CdS Core

May 28, 2006 - Nanomaterials and Nanofabrication Laboratories, P.O. Box 2168, Fayetteville, Arkansas 72702, and Department of Chemistry and Biochemist...
1 downloads 15 Views 177KB Size
Download PDF
Langmuir 2006, 22, 6341-6345

6341

Highly Luminescent, Stable, and Water-Soluble CdSe/CdS Core-Shell Dendron Nanocrystals with Carboxylate Anchoring Groups Yongcheng Liu,*,† Myeongseob Kim,‡ Yunjun Wang,† Y. Andrew Wang,† and Xiaogang Peng*,†,‡ Nanomaterials and Nanofabrication Laboratories, P.O. Box 2168, FayetteVille, Arkansas 72702, and Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed October 11, 2005. In Final Form: February 7, 2006 A dendron ligand with two carboxylate anchoring groups at its focal point and eight hydroxyl groups as its terminal groups was found to efficiently convert as-synthesized CdSe/CdS core-shell nanocrystals in toluene to water-soluble dendron-ligand stabilized nanocrystals (dendron nanocrystals). The resulting dendron nanocrystals retained 60% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene and were significantly brighter than the similar dendron nanocrystals with thiolate (deprotonated thiol group) as the anchoring group which retained just 10% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene. The carboxylate-based dendron nanocrystals survived UV irradiation in air for at least 13 days, about 9 times better than the thiolate-based dendron nanocrystals (35 h) and similar to that of the thiolate-based dendron-box stabilized CdSe/ CdS core-shell nanocrystals (box nanocrystals). Upon UV irradiation, the dendron nanocrystals became even 2 times brighter than the original CdSe/CdS core-shell nanocrystals in toluene, and the UV-brightened PL can retain the brightness for at least several months. These stable and bright dendron nanocrystals were soluble in various aqueous media, including all common biological buffer solutions tested, for at least 1.5 years. In addition to their superior performance, the synthetic chemistry of carboxylate dendron ligands and the corresponding dendron nanocrystals is relatively simple and with high yield.

Introduction Luminescent semiconductor nanocrystals are promising candidates for next generation biomedical labeling reagents.1,2 Several characteristics, such as narrow and symmetric emission spectra, broad excitation spectra, long fluorescence lifetimes, and negligible photobleaching, distinguish them from the commonly used fluorophores, inorganic/organic dyes. However, currently available high-quality semiconductor nanocrystals, at least for the most popular CdSe and related core-shell nanocrystals, are all synthesized in nonpolar organic solvents.3-5 The resulting nanocrystals are thus coated with a monolayer of organic ligands, which make them insoluble in water. Before they are conjugated onto biomolecules and used as biolabels in life science, these semiconductor nanocrystals have to be modified to become water soluble and biocompatible by converting the outer surface of the ligand/surfactant coating to be hydrophilic. Different modification strategies for semiconductor nanocrystals to become water soluble and biocompatible have been investigated in recent years. One common one is based on either micelle6 or polymer wrapping, with either synthetic7 or natural8 * Corresponding authors. Phone: 479-575-3481(Y.L.); 479-575-4612 (X.P.). Fax: 479-575-3482 (Y.L.); 479-575-4049 (X.P.). E-mail: ycliu@ nn-labs.com (Y.L.); [email protected] (X.P.). † Nanomaterials and Nanofabrication Laboratories. ‡ University of Arkansas. (1) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (2) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016-2018. (3) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-15. (4) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468-71. (5) 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, 12567-12575. (6) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759-1762. (7) (a) Wu, X.; Liu, H.; Liu, J.; Haley, K. N.; Treadway, J. A.; Larson, J. P.; Ge, N.; Peale, F.; Bruchez, M. P. Nat. Biotechnol. 2003, 21, 41-46. (b) Potapova, I.; Mruk, R.; Prehl, S.; Zentel, R.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2003, 125 (2), 320-321.

polymers. This method leaves the original surface ligands untouched except the multidental wrapping by peptides.8b Consequently, the optical properties of the nanocrystals can be largely retained. The thick polymer wrapping layer provides a sufficient steric protection for the nanocrystals, but it also makes the complex relatively large in size and limited in physical permeability. Another method is to coat a hydrophilic shell such as a silica coating1 on the surface of nanocrystals. This method results in low yield, and the sensitivity of the silica shell to pH may cause precipitation and gel formation.9,10 In addition, the silica shell is typically relatively thick and as irregular as the polymer wrapping layers. The third one is the ligand replacement strategy,2 which uses hydrophilic ligands to replace the original surface ligands from the synthesis. Among various ligands used, hydrophilic dendron ligands11 and dendron-box12,13 ligands are promising because of their exceptional stability in comparison to that of other hydrophilic ligands and relatively thin ligand layer, as thin as 1 nm in total.11 Similar to common hydrophilic ligands used for surface replacement, the dendron and dendronbox ligands developed for semiconductor nanocrystals have been based on thiol groups as the anchoring groups to the surface cations. Thiol groups, strictly speaking thiolate groups (deprotonated thiol groups), are strong bonding groups to group IIB metals ions14 on the surface of the commonly used core-shell (8) (a) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 1214212150. (b) Pinaud, F.; King, D.; Moore, H.; Weiss, S. J. Am. Chem. Soc. 2004, 126, 6115-6123. (9) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861-8871. (10) Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7070-7074. (11) Wang, Y. A.; Li, J. J.; Chen, H.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2293-2298. (12) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 31253133. (13) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2003, 125, 3901-3909.

10.1021/la052747e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006

6342 Langmuir, Vol. 22, No. 14, 2006

Liu et al.

Figure 1. Synthetic scheme of the targeted dendron ligand.

nanocrystals, such as CdSe/CdS and CdSe/ZnS core-shell nanocrystals. As a result, they can replace the original weak ligands, such as amines and carboxylates. This makes the surface chemistry relatively easy to control. However, thiol groups significantly quench the photoluminescence (PL) of the CdSe/ CdS core-shell nanocrystals,12,13 and they are subject to be photooxidized.15 In addition, the starting reagents and intermediates for the synthesis of the dendrons, thiol-containing compounds, are not pleasant to work with and are often considered to be quite toxic. In this paper, we report carboxylate groups in a dendron ligand successfully used as the anchoring groups to replace the organic alkylamine ligands coated on CdSe/CdS core-shell nanocrystals. Bidentate ligands were used previously by Mattossi’s group for thiol-based ligands8 and Bawendi’s group for phosphine oxidebased ligands,16 which were reported to be significantly more stable than a single-bonding site. To compensate the weak bonding strength of a carboxylate group, the ligand was designed to have two carboxylate groups at their focal points (Figure 1, the final product). Eight hydroxyl groups (-OH) were chosen as the outer terminal groups in the dendron ligand that were similar to our previous dendron ligand in which the thiolate group was used as the bonding group.12 This provided a comparison basis between the two different dendron ligands. Our previous results also indicated that generation three (G-3) dendron ligands were typically sufficiently bulky for providing the necessary steric protection of the inorganic nanocrystal core.11-13 Therefore, the new targeted ligand was designed to be a G-3 dendron ligand, whose linear length from the bonding site (-COOH) to the terminal group (-OH) is slightly shorter than that of stearic acid, a typical fatty acid. Materials and Methods 1. Synthesis of the Dendron Ligand. The dendron ligand was synthesized from simple starting materials and in high yields for every reaction step. The synthetic route for the dendron ligand is illustrated in Figure 1. The detailed experiments are described below. (14) Aldana, J.; Lavelle, N.; Wang, Y.; Peng, X. J. Am. Chem. Soc. 2005, 127, 2496-2504. (15) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 88448850. (16) Kim, S.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 14652-14653.

1.1. Synthesis of N-tert-Butoxycarbonylaminoethylamine (2). Ditert-butyl dicarbonate (25 mmol) dissolved in CHCl3 (125 mL) was added dropwise to a solution of ethylenediamine (250 mmol) in 250 mL of CHCl3 at 0 °C over 3 h with stirring. The reaction mixture was stirred for 16 h at room temperature and washed with water. The organic phase was dried over Na2SO4 and evaporated to dryness in vacuum to obtain a colorless oil, which was purified by silica gel chromatography (CHCl3-MeOH 5:0.1). The yield was 100%. The product was characterized as follows. 1H NMR (CDCl3): 1.17 (2H, NH2), 1.39 (9H, 3CH3), 2.74 (2H, CH2NH2), 3.10 (2H, CH2NH), 4.99 (1H, NH). FT-IR (neat): 3354 cm-1 (N-H), 2977 cm-1, 2933 cm-1 (C-H), 1693 cm-1 (CdO). 1.2. Synthesis of N,N-Bis-(2-cyanoethyl)-N-tert-butoxycarbonyl Ethylenediamine (3). To a solution of the amine 2 (10 mmol) in a large excess of acrylonitrile (50 mL), acetic acid (20 mmol) was added, and the mixture was refluxed for 24 h. The excess acrylonitrile was evaporated in vacuum to obtain a residue, which was dissolved in CHCl3 (30 mL) and added to 10 mL of concentrated NH4OH. The organic layer was separated, washed with water, and dried over Na2SO4. Evaporation of the solvent and silica chromatography of the obtained oil yielded the pure products. A yellowish oil was obtained after chromatography (CHCl3-MeOH 5:0.1). The yield was 85%. The product was characterized as follows. 1H NMR (CDCl3): 1.42 (9H, 3CH3), 2.47 (4H, 2CH2CN), 2.65 (2H, N-CH2CH2NH), 2.87 (4H, 2CH2N), 3.18 (2H, NCH2CH2NH), 4.97 (1H, NH). FT-IR (neat): 3420 cm-1 (N-H), 2977 cm-1, 2932 cm-1, 2850 cm-1 (CH2), 2250 cm-1 (CN), 1695 cm-1 (CdO). 1.3. Synthesis of N,N-Bis-(3-aminopropyl)-N-tertbutoxycarbonyl Ethylenediamine (4). The N,N-bis-(2-cyanoethyl)-N-tert-butoxycarbonyl ethylenediamine 3 (5 mmol), 20 mL of 1.4 M NaOH solution in 95% EtOH, and 1.0 g of Ni-Raney slurry in water (pH ) 10.0) were added to a 500 mL hydrogenation vessel. The mixture was placed under hydrogen (40 psi) in a Parr hydrogenation apparatus and shaken for 16 h (overnight) at room temperature. The mixture was carefully vacuum filtered through a sintered glass funnel, and the catalyst was washed with 95% EtOH. After diluting the filtrate with H2O, the EtOH was evaporated and concentrated. The residue was dissolved in a small amount of water (30 mL). After addition of NaOH pellets, the amine 4 began to separate as an oil which was extracted with CHCl3. The organic layer was dried over Na2SO4, and the solvent was evaporated to obtain a colorless oil. The yield (17) Battaglia, D.; Li, J. J.; Wang, Y.; Peng, X. Angew. Chem. 2003, 42, 5035-5039.

CdSe/CdS Core-Shell Dendron Nanocrystals

Langmuir, Vol. 22, No. 14, 2006 6343

Figure 2. Schematic illustration of the formation of the dendron-coated CdSe/CdS core-shell nanocrystals (CdSe/CdS core-shell dendron nanocrystals). was 90%. The colorless oil was characterized as follows. 1H NMR (CDCl3): 1.20 (4H, 2NH2), 1.39 (9H, 3CH3), 1.52 (4H, 2CH2CH2CH2), 2.45 (6H, 3CH2N), 2.67 (4H, 2CH2NH2), 3.13 (2H, NCH2CH2NH), 5.22 (1H, NH). FT-IR (neat): 3359 cm-1 (N-H), 2933 cm-1, 2865 cm-1 (CH), 1695 cm-1 (CdO). 1.4. Synthesis of 4-Cascade-N-tert-butoxycarbonyl Ethylenediamine [2]-(1-Azabutylidyne)-Propanitrile (5). To a solution of the amine 4 (5 mmol) in a large excess of acrylonitrile (50 mL), acetic acid (20 mmol) was added, and the mixture was refluxed for 24 h. The excess acrylonitrile was evaporated in vacuum to obtain a residue, which was dissolved in CHCl3 (30 mL) and added to 10 mL of concentrated aqueous NH4OH. The organic layer was separated, washed with water, and dried over Na2SO4. Evaporation of the solvent and silica chromatography of the obtained oil yielded the pure products. A colorless oil was obtained after chromatography (CHCl3MeOH 5:0.1). The yield was 85%. The oil was characterized as follows. 1H NMR (CDCl3): 1.43 (9H, 3CH3), 1.59 (4H, 2CH2CH2CH2), 2.46-2.57 (18H, 9CH2N), 2.84 (8H, 4CH2CN), 3.14 (2H, CH2NH), 4.99 (1H, NH). FT-IR (neat): 2980 cm-1 (CH), 2240 cm-1 (CN), 1720 cm-1, 1600 cm-1 (CdO). 1.5. Synthesis of 4-Cascade-N-tert-butoxycarbonylethylenediamine [2]-(1-Azabutylidyne)-Propanamine (6). The 4-cascade-N-tertbutoxycarbonyl ethylenediamine [2]-(1-azabutylidyne)-propanitrile 5 (5 mmol), 100 mL of a 1.4 M NaOH solution in 95% EtOH, and 1 g of Ni-Raney slurry in water (pH ) 10.0) were added to a 500 mL hydrogenation vessel. The mixture was placed under hydrogen (40 psi) in a Parr hydrogenation apparatus and shaken for 48 h at room temperature. The mixture was carefully vacuum filtered through a sintered glass funnel, and the catalyst was washed with 95% EtOH. After diluting the filtrate with 100 mL of H2O, the EtOH was evaporated and the solution was concentrated. After addition of 50 mL of saturated NaOH solution, the amine 6 began to separate as an oil, which was extracted with CHCl3. The organic layer was dried over Na2SO4, and the solvent was evaporated to obtain the amine as a yellowish oil. The yield was 90%. The product was characterized as follows. 1H NMR (CDCl3): 1.27 (9H, 3CH3), 1.42 (12H, 6CH2CH2CH2), 2.24 (18H, 9CH2N), 2.55 (8H, 4CH2NH2), 2.97 (2H, CH2NH), 4.93 (1H, NH). IR (neat): 3356 cm-1 (N-H), 2939 cm-1 (C-H), 1702 cm-1 (CON-Boc). 1.6. Synthesis of G-3 Dendron Ligand (7). To the stirred mixture of the amine 6 (5 mmol) in 150 mL of THF, K2CO3 (6 mmol) and bromoethanol (44 mmol) were added, and stirring was continued at room temperature for 2 h. The mixture was heated to 60 °C for 24 h using TLC to monitor the reaction. The mixture was centrifuged and filtered to remove unreacted K2CO3 and formed KBr. After

evaporating the solvent, a yellowish liquid was obtained. The yellowish liquid was dissolved in CHCl3 and precipitated in diethyl ether to remove the excess reactant, bromoethanol. A yellowish gel was obtained, and the yield was 80%. The product was characterized as follows. 1H NMR (CDCl3): 1.41 (9H, 3CH3), 1.9 (12H, 6NCH2CH2CH2N), 2.19 (26H, 13 NCH2), 3.12 (16H, 8NCH2-CH2OH), 3.4 (2H,NHCH2), 3.7 (16H, 8CH2-OH). IR (neat): 3378 cm-1 (NH, OH), 2947 cm-1, 2804 cm-1 (CH2), 1687 cm-1 (CdO). 1.7. Synthesis of G-3 Dendron Ligand (8). To the dendron ligand 7 (5 mmol) in 50 mL of H2O, 2 mL of concentrated HCl was added, and stirring was continued at room temperature for 0.5 h. Then the mixture was heated to 70 °C overnight. After cooling to room temperature, the solution was neutralized with NaOH. After evaporating all solvents, CHCl3 was used to extract the product from a very viscous gel. After evaporating the CHCl3, a viscous oil was obtained, and the yield was 80%. The product was characterized as follows. 1H NMR (CDCl3): 1.9 (12H, 6NCH2CH2CH2N), 2.2 (26H, 13 NCH2), 3.10 (16H, 8CH2-CH2-OH), 3.4 (2H,NHCH2), 3.9 (16H, 8CH2-OH). IR (neat): 3389 cm-1 (NH, OH), 2947 cm-1, 2804 cm-1 (CH2). 1.8. Synthesis of G-3 Dendron Ligand (9), the Target Dendron Ligand. To the stirred mixture of the amine 8 (1 mmol) in 50 mL of DMSO, K2CO3 (2 mmol) and bromoacetic acid (2.2 mmol) were added dropwise, and stirring was continued at room temperature for 48 h. The unreacted K2CO3 and the formed KBr were removed by centrifuging, and the material was washed with methanol, then DMSO and methanol were removed by evaporation. A brown gel was obtained. The brown gel was dissolved in CHCl3 to remove the unreacted raw material, BrCH2COOH. The gel was then dissolved in methanol. The solution was evaporated, and a brownish solid was obtained. The yield was 75%. The product was characterized as follows. 1H NMR (D2O): 1.73 (12H, 6NCH2CH2CH2N), 2.5-3.0 (44H, 22NCH2), 3.71 (16H, 8CH2OH), 3.91 (4H, 2COOCH2N). IR (neat): 3305 cm-1 (OH), 2945 cm-1, 2806 cm-1 (CH2), 1593 cm-1 (CdO). 2. Synthesis of the CdSe/CdS Core-Shell Dendron Nanocrystals. The CdSe/CdS core-shell nanocrystals used in this study were synthesized using the typical successive ionic layer adsorption and reaction (SILAR) or solution atomic layer epitaxy (SALE) techniques reported by us previously.5 The dendron ligand coated CdSe/CdS core-shell nanocrystals were made by surface modification of the CdSe/CdS core-shell nanocrystals with the dendron ligands. The scheme is presented in Figure 2. Typically, 1.0 mL of purified CdSe/CdS core-shell in chloroform solution (OD 1.0) and 1.0 mL of the G-3 dendron ligand in methanol

6344 Langmuir, Vol. 22, No. 14, 2006 solution (10 mg/mL) were added into an 8 mL vial. The mixture was sonicated in a sonicator (Branson) for 4 h. Then, ethyl acetate was added into the vial to precipitate the nanocrystal complexes, and the heterogeneous solution was centrifuged. After the supernatant was decanted, the precipitates were washed with ethyl acetate three times. The purified dendron nanocrystal could be dissolved in methanol, water, and various aqueous media. FT-IR spectra were recorded for the purified nanocrystals before and after the ligand exchange to verify the completion of the reaction (Supporting Information). 3. UV Irradiation and Instruments. All UV irradiation experiments were performed in a dark chamber with a model UVGL-58 Mineralight UV lamp (Upland, CA). The lamp was set to 254 nm (a short wavelength) and was placed around 4 cm directly above the samples. The samples were stored in an 8 mL glass vial, and the sample volume was 6.0 mL. The concentration of the samples was diluted to the absorbance (OD) to 0.10-0.13 at the first excitation absorption peak of the purified nanocrystals to measure the quantum yield of the nanocrystals. The UV-vis absorption spectra were recorded with an Agilent UV-vis 8453 spectrophotometer (Agilent Technologies, Foster City, CA), and the PL was measured by a spectrofluorometer (Fluorolog-3, Horiba Jobin Yvon, Irvine, CA).

Liu et al.

Figure 3. UV-vis and PL spectra of the nanocrystals of the assynthesized CdSe/CdS core-shell nanocrystals in toluene and the corresponding dendron nanocrystals in water. Inset: PL spectra of the nanocrystals before and after dendron ligand modification taken under the same conditions with the same absorbance at the excitation wavelength.

Results and Discussion Reaction Conditions for Synthesis of CdSe/CdS CoreShell Dendron Nanocrystals. Reaction conditions for synthesis of CdSe/CdS core-shell dendron nanocrystals with the new G-3 dendron ligand containing two carboxylic acid groups as anchoring groups and eight hydroxyl groups as terminal groups shown in Figure 1 are similar to those with the G-3 dendron ligand containing one thiol group as an anchoring group and hydroxyl groups as terminal groups.12,15 The new dendron ligand is active only under basic condition (pH 12). The carboxylic acid groups in the dendron ligands have to be deprotonated and changed to carboxylate groups for replacing the initial organic ligand, octadecylamine, on the surface of the CdSe/CdS core-shell nanocrystals. Otherwise, the dendron ligand cannot be coated onto the CdSe/CdS core-shell nanocrystals so that the CdSe/ CdS core-shell nanocrystals are insoluble in water. The conditions for isolation and purification of the dendron nanocrystals are also similar to the typical protocols for synthesis of hydrophilic nanocrystals. The CdSe/CdS core-shell dendron nanocrystals are precipitated from methanol/chloroform solutions by adding ethyl acetate.12 Solubility of the CdSe/CdS Core-Shell Dendron Nanocrystals. The resulting CdSe/CdS core-shell carboxylate-based dendron nanocrystals were found to be soluble in water as expected. The CdSe/CdS core-shell dendron nanocrystals were also soluble in methanol and in various aqueous solutions, such as saturated NaCl solution, 2 M NH4Ac solution, PBS buffer solution, and Tris buffer solution. The diverse solubility of the CdSe/CdS core-shell dendron nanocrystals in various aqueous media renders flexible conjugation chemistry for one to couple biofunctional species, such as DNAs, proteins, antibodies, etc., onto the surface of the nanocrystals. Optical Properties of the CdSe/CdS Core-Shell Dendron Nanocrystals. The CdSe/CdS core-shell dendron nanocrystals dissolved in water exhibit the same optical spectra as those of the as-synthesized CdSe/CdS core-shell nanocrystals dissolved in organic solvents such as toluene, chloroform, and hexane (Figure 3). The CdSe/CdS core-shell dendron nanocrystals in aqueous solution present the same peak positions and shapes of emission and absorption as those in toluene solution. However, the quantum yield (24%) of the CdSe/CdS core-shell dendron nanocrystals in the water solution is dropped to 60% of that of the initial nanocrystals (40% quantum yield) in toluene solution. In comparison to the thiol-based dendron nanocrystals (4% quantum yield), the carboxylate-based CdSe/CdS core-shell

Figure 4. Effects of UV irradiation in air on the optical properties of the CdSe/CdS core-shell dendron nanocrystals in water solution.

dendron nanocrystals present a much higher quantum yield and their brightness is the 6 times that of the thiol-based CdSe/CdS core-shell dendron nanocrystals. Photobrightening of the CdSe/CdS Core-Shell Dendron Nanocrystals. Under short-wavelength (254 nm) UV irradiation, significant photobrightening was observed. Figure 4 (top plot) shows the relationship of the PL intensity versus the UV irradiation time for the CdSe/CdS core-shell dendron nanocrystals in water solution. In the first 2 days of UV irradiation, the brightness of the dendron nanocrystal was enhanced up to 4 times in comparison to that of the CdSe/CdS core-shell dendron nanocrystals in water solution without UV irradiation. So the brightness of the CdSe/CdS core-shell dendron nanocrystals in water solution after this brightening process was actually raised to about 2 times stronger than that of the as-synthesized CdSe/CdS core-shell nanocrystals in toluene solution. Similar to the photobrightening observed previously in nanocrystals in water solution,5,12,13 the brightening process was accompanied with a slight blue-shift of the UV-vis absorption and PL spectra (Figure 4, bottom plot). This implied that the photobrightening process was likely due to the photooxidation of the surface CdS shell layer as documented previously.5 A similar photobrightening phenomenon was observed in the dendron nanocrystals water solution on a shelf with room light; however, it was very much slower. It should be pointed out that without further UV irradiation the brightened nanocrystals could be stably stored on a shelf without changing the PL intensity. It is interesting to notice that after 6 days of irradiation the PL brightness of the CdSe/CdS core-shell dendron nanocrystals

CdSe/CdS Core-Shell Dendron Nanocrystals

Langmuir, Vol. 22, No. 14, 2006 6345

Table 1. Various Properties of the CdSe/CdS Core-Shell Dendron Nanocrystals thiol-based CdSe/CdS core-shell dendron nanocrystals quantum yield/ brightness photochemical stability solubility

4% in water soln before UV irradiation; 8% after UV irradiation precipitated within 35 h under UV irradiation

carboxylate-based CdSe/CdS core-shell dendron nanocrystals 24% in water soln before UV irradiation; 75% after UV irradiation

not precipitated in 13 days under UV irradiation; 1.5 years in the shelf stored under room light soluble in methanol, water, saturated NaCl soln, soluble in methanol, water, saturated NaCl soln, 2 M NH4Ac soln, and PBS buffer soln 2 M NH4Ac soln, PBS buffer soln, and Tris buffer soln thermal stability stable at 100 °C for 1 h under vacuum stable at 100 °C for 2 h under vacuum chemical stability precipitated in 1 M HCl within 30 s stable in typical biological buffer solns above pH 4 over 1 month; and in 3% H2O2 soln for about 5 min precipitated in 1 M HCl within 5 min and in 3% H2O2 soln for about 5 min

started to decrease gradually. Until 13 days of UV irradiation, their PL brightness was dropped down to almost the same as that of the as-synthesized CdSe/CdS core-shell nanocrystals in toluene solution. Their quantum yield was about 40%. Their PL was still significantly brighter than that of the as-prepared CdSeCdS core-shell dendron nanocrystals in water solution without UV irradiation. Photochemical Stability of the CdSe/CdS Core-Shell Dendron Nanocrystals. The photochemical stability of the carboxylate-based CdSe/CdS core-shell dendron nanocrystals presented much higher than the thiol-based CdSe/CdS coreshell dendron nanocrystals. The UV-vis and PL spectra of the CdSe/CdS core-shell dendron nanocrystals shown in Figure 3 remained the same in 1.5 years of storage on the shelf under room light at room temperature. As shown in Figure 4, even under 13 days of continuous UV irradiation, the CdSe/CdS coreshell dendron nanocrystals were still soluble in water solution. However, the previous thiol-based CdSe/CdS core-shell dendron nanocrystals were precipitated from the solution after 35 h of UV irradiation under the same conditions.12 Thiols and thiolates (deprotonated thiols) have been found to be vulnerable to photooxidation that irreversibly converts them to disulfides.15 The disulfides do not bond to the nanocrystal surface, at least not sufficiently strong, so that the nanocrystals become more or less “naked” and thus precipitate from the water solution. However, carboxylate groups are known to be very stable under similar conditions. Chemically, the cationic ions of an unoxidized and oxidized surface of CdSe/CdS core-shell nanocrystals are the same, Cd2+ ions. Consequently, although the surface of the nanocrystals could be slightly photooxidized, indicated by the blue-shift in Figure 4 (bottom plot), the carboxylate groups should stay bonding to the surface of the nanocrystals. It should be pointed out that these dendron nanocrystals were substantially more stable under photooxidation conditions than any thiolbased CdSe/CdS core-shell dendron nanocrystals.12,13 In fact, their stability is comparable to that of typical CdSe/CdS coreshell box nanocrystals.12,13 Thermal Stability of the CdSe/CdS Core-Shell Dendron Nanocrystals. The thermal stability of the CdSe/CdS coreshell dendron nanocrystals was examined by sintering the nanocrystals. The purified dendron nanocrystals were heated to 100 °C in a vacuum oven for 2 h. After being cooled to room temperature, the fine powder of the dendron nanocrystals was still able to be completely dissolved in water and the various aqueous buffer media mentioned above. This excellent thermal stability makes these nanocrystals to be possibly used for certain biological applications which require high temperatures, such as PCR processes. Chemical Stability of the CdSe/CdS Core-Shell Dendron Nanocrystals. The dendron nanocrystals were found to be stable in typical biological buffer solutions above pH 4. In strong acidic conditions, such as in 1 M HCl, the nanocrystals were precipitated within 5 min, which is slightly more stable than the thiol-based

CdSe/CdS core-shell dendron nanocrystals but not as stable as the corresponding thiol-based box nanocrystals.12 The stability against chemical oxidation tested using 3% H2O2 solution was found to be similar to that of the thiol-based CdSe/CdS coreshell dendron nanocrystals, which only lasted for about 5 min in the solution.12 Various Property Comparisons of the CdSe/CdS CoreShell Dendron Nanocrystals. As discussed above, the carboxylate-based CdSe/CdS core-shell dendron nanocrystals have more improved properties including optical properties and various stabilities than the thiol-based CdSe/CdS core-shell dendron nanocrystals. To conveniently compare their properties, Table 1 lists the difference of the various properties between the carboxylate-based CdSe/CdS core-shell dendron nanocrystals and the thiol-based CdSe/CdS core-shell dendron nanocrystals.

Conclusions The dendron ligand with two carboxylic acid groups as the bonding sites and eight hydroxyl groups as terminal groups has successfully replaced the original alkylamine ligand on the assynthesized CdSe/CdS core-shell nanocrystals and converted the CdSe/CdS core-shell nanocrystals from being only soluble in nonpolar solvents to being soluble in various aqueous solvents. The resulting dendron nanocrystals are significantly more luminescent and stable than the similar thiol-based CdSe/CdS dendron nanocrystals reported previously.13 Their photochemical stability and other stabilities in biological buffer solutions are comparable to those of the hydrophilic box nanocrystals described in a previous report.13 The current synthetic chemistry for both the ligand and the nanocrystal-ligand complexes for the carboxylate-based dendron nanocrystals is significantly simpler than that for the thiol-based box nanocrystals. It should be pointed out that although the new dendron ligand worked well for CdSe/ CdS core-shell nanocrystals, it needs to be tested for other systems, such as CdSe/ZnS core-shell nanocrystals, to see if this design can become a general strategy for surface modification of colloidal nanocrystals. In addition, in comparison to thiolbased ligands, carboxylate groups are significantly weaker bonding sites. Further evidences would be needed to justify if these types of ligands can be used for nanocrystals with other types of organic ligands, such as phosphine oxides, phosphonic acids, and fatty acids. Acknowledgment. This research work was supported in part by the NIH SBIR programs at 1R43GM069065-01 and 5R43GM069065-02. Supporting Information Available: FT-IR spectra of the nanocrystals before and after the ligand exchange, and the assignments of the IR peaks. This material is available free of charge via the Internet at http://pubs.acs.org. LA052747E