give group permissions to specific device

I have a panning web camera I built, (future robot arm). The camera pans via a web interface (cgi scripts and JS interface) which access an arduino. To enable sending of commands via the interface, I added www-data (web-server user group) to the dial-out group that the arduino depends on.

Ah, well, I realize this is not what I want to do, and in very insecure! What i want to do, is give www-data group users permission to one device, /dev/ttyACM0

how can I do that, and keep the apache user out of the dial-out group??