Will phosphorothioate (PS) modifications on the ends of PCR primers provide nuclease resistance, and thus result in a longer half-life for the PCR product when transfected into a cell?

Phosphorothioate bonds (PS bonds) increase oligonucleotide resistance to exonuclease degradation. For single-stranded oligos, nuclease-resistance is highest when every phosphodiester bond is changed to a PS bond.

In general, we recommend at least 3 PS bonds on both 5' and 3' ends of the oligo, though more PS bonds will give greater exonuclease resistance.

Note, that PCR primers with PS bonds at both ends will still result in double-stranded (ds) PCR products with PS bonds only at the 5' ends of each product strand. While protected from 5' to 3' nuclease activity, because these dsDNAs lack PS bonds at the 3' strand ends, exonucleases with 3' to 5' activity will still be able to degrade them.