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Synthesis of Carbohydrate-Conjugated Nanoparticles and Quantum Dots Christopher Earhart, Nikhil R. Jana,* Nandanan Erathodiyil, and Jackie Y. Ying* Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore ReceiVed June 24, 2007. ReVised Manuscript ReceiVed March 4, 2008 Nanoparticle-based probes are emerging as alternatives to molecular probes due to their various advantages, such as bright and tunable optical property, enhanced chemical and photochemical stability, and ease of introduction of multifunctionality. This work presents a simple and general approach for functionalizing various nanoparticle systems for use as glycobiological probes. Silica-coated nanoparticles of Ag, Fe3O4, and ZnS-CdSe were synthesized and functionalized with dextran. The resulting 10–40-nm-sized particles were robust, water-soluble, colloidally stable, and biochemically active.
* Corresponding author. E-mail:
[email protected] (J.Y.Y.);
[email protected] (N.R.J.).
The synthesis of carbohydrate-functionalized nanoparticles is very important as they are excellent candidates for probing glycobiological events.10 In previous reports, carbohydratefunctionalized nanoparticles were prepared using thiol-based molecules.10 These molecules were either thiolated carbohydrate or bifunctional thiols (e.g., mercaptopropionic acid). The thiols were chemisorbed onto the nanoparticle surface, and another functional group was used to link with carbohydrate. However, high-quality QDs and iron oxide nanoparticles are typically synthesized in high-temperature organometallic route;8,9 the use of thiolated carbohydrate is incompatible under such conditions, and alternative synthesis10f in the presence of these thiols tends to produce low-quality QDs and iron oxide nanoparticles. For bifunctional thiol-based systems, adverse conjugation conditions10d are required for the coupling of the alcohol groups of carbohydrates to the functionalized nanoparticles; thiols are reactive to some of these coupling reagents (such as maleimide) that lead to nanoparticle instability. Additionally, thiols are only chemisorbed onto the nanoparticle surfaces, and the Gibbs free energy change for the formation of the thiol-nanoparticle bond is very sensitive to the particle size and type.9c Thus, thiolated nanoparticles (particularly QDs and iron oxide) are often unstable in the presence of other molecules/ions, and subsequent chromatographic purification steps and robust surface coatings are necessary for their improved protection. Ideally, welldeveloped conditions should be used for the preparation of nanoparticles, and then robust surface coating schemes should be used for the carbohydrate coating. We have developed a silica
(1) (a) Lindhorst, T. K. In Essentials of Carbohydrate Chemistry and Biochemistry; Wiley-VCH: Weinheim, 2003. (b) Lee, Y. C.; Lee, R. T. Acc. Chem. Res. 1995, 28, 321. (c) Lis, H.; Sharon, N. Chem. ReV. 1998, 98, 637. (d) Ratner, D. M.; Adams, E. W.; Disney, M. W.; Seeberger, P. H. ChemBioChem 2004, 5, 1375. (e) Oh, E.; Lee, D.; Kim, Y. P.; Cha, S. Y.; Oh, D. B.; Kang, H. A.; Kim, J.; Kim, H. S. Angew. Chem., Int. Ed. 2006, 45, 7959. (2) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (3) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435. (4) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (5) (a) Bucak, S.; Jones, D. A.; Laibinis, P. E.; Hatton, T. A. Biotechnol. Prog. 2003, 19, 477. (b) Hogemann, D.; Ntziachristos, V.; Josephson, L.; Weissleder, R. Bioconjugate Chem. 2002, 13, 116. (c) Wang, S. X.; Bae, S.-Y.; Li, G.; Sun, S.; White, R. L.; Kemp, J. T.; Webb, C. D. J. Magn. Magn. Mater. 2005, 293, 731. (6) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (7) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (b) Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125, 14280. (8) (a) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (b) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (c) Jana, N. R.; Chen, Y.; Peng, X. Chem. Mater. 2004, 16, 3931.
(9) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc. 2003, 125, 12567. (c) Aldana, J.; Lavelle, N.; Wang, Y.; Peng, X. J. Am. Chem. Soc. 2005, 127, 2496. (d) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (e) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. J. Am. Chem. Soc. 2005, 127, 17586. (10) (a) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226. (b) Hone, D.; Haines, A. H.; Russell, D. A. Langmuir 2003, 19, 7141. (c) Barrientos, A. G.; de la Fuente, J. M.; Rojas, T. C.; Fernandez, A.; Penadés, S. Chem. Eur. J. 2003, 9, 1909. (d) Zhang, J.; Roll, D.; Geddes, C.; Lakowicz, J. J. Phys. Chem. B 2004, 108, 12210. (e) Wilson, R.; Chen, Y.; Aveyard, J. Chem. Commun. 2004, 1156. (f) Osaki, F.; Kanamori, T.; Sando, S.; Sera, T.; Aoyama, Y. J. Am. Chem. Soc. 2004, 126, 6520. (g) de la Fuente, J. M.; Penadés, S. Tetrahedron-Asymmetr. 2005, 16, 387. (h) Takae, S.; Akiyama, Y.; Otsuka, H.; Nakamura, T.; Nagasaki, Y.; Kataoka, K. Biomacromolecules 2005, 6, 818. (i) Schofield, C. L.; Haines, A. H.; Field, R. A.; Russell, D. A. Langmuir 2006, 22, 6707. (j) de la Fuente, J. M.; Alcántara, D.; Eaton, P.; Crespo, P.; Rojas, T. C.; Fernández, A.; Hernando, A.; Penadés, S. J. Phys. Chem. B 2006, 110, 13021. (k) Babu, P.; Sinha, S.; Surolia, A. Bioconjugate Chem. 2007, 18, 146.
Introduction Protein-carbohydrate interactions regulate many important biological processes, including cell-cell communication.1 Identifying, quantifying, and imaging the carbohydrates, glycoproteins, and glycolipids are important both in elucidating the biological function of carbohydrates and in evaluating and designing therapeutics aimed toward manipulating carbohydrate-protein interactions. Although molecular probes are commonly used to study glycobiology events, nanoparticles are emerging as alternatives due to their advantages such as unique size-dependent physical properties, enhanced chemical and photochemical stability, controlled and enhanced endocytosis, enhanced cooperative binding activity, and easier introduction of multifunctionality for targeted delivery and imaging.2–4 For example, gold and silver nanoparticles have been previously used in the ultrasensitive detection of protein/ DNA with PCR-like sensitivity.2 Semiconductor nanoparticles or quantum dots (QDs) have been used in fluorescence-based biolabeling and imaging applications at the subcellular length scales.3 Iron oxide nanoparticles have been used in magneticbased protein harvesting, magnetic resonance imaging, and magnetic sensing of biomolecules.5 Although the synthesis of various nanomaterials is well-developed,6–9 conversion of these nanoparticles into functional nanoprobes would require the attachment of various biomolecules, high water solubility, and colloidal stability.2–4
10.1021/la800066g CCC: $40.75 2008 American Chemical Society Published on Web 05/15/2008
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Figure 1. DSC-mediated dextran conjugation to amine-functionalized nanoparticles. R represents phosphonate or PEG functional group. The nanoparticles can be Ag, Fe3O4, or ZnS-CdSe. They are first coated with silica and then functionalized with dextran.
coating scheme for various nanoparticles and QDs.11 This silica coating provides a cross-linked shell that protects the core nanoparticle from adverse experimental conditions, and the coated particles can be further functionalized with various molecules of interest.11 Herein, we report a novel dextran functionalization scheme (Figure 1) that leads to robust, water-soluble, and colloidally stable nanoparticles. This approach is simple, and widely applicable to different types of nanoparticles and dextran of different molecular weights (MWs).
Experimental Section Materials. All reagents were purchased from Sigma-Aldrich. 3-Mercaptopropyl trimethoxysilane (MPS), 2-aminoethyl-aminopropyltrimethoxysilane (AEAPS), 2-methoxy(polyethyleneoxy)propyltrimethoxysilane (PEG silane), 3-trihydroxysilylpropyl methyl phosphonate, sodium salt (phosphonate silane), disuccinimidyl carbonate (DSC), dimethylaminopyridine, tetramethylammonium hydroxide, dextran (MWs of 1K, 6K, and 40K), and Con A were used as-received without further purification. All the silane solutions (100 mM) were prepared in toluene (100 mM), while phosphonate silane (100 mM) was prepared in methanol. Tetramethylammonium hydroxide solution (100 mM) was prepared in methanol. Synthesis of Primary Amine-Functionalized Nanoparticles. Nearly monodisperse silver, iron oxide, and ZnS-CdSe nanoparticles in the size range of 2–10 nm were prepared according to standard procedures.7–9 Gold and silver nanoparticles were synthesized in toluene in the presence of fatty acid/fatty amine/tetraalkyl ammonium surfactant stabilizer.7b Fe3O4 nanoparticles were synthesized by hightemperature pyrolysis of Fe(II)-carboxylate salt in octadecene.8c The as-synthesized particles were magnetite (Fe3O4), which was not affected by the subsequent silica coating scheme.11d CdSe was prepared using Cd(II) stearate precursors by high-temperature synthesis route in octadecene solvent.9b CdSe was then capped with the ZnS shell at 200 °C in octadecene via the alternate injection of Zn stearate in octadecene and elemental S dissolved in octadecene.9b After synthesis, Fe3O4 and ZnS-CdSe nanoparticles were purified from free ligands using the standard precipitation-redispersion protocol. Typically, the as-synthesized particle solution dispersed in octadecene was mixed with an equal volume of acetone to precipitate the particles. Particle precipitate was collected and redissolved in toluene. Next, particle solution was further purified using a similar precipitation-redispersion protocol, with ethanol and toluene as the solvent for precipitation and redispersion, respectively. Finally, optically clear nanoparticle solutions were prepared in toluene. As-synthesized Ag nanoparticles were directly used for silica coating. (11) (a) Selvan, S. T.; Tan, T. T.; Ying, J. Y. AdV. Mater. 2005, 17, 1620. (b) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (c) Jana, N. R.; Yu, H. H.; Ali, E. M.; Zheng, Y.; Ying, J. Y. Chem. Commun. 2007, 1406. (d) Jana, N. R.; Earhart, C.; Ying, J. Y. Chem. Mater. 2007, 19, 5074.
Amine-functionalized, silica-coated nanoparticles were prepared with our silanization protocol.11d Briefly, AEAPS was hydrolyzed in the presence of nanoparticles in toluene, resulting in silica-coated amine-functionalized nanoparticles. In the case of iron oxide, silane was conjugated directly with the hydroxyl groups on the surface of the nanoparticles. In the cases of silver, gold, and ZnS-CdSe, 3-mercaptopropyl trimethoxysilane was used as the linker silane.11c,d Other silane or silane mixture can also be used in similar conditions to prepare different functionalized nanoparticles. Preparation of Activated Dextran. For the conjugation of dextran to amine-functionalized nanoparticles, a modified literature procedure was used.12 First, a dextran solution was prepared in dimethylsulfoxide (DMSO) (30 mM). Next, an equivalent amount of DSC dissolved in DMSO was added to the dextran solution with stirring. An equivalent amount of 4-dimethylamino pyridine (DMAP) in DMSO was then slowly added, and the solution was stirred for 6 h. Dextran was precipitated by a slow addition of acetone, and washed with acetone and DMSO using the precipitation-redispersion protocol. The activated dextran powder was dried and stored at 4 °C. The structure of activated dextran was probed by 1H nuclear magnetic resonance (NMR) spectroscopy. A typical characteristic signal from succinimidyl group was observed in proton NMR spectrum (Supporting Information Figure S1). Conjugation of Dextran with Nanoparticles. As AEAPS was used as the silane monomer, the nanoparticles were surfacefunctionalized with primary amine groups. These primary amine groups were used for the coupling with dextran alcohol groups. All nanoparticle solutions were prepared in 10 mM of phosphate buffer (pH 7.5) for subsequent coupling. The nanoparticle loading was ∼1–10 mg/mL. Next, activated dextran powder was added to the nanoparticle solution so that the dextran concentration was 6 mM. The solution was shaken overnight at 4 °C. Finally, unreacted dextran was separated from the dextran-functionalized nanoparticles through size exclusion chromatography using Sephadex (G25) column or by overnight dialysis (using 12–14 kDa molecular weight cutoff (MWCO) membrane and against deionized water) for dextran 1K and dextran 6K. Materials Characterization. Absorption spectra were obtained with an Agilent 8453 spectrophotometer using a 1-cm quartz cell. Fluorescence spectra were collected with a Fluorolog (FL 3–11) fluorometer using a 1-cm quartz cell and with 390-nm excitation. An FEI Technai G2 high-resolution transmission electron microscope was used for transmission electron microscopy (TEM) studies. Samples were prepared by placing a drop of an aqueous solution on a carbon-coated copper grid, followed by air-drying for 24 h. The purified dextran-coated silver nanoparticle solution was vaccuumdried and used for elemental analysis or dissolved in D2O for NMR study. Elemental analysis of silica-coated Ag nanoparticles was performed before and after dextran coating; ∼12 wt % of dextran was found to bind to the nanoparticles’ surface. (12) Miron, T.; Wilchek, M. Bioconjugate Chem. 1993, 4, 568.
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Results and Discussion Dextran Conjugation of Silica-Coated Nanoparticles. Figure 1 illustrates the synthesis of dextran-functionalized nanoparticles. First, silica-coated, primary amine-functionalized nanoparticles were synthesized. Next, the carbohydrate alcohol groups and the primary amine groups on the nanoparticles were linked by DSC.12 Primary amine-functionalized nanoparticles of Ag, Fe3O4, and ZnS-CdSe were prepared according to our recently developed silanization scheme. Our silanization method produced a thin and uniform silica shell surrounding the nanoparticles. The silicacoated nanoparticles have an overall size of 10–25 nm, depending on the core diameter. With this approach, single or multiple functional groups (e.g., amine, phosphonate, and poly(ethylene glycol) (PEG)) could also be introduced onto the surface of nanoparticles.11c,d Nanoparticles functionalized with AEAPS have a primary amine loading of ∼100–700/particle, as estimated by fluorescamine titrations.11c Dextran with MWs of 1K, 6K, and 40K were used, and were designated as dextran 1K, dextran 6K, and dextran 40K, respectively. DSC was reacted with the hydroxyl group of dextran to form an amine-reactive succinimidyl carbonate derivative, as confirmed by proton NMR study (Supporting Information Figure S1). Next, amine-functionalized nanoparticles were reacted with the activated dextran to form a covalent carbamate bond. The active site was ramdomly located along the dextran chain, and the number of active sites on dextran could not be precisely controlled. Since a dextran/DSC molar ratio of 1 was employed, we assumed that there was one active site on the average for each dextran molecule. As the coupling reaction was not 100% efficient, excess activated dextran (compared to nanoparticles) was used during conjugation to optimize the dextran-nanoparticle conjugation and to minimize cross-linking beween particles. The dextran-functionalized particles were separated from free dextran using size exclusion chromatography or dialysis. The dextranfunctionalized Ag nanoparticles dissolved in D2O showed a similar NMR spectrum to that of dextran dissolved in D2O (Supporting Information Figure S2), indicating the presence of dextran bound to the silver nanoparticles. The photoacoustic Fourier transform infrared (FTIR) spectrum of dextran-conjugated Ag nanoparticles exhibited a broadening of the OH band at 3000–3700 cm-1 due to the presence of dextran (Supporting Information Figure S3). Colloidal Stability of Dextran-Functionalized Nanoparticles. Colloidal stability of silica-coated nanoparticles before and after dextran coating was compared in order to understand the role of dextran coating. The as-synthesized silica-coated nanoparticles have exceptionally high colloidal stability due to the cross-linked shell. They have high water solubility, and positive or negative surface charge that is tunable by pH change.11c,d They could be precipitated and redispersed many times by simply changing the pH or ionic strength. They could be subjected to various chemical or physical environments without significant change in properties. The AEAPS-coated nanoparticles have a positive surface charge, and they have high solubility and colloidal stability in pH 3–7 buffers. As the pH was increased, the primary amine group on their surface would be deprotonated, and the particles would become neutral in charge and precipitate from solution.11c,d With the dextran coating, increased colloidal stability could be achieved with the nanoparticles in the pH range of 7–8. The high water solubility of dextran enhanced the water solubility of the nanoparticles. Zeta potential measurements showed that the positive charge of the nanoparticles was reduced from 40-50 mV to 10–20 mV after the surface modification with dextran. As dextran is neutral in charge, its covalent binding to cationic
Figure 2. Con A-induced nanoparticle aggregation of (a) dextran 1K-Ag, (b) dextran 6K-Fe3O4, and (c) dextran 40K-QD. In each sample, Con A was added with a final concentration of 10 µM, and the absorption spectrum was obtained after (black line) 0 min, (blue line) 30 min, (green line) 1 h, and (red line) 2 h. A magnet is shown to remove the aggregated Fe3O4 from the solution in (b).
nanoparticles would decrease the surface charge of the latter. However, the surface of the dextran-modified nanoparticles was still positively charged due to the presence of secondary amines present in AEAPS and the presence of some primary amines even after the dextran coating. In general, the dextran-conjugated Ag, Fe3O4, and QDs were soluble in 1–10 mM of Tris buffer, borate buffer, and phosphate buffer in the pH range of 7–8, and were stable for several days to several weeks (Figure 2; Supporting Information Figure S4). However, in the presence of salt concentrations of >50 mM of NaCl or phosphate buffer concentrations of >10 mM, the nanoparticles would be slowly precipitated from the solution. Dextran-conjugated nanoparticle solutions were passed through a 0.2-µm syringe filter without any loss in materials. The size of the dextran-conjugated nanoparticles was examined by TEM and dynamic light scattering (DLS). TEM showed that the core diameter of nanoparticles was 4–8 nm, which remained essentially unchanged after dextran coating. The nanoparticles were not aggregated (Figure 3a), and have a uniform size distribution. The overall nanoparticle size in aqueous solution was determined by DLS to be 10–40 nm, depending on the core diameter and the dextran molecular weight (Supporting Information Figure
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Figure 3. TEM micrographs of dextran 6K-functionalized nanoparticles before (a) and 1 h after (b) Con A addition. (c) A scheme illustrating the particle aggregation with Con A addition. Con A was added with a final concentration of 10 µM to the dextran-coated nanoparticle solution.
S5). If the average sizes13 of dextran 1K, dextran 6K, and dextran 40K were approximated as 1, 2, and 6 nm, respectively, then the respective dextran coating should increase the overall size of the 8-nm silica-coated nanoparticles to 10, 12, and 20 nm. However, the respective DLS particle sizes were 5–18 nm larger, suggesting some aggregation of particles during dextran conjugation. Biochemical Activity. Biochemical activity of dextranfunctionalized nanoparticles was tested using Concanavalin A (Con A) and glucose (Figures 2, 3, and 4). Con A is a lectin with binding specificity toward the mannose and glucose residues of polysaccharides. At a pH above 7.0, Con A exists as a tetramer with four mannosyl binding sites. Upon the addition of Con A to dextran-functionalized nanoparticles, visible aggregates of particles were observed within minutes. Over time, the suspended aggregates would precipitate from the solution. Complete precipitation of all nanoparticle systems was observed within 4 h. This precipitation process was associated with a significant red-shifting of the plasmon absorption band and reduced peak absorbance for the Ag nanoparticles (Figure 2a). No significant red shift was observed with Con A-dextran-ZnS-CdSe aggregation, but both the absorbance and fluorescence were reduced with time as the particle precipitated from the solution (Figure 2c). Con A-dextran-Fe3O4 aggregates were removed from the solution over time using a magnetic field, resulting in a decrease in absorbance (Figure 2b). The dextran-Fe3O4 particles could (13) Granath, K. A. J. Colloid Sci. 1958, 13, 308.
not be separated by a magnetic field unless Con A was added. This illustrated that Con A induced the particle–particle crosslinking due to the Con A-dextran interaction (see Figure 3c), reducing the solubility of dextran-Fe3O4 particles. The aggregated particles would be gradually precipitated from the solution and could be separated by a magnet. Control experiments were conducted using bovine serum albumin (BSA) in place of Con A at equivalent or higher concentrations. Con A was also added to the nanoparticle solutions prior to dextran conjugation. In these cases, no aggregation or precipitation was observed, and the nanoparticles were stable for at least several days. Dextran-conjugated nanoparticles were also examined by TEM before and after Con A addition. They were highly dispersed before Con A addition (Figure 3a). In contrast, large aggregates were formed after Con A addition (Figure 3b). Additional experiments were conducted to investigate the competitive binding of Con A with glucose vs dextran. Precipitation of dextran-Ag was induced by micromolar concentration of Con A (Figure 4a), regardless of the dextran molecular weight. In contrast, if glucose was first mixed with the dextran-functionalized nanoparticles prior to the addition of Con A, particle aggregation was not observed even after 1 day (Figure 4b). This showed that, if a sufficient amount of free glucose was present, Con A would preferentially bind with free glucose rather than with Ag-dextran, thus, dextran-Ag nanoparticles would not precipitate.
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In another experiment, we isolated the Con A-dextran-Ag aggregates, and mixed them with different amounts of glucose (Figure 4c). 40–50% of the aggregated nanoparticles would redissolve/redisperse in the solution, while the rest remained aggregated even if a very high concentration of glucose was introduced. This indicated that the binding between dextran-Ag and Con A was only partially reversible. Similar results were observed by others.10d This could be due to the formation of large aggregates associated with multiple Con A-dextran bindings, creating a large kinetic barrier for breaking the network of aggregates. These particle aggregation experiments demonstrated that the dextran-conjugated nanoparticles were biochemically active and interacted with glycoproteins. Thus, they could potentially be used as optical and magnetic resonance imaging (MRI) probes in the detection and imaging of glycoproteins.
Conclusions In this work, we have presented a general and simple approach for conjugating a wide variety of nanoparticle systems with carbohydrates. With this approach, one can choose any nanoparticle system suitable for a given application and conjugate the nanoparticle with virtually any carbohydrate via carbohydrate activation with DSC. Water-soluble, dextran-conjugated nanoparticles are obtained with an overall size of 10–40 nm. These carbohydrate-functionalized nanoparticles are robust, biochemically active, and selectively bind to glycoprotein. They can be used to monitor/detect the protein-carbohydrate interactions, or to image the glycoproteins and their cellular activities.
Figure 4. (a) Precipitation of dextran 6K-Ag in the presence of (black line) 0.0 µM, (blue line) 0.1 µM, and (red line) 0.2 µM of Con A. (b) Precipitation of dextran 6K-Ag could not be induced with 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 µM (curves from top to bottom) of Con A in the presence of 0.5 mM of glucose. (c) Solubilization of Con A-dextran 6K-Ag aggregates in the presence of (black line) 0.0 µM, (blue line) 25 µM, (green line) 250 µM, and (red line) 2.5 mM of glucose. (magenta line) The absorption spectrum of dextran 6K-Ag before binding with Con A.
Acknowledgment. The authors would like to thank Dr. S. Tamil Selvan and Dr. Hsiao-hua Yu for helpful discussions, and Dr. Yu Han for TEM characterization. This work was supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore). Supporting Information Available: 1H NMR spectra, FTIR spectra, UV–vis spectrum, DLS data, and X-ray diffraction pattern. This material is available free of charge via the Internet at http://pubs.acs.org. LA800066G
