Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 350

7.7 Quantum Clusters as Metal Ion Sensors
The inherent luminescence of the clusters can be used as a sensing-tool for metal
ions. The luminescence effect of glutathione capped Au 23 in the presence of various
metal ions was studied [ 15 ]. The ions selected were Au 3+ ,Ag + ,Cu 2+ ,Ni 2+ ,Ca 2+ ,
Mg 2+ ,Na + ,Pb 2+ ,Hg 2+ , and Cd 2+ as their nitrates or chlorides. Next, 50 ppm of the
aqueous solutions of Au 23 was treated with metal ions so that its final concentration
was 10 ppm, and the emission of the clusters was measured immediately after the
addition of the ions. Au 23 was found to be reactive toward Cu 2+ . The emission of
the cluster quenched significantly. Although cluster emission enhanced a bit in the
presence of Ag + ions, it remained unaltered in the presence of other metal ions.
Figure 11 is the plot of cluster-emission intensity against the metal ions added. Inset
of the figure is a photograph of the aqueous solutions of Au 23 under UV light after
the addition of various metal ions. The specific reactivity toward Cu 2+ can be used
as a detection tool to find the presence of Cu 2+ ions. Luminescence quenching in the
presence of Cu 2+ may be due to the aggregation of clusters by the addition of Cu 2+
ions. However, Au 23 did not show any particular reactivity toward anions.
7.8 Quantum Clusters for In Vitro Imaging
Since highly f luorescent water soluble gold clusters have been synthesized by various
routes, they can be used in biology-related experiments such as imaging. These
clusters may possess additional benefits over organic fluorophores and fluorescent
semiconductor quantum dots. Organic fluorophores are prone to photobleaching,
Fig. 11 Plot of the luminescence intensity of an aqueous solution of Au 23 against the ions added
externally to the solution. The concentration of Au 23 was the same for all the experiments. Clusters
showed specific reactivity toward Cu 2+ ions with significant quenching of luminescence. Photo-
graphs of the aqueous solution of Au 23 in the presence of externally added ions under UV light
irradiation are also given. The photograph was collected immediately after the addition of metal
ions [ 15 ]