Synthesis and characterization of human transferrin-stabilized gold nanoclusters

Human transferrin has been biolabelled with gold nanoclusters (Au NCs) using a simple, fast and non-toxic method. These nanocrystals (<2 nm) are stabilized in the protein via sulfur groups and have a high fluorescence emission in the near infrared region (QY=4.3%; λem=695 nm). Structural investigation and photophysical measurements show a high population of clusters formed of 22-33 gold atoms covalently bound to the transferrin. In solutions with pH ranging from 5 to 10 and in buffer solutions (PBS, HEPES), those biolabelled proteins exhibit a good stability. No significant quenching effect of the fluorescent transferrin has been detected after iron loading of iron-free transferrin (apoTf) and in the presence of a specific polyclonal antibody. Additionally, antibody-induced agglomeration demonstrates no alteration in the protein activity and the receptor target ability. MTT and Vialight® Plus tests show no cytotoxicity of these labelled proteins in cells (1 µg ml(-1)-1 mg ml(-1)). Cell line experiments (A549) indicate also an uptake of the iron loaded fluorescent proteins inside cells. These remarkable data highlight the potential of a new type of non-toxic fluorescent transferrin for imaging and targeting.