Abstract
In recent years, fluorescent metal nanoclusters have been used to develop bioimaging and sensing technology. Notably, protein-templated fluorescent gold nanoclusters (AuNCs) are attracting interest due to their excellent fluorescence properties and biocompatibility. Herein, we used an exosome template to synthesize AuNCs in an eco-friendly manner that required neither harsh conditions nor toxic chemicals. Specifically, we used a neutral (pH 7) and alkaline (pH 11.5) pH to synthesize two different exosome-based AuNCs (exo-AuNCs) with independent blue and red emission. Using field-emission scanning electron microscopy, energy dispersive X-ray microanalysis, nanoparticle tracking analysis, and X-ray photoelectron spectroscopy, we demonstrated that AuNCs were successfully formed in the exosomes. Red-emitting exo-AuNCs were found to have a larger Stokes shift and a stronger fluorescence intensity than the blue-emitting exo-AuNCs. Both exo-AuNCs were compatible with MCF-7 (human breast cancer), HeLa (human cervical cancer), and HT29 (human colon cancer) cells, although blue-emitting exo-AuNCs were cytotoxic at high concentrations (≥5 mg/mL). Red-emitting exo-AuNCs successfully stained the nucleus and were compatible with membrane-staining dyes. This is the first study to use exosomes to synthesize fluorescent nanomaterials for cellular imaging applications. As exosomes are naturally produced via secretion from almost all types of cell, the proposed method could serve as a strategy for low-cost production of versatile nanomaterials.
Highlights
Fluorescent metal nanoclusters (NCs), composed of several to hundreds of atoms with a sub-nanometer size, are formed by using biomolecules as a template [1]
As the tyrosine residue exhibits a strong reducing ability at pH values higher than its pKa and as the thiol group of the cysteine residue binds tightly to the Au ions [29,30,31], we expected that exosomes containing various proteins with tyrosine and cysteine residues would mediate the formation of fluorescent AuNCs
Blue- and red-emitting exo-AuNCs with high fluorescence signals were generated only when both exosomes and Au ions were present (Figure S1), and their fluorescence emission spectra matched well with the one of BSAAuNCs (Figure S2), indicating that the protein in exosome is the major component for the synthesis of exo-AuNCs
Summary
Fluorescent metal nanoclusters (NCs), composed of several to hundreds of atoms with a sub-nanometer size, are formed by using biomolecules as a template [1]. Because biomolecule-templated metal NCs exhibit excellent fluorescence properties and require lesstoxic conditions for their synthesis than organic fluorophores, they have been widely used for various purposes, such as bioimaging, sensing, and drug delivery [2,3]. Representative examples of these types of nanomaterial are gold, silver, copper, and platinum NCs that are synthesized via their interactions with proteins or nucleic acids [4]. This study on the use of exosomes as fluorescence imaging probes may broaden the potential utility of exosome-based NCs as tools for applications in diagnosis, therapy, and drug delivery
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