Assembly of Protein-Templated Gold Nanoclusters for Enhanced Fluorescence Emission and Multifunctional Applications

Abstract

Protein-templated gold nanoclusters (AuNCs) have attracted attention in fluorescence imaging due to their simple synthesis process and good biocompatibility. However, limitations still exist such as undesirable fluorescence intensity and poor colloid stability. In recent years, assembly-induced fluorescence enhancement via an aggregate approach was proved efficient on peptides- or other small molecules-templated AuNCs, while few were achieved on the protein-templated AuNCs. We hypothesized that the reported self-assembly strategy gave a loose aggregate of the protein-templated AuNCs, thus resulting in weak aurophilic interaction and no obvious enhancement in the fluorescence intensity. Our group has prepared a red emission AuNCs using keratin, a type of naturally occurring cysteine-rich protein, as the template (AuNCs@Keratin). Inspired by the unique characteristic of keratins, in this work, we develop a self-assembly strategy via a simple and mild approach, including silver ions modification and gadolinium ions-induced aggregation of AuNCs@Keratin. It should be mentioned that the silver modification of AuNCs@Keratin facilitated the formation of a dense aggregate after Gd ions-induced assembly, thus generating an enhanced fluorescence intensity. Such a mechanism was confirmed by the fluorescence correlation spectroscopy analysis. We demonstrated that the assembly of AuNCs@Keratin obtained an enhanced fluorescence intensity (6.5 times that of AuNCs@Keratin), excellent colloid stability for more than 4 months, and good biocompatibility. Moreover, such nanoparticles hold promise in multifunctional applications including near-infrared (NIR) fluorescence imaging, magnetic resonance (MR) imaging, and redox-responsive drug delivery, extending the applicability of fluorescent gold nanoclusters, especially in biomedical fields.