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Langmuir 2005, 21, 7528-7532

Inside-Out Disruption of Silica/Gold Core-Shell Nanoparticles by Pulsed Laser Irradiation V. Prasad, A. Mikhailovsky,† and J. A. Zasadzinski* Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5080 Received April 18, 2005. In Final Form: May 25, 2005 Near-infrared (NIR) femtosecond laser irradiation of metallodielectric core-shell silica-gold (SiO2Au) nanoparticles can induce extreme local heating prior to the rapid dissipation of energy caused by the large surface area/volume ratio of nanometer-scale objects. At low pulse intensities, the dielectric silica core is removed, leaving an incomplete gold shell behind. The gold shells with water inside and out still efficiently absorb NIR light from subsequent pulses, showing that a complete shell is not necessary for absorption. At higher pulse intensities, the gold shell itself is melted and disrupted, leading to smaller, ∼20-nm gold nanoparticles. Spectroscopic measurements show that this disruption is accompanied by optical hole burning of the peak at 730 nm and formation of a new peak at 530 nm. The silica removal and gold shell disruption confirms significant temperature rise of the core-shall nanoparticle. However, the entire process leads to minimal heating of the bulk solution due to the low net energy input.

Introduction External targeting and release from nanoscale drug carriers is an important goal for enhanced drug delivery in vivo.1 In current practice, a relatively large volume that contains the site for delivery is heated externally1 or internally;2 collateral tissue damage in vivo limits the maximum increase in temperature to about 10° C. An alternative approach is to induce large temperature changes in nanometer-scale volumes to disrupt individual carriers without inducing significant heating of the surrounding tissues. However, heat dissipation processes are extremely rapid from nanometer-scale particles;3-5 100-nm diameter particles thermally equilibrate with their surroundings in microseconds.6 Hence, unless the energy deposition occurs on faster time scales than dissipation, the nanoparticle temperature does not rise significantly above the background.2-4,7-9 To initiate a large temperature jump requires nano- to femtosecond light pulses to heat the nanoparticles; in this case, temperatures can easily reach the melting or even the boiling point of gold.3,10-14 * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry, University of California Santa Barbara. (1) Needham, D.; Dewhirst, M. W. Adv. Drug Delivery Rev. 2001, 53, 285-305. (2) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549-13554. (3) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226-1232. (4) Kurita, H.; Takami, A.; Koda, S. App. Phys Lett. 1998, 72, 789791. (5) Rabin, Y. Int. J. Hyperthermia. 2002, 18, 194-202. (6) Hu, M.; Wang, X.; Hartland, G. V.; Salgueirino-Maceira, S.; LizMarzan, L. M. Chem. Phys. Lett. 2003, 372, 767-772. (7) O’Neal, D. P.; Hirsch, L. R.; Halas, N. J.; Payne, J. D.; West, J. L. Cancer Lett. 2004, 209, 171-176. (8) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. J. Biomed. Mater. Res. 2000, 51, 293-298. (9) Hirsch, L. R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. Analy. Chem. 2003, 75, 2377-2381. (10) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589-2591. (11) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123-3128.

In this work, silica-gold nanoshells that strongly adsorb near-infrared (NIR) light are irradiated with multiple femtosecond light pulses. The laser pulses can induce large changes in the nanoshell temperature because the power deposition is much faster (nanoseconds) than energy dissipation (microseconds). Tissue, blood, etc. are relatively transparent to NIR light, so even relatively deep tissues could, in principle, be illuminated and initiate the temperature jumps in the nanoshells.7 The nanoshells are rapidly heated to very high temperatures, sufficient to first chemically disrupt and then eliminate the silica core. The resulting incomplete gold shells with water inside and out can still efficiently absorb NIR light from subsequent pulses. Hence, as has been observed in other studies,10,15,16 a complete gold shell is not necessary for absorption in the near-infrared; aggregates of gold nanoparticles also have their absorption maximum shifted to the near-infrared. With sufficient pulses or higher energies, the gold shells/aggregates are disrupted into smaller and smaller particles, and the absorption maximum shifts to higher wavelengths characteristic of isolated gold nanoparticles.10,15 However, the total energy input is small enough that the bulk temperature of the solution increases by only a few degrees. These nanoshells might be encapsulated within lipid bilayer vesicles or other nanostructures17,18 to act as an externally actuated trigger. Bilayer compartments are quite deformable, but pop like balloons if stretched too far.19 Energy transfer from the nanoshells should cause a minute amount of water to boil, in addition to the other physical changes in the nanoshells, (12) Hodak, J. H.; Henglein, A.; Giersig, M.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 11708-11718. (13) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152-6163. (14) Aguirre, C. M.; Moran, C. E.; Young, J. F.; Halas, N. J. J. Phys. Chem. B 2004, 104, 7040-7045. (15) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D.; Bridges, F.; Van Buuren, A. J. Phys. Chem. B 2002, 106, 70057012. (16) Grant, C. D.; Schwartzberg, A. M.; Norman, T. J.; Zhang, J. Z. J. Am. Chem. Soc 2003, 125, 549-553. (17) Kisak, E. T.; Coldren, B.; Zasadzinski, J. A. Langmuir 2002, 18, 284-288. (18) Kisak, E. T.; Coldren, B.; Evans, C. A.; Boyer, C.; Zasadzinski, J. A. Curr. Med. Chem. 2004, 11, 199-219. (19) Evans, E.; Rawicz, W. Phys. Rev. Lett. 1990, 64, 2094-2097.

10.1021/la051036d CCC: $30.25 © 2005 American Chemical Society Published on Web 07/06/2005

Disruption of Silica/Gold Core-Shell Nanoparticles

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Figure 1. (a) TEM image of nanoshells having a core diameter of 130 nm (r1 ) 65 nm in eq 1) and a shell thickness of 10 nm (r2 ) 75 nm in eq 1). The images show that the nanoshells are rather porous and that the shell is made up of aggregated gold nanoparticles rather than a continuous film. (b) Extinction spectra from the core-shell particles shows a plasmon resonance peak at λ ) 725 nm consistent with the Mie scattering calculations (eq 1) for a silica-gold-water nanoshell with r1 ) 65 nm and r2 ) 75 nm. The absorption spectrum is also similar to that of simple gold aggregates as seen in refs 10 and 15. Regardless of the origin of the shift, absorption in the near-IR portion of the spectrum facilitates in vivo applications because of the near-transparency of physiological materials to near-IR light.2,9

which will likely lead to sufficient volume increase to pop the lipid carrier and trigger drug release only where the laser light is focused. This could give both spatial and temporal control of drug delivery from liposomes without the need for specialized liposome compositions or largescale heating of the tissue itself.1 In this paper, we show the effects of intense femtosecond laser pulses on the stability of silica-gold core-shell spheres. These pulses are repeated for extended periods of time, up to 600 s.14 At low pulse intensities, the dielectric silica core of the nanoshell disappears first, while the gold nanoshell remains intact and hollow, but distorted and incomplete (See Figure 3). At higher pulse intensities, the gold nanoshell is destroyed, and smaller gold nanoparticles are formed. This is confirmed by spectroscopic measurements, which show optical hole burning of the peak at λ ) 730 nm and formation of a new peak in the extinction coefficient at λ ) 530 nm. The height of this peak increases with increasing pulse intensity, indicating formation of increasing number of ∼20-nm gold nanoparticles. This stepwise destruction of the nanoshells is much different than reported earlier in the literature.14 Such nanoshells could be used to disrupt tissues or drugdelivery vehicles in vivo by either the high temperatures or the physical changes in the particles. Materials and Methods Nanoshells with a plasmon resonance in the near-infrared region are fabricated according to techniques described previously.2,14 Briefly, monodisperse silica spheres are prepared by the Sto¨ber method;20 they are then coated with 3-aminopropyl triethoxysilane, and the solution is refluxed for 1 h.21 Gold nanoparticles (1-3 nm) are synthesized22 and mixed with the silica solution. The gold particles react with the amine groups on the organosilane and bind to the surface of the silica sphere. Reduction of chloroauric acid to gold is performed in the presence of the silica-gold particles with a reducing agent such as (20) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-68. (21) Blaaderen, A. v.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1-18. (22) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 23012309.

formaldehyde. The gold particles on the surface of the silica spheres act as nucleation sites for the reduction of chloroauric acid, and the particles grow and coalesce to form a shell around the silica sphere. The nanoparticles used in this study have a mean diameter of 130 ( 10 nm, with the shell thickness approximately 18 nm (Figure 1a). From Figure 1a, it appears that the shell is made up of aggregated gold particles rather than a continuous gold shell. The extinction spectrum from the particles, measured using a UV-vis spectrophotometer (Varian Cary 5000) shows a peak at λ ) 730 nm (Figure 1b), consistent with the predicted relationship between dielectric constants and the ratio of core radius to shell thickness given by:23

[

]

r1 2′(λ)(1 + 23) 3 ) 1+ r2 2 [2′(λ)]2 - 2′(λ)(1 + 3) + {13 - [2′′(λ)]2}

(1)

in which r1 and r2 are the inner and outer radii of the nanoshell, and 1 and 3 are the (real and constant) dielectric constants of the core and external media, respectively (silica, 1 ) 2.07, and water, 3 ) 1.77). 2′(λ) and 2′′(λ) are the real and complex parts of the frequency-dependent dielectric constant of gold at the wavelength of interest, λ.24 However, Figure 1b is also consistent with spectra of simple gold aggregates as observed in earlier work;10,15 a uniform and complete gold shell does not seem to be necessary to move the absorbance peak to the near-infrared. The important consequence of either the choice of core and shell dimensions23 or gold aggregation10,15 is that the absorption peak moves to the near-infrared, which is essential to eventual applications in vivo. However, unlike previous work,14 there is no peak arising from multipolar fields at lower wavelength, indicating an incomplete gold shell, as is observed in electron micrographs (Figures 1a, 3a). Using Mie scattering theory,14 the concentration of particles in the suspensions is estimated from the calculated extinction cross section of the nanoparticles to be ∼109 particles/ml. The particle suspensions are irradiated with a Ti:Sapphire laser (Spectraphysics Spitfire) that generates 90 fs pulses at a 1 kHz repetition rate with energies up to 1 mJ/pulse. The energy incident on the samples is varied using neutral density filters. Typically, 3 mL of the sample is irradiated in a quartz cuvette cell with a 1-mm path length. The cell is placed atop a magnetic (23) Averitt, R. D.; Westcott, S. L.; Halas, N. J. J. Opt. Soc. Am. B 1999, 16, 1824-1832. (24) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370-4379.

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Prasad et al. min, and the transmitted intensity is measured with a power meter. This irradiation time is sufficient for the measured transmitted intensity to reach a steady value. After irradiation, a drop of each sample is deposited on a Formvar-coated electron microscope grid. The dried samples are later viewed in a transmission electron microscope (FEI Cryotrans) under high magnification. The extinction spectra of the samples before and after irradiation are also measured with the UV-vis spectrophotometer.

Figure 2. Extinction spectra from nanoparticles irradiated for 600 s by femtosecond pulses with λ ) 730 nm at different total energies: 0 µJ (hexagons), 123 µJ (triangles), 180 µJ (stars), 300 µJ (diamonds), and 740 µJ (squares). Increased irradiation leads to the eventual elimination of the plasmon resonance at 725 nm in favor of a second peak at 530 nm. This is consistent with the disruption of the silica-gold nanoshells into ∼20 nm gold particles (See Figure 3, parts c, d). plate, and a stir bar is used to mix the sample as it is irradiated. The width of the beam is 2 mm. Samples are irradiated for 10

Results and Discussion The extinction spectra of the nanoparticles for increasing energies of irradiation, as shown in Figure 2, is surprising; the amplitude of the absorption peak at λ ) 725 nm decreases, while there is a corresponding increase in a new absorption peak at λ ≈ 530 nm. This optical hole burning has been seen before in femtosecond laser pulsing of gold nanorods,13 and to a lesser extent in core-shell nanoparticles.14 In both cases, this was indicative of the restructuring of the nanoparticles, with formation of spherical gold particles from the nanorods, and separation of the core and the shell, and destruction of the shell in the latter case. Further, the evolution of the peak at lower wavelengths implies formation of smaller nanometer-sized gold particles.3,10 Our results are in contrast to what Aguirre et al.14 observed, where the red-shifted peak was observed at λ ) 577 nm. The 530-nm adsorption peak in our experiments suggest smaller gold nanoparticles.3,10,14 TEM images of the irradiated samples are also very striking; at the lowest energy, 123 µJ, the image (Figure 3a) shows an open-shell, grid-like network of gold, with

Figure 3. High-resolution (50 000 X) TEM images of silica core-shell nanoparticles irradiated at increasing intensities (the scale bar in the figure is 50 nm). (a) The particles irradiated with 123 µJ total energy show incomplete gold shells with a hollow center. The silica core has disappeared, and there appears to be some fusion of the gold nanoparticles into a lacy network. (b) The 180 µJ irradiation leads to somewhat more distortion of the gold shells; not all the shells remain intact. The shells are incomplete and there is no silica core, although there is not much difference with the shells shown in (a). (c) The 300 µJ irradiation shows the formation of smaller gold spheres and an absence of silica particles; most of the nanoshells have been destroyed, although there are some shell fragments remaining (not shown). (d) The 740 µJ irradiation leads to the elimination of all the gold shells and formation of 20-50 nm gold nanoparticles.

Disruption of Silica/Gold Core-Shell Nanoparticles

no detectable silica core. The gold shell is distorted from spherical, and there appears to be some fusion of the original gold nanoparticles into extended lacy aggregates. This is a surprising result, as the metallic gold shell absorbs all the energy rather than the dielectric core. As the energy of irradiation is increased to 177 µJ, we observe the same behavior, with the silica core completely eliminated (Figure 3b). At higher pulse energies, 300 µJ (Figure 3c), the distorted gold shell is destroyed, leaving gold fragments in place. These fragments have a characteristic size of ∼20 nm with a broad size distribution, which is why the extinction spectrum shows an increase in the peak at λ ≈ 530 nm. This trend continues at higher energies; at 700 µJ, the fraction of fragmented nanoshells increases, as can be seen from the further increase in the height of the extinction peak at 530 nm. Figure 3d shows that the gold particles are very polydisperse but are significantly reduced in size from the original nanoshells. However, the absorbance decreases gradually with increasing energy of irradiation, and the peak at 730 nm is not completely gone until the nanoshells are completely reduced to the smaller nanoparticles seen in Figure 3, parts c and d. Apparently, a complete gold shell is not required for the nanoparticles to continue to adsorb nearinfrared light.15 In none of the TEM images can any trace of the silica core be found, which is surprising considering that most of the irradiated energy is absorbed by the metallic shell rather than the dielectric core, which is a much less conductive than the gold shell. The observed results can be explained by considering transient heat conduction from the gold shell to the surrounding medium and the dielectric core. Timeresolved transient absorption spectroscopy experiments on metal nanoparticles have given insights into electronphonon relaxation dynamics.25 Typically, heat transfer from the hot conduction electrons to the gold lattice occurs over a few picoseconds. In contrast, the lattice dissipates heat to the surrounding medium by phonon-phonon interactions over the course of hundreds of picoseconds.6,26 In the case of a dielectric-metal nanoshell, the process is more complicated. At short time scales (approximately a few picoseconds), the dominant process is still electronphonon mediated heating of the gold nanoshell lattice. At longer time scales, however, there are two competing processes: heat transfer from the gold lattice to the silica core, and heat transfer from the shell to the aqueous medium that surrounds the nanoparticles. The time for the system to reach steady state can be estimated from the Fourier number for conductive heat transfer, F0 ) Rτ/ R20, for silica and water (R is the thermal diffusivity, τ is the characteristic time for heat transfer, and R0 is the characteristic dimension, here, the sphere radius). For large F0, typically F0 > 100, the system can be considered to have reached a steady state. Using the thermophysical properties of silica and water, the characteristic time for F0 ) 100 is τSi ) 5.10-7 s for silica and τw ) 3.10-6 s for water. Therefore, for all practical purposes, the silica core and the gold shell reach thermal equilibrium (equal temperatures) first, and further cooling is by heat transfer to the surrounding water. At the temperatures expected, a layer of water vapor (steam) likely forms around the particles, with the heat-transfer mechanism being either transition boiling or film boiling, depending on the temperature difference.27 The time interval between two pulses is 10-3 s, many orders of magnitude greater than (25) Averitt, R. D.; Westcott, S. L.; Halas, N. J. Phys. Rev. B 1998, 58, R10203-R10206. (26) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. Chem. Phys. Lett. 1999, 315, 12-18.

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τSi and τw; therefore. the silica core, the gold shell, and the water around the nanoparticle likely reach thermal equilibrium. In a single femtosecond pulse, the temperature of the gold shell, Tg, can be estimated by3,28

Tg )

Eabs + 295 mgcg

Tg )

(Eabs - ∆Hm) + 295 mgcg

Tg )

(Eabs - ∆Hm - ∆Hb) + 295 mgcg

Tg < Tm

(2a) T m < Tg < Tb T b < Tg

(2b) (2c)

in which Eabs is the energy absorbed by a single sphere (Mie scattering calculations show that ∼18% of the incident light is absorbed, the remainder being scattered), mg is the mass of the gold shell; cg is the heat capacity/ mass, and ∆Hm is the latent heat of melting and ∆Hb is the latent heat of vaporization of the gold shell. The core and the shell reach an equilibrium temperature, which can be determined by a simple energy balance, assuming much slower dissipation to water outside the shell:

FgVgCg(Tg - Tf) + ∆Hg ) FSiVSiCSi(Tf - 295) (3) Tf is the maximum temperature reached by the core (prior to heat dissipation to the water), and ∆Hg is the latent heat of gold if Tf is above a gold phase-transition temperature (melting or boiling). The initial temperature of the gold shell and the temperature of the silica core between pulses, as estimated by these calculations, are shown in Figure 4 and are consistent with the observed blackbody spectrum of similar gold particles 3. As in eqs 2 and 3, the temperature estimates plotted in Figure 4 are based on the absorbed energies, which are ∼18% of the incident energy. For incident pulse energies lower than 300 µJ, the temperature of the silica shell is below ∼800 °C. It is therefore surprising that the silica core disappears, as the temperature is well below 1713 °C, the silica melting temperature, much less than the boiling (2230 °C) point. However, chemical reactions between the exposed silica core and high-temperature water vapor are a possibility. The presence of high-temperature steam near the particle surfaces at these elevated temperatures can lead to volatile hydroxides or oxyhydrides.29 One reaction that is possible to occur at these high temperatures (T ∼ 600-800 °C) is30

SiO2(s) + H2O(g) ) SiO(OH)2(g)

(4)

Other possible reactions involve the formation of oxyhydroxides such as Si(OH)4 and Si(OH)6 at these temperatures. These silicon compounds are volatile under these conditions and may not condense into solution. This is one possible reason the silica core disappears at low energies of irradiation, even though the temperature is well below the melting point of silica. From the estimated particle concentration and size, the volume fraction of silica in the suspension is ∼10-7, so detecting these species would be difficult and well beyond the scope of this paper. (27) Incropera, F. P.; DeWitt, D. P., Fundamentals of Heat and Mass Transfer. John Wiley & Sons: New York, 1990. (28) Link, S.; El-Sayed, M. A. J. Chem. Phys. 2001, 114, 2362-2368. (29) Chang, M. C.; Cutler, I. B. J. Am. Ceram. Soc. 1979, 62, 593596. (30) Opila, E. J.; Fox, D. S.; Jacobson, N. S. J. Am. Ceram. Soc. 1997, 80, 1009-1012.

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Figure 4. Calculated temperatures of the shell (solid line, eq 2) during a single femtosecond pulse, and the maximum temperature of the core reached in the interim period between two pulses (dotted line, eq 3) for increasing absorbed pulse energy. Mie scattering calculations show that ∼18% of the incident energy is absorbed by the particles. Symbols represent actual incident energies used, 123 µJ (triangles, 23 µJ absorbed), 180 µJ (stars, 32 µJ absorbed), 300 µJ (diamonds, 53 µJ absorbed), and 740 µJ (squares, 130 µJ absorbed). Open symbols are for the maximum silica core temperatures and the solid symbols are for the maximum shell temperature. The discontinuities arise from the melting and boiling phase transitions of gold (the phase transitions of silica have not been included in this analysis).

At high irradiation energies, the pathway toward the disintegration of the gold shell (Figure 3, parts c and d) is clear. After the silica core reacts away, the fragmented gold shell continues to absorb energy, with a similar absorption profile as before. This could be due to the partial nanoshell geometry with water, which has similar dielectric properties as silica (eq 1), taking the place of silica in the core. This is borne out by preliminary Mie scattering calculations (not shown) that show that the absorption characteristics of a silica-gold-water nanoshell are not significantly different from a water-gold-water nanoshell.

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However, simple aggregates of gold nanoparticles also absorb in the near-infrared, so the nanoshell geometry may not be essential.10,15 After the silica core is removed, all the absorbed energy goes toward heating the gold shell and the water layer around it. The temperature of the shell can exceed the melting point of gold in a single pulse, as shown in Figure 4. The gold shell is disrupted and dispersed into nanoparticles (∼20 nm), as seen in Figure 3, parts c and d, which results in the increase in absorption at 530 nm 3,10 (Figure 2) and elimination of the peak at 730 nm. The silica-gold core-shell nanoparticles clearly reach high temperatures after femtosecond pulses of nearinfrared light. This leads to complete removal of the silica core; we speculate that the nanoshells become microreactors in which the silica is decomposed into hydroxides and oxyhydroxides of silica following the reaction with water vapor at elevated temperatures. The difference in our results and those of Aguirre et al.14 are probably due to the difference in surface coverage of the silica spheres in the two cases. More complete gold coverage leads to less contact with water, and therefore, less silica can react. Preliminary experiments with core-shell structures that have a more complete gold shell and, therefore, show higher multipole moments in the extinction spectra than the particles used in this study,14 confirm this hypothesis, as the silica core remains stable in solution. As suggested by calculations, the gold shells with water inside and out continue to absorb light in the near-IR because of the similar dielectric properties of silica and water. Finally, the destruction of the core, as well as the gold shell, has potential medical applications in drug-delivery applications. The high temperatures reached by the nanoshells after pulsed laser irradiation in the NIR could trigger controlled release from lipid vesicles or other nanostructures.1,17,18 Acknowledgment. We gratefully acknowledge funding from the National Science Foundation Nanotechnology Interdisciplinary Research Team program, Grant no. 0103516, and the Institute for Collaborative Biotechnology at UCSB. LA051036D