Spherical gold nanoparticles are the most commonly used agent for lateral flow assays (LFAs). They strongly absorb the blue/green/yellow part of the visible spectra and give a nice red appearance. The question then becomes, which size do you choose?
Is there a "right" size?
Small diameter nanospheres (<20nm) are not as efficient absorbers as larger nanospheres and will provide slightly lower contrast than larger spheres. Furthermore, small spheres have a lot more surface area than equally absorbing larger spheres, requiring more targeting moeities (e.g., antibodies, proteins, peptides, ligands, affibodies, etc.) to adequately cover their surfaces for molecularly-specific capture (increasing your operation costs). On the other hand, large nanospheres (>60nm) tend to scatter more light and therefore will appear lighter (lower contrast) in appearance on the white background found on most LFA test strips.
Although both small and large gold nanospheres could be used, most LFAs utilize gold nanoparticles with diameters between 30-60 nm, with the most common being 40 nm.
The importance of monodispersity and low variation
No matter what size you choose, the best results are obtained with nanoparticles with high monodispersity (low size variation, resulting in a strong consistent color) and a manufacturer low batch-to-batch variation (so your bioconjugation and assay contrast remains consistent and repeatable). More details here.
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