Cryo-EM Grid Preparation SOP

Guidance for vitrifying a polished sample onto grids, reflecting current best practice for minimizing air–water interface (AWI) damage and ice-thickness variability. Start here, then optimize per construct — grid prep is the single most iterated step in the pipeline.

1. Grid selection & glow discharge

• Default to gold holey-foil grids (e.g. UltrAuFoil, 1.2/1.3 or 0.6/1 hole spacing) over amorphous carbon foil — gold suppresses beam-induced foil movement and improves resolution; reserve carbon foil for cost-sensitive screening.
• For small or beam-sensitive particles, or when preferred orientation is a known problem, consider a thin (~2 nm) continuous support (graphene, graphene oxide, or ultrathin carbon) rather than defaulting to bare holey foil.
• Glow discharge immediately before use (typically 15–60 s, low-pressure air or amine plasma) to render the foil hydrophilic; over-discharging can damage graphene/graphene-oxide supports, so use gentler settings for those.
• Match hole size to particle size and expected concentration — smaller holes thin faster and give more uniform ice for small particles.

2. Sample loading

• Equilibrate chamber to 4 °C (or the temperature the sample tolerates best) and ≥95% relative humidity before loading grids, to slow evaporation during the blot/plunge cycle.
• Apply 3 µL as a starting point and titrate — both concentration and volume affect final ice thickness and particle density; a short dilution/volume screen is expected, not a failure.
• Particles reach the air–water interface within milliseconds of thinning, and the great majority of protein at that interface denatures or preferentially orients there — this is the dominant failure mode in modern grid prep, more so than concentration or buffer per se.
• Where interface damage is suspected (poor particle count, one dominant view, streaked/denatured particles at high resolution), add a mild detergent/surfactant (e.g. 0.005–0.02% amphipol, CHAPSO, or fluorinated octyl maltoside) just before freezing to outcompete protein for the interface, or use a support film (Section 1) to keep particles away from it entirely.

3. Blotting & vitrification

• On a standard blotter (e.g. Vitrobot), start from blot force 0, blot time 3–5 s, single-sided or double-sided blot, and iterate — the ideal ice layer is only as thick as the particle itself, and blot time is the parameter most commonly tuned to get there.
• Minimize the sample-to-plunge dead time: particles begin migrating to the AWI as soon as the film thins, so a shorter blot-to-plunge interval reduces interface exposure for interface-sensitive samples.
• Where standard blotting keeps failing a given sample (excess particle loss, denaturation, or orientation bias), consider a blot-free alternative — self-wicking/self-blotting grids, spray/jet deposition, or a climate-jet system — which cut the dead time between application and vitrification from seconds to milliseconds and avoid the shear of filter paper contact entirely.
• Plunge into liquid ethane (or an ethane/propane mix) held just above its freezing point, not liquid nitrogen directly, to ensure a cooling rate fast enough to vitrify rather than crystallize the water.
• Prepare a small grid of blot-time × concentration conditions per session rather than committing an entire box to one setting.

4. Clipping & storage

• Clip grids under liquid nitrogen immediately after freezing; never allow a frozen grid to warm above the devitrification point (~-135 °C) at any point in handling.
• Label/orient the clip consistently (notch position convention) so autoloader mapping matches your records.
• Store clipped grids in a labeled grid box under liquid nitrogen; log grid ID, freezing conditions (blot time/force, concentration, additive), and screening outcome together so failed conditions aren't repeated blind.
• Avoid unnecessary transfers between dewars — each warm-up/cool-down cycle risks ice contamination and devitrification.

5. Screening before a full session

• Screen on a lower-end microscope or a short session before booking full collection time: check ice thickness across the grid, particle density and monodispersity in holes, and orientation distribution in a quick 2D-class pass.
• A good grid shows thin, continuous ice, evenly distributed monodisperse particles, and more than one dominant view; a single strong orientation is a sign of AWI-driven bias and should be addressed (Section 2) before collecting.
• Ice that is thick and uneven, or holes with few/aggregated particles, usually means revisiting concentration or blot time rather than pushing ahead to full collection.