If your substrate is glass, you can choose to use a functionalized glass slide. NHS slides and epoxy slides should react with amine groups on antibodies straight away.

If I am not mistaken, PDMS will bind organic molecules spontaneously.

It will be helpful if you are more specific. Do you want to fix the antibody everywhere (non-specifically) inside the PDMS chip or you want to fix it in a small area inside the device?

I am going to fix antibodies within PDMS without any particular array. The only thing which is matter is to do this with the highest density possible.