The method of conjugation is a critical element in the development and manufacture of any lateral flow assay. The potential conjugation technologies have grown over recent years as the demand for quantitative and more sensitive assays has increased. In this blog, we explore the options open to you.

lateral flow rapid test using gold at its 2 test lines

The first lateral flow assay

The first commercially available lateral flow assay, Unipath’s Clearview home pregnancy test, was launched in 1988 and used blue latex as the signal reagent, generating a blue coloured line. Whilst latex continues to be used effectively in a number of lateral flow assays, innovation has led to a range of alternative options being available including gold, silver, platinum, carbon, fluorescence and magnetic.

Which label to use?

The decision as to which label to use as a signal reagent depends on a number of factors:

  • Whether the test is to be read visually
  • If the test is intended to be used with a lateral flow device reader
  • If the test is to multiplexed and needs a clear differentiation between lines
  • Enhanced sensitivity requirements
  • Ease of scaling up for manufacturing

For visually-read assays the detection particle must be large enough to be seen but not so large as to overwhelm the antibody (or antigen) conjugated to its surface through steric hindrance. These particles usually run from 10-100 nm in diameter, but there are always exceptions.

Gold, Coloured Latex and Carbon are commonly used for visual assays.

Gold nanoparticles are popular due to their high affinity for antibody binding, relatively low antibody usage, and good stability in optimised assays. A variety of sizes are available and gold can be read both visually and is compatible with most reflectance, absorbance and camera system lateral flow readers. Binding of gold to proteins other than antibodies can require optimisation as instability can be seen in un-optimised assays.

Latex is flexible as both covalent coupling and passive conjugation are possible and is increasingly available in a wide variety of colours which lends itself as a good choice for visual multiplexing. Latex is available in a range of sizes (100nm - 300nm) and can be manufactured in large volumes, the required concentration for conjugates can be higher than gold – usually, mg/ml but overall costs can be similar. But latex can show batch to batch variability and due to the size of latex particles, flow up the strip can be more problematic and membranes can require treatment to improve flow.

Fluorescent lateral flow assay labels are becoming increasingly popular and a large range are commercially available. Fluorescence can offer advantages in sensitivity and lend themselves to higher end quantitative and multiplexed assays. Fluorescent assays are dependent on reader systems for device interpretation as there is no visual signal.

A more recent choice for a label is carbon nanoparticles, carbon assays are easy to interpret and is a relatively cheap option, its use is likely to increase in popularity in cost-sensitive applications.

Advice on lateral flow assay labels and conjugation

The key message is to seek advice early when it comes to lateral flow assay labels and conjugation. Each potential label choice comes with its own set of advantages and disadvantages and the label choice needs to be fully aligned with the specification of the lateral flow assay being developed.

Abingdon Health has significant experience of developing assays with a wide variety of labels, either for qualitative or quantitative applications and can offer this objective expertise as part of your contract development project.





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