pubs.acs.org/Langmuir © 2010 American Chemical Society
The Visualized Polarity-Sensitive Magnetic Nanoparticles Tian-Long Zhang†,‡ and Bao-Hang Han*,† †
National Center for Nanoscience and Technology, Beijing 100190, China, and ‡Graduate University of Chinese Academy of Sciences, Beijing 100049, China Received December 10, 2009. Revised Manuscript Received January 24, 2010
Three polarity-sensitive organic molecules (DIAA, DIUA, and DISA) were designed and synthesized for functionalizing high-quality superparamagnetic Fe3O4 nanoparticles (NPs) via the ligand exchange strategy to prepare polarity-sensitive Fe3O4 NPs. The functional group is chosen to be the carboxyl group (one for DIAA and DIUA, two for DISA) that is a universal coordinating site for iron oxide NPs. The method for binding these functional molecules onto the surface of the NPs is simple and straightforward. Among the three molecules, the DISA molecules passivate the NPs’ surface most efficiently owing to their particular structure with two carboxyl groups and a general good solubility. The DISA-functionalized Fe3O4 NPs (DISA-Fe3O4 NPs) display distinctly different fluorescence emissions in various solvents of different polarities with the magnetism well preserving. The prepared polarity-sensitive Fe3O4 NPs that are dual functional can be used as a visualized polarity sensor and perform NPs’ superparamagnetic properties simultaneously. It also provides a conceptual design for preparing the polarity-sensitive nanomaterials with multifunction.
Introduction A great drive in the progress of nanoscience has been elicited by the recent outstanding advances in the search for new materials with applications in many fields.1-3 Among them, to explore nanoparticles (NPs) as a specific probe or sensor has been a focus of research for many years.4-6 In general, the fabrication of highquality hybrid organic/inorganic NPs endowed with excellent sensing ability using inherent or surface derived optical and magnetic properties represents a promising avenue to the development of a novel generation of probe and sensor. In particular, superparamagnetic Fe3O4 NPs hold tremendous potential applications in magnetic bioseparation,7,8 cell targeting,9,10 and biosensing11,12 thanks to a favorable combination of unique chemical and physical properties.13,14 Furthermore, we have noted that the polarity of the environment is so vital to biology and ecology that any minor alteration would induce an evident change in the polarity, which often reveals abundant information *Corresponding author: Fax þ86-10-82545576; e-mail
[email protected].
(1) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. (2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4 (6), 455-459. (3) Lee, J. H.; Huh, Y. M.; Jun, Y.; Seo, J.; Jang, J.; Song, H. T.; Kim, S.; Cho, E. J.; Yoon, H. G.; Suh, J. S.; Cheon, J. Nat. Med. 2007, 13 (1), 95-99. (4) Snee, P. T.; Somers, R. C.; Nair, G.; Zimmer, J. P.; Bawendi, M. G.; Nocera, D. G. J. Am. Chem. Soc. 2006, 128 (41), 13320-13321. (5) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7 (6), 442-453. (6) Lee, H.; Yu, M. K.; Park, S.; Moon, S.; Min, J. J.; Jeong, Y. Y.; Kang, H. W.; Jon, S. J. Am. Chem. Soc. 2007, 129 (42), 12739-12745. (7) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Chem. Commun. 2006 (9), 941-949. (8) El-Boubbou, K.; Gruden, C.; Huang, X. J. Am. Chem. Soc. 2007, 129 (44), 13392-13393. (9) Josephson, L.; Tung, C. H.; Moore, A.; Weissleder, R. Bioconjugate Chem. 1999, 10 (2), 186-191. (10) Babic, M.; Horak, D.; Trchova, M.; Jendelova, P.; Glogarova, K.; Lesny, P.; Herynek, V.; Hajek, M.; Sykova, E. Bioconjugate Chem. 2008, 19 (3), 740-750. (11) Perez, J. M.; Josephson, L.; O’Loughlin, T.; Hogemann, D.; Weissleder, R. Nat. Biotechnol. 2002, 20 (8), 816-820. (12) Hsing, I. M.; Xu, Y.; Zhao, W. T. Electroanalysis 2007, 19 (7-8), 755-768. (13) Lu, A. H.; Salabas, E. L.; Sch€uth, F. Angew. Chem., Int. Ed. 2007, 46 (8), 1222-1244. (14) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Elst, L. V.; Muller, R. N. Chem. Rev. 2008, 108 (6), 2064-2110.
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about them.15-18 Because of the fact that polarity is highly sensitive to the external changes, it can often disclose much important prospective information on the intrinsic reactions happen to the environment, which may be difficult to be revealed via monitoring the other parameters. Since polarity is so important with outstanding qualities that it has been a significant marker of environmental balance, it is of high effectiveness and great magnitude to monitor it.19-21 Among various means of sensing polarity, fluorescence is undoubtedly one of the most valuable tools,22,23 which has also been extensively used for monitoring ions,24 small molecules,25 and biological processes26 with high sensitivity. Thus, a polarity sensor based on fluorescence incorporating Fe3O4 NPs that we are dedicated to prepare is extraordinarily intriguing, since they can perform the sensing ability as well as all the distinguishable functions that the superparamagnetic Fe3O4 NPs have, which are generally defined as “dual-functional” NPs. In this paper, we designed three polarity-sensitive organic molecules with binding groups as shown in Scheme 1, which decorated the Fe3O4 NPs through surface modification to yield polaritysensitive NPs that were schematically illustrated in Scheme 2. The behaviors of the three different molecules during the surface modification were compared and analyzed. The prepared polarity-sensitive NPs display distinct colors with the inherent (15) Piao, H. L.; Machado, I. M. P.; Payne, G. S. Mol. Biol. Cell 2007, 18 (1), 57-65. (16) Brenman, J. E. Cell Cycle 2007, 6 (22), 2755-2759. (17) Graf Von Stosch, A.; Kinzel, V.; Pipkorn, R.; Reed, J. J. Mol. Biol. 1995, 250 (4), 507-513. (18) Samaj, J.; Baluska, F.; Hirt, H. J. Exp. Bot. 2004, 55 (395), 189-198. (19) Epand, R. M.; Kraayenhof, R. Chem. Phys. Lipids 1999, 101 (1), 57-64. (20) Wang, X. C.; Wang, S. J.; Ma, H. M. Analyst 2008, 133 (4), 478-484. (21) Wang, S. J.; Wang, X. C.; Shi, W.; Wang, K.; Ma, H. M. Biochim. Biophys. Acta 2008, 1784 (2), 415-422. (22) Tanaka, K.; Inafuku, K.; Chujo, Y. Bioorg. Med. Chem. 2008, 16 (23), 10029-10033. (23) Vazquez, M. E.; Blanco, J. B.; Imperiali, B. J. Am. Chem. Soc. 2005, 127 (4), 1300-1306. (24) Nagai, T.; Sawano, A.; Park, E. S.; Miyawaki, A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98 (6), 3197-3202. (25) Chan, P. H.; Liu, H. B.; Chen, Y. W.; Chan, K. C.; Tsang, C. W.; Leung, Y. C.; Wong, K. Y. J. Am. Chem. Soc. 2004, 126 (13), 4074-4075. (26) Lippincott-Schwartz, J.; Patterson, G. H. Science 2003, 300, 87-91.
Published on Web 02/09/2010
DOI: 10.1021/la9046512
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Zhang and Han Scheme 1. Synthetic Route to Functional Molecules DIAA, DIUA, and DISA
Scheme 2. Schematic Process for Preparing Polarity-Sensitive Magnetic NPs
superparamagnetic properties of Fe3O4 NPs intact in various solvents depending on the polarity of the local environment. We believe confirmedly that our research not only develops a kind of dual-functional Fe3O4 NPs with polarity sensitivity but also expands the chemistry of surface modification, showing a conceptual design for the preparation of polarity sensor. To our best knowledge, it is the first report to endow the superparamagnetic NPs with polarity-sensitive properties.
Experimental Section Materials. Iron(III) acetylacetonate (99.9%þ), 1,2-hexadecanediol (90%), oleylamine (70%), L-aspartic acid (99%þ), phenyl ether (99%), and palladium on activated charcoal (10% Pd basis) were purchased from Sigma-Aldrich Chemical Co. Oleic acid (90%), 4-nitrophthalic acid (97%), and glycine (99%) were purchased from Alfa Aesar Chemical Co. Hydrazine hydrate (80%) and 11-aminoundecanoic acid (99%) were purchased from 8894 DOI: 10.1021/la9046512
Acros Organics. The solvents used for the organic synthesis and spectral analysis were rigorously purified according to standard procedures.27 Pure water with a resistivity equal to 18 MΩ cm was obtained using a Millipore system. Characterization. Proton nuclear magnetic resonance (1H NMR) spectra were acquired with a Bruker DMX300 spectrometer. Ultraviolet-visible (UV-vis) absorption spectra of the samples were measured with a Perkin-Elmer Lambda 900 UV-vis spectrometer and quartz cuvettes with 1 cm path length. The fluorescence spectra were acquired on a Perkin-Elmer LS55 luminescence spectrometer. Hydrodynamic sizes of NPs were measured by a Malvern Zetasizer Nano ZS dynamic light scattering instrument. Infrared (IR) spectra of dried powder pressed into KBr pellets were obtained on a Perkin-Elmer Paragon 1000 Fourier transform infrared (FTIR) spectrometer. Thermogravimetric analysis (TGA) was performed for powder samples (27) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1986, and references therein.
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(∼10 mg) with a heating rate of 10 °C/min using a Pyris Diamond thermogravimetric/differential thermal analysis instrument (Thermo) in a nitrogen atmosphere from 25 to 800 °C. The shape and structure of the Fe3O4 NPs were characterized using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) at an accelerating voltage of 200 kV on a JEOL JEM2010 transmission electron microscope. Magnetic studies were carried out using a PPMS-9 magnetometer (Quantum Design) with a field up to 9 T at 10 and 300 K, respectively.
Synthesis of Surfactant-Coated Fe3O4 NPs by Solvothermal Decomposition. High-quality Fe3O4 NPs (5.4 ( 0.6 nm) were obtained by the method developed by Sun and co-workers with minor modification.28 A typical synthesis is as follows. Iron(III) acetylacetonate (2.0 mmol), 1,2-hexadecanediol (10.0 mmol), oleic acid (6.0 mmol), oleylamine (6.0 mmol), and phenyl ether (20 mL) were mixed and magnetically stirred under a flow of nitrogen. The mixture was heated to 200 °C for 30 min and then, under a blanket of nitrogen, heated to reflux (265 °C) for another 30 min. The black-brown mixture was cooled to room temperature by removing the heat source. Under ambient condition, ethanol (40 mL) was added to the mixture, and a black material was precipitated and separated via centrifugation (8000 rpm, 10 min). The obtained precipitate containing Fe3O4 NPs was repeatedly washed five times with ethanol and finally dried under a vacuum at 40 °C for 6 h. The resulting dry black powder was stored for future use.
Synthesis of 5-(Dimethylamino)isobenzofuran-1,3-dione (DBFD).29-31 4-Nitrophthalic acid (4.2 g, 20.0 mmol) was
dissolved in 100 mL of ethanol. After warming to 50 °C, 0.1 g of palladized charcoal catalyst was added and the stirrer was started. Then, about 4.0 mL of hydrazine hydrate was added dropwise for 30 min. At this point an additional 0.1 g of catalyst was added, and the mixture was heated until the alcohol refluxed gently. The reaction was stirred overnight. The catalyst was removed by filtration with gentle suction through a thin layer of Celite. The flask was rinsed with 30 mL of hot alcohol which was then used to wash the catalyst and Celite. The combined filtrate was evaporated under a reduced pressure to obtain a dry powder of 4-aminophthalic acid (2.4 g, 13.2 mmol). The slurry of 4-aminophthalic acid and finely crushed sodium borohydride (2.91 g, 77 mmol) in tetrahydrofuran (50 mL) was added dropwise to an efficiently stirred solution of 3.0 M sulfuric acid and 35% aqueous formaldehyde (4.0 mL, 54.0 mmol) in tetrahydrofuran below 30 °C. After the first half of the addition, the mixture was acidified with 3.0 M sulfuric acid (3.4 mL, 10.0 mmol), and the addition then continued. To the resultant mixture, water (50 mL) was added with stirring and was extracted with ethyl acetate (4 50 mL). The organic phase was combined. The solvent was removed by rotary evaporation to yield a dry powder of 4-(dimethylamino)phthalic acid which was subsequently sublimated in vacuum at 150 °C to give DBFD (1.1 g, yield 29%) as a light yellow powder. 1H NMR (300 MHz, CDCl3, TMS): δ 3.16 (s, 6H), 6.96 (dd, 1H, J1 = 2.4 Hz, J2 = 8.7 Hz), 7.09 (d, 1H, J = 2.4 Hz), 7.76 (d, 1H, J = 8.7 Hz).
Synthesis of 2-(5-(Dimethylamino)-1,3-dioxoisoindolin2-yl)succinic Acid (DISA).32 A solution of DBFD (191 mg, 1.0 mmol) and L-aspartic acid (400 mg, 3.0 mmol) in pyridine (50 mL) was heated at 90 °C for 2 days under nitrogen in the presence of a catalytic amount of 4-(dimethylamino)pyridine. Pyridine was removed under vacuo, and the residue was purified by column chromatography using silica gel (eluent: methanol: (28) Sun, S. H.; Zeng, H. J. Am. Chem. Soc. 2002, 124 (28), 8204-8205. (29) Giumanini, A. G.; Chiavari, G.; Musiani, M. M.; Rossi, P. Synthesis 1980 (9), 743-746. (30) Vazquez, M. E.; Rothman, D. M.; Imperiali, B. Org. Biomol. Chem. 2004, 2 (14), 1965-1966. (31) Kuhn, W. E. Org. Synth. 1933, 13, 74-76. (32) Das, S.; Incarvito, C. D.; Crabtree, R. H.; Brudvig, G. W. Science 2006, 312, 1941-1943.
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Figure 1. FTIR spectra of DISA (A), DISA-Fe3O4 NPs (B), and as-prepared Fe3O4 NPs (C). ethyl acetate = 1:1) to give 221 mg (72% yield) of DISA as an orange powder. 1H NMR (300 MHz, CDCl3, TMS): δ 2.78 (q, 1H, J1 = 16.2 Hz, J2 = 5.1 Hz), 2.97 (t, 1H, J = 7.5 Hz), 3.1 (s, 6H), 4.96 (t, 1H, J = 14.1 Hz), 6.96 (dd, 1H, J1 = 2.4 Hz, J2 = 8.7 Hz), 7.05 (d, 1H, J = 2.1 Hz), 7.63 (d, 1H, J = 8.7 Hz). The synthetic procedures for DIAA and DIUA molecules are similar to that of DISA. DIAA: 70% yield. 1H NMR (300 MHz, CDCl3, TMS): δ 3.07 (s, 6H), 3.8 (s, 2H), 6.91 (dd, 1H, J1 = 2.1 Hz, J2 = 8.4 Hz), 7 (d, 1H, J = 2.1 Hz), 7.57 (d, 1H, J = 8.7 Hz). DIUA: 68% yield. 1H NMR (300 MHz, CDCl3, TMS): δ 1.21-1.71 (m, 16 H), 2.32 (t, 2H, J = 15 Hz), 3.11 (s, 6H), 3.61 (t, 2H, J = 14.4 Hz), 6.78 (dd, 1H, J1 = 2.4 Hz, J2 = 8.4 Hz), 7.07 (d, 1H, J = 2.4 Hz), 7.63 (d, 1H, J = 8.4 Hz). Surface Functionalization of Fe3O4 NPs. As shown in Scheme 2, previously exsiccated bare Fe3O4 NPs (5.0 mg) were suspended in 5.0 mL of a solution of the required ligand (1%) in the appropriate solvent (hexane or ethanol) under a nitrogen atmosphere. The resulting unstable dispersion was sonicated for ∼10 h at 28 °C, until a homogeneous black suspension was obtained. The mixture was then centrifuged at 6000 rpm for 10 min and the solvent discarded. The collected precipitate was washed with dichloromethane and hexane (1/5, v/v) or ethanol three times, affording the modified NPs free of excess ligand, which were dried under vacuum to remove the solvents. The surface-modified Fe3O4 NPs were then dispersed in various solvents for further analysis.
Results and Discussion Preparation of Magnetic NPs. High-quality magnetic NPs were synthesized through the thermal decomposition of iron(III) acetylacetonate in a high-boiling point organic solvent (phenyl ether) containing stabilizing surfactants (oleic acid and oleylamine). It is well-known that both magnetite and maghemite phases can be obtained from the thermal decomposition.33 The good magnetite phase of the NPs can be confirmed by the IR spectra, as both magnetite and maghemite can be easily differentiated on symmetry grounds.34 The IR spectra of stoichiometric magnetite display one peak at around 550-700 cm-1, whereas the IR spectra of maghemite are more complicated due to the sensitivity of IR analyses to vacancies ordering. As shown in Figure 1, both of the IR spectra of the as-prepared Fe3O4 NPs and DISA-Fe3O4 NPs display an intense band at around 628 cm-1 with a shoulder up to 780 cm-1. Considering the reported IR spectroscopic studies34 and the microstructure of magnetite NPs (33) Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. J. Am. Chem. Soc. 2004, 126 (1), 273-279. (34) Daou, T. J.; Greneche, J. M.; Pourroy, G.; Buathong, S.; Derory, A.; Ulhaq-Bouillet, C.; Donnio, B.; Guillon, D.; Begin-Colin, S. Chem. Mater. 2008, 20 (18), 5869-5875.
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Zhang and Han Table 1. Absorption and Fluorescence Emission Peaks of DIAA, DIUA, and DISA in Solvents with Varying Polarity DIAA/nm λabs
λem
DIUA/nm λabs
λem
DISA/nm λabs
λem
THF 396 465 385 475 382 471 396 487 394 494 392 490 CHCl3 386 506 391 504 392 502 CH3CN n-butanol 390 523 394 533 390 521 393 540 397 548 393 536 CH3OH a a 416 591 414 545 415 580 H2O a The inclusion complex of DIUA with β-cyclodextrin in water was measured.
Figure 2. Absorption and fluorescence spectra of DIAA (A), DISA (B), and DIUA (C) in various solvents. The fluorescence of DIUA in water was measured for its inclusion complex with β-cyclodextrin. The intensities of fluorescence and absorption were normalized.
with a magnetite core and an oxidized layer, the intense band at 628 cm-1 is attributed to magnetite and the shoulder to the oxidized layer with a composition close to a disordered maghemite. By controlling the reaction condition, high-quality magnetitephased Fe3O4 NPs of 5.4 nm in diameter with a standard deviation of 0.6 nm were acquired. Typical TEM images of as-prepared Fe3O4 NPs deposited from their hexane dispersion and dried under ambient condition are shown in Figure S1 (Supporting Information). It can be seen in Figure S1 that the NPs based on which the polarity-sensitive NPs of high quality 8896 DOI: 10.1021/la9046512
were acquired possess a narrow size distribution and a uniform shape with good crystallinity. Design and Synthesis of Polarity-Sensitive Organic Molecules. Polarity-sensitive fluorophores are a special class of chromophores showing spectroscopic behavior that is dependent on the polarity of the local environment. 4-(Dimethylamino)phthalimide35 generally exhibits a low quantum yield in aqueous solution but becomes highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. 6-(N, N-Dimethylamino)-2,3-naphthalimide- and 4-(N,N-dimethylamino)phthalimide-based polarity-sensitive fluorophores have been incorporated into peptides that show great potential for biological applications in sensing protein-protein interaction.23,30 Herein we designed and synthesized a series of polarity-sensitive compounds (DIAA, DIUA, and DISA: Scheme 1) derived from DBFD, all of which have polarity-sensitive moiety of 4-(dimethylamino)phthalimide. They were subsequently modified onto the Fe3O4 NPs’ surface to develop dual-functionalized Fe3O4 NPs through carboxyl group that was well established as a good ligand for binding to the surface of Fe3O4 NPs.36 Optical Properties of DIAA, DIUA, and DISA. All of the three compounds display broad absorption and fluorescence spectra that are typically resulted from an intramolecular charge transfer transition (ICT).37 An ICT compound possesses electron-donating group(s) and electron-accepting group(s) that are connected directly or through π-conjugated linker. Solvatochromic effects and spectral changes under various physical and biological environments make the ICT compounds polaritysensitive.23,35,37,38 A few representative spectra are shown in Figure 2 for the three polarity-sensitive fluorescent molecules. The absorption and fluorescence spectral data of these fluorescent molecules in various solvents are summarized in Table 1. As shown in Table 1, with an increase in the polarity of the medium, both the absorption and the fluorescence maxima generally display a gradual red shift. The photophysical behavior of DIUA in homogeneous media and micellar environment has been studied by Samanta et al.,37 who concluded that DIUA is more sensitive than 11-(4-aminophthalimido)undecanoic acid to the solvent polarity since the fluorescence peak position shift (one of the parameters that determine the efficiency of a polarity probe) is more evident. Table 1 also shows that DIAA and DISA are more suitable to act as polarity probes, considering the fluorescence peak position shift in the solvents with different polarities. (35) Saroja, G.; Soujanya, T.; Ramachandram, B.; Samanta, A. J. Fluoresc. 1998, 8 (4), 405-410. (36) Li, Z.; Tan, B.; Allix, M.; Cooper, A. I.; Rosseinsky, M. J. Small 2008, 4 (2), 231-239. (37) Saroja, G.; Ramachandram, B.; Saha, S.; Samanta, A. J. Phys. Chem. B 1999, 103 (15), 2906-2911. (38) Shin, D. M. Liq. Cryst. 2008, 491, 32-39.
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Confirmation of Ligand Exchange. The FTIR spectra of functional molecules, as-prepared Fe3O4 NPs, and surfacemodified Fe3O4 NPs are shown in Figure 1 and Figures S2 and S3 (Supporting Information). As shown in Figure 1, the approximate disappearance of the CdO stretching band of the carboxyl group, which was present at 1705 cm-1, seen for the free DISA indicates that the carboxylic acid on the DISA forms a direct bond with the surface of Fe3O4 NPs. The bond between the carboxylic acid group and the surface of iron oxide NPs has been found to be an organometallic covalent bond by Turro and co-workers.39 In Figure 1 (traces A and B), two characteristic stretching were observed in IR spectrum at 1766 cm-1 (1616, 1529, and 1386 cm-1), corresponding to the CdO stretching of amide and CdC stretching of phenyl40 of DISA molecules. In contrast, the asymmetric and symmetric stretching bands of carboxylate (-COO-) are shifted from 1563 and 1439 cm-1 to 1599 and 1452 cm-1, respectively. Combined with previous studies, the interaction between the carboxylate head and the metal atom was categorized as four types: unidentate, bridging (bidentate), chelating (bidentate), and ionic interaction.41,42 The wavenumber separation, Δν, between the asymmetric νas(-COO-) and symmetric νs(-COO-) IR bands can be used to distinguish the type of the interaction between the carboxylate head and the metal atom. The largest Δν (200-320 cm-1) was corresponding to the unidentate interaction, and the smallest Δν (
