Imaging C. elegans with thiolated tryptophan-based NIR fluorescent gold nanoclusters

Multidentate, thiolated, tryptophan-containing peptide conjugates were synthesized for the preparation of gold nanoclusters (AuNCs). Precursor Au₁₁(PPh₃)₈Cl₃ was prepared by the reduction of HAuCl₄, followed by the use of tryptophan-containing peptide conjugates in ligand displacement reactions, to afford near-infrared fluorescent AuNCs. The emission maxima for these newly synthesized AuNCs were ~715 nm. AuNCs were characterized with the help of UV–Vis, FTIR, fluorescence and MALDI analysis. FTIR spectra showed that the ligands bind to Au atoms through Au–S bonds, while MALDI mass spectra revealed that the clusters consisted of 20–23 Au atoms. Introduction of hydrophilic –COOH groups engendered water solubility to these AuNCs, enabling bioimaging applications. We demonstrate fluorescence imaging of the nematode C. elegans and confirm distribution of these AuNCs in nematode gut with the help of green fluorescent protein co-localization experiments.