506 Nanosensors: Physical, Chemical, and Biological
1/R
4
distance dependence. Because a 1.4 nm gold particle is below the nor-
mal 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, it is expected that the electrons spend all of their time at the sur-
face of the particle. If a common uorophore such as uorescein (C
20
H
12
O
5
) is
placed near such a particle, then the uorescence quantum efciency of the
dye begins to decrease with a 1/R
4
distance dependence. The basis of NSET
arises from the damping of the uorophore’s oscillating dipole by the free
electrons of gold metal as a through-space mechanism. Because the electrons
of the