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J. Phys. Chem. C 2008, 112, 17844–17848
Preparation and Optical Properties of Metallodielectric Core-Shell-Corona Particles Thomas C. Preston and Ruth Signorell* Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, BC, V6T 1Z1, Canada ReceiVed: August 19, 2008
The preparation of metallodielectric core-shell-corona particles is described, and their optical properties are analyzed using the discrete dipole approximation. These particles consist of a spherical gold core coated with a thin dielectric layer and an outer gold layer (the corona). We demonstrate that stable dispersions of these particles possess plasmon modes in the near-infrared that can be tuned in a controlled fashion, while particle diameters can be kept below 100 nm. 1. Introduction Metallic and metallodielectric particles that strongly absorb and scatter near-infrared (NIR) radiation have begun to emerge as important tools in thermal therapy,1-4 imaging,4-7 controlled drug release,8,9 and surface enhanced spectroscopy.10 Nanoscale objects such as prisms,11 rods,4,12 cages,7,13 and especially shells1,2,5,6,10 have all received attention because of this characteristic. They derive their particular appeal in biological applications from properties such as biological inertness,14 robustness, sizes below 1 µm (the limit for internalization of particles by nonphagocytic eukaryotic cells15), potential for surface functionalization (e.g., using well-developed gold-sulfur chemistry16-18), and water solubility. In this report, we describe the preparation and optical properties of metallodielectric core-shell-corona (CSC) particles. These consist of a metallic core coated with a dielectric shell that is in turn coated by a metallic outer layersthe corona. Figure 1 depicts the target particle. The motivation for preparing this structure is fourfold. First, hybridization of modes can occur when two or more plasmonic objects couple.19,20 This mixing can result in new modes that possess different energies than those found in the isolated, uncoupled components. For nanoscale Au objects, this can potentially result in the emergence of plasmon resonances in the near-infrared (NIR). A Au particle coated with a thin, positively charged dielectric layer is an ideal platform or “scaffold” onto which a metallic corona can be adsorbed or plated. As we demonstrate in the present contribution, this leads to new, lower energy modes due to the close proximity of the objects that form the corona with each other and the Au core. Second, these particles can easily possess overall diameters less than 100 nm. It has been found that the cellular uptake of Au nanoparticles is optimal when their diameter is ∼50 nm.21 Therefore, the ability to prepare particles in this size regime is important for applications. This is difficult to achieve with many of the currently available structures. For instance, to employ Au nanoshells that absorb in the NIR and whose diameter approaches this size would be challenging. Although in principle it should be possible to prepare nanoshells with diameters less than 100 nm that have plasmon resonances anywhere in the NIR, the very thin Au coatings required pose a major technical problem. For instance, an 80 nm silica core would require a * Corresponding author. Fax: +1 604 822-2847. E-mail: signorell@ chem.ubc.ca.
Figure 1. Illustration of a core-shell-corona (CSC) particle. Here, region I (the core) and III (the corona) are composed of Au, region II (the shell) is composed of a mixture of poly(allylamine hydrochloride) (PAH) and poly(sodium-4-styrenesulfonate) (PSS), and region IV, the medium in which the particle is dispersed, is water.
layer of Au less than 4 nm thick to have a plasmon resonance at wavelengths longer than 1000 nm. The fabrication of such a layer would represent a considerable challenge. Third, with the structure shown in Figure 1, tuning the plasmon resonance simply becomes a matter of adjusting the corona thickness. As we discuss, this is straightforward. Finally, in our approach to fabricating these CSC particles, we combine existing, well-developed procedures. Therefore, the preparation of particles with strong NIR modes is relatively facile, which represents an important point for applications. 2. Experimental Section 2.1. Prepartion of CSC Particles. The strategy for the preparation of the CSC particles is outlined in Scheme 1 (see the Supporting Information for further details). Colloidal Au 1 (18 ( 4 nm) that forms the particle core (Figure 1, region I) was prepared by reducing tetrachloroauric(III) acid with sodium citrate.22-25 The dielectric layer of the CSC particles (Figure 1, region II) was formed using the layer-by-layer deposition of polyelectrolytes onto Au cores.26-29 In total, five polyelectrolyte layers were deposited onto the surface of the particles 1 yielding a dielectric layer 2.3 ( 0.4 nm thick. For the outer Au layer or corona (Figure 1, region III), the popular approach of Oldenberg et al.30 was used. As illustrated in Scheme 1, this involves first attaching small Au particles (5) to the positively charged particle surface of 4 and then reducing tetrachloroauric acid with formaldehyde to grow a completed
10.1021/jp807434h CCC: $40.75 2008 American Chemical Society Published on Web 10/23/2008
Metallodielectric Core-Shell-Corona Particles
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17845
SCHEME 1: Preparation of CSC Particlesa
a
The shaded regions indicate Au.
layer. Colloidal Au 5 (2.4 ( 0.5 nm) was prepared using the method of Duff et al.31,32 The amount of 5 added to 4 was such that the available cross-sectional area of the particles in solution 5 was greater than the available surface area of the particles in solution 4. To separate 6 from this excess of 5, centrifugation was employed. This was found to be quite effective as product 6 readily precipitates at 8000 rpm while 5 remains disperse and can be removed with the supernatant. Transmission electron microscopy (TEM) indicated the successful adsorption of 5 onto the surface of 4. Using a Au salt solution and formaldehyde, increasing amounts of Au were then reduced in the presence of 6 to yield either 7, 8, or 9 (i.e., the preparation of 7 was performed with the smallest amount of reducible Au salt present while 9 had the largest amount). In a polypropylene container, this reduction proceeded quite slowly and its progress could easily be monitored. Particles could be precipitated at 8000 rpm and easily redispersed in water. 2.2. Calculations. In order to evaluate the optical properties of such a target, a numerical method is required. The discretedipole approximation (DDA) has been demonstrated to be an effective method for determining the scattering and absorption of metallic nanostructures with arbitrary geometries.33 Here, we adapted the DDSCAT code written by Draine and Flatau.34 For these calculations, the refractive index of Au was taken from the literature35 and no corrections for the mean free path of the conduction electrons were made. The index of refraction of the dielectric layer and the embedding medium were taken to be nondispersive. These values were set equal to 1.47 (the refractive index typically taken for a poly(allylamine hydrochloride)/poly(sodium-4-styrenesulfonate) dielectric layer36) and 1.33 (the refractive index of water; the medium in which the particles were dispersed when ultraviolet-visible (UV-vis) extinction measurements were taken), respectively. 3. Results and Discussion 3.1. Characterization of CSC Particles. Figure 2 shows TEM images and corresponding UV-vis extinction spectra for 7-9. Particle diameters from TEM images and hydrodynamic diameters from dynamic light scattering (DLS) measurements are listed in the caption of Figure 2. Subtracting the particle diameter of 6 from the TEM diameters at the indicated times, we calculate the corona thicknesses of 7, 8, and 9 to be 6 ( 3, 10 ( 5, and 16 ( 6 nm, respectively. The samples shown in Figure 2 are representative of the stages in which the corona growth was found to occur. When less
Figure 2. TEM images [(left) a group of particles, (right) an isolated particle] and UV-vis extinction spectra of (a) 7 at 48 h (λmax ) 543 nm, diameter from TEM ) 33 ( 4 nm, hydrodynamic diameter ) 40 ( 13 nm), (b) 8 at 48 h (λmax ) 565 nm, diameter from TEM ) 43 ( 9 nm, hydrodynamic diameter ) 55 ( 17 nm), and (c) 9 at 72 h (λmax ) 524 and 835 nm, diameter from TEM ) 54 ( 11 nm, hydrodynamic diameter ) 74 ( 10 nm).
reducible Au salt is present in solution, as in the case of 7, then an incomplete corona is observed (Figure 2a). Here, no matter how long the reaction was left to proceed (>240 h), there was little change to either particle morphology (observed using TEM) or to the UV-vis extinction spectrum. The spectra of these particles never showed any NIR extinction modes. In contrast, when larger amounts of the reducible Au salt were present, as in the case of 9, then complete coronas formed over time (Figure 2c). These coronas typically contain rods, approximately 8 nm in diameter, growing away from the core before branching in various directions. This gave the particle surface an overall “brushlike” appearance. The spectra of these particles are very sensitive to corona growth. As can be seen in Figure 3, the spectrum of 9 changed extensively over time and TEM showed continued corona growth. At 192 h, where the NIR peak is located beyond 1000 nm, TEM gave an average particle diameter of 94 ( 15 nm (which corresponds to a corona thickness of 36 ( 8 nm) and the hydrodynamic diameter of the particles was 85 ( 15 nm. This represents an increase of 20 ( 10 nm in corona thickness from the measurements made at 72 h. Colloid 8 in Figure 2b is an intermediate between the cases of 7 and 9. Coronas were often complete, but there was only a small amount of rod growth. Over time, the spectrum of 8 simply increased in overall extinction but no distinct NIR peaks were observed (although the spectra did possess a tail into the NIR). Furthermore, unlike 9, TEM images of 8 never showed the emergence of an extensive brushlike corona.
17846 J. Phys. Chem. C, Vol. 112, No. 46, 2008
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Figure 4. (left) Model particle used in DDA calculations. (right) Silhouette indicating variables. The core diameter, d, the dielectric layer thickness, a, and the cylinder diameter, s, were held constant at 18, 2, and 8 nm, respectively. The cylinder height, h, was varied in the calculations.
Figure 3. UV-vis extinction spectra of 9 recorded at several time intervals over 192 h. TEM images of particles which are characteristic of the observed morphology for these solutions at the indicated times are shown to the right.
Reactions with volumes of the reducible Au salt solution larger than that used for 9 were also performed. These resulted in a rapid emergence of a NIR peak (within 24 h) and subsequent redshift.37 Similar to 9, TEM images indicated the formation of a brushlike corona. We summarize the above observations as follows. On the basis of TEM images and UV-vis extinction spectra, there is a strong connection between the thickness of this brushlike corona/length of the rods found on the particle surface and the presence of low energy extinction modes. As the rods that form this corona increase in length, the NIR extinction peak redshifts. In principle, the observed NIR peaks could arise from interparticle plasmon coupling38–40 between agglomerated particles in solution instead of single, isolated CSC particles. This, however, can be ruled out by the following arguments. First, particle diameters measured using both DLS and TEM were always in reasonable agreement. A clear indication that isolated particles, not clusters, are the dominant species in solution. Furthermore, solutions showed no precipitation and, as can be seen in Figure 3, extinction never decreased over time. For this type of colloid, this is a strong indication that aggregation has not occurred. We thus conclude that the observed resonances result from the morphology of isolated single CSC particles and not from aggregated CSC particles. 3.2. Modeling of Optical Properties. We focus on the modeling of the UV-vis extinction spectra of 9 (Figure 3), because they possess the desired NIR peaks at 835, 900, and beyond 1000 nm at 72, 96, and 192 h, respectively. As was discussed, the factor which distinguishes these particles from 7 and 8 (at any time) is the presence of a brushlike corona. This structure is composed of a rough metallic surface that contains rods (with typical diameters around 8 nm) growing away from the surface. As this corona distinguishes particles with extinction modes in the NIR from those without, realistic targets used in scattering calculations must include these features. We thus model the corona with an array of packed cylinders whose major axes are normal to the surface of the spherical core, as depicted in Figure 4. The cylinders are placed at random, nonoverlapping locations on the surface of the sphere. The diameter of the core
(d ) 18 nm), the thickness of the PAH/PSS dielectric layer (a ) 2 nm) and the diameter of the cylinders (s ) 8 nm) were chosen to reflect particle characteristics observed in the TEM images. With these parameters and the random placement of the cylinders on the surface, the number of cylinders per spherical core was 21 in each calculation. While this model certainly represent a simplified view of the observed CSC structure, it seems appropriate as the presence of these closely packed rods is what differentiates these particles from those that do not possess modes in the NIR. For the calculations, the thickness of the dielectric layer, the spherical core diameter, and the diameter of the cylinders were kept constant, while the cylinder length, h, was varied. This was done in order to understand how the spectra behave as a function of corona thickness, which is the parameter that changes in Figure 3. The extinction spectra calculated for particles with various cylinder lengths are shown in Figure 5. With only minor contributions from scattering, the extinction is almost completely dominated by absorption which is typical for particles of these dimensions. Thus, we do not show the traces for absorption and scattering. The orientation of the particle relative to the polarization of the incident light and the random position of the cylinders on the surface have a slight influence on the calculated spectrum. To account for this, all calculated spectra are averaged over these two parameters. With the model used here, computing spectra of particles with cylinders lengths longer than 20 nm is difficult as the number of dipoles required to simulate smooth spherical and cylindrical surfaces (i.e., shapes for which convergence has been reached) quickly exceeds memory limitations. As expected, for h ) 0 (Figure 5a), only the plasmon resonance of the spherical core is observed. For h > 0, a lower energy peak emerges, which is likely made up of several modes of varying energies, given the many components used in generating the target particle (21 cylinders and one sphere). As the lengths of the cylinders increase, the lower energy peak redshifts. This trend is consistent with what is observed for the CSC particles as corona growth takes place (i.e., the observed redshift over time in Figure 3). This lower energy peak is significantly more sensitive than that of a single, isolated Au cylinder. Dispersed in water, such a cylinder (s ) 8 nm) would have its lower energy mode shift from 548 to 720 nm when the length, h, is increased from 5 to 20 nm.41 The larger wavelength shift of CSC particles (553-795 nm as the length of the cylinders, h, increases from 5 to 20 nm) originates from a combination of coupling between the cylinders on the surface with each other as well as the core. Despite the large redshift that occurs in Figure 5b-e, when attempting to match 9 at 72 h (corona thickness of 16 ( 6 nm and λmax ) 835 nm; Figure 2c), it appears that the parameters
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Figure 6. Calculated extinction spectra using DDA for the model particle shown in Figure 4. The core diameter, d, the dielectric layer thickness, a, and the cylinder diameter, s, were held constant at 18, 0, and 8 nm, respectively. Cylinder height, h, is varied (see insets for values). The number of dipoles used in the calculations are (a) that calculated using Mie theory, (b) 50 709, (c) 73 844, and (d) 95 855.
Figure 5. Calculated extinction spectra using DDA for the model particle shown in Figure 4. The core diameter, d, the dielectric layer thickness, a, and the cylinder diameter, s, were held constant at 18, 2, and 8 nm, respectively. Cylinder height, h, is varied (see insets for values). The number of dipoles used in the calculations are (a) that calculated using Mie theory, (b) 83 057, (c) 117 450, (d) 151 627, and (e) 186 028.
used to calculate the spectra in Figure 5 do not fully predict the extent of the observed redshift. The calculated maxima for h ) 15 and 20 nm were 680 and 795 nm, respectively. Therefore, a corona thickness/cylinder height, h, greater than 20 nm is required to match the peak at 835 nm. One source for this discrepancy may be the oversimplified corona used in Figure 4. Factors not considered by this modelssuch as the branching of rods and the distribution of rod lengths observed in TEM imagesscould be the reason for the discrepancy between the calculations and our observed experimental results. An additional point is the thickness of the dielectric layer which we might have chosen to be too thick in our calculations. In the prepared particles, the base of the rods that form the corona might actually be closer to the surface of the Au core if the small Au particles (5) used to seed corona growth were not adsorbed directly onto the surface of the dielectric layer but were instead able to penetrate inside this polyelectrolyte coating. This is very likely as it has been previously demonstrated that PAH/PSS shells are permeable to
