Ultrafast Laser Studies of the Photothermal Properties of Gold

Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700, INEST ... peak depends on the material, morphology, and geometry of...
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2006, 110, 1520-1524 Published on Web 01/10/2006

Ultrafast Laser Studies of the Photothermal Properties of Gold Nanocages Min Hu,†,‡,∇ Hristina Petrova,§,∇ Jingyi Chen,† Joseph M. McLellan,† Andrew R. Siekkinen,† Manuel Marquez,‡,| Xingde Li,⊥ Younan Xia,*,† and Gregory V. Hartland*,§ Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700, INEST Group, Research Center, Philip Morris USA Inc., Richmond, Virginia 23234, Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670, NCTCN Center, Physical and Chemical Properties DiVision, NIST, Gaithersburg, Maryland 20899, and Department of Bioengineering, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: December 8, 2005; In Final Form: December 27, 2005

Au nanocages were synthesized via a galvanic replacement reaction. The extinction peak of these hollow structured particles is shifted into the near-IR compared with the Ag nanocube templates. Energy transfer from the Au nanocages into the surrounding environment (water) as well as the coherently excited vibrational modes of the nanocages were studied by femtosecond pump-probe spectroscopy. The time scale for energy relaxation was found to increase with the size of the particles, with the relaxation time being independent of the laser intensity. The time scales for relaxation are comparable to those for solid spherical gold particles and are consistent with energy relaxation being controlled by heat dissipation in the solvent. The period of the coherently excited vibrational mode is proportional to the dimensions of the nanocages. Intensity-dependent measurements show that in solution the nanocages maintain their integrity up to lattice temperatures of 1100 ( 100 K.

1. Introduction Metal nanoparticles have drawn intense research interest because of their unique optical properties.1-3 The brilliant colors of colloidal metal nanoparticles arise from the collective oscillation of conduction band electrons known as the surface plasmon resonance (SPR).4 The position and shape of the SPR peak depends on the material, morphology, and geometry of the nanoparticles. For example, spherical Au nanoparticles strongly absorb incident light at 520 nm, while cubic-shaped Ag nanoparticles have an intense absorbance at 450 nm.5 Recently, Halas et al. demonstrated that they could tune the SPR peak of spherical Au shell particles into the near-IR region6 and proposed biomedical applications based on photothermal effects.7 We have also investigated the possibility of using Au nanocages as potential candidates for optical coherence tomography (OCT) imaging,8,9 with a focus on targeting breast cancer cells. Au nanocages are synthesized by a galvanic replacement reaction between Ag nanocube templates and HAuCl4.10 By controlling the ratio of Ag to HAuCl4, the SPR peak of the nanocages can be tuned into the near-IR region, which is the spectral window of interest for biomedical imaging.11 * Corresponding authors: G. V. Hartland ([email protected]) and Y. Xia ([email protected]). † University of Washington. ‡ Philip Morris USA Inc. (INEST ) Interdisciplinary Network of Emerging Science and Technologies, a Philip Morris USA Postgraduate Research Program.) § University of Notre Dame. | NIST. ⊥ University of Washington. ∇ These authors contributed equally to this work.

10.1021/jp0571628 CCC: $33.50

In initial studies of the photothermal effects for the nanocages, we found that nanocages on a TEM grid melted and transformed into nanospheres when exposed to a camera flash.9 This is a surprising result and motivated us to investigate the dynamics of energy relaxation following optical excitation. Energy relaxation in metal nanoparticles has been extensively studied over the past several years.12,13 The question of how electrons and phonons exchange energy with each other and with their environment is of fundamental interest to scientists. Most of the work has been done on spherical metal nanoparticles.12,13 For spherical particles larger than approximately 10 nm in diameter, the rate-limiting step for energy relaxation is heat dissipation into the environment.14-16 Here, we present ultrafast laser studies of the energy relaxation and vibrational dynamics of Au nanocages. We expect that the time scale for energy relaxation is the key factor for determining whether structural transformations occur in metal nanostructures: slow relaxation times should favor restructuring. 2. Experimental Section Preparation of Silver Nanocubes. Monodispersed silver nanocubes were prepared using a slightly modified version of a previously described polyol reduction procedure.17 Briefly, 5 mL of ethylene glycol (EG; J. T. Baker) was added to a disposable 6-dram glass scintillation vial (VWR International) and heated in an oil bath at 145 °C for 1 h, followed by the addition of 0.5 mL of a 3 mM solution of HCl (J. T. Baker) in EG. After 10 min, solutions of AgNO3 (1.5 mL of a 94 mM solution in EG; Aldrich) and poly(vinylpyrrolidone) (PVP, 1.5 mL of a 147 mM solution in EG in terms of the repeating unit; © 2006 American Chemical Society

Letters Mw ≈ 55 000, Aldrich) were injected at a rate of 22.5 mL/h using a dual channel syringe pump (KDS-200, Stoelting, Wood Dale, IL). Following injection, the vial was loosely capped. Upon injection of the AgNO3 and PVP, the color of the solution became milky white, then took on a pinkish hue, and finally turned clear over a period of about 6-7 h. The solution was left in this state until 20 h, when the vials were sealed. After sealing, the solution remained clear for an additional 1-2 h, then it gradually became yellow. This color intensified over the next hour, going from dark yellow to reddish-brown and finally a thick greenish-gray. The total reaction time was 22-24 h. Magnetic stirring at 350 rpm was applied throughout the synthesis. The final product was washed and collected by centrifugation for 30 min at 3900 rpm, first with acetone to remove EG then at least twice with water to remove excess PVP. Cubes were the primary product of this synthesis (>95%). They formed stable suspensions in water without any additional stabilizers and were used as templates for the synthesis of Au nanocages. The size of the cubes is controlled by the reaction time.9,17 Preparation of Au Nanocages Using Ag Nanocubes as Templates. In a typical procedure, a 75-µL aliquot of the asobtained dispersion of Ag nanocubes was added to a solution of 5 mg PVP (Mw ≈ 55 000) in 5 mL of deionized water. This solution was then refluxed for 2 min. A specific amount of a 0.2 mM HAuCl4 aqueous solution was added dropwise into the reaction system. Vigorous magnetic stirring was maintained in the entire process until the color of the solution became stable. The solution was cooled to room temperature, and the white AgCl precipitate was removed by dissolving with a saturated NaCl solution.10 The solution was then centrifuged at 10 000 rpm for 15 min, and the supernatant containing the dissolved AgCl was removed using a pipet. The solid was rinsed with water and centrifuged three more times, and finally redispersed in water for further characterization and usage. Characterization of Au Nanocages. The samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were prepared by placing small drops of the dispersions of metal nanostructures on silicon substrates (Silicon Valley Microelectronics, San Jose, CA) and copper grids (Ted Pella, Redding, CA), respectively. The samples were allowed to dry at room temperature in a fume hood. The SEM images were obtained using a field emission microscope (Sirion XL, FEI, Hillsboro, OR) operated at 15 kV. TEM images were taken using a Philips 420 electron transmission microscope operated at 120 kV. The composition of the Au nanocages were analyzed using an atomic emission spectrophotometer (Thermo Jarrell Ash Corp., Franklin, MA) equipped with a Jarrell Ash 955 inductively coupled plasma system. The emission lines at 328.0 and 242.8 nm were used to measure the contents of Ag and Au, respectively. Absorbance spectra were recorded at room temperature on an HP 8453 UV-vis-near-IR spectrometer using disposable semimicro methacrylate cuvettes with an optical path of 1 cm. Photothermal Studies: Transient Absorption Apparatus. A commercial regeneratively amplified Ti:sapphire laser system (Clark-MXR CPA-1000) was used for the transient absorption experiments. This laser produces pulses with 0.4 mJ energy at a 1 kHz repetition rate. The central wavelength is located at 790 nm with a full width at half-maximum of ∼150 fs. A 90: 10 beam splitter was used to produce pump and probe beams. The 10% portion generated a white light continuum in a 3-mm sapphire window, while the 90% portion was frequency doubled in a 1-mm BBO crystal to produce 395-nm pump pulses. The

J. Phys. Chem. B, Vol. 110, No. 4, 2006 1521 intensity of the pump was adjusted by a λ/2-waveplate/polarizer combination. The time delay between the pump and probe pulses was controlled by a stepper motor driven translation stage (Newport UTM150PP.1). The pump and probe beams were gently focused at the sample by separate 8-in lenses. A JobinYvon Spex H-10 monochromator was placed after the sample, so that specific probe wavelengths can be selected to monitor the sample response. The probe wavelengths used were in resonance with the long-wavelength band observed in the UVvis absorption spectra. This arrangement of near-UV pump and near-IR probe gives the maximum sensitivity and allows us to work at both very low and very high excitation levels.13 The damage threshold of the nanocages in aqueous solution was determined to be 20 µJ/pulse by observing whether the pump laser induces a color change in the sample. In all the transient absorption traces presented below, the pump laser intensity was kept below the damage threshold. The pump laser intensity at the sample was measured by a Molectron J3-02 Energy Detector, and the pump laser spot size was 0.2 mm2 for these experiments. The spot size was measured using standard laser burn paper, and the measurement was done in Adobe Photoshop software (the error in the spot size is approximately 40%). Fits to the transient absorption data were performed by using the “solver” function in Microsoft Excel for Windows. The temperatures created by laser excitation were estimated by measuring the period of the breathing mode for spherical gold particles under identical experimental conditions. Details of this procedure are given in the Supporting Information for this paper. Briefly, the measured periods for the spheres are compared to periods calculated using the temperature-dependent elastic constants of gold.18 This allows us to determine a relationship between intensity and temperature, which is then used to determine the temperature of the nanocages following ultrafast excitation. 3. Results and Discussion Characterization of Nanocages. Representative TEM and SEM images of two different-sized nanocages are shown in Figure 1. These samples are termed “nanocages” because the particles are empty boxes with holes at the corners or on the facets of the box. The thin walls of the box can be clearly seen in the TEM images. The average dimensions of the nanocages (measured by counting approximately 100 particles) are 36 ( 4 nm and 68 ( 12 nm, where the errors indicate the standard deviation in the dimension. The nanocages actually consist of an alloy of Au and Ag. Analysis using atomic emission spectroscopy shows that the Au/Ag ratios are 3:1 for the 36nm nanocages and 2:1 for the 68-nm nanocages. UV-vis spectra of the nanocages used in our experiments are shown in the insert of Figure 2. The surface plasmon band of the nanocages occurs in the near-IR region, with an absorption maximum near 800 nm for both samples. Photophysics of Nanocages. Laser-induced heating in metal nanoparticles has been investigated previously by time-resolved experiments. The intense pump laser pulse excites the electrons and induces a hot electron distribution.12,13,19 The temperature of the electron distribution can be very high, because of the small value of the heat capacity of electrons.19 The electrons subsequently equilibrate with the lattice through the electronphonon (e-ph) coupling process. This usually happens on a picosecond time scale and has been extensively studied for spherical particles.12,13,19 Once the temperature of the electrons and lattice have equilibrated, the energy is transferred to the surroundings, which is water in our experiments, on a 10-100 ps time scale.12-14

1522 J. Phys. Chem. B, Vol. 110, No. 4, 2006

Letters

Figure 2. Transient absorbance traces for the different Au nanocages. The initial decay corresponds to the electron-phonon (e-ph) coupling process. This is followed by a modulation and a decay due to the coherently excited vibrational modes and heat dissipation into the environment, respectively. The pump laser power was 2 µJ/pulse for each scan, and the probe wavelength was 740 nm. The insert shows the absorption spectra of the samples.

Figure 1. SEM images of Au nanocages synthesized via galvanic replacement reaction. The average sizes were measured as (A) 36 ( 4 nm and (B) 68 ( 12 nm. The errors indicate standard deviations. The inserts are the corresponding TEM images.

Figure 2 shows representative transient absorption traces for different gold nanocage samples. The probe wavelengths were between 740 and 760 nm in these experiments and are close to the plasmon band maximum of the sample. The fast decay immediately after excitation corresponds to the e-ph coupling process, and the slower decay is energy transfer from the lattice to the environment.13,14 Superimposed on the slower decay is a modulation due to coherently excited vibrational motion of the nanocages.20,21 These modulations have been extensively studied for spherical nanoparticles and nanorods,20-22 as well as for more exotic structures such as prisms23 and triangles.24 They are excited in the transient absorption experiments because lattice heating is faster than the period of the vibrational mode that correlates with the expansion coordinates of the particles.13 By probing these modes, one can get information on, for example, the temperature of the lattice25 or the elastic properties of the particles.26 In this paper, we will focus on both the vibrational modes and the slow decay in the transient absorption spectra, as this decay determines how long the particles stay hot. All the traces were fitted using a damped cosine function plus an exponential decaying background as

S(t) ) A cos

(2πtT + φ) e

-t/τv

+ Be-t/τe-ph + Ce-t/τd + D (1)

where T is the vibrational period, φ is the phase for the vibration,

and τv is the vibrational damping time. The first exponential term (time scale τe-ph) accounts for the electron-phonon coupling process, while the last two terms (the exponential with time constant τd and the offset D) describe energy relaxation. This function allows us to define a characteristic time scale for energy transfer from the particle to the environment in terms of a half-time t1/2 ) -τd ln[(C- D)/2C]. The data in Figure 2 give vibrational periods of T ) 39 ( 1 ps for the 36-nm nanocages and T ) 72 ( 1 ps for the 68-nm nanocages. The half-times for energy relaxation are t1/2 ) 40 ( 10 ps for the 36-nm nanocages and t1/2 ) 100 ( 20 ps for the 68-nm nanocages. For the experiments in Figure 2, the pump laser power was ∼2 µJ/pulse, well below the damage threshold of the sample. To investigate the excitation power dependence of the signal, we increased the pump laser power from 2 µJ/pulse to 17 µJ/ pulse. Note that, even at 17 µJ/pulse, the samples do not show any observable change over the course of the experiments. Figure 3 shows transient absorption traces for the 68-nm nanocages under different excitation powers. The probe wavelength was 740 nm for these experiments. The data were fitted using eq 1 to obtain the energy relaxation times and the vibrational periods. A plot of the vibrational period versus the pump laser intensity is presented in the insert of Figure 3. The period increases approximately linearly with the pump laser intensity from 72 ps at low intensity to 77 ps at high intensity, i.e., ∆T/T ≈ 7%. This is similar to the changes observed for spherical particles at high excitation powers.27 There is no significant change in the energy relaxation time. These data shows that the nanocages maintain their structural integrity at high laser intensities. That is, they still have well-defined vibrational modulations, with no significant change in form or degradation with time of the transient signal. Note that the time constant for energy relaxation is very hard to accurately measure in these experiments because of the presence of the pronounced vibrational modulations. In our previous studies of solid spherical Au nanoparticles, the probe laser was tuned to the peak of the plasmon resonance,14 where

Letters

J. Phys. Chem. B, Vol. 110, No. 4, 2006 1523

Figure 3. Intensity dependence of the transient absorption signal for 68 nm nanocages. The different traces (from bottom to top) correspond to pump laser intensities of 3, 7, and 9.5 µJ/pulse. The data have not been normalized. The insert shows how the period of the coherently excited vibrational mode depends on the pump intensity.

the signal from the coherently excited vibrational modes essentially disappears.28 This allowed us to accurately determine how the energy relaxation process depends on the size.14 However, this does not work well for nonspherical particles,24 where modulations appear at all wavelengths. The data in Figures 2 and 3 show that the energy relaxation times for nanocages are similar to the time scales for spheres: tens to hundreds of picoseconds.14 Furthermore, the time scale for energy relaxation increases with the size of the nanocages and is independent of the pump laser intensity, again consistent with the results for spheres. For spherical particles larger than 10-nm diameter, the relaxation process is controlled by heat dissipation in the surroundings,14 rather than heat transfer at the particle-solution interface.15 The time scale for heat dissipation can be estimated by equating the heat capacity of the particle to the heat capacity of a liquid shell on both the internal and external surfaces with a thickness equal to the thermal diffusion length ld.15 Using ld ) xksτd/FsCs where ks, Fs, and Cs are the thermal conductivity, density, and heat capacity, respectively, of the solvent, and τd is the time scale for thermal diffusion yields

F m Cm h2L2 × FsCsks 4(L - 2h)2 2

τd ≈

2

(2)

where Fm and Cm are the density and the heat capacity of the metal, and we have assumed that the edge length (L) of the nanocages is much greater than the wall thickness (h). This equation gives thermal diffusion times of ∼10 ps for the 36nm nanocages and ∼40 ps for the 68-nm nanocagessfaster than the experimental relaxation times. This indicates that heat conduction at the particle-solution interface may actually be the rate-limiting step for energy relaxation for the nanocages.15 However, this analysis is very approximate (it assumes, for example, that the particles are perfect boxes); more rigorous experiments and analysis are needed to quantitatively understand energy relaxation in these systems. Note that eq 2 shows that the wall thickness is very important in determining the thermal diffusion times for the nanocagessessentially because it determines the volume of metal and therefore the total heat capacity of the particle.

It was previously shown that nanocages deposited on a TEM grid can be transformed into spheres under even moderate light intensities, such as a camera flash.9 In our solution-phase transient absorption experiments, we do not observe any thermal reshaping of the nanocages for pump powers up to 17 µJ/pulse, which is clearly much higher intensity than the camera flash. We believe that this is because energy relaxation in solution is fast enough to circumvent thermal reshaping: The particles do not stay hot for long enough after laser excitation in water for significant structural changes to occur. Heat dissipation is much slower in dry films compared to solution because of the lower thermal conductivity of air compared to liquids.29 For example, for 15-nm diameter Au spheres with a 5-nm SiO2 shell, the relaxation time is six times longer for the particles in a dry film compared to water.29 Thus, we expect slower relaxation times for the nanocages on a TEM grid, which should assist photothermal structural changes. However, it is not clear if other effects (such as the wavelength and pulse width of the excitation source) need to be considered to explain the camera flash results. At pump laser powers of 20 µJ/pulse or above, the nanocages are thermally reshaped in solution. Experiments were performed with spherical gold particles under the same experimental conditions to determine the temperature created by pump excitation (see Supporting Information). The results show that, for the spheres, pulse energies of 20 µJ produce lattice temperatures very close to the melting point of gold. The temperature created in the nanocages will be different, however, because the nanocages absorb light differently. At the 400-nm pump wavelength used in our experiments (which is significantly displaced from the plasmon resonances of either the spheres or the nanocages), the differences will mainly arise from the fact that the nanocages are made of an alloy of silver and gold rather than pure gold and therefore have different dielectric constants. Using tabulated dielectric constant data for silver and gold,30 and simply averaging to obtain the dielectric constant of the silver-gold alloy, we estimate that the ratio of absorption coefficients for the 68-nm nanocages (gold/silver composition ) 2:1) to pure gold particles is 0.74 ( 0.02. This implies that pump energies of 20 µJ will create a lattice temperature of 1200 ( 100 K in the nanocagessslightly below the melting point of the gold-silver alloy (∼1320 K).31 The lattice temperature at 17 µJ/pulse (where the nanocages are still intact) is 1100 ( 100 K. 4. Summary and Conclusions Au nanocages were formed from a galvanic replacement reaction with Ag cubes. The SPR peak of the nanocages can be tuned into the near-IR region, which makes them promising candidates for photothermal therapy and biological imaging applications.7-9,11 Time-resolved experiments have revealed that the time constant for energy relaxation for nanocages in water is on the order of tens to hundreds of picoseconds. Two different-sized nanocages were studied in detail, and the results show that the energy relaxation time increases as the size of the particles increases. Experiments with different excitation powers for the nanocages indicate that the relaxation time is independent of the initial particle temperature. Both these observations are consistent with our previous studies of solid spherical particles in water.14 It is not clear at this stage if the relaxation time for the nanocages is controlled by heat dissipation in the surroundings or heat transfer at the particle-solution interface.15 Modulations due to coherently excited vibrational modes were also observed in the transient absorption traces. The period of

1524 J. Phys. Chem. B, Vol. 110, No. 4, 2006 the excited vibrational motion is proportional to the dimensions of the particles; specifically, the periods are T ) 39 ( 1 ps for the 36-nm nanocages and T ) 72 ( 1 ps for the 68-nm nanocages. The excited vibrational mode shows significant softening with increasing pump laser power, up to pump intensities of ∼17 µJ/pulse. At higher intensities (g20 µJ/pulse), the nanocages are destroyed by laser-induced heating. We estimate that the 20-µJ pump energies produce a lattice temperatures of 1200 ( 100 K in the nanocages. The fact that the nanocages maintain their structure up to high lattice temperatures is good for photothermal therapy applications. The goal in photothermal therapy is to use nearIR light to image and kill cancer cells (for example) via selective heating.7 For this to work with metal nanoparticles, the particles must be stable under laser irradiation. The present work shows that, for nanocages in solution, ultrafast laser excitation can create very high lattice temperatures without destroying the particles: The particles are still intact at 17 µJ/pulse, which corresponds to a lattice temperature of 1100 ( 100 K. This stability arises because of the rapid heat dissipation in solution. Note that thermally reshaping nanocages to spheres would greatly reduce their absorbance in the near-IR and make them poor choices for photothermal therapy. In biomedical applications, the energy deposited by the laser will be transferred to the cell, eventually resulting in cell death.7 Thus, the combination of high repetition rate near-IR, ultrafast laser sources with gold nanocages is very promising for photothermal therapy. Acknowledgment. This work has been supported in part by a DARPA-DURINT subcontract from Harvard University and a fellowship from the David and Lucile Packard Foundation. Y. X. is an Alfred P. Sloan Research Fellow and a Camille Dreyfus Teacher Scholar. X. L. is supported by a CAREER Award from the NSF. Instrumentation was provided by the Nanotech User Facility (NUTF), a member of the National Nanotechnology Infrastructure Network (NNIN) supported by NSF. G. V. H. acknowledges the support of the NSF (grant number CHE02-36279), and the Petroleum Research Fund (grant number PRF-39761AC) administered by the American Chemical Society. We thank a referee for suggesting the analysis that led to eq 2. Supporting Information Available: Details of how the lattice temperatures for the nanocages are determined for a given pump laser intensity. This material is available free of charge via the Internet at http://www.pubs.acs.org.

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