Quick answer
Choose the method by molecular weight. For antigens above 10 kDa, passive adsorption on a High Binding and Medium Binding plates at 3–5 µg/mL works for most indirect ELISAs. Between 1 and 10 kDa, passive adsorption starts to fail, so biotinylate the antigen and capture it on a High Binding Streptavidin plate or couple it covalently. For small peptides and haptens with a reactive group, use covalent coupling: Maleimide surface for sulfhydryl, Carboxylated or Aminated surfaces for amines and carboxyl.
The hard part of coating an antigen is not only the protocol, it is a high molecular weight antigen (>> 10 kDa) and a low molecular weight antigen (<< 10 kDa) behave nothing alike on a polystyrene surface. The large antigen adsorbs and stays. The small antigen washes off, because it lacks the contact area to hold by physical adsorption. This guide suggests the immobilization of the antigen based on its molecular and chemical structure: by size first, then by the functional groups available for binding.
What ELISA plates are coated with, and why size decides the method
In an indirect ELISA the antigen is immobilized on the well surface, then detected by an antibody. How you immobilize it depends on two properties: molecular weight and the reactive chemical groups present in its primary structure.
Above roughly 10 kDa, an antigen has enough surface area to adsorb passively to polystyrene through ionic or hydrophobic interactions. Adsorption flattens the molecule against the surface and masks part of its epitopes, but a protein with multiple epitope sites usually keeps enough of them exposed for an antibody to bind. Below 10 kDa that trade stops working: there is not enough contact area for stable adsorption, and what little binds tends to wash away. That is where affinity and covalent methods take over, holding the antigen by a single defined point and leaving the rest of it free.
Antigens above 10 kDa: passive adsorption and streptavidin
For larger antigens, three methods cover most indirect assays.
Passive adsorption is the default for well-behaved antigens with several epitopes. Use Medium Binding instead of High Binding when the antigen is lipid-rich since high backgrounds may occur due to the hydrophilicity of the HB microplate. Move to a biotinylated antigen on Streptavidin when you need oriented, wash-stable capture, or when passive adsorption denatures the antigen and you lose the conformational epitope. Because biotinylation targets the lysine εNH₂ groups spread across the antigen, place the biotin away from the epitope where the primary structure sequence allows it.
Antigens between 1 and 10 kDa: streptavidin and covalent coupling
This is the range where most coating problems show up, and where the companion methods earn their place. Small antigens, peptides, and haptens need either a high-capacity affinity surface or a covalent bond.
Two points change how you run these. First, with covalent coupling the amount of antigen bound is inversely proportional to its molecular weight, so a smaller peptide loads more densely than a larger one at the same concentration. Second, the covalent surfaces that need EDC activation are post-coated after the antigen is bound, not before, which reverses the usual order of operations.
The functional group drives the surface choice. A peptide with a terminal cysteine goes on Maleimide, and because the bond targets the –SH, you can place the cysteine away from the epitope and keep that epitope exposed. A peptide presenting a free amine goes on Carboxylated; one presenting a free carboxyl goes on Aminated.
Not sure which surface fits your peptide? See all Biomat 96-well microplates, or request a quote with the molecule’s size and reactive groups.
How to choose the method for your antigen
- Above 10 kDa, multiple epitopes, conformation not critical? Passive adsorption on High Binding. Drop to Medium Binding if background is high or the antigen is lipid-rich.
- Above 10 kDa but passive adsorption denatures it or buries the key epitope? Biotinylate and capture on Streptavidin for oriented, gentle immobilization.
- Between 1 and 10 kDa? Biotinylate onto High Binding Streptavidin, or couple covalently through whichever reactive group sits away from the epitope.
- A short synthetic peptide or hapten with a known reactive group? Covalent coupling, matched to the group: –SH to Maleimide, –NH₂ to Carboxylated, –COOH to Aminated.
The payoff of getting this right is measured in failed lots avoided. A peptide that washes off a passive surface produces low signal and high variability that no amount of buffer optimization will fix; a covalent bond holds it through rigorous washing and stabilizes the assay before validation.
Coating buffer and pH
The buffer changes with the method, and this is where antigen coating differs most from the textbook ELISA. Passive adsorption and streptavidin capture run in the mild pH 6–8 range on Biomat surfaces, rather than the carbonate–bicarbonate pH 9.6 that many published protocols use for adsorption on untreated polystyrene. If you are porting a protocol optimized at pH 9.6, re-check the buffer on the Biomat surface rather than carrying it over unchanged.
Covalent coupling uses its own conditions and they are not interchangeable. Carboxylated and aminated surfaces bind in 0.1 M MES at pH 6.0 with EDC, which activates a carboxyl group for reaction with an amine; the plate is then post-coated after the antigen is attached. Maleimide coupling runs in PBS around pH 6.6, where the maleimide ring reacts selectively with sulfhydryl to form a thioether. Above roughly pH 7.5 that selectivity drops, so keep maleimide reactions on the acidic side of neutral. Dispense 100 µL per well throughout, and confirm incubation time, temperature, and storage stability experimentally for your antigen.
What to do when you require a different surface for your application?
Most antigens fall cleanly into the size bands above. The exceptions are antigens that lose a conformational epitope on every surface tried, or peptides where the only reactive group sits right on the epitope. Those need an adjusted surface chemistry rather than another protocol change.
Biomat irradiated and coating in-house and runs pilot batches for custom surfaces, typically 10-20 plates for validation. For the full decision logic across antibodies, antigens, and small molecules, see the microplate surface selection guide, or read the companion guide on how to coat an antibody instead. To match a surface to a specific antigen, request a quote with the molecule’s size, reactive groups, and detection method.
Not sure which microplates surface best fits you needs?
Talk to our R&D team
References
- Engvall E, Perlmann P. Enzyme-linked immunosorbent assay (ELISA). Quantitative assay of immunoglobulin G. Immunochemistry, 1971. (Foundational ELISA method – link at publish, target=”_blank” rel=”noopener”.)
- Hermanson GT. Bioconjugate Techniques, 3rd ed. Academic Press. (Standard reference for EDC/amine, maleimide and streptavidin–biotin coupling chemistry – link at publish.)
Biomat is a specialist in surface modification and custom plate engineering, delivering high-performance microplates and tailored plastic consumables for demanding diagnostic and research applications. Produced under an ISO 9001:2015 quality management system. Made in Italy, since 1994.
