Fluorescent Sensors for Nitroaromatic Compounds Based on

Gang He , Guofang Zhang , Fengting Lü and Yu Fang. Chemistry of ..... Chu-Hua Lu , Chia-Hua Tsai , Feng-Chih Chang , Kwang-Un Jeong , Shiao-Wei Kuo...
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Langmuir 2007, 23, 1584-1590

Fluorescent Sensors for Nitroaromatic Compounds Based on Monolayer Assembly of Polycyclic Aromatics Shujuan Zhang,†,‡ Fengting Lu¨,† Lining Gao,† Liping Ding,† and Yu Fang*,† Key Laboratory of Macromolecular Science of Shaanxi ProVince and the School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, P. R. China, and Xi’an High Technique Institute, Xi’an 710025, P. R. China ReceiVed September 22, 2006. In Final Form: NoVember 1, 2006 A class of fluorescent films in which pyrene was assembled, in a monolayer manner, on glass slide surfaces via various flexible spacers of different lengths and substructures was used for the detection of nitroaromatic compounds (NACs) in vapor phase. This design strategy offers several advantages for thin film fluorescent sensory materials. These advantages have been demonstrated experimentally by the sensitive response of the films to the presence of trace amounts of NACs in vapor phase. The fluorescence quenching of the films upon exposure to NACs vapors depends on several factors, including the evaporate rate of the NAC detected, the length of the spacers connecting the sensing element and the substrate surface, and the density of the sensing element on the substrate surface. Further experimentation showed that the sensing process is reversible and free of commonly encountered interference. The sensitive response, reversibility of the sensing process, and freedom from commonly encountered interference of the specially designed films to NACs qualify these materials as promising NACs fluorescent sensory materials.

Introduction The fast and on-site detection of 2,4,6-trinitrotoluene (TNT) and other nitroaromatic compounds (NACs) would be one way to enhance the protection of society against terrorist attacks,1 be a key component in forensics investigations of such things as arson or postblast residue determinations,2 and be used to alleviate the acute, urgent worldwide problem of locating buried landmines.3 Up to now only sniffers (sensors working as an artificial dog’s nose) using advanced materials are able to search large areas and to locate explosives by smelling the compounds. Among these sniffers, fluorescence-based sensors were generally employed due to their sensitivity, selectivity, simplicity, and low cost in instrumentation. Materials used for the preparation of the fluorescent sensors include conjugated organic polymers4 and conjugated inorganic polymers.5 All the sensors were prepared by spin coating one of the polymers on a solid substrate surface. It was reported that low permeability of the NACs in the thin films, the weak binding strength of the films to the analytes, and the energy migration between polymer chains restrict the sensing performance of the films.6 Swager and co-workers6 found that it is possible to improve the sensing performance of a conjugated polymer-based fluorescent film to NACs by introducing bulky structures on the backbone of the polymer. For example, they incorporated rigid three-dimensional pentiptycene moieties in * Corresponding author. Phone: 86-29-85310081. Fax: 86-29-85310097. E-mail: [email protected]. † Shaanxi Normal University. ‡ Xi’an High Technique Institute. (1) Krausa, M.; Reznev, A. A. Vapour and Trace Detection of ExplosiVes for Anti-Terrorism Purposes; Kluwer Academic Publishers: Boston, 2004. (2) (a) Barshick, S.-A. J. Forensic Sci. 1998, 43, 284-293. (b) Smith, K. D.; McCord, B. R.; MacCrehan, W. A.; Mount, K.; Rowe, W. F. J. Forensic Sci. 1999, 44, 789-794. (3) Yinon, J. Trends Anal. Chem. 2002, 21, 292-301. (4) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (5) (a) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 36933702. (b) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125, 3821-3830. (c) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871-2883. (6) (a) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321-5322. (b) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-11873.

the backbone of poly(phenylene-ethynylene) that prevent π-stacking or excimer formation, increase the porosity of the polymer films, and significantly improve the sensing response of the film to TNT. Similar findings have been reported by other groups.7 Adulteration of surfactant into the polymer layer is another way to improve the sensing performances of the conjugated polymer sensors.8 It is believed that introduction of the surfactant enhances the compatibility of the film to the analyte. Inorganic polymers with Si-Si, Ge-Ge, and Si-Ge backbones, in which the monomer units are silicon-, germanium-, or tin-containing metallacyclopentadienes (metalloles), are other kinds of important conjugated polymers because they also exhibit high electron affinity and fast electron mobility.9 It has been demonstrated that the polymetalloles and metallole copolymers are effective materials for building up NACs sensors used both in air and in solution. Unlike conjugated organic polymers, film sensors from conjugated inorganic polymers are robust and insensitive to common interferents, such as organic solvents, inorganic acids, and oxygen in air.10 Furthermore, aggregation of conjugated organic polymers is almost an unavoidable problem for the fabrication of thin films which decreases the emission of the films and inhibits the diffusion of analytes in the films, leading to poor sensing performance. For various silole derivatives, which (7) (a) Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K. S. Langmuir 2001, 17, 7452-7455. (b) Chang, C. P.; Chao, C. Y.; Huang, J. H.; Li, A. K.; Hsu, C. S.; Lin, M. S.; Hsieh, B. R.; Su, A. C. Synth. Met. 2004, 144, 297-301. (8) Chen, L.; McBranch, D.; Wang, R.; Whitten, D. Chem. Phys. Lett. 2000, 330, 27-33. (9) (a) Tracy, H. J.; Mullin, J. L.; Klooster, W. T.; Martin, J. A.; Haug, J.; Wallace, S.; Rudloe, I.; Watts, K. Inorg. Chem. 2005, 44, 2003-2011. (b) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535-1546. (10) Saxena, A.; Fujiki, M.; Rai, R.; Kwak, G. Chem. Mater. 2005, 17, 21812185. (11) (a) Chen, H. Y.; Lam, W. Y.; Luo, J. D.; Ho, Y. L.; Tang, B. Z.; Zhu, D. B.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574-576. (b) Murata, H.; Kafafi, Z. H.; Uchida, M. Appl. Phys. Lett. 2002, 81, 189-191. (c) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Tang, B. Z.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D. Chem. Commun. 2001, 1740-1741. (d) Chen, J.; Xie, Z.; Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z. Macromolecules 2003, 36, 11081117. (e) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319-4327. (f) Lee, M. H.; Kim, D.; Lam, J. W. Y.; Tang, B. Z. J. Korean Phys. Soc. 2004, 45, 329-332.

10.1021/la062773s CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006

Fluorescent Sensors for Nitroaromatic Compounds

can be used for preparing conjugated polysiloles, however, aggregation of them enhances photoluminescence from them.9b,11 For example, polymers incorporating metalloles have been prepared in attempts to construct simple, efficient photoluminescent and electroluminescent devices.11d,e,12 While some linear polysiloles have been observed to exhibit the aggregation-induced enhancement effect,11d,e hyperbranched poly(phenylenesilolene)s and poly(phenylenegermole)s do not show enhanced emission when aggregated, in sharp contrast to that of organic conjugated polymers.11e,12 These two characteristics may distinguish the inorganic conjugated polymers from their counterparts, the organic ones, and make them even useful in a variety of ophotoelectronic applications. Clearly, the interest in these materials has been driven by the ability of the conjugated polymers to create large signal amplification relative to small molecule chemosensors due to the delocalization and rapid diffusion of excitons throughout the individual polymer chains, the so-called molecular wire effect, or one point contact and multipoint response effect, in solution13 and in thin films.6 The limitations of the film sensors are also obvious, although some of them could be overcome by modification of the polymers or by improving the fabricating techniques of the polymer films. But generally speaking, (1) the thickness of the film has a significant effect upon the performance of the sensors, both due to steric resistance to the diffusion of the analytes within the films and due to too many fluorophore molecules per unit area, and (2) leaking of the polymers to the medium is almost unavoidable, particularly when the film sensors are used in solution, leading to contamination of the analysis systems and shortening the lifetime of the film sensors. Considering the limitations of the conjugated polymer-based fluorescent film sensors, we started to seek to develop a new family of fluorescent film sensors via monolayer assembly of polycyclic aromatics on substrate surfaces a few years ago. The methodology adopted for the assembly of the aromatics on substrate surfaces is chemical immobilization of small molecules onto a substrate surface, which is a commonly used approach to prepare monomolecular assemblies and provides a convenient way to produce surfaces with specific chemical functionalities, leading to precise turning of surface properties.14 On the basis of this methodology, a series of fluorescent film sensors for various analytes (including nitrite,15 nitromethane,16 dicarboxylic acids,17 organic copper salts,18 and nitrobenzene,19 in aqueous phase), for the composition of the mixtures of ethanol and water,20 and for the purity of water21 have been prepared in our laboratory. It may be worthwhile to mention that the sensing elements (12) Law, C. C. W.; Chen, J.; Lam, J. W. Y.; Peng, H.; Tang, B. Z. J. Inorg. Organomet. Polym. 2004, 14, 39-51. (13) (a) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017-7018. (b) Kim, Y.; Whitten, J. E.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 1212212130. (14) (a) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (b) Crego-Calama, M.; Reinhoudt, D. N. AdV. Mater. 2001, 13, 1171-1174. (15) Wang, H.; Fang, Y.; Cui, Y.; Hu, D.; Gao, G. Mater. Chem. Phys. 2002, 77, 185-191. (16) Wang, H.; Fang, Y.; Ding, L.; Gao, L.; Hu, D. Thin Solid Films 2003, 440, 255-260. (17) (a) Gao, L.; Fang, Y.; Wen, X.; Li, Y.; Hu, D. J. Phys. Chem. B 2004, 108, 1207-1213. (b) Gao, L.; Fang, Y.; Lu¨, F.; Ding, L. Sci. China, Ser. B 2004, 47, 240-250. (c) Gao, L.; Fang, Y.; Lu¨, F.; Cao, M.; Ding, L. Appl. Surf. Sci. 2006, 252, 3884-3893. (d) Lu¨, F.; Fang, Y.; Gao, L.; Ding, L; Jiang, L. J. Photochem. Photobiol. A 2005, 175, 207-213. (18) Lu¨, F.; Gao, L.; Ding, L.; Jiang, L.; Fang, Y. Langmuir 2006, 22, 841845. (19) Ding, L.; Fang, Y.; Kang, J.; Lu¨, F.; Gao, L.; Yin, X. Thin Solid Films (doi: 10.1016/j.tsf.2006.05.035). (20) Ding, L.; Fang, Y.; Jiang, L.; Gao, L.; Yin, X. Thin Solid Films 2005, 478, 318-325. (21) Fang, Y.; Ning, G.; Hu, D.; Lu, J. J. Photochem. Photobiol. A 2000, 135, 141-145.

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employed in the sensing films reported are all the derivatives of pyrene, dansyl, and anthracene. These aromatics or aromatic derivatives are chosen because they can be excited at a wavelength around 350 nm. This will bring simplicity later on for instrumentation and a guaranteed low cost of the instrument. With this consideration in mind, the films have been designed by varying the length, flexibility, and substructure of the spacers and the rigidity of the substrate. Furthermore, the concepts such as host-guest interaction and molecular recognition have also been introduced in the design of the films. New concepts like “two-dimensional solution” or “spacer layer screening effect” have been proposed and successfully applied to the design of new film sensors.18 For example, organic copper salts, like copper(II) acetate, copper(II) citrate, and copper(II) tartrate; nitrobenzene; and TNT can be selectively determined in aqueous phase. Interference from inorganic compounds can be efficiently avoided due to the spacer layer screening effect. Trace amounts of benzene, acetone, ethanol, and other common organic solvents have little effect upon the determination of NACs like nitrobenzene and TNT. Similar studies have been carried out by some other groups. Reinhoudt and co-workers22 studied the preparation and sensing properties of several fluorescent thin films with dansyl, pyrene, or coumarin immobilized on glass surfaces via silane coupling reagents. It was reported that the films are sensitive to the presence of Na+, Pb2+, or β-cyclodextrin and have a good selectivity to them. Gulino et al.23 reported a sensitive, reversible, and fastresponse optical NO2 sensor by covalently binding porphyrin molecules onto silica substrates through a spacer. Fre´chet and co-workers24 prepared a mixed fluorescent sensing film by simultaneously immobilizing two chromophores (donor and acceptor) onto silica wafers. Bandyopadhyay and his colleagues25 designed a fluorescent film sensor with high selectivity to K+. The selectivity is reached by introduction of crown-ether, which functions as the host of the guest, on the substrate surface. Recently, Leblanc et al.26 synthesized two surfactant molecules with unique head structures that can bind Cu2+ selectively, and the surfactants were assembled into films by employing a LB technique. It was found that the film is a quite ideal Cu2+ sensor. Considering the works depicted above, it is safe to say the following: (1) Monolayer assembly of small fluorophores on substrate surfaces is still an effective way to fabricate fluorescent film sensors. (2) The structures of spacers, the chemical nature of sensing elements and substrates, and the density of fluorophore molecules on substrate surfaces, etc., can be alternated, and besides intensity, parameters from other photophysical phenomena, including excimer formation, fluorescence resonance energy transfer (FRET), and intramolecular charge transfer (ICT), can be employed, and thereby various designs can be made. In short, this methodology provides people more freedom for film design. (3) In this design, sensing elements are directly exposed to the medium, and the diffusion problem, which is commonly encountered in conjugated polymer-based films, should be automatically avoided (at least in theory). (4) The polycyclic(22) (a) van der Veen, N. J.; Flink, S.; Deij, M. A.; Egberink, R. J. M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2000, 122, 6112-6113. (b) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 2229-2230. (c) Crego-Calama, M.; Reinhoudt, D. N. AdV. Mater. 2001, 13, 1171-1174. (23) Gulino, A.; Mineo, P.; Scamporrino, E.; Vitalini, D.; Fragala`, I. Chem. Mater. 2004, 16, 1838-1840. (24) Chrisstoffels, L. A. J.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem. Int. Ed. 2000, 39, 2163-2167. (25) Bandyopadhyay, K.; Shu, L.; Liu, H.; Echegoyen, L. Langmuir. 2000, 16, 2706-2714. (26) Zheng, Y.; Cao, X.; Orbulescu, J.; Konka, V.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. Anal. Chem. 2003, 75, 1706-1712.

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Zhang et al. Table 1. Sensing Molecular Densities of the Films To Be Examined films

pyrene density (molecules/100 Å2)

film 2-1 film 2-2 film 2-3

2.028 2.089 2.982

Figure 1. The general structure of the pyrene functionalized fluorescent films: film 1, n ) 2, m ) 1; film 2, n ) 3, m ) 1; film 3, n ) 4, m ) 1; film 4, n ) 2, m ) 2.

aromatics-based films would enrich NACs when they are used in vapor-phase due to possible electron transfer from the aromatics to the NACs, because the former are electron-rich but the later are poor. (5) In the design, very limited fluorophore molecules can be immobilized on a substrate surface per unit area, which may guarantee the sensitivity of the film sensors.18,22b As for the measurement, it should not be difficult to monitor weak luminescence, since big progress has been made in electronic technology during the last few decades. On the basis of the above discussions, a series of pyrenefunctionalized films with different densities and/or different spacer lengths were prepared via monolayer assembly of it on glass slide surfaces, and their sensing performances to some typical NACs in vapor phase were investigated. This paper reports the details. Experimental Section Materials. Pyrene (Acros, 96%) was recrystallized from an ethanolic solution and then extracted with a Soxhlet extractor with ethanol. Its derivative, pyrenesulfonyl chloride (PSC), was synthesized by adopting a literature method.27 3-Glycidoxypropyltrimethoxysilthoxysilane (GPTS, Acros, 97%), 1,3-diaminopropane (DAP; Fluka, 99%), 1,4-diaminobutane (DAB, Acros, 99%), and diethylenetriamine (DETA, Acros, 98.5%) were used as received. Ethylenediamine (EDA) was washed with solid KOH and then distilled. All the solvents were of analytical grade and used after purification by standard literature methods. NACs, including TNT (2,4,6-trinitrotoluene), DNT (2,4-dinitrotoluene), DNTf(2,6-dinitrotoluene), p-DNB (p-dinitrobenzene), m-DNB (m-dinitrobenzene), NT (p-nitrotoluene), CNB (4-nitrochlorobenzene), and NB (nitrobenzene), were of analytical grade and used directly without further purification. Caution: TNT and other NACs used in the present study are high explosiVes and should be handled only in small quantities. Fluorescence Measurements. Fluorescence measurements were performed at room temperatures on a time-correlated single photon counting fluorescence spectrometer (Edinburgh Instruments FLS 920) with a front face method. The fabricated film was inserted into a quartz cell with its surface facing the excitation light source. The cell was fixed in the solid sample holder of the instrument. The position of the film was kept constant during each set of measurements. Film Preparation and Characterization. The pyrene-functionalized glass slides were prepared in a similar way as that described before.17 However, the fluorescent films prepared during this study are characterized by different lengths of spacers and different densities of the sensing molecules on the substrate surfaces. The densities of were measured by a spectroscopy method,18 of which the details are listed in the Supporting Information. The successful coupling of pyrene on the substrate surfaces was confirmed by employing XPS (27) Ezzell, S. A.; McCormick, C. L. In Water-Soluble Polymers; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; Chapter 8.

Figure 2. Schematics of the designed apparatus. measurements, contact angle measurements, and fluorescence measurements as reported earlier.17 The general structure and main characteristics of the films are shown in Figure 1 and Table 1. Fluorescence Quenching. The fluorescence quenching experiments with NACs vapor were conducted by using three different methods. (a) The film was inserted into a cell with a lid, about 100 mg of TNT or another NAC (except nitrobenzene, which is a liquid at room temperature and instead a few drops were used) was carefully added into the cell, the cell was sealed immediately, and finally the fluorescence spectrum was recorded every 2 min. The quenching efficiency was calculated using eq 1 η)

I0 - I × 100% I0

(1)

(b) The film was inserted into a sealed glass bottle (5 mL in volume) that contains solid NACs. Cotton gauze was used to prevent the direct contact of the film with the NACs, and the vapor pressure of the NACs was maintained constant during the measurement. After a period of exposure, the film was taken out and the fluorescence spectrum was recorded immediately. (c) The fluorescence quenching was also measured by using a specially designed apparatus, which is schematically shown in Figure 2. It is to be noted that all the materials used for fabricating the apparatus are Teflon, except the cell and the pump, to minimize the absorption of NACs by the inner wall of the system. It was expected that the comprehensive sensing performance of the film would be examined more genuinely in this method. The methods designed for the measurement of fluorescence quenching are denoted as method a, method b, and method c, respectively.

Results and Discussion 1. Steady-State Excitation and Emission Spectra of the Films. As expected, the excitation spectra of all the films are similar to each other, irrespective of the loading densities of the fluorophore and the structures of the spacers in the film, and all of them are characterized by that of the fluorophore, pyrene. The profiles of the emission spectra of the films, however, are different from each other. The differences are mainly reflected by the relative intensities of the excimer emission and the monomer emission of the fluorophore. The ratio of the two emissions depends upon the loading density of the fluorophore on the substrate. It is clear that the higher the loading density, the greater the ratio of IE/IM, where IE and IM stand for the intensity of the

Fluorescent Sensors for Nitroaromatic Compounds

Figure 3. Fluorescence excitation and emission spectra of film 2-3 in dry state recorded at various excitation and analysis wavelengths.

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Figure 5. Quenching efficiency of nitrobenzene to the fluorescence emission of film 3 examined in nitrobenzene-saturated air.

three NACs (2.7 × 10-1, 1.74 × 10-4, 8.02 × 10-6 mmHg, respectively at 25 °C). Considering the method adopted for the quenching measurement, it should be reasonable to think that the “nonideal” performance of the film might be a result of either the slow response of the film or slow evaporation of the NACs tested. To clarify the main reason, an additional experiment for NB was conducted in which the cell was presaturated with NB vapor, the film was inserted in it, and the fluorescence emission of it was monitored immediately. The result is shown in Figure 5. Clearly, the response is almost instantaneous, and the quenching efficiency reaches more than 92%, suggesting that the slow evaporation of the NACs might be the main reason. On the basis of the discussions made above, eq 2 could be established to describe the process mathematically (The details for the process can be found in the Supporting Information.), Figure 4. Quenching efficiencies of different NACs to the fluorescence emission of film 3 examined in method a.

excimer emission and that of the monomer emission. As an example, Figure 3 shows the excitation and emission spectra of film 2-3, which is the most typical one and has been studied thoroughly. Upon further examining the excitation and emission spectra of film 2-3, which were recorded at different analysis wavelengths or different excitation wavelengths, it may be supposed, according to Kasha’s rule,28 that the sensing molecules were quite homogeneously distributed on the substrate surfaces, because the profiles of the two spectra are analysis wavelength or excitation wavelength independent. It is to be noted that all the quenching data reported in this paper were calculated from the maximum emission measurements. 2. Evaporation Effect upon the Sensing Performance of Film 3. The evaporation effect of the NACs upon the sensing performance of the films was examined in method a by taking film 3 as a representative of the films and TNT, DNT, and NB as examples of the NACs. As indicated, film 3 was fabricated with high pyrene density (2.8 molecules/100 Å2, more than 48% of the theoretical value18). Figure 4 shows the plots of the quenching efficiency as a function of time for each NAC. It can be observed that (1) it takes hours for the emission of the film to reach equilibrium and (2) the response rate of the film to the three NACs follows an order of NB > DNT > TNT, which is just in accordance of the saturated vapor pressure order of the (28) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publisher: New York, 1999.

I0RTS k1 I ln ) ‚ t I - I∞ I VP 2 K ∞

(2)

0

where I stands for the fluorescence intensity of the film at time t, I0 the fluorescence intensity in the absence of the quencher, and I∞ the fluorescence intensity of the film at t ) ∞ or at equilibrium. P0 is the equilibrium vapor pressure of the compound, and S is the total surface area of the NACs sample. k1 and K are the rate constants and equilibrium constant, respectively. Clearly, the values of I0, I∞, V, P0, R, T, S, k1, and K are all constants or can be kept as constants. Figure 6 depicts the plot of ln[I/(I - I∞)] against t. Reference to the figure reveals that the plots for TNT and DNT are almost perfect straight lines, but the one for NB1 is obviously curved up, indicating that the quenching processes for the former two systems may be really evaporation controlled; in other words, the response rate of the film to the two NACs is dictated by the evaporation rate of them. For NB1, however, the situation might be more complicated. This is because NB1 is liquid at room temperature, and it is almost impossible to keep the total surface area, S, constant during the measurement by simply adding a few drops of it to the cell. To verify of the poor linearity of NB1, the quenching experiment was repeated, but this time the quencher, NB, was contained in a capillary tube, and it was hoped that the surface area responsible for the evaporation would not change very much during the measurement. The plot for this experiment is also shown in the figure (cf. NB2). Compared with the plot for NB1, the linearity of this plot is significantly improved, indicating that the response of the film to NB might be also evaporation controlled.

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Figure 6. Evaporation rate effect upon the sensing performance of film 3 to different NACs examined in method a.

3. Spacer Length Effects. The spacer length effects upon the sensing performances of the films to TNT and NB were examined by employing methods c and a, respectively. It was believed that with method c the experimental efficiency could be accelerated and that the possible variations like change in the position of the sensing films could be avoided. To minimize contamination, the system was completely washed with ethanol several times and left to dry for several hours after each measurement. For the liquid sample, NB, method a may be more suitable for the test, because with method c the NB could be pumped through the pipeline into the corvette, which can distort the experimental results and may result in wrong conclusions. Figure 7a,b shows the experimental results. It is clearly seen that film 2 (film 2-3), which is the one containing diaminopropane as a subunit in its spacer with a high density of pyrene on its surface, is more sensitive to the presence of both TNT and NB tested. For example, it is at least 20% more sensitive to TNT than other sensing films. No doubt, the surface of the substrate will affect the interaction between the sensing element, pyrene, and the quencher, TNT. A short spacer means that the sensing molecules may locate near the substrate surface and should be affected more seriously. For the long spacer, however, it may adopt coiled conformation and wrap the sensing molecule within it and thereby inhibit binding of the quencher molecules. But more work is needed to make clear how the surface affects the sensing performance of this kind of fluorescent film. As for the effect of spacer conformation on the sensing performance of the films, a more general work has been planned, and the study of a monolayer assembly with rigid spacers is in progress. Anyway, for the following studies, all the films used are the ones with diaminopropane as their spacer’s subunit. 4. Sensing Molecule Density Effects. To examine how the density of the sensing molecules, pyrene, immobilized on the substrate surface affects the sensing performance of the films to the NACs, films 2-1, 2-2, and 2-3 were prepared, and the density data are also given in Table 1. Figure 8 depicts the emission spectra of the three films. It can be seen that the excimer emission around 480 nm (perfect excimer) increases upon increasing the loading density of the sensing molecules on the substrate surface. In fact, for film 2-1, the emission is dominated by the monomer emission of the sensing element. For film 2-2, however, the emission is characterized by both monomer and excimer emissions, and the excimer emission is centered at 450 nm, indicating formation of distorted excimers. This fact may be rationalized by considering that the low density of the sensing molecules and the limited length of the spacer connecting the

Figure 7. (a) Quenching efficiencies of TNT to the fluorescence emissions of films 1-4 of high pyrene densities examined in method c. (b) Quenching efficiencies of NB to the fluorescence emissions of films 1-4 of high pyrene densities examined in method a.

Figure 8. Fluorescence emission spectra of films 2-1, 2-2, and 2-3 in dry state (λex ) 350 nm).

sensing molecules and the substrate surface restricted the sensing molecules to adopt a position that may be necessary for the formation of a perfect excimer. In other words, the final structures of the excimers formed are balances of thermostability and dynamical possibility. Similarly, for film 2-3, the high density of the sensing molecules guaranteed them having more opportunities to form more stable excimers like that characterized by the emission at longer wavelength. In contrast, for film 2-1, the low loading density of the sensing molecules minimized

Fluorescent Sensors for Nitroaromatic Compounds

Figure 9. Quenching efficiencies of TNT to the fluorescence emissions of films 2-1, 2-2, and 2-3 examined in method b. For the three films, the analyses were conducted at λex ) 350 nm and λem ) 380, 450, and 490 nm, respectively.

their chances to reach their neighbors, and thereby they have little chance to form excimers, even distorted excimers. The sensing performances of the three films to NACs were examined in method b and by taking TNT as an example NAC. The results are shown in Figure 9. Upon examination of the figure, it can be observed that film 2-1 is more sensitive to the presence of TNT. One-minute exposure of the film to saturated TNT vapor causes nearly 22% quenching of the fluorescence emission (λem ) 380 nm). The quenching increases along with further exposure of the film to the vapor. Nearly 80% of the emission was quenched when the integrated exposure time reaches 10 min. The performance of film 2-2 (λem ) 450 nm) is comparable with that of film 2-1, except its response at longer times is weaker. For the film 2-3 (λem)490 nm), however, it takes much longer time to reach equilibrium, as shown by the large positive slope of the plot. This result is not difficult to understand, because in theory the quenching occurs on the basis of one (fluorophore molecules) to one (quencher molecules) and the saturated vapor pressure of TNT is very low (∼5 ppb, 25 °C), and thereby the film with the lower pyrene density should consume less quencher molecules, leading to greater quenching efficiency. 5. Reversibility of the Quenching Process. The reversibility of the sensing of the films to NACs was examined with film 2-1 as an example film and TNT as an example NAC. The film was first exposed to a saturated TNT vapor at room temperature for 9 min and then its emission spectrum was recorded. After the measurement, the film was cleaned by immersing it in an ethanol solvent for 5 min and rinsing with distilled water several times. The emission spectrum of the film was recorded again after blower drying. The whole process was repeated for several times, and the results are shown in Figure 10. Clearly, the response of the film to the presence of TNT vapor tends to stabilize after three uses. 6. Quenching Mechanism of the Sensing. The quenching mechanism of the sensing was also studied by comparing the quenching results from steady-state measurement and those from time-resolved measurement. Upon examination of the plots shown in Figure 11, it is obvious that the lifetime of the sensing molecules did not change very much with increasing exposure time of the film in TNT vapor, a sharp contrast to that of the emission intensity, indicating that the quenching was mainly caused by formation of a nonfluorescent complex, F-Q, in support of the assumption that the fluorophore and the quencher may form an electron-transfer complex. But the stability of the complex should not be high, since the quencher is easy to be freed from the film

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Figure 10. The reversibility of the response of film 2-1 to the presence of TNT examined in method b (λex/λem ) 350/380 nm).

Figure 11. The ratios of I0/I and τ0/τ as functions of exposure time in TNT vapor for film 2-3 examined in method b.

surface, suggesting that the film forms a new kind of sensing materials for NACs. 7. Interference from Other Chemicals. To test the effect of common interferents and environmental contaminants, a series of control experiments, using film 2-1 as an example film, were conducted. No significant change in fluorescence emission intensity of the film was observed upon exposing it to the vapor of organic solvents and daily chemicals such as benzene, toluene, ethanol, and perfume. Similarly, exposure of the film to the vapor phase of solids like 2,4-dinitrophenylhydrazine, 2,4,6trinitrophenol, o-nitroaniline, and apple has little effect upon the emission of the film. More interestingly, smoke has also no effect upon the emission of the film. All these results reveal the insensitivity of the film to such interferents.

Conclusions In the present work, a class of fluorescent films in which pyrene was assembled on glass slide surfaces in a monolayer manner was revealed to be sensitive to the presence of traces amount of NAC vapors. The response rate is determined by the evaporation rate of the NACs detected. Spacer length effect studies demonstrated that within the films studied the sensing performances of the one with a spacer containing diaminopropane as a subunit is superior to those of others. Furthermore, for films with the same spacer structure, the sensing performance of the one with lower loading density of pyrene is much better than the others. Further examination showed that the responses of the

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films to NACs are reversible and their sensing performances are not affected by common interferents. The sensitive response, reversibility of the sensing process, and freedom from commonly encountered interference of the specially designed films to NACs may qualify these materials as promising NACs fluorescent sensory materials. Acknowledgment. We would like to thank the National Natural Science Foundation of China (Nos. 20373039, 20543002) and the Ministry of Education of China (Nos. of China

Zhang et al.

20040718001, 306015) for financial support. We gratefully thank Mr. Jinbo Yang for his kind assistance with the process of solving the mathematical questions. Supporting Information Available: Details of the process for determining eq 2 and of calculating the loading density of pyrene on the glass slide surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA062773S