J. Phys. Chem. C 2008, 112, 3267-3271
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pH-Dependent Aggregation of Histidine-Functionalized Au Nanoparticles Induced by Fe3+ Ions Jian Guan, Lin Jiang, Jun Li, and Wensheng Yang* Key Laboratory of Surface and Interface Chemistry of Jilin ProVince, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: October 5, 2007; In Final Form: December 1, 2007
The aggregation of histidine-functionalized Au nanoparticles induced by Fe3+ was identified to show pHdependent character. At low pH value, the Fe3+ ion was effective to induce aggregation of the Au nanoparticles via its coordination with imidazole group of the histidine ligand. At pH value higher than 9, the histidine ligand provided an additional coordination site for the Fe3+ ion due to deprotonization of its R-NH2 group. Therefore, the Fe3+ ion-induced aggregation of the Au nanoparticles was suppressed as a result of the transformation of the histidine from monodentate to bidentate ligand upon increased pH value.
Introduction Self-assembly is generally regarded as the most promising means for designing and controlling the bottom-up assembly of molecules and nanoparticles.1-4 The weak interactions are known to be responsible for ordering of molecules or nanoparticles in self-assembly systems,5,6 including hydrogen bond,7,8 aromatic stacking,9 electrostatic interaction,10 van der Waals force,11 hydrophobic interaction,12 and so forth. Coordination offers simplicity, stable bonding, and metal-ligand specificity, allowing the ligand-bearing molecular components to be assembled into supramolecular structures connected by metal ions.13 It is inferred that the aggregation of nanoparticles can also be efficiently assessed after the functionalization of the particles by ligand that can coordinate with the metal ions specifically. Such special responses of ligand-functionalized nanoparticles to certain metal ions give rise to much research on the application of detection of metal ions through coordination. For example, Mirkin and co-workers showed that Au nanoparticles functionalized by oligo-DNA are sensitive to Hg2+ ion in aqueous media.14 Obare and co-workers demonstrated that 1,10-phenanthroline derivative-functionalized Au nanoparticle bonds selectively to Li+ ion.15 Lu and co-workers reported that the DNA-zyme directed aggregation of Au nanoparticles is initiated selectively by Pb2+ ion,16-20 and more recently Thomas and co-workers used gallic acid-functionalized Au and Ag nanoparticles for selective detection of Pb2+ ion.21 In addition, such a strategy is also proven to be powerful to assemble nanoparticles into monolayer and multilayer structures.22-26 Amino acids are the building blocks of protein, and some of them play important roles in holding metal ions in the proper position of protein. Among the 20 natural amino acids, histidine, the sole amino acid with pKa near neutrality,27 usually interacts with metal ions via its imidazole side chain. In this study, we show that the histidine-functionalized Au nanoparticles experience pH-dependent aggregation in the presence of the Fe3+ ion. It was identified that the transformation of the histidine * To whom correspondence should be addressed. Telephone: +86-431-85168185. Fax: +86-431-85168086. E-mail:
[email protected].
Figure 1. TOF-MS spectra of (A) citrate (Cit) and citrate-functionalized Au nanoparticles (Au-Cit), and (B) histidine (His) and histidinefunctionalized Au nanoparticles (Au-His). After the ligand exchange, a peak of histidine at 156 was observed, whereas that of citrate at 63 almost disappeared.
from monodentate to bidentate ligand for the Fe3+ ion is responsible for the pH-dependent behavior of the Au nanoparticles.
10.1021/jp7097763 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/09/2008
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Figure 2. UV-visible spectra of the dispersions of the histidinefunctionalized Au nanoparticles at neutral (pH ) 7.74) and basic (pH ) 9.47) pH values with and without the addition of 5 µM Fe3+. (Insets) Photographs of the Au colloids to illustrate the color change at the different pH values.
Experimental Section Materials. All reagents were of analytical grade and prepared using high pure water with a resistivity of 18 MΩ‚cm. L-Histidine (99.5%, C6H9N3O2, FW 155.2), hydrogen tetrachloroaurate (III) trihydrate (99.9+%, HAuCl4‚3H2O, FW 393.8), sodium citrate tribasic dihydrate (ACS reagent, g99.0%, Na3C6H5O7‚2H2O, FW 294.1), FeCl3‚6H2O (FW 270.3), and other metal ion containing reagents were purchased from the Sigma-Aldrich and were used as received. All solutions of metal ion containing reagent were prepared fresh before the experiments. Reagent-grade NaOH and HCl were used to adjust the pH value. Preparation of Histidine-Functionalized Au Nanoparticle. Au colloids were prepared by sodium citrate reduction of chloroauric acid solution according to the modified Frens method.28 Typically, an aqueous solution of HAuCl4‚3H2O (0.37 mM) in 100 mL of water was brought to reflux under vigorous stirring in a round-bottom flask. A trisodium citrate (1.9 mM) solution was added, resulting in change in solution color from pale yellow to deep red. After the color change, the solution was heated for an additional 15 min. The suspension was filtered using a 0.22-µm filter. After being isolated by centrifugation
Guan et al. (8000 rpm for 30 min), the diameter of the resulting particle was determined to be 18 nm as indicated by transmission electron microscopy (TEM) observations. After being redispersed in water, the original pH value of the Au nanoparticles sol was 6.5. To prepare histidine-functionalized Au nanoparticles, 100 µM histidine was added into the as-prepared colloid whose pH value was previously adjusted to about 9.6 using 0.1 M NaOH. After addition of histidine and aging the Au colloid for 1 h, the mixture was washed twice with pure water to remove any uncoordinated histidine by centrifugation. The concentration of the histidine-functionalized Au nanoparticles was 0.25 mM. The final pH value of the nanoparticle colloid was about 7.7 and adjusted to a different pH value (6.02-10.7) with 0.1 M NaOH and 0.1 M HCl before the addition of 5 µM metal ions ([Mx+]/[Au] ) 1:50). Characterization. The MALDI-TOF MS analyses were performed on Kompact Maldi (Kratos Analytical Shimadzu Group Company) equipped with a nitrogen laser (λ ) 337 nm). One microliter portion of the final solution was deposited onto the stainless steel sample slide and allowed to dry at room temperature. Analyses were carried out in the linear mode at a mass range of 50-400 m/z. The Raman spectra were performed with a microscopic confocal Raman spectrometer (Renishaw, RM 2000) operated with a He-Ne laser (632.8 nm, 4.7 mW). Absorption spectra were collected with a Varian Cary-100 scan UV-vis spectrophotometer. Dynamic light scattering (DLS) measurements were performed using a particle size analyzer (BI-90Plus, Brookhaven Instruments) with a scattering angle of 90°. TEM images were observed using a Hitachi H-8100 IV electron microscope by using a carbon-coated copper grid as substrates. The photographs of the samples were caught by a Konica Minolta DiMAGE Z2 digital camera. All the experiments were carried out at room temperature (25 ( 2 °C). Results and Discussion MS spectra show that the citrate-capped Au nanoparticles present an m/z peak of citrate at 63. After ligand exchange, the peak of citrate at 63 almost disappears and a peak corresponding to histidine at 156 is observed (Figure 1). These results mean that the citrate molecules on the particle surface are replaced almost completely by histidine once after the ligand exchange. The histidine-functionalized Au nanoparticles show a characteristic surface plasmon (SPR) band at around 520 nm, which experiences little change upon pH change from neutral to basic. At neutral pH value (7.74), intensity of the plasmon band
Figure 3. TEM images of the histidine-functionalized Au nanoparticles in the presence of 5 µM Fe3+ observed under (A) neutral (pH ) 7.74) and (B) basic (pH ) 9.47) pH values.
Histidine-Functionalized Au Nanoparticles
Figure 4. (A) UV-visible spectra of the dispersions of the histidinefunctionalized Au nanoparticles with different pH values (a-j, pH 7.74-10.70) in the presence of 5 µM Fe3+. Dotted line is the UVvisible spectrum of the Au nanoparticles in the absence of Fe3+ at pH ) 7.74, which experiences almost no change with increased pH value. (B) Ratio of E620/E520 of the histidine-functionalized Au nanoparticle in the presence of 5 µM Fe3+ versus pH value.
decreases significantly and a new band appears at longer wavelength after the addition of 5 µM Fe3+, suggesting the formation of anisotropic aggregates of the particles.29 However, the Au nanoparticles show no color change in the presence of 5 µM Fe3+ at high pH (pH ) 9.47), as seen in Figure 2. TEM observations show that, at the neutral pH value, the addition of Fe3+ induces the formation of fractal-like aggregates of the Au nanoparticles (Figure 3A). While at high pH value, the Au nanoparticles are nearly dispersed (Figure 3B), meaning that
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3269 the Fe3+-induced aggregation of the histidine-functionalized Au nanoparticles is suppressed effectively at the high pH value. Such a conclusion can also be supported by the DLS measurements (Figure S1 in Supporting Information). Figure 4A shows the absorption spectra of the dispersions of the histidine-functionalized Au nanoparticles recorded under different pH values after the addition of 5 µM Fe3+ ion. With increased pH value, the peak at longer wavelength experiences blue-shift and its intensity decreases gradually. When the pH value is higher than 9, the two peaks combined into one, which is almost identical to the histidine-functionalized Au nanoparticles in the absence of the Fe3+ ion. We used the ratio of extinction at 620 and 520 nm (E620/E520), which represent aggregated and free Au nanoparticles, respectively, to evaluate the degree of the aggregation.18,19 It is seen that the ratio decreases upon increased pH value and the curve shows a transition point near pH ) 9 (Figure 4B), which is close to the pKa value of R-NH2 (9.1) of histidine.30 Thus, it is supposed that the pH-dependent aggregation of the Au nanoparticles induced by Fe3+ is correlative to deprotonization of the R-NH2 group of histidine. Raman spectra were employed to study the effect of pH on the Fe3+ aggregation of the Au nanoparticles. In Raman spectra, the C4dC5 stretching vibration of the imidazole ring can be used as a marker to determine the coordinating state of N1and N3- of the imidazole.31 After the imidazole was loaded onto the Au nanoparticle surface, observation of the sharp band at 1572 cm-1 indicates that the N1- group of the imidazole is involved in coordination of the histidine with the Au nanoparticle surface.31 The appearance of the ν(COO-) vibration mode at 1394 cm-1 indicates that the carboxyl group of histidine also binds to the Au surface (Figure 5A).32,33 These results suggest that, after the ligand exchange, the Au nanoparticles are coated and stabilized by the histidine ligands with their carboxyl and N1- of imidazole binding to their surface. This conclusion is consistent with the previous reports.32,34 At neutral pH value, the histidine is a monodentate ligand since the R-NH2 group is protonized under such low pH values and N3- group of imidazole is expected to be the sole possible coordination site for the Fe3+ ion. As seen in Figure 5A, in addition to 1572 cm-1, a shoulder peak at around 1597 cm-1 is identified in the Raman spectrum after the addition of the Fe3+ ion, which is characteristic of the imidazole N3- of histidine coordinated with metal ion.31 At this time, it is reasonable for the Fe3+ ion to be effective to induce the aggregation of the Au nanoparticles since it can coordinate with two or more monodentate histidine ligands from different Au nanoparticles. At high pH value (>9.1), both the N3- and the deprotonized R-NH2 group of histidine may act as coordination sites of the Fe3+ ion. Figure 5B shows the Raman spectra of the histidine-function-
SCHEME 1: Schematic Representation of the Coordination of Fe3+ Ion with the Histidine Ligand on the Au Nanoparticle Surface at Low (pH ) 7.74) and High (pH ) 9.47) pH Values
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Figure 5. Raman spectra of the histidine-functionalized Au nanoparticle colloid in the absence (Au-His) and presence (Au-His-Fe) of Fe3+ under (A) neutral (pH ) 7.74) and (B) basic (pH ) 9.47) pH values. (C and D) Enlarged spectra and fitting results (dotted lines) of corresponding marker region for histidine-functionalized Au nanoparticles with and without addition of Fe3+ at pH value of 9.47 in (B), respectively.
Figure 6. Ratios of E620/E520 of the (A) histidine- and (B) citrate-functionalized Au nanoparticles in the presence of different metal ions (Ag+, Ba2+, Cd2+, Cr3+, Cu2+, Fe2+, Ni2+, Pb2+, Sn4+). The concentration of each metal ion is 5 µM.
alized Au nanoparticles taken at pH of 9.47. A weak shoulder at 1601 cm-1 attributed to the bending vibration of the R-NH2 comes out, indicating the presence of deprotonized R-NH2 groups under this high pH value.31 After the addition of Fe3+, this peak becomes stronger and experiences a blue-shift to 1626 cm-1, indicating the coordination of Fe3+ with the R-NH2 group.34,35 The coordination of N3- with Fe3+ can also be identified as indicated by the C4dC5 stretching vibration of the imidazole ring that appeared around 1607 cm-1.31 On the basis of the above results, the coordination of Fe3+ with histidine
ligand on the particle surface under different pH value is proposed in Scheme 1. At neutral pH value (pH ) 7.74), histidine acts as a monodentate ligand. The addition of the Fe3+ ion is effective to induce the aggregation of the gold nanoparticles since an Fe3+ ion may act as bridge between two particles via its coordinate with two or more ligands from different particle surfaces. With increased pH value (>9.1), histidine becomes a bidentate ligand after deprotonization of the R-NH2 group. The aggregation of the gold nanoparticles is suppressed effectively, indicating that, instead of the interparticle coordina-
Histidine-Functionalized Au Nanoparticles tion (Fe3+ with two or more N3- sites from different particle surfaces), the intraparticle fashion (Fe3+ with N3- and R-NH2 groups from the same ligand) becomes dominant at high pH value. Control experiments with other metal ions, such as Ag+, Ba2+, Cd2+, Cr3+, Cu2+, Fe2+, Ni2+, Pb2+, Sn4+, were also carried out under the same experimental conditions. The histidinefunctionalized Au nanoparticles show pretty good selectivity to the Fe3+ ion. The ratio of E620/E520 in the presence of the Fe3+ ion is as high as 1.2, while those of other metal ions are lower than 0.2 at the neutral pH value (Figure 6A). We also used the citrate-capped Au nanoparticles for control under the same experimental conditions. It is identified that the citratecapped Au nanoparticles are insensitive to all the metal ions with a concentration of 5 µM (Figure 6B). All the results of the control experiments further support the conclusion that the pH-dependent aggregation of the histidine-functionalized Au nanoparticle is induced by the coordination of the histidine ligand with the Fe3+ ion. Conclusions In this work, we report the pH-dependent aggregation of the histidine-functionalized Au nanoparticles induced by Fe3+. Histidine is anchored to the Au nanoparticle surface via its carboxyl group and N1- of imidazole. At low pH value, histidine acts as a monodentate ligand, and Fe3+ is expected to coordinate with N3- sites of histidine ligands from neighboring particle surfaces, which induces aggregation of Au nanoparticles. At high pH value, histidine is transformed into a bidentate ligand after the deprotonization of its R-NH2, and thus the aggregation process is suppressed greatly. The pH-dependent aggregation of histidine-functionalized Au nanoparticles is identified to show pretty good selectivity to the Fe3+ ion. Acknowledgment. This work was supported by the National Nature Science Foundation of China, the Key Project, and the Program for NCET in University of Chinese Ministry of Education. Supporting Information Available: Histograms presenting size distribution of the Au nanoparticles in the presence of Fe3+ under different pH values. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Kamien, R. D. Science 2003, 299, 1671.
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3271 (3) Zhang, Z.; Horsch, M. A.; Lamm, M. H.; Glotzer, S. C. Nano Lett. 2003, 3, 1341. (4) Jiang, L.; Zhang, H. X.; Zhuang, J. Q.; Yang, W. S.; Li, T. J.; Sun, C. C. AdV. Mater. 2005, 17, 2066. (5) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; Wiley-VCH: Weinheim, Germany, 1995. (6) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692. (7) Lehn, J.-M.; Mascal, M.; Decian, A.; Fischer, J. Chem. Commun. 1990, 479. (8) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849. (9) Jin, J.; Iyoda, T.; Cao, C. S.; Song, Y. L.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew .Chem., Int. Ed. 2001, 40, 2135. (10) Galow, T. H.; Boal, A. K.; Rotello, V. M. AdV. Mater. 2000, 12, 576. (11) Bruch, L. W. Phys. ReV. B 2005, 72, 033410. (12) Southall, N. T.; Dill, K. A.; Haymet, A. D. J. Phys. Chem. B 2002, 106, 521. (13) McCleverty, J. A.; Meyer, T. J. ComprehensiVe Coordination Chemistry II: From Biology to Nanotechnology; Elsevier: Amsterdam, 2004. (14) Lee, J.-S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093. (15) Obare, S. O.; Hollowell, R. E.; Murphy, C. J. Langmuir 2002, 18, 10407. (16) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2003, 125, 6642. (17) Liu, J.; Lu, Y. Anal. Chem. 2004, 76, 1627. (18) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2004, 126, 12298. (19) Liu, J.; Lu, Y. J. Am. Chem. Soc. 2000, 122, 10466. (20) Swearingen, C. B.; Wernette, D. P.; Cropek, D. M.; Lu, Y.; Sweelder, J. V.; Bohn, P. W. Anal. Chem. 2005, 77, 442. (21) Yoosaf, K.; Ipe, B. I.; Suresh, C. H.; Thomas, K. G. J. Phys. Chem. C 2007, 111, 12839. (22) Yuan, M. J.; Li, Y. L.; Li, J. B.; Li, C. H.; Liu, X. F.; Lv, J.; Xu, J. L.; Liu, H. B; Wang, S.; Zhu, D. B. Org. Lett. 2007, 9, 2313. (23) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682. (24) Zamborini, F. P.; Smart, L. E.; Leopold, M. C.; Murray, R. W. Anal. Chim. Acta 2003, 496, 3. (25) Chen, S. W.; Pei, R. J.; Zhao, T. F.; Dyer, D. J. J. Phys. Chem. B 2002, 106, 1903. (26) Wanunu, M.; Popovitz-Biro, R.; Cohen, H.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2005, 127, 9207. (27) Stryer, L. Biochemistry, 4th ed.; W. H. Freeman: New York, 1995. (28) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. J. Am. Chem. Soc. 2007, 129, 13939. (29) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. J. Phys. Chem. B 2004, 108, 4046. (30) Richard, J. S.; Martin, R. B. Chem. ReV. 1974, 74, 471. (31) Miura, T.; Satoh, T.; Hori-i, A.; Takeuchi, H. J. Raman Spectrosc. 1998, 29, 41. (32) Martusevicˇius, S.; Niaura, G.; Talaikyte´, Z.; Razumas, V. Vib. Spectrosc. 1996, 10, 271. (33) Hashimoto, S.; Ono, K.; Takeuchi, H. J. Raman Spectrosc. 1998, 29, 969. (34) So´va´go´, I.; ?sz, O K. Dalton Trans. 2006, 3841. (35) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460.
