NANO LETTERS
Gold Nanoshells Improve Single Nanoparticle Molecular Sensors
2004 Vol. 4, No. 10 1853-1857
G. Raschke,* S. Brogl, A. S. Susha, A. L. Rogach, T. A. Klar, and J. Feldmann Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Amalienstr. 54, D-80799 Munich, Germany
B. Fieres, N. Petkov, and T. Bein
Downloaded via UNIV OF TOLEDO on June 18, 2018 at 19:28:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Chemistry and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstr. 5-13, D-81377 Munich, Germany
A. Nichtl and K. Ku1 rzinger Roche Diagnostics GmbH, Nonnenwald 2, D-82372 Penzberg, Germany Received June 21, 2004; Revised Manuscript Received August 4, 2004
ABSTRACT Molecular sensors based on scattering spectroscopy of single gold nanoparticles can be improved three-fold by the use of gold nanoshells instead of solid gold nanoparticles. The particle plasmon resonance responds more sensitively to changes in the environment, the biological spectral window is accessible, and the scattering spectra show sharper resonances. In particular, we focus our discussion on the narrow homogeneous line width of only 180 meV.
Gold nanoshells are fascinating nanoparticles composed of a spherical dielectric core coated with a nanometer thin gold layer. Their scattering spectra show a pronounced resonance in the visible range,1,2 similar to solid noble-metal nanospheres. The origin of this resonance behavior is a collective oscillation of the conduction band electrons, which is known as the nanoparticle plasmon (NPP).3 Au nanoshells are qualified for use in various applications because of the wellestablished functionalization chemistry of gold surfaces. These applications include environmental,4 Raman,5 chemical,4 and biological sensing.6 An important functional principle employed in chemical and biological sensing relies on the strong dependence of the NPP resonance position on the refractive index of the particles’ surroundings.3,7,8 In particular, an increase of the dielectric function of the environment surrounding the nanoparticles induces a shift of the NPP resonance to lower energies. Such a change can be caused by the adsorption of molecules directly to the surface of noble-metal nanoparticles,9,10 or by the specific binding of analyte molecules to nanoparticles functionalized with molecular recognition sites such as antibodies.11-16 Recently, this approach has been extended to a single nanoparticle biosensor, which utilizes the scattered light from a single nanoparticle as a reporter signal for molecular * Corresponding author. E-mail:
[email protected]. 10.1021/nl049038q CCC: $27.50 Published on Web 08/21/2004
© 2004 American Chemical Society
binding. By these means the total number of biomolecular binding events necessary for a detectable signal is drastically reduced.17,18 In this letter we show that molecular sensors based on single gold nanoparticle light scattering can be improved by the use of gold nanoshells instead of solid gold nanospheres. The improvement is three-fold. First, the particle plasmon resonance of nanoshells appears at lower energies compared to the plasmon resonance of nanospheres of the same diameter. This means that the particle plasmon resonance is closer to or even coincides with the “biological window” of high optical transmission in blood and tissue between 700 and 1100 nm.6 The second advantage is that the particle plasmon of nanoshells exhibits a larger spectral shift than the particle plasmon of nanospheres, provided that they experience the same change of refractive index in their direct vicinity. Finally, the third advantage is the much smaller full width at half-maximum (fwhm) of the scattering spectrum. It is obvious, that a spectral shift can be more accurately detected if the scattering spectra are sharp. Up to now, the values for the homogeneous line widths of nanoshells were unknown because only ensemble spectra have been taken from nanoshells. Some rough estimations suggested that the homogeneous line width is ∼420 meV and the ensemble spectra are further broadened to 560 meV due to an inhomogeneous size distribution.19 However, it is hard to
Figure 1. (a) Successively recorded ensemble extinction spectra measured during the synthesis of Au2S/Au nanoshells. The peak centered at 2.33 eV originates from the absorption of solid gold nanospheres with a diameter of ∼5 nm. The second plasmon peak, shifting across the visible spectrum during the synthesis, arises from Au2S/Au nanoshells. The given reaction times refer to the second addition of Na2S (t ) 0, 10, 30, 150 min, 140 h). (b) Scattering spectra of three individual nanoparticles: A Au2S/Au nanoshell (red line) and solid nanospheres with diameters of 40 nm (solid black line) and 150 nm (dashed black line).
determine the true homogeneous line width without performing spectroscopic measurements on single nanoshells. In this letter we use single-nanoparticle spectroscopy and find a homogeneous line width of only 180 meV, less than 50% of the line width expected in previous work by Halas et al.2,19 We compare our experimental results with Mie theory including surface scattering and show that approximately 50% of the damping associated with a line width of 180 meV is due to surface scattering of electrons. Gold nanoshells with a gold sulfide core and a gold shell (Au2S/Au nanoshells) are prepared in water analogous to the synthesis described by Zhou et al.1 using a two-step reaction of Na2S with HAuCl4. A typical sample is made by addition of 10 mL of 1 mM Na2S to 10 mL of 2 mM HAuCl4, waiting for 5 min, and a second addition of 5 mL of 1 mM Na2S. The Na2S solution was aged for 24-48 h before use. Au2S/ Au nanoshells with an average diameter of 30-50 nm and much smaller Au nanospheres (∼5 nm) grow simultaneously during the synthesis. Figure 1a shows the time evolution of the UV-vis extinction spectrum during the nanoparticle growth. At t ) 0 the second aliquot of Na2S is added. The spectral feature centered at 2.34 eV is due to the extinction caused by solid gold nanospheres; the peak at lower energies is due to the extinction caused by Au/Au2S nanoshells.1,2 The samples for single nanoparticle spectroscopy are prepared by drop casting a dilute suspension of the fully reacted solution onto a cleaned microscope cover slip. Unless noted, the dried samples are covered with index matching fluid to ensure a uniform dielectric environment (n ) 1.52). The solid 1854
Au nanospheres used for comparative measurements were obtained from BBI.20 The scattering spectra of individual nanoparticles are recorded by a dark-field microscope setup with an attached grating spectrometer coupled to a cooled CCD.8,21,22 Unpolarized white light from a 100 W halogen lamp is used for sample illumination. The polarization of the light scattered by a single nanoparticle is monitored with the help of a polarizer in front of the spectrometer. With this setup we are able to measure the scattering spectra of single gold nanospheres down to a diameter of ∼20 nm. Therefore, the setup automatically suppresses the scattering from the ∼5 nm small nanospheres formed during the synthesis of the nanoshells. Figure 1b shows measured single nanoparticle scattering spectra of an individual Au2S/Au nanoshell (red line) and for two nanospheres with diameters of 40 nm (solid black line) and 150 nm (dashed black line). While the NPP resonance peak of the nanoshell appears at 1.84 eV, the resonance peak of the 40 nm sized nanosphere is centered at 2.22 eV, far away from the biological window. The only possibility to tune the NPP resonance position of a solid gold nanosphere in aqueous solution to lower energies is to enlarge the particle diameter. Inescapably, this substantially broadens the NPP resonance due to enlarged radiation damping.22 Additionally, the appearance of the quadrupole resonance further broadens the spectrum. In particular, the 150 nm sized nanosphere in Figure 1b shows a plasmon resonance peak at 1.82 eV and a line width of 880 meV. In remarkable contrast, the NPP resonance of the Au2S/Au nanoshell shows a line width of only 180 meV. This is even narrower than the 250 meV line width of the 40 nm nanosphere due to decreased interband and radiation damping.22 The reduction in radiation damping occurs because the gold volume in a nanoshell compared to a solid nanosphere is smaller. As a result, the detection of a spectral shift of the plasmon resonance due to molecular binding events is expected to be significantly simplified, in particular, if the shift is small as it is the case for low molecular weight analytes under physiological conditions. A small line width is also a prerequisite for parallel detection of several different analytes because it facilitates multiplexed assays by multicolor coding. Furthermore, the inverse of the line width of the NPP is directly proportional to the field strength at the surface of the nanosphere. A detailed analysis of the homogeneous line width is therefore also important for surface enhanced Raman cross sections23 and third-order susceptibilities.24 In general, the NPP line width of ensembles of metal nanoparticles is determined by three contributions: (i) a homogeneous broadening due to intrinsic damping of the plasmon via single carrier excitation inside the nanoparticles or emission of photons; (ii) homogeneous broadening due to scattering at the surface or interaction of the NPP with adsorbates, and (iii) inhomogeneous line broadening due to particle-to-particle variations in size and shape. Intrinsic broadening effects (i) are fully covered by Mie theory calculations using the experimentally determined dielectric function of gold.22,25 However, contributions to the line width Nano Lett., Vol. 4, No. 10, 2004
Figure 2. Measured resonance line width of the NPP resonances in single gold nanoshells (triangles) plotted versus resonance energy. Solid and dashed lines are Mie theory calculations for constant core radius (RC ) 15 nm) and increasing shell thickness, and for constant total particle diameter (Rtot ) 40 nm) with different core/shell aspect ratios, respectively. The calculations were made for three different values of the surface scattering parameter A = 0, 0.5, 1.0 (see text). Inset: Schematic sketch of an Au2S/Au nanoshell composed of a dielectric core with radius RC and a gold shell of thickness dS.
of gold nanoshell NPPs stemming from surface effects (ii) or inhomogeneities (iii) are difficult to separate from each other, because up to now only ensemble measurements have been available. One possibility to dissolve this ambiguity is to quantify the inhomogeneous size distribution of the nanoparticles with the help of transmission electron microscopy (TEM).2,19 However, in case of the nanoshells, only the total diameter is accessible to TEM measurements, while the aspect ratio of the core and the shell size remains unresolved. Consequently, the total inhomogeneity of the sample may be underestimated and therefore surface scattering effects may be overestimated when ensemble NPP line widths are investigated. Since we measure the homogeneous scattering spectra of single nanoshells, we can clearly separate between homogeneous and inhomogeneous contributions to the NPP line width. Therefore, we are able to quantify the contribution of surface scattering to the total homogeneous line width more accurately than it is possible using ensemble spectra and TEM micrographs. The observation of several gold nanospheres in the dark field microscope reveals that there is an inhomogeneous distribution of the plasmon peak energies Eres. The triangles in Figure 2 show the experimentally determined homogeneous line widths Γ (fwhm) plotted against the corresponding Eres. The various observed resonance positions are either due to different particle sizes or due to variations in the aspect ratio of the particle core size and of the shell thickness. In principle, elipticity of the particles or uneven or incomplete gold shells could also lead to a variation of the NPP resonance frequency. However, all data points displayed in Figure 2 are taken from spectra that are almost independent of the orientation of the polarizer in front of the detector. Therefore, the data points in Figure Nano Lett., Vol. 4, No. 10, 2004
2 represent only more or less spherical nanoshells with homogeneous gold coatings. To distinguish between intrinsic (i) and extrinsic (ii) contributions to the line width Γ, we calculate the scattering spectra according to Mie theory extended to spherical multilayered particles.3,26 A single nanoshell is modeled by a dielectric sphere with radius Rc surrounded by a concentric gold shell with thickness dS (inset of Figure 2). The dielectric constants of the core and host material are real and assumed to be Core ) 5.42 and Host ) 2.3 (oil). We use a fit to the data of Johnson and Christy25 for the bulk dielectric function Au(ω) of gold. For nanoshells Au(ω) depends on the shell thickness.2,19 Empirically this can be described using the “limited mean free path model” following Kreibig et al.3 In this model it is assumed that a substantial amount of the collectively excited electrons in the NPP resonance scatter at the surface of the Au2S/Au nanoparticles if the gold shell is thinner than the Drude mean free path l∞ ) 42 nm3,27 of the conduction electrons in bulk gold. A surface-induced contribution ΓS to the damping is given by ΓS ) A × υF/l, where υF ) 1.4 × 106 m/s is the Fermi velocity of conduction electrons in gold and l is a characteristic dimension depending on the geometry of the nanoparticle. The empirical parameter A describes the loss of coherence by scattering events. In case of a thin film, the 1/l dependence still holds if l is the film thickness.28-30 It has been predicted that for a thin film the A parameter AFilm is close to the A parameter of a nanosphere ASphere.28-30 Because the gold shell of the Au2S/Au nanoparticles is, from the viewpoint of an electron, more comparable to a thin gold film than to a solid gold nanosphere, we expect an A parameter similar to AFilm and l is assumed to be equal to the shell thickness dS. Incorporating ΓS into the bulk dielectric function of gold leads to2,3
Au(l,ω) ) Au(ω) +
ωp2 ω2 + iωΓ∞
-
ωp2 ω2 + iω(Γ∞ + ΓS)
(1)
where ωp ) 1.37 × 1016 s-1 is the plasma frequency and Γ∞ ) υF/l∞ is the bulk collision frequency of conduction electrons in gold. To address both possible origins for the experimentally observed inhomogeneous distribution of resonance positions, namely variations of the size and the core/shell ratio, Figure 2 shows two different sets of calculations. Each set consists of three lines calculated for different values of A. The solid lines are calculated for a fixed core size of Rc ) 15 nm and an increasing gold shell according to the growth model presented by Averitt et al.2 For the second set of calculations (dashed lines) a fixed total diameter of Rtot ) Rc + dS ) 40 nm with varying core/shell ratio Rc/dS has been assumed. The best agreement between experiment and both theoretical calculations is found for A ) 0.5. This is much less than the previously reported values of A ) 12,19 and A ) 2-331 found by measurements carried out on ensembles of nanoshells. In those studies, inhomogeneous broadening is accounted for by determining the statistical size distribution 1855
Figure 3. (a) Scattering spectra of a single gold nanoshell in air (blue curve) and in oil with a refractive index of 1.52 (red curve). The change in refractive index results in a 173 meV red shift of the resonance peak. (b) Absolute plasmon peak shift plotted against the refractive index of the medium. Open and full circles represent averaged plasmon shifts from different single nanoshells and spheres, respectively. The lines are linear regressions to the data.
of the total diameter of the nanoshells with TEM measurements. However, the neglected inhomogeneous distributions of the core/shell ratio may have led to an overestimation of the surface scattering. Furthermore, large A parameters may be caused by rough and incomplete gold shells where electrons may additionally scatter at domain boundaries.31 Roughness and incompleteness of gold shells may also lead to a lift of the energetic degeneracy of orthogonal plasmon polarizations. This gives rise to an additional contribution to an inhomogeneous broadening of the line width exceeding the inhomogeneity estimated from TEM measurements alone. We will now turn to the NPP resonance shift induced by changes of the refractive index of the nanoparticles’ surroundings. Figure 3a shows the scattering spectra of the same nanoshell in the two different host materials air and oil (n ) 1.52). The plasmon resonance shifts 173 meV to the red due to the increase of the refractive index. For a 40 nm sized gold nanosphere, the plasmon peak shifts only by 87 meV (data not shown). To compare the sensitivity to a change in refractive index for Au2S/Au nanoshells and Au nanospheres, we cover the nanoparticles with purified water and increase the refractive index stepwise from 1.33 to 1.44 by adding sucrose. Figure 3b shows the red shift of the plasmon energy plotted against the refractive index change of the embedding medium. The open and full circles are measurements from individual nanoshells and individual nanospheres, respectively. For each data point the shifts detected for several single nanoparticles are averaged. The straight lines are linear regressions to the data points. Their gradients give a sensitivity of 3.3 meV per 0.01 change in refractive index for the nanoshells and of 2.0 meV/0.01 ∆n for nanospheres of 40 nm in diameter. This is well in agreement with Mie theory calculations, which predict an NPP resonance shift of 2.7 meV for a spherical gold nanoparticle with 40 nm in diameter surrounded by a homogeneous dielectric medium. Corresponding calculations for gold nanoshells show an increased sensitivity to the refractive index of the environ1856
Figure 4. Absolute NPP resonance shift versus incubation time of a single Au2S/Au nanoshell in aqueous solution. Upon addition of 16-mercaptohexadecanoic acid at t ) 100 s the NPP starts to red shift until it reaches a saturation level of 19 meV. The inset shows two plasmon resonance spectra taken before (blue curve) and after (red curve) the binding of 16-mercaptohexadecanoic acid to the surface of the Au2S/Au nanoshell.
ment and lead to a resonance shift of 4.3 meV per 0.01 change in refractive index. Both values are higher than experimentally found, which can be easily attributed to the influences of the glass substrate (n ) 1.5) unaccounted for in the calculations.8 Overall, our measurements show yet another advantage of nanoshells compared to nanospheres, namely an increased sensitivity to changes of the surrounding refractive index. Apart form applications in molecular sensing, this may also be relevant for optoelectronic switching devices containing noble metal nanoparticles.32 We now demonstrate that single gold nanoshells can indeed be used as a molecular sensor. Figure 4 monitors the absolute value of the NPP resonance shift of a single Au nanoshell put on a microscope slide, covered with water, and imaged in a dark-field microscope. At t ) 100 s, 50 µL of 16-mercaptohexadecanoic acid (MW: 288.5 D) is added to a final concentration of 2.6 × 10-6 mol/L in the solution. Directly after the addition, the 16-mercaptohexadecanoic acid starts to bind to the gold surface of the nanoshell. In consequence the refractive index close to the surface is changed. Therefore the NPP resonance starts to shift and reaches a constant total red shift of 19 meV after ∼800 s. In conclusion, we have investigated the scattering properties of single Au2S/Au nanoshells with respect to their applicability as single nanoparticle molecular sensors. Compared to solid gold nanospheres, Au2S/Au nanoshells have the advantage of a narrow NPP resonance in the biological window of the optical spectrum and they show a larger plasmon shift for the same amount of change in refractive index of the surrounding nanoenvironment. These features are important improvements for single gold nanoparticle molecular sensors. We have shown that roughly half of the homogeneous line width of 180 meV, which is unexpectedly small, is due to surface scattering of electrons. A surface Nano Lett., Vol. 4, No. 10, 2004
scattering parameter A of only 0.5 also shows that gold nanoshells may improve SERS and nonlinear optical effects to a larger extent than expected previously.19 Acknowledgment. We thank A. Helfrich and W. Stadler for excellent technical assistance. Financial support by the Bayerische Forschungsstiftung through the funding program ForNano and by the DFG through the Gottfried-WilhelmLeibniz Award is gratefully acknowledged. References (1) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. ReV. B 1994, 50, 12052. (2) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (4) Sun, Y.; Xia, Y. Anal. Chem. 2002, 74, 5297. (5) Jackson, J. B.; Westcott, S. L.; Hirsch, L. R.; West, J. L.; Halas, N. J. Appl. Phys. Lett. 2003, 82, 257. (6) Hirsch, L. R.; Jackson, J. B.; Lee, A.; Halas, N. J.; West, J. L. Anal. Chem. 2003, 75, 2377. (7) Mulvaney, P. Langmuir 1996, 12, 788. (8) Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 485. (9) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (10) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (11) Englebienne, P. Analyst 1998, 123, 1599. (12) Englebienne, P.; Van Hoonacker, A.; Verhas, M. Analyst 2001, 126, 1645.
Nano Lett., Vol. 4, No. 10, 2004
(13) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (14) Haes, A. J.; Zou, S. L.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2004, 108, 109. (15) Riboh, J. C.; Haes, A. J.; McFarland, A. D.; Yonzon, C. R.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 1772. (16) Nath, N.; Chilkoti, A. Anal. Chem. 2002, 74, 504. (17) Raschke, G.; Kowarik, S.; Franzl, T.; So¨nnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kurzinger, K. Nano Lett. 2003, 3, 935. (18) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057. (19) Averitt, R. D.; Westcott, S. L.; Halas, N. J. J. Opt. Soc. Am. B-Opt. Phys. 1999, 16, 1824. (20) BBInternational; United Kingdom. (21) Yguerabide, J.; Yguerabide, E. E. Anal. Biochem. 1998, 262, 157. (22) So¨nnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Phys. ReV. Lett. 2002, 88, 077402. (23) Messinger, B. J.; von Raben, K. U.; Chang, R. K.; Barber, P. W. Phys. ReV. B 1981, 24, 649. (24) Heilweil, E. J.; Hochstrasser, R. M. J. Chem. Phys. 1985, 82, 4762. (25) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6 (12), 43704379. (26) Bohren, C.; Huffmann, D. Absorption and scattering of light by small particles; John Wiley & Sons: New York, 1983. (27) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Saunders College Publishing: Philadelphia, 1976. (28) Kreibig, U.; von Fragstein, C. Z. Phys. A 1969, 224, 307. (29) Ruppin, R.; Yatom, H. Phys. Status Solidi B 1976, 74, 647. (30) Barma, M.; Subrahmanyam, V. J. Phys.-Condens. Matter 1989, 1, 7681. (31) Westcott, S. L.; Jackson, J. B.; Radloff, C.; Halas, N. J. Phys. ReV. B 2002, 66. (32) Mu¨ller, J.; So¨nnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T. A.; Feldmann, J. Appl. Phys. Lett. 2002, 81, 171.
NL049038Q
1857
