogen bonding interactions between the phosphonic acid
and the nitro groups of TNT, ultimately enhancing sensitivity
and selectivity. The formation of a porous and fibril-type layer
of thin films of PO and PE opens the possibility of fabricating
promising, photo-stable and highly responsive materials
towards the detection of NACs. We believe that the phosphonic
acid groups can greatly facilitate the construction of next-generation
materials for the detection of NACs, some of them based on metallic
centres (such as metal–organic frameworks), while simultaneously
providing new avenues to design more eco-friendly heterogeneous
catalysts for degradation of toxic NACs.
˜
ˆ
We would like to thank Fundaçao para a Ciencia e a Tecno-
logia (FCT, Portugal), the European Union, QREN, FEDER through
the COMPETE program, QOPNA research unit (project PEst-C/QUI/
UI0062/2013; FCOMP-01-0124-FEDER-037296) and CICECO asso-
ciated Lab (PEst-C/CTM/LA0011/2013; FCOMP-01-0124-FEDER-
037271) for their general funding scheme. We further wish to
Fig. 3 (a) Time-dependent emission spectra of a PO thin film upon
exposure to saturated vapours of TNT. The inset shows the fluorescence
quenching of PE and PO as a function of time. (b) Quenching efficiency of
PE and PO films to the saturated vapours of NACs. (c) Photograph of the
PO thin film under UV light at 365 nm before and after exposure to the TNT
vapours at different time intervals. (d) SEM image of PE and PO thin films.
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