Electric field dependence of quantum dot (QD) photoluminescence under single ph oton and two-photon illumination: towards membrane voltage QD sensors

Optical stimulation and detection of neuronal signals with high spatio-temporal resolution are of major importance in neuroscience. Quantum dots (QDs) have been used in biological applications primarily in the imaging field, where the interfaces created between such particles and cells were mainly passive (no influence of the QDs on the cellular properties) [1]. On the other hand, semiconductor nanoparticles (NPs) and nanowires (NWs) display optoelectronic properties that have motivated their use in photovoltaic devices, lasers, light emitting diodes and biosensors [2]. Recent reports showed the possibility to induce action potentials in electrogenic cells by QDs under light activation [3,4]. The electric field-dependence of their optical properties is hinting that semiconductor nanostructures might act as voltage sensitive probes with superior performance as compared to organic voltage-sensitive molecules. QDs have a broader absorption, narrower emission peaks and larger single- and two-photon absorption cross-sections. Two-photon based techniques, like 2P fluorescence and second harmonic generation (SHG) imaging, are particularly interesting in the context of in vivo imaging. Due to the electric-field induced changes in the QD electronic structure, it is expected that also 2P fluorescence and SHG signals originated by QDs would be voltage sensitive. Therefore, in this project, we aim to study the electric-field influence on the optical properties of QDs in the context of detecting transmembrane voltages in electrogenic cells. Recent calculations by Marshall et.al. [5] have revealed that transmembrane electric fields, responsible for neuronal spiking, might be able to induce significant changes in the QD fluorescence intensity and its emission wavelength. Experiments performed by our group reveal also chargeinduced changes in the fluorescence intensity and emission wavelength (unpublished data). Moreover, calculations of conductance changes induced by the alteration of the surface charge of the nanowire are indicating the possibility to reach single charge sensitivity [6] in field-effect NW-based sensing. These properties are particularly relevant for the detection of the membrane voltages in electrogenic cells. The localization of the semiconductor nanoprobe into the cell membrane is essential for the voltage detection sensitivity. Shape, size and surface properties are determining the particle/membrane interface. In this respect, vertical semiconductor nanowires or surface-bound nanoparticles functionalized with cell-adhesion molecules might results in more robust contacts with the cell membrane (i.e. be more tightly engulfed by the cell membrane, without internalization). Our own work as well as reports by other groups demonstrated very tight membrane-nanopillar interfaces that may result in higher signal-to-noise ratio detection of cellular activity [7-9]. Nevertheless, in all these reports the detection of action potentials was performed electrically. In this project specifically, we aim to investigate the voltage sensitivity of the photoluminescence (PL) and SHG of semiconductor nanostructures in function of their composition and size. For this purpose, various QD nanoparticles will be prepared and their voltage sensitivity will be evaluated under dry conditions as well as in aqueous environments under single- and two-photon illumination in the presence of external electric fields. We would use both commercially available core-shell CdSe/ZnS colloidal particles as well as self vacuum-produced nanoparticles and nanowires (e.g. SiC), Also strategies to enhance the absorption cross-section and quantum efficiency of QDs will be investigated. Furthermore, the QDs exhibiting the highest voltage sensitivity will be functionalized for efficient cell membrane targeting and their voltagesensitivity will be evaluated in in vitro cellular preparations (primary neurons and cell lines) by a combination of optical and electrophysiology techniques (i.e. patch clamp).