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Langmuir 1997, 13, 3944-3947
On the Stability of Carboxylic Acid Derivatized Gold Colloidal Particles: The Role of Colloidal Solution pH Studied by Optical Absorption Spectroscopy K. S. Mayya, V. Patil, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received December 31, 1996. In Final Form: April 29, 1997X Results of an investigation into the influence of hydrosol pH on the optical properties of gold colloidal particles capped with a novel aromatic bifunctional molecule, 4-carboxythiophenol (4-CTP), are presented. Changes in the optical properties of the carboxylic acid derivatized clusters have been interpreted as arising due to flocculation of the clusters and quantified using a “flocculation parameter” (Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763). It is observed that there is a large fall in the flocculation parameter above hydrosol pH ) 4 which then is constant above pH ) 7. This indicates that the cluster distribution is very stable at high pH due to complete charging of the clusters and maximization of the repulsive electrostatic interaction. Contact angle titration measurements on a selfassembled monolayer of 4-CTP on gold revealed an analogous trend with the contact angle falling above pH ) 6 and then remaining constant above pH ) 8. This indicates that monolayer formation of 4-CTP on planar and curved surfaces is similar. However, these results are at variance with earlier similar studies on carboxylic acid functionalized alkanethiols where a decrease in the flocculation parameter was observed for intermediate pH values (3 to 7) (Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763) and a possible explanation is presented.
Introduction There is considerable interest in the area of colloidal particles motivated partly by the different and exotic physicochemical properties exhibited by this “intermediate state of matter”1 and in an equal measure due to the fact that the colloidal route represents an experimentally versatile method for obtaining colloidal particle monolayer films by self-assembly.2-4 One approach for the immobilization of colloidal particles on solid substrates is based on derivatization of the particles with suitable bifunctional molecules and then using the terminal functionality of the capping molecule to covalently bind the molecule to the substrate surface.5,6 An important prerequisite for the above is an understanding of the capping of the colloidal particles with the derivatizing molecule and the stability of the derivatized particles in solution. The process of capping colloidal metal particles such as gold and silver with bifunctional molecules containing thiol groups may be viewed as “threedimensional self-assembly”,7,8 which is expected to be different from self-assembly on planar surfaces, a wellinvestigated area.9 As recently demonstrated by Whitesides et al.,8 optical absorption spectroscopy is ideal for studying problems relating to three-dimensional selfassembly and stability of derivatized colloidal gold par* Author for correspondence: Ph, 0091-212-337044; fax, 0091212-337044; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (2) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (3) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (4) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (5) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (6) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (7) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (8) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (9) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir Blodgett to Self Assembly; Academic Press: New York, 1991.
S0743-7463(96)02140-3 CCC: $14.00
ticles due to the strong surface plasmon resonance of this metal.10 They have presented detailed results of the effect of colloidal solution pH on the stability of gold particles capped with alkanethiol derivatives. The stability of the gold particles was based on a “flocculation” parameter defined as the integrated extinction between 600 and 800 nm in the optical absorption spectra. An interesting aspect of the study was the pH dependent flocculation of carboxylic acid derivatized clusters which showed a reduction in the flocculation between pH values of 3 and 7. This anomalous hump in the flocculation between pH ) 3 and 7 agreed with contact angle titration measurements of carboxylic acid terminated planar surfaces11,12 indicating similarity of the monolayers on the gold particle surface. In this communication, we investigate the pH dependence of the stability of gold colloidal particles in solution capped with an aromatic bifunctional molecule, 4-carboxythiophenol (4-CTP). The analysis is based essentially on that outlined by Whitesides et al.8 with some minor modification. Our investigations do not show a dip in the flocculation parameter at intermediate pH values observed for alkanethiol carboxylates.8 Contact angle titration studies of a self-assembled monolayer (SAM) of 4-CTP on gold showed a reduction in the contact angles above pH ) 6, a result which shows close correspondence with the flocculation data indicating that monolayer formation of 4-CTP is similar on planar and curved surfaces. Experimental Details The gold colloidal particles were prepared by the procedure of Lee and Meisel.13 HAuCl4 (240 mg) was dissolved in 500 mL of water, and the solution brought to a boil. A solution of 1% sodium citrate (50 mL) was added and the boiling continued for 1 h. The resulting colloidal solution had a deep violet color and was exceptionally stable for periods for many months. The asprepared colloidal solution had a pH ) 3. After the gold colloidal (10) Mulvaney, P. Langmuir 1996, 12, 788. (11) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (12) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (13) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.
© 1997 American Chemical Society
Stability of Acid Derivatized Gold Particles
Figure 1. Optical absorption spectra with time from capping with 4-CTP at hydrosol pH ) 7 together with uncapped hydrosol; the times are shown by the curves. The inset shows the flocculation parameter with time for hydrosols at pH ) 7 (filled circles) and 4.2 (filled squares).
Langmuir, Vol. 13, No. 15, 1997 3945
Figure 2. Optical absorption spectra with time from capping with 4-CTP at hydrosol pH ) 4.2 together with uncapped hydrosol; the times are shown by the curves.
The optical absorption spectra of the gold hydrosol capped with 4-CTP at pH ) 7 as a function of time after capping are shown in Figure 1. The spectrum for the as-prepared uncapped gold sol is also shown for comparison. A shift in the surface plasmon resonance from 525 to 533 nm along with a reduction in intensity of the resonance is observed after mixing the bifunctional molecule solution indicating capping of the clusters.10 There are small changes with time in the absorption spectra which can attributed to “flocculation” of the
clusters. This is more clearly observed for the spectra at lower pH values. Figure 2 shows the optical absorption spectra with time from capping of the gold colloidal particles at pH ) 4.2. A large red shift in the surface plasmon resonance and a reduction in intensity are observed for the low pH spectra as well when compared with the optical spectrum of the uncapped gold sol. However, there are additional features not observed in the spectra at pH ) 7. It is seen that immediately after capping (curve 0 min), there is growth of an additional component at ∼680 nm which continues to increase in intensity and shift toward larger wavelengths with time. This is clear indication of “flocculation” of the clusters in quasi-two-dimensional strands15 which leads to elongation of the clusters (an increase in the aspect ratio) and a consequent lifting of the degeneracy in the plasmon vibration modes. Following Whitesides et al.,8 we use the term “flocculation” since the terms “aggregation” or “agglomeration” have more restricted usage. As shall be seen below, it may be possible to differentiate between the nature of the flocs observed in this communication by means of a simple experiment. A comparison of Figures 1 and 2 shows that flocculation is much more rapid at lower pH values. This is due to the fact that at pH ) 4.2, the carboxylic acid groups are expected to be almost completely un-ionized, and hence the repulsive coulombic stabilization interactions are absent while complete charging at pH ) 7 leads to the long term stability observed. While these observations are qualitative in nature, Whitesides et al.8 have defined a semiempirical spectroscopic flocculation parameter which enables quantitative comparisons to be made for colloidal particles under different conditions. The flocculation parameter was defined as the integrated area between 600 and 800 nm in the optical absorption spectrum. We would like to stress here that this semiempirical parameter as defined above is not arbitrary. On the basis of Mie theory calculation of the optical absorption of colloidal particles, Blatchford et al. have shown that as the clusters flocculate into strands, there is a growth of a higher wavelength component which shifts to the red and increases in intensity as flocculation proceeds.15 Concomitantly, there is a small reduction in intensity of the primary, transverse vibration component.15 Therefore, determination of the area under the optical absorption spectrum with suitably chosen wavelength limits encompassing the higher wavelength longitudinal component
(14) Sastry, M.; Patil, V.; Mayya, K. S. J. Phys. Chem. B 1997, 101, 1167.
(15) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435.
solution cooled to room temperature, a drop of the sol was dried on a transmission electron microscope (TEM) grid and the sol imaged at a magnification of 57 000. The average cluster size was determined from the TEM micrograph to be 126 Å while the standard deviation of particle size distribution (fitted to a Gaussian) was 30 Å. The pH of the colloidal solution in the range 3-10 was adjusted using NaOH prior to capping with 4-CTP. This was done since the capped clusters at low pH had a very high flocculation rate and this would lead to problems in interpretation. The capping of the gold colloidal particles was effected by mixing 9 mL of the hydrosol with 1 mL of 4-CTP in absolute ethanol to yield an overall bifunctional molecule concentration of 10-5 M. Assuming an area/molecule of 25 Å2 and total reduction of HAuCl4, this concentration of 4-CTP would lead to complete surface coverage of ∼120 Å gold clusters. The optical absorption spectra were recorded as a function of time for the capped hydrosols at different pH values using a UV-vis Hewlett-Packard 8452 diode array spectrometer (2 nm resolution). The hydrosol pH was adjusted using ammonia and HCl. SAMs of 4-CTP were grown on gold films thermally evaporated onto Si (111) wafers by immersion of the substrate in 10-3 M solution of the bifunctional molecule in absolute ethanol. The time taken for monolayer formation as determined from quartz crystal microbalance studies was typically 4-5 h.14 After thorough washing of the SAM with absolute ethanol and water, advancing contact angles of a sessile water drop were measured using a Rame Hart 100 goniometer. Contact angle measurements were made in air since we did not observe complete wetting of the SAM-covered surface as expected for such hydrophilic surfaces.12 This may be due to the small size of 4-CTP and the delocalized π electrons in the aromatic ring which do not effectively decouple surface carboxylic acid groups from the gold film surface. The contact angle of the bare gold surface was ∼100° indicating a hydrophobic gold surface. The pH of the water used in the titration was adjusted using NaOH and HCl.
Results and Discussion
3946 Langmuir, Vol. 13, No. 15, 1997
Figure 3. Optical absorption spectra of the uncapped gold hydrosol (curve 1, gold sole capped at pH ) 10 (curve 2), capped sol shown in curve 2 with pH reduced to 3 (curve 3), and capped sol of curve 3 with pH increased back to 100 (curve 4).
(value of the “flocculation” parameter) should track the flocculation process accurately. However, from Figure 2 we observe that using the definition for the flocculation parameter in this form for the spectrum recorded after 45 min of aging would lead to a flocculation parameter less than that calculated for the 7 min spectrum. This is counterintuitive since a comparison of the 7 min and 45 min optical spectra shows clearly that the higher wavelength component has red shifted further in this time span indicating additional flocculation of the gold particles. In order to account for such problems, we suggest the following minor modification in the definition of the flocculation parameter. In the first step, all the spectra (including the uncapped sol spectrum) have been normalized to the intensity at the surface plasmon resonance wavelength. The areas of the various absorption spectra were then calculated between the wavelength limits 600 and 800 nm following which the area of the uncapped gold sol was subtracted to yield a modified flocculation parameter. Defined in this fashion, capped clusters which are exceptionally stable and do not flocculate (spectra shown in Figure 1, pH ) 7) would yield a negligibly small flocculation parameter and at the same time this definition would account for problems such as those pointed out above. Unless otherwise mentioned, the flocculation parameters for all the spectra analyzed in this communication are based on the modified definition given above. The inset of Figure 1 shows the flocculation parameters obtained as a function of time for the spectra of 4-CTP capped clusters shown in Figures 1 and 2 at pH ) 7 (filled circles) and pH ) 4.2 (filled squares). It is seen that the flocculation parameter is very small over the time scale of measurement of the absorption spectra (45 min) for clusters at pH ) 7 while the clusters at pH ) 4.2 exhibit a large rate of change in the flocculation parameter until 5 min of aging after which the flocculation parameter increases slowly with time until 45 min to reach a maximum value of ∼200. The hydrosol was still clear with no sign of precipitation after this time period. In order to check whether the flocculation was reversible, a simple experiment was performed with the results shown in Figure 3. The as-prepared gold sol (curve 1) was capped with the bifunctional molecule at pH ) 10 (curve 2) and the pH reduced to 2 (curve 3). After measurement of the optical absorption spectrum at this pH, the pH was quickly increased back to 10 (curve 4). From curves 2-4, it appears that the flocculation which commences at low pH
Mayya et al.
Figure 4. Flocculation parameters calculated for different times from capping with 4-CTP as a function of colloidal solution pH: filled squares, t ) 0.5 min; filled triangles, t ) 2 min; filled circles, t ) 45 min.
cannot be reversed. In fact, the flocculation continued for some time even after the pH stabilized at 10. This indicates that the flocculation is irreversible and thus may be termed “agglomeration” (close, irreversible association of the clusters).8 Figure 4 shows the flocculation parameter as a function of the colloidal solution pH for three different times of aging after capping of 0.5 min (filled squares), 2 min (filled triangles), and 45 min (filled circles). For pH values e6, there is an appreciable increase in the flocculation parameter with time for a particular pH value, and above pH ) 6, the clusters are quite stable. Perhaps the most interesting aspect of this study is the pH dependence of the flocculation parameter for a given time of aging. From Figure 4, a monotonic decrease in the flocculation parameter is observed above pH ) 4.2 with small fluctuations above pH ) 7. We do not observe a fall in the flocculation parameter in the pH range 3-7 followed by an increase at higher pH values as obtained by Whitesides et al. for carboxylic acid functionalized alkanethiols with seven or more carbons in the chain.8 The size of the molecule used in this study would be comparable to the HS(CH2)2COOH molecule used by Whitesides et al.,8 which showed absolutely no variation in the flocculation parameter with pH. These are the main points of deviation between the two studies and may contain important information regarding SAM formation on planar and curved metal surfaces. The similarity between the features observed in the pH dependence of the flocculation parameter and contact angle titration results of carboxylic acid terminated alkanethiol SAMs12 was interpreted as indicating the similarity between SAM formation on clusters and planar surfaces.8 The increase in the contact angle observed for the SAMs in the pH interval 3-7 was explained as arising due to hydrogen bond formation in the regime corresponding to partial charging of the SAM which leads to removal of the hydrophilic COO- and COOH groups from the surface and exposure of the more hydrophobic carbonyl groups.11,12 We attempted a contact angle titration of a SAM formed on gold films as mentioned earlier, and the results obtained are shown in Figure 5. It is seen that up to pH ) 6, the contact angle is nearly constant at ∼77° (the contact angle of the gold surface was 100°) above which it falls to 68° at pH ) 8. Thereafter, the contact angle remains constant. The reduction in contact angle above pH ) 6 may be explained as arising due to charging of the carboxylic acid groups with increasing pH thus
Stability of Acid Derivatized Gold Particles
Figure 5. Advancing contact angles with water drop pH for the SAM of 4-CTP on gold.
leading to increased hydrophilicity.12 The charging of the carboxylic acid groups was earlier explained to lead to increased stability of the gold clusters (reduced flocculation parameter, Figure 4). On the basis of the similarity in the pH dependence of the flocculation data presented in Figure 4 and the contact angle titration results, we conclude that monolayer formation on planar and curved gold surfaces for 4-CTP is similar. While the nature of the curves presented in Figures 4 and 5 is similar, it is to be pointed out that there is a difference in the pH value at which reduction in flocculation and reduction in contact angles takes place. This aspect is not understood at the moment. The behavior seen for SAMs of 4-CTP is similar to that observed for polyethylene carboxylic acid8 but different than for SAMs of carboxylic acid derivatized alkanethiols.12 A possible explanation could be that hydrogen bond formation between carboxylic acid groups in SAMs of aromatic molecules is energetically unfavorable, possibly due to steric and packing constraints. In alkanethiols, distortions in the geometry of the molecule required for hydrogen bonding can be accommodated in the hydro-
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carbon chain which would not be possible for 4-CTP. Studies on the surface crystallography of SAMs of this molecule together with molecular modeling would help in understanding this point of deviation. We believe this is important in the light of current interest in the formation of SAMs of phenyl derivatives16,17 and is currently being pursued. Another difference between SAMs of carboxylic acid derivatized alkanethiols and 4-CTP is the difference in the contact angle in the titration curve at the high and low pH limits. While a well-defined titration curve is obtained for 4-CTP (Figure 5), the magnitude of the difference is small. This may be due to the small size of the molecule (∼7 Å) as well as inefficient decoupling of the SAM surface from the gold support mediated by the π-electron phenyl group. It has also been observed that thioaromatic molecules with one phenyl group form monolayers in which the molecules are tilted and less densely packed than molecules containing larger number of phenyl groups.17 Here again, molecular modeling studies will be important in arriving at a complete understanding of these differences. In conclusion, it has been shown through studies on the optical properties of carboxylic acid derivatized gold clusters and contact angle titration measurements that monolayer formation on planar and curved surfaces for 4-CTP is similar. However, hydrogen bonding between carboxylic acid groups in SAMs for 4-CTP is not energetically favorable unlike for SAMs of alkanethiol carboxylates. pH-induced flocculation of the gold clusters appears to be irreversible. Acknowledgment. K.S.M. and V.P. would like to thank the Council for Scientific and Industrial Research (CSIR), Government of India, for financial assistance. Experimental facilities extended by the Polymer Sciences Division, NCL Pune, are gratefully acknowledged. LA962140L (16) Mohri, N.; Inoue, M.; Arai, Y.; Yoshikawa, K. Langmuir 1995, 11, 1612. (17) Sabatani, E.; Cohen-Boulakia, J.; Bruenig, M.; Rubinstein, I. Langmuir 1993, 9, 2974.
