Arginine-Modified Fluorescent Gold Nanoclusters for Förster Resonance Energy Transfer with a Hemicyanine Dye: A Biofriendly Approach

Ligand-protected gold nanoclusters (Au NCs), generally being less fluorescent, are occasionally used as energy donors in Förster resonance energy-transfer (FRET) processes. Although several recent reports have stated methods to enhance the fluorescence quantum yield (QY) of Au NCs, these have limited applications. Ultrasmall Au NCs are reportedly nontoxic to biological cells limited to the protecting ligands. Herein, we have used a testified protocol to substantially enhance the fluorescence QY of the ligand-protected water-soluble Au NCs by rigidifying the protecting ligands using arginine, a physiologically essential amino acid. This has enabled us to use the Au NCs as energy donors to do FRET with a hemicyanine dye belonging to a family that is prospectively used in neuronal and mitochondrial centers in biological cells. We could hardly find applications of FRET to transfer photon energy to hemicyanine dyes using Au NCs or any metal NCs in the literature. This biofriendly FRET-based approach, creating an intrinsic antenna effect, could be useful in controlling the fluorescence emission from nanomaterials and conversion to energy useful for cellular functioning.