|
|
Your Optics Solution
2582 Island View Ln.
Lummi Island, WA
email:
         |
Tuning the Optical Properties of Gold Nanoparticles by Shape and Orientation
INTRODUCTION We propose to explore the surface plasmon resonance of gold nanorods as a function of angle of incidence and polarization of the light field. We have a unique method to predict the total extinction due to scattering and absorption [including absorption due to the plasmon excitation] even in the presence of significant multiple scattering [large particle number density]. An extremely strong absorption exists for metallic nanoparticles. This absorption is associated with excitations of surface plasmon modes in the metal (i.e., surface plasmon resonance). Mathematically, the resonance corresponds to a pole [zero in the denominator] in the complex refractive index plane of the Mie solutions for electro-magnetic scattering and absorption. The precise location of the pole depends on the probe wavelength, the particle size, the particle shape, and the medium the particle is embedded in. It is therefore possible to tune the plasmon resonance by changing these parameters. We are interested in exploring the surface plasmon resonance properties of gold nanorods. Preliminary calculations [Mie solutions to first order in size parameter] for needle-shaped particles indicate that there is a plasmon resonance located at m2+1=0, where m is the complex refractive index, particle to medium. The plasmon resonance occurs only when the incident light is polarized parallel to the short axis, but not when the light is polarized parallel to the long axis of the cylinder. Ordinary scattering and absorption takes place regardless, but the plasmon resonance is present only in the first case. Since the resonance is very strong, the difference is easily detectable, even when there are a large number of particles present [large optical thickness] contributing to the ordinary scattering and absorption. METHOD We have previously developed a technique for measuring the relevant optical parameters of suspensions of nanoparticles, even in the presence of significant multiple scattering. [1-3] Specifically, we can experimentally determine the optical thickness [t, the scattering + absorption coefficient times the optical path length] and the Mie extinction parameter [Q the total scattering + absorption cross section divided by the geometric cross section]. The optical thickness can be expressed as a function of the extinction parameter. The extinction parameter can be calculated for a single particle from the Mie theory and t can be calculated in turn. We therefore have a means of predicting and measuring the relevant parameters for a wide range of t [ 0< t < 10]. This research was planned in collaboration with Dr. Steven Emory at Western Washington University. Dr. Emory has been successful in synthesizing colloidal gold nanorods and nanospheres using wet chemical based synthetic routes. In particular, the gold atom in hydrogen tetrachloroaurate(III) [HAuCl4] is reduced to gold metal using reducing agents such sodium borohydride and ascorbic acid. The aspect ratio of these gold particles is controlled using a "seeding growth" method described by Murphy and coworkers. [4] In this procedure, the concentration of preformed gold nanosphere seeds and HAuCl4 is varied in the presence of a "rodshaped" micellar template to produce gold nanorods of desired aspect ratio ranging from 1 to 7. The research is planned in the following steps: 1. Develop a scattering code for finite cylinders with arbitrary incidence angle and polarization states. Such codes are available, although we anticipate having to modify the code to average over all incidence angles for the case of randomly oriented cylinders. From the results of the code and our analysis for t, we will determine the optimum aspect ratios for the gold nanorods. 2. Synthesize gold nanorods of various aspect ratios. The size of the nanorods will be verified by transmission electron microscopy. 3. Techniques to align the nanorods must be developed. One approach is to apply an external field to the nanorod suspension. This will induce a torque on the nanorods causing them to align along the direction of the applied field. An alternative approach is to align the nanorods using liquid crystals as described by Patrick et al. [5] 4. Extinction data will be taken for the nanorods in each of the following configurations:
5. Determine the sensitivity of these measurements. SCIENTIFIC MERIT Results of this research will contribute to the fundamental understanding of optical resonances for very small particles. The ability to switch the resonance on and off by changing either the polarization of the laser or the orientation of the nanorods may be useful for some applications, though the switching time will not be fast. For these nanorods, the surface area to volume ratio becomes large and surface effects will dominate over the bulk properties of the material. The outcome of the proposed research will impact chemical and biological detection methodology. Some current biological detection devices use microparticles to condense, purify and detect biological species in water samples.[6] This detection scheme requires PCR processing and detection by fluorescence. Also, work by Mirkin et al has demonstrated the use of gold nanospheres for the sensitive detection of specific DNA sequences.[7] Alternatively, gold nanorods could be used for ultrasensitive optical detection. The surface of the rods can be chemically modified with a molecule that is the complement to the target molecule. This, by itself, will change the plasmon resonance but the extinction can still be carefully calibrated. A possible measurement might be the ratio of the extinction when the light is polarized parallel to the long axis versus the short axis. When the target molecule combines with its complement, the optical properties should change. If we choose a wavelength that is at peak resonance, the difference should be easily detectable. Use of aligned nanorods for detection would be smaller, faster, and possibly more sensitive. One could visualize gold nanowires [cylinder with and infinite long axis] for detection of biological particles in air or water samples.
There are a few commercially available surface plasmon resonance [SPR] biodetectors [e.g. BioCore, Texas Instruments] that are based on changes in the plasmon resonance of gold thin films as molecules attach to the gold surface. The SPR is sensitive to the angle of incidence. When the molecules attach to the gold surface, the angle of plasmon excitation changes. The angle can be accurately measured to 0.001 degree. The concentration corresponding to this change in angle will depend on the properties of the sensor surface and the molecule responsible for the change. These detectors have high sensitivity but require liquid samples. We believe the nanorods have potential for higher sensitivity with a wider range of sample types. REFERENCES:
Optimizing the IR Reflectivity of Paint by the Addition of Metallic Particles.
Note: For one reason or another, this project didn't take off, but the information may still be of interest. The idea of adding metallic particles to paint is not new. It is used extensively for the metallic look in car paint. It has also been developed [ChemRex] as a low-e paint for the inside of exterior walls to reduce losses of radiant heat. What we bring to this is the optimization of the particle size for maximum reflectivity. We have perfected a technique to select a particle size that optimizes the optical properties of scattering and absorption that is valid for high particle number density. [1-4]
Heat [infrared radiation] is transferred in three ways: convection, conduction, and radiation. Insulation is added to walls to reduce heat transfer by convection and conduction but it does little for transfer by radiation. It is well known that metallic particles [Ag, Al, Au, Cu, Rh] have reflectivities of 98-99% in the infrared [IR] portion of the spectrum. [5] Adding these particles to white primer will increase the reflection of radiant heat. Therefore painting this primer on the inside of exterior will reduce heat loss by radiation. Current paints manufactured by ChemRex are claimed to reflect 30% of the radiant incident heat. We believe we can double this value with our optimization technique.
The radiation of a room at 70oF will peak at a wavelength of 10 microns, according to the black-body equation. Using the refractive indices of the particles and the paint, and a wavelength of 10 um, we can calculate a scattering curve as a function of particle size. We then choose the size that maximizes the scattering [reflectivity]. The reflectivity increases linearly with particle number density [adding more particles] but not with particle size.
This research has wide application to increasing the energy efficiency of buildings or other structures.
N.L.Swanson and B.D. Billard, "Electively maximizing and minimizing the scattering and absorption of electromagnetic waves," U.S. Patent no. 6704105, March 9, 2004 see, for example, W.L. Wolfe and G.J. Zissis, Eds., The Infrared Handbook, Office of Naval Research, U.S. Govt. Printing Office, 1978, p. 7-81. |
|
Abacus Enterprises is a small, woman-owned business providing consulting services in the physics and engineering of optical systems. Please call 360 758-4255 or e-mail
for inquiries. |