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Conjugated polymer-grafted silica nanoparticles for the sensitive detection of TNT
Sensors and Actuators B 145 (2010) 438–443
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Conjugated polymer-grafted silica nanoparticles for the sensitive detection of TNT Jichang Feng, Yang Li ∗ , Mujie Yang ∗ Department of Polymer Science and Engineering, Zhejiang University, Key Laboratory of Macromolecule Synthesis and Functionalization, Ministry of Education, Hangzhou 310027 China
a r t i c l e
i n f o
Article history: Received 11 October 2009 Received in revised form 14 December 2009 Accepted 17 December 2009 Available online 29 December 2009 Keywords: Conjugated polymer Silica nanoparticle TNT Fluorescent chemosensor
a b s t r a c t Conjugated polymer-grafted silica nanoparticles were prepared by Sonogashira coupling reaction involving a two-step procedure. The resulting conjugated polymer-grafted silica nanoparticles were characterized by infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, and absorption and fluorescence spectroscopy. The conjugated polymer-grafted silica nanoparticles are highly fluorescent, and the fluorescent response of the nanoparticles of different size towards electrondeficient trinitrotoluene (TNT) was studied by examining how the steady state fluorescence intensity changes with the concentration of TNT. The conjugated polymer-grafted silica nanoparticles show high sensitivity toward TNT, with a detection limit down to 1 M. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the fast and sensitive detection of nitroaromatic compounds, which have been widely used as explosives both in military and engineering, has aroused great attention due to increasing concerns for anti-terrorism and environmental protection [1–7]. Traditional methods for nitroaromatics assays such as gas chromatography coupled to a mass spectrometer, ion mobility spectrometry, and neutron activation analysis, are time-consuming. Moreover, these techniques are cumbersome and cannot be easily fielded in a small and low-power package [4]. Thus, the development of new, highly sensitive, and real-time analytical materials for explosives remains a challenge. It is known that fluorescence chemosensors show superiority of high sensitivity and selectivity in the detection of trace amount of analyte [8–11]. Specifically, conjugated polymers have been explored as chemosensory materials for the fluorescence detection of nitroaromatic explosives such as trinitrotoluene (TNT) [12–15]. Swager et al. reported the synthesis of several poly (pphenyl acetylenes) containing pentiptycene groups, which was demonstrated as highly sensitive chemosensors materials for TNT and dinitrotoluene (DNT) [16]. Recently, Smith et al. synthesized a new poly(p-phenylene ethynylene) incorporating sterically
∗ Corresponding authors. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail addresses: [email protected] (Y. Li), [email protected] (M. Yang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.12.056
enshrouding m-terphenyl oxacyclophane canopies, which showed high sensitivity over the common poly(p-phenylene ethynylene) toward vapors of nitro-organics such as TNT [17]. Other photoluminescent materials such as poly(tetraphenylsilole) [18,19], poly(tetraphenylgermole) [20] and photoluminescent silica films [21] have also been used for the fluorescence detection of nitroaromatic explosives. The incorporation of conjugated polymers into an inorganic matrix and the development of organic–inorganic hybrid materials is an efficient method to finetune the optical and electronic properties as well as to improve the environmental stability. The hybrids exhibit improved sensitivity and selectivity compared with conjugated polymers alone in chemosensor systems. There have been some reports on the incorporation of conjugated polymers into silica matrix [22–24]. The silica nanoparticles are particularly suited for the realization of the fluorescent-based chemosensors, because they are optically transparent and photophysically inert, and their surface can easily be modified by the coupling reactions with alkoxysilane derivatives. In this paper, conjugated polymer-grafted silica nanoparticles of different sizes were prepared and characterized. These polymergrafted silica nanoparticles are of interest as explosive fluorescent chemosensor materials. The fluorescent response of the nanoparticles of different sizes towards TNT of various concentrations was examined in various solvents. The conjugated polymer-silica nanoparticles exhibited high sensitivity towards TNT, and showed great potential in the fabrication of sensitive fluorescent chemical sensors.
J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443
2. Experiment 2.1. Materials (3-Aminopropyl)trimethoxysilane (APTS) and 4-iodobenzoic acid were purchased from Alfa Aesar. 1,4-Diethynylbenene was purified by reduced pressure sublimation. Uniform silica nanoparticles with diameter in the range of 78–198 nm were synthesized by the hydrolysis of TEOS with aqueous ammonia, according to the reported Stöber method [25]. 1,4-Diiodo2,5-di(propyloxysulfonate)benzene (2I-Ben-SO3 ) was synthesized according to the reference [26]. All the solvents used in this study were purified according to standard methods prior to use, and other reagents were commercially obtained and used without purification. 2.2. Measurements FT-IR spectra were measured on a Bruker Vector 22 IR spectrometer. 1 H NMR spectra were recorded using a Bruker Avance DMX500 spectrometer, operating at 500 MHz (solvent: CDCl3 , or DMSO-d6 ; internal standard: tetramethylsiliane). TEM images were obtained using JEM-1230 operating at an acceleration voltage of 60 kV. A drop of 0.5 mg/mL solution was placed onto a copper grid with carbon film and dried before measurement. The diameter and size distribution of the nanoparticles were determined by DLS at 90◦ angle to the incident beam and at 25 ◦ C on a Brookhaven 90 Plus particle size analyzer. Thermogravimetric analyses (TGA) were conducted on a PerkinElmer Pyris 6 TGA at a heating rate of 20 ◦ C/min. The elemental analyses were performed on ThermoFinnigan Instrument Flash EA1112. The UV–vis spectra were recorded on a Cary 100 Bio UV–vis spectrophotometer. Photoluminescence (PL) measurements were carried out on a PerkinElmer LS55 Luminescence spectrometer. 2.3. Synthesis of monomers and polymer 2.3.1. Preparation of 4-iodobenzoyl chloride (IBC) To an excess of thionyl chloride (30 mL) was added 5 g (20 mmol) of 4-iodobenzoic acid. The solution was heated at reflux for 2 h, afterwards thionyl chloride was distilled. A white power was obtained. Yield: 85%. 1 H NMR (CDCl3 ): ı = 7.90 (d, J = 8.6 Hz, 2H), 7.80 (d, J = 8.7 Hz, 2H). FT-IR (KBr): 2943, 2878, 1776, 1723, 1678, 1579, 1463, 1378, 1195, 1178, 1054, 1009, 853, 812, 710, 689, 617, 842 cm−1 . 2.3.2. 4-Iodo-N-[3-(trimethoxysilyl)propyl]benzamide (APTS-I) A solution of APTS (1.78 mL, 10 mmol) in 10 mL of pyridine was added dropwise to a suspension of IBC (1.33 g, 5 mmol) in 20 mL of pyridine at 0 ◦ C. The mixture was stirred for 1 h, and the solvent was evaporated in vacuum. The residue was purified by column chromatography on silica with ethyl acetate/n-hexane (v/v, 3/1) as eluent. The solvent was removed by rotary evaporation to give viscous oil. Yield: 78%. 1 H NMR (CDCl3 ): ı = 7.77 (d, J = 8.5 Hz, 2H), 7.49 (d, J = 8.5 Hz, 2H), 6.55 (s, 1H), 3.57 (s, 9H), 3.45 (m, 2H), 1.75 (m, 2H), 0.72 (t, J = 8.1 Hz, 2H). (C13 H20 INO4 Si) (409.29): Calcd. C 38.15, H 4.93, N 3.42; Found: C 38.24, H 4.95, N 3.49. 2.3.3. Preparation of polymer PPS A mixture of 20 mL of water, 30 mL of DMF, and 10 mL of diisopropylamine were placed in an isobarically funnel which was connected to a Schlenk flask. And then 1.008 g (1.55 mmol) of monomer 2I-Ben-SO3 , 0.189 g (1.50 mmol) of 1, 4diethynylbenzene, 50 mg of Pd(PPh3 )2 Cl2 , and 10.0 mg of CuI were added to the flask rapidly. The whole system was deoxygenated by a gentle flow of argon for 30 min, and the solvent mixture was
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dropped into the reaction flask slowly. The final mixture was then stirred at 50 ◦ C for 24 h. After cooled to room temperature, the reactant was slowly added to 1 L of a methanol/acetone/ether mixture (volume ratio = 1:4:5). The resulting precipitation was filtered, then redissolved in 200 mL of water/methanol (volume ratio = 7:3) and treated with 0.1 g of sodium sulfide (Na2 S). Afterwards the solution was filtered with quantitative filter paper, and reprecipitated by addition to a large volume of methanol/acetone/ether mixture (1:4:5). The precipitation was centrifugated to collect the polymer PPS as a brown powder. Yield: 67%. 1 H NMR (DMSO-d6 ): ı = 7.59 (4H), 7.21 (2H), 4.16 (4H), 2.71 (4H), 2.09 (4H). FT-IR (KBr): 2943, 2878, 1626, 1515, 1467, 1413, 1281, 1191, 1049, 1003, 878, 835, 735, 666, 614, 532 cm−1 . 2.4. Surface modification of silica nanoparticles (SiO2 -I) To a solution of silica particles (200 mg) in 10 mL of toluene, compound APTS-I (200 mg, 0.53 mmol) in 5 mL THF solution was added under argon atmosphere. The reaction solution was refluxed for 4 h. The surface-modified silica particles SiO2 -I were collected via centrifugation and washed with toluene and acetone several times. The particles were dried under vacuum overnight. 2.5. Grafting of polymer onto silica nanoparticles (SiO2 -PPS) 1,4-Diethynylbenzene (23.7 mg, 0.188 mmol), surface-modified silica particles (SiO2 -ArI) (100 mg), CuI (4.5 mg), and Pd(PPh3 )2 Cl2 (11.2 mg) were dissolved in a mixture of DMF (30 mL) and triethylamine (10 mL). The reaction mixture was refluxed for 2 h, and then 2I-Ben-SO3 (119 mg, 0.184 mmol) in water (20 mL) was added dropwise. The reaction mixture was refluxed overnight. The polymer-grafted silica particles were collected via centrifugation and washed with methanol several times. The particles were washed successively with multiple portions of THF, methanol, and water. Washing with methanol and water was repeated until the solution exhibited no yellow color with blue fluorescence under a UV lamp. Finally, the particles were washed with acetone and dried under vacuum overnight. 3. Results and discussion 3.1. Synthesis and characterization of SiO2 -PPS nanoparticles The synthetic routes of SiO2 -PPS and corresponding monomers are illustrated in Scheme 1. In order to introduce aryl iodide functionality onto the SiO2 surface, APTS was reacted with 4iodobenzoyl chloride to form the corresponding amide (APTS-I) firstly. For the surface modification reactions, the number of equivalents of trialkoxysilane to silica particles was calculated according the Schanze method [27], and >50 equiv of APTS-I was reacted with silica nanoparticles. The conjugated polymer PPS containing diethynylbenzene and 2I-Ben-SO3 units was synthesized via Sonogashira coupling reaction. The polymer was characterized by FT-IR, 1 H NMR, and viscometry. Polymer solutions with various concentrations ranging between 1.0 and 0.2 g/dL were prepared in DMSO. Following a literature procedure [28], the intrinsic viscosity of the polymer was found to be 0.15 dL/g. From this value, the molecular weight of the polymer was estimated to be ∼20 kD. To graft the polymer onto the surface of the SiO2 particles, solution polymerization under the same conditions was carried out in the presence of APTS-I-modified silica particles (SiO2 -I). A substantial quantity of the PPS has been grafted to the surface of the silica nanoparticles, as discussed below. The FT-IR spectra of silica, PPS and their hybrids are shown in Fig. 1. It can be seen that the spectra are in agreement with the reference report [27]. Specifically, in the spectrum of SiO2 -PPS,
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J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443
Scheme 1. Synthetic routes of the monomers and SiO2 -PPS.
the enhanced shoulder at around 1210 cm−1 is observed (Fig. 1d), which is associated with the sulfonate group on the polymer side chain. Moreover, the expansion of the spectrum shows a band at 2201 cm−1 , confirming the presence of the ethynyl bonds in the polymer backbone. The spectrum observations indicate the existence of PPS chain on the surface of silica particles. Fig. 2 compares the TGA traces of unfunctionalized silica nanoparticles, surface-modified silica nanoparticles, and the PPSgrafted silica nanoparticles. The unmodified particles exhibit a weight loss of approximately 10% over the temperature range of 150–800 ◦ C, which can be attributed to loss of strongly adsorbed water and dehydration of residual silanol units. In the TGA trace of PPS, the maximum decomposition rate is around 350 ◦ C and an onset with 5% mass loss occurs at around 312 ◦ C. The surfacemodified sample exhibits a greater weight loss with temperature compared with the unmodified silica nanoparticles. It is proposed that the increased loss is associated with the presence of organic material on the particle surfaces. Thus, the TGA results further sup-
Fig. 1. Infrared spectra of silica particles: (a) unfunctionalized silica particles (SiO2 ), (b) aryl iodide-modified silica particles (SiO2 -I), (c) PPS-grafted silica particles (SiO2 PPS), and (d) free polymer (PPS).
port that PPS has been successfully grafted onto the surface of SiO2 particles. The surface texture of the particles caused by surface grafting of the polymer was observed by TEM. As observed in Fig. 3a, the unfunctionalized silica particles (SiO2 , 164 nm) are featured with smooth surface. In contrast, the PPS-grafted silica particles exhibit a rough surface texture (Fig. 3b and c), which is associated with the presence of the polymer on the surface. Moreover, the numerical statistics (>100 nanoparticles were selected) revealed that the average particle size is increased from original 164 to 188 nm after grafting. 3.2. Absorption and fluorescent properties The absorption and fluorescence of PPS were characterized in methanol and water, and the results are summarized in Table 1. The polymer exhibits a 30 nm hypochromatic shift in its absorption maximum upon switching of the solvent from water to methanol. It
Fig. 2. Thermogravimetric analysis of unfunctionalized silica particles (SiO2 ), PPSgrafted silica particles (SiO2 -PPS), and free polymer (PPS).
J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443
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Table 1 Photophysical properties of PPS and SiO2 -PPS. Solvent
max (nm)
em (nm)
PPS
Methanol H2 O DMSO
439 468 431
444 532 456
SiO2 -PPS
Methanol DMSO
359 336
500 460
matic shifted by almost 90 nm upon switching of the solvent from water to methanol. Suspensions of PPS-grafted silica particles in methanol also exhibit strong fluorescence. As shown in Fig. 4, the fluorescence of the surface-grafted particles (SiO2 -PPS) appears as a very broad and structureless band, with max = 500 nm (ex at 336 nm). The redshift of the fluorescence of SiO2 -PPS compared to that of PPS in methanol suggests that the polymer is strongly aggregated on the surface of silica. Moreover, the absorption spectrum of SiO2 -PPS shows a max at 359 nm, which is blue-shifted from that of free polymer in methanol. The blue shift may be ascribed to the fact that the length of the surface-grafted polymer is lower than that of the solution-polymerized polymer on average. Alternatively, it may result from the distortion of conformation of the conjugated backbone in the surface-grafted polymer layer, just as reported by Ogawa et al. [27]. The fluorescence quantum efficiency of SiO2 -PPS in methanol solution was 0.27, which was measured by excitation of the corresponding solution and compared with the solution emission of quinine sulfate in 0.05 M H2 SO4 (standard, ϕ = 0.546) [29]. 3.3. Fluorescence response of PPS and SiO2 -PPS towards TNT The fluorescence quenching properties of the PPS and SiO2 PPS nanoparticles were studied by systematically examining the steady state fluorescence intensity changes in response to electrondeficient TNT in solutions. Fig. 5 shows the emission spectra of PPS in DMSO solution upon successive addition of TNT of different concentrations with an excitation wavelength of 415 nm. The quenching of PPS shows a linear curvature in the Stern–Volmer plot (I0 /I − 1 vs. [TNT], where I and I0 are the fluorescence intensity measured with and without the addition of TNT) [30] in the concentration range of (5–270) × 10−5 M (inset in Fig. 5). The Stern–Volmer constant (KSV ) is found to be 73 M−1 . Fig. 6 shows the emission spectra of SiO2 -PPS (88 nm) in DMSO upon successive addition of TNT ([TNT] = (5–18.5) × 10−5 M) with
Fig. 3. Transmission electron microscopy of silica particles: (a) unfunctionalized silica nanoparticles, (b) SiO2 -PPS, and (c) SiO2 -PPS.
is due to aggregation of the polymer chains in water. In comparison, aggregation in water has a more significant effect on the fluorescence properties of the polymer. The fluorescence spectrum of PPS shows a broad band with max = 532 nm in water (ex at 438 nm). On the other hand, the emission spectrum becomes a narrow band with max = 444 nm and a shoulder at about 470 nm in methanol (ex at 415 nm). Moreover, the fluorescence max for PPS is hypochro-
Fig. 4. Absorption and emission spectra of PPS in methanol, and SiO2 -PPS as a suspension in methanol.
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J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443 Table 2 KSV Values for free polymer PPS and SiO2 -PPS. KSV (M−1 )
Solvent
Sample
MeOH
PPS PPSa SiO2 -PPSa (188 nm)
196 323 732
DMSO
PPS SiO2 -PPS (216 nm) SiO2 -PPS (188 nm) SiO2 -PPS (139 nm) SiO2 -PPS (88 nm)
73 1300 2290 3490 3110
a 10 L of hexadecyl trimethyl ammonium bromide was added into 2 mL of detection solution.
Fig. 5. Fluorescence spectra and KSV plot of PPS (inset) upon addition of variable concentrations of TNT in DMSO. [PPS] = 1 × 10−5 M in RU, [TNT] = 0–2.7 × 10−3 M.
an excitation wavelength of 336 nm. Herein, the quenching of SiO2 PPS shows an upward non-linear curvature in the Stern–Volmer plot when TNT concentration is higher than 7.5 × 10−4 M (inset of Fig. 6). The curve deviates from the linear trend and bends upward, indicating the occurrence of super-quenching due to the involvements of self-absorption and/or energy transfer [30]. At lower concentration of TNT (0–7.5 × 10−4 M), a linear Stern–Volmer plot is obtained with a Stern–Volmer constant (KSV ) of 3.11 × 103 M−1 , which is much higher than that of the PPS polymer alone. The linear plot of the relative fluorescence intensity of the SiO2 -PPS versus the concentration of TNT (inset of Fig. 6) can be used to detect TNT lower than 75 × 10−5 M. The florescent response of different sized PPS-grafted silica nanoparticles (88–216 nm) were examined towards TNT in DMSO, and the results are summarized in Table 2. It is worth noting that, for all cases, the PPS-grafted silica nanoparticles systems show larger KSV values than solutions of PPS alone. Interestingly, the value of KSV increases with the decrease in the size of SiO2 -PPS. It may be explained as follows: first, the probability of the contact of TNT with SiO2 -PPS is enhanced with the increase in the surface area of nanoparticles; second, the polymer chain attached to silica parti-
Fig. 7. Evolution of fluorescent intensity with increasing amounts of TNT into 2 mL of the SiO2 -PPS suspension.
cles may get closer when the size of SiO2 -PPS is reduced, which enhances the interchain electron/energy migration and results in more efficient quenching. The data in Table 2 suggest that the SiO2 -PPS nanoparticles are applicable to detect trace amount of TNT in solution. Fig. 7 shows the change of fluorescent intensity with increasing amounts of TNT in the solution of SiO2 -PPS nanoparticles. Apparently, the fluorescence of nanoparticles was gradually quenched with the addition of increasing amount of TNT. It can be seen that 1 M of TNT in solution resulted in ∼2% decrease in the fluorescence intensity of the nanoparticles, which is five times the noise level of the spectrometer system. Accordingly, the attenuation of fluorescence intensity of SiO2 -PPS can be used to detect TNT in solution down to 1 M at least, suggesting its high sensitivity. 4. Conclusions Conjugated polymer-grafted silica nanoparticles were prepared by Sonogashira coupling reaction. The conjugated polymer-grafted silica nanoparticles are highly fluorescent, and the fluorescence of the polymer-grafted silica nanoparticles can be effectively quenched by electron-deficient TNT. Both the solvents and the size of nanoparticles have effect on its florescent response towards TNT. The conjugated polymer-grafted silica nanoparticles show high sensitivity toward TNT in solution, and the detection limit could be as low as 1 M. Acknowledgement
Fig. 6. Fluorescence spectra and KSV plot of SiO2 -PPS (inset) upon addition of variable concentrations of TNT in DMSO. (diameter of PPS-grafted silica nanoparticles 88 nm, [TNT] = 0–1.85 × 10−3 M).
The authors thank Ms. Qian Yang of Zhejiang University for help in materials preparation and characterization.
J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443
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Biographies Jichang Feng is a Ph.D. student in the department of polymer science and engineering, Zhejiang University. His research interests include synthesis and applications of functional polymers. Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic composites for chemical sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests include functional polymers with optical and electrical characteristics.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Conjugated polymer-grafted silica nanoparticles for the sensitive detection of TNT Jichang Feng, Yang Li ∗ , Mujie Yang ∗ Department of Polymer Science and Engineering, Zhejiang University, Key Laboratory of Macromolecule Synthesis and Functionalization, Ministry of Education, Hangzhou 310027 China
a r t i c l e
i n f o
Article history: Received 11 October 2009 Received in revised form 14 December 2009 Accepted 17 December 2009 Available online 29 December 2009 Keywords: Conjugated polymer Silica nanoparticle TNT Fluorescent chemosensor
a b s t r a c t Conjugated polymer-grafted silica nanoparticles were prepared by Sonogashira coupling reaction involving a two-step procedure. The resulting conjugated polymer-grafted silica nanoparticles were characterized by infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, and absorption and fluorescence spectroscopy. The conjugated polymer-grafted silica nanoparticles are highly fluorescent, and the fluorescent response of the nanoparticles of different size towards electrondeficient trinitrotoluene (TNT) was studied by examining how the steady state fluorescence intensity changes with the concentration of TNT. The conjugated polymer-grafted silica nanoparticles show high sensitivity toward TNT, with a detection limit down to 1 M. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the fast and sensitive detection of nitroaromatic compounds, which have been widely used as explosives both in military and engineering, has aroused great attention due to increasing concerns for anti-terrorism and environmental protection [1–7]. Traditional methods for nitroaromatics assays such as gas chromatography coupled to a mass spectrometer, ion mobility spectrometry, and neutron activation analysis, are time-consuming. Moreover, these techniques are cumbersome and cannot be easily fielded in a small and low-power package [4]. Thus, the development of new, highly sensitive, and real-time analytical materials for explosives remains a challenge. It is known that fluorescence chemosensors show superiority of high sensitivity and selectivity in the detection of trace amount of analyte [8–11]. Specifically, conjugated polymers have been explored as chemosensory materials for the fluorescence detection of nitroaromatic explosives such as trinitrotoluene (TNT) [12–15]. Swager et al. reported the synthesis of several poly (pphenyl acetylenes) containing pentiptycene groups, which was demonstrated as highly sensitive chemosensors materials for TNT and dinitrotoluene (DNT) [16]. Recently, Smith et al. synthesized a new poly(p-phenylene ethynylene) incorporating sterically
∗ Corresponding authors. Tel.: +86 571 87952444; fax: +86 571 87952444. E-mail addresses: [email protected] (Y. Li), [email protected] (M. Yang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.12.056
enshrouding m-terphenyl oxacyclophane canopies, which showed high sensitivity over the common poly(p-phenylene ethynylene) toward vapors of nitro-organics such as TNT [17]. Other photoluminescent materials such as poly(tetraphenylsilole) [18,19], poly(tetraphenylgermole) [20] and photoluminescent silica films [21] have also been used for the fluorescence detection of nitroaromatic explosives. The incorporation of conjugated polymers into an inorganic matrix and the development of organic–inorganic hybrid materials is an efficient method to finetune the optical and electronic properties as well as to improve the environmental stability. The hybrids exhibit improved sensitivity and selectivity compared with conjugated polymers alone in chemosensor systems. There have been some reports on the incorporation of conjugated polymers into silica matrix [22–24]. The silica nanoparticles are particularly suited for the realization of the fluorescent-based chemosensors, because they are optically transparent and photophysically inert, and their surface can easily be modified by the coupling reactions with alkoxysilane derivatives. In this paper, conjugated polymer-grafted silica nanoparticles of different sizes were prepared and characterized. These polymergrafted silica nanoparticles are of interest as explosive fluorescent chemosensor materials. The fluorescent response of the nanoparticles of different sizes towards TNT of various concentrations was examined in various solvents. The conjugated polymer-silica nanoparticles exhibited high sensitivity towards TNT, and showed great potential in the fabrication of sensitive fluorescent chemical sensors.
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2. Experiment 2.1. Materials (3-Aminopropyl)trimethoxysilane (APTS) and 4-iodobenzoic acid were purchased from Alfa Aesar. 1,4-Diethynylbenene was purified by reduced pressure sublimation. Uniform silica nanoparticles with diameter in the range of 78–198 nm were synthesized by the hydrolysis of TEOS with aqueous ammonia, according to the reported Stöber method [25]. 1,4-Diiodo2,5-di(propyloxysulfonate)benzene (2I-Ben-SO3 ) was synthesized according to the reference [26]. All the solvents used in this study were purified according to standard methods prior to use, and other reagents were commercially obtained and used without purification. 2.2. Measurements FT-IR spectra were measured on a Bruker Vector 22 IR spectrometer. 1 H NMR spectra were recorded using a Bruker Avance DMX500 spectrometer, operating at 500 MHz (solvent: CDCl3 , or DMSO-d6 ; internal standard: tetramethylsiliane). TEM images were obtained using JEM-1230 operating at an acceleration voltage of 60 kV. A drop of 0.5 mg/mL solution was placed onto a copper grid with carbon film and dried before measurement. The diameter and size distribution of the nanoparticles were determined by DLS at 90◦ angle to the incident beam and at 25 ◦ C on a Brookhaven 90 Plus particle size analyzer. Thermogravimetric analyses (TGA) were conducted on a PerkinElmer Pyris 6 TGA at a heating rate of 20 ◦ C/min. The elemental analyses were performed on ThermoFinnigan Instrument Flash EA1112. The UV–vis spectra were recorded on a Cary 100 Bio UV–vis spectrophotometer. Photoluminescence (PL) measurements were carried out on a PerkinElmer LS55 Luminescence spectrometer. 2.3. Synthesis of monomers and polymer 2.3.1. Preparation of 4-iodobenzoyl chloride (IBC) To an excess of thionyl chloride (30 mL) was added 5 g (20 mmol) of 4-iodobenzoic acid. The solution was heated at reflux for 2 h, afterwards thionyl chloride was distilled. A white power was obtained. Yield: 85%. 1 H NMR (CDCl3 ): ı = 7.90 (d, J = 8.6 Hz, 2H), 7.80 (d, J = 8.7 Hz, 2H). FT-IR (KBr): 2943, 2878, 1776, 1723, 1678, 1579, 1463, 1378, 1195, 1178, 1054, 1009, 853, 812, 710, 689, 617, 842 cm−1 . 2.3.2. 4-Iodo-N-[3-(trimethoxysilyl)propyl]benzamide (APTS-I) A solution of APTS (1.78 mL, 10 mmol) in 10 mL of pyridine was added dropwise to a suspension of IBC (1.33 g, 5 mmol) in 20 mL of pyridine at 0 ◦ C. The mixture was stirred for 1 h, and the solvent was evaporated in vacuum. The residue was purified by column chromatography on silica with ethyl acetate/n-hexane (v/v, 3/1) as eluent. The solvent was removed by rotary evaporation to give viscous oil. Yield: 78%. 1 H NMR (CDCl3 ): ı = 7.77 (d, J = 8.5 Hz, 2H), 7.49 (d, J = 8.5 Hz, 2H), 6.55 (s, 1H), 3.57 (s, 9H), 3.45 (m, 2H), 1.75 (m, 2H), 0.72 (t, J = 8.1 Hz, 2H). (C13 H20 INO4 Si) (409.29): Calcd. C 38.15, H 4.93, N 3.42; Found: C 38.24, H 4.95, N 3.49. 2.3.3. Preparation of polymer PPS A mixture of 20 mL of water, 30 mL of DMF, and 10 mL of diisopropylamine were placed in an isobarically funnel which was connected to a Schlenk flask. And then 1.008 g (1.55 mmol) of monomer 2I-Ben-SO3 , 0.189 g (1.50 mmol) of 1, 4diethynylbenzene, 50 mg of Pd(PPh3 )2 Cl2 , and 10.0 mg of CuI were added to the flask rapidly. The whole system was deoxygenated by a gentle flow of argon for 30 min, and the solvent mixture was
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dropped into the reaction flask slowly. The final mixture was then stirred at 50 ◦ C for 24 h. After cooled to room temperature, the reactant was slowly added to 1 L of a methanol/acetone/ether mixture (volume ratio = 1:4:5). The resulting precipitation was filtered, then redissolved in 200 mL of water/methanol (volume ratio = 7:3) and treated with 0.1 g of sodium sulfide (Na2 S). Afterwards the solution was filtered with quantitative filter paper, and reprecipitated by addition to a large volume of methanol/acetone/ether mixture (1:4:5). The precipitation was centrifugated to collect the polymer PPS as a brown powder. Yield: 67%. 1 H NMR (DMSO-d6 ): ı = 7.59 (4H), 7.21 (2H), 4.16 (4H), 2.71 (4H), 2.09 (4H). FT-IR (KBr): 2943, 2878, 1626, 1515, 1467, 1413, 1281, 1191, 1049, 1003, 878, 835, 735, 666, 614, 532 cm−1 . 2.4. Surface modification of silica nanoparticles (SiO2 -I) To a solution of silica particles (200 mg) in 10 mL of toluene, compound APTS-I (200 mg, 0.53 mmol) in 5 mL THF solution was added under argon atmosphere. The reaction solution was refluxed for 4 h. The surface-modified silica particles SiO2 -I were collected via centrifugation and washed with toluene and acetone several times. The particles were dried under vacuum overnight. 2.5. Grafting of polymer onto silica nanoparticles (SiO2 -PPS) 1,4-Diethynylbenzene (23.7 mg, 0.188 mmol), surface-modified silica particles (SiO2 -ArI) (100 mg), CuI (4.5 mg), and Pd(PPh3 )2 Cl2 (11.2 mg) were dissolved in a mixture of DMF (30 mL) and triethylamine (10 mL). The reaction mixture was refluxed for 2 h, and then 2I-Ben-SO3 (119 mg, 0.184 mmol) in water (20 mL) was added dropwise. The reaction mixture was refluxed overnight. The polymer-grafted silica particles were collected via centrifugation and washed with methanol several times. The particles were washed successively with multiple portions of THF, methanol, and water. Washing with methanol and water was repeated until the solution exhibited no yellow color with blue fluorescence under a UV lamp. Finally, the particles were washed with acetone and dried under vacuum overnight. 3. Results and discussion 3.1. Synthesis and characterization of SiO2 -PPS nanoparticles The synthetic routes of SiO2 -PPS and corresponding monomers are illustrated in Scheme 1. In order to introduce aryl iodide functionality onto the SiO2 surface, APTS was reacted with 4iodobenzoyl chloride to form the corresponding amide (APTS-I) firstly. For the surface modification reactions, the number of equivalents of trialkoxysilane to silica particles was calculated according the Schanze method [27], and >50 equiv of APTS-I was reacted with silica nanoparticles. The conjugated polymer PPS containing diethynylbenzene and 2I-Ben-SO3 units was synthesized via Sonogashira coupling reaction. The polymer was characterized by FT-IR, 1 H NMR, and viscometry. Polymer solutions with various concentrations ranging between 1.0 and 0.2 g/dL were prepared in DMSO. Following a literature procedure [28], the intrinsic viscosity of the polymer was found to be 0.15 dL/g. From this value, the molecular weight of the polymer was estimated to be ∼20 kD. To graft the polymer onto the surface of the SiO2 particles, solution polymerization under the same conditions was carried out in the presence of APTS-I-modified silica particles (SiO2 -I). A substantial quantity of the PPS has been grafted to the surface of the silica nanoparticles, as discussed below. The FT-IR spectra of silica, PPS and their hybrids are shown in Fig. 1. It can be seen that the spectra are in agreement with the reference report [27]. Specifically, in the spectrum of SiO2 -PPS,
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Scheme 1. Synthetic routes of the monomers and SiO2 -PPS.
the enhanced shoulder at around 1210 cm−1 is observed (Fig. 1d), which is associated with the sulfonate group on the polymer side chain. Moreover, the expansion of the spectrum shows a band at 2201 cm−1 , confirming the presence of the ethynyl bonds in the polymer backbone. The spectrum observations indicate the existence of PPS chain on the surface of silica particles. Fig. 2 compares the TGA traces of unfunctionalized silica nanoparticles, surface-modified silica nanoparticles, and the PPSgrafted silica nanoparticles. The unmodified particles exhibit a weight loss of approximately 10% over the temperature range of 150–800 ◦ C, which can be attributed to loss of strongly adsorbed water and dehydration of residual silanol units. In the TGA trace of PPS, the maximum decomposition rate is around 350 ◦ C and an onset with 5% mass loss occurs at around 312 ◦ C. The surfacemodified sample exhibits a greater weight loss with temperature compared with the unmodified silica nanoparticles. It is proposed that the increased loss is associated with the presence of organic material on the particle surfaces. Thus, the TGA results further sup-
Fig. 1. Infrared spectra of silica particles: (a) unfunctionalized silica particles (SiO2 ), (b) aryl iodide-modified silica particles (SiO2 -I), (c) PPS-grafted silica particles (SiO2 PPS), and (d) free polymer (PPS).
port that PPS has been successfully grafted onto the surface of SiO2 particles. The surface texture of the particles caused by surface grafting of the polymer was observed by TEM. As observed in Fig. 3a, the unfunctionalized silica particles (SiO2 , 164 nm) are featured with smooth surface. In contrast, the PPS-grafted silica particles exhibit a rough surface texture (Fig. 3b and c), which is associated with the presence of the polymer on the surface. Moreover, the numerical statistics (>100 nanoparticles were selected) revealed that the average particle size is increased from original 164 to 188 nm after grafting. 3.2. Absorption and fluorescent properties The absorption and fluorescence of PPS were characterized in methanol and water, and the results are summarized in Table 1. The polymer exhibits a 30 nm hypochromatic shift in its absorption maximum upon switching of the solvent from water to methanol. It
Fig. 2. Thermogravimetric analysis of unfunctionalized silica particles (SiO2 ), PPSgrafted silica particles (SiO2 -PPS), and free polymer (PPS).
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Table 1 Photophysical properties of PPS and SiO2 -PPS. Solvent
max (nm)
em (nm)
PPS
Methanol H2 O DMSO
439 468 431
444 532 456
SiO2 -PPS
Methanol DMSO
359 336
500 460
matic shifted by almost 90 nm upon switching of the solvent from water to methanol. Suspensions of PPS-grafted silica particles in methanol also exhibit strong fluorescence. As shown in Fig. 4, the fluorescence of the surface-grafted particles (SiO2 -PPS) appears as a very broad and structureless band, with max = 500 nm (ex at 336 nm). The redshift of the fluorescence of SiO2 -PPS compared to that of PPS in methanol suggests that the polymer is strongly aggregated on the surface of silica. Moreover, the absorption spectrum of SiO2 -PPS shows a max at 359 nm, which is blue-shifted from that of free polymer in methanol. The blue shift may be ascribed to the fact that the length of the surface-grafted polymer is lower than that of the solution-polymerized polymer on average. Alternatively, it may result from the distortion of conformation of the conjugated backbone in the surface-grafted polymer layer, just as reported by Ogawa et al. [27]. The fluorescence quantum efficiency of SiO2 -PPS in methanol solution was 0.27, which was measured by excitation of the corresponding solution and compared with the solution emission of quinine sulfate in 0.05 M H2 SO4 (standard, ϕ = 0.546) [29]. 3.3. Fluorescence response of PPS and SiO2 -PPS towards TNT The fluorescence quenching properties of the PPS and SiO2 PPS nanoparticles were studied by systematically examining the steady state fluorescence intensity changes in response to electrondeficient TNT in solutions. Fig. 5 shows the emission spectra of PPS in DMSO solution upon successive addition of TNT of different concentrations with an excitation wavelength of 415 nm. The quenching of PPS shows a linear curvature in the Stern–Volmer plot (I0 /I − 1 vs. [TNT], where I and I0 are the fluorescence intensity measured with and without the addition of TNT) [30] in the concentration range of (5–270) × 10−5 M (inset in Fig. 5). The Stern–Volmer constant (KSV ) is found to be 73 M−1 . Fig. 6 shows the emission spectra of SiO2 -PPS (88 nm) in DMSO upon successive addition of TNT ([TNT] = (5–18.5) × 10−5 M) with
Fig. 3. Transmission electron microscopy of silica particles: (a) unfunctionalized silica nanoparticles, (b) SiO2 -PPS, and (c) SiO2 -PPS.
is due to aggregation of the polymer chains in water. In comparison, aggregation in water has a more significant effect on the fluorescence properties of the polymer. The fluorescence spectrum of PPS shows a broad band with max = 532 nm in water (ex at 438 nm). On the other hand, the emission spectrum becomes a narrow band with max = 444 nm and a shoulder at about 470 nm in methanol (ex at 415 nm). Moreover, the fluorescence max for PPS is hypochro-
Fig. 4. Absorption and emission spectra of PPS in methanol, and SiO2 -PPS as a suspension in methanol.
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J. Feng et al. / Sensors and Actuators B 145 (2010) 438–443 Table 2 KSV Values for free polymer PPS and SiO2 -PPS. KSV (M−1 )
Solvent
Sample
MeOH
PPS PPSa SiO2 -PPSa (188 nm)
196 323 732
DMSO
PPS SiO2 -PPS (216 nm) SiO2 -PPS (188 nm) SiO2 -PPS (139 nm) SiO2 -PPS (88 nm)
73 1300 2290 3490 3110
a 10 L of hexadecyl trimethyl ammonium bromide was added into 2 mL of detection solution.
Fig. 5. Fluorescence spectra and KSV plot of PPS (inset) upon addition of variable concentrations of TNT in DMSO. [PPS] = 1 × 10−5 M in RU, [TNT] = 0–2.7 × 10−3 M.
an excitation wavelength of 336 nm. Herein, the quenching of SiO2 PPS shows an upward non-linear curvature in the Stern–Volmer plot when TNT concentration is higher than 7.5 × 10−4 M (inset of Fig. 6). The curve deviates from the linear trend and bends upward, indicating the occurrence of super-quenching due to the involvements of self-absorption and/or energy transfer [30]. At lower concentration of TNT (0–7.5 × 10−4 M), a linear Stern–Volmer plot is obtained with a Stern–Volmer constant (KSV ) of 3.11 × 103 M−1 , which is much higher than that of the PPS polymer alone. The linear plot of the relative fluorescence intensity of the SiO2 -PPS versus the concentration of TNT (inset of Fig. 6) can be used to detect TNT lower than 75 × 10−5 M. The florescent response of different sized PPS-grafted silica nanoparticles (88–216 nm) were examined towards TNT in DMSO, and the results are summarized in Table 2. It is worth noting that, for all cases, the PPS-grafted silica nanoparticles systems show larger KSV values than solutions of PPS alone. Interestingly, the value of KSV increases with the decrease in the size of SiO2 -PPS. It may be explained as follows: first, the probability of the contact of TNT with SiO2 -PPS is enhanced with the increase in the surface area of nanoparticles; second, the polymer chain attached to silica parti-
Fig. 7. Evolution of fluorescent intensity with increasing amounts of TNT into 2 mL of the SiO2 -PPS suspension.
cles may get closer when the size of SiO2 -PPS is reduced, which enhances the interchain electron/energy migration and results in more efficient quenching. The data in Table 2 suggest that the SiO2 -PPS nanoparticles are applicable to detect trace amount of TNT in solution. Fig. 7 shows the change of fluorescent intensity with increasing amounts of TNT in the solution of SiO2 -PPS nanoparticles. Apparently, the fluorescence of nanoparticles was gradually quenched with the addition of increasing amount of TNT. It can be seen that 1 M of TNT in solution resulted in ∼2% decrease in the fluorescence intensity of the nanoparticles, which is five times the noise level of the spectrometer system. Accordingly, the attenuation of fluorescence intensity of SiO2 -PPS can be used to detect TNT in solution down to 1 M at least, suggesting its high sensitivity. 4. Conclusions Conjugated polymer-grafted silica nanoparticles were prepared by Sonogashira coupling reaction. The conjugated polymer-grafted silica nanoparticles are highly fluorescent, and the fluorescence of the polymer-grafted silica nanoparticles can be effectively quenched by electron-deficient TNT. Both the solvents and the size of nanoparticles have effect on its florescent response towards TNT. The conjugated polymer-grafted silica nanoparticles show high sensitivity toward TNT in solution, and the detection limit could be as low as 1 M. Acknowledgement
Fig. 6. Fluorescence spectra and KSV plot of SiO2 -PPS (inset) upon addition of variable concentrations of TNT in DMSO. (diameter of PPS-grafted silica nanoparticles 88 nm, [TNT] = 0–1.85 × 10−3 M).
The authors thank Ms. Qian Yang of Zhejiang University for help in materials preparation and characterization.
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Biographies Jichang Feng is a Ph.D. student in the department of polymer science and engineering, Zhejiang University. His research interests include synthesis and applications of functional polymers. Yang Li received his Ph.D. degree in polymer chemistry and physics from Zhejiang University in 2000. He has been working in Department of Polymer Science and Engineering, Zhejiang University since 2000 and was appointed associate professor in polymer science in 2002. His research interests include polymer materials and organic/inorganic composites for chemical sensors. Mujie Yang graduated from Zhongshan University, China in 1963. She has been working in Zhejiang University since 1963. She was promoted to full professor in polymer science in 1992. Her research interests include functional polymers with optical and electrical characteristics.