Hi Weiwei, I am not sure that I understand your assay, but I know that linking Abs directly to microbeads without using a spacer arm can denature them and render them useless..

Dear Dr Cui, please contact the dept Chemistry University Twente. typical microbeads loading with Ab is done at several depts.

I m not sure what kind of bead material you are dealing with. The large size seems to be required excessive amount of Ab and relatively longer spacer/linker chains to link them. The weight/density of beads may be also considered (light-weighted ones will be preferred) in my experience.