pubs.acs.org/Langmuir © 2010 American Chemical Society
PEGylation of Carboxylic Acid-Functionalized Germanium Nanowires Vincent C. Holmberg, Michael R. Rasch, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, and Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712-1062 Received May 26, 2010. Revised Manuscript Received July 20, 2010 Germanium (Ge) nanowires were produced in solution by supercritical fluid-liquid-solid (SFLS) growth and then functionalized with carboxylic acid groups by in situ thermal thiolation with mercaptoundecanoic acid. Polyethylene glycol (PEG) was grafted to the carboxylic acid-terminated Ge nanowires using carbodiimide coupling chemistry. The nanowires were characterized using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and transmission electron microscopy (TEM) to confirm the surface modification of the nanowires. Dispersions of PEGylated Ge nanowires in dimethylsulfoxide (DMSO) were stable for days. The PEGylated Ge nanowires were also dispersible in aqueous solution over a wide range of pH and ionic strength.
1. Introduction Solvent dispersions of semiconductor nanowires have many potential applications: as electrorheological fluids,1 inorganic liquid crystals,2-4 and inks for printed electronics5 to name a few. Nanowires might also be blended with polymers to create new composite materials with unique combinations of mechanical and optoelectronic properties.6 There is also interest in using semiconductor nanowires in biological applications,7-9 as sensor materials,10,11 and as scaffolds for cell growth.12-20 In all of these cases, the surface chemistry of the nanowires is important and *Corresponding author. (Tel) þ1-512-471-5633. (Fax) þ1-512-471-7060. E-mail:
[email protected]. (1) Lozano, K.; Hernandez, C.; Petty, T. W.; Sigman, M. B.; Korgel, B. J. Colloid Interface Sci. 2006, 297, 618–624. (2) Ghezelbash, A.; Koo, B.; Korgel, B. A. Nano Lett. 2006, 6, 1832–1836. (3) Marshall, B. D.; Davis, V. A.; Lee, D. C.; Korgel, B. A. Rheol. Acta 2009, 48, 589–596. (4) Baker, J. L.; Widmer-Cooper, A.; Toney, M. F.; Geissler, P. L.; Alivisatos, A. P. Nano Lett. 2010, 10, 195–210. (5) Korgel, B. A. AIChE J. 2009, 55, 842–848. (6) Smith, D. A.; Holmberg, V. C.; Korgel, B. A. ACS Nano 2010, 4, 2356–2362. (7) Yi, C.; Fong, C.-C.; Chen, W.; Qi, S.; Lee, S.-T.; Yang, M. ChemBioChem 2007, 8, 1225–1229. (8) Berthing, T.; Sørensen, C. B.; Nyga˚rd, J.; Martinez, K. L. J. Nanoneurosci. 2009, 1, 3–9. (9) Jiang, K.; Coffer, J. L.; Gillen, J. G.; Brewer, T. W. Chem. Mater. 2010, 22, 279–281. (10) Wang, S.; Wang, H.; Jiao, J.; Chen, K.-J.; Owens, G. E.; Kamei, K.-I.; Sun, J.; Sherman, D. J.; Behrenbruch, C. P.; Wu, H.; Tseng, H.-R. Angew. Chem., Int. Ed. 2009, 48, 8970–8973. (11) Patolsky, F.; Zheng, G.; Lieber, C. M. Nanomedicine 2006, 1, 51–65. (12) Nagesha, D. K.; Whitehead, M. A.; Coffer, J. L. Adv. Mater. 2005, 17, 921–924. (13) Kim, W.; Ng, J. K.; Kunitake, M. E.; Conklin, B. R.; Yang, P. J. Am. Chem. Soc. 2007, 129, 7228–7229. (14) Qi, S.; Yi, C.; Chen, W.; Fong, C.-C.; Lee, S.-T.; Yang, M. ChemBioChem 2007, 8, 1115–1118. (15) Hallstrom, W.; Ma˚rtensson, T.; Prinz, C.; Gustavsson, P.; Montelius, L.; Samuelson, L.; Kanje, M. Nano Lett. 2007, 7, 2960–2965. (16) Chen, L.; Liu, M.; Bai, H.; Chen, P.; Xia, F.; Han, D.; Jiang, L. J. Am. Chem. Soc. 2009, 131, 10467–10472. (17) Qi, S.; Yi, C.; Ji, S.; Fong, C.-C.; Yang, M. ACS Appl. Mater. Interfaces 2009, 1, 30–34. (18) Jiang, K.; Fan, D.; Belabassi, Y.; Akkaraju, G.; Montchamp, J.-L.; Coffer, J. L. ACS Appl. Mater. Interfaces 2009, 1, 266–269. (19) H€allstr€om, W.; Lexholm, M.; Suyatin, D. B.; Hammarin, G.; Hessman, D.; Samuelson, L.; Montelius, L.; Kanje, M.; Prinz, C. N. Nano Lett. 2010, 10, 782–787. (20) Shalek, A. K.; Robinson, J. T.; Karp, E. S.; Lee, J. S.; Ahn, D.-R.; Yoon, M.-H.; Sutton, A.; Jorgolli, M.; Gertner, R. S.; Gujral, T. S.; MacBeath, G.; Yang, E.-G.; Park, H. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 1870–1875.
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typically requires modification, especially when the nanowires must be dispersed in aqueous solutions. Semiconductor nanowires without surface passivation do not disperse well in aqueous solution and biological applications necessitate nanowires that are chemically stable, dispersible in aqueous solvents, and biocompatible; functional molecules such as proteins are also often desired on the nanowires.20 To date, there are few methods available for forming stable aqueous dispersions of semiconductor nanowires, and there is a need for general methods to modify semiconductor nanowire surfaces chemically with organic molecules that prevent chemical degradation, provide dispersibility in aqueous media, enhance biocompatibility and that can provide a docking platform for further conjugation with a sophisticated combination of biomolecules. In this article, we focus on the chemical modification of germanium (Ge) nanowires with passivating organic monolayers that impart hydrophilic carboxyl groups for dispersibility in water and as a conjugation platform. Attachment of polyethylene glycol (PEGylation) to the carboxyl-functionalized nanowires is also demonstrated. Underlying these studies is the use of the supercritical fluid-liquid-solid (SFLS) process to grow Ge nanowires. This solvent-based approach provides significant quantities (i.e., >100 mg) of nanowires and is a convenient way to modify the nanowire surface chemically after synthesis because reactants can be fed directly into the reactor for in situ surface modification without the need to remove the nanowires from the reactor. This technique has been used to graft alkenes and thiols to Ge nanowire surfaces via in situ thermal hydrogermylation and thiolation, forming hydrophobic alkyl monolayers that prevent oxidation and provide dispersibility in organic solvents.21-23 In aqueous media, surface passivation is even more important because “bare” Ge nanowires oxidize and rapidly dissolve in water.21 PEGylation chemistry has been developed for bulk Ge substrates; however, these methods are not readily applied to nanowires because they have relied on the use of alkyl trichlorosilane or trimethoxysilane as linkages to the substrate.24,25 (21) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466–15472. (22) Holmberg, V. C.; Korgel, B. A. Chem. Mater. 2010, 22, 3698–3703. (23) Wang, D.; Chang, Y.-L.; Liu, Z.; Dai, H. J. Am. Chem. Soc. 2005, 127, 11871–11875. (24) Voue, M.; Goormaghtigh, E.; Homble, F.; Marchand-Brynaert, J.; Conti, J.; Devouge, S.; De Coninck, J. Langmuir 2007, 23, 949–955.
Published on Web 08/10/2010
DOI: 10.1021/la102124y
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These silanization agents do not react directly with the Ge surface and require pretreatment with nitric acid, hydrogen peroxide, and either oxalic or ethanedioic acid.24,25 Ge nanowires cannot withstand these chemically harsh pretreatments. Consider, for example, that hydrogen peroxide etches Ge at a rate of 40 nm/min.26,27 It is very difficult to control this kind of etching chemistry on nanowires that are about 40-50 nm in diameter. Therefore, direct surface functionalization is needed in the case of nanowires. Here, we exploit in situ thermal thiolation to anchor carboxylic acid groups onto the surfaces of Ge nanowires for subsequent molecular conjugation. As an example, Ge nanowires were passivated with 11-mercaptoundecanoic acid and then PEGylated using carbodiimide coupling chemistry. The PEGylated nanowires form very stable dispersions in DMSO and aqueous solutions over a wide range of pH and ionic strength. Carboxylic acidfunctionalized nanowires should provide a general platform for conjugating a wide array of molecules, including polymers, proteins, and other biomolecules following similar procedures.
2. Experimental Section A. Nanowire Synthesis and Surface Functionalization. a. Nanocrystal Seed Preparation. Two-nanometer-diameter
Au nanocrystals were prepared by the method of Brust et al.28 A 12.5 mL solution of 200 mM tetraoctylammonium bromide dissolved in toluene was combined with an 18 mL solution of 27 mM hydrogen tetrachloroaurate(III) trihydrate dissolved in deionized water and stirred vigorously for 30 min. The aqueous phase was then removed and discarded, and 0.5 mmol of 1-dodecanethiol was added to the solution, followed by a 15 mL solution of 0.4 M sodium borohydride in deionized water. The solution was allowed to stir for 2 h, and the organic phase was separated from the aqueous phase, which was discarded once again. The 2-nmdiameter Au nanocrystals were precipitated via centrifugation using methanol as an antisolvent, redispersed in anhydrous benzene, and stored in a nitrogen glovebox. b. Germanium Nanowire Growth. Ge nanowires were prepared by the supercritical fluid-liquid-solid (SFLS) growth process.29,30 A 30 mL solution of anhydrous benzene containing 15 mg/L Au nanocrystals and 35 mM diphenylgermane was prepared in a nitrogen-filled glovebox. A 10 mL nitrogenfilled titanium tubular reactor was heated to 380 °C, filled with anhydrous benzene, and pressurized to 6.5 MPa using a highperformance liquid chromatography (HPLC) pump. Nanowire growth was carried out by using the HPLC pump to inject the Au nanocrystal and diphenylgermane precursor solution into the reactor at a rate of 0.5 mL/min for 40 min. After injection, the reactor was cooled to 80 °C for passivation and functionalization.
c. Mercaptoundecanoic Acid Surface Functionalization. Mercaptoundecanoic acid was attached to the Ge nanowire surface by in situ thermal thiolation.21,22 In an inert atmosphere, 1 g of 11-mercaptoundenoic acid was dissolved in anhydrous benzene to form a 12 mL solution. Ten milliliters of this solution was injected into the nanowire reactor, the pressure was raised back to 6.5 MPa, and the reactor was allowed to incubate at 80 °C for 2 h. The reactor was then cooled to room temperature, and the mercaptoundecanoic acid-functionalized nanowires were (25) Devouge, S.; Contri, J.; Goldsztein, A.; Gosselin, E.; Brans, A.; Voue, M.; De Coninck, J.; Homble, F.; Goormaghtigh, E.; Marchand-Brynaert, J. J. Colloid Interface Sci. 2009, 332, 408–415. (26) Onsia, B.; Conrad, T.; De Gendt, S.; Heyns, M.; Hoflijk, I.; Mertens, P.; Meuris, M.; Raskin, G.; Sioncke, S.; Teerlinck, I.; Theuwis, A.; Van Steenbergen, J.; Vinckier, C. Solid State Phenom. 2005, 103, 27–30. (27) Ardalan, P.; Musgrave, C. B.; Bent, S. F. Langmuir 2009, 25, 2013–2025. (28) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Am. Chem. Soc., Chem. Commun. 1994, 7, 801–802. (29) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424–1429. (30) Hanrath, T.; Korgel, B. A. Adv. Mater. 2003, 15, 437–440.
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Figure 1. ATR-FTIR spectra of 1-dodecanethiol-functionalized Ge nanowires (black), 11-mercaptoundecanoic acid-functionalized Ge nanowires (red), neat PEG-amine (green), and PEGylated Ge nanowires (blue). collected and washed with a 2:1:1 volume mixture of chloroform/ toluene/ethanol in order to remove any unreacted phenylgermanes and excess mercaptoundecanoic acid. The nanowire dispersion was centrifuged at 8000 rpm for 5 min, and the resulting nanowire precipitate was redispersed in dimethylsulfoxide (DMSO). The nanowires were transferred from DMSO to water by evaporating the DMSO solvent at 500-700 mTorr for 24 h. The 1-dodecanethiol-functionalized Ge nanowire control shown in Figure 1 was prepared using the above procedure using a solution consisting of 4 mL of 1-dodecanethiol and 8 mL of anhydrous benzene in place of the mercaptoundecanoic acid solution. d. Germanium Nanowire PEGylation. PEGylation of the carboxylic acid-terminated Ge nanowires was performed using EDC/NHS chemistry.31 PEGylation was carried out using a 1000 molecular weight poly(oxyethylene)-poly(oxypropylene) amine polymer, CH3(OCH2CH2)19(OCH2CH(CH3))3NH2, also known as Jeffamine M-1000 (Huntsman Advanced Materials). On the benchtop at room temperature, 4 mg of mercaptoundecanoic acid-functionalized Ge nanowires was stirred in 4 mL of DMSO. N-Hydroxysuccinamide (NHS, 25 μmol) and Jeffamine M-1000 (31) Sehgal, D.; Vijay, I. K. Anal. Biochem. 1994, 218, 87–91.
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Figure 2. Schematic of Ge nanowire functionalization with mercaptoundecanoic acid, followed by PEGylation.
Figure 3. Transmission electron microscopy (TEM) images of a PEGylated Ge nanowire (left) and a mercaptoundecanoic acid-functionalized Ge nanowire (right). (250 μmol) were added to the solution, followed by N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide (EDC) hydrochloride (25 μmol). After the solution was stirred for 3 h, the nanowires were precipitated by centrifugation at 12 000 rpm for 5 min and redispersed in DMSO. As with the carboxylated nanowires, the PEGylated nanowires were transferred from DMSO to water by evaporating the DMSO solvent at 500-700 mTorr for 24 h.
B. Nanowire Characterization. a. ATR-FTIR Spectroscopy. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained on a Thermo Mattson Infinity Gold FTIR spectrometer equipped with a Spectra-Tech Thermal ARK attenuated total reflectance module. FTIR spectra were recorded for dodecanethiol-functionalized Ge nanowires, mercaptoundecanoic acid-functionalized Ge nanowires, neat Jeffamine Langmuir 2010, 26(17), 14241–14246
M-1000, and PEGylated Ge nanowires. Prior to FTIR spectra collection, the dodecanethiol- and mercaptoundecanoic acid-functionalized nanowires were washed repeatedly in a 2:1:1 volume mixture of chloroform/toluene/ethanol in order to remove any residual unreacted dodecanethiol or mercaptoundecanoic acid and then redispersed in chloroform. To remove any excess PEG, the PEGylated Ge nanowires were washed by repeatedly redispersing the nanowires in fresh DMSO and reprecipitating them by centrifugation at 12 000 rpm. It was then necessary to remove all of the residual DMSO from the nanowire precipitate by placing the nanowires under high vacuum overnight and then redispersing the resulting product in chloroform. Spectra were recorded under a nitrogen atmosphere after forming thin films of material on the zinc selenide ATR crystal. Ge nanowire thin films were formed by drop DOI: 10.1021/la102124y
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casting the chloroform dispersions directly onto the surface of the ATR crystal and allowing the chloroform to evaporate. The Jeffamine M-1000 spectrum was recorded from a thin film of PEG-amine swabbed directly onto the surface of the ATR crystal.
b. High-Resolution Transmission Electron Microscopy (HRTEM). Samples were prepared by forming a dilute dispersion (1.5 M) salt concentrations. The ability to attach organic macromolecules to the Ge nanowires and make them dispersible in aqueous solution is an important step toward designing implantable nanowire scaffolds to modify natural biological structures. Acknowledgment. This work was funded in part by the Robert A. Welch Foundation (grant no. F-1464) and the National Institutes of Health (grant no. R01 CA132032). V.C.H. also acknowledges the Fannie and John Hertz Foundation and the National Science Foundation Graduate Research Fellowship Program for financial support. We thank the Huntsman Petrochemical Corporation for the free sample of Jeffamine M-1000.
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