Structure-Correlated Dual Fluorescent Bands in BSA-Protected Au25 Nanoclusters

In this work, we investigated the temperature-dependent luminescence of bovine serum albumin-protected Au-25 nanoclusters and the correlation with their structure. Our experiments reveal that the red luminescence consists of two bands, namely, band I at 710 nm and band II at 640 nm. The temperature dependence of band I exhibits similarity to semiconductors, such as a red shift of emission and bandwidth broadening upon increasing temperatures due to electron-phonon and electron-defect/surface scattering. In contrast, band II exhibits different temperature dependence. It is concluded that band I exclusively originates from the icosahedral core of 13 Au(0) atoms and band H dominantly arises from the [-S-Au(I)-S-Au(I)-S] staples. Moreover, with increasing temperatures, the intensity of band I and band II decreases. A similar activation energy was extracted, which is attributed to thermally activated defect/surface trapping. In addition, the relaxation from band II to band I was found to be inactive from 300 K down to 77 K.