PEEK is considered a high-performance polymer but is not typically used in staining applications due to its high cost. In typical staining applications, glass or less expensive plastics are utilized. There is no available information on why the F-ctalin phalloidin labeled with TRITC would interact with PEEK, nor is there information on whether switching to an ATTO 565 would eliminate the problem related to the interaction of the PEEK for the same application.
Another consideration might be whether a coating was applied to the PEEK. There have been cases in the past where a particular coating would provide a high background when viewed under either a light or fluorescent microscope. Glass slides and plastics are sometimes coated with a substance that provides a positive charge to the slide. If cells and the solid substrate both have a negative charge, the tissue will not remain attached to the solid surface during staining. While a light coating is typically applied, if the amount of coating is too thick, it could cause a high background on the slides when viewed with either a light or fluorescent microscope. It is advisable to check the PEEK surface with no tissue or cells attached, as a complaint on a high background observed in the past was originally thought to be due to the coating on the slide but turned out to be from the slides themselves.