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Modulation of the Photophysical Properties of Gold Nanoparticles by Accurate Control of the Surface Coverage
is really relevant is that by changing the degree of coverage also the distance between the active units bound to the surface can be controlled.
Marco Montalti,* Luca Prodi, Nelsi Zaccheroni, and Gionata Battistini
Experimetal Section
Dipartimento di Chimica “G.Ciamician”, Universita` di Bologna, Via Selmi 2, 40126 Bologna, Italy Received April 8, 2004. In Final Form: June 22, 2004
Introduction Versatility of gold nanoparticles has been largely exploited since stabilization by surface modification was achieved by Brust and co-workers.1 Passivation with thiols allowed binding of almost any kind of molecule to metal nanoclusters and to take advantage of their unusual optical and electronic properties in the design of new nanostructured materials.2 In this context, great interest was devoted to systems appended with fluorescent units with particular attention for pyrene.3 Kamat and Thomas have shown that the properties of the pyrene fluorophore can be efficiently switched by binding to the gold cluster surface and by means of redox input.3a Enhancement of the excimeric fluorescence of pyrene was observed by Katz and co-workers after surface cluster saturation with protected thiol derivatives.3b Also the effect of aging on the photophysical properties was examined3c and more recently the decoupling of a pyrene dimer on the surface of gold nanoparticles has been investiged.3d In this paper we show how fluorescence properties can be modulated by controlling the degree of coverage of the gold surface, suggesting a different approach to optical signal switching in nanosystems. The assembly of a monolayer of a pyrene derivative on the surface of preformed gold nanoclusters was followed in real time by means of absorption and fluorescence spectroscopy. In this way we were able to investigate the interaction of the pyrene fluorophores with gold nanoclusters in situations in which their surface is only partially saturated. Moreover we found out that at low surface densities the pyrene moieties find themselves quite far from each other and no interchromophoric interaction is observed. On the contrary, when the nanocluster surface is saturated, such an interaction takes place. As a result, big changes in the photophysical properties of the system were observed as soon as the surface density of the fluorophore increases. In such a perspective, nanoparticles can be seen as scaffoldings which can be loaded in controlled way. What * To whom correspondence may be addressed. E-mail:
[email protected]. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) (a) Thomas, K. J.; Kamat, P. Acc. Chem. Res. 2003, 36, 888. (b) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (c) Templeton, A. C.; Wuelfing, W. P., Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (d) Labande, A.; Astruc, D. Chem. Soc., Chem. Commun. 2000, 1007. (e) Patolsky, F.; Ranjit, K. T.; Lichtenstein, A.; Willner, I. Chem. Soc., Chem. Commun. 2000, 1025. (f) Chen, S. J. Phys. Chem. B 2000, 104, 663. (g) Fitzmaurice, D.; Rao S. N.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. 1999, 38, 1147. (3) (a) Kamat, P. V.; Barazzouk, S.; Hotchandani, S. Angew. Chem., Int. Ed. 2002, 41, 2764. (b) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 2413. (c) Wang, T.; Zhang, D.; Xu, W.; Yang, J.; Han, R.; Zhu, D. Langmuir 2002, 18, 1840. (d) Werts, M. H. V.; Zaim, H.; BlanchardDesce M. Photochem. Photobiol. Sci. 2004, 3, 29.
Synthesis of Gold Nanoparticles. All the reagents and solvents were purchased from Aldrich and used without further purification. For the preparation of the gold clusters, 1 mL of a water solution of HAuCl4 (1.3 × 10-2 M) was stirred together with 1 mL of toluene solution of TOAB (4.0 × 10-2 M). After 1 h the complete transfer of gold to the organic phase was confirmed by color changes of the two phases: the water solution, initially yellow, became completely uncolored while toluene turned to deep red. After separation of the two phases, 1 mL of NaBH4 water solution (0.1 M) was stirred for 3 h together with the toluene solution containing the [AuCl4]- anion. After gold reduction, a deeply purple solution of metal nanoparticles resulted and their average diameter was measured by transmission electron microscopy (TEM). TEM Measurements. For TEM investigations a drop of nanoparticle in ethanol solution was transferred onto holey carbon foils supported on conventional copper microgrids. A Philips CM 100 transmission electron microscope operating at 80 kV was used. Photophysical Measurements. Absorption spectra were recorded with a Perkin-Elmer Lambda 40 spectrophotometer. Fluorescence spectra were recorded with the Jobin-Yvon Fluorolog spectrofluorometer. The fluorescence lifetimes (uncertainty, (5%) were obtained with an Edinburgh single-photon-counting apparatus, in which the flash lamp was filled with D2. Since absorbance of the solutions at the excitation wavelength changes because of the addition of pyrene derivative 1, inner filter effects should be taken into consideration.4 We preferred to avoid spectral correction by performing a reference experiment where the same changes in the absorbance at the excitation wavelength were due to the addition of a pyrene derivative unable to bind to the gold surface such as 1-pyrenebutanol. We evaluate anyway the effect of spectral correction on our experiments and found out that they left our result unaltered.
Results and Discussion We prepared gold clusters with an average diameter of 7 nm and diluted them in acetonitrile until a concentration of 8 × 10-9 M. The stability of the resulting solution was checked via UV-vis spectroscopy. No changes in the plasmon resonance band at 522 nm were detected over a period of 3 days.3 The pyrene derivative 1 (Scheme 1) was prepared as previously reported.5 Compound 1 presented, in aerated acetonitrile solution, the typical photophysical properties of the pyrene moiety with absorption and fluorescence bands at 341 and 374 nm, respectively, both with sharp vibrational structure. A lifetime of 16 ns was measured for the monoexponential decay of the fluorescent excited state. The gradual coverage of the nanoparticle surfaces was achieved simply by adding increasing amounts of 1 to an acetonitrile nanoparticle dispersion (1 × 10-8 M).6 As reported in Figure 1, we observed only a very weak luminescence for a pyrene concentration lower than 5 × 10-6 M. At higher concentration, on the other hand, the luminescence intensity increased linearly with the amount (4) Credi, A.; Prodi, L. Spectrochim. Acta, Part A 1998, 54, 159. (5) Montalti, M.; Prodi, L.; Zaccheroni, N.; Baxter, R.; Teobaldi, G.; Zerbetto, F. Langmuir 2003, 19, 5172. (6) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. Nanoparticles obtained by reduction of [AuCl4]- in the absence of stabilizing thiols have tetraoctylammonium bromide (TOAB) weakely bound to the surface.
10.1021/la0491044 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004
Notes
Langmuir, Vol. 20, No. 18, 2004 7885
Scheme 1. Simplified Representation of the Organization of the Fluorescent Thiols on the Nanoparticle Surfaces
Figure 1. Changes in the fluorescence intensity at 394 nm of two acetonitrile solutions of gold nanoparticles (8 × 10-9 M) during the addition of 1 (3) or 1-pyrenebutanol (b). Ratio (0) of the intensity at 473 nm with respect to 394 nm (λexc ) 341 nm) in the case of the addition of 1.
of compound 1 added. Such a behavior is consistent with an efficient binding of the thiol to the gold surface and a consequent quenching of the pyrene fluorescence by means of electron-transfer processes.7 Of course this occurs only in the concentration range in which the available gold surface is not completely saturated. The fluorescence quenching is confirmed by the shortening of the lifetime of the excited state which, in such a situation, is τ < 0.5 ns, while in the absence of gold colloids the lifetime is 16 ns. After surface saturation, on the other hand, the pyrene added cannot bind to gold clusters any more and gives a strong fluorescence signal which increases with the concentration. To confirm this result we replaced 1 with 1-pyrenebutanol in the titration experiment; this alcohol derivative presents in fact the same photophysical properties as 1 but it cannot bind to the metal surface.3b Figure 1 shows the two very different titration profiles obtained in the two cases aforementioned. As expected fluorescence intensity of 1-pyrenebutanol increased linearly with the concentration even at low concentration showing no interaction with the gold nanoparticles. Moreover the direct comparison between the two experimental titration curves allowed us to rule out any kind of misinterpretation which could arise from inner filter effects.8 The absence of pyrene quenching in the case of addition of 1-pyrenebutanol was confirmed by its excited-state lifetime which was the same in the presence or in the absence of gold nanoparticles (τ ) 17 ns). From the plot in Figure 1 we could also evaluate the concentration of 1 corresponding to the complete coverage and calculate the thiol density in the saturation condition finding a value (23 Å2 per molecule) compatible with that reported for similar systems.2g We would like to underline that the anchoring of 1 to the gold nanoclusters is, for concentrations lower than the saturation level, fast9 and quantitative. The shortening of the lifetime recorded in the “flat” region of (7) Ipe, B. I.; Thomas, K. G.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 2002, 106, 18. (8) Each pair of points in Figure 1 (triangle and filled circles) correspond in fact to two solutions which have the same absorption at the excitation wavelength and at the emission wavelength. As a consequence their comparison allows a direct quantitative evaluation of the degree of quenching of 1 because of the interaction with the gold nanoclusters (see ref 4). (9) The linkage of the thiol to the gold nanoparticles takes place in less than 1 min.
Figure 2. Fluorescence spectra upon excitation at 341 nm of two acetonitrile solutions of gold nanoparticles (8 × 10-9 M) after the addition of the same amount of 1 (a) or 1-pyrenebutanol (b) (2.5 × 10-7 M).
the titration curve reported in Figure 1 confirms in fact that the residual fluorescence we observe in that region is really due to the fluorophore linked to the nanoparticles. This fluorescence, even if much weaker than the free fluorophore one, is still clearly detectable and suitable for applicative purposes. For this reason we examined more in detail the fluorescence spectra recorded in this “quenched” condition and found out that they show a profile which is strongly concentration dependent. At very low concentrations (2.5 × 10-7 M), the spectrum presents a single narrow band at 394 nm (Figure 2) while a new broad band rises at 473 nm when the concentration of 1 is increased. In particular, as it can be seen from Figures 1 and 3, the relative contribution of this second band is stronger at higher pyrene concentrations. Such a low energy band can be easily attributed to the formation of excimeric or dimeric pyrene species.10 We investigated the dynamics of the formation and deactivation of the excited state responsible for this new emission band by a time-correlated single photon counting technique, and (10) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum Publishers: New York, 1999. (b) Valeur, B. New Trends in Fluorescence Spectroscopy: Applications to Chemical and Life Sciences; Wiley VCH: Weinheim, 2001. (c) Prodi, L.; Ballardini, R.; Gandolfi, M. T.; Roversi, R. J. Photochem. Photobiol., A 2000, 136, 49.
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Figure 3. Changes in the fluorescence spectra (λexc ) 341 nm) of a solution containing gold nanoparticles (8 × 10-9 M) for addition of 1. Each curve corresponds to an additional 2.5 × 10-7 M.
we found a rise time shorter than 0.5 ns and a monoexponential decay indicating an excited-state lifetime of 0.7 ns. Such a lifetime is short with respect with what observed for other pyrene excimers and indicates that also the fluorescence of the low energy excited state is quenched by interaction with the gold nanoparticles through electron-transfer processes.7 The short rise time observed, on the other hand, suggests that the formation of the excimer must be very fast in order to compete with the electron transfer process that deactivates the monomeric excited state. Such a condition implies that in order to have excimeric emission the pyrene molecules must be very close to each other requiring some degree of interaction even at the ground state that could lead to the formation of some dimer-like structures. Even if, because of the complexity of the system, it is extremely difficult to distinguish between excimer-like and dimer-like11 fluorescence, the absorption spectra recorded during the titration of the gold nanoparticles with 1 show a clear perturbation of the electronic properties of the ground state of pyrene. This can be attributed to interchromophore interactions occurring when the surface density of thiols is high enough (Figure 4). At very low concentration, in fact, the absorption maximum of 1 is localized at 341 nm exactly as for the free thiol, while for higher concentrations this peak is red-shifted up to 347 nm. This shift is a clear indication of the environmental changes around the fluorophores due to the increasing of the density of organic molecules on the surface. For concentrations higher than 5 × 10-6 M, namely, after surface saturation, the peak (11) Since the interaction may involve more than just a pair of fluorophores, the use of the term “dimer” could be inappropriate.
Notes
Figure 4. Changes in the absorption spectra of a solution containing gold nanoparticles (8 × 10-9 M) for addition of 1. Each curve corresponds to an addition of 2.5 × 10-7 M going from a to b and 5.0 × 10-7 M going from b to c.
shifts back to 341 nm because of the increase of the fraction of free pyrene. On the basis of the absorption and fluorescence data, the processes of cluster surface modification can be depicted as in Scheme 1. At very low concentrations, 1 moieties bind to the gold colloids without interacting with each other; their fluorescence is quenched and the corresponding band profile is distorted because of the interaction with the metal (Scheme 1a). This interaction does not lead to any shift of the absorption maximum. As soon as the fluorophore density on the surface increases, interactions occur between the pyrene moieties (Scheme 1b) that lead to excimer-like emission. These interactions are also proved by a red shift of the absorption maximum. After surface saturation, absorption and fluorescence spectra are dominated by the contribution of free 1 (Scheme 1c). Summarizing, we demonstrated that it is possible to control the degree of modification of the surface of gold nanoclusters with a fluorescent moiety and in such a way to modulate the fluorescence properties of the resulting assembly by taking advantage from the distance dependency of the intermolecular interaction of the active units on the gold surfaces. In such a perspective nanoparticles can be seen as scaffoldings on which the interactions between different active units can be controlled simply by changing the surface loading degree. This offers a very simple and versatile approach to new functional nanodevices. Acknowledgment. The authors wish to thank MIUR (FISR, project SAIA) and the University of Bologna (funds for Selected Topics) for funding. LA0491044
