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Research of Professor M. A. El-Sayed Group The following research projects are carried out in the Laser Dynamics Laboratory (LDL) directed by Professor El-Sayed. LDL houses the most recent lasers and laser spectroscopic equipment for time resolved studies (transient absorption, fluorescence, Raman, FTIR, and single photon correlation lifetime system) in the femto-to-millisecond time scale. The present research interests are: 1. NanoScience: Properties of Material Confined in Time and Space of Different Shape The type of electronic motion in matter determines its property and thus its uses in our everyday life. This motion itself is determined by the forces acting on the electrons and thus the space in which they are allowed to move. The difference between a metal, a semiconductor and an insulator lies in the fact that the electronic motion is highly delocalized, slightly confined, and highly confined, respectively. One thus expects that if we reduce the size of material to below its naturally allowed characteristic length scale new properties should be observed which are different from that of the macroscopic material as well as of their building blocks (atoms or molecules). This size is on the nanometer length scale. A. Ultrafast Electron-Hole Dynamics in Semiconductor Nanoparticles Charge separation is used in many important technological applications such as the conversion of solar energy, detoxification of pollutants, imaging, sensors and optoelectronics. Semiconductor nanoparticles offer an important potential tunable source for charge separation. Changing their size, their shape, and their composition greatly changes the dynamics of their electrons and holes as well as their reduction and oxidation potentials. Before we use them effectively in different applications, the ultrafast relaxation dynamics of their hot electrons and holes confined in different sizes and shapes need to be studied. The femtosecond lasers present in the Laser Dynamics Lab are well suited for these studies. B. Shape Controlled Synthesis, Stability and Self Assembly of Metallic Nanoparticles Different methods are developed to synthesize, reshape and study the self assembly characteristics of gold, silver and transition metal nanoparticles. High resolution TEM is used to follow the shape distribution and their dependence on the preparation condition and to study their thermal stability. From the results, the mechanism of shape controlled growth and self assembly are elucidated. C. Photothermal Stability of Metallic Nanoparticles Metallic nanoparticles of non-spherical shape have been found to undergo a sharp transformation into the more thermodynamically stable spherical shape induced by pulsed laser excitation. A study is directed to understand mechanism and relevant time scales involved in this photothermal shape transformation. By adjusting the power and wavelength of the pulsed laser, the size and shape distribution of non-spherical nanoparticles can be changed and the nanoparticles can be reshaped. D. Optical and Nonradiative Properties of Assembled Metallic Nanoparticles Three methods are used for the assembly of metallic nanoparticles: 1) From colloidal solution of highly mono-dispersed sample, 2) Nanosphere lithography, and 3) Electron beam Lithography. Metallic nanoparticles assembled in monolayer periodic arrays present opportunities to study both unique properties of individual nanoparticle and collective properties of coupled nanoparticles. The current fundamental research in the Laser Dynamics Laboratory on metallic nanoparticle arrays includes: 1) Ultrafast dynamics of coherent vibration induced by femtosecond laser; 2) Effects of electronic coupling between nanoparticles on the optical and electronic properties; 3) Medium effects on electron-phonon and phonon-phonon dynamics. 4) The laser photothermal heating of gold and silver nanoparticles can result in the heating or melting of the medium surrounding the nanoparticle. Alternately, the nanoparticles may themselves melt or atomic ablation may take place. We are studying the dependence of the nature of photothermal heating of gold and silver nanoparticles on the rate of heat deposition by varying the photothermal laser pulse energy and the pulse duration.
2. NanoTechnology: Potential Applications of Nanoparticles A: Nanomedicine The use of nanoparticles in nanomedicine is one of the important directions that nanotechnology is taking at this time. Gold nanoparticles have great potential for diagnostic and therapeutic applications due to their strongly enhanced surface plasmon scattering and absorption . Additionally, the enhanced absorption of the nanoparticles can be rapidly converted into heat which can be used in selective photothermal therapy if the nanoparticles are conjugated to antibodies (e.g. anti-EGFR) specifically targeted to malignant cells.
Our research interests include: • For diagnosis of surface (skin) type cancer, spherical gold or silver nanoparticles conjugated to antibodies specifically targeted to cancer cells have been used to detect single malignant cells by dark field microscopy and spectrophotometry (see Fig. 1). Photothermal therapy of surface (skin) cancers can be accomplished by use of the spherical gold or silver nanoparticles by exposure to low energy visible CW lasers. • For in vivo detection and therapy, gold nanorods having optical resonance in the near-infrared (NIR) region, where tissue transmissivity is maximum, is used (see Fig. 2).
Figure 1. Dark-field microscopy images (top) and micro-absorption spectra (bottom) of HaCaT noncancerous cells (left) , HOC cancerous cells (middle), and HSC cancerous cells (right) labeled with anti-EGFR-conjugated gold nanoparticles. The figure shows clear difference of the scattering images and the absorption maxima for the noncancerous cells versus the cancerous cells. The conjugated nanoparticles bind specifically with high concentrations to the surface of the cancerous cells.
Figure 2. Light-scattering images of anti-EGFR/Au gold nanorods after incubation with cells (top). Selective photothermal therapy of cancer cells labeled with anti-EGFR/Au nanorods (bottom). When exposed to a similar low energy of a near-infrared CW laser, the HSC (middle) and HOC malignant (right) cells within the circular laser spots are destroyed while the HaCat normal cells (left) are not affected. Ivan H. El-Sayed, Xiaohua Huang and Mostafa A. El-Sayed, Nano Letters, 2005, 5(5), 829-834; Ivan H. El-Sayed, Xiaohua Huang and Mostafa A. El-Sayed, Cancer Letters, 239(1), 129-135, (2006); Xiaohua Huang, Ivan El-Sayed, Wei Qian and Mostafa A. El-Sayed, Journal of the American Chemical Society, 128(6), 2115-2120, (2006). B: Nanocatalysis Nanoparticles are potentially attractive catalysts since they have a large surface-to-volume ratio and high surface energy compared to bulk catalytic materials. In addition, metal nanoparticles of different shapes have different crystallographic facets and different fraction of atoms located on their corners and edges, which makes it interesting to study the effect of nanoparticle shape on the catalytic activity of various reactions. It is also important to note that having very active surface atoms could make the nanoparticles unstable during the course of its catalytic function. In the case of the early stages of the electron transfer reaction, we found that tetrahedral platinum nanoparticles are the most catalytically active and have the greatest fraction of surface atoms on their corners and edges, while the cubic platinum nanoparticles are the least catalytically active and have the lowest fraction of surface atoms on their corners and edges. During the full course of the reaction (2 days), it is observed that distortions in the corners and edges of both the tetrahedral and cubic platinum nanoparticles take place. In addition, the rate of dissolution of corner and edge atoms is found to be faster for the tetrahedral nanoparticles.
Shape dependence of the catalytic rate on the fraction of atom on corners and edges observed at the early stage of catalytic electron transition reaction
Radha Narayanan and Mostafa A. El-Sayed, Nano Letters, 2004, 4(7), 1343-1348; Radha Narayanan and Mostafa A. El-Sayed, Journal of the
American Chemical Society, 126(23), 7194-7195 (2004).)
As the catalytic reaction continues in colloidal solution, shape changes take place eliminating sharp edges and corners (to eventually make spherical shape).
Radha Narayanan and Mostafa A. El-Sayed, Journal of Physical Chemistry B, 2004, 108(18), 5726-5733; Radha Narayanan and Mostafa A. El-Sayed, Journal of the American Chemical Society, 125(27), 8340-8347 (2005). C: Nanophotonics When photons interact with nanoscale materials (semiconductor and metal), many new physical phenomena will be observed which are not present in corresponding bulk materials. Nanophotonics provides opportunities for making ultra-small optoelectronic devices which have great performance in lasing, sensing, and communication. In our group, we have made two-dimensional metallic nanoprism arrays with well-defined size, shape, and interparticle separation using nanosphere lithography technique. These samples could be used in surface-enhanced spectroscopy, plasmonic devices, and sensors due to its tunable optical properties. We have observed modulation of the color of plasmonic nanoparticles with coherent lattice vibration due to the dependence of the surface plasmon frequency on lattice volume change with the modulation period proportional to the size of nanoparticle.
SEM images (Left) of the prismatic silver (a-c) and gold (d-f) nanoparticles monolayer arrays (in white color) made with nanosphere lithograph technique and the absorption spectra (Right) of the silver (a) and gold (b) periodic array samples made with the 0.26 (blue spectrum), 0.36 (green spectrum) and 0.45 μm (red spectrum) PS spheres, respectively.
Expected dependence of surface plasmon absorption spectra on volume during the lattice oscillation of a gold or silver nanoparticle.
Optically detected lattice phonon oscillations induced in the prismatic Ag (a,b,c) and Au (d,e,f) nanoparticles monolayer arrays with a 100 fs laser pulse at 400 nm and monitored near the absorption maximum of each nanoparticle (solid dots). The size (bisector) of the silver nanoparticles is around (a) 52.4, (b) 79.6 and (c) 99.3 nm. The size of the Au nanoparticles is around (d) 60.5, (e) 85.7 and (f) 103.7 nm. Wenyu Huang, Wei Qian and Mostafa A. El-Sayed , Nano Letters, 2004, 4 (9), 1741-1747; Wenyu Huang, Wei Qian and Mostafa A. El-Sayed , The Journal of Physical Chemistry B, 2005, 109 (40), 18881-18887.
D: Nanomotors
Moving nanosystems on the nanometer scale to carry out a useful function is one of the important nanotechnology efforts at the moment. Most biological functions involve movement on the nanoscale. There are different mechanisms for the movement of nanosystems: 1) catalytic chemical reactions which change the surface energy in the medium thus inducing motion. 2) electrical 3) magnetic 4) the melting of a nanoprism into sphere which leads to the flying away of the nanoparticle from its substrate. We are studying the technique in which heating the gold nanoparticle is so fast as not to allow the nanoprism to melt but to cause atomic ablation. On a substrate, the rapid ablation causes a rapid build-up of pressure under the nanoprism with “jet-like” speeds. We are now studying ways to control the flying pattern. We are also studying the effect of gold nanoparticle-substrate interfacial interaction in controlling the mechanics of motion. Other methods of making nanomotors are also being examined.
Further information is available from Dr. Mostafa A. El-Sayed, Director of the Laser
Dynamics Laboratory, Dr. Wei Qian, Assistant-Director of the Laser
Dynamics Laboratory, |
