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Dye-Capped Gold Nanoclusters: Photoinduced Morphological Changes in Gold/Rhodamine 6G Nanoassemblies Nirmala Chandrasekharan and Prashant V. Kamat*,† Notre Dame Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556-0579
Jingqiu Hu and Guilford Jones II* Chemistry Department, Boston UniVersity, Boston, Massachusetts 02215 ReceiVed: June 16, 2000; In Final Form: September 18, 2000
Au nanoparticles (particle diameter ∼2 nm) prepared by thiocyanate reduction method are too small to exhibit the characteristic surface plasmon band. Addition of Rhodamine 6G (Rh 6G) to the colloidal gold solution brings about significant changes in the absorption spectrum. Two distinct peaks appear at 507 and 537 nm. Close packing of the cationic dye molecules on the gold surface induces intermolecular and intercluster interactions. Furthermore, the adsorption of the cationic dye on the gold surface results in surface charge neutralization causing the Au/dye assembly to aggregate. When the Au/Rh-6G solution was subjected to 532 nm laser pulse irradiation for long time intervals, we observed distinct changes in the absorption spectrum due to morphological changes. A growth in the particle size (5-20 nm) is observed as a result of melting and fusion of gold nanoparticles. The photoinduced morphological changes have been elucidated using picosecond laser flash photolysis. The multiphoton process leading to fusion of particles occurs with a rate constant of 7 × 108 s-1. The indirect role of Rh-6G in assisting the fusion of gold nanoparticles is discussed.
Introduction Significant efforts have been made in recent years to investigate the photophysical and photochemical behavior of multicomponent nanostructured assemblies consisting of metals, semiconductors, and photoactive dyes.1-11 Such composite materials are especially useful for developing efficient light energy conversion systems, optical devices, and sensors. Of particular interest are the organic-inorganic hybrid assemblies that have applications in biological sensing and imaging applications.12-14 Engineering of the nanocluster surfaces, with electroactive or photoactive molecules, can provide threedimensional molecular arrangements around the metal oxide nanoparticles. By using an amine-tethering group it has been possible to organize 1-methylaminopyrene chromophores on to gold nanoparticles.15 The surface binding of amine groups on gold nanoparticles suppressed the intramolecular charge-transfer interactions between the amine group and the pyrene chromophore, thereby enhancing the fluorescence yield. Binding of dye molecules to metal or semiconductor nanoparticles can significantly enhance their photoactivity. The ability of organic dyes to sensitize large band gap semiconductor materials such as TiO2 has made them useful for the design of light-energy conversion devices (e.g., photoelectrochemical cells).16-23 Furthermore, the composites of dye aggregate and metal halide nanoclusters have been extensively used in color photography.24,25 In all these instances the interaction between the dye and the semiconductor nanocluster plays an important role in dictating the excited-state properties of dye molecules. Close packing of the dye molecules on the particle surface often leads to aggregation effects.26-28 Such dye aggregates are * Corresponding authors. † E-mail:
[email protected]; or http://www.nd.edu/∼pkamat.
capable of interacting with the support material when subjected to photoexcitation.29 To date, most of these studies are limited to semiconductor nanocluster systems. There is a great deal of interest in understanding the interactions between organic dye and metal nanoclusters and the effect of excited-state quenching and surface-enhanced Raman emission of the surface bound dye molecules.30,31 Often excited-state quenching of the dye molecules on metal surfaces is attributed to energy transfer process.32-36 For silver particles coated with Rhodamine 6G, a blue-shift has been observed for both the silver surface plasmon band and the dye visible absorption band.37 Metal nanoclusters (copper, silver, and gold) embedded in a copper phthalocyanine matrix have been shown to enhance nonlinear optical processes.38 Recently, Makarova et al.35 and Templeton et al.39 have modified the surface of gold nanoparticles with fluorescein isothiocyanate. They showed that the dye is chemisorbed onto the gold particle surfaces without inducing any aggregation effects.35 Laserinduced morphological changes have also been reported for silver and gold nanoparticles.40-43 Fundamental understanding of such events becomes important if one hopes to use metal nanoclusters or composite assemblies for optoelectronic or light energy conversion devices. To probe the photoeffects in a dye-metal nanocluster assembly, we have chosen Rhodamine 6G (Rh-6G) and gold nanocluster systems. Rh-6G is a laser dye with strong absorption in the visible and high fluorescence yield. Its well-characterized photophysical properties have enabled several researchers to probe the microenvironment of a variety of heterogeneous surfaces.27,44-49 This dye readily undergoes aggregation in neat and mixed solvents,45,50-52 and heterogeneous media such as micelles, vesicles,53 clays,54,55 and silica.27,56,57 In the present study we were able to induce H-type aggregation of Rh-6G in
10.1021/jp002171w CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000
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colloidal gold solution. The spectral and morphological changes in the Rh-6G and gold nanocluster assemblies observed during visible laser (532 nm) excitation are elucidated in the present work.
Figure 1. Absorption spectra of Rhodamine 6G (4.35 µM) in aqueous solution containing different concentrations of Au@SCN. The [Au]/ [dye] ratio was maintained at values of (a) 0, (b) 0.8,(c) 3.75, (d) 6.0, (e) 11.7, (f) 26.7, and (g) 32.0
Experimental Section Preparation of Au@SCN Colloids. Gold nanoparticles in aqueous solution were synthesized using SCN- as reductant.58 The reduction of HAuCl4 was carried out as follows: 0.24 mL of 1 M NaSCN was added with stirring to 20 mL of deionized water containing 0.72 mL of 0.014 M HAuCl4 and 0.3 mL of 0.2 M K2CO3. The reduction of the [AuCl4]- by SCN - led to the formation of very small gold nanoparticles (particle diameter 2-3 nm).58 The yellow-colored solution was stabilized at room temperature for 20 min before conducting spectroscopic or laser photolysis experiments. We refer to these particles as Au@SCN in the foregoing discussion. A known concentration of Rh-6G perchlorate (Exciton laser grade) in water was mixed with the Au@SCN suspension to produce dye-Au nanocluster assemblies. The absorption spectra were recorded using a Shimadzu UV3101PC spectrophotometer. The emission spectra were recorded with an SLM S-8000 spectrofluorometer. TEM Measurements. For transmission electron microscopic examination of the samples, a small drop of the freshly prepared solution was applied to carbon-coated copper grids. Particle sizes were determined from the photographs taken at a magnification of 170000 using a Hitachi H600 transmission electron microscope. Picosecond Laser Flash Photolysis. Picosecond laser flash photolysis experiments were performed using 532 nm laser pulses from a mode-locked, Q-switched Continuum YG-501 DP Nd:YAG laser system (output 2 mJ/pulse, pulse width ∼18 ps). Passing the fundamental output through a D2O/H2O solution generated the white continuum picosecond probe pulse. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with a PC. The details of the experimental setup and its operation are described elsewhere.59,60 Time zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported in this study have an experimental error of (5%. The same laser system was employed for longterm irradiation experiments. Typically a solution in a quartz cuvette was subjected to 532 nm laser pulses (4 mJ/pulse, 10 Hz) irradiation with constant stirring for 5-30 min. Results and Discussion Aggregation Effects. Rh-6G exhibits a sharp monomeric absorption band in the visible region with a maximum at 525
nm in aqueous solutions. Significant changes in the absorption characteristics of this dye are seen in the presence of Au@SCN colloids. Figure 1 shows the absorption spectra of Rhodamine 6G (4.35 µM) in aqueous solution containing Au@SCN colloids at different [Au]/[dye] ratios. The concentration of the dye was kept constant in this experiment. With an increasing [Au]/[dye] ratio the absorption at 525 nm decreases and broadens. At an [Au]/[dye] ratio of g11.7, the peak of the monomeric absorption band at 525 nm disappears and two new bands at 507 and 537 nm emerge (spectra e-g in Figure 1). No such absorption changes were observed when the same amount of thiocyanate was added to a Rh-6G solution. It is obvious that the strong electrostatic interaction between the cationic dye and the negatively charged Au@SCN is responsible for these absorption changes. We determined the number of dye molecules associated with each gold nanoparticle by first determining the particulate concentration of gold colloids. The number of gold atoms per particle (NAu) is determined from the expression,61 NAu ) (59 nm-3)(π/6)(DMS)3 where DMS is the mean diameter of the particle. Thus, a gold particle of 2.5 nm diameter will be composed of 483 gold atoms. The [Au]/[dye] ratio of 11.7 would then correspond to 41 dye molecules per gold nanoparticle. The spectra recorded in Figure 1 indicate that the absorption changes at higher gold concentrations ([Au]/[dye] g 11.7) arise from the aggregation of Rhodamine 6G. In an earlier study we have shown that H-aggregates of Rh-6G with a blue-shifted absorption band (abs. max. around 500 nm) are formed upon adsorbing the dye molecules on SiO2 colloids.27 In a similar way, we observe aggregation effects when we maintain the ratio of [Au]/[dye] equal to or greater than 11.7. The spectral changes observed in Figure 1 (spectra e-g) show the appearance of absorption maxima at 507 and 537 nm. These aggregation effects dominate as close packing of the cationic dye molecules around the gold surface induce intermolecular interactions. Although H-type aggregation represents a sandwich-type stacking of dye molecules around the gold nanoparticles, we cannot completely rule out the existence of a herringbone-type aggregation of dye molecules.62,63 Such aggregates are known to exhibit two absorption bands corresponding to two different transitions that are polarized along perpendicular directions. If such aggregation should occur in the present case they can contribute to the absorption bands at 507 and 537 nm. Furthermore, the clustered gold aggregates can also contribute to the 537 nm absorption band. Since the dye adsorption results in surface charge neutralization it renders the gold nanoparticles
Dye-Capped Gold Nanoclusters
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to coalesce. Thus, the clustering of these small gold nanoparticles collectively can induce the surface plasmon resonance. The peak observed at 537 nm (spectrum e in Figure 1) can also arise from the surface plasmon band of the gold cluster assemblies. The absorption arises from the surface plasmon oscillation modes of conduction electrons in the gold nanoparticles that oscillate with a characteristic frequency upon coupling to the incident electromagnetic field. A red-shift as well as broadening of the peak is seen with increasing particle size.5,61,64,65 Although the 2.5-nm-diameter gold nanoparticles do not exhibit any surface plasmon absorption band in the visible, the clustered aggregates can exhibit properties similar to the one observed for larger particles (diameter > 5 nm). The overall aggregation phenomenon can be considered as a two-step process (equilibria 1 and 2):
(Au)n + 2D a {(Au)n ... D2)}
(1)
m{(Au)n ... D2)} a {(Au)n ... D2)}m
(2)
Initially the dye (D) molecules interact with the gold nanoparticles to form dye aggregates on the surface. This phenomenon is evident from the disappearance of the monomeric band. As we increase the ratio of [Au]/[dye] (by increasing the gold colloid concentration), the second step leads to the formation of Au/Rh-6G cluster assembly. The dye aggregation band and the gold surface plasmon band at 507 and 537 nm appear as the gold-dye assemblies coalesce to form larger clusters. The charge neutralization around the gold nanoparticles is likely to facilitate clustering of the Au/Rh-6G cluster assemblies. Photoinduced Changes in the Gold/Rhodamine 6G Cluster Assemblies. Colloidal gold solutions prepared by the SCN reduction method produce very small nanoclusters (particle diameter 2-3 nm).58 These particles are too small to exhibit any noticeable surface plasmon absorption features (spectrum a in Figure 2A). As demonstrated by Henglein and coworkers,66,67 the surface plasmon absorption band evolves only when particles attain a diameter of g5 nm. They reported featureless absorption below 500 nm for very small gold nanoparticles similar to the one observed in the present study. Upon laser (532 nm) excitation of Au@SCN colloids (i.e., in the absence of Rh-6G) we see little changes in the absorption characteristics during laser irradiation for 30 min. The changes are merely restricted to a red-shift (5-10 nm) in the absorption band. Since the absorption of these 2.5 nm diameter particles at 532 nm is very small, we do not expect them to show significant changes under 532 nm laser irradiation. When the Au/Rh-6G solution ([Au]/[dye] ) 11.7), was subjected to 532 nm irradiation for longer intervals, we observed distinct changes in the absorption spectrum. Figure 2B shows the spectra of Au/Rh-6G recorded at different duration of laser pulse irradiation (pulse width 18 ps, 2 mJ/pulse). With increasing exposure time of pulsed laser irradiation, one observes a decrease in the dye aggregation peak at 507 nm and a broadening of the peak in the red region. The inset in Figure 2B shows the difference absorbance spectrum of the laserirradiated solution after 25 min and the nonirradiated sample. The increased absorption in the red region with a broad maximum at 590 nm is seen in this difference absorption spectrum. Such a broadening of the absorption in the red region indicates the formation of larger particles as well as aggregation of gold particles following the laser irradiation. Similar broadening of the absorption spectrum caused by gold particle aggregation has also been observed upon neutralization of surface charge with thio compounds.41
Figure 2. (A) The absorbance spectra of the Au@SCN (0.25 mM) in aqueous medium (a) before irradiation and (b-d) after 532 nm laser irradiation. The irradiation times were, (b) 5 (c)10, and (d) 30 min. (B) Absorption spectra for Rhodamine 6G (4.35 µM) and [Au]/[dye] ) 11.7 in aqueous solution. The spectra were recorded (a) before and (b-d) after 532 nm laser pulse irradiation. The laser irradiation times were (b) 5, (c) 10, and (d) 25 min. The inset shows the difference spectrum of the absorbance for the irradiated sample at 25 min and the non- irradiated sample.
Transmisson Electron Microscopy. The morphological changes induced by 532 nm laser excitation were further examined by transmission electron microscopy. Figure 3, a and b, show the TEM images of Au@SCN before and after interaction with Rh-6G dye, and Figure 3c shows the same suspension (as in Figure 3b), but after subjecting it to 532 nm laser pulse irradiation for 25 min. The gold colloids prepared by SCN- reduction are extremely small with particle diameter in the range of 2-3 nm. Because of the limitations of the TEM facility we could not magnify these particles further. The gold nanoparticles bound to Rh-6G also exhibit similar particle diameter but exist as close-packed clusters (Figure 3b). The laser-irradiated sample (Figure 3c) shows relatively large-size particles of diameter 5-20 nm. The formation of larger-size particles during the laser irradiation of Au/Rh-6G cluster assembly thus supports the conclusion drawn from the absorption spectra in Figure 2B. As shown earlier,41 the laser-induced fusion process can dominate when the gold particles exist as small aggregated clusters. Picosecond Laser Flash Photolysis. To assess the photoprocesses in the gold-dye cluster assembly, we carried out picosecond laser flash photolysis experiments using a 532 nm laser pulse as the excitation source and a white continuum as
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Figure 3. Transmission electron micrographs: (a) Au@SCN colloids with an average particle size of 2-3 nm. The other two micrographs correspond to Rh-6G/Au@SCN colloids ([Au]/[dye] ) 11.7), (b) before and (c) after 532 nm laser irradiation for 25 min.
Figure 4. Time-resolved absorption spectra recorded following 532 nm laser pulse excitation of Rh-6G/Au@SCN solution containing 6.35 µM of Rh-6G ([Au]/[dye] ) 36.5). The spectra were recorded at ∆t ) 0, 750, 1500, and 2500 ps.
the probe. The time-resolved transient absorption spectra recorded at various delay times after the 532 nm laser pulse excitation of Au/Rh-6G solution with [Au]/[dye] ) 36.5 are shown in Figure 4. The transient spectra exhibit an increase in the absorbance around 500 nm as well as in the red region (>600 nm). The strong interference from the emission of Rhodamine 6G prevented us from probing the spectral region of 525-560 nm. The broad transient absorption seen in these spectra increased over a period of 2-3 ns. As shown earlier,40,41,68 with silver and gold nanoclusters, such absorption growth can be correlated with the morphological changes occurring within the cluster assembly. The transient absorption-time profiles at 500 and 680 nm are shown in Figures 5A and B, respectively.
Both the monomer and aggregate forms of Rh-6G absorb strongly in the 400-550 nm region.27 The excited singlet of Rh-6G aggregates has relatively short lifetime (240 ps) compared to that of its monomer form (4.8 ns).27 The primary species that are formed following the laser pulse excitation of the Rh6G solution is the singlet excited state. The decay of the excited singlet state can be conveniently monitored from the recovery of the ground state. Thus, the bleach recovery shown in Figure 5A (trace b) essentially represents the decay of the excited singlet state of Rh-6G as it is being recorded in the absence of gold colloids. In contrast to this behavior, a slow absorption growth (rate constant of ∼7 × 108 s-1) is seen following the laser pulse excitation of the gold/Rh-6G cluster assembly. This growth in the transient absorption indicates that a photoinduced process other than the formation of the excited singlet state of the Rh-6G is occurring in the gold/Rh-6G cluster assembly. The absorption growth at 500 nm is likely to arise either from the increase in the ground-state dye absorption or from the increase in the surface plasmon absorption of the gold clusters. To sort out these two processes we measured the absorption growth at 680 nm. (Note that Rh-6G has no absorption at the monitoring wavelength of 680 nm.) Figure 5B shows the growth of the transient absorption at 680 nm with a rate constant of 7 × 108 s-1. This rate constant is similar to the one observed at 500 nm. Since the gold aggregates are the only species that absorb at 680 nm monitoring wavelength, one can assign this rate constant to the aggregation of gold particles following the laser pulse excitation. Neither the Rh-6G solution nor Au@SCN nanoclusters alone showed any detectable transient absorption at the monitoring wavelength of 680 nm (traces b and c in Figure 5B, respectively). We also probed the laser dose dependence of transient absorbance. The transient absorption spectra recorded 2 ns after the pulse at different laser intensities are shown in Figure 6. At
Dye-Capped Gold Nanoclusters
Figure 5. Absorption-time profiles recorded following 532 laser pulse excitation using probe wavelengths of (A) 500 and (B) 680 nm. The colloidal suspensions were (a) Rh-6G/Au@SCN (Rh-6G ) 8 µM and [Au]/[dye] ) 31.25); (b) Rhodamine 6G (8 µM) alone in the presence of 12 mM NaSCN and 3 mM K2CO3, and (c) Au@SCN (0.5 mM) in aqueous medium.
Figure 6. Transient absorption spectra recorded 2.5 ns after 532 nm laser pulse excitation of Rh-6G/Au@SCN (Rh-6G ) 8 µM, and [Au]/ [dye] ) 31.25) colloids at different laser intensities. The inset shows a plot of the absorbance as a function of the square of the laser intensity at 610 nm.
intensities below 1.6 mJ/pulse, we do not observe any significant change in the absorption. However a dramatic increase in the absorbance is seen at higher laser intensities. A closer examination of the absorption change indicates that this photoinduced
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11107 change occurs via a multiphotonic process. The inset of Figure 6 shows the linear dependence of absorbance (610 nm) on the square of the incident laser intensity. Thus, a simultaneous absorption of at least two photons is necessary to induce the morphological changes in gold-dye cluster assemblies. Since the probability of a multiphoton event (simultaneous absorption of photons) is greater at higher laser intensities, we observe these photoeffects only at higher laser intensities. Similar multiphotonic processes dominate in the laser-induced fragmentation and melting of silver40,69 and gold nanoparticles.43,68,70,71 Role of Rhodamine 6G in the Photoinduced Fusion and Aggregation of Gold Nanoparticles. Because of the significant absorption of the dye aggregates at the excitation wavelength, it is tempting to assign the observed optical effects for Au/Rh6G cluster assemblies to the direct excitation of dye molecules. Several researchers have proposed the possibility of observing energy transfer between excited dye and gold film32-34 or nanoparticles.35,36 Although some direct energy transfer from excited dye into the bound gold nanoparticles is possible, this mechanism alone cannot induce the morphological changes in the Au/Rh-6G cluster assembly. Furthermore, the picosecond spectra recorded in Figure 4 or the transient profiles in Figure 5A do not indicate any direct participation of the excited dye molecules in the laser-induced growth and aggregation of gold nanoparticles. These observations lead us to consider the alternate possibility of direct excitation of the gold nanoparticles. Since the Au/Rh-6G cluster assembly has absorption at 537 nm corresponding to the surface plasmon band, laser excitation would lead to the generation of hot electrons. These hot electrons are rapidly thermalized by electron-phonon scattering. The time scale for this process is around 1 ps in bulk metals, and is similar to that observed for 50 nm particles.72,73 The energy deposited into the phonon modes is subsequently transferred to the surrounding medium on a 10-100 ps time scale.74 When gold particles are dispersed in solution the thermal energy gets dumped into the surrounding solvent. On the other hand, when these particles exist as clustered aggregates, the energy gained from the absorbed photons is dispersed as excess heat into the neighboring particles. Increased temperature of the cluster assembly thus paves the way to the melting of gold nanoparticles to form bigger clusters. These observations are in agreement with the recent report of laser-induced melting of gold nanoclusters and rods.41,68,70 Obviously, such a fusion of clusters requires absorption of several photons by the gold-dye cluster assembly. Indeed, the square of laser intensity dependence observed for the absorption growth in the picosecond laser flash experiments suggests the necessity of more than one photon to induce the morphological changes. At low laser intensities (
