Quantum Dot-Peptide-Fullerene Bioconjugates for Visualization of in Vitro and in Vivo Cellular Membrane Potential

We report the development of a quantum dot (QD)-peptide-fullerene (C₆₀) electron transfer (ET)-based nanobioconjugate for the visualization of membrane potential in living cells. The bioconjugate is composed of (1) a central QD electron donor, (2) a membrane-inserting peptidyl linker, and (3) a C₆₀ electron acceptor. The photoexcited QD donor engages in ET with the C₆₀ acceptor, resulting in quenching of QD photoluminescence (PL) that tracks positively with the number of C₆₀ moieties arrayed around the QD. The nature of the QD-capping ligand also modulates the quenching efficiency; a neutral ligand coating facilitates greater QD quenching than a negatively charged carboxylated ligand. Steady-state photophysical characterization confirms an ET-driven process between the donor-acceptor pair. When introduced to cells, the amphiphilic QD-peptide-C₆₀ bioconjugate labels the plasma membrane by insertion of the peptide-C₆₀ portion into the hydrophobic bilayer, while the hydrophilic QD sits on the exofacial side of the membrane. Depolarization of cellular membrane potential augments the ET process, which is manifested as further quenching of QD PL. We demonstrate in HeLa cells, PC12 cells, and primary cortical neurons significant QD PL quenching (ΔF/F₀ of 2-20% depending on the QD-C₆₀ separation distance) in response to membrane depolarization with KCl. Further, we show the ability to use the QD-peptide-C₆₀ probe in combination with conventional voltage-sensitive dyes (VSDs) for simultaneous two-channel imaging of membrane potential. In in vivo imaging of cortical electrical stimulation, the optical response of the optimal QD-peptide-C₆₀ configuration exhibits temporal responsivity to electrical stimulation similar to that of VSDs. Notably, however, the QD-peptide-C₆₀ construct displays 20- to 40-fold greater ΔF/F₀ than VSDs. The tractable nature of the QD-peptide-C₆₀ system offers the advantages of ease of assembly, large ΔF/F₀, enhanced photostability, and high throughput without the need for complicated organic synthesis or genetic engineering, respectively, that is required of traditional VSDs and fluorescent protein constructs.