J. Phys. Chem. B 2001, 105, 1683-1686
1683
Fabrication of Nanocrystal Tube Using Peptide Tubule as Template and Its Application as Signal-Enhancing Cuvette Hiroshi Matsui,* Su Pan, and Gary E. Douberly, Jr. UniVersity of Central Florida, Department of Chemistry, Orlando, Florida 32816 ReceiVed: August 31, 2000; In Final Form: December 5, 2000
A method of fabricating nanocrystal tubules was developed by coating carboxylic acid-thiol capped Au nanocrystals on peptide tubule templates to form the Au nanocrystal tubules. Hydrogen bonding between the amide groups of the tubules and the carboxylic acid groups of the nanocrystals was used as a driving force for this assembly process. The Au nanocrystal tubules with hollow structures were applied to hold and analyze trace molecules as signal-enhancing cuvettes for Raman spectroscopy. The Raman scattering of trace Rhodamine B on the Au nanocrystal-coated tubule was enhanced by a factor of 105.
The application of the quantum size effect of nanocrystals has led to state-of-the-art in optical and electrical devices.1,2 To build nanocrystal-based devices, it is necessary to arrange the nanocrystals into three-dimensional superlattices.3 It is useful to construct nanocrystals into wires, which can cross-link electric components or be assembled as arrays. If these types of wire placements are possible, the nanocrystal tubules could potentially serve as building blocks to assemble chemical sensors, catalysts, and data storage devices.4 This paper introduces a new method to produce nanocrystal tubules using self-assembled bolaamphiphile peptide tubules as templates. While there has been rapid progress in the area of nanostructure fabrication using lithographic techniques, precise control and reproducibility of the fabrication are more achievable by molecular self-assemblies via chemical interactions.5-7 Assembled structures of bis(N-R-amido-glycylglycine)-1,7heptane dicarboxylate, one of the peptide bolaamphiphiles, display sensitivity to pH: the bolaamphiphile molecules grow to a tubule in an acidic solution (pH 6) with the diameters of 20 nm to 3 µm.8,9 This tubular structure is assembled via intermolecular hydrogen bonds between amide groups and between carboxylic acid groups (Figure 1a). Interestingly, the free amide groups in bolaamphiphile tubules can intercalate metal ions.10 These neighboring amide groups capture metal ions such as Pt, Pd, Cu, Co, and Ni to form square planar complexes (Figure 1a), and the continuing electroless plating results in stable metallic coatings.10 These free amide sites on the tubules can also form hydrogen bonds with molecules containing amide or carboxylic acid groups. Thus, nanocrystals decorated with these functional groups may organize on the bolaamphiphile peptide tubule surface and produce nanocrystal coatings by using the hydrogen bonds as driving forces for this assembly process. This process is illustrated in Figure 1b. One bolaamphiphile molecule contains at least one free amide site in the tubule assembly,8 and a large number of the binding sites should be available on the tubule to form a uniform nanocrystal coating. Carbon nanotubes were also coated by inorganics nucleated at carboxylic acid groups, which were created by oxidation of the carbon nanotube surfaces at the defect sites.11 But, the oxidized areas are localized to the * To whom correspondence should be addressed. E-mail: hmatsui@ pegasus.cc.ucf.edu.
Figure 1. (a) Assembled structure of bolaamphiphile peptide tubules: A pair of the bolaamphiphile molecules are connected by hydrogen bonds between two COOH groups and via acid-acid dimer interactions in the y direction. An intermolecular amide-amide hydrogen bond is formed along the z and along the x directions. Yellow lines represent hydrogen bonds. Free amide groups of the bolaamphiphile peptide tubules are shown with arrows. The representation of atoms is in the following colors: red-oxygen, blue-nitrogen, white-hydrogen, graycarbon. (b) Schematic representation of the carboxylic acid-thiol capped nanocrystal attachment on the bolaamphiphile peptide tubule: Hydrogen bonds between the nanocrystals and the bolaamphiphile peptide tubule are highlighted by purple circles.
edge of carbon nanotubes due to the defect distribution.12 Thus, the bolaamphiphile tubule is more effective in producing the uniform nanocrystal coating. To test this scheme, Au nanocrystals, capped by thiols with carboxylic acid end groups, were deposited on the bolaamphiphile tubule. Au nanocrystals were synthesized by using 11mercaptoundecanoic acid [HS(CH2)10COOH] as a stabilizer.13 The diameters of the Au nanocrystals are in the range from 10 to 20 nm. The carboxylic acid-terminated, thiol-capped Au nanocrystals in ethanol solution were then mixed with the bolaamphiphile
10.1021/jp003166v CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
1684 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Figure 2. Scanning electron micrograph of the carboxylic acidterminated, thiol-capped Au nanocrystal tubule.
Figure 3. Transmission electron micrographs of (a) the carboxylic acid-terminated, thiol-capped Au nanocrystal tubule with a magnification of ×20 000 (b) the alkylthiol-capped Au nanocrystal tubule with a magnification of ×20 000. The insets in (a) and (b) show the electron diffraction patterns.
peptide tubules, prepared separately by procedures previously published.8,14 This solution sat overnight to complete the Au nanocrystal deposition without disturbance. Figure 2 is a scanning electron micrograph of the Au nanocrystal tubule. The hollow structure of the bolaamphiphile tubule was preserved, and nanocrystals did not fill the cavity completely. A transmission electron micrograph of the nanocrystal tubule (Figure 3a) indicates that the Au nanocrystals are deposited on the bolaamphiphile peptide tubule template. An electron diffraction pattern of the carboxylic acid-terminated, thiol-capped Au nanocrystals on the bolaamphiphile peptide tubule (Figure 3a) consists of discrete dots and rings. This pattern indicates that the Au nanocrystals have preferred orientations. The (111), (200), (222), and (311) rings are identified as shown in Figure 3a. An electron diffraction pattern of the neat Au nanocrystal is identical with the pattern of the Au nanocrystal on the bolaamphiphile peptide tubule.
Letters Au nanocrystals, capped by alkylthiols, were also examined as a control experiment to verify the proposed hydrogen-bonding scheme between the capped Au nanocrystals and the bolaamphiphile tubule. If the uniform coatings of the alkylthiol-capped Au nanocrystals, as observed in the carboxylic acid-thiol capped Au nanocrystal coatings, are not obtained on the bolaamphiphile peptide tubule, the hydrogen bonding between the carboxylic acid group of the modified Au nanocrystals and the amide group of the bolaamphiphile peptide tubules plays a key role for the Au nanocrystal coatings. Dodecanethiol-capped Au nanocrystals were synthesized as a model for this control experiment in published procedures.15,16 The bolaamphiphile tubules were then mixed with the alkylthiolcapped Au nanocrystal solution. A transmission electron micrograph of the bolaamphiphile peptide tubules with the alkylthiol-capped Au nanocrystals is shown in Figure 3b. Uniform coating of the alkylthiol-capped Au nanocrystals was not formed, and the aggregation of the alkylthiol-capped Au nanocrystals was observed as black dots on the tubule surface. An electron diffraction of the alkylthiol-capped Au nanocrystals on the bolaamphiphile peptide tubules (Figure 3b) shows the same pattern as the neat Au nanocrystals, while the rings are not as distinguishable as those of the carboxylic acid-capped Au nanocrystals on bolaamphiphile peptide tubules. This continuous bright background arises from the bolaamphiphile peptide tubules. The results of this control experiment support the proposed coating mechanism that Au nanocrystals can be modified by thiols with carboxylic acid end groups to obtain uniform Au nanocrystal coatings on the bolaamphiphile peptide tubules via hydrogen bonds between the carboxylic acid and the amide groups of the tubule. We examined the potential application of the Au nanocrystal tubules as cuvettes for Raman microscopy. While carbon nanotubes have been the material of choice for application as nanocapillary tubes,17 the Au nanocrystal tubule can also hold a trace of samples for Raman analysis due to its hollow structure. Because the tubules are coated by the Au nanocrystals, Raman scattering intensities of trace samples on the rough Au nanocrystal surfaces are expected to be enhanced due to the surface enhanced Raman effect.18 This enhancement should make weak signals from trace molecules detectable and vibrational analysis of the spectra will enable us to do structural and dynamic studies. First, we examine Rhodamine B on the Au nanocrystal tubules. A positive charge of the Rhodamine B should have a strong electrostatic interaction with the carboxylic acid ion group, COO-, of the stabilizer on Au nanocrystals, and, thus, the Rhodamine B can be adsorbed on the Au nanocrystal tubule surfaces. To contain Rhodamine B molecules onto the Au nanocrystal-coated tubule surfaces, we mixed the Rhodamine B in ethanol solution (7.0 µM) with Au nanocrystal-coated tubules and kept the mixture without disturbance overnight. Figure 4a shows Raman spectrum of the Rhodamine B on the Au nanocrystal-coated tubule. The dotted line in Figure 4 represents Raman spectrum of the neat bolaamphiphile tubules. From comparison between observed Raman intensities of Rhodamine B peaks in Figure 4a and a neat Rhodamine B Raman spectrum, the enhancement of the Rhodamine B Raman intensity on this tubule was estimated on the order of 105. The Raman spectrum of Rhodamine B with the carboxylic acid thiolcapped Au nanocrystal colloid is shown in Figure 4b. The Raman signal enhancement of the Rhodamine B in the colloid was on the order of 104. Second, DNA was examined on the Au nanocrystal tubules. The electrostatic interaction of DNA with the stabilizer is weaker
Letters
Figure 4. Raman spectra of Rhodamine B (a) on the carboxylic acidterminated, thiol-capped Au nanocrystal tubules (b) with the carboxylic acid-terminated, thiol-capped Au nanocrystal colloid. The dotted line represents Raman spectrum of a neat bolaamphiphile tubule, a template of the Au nanocrystal tubule.
J. Phys. Chem. B, Vol. 105, No. 9, 2001 1685 Because the tubule diameter is much larger than the molecular sizes, the adsorptions probably occur on both sides. To understand the large enhancements of Raman signals on the Au nanocrystal tubules, systematic experimental and theoretical studies are necessary. There have been many works to study the enhancement factors affected by thickness of the metal nanoparticles on substrates,21 particle size of nanocrystals,22 and distance between nanoparticles and adsorbates.23 In addition to these effects, the cylindrical geometry of the nanocrystal tubule may have some effect on electromagnetic field distribution on the surfaces and contribute to the additional enhancement factors. Optimization of coating thickness of the Au nanocrystals on the bolaamphiphile peptide tubules, the Au nanocrystal diameter, and chain length of the stabilizer will achieve higher enhancement of Raman signals. Coating Ag nanocrystals on the bolaamphiphile peptide tubules will also help increase Raman signals because enhancement factors for Ag surfaces are substantially higher than for Au.24,25 In conclusion, Au nanocrystals capped with the carboxylic acid-thiol coated the bolaamphiphile peptide tubules to form the Au nanocrystal tubules. Hydrogen bonding between the amide groups of the tubule and the carboxylic acid groups of the nanocrystals was used as a driving force for this assembly process. To apply nanocrystal tubules to optoelectronic devices or magnetic recording media, uniform size distributions of the nanocrystals is necessary for spatial organization on the template tubules.26 The Au nanocrystal tubules with a hollow structure were applied as cuvettes for Raman spectroscopy. The Raman scattering of trace Rhodamine B on the Au nanocrystal-coated tubule was enhanced by a factor of 105. Methods
Figure 5. Raman spectra of poly(dA-dT)‚poly(dA-dT) (a) on the carboxylic acid-terminated, thiol-capped Au nanocrystal tubules (b) with the carboxylic acid-terminated, thiol-capped Au nanocrystal colloid. The dotted line represents Raman spectrum of a neat bolaamphiphile tubule, a template of the Au nanocrystal tubule.
than the Rhodamine B due to negative charges of DNA. The Raman spectrum of poly(dA-dT)‚poly(dA-dT), a model of DNA, on the Au nanocrystal-coated tubule is shown in Figure 5a. The bolaamphiphile tubules (0.1 mg) were dispersed into 50 µL aqueous solution of the poly(dA-dT)‚poly(dA-dT) (1.25 mg/ mL) (Sigma). The mixtures were incubated overnight at 4 °C. Concentration of the adsorbed poly(dA-dT)‚poly(dA-dT) on the tubule was estimated as 10-14 g/µm2 by assuming that all poly(dA-dT)‚poly(dA-dT) molecules attach on the tubule surface uniformly.19 Compared with a neat poly(dA-dT)‚poly(dA-dT) Raman spectrum, we obtained the enhancement factor of poly(dA-dT)‚poly(dA-dT) on the tubule as an order of 103. The Raman spectrum of the DNA with the Au nanocrystal colloid is shown in Figure 5b. The Raman signal enhancement in the colloid was on the order of 102. The large discrepancy of the enhancement factors between Rhodamine B and poly(dA-dT)‚ poly(dA-dT) indicates that chemical interactions between the stabilizers/Au nanocrystals and adsorbed molecules play important roles for the enhancement.20 It should be noted that these Raman studies do not reveal whether these molecules bind to the inside or outside of the Au nanocrystal-coated tubules.
Synthesis of Carboxylic Acid-Thiol Capped Au Nanocrystals. The carboxylic acid-thiol capped Au nanocrystals were synthesized using a two-phase arrested growth method at room temperature. HAuCl4 (0.5960 g) was dissolved in 57 mL deionized water to form a bright yellow solution. A phasetransfer catalyst solution ([CH3(CH2)7]4NBr (1.0464 g) in chloroform (45 mL)) was added and stirred for 10 min to obtain a red organic layer. The organic layer was recovered then mixed with 0.0870 g of stabilizer (11-mercaptoundecanoic acid in 15 mL CHCl3). After complete stirring, the reducing agent (NaBH4, 0.4500 g in 51 mL deionized water) was added drop by drop through a separator funnel and stirred vigorously overnight. The precipitated nanocrystals were isolated by centrifugation (15 000 rpm) for 30 min using a 200 nm pore size membrane. The driedin-air nanocrystals were redissolved in ethanol until the solution turned to a dark purple color. Raman Microscopy. A confocal Raman microscope (LabRam, Jobin Yvon/Horiba) was used to obtain two-dimensional Raman images. A 632.8-nm line of an air-cooled He-Ne laser was injected into an integrated Olympus BX 40 microscope and focused on a spot size of approximately 0.7 µm by a 80× long working distance objective, and a holographic notch filter rejected the excitation laser line. A combination of an 1800 g/mm holographic grating and a slit size of 250 µm provided the spectral resolution at 1.8 cm-1. Raman scattering was collected by an air-cooled 1024 × 256 pixels CCD detector. Acknowledgment. This work was supported by Office of the Vice President for Research and Graduate Studies and Advanced Materials Processing and Analysis Center (AMPAC) at University of Central Florida. H.M. acknowledges Mr. Zia
1686 J. Phys. Chem. B, Vol. 105, No. 9, 2001 Ur Rahman at the University of Central Florida for the assistance in the SEM and TEM studies. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (4) Kastner, M. A. Phys. Today 1993, 24. (5) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (6) Xu, S.; Laibinis, P. E.; Liu, G. Y. J. Am. Chem. Soc. 1998, 120, 9356. (7) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665. (8) Matsui, H.; Gologan, B. J. Phys. Chem. B. 2000, 104, 3383. (9) Matsui, H.; Gologan, B.; Pan, S.; Douberly, G. E., Jr. Eur. Phys. J. D., accepted (2001). (10) Matsui, H.; Pan, S.; Gologan, B.; Jonas, S. J. Phys. Chem. B. 2000, 104, 9576. (11) Ebbesen, T. W.; Hiura, H.; Bisher, M. E.; Treacy, M. J.; ShreeveKeyer, J. L.; Haushalter, R. C. AdV. Mater. 1996, 8, 155. (12) Yao, N.; Lordi, V.; Ma, X. C.; Dujardin, E.; Krishnan, A.; Treacy, M. M. J.; Eddesen, T. W. AdV. Mater. 1998, 13, 2432. (13) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (14) Kogiso, M.; Ohnishi, S.; Yase, K.; Masuda, M.; Shimizu, T. Langmuir 1998, 14, 4978.
Letters (15) Aherne, D.; Rao, N.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 1821. (16) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (17) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (18) Emory, S. R.; Nie, S. J. Phys. Chem. B 1998, 102, 493. (19) This concentration was estimated by calculating weight of the poly(dA-dT)‚poly(dA-dT) per area of the laser spot as assuming that all poly(dA-dT)‚poly(dA-dT) molecules attach on the tubule uniformly. From the scanning electron micrograph of the carboxylic acid-thiol coated the bolaamphiphile tubules in Figure 2, the inner diameter of the tubule was measured as 2 µm. Because 0.025 mg of the poly(dA-dT)‚poly(dA-dT) and 0.1 mg of the bolaamphiphile molecules were assembled, we calculated 4.70 × 10-11 mg of the poly(dA-dT)‚poly(dA-dT) molecules on the tubule surface in the 1 µm2 of the laser diameter using the density of the bolaamphiphile tubules as 1.33 g/cm3. (20) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932. (21) Lee, K. C.; Pai, S. T.; Chang, Y. C.; Chen, M. C.; Li, W.-H. Mater. Sci. Eng. B 1998, 52, 189. (22) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729. (23) Keating, C. D.; Kovaleski, K. M.; Natan, M. J. J. Phys. Chem. B 1998, 102, 9404. (24) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634. (25) Ahern, A. M.; Garrell, R. L. Langmuir 1991, 7, 254. (26) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397.
