NSET - Nano-Surface Energy Transfer

Nanoparticles represent a new frontier in technology, blurring the lines and defining a sort-of conjugated system between physics, chemistry, biology and materials. A materials scientist may be interested in the ability to construct a metal or semiconductor with certain geometries for the pure challenge of the work, while at the same time, a physicist or spectroscopist may be very interested in studying that same nanomaterial to understand how size and geometry have influenced the behavior of its electrons. By taking advantage of well-characterized biological systems (eg. proteo-nucleic interactions) we are able to probe the electrons of small gold nanoparticles by observing the behavior of strategically placed nearby fluorophores at specific distances from the nanoparticle surface. Although a nanomaterial is very different from organic molecules or inorganic coordination compounds, there are certain expectations we may still hold as to the interaction a nanomaterial may have with such a system. If a metallic nanoparticle such as 1.4nm gold maintains band structure similar to the bulk metal, there would still be a population of free electrons available in the conduction band, each electron moving through the lattice with the normal scattering phenomena, (ie. electron-electron scattering or surface potential scattering). Because a 1.4nm particle is below the normal mean free-path of an electron in gold at normal temperatures, the only scattering event an electron will feel is that of the surface potential. In other words, we expect that the electrons spend all of their time at the surface of the particle. (See Figure 7.) If we place a common fluorophore such as fluorescein nearby such a particle, then we see that the fluorescence quantum efficiency of the dye begins to decrease with a 1/R^4 distance dependence (see Figure 8.) The basis of Nano-Surface Energy Transfer (NSET) comes from the damping of the fluorophore's oscillating dipole by the gold metal's free electrons as a through-space mechanism. Because the gold particle's electrons are homogenously oriented, the constraint on dipole-dipole coupling has been greatly relaxed and thus gives rise to energy transfer efficiency at much larger distances. The experiments and math describing the interactions of fluorophores above metal surfaces have already been described by many experimenters and mathematicians, where we are only taking advantage of the groundbreaking work others have laid down for us, (see JACS 2005.)

Interested in learning more?
Contact Travis Jennings

or read the published articles:

"Nanometal Surface Energy Transfer in Optical Rulers, Breaking the FRET Barrier" C.S. Yun, A. Javier, T. Jennings, M. Fisher, S. Hira, S. Peterson, B. Hopkins, N.O. Reich, and G.F. Strouse, J. Am. Chem. Soc.127(9), 3115-3119 (2005). [view article-PDF]

"Assembly of Nanomaterials Using Bio-Scaffolding." Yun, C.S.; Major, J.L.; Strouse, G.F. Mat. Res. Soc. Symp. Proc., 642, J2.3 (2001). [ view article - PDF ]