Persisters are a subpopulation of bacterial cells that are refractory to the action of antibiotics. I believe these can be seen even among planktonic cells. You can take a look at "Efficient Killing of Planktonic and Biofilm-Embedded Coagulase-Negative Staphylococci by Bactericidal Protein P128" where studies were done with persisters of CoNS strains.

Hope this information is helpful.

Persisters can also be present in biofilm and planktonic cells when the medium is not frequently agitated to bring them out of dormancy state thereby preventing the staining action or antibacterial activity.

Yes, persisters can be both seen in biofilm as well as planktonic cells and are resistant to the action of antibiotics