The use of semiconductor quantum dots (QDs) for bioimaging and sensing has progressively matured over the past decade. QDs are highly sensitive to charge-transfer processes, which can alter their optical properties. Here, we demonstrate that QD-dopamine-peptide bioconjugates can function as charge-transfer coupled pH sensors. Dopamine is normally characterized by two intrinsic redox properties: a Nernstian dependence of formal potential on pH and oxidation of hydroquinone to quinone by O(2) at basic pH. We show that the latter quinone can function as an electron acceptor quenching QD photoluminescence in a manner that depends directly on pH. We characterize the pH-dependent QD quenching using both electrochemistry and spectroscopy. QD-dopamine conjugates were also used as pH sensors that measured changes in cytoplasmic pH as cells underwent drug-induced alkalosis. A detailed mechanism describing the QD quenching processes that is consistent with dopamine's inherent redox chemistry is presented.

Plants and photosynthetic bacteria contain protein−molecular complexes that harvest photons with nearly optimum quantum yield and an expected power conversion efficiency exceeding 20%. In this work, we demonstrate the integration of electrically active photosynthetic protein− molecular complexes in solid-state devices, realizing photodetectors and photovoltaic cells with internal quantum efficiencies of approximately 12%. Electronic integration of devices is achieved by self-assembling an oriented monolayer of photosynthetic complexes, stabilizing them with surfactant peptides, and then coating them with a protective organic semiconductor.

Colloidal quantum dots (QDs) have a large fraction of their atoms arrayed on their surfaces and are capped with bifunctional ligands, which make their photoluminescence highly sensitive to potential charge transfer to, or from, the surrounding environment. In this report, we used peptides as bridges between CdSe-ZnS QDs and metal complexes to promote charge transfer between the metal complexes and QDs. We found that quenching of the QD emission is highly dependent on the relative position of the oxidation levels of QDs and metal complex used; it also traces the number of metal complexes brought in close proximity of the nanocrystal surface. In addition, partial bleaching of the absorption was measured for the QD-metal complex assemblies. These proximity driven interactions were further used to construct sensing assemblies to detect proteolytic enzyme activity.

The results of a kinetic study are reported for cross-surface electron transfer in the reversible oxidation of [Os II (bpy) 2 (4,4′-(CO 2 H) 2 bpy)](PF 6 ) 2 to Os III surface adsorbed to nanocrystalline TiO 2 films on optically transparent ITO (tin-doped indium oxide) electrodes. The kinetics are sensitive to the extent of surface loading. From monolayers adsorbed from CH 3 CN, a percolation threshold of ∼60% was measured. Adsorption isotherms for [Os II (bpy) 2 (4,4′-(CO 2 H) 2 bpy)](PF 6 ) 2 in EtOH and CH 3 CN show that surface coverage from EtOH is ∼60% of that from CH 3 CN at similar adsorbate concentrations. Apparent charge-transfer diffusion coefficients in 0.1 M [N(n-C 4 H 9 ) 4 ](PF 6 ) in CH 3 CN measured by chronoabsorptometry are D app ) 1.4 × 10 -9 cm 2 /s (Γ ) 1.1 × 10 -10 mol/cm 2 , CH 3 CN) and 1.4 × 10 -11 cm 2 /s (Γ ) 7 × 10 -11 mol/cm 2 , EtOH). Electron-transfer mediation of the oxidative component of the Os III/II surface couple occurs in the presence of added Ru(bpy) 3 2+ by stepping the potential past the Ru(bpy) 3 3+/2+ wave. Mediation of both oxidative and reductive components occurs with added Os(bpy) 3 2+ . The apparent diffusion coefficient for oxidation of the Os III/II surface couple in the presence of Ru(bpy) 3 2+ increases linearly with the concentration of added mediator.