Controlled Interparticle Spacing for Surface-Modified Gold

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Langmuir 2008, 24, 5562-5568

Controlled Interparticle Spacing for Surface-Modified Gold Nanoparticle Aggregates Soumen Basu,† Surojit Pande,† Subhra Jana, Sreenath Bolisetty,‡ and Tarasankar Pal*,† Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India, and Physikalische Chemie I, UniVersity of Bayreuth, D-95440 Bayreuth, Germany ReceiVed January 9, 2008. ReVised Manuscript ReceiVed February 21, 2008 Aggregation of gold nanoparticles of increasing size has been studied as a consequence of adsorption of 2-aminothiophenol (ATP) on gold nanoparticle surfaces. The capping property of ATP in the acidic pH range has been accounted from UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS) studies. The effect of nanoparticle size (8-55 nm) on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects among the gold particles have been critically examined. Various techniques such as transmission electron microscopy, X-ray diffraction, ζ-potential, and average particle size measurement were undertaken to characterize the nanoparticle aggregates. The aggregate size, interparticle distances, and absorption band wavelengths were found to be highly dependent on the pH of the medium and the concentration of the capping agent, ATP. The acquired SERS spectra of ATP relate the interparticle spacing. It has been observed that the SERS signal intensities are different for different sized gold nanoparticles.

Introduction Bottom-up fabrications of metals into two- and threedimensional (2D/3D) nanoarchitectures have recently stimulated great interest of the researchers in many fields and inspired the development of various nanodevices for future technical applications.1–5 By creating metal nanostructures, it is possible to control fundamental optical and electrical properties of metals. Metal nanoclusters of desired size are of enormous importance in nanotechnology because of their size dependent physical and chemical properties, which are significantly different from those of the corresponding bulk materials.6 As the surface plasmons are localized in a confined volume, such as in nanosized metal particles, the local electromagnetic field can be remarkably amplified.7–9 Under resonance conditions, the localized surface plasmon resonance in turn enhances various optical processes, including Raman scattering,10,11 fluorescence,12,13 second harmonic generation,14 photochemical reactions,15 and so forth. One of the most widely studied nanoparticles, gold, has attracted great attention due to its potential applications in optoelectronics, * E-mail: [email protected]. † Indian Institute of Technology. ‡ University of Bayreuth.

(1) Kovtyukhova, N. I.; Mallouk, T. E. Chem. Eur. J. 2002, 8, 4354–4363. (2) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549–561. (3) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (4) Abdelrahman, A. I.; Mohammad, A. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2006, 110, 2798–2803. (5) Lu, L.; Randjelovic, I.; Capek, R.; Gaponik, N.; Yang, J.; Zhang, H.; Eychmuller, A. Chem. Mater. 2005, 17, 5731–5736. (6) (a) Fendler, J. H. Nanoparticles and Nanostructured Films; Wiley-VCH Weinheim, 1998. (b) Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Nano Lett. 2001, 1, 319–322. (7) Kahl, M.; Voges, E. Phys. ReV. B 2000, 61, 14078–14088. (8) Shalaev, V. M.; Sarychev, A. K. Phys. ReV. B 1998, 57, 13265–13288. (9) Hayakawa, T.; Selvan, S. T.; Nogami, M. Appl. Phys. Lett. 1999, 74, 1513–1515. (10) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241–250. (11) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783–826. (12) Sokolov, K.; Chumanov, G.; Cotton, T. M. Anal. Chem. 1998, 70, 3898– 3905. (13) Yokota, H.; Saito, K.; Yanagida, T. Phys. ReV. Lett. 1998, 80, 4606–4609. (14) Goetz, T.; Buck, M.; Dressler, C.; Eisert, F.; Traeger, F. Appl. Phys. A 1995, 60, 607–612. (15) Kidd, R. T.; Lennon, D.; Meech, S. R. J. Chem. Phys. 2000, 113, 8276– 8282.

electronics, catalysis, and other areas.6a There has been much experimental interest in the optical properties of noble metal nanoparticle aggregates to construct assemblies of perfect nanocrystallites, identically replicated in unlimited quantities.16–20 This interest has arisen in part because of new methods for linking nanoparticles into clusters and complicated aggregates and in part because of advances in the technology for characterizing the nanoparticles using AFM, STM,21,22 surface-enhanced Raman scattering,21–24 optical measurements,25 and near-field methods26 that illuminate only a few nanoparticles. Aggregation processes in colloids have been the subjects of numerous experimental, theoretical, and computational studies.27–29 For example, aminothiophenol has attracted significant attention recently and has been used for 2D/3D assembly of nanoparticles using covalent or electrostatic interactions.30 Further, these molecules are used to assemble nanoparticles advantageously by “soft-soft” in(16) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316–3320. (17) Basu, S.; Pal, T. J. Nanosci. Nanotech 2007, 7, 1904–1910. (18) (a) Basu, S.; Panigrahi, S.; Praharaj, S.; Ghosh, S. K.; Pande, S.; Jana, S.; Pal, T. New J. Chem. 2006, 30, 1333–1339. (b) Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Jana, S.; Pande, S.; Vo-Dinh, T.; Jiang, H.; Pal, T. J. Phys. Chem. B 2006, 110, 13436–13444. (19) Mucic, R. C.; Stohoff, J. J.; Letsinger, R. L.; Mirkin, C. A. Nature 1996, 382, 607–609. (20) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499–1501. (21) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Mater. 1992, 4, 1143–1212. (22) Moskovits, M. ReV. Mod. Phys. 1986, 57, 783–826. (23) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 4, 241–250. (24) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435–455. (25) Van Duyne, R. P.; Haller, K. L.; Altkorn, R. I. Chem. Phys. Lett. 1986, 126, 190–196. (26) Kerker, M. Acc. Chem. Res. 1984, 17, 271–277. (27) (a) Myers, D. Wiley-VCH: New York, 1999; Chapters 4, 5, and 10. (b) Bradley, J. S., Schmid, G., Eds.; VCH: Weinheim, Germany, 1994; p 465. (28) Creighton, J. A., Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982; p 315. (29) Weitz, D. A.; Oliveria, M. Phys. ReV. Lett. 1984, 52, 1433–1436. (30) (a) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Chem. Phys. Lett. 1999, 300, 651–655. (b) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234–1239. (c) He, H. X.; Zhang, H.; Li, Q. G. ; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846–3851. (d) Zhu, T.; Fu, X.; Mu, T.; Wang, J.; Liu, Z. Langmuir 1999, 15, 5197–5199.

10.1021/la8000784 CCC: $40.75  2008 American Chemical Society Published on Web 04/22/2008

Surface-Modified Gold Nanoparticle Aggregates

teraction of terminal functional groups of thiolates adsorbed on the surfaces of nanoparticles. Surface-enhanced Raman scattering (SERS) is a wellestablished phenomenon that can enhance Raman signals of nonresonant molecules adsorbed on noble metal particles by 5 to 6 orders of magnitude.21–24,31 Significant SERS enhancement is often present at the junction of two or more metal nanoparticles as shown recently by Brus and co-workers.32 As two metal nanoparticles are brought together to form aggregates, their transition dipoles couple to each other. The enhanced fields of each particle begin to coherently interfere at the junction site between the particles. It is known that the optical excitation of the surface plasmon band in an ensemble of individual colloidal metal nanoparticles can result in a significant increase in the average electromagnetic field intensities at the surface of the particles.33 This enhancement decreases as the excitation wavelength is shifted away from the surface plasmon absorption band.34 In the case of ensembles of particles in aggregated form, where the plasmon modes can interact,35 it is predicted that the excitation energy is not distributed uniformly over the particles but is localized in “hot spots”, often much smaller than the excitation wavelength.36 Calculations by Xu et al. have shown that electromagnetic enhancements of 1010 are present between two nanospheres separated by 1 nm.37 These results suggest that aggregates are better substrates for SERS applications than individual nanoparticles because large enhancements can be achieved at particle junctions of aggregates.38 In this paper, we examined adsorption behavior of 2-aminothiophenol (ATP) on gold nanoparticle surfaces of varying mean diameter from 8 to 55 nm by UV-vis absorption spectroscopy and SERS studies. We have investigated the nanoparticle size effect on the nature of aggregation of gold particles. This nanoparticle aggregation process occurs supplementing a concomitant color change from red (dispersed gold nanoparticles) to blue (aggregated networks), which can be monitored spectrophotometrically in solution. From the kinetic experiments, it becomes clear that the interparticle spacing between the nanoparticles decreases with the progress of the place exchange reaction. The aggregate size, interparticle distances, and absorption band wavelengths are found to be highly dependent on the pH of the medium and the concentration of the ATP molecules. Aggregation and crystallinity of different size gold nanoparticles were examined by transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies. The acquired SERS spectra of ATP relate the nanoparticle size and the interparticle spacing within the aggregates. We have observed that the SERS signal intensities are different for different size gold nanoparticles.

Experimental Section Reagents and Instruments. All the reagents used were of AR grade. Chloroauric acid (HAuCl4 · 3H2O) was purchased from Aldrich. Trisodium citrate (Mallinckrodt) and ATP (Aldrich) were used as (31) Chang, R. K.; Furtak, T. E. Plenum Press: New York, 1982. (32) Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, 11965– 11971. (33) Kerker, M.; Wang, D. S.; Chew, H. Appl. Opt. 1980, 19, 4159 4174. (34) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695– 5701. (35) Fornasiero, D.; Grieser, F. J. Chem. Phys. 1987, 87, 3213–3217. (36) Shalaev, V. M.; Botet, R.; Mercer, J.; Stechel, E. B. Phys. ReV. B. 1996, 54, 8235–8242. (37) Xu, H. X.; Bjerneld, E. J.; Kall, M.; Borjesson, L. Phys. ReV. Lett. 1999, 83, 4357–4360. (38) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667–1670.

Langmuir, Vol. 24, No. 10, 2008 5563 Table 1. Details the Size-Selective Synthesis of Gold Nanoparticles by Frens Method volume of volume of citrate set HAuCI410-2 (1% by weight) (µL) no. M (µL) A B C D E

1250 1250 1250 1250 1250

2000 1300 875 625 400

color dark red red red pinkish red pink

particle λmax size (nm) (nm) ((3 nm) 521 523 528 532 538

8 13 20 32 55

received. Double distilled water was used throughout the course of this investigation. The absorption spectrum of each solution was recorded in a Spectrascan UV 2600 digital spectrophotometer (Chemito, India) in a 1 cm well-stoppered quartz cuvette. TEM was carried out on a Hitachi H-9000 NAR transmission electron microscope, operating at 200 kV. Samples were prepared by placing a drop of solution on a carbon-coated copper grid and allowing the liquid to dry in air at room temperature. Fourier transform IR (FTIR) spectral characteristics of the samples were collected in reflectance mode with Nexus 870 Thermo-Nicolet instrument coupled with a ThermoNicolet Continuum FTIR microscope. One drop of the test solution was placed on a KBr pellet and was dried under vacuum for 6 h before analysis. Raman spectra were obtained with a Renishaw Raman microscope, equipped with a He-Ne laser excitation source emitting at a wavelength of 633 nm, and a Peltier cooled (-70 °C) charge coupled device camera. For the measurement of Raman signal, samples were prepared by placing a drop of solution on glass slide. Because of the small spot size of the laser compared with the large surface area of the aggregates, it was necessary to obtain spectra at different points on a surface to find a spectrum with the best signalto-noise ratio. SERS spectra from different points on the surface were the same, differing only in intensity; the spectrum that gave the most intense signal was retained. For better result, we have represented the average out intensity (average of 5 determinations) in the figure. The XRD pattern was recorded in an X’pert pro diffractometer with Co (KR ) 1.78891) radiation. ζ-potential and the average particle size were measured with Malvern Nano ZS Zetasizer. Preparation of Gold Nanoparticles. To study the size effect on the process of aggregation of the metallic gold particles in the nanometer size regime, it is important to have sets of nanoparticles within the range of 1-100 nm with a tight size distribution. The well-documented Frens method has been used to obtain monodispersed gold colloids over a wide size range.39 In a typical preparation, an aliquot of 50 mL aqueous solution of HAuCl4 (0.25 mM) was heated to boiling, and 2 mL of trisodium citrate (1%) was added for 8 nm gold colloid. In about 25 s, the boiling solution turned faintly blue. After ∼70 s, the blue color changed to deep red. The solution was set aside to cool down to room temperature. The details of the preparation and the UV-vis spectral characteristics of variable sizes for different sets of gold particles are summarized in Table 1. Synthesis of ATP/Gold Assemblies. A stock solution of ATP (10 mM) was prepared in ethanol. Then, an aliquot of ethanolic ATP solution (10 mM, 30 µL) was added to that deep red acidic gold colloidal solution (3 mL). The color change of the gold sol from red to blue indicates the formation of the aggregates among the gold particles, and the changes in the surface plasmon resonance was measured with the UV-vis spectrophotometer.

Results and Discussion Uv-vis and ζ-Potential Studies. Five different sizes of gold nanoparticles have been employed to investigate the size effect on the aggregation behavior (i.e., size, shape, and morphology of the produced aggregates) of metallic particles in the nanometer size regime. The particle size was varied within the size range from 8 to 55 nm (set A-E) where the concentrations of gold (39) Frens, G. Nat. Phys. Sci. 1973, 241, 20–22.

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Basu et al. Table 2. Average Diameter and ζ-Potential of the Gold Nanoparticles before and after the Addition of ATP before

Figure 1. (A) UV-vis absorption spectra of different size gold nanoparticles. (B) UV-vis absorption spectra of different size gold nanoparticle aggregates induced by the ATP addition.

remain the same in all cases. The gold particles have been prepared by Frens method (employing HAuCl4 as the precursor salt and trisodium citrate as the surface capping as well as reducing agent), which offers us the ease of achieving monodispersed gold colloids over a wide size range.39 In this method, it is possible to control the size of the particles by varying [Au(III)]/[citrate] ratio during the reduction step as has been listed in Table 1. The color of the solution varies from red to pink depending on the size of the particles. In accordance to Mie theory40 the surface plasmon resonance of the gold particles is red-shifted with the increase in particle size, which is shown in Figure 1A. Now, upon the addition of ATP to the citrate-stabilized gold particles, the color of all the solutions becomes blue indicating the formation of aggregates of gold nanoparticles. To study the size effect of metal nanoparticles on the surface plasmon resonance due to aggregation, we measured the changes in the UV-vis spectra of the resultant colloids. It is observed that different sets of gold colloids show distinctly different absorption profile in the presence of ATP molecules. The salient feature of physical significance is that a new plasmon band is developed at longer wavelength and a clear bathochromic shift in λmax is observed with increasing particle size from 8 to 55 nm, which is shown in Figure 1B. The UV-vis extinction spectra of colloidally stable citratecapped gold nanoparticles displayed in Figure 1A presents a strong extinction band with a maximum at ∼523 nm, characteristic of the collective excitation of the free conduction band electrons of the particles known as the surface plasmon resonance. However, after the addition of ATP a significant difference in extinction spectra was observed as shown in Figure 1B. A second absorption band appeared on the red side of the spectrum. Established theoretical descriptions of Mie scattering from similar small aggregate clusters suggested that the plasmon resonance absorption of the aggregates would have an additional long wavelength component in the optical absorption spectrum relative to the absorption from isolated nanoparticles dispersed in solutions.41 According to this theory, this new long wavelength band is then (40) Mie, G. Ann. Phys. 1908, 25, 377–445. (41) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. J. Phys. Chem. B 1999, 103, 8706–8710.

after

sample

diameter (nm)

ζ-potential (mV)

diameter (nm)

ζ-potential (mV)

A B C D E

13.1 20.9 31.7 44.6 69.8

-50.8 -47.3 -35.7 -29.7 -26.1

69.9 123.2 143.7 187.9 246.8

-57.1 -55.3 -42.8 -38.6 -30.3

associated with the longitudinal mode of the electronic plasma oscillation along the long axis of the gold nanoparticle chains. The capping agent, ATP has been chosen as it supports a chelating or bridging like structure presumably with gold nanoparticles because of the proximity of the two functionalities that is, -SH and -NH2. In solution phase, chelating or bridging cannot be distinguished. Uncovered portion of gold nanoparticles (but chelated) can be bridged by extra 2-ATP that brings the aggregated picture. So 2-ATP serves as a bridging as well as chelating ligand, which is not possible when 4-ATP is used. Again possibility of hydrophobic interaction is ruled out from the observation noted out of pH effect. All chelating ligands can act as a bridging ligand but the reverse is not true. However, 2-ATP and any other bidentate ligand can act as flexidentate ligand.42 This would not be the case when 4-ATP is taken into consideration. Upon the addition of ATP, a competitive but preferential adsorption is expected to occur between citrate and ATP molecules. Because the ATP concentration (10-4 M) is much higher than that required for its full monolayer coverage, it is expected that most of the citrate ions are replaced by thiolates.43 Although Au-S bond formation is energetically the most favorable bond formation, it might be difficult for ATP molecules to completely replace the entire capping citrate molecules bound to the gold surfaces. In the Raman study, the unshifted ν(C-N) mode at 1305 cm-1 for adsorbed ATP also indicates that some of the nitrogen atoms of the ATP moiety are not bound to the gold surfaces (Figure 8). Table 2 lists the average diameter and ζ-potential of the gold particles prepared by adding reducing agent (i.e., trisodium citrate). According to the model by Chow et al.,44 an increase in the concentration of citrate ions should increase the ξ-potential of particles, whereas it decreases the final size of colloidally stable particles. This is consistent with our experimental results in Table 1. But after the addition of ATP, both citrate and ATP molecules remain present on the gold surfaces. This is authenticated from the surface potential values. Addition of ATP molecules to the citrate-capped gold solution increases the surface potential of the particles (due to the presence of both trivalent citrate ions and monovalent ATP ions as ATP can not replace the entire citrate ion), which is reflected from the ζ-potential measurement and that is presented in Table 2. We believe the increase in the ζ-potential value from our study is due to the partial replacement of citrate ions, whereas Joo’s group noticed a decrease in the ζ-potential value owing to the quantitative place exchange reaction.45 This could be due to the higher affinity of benzyl mercaptan for gold nanoparticle surfaces and hence all the citrate ions are replaced. From Table 2, it is clear that the replacement of the citrate with ATP molecules destabilize the (42) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; Interscience: New York, 1962. (43) Jang, S.; Park, J.; Shin, S.; Yoon, C.; Choi, B. K.; Gong, M.-S.; Joo, S.-W. Langmuir 2004, 20, 1922–1927. (44) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97–109. (45) Taehoo, Kim; Kangtae, Lee; Myoung-seo, Gong; Sang-Woo, Joo Langmuir 2005, 21, 9524–9528.

Surface-Modified Gold Nanoparticle Aggregates

Figure 2. Time-dependent UV-vis spectra of 20 nm gold nanoparticles (0.25 mM, 3 mL) at pH ∼ 5 recorded at various times after addition of ATP (10 mM, 30 µL).

particles. The destabilization is authenticated from the aggregate formation. The average diameter of the aggregates is also measured. The larger the aggregate formation is, the greater the destabilization will be. This becomes pronounced in the case of particles with larger size. The average particle diameter increases with time after the addition of ATP molecules, which is represented in Table 2. The gold nanoparticles acquired a negative surface charge due to weakly bound citrate molecules. The negatively charged gold nanoparticles repel each other and inhibit aggregation. Because the larger size gold particles have a less negative charge due to a smaller amount of citrate (supplied) adsorbed onto the surface, a cross-link between the gold nanoparticles should be more facile through the sulfur and nitrogen atom of the ATP moiety. Kinetics Study of the Aggregates. UV-vis extinction spectrum was also used to study the kinetics of the aggregation to investigate the evolvement of the chainlike nanoparticle aggregates in solution. We have monitored the shift in the surface plasmon resonance of gold nanoparticles as a function of time to investigate the kinetics of the aggregation process. Ethanolic solution of ATP was introduced into a prechosen gold nanoparticle solution containing 20 nm particles at room temperature, and the spectra were rcorded over different time intervals that have been shown in Figure 2. A distinctive red shift of the dipole plasmon resonance from 520 to 665 nm with a decrease in intensity at 520 nm is observed that indicates that interparticle spacing decreases with the progress of the place exchange reaction. In addition, it is obvious that the red shifting of the dipole plasmon indicates that the nanoparticle chains get longer and longer with the elapse of time. Therefore, Figure 2 describes the growth of nanoparticle chains, which begin from a single gold nanoparticle and go to chainlike aggregates. Effect of Ligand Concentration. To evaluate the morphological effect of particles on the introduced ligand (ATP) concentration, a set of gold aggregates was prepared by using ATP with variable molar ratios of [ATP]/[Au(0)] from 0 to 4. The optical spectra of these aggregates are shown in Figure 3. For the sample (20 nm gold nanoparticles) with a very low ATP content, that is, [ATP]/[Au(0)] ) 0.0004, no assembly is observed. When we increased ATP concentration, the λmax shifted to higher wavelength region, indicating a change in the morphology of the nanoparticles from isolated domains to compact domains. On the contrary, gold colloidal particles will spontaneously reorganize into chainlike aggregates at [ATP]/[Au(0)] ) 0.004, which can be indeed reflected from Figure 3. But a further increase in concentration [ATP]/[Au(0)] ) 4 of ATP indicated a broad peak. At this high concentration (10-3 M) of ATP, the gold particles were precipitated.

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Figure 3. UV-vis spectra of 20 nm gold nanoparticle aggregates at different concentrations of ATP (a) 0; (b) 10-7; (c) 10-6; (d) 10-5; (e) 10-4; and (f) 10-3 measured after 5 min of the addition.

Figure 4. Absorption spectra for 20 nm Au aggregates with ATP (0.1 mM) at pH (a) 7.2; (b) 7; (c) 6; (d) 5; (e) 4; and (f) 3 after 5 min of the addition.

Effect of pH of the Solution. It can be anticipated that because ATP consists of the -NH2 and -SH functionalities, binding modes are pH sensitive. The evolution of chainlike aggregates of gold nanoparticles from the nonaggregated constituents is found to be dependent on the pH of the colloidal solution. We have studied the effect of pH of the solution on the adsorption of ATP onto the gold nanoparticle surfaces. In alkaline solution, where the amino group of the ATP is not protonated, it could not bind to the negatively charged gold nanoparticle surfaces. But under acidic conditions, protonation makes the amino group positively charged and the binding becomes facile by strong electrostatic interaction between negatively charged gold nanoparticles and positively charged amino groups.30d As a preliminary controlled experiment, we observed that a deliberate pH change (3-12) upon the addition of aqueous HCl and/or NaOH solution could not bring out any such aggregates of gold naoparticles. It has been noted that the generation of aggregates with a perfect blue color solution is observed at a pH e 6.0 upon the addition of ATP. The protonation of the amino group of ATP will promote the adsorption of ATP on the negatively charged gold nanoparticle surface by strong electrostatic interaction and caused aggregation. Figure 4 shows the optical spectra of a typical 20 nm size gold colloids at different pH for the ATP solution. There is clearly a progressive suppression of new color development and a red shift of surface plasmon resonance band with increasing pH. At pH g 7.0, no spectral changes occur but as the pH decreases then the dipole plasmon resonance shifts to the higher wavelengths. FTIR and Raman Studies. FTIR and Raman spectra revealed the characteristic bands of ATP after gold nanoparticle conjugation. Figure 5 shows the FTIR spectra recorded for the gold aggregates (curve b) in the spectral window 1200-3000 cm-1 along with the spectrum recorded from pure ATP (curve a). The evidence for the presence of surface-bound ATP is provided by FTIR measurements of the gold particles in the range of

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Figure 5. FTIR spectra of pure ATP (curve a) and ATP-induced 20 nm gold nanoparticle aggregates at pH ∼5 (curve b) in the spectral windows 1200-3000 cm-1.

1600-1700 cm-1. The NH2 bending of the ATP molecules is observed at 1608 cm-1. But upon complexation of ATP with gold nanoparticles, this band becomes sharper and shifts to 1639 cm-1. This clearly indicates that ATP molecules are engaged to bind gold nanoparticles through the nitrogen atom. The prominent -SH vibrational band centered at ca. 2523 cm-1 is clearly seen in the pure ATP molecules (curve a) and that vanishes upon coordination with colloidal gold (curve b).This is a strong evidence of surface binding of ATP to the gold particles via thiolate linkage (soft-soft interaction),46 which agrees well with the earlier studies on alkanethiol modification of gold nanoparticles by Murray and co-workers.47 The binding of the ATP molecule to the gold surface is also authenticated from the Raman studies. The ATP molecules are adsorbed to the gold surface via their nitrogen and sulfur atoms. This conclusion is based on the facts that the in-plane vibrations ν(C-N) at 1305 cm-1 and ν(C-S) at 474 cm-1 have been observed to be shifted to 1285 and 470 cm-1, respectively, and weakly enhanced after ATP adsorption onto the gold surfaces (Figure 8). TEM Studies. Because of the random nature of aggregate formation, the synthesized chainlike gold nanoparticle aggregates have a broad distribution of sizes and shapes. This variety of sizes and shapes are apparent from the TEM images. Figure 6A-C show the TEM images of chainlike aggregates of 8, 20, and 55 nm gold nanoparticles, respectively. Inset shows the magnified details of their respective panels. From the TEM images, it is evident that gold nanoparticles are found to spontaneously assemble into chainlike assemblies. It should be noted that no separate nanoparticle has been found on the microgrid, which reveals that gold particles prefer linear aggregate formation. XRD Studies. The as-prepared sample was centrifuged, and the precipitate was dried under vacuum and taken for XRD analysis. XRD pattern for the ATP-capped gold aggregates is shown in Figure 7. Several peaks are observed with these being at 38.2, 44.5, 64.6, 77.7, and 81.8°, which correspond to the {111}, {200}, {220}, {311}, and {222} facets of the fcc crystal structure of gold, respectively. The observation of diffraction peaks for the gold nanoparticles indicates that these are crystalline in this size range, and its broadening is related to the particles in the nanometer size regime. Although XRD patterns of the 8, 20, and 55 nm gold particles were measured to examine a difference in their crystallinity, their diffraction patterns showed similar structure, as shown in Figure 7. SERS Studies. Metal colloids have been widely employed in SERS because they have a number of advantages, such as ease (46) Nath, S.; Ghosh, S. K.; Kundu, S.; Praharaj, S.; Panigrahi, S.; Pal, T. J. Nano. Res. 2006, 8, 111–116. (47) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66–76.

Figure 6. (A) TEM images of 8 nm gold nanoparticle aggregates. Inset shows the magnified details panel A. (B) TEM images of 20 nm gold nanoparticle aggregates. Inset shows the magnified details of panel B. (C) TEM images of 55 nm gold nanoparticle aggregates. Inset shows the magnified details of panel C.

of formation and manipulation, ability to control and vary particle size and shape, and a more tractable morphology for theoretical analysis, and so forth.31 Aggregation among the metal particles offers a strong influence on SERS because rough or fractal surfaces, which can give rise to a stronger coupling of the electric field that happens to be resonantly excited by the illuminating laser light, are called “hot spots”.36,48 To generate greater SERS signals, aggregation of the metal colloids induced by an analyte itself or by the addition of coadsorption ions such as Cl- and NO3- are essential.49 However, aggregated metal colloids tend (48) Xu, H.; Aizpurua, J.; Kall, M. Phys. ReV. E 2000, 62, 4318–4324. (49) (a) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790–798. (b) Dou, X.; Jung, Y. M.; Cao, Z.; Ozaki, Y. Appl. Spectrosc. 1999, 53, 1440–1447.

Surface-Modified Gold Nanoparticle Aggregates

Figure 7. XRD pattern of (a) 8 nm; (b) 20 nm; and (c) 55 nm gold aggregates.

Langmuir, Vol. 24, No. 10, 2008 5567

Figure 9. Plot of apparent enhancement factor (AEF) vs size of nanoparticles.

concentration of ATP was kept above 10-4 M to obtain stable aggregates of the particles in the 8–55 nm size range. Qualitatively, Figure 3 accounts for this fact. So a stable aggregate formation for larger particles with lower amount of ATP becomes possible and that qualifies for better SERS intensity. In this report we have considered ATP as a probe to obtain the SERS spectra. It has been observed that the1030.9 and 558 cm-1 peaks are enhanced to the maximum extent. Again, we have calculated the apparent enhancement factor (AEF) using the following equation keeping an attention to these peak positions. Figure 9 clearly represent the size dependent enhancement with AEF of SERS signal intensity due to ATP. Figure 8. Normalized (to 1030 cm-1) SERS spectra of 10-4 M ATP for different sized gold aggregates after 5 min of the addition.

AEF ) σSERS[CNRS] ⁄ σNRS[CSERS]

(1) 52

to coagulate. This makes them unstable resulting in poor reproducibility of SERS signals. The problem can be reduced by the addition of stabilizers such as poly(vinyl alcohol), poly(vinylpyrrolidone), and sodium dodecyl sulfate into the metal colloidal systems that prevent further aggregation.50 Sometimes the stabilizers cause problems as they diminish the SERS signal. In this case, the aggregates synthesized by the proposed method become SERS active without the need of any aggregating agent such as Cl- or NO3-. Ethanolic solution of ATP was used as the SERS probe. It adsorbed well on the surface of gold nanoparticles and showed good SERS signal intensity. Here, ATP served the purpose of a capping agent as well as aggregating reagent. Interparticle distance is an important factor for SERS studies due to its direct correlation to electromagnetic enhancement. Figure 8 shows the normalized (to 1030 cm-1) SERS spectra taken in 8, 13, 20, 32, and 55 nm gold particles, respectively, at a high bulk concentration of ATP (10-4 M). We found that the SERS signal intensities are different for different size gold nanoparticles. We could obtain reasonably stable citrate-capped Au colloid until we reach 55 nm in size. After that, precipitation takes place. Because the larger size gold particles have less amount of citrate onto the particle surface in comparison to the smaller particles, a cross-link between the larger gold nanoparticles should be more facile through the sulfur and nitrogen atom of the ATP moiety. Thus ATP becomes a better linker for larger particles. Moreover, the most pronounced red-shifted peak position (low energy condition) happens to be the more favorable arrangement in the case of larger particles. It has also been reported that a higher concentration of aggregating agents is needed to always cause a stable aggregate of smaller particles.51 Experiments show that a reasonable aggregate of ∼8 nm particles are difficult to obtain with a concentration lower than 10-4 M. Hence, a (50) (a) Lee, P. C.; Meisel, D. Chem. Phys. Lett. 1983, 99, 262–265. (b) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014–1023. (51) Basu, S.; Ghosh, S. K.; Kundu, S.; Panigrahi, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, T. J. Colloid Interface Sci. 2007, 313, 724–734.

As shown previously in many reports on SERS, the SERS enhancement is the result of a combination of electromagnetic effect and chemical effect. But chemical effect is generally thought to contribute only a factor of 10-102, compared to factors of 104-107 for electromagnetic effect.52 In the present study, the SERS enhancement of our samples must also come from both electromagnetic effect and chemical effect. The chemical effect involved in the SERS process is supported by the facts that the δ(C-S) bending mode at 371 cm-1 and ν(C-S) stretching mode at 483 cm-1 in the normal Raman spectrum of pure ATP have been shifted to 388 and 474 cm-1, respectively, in the SERS spectra of adsorbed ATP (Figure 8). The lower degree of enhancement is also observed for in-plane vibrations such as ν(C-C) at 1585 and 1478 cm-1, and γ(C-C) and γ(C-H) bending modes at 676 and 1158 cm-1, respectively. There are also some in plane bending modes such as δ(C-S), δ (C-C), and δ (C-H) at 371, 558, and 1030 cm-1 respectively, which have been strongly enhanced. The unshifted ν(C-N) mode at 1305 cm-1 for adsorbed ATP also indicates that some of the nitrogen atoms are not bound to the gold surface. In the electromagnetic description of SERS, the enhancement is caused by an amplification of the electric field due to the response of the material and the coupling between different surfaces. The enhancement of the local field can vary by several orders of magnitude. Figure 10 shows the SERS spectra of ATP obtained from the 20 nm gold particles at their different stages of aggregation. It is observed that the SERS intensity increases as the interparticle distance between the gold aggregates decreases. It can be seen from Figure 2 that the surface plasmon peak gradually red-shifted with time indicating the decrease in interparticle separation. A red-shifted plasmon band indicates that the size of the aggregates also increases. As the size of the aggregates increases, the number of “hot junctions” increases and thus provides a much more intense SERS band. From this (52) (a) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544– 12548. (b) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702–12707. (c) Doering, W.; Nie, S. J. Phys. Chem. B 2002, 106, 311–317.

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Basu et al.

1B) due to the surface plasmon resonances characteristic of each specific configuration. Because of the presence of these peaks, the enhancement factor is very sensitive to the wavelength of the incident and scattered radiation. It is already reported that the small decrease in diameter yields a dramatic increase in MEM and that the dimer configuration always leads to a higher enhancement than a single particle, irrespective of wavelength.52,53

Conclusion

Figure 10. Successive increases in the SERS intensity of ATP at the different stages of aggregation of 20 nm gold nanoparticles at definite time intervals: (a) 2 min; (b) 4 min; (c) 6 min; (d) 8 min; (e) 10 min; (f) 12 min; and (g) 14 min.

result, we can conclude that the enhancement was due to electromagnetic effect. Because in this study we do not need to add any extra aggregating agent from outside, we can conclude that the electromagnetic field enhancement plays the major role. The EM SERS effect can be described as a consequence of the enhancement of both the incident field and the scattered field. If this enhancement is assumed to be independent of the absolute photon fluxes and polarizations involved, the EM enhancement factor can be expressed as (Stokes case)52

MEM ) [EL(ωI) ⁄ EI(ωI)]2 · [EL(ωI - ων) ⁄ EI(ωI - ων)]2 (2) Here EI and EL are the modulus of the incident electric field EI and the total local electric filed EL in the presence of the metal, respectively. The frequency of the incident light and the vibrational frequency are denoted by ων and ων, respectively. Furthermore, if ων , ωI, then the above equation can be approximated by

MEM ) [EL(ωI) ⁄ EI(ωI)]4

(3)

which shows that the local field EL(r, ω), generated by the response of the electrodynamical environment: I

Acknowledgment. Authors are thankful to IIT Kharagpur, CSIR, UGC, and DST New Delhi for financial assistance and to Professor Matthias Ballauff for providing instrumental facilities. We are thankful to the reviewers for suggestions and constructive comments. LA8000784

E (r, ω) ) E (r, ω) + E (r, ω) L

We have demonstrated a very simple approach to synthesize nanoscale chainlike gold aggregates by linking individual Au colloidal particles with ATP molecules. This facile synthesis of gold nanoparticle aggregates has been shown to be particularly favorable through easy manipulations via place-exchange reaction. Interparticle coupling effects on the surface plasmon resonance of gold particles with variable sizes in the nanometer regime have been investigated, and the optical absorbance behavior of the resulting nanoscale aggregates has been enlightened. This experiment reveals that metal sols can be induced to aggregate by replacing the charged surface species by uncharged adsorbates containing various functional groups. Experiments demonstrate that the surface chemistry of colloidal gold is dominated by electrodynamic factors related to its surface negative charge. Several factors, such as ligand concentration, pH adjustment and reaction kinetics of the judicious intermixing, are seen to regulate or tune the interactions between the gold nanoparticles. The reactivity of the gold nanoparticles assembled into well-defined architectures might be useful for excellent sensory applications as a result of tuned electrochemical characteristics and surface plasmon resonance of gold nanoparticles with the ATP molecules.

ind

(4)

When the aggregation occurs, we observed a distinct peak (Figure

(53) Otto, A. Topics in Applied Physics; Springer-Verlag: Berlin, 1984; Vol. 54.