This may sound rather selfish in Ai's side but I think she can create a permanent antidote for Conan considering that she didn't actually lose the data about the APTX. If the data is there, then most probably, with her knowledge could synthesized an antidote- a permanent one for their convenience. However, she deliberately lied about the data and so let us wondering for what reason. Personally, I think she doesn't want Conan to turn back to Shinichi just yet. Two possible motives might cover the supposition. One, she wants to protect Conan and herself from the organization. Or, she doesn't want him to be Shinichi once again as it would mean that he finally has a chance of being with Ran which if viewed by most of us would be quite difficult for Ai seeing that she must like Conan.