Glow discharge or plasma treatment is always recommended before using QUANTIFOIL® holey carbon film supports as it increases their hydrophilicity and wettability. Hydrophilic grids wet more evenly, resulting in a more even layer of ice and better quality grids for data collection.
In addition, you may very rarely observe unwanted material on your Quantifoil supports which may originate from a variety of sources, such as the way they have been stored, or, very occasionally, as a residual of the production process. Glow-discharge or plasma treatment can also help to remove these deposits, but if they persist we recommend washing supports with an organic solvent.
Glow discharge and plasma cleaning protocols1
PLEASE NOTE: Care should be taken when treating grids with an additional ultrathin carbon layer, as these methods may remove that layer as well as improving grid performance: we suggest choosing lower power settings and/or shorter exposures for these supports.
Glow discharge and plasma cleaning makes the surface of your supports more hydrophilic, so improving the wetting properties of the grid and therefore sample dispersion. It can also burn off any carbonaceous deposits on the surface.
There are several different protocols for plasma cleaning but in all cases the following precautions should be observed:
- Prior to the treatment the supports should be inspected under a microscope. All supports should be flat, continuous and without dust or lint. Discard any that do not reach this standard.
- Plasma treated grids should always be used within 1 hour to prevent contamination accumulating after surface modification.
- Gloves should be worn when handling anything to be placed in the vacuum chamber of a plasma cleaner to prevent contamination and fingerprints.
Although plasma generated from residual air (glow discharging) can be used, plasmas of controlled composition are recommended, as it they give more reproducible results. We provide protocols utilizing both methods:
- Glow discharging
- Treatment with a 9:1 argon:oxygen plasma
- Plasma treatment with amylamine to increase hydrophilicity
- Treatment with low-energy hydrogen plasma to control protein adsorption on graphene
Protocol 1: Glow discharging
- Place supports in the chamber of a glow discharging apparatus. Supports should be placed in the centre of the stage on a clean glass slide, foil side up.
- Pump chamber to 200 mTorr
- Turn on HT to 7 kV, current 28-30 mA
- Expose supports for 30 sec.
- Turn off the HT and slowly vent the chamber.
- Store treated supports in a clean glass Petri dish and use within 1 hour.
Protocol 2: Argon-oxygen plasma treatment
Caution: Poorly-shielded plasmas, or those too high in energy, may damage gold supports by sputtering.
- Supports should always be placed in the plasma cleaner in the same location relative to the plasma to maximize reproducibility of exposure between batches. Some facilities have created custom holders to aid with this, but supports can simply be placed directly in the chamber on a clean glass slide, foil side up.
- Evacuate chamber to << 0.1 mTorr.
- Admit high purity argon and oxygen, in a ratio of 9:1 to a pressure of 21 mTorr
- Apply radio frequency plasma with 38 W forward power and ≤ 2 W reverse power.
- Exposure time should be optimized for desired hydrophilicity, but will be 10-60 sec. For most supports, around 20 sec works well.
- To prevent damage to very thin films of continuous carbon, use 19:1 argon:oxygen ratio at 35 W forward power for 5-15 sec.
- Vent plasma chamber and remove supports, use within 1 hour.
Protocol 3: Amylamine plasma treatment
- Place supports on a clean glass slide, foil side up, in the chamber in the center of the stage.
- Place a small glass vial containing 0.5 mL of amylamine in the chamber at the edge of the stage.
- Pump chamber to below 400 mTorr
- Turn on HT and adjust power to form uniform plasma (~90 V, 1.5 A)
- Expose supports for 30-60 sec.
- Turn off HT and slowly vent the chamber. Discard the vial.
- Store supports in a clean glass Petri dish and use within 1 hour.
Protocol 4: Hydrogen plasma treatment
For this protocol, a hydrogen generator should be attached to the input port of the plasma generator.
- Use only ultra-high purity hydrogen (>99.999% pure), all stainless steel tubing and fittings (NO plastic), ultra-high purity grade regulators.
- Distance of coils to the sample is important, as it affects the energy of the hydrogen species. To avoid damaging a graphene top layer, atoms and ions striking the surface must deliver energy <21 eV. To determine whether chamber this sputter threshold is exceeded, either image test graphene supports at increasing hydrogen doses or use a Langmuir probe to directly measure the energy at the sample.
- Prior to the treatment, insert the empty sample holder and evacuate the plasma chamber to << 0.1 mTorr.
- Burn the empty holder for 10 min at 100% pure hydrogen (35 W forward power, < 2 W reverse power, pure hydrogen).
- Mount the graphene supports in the holder and place into the chamber.
- Evacuate the chamber to << 0.1 mTorr.
- Treat supports for 5-40 seconds hydrogen plasma, using the same settings as (4).
- Remove supports and use immediately.
Protocol for washing grids in organic solvent2
PLEASE NOTE: this treatment is not suitable for grids with an additional ultrathin Carbon layer, as some solvents may damage the additional layer.
We recommend the use of chloroform, ethyl acetate or acetone for washing grids.
- Place 4-5 layers of filter paper in a glass Petri dish.
- Soak the paper with your choice of organic solvent using a glass Pasteur pipette.
- Place the grids, QUANTIFOIL® holey carbon foil side upward, on the saturated filter paper.
- Cover the Petri dish and leave the grids in a fume hood overnight.
- Inspect the grids and repeat as required.
- An additional washing step with deionized water may be desirable to remove any water soluble contaminants.