Western blot supplies: membranes, blotting paper and cassettes

Choosing the consumables and the western blot equipment around them

1. Choose the membrane chemistry for the detection and the target. Nitrocellulose binds well, has low background and is fragile. Polyvinylidene fluoride is mechanically robust, binds more protein, requires methanol wetting and is the usual choice for fluorescent detection and for stripping and reprobing.
2. Pick the pore size from the protein size. A nought point four five micrometre pore suits most proteins; a nought point two micrometre membrane retains small proteins and peptides that otherwise pass straight through. A small target lost in transfer is very often a pore size choice nobody revisited.
3. Use thick blotting paper, cut exactly to the gel. Blotting paper holds the buffer reservoir that carries the current through the sandwich. Thin or unevenly cut paper gives uneven current and patchy transfer, and paper larger than the gel lets current bypass it entirely at the edges.
4. Retire cassettes and pads when they stop compressing. Foam pads compress permanently with use and a cassette that no longer applies even pressure gives exactly the patchy transfer people blame on the gel. Date the pads and replace them; they are cheap and they are consumables.
5. Hold the stock that stops work. Membrane, thick paper, pads, cassettes and blocking reagent for a fortnight. None is expensive and each of them has stopped a blot on a Friday afternoon somewhere.

Proving the transfer happened before you blame the antibody

A reversible total protein stain on the membrane after transfer, photographed and kept, answers the question that otherwise takes a week of antibody troubleshooting. It also gives a total protein normalisation better than most housekeeping proteins.

A prestained ladder visible on the membrane is the quick version of the same check and should be marked in pen immediately, because several dyes fade during blocking.

Wet, semi-dry and dry systems, and the western blot cassette each needs

Wet transfer is slow, gentle and the most reliable for large proteins. Semi-dry is much faster and can dry out on long transfers. Dry systems are fastest and use proprietary consumables, which is a lock-in worth pricing over five years.

Whichever is used, keep one method and one set of consumables for a study. Comparing blots transferred by different systems introduces a variable nobody records.

Western blot blocking buffer and other buffers

Milk is cheap and interferes with phospho-specific antibodies; bovine serum albumin is the usual alternative and costs more. Test both for a new antibody rather than inheriting a choice.

Prepare buffers fresh and label them. A transfer that failed because the buffer was last week's, with the methanol evaporated, is a recurring and entirely preventable waste of a day.

Membrane, and matching it to the protein

Nitrocellulose binds protein well, gives low background and tears. The fluoropolymer membrane binds more, survives stripping and reprobing, needs wetting in alcohol first and gives higher background with some detection chemistries. Neither is generally better.

Pore size matters for small proteins, which pass straight through a coarse membrane during transfer. If a small protein disappears and everything else transfers, the membrane is the first thing to change, not the antibody.

A western blot protein ladder tells you two different things

A pre-stained ladder is for watching the transfer and judging position approximately; its apparent masses shift with the dye attached and with the gel system, which is why two ladders disagree. A ladder of unstained standards run alongside gives a more accurate mass estimate and is invisible on the membrane unless it is detected.

Use the pre-stained one routinely to confirm the transfer worked and pick its range to bracket the target. Where the mass itself is the claim, run an unstained standard and say which was used, because a figure read off a pre-stained ladder is an approximation.

Chemiluminescence or alkaline phosphatase western blot detection: the dynamic range

Enhanced chemiluminescence is sensitive and its signal decays, so quantification depends on catching the right exposure and saturates easily. A fluorescent secondary has a wider linear range, allows two targets at once and needs an imager that reads it. A phosphatase-based substrate is slower and very stable, which suits a faint target and a long development.

Choose the chemistry from what the result has to support. For presence or absence, any of them works; for a quantitative comparison, the linear range and a loading control measured in the same range are what make the number defensible.

Western blot reagents, the western blot incubation box, washing and the background nobody blames

Most background is a washing problem: too few washes, too short, or a detergent concentration that drifted because the stock was made up by eye. Making running, transfer and wash buffers from a recorded recipe, and recording the lot of the detergent, removes a whole class of irreproducibility.

Lysis buffer choice is the other half. A strong detergent buffer extracts membrane and nuclear proteins and destroys complexes; a gentle one preserves complexes and may not solubilise your target at all. Choose it from what you are measuring rather than from what is in the fridge.

When a blot is the wrong assay

A blot gives molecular weight, which is its real advantage, and it is slow, semi-quantitative and hard to run at scale. Where the question is how much of a known protein is present across many samples, an immunoassay in a plate is faster, more quantitative and easier to validate.

Where it is where the protein is, imaging answers better; where it is which proteins are present at all, mass spectrometry does. The blot earns its place when size, isoform or modification state is part of what has to be shown.

Common questions

Nitrocellulose or PVDF?

Nitrocellulose for low background and straightforward chemiluminescence. PVDF for mechanical robustness, higher binding capacity, fluorescent detection and any blot you intend to strip and reprobe. PVDF must be wetted in methanol first.

Which pore size should I use?

0.45 micrometre for most proteins, 0.2 micrometre for small proteins and peptides. If a small target is transferring poorly, the pore size is the first thing to change.

Does western blot paper really matter?

Yes. It is the buffer reservoir that carries current through the sandwich, so thickness and an exact cut to the gel size decide how evenly the transfer happens. Paper overhanging the gel lets current bypass the sandwich.

How do I know a western blot transfer cassette is worn?

Patchy transfer that moves with the cassette rather than with the gel, and pads that no longer spring back. Date the pads when they are first used and replace them on that basis rather than waiting for a bad blot.

Which membrane should I use?

Nitrocellulose for low background on a single probe; the fluoropolymer membrane where you need higher binding capacity or intend to strip and reprobe. If a small protein disappears while others transfer, change the pore size first.

Why do two ladders disagree about a band's size?

Pre-stained ladders carry dye that shifts their apparent mass and behaves differently between gel systems. Use them to confirm the transfer and bracket the target; where the mass is the claim, run unstained standards.

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