pubs.acs.org/Langmuir © 2009 American Chemical Society
Enhanced Stability of Gold Colloids Produced by Femtosecond Laser Synthesis in Aqueous Solution of CTAB Mushtaq A. Sobhan,*,† Michael J. Withford,†,‡ and Ewa M. Goldys† †
MQ Photonics Research Centre and ‡Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Department of Physics & Engineering, Macquarie University, NSW, Sydney 2109, Australia Received August 18, 2009. Revised Manuscript Received October 12, 2009
Femtosecond laser ablation of gold in an aqueous solution of cetyl trimethylammonium bromide (CTAB) is shown to produce nanoparticle suspensions with superior colloidal stability compared to other surfactants, with shelf lives exceeding 2 months even at low concentrations of CTAB, below 1 mM. CTAB also helps control nanoparticle size with mean diameters of 6.3, 5.6, and 4.7 nm obtained in 0.1, 0.5, and 1 mM concentrations of CTAB respectively, compared to 11.9 nm obtained in pure deionized water under same ablation conditions. The size distributions produced with low concentrations of CTAB are comparable to those produced by other surfactants, typically used at high concentrations.
Introduction Gold nanoparticles are frequently used in biochemical analysis due to their unique properties including exceptionally high optical scattering cross section, excellent electron contrast, and ease of bioconjugation.1,2 Chemical synthesis is the most popular method for their production;3 however, contamination associated with chemical precursors and other additives limits their use for many applications.4 In recent years, femtosecond laser ablation has emerged as a promising method for producing gold nanoparticles, since it is able to minimize the contamination problem.5 The use of surfactants, which cover the particles during their condensation, promotes improved size uniformity as well as reduces their coalescence.6-8 As the separation of the particles from the surfactant tends to be difficult, it is important that the surfactant is compatible with the end-use procedure. In this context, the cationic surfactant cetyl trimethylammonium bromide (CTAB) is an appropriate choice for many biochemical assays. For example, it has been demonstrated that Au nanoparticles capped with CTAB assemble into a DNA template owing to the electrostaticdriving force between the positive charge of CTAB-capped gold nanoparticles and the negative charge of phosphate groups in DNA molecules.9 Such CTAB-capped Au nanoparticles have been employed as a plasmon resonance light scattering (PRLS) probe to study cysteine.10 CTAB has also been used in chemical synthesis of gold nanoparticles where it improves control over the *Corresponding author. E-mail:
[email protected]. Telephone: þ61 02 9850 6598. Fax: þ61 02 9850 8115.
(1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Hayat, M. A. Colloidal Gold: Principles, Methods and Applications; Academic press: San Diego, 1989. (3) Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55– 75. (4) Srnova, I.; Prochazka, M.; Vlckova, B.; Stepanek, J.; Maly, P. Langmuir 1998, 14, 4666–4670. (5) Kabashin, A. V.; Meunier, M. J. Appl. Phys. 2003, 94(12), 7941–7943. (6) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2002, 106 (31), 7575–7577. (7) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2003, 107, 4218–4223. (8) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2001, 105 (22), 5114–5120. (9) Wu, A.; Cheng, W.; Li, Z.; Jiang, J.; Wang, E. Talanta 2006, 68, 693–699. (10) Wang, J.; Li, Y. F.; Huang, C. Z.; Wu, T. Anal. Chim. Acta 2008, 626, 37– 43.
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size distribution,11-14 and also as postprocessing in nanosecond laser ablation of silver15 and gold.16 Femtosecond and nanosecond laser ablation represent two significantly different experimental regimes.17 Femtosecond laser ablation creates an environment very far from thermodynamic equilibrium where individual gold atoms are able to directly interact with other in situ molecules such as CTAB, and such a situation has been unexplored so far. In this study, we demonstrate that using CTAB as a surfactant in femtosecond laser ablation makes it possible to reduce the mean diameter as well as the size distribution of the produced nanoparticles. We also show and quantitatively analyze enhanced colloidal stability of colloids produced using CTAB at various concentrations (0.1-3 mM). Benchmarking to the literature reports indicates that the colloidal stability achieved with CTAB is superior compared to that for aqueous solutions of other surfactants.
Experimental Section Figure 1 shows a schematic diagram of the experimental setup used to synthesize the gold nanoparticles. The output of a Ti: sapphire femtosecond laser (Hurricane, Spectra Physics) at 800 nm was used in this study. The maximum energy used in the present work was 300 μJ/pulse, with a pulse duration of 100 fs at a repetition rate of 1 kHz. The laser beam was focused using a microscope objective (NA = 0.1, focal length 25 mm) onto the surface of a gold disk (2.5 mm thick, 6 mm diameter, 99.99% pure). The gold disk was placed inside a quartz cuvette filled with 2 mL of either aqueous solution of CTAB or pure deionized water (18 MΩ) as a control. The cuvette was attached to a 3-D motion control stage (Aerotech) that could move in the x, y, and z directions, controlled through software. The beam was focused down to a spot diameter of approximately 8 μm onto the gold (11) Chen, F.; Xu, G.-Q.; Hor, T. S. A. Mater. Lett. 2003, 57, 3282–3286. (12) Lin, J.; Zhou, W.; O’Connor, C. J. Mater. Lett. 2001, 49, 282–286. (13) Yuan, H.; Ma, W.; Chen, C.; Zhao, J.; Liu, J.; Zhu, H.; Gao, X. Chem. Mater. 2007, 19(7), 1592–1600. (14) Perez-Juste, J.; Liz-Marzan, L. M.; Carnie, S.; Chan, D. Y. C.; Maulvaney, P. Adv. Funct. Mater. 2004, 14, 571–579. (15) Chen, Y.-H.; Yeh, C. S. Colloids Surf., A 2002, 197, 133–139. (16) Muto, H.; Yamada, K.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2007, 111(46), 17221–17226. (17) Chin, A.; Schoenlein, R.; Glover, T.; Balling, P.; Leemans, W.; Shank, C. Phys. Rev. Lett. 1999, 83, 336–339.
Published on Web 11/16/2009
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Figure 1. Experimental setup for production of gold nanoparticles by laser ablation.
Figure 2. Optical absorption spectra of gold colloids produced in various concentrations of CTAB. surface. The laser beam was scanned over the surface of the gold disk at a speed of 5 mm/min, and the ablation was performed for 20 min. Transmission electron microscopy (TEM) measurements of the colloidal gold nanoparticles were performed with a Philips CM-10 microscope operating at 100 kV. A small drop of the colloidal solution was placed on a carbon film supported on a 300 mesh copper grid and dried at room temperature. Particle size distributions were determined from the TEM data using image J analysis software. The zeta potential was measured using a zetasizer (Malvern nano series). The surface plasmon resonance spectra measurements of the colloidal gold nanoparticles were evaluated by using a UV-vis spectrophotometer (Varian Inc.). The solution was kept inside a 1 cm path length quartz cuvette, and the investigated spectral range was 300-800 nm.
Results and Discussion The ablation led to visible changes of the solution color after several minutes in the experiment in both CTAB and in pure deionized water. The optical absorption spectra of gold colloids exhibited the characteristic peak of the surface plasmon resonance at 520-530 nm, indicative of the formation of colloidal gold nanoparticles (Figure 2).The absorbance at the surface plasmon resonance peak decreased as the concentration of CTAB was increased. For example, the value of peak absorbance of gold nanoparticles ablated in pure water and in 0.1 mM concentration of CTAB was found to be 0.77 and 0.61, respectively. We also observed an associated change in sample color from red to pink. This implies that the samples produced at higher CTAB concentration contained lower gold colloidal concentration, as the value of the peak absorbance is known to be proportional to the concentration of the gold nanoparticles with sizes in the range of 5-50 nm.18 We attribute the reduced gold nanoparticle density at higher CTAB concentrations to energy loss in the laser beam arising from an associated formation of bubbles. The bubble formation was more intense at higher CTAB concentrations (18) Tarasenko, N. V.; Butsen, A. V.; Nevar, E. A.; Savastenko, N. A. Appl. Surf. Sci. 2006, 252, 4439–4444.
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Figure 3. TEM image and corresponding size distribution of gold nanoparticles prepared by femtosecond laser ablation in (a) 0.5 mM CTAB and (b) pure deionized water.
which we believe contributed to the scattering/screening of the laser beam during ablation. Figure 3a shows the TEM micrograph and size distribution histogram of gold nanoparticles produced in 0.5 mM concentration of CTAB. The micrograph shows that the particles are quite monodisperse with a mean diameter and standard deviation of 5.3 and 2.1 nm, respectively. On the other hand, ablation in pure deionized water produced particles with a wide size distribution, with the mean diameter and the standard deviation of the particles being 11.9 and 7.9 nm, respectively (Figure 3b). In addition, some larger particles in the range of 30-50 nm were also observed. To compare our results with those in the literature, we constructed a histogram of mean diameter and standard deviation of gold nanoparticles, produced in various surfactants by other workers using laser ablation technique (Figure 4b). It shows that particles produced in dextran,19 acetone,20 and acetonitrile21 show similar sizes and standard deviations to the nanoparticles produced with CTAB (Figure 4a). However, ablation in tetrahydrofuran (THF),21 cyclodextrin (CD),22 and PAMAM G523 gave larger values of average size and standard deviation compared to our results. In addition, different salts24 have also been used for reducing the size distribution of the gold nanoparticles. A similar effect was observed with single strand nucleotides.25 No correlation is observed in refs 19-24 between larger pulse wavelength/duration and nanoparticle size (see Table 1, Supporting Information). Typically, the nanoparticle size depends not only on the type of surfactant used but also on the surfactant concentration. Such (19) Besner, S.; Kabashin, A. V.; Meunier, M.; Winnik, F. W. Proc. SPIE 2006, 5969, 59690B-7. (20) Tilaki, R. M.; Irajizad, A.; Mahdavi, S. M. Appl. Phys. A: Mater. Sci. Process. 2006, 84, 215–219. (21) Amendola, V.; Polizzi, S.; Meneghetti, M. J. Phys. Chem. B 2006, 110(14), 7232–7237. (22) Kabashin, A. V.; Meunier, M.; Kingston, C.; Luong, J. H. T. J. Phys. Chem. B 2003, 107, 4527–4531. (23) Giorgetti, E.; Giusti, A.; Laza, S. C.; Marsili, P.; Giammanco, F. Phys. Status Solidi A 2007, 204, 1693–1698. (24) Sylvestre, J. P.; Poulin, S.; Kabashin, A. V.; Sacher, E.; Meunier, M.; Luong, J. H. T. J. Phys. Chem. B 2004, 108, 16864–16869. (25) Petersen, S.; Barcikowski, S. Adv. Funct. Mater. 2009, 19, 1167–1172.
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Figure 4. (a) Mean diameter and standard deviation of gold nanoparticles at various CTAB concentrations and (b) mean diameter and standard deviation of gold nanoparticles in CTAB compared to aqueous solutions of various compounds: acetonitrile (CH3CN) (ref 21), dextran (ref 19), acetone (ref 20), ethanol (ref 20), THF (ref 21), CD (ref 22), SDS (ref 6, (i) 0.9 mM, (ii) 50 mM), PAMAM G5 (ref 23), NaOH (ref 24), KCL (ref 24), and NaCl (ref 24).
strong dependence on the surfactant concentration was observed, for example, when the ablation was performed in the presence of sodium dodecyl sulfate (SDS).6 In that work, the average particle size was reduced from 14.4 to 4.6 nm when the concentration of SDS was increased from 0.1 to 10 mM. This dependence of particle size on surfactant concentration was explained by the dynamic formation mechanism.26 In brief, during the laser ablation the small gold atoms aggregate to form embryonic nanoparticles which continue to grow until all atoms in close proximity (∼40 nm) to the embryonic nanocrystals are depleted. This growth continues slowly even after the ablation as the nanoparticles diffuse through the solution and interact with other nanoparticles or atoms to form larger clusters. The latter growth can be controlled by using surfactants. If the surfactants used have strong affinity with the gold nanoparticle surface, they cover the surface very early in the ablation and growth process and thus limit the interaction between free atoms or clusters, and reduce the final size of the nanoparticles. In our case, the average particle size remained within the range of 4-6 nm when the CTAB concentration was varied between 0.1 to 3 mM, reflecting the strong affinity of CTAB molecules toward the surface of gold nanoparticles. In the experiment with 0.1 mM CTAB, the concentration of gold nanoparticles estimated from the peak surface plasmon resonance absorption was in the order of 1.5 10 -4 mM, comparable to commercially available samples of gold colloids. This indicates that the ratio of CTAB per nanoparticle of 600:1 and beyond was sufficient to achieve adequate coverage. Finally, we investigated the colloidal stability of the gold nanoparticles produced in an aqueous solution of CTAB compared to those in deionized water. To this aim, the surface plasmon resonance spectra of the nanoparticles were measured over a period of 2 months under ambient laboratory conditions. Figure 5a shows the plot of peak absorbance (indicative of nanoparticle density) prepared in pure deionized water as a function of time. In this case, a number of different pulse energies, namely, 100, 300, and 500 μJ, were investigated. These different pulse energy conditions yield different nanoparticle production rates in those samples. As a result, the peak absorbance of the nanoparticles at the starting point (0 day) differs. It is evident from the figure that the gradients are different for the three pulse energies and a higher gradient is observed for the 500 uJ/pulse, implying faster degradation of the colloids ablated at higher pulse energies. Figure 5b shows the peak absorption measurements of gold nanoparticles produced in three different concentrations (0.1, 1, (26) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104(35), 8333–8337.
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Figure 5. Stability of gold nanoparticles in (a) in pure water at various laser energies and (b) in various CTAB concentrations.
and 3 mM) of CTAB and in pure deionized water, monitored over a period of 2 months. The gradients are the same even though the concentrations of nanoparticles are different, in contrast to the data shown in Figure 5a. This absence of correlation between nanoparticle concentration and agglomeration implies that the nanoparticles are adequately encapsulated for all concentrations of CTAB examined. The stability of the nanoparticles produced in CTAB can be explained as follows: CTAB has a positively charged trimethyl ammonium group (-N(CH3)3þ) and a long carbon chain (C16H33-). The trimethyl ammonium group attaches to the gold nanoparticle surface due to electrostatic interaction with the trimethyl ammonium group pointing inward. The second layer of CTAB molecules will have the trimethyl ammonium group pointing outward. The formation of such double surfactant layers on particle surfaces leads to improved colloidal stability. By comparison, the nanoparticles produced in pure water were less stable and the peak absorbance decreased by 48% over the 2 month period. We have also measured the electrostatic stability of the gold nanoparticles by measuring the ζ-potential immediately after the particles had been synthesized. Figure 6 shows the ζ-potentials of the gold nanoparticles in pure water and in different concentrations (0.1, 0.5, and 1 mM) of CTAB. The ζ-potential of the particles in pure water (-28 mV) indicates that they are negatively charged16 and the colloids are not very stable. However, the ζ-potential of the particles in CTAB with values in the range from þ32 to þ 40 mV, indicating positive charge, shows the nanoparticles produced in CTAB are stable. It is interesting to compare these stability measurements with those of nanoparticles produced in other surfactants. For example, Besner et al.19 reported that only a small fraction of the particles produced in dextran (2 g/L) sediment after 1-2 weeks. In contrast, we observed only small sedimentation in the particles produced in CTAB over a period of 2 months. Other reports of Langmuir 2010, 26(5), 3156–3159
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Figure 6. ζ-potential of gold nanoparticles in pure water and in various concentration of CTAB.
gold nanoparticles formed in CDs had varying stabilities depending on the type of CDs employed and the concentration used. Kabashin et al.19 mentioned that a loss of 7% in the peak absorbance was observed after 45 days when the ablation was performed in 10 mM β-CD. In our case, a similar loss of 10% was recorded after 60 days by using 100 times less concentration (0.1 mM) of CTAB, which again demonstrates the efficiency of small concentrations of CTAB. The understanding of the role of CTAB and other surfactants have on the colloidal stability of nanoparticle suspension produced by laser ablation extends beyond a simple “cause and effect” relationship. One of the complexities is the effect of particle concentration. This quantity is expected to have profound implications for the understanding of colloidal stability measurements. For example, it is reasonable to expect that the rate of aggregation will be concentration dependent, with higher concentration of nanoparticles in the colloid more likely to lead to aggregation. As a result, we expect that there would be a relationship between nanoparticle concentration and the stability. The results presented herein showing the effect of CTAB on (27) Besner, S.; Kabashin, A. V.; Meunier, M. Appl. Phys. A: Mater. Sci. Process. 2007, 88, 269–272.
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nanoparticulate concentrations within the range comparable with commercially available preparations substantiate our claim that this surfactant plays a dominant role in our experiment as opposed to the nanoparticle concentration, but this is not necessarily so in the case of other works, especially those quoted in Figure 4. We have also observed other, more complex effects affecting colloidal stability, such as size distribution of nanoparticles. In a similar experiment to that reported here, we examined two samples made in pure water with two different pulse repetition rates over the same exposure time, associated with different amounts of incident energy on the sample. In this experiment, we found that the sample produced at a high repetition rate (1 kHz) degraded at a slower rate over time compared to the sample of low (0.1 kHz) repetition rate, despite the fact it had a higher nanoparticle concentration. In this case, it was also noted that the high repetition rate produced a narrower size distribution, which is consistent with the postprocessing mechanism reported by Besner et al.27 in which larger colloids are fragmented by subsequent laser pulses, thereby producing smaller colloids with reduced aggregation.
Conclusion Gold nanoparticles were prepared by femtosecond laser ablation of a gold target in pure deionized water and in aqueous solutions of cetyl trimethylammonium bromide (CTAB). The mean diameter of the particles produced in CTAB was smaller (in the range of 4-6 nm) compared to that produced in pure deionized water. The gold nanoparticles in different concentrations of CTAB showed excellent colloidal stability over a test period of 2 months. Most importantly, small concentrations of CTAB (∼0.1 mM) were sufficient to obtain comparable size distribution and superior stability to that of other surfactants, where significantly higher concentrations were typically utilized. Supporting Information Available: Relationship between nanoparticle size and laser ablation conditions in refs 19-24 (Table 1). TEM image of gold nanoparticles produced in 0.1 and 1 mM concentrations of CTAB. This material is available free of charge via the Internet at http://pubs.acs.org.
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