The photoluminescence of inorganic QDs is inherently voltage sensitive due to the quantum confined Stark effect. Incident electric fields present at the neuron membrane surface influence the luminescence energy, which can be read out using steady state luminescence measurements. When coupled with a transition metal dopant QD, the change in energy can be readout from the ratio of the red and green emission bands (a,b). We have confirmed coupling of the states in our system by measuring temperature-dependent photoluminescence as well as emission lifetimes, which confirms an equilibrium exchange of excited states in the dopant and semiconductor. The voltage-sensitivity of this excited state equilibrium is being measured ex vivo on QD ensembles and the single-QDs.
Lastly, we are pursuing two strategies to stain neurons using these materials, one of which involves water-soluble QDs that target neuron membranes, and the second utilizes QDs implanted in the lipid-membrane bilayer. In the first strategy we have synthesized ligands that bind to the QD surface and result in stable dispersions in biological media (c,d). Our current ligand design utilizes a multidentate thiol binding group to append water-soluble polyethylene glycol oligomers. We are currently exploring the use of alternate binding groups at the QD surface to maintain strong photoluminescence and vary the terminal group at the end of the polymer to target specific sites on the neuron surface. Our second approach involves the preparation of liposomes containing the two color emitting QDs. Fluorescence images and transmission electron microscope images demonstrate fluorophore localization within the lipid membrane. Staining of neurons with these lipids is ongoing.