Squaramide-based lab-on-a-molecule for the detection of silver ion and nitroaromatic explosives

Abstract

A squaramide based chemosensor SA was developed as a quantitatively operating lab-on-a-molecule, for the detection of silver ions and nitroaromatic explosives in aqueous solution, showing significant emission quenching and absorption enhancement in dual channel along with a distinct solution color change.

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The field of lab-on-a-molecule¹ was proposed by de Silva² as a molecular logic gate producing an output signal (generally fluorescent emission) in response to various chemical input signals, which has been employed to overcome the boundary of single analyte detection and to initiate highly selective orthogonal signaling protocols for multi channel ion sensing in large varieties.³

Dramatic improvements in the detection of nitroaromatic compounds such as 2,4,6-trinitrotoluene (TNT) and its derivatives picric acid (TNP), 1-methyl-4-nitrobenzene (NT), 1-methyl-2,4-dinitrobenzene (DNT) have aroused wide public concern since they are highly explosive and environmentally deleterious substances.⁴ Similarly, trace amount of toxic metal ions such as Ag⁺ can pose serious threats to human health and environment.

The presently used detections of nitroaromatics are usually time-consuming with the employment of cumbersome⁵ and expensive gas chromatography coupled to a mass spectrometer,⁶ ion mobility spectrometry⁷ and surface acoustic wave method,⁸ which requires frequent instrument calibration, sophisticated vapor sampling and preconcentration procedures.⁹ In contrast, the fluorescence-based chemosensors have been of significant importance with remarkable advantages over the other newly developed detection technologies owing to high sensitivity, portability, short response time,¹⁰ low cost and dual compatibility in solid and solution media.¹¹

Therefore, the development of robust and sensitive platforms for their real-time analytical detection has attracted considerable research efforts in recent years.¹² Fluorescence quenching-based chemosensors,¹³ where analyte is binding with producers and attenuates in the light emission,¹⁴ are believed to be the most suited for the detection of nitroaromatics because the electron-deficient nitro compounds are the strong quenchers of electron-rich fluorophores via an electron-transfer mechanism.¹⁵

Until now, most of the fluorescence resonance energy transfer (FRET) based chemosensors usually employ two photoluminescent elements with an approximate energy band gap, in which the fluorescent emission of a donor is transferred to an acceptor and thus activates the fluorescence of the acceptor. However, the FRET from a photoluminescent donor to a non-emissive receptor for direct chemodetection of analyte has thus been rarely explored. In principle, if the absorption band of an analyte (or its derivatives) overlaps with the emission band of a fluorescent dye, the resonance energy transfer due to the polar–polar interaction at a spatial proximity will result in the quenching of donor fluorescence, which may provide a facile and sensitive detection for the target analyte.

On the other hand, given the toxic mental ions such as silver posing serious threats to human health and environment, there is a growing demand to develop the up-to-date technologies and methods for the qualitative and quantitative detection of silver. Although different analytical methods are developed for the detection of Ag⁺, namely, quantum-dot-based assay,¹⁶ electrochemical¹⁷ and fluorescent sensor,¹⁸ which can detect Ag⁺ ion with high sensitivity and selectivity but need highly sophisticated expensive instrumentation,¹⁹ the absorption-based colorimetric chemosensors are greatly favored due to the distinct visual color change response for fast and easy detection by naked-eye.²⁰

Herein, we reported the resonance energy transfer-amplifying fluorescence quenching of dye for the sensitive detection of TNT and its derivatives²¹ and competitive sensing of silver ions. The specific binding of TNT with the amine ligands leads to the formation of TNT–amine complexes, which strongly suppressed the fluorescence emission of the proximally positioned marker dye through the resonance energy transfer from the chosen dye to the TNT derivatives. The amplifying fluorescence quenching can sensitively detect the ultratrace TNT in the solution,²² and selectively distinguish different types of nitrocompounds. Notably, the title probe also performed high competitive selectivity of Ag⁺ ion over other metal ions by its outstanding “turn-on” effect in absorption channel, even in the presence of other metal ions.²³

Experimental

Synthetic materials and instrumentation

All the chemicals used for the synthesis were of AR grade from Sigma-Aldrich unless otherwise stated. All the reagents and solvents were purchased from commercial sources and used without further treatment. The aqueous stock solutions of cations were prepared from their perchlorate salts. The stock solution of squaramide sensor SA was prepared by dissolving an accurately weighed SA in DMSO, and then diluted to 5 × 10⁻⁵ mol L⁻¹ with buffer solution (pH = 7.24, DMSO : buffer = 90/10, v/v, aq. buffer = 0.1 M Tris–ClO₄).

The ¹H-NMR was recorded on a Varian mercury-plus 400 NMR spectrometer at room temperature using d₆-DMSO as solvent according to the internal standard. Absorption spectra were performed on a TU-1810 ultraviolet spectrophotometer. Fluorescence emission spectra were measure with a Hitachi F-4500 luminescence spectrometer.

Synthesis of 3,4-bis(phenylamino)cyclobut-3-ene-1,2-dione (SA) (Scheme 1)

Typically, to a stirred solution of 3,4-diethoxycyclobut-3-ene-1,2-dione (81.3 μL, 0.55 mmol, 1.0 equiv.) and zinc trifluoromethanesulfonate (40 mg, 0.11 mmol, 20 mol%) in toluene/DMSO 19 : 1 (1 mL) was added aniline (92.6 μL, 1.15 mmol, 2.1 equiv.). The solution was heated to 100 °C and stirred for 12 h. When the solution was cooled down to room temperature, a white precipitate was observed and isolated by decanting the solvent. The solid was further washed with methanol (3 × 5 mL) to acquire white solid (138 mg, 99% yield).^{24 1}H NMR (400 MHz, DMSO-d₆) δ 7.50 (4H, d, J = 7.3 Hz), 7.38 (4H, app t, J = 7.4 Hz), 7.90 (2H, app t, J = 7.4 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 181.5, 165.6, 138.5, 129.3, 123.2, 118.4; HRMS (EI) calcd for C₁₆H₁₂N₂O₂ 264, found m/z 264.

Results and discussion

Sensing of Ag⁺ by SA

The colorimetric sensing properties of SA combine with various cations were investigated by UV-Vis absorption spectroscopy. In buffer solution (pH = 7.24, DMSO : H₂O = 90 : 10, v/v), SA showed one characteristic peak at 422 nm (Fig. S1†) which was due to the n → π* electronic transition. Upon the addition of equivalent cations into the buffered aqueous solution of SA, the dramatic absorption enhancement at 422 nm (Fig. S1†) accompanied with the obvious solution color change from yellow to dark red could only be observed in the case of Ag⁺, clearly indicating the formation of a complex species between SA and Ag⁺. In contrast, the addition of other cations, such as Ba²⁺, Zn²⁺, Ni²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cd²⁺, Co²⁺, Hg²⁺, and K⁺ did not cause either significant UV-Vis spectroscopic change or solution color change. Therefore, it can be concluded that SA can recognize Ag⁺ selectively over other anions not only by naked eyes but also by absorption spectroscopy.

In addition, the sensitive recognition and binding mode of SA with Ag⁺ was demonstrated by carrying out absorption titration experiments. With the constant addition of Ag⁺ from 0–10 equivalents to the solution of SA (5 × 10⁻⁵ mol L⁻¹) in aqueous buffer (pH = 7.24, DMSO : H₂O = 90 : 10, v/v), the absorption intensity at 422 nm was increased sharply due to the formation of complexes between probe SA and Ag⁺.

As a rigorous test, the selective detection of silver ion using SA was challenged in competition experiments (Fig. 1). In none of the cases, the presence of other anions interfered substantially. The good linear relationship between the absorption intensity and concentration of Ag⁺ (Fig. 2 inset) allowed for a facile quantification of Ag⁺ with a detection limit of 4.6 μM according to the literature method.²⁴

Fig. 1 Absorption of SA in the presence of (a) 10 equiv. of cations (inset: naked-eye detection of Ag⁺ ion by probe SA under visible) and (b) 10 equiv. of Ag⁺ + 10 equiv. of other cations at λ = 422 nm in 0.1 M Tris–ClO₄ : DMSO = 10/90, v/v, pH 7.24 [X⁻ = Ag⁺ (0), Ba²⁺ (1), Zn²⁺ (2), Ni²⁺ (3), Ca²⁺ (4), Mg²⁺ (5), Pb²⁺ (6), Cd²⁺ (7), Co²⁺ (8), Hg²⁺ (9), K⁺ (10)].

Fig. 2 The UV-Vis titration of 5 × 10⁻⁵ mol L⁻¹ SA with 1–10 equiv. addition of Ag⁺ ions. Inset: the absorption intensity of probe SA at λ = 422 nm as a function of mole equivalents of Ag⁺ ions.

FRET mechanism between SA and TNT derivatives

TNT is almost faint yellow in solution and does not absorb any visible light. Our work showed that the solution of TNT could change from faint yellow to deep red by adding organic amines such as SA. A strong charge transfer interaction, as a predominant process, occurring between the electron-deficient aromatic ring of TNT and the electron-rich amine group of SA, could lead to the formation of a Meisenheimer complex as zwitterions of TNT⁻ and RNH₂⁺, which is resonance-stabilizing form due to three electron-withdraw nitro groups (Scheme 2).

Scheme 1 The synthetic route for the preparation of SA.

Scheme 2 The interaction between TNT and SA and Meisenheimer complex formed between TNT and the amine.

The absorbance spectrum of TNT solution shows a strong visible absorption with λ at 264–600 nm, and has a spectral overlapping with the fluorescence emission of SA solution at 285–540 nm, suggesting that the resonance energy transfer from SA to TNT derivatives may occur when the fluorophores and TNT derivatives are spatially close enough to each other. As a result, the non-emissive resonance transfer-based fluorescence quenching of SA will occur, and the quenching efficiency mainly depends on the degree of spectral overlapping between the emission of donor and the absorption of analyte acceptor and on the distance between these two, as depicted by following equation,²⁵

Φ_T = 1/Q, Q = 1 + (R/R₀)⁶

where R is the distance of the two fluorescent groups; R₀ is the distance of the Förster (the critical transfer distance of the donor and acceptor).

UV/Vis response of SA and nitroaromatic explosives

The UV-Vis selective response of SA upon the presence of nitroamine explosives can be seen from the observed color change of SA solution from pale-yellow to dark red (Fig. S3†). In order to perform these naked-eye experiments, 100 equivalents of various nitroamine explosives (1.5 × 10⁻¹ mol L⁻¹, NT, DNT, TNT, TNP) were added to the solution of SA (5 × 10⁻⁵ mol L⁻¹) in buffer solution (pH = 7.24, DMSO : H₂O = 90 : 10, v/v). As show in Fig. 3, the distinct solution color change was only observed in the presence of TNT along with the significant absorption intensity enhancement in visible band centered at 512 nm (Fig. S4†).

Fig. 3 Naked-eye color changes of SA (5 × 10⁻³ mol L⁻¹) with subsequent addition of TNT from 0.5 to 100 equiv. under visible.

In addition, the sensitive recognition of SA with TNT was demonstrated by carrying out absorption titration experiment. With the progressive addition of TNT from 0–100 equivalents to the solution of SA (5 × 10⁻³ mol L⁻¹) in buffer solution (pH = 7.24, DMSO : H₂O = 90 : 10, v/v), the absorption intensity at 512 nm was increased gradually. The linear relationships between the absorption intensity and amount of TNT was established for SA (Fig. 4) that allow for quantification over a wide range of concentration with a detection limit of 5.0 μM.

Fig. 4 UV-Vis absorption titration of SA (10 μM) upon addition of various equiv. of TNT in DMSO buffered solution (pH = 7.24, DMSO : H₂O = 90 : 10, v/v). Inset: linear correlation of absorption intensity at λ = 512 nm versus concentration of TNT.

Fluorescence response of SA and nitroaromatic explosives

As expected, upon the addition of TNT, a charge-transfer interaction occurred between the electron-deficient aromatic ring of TNT and the electron-rich amino group, leading to the formation of a Meisenheimer complex and exhibiting a strong absorption band at approximate 338 nm. The well overlap (see Fig. S4†) between the absorption of TNT and the emission band of the chosen SA fluorophore facilitated the FRET mechanism by intermolecular polar–polar interaction in the spatial proximity, as a result the efficient quenching effect could be found on the emission of the SA donor.²⁶

The probe SA also possesses the ability to react with TNT in a dose-response manner, which can be utilized for quantification of the analyte. As is shown in Fig. 5, the emission intensity of SA at 338 nm gradually decreased with the successive addition of TNT.

Fig. 5 (a) PL intensity of SA (10 μM) at λ_exc = 278 nm with respect to the concentration of TNT from 0 to 20 equivalents in DMSO buffered solution (pH = 7.24, DMSO : H₂O = 90 : 10, v/v); (b) Stern–Volmer plot of SA treated with various explosives at different concentrations at λ_em = 338 nm.

Similar quenching effect was also found in the case of other aromatic nitroamines such as NT, DNT and TNP with a relative weaker degree. Therefore, it can be concluded that SA could selectively response the aromatic nitroamines through emission quenching.

To evaluate the quenching efficiency of different kinds of nitroaromatic analytes, the Stern–Volmer equation is applied as follows: (I₀/I) − 1 = K_SV[TNT], where I₀ is the initial fluorescence intensity in the absence of analyte, I is the fluorescence intensity in the presence of [TNT], and K_SV is quenching constant with TNT. By employing the emission intensity data of SA at λ_exc = 278 nm into Stern–Volmer equation, it is calculated that the quenching efficiency of NT, DNT, TNT and TNP is 6.01 × 10⁴, 6.14 × 10⁴, 6.30 × 10⁴ and 6.65 × 10⁴ M⁻¹, respectively.

Conclusions

We have successfully demonstrated a novel lab-on-a-molecule of nitroaromatic compounds and silver ions by using UV-Vis and emission channels. In UV-Vis channel, the remarkable absorption intensity enhancement was observed only in the presence of silver ions, while in emission channel the selective emission quenching effect was found towards nitroaromatic compounds such as TNT, DNT, NT and TNP through resonance energy transfer mechanism by the formation of a Meisenheimer complex. To the best of our knowledge, this is the first time reporting dual channel chemosensor for the selective detection of aromatic nitroamines and Ag⁺ ions, even in the presences of other competing analogues, suggesting that probe SA could be used as dual channel lab-on-a-molecule for detecting nitroaromatic explosives and metal ions by simply achieved probes.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (NSCF21507005).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18754k

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