Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 347

glutathione (-SG-D) when the Au 25 SG 18 was stirred with dansyl glutathione.
Both the methods led to the formation of Au 25 protected with a mixture of
glutathione and N -dansyl glutathione. The donor-acceptor separation observed
in the two synthetic routes reflects the asymmetry in the ligand binding on the
cluster core, which is in agreement with the recent theoretical and experimental
results on the structure of Au 25 .
7.4 Two-Photon Emission
Two-photon emission is a process in which electronic transition between quantum
levels occurs through the simultaneous emission of two photons. [ 27 ] presents more
details about two-photon emission. Since metal clusters emit in the near-infrared
region, they can be made useful for two-photon imaging with infrared excitation.
Two-photon emission of Au 25 clusters is observed at 830 nm by exciting at
1,290 nm (Fig. 8 ).[ 23 ] Two-photon absorption (TPA) cross-section of Au 25 at
1,290 nm in hexane was determined to be 2,700 GM, which is superior to the
TPA cross-sections of many organic chromophores with emission in the near-
infrared region. In addition to the near-infrared luminescence for Au 25 clusters,
additional luminescence in the visible region with a maximum around 510 nm was
observed, which is also two-photon allowed (excited at 800 nm). Present measure-
ments on Au 25 gold clusters suggest that they can be used as multiphoton imaging
agents with near-infrared luminescence.
Fig. 8 Optical absorption of Au 25 and two-photon emission spectrum after excitation at 1,290 nm
for Au 25 clusters [ 23 ]