Axopatch 200B
| Axopatch 200B | |
|---|---|
| Developer | Axon Instruments |
| Manufacturer | Molecular Devices |
| Type | Patch-clamp amplifier |
| Connectivity | Analog and digital BNC outputs |
| Power | 85 to 264 V AC, 30 W at most |
| Dimensions | 8.9 × 48.3 × 31.7 cm (2U rack); headstage 1.8 × 1.9 × 10.5 cm |
| Mass | 5.1 kg |
The Axopatch 200B is a patch clamp amplifier made by Axon Instruments and sold by Molecular Devices. It holds a voltage across a membrane or a pore and measures the current that flows, in ranges from fractions of a picoampere to hundreds of nanoamperes. It was designed to record single ion channels in cells, and it appears in the methods of many nanopore papers; the authors of one called it "the commonly used Axopatch 200B".[1]
The instrument is a box for a 19-inch rack that weighs 5.1 kg, with a headstage 10.5 cm long that stands beside the sample and holds the first stage of amplification.[2] The maker says the design of that stage "provides the lowest-noise single-channel recording available". In the mode for single channels its feedback element is a capacitor, and its critical components are cooled to about −15 °C.[2]
Capacitor feedback
The first stage of a patch-clamp amplifier is a transimpedance amplifier, and whatever sits in its feedback loop sets the noise floor. A resistor turns current into voltage by Ohm's law. For currents of a picoampere it must be of the order of a gigaohm, and resistors that large are imperfect parts whose noise grows with their value. A capacitor avoids the problem: the voltage across it is the integral of the current passed into it, so a circuit built around one reads the current as the rate at which the voltage climbs. The Axopatch 200B sends the current into a capacitor of 1 pF. In the words of the data sheet, "Noiseless capacitors, on the other hand, are easily manufactured."[2]
A voltage cannot climb for ever. When the output reaches the limit of the supply the headstage resets, which takes 50 µs ±10% and holds the output at the last value measured. For a constant current the time between resets is 10/(I − Ibias) seconds, with the currents in picoamperes and the bias current typically 0.3 to 1.0 pA. A reset also occurs whenever the headstage has to deliver more than 10 pC of charge to the membrane.[2][a]
The cooling is active. A solid-state element holds the input circuitry at about −15 °C, and the front-panel meter can display the temperature. With the input open, the largest noise the data sheet allows is 0.130 pA rms in a bandwidth of 0.1 to 10 kHz, and 0.145 pA with a pipette holder attached.[2] How those figures compare with a resistor is worked out in Patch clamp and in Nanopore noise.
Resistor feedback
In whole-cell recording the cell and its surroundings make more noise than the amplifier, and the quiet capacitor no longer pays. The same headstage therefore carries two resistors, each in parallel with the 1 pF capacitor. The 500 MΩ setting passes up to 20 nA and the 50 MΩ setting up to 200 nA.[2]
| Setting | Feedback element | Gain | Largest noise, 0.1 to 10 kHz |
|---|---|---|---|
| Patch | 1 pF | 1 mV/pA | 0.130 pA rms |
| Whole cell, β = 1 | 500 MΩ in parallel with 1 pF | 1 mV/pA | 1.10 pA rms |
| Whole cell, β = 0.1 | 50 MΩ in parallel with 1 pF | 0.1 mV/pA | 3.0 pA rms |
A resistor of 500 MΩ with 1 pF across it has a corner at 320 Hz. The main unit therefore contains a tuning circuit "to idealize response of the feedback resistor", and the internal bandwidth of the headstage is given as 140 kHz in patch mode and 70 kHz in whole-cell mode.[2]
Output
The signal passes through a four-pole Bessel filter with five settings, 1, 2, 5, 10 and 100 kHz, and a gain stage with ten settings from 0.5 to 500, and leaves on an output that swings ±10 V. Telegraph outputs report the gain, the filter and the headstage mode as voltages, so that acquisition software can scale what it records. The amplifier does not digitize. The output goes to a separate converter, for which the maker sells the Axon Digidata, and the usable bandwidth outside the instrument is the 100 kHz of the filter.[2][3]
Most of the front panel is for cells. It has controls to cancel the capacitance of the pipette and of the whole cell, the second in ranges of 100 pF and 1,000 pF, and to correct and predict the voltage lost across the series resistance of the pipette. It has a seal test, leak subtraction, a current clamp, and a ZAP button that applies +1.3 V at the pipette for 0.5 to 50 ms, or for as long as the button is held, to rupture a patch; see The zapper. Two model cells come in the box, a 10 GΩ resistor to stand for a patch and a 10 kΩ resistor in series with 100 pF to stand for a bilayer, so that the amplifier can be tested against something known before it is blamed for an experiment.[2]
Use with nanopores
A nanopore experiment is a patch-clamp experiment with a different membrane, and the amplifiers came with it. The 1996 paper that drove single strands of DNA through a protein pore used "an Axopatch 200A (Axon Instruments, Foster City, CA)" or another maker's amplifier. It set a Bessel filter at less than three eighths of the sampling rate, which was typically 64,000 samples a second, and digitized with a 16-bit board.[4] The 200B is the model in later work. A study of the noise of solid-state pores in 2008 recorded with it in resistive feedback mode, passed the current through an external eight-pole Bessel filter with a cutoff of 49.9 kHz, and digitized at 250 or 500 kHz.[5] In 2014 the group that introduced controlled dielectric breakdown recorded DNA through its pores on a 200B with the filter at 100 kHz and a sampling rate of 250 kHz, and wrote that apart from a custom current amplifier its setup was identical to "the instrumentation used to study DNA or proteins translocation through nanopores".[1] A 2020 comparison of noise in biological and solid-state pores recorded the example traces of both kinds with one.[3]
The limit is bandwidth. The 500 MΩ resistor in the Axopatch contributes white current noise of 33 fA2/Hz. In 2012 a group that placed a CMOS amplifier beside the pore measured 9 pA rms with the Axopatch at its highest bandwidth, 100 kHz, against 3.2 pA rms for its own amplifier. DNA crosses a solid-state pore at 10 to 1,000 ns per base, and the group went on to record at 1 MHz.[6]
At Molecular Reality
The recordings of DNA barcodes that Bell and Keyser published in 2016, and that Molecular Reality Corporation has used to train its machine-learning models, were made on an Axopatch.[7] The company's own bench uses a four-channel amplifier from Elements srl, the e4, which is read by the software EDR4. It weighs 106 g and takes its power from a USB cable.[8] The Axopatch weighs 5.1 kg and draws up to 30 W from the mains.[2]
History
Axon Instruments was established in 1983. On March 21, 2004 Molecular Devices announced that it would buy the company, which then had 128 employees, most of them at its headquarters in Union City, California, for about $70 million in cash and about $70 million in Molecular Devices stock.[9] The data sheet for the 200B, revised in July 2012, calls it "the latest version of our premier patch-clamp amplifier" and says that "the recording bandwidth has been doubled to up to 100 kHz". It is printed with the notice "FOR RESEARCH USE ONLY. NOT FOR USE IN DIAGNOSTIC PROCEDURES."[2]
See also
Notes
- ^ Ignoring the bias current, a constant 100 pA gives a reset every 0.1 s. A constant 1 nA, the order of the open-pore current of a pore a few nanometers wide in 1 M potassium chloride at 200 mV, gives one every 10 ms.
References
- ^ a b Kwok, H.; Briggs, K.; Tabard-Cossa, V. (2014). "Nanopore fabrication by controlled dielectric breakdown". PLOS ONE 9 (3): e92880. doi:10.1371/journal.pone.0092880.
- ^ a b c d e f g h i j k l "Axon Axopatch 200B Microelectrode Amplifier (data sheet)". Molecular Devices. July 2012.
- ^ a b Fragasso, A.; Schmid, S.; Dekker, C. (2020). "Comparing current noise in biological and solid-state nanopores". ACS Nano 14 (2): 1338-1349. doi:10.1021/acsnano.9b09353.
- ^ 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.
- ^ Smeets, R. M. M.; Keyser, U. F.; Dekker, N. H.; Dekker, C. (2008). "Noise in solid-state nanopores". Proceedings of the National Academy of Sciences 105 (2): 417-421. doi:10.1073/pnas.0705349105.
- ^ Rosenstein, J. K.; Wanunu, M.; Merchant, C. A.; Drndic, M.; Shepard, K. L. (2012). "Integrated nanopore sensing platform with sub-microsecond temporal resolution". Nature Methods 9 (5): 487-492. doi:10.1038/nmeth.1932.
- ^ "Company records". Molecular Reality Corporation. October 2026.
- ^ "e4: 4-channel current amplifier". Elements srl. Retrieved October 25, 2026.
- ^ "Molecular Devices announces definitive agreement to acquire Axon Instruments". Molecular Devices. March 21, 2004.