J. Phys. Chem. C 2010, 114, 7521–7526
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Abnormally Large Plasmonic Shifts in Silica-Protected Gold Triangular Nanoprisms† Matthew J. Banholzer, Nadine Harris, Jill E. Millstone, George C. Schatz,* and Chad A. Mirkin* Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 ReceiVed: December 16, 2009; ReVised Manuscript ReceiVed: February 25, 2010
The synthesis of silica-encapsulated gold nanoprisms (AuNP@SiO2) is reported. These nanostructures are remarkably stable and resist etching and rounding of their sharp vertices (a process which begins on unprotected Au nanoprisms in a matter of hours) in many chemical environments (water, ethanol, dimethyl sulfoxide, and tetrahydrofuran). The silica growth process has been studied and occurs according to the shape of the particle, where the edges of the prisms are coated less than the large triangular facets. The AuNP@SiO2 particles have dielectric sensitivities that are as large as 737 nm/RIU. Discrete dipole approximation calculations have been used to investigate the effects of this variable thickness on dielectric sensitivity and show that for the anisotropic coatings it is significantly higher than for a uniform coating due to the location of electromagnetic hot spots near the tips and edges of the particles. These calculations also show that dipole resonances exhibit greater sensitivity than multipole resonances, due to the shorter range of the multipolar electromagnetic fields. Introduction The localized surface plasmon resonance (LSPR) of a noble metal nanostructure is extremely sensitive to the refractive index of the nanostructure’s surrounding dielectric environment. This observation has been used to create a suite of chemical and biological detection systems, which function based upon plasmonic shifts that accompany target binding events.1-9 Compared to many other techniques, including amplification via polymerase chain reaction and a variety of nanoparticle-based assays,10 the sensitivities of many LSPR-based techniques are often quite low.6 Consequently, there has been much interest in developing nanostructures with enhanced dielectric sensitivity that can be readily functionalized with biorecognition agents in a straightforward fashion.7,11-19 Of the nanoparticles studied thus far, both theoretical and empirical work suggest that anisotropic gold nanoparticles (e.g., prisms, rods, bifrustrums) possess the highest inherent sensitivity.13,19,20 However, a major problem with many polyhedra noble metal nanoparticle systems is the instability of the vertices, which are the high-energy points for the structures.21 Indeed, over time, they often tend to round or truncate and adopt more stable morphologies with a concomitant broadening and blue shift in their plasmon resonances. This shape change decreases the sensitivity of the LSPR to changes in dielectric environment and leads to an unstable assay.21,22 Therefore, the inherent dielectric sensitivity of a plasmonically active nanostructure is not enough to make a useful detection system. Indeed, the system must be structurally robust and amenable to chemical functionalization without corresponding changes in shape before it can be utilized for many sensing applications. Although there have been some successful attempts to address this issue (in particular through the use of atomic layer deposition),23 the methods developed are limited to lithographically patterned nanostructures as †
Part of the “Martin Moskovits Festschrift”. * To whom correspondence should be addressed: Chad Mirkin for experimental work,
[email protected]; George Schatz for theoretical work,
[email protected].
opposed to dispersible probes and require the use of sophisticated and costly instrumentation. Herein, we report the synthesis of silica-encapsulated gold nanoprisms (AuNP@SiO2), which exhibit high sensitivity to their dielectric surroundings. The AuNP@SiO2 nanostructures are remarkably stable and resist etching and rounding of their sharp vertices (a process which begins on unfunctionalized (e.g. with thiolated compounds) Au nanoprisms in a matter of hours) in many chemical environments (water, ethanol, dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF)). We have investigated the dielectric sensitivity of the AuNP@SiO2 particles with varying shell thickness (3-20 nm) using both empirical and theoretical methods and found that sensitivities as high as ∼740 nm per refractive index unit (RIU) are attainable. Unmodified particles typically exhibit sensitivities in the 200-400 nm/RIU range. In addition to the study of the LSPR dielectric sensitivity, we also report an interesting aspect of the silica coating process itself. Specifically, we have discovered that the silica shells grow anisotropically, according to the shape of the particle, where the edges of the prisms are coated less than the large triangular facets. We use electromagnetic theory (DDA calculations) to investigate the effects of this variable thickness on dielectric sensing and show that for the anisotropic coatings, the dielectric sensitivity is significantly higher than for a uniform coating due to the location of electromagnetic hot spots near the tips and edges of the particles. We have also explored the dielectric sensitivity of higher multipole resonances for these particles, showing that they have less dielectric sensitivity than dipole resonances due to the shorter range of the plasmon-enhanced local fields near the nanoparticle surfaces. Experimental Section Nanoprisms were prepared according to a modified literature procedure.24,25 All glassware was washed with aqua regia (3:1 ratio by volume of HCl and HNO3; CAUTION: Aqua Regia is highly toxic and corrosiVe), and rinsed copiously with Nanopure (18.1 MΩ) water. Au nanoparticle seeds were prepared by
10.1021/jp911889a 2010 American Chemical Society Published on Web 03/19/2010
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J. Phys. Chem. C, Vol. 114, No. 16, 2010
Figure 1. (A-C) TEM images of Au nanoprisms with 3.0, 13.3, and 23.4 nm SiO2 coating. (D) Linear regression of silica thickness as a function of TEOS concentration. Shell thickness (nm) ) 984[TEOS] (µM) - 6.60.
reducing 1 mL of 10 mM HAuCl4 with 1 mL of 100 mM NaBH4 while stirring vigorously. The reduction was done in the presence of 1 mL of 10 mM sodium citrate and 36 mL of Nanopure water. Upon addition of the NaBH4, the solution turned a reddish-orange color and was allowed to continue stirring for 1 min. The resulting mixture was aged for 2-6 h in order to allow for the hydrolysis of unreacted NaBH4. The gold nanoparticle seeds exhibited a plasmon resonance band at 500 nm and an average diameter of 5.1 ( 0.7 nm. After the aging period, three growth solutions were prepared for the seedmediated growth step. The first two solutions (1 and 2) contained 0.25 mL of 10 mM HAuCl4, 0.05 mL of 100 mM NaOH, 0.05 mL of 100 mM ascorbic acid, and 9 mL of supersaturated cetyl trimethylammonium bromide (CTAB) solution.24,25 The final growth solution (designated 3) contained 2.5 mL of 10 mM HAuCl4, 0.50 mL of 100 mM NaOH, 0.50 mL of 100 mM ascorbic acid, and 90 mL of the CTAB solution. Nanoprism formation was initiated by adding 1 mL of seed solution to growth solution 1. The solution was gently shaken, and after 1.5 s elapsed, 1 mL of growth solution 1 was immediately added to 2. The resulting solution was shaken, and after 1.5 s elapsed, all of the resulting growth solution was added to 3. After 30 min, nanoprisms were purified by separating aliquots of the reaction mixture into 1.5 mL centrifuge tubes in 1.5 mL aliquots, and allowing the prisms to precipitate overnight. After separation, the supernatant was removed, and the nanoprism pellet was resuspended in Nanopure water. Silica-coated nanoprisms were prepared by adapting literature procedures.26 Purified AuNPs were functionalized with 1-mercaptohexadecanoic acid (MHA) (10 mM MHA in ethanol was added to 1 mL aqueous solution of AuNPs and mixed overnight), and twice centrifuged and resuspended in 1 mL of absolute ethanol. A certain amount of tetraethylorthosilicate (TEOS) was added corresponding to a desired thickness (Figure 1), and 65 µL of 40% dimethylamine (DMA) was added to catalyze the condensation reaction. After mixing for 3 h, the AuNP@SiO2 particles were washed two times with ethanol.
Banholzer et al. The optical features of Au nanoprisms were monitored by absorbance spectroscopy (UV-vis-NIR) using a Varian Cary 5000 spectrophotometer, in double-beam mode, baselined to the spectrum of 99.99% deuterated solvent so that extinction measurements could be taken at wavelengths >1300 nm (water, ethanol, DMSO, or THF, depending on the solvent, Cambridge Isotope Laboratories). Absorbance cuvettes were 10 mm path length, single crystalline quartz (QS, 284, Hellma, Inc.), and contained 1.5 mL sample chambers. All nanostructures were characterized using a Hitachi-8100 transmission electron microscope (TEM) at 200 kV using 400-mesh, Formvar-coated, copper grid supports (Ted Pella, Inc.). Measured structure properties of the particles are summarized in Table S1 (Supporting Information). The discrete dipole approximation (DDA) technique was used to model the silica-coated nanoprisms. This technique was first developed by Purcell and Pennypacker27 and then latter modified and implemented by Draine and co-workers28-32 into DDSCAT.33 The DDA technique has been extensively used, and the mathematical formalism, advantages, disadvantages, and recent developments of this technique have been thoroughly described elsewhere.28,34 In the present application, the DDA technique calculates the extinction spectrum by representing the SiO2 encapsulated Au prism structure with a cubic array of polarizable dipoles. The polarizability of each dipole is a function of the local dielectric constant, either gold or SiO2. Each dipole is polarized upon application of an electromagnetic plane wave, and then self-consistently coupled to determine overall particle optical properties such as extinction. Au prisms of 125 nm edge length and 7 nm thickness were modeled with three different thicknesses of SiO2 coatings: 3 nm (AuNP1), 13 nm (AuNP2), and 21 nm (AuNP3). The dimensions for the fully coated prisms are shown in Table S1 (Supporting Information). Additional calculations were performed on AuNP2 to examine the anisotropic coatings where only the face or edge of the Au prism is coated with SiO2. These dimensions can also be seen in Table S1 (Supporting Information). Schematic figures of each of the modeled prisms are shown in Figure S2 (Supporting Information). Prism extinction efficiencies were calculated over orientationally averaged angles with an interdipole spacing of 1 nm (see Supporting Information for more information on orientation averaging). The Au prisms were described with the frequency-dependent refractive index from Johnson and Christy,35 and the refractive index of SiO2 was assumed to be constant with a value of 1.47.36 The refractive indices of Au and SiO2 were then divided by the refractive index of the appropriate solvent to get the relative refractive indices. Results and Discussion Building on our past work with Ag nanoprisms,26 we have used a modified procedure based upon the Sto¨ber-inspired synthesis of Kobayaski to synthesize silica-coated nanoprisms.37 Au nanoprisms (and many other anisotropic nanostructures generated by similar methods) present a new challenge since the CTAB bilayer that exists on as-made nanostructures can disrupt silica condensation around the metal nanoparticle and preclude high-yield formation of encapsulated particles. However, we have previously shown that thiol-containing moieties may displace this tightly bound bilayer under certain conditions.38 The critical issue with this type of procedure is maintaining prism stability in solution while removing the original ligand layer. We have found that charged moieties with terminal thiol groups such as MHA or thiolated oligomers of ethylene glycol are able to disrupt the CTAB bilayer and allow
Silica-Encapsulated Gold Nanoprisms silica coatings to form while simultaneously preventing both agglomeration and etching of nanostructure features during shell formation. The silica coating thickness is easily controlled from approximately 3 to 30 nm by varying the concentration of TEOS in the reaction mixture (Figure 1). This shell effectively prevents prism degradation (Figure S1, Supporting Information). However, after a critical concentration (approximately 40 nM), enough TEOS is present in the reaction mixture that the formation of pure silica spheres competes effectively with the growth of silica shells on the Au nanoprisms, which creates a practical upper limit on the shell thickness in a single step reaction (∼40 nm). Nevertheless, the process is quite predictable and results in relatively uniform shell thickness from prism-toprism, face-to-face, and edge-to-edge (
