Traditional methods of measuring the affinity of lectins (or other carbohydrate-binding proteins) to their target carbohydrate ligand rely on the use of chemically/recombinantly modified proteins in sorbent assays, microarrays or the use of expensive label-free methods such as surface plasmon resonance spectrometry. In this work we exploit the extremely high extinction coefficient (i.e. colour) of gold nanoparticles as resolving agents in sorbent assays. The anionic nanoparticles adhere strongly to immobilized proteins, but not to the carbohydrate-surfaces allowing investigation of protein binding and screening of novel multivalent inhibitors. Furthermore, the use of a simple digital camera (or mobile phone) to obtain the data is shown, providing a simple ultra-low cost route to the detection of unmodified, carbohydrate-binding proteins.
