Immobilization of Gold Nanorods onto Acid-Terminated Self

ACS Applied Materials & Interfaces 2016 8 (1), 634-642. Abstract | Full Text ... Wenqiong Tang , D. Bruce Chase , and John F. Rabolt. Analytical .... ...
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Langmuir 2004, 20, 7117-7122

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Immobilization of Gold Nanorods onto Acid-Terminated Self-Assembled Monolayers via Electrostatic Interactions Anand Gole, Christopher J. Orendorff, and Catherine J. Murphy* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Received April 14, 2004. In Final Form: June 10, 2004 We report the immobilization of gold nanorods onto self-assembled monolayers (SAMs) of 16mercaptohexadecanoic acid (16-MHA). The simple two step protocol involves formation of a SAM of 16MHA molecules onto gold-coated glass slides and subsequent immersion of these slides into the gold nanorod solution. The nanorods, formed by a seed-mediated, surfactant-assisted synthesis protocol, are stabilized in solution due to surface modification by the surfactant cetyltrimethylammonium bromide (CTAB). Attractive electrostatic interactions between the carboxylic acid group on the SAM and the positively charged CTAB molecules are likely responsible for the nanorod immobilization. UV-vis spectroscopy has been used to follow the kinetics of the nanorod immobilization. The nature of interaction between the gold nanorods and the 16-MHA SAM has been probed by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the immobilized rods is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. SEM was also used to determine the density of the immobilized nanorods as a function of the pH of immobilization. Control over the surface coverage of the immobilized gold nanorods has been demonstrated by simple pH variation. Such well-dispersed immobilized gold nanorods with control over the surface coverage could be interesting substrates for applications such as surfaceenhanced Raman spectroscopy (SERS).

Introduction Currently there is a great deal of interest in metallic nanorods due to their shape-dependent optoelectronic properties.1 Appearance of a longitudinal plasmon resonance,1 strongsurface-enhancedRamanscattering(SERS),2 fluorescence,3 and anisotropic chemical reactivity4 are important properties of metallic nanorods that can be exploited for various applications. Metallic/semiconductor nanorods can be synthesized either by using rigid templates such as membranes, zeolites, and nanotubes,5-8 or by using surfactants as directing agents for electrochemical9 and seed-mediated growth methods.10-15 Our group10-14 is actively involved in fine-tuning the colloidal synthesis of gold and silver nanorods. In this route, it was observed that fairly monodisperse, stable, gold nanorods with different aspect ratios could be synthesized in water by the seed-mediated protocol in the presence of a directing agent such as cetyltrimethyl-

ammonium bromide (CTAB). In such a process, the nanorods are stabilized by a bilayer of CTAB, which gives them a net positive charge.16 For many sensing/catalytic applications in nanotechnology, one requires a programmed assembly of nanospheres/nanorods onto 2D surfaces. To date a number of strategies have been adopted to assemble metal/semiconductor nanospheres onto planar surfaces. Some such methods include organization of nanospheres via simple solvent evaporation,17,18 attachment onto self-assembled monolayers (SAMs) via covalent interactions,19-23 and by the Langmuir-Blodgett (LB) technique.24-26 Immobilization of nanorods, surprisingly, has not received comparable attention. El-Sayed et al. 27 have demonstrated that simple solvent evaporation leads to the organization of gold nanorods into one, two, and three-dimensional structures. Others have used the versatile Langmuir-Blodgett (LB) technique for the assembly of hydrophobized nanorods of

* Corresponding author e-mail: [email protected].

(16) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (18) Connolly, S.; Fullam, S.; Korgel, B.; Fitzmaurice, D. J. Am. Chem. Soc. 1998, 120, 2969. (19) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (20) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (21) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (22) Bandyopadhyay, K.; Patil, V.; Vijjayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (23) Rizzo, R.; Fitzmaurice, D.; Hearn, S.; Hughes, G.; Spoto, G.; Ciliberto, E.; Kerp, H.; Schropp, R. Chem. Mater. 1997, 9, 2969. (24) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (25) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (26) Sastry, M. Nanoparticle thin films: An approach based on selfassembly. In Handbook of Surfaces and Interfaces of Materials; Academic Press: San Diego, 2001; Vol. 3. (27) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635.

(1) El-Sayed, M. Acc. Chem. Res. 1999, 34, 257. (2) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17. (3) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517. (4) Jana, N. R.; Gearheart, L.; Obare, S. O. Murphy, C. J. Langmuir 2002, 18, 922. (5) Martin, C. R. Chem. Mater. 1996, 8, 1739. (6) Wu, C.-G.; Bein, T. Science 1994, 266, 1013. (7) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M. L.; Lyon, A.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 1999, 11, 1021. (8) Wong, E. W.; Maynor, B. W.; Burns, L. D.; Lieber, C. M. Chem. Mater. 1996, 8, 2041. (9) Ying, Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (10) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (11) Jana, N. R.; Gearheart, L. A.; Murphy, C. J. Chem. Commun. 2001, 617. (12) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (13) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (14) Busbee, B. D.; Obare, S. O.; Murphy, C. J. Adv. Mater. 2003, 15, 414. (15) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957.

10.1021/la049051q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/21/2004

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Scheme 1: Cartoon of the Immobilization of CTAB-Modified Gold Nanorods onto SAMs of 16-MHA

BaCrO4, BaWO4, and gold at the air-water interface.28 Alivisatos et al.29 demonstrated the macroscopic alignment of CdSe nanorods in a nematic liquid-crystalline phase and the superlattice structures of these nanorods formed upon deposition on a substrate. We have recently demonstrated a spontaneous self-assembly of gold nanorods in concentrated solutions to produce liquid crystalline arrays.30 A microfluidic-based approach for the patterned assembly of selectively surface functionalized magnetic nanowires has been shown by Salem et al.31 Specific interactions such as the DNA-directed method for the assembly of gold nanowires has been demonstrated by Mbindyo et al.32 Most of the above-mentioned methods use either nonspecific interactions27-30 or specialized techniques31,32 for the organization of nanorods onto 2D surfaces. A simple method for programmed assembly of nanorods using noncovalent weak interactions is yet to be investigated. In this paper, we demonstrate that electrostatic interactions can be successfully employed for the organization of nanorods onto surfaces. Earlier, electrostatic interactions have been used to organize nanospheres onto SAMs,33 and we have extended this approach for organization of nanorods. A simple two-step protocol for immobilization involves SAM formation onto gold-coated glass slides and their subsequent immersion into an aqueous solution of gold nanorods (Scheme 1). We believe that attractive electrostatic interactions between the positively charged CTAB bilayer on the gold nanorods and the negatively charged carboxylic acid functionality of the SAM are primarily responsible for the immobilization. Furthermore, the beauty of using electrostatic interactions is that the density of immobilized rods can be controlled by simple pH variation. Experimental Section Materials. Chloroauric acid (HAuCl4 • 3H2O), trisodium citrate, sodium borohydride (NaBH4), ascorbic acid, and 16mercaptohexadecanoic acid (16-MHA) were obtained from Aldrich and used as received. Cetyltrimethylammonium bromide (CTAB) was obtained from Sigma and used without further purification. All the glassware was cleaned by aqua regia and rinsed with deionized water prior to experiments. (28) Yang, P.; Kim, F. ChemPhysChem 2002, 3, 503. (29) Li, L. S.; Alivisatos, A. P. Adv. Mater. 2003, 15, 408. (30) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909. (31) Salem, A. K.; Chao, J.; Leong, K. W.; Searson, P. C. Adv. Mater. 2004, 16, 268. (32) Mbindyo, J. K. N.; Reiss, B. D.; Martin, B. R.; Keating, C. D.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 2001, 13, 249. (33) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234.

Gole et al. Synthesis of Gold Nanorods. Gold nanorods were synthesized by the seed-mediated-template assisted protocol as has been described earlier.10 Typically, this involves the synthesis of a gold nanoparticle seed solution by reduction of an aqueous solution containing 2.5 × 10-4 M HAuCl4 and 2.5 × 10-4 M trisodium citrate by 0.1 M of ice-cold aqueous solution of sodium borohydride (NaBH4). After 3 h, this seed solution was used for the synthesis of gold rods by a three-step seeding method. Three test tubes (labeled A, B, and C), each containing 9 mL of growth solution, consisting of 2.5 × 10-4 M HAuCl4 and 0.1 M CTAB, were mixed with 0.05 mL of 0.1 M freshly prepared ascorbic acid. Next, 1.0 mL of seed solution was mixed with sample A. After 15 s, 1.0 mL of sample A was mixed with sample B. After 30 s, 1.0 mL of sample B was further added to sample C. Solution C was kept at 25 °C for a period of 12 h and further centrifuged, and the pellet was resuspended in deionized water to get rid of excess CTAB. This method gave 16 ( 2.5 aspect ratio (∼400 nm long x ∼25 nm wide) gold nanorods surface-modified with a bilayer of CTAB. Our protocol for the nanorod synthesis uses a high concentration of CTAB (0.1 M). For most of the characterization and further applications, it is essential to remove excess uncoordinated CTAB molecules. This can be achieved by centrifugation (twice) of the gold nanorod solution at 7000 rpm. Experimentally, it was observed that this might lead to aggregation or “crashing out” of the gold nanorods if continued more than two times. This indicates that some amount of CTAB is essential to prevent aggregation of the gold nanorods. This concentration of CTAB is low as shown by FTIR measurements and decreases as the number of cycles of centrifugation is increased (Supplementary information Figure S1). Formation of SAM-coated Slides. Glass slides (dimensions: 3 × 1 in. surface area) used for immobilization were cleaned thoroughly using piranha solution (H2SO4: H2O2 ) 3:1), rinsed in deionized water and dried in flowing nitrogen. After initially depositing a layer (10 nm) of chromium, 200-nm thick gold films were sputter-coated onto these slides. These slides were immersed in 10-3 M ethanolic solution of 16-mercaptohexadecanoic acid (16-MHA) for a period of 36 h. After this time, the slides were rinsed in ethanol, dried in flowing nitrogen, and characterized by Fourier transform infrared spectroscopy (FTIR) measurements in the grazing incidence mode. The methylene symmetric and antisymmetric vibrations were used to detect the formation of the SAM. Immobilization of Gold Nanorods onto SAM Films. The SAM-coated glass slides were immersed into the gold nanorod solution held at pH 6.5 for a period of 4 h. Thereafter, the films were rinsed in deionized water (to remove unbound gold rods) and dried in flowing nitrogen. These nanorod films were used for all further characterization. A pH-dependent immobilization study was also performed wherein the gold nanorod solution pH was varied between 1 and 11 pH units and subsequently used for immobilization onto 16-MHA SAMs. UV-vis Spectroscopy Measurements. The kinetics of immobilization of gold nanorods onto 16-MHA SAMs was performed using UV-vis spectroscopy measurements on a Varian model Cary 500 Scan UV-vis spectrophotometer. The 16-MHAcoated SAM surface (formed on a gold-coated glass slide) was immersed into gold nanorod solution (pH 6.5). The transverse plasmon resonance band wavelength at ca. 500 nm was used to monitor the kinetics. At appropriate time intervals, aliquots of gold nanorod solution were drawn and characterized by UV-vis spectroscopy. An immersion time of 4 h was determined from the UV-vis spectroscopy measurements to be suitable for the immobilization experiments. As a control experiment, UV-vis studies were also performed on the gold nanorod solution in which a 200-nm thick as-deposited gold film (without 16-MHA SAM) was immersed. Fourier Transform Infrared Spectroscopy (FTIR) Measurements. FTIR measurements were used as a tool to probe the nature of the interaction between the SAM and the gold nanorods and to confirm the presence of CTAB on the gold nanorods. The SAM-coated films were characterized by FTIR measurements before and after immobilization of gold nanorods, using a Matteson model Infinity series FTIR instrument. Nitrogen

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purging for 1 h prior to measurements in the compartment and the detector was crucial to obtain a good signal-to-noise ratio. The samples were scanned in the grazing incidence mode, and 1000 scans of each sample were recorded. The methylene symmetric and anti-symmetric modes of vibration arising from the CTAB molecules were used as an indicator for the presence of CTAB. In addition, the loss of CTAB molecules upon washing was followed by FTIR. Specifically, gold nanorod solutions before and after centrifugation (1-3 times at 7000 rpm), were drop-dried on 4-cm2 Si (111) wafers. Si (111) wafers were used in these experiments because they are relatively inert to interactions with CTAB. The samples were analyzed on a Nexus Thermo-Nicolet 470 series FTIR instrument coupled with a Thermo-Nicolet Continuum FTIR microscope in the reflection mode. Using a resolution of 4 cm-1, 128 scans of each sample were recorded. Surface Morphology Studies. Scanning electron microscopy (SEM) measurements were performed onto the nanorod films using the Philips XL 30 ESEM instrument. Images were recorded at various magnifications. SEM was also used to determine the density of immobilized nanorods as a function of pH, by individual counting of the nanorods. To study the removal of gold nanorods from a gold nanorod film (formed at pH 6.5), the film was immersed in a pH-3 solution (aqueous) for a period of 4 h and further analyzed by SEM. The immobilized nanorods were also characterized by atomic force microscopy (AFM) using a commercial Digital Instruments (Santa Barbara, CA) Multimode scanning probe microscope operated in the contact mode, with sharpened silicon nitride tips.

Results and Discussion The seed-mediated surfactant-assisted growth mechanism yields gold nanorods with CTAB molecules preferentially binding to the (100) crystal face of the gold nanorods.10-13,30 El-Sayed and co-workers15,34 have suggested that the CTAB binding takes place via the AuBr-surfactant complex. The bromide ion is the bridge between the gold surface and the positively charged quaternary nitrogen of the surfactant. Theoretical studies show that the bound bromide ions to the metal surface possess high electron density, which are suitable for binding to the electron deficient quaternary nitrogen of the headgroups.35 It has been previously established by Sastry et al.36 that bilayer formation is more facile on nanocurved surfaces compared to 2D surfaces. Similarly, CTAB molecules are likely to form a bilayer on the gold nanorod surfaces. It was found by El-Sayed and Nikoobakht16 by TGA analysis that the temperature required to decompose CTAB bound to gold nanorods was higher compared to free CTAB molecules. This suggests that the bilayer on the nanorods is highly stable. The bilayer capping of the gold nanorods leads to a net positive charge on the gold nanorods.16 Electrostatic attachment of the positively charged rods to negatively charged silica particles has been previously studied by El-Sayed et al.34 Herein we demonstrate the electrostatic attachment of CTAB-coated gold nanorods onto 2-D SAM surfaces. As mentioned in the Experimental Section, UV-vis spectroscopy was used as a tool to monitor the kinetics of the gold nanorod immobilization process. SAM-coated glass slides were immersed in the gold nanorod solution and aliquots of the solution were drawn and monitored by UV-vis spectroscopy to measure depletion of gold nanorods from solution and thereafter presumed adsorption to the SAM surface. Figure 1A shows the UV-vis kinetics (34) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17. (35) Koglin, E.; Tarazona, A.; Kreising, S.; Schwuger, M. J. Colloids Surf., A 1997, 123-124, 523. (36) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.

Figure 1. (A) UV-vis spectra of supernatant solution of gold nanorods as a function of immersion time of 16-MHA SAMcoated slide in the nanorod solution. The inset shows the data for variation of absorbance (at 500 nm) for supernatant solutions as a function of time when the 16-MHA SAM surface (circles) and gold-coated glass slide (squares) are immersed in the nanorod solutions. (B) and (C) FTIR spectra of 16-MHA SAM before (curve 1) and after immobilization of gold nanorods (curve 2). Curve 3 corresponds to CTAB-coated gold nanorods on a Si (111) surface with no SAM. Assignments: a: 2953 cm-1, b: 2920 cm-1, c: 2850 cm-1, d: 1731 cm-1, e: 1548 cm-1, f: 1460 cm-1, g: 1186 cm-1, h: 1128 cm-1.

data (curves 1-6: time t)0 to t ) 275 min). A clear drop in the absorbance intensity as a function of time is observed, suggesting presumable adsorption of the gold nanorods onto the SAM surface, which is completed in ca. 4 h. The plasmon intensity variation at 500 nm as a function of time has been shown in the inset of Figure 1A (circles). A control experiment was also performed wherein a gold-coated glass slide without 16-MHA SAM was immersed in gold nanorod solution for a period of ca. 4h. As in the previous case, aliquots of the gold nanorod solution were drawn at appropriate time intervals and studied by UV-vis spectroscopy. A small decrease in the absorbance intensity at some points followed by increase in intensity at some other points is observed (Inset of Figure 1A, squares). But there is an overall decrease in the absorbance intensity suggesting some nonspecific binding of CTAB to the gold film surface as also observed by SEM measurements (see below). The overall intensity change in this case is less compared to that observed for gold nanorod immobilization on the 16-MHA SAM film (Inset of Figure 1A, circles) indicating the influence of electrostatic interactions in the previous case. After rinsing (in deionized water) and drying (in flowing nitrogen), the films were further characterized by FTIR

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spectroscopy measurements. Figure 1B shows the FTIR spectra of 16-MHA SAM before (curve 1) and after (curve 2) immobilization of gold nanorods. A number of spectral features can be seen in both the curves. The methyl band at 2953 cm-1 (Figure 1B, feature a, curve 2) is due to the presence of methyl groups of the CTAB molecules on the gold nanorods and compares well with the FTIR spectra of Nikoobakht and El-Sayed.16 The methylene symmetric and antisymmetric vibrations can be seen at 2920 and 2850 cm-1, respectively, for the SAM surface before and after gold nanorod immobilization (Figure 1B, features b and c, curves 1 and 2). Small features in the 3300-3500 cm-1 range (Figure 1B, curve 2) arise possibly due to the water molecules coordinated to the ammonium headgroup of the gold nanorods. A few broad C-N+ bands can also be seen in the 1200-1000 cm-1 range (Figure 1C, features g and h, curve 2) and are close to that reported by Nikoobakht and El-Sayed16 further indicating the presence of CTAB-coated gold nanorods. The nature of interaction between the gold nanorods and the SAM surface can be probed by FTIR measurements in the 1800-1400 cm-1 range (Figure 1C). A band at ca. 1730 cm-1 (Figure 1C, feature d, curve 1) is assigned to the carbonyl stretch vibration mode of the carboxylic acid group on the SAM surface.37,38 This band disappears after immobilization of nanorods (Figure 1C, curve 2); new bands centered at ca. 1548 cm-1 (Figure 1C, feature e, curve 2) and a very weak peak at 1513 cm-1 appear that correspond to the carboxylate asymmetric stretch modes.37,39 This indicates formation of a salt, possibly between the ammonium headgroup of the CTAB-coated gold rods with the carboxylic acid groups of the SAM.39,40 Such shifts have been observed previously during electrostatic attachment of ions/nanoparticles with fatty acid films and SAMs.39-41,43 This is consistent with electrostatic interaction in our case as at pH 6.5 the rods are highly positively charged due to the tetramethylammonium ion (NR4+) and the SAM bears a net negative charge due to the deprotonation of carboxylic acid group (pKa of 16-MHA SAM ∼ 5.2).42 It is essential to compare the spectral features for the SAM film (Figure 1, curve 1), gold nanorods immobilized onto SAM film (Figure 1, curve 2), and the gold nanorod solution on the non-SAM surface (Figure 1, curve 3). As explained in the Experimental Section, a gold nanorod solution (twice centrifuged) was drop-dried on a Si (111) substrate (Figure 1, curve 3). It is interesting to note that features d and e are completely absent in the FTIR spectra of gold nanorods in the absence of SAM (Figure 1, curve 3). This clearly indicates that the interactions between the SAM surface and the gold nanorods result in the observed spectral features (features d and e) further strengthening the notion of the electrostatic nature of interaction. Furthermore, a strong peak centered at 1460 cm-1 in the 16-MHA SAM film (Figure 1C, feature f, curve 1) corresponds to the methylene scissoring mode. The intensity of this band reduces significantly upon immobilization of gold rods. We do not completely understand the reason for this at this moment. A band at 1646 cm-1 (37) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (38) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Nelson, A.; Terminello, L. J.; Fadley, C. S. Langmuir 2004, 20, 2746. (39) Gole, A.; Kaur, J.; Pavaskar, N. R.; Sastry, M. Langmuir 2001, 17, 8249. (40) Jiang, P.; Liu, Z.-F.; Cai, S.-M. Langmuir 2002, 18, 4495. (41) Damle, C.; Kumar, A.; Sastry, M. J. Phys. Chem. B 2002, 106, 297. (42) Schweiss, R.; Welzel, P. B.; Werner, C.; Knoll, W. Langmuir 2001, 17, 4304. (43) Damle, C.; Gole, A.; Sastry, M. J. Mater. Chem. 2000, 10, 1389.

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Figure 2. Scanning electron micrographs at different magnifications of the gold nanorods immobilized onto a 16-MHA SAM surface at pH 6.5. Images A-D are high to low magnification images.

can be seen (Figure 1C, curve 2) and at present we are unable to interpret this peak. This peak is absent in the FTIR spectra of gold nanorods on the non-SAM surface (Figure 1C, curve 3) and further suggests its origin to be due to the interaction between the MHA SAM and the CTAB molecules on the gold nanorods. Such a peak has been previously observed at ca. 1645 cm-1 in LB films of metal salts of fatty acids.43 The interaction between rods and the monolayer can be looked upon in two different ways. One can compare the interaction between tetraalkylammonium (NR4+) and carboxylic acid (COO-) groups to the ammonium (NH4+)COO- interaction; or one can consider (NR4+) as a giant cation and study the cation-carboxylic acid interactions. In the first case, the reported literature discusses mainly the formation of NH4+ (from NH3+ in a polyelectrolyte) and its interaction with carboxylic acid to form an amide bond at ca. 1670 and 1530 cm-1 upon heating.44 We found the second case more reasonable for CTAB, wherein we assume the surfactant as a bulky cation (NR4+) and study its interaction with carboxylic acid molecules of the SAM based on purely electrostatic terms. Also, such cationfatty acid interactions are well documented.43 The FTIR data of the SAM film before and after rod immobilization and its comparison with gold nanorods on the non-SAM surface supports this assumption. The immobilized rods were further characterized by scanning electron microscopy (SEM) as shown in Figure 2. Micrographs were recorded at different magnifications and a large number of rods can be clearly seen in a large area scan image (Figure 2D). Other nanoparticle shapes are also observed at high magnification (Figure 2A). Compared to the control experiments (see below), the gold nanorods are well dispersed on the SAM surface. We would like to point out that the work reported here is a first step toward immobilization of rods via electrostatic interactions, and simple electrostatics does not induce any ordering of the nanorods vis-a`-vis rods immobilized by the LB techniques. Figure 3A shows a 1.0-µm2 topographical AFM image of two immobilized nanorods rotated 40° with 20° pitch in a region specifically chosen because it is devoid of other nanoshapes. Line scans are measured across the width and length of these nanorods shown in Figure 3B and 3C. From the line scans, the dimensions of these rods are (44) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931.

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Figure 3. Atomic force microscopy (AFM) images of selected region of gold nanorods immobilized onto 16-MHA SAM. (A) 1.0-µm2 topographical image of two immobilized nanorods rotated 40° with 20° pitch, (B) the same 1.0-µm2 topographical image with the line scan traces marked, and (C) line scans across the width and length of two nanorods.

estimated to be ∼25 nm wide and ∼400 nm long, which are similar to the as-prepared nanorods according to the TEM measurements (data not shown for brevity). Over the course of imaging the surface for ca. 3 h, the immobilized rods were stable, and we did not observe any tip-induced displacement of the rods. To further demonstrate the electrostatic nature of the interaction used for nanorod assembly, we have performed control experiments. Immobilization of the nanorods onto 16-MHA SAM was studied at 5 different pH values (pH of 1, 3.3, 6.5, 8, and 11). The density of the immobilized nanorods was calculated by counting individual rods by SEM over the scanned surface area. At pH values lower than that of the pKa of 16-MHA, we would expect to see fewer rods on the surface, because the acid groups are protonated. At higher pH values, the acid groups of the SAM would be increasingly deprotonated and would thus lead to a strong attractive interaction between the -NR4+ ions on the rod surface and the carboxylic acid groups. As can be seen from Figure 4A (circles), the density of rods immobilized on the surface is a maximum at pH 6.5 and falls off at other pH values. It was found that at extreme pH values, the number of countable rods is reduced considerably due to clustering/aggregation of the rods on the SAM surface. Ideally, there would be maximum electrostatic interactions prevailing at pH 8. Experimentally, a large density of rods was observed on the SAM surface at pH 8, most of which were in a clumped/ aggregated state. This might be due to the increasing ionic strength of solution, which screens electrostatic repulsion between nanorods and “salts them out”. The gold nanorods were found to aggregate or “crash out of solution” at high pH (>10). Hence, the choice of pH 6.5 was made because of optimum conditions prevailing at this pH, resulting in a high density of well-dispersed nanorods. A control experiment was also performed wherein the gold nanorod film (formed at pH 6.5) was immersed into an aqueous solution at pH 3 for a period of 4 h. SEM images before and after the immersion of the film in pH 3 solution were recorded and rod density was calculated. The rod density was found to decrease and has been displayed in Figure 4A (inverted triangle symbol). This indicates that alter-

Figure 4. (A) Control experiments: Density of immobilized nanorods as measured by SEM images. Circles: pH profile for immobilization of nanorods using 16-MHA SAM. Square: immobilization of nanorods onto 1-ODT SAM, Up Triangle: immobilization of nanorods onto 4-ATP SAM, star: nanorod immobilization onto gold-coated glass slide (without SAM), Down triangle: removal of gold nanorods by immersion in pH 3 from a fully loaded gold nanorod film. (B) SEM image of gold nanorods immobilized on 1-ODT SAM.

ation of electrostatic interactions by pH variation leads to the modulation of rod density on the SAM surface. Figure 4A also shows data for gold nanorod immobilization at pH 6.5 onto SAMs of 1-octadecanethiol (ODT, square symbol) and 4-aminothiophenol (4-ATP, star symbol), respectively, on a gold surface. These SAMs were formed by the method similar to that employed for 16-MHA. The 1-ODT SAM exposes a hydrophobic surface whereas the 4-ATP SAM surface has positive surface charge (due to the protonated amine groups at pH ∼6.5). Some rods do seem to bind on these SAM surfaces, probably due to nonspecific adsorption. A blank control experiment was also performed wherein a gold-coated glass slide (without any SAM) was immersed in the gold nanorod solution held at pH 6.5 (triangle symbol). Some gold rods do adsorb on this surface. A number of images were recorded to calculate the density of rods. This was done to avoid areas where the rods were aggregated/clustered. The control experiments indicate that the density and packing of nanorods is high where favorable electrostatic interactions are used for immobilization. Furthermore, nonspecific interactions lead to mere adsorption and clustering/ aggregation (as opposed to well-dispersed nanorods) of the nanorods and could be washed out by repetitive rinsing

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of the substrate. A typical image showing aggregated/ clumped rods due to nonspecific interactions (immobilization of nanorods on 1-ODT SAM) is shown in Figure 4B. Similar aggregated/clumped rods could be seen when immobilization was carried out at extreme pH conditions. This clearly points out the importance of electrostatics and optimum pH conditions for the immobilization process.

pH. Such substrates with well-dispersed immobilized gold nanorods could be used for applications such as surfaceenhanced Raman spectroscopy (SERS) measurements. Furthermore, the possibility to organize patterned nanorods by simple patterning of the substrates makes this approach versatile.

Conclusion

Acknowledgment. The authors thank Dr. M. L. Myrick for making available the sputtering unit, AFM, and FTIR for our use.

To summarize, a simple two-step protocol for the immobilization of gold nanorods onto 2-D surfaces has been demonstrated. The immobilization is driven primarily by attractive electrostatic interactions between the ammonium headgroups of the CTAB-capped gold rods and the carboxylic acid groups of the SAM. The beauty of using electrostatic interaction for organization of nanorods is that the density can be controlled by simple variation of

Supporting Information Available: FTIR measurements showing the concentration of CTAB as the number of cycles of centrifugation is increased. This material is available free of charge via the Internet at http://pubs.acs.org. LA049051Q