Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 344

further enhance their application potential. They also exhibit electroluminescence at
room temperature and hence provide facile routes to produce strong single-photon
emitters. The photoluminescence of quantum clusters arises due to their molecular-
like electronic structure. The emission originates from radiative intraband transitions
within the sp bands, across the HOMO-LUMO gap. As the size of the cluster
decreases, the spacing between the discrete states increases. This leads to a blue shift
in the emission of smaller clusters when compared with the larger analogues.

7.1 Photostability

Quantum clusters are highly photostable when compared with organic fluorophores.
A study was conducted to check the photostability of clusters in comparison to
organic fluorophores and semiconductor quantum dots [ 12 ]. Photostability of a gold
cluster capped with dihydrolipoic acid (AuNC@DHLA) was compared with poly-
mer coated CdSe/ZnS semiconductor quantum dots and two different organic
fluorophores namely fluorescein and rhodamine 6G (Fig. 5 ). For the study, 20 l

of fluorescent AuNC@DHLA was dissolved in sodium borate buffer of pH 9. The
sample was loaded into a quartz cuvette and was exposed to blue-light (480 nm)

m

Fig. 5 Photostability of fluorescent Au nanoclusters (AuNC@DHLA) compared with semi-
conductor quantum dots (polymer-coated QD 520 from Invitrogen) and organic fluorophores
(fluorescein, rhodamine 6G) [ 12 ]