Figure 1.

QD–peptide–C₆₀ bioconjugates. (A) Modular design of QD–peptide–C₆₀ bioconjugates. The core/shell QD serves as a central scaffold to which is appended peptides bearing a C₆₀ fullerene at discrete positions. A unique lysine (K) allows for the controlled placement of the C₆₀ moiety at different distances from the QD surface. An N-terminal polyhistidine tract (H₆) mediates self-assembly of the peptide to the QD shell. Peptide sequences are written in N → C orientation. (B) Molecular models of QD–peptide–C₆₀ bioconjugates. Shown is the 605 nm emitting core/shell QD (yellow sphere; 100 Å in diameter) capped with DHLA (left panel) and DHLA–PEG₇₅₀–OMe (right panel) ligands for water solubility. The ligand layer is shown in gray and is 11 Å (DHLA) or 33 Å thick (DHLA–PEG₇₅₀–OMe). Each QD is shown appended with peptides JBD1, JBD2, and JBD3 wherein the C₆₀ electron acceptor is iteratively positioned at increasingly further distances from the QD surface. The predicted surface-to-surface distances from the QD to the C₆₀ surface are as follows: JBD1, 10 Å; JBD2, 24 Å; JBD3, 42 Å. (C) Schematic of the response of QD–peptide–C₆₀ bioconjugates to changes in membrane potential. Multiple peptide–C₆₀ conjugates are arrayed around the central QD (electron donor), and they all engage in electron transfer (ET) with the QD; a percentage of the arrayed peptide–C₆₀ are inserted into the membrane bilayer and contribute to depolarization-induced PL quenching. At resting potential, minimal ET from the photoexcited QD to those C₆₀ embedded in the membrane bilayer results in bright QD emission (top). Depolarization of the membrane potential augments the rate of ET, causing a decrease in QD PL (bottom).