Underlying Mechanisms for the Modulation of Self-Assembly and the Intrinsic Fluorescent Properties of Amino Acid-Functionalized Gold Nanoparticles.

The origin of the blue fluorescence of proteins and peptides in the visible region has been a subject of intense debate despite several efforts. Although aromatic amino acids, namely tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe) are responsible for the intrinsic luminescence of proteins and peptides, the underlying mechanism and contributions of these amino acids to the unusual blue fluorescence are still not well resolved. In the present endeavor, we show that the clusterization of both aromatic and aliphatic amino acids on the surface of the gold nanoparticles (Au NPs) leads to clusteroluminescence, which could be linked to the unusual fluorescence properties of the proteins and peptides and have been ignored in the past. The amino acid monomers initially form small aggregates through clusterization, which provides the fundamental building blocks to establish the amyloid structure as well as the luminescence property. Because of the clusterization, these Au NPs/nano-aggregate systems are also found to exhibit remarkable stability against the freeze-thaw cycle and several other external stimuli, which can be useful for biological and biomedical applications.