Nanoscale optical voltage sensing in biological systems

Local electric fields modulate a variety of cellular processes and are particularly relevant in the nervous system. Signal propagation along an axon occurs through small (around 100 mV) rapid changes in the potential across the cell membrane. Experimental techniques capable of measuring voltage changes in cells, tissue, and animal models have been instrumental in understanding cell signaling in the nervous system. Optical approaches relying on fluorescent voltage sensors have emerged as a minimally invasive – and less technically demanding – alternative to traditional, electrode-based techniques. Combined with state-of-the-art optical microscopy techniques, luminescent voltage sensors allow voltage sensing at relevant timescales (sub-millisecond) and diffraction-limited spatial resolutions. Voltage-sensitive dyes and proteins are already well-established fluorescent voltage sensors, while semiconductor nanostructures and nitrogen vacancy-containing diamonds are attracting increasing attention due to their superior photoluminescent properties. In this review, we describe the working mechanism of the different classes of fluorescent voltage sensors, discuss their advantages and limitations, and compare their performance based on sensitivity and temporal resolution. Further, we discuss the more recent advances in optical voltage sensing and identify the knowledge gaps that need to be addressed for this technique to realize its full potential as a valid replacement to electrode-based sensing.