Picoampere
The picoampere (symbol pA) is a unit of electric current equal to 10−12 ampere. Since the ampere was tied to the elementary charge in 2019, a current of one picoampere is exactly 1 / (1.602176634 × 10−7) elementary charges a second, or 6,241,509 of them.[1] It is the natural unit for the currents of single ion channels and for the changes in ionic current that mark a molecule in a nanopore, and it is the unit in which the amplifier at the front of such an instrument is judged.
Scale
A picoampere is a small current, but the charge in it is not small by the standards of a single ion. At 6.2 million charges a second, about six pass a point in each microsecond. A current of this size is therefore a count of discrete events, and it cannot be steadier than counting allows: the shot noise of 1 pA in a 10 kHz band is about 57 fA, or 5.7 percent of the current itself.
| Current | Charges a second | Example |
|---|---|---|
| 3 fA | 19 thousand | Typical input bias current of the LMP7721 amplifier[2] |
| 1 pA | 6.2 million | The unit |
| 13 pA | 80 million | Root-mean-square thermal noise of a 1 MΩ resistor in a 10 kHz band |
| 120 pA | 750 million | Alpha-hemolysin at 120 mV in 1 M potassium chloride[3] |
| 3,100 pA | 19 billion | The modeled current of a 5 nm pore through 10 nm of membrane at 200 mV (see Pore conductance) |
The thermal noise in the third row is the reason a picoampere signal is hard to find. A resistor of 1 MΩ, the feedback resistor in the first stage of the MR1 Nanopore Reader as drawn, produces a noise current larger than the signal it is meant to read, and the remedy is a larger resistor, as described in Transimpedance amplifier.
Measurement
Two circuits read picoamperes. A shunt ammeter passes the current through a resistor and measures the voltage across it; because the voltage is a few microvolts, the resistor must be very large and the measurement slow. A feedback ammeter, the circuit used in the Axopatch 200B and its descendants, holds its input at a fixed potential and lets the current flow through a feedback resistor, so that the input never moves. Both are called picoammeters when they read below a nanoampere.
The harder part is keeping the current in the wire. A volt across 1012 Ω drives a picoampere, so any leakage path with a resistance of a teraohm is as loud as the signal: a film of dried salt on a circuit board, a damp fingerprint, a connector that has been cleaned with the wrong solvent. Careful designs surround the input with a guard ring held at the same potential, which leaves no voltage across the leakage to drive current through it.
Electrophysiologists reached the unit first. In 1976 Erwin Neher and Bert Sakmann recorded the current of single acetylcholine channels in muscle fibers of the frog, and in 1981 their group described the high-resistance seal between pipette and membrane that made such recordings routine.[4][5] The seal is called a gigaseal because its resistance exceeds a gigaohm, which is what lets a picoampere stand out from the noise of the seal itself.
In converters
A converter reports counts, and a count is a fraction of a picoampere or a few of them, depending on the gain in front of it. Oxford Nanopore's sequencing files store each raw sample as an integer, with three numbers beside it, the digitisation, the offset and the range, that turn the integer into picoamperes.[6] The data-acquisition specification of the MR1 Nanopore Reader gives one count of its 16-bit converter as 62.5 fA, which makes a picoampere 16 counts. The amplifier as drawn in the same repository has a gain 100 times lower, and behind it one count is 6.25 pA.[7]
At Molecular Reality
Molecular Reality Corporation has worked at this scale since its first runs. The sliding pores of 2015 and 2016 were read on a benchtop picoammeter, and the demonpore 64 of 2020 and 2021 was specified for currents from picoamperes to nanoamperes at 125 kHz.[7] The control that sets the vertical range of the live molecular stream runs from plus or minus 10 femtoamperes to plus or minus 600 nanoamperes, a span of 7.8 decades.[7]
A company video about the older MR1 Molecular Streaming Device describes signals of this size, in its caption, as being "at the scale of a gnat's whisper".[8] The gnat is not a unit; the phrase appears with the company's other measures in the List of unusual units used at Molecular Reality. The reader's input amplifier has a datasheet bias current of 3 fA, which is 0.3 percent of a picoampere.
See also
References
- ^ "The International System of Units (SI), 9th edition". Bureau International des Poids et Mesures. 2019.
- ^ "LMP7721 datasheet". Texas Instruments. Retrieved October 25, 2026.
- ^ Kasianowicz, J. J.; Brandin, E.; Branton, D.; Deamer, D. W. (1996). "Characterization of individual polynucleotide molecules using a membrane channel". Proceedings of the National Academy of Sciences 93 (24): 13770-13773. doi:10.1073/pnas.93.24.13770.
- ^ Neher, E.; Sakmann, B. (1976). "Single-channel currents recorded from membrane of denervated frog muscle fibres". Nature 260: 799-802. doi:10.1038/260799a0.
- ^ Hamill, O. P.; Marty, A.; Neher, E.; Sakmann, B.; Sigworth, F. J. (1981). "Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches". Pflügers Archiv 391: 85-100. doi:10.1007/BF00656997.
- ^ "fast5_research". Python Package Index. Oxford Nanopore Technologies. Retrieved October 25, 2026.
- ^ a b c "Company records". Molecular Reality Corporation. October 2026.
- ^ Keltar explains MR1 (Video). EpicQuestBio.