NANO LETTERS
Composite Plasmon Resonant Nanowires
2002 Vol. 2, No. 5 465-469
J. J. Mock,† S. J. Oldenburg,‡ D. R. Smith,† D. A. Schultz,† and S. Schultz*,† Department of Physics, UniVersity of California, San Diego, La Jolla, California 92093-0319, and Seashell Technology, La Jolla, California 92037 Received February 11, 2002; Revised Manuscript Received March 26, 2002
ABSTRACT We present an experimental study of the polarization-dependent scattering of light from homogeneous and multisegment silver, gold, and nickel nanowires. The metallic nanowires are prepared within a polycarbonate membrane template by a combination of electroplating (gold and nickel) and electroless (silver) growth processes. The size range of the nanowire segments is such that surface plasmon resonances are supported, dominating the optical spectra. We characterize the light scattering properties of individual composite nanowires using an optical microscope configured for single particle spectroscopy. Because of the scattering efficiency associated with the plasmon resonance, very narrow (∼30 nm diameter) nanowires can be readily observed under white-light illumination, with the spectral characteristics of each subsection easily distinguishable. Because of their compactness, these simply prepared multiisegment plasmon resonant nanowires are capable of hosting a large number of segment sequences over a comparatively small spatial range, suggesting their possible application as unique nanolabels in biological assays.
There has been recent interest in the production of microscopic and nanoscopic labels that have a distinct optical signature, or code.1-3 Populations of coded nanoparticles that are functionalized with various nucleic acid sequences, antibodies, drugs, or antigens can be used to multiplex many different assays in a single reaction vessel. The read-out of such assays requires the identification of the nanoparticle and the presence or the identification of any molecules that have specifically bound to the nanoparticle surface. Recently, segmented metallic rodsstermed “barcodes”shave been demonstrated as uniquely identifiable coded substrates in standard biological assays.3 The metallic barcode introduced in ref 3 is an example of a fabricated composite nanostructure, the optical properties of which are determined by the wavelength-dependent bulk reflectance of the metals used in each segment. The typical overall dimension of one of these barcodes is ∼200 nm in diameter by ∼10 µm long. Since metals such as silver and gold possess large scattering cross-sections at optical wavelengths, the metallic barcode is easily observable under a microscope with unpolarized white-light illumination. However, the scattering efficiency of composite metallic structures can be significantly enhanced by reducing the dimensions of the segments such that the excitation of surface plasmon resonances occurs. Nanoscale (50-80 nm) plasmon resonant particles are highly efficient elastic scatterers, readily observable individually in a standard light microscope equipped * Corresponding author. E-mail:
[email protected]. † University of California. ‡ Seashell Technology. 10.1021/nl0255247 CCC: $22.00 Published on Web 04/20/2002
© 2002 American Chemical Society
with dark-field illumination, and have previously been utilized as labels in single particle labeling biological assays.4 The particles used in these assays were roughly spherical; however, other geometries of nanosized metal particles, such as cylindrical,5 ellipsoidal, tetrahedral, and hexagonal,6-8 can also give rise to plasmon resonances and may find application as elements in a variety of optical devices. In particular, we show how combinations of cylindrically shaped plasmon resonators can yield composite structures that retain the large scattering efficiencies of the components, yet have unique spectral signatures distinct from the individual components. Here we study the scattering properties of homogeneous and multisegment nanowires of silver, gold, and nickel that have diameters of ∼30 nm and are up to ∼7 µm in length, synthesized by controlled growth in polycarbonate membrane templates.5,9 The filter pore size determines the diameter of the wire, while the length can be controlled by adjusting the quantity of deposited metal within the pore.9,10 The membrane can subsequently be selectively dissolved, releasing the wires into solution. The multisegment nanowires are analogous to the larger sized metal barcodes previously reported;3 however, the scattering properties are dominated by the polarization dependent plasmon resonance of the silver and gold subsections rather than by the bulk reflectance of the metal. The nanowires were fabricated using a combination of electroless and electrochemical deposition techniques in a 7 µm thick polycarbonate membrane (Costar, Nuclepore) with a 15 nm stated pore size. We chose to use an electroless plating (enhancement) technique for the silver segments
Figure 1. Single particle spectra taken with a 40× UPlanApo oil immersion objective for five individually selected (A) silver, (B) gold, and (C) nickel nanowires immobilized on a glass slide and illuminated with white light polarized along the short axis of each nanowire. The corresponding digital camera color images (Nikon Coolpix 950, 1/30 s exposure) of individually selected single silver, gold, and nickel nanowires were taken using a 100× oil immersion objective. Next to each optical microscope image is a TEM image of a ∼1 µm segment from a typical silver, gold and nickel nanowire, respectively.
because the silver plating solution tended to dissolve the polycarbonate membrane. One side of the filter was sputtered with a gold layer (15 min at 15 mTorr and 15 mA, Technics Hummer V). The filters were mounted gold side down on the aluminum base of a custom Teflon electrochemical cell containing a platinum counter electrode. Gold and nickel nanowires were produced by electroplating (RTU solutions, 434 gold and high-speed nickel sulfamate, Technic Inc.) at a constant current density of 0.4 mA, for 4-10 min. Silver nanowires were formed by electroless growth using a 50:50 mixture of silver initiator and enhancer (BBI International) for 15-90 min depending on the average length desired. The membranes were sonicated for 30 s in ethanol to remove the gold layer on the bottom side of the membrane, rinsed in ethanol, and then dissolved in chloroform. A drop of this solution was placed on a glass slide and air-dried, followed by successive rinses in chloroform and ethanol. The optical investigation setup is configured specifically for nanocylinder characterization. As we are using brightfield illumination, the cylinders must be embedded in a media with matched indices of refraction to reduce background light scattering. Therefore the slide was placed on a fused silica prism with index matching oil (n ) 1.48). The sample was illuminated with polarized light using either a Nikon 1.25 NA PLAN 100× oil immersion objective for the color images, or an 466
Olympus 1.00 NA UPlanApo 40× objective chosen to reduce chromatic aberration when taking spectra from individual rods. The focused polarized beam illuminates the particles in a transmission mode. The backscattered light from the particles can be collected with the same oil immersion lens and directed either into the color digital camera (Nikon Cool Pix 950) or into the SPEX 270M grating spectrometer system with Princeton Instruments CCD detector. Single particles were selected for investigation using an adjustable image plane aperture. Using this aperture, much of the background scattered light was deflected away from the detection system. By using the index matched interfaces arrangement, the illumination system provides very high contrast, as only light backscattered by the nanowire is collected by the optical detection system, while surface defects and other imperfections are invisible due to index matching. The single particle microscope, spectrometer, and detection system have been described in previous publications.5,6 When silver or gold nanowires are illuminated with light of appropriate wavelength that is polarized along the short axis, the plasmon resonance of the nanowires is excited, resulting in an optical scattering spectrum with a distinct color. When this polarization is used, the diameter of the nanowire determines the plasmon resonance and we expect the color to be independent of rod length for the long aspect Nano Lett., Vol. 2, No. 5, 2002
Figure 2. Calculated spectra for an individual 30 nm gold, silver, or nickel nanowire when illuminated with light polarized along the short axis. The measured spectra are the normalized average of the five individual nanowire spectra shown in Figure 1. The nanowire spectra were calculated using the solution for an infinite cylinder and were independently scaled to fit the measured data. The dielectric constant of the medium is taken as 1.48.
ratios used. Figure 1 shows the short axis spectra of five homogeneous silver, gold, and nickel nanowires. The average peak of the gold spectra occurs at ∼525 nm, whereas the nickel nanowire spectra are nearly flat across the visible wavelengths. The average peak of the silver nanowire spectra, previously determined to be ∼380 nm,5 was below the spectral limit of our microscope for the measurements presented here but for those particles on the long wavelength region of the distribution, their scattering intensity (normalized to the white light source) is ∼15 or ∼30 times greater than that of the gold and the nickel nanowires, respectively. Silver nanowires appear violet/blue, gold nanowires appear green, and nickel nanowires appear gray in color when illuminated with white light polarized parallel to the short axis (Figure 1). The corresponding transmission electron microscope (TEM, JEOL 2000) images, representing a portion of the full nanowire length, permit measurement of the actual diameter for the silver, gold, and nickel nanowires (30 ( 8 nm). The variation in the scattering spectra of the nanowires can be attributed to the spread of possible nanowire diameters achievable from a single polycarbonate membrane. We also found that there was typically some variation in the nanowire diameter along the rod length ((4 nm), which we attribute to the nonuniformity of the membrane pores. These radial nonuniformities may be the cause of the variation in scattering intensity along the nanowires (such as is observed in Figure 1). We note that some nanoparticles appear uniformly bright along the rod length, including their ends. The aspect ratio of the nanowires in the optical images appears smaller than the actual aspect ratio since the optically imaged width of the nanowires is Nano Lett., Vol. 2, No. 5, 2002
Figure 3. Digital camera color images (1/30 s exposure) of selected coded nanowires, illuminated with white light polarized along the short axis of each nanowire, using the 100× oil immersion objective: (A) silver/gold; (B) gold/silver/gold; (C) gold/nickel/ gold/nickel; (D) silver/gold/nickel. (E) is for a silver/gold/nickel nanowire with light polarized along the long axis. (F) is the same silver/gold/nickel particle as in (E) with white light polarized along short axis.
diffraction-limited (NA ) 1.25) to approximately Λ/2NA or 400-600 nm/(2 × 1.25) ) 160-240 nm of the peak scattered light, which is ∼10 times larger than the physical diameter of the nanowires. The aspect ratio of the nanowires is sufficiently large that the plasmon resonances associated with the long and short axes are entirely decoupled. In this limit, the spectrum of a nanowire should correspond closely to that of an infinite cylinder excited along the short axis. In Figure 2, we show the results from Mie calculations (dashed lines) of the scattered power from infinitely long 30 nm diameter silver, gold, or nickel cylinders,11 assumed to be immersed in index matching oil (n ) 1.48). The calculated results are in close agreement with the averaged measured spectra (solid lines). The relative scattering cross-section for the different wires varies significantly as a function of wavelength, such that at certain wavelengths very large scattering ratios between different metals can be observed. For instance, a 30 nm diameter silver cylinder is predicted to be more than 30 times as bright as a similar sized gold cylinder when illuminated with 366 nm light, and a gold cylinder is ∼3 times brighter than a silver cylinder when illuminated at 522 nm. Figure 3 shows composite plasmon resonant nanowires having silver, gold, and or nickel segments. Nanowires with four different patterns, or codes, are shown in microscope 467
Figure 4. Illustration of the use of narrow band-pass filtered light illumination to improve contrast and characterize a short silver segment embedded in a gold nanowire. Illumination polarized along the short axis of the nanowire, 100× oil immersion objective. (A) Nanowire #1, 1/30 s exposure digital camera color image of gold/silver/gold nanowire illuminated with white light. (B) Nanowire #1, black and white CCD (Princeton Instruments) image, illuminated with 410 nm centered 10 nm band-pass filtered light. (C) Nanowire #2, same conditions as in (A). (D) Nanowire #2 same conditions as in (B).
images in Figure 3A-D. We found that the spectral characteristics of a segment of a composite nanowire retained the plasmon resonance property exhibited in the homogeneous nanowire (compare with Figure 1). As can be seen from Figure 3, each segment of the composite nanowire has a specific spectral signature (length and intensity) that readily allows for optical identification. Note that the plasmon resonance properties are polarization dependent: polarizing the illumination along the long axis of the rod results in a loss of plasmon resonant information (Figure 3E) as compared to polarization along the short axis (Figure 3F), as demonstrated for a composite nanowire constructed of nickel/ gold/silver. By illuminating with narrow band-pass filters matched to the plasmon resonance peak of one of the metals in a composite nanowire, we can improve the spatial imaging resolution of individual segments. The improvement achievable by this process is demonstrated in Figure 4. A gold/ silver/gold nanowire is illuminated with polarized white light and imaged with the digital color camera in Figure 4A,C. In Figure 4B,D, 410 nm centered narrow band-pass filter illumination of the same rods provides a much higher contrast representation of the embedded silver segment due to its greater scattering efficiency at that wavelength. The same 468
filtering technique can be used with a 530 nm narrow bandpass filter to emphasize the gold scattering relative to the silver scattering. The nickel nanowire, being a weak scatterer at the gold and silver plasmon resonant wavelengths, can be used to distinguish the nickel segment when using filtered light illumination. The large contrast between the silver and gold segments is evident in the line plots of Figure 4, taken through the silver segments using a 10 nm band-pass filter centered at 410 nm. The shorter silver segment is ∼2.5 times brighter and the longer segment is ∼6 times brighter than the rest of the gold rod. The reduced intensity of the shorter silver segment is directly related to the physical segment length. Using this relationship may allow us to determine segment lengths that are shorter than the diffraction limit. Note that, in theory, with a spatial density of segments that permits diffraction-limited imaging, we could expect to fit five three-color equal-sized segments in a 1 µm wire length. As a barcode label, such a nanowire would be able to host approximately 35/2 ) 121 unique patterns and be relatively stable in solution. In practice, we were unable to control the segment lengths and uniformity of batches to make such codes. We also found that there were a large number of broken or bent nanowires in our samples (likely due to the high aspect ratios of the long (∼6 µm) nanowires used), Nano Lett., Vol. 2, No. 5, 2002
which would certainly confuse characterization of coded populations. While our current preparation setup failed to produce highly controllable segment lengths, improved fabrication procedures, or novel preparation methods,12 may allow testing this coding density limit soon and create truly nanoscale plasmon resonant codes. In summary, we have shown that plasmon resonant metallic segments can be combined to form composite particles that have unique optical characteristics. Because the plasmon resonance occurs for nanosized metallic particles, the resulting composite particles can also be nanoscale yet can host a large number of unique identifying patterns, possibly useful as labels in biological assays or as embedded identification tags. In the experiments presented here, the plasmon resonance characteristics associated with infinitely long cylinders were also observed for the individual segments of the composite nanowires, suggesting that the interface between metal segments plays a negligible role at these dimensions. As the length of the segments is decreased, however, the influence of the interface should increase, as should the electromagnetic interaction between segments. These effects should lead to particles with more complicated and interesting spectral features. Acknowledgment. We thank Professor Mark Ellisman and acknowledge the use of the UCSD National Center for Microscopy and Imaging Research. We also thank Dr. Mladen Barbic of the California Institute of Technology and Andy Pommer of the UCSD Physics Machine Shop for their valuable technical help. This work was supported by the National Science Foundation (Grant No. DBI-98-76651 and DMR-0100962), the Richard Lounsbury Foundation, and the
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