Given that the biotin-streptavidin interaction is commonly regarded as the highest affinity interaction, the comparable performance of FluoroPerAuNPs was further rationalised based on the availability of fluorescein tags on the particle surface. Using mass spectrometry analysis, the average fluorescein-to-antibody ratio was determined to be 4:1, enabling functionalised particles to present numerous tags across its surface for rebinding by test line receptors (Supplementary Information Figure ).