I am doing the optimization of Octet assay condition. Some of my koff ranking experiments need to load antigen on sensors and use supernatant as the analyte(such as same tag,or biotin-Ag) . But almost all the sensors had non-specific binding to the supernatant,except ProA(just a little bit).

1.How is proA different from other sensors? I thought all the sensors are modified from SA sensors. Am I right?

2.Does anyone have an idea to remove the non-specific binding(fig1.)? I tried to block with biotin(fig.2),but still can observe the non-specific binding signal and I cannot even see the binding signal at all! Please see the figures below. The sensor is HIS1K.

Thank you all.