pubs.acs.org/Langmuir © 2011 American Chemical Society
Immobilized β-Cyclodextrin on Surface-Modified Carbon-Coated Cobalt Nanomagnets: Reversible Organic Contaminant Adsorption and Enrichment from Water Roland Fuhrer, Inge K. Herrmann, Evagelos K. Athanassiou, Robert N. Grass, and Wendelin J. Stark* Institute for Chemical and Bioengineering, ETH Zurich, Zurich CH-8093, Switzerland Received September 27, 2010. Revised Manuscript Received November 24, 2010 Surface-modified magnetic nanoparticles can be used in extraction processes as they readily disperse in common solvents and combine high saturation magnetization with excellent accessibility. Reversible and recyclable adsorption and desorption through solvent changes and magnetic separation provide technically attractive alternatives to classical solvent extraction. Thin polymer layered carbon-coated cobalt nanoparticles were tagged with β-cyclodextrin. The resulting material reversibly adsorbed organic contaminants in water within minutes. Isolation of the immobilized inclusion complex was easily carried out within seconds by magnetic separation due to the strong magnetization of the nanomagnets (metal core instead of hitherto used iron oxide). The trapped molecules were fully and rapidly recovered by filling the cyclodextrin cavity with a microbiologically well accepted substitute, e.g., benzyl alcohol. Phenolphthalein was used as a model compound for organic contaminants such as polychlorinated dibenzodioxins (PCDDs) or bisphenol A (BPA). Fast regeneration of nanomagnets (compared to similar cyclodextrin-based systems) under mild conditions resulted in 16 repetitive cycles (adsorption/desorption) at full efficiency. The high removal and regeneration efficiency was examined by UV-vis measurements at chemical equilibrium conditions and under rapid cycling (5 min). Experiments at ultralow concentrations (160 ppb) underline the high potential of cyclodextrin modified nanomagnets as a fast, recyclable extraction method for organic contaminants in large water streams or as an enrichment tool for analytics.
1. Introduction Magnetic nanocarriers have received tremendous interest for the selective removal or manipulation of chemicals.1-3 Largescale applications of chemically functionalized nanomagnets4 are attractive for the treatment of wastewater as well as process and drinking water where noxious contaminations are present at (ultra)low concentration.5-7 In this regard, the high mobility of nanomagnets may in principle enable fast and efficient reagent recycling by rapid adsorption/desorption. Such a process would allow economic operation and may be used for concentrating contaminants from a large stream into small volumes, where they could then be treated separately. In the following study, we would like to address, as a most prominent example, contaminated process water as it occurs in various industries such as textile coloring, ore refining, machining, fine chemistry, food industry, pharmacy, or cosmetics.8-11 The presence of organic contaminants *To whom correspondence should be addressed. E-mail: wendelin.stark@ chem.ethz.ch.
(1) Schaetz, A.; Reiser, O.; Stark, W. J. Chem.;Eur. J. 2010, 16, 8950–8967. (2) Lu, A. H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222– 1244. (3) Stark, W. J. Angew. Chem., Int. Ed. 2011, in press (DOI: 10.1002/anie.200906684). (4) Koehler, F. M.; Rossier, M.; Waelle, M.; Athanassiou, E. K.; Limbach, L. K.; Grass, R. N.; Gunther, D.; Stark, W. J. Chem. Commun. 2009, 4862–4864. (5) Dhaouadi, A.; Monser, L.; Adhoum, N. J. Hazard. Mater. 2010, 181, 692– 699. (6) Shue, M. F.; Chen, F. A.; Chen, T. C. Environ. Monit. Assess. 2010, 168, 91–101. (7) Ambashta, R. D.; Sillanpaa, M. J. Hazard. Mater. 2010, 180, 38–49. (8) Sardessai, Y. N.; Bhosle, S. Biotechnol. Prog. 2004, 20, 655–660. (9) Diacono, M.; Montemurro, F. Agron. Sustain. Dev. 2010, 30, 401–422. (10) Latif, A.; Noor, S.; Sharif, Q. M.; Najeebullah, M. J. Chem. Soc. Pak. 2010, 32, 115–124. (11) Singleton, I. J. Chem. Technol. Biotechnol. 1994, 59, 9–23.
1924 DOI: 10.1021/la103873v
often hinders state of the art biological sludge treatment due to toxicity affecting the microbiota or bacteria (i.e., the contaminants kill the bacteria and thus decrease water cleaning efficiency)12,13 and therefore demands a preprocessing step. Furthermore, contaminated sludge in a landfill can leach organic contaminants back to water.14 The present work shows how an exchange of an eco-toxicologically problematic contaminant (e.g., dyes, dioxins, phenols) through a microbiologically well accepted substitute (e.g., benzyl alcohol) can be used for contaminant enrichment (from low to higher concentration) and, therefore, offers a potential way to treat contaminated wastewater prior to release or biological sludge treatment. The concept makes use of host/guest interactions in cyclodextrins, which have been shown to reliably form rapid and reversible inclusion complexes with small organic molecules.15,16 Cyclodextrins (CD) have reached significant attention as efficient and selective adsorbents in a wide range of processes, ranging from cholesterol extraction from foods,17,18 encapsulation of flavors in foods or cosmetics,16 and environmental (12) Sponza, D. T.; Oztekin, R. J. Chem. Technol. Biotechnol. 2010, 85, 913–925. (13) Munoz, R.; Guieysse, B. Water Res. 2006, 40, 2799–2815. (14) Yamamoto, T.; Yasuhara, A.; Shiraishi, H.; Nakasugi, O. Chemosphere 2001, 42, 415–418. (15) Inoue, Y.; Hakushi, T.; Liu, Y.; Tong, L. H.; Shen, B. J.; Jin, D. S. J. Am. Chem. Soc. 1993, 115, 475–481. (16) Del Valle, E. M. M. Process Biochem. 2004, 39, 1033–1046. (17) Astray, G.; Gonzalez-Barreiro, C.; Mejuto, J. C.; Rial-Otero, R.; SimalGandara, J. Food Hydrocolloids 2009, 23, 1631–1640. (18) Szejtli, J.; Szente, L. Eur. J. Pharm. Biopharm. 2005, 61, 115–125. (19) Cathum, S. J.; Dumouchel, A.; Punt, M.; Brown, C. E. Soil. Sediment. Contam. 2007, 16, 15–27. (20) Yamasaki, H.; Makihata, Y.; Fukunaga, K. J. Chem. Technol. Biotechnol. 2008, 83, 991–997.
Published on Web 01/18/2011
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contaminant isolation such as dioxin,19 trichlorfon,16 or phenols20,21 to cytostatic agents in drug delivery or general medicinal applications.22-26 These cone-shaped sugars also gain rapid interest in biotechnological applications, i.e., as biocatalysts of antimalarial drug artemisin.27-29 This fascinating properties are based on cyclodextrin (hydrophobic) cavities (various sizes available, e.g., R-CD, β-CD, γ-CD) which can encapsulate hydrophobic compounds. Substituents are often introduced for tuning of the hydrophobicity/hydrophilicity or used as quality control by color change.30,31 Because cyclodextrins are highly soluble in water,32 they need to be immobilized for an efficient recovery of the adsorbed target compound.33-35 Many approaches36-40 have investigated the use of cyclodextrin/polymer composites: Once the cyclodextrins have captured their target compound in their cavity, the polymer-based particles are filtered off. Banerjee and Chen were among one of the first to propose magnetic particlebound anticancer drug delivery making use of the hydrophobic cavity of cyclodextrins to reversibly adsorb retinoic acid.23,41,42 In this work, we used carbon-coated cobalt metal nanomagnets (20 nm in average BET surface equivalent area diameter, see Supporting Information Table S1) as a stable (pH, temperatureresistant) and highly magnetic carrier for β-cyclodextrin and demonstrate the reversible and rapid removal of a triphenylmethane dye molecule. This dye represents a large class of organic contaminants and may serve as a suitable model compound as shown earlier.43-45 β-Cyclodextrin was chosen to get an optimal host-guest interaction due to the best possible cage size.30,31 The procedure shown here allows the reliable use of nanosized metal nanomagnets carriers (high external surface area) while the metal core maintains good handling (the nanomagnets can be rapidly isolated by applying an external magnetic field gradient rather (21) Yeom, S. H.; Daugulis, A. J.; Lee, S. H. Process Biochem. 2010, 45, 1582– 1586. (22) Badruddoza, A. Z. M.; Hidajat, K.; Uddin, M. S. J. Colloid Interface Sci. 2010, 346, 337–346. (23) Banerjee, S. S.; Chen, D. H. Int. J. Appl. Ceram. Technol. 2010, 7, 111–118. (24) Bonacchi, D.; Caneschi, A.; Dorignac, D.; Falqui, A.; Gatteschi, D.; Rovai, D.; Sangregorio, C.; Sessoli, R. Chem. Mater. 2004, 16, 2016–2020. (25) Chung, J. W.; Kwak, S. Y. Langmuir 2010, 26, 2418–2423. (26) Pun, S. H.; Bellocq, N. C.; Liu, A. J.; Jensen, G.; Machemer, T.; Quijano, E.; Schluep, T.; Wen, S. F.; Engler, H.; Heidel, J.; Davis, M. E. Bioconjugate Chem. 2004, 15, 831–840. (27) Singh, M.; Sharma, R.; Banerjee, U. C. Biotechnol. Adv. 2002, 20, 341–359. (28) Usuda, M.; Endo, T.; Nagase, H.; Tomono, K.; Ueda, H. Drug Dev. Ind. Pharm. 2000, 26, 613–619. (29) Singleton, M. L.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 2010, 132, 8870–8871. (30) Basappa, C.; Rao, P.; Rao, D. N.; Divakar, S. Int. J. Food Sci. Technol. 1998, 33, 517–520. (31) Ueno, A.; Kuwabara, T.; Nakamura, A.; Toda, F. Nature 1992, 356, 136–137. (32) Connors, K. A. Chem. Rev. 1997, 97, 1325–1357. (33) Hu, J.; Shao, D. D.; Chen, C. L.; Sheng, G. D.; Li, J. X.; Wang, X. K.; Nagatsu, M. J. Phys. Chem. B 2010, 114, 6779–6785. (34) Topchieva, I. N.; Spiridonov, V. V.; Kataeva, N. A.; Gubin, S. P.; Filippov, S. K.; Lezov, A. V. Colloid Polym. Sci. 2006, 284, 795–801. (35) Cao, H. N.; He, J.; Deng, L.; Gao, X. Q. Appl. Surf. Sci. 2009, 255, 7974– 7980. (36) Krause, R. W. M.; Mamba, B. B.; Dlamini, L. N.; Durbach, S. H. J. Nanopart. Res. 2010, 12, 449–456. (37) Yudiarto, A.; Kashiwabara, S.; Tashiro, Y.; Kokugan, T. Sep. Purif. Technol. 2001, 24, 243–253. (38) Harada, A.; Li, J.; Kamachi, M. Nature 1994, 370, 126–128. (39) Forrest, M. L.; Gabrielson, N.; Pack, D. W. Biotechnol. Bioeng. 2005, 89, 416–423. (40) Bibby, D. C.; Davies, N. M.; Tucker, I. G. Int. J. Pharm. 1999, 187, 243– 250. (41) Banerjee, S. S.; Chen, D. H. Chem. Mater. 2007, 19, 6345–6349. (42) Banerjee, S. S.; Chen, D. H. Nanotechnology 2008, 19, 265602. (43) Afkhami, A.; Madrakian, T.; Khalafi, L. Anal. Lett. 2007, 40, 2317–2328. (44) Mohamed, M. H.; Wilson, L. D.; Headley, J. V. Carbohydr. Polym. 2010, 80, 186–196. (45) Uyar, T.; Havelund, R.; Nur, Y.; Hacaloglu, J.; Besenbacher, F.; Kingshott, P. J. Membr. Sci. 2009, 332, 129–137.
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Article
than filtration). Metal-core nanomagnets have one of the highest saturation magnetizations (162 emu/g)50 if compared to commonly used metal oxide beads. Since the separation force strongly depends on magnetization, this enables product isolation within a few seconds.4 A thin polymer layer (polyethylenimine) was physisorbed on the compact carbon layer to achieve the covalent immobilization of β-cyclodextrin. This polymer layer is not used as in classical polymer particles to load the adsorbent but as a cross-linking agent. This covalent surface functionalization enables a stable linkage of the capturing agent (CD) to the magnetic force anchor (metal core). The maximum amount of immobilized β-CD was measured to be 27 μmol/g nanoparticles (Supporting Information Table S2). Single-step recovery efficiencies of well above 90% are reached by combining these two separation concepts. In order to show the importance of careful surface modification, we included control experiments using nanoparticles without a CD cavity. In line with most recent studies,46-48 these control experiments have shown rapid, but irreversible, adsorption on naked nanoparticles (this impedes efficient recycling). Reversibility allows organic contaminant enrichment and up-concentration. Studies on magnetic extraction showed that the principle of magnetic extraction offers economical advantages also in large scale46 or medicinal applications.49 Similar to them, our study demonstrates that the same principles can be successfully applied even at very low concentration (μg/L, ppb level) of pollutant. Very fast (few minutes) and highly efficient (0.5 g particles/L water) adsorption/desorption of the model compound and the ability to run over 16 iterations without losing the efficiency (>90%) could make such nanomagnets interesting for wastewater treatment or organic contaminant enrichment.
2. Experimental Section 2.1. Nanoparticle Polyethylenimine Functionalization.4 Magnetic carbon-coated cobalt metal nanoparticles (C/Co, Turbobeads LLC, Switzerland, see Supporting Information for characterization) were washed in aqueous 1 wt % HCl (Fluka, puriss) using deionized water (Milipore) until the pH remained stable to remove free metal ions or poorly coated magnetic particles. The treatment of the cobalt nanoparticles with 1 M HCl removes not completely coated cobalt particles to avoid a subsequent leaching as shown by Rossier et al.46 With the aid of a commercial neodymium-based magnet (N42, Q-40-40-20-N, Webcraft GmbH), the nanomagnets could be rapidly (seconds) accumulated at the wall of the reaction flask. After decanting the remaining liquid, the particles were washed three times each with deionized water (Millipore) and acetone (puriss, Fluka). In each cleaning step, the nanomagnets were suspended by sonication for 5 min and recovered with the aid of the magnet at the flask wall. At the end, the particles were dried in vacuo at 50 °C. For the functionalization of the nanomagnets with β-cyclodextrin the particles were initially coated with polyethylenimine (PEI). The physisorption of the polymer was done by dispersing 1 g of acid-treated particles in 20 mL of N,N-dimethylformamide (DMF) (puriss 99.8%, Fluka) and then adding 100 mL of a polyethylenimine (PEI) solution prepared from dissolving 8 g of PEI (99%, Mw: 10 000, Polysciences Inc.) in 120 mL of dry DMF. The particles were stirred with the polymer at room temperature for 48 h. The functionalized nanomagnets were (46) Rossier, M.; Koehler, F. M.; Athanassiou, E. K.; Grass, R. N.; Aeschlimann, B.; Gunther, D.; Stark, W. J. J. Mater. Chem. 2009, 19, 8239–8243. (47) Wittmann, S.; Schatz, A.; Grass, R. N.; Stark, W. J.; Reiser, O. Angew. Chem., Int. Ed. 2010, 49, 1867–1870. (48) Yang, K.; Xing, B. Chem. Rev. 2010, 110, 5989-6008. (49) Herrmann, I. K.; Urner, M.; Koehler, F. M.; Hasler, M.; Roth-Z’Graggen, B.; Grass, R. N.; Ziegler, U.; Beck-Schimmer, B.; Stark, W. J. Small 2010, 6, 1388– 1392.
DOI: 10.1021/la103873v
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Article Scheme 1. β-Cyclodextrins Can Be Loaded with Dye Contaminants and Removed from Bulk Solution in a First Step, and Subsequently the Dye Can Be Replaced with an Eco-Friendly Reagent and the Cyclodextrin Nanomagnets Can Be Recycleda
Fuhrer et al. Scheme 2. Metallic, C/Co Nanoparticles (Left) Were Chemically Functionalized Using Physisorption of Polyethylenimine (PEI), Reaction with the Cross-Linker Hexamethyl Diisocyanate (HMDI), and Final Attachment of β-Cyclodextrin
a The repetitive manner allows a cost-effective reuse of the nanomagnets for contaminant enrichment.
washed as described above three times with DMF and dried in vacuo at 50 °C. 2.2. Nanoparticle β-Cyclodextrin Functionalization. The polyethylenimine-functionalized particles were further reacted with hexamethylene diisocyanate (HMDI),20,42 a strong crosslinker of amino or hydroxyl groups. Under nitrogen flow, HMDI (2 mL, purum, g98%, Sigma-Aldrich) and extra dry DMF (100 mL, with molecular sieve, water
