Mermaid

FRET-based GEVI using a VSD paired with coral-derived fluorescent proteins for ratiometric voltage.

Category: VSD-FRET GEVI
Published: 2008-07-11 — Nature Methods 2008
Response polarity: positive-going (fluorescence increases on depolarization)
Spectrum: excitation peak 483 nm, emission peak 564 nm (Mermaid)
Plasmid: Addgene #53617 (Addgene #53617 is 'Chicken Mermaid S188' (deposited by V. Pieribone) — a chicken-codon-optimized derivative of the canonical Mermaid (Tsutsui et al. 2008), which does not appear to have its own Addgene deposit. Added as the closest available plasmid for this sensor; flagged as a variant, not the exact 2008 construct.)
Tags: Ci-VSP, FRET, Positive-going

Headline values (as shown in the GEVIBench table)

Metric	Value	Definition
Brightness (B/B_EGFP)	1.29× EGFP	Molecular brightness relative to EGFP (extinction coefficient × quantum yield, or a measured ratio). Sensors not measured against EGFP directly are placed through a chain of pairwise brightness comparisons, combined by geometric mean.
Kinetics τ_on / τ_off (ms)	10 / 19 ms	Activation / decay time constants in ms (amplitude-weighted mean τ for multi-exponential fits). The value shown averages the measurements made at physiological temperature (≥33 °C); if there are none, it is the measurement closest to 33 °C.
ΔF/F per 100 mV	+40% (median of 2 measurements: +26.4%)	Steady-state fluorescence change for a 100 mV depolarization step (−70 → +30 mV); the sign gives the polarity. The table shows the first listed measurement; the median across measurements (one-photon preferred) is used for sorting.
ΔF/F per mV	0.25% (estimated from the F–V curve)	Subthreshold sensitivity: fluorescence change per mV near rest (−90 to −50 mV). Where no value is reported directly, it is estimated from the F–V curve.
ΔF/F per AP	1.6%	Fluorescence change per single action potential in neurons. The table shows the first listed measurement; the median across measurements (one-photon preferred) is used for sorting.
AP width (FWHM, ms)	—	Full width at half maximum of the optical spike waveform.
Photostability t75% @ 100 mW/mm²	4.1 s	Time for fluorescence to fall to 75% of its initial value under one-photon illumination, linearly dose-scaled to 100 mW/mm² (t50% is used where the 75% crossing falls inside a fast initial transient). Two-photon curves are not dose-scaled and are excluded.
Papers using it (N_used)	9	Number of published research papers, tracked by GEVIBench, that used the sensor for an experiment (a popularity / track-record measure).

Measurements and sources

Kinetics

τ_on 8 ms, τ_off 15 ms at 33°C — Tsutsui 2008, Fig. 1f
τ_on 12 ms, τ_off 23 ms at 35-37°C — Barnett 2012, Fig. 5B

ΔF/F per 100 mV

+40%, 1P — Tsutsui 2008, Fig. 1e
+12.9%, 1P — Mutoh 2009, Table 1

ΔF/F per action potential

1.6%, 1P — Tsutsui 2008, Table S1

Brightness comparisons

1.286× the brightness of EGFP — Tsutsui 2008, Main text

Photobleaching

t75% = 33 s, 1P — Tsutsui 2008, Fig. S2a

Fluorescence–voltage (F–V) curve

ΔF/F (%) at each membrane potential, normalized to 0 at −70 mV: -120 mV: -5%; -100 mV: -5%; -80 mV: -3%; -70 mV: 0%; -60 mV: 2%; -40 mV: 16%; -20 mV: 26%; 0 mV: 33%; 20 mV: 36%; 40 mV: 37% — Tsutsui 2008, Fig. 1e

Research papers using Mermaid (9)

Improving membrane voltage measurements using FRET with new fluorescent proteins — Tsutsui H, Karasawa S, Okamura Y, Miyawaki A, Nature Methods 2008 (HEK293 cells, Neuro2A cells, rat cortical neurons, Xenopus oocytes, in vitro)
Visualizing voltage dynamics in zebrafish heart — Tsutsui H, et al., The Journal of Physiology 2010 (Zebrafish, heart, in vivo)
Optical action potential screening on adult ventricular myocytes as an alternative QT-screen — Tian Q, et al., Cellular Physiology and Biochemistry 2011 (Rat, ventricular myocytes, in vitro)
Neural coding in a single sensory neuron controlling opposite seeking behaviours in Caenorhabditis elegans — Kuhara A, et al., Nature Communications 2011 (C. elegans, in vivo)
Rapid evaluation of a protein-based voltage probe using a field-induced membrane potential change — Tsutsui H, et al., Biochimica et Biophysica Acta 2014 (HEK293 cells, in vitro)
INaP selective inhibition reverts precocious inter- and motorneurons hyperexcitability in the Sod1-G93R zebrafish ALS model — Benedetti L, et al., Scientific Reports 2016 (Zebrafish, spinal motor neurons, in vivo)
Biosensing Motor Neuron Membrane Potential in Live Zebrafish Embryos — Benedetti L, et al., Journal of Visualized Experiments 2017 (Zebrafish, motor neurons, in vivo)
Tracking the sarcoplasmic reticulum membrane voltage in muscle with a FRET biosensor — Sanchez C, et al., The Journal of General Physiology 2018 (Mouse, skeletal muscle fibers, in vitro)
Use of FRET-Sensor 'Mermaid' to Detect Subtle Changes in Membrane Potential of Primary Mouse PASMCs — Dartsch RC, et al., Cells 2024 (Mouse, pulmonary arterial smooth muscle cells, in vitro)

Data and citation

llms.txt — summary for language models
llms-full.txt — every sensor in full (Markdown)
gevis.json — all values and measurements with sources (JSON)
gevis.csv — one row per sensor (CSV)

Cite GEVIBench as: Adam E. Cohen and Daozheng Gong (2026). Voltage imaging: A practical guide. Neuron. https://doi.org/10.1016/j.neuron.2026.08.002

Generated from the GEVIBench database on 2026-10-12.