Streptavidin adsorption and binding to carboxylated chip

Hello everyone !

I am trying to immobilize/bind streptavidin onto a gold carboxylated biochip (equivalent of sensor chip C1 from another distributor, without dextran), and i get a very low level of immobilization after adsorption or amine coupling (+100 RU or less).
The conditions used are a streptavidin concentration of 200 µg/mL, diluted in acetate buffer pH 4,5 or 5.

I saw that this situation seems common, as the question already arose in the forum .

I checked the publication on which is based the protocol detailled in the website (attached file). It appears that the streptavidin binds poorly on the surface by using amine coupling and a 40 µg/mL concentration(less than avidin, explained by the fact that "avidin contains 36 lysines per tertamer; streptavidin contains 12 lysines per tetramer").

Does anyone know a protocol/tips to enhance streptavidin binding on a carboxylated chip (higher concentrations ?) or could provide feedbacks from similar manipulations ?

Thanks