I do this kind of experiments with AuNPs and streptavidin. I employed a simple incubation method. As far as I know, the gold layer should be the same as Au nanoparticles. I mean that in order to obtain a self assembled monolayer, the  incubation process should be adequate. I think what you need to do is to coat streptavidin on gold layer followed by incubating in 37C for almost 1 hour. 

I did this kind of streptavidin and bioting binding studies on silica plate for biosensor fabrication. Simply you can disperse the streptavidin with suitable concentration in buffer solution and spread over 50nm Au coated plate as your mentioned. And then you follow an incubation in 37C for an hours. Then you go for imaging in AFM for the confirmation of surface modification.

The secret of assembling is to know how nanoparticles can interact with biomolecules such as streptavidin which is the electrostatic interation.

The pH values is most important parameter

This article maybe useful

https://www.researchgate.net/file.PostFileLoader.html?id=54f790b8f15bc775018b456e&assetKey=AS%3A273722710134784%401442271987189

Why not put the biotin on the gold? Creating a mixed SAM of for example thiol-PEG-Biotin and thiol-PEG. 

Streptavidin/biotin should be immobilized onto the sensor chips surface as opposed to just  being adsorbed onto a plan gold surface as the non-specific binding will be significant.  You can create a sef-assembled monolayer on the gold surface using an alkanethiol that is derivatized with a COOH group, which you can then covalently immobilize streptavidin to using EDC/NHS amine coupling chemistry.

I was looking for Streptavidin protein to order. I found out that the molecular weight is different from two different companies like 13500 da and 60000 da. Which one is more suitable for SPR experiment in order to form Streptavidin/biotin interaction.