pubs.acs.org/Langmuir © 2009 American Chemical Society
Controllable Aggregation and Reversible pH Sensitivity of AuNPs Regulated by Carboxymethyl Cellulose )
Junjun Tan,†,‡ Ruigang Liu,*,† Wen Wang,†,‡ Wenyong Liu,†,‡ Ye Tian,†,‡ Min Wu,*,§ and Yong Huang*,†,§, †
)
State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, ‡Graduate University, Chinese Academy of Science, Beijing 100039, China, §National Engineering Research Center of Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Science, Guangzhou 510650, China Received July 15, 2009. Revised Manuscript Received August 12, 2009
A pH-sensitive gold nanoparticle-cysteamine/carboxymethyl cellulose (Au-CA/CMC) dispersion system was prepared by a simple approach. Gold nanoparticles (AuNPs) were first synthesized by directly reducing chloroauric acid (HAuCl4) with sodium carboxymethyl cellulose (CMC). Then the AuNPs were decorated by an electrostatic compound of cysteamine hydrochloride (CA) and sodium carboxymethyl cellulose (CMC) through ligand exchange to get the assembly of Au-CA/CMC. The Au-CA/CMC dispersion system exhibits strongly reversible pH-responsive behavior with the aggregation of AuNPs caused by the combined action of the chain conformation change of CMC and electrostatic interactions between CA and CMC at different pH values. Finally, the reversible aggregation mechanism of AuNPs in the Au-CA/CMC dispersion system has been investigated by transmission electron microscopy (TEM) and ultraviolet-visible spectroscopy (UV-vis spectroscopy). This study provides a new method to fabricate a stimuliresponsive system free from complicated organic synthesis without using a toxic reducing agent.
Introduction Gold nanoparticles (AuNPs) have attracted increasing interest in recent years because of their unusual properties and potential applications in optoelectronics,1,2 sensors,3-6 catalysis,7-9 biotechnology,10 and medicine.11,12 Modification of AuNPs with smart polymers such as pH-sensitive13-15 or thermosensitive15,16 polymers can tailor the dispersity of AuNPs with pH- or temperature-responsive properties. As a result, the modified AuNPs show interesting reversible changes in surface plasmon spectroscopy, which may provide a new avenue for nanosensors. These AuNP intelligent polymer-dispersed systems are smart nanoma*To whom correspondence should be addressed. Tel: þ8610-82618573. Fax: þ8610-62554670. E-mail:
[email protected] (R.L.);
[email protected] (Y.H.). (1) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 13, 1501. (2) Chen, S.; Yang, Y. J. Am. Chem. Soc. 2002, 124, 5280. (3) Meriaudeau, F.; Downey, T. R.; Passian, A.; Wig, A.; Ferrell, T. L. Appl. Opt. 1998, 37, 8030. (4) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (5) Tanabe, T.; Touma, K.; Hamasaki, K.; Ueno, A. Anal. Chem. 2001, 73, 1877. (6) Raj, C. R.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2003, 543, 127. (7) Maye, M. M.; Lou, Y.; Zhong, C. J. Langmuir 2000, 16, 7520. (8) Jaramillo, T. F.; Baeck, S. H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (9) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; BerenguerMurcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Nature 2008, 454, 981. (10) Xu, C.; Xie, J.; Ho, D.; Wang, C.; Kohler, N.; Walsh, E. G.; Morgan, J. R.; Chin, Y. E.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 173. (11) Lai, M. K.; Chang, C. Y.; Lien, Y. W.; Tsiang, R. C. C. J. Controlled Release 2006, 111, 352. (12) Chompoosor, A.; Han, G.; Rotello, V. M. Bioconjugate Chem. 2008, 19, 1342. (13) Li, D.; He, Q.; Cui, Y.; Li, J. Chem. Mater. 2007, 19, 412. (14) Li, D.; He, Q.; Yang, Y.; Mohwald, H.; Li, J. Macromolecules 2008, 41, 7254. (15) Nuopponen, M.; Tenhu, H. Langmuir 2007, 23, 5352. (16) Zhu, M. Q.; Wang, L. Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656.
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terials that have promising applications in the fields of biosensors and biotechnology.17 To prepare responsive AuNPs, thiolated terminal groups are generally needed to anchor the responsive polymer chains to the surface of AuNPs by using a complicated synthesis route. Moreover, residue toxic reducing agents such as sodium borohydride (NaBH4) and hydroxyl amine hydrochloride that were used to prepare AuNPs need to be removed for further application of the responsive AuNPs. Sodium carboxymethyl cellulose (CMC) (Scheme 1), a nontoxic and biodegradable cellulose derivative,18 has been used widely in biotechnology,19,20 medicine,21,22 hydrogels,23-26 and other materials.27-29 Recently, CMC has been used as a good stabilizer in the synthesis of metal nanoparticles in an aqueous medium.30-32 However, reducing agents such as NaBH4 were still used in these studies. More recently, cellulose was used as the reducing regent in Au(III)-Au(0) reduction and also as a (17) Biju, V.; Itoh, T.; Anas, A.; Sujith, A.; Ishikawa, M. Anal. Bioanal. Chem. 2008, 391, 2469. (18) vanGinkel, C. G.; Gayton, S. Environ. Toxicol. Chem. 1996, 15, 270. (19) Lali, A.; Balan, S.; John, R.; D’Souza, F. Bioseparation 1998, 7, 195. (20) Chen, H. Q.; Fan, M. W. J. Bioact. Compat. Polym. 2007, 22, 475. (21) Rokhade, A. P.; Agnihotri, S. A.; Patil, S. A.; Mallikarjuna, N. N.; Kulkarni, P. V.; Aminabhavi, T. M. Carbohydr. Polym. 2006, 65, 243. (22) Bajpai, A.; Mishra, A. J. Mater. Sci.: Mater. Med. 2008, 19, 2121. (23) Liu, P. F.; Zhai, M. L.; Li, J. Q.; Peng, J.; Wu, J. L. Radiat. Phys. Chem. 2002, 63, 525. (24) Feng, X. H.; Pelton, R. Macromolecules 2007, 40, 1624. (25) Mitsumata, T.; Suemitsu, Y.; Fujii, K.; Fujii, T.; Taniguchi, T.; Koyama, K. Polymer 2003, 44, 7103. (26) Chen, H. Q.; Fan, M. W. J. Bioact. Compat. Polym. 2008, 23, 38. (27) El-Saied, H.; Basta, A. H.; Hanna, A. A.; El-Sayed, A. M. Polym. Plast. Technol. Eng. 1999, 38, 1095. (28) Bourlinos, A. B.; Petridis, D. Chem. Commun. 2002, 2788. (29) Nadagouda, M. N.; Varma, R. S. Biomacromolecules 2007, 8, 2762. (30) Si, S.; Kotal, A.; Mandal, T. K.; Giri, S.; Nakamura, H.; Kohara, T. Chem. Mater. 2004, 16, 3489. (31) Liu, J.; Sutton, J.; Roberts, C. B. J. Phys. Chem. C 2007, 111, 11566. (32) Liu, J. C.; He, F.; Durham, E.; Zhao, D.; Roberts, C. B. Langmuir 2008, 24, 328.
Published on Web 09/01/2009
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Scheme 1. Structural Formula of Carboxymethyl Cellulose Sodium Salt
moderately efficient template for the formation of various micrometer-sized gold particles.33 Meanwhile, CMC was also reported to be used as the reducing agent for the preparation of metal nanocomposites under microwave irradiation.29 These published results suggested that abundant hydroxyl groups on CMC chains can be used as the reducing agent in preparing metal nanoparticles.29,33 The more attractive aspect is that CMC with a high degree of substitution (DS) of carboxymethyl groups, (e.g., DS = 1.2) is an interesting pH-sensitive polyelectrolyte similar to poly(acrylic acid). CMC chains can collapse and aggregate to form micropheres (Supporting Information Figure S1) at low pH in aqueous solution. If the features of CMC as both the reducing agent and the pH-responsive polyelectrolyte for the decoration of nanoparticles can be combined, this may provide a facile point of entry for the production of responsive nanoparticles without using any toxic reducing agent, such as sodium borohydride (NaBH4) or hydroxylamine hydrochloride, or capping/surfactant agent. Moreover, the use of benign biodegradable polymer CMC for the decoration of nanoparticles could find various technological and medicinal applications. In this work, we present a simple approach to preparing pHresponsive AuNPs by using CMC as the reducing agent and the stabilizer in an aqueous medium, in which the tedious organic synthesis procedure was avoided without using a toxic reducing agent. Herein, a gold nanoparticle dispersion in a CMC aqueous medium (Au/CMC) was first prepared by a one-pot, one-step method by reducing HAuCl4 with CMC. AuNPs-cystamine/ carboxymethyl cellulose (Au-CA/CMC) was then assembled by adding cysteamine hydrochloride to a Au/CMC dispersion through ligand exchange. The reversible aggregation of AuNPs was achieved by carboxymethyl cellulose at different pH values in the Au-CA/CMC system.
Experimental Section Materials. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4 3 4H2O) (Shenyang Jinke Reagent Plant, China), sodium carboxymethylcellulose (CMC, DS = 1.2, Mw = 2.5 105 g/mol) (Acros Organics), and cysteamine hydrochloride (CA) (Alfa Aesar) were used as received. Preparation of CMC-Stabilized Gold Nanoparticles (Au/CMC). Typically, 600 μL of a 0.05 M chlorauric acid (HAuCl4) aqueous solution was added to 200 mL of a 0.2 wt % CMC aqueous solution in a reaction vessel, followed by constant stirring. The mixture was kept at 110 °C for 12 h to obtain colloidal AuNPs stabilized by CMC (Au/CMC). The obtained colloidal solution was used for characterization without further treatment. Fabrication of the Au-CA/CMC Assembly. A 5 mg/mL cysteamine hydrochloride (CA) aqueous solution was added to the above-obtained Au/CMC solutions. The final concentration of CA in the mixture was kept at about 8.6 10-3 mg/mL. The mixure solution was further stirred for 2 days at room temperature to obtain the dispersion of the Au-CA/CMC assembly. (33) Li, Z. G.; Friedrich, A.; Taubert, A. J. Mater. Chem. 2008, 18, 1008.
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Figure 1. HRTEM of AuNPs dispersed in CMC solution (Au/CMC).
Characterization. The pH value of the dispersion, which was adjusted by adding hydrochloric acid or sodium hydroxide solution, was measured by a Shanghai Leici digital pHS-2F acidic meter. The UV-vis spectroscopy measurements were performed on a Shimadzu UV-1601PC spectrophotometer. The morphology of AuNPs and their hybrids was observed on a Hitachi H-800 transmission electron microscope (TEM) operated at 100 keV and a JEOL FS-2200 high-resolution transmission electron microscope (HRTEM) operated at 200 keV. The samples for TEM and HRTEM observation were prepared by dropping the colloidal solution onto the carbon-coated copper grid and drying in air.
Results and Discussion Preparation of CMC-Stabilized Gold Nanoparticles (Au/ CMC) and Further Modification by CA. When CMC reacted with HAuCl4 at 110 °C for 12 h, the nearly colorless original solution turns pink and finally bright ruby red. The phenomena are typical during the preparation of AuNPs and the complete reaction of HAuCl4 (Supporting Information Figures S2 and S3). Figure 1 shows HRTEM images of the obtained AuNPs dispersed in CMC solution. The results show that dispersed spherical AuNPs with a fairly uniform diameter of around 20 nm were obtained. The boundary between the neighboring AuNPs is clear, which indicates that there are no aggregates in the samples. The AuNPs are stable for several months in the aqueous reaction system with 0.2 wt % CMC. This phenomenon demonstrates that CMC served as both a reducing agent for gold ions and a stabilizer for AuNPs. The rich carboxylic ions (-COO-) on CMC chains have strong interactions with metal nanoparticals, which made CMC a good stabilizer.31,32 Moreover, the hydroxyl groups on CMC can be used as the reducing agent for the synthesis of AuNPs,29 which makes CMC one of the candidates for the onepot, one-step method of preparation of AuNPs. One may question how the aldehyde groups of the cellulosic chain ends can act as the reducing agent. The Mw of CMC is 2.5 105 g/mol, and the concentration of CMC in our work is 0.2 wt %. Therefore, the concentration of chain ends is quite low and the reducing properties of the end aldehyde groups can be omitted, and the hydroxyl groups on the CMC chain act as the main reducing agents for the formation of gold nanoparticles in this work. These results Langmuir 2010, 26(3), 2093–2098
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Figure 3. Au-CA/CMC dispersion solution at different pH values.
Figure 2. UV-vis spectra of Au/CMC and Au-CA/CMC dispersion solutions.
suggest that CMC can act as the stabilizer as well as the reducing agent to avoid the procedure of removing an additional reducing agent such as sodium borohydride (NaBH4). The wavelength of surface plasmon resonance (SPR) absorbance appears at 519 nm in the UV-vis spectrum (Figure 2, solid line). When a suitable amount of cysteamine hydrochloride (CA) (e.g., 8.6 10-3 mg/mL) was added to the Au/CMC solution, the mercapto group (-SH) would anchor on the surfaces of AuNPs instead of carboxylic groups via ligand exchange.31 Meanwhile, the ammonium ions (-NH3þ) on CA molecules have electrostatic interactions with the carboxylic ions (-COO-) of CMC. The wavelength of the surface plasmon resonance (SPR) absorbance of the AuNPs shifts slightly from 519 to 520 nm (Figure 2). The red shift may be attributed to the changes in the AuNPs surface surrounding by the bounding of the mercapto group, which could affect the plasmon band because of charge screening effects and a change in the dielectric constant of the medium.34 pH-Induced Reversible Aggregation of the AuNPs in CA/CMC Solution. The initial pH value of Au-CA/CMC solution is 6.4, and the solution is a bright ruby red color. The pH value of the Au-CA/CMC solution was adjusted by adding hydrochloric acid, and the color of the dispersion solution changes accordingly (Figure 3). The original ruby red color turns into violet in a narrow pH range of 2.6-2.1 and blue at pH < 2.1. It should be noted that the color change of the AuNPs dispersion is reversible. When the pH value of the AuNPs dispersion was adjusted from 1.5 back to 6.4, the color reverted to red. This phenomenon is due to the fact that the dissociation of CMC in aqueous solution is pH-dependent. Generally, the apparent dissociation constant of polyelectrolytes can be defined as35-37
R pKa ¼ pH -n log 1 -R
ð1Þ
where pKa is dissociation constant, R is the degree of dissociation, and n is an empirical index of the intramolecular electrostatic interactions. The values of pKa and n have been found to depend on the nature of the polymer, the concentration of neutral salts in the system, the concentration of the polyion itself, and the (34) (35) (36) (37)
Mulvaney, P. Langmuir 1996, 12, 788. Katchalsky, A.; Spitnik, P. J. Polym. Sci. 1947, 2, 432. Katchalsky, A.; Shavit, N.; Eisenberg, H. J. Polym. Sci. 1954, 8, 69. Trivedi, H. C.; Patel, C. K.; Patel, R. D. Makromol. Chem. 1981, 182, 3561.
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distance between the carboxyl groups along the polymer backbone.35,37 For CMC, pKa and n are in the ranges of 4.85-5.38 and 1.75-1.82, respectively, depending on the CMC concentration.37 Using this data in eq 1, in the original state (pH 6.4) we find that almost all of the carboxyl groups on CMC chains are in their dissociated state, which leads to good stability for AuNPs in the system. At pH values of 2.1-2.6, most of the -COO- groups (>80%) on the CMC chain are converted to carboxylic acid groups (-COOH). As a result, the electrostatic interaction between -NH3þ of CA and -COO- of CMC is minimized and leads to less stability for AuNPs. Further decreases in the pH value lead to the aggregation of the AuNPs. The turning point is almost consistent with the published literature,38 which indicates that the ionization of carboxylic ion groups was minimized obviously at pH 2.5. Figure 4a shows the UV-vis spectroscopy curves of the AuCA/CMC solution at different pH values. It is found that the SPR peak position changes from 520 nm at pH 6.4 to 598 nm at pH 1.5 to exhibit a critical point at about pH 2.6. When the pH value of the system decreased from 6.4 to 2.6, the SPR band of AuNPs shifted slightly from 520 to 526 nm. When the pH value is below 2.6, then the SPR band is broadened and red-shifted sharply from 527 to 598 nm. This process is reversible. The SPR band of the AuNPs gradually recovered almost to its initial position when the pH value of the system was adjusted gradually from 1.5 to 6.4 (Figure 4b). Moreover, the absorption intensity becomes smaller than that of the initial Au-CA/CMC dispersion solution, which is due to the dilution of the solution by adding HCl and NaOH solution during the adjustment of the pH of the solution (Figure 4a). A different CA concentration was investigated in the Au-CA/CMC system. It was found that at a CA concentration of 4.3 10-3 mg/mL the absorbance wavelength of AuNPs cannot be fully reversed when in the 6.4 f 1.5 f 6.4 pH cycle in the system (Figure 5), which may be due to the fact that there are not enough CA molecules to cover the surfaces of the AuNPs sufficiently. When the CA concentration is 4.3 10-3 mg/mL or as high as 4.3 10-3 mg/mL, the absorbance wavelength of AuNPs is fully reversible, indicating that the prepared AuNPs are efficiently covered and protected by CA. Therefore, 4.3 10 -3 mg/mL is a suitable concentration for CA in our feed. The morphology of the Au-CA/CMC assembly was observed by TEM as shown in Figure 6. The samples for TEM observation were prepared by dropping a small amount of the dispersion at the indicated pH values onto a carbon-coated copper grid and air drying. The AuNPs tended to aggregate with the decrease in the pH of the dispersion. The lower the pH, the larger the AuNPs aggregates formed. Moreover, gray particles without embedded AuNPs were also observed in Figure 6c-e, which should be the aggregates of the collapsed CMC chains that detached from the AuNPs at low pH. When the pH of the dispersion with aggregated AuNPs at pH 1.5 was adjusted to 6.4 by adding NaOH solution, (38) Bhattacharjee, R. R.; Chakraborty, M.; Mandal, T. K. J. Phys. Chem. B 2006, 110, 6768.
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Figure 6. TEM of Au-CA/CMC at different pH values. (a) pH = 6.4, (b) pH = 2.6, (c) pH = 2.1, (d) pH = 1.9, (e) pH = 1.5, and (f) the pH value recovered from 1.5 to 6.4.
Figure 4. UV-vis spectra at different pH values (a) and the effect of pH on the surface plasma absorbance wavelength (b).
Figure 5. Dependence of the surface plasma absorbance wavelength of the prepared AuNPs on the pH values at different CA concentrations. The solid symbol indicated the absorption band of the samples whose pH value was adjusted from 1.5 to 6.4.
the AuNPs redispersed. The TEM results are consistent with those from UV-vis spectroscopy. The results obviously indicate the dependence of the morphology of the AuNPs on the pH values of the systems. No obvious changes in the CMC chain 2096 DOI: 10.1021/la902593e
conformation occurred in the pH range from 6.4 to 2.6, whereas at pH
