Article pubs.acs.org/ac
Highly Selective and Sensitive Fluorescent Paper Sensor for Nitroaromatic Explosive Detection Yingxin Ma, Hao Li, Shan Peng, and Leyu Wang* State Key Laboratory of Chemical Resource Engineering, School of Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Rapid, sensitive, and selective detection of explosives such as 2,4,6trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP), especially using a facile paper sensor, is in high demand for homeland security and public safety. Although many strategies have been successfully developed for the detection of TNT, it is not easy to differentiate the influence from TNP. Also, few methods were demonstrated for the selective detection of TNP. In this work, via a facile and versatile method, 8hydroxyquinoline aluminum (Alq3)-based bluish green fluorescent composite nanospheres were successfully synthesized through self-assembly under vigorous stirring and ultrasonic treatment. These polymer-coated nanocomposites are not only water-stable but also highly luminescent. Based on the dramatic and selective fluorescence quenching of the nanocomposites via adding TNP into the aqueous solution, a sensitive and robust platform was developed for visual detection of TNP in the mixture of nitroaromatics including TNT, 2,4-dinitrotoluene (DNT), and nitrobenzene (NB). Meanwhile, the fluorescence intensity is proportional to the concentration of TNP in the range of 0.05−7.0 μg/mL with the 3σ limit of detection of 32.3 ng/mL. By handwriting or finger printing with TNP solution as ink on the filter paper soaked with the fluorescent nanocomposites, the bluish green fluorescence was instantly and dramatically quenched and the dark patterns were left on the paper. Therefore, a convenient and rapid paper sensor for TNP-selective detection was fabricated.
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Mao reported a simple but sensitive method for the colorimetric visualization of TNT.62 Compared with TNT, 2,4,6-trinitrophenol (TNP) is also a common and powerful explosive with stronger explosion ability and lower safety coefficient, as well as being capable of explosion without any triggers. Because of the extremely similar structure and character, the current TNT analysis schemes based on the colorimetric and fluorescent methods are not suitable for distinguishing between TNT and TNP. Although various analytical methods for the detection of TNT have been reported, the detection of its analogue, TNP, is still in high demand for homeland security needs in many situations.63−65 Here, we present a novel nanocomposite-based fluorescence sensing platform to meet this challenge. As reported by Zhang,47,51 via the charge-transfer from electron-rich amine functionalized fluorescent donors to electron-deficient aromatic rings of TNT, the colored TNT−amine complex can strongly suppress the fluorescence of donors; thus, a series of fluorescence quenching methods for TNT detection has been successfully developed. As we know, TNP has a strong absorption at 365 nm; if a fluorescent donor demonstrates an excitation peak at ∼365 nm, the charge transfer from donor to TNP will result in the fluorescence quenching and, consequently, a fluorescence quenching senor is desirable for TNP detection. To the best of our knowledge, 8-hydroxyquino-
apid, sensitive, and selective detection of nitroaromatic explosives such as 2,4,6-trinitrotoluene (TNT) has already attracted wide attention and is gaining increasing concern, because these explosive materials are of great interest, with regard to homeland security and public safety.1,2 Despite the significant advances that have been made, a portable and reliable method for sensitive detection in a field environment still faces great challenges, including multiple interferences from common household and personal care products, high-cost, and the need of complicated and expensive instruments. Current analysis methods are relying heavily on LC-MS,3,4 GC-MS,5 HPLC,6,7 ion mobility spectrometry,8−10 and proton transfer reaction−mass spectrometry (PTR-MS).11 Meanwhile, other methods including electrochemical method,12−19 surface plasmon resonance (SPR), 20,21 field-effect transistors (FETs), 22,23 and surface-enhanced Raman scattering (SERS)24,25 were also developed recently. Indeed, because of their sensitivity and portability, optical and fluorescence-based sensors have been extensively studied for portable applications.26−40 During the past years, nanomaterials (especially fluorescent quantum dots (QDs)) have drawn great attention41−44 and have been widely used for the analysis of TNT, because of their novel optical properties and sensitive response to TNT.45−49 Based on nanotechnology, Zhang and coworkers47,50−57 have successfully developed many novel strategies for TNT detection. By means of the specific recognition of TNT antibodies, Goldman and co-workers fabricated some fluorescence immunoassays for TNT.48,49,58−61 Via the aggregation of gold nanoparticles induced by TNT, © 2012 American Chemical Society
Received: July 26, 2012 Accepted: September 4, 2012 Published: September 4, 2012 8415
dx.doi.org/10.1021/ac302138c | Anal. Chem. 2012, 84, 8415−8421
Analytical Chemistry
Article
line aluminum (Alq3), emitting strong bluish green fluorescence with an excitation peak at 365 nm, is a highly favorable fluorescent donor for TNP detection. However, because of its low solubility in water, the Alq3 should be functionalized to make it water-stable before utilization as fluorescent donors for TNP detection in aqueous media. Herein, via a facile strategy, the hydrophobic Alq3 was transferred to water by coating a hydrophilic polymer shell on the Alq3 core to form a fluorescent composite nanosphere. These luminescent nanospheres are monodispersed in size and highly water-stable. By adding the TNP into the nanosphere colloidal solution, the strong bluish green fluorescence of the nanospheres was quenched dramatically. Meanwhile, the fluorescence decreased step by step with the increase of TNP concentration and the fluorescence intensity is negatively proportional to the TNP concentration. However, the fluorescence was hardly influenced by other nitroaromatics, including 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and nitrobenzene (NB). Therefore, a novel fluorescence quenching method was successfully developed for the selective detection of TNP without the interference of other nitroaromatics. A fluorescent filter paper was also obtained by immersing the paper into the nanosphere colloidal solution and then drying at 70 °C. After pressing the fingerprint, which has been dipped with TNP solution, or writing with the TNP solution on the luminescent paper, the bluish green fluorescence of the part stained with TNP was immediately and dramatically quenched, and the dark patterns were left on the filter paper (under irradiation with a portable UV lamp). However, if the TNP ink was replaced with other nitroaromatic solutions, the green luminescence was almost not influenced. In addition, by increasing the concentration of TNP in the ink, the pattern became darker and darker. This fluorescence quenching process is pretty quick and can be accomplished within half a minute. Therefore, a rapid, facile, and selective fluorescent paper sensor has been successfully developed for TNP detection.
Characterization. The absorption spectra were conducted on a UNICO 2802PC spectrophotometer with a spectral window range of 190−800 nm. The shape and size of the composite nanospheres were carried out using an H-800 transmission electron microscopy (TEM) system with a tungsten filament at an accelerating voltage of 100 kV. The photoluminescence measurements were performed on an F4600 fluorospectrometer (Hitachi). Synthesis of 8-Hydroxyquinoline Aluminum (Alq3). To synthesize 8-hydroxyquinoline aluminum (Alq3), 1.67 g of aluminum potassium sulfate dodecahydrate (KAl(SO4)2·12H2O) and 1.533 g of 8-hydroxyquinoline were dissolved in 30 mL of deionized (DI) water and 40 mL of ethanol, respectively. These two types of solution were then mixed together under stirring for 2 h at room temperature to form Alq3. Thereafter, the pH was adjusted to 1−2 with HCl to dissolve the byproduct Al(OH)3 formed during the synthesis process and then the colloidal solution was filtered immediately. The obtained products were then washed with DI water for three circles. Finally, the as-prepared products were treated at 120 °C for 8 h and stored for later use. Synthesis of Composite Nanospheres. The composite nanospheres containing a bluish-green fluorescence Alq3 core and an amphiphilic polymer shell were synthesized by an ultrasonication assistant emulsion strategy. Typically, 1.0 mg of Alq3 and 30 mg of poly(St-Co-MAA) polymer were dissolved in 1.0 mL of chloroform. Then, the mixture solution was added to 10 mL of NaOH aqueous solution (0.07 mg/mL). Under vigorous stirring and ultrasonic treatment, the mixture solution changed into the white emulsion. The emulsion was then stirred and heated at 60 °C for 2 h to remove the chloroform. Thereafter, the composite nanospheres were washed with DI water and collected by centrifuging. Then, the stock solution of the nanocomposite with a final concentration of 0.9 mg/mL was obtained by dispersing the dried nanosphere with known weight in DI water. 2,4,6-Trinitrophenol (TNP) Analysis. In brief, 100 μL of the nanocomposite colloidal solution (0.9 mg/mL) was mixed with various concentrations of TNP, and then the mixed solution was diluted to 1.0 mL with NaH2PO4−Na2HPO4 (pH, 7.0, 0.02 mol/L) phosphate buffer solution (PBS). The fluorescence spectra of the mixture solution were measured using an excitation wavelength of 365 nm. Paper Sensor for Visual Detection of TNP Residues. A piece of filter paper (36 mm in diameter) was immerged in the fluorescent nanocomposite colloidal solution for 10 min. The filter paper then was removed from the solution and dried at 70 °C for 10 min. Under the irradiation of 365 nm with a UV lamp, this filter paper emitted strong bluish green fluorescence. To demonstrate its application as a fluorescence paper sensor for TNP detection, the handwriting or fingerprint with different concentration of TNP solution as ink was pressed on the filter paper. Thereafter, the paper with fingerprints or handwritings was imaged under 365 nm of irradiation with a UV lamp and the luminescent photos were taken by a digital camera.
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EXPERIMENTAL SECTION Chemicals and Materials. 2,4,6-Trinitrophenol (TNP) and 2,4,6-trinitrotoluene (TNT) were supplied by National Security Department of China and was recrystallized with ethanol before use. For safety considerations, all of the explosives must be kept away from fire, striking, and friction, and they should be handled carefully. Poly(styrene-comethacrylic acid) (poly st-co-MAA) was synthesized according to previously published work.66,67 All other reagents are analytical grade and used as received without further purification. 2,4-Dinitrotoluene (DNT) and nitrobenzene (NB) were purchased from Aladdin Chemistry Co., Ltd. (China). These four nitroaromatics were dissolved in the mixed solvent of ethanol and acetonitrile (volume ratio of 4:1) to obtain the stock solution before use, respectively. In brief, 5 mg of nitroaromatics was dissolved in 25 mL of a mixture solvent containing ethanol (20 mL) and acetonitrile (5 mL) to get the stock solution with a final concentration of 0.2 mg/mL. By diluting 2 mL of this stock solution with 18 mL of mixture solvent (14.4 mL of ethanol and 3.6 mL of acetonitrile), the work solution with the concentration of 0.02 mg/mL was obtained. NaOH, ethanol, acetonitrile, cyclohexane, chloroform, aluminum potassium sulfate dodecahydrate (KAl(SO4)2·12H2O), hydrochloric acid, and methanol were received from Beijing Chemical Factory (China). Filter paper was purchased from Hangzhou Special Paper Co., Ltd. (China).
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RESULTS AND DISCUSSION As mentioned previously, the strong fluorescence Alq3 was encapsulated in a poly(St-Co-MAA) polymer nanosphere via a facile fabrication strategy. These nanospheres are highly waterstable because of the enriched carboxylic groups in the polymer shell, facilitating their applications as luminescent sensors in aqueous media. Under irradiation at 365 nm, the nano8416
dx.doi.org/10.1021/ac302138c | Anal. Chem. 2012, 84, 8415−8421
Analytical Chemistry
Article
composites emitted the strong bluish green fluorescence that can be quenched dramatically by the added TNP (Scheme 1). Scheme 1. Scheme for the Selective Fluorescence Quenching Detection of TNP, Based on the Fluorescent Nanocompositesa
a
Right column shows the digital photos obtained from the nanocomposite colloidal solution in the presence of different nitroaromatics (7.0 μg/mL) under UV light (365 nm).
The intensity of this bluish green fluorescence, however, was not influenced by the addition of other nitroaromatics including TNT, DNT, and NB (see the digital photos shown in Scheme 1). It should be mentioned that these fluorescence photos were taken from the nanocomposite colloidal solution in the presence of different explosives (7.0 μg/mL) with a digital camera. Because of this dramatic and selective fluorescence quenching by TNP, the as-fabricated fluorescent composite nanospheres are capable of being utilized as fluorescence nanosensors for TNP detection. The shape and size of the composite nanospheres were carried out using transmission electron microscopy (TEM). From the TEM images shown in Figure 1, it can be seen that the as-prepared nanocomposites are somewhat monodispersed nanospheres with an average size of
