PI of Streptavidin (STA) is about 5.5, buffers with pH 6-7.5 are OK for coating. I usually coate ELISA sripts with STA in 10 mM phosphate buffer, pH:7.5 (overnight 4C or 0.5 h 37C). 10 mM acetate buffer (pH:5.5) also works.

Thanks Mohammad.

the pH is fine, you can use concentrations between 1 µg/mL ans 10 µg/mL. 

You can calculated the concentration needed depending on the latex content (number of particles in your solution), the size of the latex particles resulting in the available surface of the single latex sphere multiplied with the number of particles, expecting the 1 cm² will bind about 100 ng of protein ... or simply try this. Overnight will work much better because of the binding will be realized via hydrophobic interactions using multipoint attachments (to get all available hydrophobic amino acids attached). And this will take some time to get a stable binding.