Understanding the evolution of luminescent gold quantum clusters in protein templates

Abstract

We show that the time-dependent biomineralization of Au³⁺ by native lactoferrin (NLf) and bovine serum albumin (BSA) resulting in near-infrared (NIR) luminescent gold quantum clusters (QCs) occurs through a protein-bound Au¹⁺ intermediate and subsequent emergence of free protein. The evolution was probed by diverse tools, principally, using matrix-assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The importance of alkaline pH in the formation of clusters was probed. At neutral pH, a Au¹⁺-protein complex was formed (starting from Au³⁺) with the binding of 13-14 gold atoms per protein. When the pH was increased above 12, these bound gold ions were further reduced to Au(0) and nucleation and growth of cluster commenced, which was corroborated by the beginning of emission; at this point, the number of gold atoms per protein was ~25, suggesting the formation of Au₂₅. During the cluster evolution, at certain time intervals, for specific molar ratios of gold and protein, occurrence of free protein was noticed in the mass spectra, suggesting a mixture of products and gold ion redistribution. By providing gold ions at specific time of the reaction, monodispersed clusters with enhanced luminescence could be obtained, and the available quantity of free protein could be utilized efficiently. Monodispersed clusters would be useful in obtaining single crystals of protein-protected noble metal quantum clusters where homogeneity of the system is of primary concern.