When activating carboxylic acids on surfaces, I just used water. I think you can use any buffer so long as the buffering agent doesn't contain -COOH or -NH3 groups.
as I just found out pH 8.9 might not be the best idea, as it says the higher the pH the lower stable is the NHS ester. ("Although prepared NHS or Sulfo-NHS esters are
sufficiently stable to process in a two-step reaction scheme, both groups will hydrolyze within hours or minutes, depending on water-content and pH of the reaction solution. (NHS esters have a half-life of 4-5 hours at pH 7, 1 hour at pH 8 and only 10 minutes at pH 8.6.)) Thermo Fisher Scientific Inc. Maybe the file attached can be helpful.