DOI: 10.1002/chem.201304390
Communication
& Fluorescent Probes
Water-Soluble Polymer Functionalized CdTe/ZnS Quantum Dots: A Facile Ratiometric Fluorescent Probe for Sensitive and Selective Detection of Nitroaromatic Explosives Bingxin Liu,[a] Cuiyan Tong,*[a] Lijuan Feng,[a, b] Chunyu Wang,[a] Yao He,[a] and Changli L*[a]
Chem. Eur. J. 2014, 20, 2132 – 2137
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Communication Abstract: A ratiometric fluorescent probe based on dual luminescence QD/CPL for selective sensing of the nitroaromatic explosive picric acid (PA) was constructed. The observed ratiometric fluorescence intensity change allows the quantitative detection of PA with a detection limit of 9 nm.
Nitroaromatic compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and picric acid (PA) are always known as homemade explosives. Selective and sensitive detection of these explosives is very important in countering terrorist threats and pollution of soil and groundwater.[1] Fluorescence detection based either on Fçrster resonance energy transfer (FRET) or electron transfer (ET) mechanism has attracted much attention in recent years owing to its high sensitivity, simplicity, short response time, and its ability to be employed both in solution and solid phase.[1–3] However, there are few reports about rapid and selective detection of nitroexplosives, especially PA.[1] The aggregates of hexaphenylbenzene derivatives could be used as an efficient and sensitive fluorescent chemosensor for PA.[4] Fang and co-workers explored a fluorescent self-assembled monolayer film sensor for the detection of PA among other analogues.[5] Organic dyes and conjugated polymers also show high selectivity for PA.[6] However, all of them exhibit a turn-off fluorescent response to PA in nonaqueous solutions. Xu et al. constructed a ratiometric NIR fluorescent probe for PA based on the intramolecular charge transfer in CH3CN[3] (various fluorescence methods for the detection of PA are summarized in Table S1 in the Supporting Information). To the best of our knowledge, there are yet no reports on the direct exploration of ratiometric response to PA in visible region in aqueous solution. Quantum dots (QDs) have special physical dimensions that lay between bulk and discrete molecules.[7] Many studies have focused on the control of optical and photophysical properties of QDs by surface modification with functional molecules.[8] Chemically modified CdSe/ZnS QDs were used as fluorescent probes by Willner and co-workers to detect TNT or trinitrotriazine (RDX) with a low limit of detection (LOD) of 1 nm.[9] It is known that 8-hydroxyquinoline (HQ) and its derivatives can coordinate the surface of nanoparticles such as ZnS and CdS to form stable fluorescent complexes because these particles have a great number of surface metal atoms.[10] The inherent luminescent emission of QDs may have a cooperating interaction with that of the HQ–metal complexes formed on the QDs [a] B. Liu, Dr. C. Tong, Dr. L. Feng, C. Wang, Y. He, Prof. C. L College of Chemistry, Northeast Normal University Changchun 130024 (China) E-mail: [email protected] [email protected] [b] Dr. L. Feng Centre of Analytical and Test, Beihua University Jilin 132013 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304390. Chem. Eur. J. 2014, 20, 2132 – 2137
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surface. Thus, the HQ and its derivatives can be used as functional ligands to tailor the optical properties of the QDs when HQ is anchored to the semiconductor nanoparticles.[11] However, the intrinsic fluorescence of these QDs, such as ZnS and CdS, is too weak to fabricate the dual luminescence QDs needed for analyte detection. Herein, we reported for the first time water soluble macromolecular ligand functionalized high-quality QDs as a scaffold for constructing a ratiometric fluorescent nanosensor for selective detection of nitroexplosives in aqueous media. As shown in Scheme 1, 5-(2-methacryloylethyloxymethyl)-8-quinolinol
Scheme 1. Schematic illustration of QDs/CPL-2 as selective ratiometric fluorescent detection for PA through electron transfer (ET).
(MQ) was selected as functional ligand to copolymerize with Nisopropylacrylamide (NIPAM) monomer to fabricate a novel copolymer ligand (CPL). The CPL exhibits a good solubility in aqueous solution. Then the CPL was used to decorate the surface of water-soluble CdTe/ZnS QDs via coordinate bond to form QDs/CPL hybrids with a dual luminescence emission. It is found that the fluorescence quenching of QDs/CPL systems is observed due to the electron transfer (ET) from QDs to the coordinated CPL on the surface of QDs. However, picric acid (PA) can efficiently quench the inherent fluorescence of QDs through the competitive electron transfer, while the surfacecoordination emission of the Zn–CPL complex on QDs can be recovered because the ET from QDs to Zn-complex is interrupted in the presence of PA. So, the QDs/CPL systems can be used as a water-soluble ratiometric fluorescent probe for sensitive and selective sensing of PA by using the interesting luminescence property. The novel copolymer ligand (CPL) of p(NIPAM-co-MQ) was prepared by conventional free radical copolymerization of NIPAM and MQ monomers. The CPL structure has been confirmed and the molar ratio of NIPAM and MQ units in CPL was calculated to be 58:1 by 1H NMR. Gel permeation chromatogra-
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Communication phy (GPC) measurements revealed that the number average molecular weight (Mn) of the copolymer was around 26 000 with a polydispersity index (PDI) of 1.63. The CdTe/ZnS QDs were obtained through in situ growth of ZnS shell on the surface of CdTe core in alkaline solution.[12] The size of CdTe/ZnS QDs was confirmed through transmission electron microscopy (TEM) (Figure 1) with the average diameter of 3.7 nm. Here,
of QDs due to the steric hindrance effect provided by the surface of QDs and the existence of Zn S bonds as compared with the conventional small coordination compounds such as Zn(MQ)2.[13] The photoluminescence (PL) properties of the CPL functionalized QDs are shown in Figure 3. There are two emission centers, including the inherent luminescent emission (606 nm) of
Figure 1. a) TEM image and b) HRTEM image of pure QDs.
Glutathione (GSH) was used both as capping reagent and sulfur source. Then the multifunctional hybrid QDs were prepared through coordination bonds between QDs and CPL in aqueous solutions. The samples from QDs/CPL-1 to QDs/CPL-3 represent the CPL-functionalized CdTe/ZnS QDs with molar feed ratios of [QDs]/[CPL] = 3:200, 3:600, and 3:1000. The interaction between QDs and CPL was supported by UV/Vis spectra (Figure 2). The absorption signal at 259 nm for QD/CPL systems is associated with the p–p* electron transition from quinoline ring. A new absorption band, which is caused by the metal–quinolate transition on the surface of QDs,[13] can be observed at 380 nm, and the intensity of this signal increases with the increasing molar fraction of CPL. The above results indicate that an increasing amount of MQ segments on CPL chains are anchored to the surface of QDs to form the metalloquinolates.[11, 13] In addition, the new species at labs = 325 nm is similar to that of pure Zn–CPL complex (Figure 1 insertion), indicating that the new species should be attributed to the single coordination bound of Zn–CPL complex on the surface
Figure 2. UV/Vis absorption spectra of QDs/CPL systems. The inset shows the UV/Vis absorption spectrum of pure Zn–CPL complex in aqueous solution. Chem. Eur. J. 2014, 20, 2132 – 2137
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Figure 3. PL emission spectra of pure QDs and different QDs/CPL systems.
QDs and the surface-coordination emission (517 nm) of Zn– CPL complex formed on the surface of QDs. With the increasing concentration of CPL in QDs solution, the characteristic emission of QDs is suppressed and the surface coordination emission of the Zn–CPL complex on QDs is increased. Even though an obvious spectral overlap (see Figure S1 in the Supporting Information) can be observed between the luminescence emission of Zn–CPL complex and the absorption band of pure QDs, we believe that the fluorescence quenching in QDs/CPL systems is possibly caused by charge separation photoinduced electron transfer from the photoexcited QDs to the coordinated CPL on the surface of QDs and charge recombination.[14] Actually, the surface-coordination emission of the Zn– CPL complex is also obviously quenched due to the ET interaction.[15] Cyclic voltammetry (CV, see Figure S2 in the Supporting Information) studies of QDs and Zn–CPL complex confirm the electron-transfer event as the conduction band (CB, 3.9 eV) of the QDs facilitates the electron to jump to the lower energy lowest unoccupied molecular orbital (LUMO) of the Zn–CPL complex ( 4.9 eV). Besides, the full width at half maximum (FWHM) of PL spectra also gradually broadens due to the cooperating interaction between QDs and CPL. Pure QDs and QD/CPL-2 were used as fluorescent probes to study the sensing property for nitroaromatic compounds including TNT, DNT, and PA. Figure 4 shows the fluorescence ratiometric responses of pure QDs and QD/CPL-2 to trace analyte explosives in aqueous solution. When pure QDs were used as probes, the fluorescence quenching efficiency of PA and TNT resulted to be almost the same because both of them have extremely strong electron affinity to QDs.[16] Thus, the selective detection of TNT and PA is very difficult for pure QDs. Figure 4 A shows the fluorescence spectral changes in QD/CPL-2
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Figure 5. Evolution of UV/Vis absorption spectra with addition of GSH (50 mg) into 20 mL of PA solution (1 mm). Color inset shows the corresponding colors of the PA solutions before and after adding GSH.
Figure 4. Evolution of the fluorescence spectra of QDs/CPL-2 with increasing volumes of A) PA (1 10 6 mol/L), B) TNT (1 10 4 mol/L), and C) pure Zn– CPL complex and QDs with increasing concentration of PA. D) Fluorescenceintensity changes (I517/I606) for QDs/CPL-2 aqueous solution as the concentration of PA, TNT, and DNT is increased.
aqueous solution upon addition of PA. While the PL intensity at 606 nm decreases, the PL intensity at 517 nm increases with the successive increasing concentration of PA in QD/CPL-2 solution. The organic amine-capped (GSH) QDs can bind PA species from solution by the acid base pairing interaction between amino ligands and acid phenol rings. PA is a stronger acid than TNT, and has the stronger acid base pairing interaction occurred between PA and amino ligands, thus resulting in the formation of PA anions at the surface of amine-capped QDs.[17] The CV measurement (Figure S2 in the Supporting Information) confirms that the electron transfer from the conduction band of QDs to PA bound on the surface of QDs is thermodynamically feasible because the LUMO ( 3.89 eV) of PA is lower as compared to the conduction band ( 3.9 eV) of QDs.[18] Thus, the photogenerated electron is trapped by PA on the surface of QDs more easily than that of Zn–CPL complex since the LUMO of Zn–CPL complex is much lower ( 4.9 eV). Therefore, the resultant PA anions bound onto the amino monolayer can efficiently quench the photoluminescence of QDs through the competitive electron transfer.[1, 17] However, the surface-coordination emission originated from Zn–CPL complex on the QDs surface is recovered because the ET from QDs to Zn-complex is interrupted due to the presence of PA, thereby minimizing the fluorescence quenching effect of the Zn-complex. Actually, the pure Zn-complex is not sensitive for all the mentioned nitroaromatic explosives (Figure 4 C left). Moreover, the absorption band of the acid–base pairing species of GSH and PA was observed at below 470 nm when adding PA into the solution of GSH (Figure 5). Meanwhile, we can clearly see that the PA solution changes from bright yellow into colorless with the addition of cysteamine, as shown in the inset image of Figure 5. The wavelength of visible absorption of PA anions is much
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shorter than the emission wavelength of Zn–CPL complex. Thus, the FRET between PA and QD/CPL-2 can be excluded. However, both the emission sites at 517 and 606 nm decrease with the increase of TNT concentration in QD/CPL-2 solution as shown in Figure 4 B. This result suggests that the photoinduced electron transfer is not the only quenching mechanism of QD/CPL-2 for TNT. It is well known that the charge transfer from electron-rich amino groups to electron-deficient aromatic rings leads to the formation of a Meisenheimer complex with addition of organic amines. Here the amino groups from GSH capped on the surface of QDs can chemically recognize and absorb TNT molecules by the formation of the Meisenheimer complex mentioned above[7] (figure S3 in the Supporting Information shows the UV/Vis absorption spectra after adding GSH into TNT solution). Visible absorptions at 515 nm were observed. It can be seen that the Meisenheimer complex absorbs the green part of visible light, and strongly suppresses the fluorescence emission of the surface-coordination complex Zn–CPL at 517 nm through FRET.[19] As a result, the surface-coordination emission of Zn–CPL complex on the surface of QDs is gradually quenched with the adding amount of TNT in QD/ CPL-2 system. Thus, we assume that FRET and ET take place simultaneously when TNT is added into our QD/CPL system. Namely, the fluorescence emission of the surface-coordination complex Zn–CPL complex at 517 nm is suppressed through FRET while the PL quenching at 606 nm is caused by ET. Also the PL intensity of the QD/CPL-2 system shows a relatively slight decline with increasing concentration of DNT in solution (Figure 6). It is well known that the electron-accepting ability of DNT with two nitro groups is much weaker than TNT molecules with three nitro groups, and so the consequent weak electron-accepting ability towards the amino groups leads to the low quenching efficiency of DNT.[20] The quenching constant (KSV) at 606 nm is calculated with the Stern–Volmer equation to be 8.4 105 m 1 for PA, 1.1 104 m 1 for TNT, and 4.1 103 m 1 for DNT. It can be noted that the ratios (I517/I606) of the emission intensities at 517 and 606 nm for QD/CPL-2 system increase with the increasing concentration of PA (Figure 4 D). However, I517/I606 shows almost no change upon incre-
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Communication Trinitrotoluene (TNT) in methanol (1 mg mL 1) was obtained from Aladdin. 2,4-Dinitrotolene (DNT) was purchased from TCI and picric acid (PA) was obtained from Xilong Chemical Reagent Co., Ltd. Nisopropylacrylamide (NIPAM, Aldrich) was recrystallized in hexane. Other chemicals were of analytical grade without purification.
Synthesis of CdTe/ZnS QDs
Figure 6. Evolution of fluorescence spectra of QDs/CPL-2 with increasing volume of DNT (1 10 6 mol/L).
mental addition of a TNT or DNT solution. This result can render the QD/CPL-2 a highly selective ratiometric fluorescent sensing for PA. The LOD of the probe can be determined by kSB/m, where k is a constant, usually k = 3, SB is the standard deviation of the blank signal, and m is the slope of I517/I606 versus the analyte concentration plot. The calculated detection limit of PA using QD/CPL-2 as a probe can reach 9 nm. However, the detection limit of PA can only reach 9 mm when pure QDs are used as the probe. We have still evaluated the effect of different metallic ion on the QDs/CPL-2 by PL emission spectra (Figure S4 in the Supporting Information). The result shows that the presence of these ions scarcely affects the fluorescence at 606 nm and 517 nm because there are almost no free quinoline groups in the QD/CPL-2 system. Thus, these metallic ions will not lead the QDs/CPL to form fluorescent metal-chelate complexes. This result indicates that our QDs/CPL system will not exhibit the ratiometric response to these ions. Thus, the novel fluorescent probe shows highly selective optical sensing for PA. In conclusion, we have successfully fabricated functionalized QDs decorated with CPL containing 8-hydroxyquinoline and NIPAM units coordinatively bonded. The novel fluorescent probe that is based on the fluorescence turn-on mechanism can fulfil the ratiometric response to PA. The present study provides a new insight into highly selective fluorescent sensing of the nitroaromatic explosive PA with a detection limit of 9 nm. In addition, we expect this study to open up a new perspective in the design of ratiometric fluorescent sensors for various analytes based on functionalized dual-emission quantum dot hybrids.
Synthesis of the copolymer ligand of p(NIPAM-co-MQ) 5-(2-Methacryloylethyloxymethyl)-8-quinolinol (MQ) was synthesized according to a previous procedure.[22, 23] A typical synthesis of the copolymer ligand of p(NIPAM-co-MQ) is described as follows. NIPAM (5 g), MQ (0.05 g), and AIBN (0.35 g) in a tetrahydrofuran solution (125 mL) were added to a 250 mL three-necked round-bottomed flask fitted with a magnetic stirrer, a reflux condenser, and a nitrogen inlet. The solution was heated gradually and was kept at 60 8C for 20 h. The resulting products were reprecipitated using hexane and washed several times. After drying under vacuum for one day, the CPL was obtained. 1H NMR (400 MHz, CDCl3): d = 3 9.80 (1 H, Ph OH), 8.75–7.08 (5 H, Ph H), 4.85 (2 H, CH2 Ph), 3.69 (4 H, CH2 O), 1.88 ppm (9 H, CH3).
Copolymer ligand-functionalized CdTe/ZnS QDs A series of copolymers of P(NIPAM-co-MQ) as prepared above were dissolved in ultrapure water (10 mL). To the aqueous solution, the above dialyzed CdTe/ZnS QDs were slowly titrated at different molar feed ratios (3:200, 3:600, and 3:1000) of QDs to CPL. The mixtures were stirred at room temperature for 12 h. Finally, the obtained samples of CPL-functionalized CdTe/ZnS QDs with different molar feed ratios were defined as QDs/CPL-1 to QDs/CPL-3, respectively.
Nitroaromatic explosive detection experiment for QDs/CPL-2 Various volumes of PA (1 10 6 mol/L), TNT (1 10 4 mol/L), and DNT (1 10 4 mol/L) in aqueous solution were poured into QDs/ CPL-2 (5 mL). The fluorescent signals were collected after having stabilized for 5 min the QDs/CPL-2 aqueous solution containing different nitroexplosives. The test was carried out at room temperature.
Experimental Section Materials Thioglycolic acid (TGA, 99 %), NaBH4 (96 %), and tellurium powder (9.8 %) were obtained from Sinopharm Chemical Reagent Co., Ltd. ZnCl2 (99 %) and CdCl2·2.5H2O (99 %) were obtained from Shanghai Reagent Company. Reduced Glutathione (GSH, 98 %) was purchased from Beijing Jing Ke Hong Da Biotechnology Co. Ltd. 2,4,6Chem. Eur. J. 2014, 20, 2132 – 2137
Aqueous colloidal CdTe solution was synthesized by the procedures based on Zhang’s method.[21] In brief, adding freshly prepared NaHTe solution to 1.25 10 3 mol/L N2-saturated CdCl2 solution at pH 9.0 in the presence of MPA as a stabilizing agent. The molar ratio of Cd2 + /MPA/HTe was fixed at 1:2.4:0.5. The resulting mixture was then subjected to a reflux that controlled the growth of CdTe nanocrystals. The obtained CdTe colloids were dialysed for one week before the further use. The core-shell CdTe/ZnS QDs were prepared in aqueous phase following the method described elsewhere with some medications.[12] 40 mg of as-prepared CdTe sample was added to 50 mL solution (pH 8) containing 1 mmol L 1 ZnCl2 and 4 mmol L 1 GSH as a capping agent. The solution was heated to 100 8C under open-air conditions and was refluxed for 10 min. The obtained core-shell CdTe/ZnS QDs were dialyzed for one week before the further use.
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Characterization 1
H NMR spectra (in CDCl3) were recorded on a DRX-400 MHz (Bruker) spectrometer with TMS as an internal standard. The molec-
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Communication ular weights of polymer were measured at a flow rate of 1.0 mL min 1 at 25 8C by gel permeation chromatography (GPC) equipped with Waters 1515 pump and Waters 2414 differential refractive index detector. CHCl3 was used as eluent, and the molecular weights were determined vs polystyrene standards. Transmission electron microscopy (TEM) was recorded on a JEM-2100 microscope. UV/Vis absorption spectra were recorded on a Shimadzu UV-2550 UV/Vis spectrometer in the range 200–800 nm. The PL properties were measured on a Cary Eclipse fluorescence spectrometer. Cyclic voltammetry (CV) was performed on an electrochemical workstation CHI-660D. The counter electrode is platinum, the working electrode is a glassy carbon electrode and an Ag/AgCl serves as a reference electrode. All the CV experiments were carried out in aqueous solutions at room temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21074019) and the Natural Science Foundation of Jilin Province (20101539). Keywords: hydroxyquinolines · picric acid · quantum dots · ratiometric fluorescent probes · sensors [1] a) Y. Salinas, R. Martinez-Manez, M. D. Marcos, F. Sancenon, A. M. Castero, M. Parra, S. Gil, Chem. Soc. Rev. 2012, 41, 1261 – 1296; b) S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee, S. K. Ghosh, Angew. Chem. 2013, 125, 2953 – 2957; Angew. Chem. Int. Ed. 2013, 52, 2881 – 2885. [2] a) K. Zhang, H. Zhou, Q. Mei, S. Wang, G. Guan, R. Liu, J. Zhang, Z. Zhang, J. Am. Chem. Soc. 2011, 133, 8424 – 8427; b) M. E. Germain, M. J. Knapp, Chem. Soc. Rev. 2009, 38, 2543 – 2555. [3] a) Y. Xu, B. Li, W. Li, J. Zhao, S. Sun, Y. Pang, Chem. Commun. 2013, 49, 4764 – 4766; b) R. Freemana, I. Willner, Chem. Soc. Rev. 2012, 41, 4067 – 4085. [4] V. Bhalla, S. Kaur, V. Vij, M. Kumar, Inorg. Chem. 2013, 52, 4860 – 4865.
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[5] G. He, H. Peng, T. Liu, M. Yang, Y. Zhang, Y. Fang, J. Mater. Chem. 2009, 19, 7347 – 7353. [6] a) S. J. Toal, D. Magde, W. C. Trogler, Chem. Commun. 2005, 5465 – 5467; b) M. Kumar, S. I. Reja, V. Bhalla, Org. Lett. 2012, 14, 6084 – 6087; c) N. Venkatramaiah, S. Kumar, S. Patil, Chem. Commun. 2012, 48, 5007 – 5009; d) Z. F. An, C. Zheng, R. F. Chen, J. Yin, J. J. Xiao, H. F. Shi, Y. Tao, Y. Qian, W. Huang, Chem. Eur. J. 2012, 18, 15655 – 15661. [7] a) X. Zhang, S. Mohandessi, L. W. Miller, P. T. Snee, Chem. Commun. 2011, 47, 3532 – 3534; b) X. Li, Y. Zhou, Z. Zheng, X. Yue, Z. Dai, S. Liu, Z. Tang, Langmuir 2009, 25, 6580 – 6586. [8] a) H. Zhu, N. Song, T. Lian, J. Am. Chem. Soc. 2010, 132, 15038 – 15045; < lit b > H. Meng, Y. Yang, Y. Chen, Y. Zhou, Y. Liu, X. Chen, H. Ma, Z. Tang, D. Liu, L. Jiang, Chem. Commun. 2009, 2293 – 2295. [9] a) R. Freeman, T. Finder, L. Bahshi, R. Gill, I. Willner, Adv. Mater. 2012, 24, 6416 – 6421; b) R. Freeman, I. Willner, Nano Lett. 2009, 9, 322 – 326. [10] a) J. P. Phillips, Chem. Rev. 1956, 56, 271 – 297; < lit b > D. S. Bohle, C. J. Spina, J. Phys. Chem. C 2009, 113, 14435 – 14439. [11] a) X. L, J. Yang, Y. Fu, Q. Liu, B. Qi, C. L, Z. Su, Nanotechnology 2010, 21, 115702; b) B. Liu, X. L, C. Wang, C. Tong, Y. He, C. L, Dyes Pigm. 2013, 99, 192 – 200. [12] Y. F. Liu, J. S. Yu, J. Colloid Interface Sci. 2010, 351, 1 – 9. [13] C. L, J. Gao, Y. Fu, Y. Du, Y. Shi, Z. Su, Adv. Funct. Mater. 2008, 18, 3070 – 3079. [14] a) R. S. Dibbell, D. F. Watson, J. Phys. Chem. C 2009, 113, 3139 – 3149; b) K. E. Knowles, M. Malicki, E. A. Weiss, J. Am. Chem. Soc. 2012, 134, 12470 – 12473. [15] B. Liu, X. L, C. Tong, C. Wang, L. Feng, Y. He, C. L, RSC Adv. 2013, 3, 21298 – 21301. [16] B. Xu, X. Wu, H. Li, H. Tong, L. Wang, Macromolecules 2011, 44, 5089 – 5092. [17] R. Tu, B. Liu, Z. Wang, D. Gao, F. Wang, Q. Fang, Z. Zhang, Anal. Chem. 2008, 80, 3458 – 3465. [18] S. Pramanik, V. Bhalla, M. Kumar, Anal. Chim. Acta 2013, 793, 99 – 106. [19] D. M. Gao, Z. Y. Wang, B. H. Liu, L. Ni, M. H. Wu, Z. P. Zhang, Anal. Chem. 2008, 80, 8545 – 8553. [20] L. Feng, C. Wang, Z. Ma, C. L, Dyes Pigm. 2013, 97, 84 – 91. [21] H. Zhang, Z. Zhou, B. Yang, M. Gao, J. Phys. Chem. B 2003, 107, 8 – 13. [22] Q. Mei, N. Du, M. Lu, Eur. Polym. J. 2007, 43, 2380 – 2386. [23] N. Du, R. Tian, J. Peng, M. Lu, J. Polym. Sci. Part A 2005, 43, 397 – 406. Received: November 8, 2013
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& Fluorescent Probes
Water-Soluble Polymer Functionalized CdTe/ZnS Quantum Dots: A Facile Ratiometric Fluorescent Probe for Sensitive and Selective Detection of Nitroaromatic Explosives Bingxin Liu,[a] Cuiyan Tong,*[a] Lijuan Feng,[a, b] Chunyu Wang,[a] Yao He,[a] and Changli L*[a]
Chem. Eur. J. 2014, 20, 2132 – 2137
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Communication Abstract: A ratiometric fluorescent probe based on dual luminescence QD/CPL for selective sensing of the nitroaromatic explosive picric acid (PA) was constructed. The observed ratiometric fluorescence intensity change allows the quantitative detection of PA with a detection limit of 9 nm.
Nitroaromatic compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and picric acid (PA) are always known as homemade explosives. Selective and sensitive detection of these explosives is very important in countering terrorist threats and pollution of soil and groundwater.[1] Fluorescence detection based either on Fçrster resonance energy transfer (FRET) or electron transfer (ET) mechanism has attracted much attention in recent years owing to its high sensitivity, simplicity, short response time, and its ability to be employed both in solution and solid phase.[1–3] However, there are few reports about rapid and selective detection of nitroexplosives, especially PA.[1] The aggregates of hexaphenylbenzene derivatives could be used as an efficient and sensitive fluorescent chemosensor for PA.[4] Fang and co-workers explored a fluorescent self-assembled monolayer film sensor for the detection of PA among other analogues.[5] Organic dyes and conjugated polymers also show high selectivity for PA.[6] However, all of them exhibit a turn-off fluorescent response to PA in nonaqueous solutions. Xu et al. constructed a ratiometric NIR fluorescent probe for PA based on the intramolecular charge transfer in CH3CN[3] (various fluorescence methods for the detection of PA are summarized in Table S1 in the Supporting Information). To the best of our knowledge, there are yet no reports on the direct exploration of ratiometric response to PA in visible region in aqueous solution. Quantum dots (QDs) have special physical dimensions that lay between bulk and discrete molecules.[7] Many studies have focused on the control of optical and photophysical properties of QDs by surface modification with functional molecules.[8] Chemically modified CdSe/ZnS QDs were used as fluorescent probes by Willner and co-workers to detect TNT or trinitrotriazine (RDX) with a low limit of detection (LOD) of 1 nm.[9] It is known that 8-hydroxyquinoline (HQ) and its derivatives can coordinate the surface of nanoparticles such as ZnS and CdS to form stable fluorescent complexes because these particles have a great number of surface metal atoms.[10] The inherent luminescent emission of QDs may have a cooperating interaction with that of the HQ–metal complexes formed on the QDs [a] B. Liu, Dr. C. Tong, Dr. L. Feng, C. Wang, Y. He, Prof. C. L College of Chemistry, Northeast Normal University Changchun 130024 (China) E-mail: [email protected] [email protected] [b] Dr. L. Feng Centre of Analytical and Test, Beihua University Jilin 132013 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304390. Chem. Eur. J. 2014, 20, 2132 – 2137
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surface. Thus, the HQ and its derivatives can be used as functional ligands to tailor the optical properties of the QDs when HQ is anchored to the semiconductor nanoparticles.[11] However, the intrinsic fluorescence of these QDs, such as ZnS and CdS, is too weak to fabricate the dual luminescence QDs needed for analyte detection. Herein, we reported for the first time water soluble macromolecular ligand functionalized high-quality QDs as a scaffold for constructing a ratiometric fluorescent nanosensor for selective detection of nitroexplosives in aqueous media. As shown in Scheme 1, 5-(2-methacryloylethyloxymethyl)-8-quinolinol
Scheme 1. Schematic illustration of QDs/CPL-2 as selective ratiometric fluorescent detection for PA through electron transfer (ET).
(MQ) was selected as functional ligand to copolymerize with Nisopropylacrylamide (NIPAM) monomer to fabricate a novel copolymer ligand (CPL). The CPL exhibits a good solubility in aqueous solution. Then the CPL was used to decorate the surface of water-soluble CdTe/ZnS QDs via coordinate bond to form QDs/CPL hybrids with a dual luminescence emission. It is found that the fluorescence quenching of QDs/CPL systems is observed due to the electron transfer (ET) from QDs to the coordinated CPL on the surface of QDs. However, picric acid (PA) can efficiently quench the inherent fluorescence of QDs through the competitive electron transfer, while the surfacecoordination emission of the Zn–CPL complex on QDs can be recovered because the ET from QDs to Zn-complex is interrupted in the presence of PA. So, the QDs/CPL systems can be used as a water-soluble ratiometric fluorescent probe for sensitive and selective sensing of PA by using the interesting luminescence property. The novel copolymer ligand (CPL) of p(NIPAM-co-MQ) was prepared by conventional free radical copolymerization of NIPAM and MQ monomers. The CPL structure has been confirmed and the molar ratio of NIPAM and MQ units in CPL was calculated to be 58:1 by 1H NMR. Gel permeation chromatogra-
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Communication phy (GPC) measurements revealed that the number average molecular weight (Mn) of the copolymer was around 26 000 with a polydispersity index (PDI) of 1.63. The CdTe/ZnS QDs were obtained through in situ growth of ZnS shell on the surface of CdTe core in alkaline solution.[12] The size of CdTe/ZnS QDs was confirmed through transmission electron microscopy (TEM) (Figure 1) with the average diameter of 3.7 nm. Here,
of QDs due to the steric hindrance effect provided by the surface of QDs and the existence of Zn S bonds as compared with the conventional small coordination compounds such as Zn(MQ)2.[13] The photoluminescence (PL) properties of the CPL functionalized QDs are shown in Figure 3. There are two emission centers, including the inherent luminescent emission (606 nm) of
Figure 1. a) TEM image and b) HRTEM image of pure QDs.
Glutathione (GSH) was used both as capping reagent and sulfur source. Then the multifunctional hybrid QDs were prepared through coordination bonds between QDs and CPL in aqueous solutions. The samples from QDs/CPL-1 to QDs/CPL-3 represent the CPL-functionalized CdTe/ZnS QDs with molar feed ratios of [QDs]/[CPL] = 3:200, 3:600, and 3:1000. The interaction between QDs and CPL was supported by UV/Vis spectra (Figure 2). The absorption signal at 259 nm for QD/CPL systems is associated with the p–p* electron transition from quinoline ring. A new absorption band, which is caused by the metal–quinolate transition on the surface of QDs,[13] can be observed at 380 nm, and the intensity of this signal increases with the increasing molar fraction of CPL. The above results indicate that an increasing amount of MQ segments on CPL chains are anchored to the surface of QDs to form the metalloquinolates.[11, 13] In addition, the new species at labs = 325 nm is similar to that of pure Zn–CPL complex (Figure 1 insertion), indicating that the new species should be attributed to the single coordination bound of Zn–CPL complex on the surface
Figure 2. UV/Vis absorption spectra of QDs/CPL systems. The inset shows the UV/Vis absorption spectrum of pure Zn–CPL complex in aqueous solution. Chem. Eur. J. 2014, 20, 2132 – 2137
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Figure 3. PL emission spectra of pure QDs and different QDs/CPL systems.
QDs and the surface-coordination emission (517 nm) of Zn– CPL complex formed on the surface of QDs. With the increasing concentration of CPL in QDs solution, the characteristic emission of QDs is suppressed and the surface coordination emission of the Zn–CPL complex on QDs is increased. Even though an obvious spectral overlap (see Figure S1 in the Supporting Information) can be observed between the luminescence emission of Zn–CPL complex and the absorption band of pure QDs, we believe that the fluorescence quenching in QDs/CPL systems is possibly caused by charge separation photoinduced electron transfer from the photoexcited QDs to the coordinated CPL on the surface of QDs and charge recombination.[14] Actually, the surface-coordination emission of the Zn– CPL complex is also obviously quenched due to the ET interaction.[15] Cyclic voltammetry (CV, see Figure S2 in the Supporting Information) studies of QDs and Zn–CPL complex confirm the electron-transfer event as the conduction band (CB, 3.9 eV) of the QDs facilitates the electron to jump to the lower energy lowest unoccupied molecular orbital (LUMO) of the Zn–CPL complex ( 4.9 eV). Besides, the full width at half maximum (FWHM) of PL spectra also gradually broadens due to the cooperating interaction between QDs and CPL. Pure QDs and QD/CPL-2 were used as fluorescent probes to study the sensing property for nitroaromatic compounds including TNT, DNT, and PA. Figure 4 shows the fluorescence ratiometric responses of pure QDs and QD/CPL-2 to trace analyte explosives in aqueous solution. When pure QDs were used as probes, the fluorescence quenching efficiency of PA and TNT resulted to be almost the same because both of them have extremely strong electron affinity to QDs.[16] Thus, the selective detection of TNT and PA is very difficult for pure QDs. Figure 4 A shows the fluorescence spectral changes in QD/CPL-2
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Figure 5. Evolution of UV/Vis absorption spectra with addition of GSH (50 mg) into 20 mL of PA solution (1 mm). Color inset shows the corresponding colors of the PA solutions before and after adding GSH.
Figure 4. Evolution of the fluorescence spectra of QDs/CPL-2 with increasing volumes of A) PA (1 10 6 mol/L), B) TNT (1 10 4 mol/L), and C) pure Zn– CPL complex and QDs with increasing concentration of PA. D) Fluorescenceintensity changes (I517/I606) for QDs/CPL-2 aqueous solution as the concentration of PA, TNT, and DNT is increased.
aqueous solution upon addition of PA. While the PL intensity at 606 nm decreases, the PL intensity at 517 nm increases with the successive increasing concentration of PA in QD/CPL-2 solution. The organic amine-capped (GSH) QDs can bind PA species from solution by the acid base pairing interaction between amino ligands and acid phenol rings. PA is a stronger acid than TNT, and has the stronger acid base pairing interaction occurred between PA and amino ligands, thus resulting in the formation of PA anions at the surface of amine-capped QDs.[17] The CV measurement (Figure S2 in the Supporting Information) confirms that the electron transfer from the conduction band of QDs to PA bound on the surface of QDs is thermodynamically feasible because the LUMO ( 3.89 eV) of PA is lower as compared to the conduction band ( 3.9 eV) of QDs.[18] Thus, the photogenerated electron is trapped by PA on the surface of QDs more easily than that of Zn–CPL complex since the LUMO of Zn–CPL complex is much lower ( 4.9 eV). Therefore, the resultant PA anions bound onto the amino monolayer can efficiently quench the photoluminescence of QDs through the competitive electron transfer.[1, 17] However, the surface-coordination emission originated from Zn–CPL complex on the QDs surface is recovered because the ET from QDs to Zn-complex is interrupted due to the presence of PA, thereby minimizing the fluorescence quenching effect of the Zn-complex. Actually, the pure Zn-complex is not sensitive for all the mentioned nitroaromatic explosives (Figure 4 C left). Moreover, the absorption band of the acid–base pairing species of GSH and PA was observed at below 470 nm when adding PA into the solution of GSH (Figure 5). Meanwhile, we can clearly see that the PA solution changes from bright yellow into colorless with the addition of cysteamine, as shown in the inset image of Figure 5. The wavelength of visible absorption of PA anions is much
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shorter than the emission wavelength of Zn–CPL complex. Thus, the FRET between PA and QD/CPL-2 can be excluded. However, both the emission sites at 517 and 606 nm decrease with the increase of TNT concentration in QD/CPL-2 solution as shown in Figure 4 B. This result suggests that the photoinduced electron transfer is not the only quenching mechanism of QD/CPL-2 for TNT. It is well known that the charge transfer from electron-rich amino groups to electron-deficient aromatic rings leads to the formation of a Meisenheimer complex with addition of organic amines. Here the amino groups from GSH capped on the surface of QDs can chemically recognize and absorb TNT molecules by the formation of the Meisenheimer complex mentioned above[7] (figure S3 in the Supporting Information shows the UV/Vis absorption spectra after adding GSH into TNT solution). Visible absorptions at 515 nm were observed. It can be seen that the Meisenheimer complex absorbs the green part of visible light, and strongly suppresses the fluorescence emission of the surface-coordination complex Zn–CPL at 517 nm through FRET.[19] As a result, the surface-coordination emission of Zn–CPL complex on the surface of QDs is gradually quenched with the adding amount of TNT in QD/ CPL-2 system. Thus, we assume that FRET and ET take place simultaneously when TNT is added into our QD/CPL system. Namely, the fluorescence emission of the surface-coordination complex Zn–CPL complex at 517 nm is suppressed through FRET while the PL quenching at 606 nm is caused by ET. Also the PL intensity of the QD/CPL-2 system shows a relatively slight decline with increasing concentration of DNT in solution (Figure 6). It is well known that the electron-accepting ability of DNT with two nitro groups is much weaker than TNT molecules with three nitro groups, and so the consequent weak electron-accepting ability towards the amino groups leads to the low quenching efficiency of DNT.[20] The quenching constant (KSV) at 606 nm is calculated with the Stern–Volmer equation to be 8.4 105 m 1 for PA, 1.1 104 m 1 for TNT, and 4.1 103 m 1 for DNT. It can be noted that the ratios (I517/I606) of the emission intensities at 517 and 606 nm for QD/CPL-2 system increase with the increasing concentration of PA (Figure 4 D). However, I517/I606 shows almost no change upon incre-
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Communication Trinitrotoluene (TNT) in methanol (1 mg mL 1) was obtained from Aladdin. 2,4-Dinitrotolene (DNT) was purchased from TCI and picric acid (PA) was obtained from Xilong Chemical Reagent Co., Ltd. Nisopropylacrylamide (NIPAM, Aldrich) was recrystallized in hexane. Other chemicals were of analytical grade without purification.
Synthesis of CdTe/ZnS QDs
Figure 6. Evolution of fluorescence spectra of QDs/CPL-2 with increasing volume of DNT (1 10 6 mol/L).
mental addition of a TNT or DNT solution. This result can render the QD/CPL-2 a highly selective ratiometric fluorescent sensing for PA. The LOD of the probe can be determined by kSB/m, where k is a constant, usually k = 3, SB is the standard deviation of the blank signal, and m is the slope of I517/I606 versus the analyte concentration plot. The calculated detection limit of PA using QD/CPL-2 as a probe can reach 9 nm. However, the detection limit of PA can only reach 9 mm when pure QDs are used as the probe. We have still evaluated the effect of different metallic ion on the QDs/CPL-2 by PL emission spectra (Figure S4 in the Supporting Information). The result shows that the presence of these ions scarcely affects the fluorescence at 606 nm and 517 nm because there are almost no free quinoline groups in the QD/CPL-2 system. Thus, these metallic ions will not lead the QDs/CPL to form fluorescent metal-chelate complexes. This result indicates that our QDs/CPL system will not exhibit the ratiometric response to these ions. Thus, the novel fluorescent probe shows highly selective optical sensing for PA. In conclusion, we have successfully fabricated functionalized QDs decorated with CPL containing 8-hydroxyquinoline and NIPAM units coordinatively bonded. The novel fluorescent probe that is based on the fluorescence turn-on mechanism can fulfil the ratiometric response to PA. The present study provides a new insight into highly selective fluorescent sensing of the nitroaromatic explosive PA with a detection limit of 9 nm. In addition, we expect this study to open up a new perspective in the design of ratiometric fluorescent sensors for various analytes based on functionalized dual-emission quantum dot hybrids.
Synthesis of the copolymer ligand of p(NIPAM-co-MQ) 5-(2-Methacryloylethyloxymethyl)-8-quinolinol (MQ) was synthesized according to a previous procedure.[22, 23] A typical synthesis of the copolymer ligand of p(NIPAM-co-MQ) is described as follows. NIPAM (5 g), MQ (0.05 g), and AIBN (0.35 g) in a tetrahydrofuran solution (125 mL) were added to a 250 mL three-necked round-bottomed flask fitted with a magnetic stirrer, a reflux condenser, and a nitrogen inlet. The solution was heated gradually and was kept at 60 8C for 20 h. The resulting products were reprecipitated using hexane and washed several times. After drying under vacuum for one day, the CPL was obtained. 1H NMR (400 MHz, CDCl3): d = 3 9.80 (1 H, Ph OH), 8.75–7.08 (5 H, Ph H), 4.85 (2 H, CH2 Ph), 3.69 (4 H, CH2 O), 1.88 ppm (9 H, CH3).
Copolymer ligand-functionalized CdTe/ZnS QDs A series of copolymers of P(NIPAM-co-MQ) as prepared above were dissolved in ultrapure water (10 mL). To the aqueous solution, the above dialyzed CdTe/ZnS QDs were slowly titrated at different molar feed ratios (3:200, 3:600, and 3:1000) of QDs to CPL. The mixtures were stirred at room temperature for 12 h. Finally, the obtained samples of CPL-functionalized CdTe/ZnS QDs with different molar feed ratios were defined as QDs/CPL-1 to QDs/CPL-3, respectively.
Nitroaromatic explosive detection experiment for QDs/CPL-2 Various volumes of PA (1 10 6 mol/L), TNT (1 10 4 mol/L), and DNT (1 10 4 mol/L) in aqueous solution were poured into QDs/ CPL-2 (5 mL). The fluorescent signals were collected after having stabilized for 5 min the QDs/CPL-2 aqueous solution containing different nitroexplosives. The test was carried out at room temperature.
Experimental Section Materials Thioglycolic acid (TGA, 99 %), NaBH4 (96 %), and tellurium powder (9.8 %) were obtained from Sinopharm Chemical Reagent Co., Ltd. ZnCl2 (99 %) and CdCl2·2.5H2O (99 %) were obtained from Shanghai Reagent Company. Reduced Glutathione (GSH, 98 %) was purchased from Beijing Jing Ke Hong Da Biotechnology Co. Ltd. 2,4,6Chem. Eur. J. 2014, 20, 2132 – 2137
Aqueous colloidal CdTe solution was synthesized by the procedures based on Zhang’s method.[21] In brief, adding freshly prepared NaHTe solution to 1.25 10 3 mol/L N2-saturated CdCl2 solution at pH 9.0 in the presence of MPA as a stabilizing agent. The molar ratio of Cd2 + /MPA/HTe was fixed at 1:2.4:0.5. The resulting mixture was then subjected to a reflux that controlled the growth of CdTe nanocrystals. The obtained CdTe colloids were dialysed for one week before the further use. The core-shell CdTe/ZnS QDs were prepared in aqueous phase following the method described elsewhere with some medications.[12] 40 mg of as-prepared CdTe sample was added to 50 mL solution (pH 8) containing 1 mmol L 1 ZnCl2 and 4 mmol L 1 GSH as a capping agent. The solution was heated to 100 8C under open-air conditions and was refluxed for 10 min. The obtained core-shell CdTe/ZnS QDs were dialyzed for one week before the further use.
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Characterization 1
H NMR spectra (in CDCl3) were recorded on a DRX-400 MHz (Bruker) spectrometer with TMS as an internal standard. The molec-
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Communication ular weights of polymer were measured at a flow rate of 1.0 mL min 1 at 25 8C by gel permeation chromatography (GPC) equipped with Waters 1515 pump and Waters 2414 differential refractive index detector. CHCl3 was used as eluent, and the molecular weights were determined vs polystyrene standards. Transmission electron microscopy (TEM) was recorded on a JEM-2100 microscope. UV/Vis absorption spectra were recorded on a Shimadzu UV-2550 UV/Vis spectrometer in the range 200–800 nm. The PL properties were measured on a Cary Eclipse fluorescence spectrometer. Cyclic voltammetry (CV) was performed on an electrochemical workstation CHI-660D. The counter electrode is platinum, the working electrode is a glassy carbon electrode and an Ag/AgCl serves as a reference electrode. All the CV experiments were carried out in aqueous solutions at room temperature.
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