Zeptoliter
The zeptoliter (symbol zL) is a unit of volume equal to 10−21 liter, or 10−24 cubic meter. It is the volume of a cube 10 nm on a side, which is 1,000 cubic nanometers. Water at room temperature holds about 33 molecules in each cubic nanometer, so a zeptoliter of it contains about 33,000. The unit sits at the scale of the sensing region of a nanopore, and the founder of Molecular Reality Corporation titled a message to its members "Zeptoliter is My Favorite Unit of Measurement".[1]
The prefix
The SI prefix zepto- stands for 10−21. It was adopted in 1991, with yocto- (10−24) and the large prefixes zetta- and yotta-, and it takes its name from the Latin septem, seven, because 10−21 is the seventh power of 1/1,000.[2] The liter is not an SI unit but is accepted for use with them. A zeptomole, 10−21 mole, is 602 molecules.
What fits in one
In 1 M potassium chloride the ions are 0.602 per cubic nanometer of each kind, so a zeptoliter of the solution holds 602 potassium ions and 602 chloride ions, as many ions of each kind as there are molecules in a zeptomole. The other sizes below are computed from diameters, or from molecular mass and a typical protein volume of 0.73 cm3 per gram.
| Object | Volume | In zeptoliters |
|---|---|---|
| Antibody (immunoglobulin G, 150 kDa) | 180 nm3 | 0.18 |
| Cylinder 5 nm wide and 10 nm long, a small pore | 196 nm3 | 0.20 |
| Cylinder 10 nm wide and 20 nm long | 1,570 nm3 | 1.6 |
| Sphere 100 nm across, the size of many viruses | 524,000 nm3 | 520 |
| Bacterium of about 1 µm3 | 109 nm3 | 1,000,000 |
| Aperture of a Coulter counter, 100 µm wide and 100 µm long | 785 pL | 790 billion |
The last row is the reason the unit is used about nanopores at all. A Coulter counter senses a particle in a volume of hundreds of picoliters, and the Coulter principle applies just as well to a hole of a few nanometers; the volume it senses in falls by a factor of about 1012. The sensing zone of a pore is somewhat larger than its bore, because the resistance of the openings (see Access resistance) is concentrated within a few pore radii of each mouth, but for a pore a few nanometers across it remains of order one zeptoliter.
In nanopore papers
Two papers made the unit conspicuous in the literature. In 2017 a group at Imperial College London pulled a pair of glass pores 20 nm apart, so that a droplet of liquid of about 63 zL joined their tips, and trapped DNA in the bridge. Fragments of 5 kilobase pairs crossed a single pore in a mean of 0.13 ms and lingered up to 100 ms in the bridge, a slowing of up to three orders of magnitude.[3] The same year a group reported that proteins tethered to a lipid coat inside a solid-state pore could be characterized in five properties from the shape of the current during a single passage: approximate shape, volume, charge, rotational diffusion coefficient and dipole moment.[4]
At Molecular Reality
The founder wrote about both papers on March 29, 2026, in two messages to members of EQBio-OS. The first, on the bridge, carries the line that gave the unit its place in the company's vocabulary. Kent Kemmish wrote that he often brings the paper up "to spark the curious imagination about what we can do with nanopores", called it "charismatic", and said the setup behaves as one would expect: "everything at that scale is not supposed to be intuitive, and yet, if you described this experimental setup to me, the data they get is exactly what I'd expect to see." He gives the volume of the bridge as "one sextillionth of a liter".[1] The second message, "The Zeptoliter Strikes Again", is about the protein paper, which he calls the first he would discuss with anyone new to solid-state pores, and whose measurements he places in a "(zeptoliter!) sensing zone". Its lesson, he wrote, concerns entropy: "translocation control is most powerful when degrees of freedom are reduced".[5]
Both papers appear in the company's account of what a pore can do to a molecule that does not stay in place. A small molecule in free solution diffuses away from a pore within microseconds, so a pore that wants a second look at it needs a bridge, a tether or a strand held between two pores, as discussed in Active sensing.[6] The term has an entry in the Glossary of Molecular Reality jargon, and the company's other unusual measures are collected in the List of unusual units used at Molecular Reality.
See also
References
- ^ a b Kemmishtree (March 29, 2026). "Zeptoliter Nanobridge". EQBio-OS.
- ^ "The International System of Units (SI), 9th edition". Bureau International des Poids et Mesures. 2019.
- ^ Cadinu, P.; et al. (2017). "Single molecule trapping and sensing using dual nanopores separated by a zeptoliter nanobridge". Nano Letters 17 (10): 6376-6384. doi:10.1021/acs.nanolett.7b03196.
- ^ Yusko, E. C.; et al. (2017). "Real-time shape approximation and fingerprinting of single proteins using a nanopore". Nature Nanotechnology 12 (4): 360-367. doi:10.1038/nnano.2016.267.
- ^ Kemmishtree (March 29, 2026). "The Zeptoliter Strikes Again". EQBio-OS.
- ^ "Company records". Molecular Reality Corporation. October 2026.