Synthesis and characterization of gold nanoshells using poly(diallyldimethyl ammonium chloride)

Colloids and Surfaces A: Physicochem. Eng. Aspects 329 (2008) 134–141

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Synthesis and characterization of gold nanoshells using poly (diallyldimethyl ammonium chloride) Roya Ashayer, Samjid H. Mannan, Shahriar Sajjadi ∗ Division of Engineering, King’s College London, Strand, London WC2R 2LS, UK

a r t i c l e

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Article history: Received 31 July 2007 Received in revised form 2 July 2008 Accepted 4 July 2008 Available online 15 July 2008 Keywords: Metal nanoshells Silica nanoparticles Poly(diallyldimethyl ammonium chloride) Gold nanoshells

a b s t r a c t Metal nanoshells are a new class of nanoparticles consisting of dielectric cores with metal shells. In this investigation, a novel linker, polydiallyldimethyl ammonium chloride (PDADMAC), was used for formation of gold nanoshells on the surface of silica nanoparticles. In comparison with commonly used linker 3aminopropyltrimethoxysilane (APTMS) at a given amount, this linker shows much better coverage in both nucleation and shell growth stages of nanogold shell formation. Furthermore, the concentration of PDADMAC can be varied over a wider range, without causing system collapse while fine tuning shell coverage. It was also observed that the size of gold nanoparticles strongly influences the extent to which they attach themselves to the surface of silica nanoparticles. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In recent years, nanomaterials have attracted a great deal of attention for their wide range of applications. The assembly of nanoparticles with dimensions approaching those of individual molecules is one of the most exciting challenges of modern colloid chemistry [1]. Due to the numerous advantages of nanotechnology; this field of work has established itself as one of the fastest growing. Although nanoparticle technology is currently in use in only a few commercially available products, ranging from sunscreen to wrinkle-free trousers, the potential applications range from health care (medical drug delivery) to computing (electronically configurable logic circuit) to Aerospace (use of nano-structured materials in aircraft etc.) and so on. Currently, fabrication of nanostructures consisting of a dielectric core, surrounded by a thin metal shell, termed “nanoshells” [2], is a subject of extensive research due to their unique application in many areas such as nonlinear optics, catalysis, and surface-enhanced Raman scattering (SERS) [3–6]. These types of nanoparticles have received particular attention because of their stability and their ease of preparation [7]. One of the most used materials in nanotechnology is silica due to its availability and ease of production. Silica has a wide range of applications in microelectronics, optical communications and thin-film technology. Originally Stöber et al. [8] reported a method

∗ Corresponding author. Tel.: +44 20 78482322; fax: +44 20 78482932. E-mail address: [email protected] (S. Sajjadi). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.07.004

for the preparation of monodisperse spherical silica particles with sizes covering almost the whole colloidal range, by tetraethyl orthosilicate (TEOS) hydrolysis, in ethanolic medium in the presence of ammonia. After this pioneering work, Stöber silica particles have been used as model colloids in a large number of experimental investigations [9–11]. Halas’s group has used a modification of the Stöber method and shown in several papers how nanoshells composed of a gold coating grown around silica colloids can be produced and how the single-particle properties can be exploited [4,7,12,13]. There are many known techniques available for synthesizing various kinds of silica-metal-based composite materials such as metal core-silica shells [14,15], silica core-metal shells [16,17], and nanoparticles embedded in porous silica [18]. At present, the usage of silica as a core has attracted a great deal of interest because of their biomedical [19] and catalysis [20] applications. This investigation is motivated by an attempt to modify the mechanical properties of solder used in electronics interconnect applications. Ceramic nanoparticles embedded in solder have been reported to give rise to increased solder joint reliability [21]. However, in order to facilitate incorporation of the nanoparticles into the solder, a solder wettable coating of e.g. Au is required. Therefore, in this investigation, the dielectric core is comprised of silica while the metallic shell is gold. Previously, it has been shown that silica nanoparticles can be functionalized using a linker. Gold nanoparticles can be immobilised on functionalized silica nanoparticles and used as nucleation sites for the growth of a thin gold shell [7]. The most commonly used linker for production of hybrid nanoparticles is 3-aminopropyltrimethoxysilane (APTMS). In this research we investigate a novel linker, poly(diallyldimethylammonium chlo-

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of the APTMS groups to the silica nanoparticle surface [28,29], the solution was then gently boiled for 1 h. The volume of the solution was kept constant by addition of ethanol during the heating. The APTMS-coated silica nanoparticles were purified by centrifuging at 2000 rpm and redispersion in ethanol. Analysis of the purified nanoparticles by TEM showed no difference between pre- and postfunctionalization with APTMS. The same procedure was repeated for different volumes of polydiallyldimethyl ammonium chloride including 45, 75, 100, 150 and 200 ␮L per 50 mL of nanosilica solution for the production of the nanoshells. Also, the shelf life of the PDADMAC functionalized silica nanoparticles was found to be at least 4 weeks. 2.5. Assembly of colloidal gold nanoparticles

Fig. 1. Summary of the gold nanoshell process.

ride) (PDADMAC). PDADMAC is a cationic polyelectrolyte widely used, both in industrial applications and as a model for charged polymer behaviour [22]. Dilute solution properties of PDADMAC have been studied by several methods [23–26]. However, to the best of our knowledge, no previous studies have explored this linker in the production of nanoshell. 2. Experimental 2.1. Materials Hydrogen tetrachloroaurate (HAuCl4 ) (99.9%), tetraethyl orthosilicate (TEOS) (99.9%), 3-aminopropyltrimethoxysilane, Tetrakiss (hydroxymethyl) phosphonim (THPC) (80% solution in water), poly(diallyldimethyl ammonium chloride) (low molecular weight), potassium carbonate (99%), formaldehyde, ammonium hydroxide solution (33% NH3 ) and ethanol (99%) were obtained from Aldrich Chemical. All chemicals were used as received. HPLC grade water was purchased from BDH and used in all experiments unless otherwise stated. 2.2. Syntheses The gold-covered silica shells were synthesized in a multi step reaction using the method described by Pham et al. [27]. An outline of the synthesis is shown in Fig. 1. In the following sections the steps taken in synthesis of gold nanoshell are described. 2.3. Preparation of silica nanoparticles Silica nanoparticles were produced using the method developed by Pham et al. [27]. 3 mL of ammonium hydroxide solution was added to 50 mL of absolute ethanol. The mixture was stirred vigorously, and 1.5 mL of Si(OC2 H5 )4 (tetraethyl orthosilicate, TEOS) was added dropwise. The initial reaction mixture was clear. After 45 min, the reaction mixture began to turn cloudy as nanosilica particles were grown and eventually turned white. The solution was stirred overnight. Analysis by TEM indicated that the silica nanoparticles were spherical in shape with an average diameter of approximately 90 nm. 2.4. Functionalization of silica nanoparticles

Aqueous solutions of gold nanoparticles were prepared by reduction of chloroauric acid with tetrakishydroxymethylphosphonium chloride (THPC) as described by Duff et al. [30]. These colloidal Au particles are highly monodispersed. First, 0.5 mL of 1 M NaOH, 1 mL of THPC solution (prepared by adding 10 ␮L of 80% THPC in water to 1 mL of water) was added to 45 mL of water. The solution was vigorously stirred for 5 min. After which, 2.0 mL of 1 wt% HAuCl4 in water was added to the stirred solution. The THPC gold solution preparation produced a clear cola brown colour solution within 2–3 s of chloroauric acid addition. Analysis by TEM indicated that the gold nanoparticles were spherical shape with the size of 2–3 nm in diameter. Freshly prepared gold was used since changes in pH with time have been observed previously [31]. Same procedure was used with different concentration of HAuCl4 , ranging from 0.25 to 5 wt%. 2.6. Attachment of colloidal gold to silica 2 mL of functionalized silica nanoparticle solution was added to 20 mL of gold colloid in a tube. After shaking the tube for 3 min, it was left to stand for further 2 h. To remove non-attached small gold nanoclusters, the solution was centrifuged (2000 rpm), the supernatant was removed, and the remaining light brown pellet was redispersed in water. To prevent the growth of pure gold particles during the formation of the gold shell, this centrifugation/redispersion step was repeated until the supernatant contained negligible numbers of small gold nanoclusters. 2.7. Nanoshell growth In order to grow the gold overlayer on the Au/APTMS/silica and Au/PDADMAC/silica nanoparticles, 25 mg of potassium carbonate (K2 CO3 ) was dissolved in 100 mL of water. The mixture was stirred for 10 min prior to addition of 1.5 mL of a solution of 1 wt% HAuCl4 . The solution initially appeared transparent yellow and slowly became colourless over the course of 20–30 min. To a vigorously stirred 20 mL aliquot of this colourless solution, 1 mL of the solution containing the Au/APTMS/silica nanoparticles was added. The same procedure was repeated for Au/PDADMAC/silica. The colour in both cases changed to purple/pink. After addition of 40 ␮L of formaldehyde the colour changed to blue which is a characteristic of nanoshell formation [27]. In general the reproducibility of the results for PDADMAC/silica particles was always consistent, which was not the case for APTMS/silica particles. 2.8. Characterization

The amount of APTMS needed for surface functionalization is estimated [28] to be in excess of (∼50 ␮L, 0.28 mmol) for a 100 mL quantity of silica nanoparticle solution. Therefore, ∼25 ␮L of APTMS was added to 50 mL of the vigorously stirred silica nanoparticle solution and allowed to react for 2 h. In order to enhance bonding

All the prepared samples were characterized by transmission electron microscopy (TEM). TEM was performed with, a FEI TecnaiT20 electron microscope operating at a bias voltage of 200 kV. Sample preparation involved deposition of the nanoparticles in

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their original solution onto a carbon-coated copper grid. The excess solvent was absorbed with the means of filter paper. The grid was then set aside to allow drying before analysis. The size of the particles was determined with the measurement tools of the analysis software (Digital Micrograph; Gatoan Ltd.; USA). A Zetasizer 3600 (Malvern Instruments Ltd., U.K.) was also used for measuring the surface electric potential of the silica nanoparticles before and after addition of APTMS and PDADMAC. Measurements were performed at room temperature and the pH of the solutions was approximately 9.6. Zeta potential was determined by injecting 0.7ml of the solution into a clear disposable zeta cell. The zeta potential was calculated using the Huckle relationship. The average size of silica and gold particles was checked by Zetasizer 3600 (Malvern Instruments Ltd., U.K.). The volume-average diameter (Dv ) of gold nanoparticles was calculated using the following equation:

 Dv =

˙ni Di3

1/3

˙ni

3.1. Effect of amount and type of the linker (1)

where ni is the number of particles with diameter of Di . The number of gold particles, Np , is given by



Np =

6W



Dv3

charge that yield a potential of around −52.9 mV. Also the zeta potential measurements for gold nanoparticles showed a negative potential of −31.6 mV. Thus, the adsorption of the negatively charged nanogold particles to the surface of silica is unlikely. However, gold nanoparticles can be immobilised on functionalized silica nanoparticles after the functional groups at the surface of silica nanoparticles, predominantly silanol (Si–OH), are treated with linkers to modify the surface charge. Therefore, when an aqueous solution of gold nanoparticles is mixed with an ethanolic solution of functionalized silica nanoparticles, the gold nanoparticles become attached to the surface of the silica particles [32]. A very common linker is APTMS which produces a surface terminated with amine groups. In this research, in order to attach the nanogold to silica, we used a novel linker (PDADMAC). The results were then compared with those obtained with the commonly used linker; APTMS.

(2)

where, W is the total mass and  is the density of gold. The value for W was obtained using the stoichiometric amount of elemental gold (Au0 ) obtained by reduction of HAuCl4 assuming a full conversion. The surface coverage of silica particles by gold nanoparticles was estimated using image analysis software. 3. Results and discussion It is known that the silanol (Si–OH) groups of the silica nanoparticles generate a negatively charged surface. The monodispersed nanosilica particles synthesized in this research had a surface

A number of different parameters were systematically varied in order to find the optimum condition in respect to PDADMAC usage in nanoshell formation. The first parameter considered was the quantity of the linker. Fig. 2 shows TEM micrographs of the nucleation stage and growth of gold shell on silica particles for different added volumes of PDADMAC. It was observed that as the volume of the PDADMAC linker increased to 75 ␮L (data not shown) the number of nucleation sites for the attachment of gold increased. However further increase showed the opposite effect (Fig. 2a1 –c1 ). It can be seen that the number of nucleation sites decreased rapidly as the volume of PDADMAC increased to 200 ␮L. A possible explanation for this effect could be due to deposition or extension of polymer chains on nanosilica particles in a way that limits the exposure of positive sites for the attachment of negatively charged gold particles (Fig. 2c1 ). Particle size measurements indicated that the average hydrodynamic diameter of silica particles increased from 126 nm with addition of 75 ␮L of PDADMAC to 136 nm at 100 ␮L.

Fig. 2. TEM images of attachment of gold nanoparticles using: (a) 100 ␮L, (b) 150 ␮L, and (c) 200 ␮L of PDADMAC for stages one and two (indexed 1 and 2, respectively) with (a1 ) having 20 nm and (b1 –c2 ) 50 nm scale bar. Micrographs for 75 ␮L of PDADMAC are shown in Fig. 3.

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Fig. 3. TEM micrographs of attachment of gold nanoparticles to silica particles and nanoshell formation at optimum quantity, i.e. using (a) 25 ␮L APTMS and (b) 75 ␮L PDADMAC. (a1 ) and (b1 ) refer to first stage with scale bar of 20 nm, and (a2 ) and (b2 ) refer to the shell formation with 200 nm scale bar.

Fig. 2a2 –c2 shows the micrographs for nanoshell formation stage. The nanoshell coverage showed a similar trend with variation in the volume of PDADMAC as that for the nucleation stage. The shell coverage increased with increasing PDADMAC up to 75 ␮L. Above this amount, i.e. the optimum volume, the number of nucleation sites decreased and the shell coverage also diminished, as expected. In the case of the APTMS linker, any increase in the volume from 25 ␮L, i.e. the optimum volume, resulted in the collapse of the system (i.e. the nanosilica particles precipitated at the bottom with a clear effluent on the top). The attachment of the nanogold particles to silica nanoparticles seems to be strongly influenced by the nature of the functionalization of the silica nanoparticles. Fig. 3 shows TEM micrographs of gold attachment to silica nanoparticles obtained with optimum volumes of two linkers. At nucleation stage (Fig. 3a1 and b1 ); the gold nanoparticles are well separated from each other. This could be due to the electrostatic repulsion of charged metallic nanoparticles which causes wide separation on the substrate [33,34]. It can be seen that when using the optimum amount of linkers, the PDADMAC/silica particles (Fig. 3a) gives more coverage than APTMS/silica particles (Fig. 3b). The coverage for the PDADMAC/silica nanoparticles in the nucleation stage was found to be approximately 40% whereas the APTMS/silica nanoparticles give coverage of approximately 25%. This is probably due to the fact that in the case of APTMS, the alkylamines exist predominately as positively charged R–NH3 + groups at values of pH < 10 [11]. PDADMAC on the other hand has a [N+ (CH3 )2 ]n chain, therefore more positive sites exist on the polymer chain for gold nucleation to occur than the other linker. Zeta potential measurements also showed a value of +59.8 for PDADMAC whereas APTMS showed a lower value of +46.5 mV (Fig. 4).

Therefore, higher level of adsorption of negatively charged gold on the surface of silica could be due to the higher level of positively charged group of PDADMAC on the surface of silica in comparison with APTMS. In addition, since APTMS has NH3 + group, and PDADMAC has CH3 group in its chain, the change in the pH of the solution is expected to have less effect on the latter linker. This is particularly important as pH of gold dispersions may change with time [31]. PDADMAC chains immobilise the gold particles onto the substrate because of the affinity of the positive group for the gold nanoparticles. The shelf life of functionalized silica particles with PDADMAC was quite long and no variations in the affinity of the particles toward gold nanoparticles were detected within a period of 3–4 weeks. Therefore, it can be concluded that PDADMAC gives better coverage than APTMS at optimum added quantity. The degree of coverage of particles could be improved by increasing the concentration of PDADMAC up to 75 ␮L, whereas for APTMS any increase above 25 ␮L led to mass flocculation. Having established that the

Fig. 4. Comparison of zeta potential after addition of PDADMAC and APTMS to nanosilica particles.

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Fig. 5. TEM images of effect of time: (a–c) 0.5, 10, and 90 min, and (d) 7 days on attachment of gold nanoparticles using 1 wt% HAuCl4 and 75 ␮L of PDADMAC. All scale bars are 20 nm.

Fig. 6. TEM images showing effect of formaldehyde addition on the shell formation after: (a–d) 0, 1, 2, and 3 min using 1 wt% HAuCl4 and 75 ␮L of PDADMAC. All scale bars are 20 nm.

attachment of gold nanoparticles to silica nanoparticles is strongly influenced by the concentration of linker, other parameters which could have an effect on the gold nanoshell formation were also investigated as follows. 3.2. Time evolution of silica-gold morphology 3.2.1. Nucleation—attachment of colloidal gold to silica After adding the PDADMAC/silica nanoparticles solution to gold colloid in a tube, the solution was then monitored for 1 week to

follows any changes in the amount of gold attachment. At various time intervals, a drop of the sample was taken and used for TEM measurements. Although the rate of nucleation appears to be quite fast, it is not instantaneous. In a fraction of a minute, gold particles were detected on the surface of silica particles. The size of attached gold particles did not significantly change with time but the number of nucleation sites increased with time until it reached an almost constant value after about 10 min (Fig. 5). Beyond this stage, nucleation of new sites stopped but a limited growth of the existing sites occurred locally by coagulation of attached gold particles with the

Fig. 7. TEM images of gold attachment on nanosilica particles using: (a) 0.25%, (b) 0.5%, (c) 2%, (d) 3%, (e) 4%, and (f) 5% HAuCl4 . All scale bars are 50 nm.

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Fig. 8. TEM images of gold nanoparticles (a–c) 0.25, 0.5 and 2 wt% HAuCl4 with scale bar of 20 nm, (d–e) 3 and 4 wt% with scale bar of 50 nm and (f) 5 wt% HAuCl4 with 200 nm scale bar.

gold nanoparticles in the mixture (see Fig. 5d). Even after leaving the sample for 1 week there was only a small change in the amount of gold attachment to the surface of silica. This is due to the fact that negatively charged gold nanoparticles repel each other. The time evolution of particle morphology during the nucleation stage indicates the necessity for a second stage during which the shell growth can occur. 3.2.2. Shell formation Formation of the shell was carried out by reduction of HAuCl4 in the presence of Au/PDADMAC/silica nanoparticles. Samples were first taken after addition of Au/PDADMAC/silica nanoparticles to the solution containing colloidal nanogold particles. The results show hardly any difference in the amount of gold attachment to the surface of silica nanoparticles (not shown) in comparison with previous stage (i.e. attachment of gold). After addition of the formaldehyde to the solution, shell formation started rapidly and was completed within 2–3 min, as shown in Fig. 6. Formaldehyde is a known reducing agent for metals [35]. Gold was reduced onto the attached gold particles until they coalesced into a complete

Fig. 9. Number (Np ) and volume-average diameter (Dv ) of gold particles versus HAuCl4 concentration.

shell. Within a minute into the reduction, the shell resembles a fractal network of aggregated colloid which later starts to coalesce with the adjacent particles to form a complete metallic nanoshell (Fig. 6d). 3.3. Effect of gold concentration We studied the effect of HAuCl4 concentration on the formation of nucleation sites. The results for the attachment of gold particles onto silica particles are presented in Fig. 7. From this figure, it can be seen that at low concentration of HAuCl4 (0.25–2%), there is no gold cluster formation indicating that the gold nanoparticles preferably attach to the surface of the functionalized silica. The quantity of colloidal gold particles at low concentrations (i.e. 0.25–1%) is not sufficient to give a high coverage on the functionalized silica particles. However, as the concentration of the HAuCl4 increases to 2%, the attachment of the gold nanoparticles to the silica surface seems to be at its maximum (Fig. 7c). Fig. 6 shows that at low concentrations of HAuCl4 , the average size of gold particles increases from 1 to 2 nm at 0.25% to about 5 nm at 2.0% HAuCl4 concentration. Any further increase in HAuCl4 concentration favours agglomeration of the gold particles resulting in formation of large gold clusters, preventing particle attachment to the silica surface. In order to prevent the build up of large clusters, the formation of gold nanoparticles and their attachment to the silica particles were conducted simultaneously in a reaction vessel at a constant gold concentration. This led to better coverage at the nucleation stage and the formation of smaller gold clusters within the solution (data not shown). This approach can improve the efficiency of gold attachment at a given concentration of gold. Examination of gold nanoparticles as shown in Fig. 8 reveals that the size of gold nanoparticles increased with an increase in HAuCl4 concentration. The reducing agent causes Au3+ ions to be reduced to non-ionized gold atoms. After the solution becomes supersaturated, the gold gradually starts to precipitate in the form of sub-nanometer particle clusters. The resulting metallic gold

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Fig. 10. TEM images of silica particles at: (a1 ) nucleation stage using 1% HAuCl4 concentration, (b1 ) nucleation stage using 2% HAuCl4 concentration, both scale bars are 20 nm, and (a2 and b2 ) shell formation using 1% gold with 50 nm scale bar. All samples are made with optimum PDADMAC.

clusters then undergo growth to form uniform gold nanoparticles. Cross examination of Figs. 7 and 8 indicate that the size of gold nanoparticles attached to the surface of silica nanoparticles is strongly influenced by the size of gold nanoparticles in the solution. The volume-average diameter (Dv ) and the number of nanogold particles (Np ) as a function of HAuCl4 concentration at nucleation stage are shown in Fig. 9. It can be observed (Fig. 9) that as the concentration of HAuCl4 increases to about 2 wt%, there is a slight decrease in the number of the particles (Np ). Since the number of the particles seems to change slightly, the increase in the gold coverage could be due to an increase in the size of gold particles. However, further increase in the concentration of HAuCl4 causes a sharp decrease in the number of gold particles in the solution, due to the formation of large gold clusters, resulting in a poor coverage. The extent of nanogold particle attachment onto the silica particles in the nucleation stage can have a significant effect on the formation of the shell. As previously stated, the optimum HAuCl4 concentration which provides the maximum coverage at the nucleation stage was found to be at 2.0 wt% (Fig. 10b1 ). Fig. 10 shows that the best coverage can be obtained for the shell formation stage with 1% HAuCl4 if preceded by a nucleation stage using the optimum concentration of HAuCl4 (i.e. 2.0 wt%). Shell formation using 2.0 wt% HAuCl4 led to a mass coagulation of particles (data not presented). 4. Conclusion The effect of a novel linker (PDADMAC) on the formation of gold nanoshell was investigated. This linker gives much better and more uniform nanogold coverage at both nucleation and shell

formation stage in comparison with APTMS which is commonly used. The shelf life of functionalized silica particles with PDADMAC was also quite long and no variations in the affinity of the particles toward gold nanoparticles were detected within 4 weeks. The time evolution of gold silica nanoparticles revealed that the size of attached gold nanoparticles was strongly influenced by the size of gold nanoparticles in the solution. Also, the concentration of PDADMAC can be varied over a wider range, without causing system collapse while fine tuning shell coverage. The best results for gold attachment to the nanosilica particles were obtained at the HAuCl4 concentration of 2 wt% for the nucleation stage and 1 wt% for the shell growth stage. Further increase in the gold concentration caused the formation of large gold clusters, thus resulting in a poor surface coverage. Acknowledgements The authors wish to thank Dr. Tony Brain for the TEM measurements. This work was funded by IeMRC under grant code FE/05/01/01. References [1] Y. Kobayashi, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Mater. 13 (2001) 1630. [2] R.D. Averitt, D. Sarkar, N. Halas, J. Phys. Rev. Lett. 78 (1997) 4217. [3] S. Lal, S.L. Westcott, R.N. Taylor, J.B. Jackson, P. Nordlander, N.J. Halas, J. Phys. Chem. B 106 (2002) 5609. [4] S.J. Oldenburg, S.L. Westcott, R.D. Averitt, N.J. Halas, J. Chem. Phys. 111 (1999) 4729. [5] J.B. Jackson, S.L. Westcott, L.R. Hirsch, J.L. West, N. Halas, J. Appl. Phys. Lett. 82 (2003) 257. [6] J.B. Jackson, N. Halas, J. Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 17930.

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