Silica-Coated Gold Nanorods with a Gold Overcoat

Silica-Coated Gold Nanorods with a Gold Overcoat

(Parte 1 de 3)

DOI: 10.1021/la9032223 ALangmuir X, X(X), X–X ©XXXX American Chemical Society

Silica-Coated Gold Nanorods with a Gold Overcoat: Controlling Optical Properties by Controlling the Dimensions of a Gold-Silica-Gold Layered Nanoparticle

Huaiping Cong, Rasmus Toftegaard, Jacob Arnbjerg, and Peter R. Ogilby*

Center for Oxygen Microscopy and Imaging (COMI), Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Arhus C, Denmark

Received August 28, 2009. Revised Manuscript Received November 20, 2009

Silica shells were directly coated onto surfactant-capped gold nanorods by a simple one-step method. The procedure required no intermediate coating of the gold nanorod prior to the formation of the smooth silica shell, the thickness of which could be accurately controlled over the range 60-150 nm. These silica-encased gold nanorods were then covered with a gold overcoat to yield nanoparticles with unique optical properties that varied with the thicknesses of both the silica layer and the gold overcoat. Using these bulk solution-phase techniques, homogeneous distributions of gold-silica-gold layered nanoparticles with a pronounced plasmon extinction band in the near-IR (i.e., ∼900- 1700 nm) are readily and reproducibly prepared. More specifically, when using a core gold nanorod whose dimensions yield a plasmon band in the visible region of the spectrum (e.g., ∼685 nm), the effect of the gold overcoat is to produce a broad plasmon band that is red-shifted by as much as ∼1000 nm. As such, these multilaminate particles should be of interest as a convenient tool to enhance weak near-IR radiative transitions (e.g., singlet oxygen, O2(a1Δg), phosphorescence at 1270 nm).


Interest in metal nanoparticles, and their synthesis and characterization, has increased tremendously because such particles have unique properties that (1) challenge our understanding and interpretation of fundamental scientific principles and (2) have pertinent applications in a number of fields and disciplines.1-3

Theprincipal characteristicfeatureofametalnanoparticlethat defines its optical properties and behavior is the localized surface plasmon resonance.2,4,5 This is particularly true of gold and silver particles. These localized electric fields depend on the size and shape of the nanoparticle and, given the dimensions involved, are significant with respect to the interaction of the nanoparticle with visible and near-IR electromagnetic radiation.2,4,5 As such, gold and silver nanoparticles can be useful in a wide range of applications that involve light, ranging from communications to the development of new medical techniques.6-10

Nonspherical metal nanoparticles are unique in that they exhibit transverse as well as longitudinal plasmon bands, and the latter can be selectively tuned by altering the nanoparticle aspect ratio.4,1,12 Such asymmetry has been achieved, for example, through the preparation of gold nanorods and nanodisks which, in turn, have yielded plasmon resonances over a large range of visible and near-IR wavelengths.13-15 These plasmon resonances can be used in a number of different ways, one of which is to perturb a radiative transition of a nearby molecule that occurs at the same energy as that of the plasmon resonance.16-18Of particularinterest inthisregard, certainly with respect to our own research program, are (1) nonlinear twophoton transitions in large organic molecules19,20 and (2) the transition between ground state oxygen, O2(X3Σg-), and singlet oxygen, O2(a1Δg). Indeed, with respect to the latter, we have recently shown that the weak O2(a1Δg) f O2(X3Σg-)p hosphorescence at 1270 nm can be significantly enhanced by a gold nanodisk of appropriate dimensions.21

With singlet oxygen spectroscopic experiments in mind, we would like to develop methods by which particles with plasmon resonancesinthe range ∼10-1500nmcan bereadily produced in high yield. Although the surface-immobilized, lithographically produced nanodisks used in our earlier study have many advantages,21 we would now like to focus on bulk solution-phase techniques with the goal of readily producing suspensions of

*To whom correspondence should be addressed. E-mail: progilby@ (1) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729–74. (2) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (3) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. (4) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426. (5) Kelly,K.L.;Coronado, E.;Zhao,L.L.;Schatz,G.C.J. Phys.Chem. B2003, 107, 668–677. (6) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Plasmonics 2007, 2, 107–118. (7) Gobin,A.M.;Lee,M.H.;Halas,N.J.;James,W.D.;Drezek,R.A.;West,J.

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B DOI: 10.1021/la9032223 Langmuir X, X(X), X–X

Article Cong et al.

nonaggregated particles in high yield. In short, we would like to overcome many of the limitations now associated with the production and use of gold nanorods that have an aspect ratio suitable for spectroscopic experiments at ∼1270 nm.

Given the importance ascribed to asymmetric nanoparticles, particularly gold nanorods, great efforts have already been expended to develop techniques by which homogeneous populations of such nanoparticles can be readily prepared.2-26 To date, the seed-mediated, cetyltrimethylammonium bromide (CTAB)-assisted chemical approach has been an effective and reproducible way to obtain populations of gold nanorods with uniform size.27-29 In this technique, the aspect ratio of the nanoparticle is controlled by experimental conditions.

Once prepared, it is important to stabilize and protect a given gold nanoparticle, certainly if the desired optical properties are to be preserved. This stabilization should particularly preclude the tendency for such particles to aggregate.30,31 To this end, a valid methodistocoattheparticlesurfacewithamaterial suchassilica. In this way, chemically stable particles can be homogeneously dispersed in a liquid medium. Although the optical properties oftheparticleand/orparticleensembledependonthethicknessof the silica shell,32,3 this phenomenon can nevertheless be used to one’s advantage with some forethought (i.e., one can use the silica shell to “tune” the particle’s optical properties).

Since silica shells were first coated onto gold nanoparticles using amino-terminated silane coupling agents as the primer to make the gold surface vitreophilic,34 various routes have been reported to prepare such silica-coated nanocomposites.35-38 When coating CTAB-capped gold nanorods, it is well-known thatthe concentrationofthe CTAB surfactantusedmust beclose to its critical micelle concentration.39 However, large amounts of CTAB in the system make the displacement by surface-coupling silane agents difficult due to the strong binding of CTAB molecules to the gold surface. Thus, with the CTAB-based approach, it has been a challenge to develop effective methods to coat gold nanorods with uniform silica shells in a controlled fashion.39-43 These problems tend to amplify for nanorods with the comparatively large aspect ratio needed for a near-IR plasmon band.

In this paper, we present an effective one-step approach to deposit a silica shell on a CTAB-capped gold nanorod with controlled shell thickness. The key in this process is simply to remove excess CTAB surfactant by washing the gold particles once. Thereafter, direct deposition of silica shells can be successfully achieved by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in a mixture of 2-propanol and water. The tunable optical properties of silica-coated gold nanorods with different shell thickness are also revealed simply by changing the concentration of deposition reagents.

In addition, we have been able to deposit a gold overcoat onto the silica-coated nanorods to yield a new multilayered structure with unique and characteristic optical properties that depend on the thickness of both the gold overcoat and the silica shell. Although multilayer nanoparticles have been previously prepared,4-48 a bulk solution-phase-derived gold-silica-gold structure has, to our knowledge, not been reported and characterized. Most importantly, we demonstrate that, when using an easily prepared and easily handled core gold nanorod with a comparatively small aspect ratio (i.e., plasmon band in the visible region of the spectrum), the production of the gold-silica-gold laminate yields a particle with an appreciable and broad plasmon resonance in the spectral range ∼1000-1700 nm. As such, the data reported herein should be particularly relevant to those interested in methods by which one can readily obtain a homogeneousdistributionofreasonablysizedsphericalparticlesforuse in near-IR optical experiments.

Results and Discussion

1. Gold Nanorods. The morphology and size of the gold nanorods we prepared by the seed-mediated, CTAB-directed procedure are shown in the TEM images of Figure 1. The images reveal an average length and width of 59.2 ( 4.8 and 19.9 ( 2.4 nm, respectively, corresponding to an aspect ratio of ∼3.0. The extinction spectrum of these nanorods is characterized by a weak peakat523nmandamoreintenseplasmonresonancepeakat685 nm (Figure 1), which are assigned to the transverse and longitudinal plasmon bands of the gold nanorods, respectively.12,49 Moreover, the wavelengths at which the maxima of these bands occur are in good agreement with those expected given the size and aspect ratio of our gold nanorods.12

These nanorods were used as a core onto which we deposited a silica shell and, subsequently, a gold overcoat (vide infra). 2. Silica-Coated Gold Nanorods. Silica-coated gold nanorodswerepreparedbyaprocessofhydrationandcondensationof TEOS in a water-isopropanol mixture.35 We confirmed that, with high concentrations of CTAB in the system and the associated equilibrium between CTAB in solution and CTAB coated onto the gold nanorods, it was difficult to deposit a uniform silica shell onto the gold. Specifically, given the established procedures that leave a large amount of CTAB on the gold surface (vide supra), it was difficult to control the hydrolysis and condensation of TEOS. As we discussed in the Introduction, previous studies have ascertained that, to overcome this problem and obtain uniform silica deposition, one can apply an intermediate coating to the CTAB-gold particle to act as a scaffold for silica deposition.39-43

Alternatively, we found that high quality and uniform silica deposition on gold nanorods could be readily achieved simply by

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DOI: 10.1021/la9032223 CLangmuir X, X(X), X–X

Cong et al. Article removing excess CTAB from the system. In this procedure, the solution of CTAB-coated nanorods was centrifuged at a slow speed (4000 rpm for 2 min; see Experimental Section for further details). The solid CTAB surfactant was then removed and the sample centrifuged at a higher speed (13300 rpm for 8 min) to remove even more CTAB. Finally, the gold nanorods were washed once with water and, after centrifugation (13300 rpm for 8 min), were redispersed into water. In this way, excess CTAB was removed, leaving enough CTAB behind to stabilize the gold rods and prevent aggregation. Most importantly, the resultant particles presented an adequate surface for the controlled hydrolysis and condensation of TEOS (Figure 2).

To better describe this washing/centrifugation procedure, and itseffects,wequantifiedtheconcentrationofCTABafterthefinal centrifugation and redispersal into water (i.e., the system used for silica deposition). Using liquid chromatography/mass spectrometry, as previously applied by Alkilany et al.,50 we ascertained that the CTAB concentration in this aqueous system was ∼200 μM.

If the gold nanorods were washed several times, silica deposition was not ideal. Specifically, under these conditions, we tended to make more gold-free silica particles and nonspherical particles with a core of aggregated gold nanorods (see Supporting Information). If the gold nanorods were not washed, a large amount of aggregated precipitate appeared in the course of the sol-gel process. Therefore, we conclude that a single wash is an effective pretreatment step conducive for silica deposition from TEOS on CTAB-coated gold nanoparticles.

Silica shells with different thicknesses were accurately obtained by controlling the amount of TEOS introduced into the reaction system(Figure2).WhentheamountofTEOSwasincreasedfrom 4.5 to 2.5 mM (i.e., 6 to 30 μL of added TEOS solution; see Experimental Section), the silica shell thickness increased from ∼60to∼150 nm. The TEM imagesrevealthatthe hydrolysisand condensationofTEOStransformedtheanisotropicgoldnanorod into an isotropic spherical silica-coated nanoparticle. Most of the particles had a single gold core which tended to be more localized in the center of the sphere as the coating thickness increased (e.g., compare the TEM images in Figure 2a,f).

In addition to the washing treatment described above, the concentration of ammonium hydroxide used in the sol-gel process played an important role in controlling the deposition of silica on the gold nanorods (see pertinent TEM images in the

Supporting Information). This NH4OH dependence in the process of hydration and condensation of TEOS has previously been established.36 Under our conditions, with comparatively small amounts of added NH4OH (i.e., 100 μLo f a 30%s olution added to our total reaction volume of 6.0 mL), large numbers of gold- free silica spheres and distorted particles were observed. Other particles contained aggregated gold nanorods. Likewise, when a comparatively large amount of NH4OH was injected into the sol-gel reaction mixture (i.e., 150 μL of the 30% solution), similar undesirable particles were obtained. The desired result of a homogeneously coated single gold nanorod was only achieved, under our conditions, with the addition of 125 μLo f our NH4OH solution, indicating the sensitivity of this reaction to this particular reagent.

Extinction spectra of silica-coated gold nanorods are shown in

Figure 3. Data were recorded from samples prepared using 20 60 nm gold rods (i.e., Figure 1) coated with silica shells of thickness ranging from 60 to 150 nm. The spectra show that the transverse plasmon band at ∼525 nm (see Figure 1) is not appreciably affected by the silica coat, whereas the longitudinal plasmon band shifts to a longer wavelength upon the addition of thesilicacoat.Thesegeneralobservationsareconsistentwithdata previously published.35,39 Moreover, the extent to which the longitudinal band is red-shifted depends on the thickness of the silicacoat.Thelargestredshift(i.e.,from685nmforthebaregold nanorodto728nm) isobservedfromthesample with thethinnest silica shell (i.e., thickness of ∼60 nm). For our samples with a thicker silica shell, the extent of this red shift is not as large. These data are consistent with previous reports which demonstrate how the longitudinal surface plasmon band responds to changes in the local refractive index surrounding the gold nanoparticle.4,5,34 3. Gold Overcoats on Silica-Coated Gold Nanorods. a. Unique Multilayer-Dependent Optical Properties. Experiments were first performed using 20 60 nm gold nanorods with a silica shell thickness of ∼75 nm (Figure 4a). From a macroscopic perspective, the purple color of the bare gold rod became lighter when coated with silica (Figure 4f). Upon attaching goldseeds to theouter surface ofthese silica-coatednanorods, the aqueous dispersion turned brown (Figure 4f). The corresponding TEM imagesofthese particlesshoweda uniformshell of small gold seeds adsorbed on the silica surface (Figure 4b,c). Under our conditions (see Experimental Section), the seeds obtained were ∼2-3 nm in diameter, and free gold particles independent of the silica-coated structure were not observed.

When these seed coated particles were dispersed in the gold plating solution (see Experimental Section), particles with a thicker gold overcoat were prepared. The thickness of this outer gold shell could be increased by increasing the amount of added plating solution. For our experiments, samples with gold overcoats of ∼8a nd ∼15 nm were prepared (Figure 4d,e). The thickness of these gold overcoats was determined from TEM

Figure 1. (a, b) TEM images of gold nanorods grown in aqueous solution indicate an average length and width of 59.2 ( 4.8 and 19.9 ( 2.4 nm, respectively, for the preparative conditions employed (see Experimental Section). (c) Normalized extinction spectrum obtained from an aqueous dispersion of these gold nanorods.

(50) Alkilany, A. M.; Nagaria, P. K.; Hexel, C. R.; Shaw, T. J.; Murphy, C. J.; Wyatt, M. D. Small 2009, 5, 701–708.

D DOI: 10.1021/la9032223 Langmuir X, X(X), X–X

(Parte 1 de 3)