Hi Li,
Most people are still using a paper tape called Filmoplast P90 to seal their daguerreotypes. It is convenient to use and the tape itself is archival. The trouble I see, is that the P90 tape is not vapor proof. Air can travel through it, which will eventually result in the formation of tarnish on your daguerreotype, just like the tarnish seen on daguerreotypes made in the 1800s.
Another choice is a clear polypropylene tape called J-Lar which some daguerreotypists use.
I invented my own sealing tape to prevent my daguerreotypes from tarnishing. My tape is vapor proof and prevents air incursion into the daguerreotype package. It also has an additional protective layer that scavenges-out any harmful atmospheric vapors that may have been trapped inside the enclosure when it was sealed. Unfortunately, my tape is not yet available for sale so you will have to use one of the other choices.
Cheers,
Rob McElroy
Buffalo, NY