Formation of Fluorescent Metal (Au, Ag) Nanoclusters Capped in Bovine Serum Albumin Followed by Fluorescence and Spectroscopy

Fluorescent noble metal (Au, Ag) nanoclusters have been biolabeled to bovine serum albumin (BSA) by wet chemistry. Spectroscopic and fluorescence investigations relate the role of the pH and the nature of the reducing agent to the size and the oxidation state of metal clusters. Blue-emitting (λ = 450 nm) small gold nanoclusters (eight atoms) prepared at pH 8 weakly bonded to BSA grow at higher pH to form red-emitting (λ = 690 nm) bigger clusters (25 atoms) covalently bonded to BSA via the sulfur group. X-ray photoelectron spectroscopy (XPS) measurements indicate the presence of Au(I) only for the big clusters. Small silver nanoclusters labeled to the protein with a fluorescence emission in the red region are synthesized in the presence of a strong reducing agent and present only Ag(0). Steady-state and lifetime measurements confirm the crucial impact of the size and the oxidation state of Au(I) on the stabilization of the metal core inside the protein and on the presence of a long lifetime component (τ > 170 ns).