Effect of Alkanethiol Chain Length on Gold Nanoparticle Monolayers

ARTICLE pubs.acs.org/Langmuir

Effect of Alkanethiol Chain Length on Gold Nanoparticle Monolayers at the Air Water Interface Katelynn D. Comeau and M. Vicki Meli* Department of Chemistry and Biochemistry, Mount Allison University, 63C York Street, Sackville, New Brunswick, Canada

bS Supporting Information ABSTRACT: In this study, we report the effects of the alkyl chain length on alkanethiol-capped gold nanoparticle Langmuir films. Gold nanoparticles (2 3 nm) capped with CnH2n+1SH (n = 5 12, 14 16, 18) were prepared via a two-phase synthesis. The films were sampled by Langmuir Schaefer horizontal transfer at various points in the pressure area isotherm and monitored with transmission electron microscopy. Changes in surface pressure, temperature, and alkyl chain length did not lead to observable differences in the mesoscale film morphology. Pressure area isotherms at 22 °C, however, revealed that the work of compression and the collapse pressure are directly dependent on alkyl chain lengths of 14 carbons or greater. Variable temperature isotherms suggest that the work of compression is strongly affected by the phase state (i.e., crystalline vs liquid-like) of the gold thiolate self-assembled monolayer (SAM) capping the nanoparticles.

’ INTRODUCTION The study of gold nanoparticles and their applications has flourished since the synthesis of exceptionally stable alkanethiolcapped gold nanoparticles was established.1 The interesting sizedependent optical, electronic, and chemical properties of gold (and other) nanoparticles make them attractive as components in chemical/biochemical sensors, catalysts, and plasmonics applications.2 4 Several applications would benefit from the ability to form arrays in order to tune the properties from a single nanoparticle to collective optical and electronic response. Interfacial films of nanoparticles are of great interest for their more recently demonstrated mechanical behavior, also tunable via control over the nanoparticle/ligand size and nanoparticle packing.5 7 Among the various techniques used to generate such arrays,4,8 the most promising approaches can be applied to large areas, and have sufficient generality as to be applicable to a wide variety of nanoparticle shapes, sizes, and surface chemistry. Such a patterning method would allow one to fully access the anticipated range of interesting properties that nanoparticles, and their films, promise. Self-assembly of monolayer films at the air water interface (Langmuir films) remains a robust approach in forming largescale (cm2) films of amphiphiles with controllable areal density and phase. The approach is amenable to hydrophobic gold nanoparticles capped with alkanethiols to form close-packed arrays. Generally, low density films with poor surface coverage are observed for smaller (2 3 nm) nanoparticle core diameters while larger core sizes (>4 nm) tend to result in close-packed films. In studying a variety of metal nanoparticle/ligand pairs over a range of temperatures, Heath et al. found that the monolayer morphology could be classified in terms of the ratio between core diameter (d) to ligand length (l) and the resulting r 2011 American Chemical Society

free volume available to the ligand alkyl chains.9 The surface pressure at a given nanoparticle areal density was also reported to decrease with increasing temperatures. Several studies indicate that, during film spreading, the nanoparticles are “frozen” into their respective film morphologies once the solvent has evaporated,10 13 and that increasing surface pressure does not correspond with a change in the particle-to-particle spacing.12 15 While improvements to the packing of nanoparticle films have largely focused on core sizes of 4 nm or greater, to date, Langmuir films of 2 3 nm particles remain restricted to low density film morphologies despite their promising utility.16 Nonetheless, the surface pressure vs area isotherms of such Langmuir films reflect changes in the nanoparticle packing with increasing areal density and are affected by nanoparticle core size/shape, as well as the characteristics of the capping ligand. In this study, we aim to determine how the alkanethiol coating might contribute to the observed monolayer formation and behavior. Nanoparticle monolayers are known to pack such that the edge-to-edge distance between nanoparticle cores is less than twice the length of fully stretched, all trans, alkyl chains.17,18 Thomas et al. have shown that alkyl chain length can affect the organization of nanoparticle films for thiol-capped 4.5 nm Pd particles assembled from solution onto solid supports.19 To date, the effects of alkanethiol chain length seen in several studies9,20,21 have been linked to trends in d/l ratio and subsequent packing driven by van der Waals forces. However, exceptions to the trend have suggested that using d/l ratio as the operative parameter has limitations.19 Studies of gold nanoparticles in the solid state have Received: July 26, 2011 Revised: September 28, 2011 Published: November 28, 2011 377

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Table 1. Average Diameter of Nanoparticles Synthesized with Varying Alkyl Chain Lengths in Thiol Capping Agent (CnH2n+1SH) n, Number of Carbons Diameter (nm)

5 2.9

6 3.0

7 2.4

8 2.6

9 2.8

10 2.3

11 2.4

12 2.4

14 2.7

15 2.0

16 2.5

18 2.7

Standard Deviation (nm)

0.8

0.9

0.5

0.6

0.8

0.4

0.8

0.7

0.7

0.4

0.5

0.7

shown that the alkanethiol coating can exhibit different degrees of order at room temperature, depending on the alkyl chain length. Highly ordered, crystalline-like thiol monolayers have been observed for chains of 16 carbons or greater at 25 °C by infrared spectroscopy17 corresponding to alkyl chains ordered in an all trans configuration. Shorter alkyl chains contain increasingly more gauche defects, whereupon alkyl chains of 8 carbons or less are in a liquid-like state. In this study, Langmuir films of nanoparticles coated with a range of alkanethiol chain lengths were studied for effects on film morphology and compressibility. By performing the film compression experiment at different temperatures, we demonstrate that the phase state of the alkanethiol coating plays a key role in determining the film resistance to compression and collapse.

’ METHODS AND MATERIALS Materials. Gold(III) chloride trihydrate (99.9+%), sodium borohydride (reagent grade), tetraoctylammonium bromide (98%), chloroform (HPLC grade), and all thiols (reagent grade) were purchased from Sigma-Aldrich Canada. Anhydrous ethanol was obtained from Commercial Alcohols Canada. Toluene (99.5%) was purchased from ACP Chemicals Inc., and benzene (99%) was obtained from Caledon Laboratory Chemicals. All chemicals were used as received. Deionization of distilled water was performed using a Elga Purelab UHQ filtration system to yield water with 18.2 MΩ resistivity. Copper TEM substrates (coated with carbon and Formvar, 400 mesh) were purchased from Ladd Research Industries. Gold Nanoparticle Synthesis. The Brust-Schiffrin two phase synthesis was followed.1 Briefly, an aqueous HAuCl4 3 3H2O (0.03 M, 30 mL) solution was prepared, to which a toluene solution of tetraoctylammonium bromide (0.05 M, 80 mL) was added. The mixture was stirred until the toluene phase had turned a dark red color and the aqueous phase remained colorless (approximately 25 min). Next, 0.84 mmol of the desired thiol was added to the organic phase and left to stir until the solution changed from red to colorless. Within 2 4 h of thiol addition, a fresh aqueous solution of NaBH4 (0.4 M, 25 mL) was added dropwise at a rate of 2 3 drops/second. The solution quickly turned dark brown and was left to stir in an open flask for 2 3 h. The organic phase was separated, concentrated to 1000 nanoparticles.

’ RESULTS Characterization by TEM. After synthesis, the average particle diameter was estimated from the measured nanoparticle area for films prepared at 0 mN m 1 (Table 1). The mesoscopic nanoparticle film morphology was also investigated microscopically using TEM to compare nanoparticle films formed at different surface pressures, temperatures, and for various alkanethiol chain lengths.24 Generally, we observed two distinct film morphologies in all films before evidence of bilayer 378

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Figure 4. Measured collapse pressure of nanoparticle films with various thiol chain lengths at 13 °C (left), 22 °C (middle), and 40 °C (right).

Figure 2. Isotherms for various alkanethiol chain lengths at 22 °C. Collapse pressure was determined from the change in slope, shown for the heptanethiol-capped nanoparticle film (top).

Figure 5. Work required to compress a monolayer to collapse for various thiol chain lengths at 22 °C.

Also apparent in Figure 2 is the significant variation in collapse pressure and range of compression with nanoparticle ligand length. The collapse pressures were plotted as a function of alkyl chain length (Figure 3). As seen in Figure 2, the low compressibility region appears to increase significantly for the longer alkyl chains (16 and 18 carbons), while the slope appears to decrease. Measurements of slope alone, however, do not yield significant differences once the variation in polydispersity across all nanoparticle batches is considered (Supporting Information). Effect of Temperature on Isotherms. Representative isotherms for alkanethiol-capped nanoparticles at 13, 22, and 40 °C are available in the Supporting Information. The experiments were conducted by first heating or cooling the water subphase to the desired temperature, spreading the nanoparticles, allowing for solvent evaporation and finally compressing the film. In order to account for changes due to the solvent evaporation rate, a second heating/cooling method was used. Here, the solution was spread at room temperature (22 °C) with 20 min elapsed for solvent evaporation, before the subphase was heated/cooled to the appropriate temperature. The average collapse pressure obtained by each method does not change significantly (Supporting Information). Analysis by TEM also did not indicate a significant change in the overall film morphologies. The collapse pressures of gold nanoparticles with various alkyl chain lengths at various temperatures are plotted in Figure 4. The work of compression (plotted in Figure 5) was calculated for each of the isotherms by estimating the area under the curve until the collapse point.

Figure 3. Collapse pressures of varying alkyl chain lengths at 22 °C on gold nanoparticles. Error bars represent the standard deviation of three replicate measurements. Inset: Reported values for chain-melting transition temperatures, Tm (taken from ref 15).

or multilayer formation (collapsed films). As seen in Figure 1, foam-like films were characterized by the presence of numerous regions void of at least 3 nanoparticles, whereas continuously close-packed films occasionally displayed defects that would otherwise contain up to 3 nanoparticles. However, there was no discernible trend observed in the mesoscale morphologies observed for changes in the alkanethiol chain length or temperature. At all pressures and temperatures, it was common to see both monolayer morphologies. Furthermore, partially collapsed films (Figure 1) were occasionally observed for films collected at pressures 5 10 mN/m before the isotherm-determined collapse point. Effect of Thiol Alkyl Chain Length on Isotherms. Representative isotherms for all chain lengths examined can be found in Figure 2. The area/nanoparticle was estimated by including a calculation of average molecular weight for each batch of nanoparticles synthesized.25 Despite this correction, there remains much variability in the onset pressure of the isotherms from one synthetic batch of nanoparticles, and from one solution to another. The polydispersity in nanoparticle diameter and uncertainty in solution concentration, however, are likely the source of this variability, as the isotherms collected from a single solution were reproducible to within approximately (3 mN/m and (2 nm2/particle.

’ DISCUSSION Nanoparticle Size and Film Morphology. Analysis of the nanoparticle core diameter measurements from TEM images reveals that the average nanoparticle size varies between 2 and 3 nm and the polydispersities range approximately 20 30% across all batches. This range in size distributions leads to 379

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variations in the number of nanoparticles spread from one solution to the next. Given the presence of foam-like film morphologies, it is not surprising to see the large variations in nanoparticle areal density (position of the isotherms along the x-axis) apparent in Figure 2. Furthermore, the variations in collapse pressure observed for alkanethiols with less than 14 carbons (Figure 3) might be attributed to differences in polydispersity, while the work of compression data (Figure 5) closely follows the variations in average core diameter. As described earlier, the monolayers exhibit both foam-like and continuous close-packed regions for all alkyl chain lengths tested. Previous studies have shown that upon solvent evaporation, Langmuir films of thiol-capped gold nanoparticles form macroscopic islands consisting of randomly oriented, crystalline domains.11 Solvent and large changes in concentration have been shown to affect the film morphology of the island domains. However, voids are commonly observed for similar spreading conditions (0.6 mg/mL and chloroform solvent).9,10,13 During compression, the nanoparticle islands are gathered together to form a rigid continuous film, where organization and spacing are not affected by compression until surface pressures greater than the collapse pressure are reached. It follows that the marginal changes in interfacial energy as accessed by changes in alkanethiol chain length in our experiments did not result in changes in overall film morphology. It is possible, but beyond the scope of this study, that the extent of foam-like vs continuous morphologies varies across alkanethiol chain lengths, which would also account for some of the variability in areal density observed when comparing all samples. Disagreement in the collapse pressure as observed from TEM imaging and the pressure area isotherms was occasionally observed in our studies. This is likely a reflection of the inhomogeneity in the film morphology especially during the collapse process. It is not surprising, however, given that the determination of macroscopic film collapse from surface pressure measurements would not be sensitive toward the detection of microscopic or localized film collapse. Nanoparticle Film Compression. Inspection of Figure 4 and Figure 5 indicates a clear dependence of the measured collapse pressure and work of compression on the length of the capping ligand. Nanoparticles capped with alkanethiol of 14 carbons or more have distinctly higher collapse pressures over those with 5 to 12 carbons. The collapse pressure also appears to increase with the number of carbons once the minimum number of carbons has been reached (12 in this case). Moreover, the work of compression follows the same trend, and more clearly differentiates between alkyl chains with greater that 12 carbons from the rest tested. The significance of 12 carbons in alkanethiol SAMs on nanoparticulate gold can be understood by taking into account the dynamic nature of the SAMs. Differential scanning calorimetry and variable temperature NMR and FT-IR studies by Badia et al. indicate that gold nanoparticles of 2 3 nm core diameter undergo an order-to-disorder phase transition at temperatures dependent on the number of carbons in the alkyl backbone (Figure 3 inset). The transition is broad, typically spanning ∼25 °C, indicating a progressive melting process which starts at the nonbonded end and propagates toward the middle of the alkyl chain.17 At 22 °C, dodecanethiol-capped gold nanoparticles are past their associated Tm of 3 °C, while tetradecanethiol-capped gold nanoparticle SAMs are likely to be only partially disordered with a Tm of 22 °C. Certainly, deviations in nanoparticle size,

Figure 6. Work required to collapse a nanoparticle film with various thiol chain lengths at 13 °C (left), 22 °C (middle), and 40 °C (right).

shape, and purity could be expected to contribute to changes in exact values for Tm. Such effects of nanoparticle curvature on alkanethiol chain mobility have also been suggested for changes in signal strength for methylene units by 1H NMR.26 The similarity in nanoparticle preparation, size, and polydispersity, however, suggests that the reported values serve as useful guides to this study. The dependence of collapse pressure and work of compression on temperature (Figure 4 and Figure 6) show that the compression properties of 2 3 nm gold nanoparticle monolayers can be highly tuned with temperature when ΔT spans Tm. The nanoparticle films with alkanethiol already in a fully disordered state (T . Tm) exhibit a small decrease in collapse pressure with increasing temperature, in agreement with previous work of Heath and co-workers.9 In contrast is the temperature dependence, or lack thereof, for films where the alkanethiol SAM remains in a rigid, ordered state. As the alkanethiol monolayer undergoes the order-to-disorder transition, the alkyl chains gain greater conformational degrees of freedom as energy is provided to disrupt van der Waals forces which otherwise stabilize the ordered, all trans state. Increased rotational freedom can allow for more efficient packing within the nanoparticle monolayer through entanglement or by bending to fill any available space accessible to the thiol chain without requiring movement of the nanoparticle core.27 It follows that rotational freedom can be slightly increased with temperature for methylene carbons at some distance from the gold core, leading to the small temperature dependence observed for films of heptanethiol and octanethiol-capped particles, while no significant dependence is observed for octadecanethiol-capped particles. The very high collapse pressures observed for C16SH- and C18SH-capped nanoparticles, which are in a frozen, ordered state at 22 °C, suggest that the nanoparticles are more resistant to monolayer collapse. Furthermore, despite the larger variations seen in the measurements for these films (Figure 3 and Figure 5), Figure 5 clearly shows that work of compression is dependent on the number of carbons in addition to the state of the alkanethiol chains. For example, while both SAMs are likely to be in a rigid/ ordered state at 22 °C, an additional 50 kJ per mol of nanoparticles is required to collapse the C18SH-capped nanoparticle film compared to its C16SH counterpart. Similarly, at 13 °C an extra 50 kJ per mol of nanoparticles is required to collapse the C18SHcapped nanoparticle film compared to that of C14SH. Thus, we approximate that an average of 19 kJ per mol of methylene units is required to compress nanoparticle films where the alkanethiol SAM is in a rigid/ordered state. Without more detailed knowledge of the exact number of thiols involved in the nanoparticle nanoparticle interactions, we have made a simple approximation based on the following assumptions. First, we assumed that it is primarily the thiols attached to an equatorial perimeter of each 380

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nanoparticle residing at the air/water interface that are interacting with nearest neighbors. Using the truncated octahedral structure of a 2.5 nm nanoparticle as determined by Whetten and co-workers,28 such a perimeter would consist of four Au(111) lattice planes and two Au (100) planes. Using the known c(4  2) packing density of alkanethiols on planar Au (111) films, approximately 36 alkyl chains per nanoparticle could be involved in nearest-neighbor interactions.29 By estimating 1 kJ mol 1 per methylene unit needed to disrupt attractive van der Waals interactions, we calculate 36 kJ mol 1 NP 1 per methylene unit to contribute to the work of compression of nanoparticles with ordered SAMs. Given this rough estimation, the order-ofmagnitude level of agreement suggests that a model based on the disruption of alkyl chains from an all trans configuration can reasonably account for the temperature effects seen in these films.

(4) Tao, A. R.; Huang, J.; Yang, P. Acc. Chem. Res. 2008, 41, 1662–1673. (5) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.; Lin, B.; Lee, K. Y. C.; Cerda, E. Science 2008, 320, 912–916. (6) Leahy, B. D.; Pocivavsek, L.; Meron, M.; Lam, K. L.; Salas, D.; Viccaro, P. J.; Lee, K. Y. C.; Lin, B. Phys. Rev. Lett. 2010, 105, 058301. (7) Mueggenburg, K. E.; Lin, X.; Goldsmith, R. H.; Jaeger, H. M. Nat. Mater. 2007, 6, 656–660. (8) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. ChemPhysChem 2008, 9, 20–42. (9) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189–197. (10) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol., B 2001, 19, 2045–2049. (11) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. J. Vac. Sci. Technol., B 2001, 19, 115–120. (12) Pei, L.; Mori, K.; Adachi, M. Colloids, Surf. A: Physicochem. Eng. Aspects 2006, 281, 44–50. (13) Schultz, D. G.; Lin, X.; Li, D.; Gebhardt, J.; Meron, M.; Viccaro, J.; Lin, B. J. Phys. Chem. B 2006, 110, 24522–24529. (14) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Langmuir 2004, 20, 2274–2276. (15) Fukuto, M.; Heilmann, R. K.; Pershan, P. S.; Badia, A.; Lennox, R. B. J. Chem. Phys. 2004, 120, 3446–3459. (16) Bera, M. K.; Sanyal, M. K.; Pal, S.; Daillant, J.; Datta, A.; Kulkarni, G. U.; Luzet, D.; Konovalov, O. EPL (Europhysics Letters) 2007, 78, 56003. (17) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475–481. (18) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G. J. Am. Chem. Soc. 1995, 117, 12537–12548. (19) Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 2000, 104, 8138–8144. (20) Kulkarni, G. U.; Thomas, P. J.; Rao, C. N. R. Pure Appl. Chem. 2002, 74, 1581–1591. (21) Landman, U.; Luedtke, W. D. Faraday Discuss. 2004, 125, 1–22. (22) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem.—Eur. J. 1996, 2, 359. (23) Samples obtained near the end of their compression range were obtained ∼ 10 mN/m below the determined collapse pressure. (24) Microscopic changes in morphology, such as interparticle distances and orientations, were not measured in these studies. (25) For the purposes of calculation, nanoparticles were estimated to be spherical with the same density as bulk gold. (26) Hostetler, M. J.; Wingate, J. E.; Zhong, C.; 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. (27) We have not observed any evidence that the foam-like phase is lost at high surface pressures, but rather we’ve observed that a mixture of continuous films and foam-like films can persist after several (>10) compression/expansion cycles. (28) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428–433. (29) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853–2856.

’ CONCLUSIONS The effects of alkyl chain length and temperature on the compression of alkanethiol-capped gold nanoparticle Langmuir films were studied at 22 °C. Films of greater than 12 carbons in the alkyl chain were found to resist collapse to a much greater extent than shorter chain lengths, and this capacity increased with alkyl chain length. The pressure area isotherms were found to be temperature-dependent, especially for temperature changes spanning the chain order-to-disorder transition of the SAMs where changes in the collapse pressure of >30 mN/m were observed. The film compressibility is thus highly tunable with temperature, especially for temperatures approaching Tm, as well as with alkyl chain length when T , Tm. These results suggest that chain mobility and order play a dominant role in the film compression properties, where longer alkyl chains have more conformational degrees of freedom available for reorganization into more efficient packing arrangements, until eventual collapse. At temperatures below Tm, the additional work required to collapse the films is consistent with the notion that additional energy is needed to overcome attractive van der Waals interactions between ordered chains in order to reorganize. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional graphs and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Louise Weaver at University of New Brunswick for performing the TEM imaging. K.C. thanks Mount Allison University for funding (Goodridge award), and V.M. thanks NSERC and NBIF agencies for research funding. ’ REFERENCES (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. (2) Ghosh, S. K.; Pal, T. Chem. Rev. 2007, 107, 4797–4862. (3) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18–52. 381

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