Phosphonate Appended Porphyrins as Versatile Chemosensors for

Article pubs.acs.org/ac

Phosphonate Appended Porphyrins as Versatile Chemosensors for Selective Detection of Trinitrotoluene N. Venkatramaiah,†,‡ Carla F. Pereira,†,‡ Ricardo F. Mendes,‡ Filipe A. Almeida Paz,*,‡ and Joaõ P. C. Tomé*,†,§ †

QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal CICECO-Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal § Department of Organic and Macromolecular Chemistry, Ghent University, B-9000 Gent, Belgium ‡

S Supporting Information *

ABSTRACT: Fluorescent molecular probes based on phosphonate-functionalized porphyrin derivatives have been designed for selective detection of nitroaromatics. It is shown that molecular recognition is based on cooperative hydrogen bonding and π−π stacking interactions with electron-deficient molecules (nitroaromatic compounds, NACs), displaying superior detection toward trinitrotoluene (TNT). The PO functional groups decrease the lowest unoccupied molecular orbital (LUMO) energy level of the porphyrins and, consequently, facilitate the electron inoculation to TNT through a photoinduced electron transfer (PET) process. The hydroxyl groups of the phosphonates and pyrrole −NH protons are further engaged in donor− acceptor interactions with TNT by strong intermolecular hydrogen bonding interactions (as evidenced by single crystal X-ray, NMR, and density functional theory (DFT)) showing turn off fluorescence behavior. The nonplanarity of the porphyrins induced by protonation at the central core of the porphyrin H4TPPA2+ undergoes additional interactions, furnishing an anomalous increase in the selectivity of TNT at nanomolar levels in solution (limit of detection, LOD ∼ 5 nM). Porphyrindoped hybrid PMMA [poly(methyl methacrylate)] polymer films demonstrate the reversibility of the fluorescence behavior and exhibit high photostability. The formation of discrete molecular aggregates on the surface of hybrid films and efficient diffusion of TNT vapors (10 ppb) displayed high selectivity in the solid state. The hybrid films are further used to demonstrate the detection of NACs in the aqueous medium, ultimately providing a platform for a practical strategy and implementation for the detection of toxic NACs. fluorescence quantum yields. The structural modification of porphyrins can be achieved either by acting on the central core through complexation with a variety of metals or by chemical functionalization at the periphery.11 It opens a wide variety of possible interaction mechanisms to impart high selectivity toward analytes. Rakow and Suslick have employed different metalloporphyrins as artificial olfactory receptors for the detection of volatile organic compounds (VOCs) by visual color change of porphyrins.12 Interactions between porphyrins and VOCs can be achieved by way of hydrogen bonds and polarization effects and even through coordination.13 As an example, porphyrin-functionalized silica films were used as sensory materials for the detection of NACs.14−17 The sensitivity of the composites were found to be higher in comparison with other conjugated polymers and different macrocycles.18,19 A series of free base and Zn2+-porphyrins

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he design and synthesis of artificial receptors capable of rapidly detecting chemical explosives are prevalent and current research topics. These novel materials can find immediate applicability in antiterrorism, in national security, and in environmental protection.1−3 Harper et al. have classified explosive materials into three classes according to their vapor pressures and volatilities.4 Nitroaromatic compounds (NACs) are one of the major constituents of many standard explosive compositions. Among different NACs, 2,4,6trinitrophenol (TNP) and 2,4,6-trinitrotoluene (TNT) have received special attention because of their low vapor pressure and high toxicity.5 An effective detection of NACs in the vapor phase remains a great challenge, despite the excellent sensitivity offered by many spectroscopic techniques. However, fluorescence spectroscopy provides a new and versatile route for a rapid detection of ultratrace analytes from explosives, offering high sensitivity and selectivity.6−9 This method was found to be fast, cost-effective, and suitable for incorporation into inexpensive and portable microelectronic devices for on-field tests using different fluorescent polymers.10 Porphyrins are a class of compounds with high molar absorptivity and © XXXX American Chemical Society

Received: February 11, 2015 Accepted: March 26, 2015

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DOI: 10.1021/acs.analchem.5b00772 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

(H4TPPA2+), and crystallization methods were described in the Supporting Information. NMR spectra were recorded on Bruker 300 or 500 MHz spectrometers. Mass of the final compounds was obtained from MALDI-Micromass Q-TOF2 equipment. Absorption spectra in solution were recorded in dimethylformamide (DMF) (conc. 1 ×10−5 mol/L) on a PerkinElmer (Lambda 35) UV−Vis Spectrometer. Steady-state emission studies were carried out in a PerkinElmer Luminescence Spectrometer (LS-55), and time resolved fluorescence measurements were performed using the timecorrelated single-photon counting (TCSPC) with Nano LED (459 nm; fwhm H4TPPE2+ > H4TTP2+ > H2TPPA > ZnTPPE > H2TPPE > H2TTP (Figures S48−S52, Supporting Information). These results show that the protonated central core of the porphyrin exhibits a higher sensitivity than the free base and metallo porphyrins. Besides TNT, other NACs (such as DNT) also quench the fluorescence of the porphyrin derivatives. The quenching efficiency is, however, much lower than that of TNT. Selectivity was tested for different NACs and other volatile organic compounds (VOCs). Figure 8d depicts the comparative quenching efficiency of the modified porphyrins toward different NACs. The phosphonate-derived porphyrin doped films not only are sensitive toward TNT but F

DOI: 10.1021/acs.analchem.5b00772 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry also exhibit high selectivity. H4TPPA2+ exhibits the best selectivity for TNT among all tested porphyrins. The emission behavior of other porphyrins with saturated vapors of other NACs is summarized in Figure S53, Supporting Information. The sensing properties are strongly dependent on the surface morphology of the films, which is a crucial factor in the diffusion of the vapors of the analytes. The surface morphology and thickness of the thin films was, in this way, investigated. Figure 9a shows the SEM image of a H4TPPA2+ pristine film

Figure 10. (a) Fluorescence quenching behavior of H4TPPA2+ doped PMMA film placed in different concentrations of TNT (0−100 μM). (b) Stern−Volmer plot of H4TPPA2+ doped PMMA film treated with different NACs at different concentration levels; the standard deviation (SD) in the Io/I is ±0.02. (c) Quenching efficiency of H4TPPA2+ doped PMMA film with respect to various NACs.

concentration of TNT, showing a maximum of a 52% decrease in the quenching efficiency. Other NACs do not show a dramatic change in the quenching behavior; only TNP shows a 38% quenching efficiency in 100 μM solution. Figure 10b depicts the SV plot of H4TPPA2+ treated with different NACs in an aqueous medium. The appearance of the linear SV plot suggests that static quenching behavior is high for TNT, and the rate constants were found to be 2.7 × 103, 3.5 × 103, 4.1 × 103, 5.1 × 103, and 1 × 104 M−1 for H2TPPE, ZnTPPE, H2TPPA, H4TPPE2+, and H4TPPA2+, respectively. Figure 10c describes the quenching efficiency of H4TPPA2+ with different nitro analytes in aqueous solution, emphasizing its good sensitivity toward TNT.

Figure 9. SEM image of the H4TPPA2+-doped PMMA film (a) before and (b) after 840 s of exposure to the saturated vapors of TNT. The inset shows the measured thickness of the film. (c, d) Reversibility of the H2TPPA and H4TPPA2+ doped polymer films exposed to saturated vapors of TNT for 300 s. The photographs show the color of the PMMA films doped with H2TPPA (violet) and H4TPPA2+ (green).

with homogeneous morphology and a thickness of 23 ± 2 μm. Figure 9b shows the formation of discrete aggregates of TNT adsorbed on the surface of the thin film after an exposure to the saturated vapors of TNT for 840 s. The reversibility of the polymer films to NACs was examined using TNT as a representative model. The emission spectrum of the thin film was measured first; then, it was exposed to a saturated TNT vapor at ambient temperature for 420 s, and the emission of the film was measured again. After the measurement, the thin film was washed with water and dried under a nitrogen stream for 5 min and the emission of thin film was collected again.36−39 The process was repeated for several cycles, and the results are depicted in Figure 9c,d. The emission intensity of the films was found to be highly reversible after several cycles ultimately indicating a high photostability of the sensing thin films. The high selectivity and photostability of the polymer films allow one to extend these sensor materials for real-time applications in an aqueous medium. The thin films were immersed in water, and the emission spectra of the films were recorded to understand the leaching behavior of the porphyrin derivatives. Different concentrations of aqueous solutions containing NACs (0−100 μM) were prepared as stock solutions. The films were placed in the solutions containing NACs for 300 s, and the emission spectra were collected after drying the films as described. Figure 10a shows the representative emission of H4TPPA2+doped film immersed in different concentrations of TNT. The emission intensity gradually decreases with respect to

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CONCLUSIONS A series of porphyrins bearing aryl phosphonate and phosphonic acids groups at the periphery was synthesized. Modification of the substituents of porphyrins at the periphery and central core by protonation induces considerable modifications in the photophysical properties. The presence of phosphonic acid groups on the porphyrin decreases the LUMO energy levels, exhibiting an efficient photoinduced electron transfer process with NACs and showing high selectivity toward TNT. Additionally, the phosphonic acid groups undergo strong intermolecular hydrogen bonding interactions with TNT by forming a donor−acceptor based charge transfer complex. Protonation at the central core permits an additional interaction site with TNT, corroborated by singlecrystal studies with bulky anionic groups, which further results in an enhancement in its sensitivity and selectivity with concomitant high binding rate constants. The free base porphyrins show efficient π−π stacking interactions with TNT while the Zn porphyrin tends to form axial coordination with the nitro groups of TNT. This was shown to provide a strong driving force for the fluorescence quenching with static quenching behavior. Results herein summarized further demonstrate that phosphonate porphyrins are highly selective toward detection of TNT with a LOD in the nanomolar level in solution (5 ± 2 nM) and 10 ppb in the vapor phase. The order of detection is as follows: H4TPPA2+ > H4TPPE2+ > H4TTP2+ G

DOI: 10.1021/acs.analchem.5b00772 Anal. Chem. XXXX, XXX, XXX−XXX

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> H2TPPA > ZnTPPE > H2TPPE > H2TTP. We are currently exploring the potential use of these aryl phosphonic acid derivatives of porphyrins for the construction of coordination polymers with lanthanide metals as highly porous hybrid structures which can be employed as sensitizers for the fast detection of NACs. In this manner, we also aim to develop ecofriendly photocatalysts to degrade NACs and other toxic pollutants in the environment.

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ASSOCIATED CONTENT

S Supporting Information *

The structural characterization of porphyrins with various techniques (1H, 31P, 13C NMR, MALDI-TOF MS), fluorescence quenching of porphyrins with NACs, and DFT. Single crystal X-ray structures with crystallographic data summarized with CCDC 1012817, 1012815, and 1012816. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: fi[email protected]. Fax: +351 234 401470. Tel: +351 234401418. *E-mail: [email protected]. Fax: +351 234370084. Tel: +351 234370342. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Fundaçaõ para a Ciência e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER through the COMPETE program; QOPNA research unit (project PEst-C/QUI/UI0062/2013; FCOMP-01-0124FEDER-037296) and CICECO-Aveiro Institute of Materials (ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when applicable cofinanced by FEDER under the PT2020 Partnership Agreement, PEst-C/ CTM/LA0011/2013; FCOMP-01-0124-FEDER-037271 for their general funding scheme. We further wish to thank FCT for funding the R&D project (EXPL/CTM-NAN/0013/2013; FCOMP-01-0124-FEDER-041282). The authors acknowledge FCT for the postdoctoral and doctoral grants SFRH/BPD/ 79000/2011 (to N.V.), SFRH/BD/86303/2012 (to C.F.P.), and SFRH/BD/84231/2012 (to R.F.M.). We thank Prof. Satish Patil, SSCU, Indian Institute of Science (IISc), Bangalore, for useful discussions.

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REFERENCES

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DOI: 10.1021/acs.analchem.5b00772 Anal. Chem. XXXX, XXX, XXX−XXX