10320
Langmuir 2005, 21, 10320-10323
Preparation Temperature Dependence of Size and Polydispersity of Alkylthiol Monolayer Protected Gold Clusters Jørgen Møller Jørgensen, Kurt Erlacher,† Jan Skov Pedersen, and Kurt Vesterager Gothelf* Department of Chemistry and Interdisciplinay Nanoscience Center (iNANO), University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark Received July 1, 2005. In Final Form: September 7, 2005 The influence of preparation temperature on the size and size distribution of dodecylthiol monolayer protected gold clusters was studied. The monolayer protected clusters (MPCs) were synthesized by two different variations of the Brust-Schiffrin procedure. In all of the experiments, the stoichiometry of the reactants dodecylthiol, HAuCl4, and sodium borohydride was kept constant, while the temperature was varied in the range of -18 to +90 °C. Two series were performed in which an aqueous solution of NaBH4 was either added over 30 s or all in one portion. The size and size distribution of the MPCs were determined by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It has been demonstrated that in general the MPC size increases with elevated preparation temperatures.
Introduction Monolayer protected clusters (MPCs) of gold and thiols are widely applied in chemical, physical, and biological sciences due to their unique properties and the ease of their preparation.1 Depending on the size, MPCs have properties which are between those of large molecules and those of bulk material. The ability to bind a multitude of different ligands to the gold core enables the design of MPCs with broad variety of different functionalities.1 Combined with their optical and/or electronic properties, the functionalization of MPCs with biomolecules has been a very successful approach in the development of several different types of biosensors.2 The most important factor determining the properties of gold-thiol MPCs is their size and size distribution. The dimensions are typically within the range of 0.5-10 nm. One example of the relation between size and properties is that small dodecylthiol-protected gold particles are highly soluble in organic solvents such as toluene and dichloromethane and their UV spectrum shows no plasmon resonance. Larger MPCs become less soluble as their size extends 5 nm and their UV spectrum shows clear plasmon resonance absorption that increases with size.1,3 The most common method for the preparation of MPCs is the so-called Brust-Schiffrin procedure.4 The mechanism of this procedure is shown in eqs 1 and 2. It is a core nucleation-growth-passivation mechanism.5 The morphology of the monolayer consists of regions with hexagonally packed crystalline domains.6 A systematic study of the preparation conditions was reported by Murray et al.7 It was found that the average size of the MPCs can be controlled by varying the relative amounts of gold and * To whom correspondence should be addressed. Phone: +45 8942 3907. Fax: +45 8619 6199. E-mail:
[email protected]. † Present Address: Bruker AXS Inc., 5465 E. Cheryl Parkway, Madison, Wisconsin 53711. (1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346. (2) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042-6108. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Commun. 1994, 801-802. (5) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735-7741. (6) Jackson, A. M.; Myerson, J W.; Stellacci, F. Nat. Mater. 2004, 3, 330-336.
thiols. Furthermore, the addition rate of the NaBH4 was also found to have some effect on the size and in particular the size distribution. The preparation temperature also affects the size of the MPCs and in a recent study Murray et al. reported on the formation of very small (38 gold atoms) and highly monodisperse MPCs at -78 °C.8
Oct4N+AuCl4- + 3RSH f [-AuISR-]n + RSSR (1) [-AuISR-]n + RSSR + Oct4N+BH4- f Aux(SR)y (2) The importance of the size, size distribution, and shape has urged a number of researchers to develop modified or alternative approaches to obtain higher control of the formation of MPCs.1 Studies on the formation of naked gold nanoparticles using reversed micelles at different temperatures has been performed.9 However, by far the most commonly applied method for their preparation is still the Brust-Schiffrin procedure. Although this procedure has been studied in much detail, to the best of our knowledge, there have not been performed any systematic studies on the effect of elevated temperatures on the size of alkylthiol-gold MPCs. Postsynthesis annealing of clusters at elevated temperatures (>100 °C) prepared according to the Brust-Schiffrin procedure has been studied in the solid state and in solution.10-14 The general trend observed for postsynthesis annealing of alkylthiol protected gold clusters is significantly increased size and decreased polydispersity. Synthesis of MPCs at elevated (7) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L. Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (8) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 19451952. (9) Chiang, C.-L., Hsu, M.-B., Lai, L.-B. J. Solid State Chem. 2004, 177, 3891-3895. (10) Maye, M. M.; Zhong, C.-J. J. Mater. Chem. 2000, 10, 18951901. (11) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490-497. (12) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305-2311. (13) Nakamoto, M.; Yamamoto, M.; Fukusumi, M. Chem. Commun. 2002, 1622-1629. (14) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724.
10.1021/la051770x CCC: $30.25 © 2005 American Chemical Society Published on Web 10/01/2005
Letters
Langmuir, Vol. 21, No. 23, 2005 10321
Table 1. Rg of MPCs Synthesized by Procedures A and B and the Number of Atoms in the Metal Core for the Particles with Size Given by Rg entry
procedure
T (°C)
Rg (nm)
NAu
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
A A A A A A A A A A A A B B B B B B B B B B
-18 -6 0 11 21 30 46 51 60 70 83 88 -17 0 10 23 32 41 54 60 81 90
1.8 1.6 1.9 1.5 1.5 1.8 2.3 2.4 2.6 2.8 2.3 2.6 1.5 1.7 1.5 1.8 2.1 2.3 2.3 2.5 2.8 2.5
1439 1011 1693 833 833 1439 3003 3412 4338 5418 3003 4338 833 1213 833 1439 2286 3003 3003 3857 5418 3857
temperatures according to alternative procedures using other reduction conditions and agents have also been reported.15,16 In one case, preparation of MPCs at elevated temperatures according to the Brust-Schiffrin method has been reported, and it was found that slightly larger particles were obtained at 90 °C.7 In a preliminary study, we observed that elevated temperatures caused a significant increase in size and polydispersity of the MPCs. Since the size is crucial for the properties of MPCs, we have conducted a systematic study of the influence of preparation temperature on the size of the MPCs, which is presented here. Experimental Details MPC Synthesis. Two different preparation procedures have been applied for the synthesis of the MPCs. In all procedures, identical stoichiometry of reactants was applied, and the molar ratio between HAuCl4:thiol:NaBH4 was 4:1:12. In all experiments, tetraoctylammonium bromide was used as the phase transfer agent.1,7 Procedure A. The MPC formation at different temperatures was carried out in series of three experiments as described below. Oct4NBr (1.5 g, 2.74 mmol) was dissolved in toluene (80 mL) and HAuCl4‚3H2O (0.34 g, 0.79 mmol) was dissolved in water (25 mL). The two phases were mixed, and after decoloration of the aqueous phase, the phases were separated and the water phase discarded. Dodecanethiol (47µL, 0,20 mmol) was added to the toluene phase, and the reaction mixture was divided into three equally sized portions and placed in three 100 mL round-bottomed flasks each containing a magnetic stirring bar. Each portion was heated or cooled to the desired temperature, and then a NaBH4 solution (3.16 mmol, 0.4 M) in water (8 mL) was added over approximately 30 s to each flask under vigorous stirring. The reaction mixtures were stirred for exactly 30 min at the desired temperature and then adjusted to room temperature. The solvent was removed, and the resulting solid was suspended in EtOH (10 mL) by ultrasonication. The suspended MPCs were captured on a 3 cm plug of Celite and washed thoroughly with EtOH and acetone. The Celite containing the MPCs was stirred with toluene to dissolve the MPCs, and the Celite was removed by filtration. Finally the solvent was removed by evaporation in a vacuum to give the clusters as a black or dark red solid. The procedure was repeated several times at temperatures between -18 to +88 °C (see Table 1). (15) Fleming, D. A.; Williams, M. E. Langmuir 2004, 20, 3021-3023. (16) Nakamoto, M.; Yamamoto, M.; Kukusumi, M. Chem. Commun. 2002, 1622-1623.
Procedure B. Procedure B was performed as procedure A with the exception that the NaBH4 solution was added quickly in one portion. The procedure was repeated several times at temperatures between -17 to +90 °C (see Table 1) Small-Angle X-ray Scattering (SAXS). The size and the size distribution of clusters can ideally be investigated using the technique of SAXS.17-19 The experiments were performed with an original Bruker-AXS NanoSTAR instrument modified and optimized for solution scattering. The instrument is equipped with a rotating anode generator (Cu KR radiation, operated at 45 kV/90 mA), cross-coupled Go¨bel mirrors, three-pinhole collimation, evacuated beam path, and a 2D detector (HI-STAR).18 The accessible scattering range of the instrument can be varied by selecting different distances between the sample and the detector of 42.2 cm and 66.2 cm. The modulus of the scattering vector is q ) 4π sinΘ/λ, where 2Θ is the scattering angle and λ is the X-ray wavelength. Solutions of the clusters in toluene of approximate 0.1 w/v % concentration were used for the measurements. The concentration was so low that interparticle interference effects could be neglected. The gold clusters were measured in flame sealed boron silicate glass capillaries (diameter of about 1.75 mm, Hilgenberg GmbH, Germany). A capillary filled with only the solvent was used for the background correction. The particles consist of a gold core surrounded by a shell of thiols. The scattering contrast for X-rays is given by the electron density difference between the particle and the solvent. Since the gold has a much higher contrast than the thiols, only the core has to be considered in the analysis. Neglecting particle interaction, the azimuthally averaged scattering intensities of the 2D pattern can be written as I(q) ∝ ∫ D(R) V(R)2 P(q,R) dR, where P(q,R) is the particle form factor, V(R) is the particle volume, and D(R) is the number size distribution. The obtained scattering profiles were fitted using a weighted least-squares method and assuming a form factor for homogeneous spheres, P(q,R)) [3(sin(qR) - qR cos(qR))/(qR)3]2, and a Gaussian number size distribution,18 which was truncated at R ) 0. Since these nanoparticles have a high degree of polydispersity and that it is found to vary significantly, the average particle size is better described by the radius of gyration Rg calculated from the size distribution than by the average radius obtained from the fit. The radius of gyration reflects the weighting of the intensity by the volume squared of the particles and it follows from the 8th and 6th moment of the distribution Mn as
Rg2 )
3 M8 5 M6
(3)
where the nth moment is defined according to
Mn )
∫
∞
0
D(R)Rn dR
(4)
The moments were calculated numerically from the size distribution. Assuming a density FAu of the gold particles being equal to that of the bulk material, the molar mass of gold mAu can be calculated using
mAu ) VFAu
(5)
The number of gold atoms NAu follows from the molar mass of gold and Avogadro’s number to
mAu NAu ) NA MAu
(6)
Tranmission Electron Micoscopy. The contrast is also for this technique given by the electron density. Since the electron density is much higher for the gold core, the thiol layer will not (17) Pedersen, J. S. Adv. Colloid Interface Sci. 1997, 70, 171-210. (18) Pedersen, J. S. In Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter; Lindner, P., Zemb, Th., Eds.; North-Holland: New York, 2002; pp 391-420. (19) Pedersen, J. S. J. Appl. Crystallogr. 2004, 37, 369-380.
10322
Langmuir, Vol. 21, No. 23, 2005
Letters
Figure 1. Typical background-subtracted SAXS data for synthesis by procedure B at -17 (b), +41 (1), and +90 °C (9) with fits derived from Gaussian distributions and form factors of spheres. directly be observed. The TEM pictures were obtained as follows: A drop of a 0.1 w/v % solution of the clusters in toluene was deposited onto a holey carbon film on copper grids (300 mesh) and was allowed to dry in air. Bright field images were taken in a transmission electron microscope Philips CM20 operating at 200 keV.
Results and Discussion The MPCs were synthesized according to the two procedures A and B at 12 and 10 different temperatures, respectively, ranging from -18 to +90 °C. For each of the 22 experiments, SAXS data were obtained and typical SAXS data obtained for MPCs formed at three different temperatures according to procedure B are shown in Figure 1. The radii of gyration (Rg) were obtained, and from these values the average number of gold atoms/MPC was derived as shown in Table 1. Both procedures give MPCs with a minimum average radius of 1.5 nm at the lower temperatures and a maximum average radius of 2.8 nm at the higher temperatures. This corresponds to clusters consisting of below 1000 to above 5000 gold atoms/ MPC. The SAXS data listed in Table 1 were recorded using a distance between the sample and the detector of 42.2 cm. For the 12 samples prepared by procedure A, the SAXS data were also recorded using a sample-detector distance of 66.2 cm. The data obtained were almost identical to the data recorded using the shorter distance. To evaluate the relation between size of the obtained MPCs and the preparation temperature for the two procedures, Rg has been depicted as a function of the preparation temperature in Figure 2. Parts A and B of Figure 2 clearly show that the average size of the clusters increase at elevated preparation temperatures. The inserted lines show the linear regression, and it is clear that there is a significant deviation from linearity of the size/temperature relation for MPCs synthesized by procedure A in which an aqueous solution of the reductant was added over 30 s. For the MPCs synthesized by procedure B where the NaBH4 solution was added in one portion, the size/temperature relation is much closer to linearity with a correlation value R2 ) 0.88 compared to R2 ) 0.67 for procedure A. Thus, panel B provides strong evidence for a linear relationship between average size and preparation temperature, and we believe that the deviation from linearity observed for procedure B is due to variation in the rate of addition of the NaBH4 solution. The polydispersity of the Gaussian size distributions was calculated numerically and is displayed as σ values as a function of the synthesis temperature for synthesis procedures A and B (Figure 3, panels A and B). The inserted lines show the linear regression, and in general, large deviations from a general trend in the σ values are
Figure 2. Plot of MPC radius of gyration (Rg) as a function of T (b). (A) procedure A with addition of an aqueous solution of NaBH4 over 30 s and (B) procedure B with addition of an aqueous solution of NaBH4 in one portion. The Rg values obtained by TEM for the colloids synthesized according to procedure B are also inserted (0).
observed. There is, however, a clear tendency for higher polydispersity at elevated temperatures. This tendency is most outspoken for the particles obtained from procedure B (Figure 3B), and if the outlier data points obtained at 0 and 90 °C are omitted, a satisfying linear fit is observed. The higher degree of polydispersity for MPCs synthesized at elevated temperatures as observed here is significantly different from the decreased polydispersity obtained by postsynthesis annealing of MPCs at elevated temperatures.10-14 It should be noted that annealing temperatures and reaction temperatures in those studies are significantly higher and longer than the preparation conditions applied in the present study. Images obtained by TEM of MPCs synthesized at three different temperatures (-17, +41, and +90 °C) according to procedure B are displayed in Figure 4. A finite separation between the particles is observed due to the presence of the thiol layer, which, however, is invisible in the micrographs. The particle sizes increase with increasing preparation temperature in agreement with the SAXS data. The histograms (Figure 4, panels B, D, and F) are obtained by analysis of the TEM images in Figure 4, panels A, B, and C, and they provide important information about the size distribution of the particles. The data were obtained by electronically counting and determining the size of 610, 1115, and 543 clusters in the TEMs obtained at -17, +41, and +90 °C, respectively. To compare the polydispersity obtained from the TEM analysis with the polydispersity obtained by SAXS, Gaussian fits were made for the histograms in Figure 4. It is evident that the particles become more polydisperse with increased preparation temperature.
Letters
Figure 3. Polydispersity of the size distributions of MPCs displayed as σ values as a function of preparation temperature (b,O). The values are calculated numerically from the Gaussian number distribution of spherical particles used for fitting the SAXS data. Linear fits were inserted to show the general trend. (A) Procedure A with addition of an aqueous solution of NaBH4 over 30 s and (B) procedure B with addition of an aqueous solution of NaBH4 in one portion. If two strongly deviating data are omitted (O), a new linear rectification is obtained (dotted line). The σ values obtained by TEM for the colloids synthesized according to procedure B are also inserted (0).
The radii of gyration of the MPCs synthesized at -17, +41, and +90 °C according to procedure B were determined by TEM to be 1.6, 2.6, and 3.7 nm. These numbers are higher than the numbers obtained by SAXS, however, the numbers show a similar trend; the particle sizes increase with increasing preparation temperature (see Figure 3B). The average sizes obtained by TEM could be higher than the actual sizes since very small particles might not be observed in the TEM images. The polydispersity of the size distributions obtained by both TEM and SAXS also show a tendency to increase with increasing preparation temperature, and they are in good agreement (see Figure 3B). In general, the σ value measured by SAXS for the sample prepared at 90 °C is lower than expected; however, this is ascribed to experimental deviation, since the σ value derived from the linear regression of the σ values is higher. During the synthesis, the thiols work effectively as surfactants covering the surface of the gold particles. There is a dynamic exchange of the thiols and a finite concentration in the solvent between the particles. By increasing the temperature, the equilibrium is slightly shifted so that there is a higher concentration in the solvent and a smaller total amount at the surface of the particles. This effect is comparable to the effect of decreasing the thiol/ Au ratio at a fixed temperature, which in studies by
Langmuir, Vol. 21, No. 23, 2005 10323
Figure 4. TEM images and size distribution histograms of clusters prepared according to procedure B prepared at (A, B): -17 °C (histogram based on 610 particles), (C, D): T ) 41 °C (histogram based on 1115 particles) and (E, F) 90 °C (histogram based on 643 particles). Gaussian distributions were fitted to the histograms in B, D, and E.
Murray et al. led to similar changes in MPC size and polydispersity.7 In summary, we have shown that the particle size of dodecylthiol-gold MPCs increases with increasing preparation temperature and is a linear relation between the average size of the clusters and their preparation temperature. Elevated temperatures presumably favor the rate of core growth more than that of thiolate passivation leading to larger MPCs at higher temperatures. The formation is controlled by kinetic competition, which can also explain the higher polydispersity at elevated temperature. Whereas it has previously been shown that lower temperatures (>0 °C) give rise to smaller and more monodisperse MPCs,8 we have here shown that elevated temperatures give rise to larger and more polydisperse MPCs. In addition to the gold-to-thiol ratio and the method of addition of reductant,7 the temperature is also a important factor to consider in the synthesis of MPCs. Acknowledgment. Toke Krogager Hansen, Anne Roslev Bukh, and Christel Rothe Brinkmann are acknowledged for performing initial experiments as a part of their first year iNANO project. We thank Jacques Chevallier, Department of Physics at the University of Aarhus, for the assistance in the performance of the TEM measurements. This study was funded in part by the Danish Technical Research Council, the Danish National Research Foundation, and The Carlsberg Foundation. LA051770X
