Three-Dimensional Wormhole and Ordered Mesostructures and Their

Mar 15, 2008 - Large-scale cubic Pm3n silica monoliths (HOM) were fabricated in wormhole and ordered meso- structures and in shape- and size-controlle...
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Chem. Mater. 2008, 20, 2644–2654

Three-Dimensional Wormhole and Ordered Mesostructures and Their Applicability as Optically Ion-Sensitive Probe Templates Sherif A. El-Safty,* D. Prabhakaran, A. A. Ismail, H. Matsunaga, and F. Mizukami Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science & Technology (AIST), 4-2-1 Nigatake, Miyagino-Ku, Sendai 983-8551, Japan ReceiVed July 23, 2007. ReVised Manuscript ReceiVed December 22, 2007

Large-scale cubic Pm3n silica monoliths (HOM) were fabricated in wormhole and ordered mesostructures and in shape- and size-controlled cage pores by using a simple and fast strategy. The functional use of these 3D HOM monoliths as probe anchoring templates enabled the efficient designs of optical nanosensors. In this regard, the synthesized chromoionophore was physisorbed into the 3D HOM pore surface carriers without potential leaching. Results revealed that the structural features of the HOM monoliths such as ordered and worm-like cage pores substantially influenced the ion-sensing functionality in terms of their probe inclusion capacities, ion-transport diffusion, optical responsive profile, and visual color transition series during the detection of ultratraces of toxic Pb(II) ions. The nanosensors were selective in discriminating trace Pb(II) ions over multicomponent matrix species, with reliable and reproducible detection and quantification limits. A comparative study on the ion-sensing efficiency of the chromoionophore in both solution and solid phases indicated that the solid HOM monoliths show promise as probe templates to design-made nanosensors for the detection of ultratraces Pb(II) ions. Considering the environmental factors, nanosensors were solvent-free systems and had the capacity to serve as ion preconcentrators with complete reversibility and reusability. The significant features of the probe-design nanosensors led to overcoming the disposal problems, which were normally associated with the liquid probe systems.

Introduction Since the fabrication of the classic MS41 family of materials,1 synthetically designed mesoporous materials have emerged as being particularly important in diverse aspects of human activity. These mesoporous molecular sieves show promise in a wide range of applications, for example, as catalysts, supports, adsorbents, matrixes for photochemical species, optical, electronic and sensing devices, and drug delivery agents.2–5 Significant effort has been expended to create new levels of hierarchical design of materials with strong control over the structural geometry, mesophase * Corresponding author. E-mail: [email protected].

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morphology, and pore size and shape by adjusting the surfactant templates and synthesis conditions.6–9 In this respect, mesoporous molecular sieve silicas with worm-like framework structures (e.g., MSU-X,1,2 and HMS-3) are of great interest in adsorption, storage energy, heterogeneous catalysis of organic chemical transformations, and environmental remediation of water and air purification.7 These disordered channel structures with uniform diameters ranging from 2.0 to 5.8 nm have been obtained by varying the size and structure of the surfactant molecules.3a,b The 3D textural porosity associated with flexible rod- or worm-like frameworks can improve catalytic reactivity by facilitating the accessibility to the reactive sites in the frameworks. In turn, (5) (a) Yang, D.; Xu, Y.; Wu, D.; Sun, Y. J. Phys. Chem. C 2007, 111, 999. (b) Casagrande, M.; Moretti, E.; Storaro, L.; Gersich, M. L. J.; Stievano, L.; Wagner, F. E. Microporous Mesoporous Mater. 2006, 91, 261. (c) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (d) Pang, J. B.; Qiu, K.-Y.; Wei, Y. Chem. Mater. 2001, 13, 2361. (e) Zhu, W.; Han, Y.; An, L. Mater. Chem. Phys. 2006, 97, 121. (6) (a) Zhao, D.; Huo, Q.; Jianglin, F.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (b) Yu, C.; Tian, B.; Fan, J.; Stucky, G. D.; Zhao, D. J. Am. Chem. Soc. 2002, 124, 4556. (7) (a) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (b) Kim, S.-S.; Pauly, T. R.; Pinnavaia, T. J. Chem. Commun. 2000, 835. (8) (a) Che, S.; Garcia-Bennett, A. E.; Yokoi, T.; Sakamoto, K.; Kunieda, H.; Terasaki, O.; Tatsumi, T. Nat. Mater. 2003, 2, 801. (b) GarciaBennett, A. E.; Lund, K.; Terasaki, O. Angew. Chem., Int. Ed. 2006, 45, 2434. (c) Gao, C.; Sakamoto, Y.; Sakamoto, K.; Terasaki, O.; Che, S. Angew. Chem., Int. Ed. 2006, 45, 4295. (9) (a) Shen, S.; Baracia-Bennett, A. E.; Liu, Z.; Lu, Q.; Shi, Y.; Yan, Y.; Yu, W.; Cai, Y.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 6780. (e) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (f) Choi, M.; Cho, H. S.; Srivastava, R.; Venkatesan, C.; Choi, D.-H.; Ryoo, R. Nat. Mater. 2006, 5, 718.

10.1021/cm701966c CCC: $40.75  2008 American Chemical Society Published on Web 03/15/2008

Three-Dimensional Wormhole and Ordered Mesostructures

among all approaches used for fabrication mesostructures, the direct templating of lyotropic liquid crystals is a reliable approach to provide control over the microstructure phases of the templates and the final composite replicas, as gels, cast monoliths, or membranes.10–14 An important development in this direct synthesis approach is the use of instantpreformed liquid crystalline lyotropic and microemulsion phases as templates.15,16 This approach has been successfully used to fabricate monolithic structures with various unique geometries and morphologies that have uniform pores up to 15 nm in diameter. Herein, we used this synthesis protocol for engineered control of 3D ordered and wormhole pore systems of cage-types, which are of particular interest for applications where 3D uniformly sized monoliths are required.17a,b The elemental or ionic forms of lead are bioconcentrated by aquatic organisms as potentially toxic tetraethyllead and tetramethyllead species, persisting up in the life chain through fish and fish-eating animals.17,18 Lead (Pb2+) is a potential neuro- and nephro-toxin and when exposed to humans beyond its permissible level can cause chronic inflammatory to kidney and heart. In addition, lead ions can induce nervous and gastrointestinal disorders and impair immune and reproductive systems.19 Regulations and periodic monitoring for these pollutant species are desired on the basis of their toxicological significance and greater bioavailability to the children compared to the adults. However, environmental (10) (a) Attard, G. S.; Glyde, J. C.; Göltner, C. G. Nature 1995, 378, 366. (b) Attard, G. S.; Edgar, M.; Göltner, C. G. Acta Mater. 1998, 46, 751. (11) (a) Göltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew, Chem. Int. Ed. 1998, 37, 613. (b) Kramer, E.; Forster, S.; Göltner, C. G.; Antonietti, M. Langmuir 1998, 14, 2027. (c) Göltner, C. G.; Berton, B.; Kramer, E.; Antonietti, M. AdV. Mater. 1999, 11, 395. (12) (a) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, C. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332. (b) Yang, H.; Shi, Q.; Tian, B.; Xie, S.; Zhang, F.; Yan, Y.; Tu, B.; Zhao, D. Chem. Mater. 2003, 15, 536. (13) (a) Soler-Illia, G. J. D. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (b) Soler-Illia, G. J. D. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opin. Colloid Interface Sci. 2003, 8, 109. (14) (a) Smarsly, B.; Polarz, S.; Antonietti, M. J. Phys. Chem. B 2001, 105, 10473. (b) Coleman, N. R. B.; Attard, G. S. Microporous Mesoporous Mater. 2001, 44, 73. (15) (a) El-Safty, S. A.; Hanaoka, T. Chem. Mater. 2004, 16, 384. (b) ElSafty, S. A.; Hanaoka, T. AdV. Mater. 2003, 15, 1893. (16) (a) El-Safty, S. A.; Hanaoka, T.; Mizukami, F. AdV. Mater. 2005, 17, 47.; (b) Chem. Mater. 2005, 17, 3137; (c) J. Phys. Chem. B 2005, 109, 9255. (17) (a) El-Safty, S. A.; Balaji, T.; Matsunaga, H.; Hanaoka, T.; Muzukami, F. Angew. Chem., Int. Ed. 2006, 45, 7202. (b) El-Safty, S. A.; Ismail, A.; Matsunaga, T.; Mizukami, F. Chem. Eur. J. 2007, 13, 9245. (c) Ros-Lis, J. V.; Marcos, M. D.; Mártinez-Mánez, R.; Rurack, K.; Soto, J. Angew. Chem., Int. Ed. 2005, 44, 4405. (d) Balaji, T.; Sasidharan, M.; Matsunaga, H. Anal. Bioanal. Chem. 2006, 384, 488. (e) Nicole, L.; Boissiere, C.; Grosso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15, 3598. (f) Metivier, R.; Leray, I. ; Lebeau, B.; Valeur, B. J. Mater. Chem. 2005, 15, 2965. (18) (a) Francesconi, K. A. Analyst 2007, 132, 17. (b) Coronado, E.; GalanMascaros, J. R.; Marti-Gastaldo, C.; Palomares, E.; Durrant, J. R.; Vilar, R.; Gratzel, M.; Nazeeruddin, K. M. J. Am. Chem. Soc. 2005, 127, 12351. (c) Nendza, M.; Herbst, T.; Kussatz, C.; Gies, A. Chemosphere 1997, 35, 1875. (19) (a) Oken, E.; Wright, R. O.; Kleinman, K. P.; Bellinger, D.; Amarasiriwardena, C. J.; Hu, H.; Rich-Edwards, J. W.; Gillman, M. W. EnViron. Health PerspectiVes 2005, 113, 1376. (b) Manahan, S. E. EnVironmental Chemistry, 6th ed.; Lewis Publishers: New York, 1994; p 677. (c) Derelanko, M. J.; Hollinger, M. A. CRC Handbook of Toxicology; CRC Press: Boca Raton, FL, 1995; p 550.

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monitoring of heavy metal ions has always relied on sensitive techniques such as atomic absorption spectrometry,20 inductively coupled plasma atomic emission spectrometry and mass spectrometry,21 atomic fluorescence spectrometry,22 and electrochemical stripping analysis.23 These methods often combine specificity and sensitivity but are hardly in the class of routine methods available for the control laboratory usage or for the on-site field analysis. Also, these sophisticated methodologies often require some form of sample cleanup procedures to alleviate sample matrix interferences.24 Intense research activities have been focused to develop simple, reliable methods that are both sensitive and selective and also are capable of use by technicians, preferably in the field. In this context, an attractive means of improving pollution monitoring would be the use of simple, inexpensive, rapid responsive and portable sensors.25–28 In this, selective optical sensing is attracting strong interest as a result of the use of “low-tech” spectroscopic instrumentation to detect relevant chemical species in biological and environmental processes.28 Recent synthesis activity has focused on tailoring specific sensors for toxic heavy metal ions. The resulting (20) (a) Yang, L.; Saavedra, S. S. Anal. Chem. 1995, 67, 1307. (b) Ghaedi, M.; Ahmadi, F.; Shokrollahi, A. J. Hazardous Mater. 2007, 142, 272. (c) Maranhão, T. A.; Borges, D. L. G.; da Veiga, M. A. M. S.; Curtius, A. J. Spectrochim. Acta, Part B 2005, 60, 667. (d) Mena, C. M.; Cabrera, C.; Lorenzo, M. L.; Lopez, M. C. J. Agric. Food Chem. 1997, 45, 1812. (21) (a) Palmer, C. D., Jr.; Lewis, M. E.; Geraghty, C. M., Jr.; Barbosa, F.; Parsons, P. J. Spectrochim. Acta, Part B 2006, 61, 980. (b) Gomez, M. R.; Cerutti, S.; Sombra, L. L.; Silva, M. F.; Martínez, L. D. Food Chem. Toxicol. 2007, 45, 1060. (c) Xu, Y.; Zhou, J.; Wang, G.; Zhou, J.; Tao, G. Anal. Chim. Acta 2007, 584, 204. (d) Petrov, P. K.; Wibetoe, G.; Tsalev, D. L. Spectrochim. Acta, Part B 2006, 61, 50. (e) Lewen, N.; Mathew, S.; Schenkenberger, M.; Raglione, T. J. Pharm. Biomed. Anal. 2004, 35, 739. (22) (a) Wagner, E. P., II.; Smith, B. W.; Winefordner, J. D. Anal. Chem. 1996, 68, 3199. (b) Neuhauser, R. E.; Panne, U.; Niessner, R.; Petrucci, G.; Cavalli, P.; Omenetto, N. Sens. Actuators, B 1997, 39, 344. (c) Irwin, R. L.; Guor-Tzo, W.; Butcher, D. J.; Liang, Z.; Su, E. G.; Takahashi, J.; Walton, A. P.; Michel, R. G. Spectrochim. Acta, Part B 1992, 47, 1497. (23) (a) Maghasi, A. T.; Conklin, S. D.; Shtoyko, T.; Piruska, A.; Richardson, J. N.; Seliskar, C. L.; Heineman, W. R. Anal. Chem. 2004, 76, 1458. (b) Dragoe, D.; Spa˘taru, N.; Kawasaki, R.; Manivannan, A.; Spa˘taru, T.; Tryk, D. A.; Fujishima, A. Electrochim. Acta 2006, 51, 2437. (c) Salvo, F.; La Pera, L.; Di Bella, G.; Nicotina, M.; Dugo, G. J. Agric. Food Chem. 2003, 51, 1090. (d) Sherigara, B. S.; Shivaraj, Y.; Mascarenhas, R. J.; Satpati, A. K. Electrochim. Acta 2007, 52, 3137. (e) Li, G.; Ji, Z.; Wu, K. Anal. Chim. Acta 2006, 577, 178. (24) (a) Torre, M.; Marina, M. L. Crit. ReV. Anal. Chem. 1994, 24, 327. (b) Prabhakaran, D.; Subramanian, M. S. Sep. Sci. Technol. 2004, 39, 937. (25) (a) Turner, F. Science 2000, 290, 1315. (b) Chen, P.; He, C. J. Am. Chem. Soc. 2004, 126, 728. (c) Basabe-Desmonts, L.; Beld, J.; Zimmerman, R. S.; Hernando, J.; Mary, P. M.; Parajo, F. G.; van Hulst, N. F.; den Berg, A. V.; Reinhoudt, D. N.; Crego-Calama, M. J. Am. Chem. Soc. 2004, 126, 7293. (d) Boiocchi, M.; Bonizzoni, M.; Fabbrizzi, L.; Piovani, G.; Taglietti, A. Angew. Chem., Int. Ed. 2004, 43, 3847. (26) (a) Nam, J-M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. (b) Elghtanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (c) Lei, B.; Li, B.; Zhang, H.; Lu, S.; Zheng, Z.; Li, W.; Wang, Y. AdV. Funct. Mater. 2006, 16, 1883. (27) (a) Nolan, E. M.; Racine, M. E.; Lippard, S. J. Inorg. Chem. 2006, 45, 2742. (b) Wu, Z.; Zhang, Y.; Ma, J. S.; Yang, G. Inorg. Chem. 2006, 45, 3140. (c) Fakhari, A. R.; Ganjali, M. R.; Shamsipur, M. Anal. Chem. 1997, 69, 3693. (d) Yuan, M.; Li, Y.; Li, J.; Li, G.; Liu, X.; Lv, J.; Xu, J.; Liu, H.; Wang, S.; Zhu, D. Org. Lett. 2007, 9, 2313. (28) (a) Bakker, E.; Bühlmann, P.; Pretsch, E. Chem. ReV. 1997, 97, 3083. (b) Lerchi, M.; Elmar, R.; Simon, W.; Pretsch, E. Anal. Chem. 1994, 66, 1713. (c) Bühlmann, P.; Pretsch, E.; Bakker, E. Chem. ReV. 1998, 98, 1593. (d) Wolfbeis, O. S. J. Mater. Chem. 2005, 15, 2657.

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materials typically contain a platform for ion recognition as well as an optical transducer to measure the binding event.29,30 Although the chemical sensors can allow on-site, real time qualitative or semiqualitative detection and freesolvent sensing systems without complicated analytical instruments, much less attention has been devoted to fabricate naked-eye nanosensors. At this stage, such developments in the form of compact instrumental free-ion sensors are still under research level and are currently being investigated.31–33 If such optical sensors can be developed within 3D cage nanostructures that have large particle monoliths, high surface area, and uniform pores, then their application can be expanded broadly to control sensing of environmentally critical toxic species, as indicated in the present work.17 Here, we describe simple control of ordered and wormhole cage mesopores with cubic Pm3n geometry (HOM-type) by using a simple and fast strategy in which instant directtemplating phases of nonionic surfactants were used as templates. The HOM materials provide a very robust, open, and tunable periodic scaffold on the nanometer scale and large grain sizes that can show promise as ion sensors in the visual detection of ultratrace levels of lead ions. However, direct immobilization of a chromoionophore, namely, 4-ndodecyl-6-(2-thiazolylazo)-resorcinol (DTAR), onto these HOM materials led to fabrication of 3D optical nanosensors. The modified-DTAR monoliths with ordered and wormhole pores showed distinct features in terms of their probe inclusion capacities, ion-transport diffusion, optical responsive profile, and visual color transition series during the recognition of ultratraces of toxic Pb(II) ions. The sensing efficiency of the DTAR probe in both solution and solid phases indicated the advantages and superiority of designmade nanosensors in terms of selectivity, reversibility, and sensitivity of ultratraces of Pb(II) ions. The 3D optical nanosensors were selective in discriminating trace Pb(II) ions over multicomponent species in real samples. Experiments 1. Chemicals. All materials were used as produced without further purification. Tetramethylorthosilicate (TMOS) and 4-dodecylresorcinol were purchased from Aldrich Chem. Co., U.S.A. 2-Aminothiazole and 2-aminopyridine were procured from TokyoKaisei Kogyo Company, Ltd., Japan. Standard lead (Pb2+) and other metal ion concentrations were prepared from their corresponding (29) (a) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 1, 119. (b) Nicole, L.; Boissiere, C.; Grosso, D.; Hesemann, P.; Moreau, J.; Sanchez, C. Chem. Commun. 2004, 2312. (c) Lee, S. J.; Lee, S. S.; Lee, J. Y.; Jung, J. H. Chem. Mater. 2006, 18, 4713. (d) Liu, J.; Lu, Y. Chem. Mater. 2004, 16, 3231. (30) (a) Yang, Y.-K.; Yook, K.-J.; Tae, J. J. Am. Chem. Soc. 2005, 127, 16760. (b) Brumer, O.; La Clair, J. J.; Janda, K. D. Org. Lett. 1999, 1, 415. (c) Brümer, O.; La Clair, J. J.; Janda, K. D. Bioorg. Med. Chem. 2001, 9, 1067. (31) (a) Capitan-Vallvey, L. F.; Raya, C. C.; Lopez, E. L.; Ramos, M. D. F. Anal. Chim. Acta 2004, 524, 365. (b) Kalinina, M. A.; Golubev, N. V.; Raitman, O. A.; Selector, S. L.; Arslanov, V. V. Sens. Actuators, B 2006, 114, 19. (c) Oehme, I.; Wolfbeis, O. S. Mikrochim. Acta 1997, 126, 177. (32) (a) Nagel, J.; Oertel, U.; Friedel, P.; Komber, H.; Mobius, D. Langmuir 1997, 13, 4693. (b) Mayr, T.; Igel, C.; Liebsch, G.; Klimant, I.; Wolfbeis, O. S. Anal. Chem. 2003, 75, 4389. (c) Kuswandi, B. J. ILMU DASAR 2000, 1, 18. (33) (a) Riu, J.; Maroto, A.; Rins, F. X. Talanta 2006, 69, 288. (b) Valli, L. AdV. Colloid Interface Sci. 2005, 116, 13.

El-Safty et al. Scheme 1. Representative Design of the Construction of the DTAR Probe with Possible Interactions into the Cubic Pm3n pore matrices (a), Signal Sensing Responses of the Nanosensors for Pb(II) Ion with the Formation of the [Pb-DTAR]n+ Complex, and Analytical Cycles (Complexation/Decomplexation Process) of the Nanosensors

AAS grade (1000 µg/mL) solutions. These stock solutions were procured from Wako Pure Chemicals, Japan. For pH adjustments, buffer solutions (0.2 M) of KCl-HCl, CH3COOH-CH3COONa, 3-morpholinopropane sulfonic acid (MOPS)-NaOH, 2-(cyclohexylamino) ethane sulfonic acid (CHES)-NaOH, and N-cyclohexyl3-aminopropane sulfonic acid (CAPS)-NaOH were used. The MOPS, CHES, and CAPS were procured from Dojindo Chemicals, Japan, and the remaining were from Wako Pure Chemicals, Japan. The solublizing agent (dodecane, C12H26) and nonionic surfactants (CxEOy) such as Brij 35 (polyoxyethylene(23) lauryl ether, C12EO23), Brij 76 (polyoxyethylene(10) stearyl ether, C18EO10), Brij 78 (polyoxyethylene(20) stearyl ether, C18EO20) and Triton X-100 (polyoxyethylene(10) isooctylphenyl ether, 4-(C8H17)C6EO10) were obtained from Sigma-Aldrich Company Ltd., U.S.A. 2. Synthesis of the DTAR Amphiphilic Chromoionophore. The amphiphilic chromophore DTAR (see Scheme 1a) was chemically synthesized via standard diazonium chemistry. A 100 mL solution of 0.4 M H2SO4 and 2-aminothiazole (15 g, 0.15mol) was dissolved uniformly and stirred for 1 h at 2 °C. To that homogeneous mixture, an ice-chilled 100 mL solution of sodium nitrite (10.8 g, 0.16 mol) was added drop-by-drop and stirred vigorously for 2 h, under freezing conditions. The excess nitrous acid was tested using starch-iodide paper and quenched with urea. An equimolar amount of 4-dodecylrescorcinol (41.8 g, 0.15 mol), dissolved in 50 mL of C2H5OH-(0.5%) NaOH mixture (3:1), was added to the diazotate at 1–3 °C. The coupling reaction was preformed at a solution pH in the range of 5.5–6.5. The reaction mixture was refrigerated overnight, and the product was observed as a dark reddish precipitate in the mother liquor. The solid product was filtered and purified by washing with hot and cold water before recrystallization by 70% ethanol. The purity of the DTAR probe product was analyzed by CHNS elemental analyses. The data were as follows: C, 64.79; H, 8.01; N, 10.75; S, 8.20, as consistent with the C21H31O2N3S molecular formula, which requires C, 64.78; H, 7.97; N, 10.79; S, 8.22%. The product was characterized by 1H NMR and 13C NMR spectroscopies. The 1H NMR spectra were shown with the following signal resonances: δ 0.88 (3H, t, J ) 7.0), 1.26–1.37 (16H, m, 8 × CH2), 1.63 (2H, p, J ) 7.3), 2.58 (2H, t, J ) 7.6), 6.41 (1H, s, Ph), 7.21 (1H, s, Ph), 7.32 (1H, d, J

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Table 1. Efficiency of DTAR-Modified Nanosensors in Terms of Their Accessibility, Response Time, Sensitivity, and Reversibility Features during Recognition of Pb(II) Ionsa sensor reusability probe carrier

Q (µmol g-1)

LD µM

Rt min

LQ µM

1016 × D (cm2 min-1)

No.

HOM-13/DTAR (1)

25

0.017

Rt

E%

0.048-0.362

17.5

0.051

0.67

2 4 6

18 19 20

99.0 97.5 96.3

HOM-13/DTAR (2)

38

0.013

0.048–0.483

12.5

0.043

1.38

2 4 6

13 15 15

99.0 97.9 96.1

HOM-9/DTAR (3)

51

0.009

0.024–0.724

7

0.031

4.53

2 4 6

8 8 9

99.5 98.2 97.6

HOM-9/DTAR (4)

53

0.009

0.024–0.724

5

0.030

8.75

2 4 6

5 5 6

99.5 98.2 97.6

DR µM

a Adsorption capacity (Q) of the probes, detection (LD) and quantification (LQ) limits, detection range (DR), response times (Rt), diffusion coefficient (D), and the efficiency of the sensing design (E) after several times (No.) of regeneration cycle.

) 3.4, Tz), 7.88 (1H, d, J ) 3.4, Tz), 5.02 (2H, s(b), OH), where the Ph and Tz are the phenyl and thiazolyl groups, respectively. The 13C NMR spectra were shown with the following signal resonances: δ 14.1 (CH3), 22.7 (CH2), 29.0 (CH2), 29.4 (CH2), 29.5 (CH2), 29.6 (CH2), 29.6 (CH2), 29.7 (CH2), 29.7 (CH2), 29.7 (CH2), 29.7 (CH2), 31.9 (CH2), 76.2 (Ph, C), 113.1 (Ph, CH), 132.3 (Ph, C), 138.1 (Ph, CH), 119.6 (Tz, CH), 143.2 (Tz, CH), 158.6 (Tz, C), 167.1 (Ph, C-OH), 178.4 (Ph, C-OH). In addition, the product was recorded by UV–vis spectroscopy and showed an absorbance band at λmax ) 485 nm (, dm3 mol-1 cm-1 ) 24 700). 3. Synthesis of Wormhole and Ordered Cubic Pm3n Monoliths. Ordered (HOM-9) and wormhole (HOM-13) cubic Pm3n cage monoliths were fabricated via an instant direct-templating method using microemulsion systems formed by addition C12-alkane of nonionic surfactants. The addition of C12-alkane to the lyotropic hexagonal or cubic Im3m mesophases at either Brij 76 or Brij 78/ TMOS mass ratio of 50 wt %, respectively, led to the formation of ordered HOM-9 phase. In turn, disordered HOM-13 phase was fabricated using microemulsion systems of Triton X-100 and Brij 35 surfactants at surfactant/TMOS ratios of 50 wt %. The use of the instant direct-templating method and the formation of the microemulsion system were previously reported (see Supporting Information S1 and S2).15,16 4. 3D Wormhole and Ordered Nanosensors (1–4). Direct incorporation of hydrophobic DTAR molecules onto the 3D wormhole HOM-13 and ordered HOM-9 cubic monoliths was adapted for the fabrication of nanosensors 1, 2, 3, and 4 (see Scheme 1a). Note that the HOM-13 monoliths that were synthesized from the microemulsion systems of Triton X-100 and Brij 35 were used as DTAR probe templates to design wormhole nanosensors 1 and 2, respectively. In turn, the HOM-9 monoliths that were synthesized from the microemulsion systems of Brij 78 and Brij 76 were used as DTAR probe templates to design wormhole nanosensors 3 and 4, respectively. The DTAR probe (0.05 mmol) was dissolved in dehydrated ethanol and equilibrated with 0.5 g of HOM materials for 2 h at 20 °C. The impregnation procedure was performed under vacuum at 25 °C to reach probe saturation. The probe anchored HOM materials were washed with deionized water and dried at 60 °C for 45 min. The adsorption capacity (Q, mmol · g-1) of the DTAR probe at saturation was determined by the following equation; Qt ) (C0 - Ct)V/m, where Qt is the adsorbed amount at saturation time t, V is the solution volume (L), m is the mass of HOM carriers (g), and C0 and Ct are the initial concentration and the concentration at saturation time, respectively (see Table 1).

5. Recognition Procedure for Lead Ions Using Cage Nanosensors 1–4. In a typical sensing experiment, a mixture containing specific concentrations of toxic lead ions was adjusted at solution pH of 6.5 by using MOPS for all sensors. This Pb(II) analyte mixture was directly added to ∼3–5 mg of the monolithic sensors at constant volume (20 cm3) with shaking at room temperature. Studies on the sample volume also indicated that the volume at 20 cm3 was sufficient for effective interaction of Pb2+ with cage sensors to give good color separation. A blank solution was also prepared, following the same procedure for comparison. After an interval time of (5–15 min) depending on the nature of the sensor systems, the monolithic sensors were filtered after equilibration time, according to the feature of Pb ion sensors (Scheme 1), using cellulose acetate filter paper (25 mm; Sibata filter holder). It is important to note that and the monolithic sensors were grinded and packed in a cylindrical holder for visual color assessment and absorbance measurements. In a typical experiment, the Pb(II) ionsensing system was studied by the batch equilibration method at various pH values. The concentrations of toxic Pb(II) ions were calculated by comparing the color intensity of the target samples with that of the standard samples, which were prepared with known concentrations of analyte solutions. Note: The stoichiometry of the [Pb-DTAR]n+ complex was derived from the deviation from linearity at the inflection point in the calibration curve, and the Pb:DTAR stoichiometric ratio of the [Pb-DTAR]n+ complexes was 1:1 using 3D chemosensors. Further evidence of the stoichiometric [Pb-DTAR]n+ complexes was seen in the Job’s plot in which changes in the absorbance of the color complexes in solution under experimental control conditions were monitored. Results indicated no changes in the stoichiometry of the [Pb-DTAR]n+ complexes formed in both homogeneous and heterogeneous systems. The detection and quantification limits (LD and LQ) of Pb(II) analyte ions by using the chemosensors was estimated from the linear part of the calibration plot,17a,b according to the equation LD and LQ ) k1,2Sb/m, where k1 ) 3 and k2 ) 10 in the cases of the determination of detection and quantification limits, respectively, Sb is the standard deviation for the blank, and m is the slope of the calibration graph in the linear range. In addition, the stability constant (log Ks) of the formed [Pb-DTAR]n+ complex into the nanosensors at pH 6.8 was estimated as 8.56, according to the following equation: log Ks ) [ML]S/[L]S × [M], where [M] refers to the concentration of Pb(II) ions in solution that have not reacted with the DTAR chelating agent, [L] represents not only the concentration of free DTAR ligand but also all concentrations of DTAR not bound to the Pb(II) ion,

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and the subscript S refers to the total concentration and the species in the solid phase.17d 6. Analyses. Small-angle powder X-ray diffraction (XRD) patterns were measured by using an MXP 18 diffractometer (Mac Science Co. Ltd.) with monochromated Cu KR radiation with scattering reflections recorded for 2θ angles between 0.3° and 6.5° corresponding to d-spacings between 29.4 and 1.35 nm. N2 adsorption–desorption isotherms were measured using a BELSORP36 analyzer (JP. BEL Co. Ltd.) at 77 K. The pore size distribution was then determined from the adsorption curve of the isotherms by using nonlocal density functional theory (NLDFT). Transmission electron microscopy (TEM) and three-dimensional surface (3DTEM surface) images were obtained by using a JEOL TEM (JEM-2000EXII) operated at 200 kV with a side-mounted CCD camera (Mega View III from Soft Imaging System Co.). The TEM samples were prepared by dispersing the powder particles onto holey carbon film on copper grids. Thermogravimetric and differential thermal analyses (TG and DTA, respectively) were done using a Thermo Plus TG8120 (Rigaku, Japan). Energy dispersive X-ray microanalysis (EDS-130S) was used to determine the elemental compositions of the functionalized HOM carriers and sensors. 1H and 13C NMR spectra at room temperature for the DTAR probe were also measured using a Varian 400-MR model (Varian Inc., U.S.A.) with software VNMR version 6.1C operated at 500 and 125 MHz for the 1H and 13C NMR spectrometer. We used CDCl3 as a solvent. 29Si MAS NMR spectra at room temperature were measured using a Bruker AMX-500 operated at 125.78 MHz with a 90° pulse length of 4.7 µs. For all samples, the repetition delay was 180 s with a rotor spinning at 4 kHz. The absorbance spectrum of the sensor material was recorded using a Shimadzu 3150 model solid-state UV–vis spectrophotometer. The metal ion concentration after equilibration was determined with a Seiko SPS-1500 model inductively coupled plasma atomic emission spectrometer (ICP-AES). Buffer solutions were adjusted to ambient pH values using a Horiba F-24 (Kyoto, Japan) model microcomputerized pH/ion meter.

Results and Discussion 1. Synthesis of 3D Wormhole and Ordered Monolithic Probe Templates. Our simple method in the context of synthesis time (on the order of minutes) and in composition domains of (surfactant/C12-alkane/TMOS/H2O) shows key facts of control design of cubic Pm3n surfactant-silica mesophases (see S1 and S2, Supporting Information). The cubic Pm3n mesophases are either mesoscopically ordered or wormhole structures with shape- and size-controlled cage pore geometries, which indicated the versatility of this method over the real control of the surfactant phases. Results reveal that the addition of C12-alkane significantly affected both diameter and surface interfacial curvature of the surfactant micelles. These changes likely led to the cubic phase with high surface curvature preferred by these surfactants, as evidenced by the formation of large pore cage of cubic Pm3n phases (see S1 and S2, Supporting Information).15,16When the microemulsion liquid crystalline phases of Brij 35 (C12EO23) and Triton X-100 were used as templates, wormhole cubic Pm3n mesostructures could be fabricated with wormhole pores (see XRD patterns in S2, Supporting Information). However, highly ordered mesostructures (see S2, Supporting Information) were successfully fabricated by using both synthesis systems of Brij 78 (C18EO20) and Brij 76 (C18EO10), indicating the effect of the EO chain lengths of surfactants

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Figure 1. 3D surfaces of TEM micrograph of ordered (HOM-9) and wormhole (HOM-13) cage cubic Pm3n structures that were fabricated by using an instant direct-templating method of microemulsion phases of nonionic surfactants.

in the formation of long-range ordered structures. In general, ordered (HOM-9) and disordered (HOM-13) cubic Pm3n structures were only fabricated in the microemulsion systems of these surfactants.15,16 The 3D TEM surface images (Figure 1) clearly provided evidence of the formation of topologically ordered and wormhole pore surfaces associated with HOM-9 and HOM-13 monoliths (Figure 1a,b), respectively. The design of optical molecular nanosensors 1, 2, 3, and 4 occurred via direct inclusion of ethanol solution of DTAR probe into disordered and ordered cage cubic Pm3n monoliths, respectively. In fact, the removal of ethanol by vacuum at ambient temperature (Scheme 1a) led to the creation of sufficiently physisorbed interactions. However, the H-bonding interactions might occur between the abundant hydroxyl groups of pore surface silicates and the heteroatoms of DTAR probe molecules. In addition, the modified-DTAR molecules with long hydrocarbon tails (gC12) offer the required hydrophobicity to prevent the leaching of the probe receptors from both worm-like and ordered HOM monolithic carriers during the detection of Pb(II) ion in aqueous solutions.17 Such interactions are necessary to achieve retention of the DTAR probe modified monoliths during the washing cycle and potential sensing recognition of Pb2+ ions, as consistent with TG-DTA and EDS X-ray analyses (see Supporting Information, S3). Although the binding events of the Pb(II) ion with the hydrophobic DTAR probe might lead to increase

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Figure 2. XRD patterns (A) and N2 adsorption/desorption isotherms at 77 K (B) of wormhole cubic Pm3n (DTAR/HOM-13) nanosensors 1, 2 (a, b) and ordered cubic Pm3n (DTAR/HOM-9) nanosensors 3, 4 (c, d), respectively. Inset lists (A, B) are unit lattice constant (aPm3n ) d210210), mesopore size (P), volume (Vp), and surface area (SBET).

the hydrophilicity of the probe molecules, no evidence of the probe leaching in the aqueous phase was distinctive during the Pb(II)-DTAR complexation (Scheme 1b).36 Results here observed that the large spherical cavity and ordered geometry of nanosensors 3 and 4 had higher adsorption capacity and accessibility of the DTAR probe than that of 3D worm-like sensors 1 and 2 (Table 1), indicating the effect of the 3D nanoscale pore ordering and the cage characteristics on the loading capacity (Q) of DTAR probe. XRD patterns (Figure 2Aa, b) of (DTAR/HOM-13) nanosensors 1 and 2 show broad and poorly resolved diffraction peaks that tentatively led to the assignment of the structural geometries. The less resolved high-intensity reflection peaks with respective d-spacing ratios of 4:5:6 are indicative of the disordered cubic Pm3n symmetry with nanosensors 1 and 2. In turn, the high resolution peaks (Figure 2Ac,d) of (DTAR/HOM-9) nanosensors 3 and 4 were similar to that of cubic Pm3n monoliths (see S2, Supporting Information). Furthermore, the intensity and resolution of all reflections of nanosensors 3 and 4 strongly suggest a high degree of 3D architecture ordering with cubic Pm3n phase.34 Figure 2B showed a H2-type hysteresis loop and well-defined steepness of isotherms, indicating that cage-like pore structures were characteristic of both wormhole and ordered cubic Pm3n nanosensors.35 The steepness of the isotherms, in principle, decreased with the HOM monolithic templates synthesized by high EO-block lengths (g20) and Triton X-100, indicating the effect of the surfactant structures on the cage pores (see Figure 2B, inset). Moreover, with the immobilization of the DTAR organic moieties into HOMtemplates, the decrease in the width of the hysteresis loop indicated the decrease in the nanoscale pore size with all of the fabricated nanosensors 1, 2, 3, and 4. Results, in general, (34) Zhao, D.; Yang, P. ; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Commun. 1998, 2499. (35) (a) Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 1550. (b) Matos, J. R.; Kruk, M.; Jaroniec, M.; Zhao, L.; Kamiyama, T.; Terasaki, O.; Pinnavaia, T. J.; Liu, Y. J. Am. Chem. Soc. 2003, 125, 821. (36) (a) Overgaard, J.; Schiøtt, B.; Larsen, F. K.; Schultz, A. J.; MacDonald, J. C.; Iversen, B. B. Angew. Chem., Int. Ed. 1999, 38, 1239. (b) Yang, C.; Wang, Y.; Zibrowius, B.; Schüth, F. Phys. Chem. Chem. Phys. 2004, 6, 2461. (c) Gilliland, J. W.; Yokoyama, K.; Yip, W. T. Chem. Mater. 2004, 16, 3949.

indicated that the large amounts of the DTAR probe were embedded into the inner pores; however, a significant amount might be incorporated into the outer pore surfaces, as evidenced from the decrease of the structural parameters of sensors 1–4.17 The TEM images (Figure 3) show evidence of the retention in structural integrity of nanosensors with cubic Pm3n cage structures.37,38 The TEM micrographs along the zone axes of DTAR-modified HOM-9 “nanosensors 3 and 4” revealed regular straight arrays running along a large area of these directions. The estimated lattice constants along the [100] and [210] directions are about 149 and 154 Å, which agrees well with the unit-cell dimension determined from X-ray diffraction of HOM-9 nanosensors 3 and 4, respectively. 2. Ion Diffusion into 3D Wormhole and Ordered Sensors. The kinetic Pb(II)-to-DTAR binding responses were studied by continuously monitoring the colored complex spectra of ordered (HOM-9) and worm-like (HOM-13) monolithic sensors after the addition of analyte ions (100 ppb for Pb2+) as a function of time at optimal sensing conditions (such as pH of 6.5–6.8, HOM amount of 2–4 mg, volume of 20 mL, and temperature of 20 °C; Figure 4A). The observed results show that the responding times Rt (where 5 min e Rt e 7 min) were characteristic the complexation reactions of the Pb(II) ions and DTAR probe with the ion-sensing system of ordered nanosensors 3 and 4. In turn, the Rt value was extended to be >15–20 min, when wormhole sensors 1 and 2 were applied to visual detection of Pb(II) ions. This extension in the response time mainly indicated the influence of the disordered molecular orientation and nonuniformly DTAR-modified template structures in the ion-sensing systems.39 (37) (a) Huo, Q.; Margolese, D.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schüth, F.; Stucky, G. D. Nature 1994, 368, 317. (b) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (38) (a) Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G. D.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449. (b) Sakamoto, Y.; Diaz, I.; Terasaki, O.; Zhao, D.; Pérez-Pariente, J.; Kim, J. M.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 3118. (39) (a) El-Safty, S. A. J. Colloid Interface Sci. 2003, 260, 184. (b) Bajpai, J.; Shrivastava, R.; Bajpai, K. J. Appl. Polym. Sci. 2007, 103, 2581. (c) Rodman, D. L.; Pan, H.; Clavier, C. W.; Feng, X.; Xue, Z.-L. Anal. Chem. 2005, 77, 3231.

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Figure 3. Representative TEM micrographs recorded along the [110] (a), [100] (b), [111] (c), and [210] (d) zone axes of ordered cubic Pm3n (HOM-9/ DTAR) nanosensors 3 (a, b) and 4 (c, d).

Furthermore, the diffusion coefficient (D) of the intraparticle transport of the Pb(II) analyte into the spherical cavity of the cage nanosensors was calculated by using the following equation: D ) 0.03 R2/t1/2, where R is radius of nanosensors and t1/2 is the time for the half-signal response (see Figure 4).39a,b Results (Table 1) indicated that the open, uniform pore-cage architectures allowed efficient binding and diffusion of Pb(II) ions to DTAR probe. The high ion transport and the Pb(II)-DTAR binding were significantly affected by the 3D pore geometry and shape, as evidenced from the D value of the nanosensors 1–4 (Table 1). The calculated D values revealed that the Pb(II) ion-diffusion effectively appended to the ordered nanoscale structures, as evidenced by the higher D value of ordered cage compared to wormhole nanosensors (Table 1).39 Furthermore, the increase in solution temperature had significant influence on the Pb(II) ion-sensing rate of nanosensors (Figure 4B).39 However, the increase in the reaction temperature led to enhance the internal energy of the Pb(II) ion aqueous phase and to facilitate the mass transfer through the 3D wormhole or ordered pore cage sensors. Results, in general, indicated that the ordered pore geometry and the reaction temperature play a role in the design of efficient sensing systems for detection of ultratrace levels of Pb(II) ions in rapid response time. 3. 3D Wormhole and Ordered Pb(II) Ion Sensors. For the 3D DTAR-HOM nanosensors, the effects of experimental sensing factors such as the pH, temperature, sample volume,

response time (Rt), and critical amount of probe-modified HOM monoliths were studied to optimize batch equilibration conditions for a quick visual detection of Pb(II) ions (see Supporting Information, S4).40,41 The quantification procedure of Pb(II) ion sensing with 3D HOM nanosensors 1-4 was studied after equilibration time, namely, response time (Rt), in which the prominent color change and signal saturation in the nanosensor reflectance spectra were achieved (Figure 4A). The efficiency of Pb(II) ion sensing with nanosensors 1–4 was influenced by the pH solution. The reflectance spectra of the [Pb-DTAR]n+ complex was carefully monitored over a wide range of pH solutions. The 3D HOM nanosensors 1-4 were sensitive in terms of their optical “color intensity” and signal response for Pb(II) ions at pH 6.5–6.8 (see Supporting Information, S4). It was observed that the ion-sensing ability was narrowed between near neutral to slight alkaline conditions, assigning to the poor acidic property of the hydrophobic DTAR probe. In addition, the hydrolysis tendency of the Pb(II) analytes with increasing alkalinity led to diminishing the ion-sensing behavior of nanosensors after pH g8.0 (see Supporting Information, S4). In addition, the amount of adsorbed DTAR probe significantly affected the Pb(II) ion-sensing systems. Our results showed that the color intensity of the [Pb(40) (a) Ros-Lis, J. V.; Marcos, M. D.; Mártinez-Mánez, R.; Rurack, K.; Soto, J. Angew. Chem., Int. Ed. 2005, 44, 4405. (b) Qi, X.; Jun, E. J.; Xu, L.; Kim, S.-J.; Hong, J. S. J.; Yoon, Y. J.; Yoon, J. J. Org. Chem. 2006, 71, 2881. (41) (a) Liu, B.; Tain, H. Chem. Commun. 2005, 3156. (b) Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386. (c) Yang, Y.-K.; Yook, K.-J.; Tae, J. J. Am. Chem. Soc. 2005, 127, 16760.

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Figure 5. Changes in the UV–vis absorption spectra and the colorimetric response profile of 30 µM DTAR solutions in ethanol medium upon titration with standardized Pb(II) ions, at 20 °C. The pH of the medium was adjusted to 6.5 for optimum spectral and colorimetric isolation. A constant stay time of 15 min was maintained for stable color development for all solutions with 25 mL volume.

Figure 4. Time course changes (A) observed for nanosensors 1–4 during the recognition of [100 ppb] Pb(II) ions, where the used amount of HOM/ DTAR was 0.2 and 0.4 mg for nanosensors 3 and 4, and 1 and 2, respectively, and the sensing experiments performed at 20 °C and pH 6.5–6.8. Temperature-dependent kinetic responses (B) of nanosensor 2 during the recognition of [100 ppb] Pb(II) ions at pH 6.5–6.8 and HOM/ DTAR amount of 0.2 mg.

DTAR]n+ complex also depended on the amount of solid HOM-DTAR nanosensor used. However, the amount of DTAR probe significantly influenced the formation of the [Pb-DTAR]n+ complex (see Supporting Information, S4). Our findings revealed that 2 and 4 mg of HOM-DTAR monoliths was sufficient to achieve a good color separation between the blank and the Pb(II) ion sample using ordered DTAR/HOM-9 and worm-like DTAR/HOM-13 nanosensors. The variation in these ion-sensing conditions was mainly dependent on the physical properties of the nanosensors with respect to the ion-chromophore complex. To demonstrate the sensing applicability of the design of DTAR-modified nanosensors, we carried out sensing experiments of the Pb(II) ion using the DTAR molecular receptor in solution (Figure 5). The absorption spectrum of the DTAR probe in the ethanol phase shows a maximum absorption at λmax of 476 nm (Figure 5). The Pb(II) quantification experiments were carried out on a quartz flask with a total volume of 25 mL. The Pb(II) aliquots were injected from a 10 mM aqueous stock solution into the flask containing a mixture solution of 30 µM DTAR and pH 6.5 at constant temperature of 20 °C. The addition of specific amounts of Pb(II) ions to the DTAR probe induced the changes in the

DTAR absorption spectrum. However, the binding of Pb(II) ions with DTAR probe led to the formation of the Pb(II)DTAR chelate [Pb-DTAR]n+ complex (Scheme 1).17,26 In general, the optically visible and spectral intense transitions were observed when the aqueous medium was maintained within the pH range of 6.5–6.8, using 0.2 M MOPS-NaOH buffer, at signal response time (Rt g 15 min), Figure 5. The broadening in the absorption bands of the complex was due to the aromatic rings and the appearance of a new low-energy band at 647 nm. The well-defined isosbestic points at 495 nm clearly indicate the presence of a unique [Pb-DTAR]n+ complex in equilibrium with the free DTAR ligand. The new low energy band is responsible for naked-eye detection in the range of 0.2–5.0 ppm, as evidenced by the color change from reddish-orange to a deep blue (Figure 5, inset). The key design of DTAR-based nanosensors is that the significant Pb(II)-to-DTAR binding affinity provided a high degree of sensitivity and stability of the formed [PbDTAR]n+ complex under our simple sensing procedure. However, the nanosensors 1–4 with highly physical properties (see Figure 2B, inset data) could be used as efficient preconcentrators for simultaneously visual inspection “naked eye” and simple detection of Pb(II) ions over a wide, adjustable range of 5.0–150 ppb.42 The color transition corresponding to the formation of the [Pb-DTAR]n+ complex (Scheme 1b) provided a simple procedure for sensitive and selective detections of Pb(II) ions by the naked eye without the need for sophisticated instruments (Figure 6, inset).17–26,31 The color intensity and homogeneity increased with increasing Pb(II) ion concentration. The quantification process of nanosensors 1–4 for Pb(II) ion sensing over a wide range of concentration was monitored by UV/vis spectroscopy. The reflectance spectra of the DTAR design nanosensors at (λmax) 451 nm exhibited a bathochromic shift to 597 nm by addition of Pb(II) ions, indicating the formation of the charge-transfer (42) (a) Sandell, E. B. Colorimetric Determination of Traces of Metals, 3rd ed.; Interscience Publisher, Inc.: New York, 1959; p 326. (b) Christian, G. D. Analytical Chemistry, 6th ed.; John Wiley & Sons Inc.: New York, 2003.

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Figure 6. Concentration proportionate color transition and reflectance spectra observed for sensor 4 with increasing concentrations of Pb(II) ions within the pH range of 6.5–6.8, after equilibrating for 5 min, and at a sensor amount of 2.0 mg, solution volume 20 mL, and 20 °C.

[Pb-DTAR]n+ complex. The colorimetric determination of Pb(II) ions was revealed in the detection range of 10–150 ppb for worm-like (HOM-13) nanosensors 1 and 2 (see Supporting Information S5), and 5.0–150 ppb for ordered (HOM-9) nanosensors 3 and 4 (Figure 6). Our extensive study of ion-sensing systems revealed that the relatively high absorptivity of DTAR probe into a uniformly sized cavity of 3D cubic Pm3n carriers (HOM-9) led to high affinity DTAR receptor-Pb(II) ion binding events, as clearly evidenced by the significant sensing ability of 3D nanosensors 3 and 4 for Pb(II) ions in terms of sensitivity and response time (Figure 6 and Table 1). The calibration plots of the DTAR probe nanosensors, in general, show a linear correlation at low concentration ranges of Pb(II) analyte ions (Figure 7). The linear curves indicated that the Pb(II) analyte can be detected with highest sensitivity over a wide range of concentrations.42 In fact, the quality of the calibration methods is necessary to ensure both accuracy and precision of the Pb(II) ion sensing systems. Several quantification measurements (g10 times) were carried out using wide-range concentrations (2.4 × 10-8 to 7.24 × 10-7 M) of the standard “well-known” solutions of Pb(II) ions at the specific sensing conditions. The standard deviations for the analysis of Pb(II) ions using all nanosensors S1-S4 were in the range of 0.1–0.16%, as evidenced for the fitting plot of the calibration graphs (Figure 7, insets). The quantification limit (LQ) signifies the precise correlation of our experimental sensing procedure of Pb(II) ion-sensing data obtained from the fabricated nanosensors (Table 1). Furthermore, the LD value (Table 1) indicated that the ordered HOM-9 nanosensors enabled an effective detection of Pb(II) target ions. The reusability and reproducibility of the nanosensors are of particular interest in developing recyclable inorganic–organic hybrid optical sensors. In such a treatment procedure, we used the stripping agent Cl- anion with 0.2 M concentration to effectively remove the Pb(II) ions (i.e., decomplexation) after a complete detection process.43,44 After several times of the liquid-exchange process, the nanosensor was (43) (a) Nazeeruddin, M. K.; Censo, D. D.; Humphry-Baker, R.; Gratzel, M. AdV. Funct. Mater. 2006, 16, 189. (b) Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145.

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Figure 7. Calibration plots (A-A0 and [Pb(II)] concentration, where A0 and Ac are the absorbance of the DTAR probe and [Pb-DTAR]n+ complex at λ ) 597 nm) of nanosensors 4 in the presence and absence of the interference species. The nanosensor working conditions were within the pH range of 6.5–6.8, with a contact time of 15 min, at 20 °C. A normalized plot was achieved by measuring the relative absorbance of the [Pb(II)DTAR]n+ complex formed with HOM/DTAR solid with respect to the solid sensor blank at λ597 nm in the presence and absence of interference species. The inlet in the graph shows the amplification for the low-limit colorimetric response for Pb(II) ions. The graphs were represented by a linear fit line in the linear concentration range before saturation. The error bars denote a relative standard deviation of >4.5% for the analytical data of 10 replicate analyses.

collected and washed by deionized H2O. The UV/vis reflectance spectrum of DTAR-modified nanosensor at λ ) 451 nm was measured and revealed no change in the signal intensity, indicating no potential leaching of DTAR-modified HOM carriers during the regeneration process. Results observed that the decomplexation process of the Pb(II) analyte could be carried out for up to six repeated cycles, without significant loss in the sensing efficiency of the nanosensors (Table 1). However, as a consequence of the regeneration/reuse cycles, the stripping agent mainly affected the deactivation behavior of the specific activity of the probe binding affinity, as evidenced from the slight influence of the Pb(II) ion nanosensor utilities in terms of the sensitivity and response time (Table 1).17 4. Optical Pb(II) Ion-Selective Nanosensors. The scale of nanosensor ion selectivity for Pb(II) ion was investigated with possible interfering foreign ions. The key result in our study here is that the DTAR-modified nanosensors exhibited significant ion-selective ability, particularly at low concentrations of Pb(II) ions, compared with the Pb(II) ion-selective recognition in solution (see Supporting Information, S6).45 The selectivity profiles of nanosensors for Pb(II) ion were examined by two significant methods that we called specifically (Figure 8A,B) and simultaneously (Figure 8C) selective recognition methods. The first method was carried out by adding a series of cations, anions, and surfactants with high concentration range of 2–6000 ppm to DTAR/HOM sensors (44) (a) Bossi, M.; Belov, V.; Polyakova, S.; Hell, S. W. Angew. Chem., Int. Ed. 2006, 45, 7462. (b) Carringto, N. A.; Thomasa, G. H.; Rodmana, D. L.; Beach, D. B.; Xue, Z.-L. Anal. Chem. Acta 2007, 581, 232. (45) Zheng, Y.; Orbulescu, J.; Ji, X.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 2680.

Three-Dimensional Wormhole and Ordered Mesostructures

Figure 8. Representative selectivity profiles of nanosensor 4 for [0.1 ppm] Pb(II) ions by using specifically (A and B) and simultaneously selective recognition (C) methods. The interfered cations are listed in the order (left to right), (1) 6000 ppm Na+, (2) 5000 ppm K+, (3) 20 ppm Ca2+, (4) 15 ppm Sr2+, (5) 4 ppm Cr6+, (6) 3.8 ppm Al3+, (7) 1.5 ppm Cu2+, (8) 1.8 ppm Ni2+, (9) 2.5 ppm Mn2+, (10) 2.2 ppm Zn2+, (11) 2 ppm Co2+, (12) 0.5 ppm Cd2+, (13) 0.5 ppm Hg2+, (14) 3.5 ppm Fe2+, (15) 3.5 ppm Bi3+, (16) 4.5 ppm Sn2+, and (17) 4 ppm Sb3+. The surfactants and anions are listed in the order (left to right), (1) 10 ppm SDS, (2) 20 ppm CTAB, (3) 30 ppm Triton X-100, (4) 35 ppm tartrate, (5) 20 ppm citrate, (6) 15 ppm oxalate, (7) 300 ppm Cl-, (8) 50 ppm acetate, (9) 250 ppm nitrite, (10) 32 ppm nitrate, (11) 200 ppm sulfite, (12) 255 ppm sulfate, (13) 100 ppm phosphate, and (14) 250 ppm carbonate. Both selective recognition methods were studied at a specific sensing condition of pH 6.8, equilibrating time of 5 min, and solution volume of 20 mL and at 20 °C, respectively.

(blank) at the normal sensing conditions (pH of 6.5–6.8, time g Rt, HOM amount of 2 mg, volume of 20 mL, and temperature of 20 °C; see Figure 8A,B). Results showed that no significant changes in reflectance spectra at λ ) 597 nm and visible color patterns of sensors were observed. In turn, the addition of [0.1 ppm] Pb(II) analyte to DTAR-based sensors showed a prominent color change and signal intensity (Figure 8A,B). The simultaneously selective method was carried out by studying the effect of the addition of high concentrations of interfered species prior to the addition of Pb(II) ions [0.1 ppm] in the recognition sensing systems

Chem. Mater., Vol. 20, No. 8, 2008 2653

(Figure 8C). The latter method effectively examined the selectivity of ion sensors for Pb(II) ion over the active multicomponent ions and species. However, in real samples, the Pb(II) analyte is normally encapsulated by a variety of matrix compounds (see below). Results from both selectivity profiles of nanosensors showed that alkali and alkaline-earth metal ions were practically noninterfered with a tolerable concentration of g30 ppm for Mg2+ and Ca2+. Although, with addition of transition metal ions, a slight reduction in the sensing kinetics for Pb2+ ions was recorded, the DTARmodified nanosensor exhibited a good tolerance for major and active foreign cations. These diverse ions (such as Cu2+, Ni2+, Co2+, and Zn2+ ions) were found to be competitive for the active chelating sites beyond 0.38 ppm, at pH 6.5–6.8. The addition of a mixture of 0.15 mM of citrate, thiosulphate, and tartrate to the buffer solution enhanced the matrix tolerance concentrations up to 150-fold excess over Pb(II) ions. The metal ions of the lanthanide series were totally noncompetent for the chelating sites. Other heavy metal ions such as, Al3+, Bi3+, Cr6+, La3+, Ir3+, Sn2+, and Sb3+ did not interfere in the Pb(II) ion-sensing procedure by nanosensors, even with a tolerable concentration e 4 ppm. Interestingly, the Hg2+ and Cd2+ ions showed interference, which was eliminated by using 0.2 mM thiosulphate and thiocyanide. The influence of organic matters like humic acid and surfactants (cationic, anionic, and nonionic) that might impede the sensing process was also investigated. The tolerance toward complexants and surfactants showed satisfactory findings with a permissible tolerance limit of (5%, as presented in the tolerance Tables (see Supporting Information, S6). In fact, the selectivity of the nanosensors for Pb(II) ions over active multicomponent ions and species (Figure 8C) indicated the high thermodynamic binding of Pb(II) ion for S, N, and O-chelate DTAR ligand and the fast Pb-to-DTAR binding kinetics at the optimized pH 6.5–6.8 range.45,46 To explore further applicability of the DTAR-design nanosensors as Pb(II) ion-selective systems, the quantification of 0.05 mg dm-3 of Pb2+ was tested with stimulated synthetic seawater composite mixtures of 3.5% salinity. The quantification data of the Pb(II) ion sample examined five times were fitted the calibration plot (Figure 7). Spectral analyses and naked-eye detection of the nanosensors revealed that the calibrated concentration of Pb(II) ions in such multicomponent mixtures was 0.049 mg dm-3, with a standard deviation value of e4.3%. The practically important issue is that the pretreated effluents (Table 2) were used as precursor sources to test the practical implementation of the DTAR nanosensors with real samples. The real samples were identified by the ICP-AES analysis. The ICP data indicated that the samples contained about 15.7–265 mg dm-3 of alkali and alkaline earth metal ions, in addition to traces (0.02– 0.083 mg dm-3) of Zn2+, Mn2+, and Fe2+/3+ ions. To these samples, various trace levels of Pb(II) ions were spiked along (46) (a) Kramer, R. Angew. Chem., Int. Ed. 1998, 37, 772. (b) Shao, N.; Zhang, Y.; Cheung, S.; Yang, R.; Chan, W.; Mo, T. Anal. Chem. 2005, 77, 7294. (c) Winker, J. D.; Bowen, C. M.; Michelete, V. J. Am. Chem. Soc. 1998, 120, 3237.

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Table 2. Sensing Analysis of Pb(II) Ion in Spiked Environmental Samples by Using DTAR/HOM Nanosensor 4

sample source food processing factory semiconductor discharge local hospital effluent

spiked amount (mg dm-3) 2+

2+

2+

1.3 - Co , Cu , Ni , Zn2+, Mn2+, Fe2+; 0.5 - Cr6+, Sn2+, Sb2+; 10 - Ca2+, Mg2+ 0.25 - Ga3+; 0.5 - La3+, Ce4+, Nd3+, Sm3+; 0.05 - W6+, Mo6+; 0.5 - Si4+ 1.2 - Cu2+, Ni2+, Zn2+, Fe2+, Mn2+, Co2+; 25 - Ca2+, Mg2+

spiked amount (µg dm-3)

analyzed amount (µg dm-3)

Pb2+

Pb2+

0 10 25

09.7 ( 2.3a 19.6 ( 2.0a

15 50

14.8 ( 2.1 50.3 ( 3.8

5 15

4.7 ( 2.1 14.2 ( 3.0a

were solvent-free systems and had the capacity to serve as ion preconcentrators with complete reversibility and reusability (Table 1).17,29,42 These significant features of the probe-design nanosensors led to overcoming the disposal problems, which normally associated with the liquid probe systems. In addition, the behavior of the solid-probe nanosensor with the Pb(II) analytes and its matrix environment showed conspicuous divergence from its solution chemistry. The specific utility of the nanosensor was interesting on its application as an ionsensing tool (see Table 2). Conclusion

a

a Data obtained after eliminating the interfering foreign ions; at pH 6.5–6.8, temperature ) 20 °C; Rt ) 15 min.

with other matrix composites, as listed in Table 2. The quantification procedures of the spiked samples were carried out under default conditions, at real-time response (i.e., gRt), and at 20 °C. The analytical data (Table 2) indicated that the design made of DTAR-HOM carriers could be used as potential ion-selective sensors for monitoring of Pb(II) target from environmental samples and waste disposals, as confirmed from excellent correlations between the analytical results obtained with the proposed method. 5. Assessment of 3D Wormhole and Ordered Nanosensors. Key to our development design of nanosensors based 3D wormhole and ordered HOM monoliths is that although the DTAR receptor was successfully used for a Pb(II) ionsensing recognition in solution up to ∼10-7 M, our nanosensors enabled the creation of ion-sensitive responses with revisable, selective, and sensitive recognitions of a wide range of detectable Pb(II) ions down to nanomolar (∼10-9 M) in rapid sensing responses (on the order of minutes). However, when the DTAR probe was anchored on a structurally designed mesoporous silica HOM monolith, selective and sensitive designs for Pb(II) ion sensing were substantially distinctive, as evidenced from the significant variation in the sensing behavior of probe molecules in the solid medium compared to the liquid phase (see Figures 5 and 6). As a result of the probe accommodation into nanoscale HOM monoliths, the intrinsic property and functionality of the probe receptor in terms of absorption wavelength (λmax), selectivity, sensitivity, and binding response kinetics were significantly changed with its respect to the physical properties of the HOM monoliths. On subsequent studies, it was found that the use of HOM monoliths as probe templates had profound sensing advantages over the liquid colorimetric determination using the DTAR receptor. Considering the environmental factors, the solid-state probe nanosensors

The development of materials at the nanoscale level to create regularly spaced pores and 3D geometries had led to widespread advances in the field of optical nanosensors. The functional use of 3D HOM monoliths fabricated via a simple and fast strategy enhanced the functionality of the materials as probe carriers. In general, the cage character and the degree of pore uniformity of 3D HOM carriers substantially influenced the ion-sensing functionality in terms of their probe inclusion capacities, ion-transport diffusion, optical responsive profile, and visual color transition series during the recognition of ultratraces of toxic Pb(II) ions. Results show that the 3D HOM wormhole pores might contribute to a bulk mass hindrance and a difficult homogeneity of the ion-percolation process. However, in case of ordered cage structures, the high metal flux, namely, ion transport, and the affinity of the Pb(II)-DTAR binding events were significantly affected by the mesopore uniformity and by the DTAR-templated orientation into the monolith nanosensors. Results show that although the DTAR receptor was successfully used for a Pb(II) ion-sensing recognition in solution up to ∼10-7 M, our nanosenors enabled us to create ionsensitive responses with revisable, selective, and sensitive recognitions of a wide range of detectable Pb(II) ions down to nanomolar (∼10-9 M) in rapid sensing responses (on the order of minutes). The key result in our study is that the DTAR-design nanosensors exhibited significant ion-selective ability, particularly at low concentrations of Pb(II) ions, compared with the Pb(II) ion-selective recognition in solution. Moreover, the nanosensors show evidence as potential ion-selective candidates for an efficient Pb(II) ion detection from environmental samples and waste disposals. Supporting Information Available: Fabrication procedures and characterization of ordered 2D and 3D HOM monoliths, TG-DTA and EDS X-ray analyses, pH and amount dependent response profile of 3D nanosensors, colorimetric and visual recognition, and calibration plot of Pb(II) ion by using sensors 1 and 2, Pb(II) ionselective sensor in solution, and solid (PDF). This information is available free of charge via the Internet at http://pucs.acs.org. CM701966C