Optical Strategies for Sensing Neuronal Voltage Using Quantum Dots and Other Semiconductor Nanocrystals

Abstract

Biophysicists have long sought optical methods capable of reporting the electrophysiological dynamics of large-scale neural networks with millisecond-scale temporal resolution. Existing fluorescent sensors of cell membrane voltage can report action potentials in individual cultured neurons, but limitations in brightness and dynamic range of both synthetic organic and genetically encoded voltage sensors have prevented concurrent monitoring of spiking activity across large populations of individual neurons. Here we propose a novel, inorganic class of fluorescent voltage sensors: semiconductor nanoparticles, such as ultrabright quantum dots (qdots). Our calculations revealed that transmembrane electric fields characteristic of neuronal spiking (~10 mV/nm) modulate a qdot's electronic structure and can induce ~5% changes in its fluorescence intensity and ~1 nm shifts in its emission wavelength, depending on the qdot's size, composition, and dielectric environment. Moreover, tailored qdot sensors composed of two different materials can exhibit substantial (~30%) changes in fluorescence intensity during neuronal spiking. Using signal detection theory, we show that conventional qdots should be capable of reporting voltage dynamics with millisecond precision across several tens or more individual neurons over a range of optical and neurophysiological conditions. These results unveil promising avenues for imaging spiking dynamics in neural networks and merit in-depth experimental investigation. Toward addressing these challenges, we studied whether tools from nanotechnology, specifically, bright, multifunctional semiconductor quantum dots (qdots), 22,23 might be useful for imaging neuronal membrane potentials. Qdots are known to possess voltagesensitive optical properties, including a red-shifting of the qdot's fluorescence emission peak, spectral broadening, and a decrease in the intensity of fluorescence emission ( Figure 1C,D). [24][25][26][27] Qdots also are highly resistant to photobleaching and exhibit large quantum yields and absorption cross sections for one-(σ 1 ) and two-photon (σ 2 ) excitation. For example, CdSe qdots with radius R ~ 3 nm exhibit cross sections of σ 1 > 1 nm 2 and σ 2 > 3 × 10 4 GM, as compared to σ 1 ~ 10 −2 nm 2 and σ 2 ~ 3 × 10 2 GM for green fluorescent protein. 28,29 Marshall and Schnitzer Page 2 ACS Nano. Author manuscript; available in PMC 2017 December 15. Graphical Abstract HHMI Author Manuscript HHMI Author Manuscript HHMI Author ManuscriptTo evaluate the spike detection capabilities of qdot indicators quantitatively, we applied a theoretical approach that incorporates the physical, biophysical, and statistical components of the problem. We calculated the effect of applied electric fields on the qdot's electronic structure and examined how this modulates a qdot's fluorescence intensity and emission spectra. We also studied nonspherical, nonhomogenous qdots, which we refer to more generally as semiconductor nanocrystals, and found that these nanoparticles can demonstrate muc...

Electric Field Modulation of Semiconductor Quantum Dot Photoluminescence: Insights Into the Design of Robust Voltage-Sensitive Cellular Imaging Probes

“…Finally, in structures with strong spatial confinement, the electric field results in QCSE, 26 which always decreases the energy of electron and hole levels, resulting in the Stokes shift, ΔE SE , of the PL. Straightforward calculations show that the magnitude of the Stokes shift (ΔE SE ∼ a 4 ) and the change of the overlap integral (Δ|I e−h | 2 ∼ a 6 ) caused by an e-field decrease rapidly with QD radius, a (see, for example, ref 27).…”

Voltage-sensitive rhodol with enhanced two-photon brightness

“…The challenge of achieving optical voltage sensing is a longstanding goal within the scientific community (4,5), and recent approaches have included fluorinated styryl dyes (6), annulated hemicyanines (7,8) and cyanines (9), lipophilic anions (10,11), hybrid small-molecule/fluorescent protein probes (12,13), porphyrins (14), and nanoparticles (15,16). However, combinations of poor sensitivity, slow kinetics, ineffective membrane localization, rapid photobleaching, and/or limited two-photon crosssection, which is important for imaging in thick tissue, have hampered rapid progress toward a general solution for optical voltage imaging.…”

Monoclinic Copper(I) Selenide Nanocrystals and Copper(I) Selenide/Palladium Heterostructures: Synthesis, Characterization, and Surface‐Enhanced Raman Scattering Performance

“…Undecylenic acid (95 %), terpineol (95 %), absolute alcohol, 2-propanol, and 4-Mpy 4 ] solution. After 180°C for 1 h, the reaction mixture was naturally cooled to room temperature.…”

“…Micro-and nanoscaled transition-metal chalcogenides (TMCs) have attracted much attention because of their unique chemical and physical properties, which allow for widespread applications in energy devices, [1][2][3] sensors, [4,5] thermoelectric devices, [6,7] and memory devices. [8,9] Among the various TMCs, copper selenides (Cu x Se, x = 1-2) are very important semiconductors and exist in a wide range of compositions (CuSe, Cu 2 Se, CuSe 2 , Cu 3 Se 2 , Cu 2-x Se etc.)…”