J. Phys. Chem. C 2008, 112, 19279–19282
19279
Dielectric Sensing with Deposited Gold Bipyramids Julien Burgin,* Mingzhao Liu,† and Philippe Guyot-Sionnest* James Franck Institute, The UniVersity of Chicago, 929 East 57th Street, Chicago, Illinois 60637 ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: October 18, 2008
Colloidal gold bipyramids with a narrow ensemble plasmon resonance in the near-infrared are adhered on glass or silicon substrates using polyelectrolytes. Atomic force microscopy and scanning electron microscopy show a monolayer of the deposited colloids that remains nonaggregated, with an optical density of ∼0.1 at the peak plasmon resonance. The substrates can be repeatedly immersed in various solvents. The near-infrared resonance shifts with the optical index of the solvent by ∼-0.62 eV/refractive index unit. The figure of merit for the ensemble absorption shift is comparable to the best values reported for single metallic colloidal particles. Introduction The Surface Plasmon Resonance (SPR) of metal nanoparticles is sensitive to size, shape, and dielectric environment1 and can be tuned in the near-infrared (NIR) for potential biological applications.2 Metal nanostructures have been used as sensors,3-8 and there are examples of surface-plasmon-based biosensors.2,9-11 Previous sensing studies have been reported for ensembles of nanoparticles in colloidal solution3,7,12 or single objects deposited on a substrate8,13-15 for shapes such as triangles, shells, or rods. In this work, we report the formation of a dense but nonaggregating monolayer of colloidal gold bipyramids16 deposited on glass or silicon substrates. We show that the supported bipyramid samples, which have the narrowest SPR in the NIR for metal colloids, result in a high sensitivity to the dielectric environment. In this NIR region, the bipyramid monolayers provide actually the best sensitivity for ensemble transmission measurements for colloidal systems, comparable to results obtained with single particle measurement. We then demonstrate the chemical accessibility of the supported bipyramids and the local sensitivity of the SPR by the shift induced by the adsorption of a partial monolayer of quantum dots. Experimental Section The gold bipyramids are synthesized using a seed-mediated approach16 as follows: Spherical gold seed particles are first prepared by adding 0.5 mL of 0.25 mM sodium citrate to 20 mL of a 0.125 mM HAuCl4 solution at room temperature and by quickly injecting 0.3 mL of a freshly prepared 10 mM NaBH4 solution under vigorous stirring. The growth solution is made by mixing 0.5 mL of 10 mM HAuCl4 and 0.1 mL of 10 mM AgNO3 with 10 mL of a 0.1 M cethylmethylammonium bromide (CTAB) solution. The solution is then acidified with 0.2 mL of 1.0 M HCl, and 0.08 mL of 0.1 M L-ascorbic acid is added to reduce Au(III) to Au(I). The growth is initiated by injecting from 20 to 100 µL of gold seed solution, and the reaction is performed at 35 °C under gentle stirring for about 2 h. The supported bipyramid samples consist of the deposition of gold bipyramids colloids on a dielectric substrate using adsorbed charged polymers. Glass or silicon slides, cleaned with * Corresponding authors. (J.B.) E-mail:
[email protected] (P.G.-S.) E-mail:
[email protected]. † Current address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138.
a piranha solution (50/50 mixture of 30% H2O2 and concentrated H2SO4, Caution: exothermic reaction) for 30 min, are rinsed with deionized water and dipped in 0.1% poly(dimethylammonium chloride) (PDC), a positively charged polymer, for 1 h. The gold bipyramids are treated to carry negative charges by capping them with poly(lithium-4-styrenesulfonate acid) (PSS). The colloidal solution is cooled for 12 h at 8 °C to reduce the solubility of the CTAB and centrifuged once at 2000g. Tubes of 1.5 mL of solution are centrifuged once more at 5000 g for 5 min. The colored precipitate is dissolved in 1 mL of 1 mM NaCl solution before the addition of 0.5 mL of 0.1% PSS. The mixture is shaken and left in the tube for 2 h at room temperature. The solution is then centrifuged again at 5000 g for 5 min, and the precipitate is diluted in 1.5 mL of 1 mM NaCl; this step is repeated once. The PDC-covered slide is rinsed with deionized water and immersed in the PSS-bipyramid solution in a Petri dish for 1 day at room temperature. The slide is then removed from the solution and cleaned with deionized water. The substrate is allowed to dry in air while a part of it is cleaned with a cotton swab to remove nanoparticles and serve as blank for optical measurements. To demonstrate local sensing and the chemical accessibility of the surface, CdSe/CdS quantum dots of ∼6 nm diameter17 are adsorbed onto the deposited pyramids. To this end, the supported bipyramid glass slides are first dipped into a 10 mM mercaptoundecanoic acid (MUA) in ethanol solution for 15 min, rinsed with ethanol, and N2-dried. Then the slide is dipped into a solution of quantum dots (0.1 nM, typically) in a 10:1 toluene/ ethanol mix for at least 6 h and then rinsed with ethanol. Extinction spectra are measured using an Ocean Optics USB4000 VIS-NIR spectrometer and a 50 W tungsten lamp. Transmission electron microscopy (TEM) is performed with a FEI Tecnai F30 operating at 300 kV. The sample is a copper grid on which a droplet is allowed to dry in air for 30 min. Scanning electron microscopy (SEM) is performed with a FEI Nova 200 NanoLab operating at 3 kV. SEM measurements are performed on silicon subtrates. Atomic force microscopy (AFM) is performed with a NanoScope III in tapping mode, on either glass or silicon substrate. Results and Discussion Figure. 1 shows a TEM image of the colloidal solution. It contains several nanostructures, including ∼30% of the monodispersed bipyramids (length of 90 nm and diameter of 30 nm),
10.1021/jp807465p CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
19280 J. Phys. Chem. C, Vol. 112, No. 49, 2008
Figure 1. (a) High-resolution TEM image of the synthesis product made using 45 µL of seed solution in the growth solution. The inset shows a gold bipyramid under higher magnification (390 kX). (b) Extinction spectrum of the colloidal solution (solid line, arbitrary unit scale) and gold nanoparticles deposited on a glass slide (dotted line, optical density scale).
spheres of ∼50 nm diameter, and a small number of nanorods. The extinction spectrum of the colloidal solution shown in Figure 1b has two features. The 2.25 eV peak is the SPR of the spheres, and the NIR peak at 1.55 eV is the longitudinal SPR of the bipyramids. Their transverse resonance is weak and hidden in the background.18 The NIR resonance is narrow16 due to the highly monodispersed bipyramid shape. When the colloid is adsorbed on a glass slide, it shows the same two peaks (Figure 1b), which are blue-shifted because of the lower refractive index of air. There is also a new peak near 1.45 eV, which is attributed to interacting particles. The relative amplitude of this peak can be lowered by reducing the concentration of the deposited particles. The optical density with a single layer deposition is ∼0.1 for the bipyramid SPR, which is comparable to previous studies.19 AFM images show that the bipyramids appear with an asymmetric profile, with one side lower than the other, as shown in Figure 2. This is seen for most bipyramids. The pentagonal section of the bipyramid implies that they are standing on only one of the 10 facets, which is a priori interesting and appropriate for sensing. The dielectric sensing was quantified by measuring extinction spectra with the glass substrates immersed in water (n ) 1.333),
Burgin et al.
Figure 2. (a) Surface topography of gold nanopyramids deposited on a glass slide measured using an AFM in tapping mode (image size 2 µm). The inset shows an expansion over few particles (image size 275 nm). (b) Cross section profile measured along the principal direction of the gold bipyramid shown in inset of part a with a schematic drawing of its position on the horizontal substrate.
Figure 3. Extinction spectra of gold bipyramids deposited on a glass slide in different dielectric environments: air (solid line), water (dashed line), ethanol (dotted line), and toluene (short dashed line).
ethanol (n ) 1.361), and toluene (n ) 1.467). As shown in Figure 3, there is a red shift with increasing optical index.12 Measurements on several bipyramid samples with different aspect ratios, defined as the length divided by the largest width, are summarized in Figure 4. For all samples, the SPR peak follows linearly the changes in refractive index units (RIU) with a sensitivity of -0.62 eV/RIU (standard deviation ∼ 5% over several samples). Sherry et al.8 proposed another relevant factor for sensing applications, the “figure of merit” (FOM) defined as FOM ) slope (eV/RIU)/fwhm (meV). These supported bipyramids have the highest FOM, around 4.5, found for any
Dielectric Sensing with Deposited Gold Bipyramids
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Figure 5. SPR energy shift measured in several solvents and for several steps of the dots-anchoring procedure, showing a stable 0.1 eV red shift in air after anchoring a partial monolayer of quantum dots. Inset: SEM image of a bipyramid with anchored quantum dots (scale bar 50 nm).
Figure 4. (a) Surface plasmon energy shift of colloidal bipyramids with different aspect ratios deposited on glass substrates (2 (2), 2.75 (b), 3 (9), and 3.5 (1)) as a function of the dielectric environment (air, water, ethanol, and toluene). The solid lines are linear fits of the data (correlation ∼ 0.998). (b) PSS-capped bipyramids with an aspect ratio of 2.75 deposited on glass slide (b) or in colloidal solution (O). The solid line is a quasistatic calculation for an ellipsoid of aspect ratio 3.76 chosen to match the bipyramids’ SPR resonance at the index matching condition with the polymer layer (nPSS ) 1.398); that is, when the dielectric environment is homogeneous. The segmented lines are calculations using an effective index whose weighting factor, R, is estimated in order to fit the experimental data: R ) 0.77 for the dashed line, R ) 0.65 for the dotted line.
ensemble measurement previously reported due to their narrow ensemble SPR width (∼140 meV).16 This FOM value is similar to the one reported for single silver nanocubes,20 but it has the simplicity of being available with an ensemble measurement and the advantage of being in the NIR domain. As a comparison, supported nanorods synthesized by an analogous seed growth were measured to have a much lower FOM of ∼1.2 (-0.52 eV/RIU, 287 nm/RIU) consistent with their larger inhomogeneous plasmon width as well as the likely smaller surface exposure. The linear behavior of the SPR with the optical index is verified in the analytical case of the quasistatic approximation for an ellipsoid with the gold bulk dielectric constant.21 The SPR peak position ΩSPR is given by εAu′(ΩSPR) ) -(1 - L)ε/L, where εAu′(ω) is the real part of the gold dielectric function, L is the depolarization factor,4,22 and ε is the dielectric constant of the environment (ε ) n2). Figure 4b shows that the energy shifts quite linearly with the optical index, n. It is calculated for an ellipsoid of aspect ratio (3.76) chosen to match the bipyramid resonance at n ) nPSS ) 1.398. We note that the linearity results from the specific energy dependence of the gold dielectric constant in the spectral range considered and that it should not be generally expected. We also note that the shift, expressed in terms of wavelength, will correspondingly be parabolic. We therefore prefer to report sensitivity as electron volts per RIU.4,23,24 In Figure 4b, the calculated sensitivity of the ellipsoid is ∼-0.96 eV/RIU. The sensitivity of the supported bipyramids is ∼65% of this calculated value. The lower experimental value
is not likely an effect of shape, although 3D computations would be needed to confirm this point. Instead, it is likely due to the glass substrate and the adsorbed polymer layer, which shield the bipyramids from the solvent dielectric. To address the relative roles of the substrate and the polymer, we measured the dielectric sensitivity directly in solution by changing from water (n ) 1.333), to ethylene glycol (n ) 1.432) and glycerol (n ) 1.473). The results are shown in Figure 4b (open circles). The sensitivity of the colloid in solution is now 0.77 of the ideal ellipsoid with index matching at n ) nPSS. The reduction of sensitivity can be explained by an average effective refractive index, neff, defined as15 neff ) Rn + (1 - R)n0, where n is the bulk environment index, n0 is the local environment index, and R is a weighting factor of 0.77. Effectively, the polymer layer around the nanoparticles reduces the index change by ∼23%, and this is similar to the ∼20% effect reported for thiols on silver nanoprisms.25 For the supported bipyramids on the glass substrate, we modify the effective index to account for the glass using neff ) β(0.77n + 0.23nPSS) + (1 - β)nglass. From the data in Figure 4b, β ) 0.15. This is close to the expectation of 1/10, considering that one facet is in contact with the glass. Overall, the effective index seen by polymer-capped and deposited bipyramids is quite high, ∼65% of the solvent index, similar to the best value that was reported for single gold nanostructures deposited on quartz slides without surrounding polymer material.15 To test that these substrates allow local chemical sensing, we investigated the SPR shift of gold bipyramids when functionalized with semiconductor quantum dots. We anchored CdSe/CdS core/shell quantum dots to gold bipyramids using MUA, where the thiol group binds to gold while the acid group binds to the chalcogenide dots. SEM images shown in the inset of Figure 5 confirm the deposition of a partial monolayer of dots on the bipyramids and, therefore, that the surface is chemically accessible. The SPR of the supported bipyramids is shown in Figure 5 for the sample in air, inside the MUA/ethanol solution, after drying with N2, in a toluene/ethanol mix without dots, after drying, and after immersion in a solution of dots for 6 h and drying. The anchoring of quantum dots at the surface of the bipyramids results in a permanent red shift from 1.87 to 1.74 eV. This confirms that the sensing is quite local. Using the results above, the effective index change due to the quantum dot monolayer is ∼1.23. This is already a sizable fraction of
19282 J. Phys. Chem. C, Vol. 112, No. 49, 2008 the index of refraction (∼1.75) for thick and smooth quantum dot films measured by ellipsometry. Therefore, the supported bipyramids are quite sensitive to the local dielectric environment, with surfaces that are chemically accessible, both of which are requirements for further development as a sensor.
Burgin et al. of their dielectric environment for prolate ellipsoids using the quasistatic approximation. It also contains SEM pictures of deposited gold particles on a silicon slide; quantum dots functionalized samples are also shown. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Conclusion Polyelectrolytes were used to irreversibly adsorb a gold colloidal bipyramid monolayer homogeneously on a substrate, leading to uniform substrates with ∼0.1 optical density, a good stability in several solvents, and a narrow plasmon near-infrared resonance. The energy of the plasmon resonance varies linearly with the optical index of the contacting solution, in agreement with the quasistatic model for an ellipsoid shape. The narrow plasmon width leads to a figure of merit of ∼4.5, defined as the energy tuning of the plasmon resonance with the refractive index divided by the plasmon width. This is the highest figure of merit for ensemble measurements, and it is competitive with single particle results.10 Using the expectation for an ideal ellipsoid as a reference, the sensitivity to the liquid optical index is ∼77% in colloidal form and ∼65% for the supported bipyramids. The supported bipyramids are also shown to be chemically accessible. An example of nanosensing with anchored quantum dots is given. The gold bipyramids should be of interest in biological applications because of their high sensitivity to the local dielectric environment in the nearinfrared, achievable without the need for single particle measurements. Acknowledgment. The work is supported by NSFCHE0718718 and made use of the common facilities supported by the NSF MRSEC program under DMR-0213745. The authors thank Qiti Guo for help in using the MRSEC Material Preparation Laboratory and Alexandre Pourret for AFM instrumentation aid. Supporting Information Available: SI contains plots of SPR wavelength and energy dependence over the optical index
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