An All-Optical Diamond Voltage Microscope

The development of fluorescent molecular sensors for imaging voltage changes in biological systems has revolutionized neuroscience, providing a tool to capture neuronal activity over large areas with sub-neuron resolution both <i>in vitro</i> and <i>in vivo</i> [1–3]. However, the poor photostability of molecular voltage sensors limits recording times to a few minutes [1–3], posing problems for longitudinal studies of network evolution and disease processes. These limitations have led to the uptake of lower-resolution extracellular recording techniques such as multi-electrode arrays (MEAs) for long term neurological research[4,5].<br/> <br/>Here, we present an alternate platform for sensitive high resolution voltage imaging using fluorescent, charge-sensitive defects in a transparent diamond substrate [6]. The nitrogen vacancy (NV) defect in diamond possesses three optically distinguishable charge states known to be responsive to voltage changes in solution [7-9]. In this work, we will show that precise electrochemical control of the diamond surface termination, can effectively tune the ensemble charge state population to an optimal composition for voltage sensing consisting exclusively of the fluorescent neutral (NV0) and non-fluorescent positive (NV+) states. Using these charge state sensors, we establish an all-optical diamond voltage imaging microscope (DVIM) capable of real-time imaging of capacitive charge injection by a microelectrode in solution. Finally, we show that this sensing mechanism can be replicated and enhanced in an array of diamond nanopillars, possessing sub-millisecond fluorescence response times and sub-millivolt sensitivity suitable for a host of excitable cell imaging applications.<br/> <br/><b>References:</b><br/>1. Knöpfel, T. & Song, C. Optical voltage imaging in neurons: moving from technology development to practical tool. Nature Reviews Neuroscience 20, (2019).<br/>2. Piatkevich, K. D. et al. Population imaging of neural activity in awake behaving mice. Nature 574, (2019).<br/>3. Wang, W., Kim, C. K. & Ting, A. Y. Molecular tools for imaging and recording neuronal activity. Nature Chemical Biology 15, (2019).<br/>4. Abbott, J. et al. Extracellular recording of direct synaptic signals with a CMOS-nanoelectrode array. Lab on a Chip 20, (2020).<br/>5. Emmenegger, V., Obien, M. E. J., Franke, F. & Hierlemann, A. Technologies to Study Action Potential Propagation With a Focus on HD-MEAs. Frontiers in Cellular Neuroscience 13, (2019).<br/>6. McCloskey, D, Dontschuk, N. et al. A diamond voltage imaging microscope. Nature Photonics (accepted 13th June 2022).<br/>7. Karaveli, S. et al. Modulation of nitrogen vacancy charge state and fluorescence in nanodiamonds using electrochemical potential. Proceedings of the National Academy of Sciences 113, (2016).<br/>8. Grotz, B. et al. Charge state manipulation of qubits in diamond. Nature Communications 3, (2012).<br/>9. Krečmarová, M. et al. A Label-Free Diamond Microfluidic DNA Sensor Based on Active Nitrogen-Vacancy Center Charge State Control. ACS Applied Materials & Interfaces 13, (2021).