Exciton Energy Transfer in Self-Assembled Quantum Dots on Bioengineered Bacterial Flagella Nanotubes

Abstract

The bacterial flagellin fusion protein, FliTrx, was modified by inserting a peptide containing 24 histidine residues and shown to successfully export and self-assemble to form flagella nanotubes. The flagella of this “His-loop” protein were harvested and employed as scaffolds for the self-assembly of 3 nm ZnS/Mn (ZnS quantum dots doped with Mn2+ ions) and CdTe quantum dots (QDs). The QDs were shown to form an ordered array by transmission electron microscopy (TEM) and were separated by an average distance of 2.4 nm. The absorption and emission spectra of uncapped ZnS/Mn QDs exhibited no change upon self-assembly on the His-loop flagella compared to those in solution. Band gap excitation and subsequent Mn-centered emission were insensitive to self-assembly on the flagella nanotubes. By contrast, the l-cysteine-capped CdTe QDs exhibited a red shift in both the absorption and emission spectra upon binding to the His-loop flagella compared to those of the free QDs in solution. This shift was consistent with exciton energy transfer from smaller to larger CdTe QDs on the flagella nanotube, indicating that the self-assembly facilitated such an energy transfer. The 5 nm (18.4 meV) shift in the emission spectrum was similar to previous observations with organically linked CdTe QDs and theoretical calculations reported in the literature.