Until you post the original source procedure, my answer is going to be generic.
When I am repeating an experiment I found in the literature, why do I need to do [insert step that seems unnecessary, difficult, or just plain long]?
The short answer is, because it works.
You need to incubate the slides at 60 oC for 18 hours because that's what the author of the paper found to be necessary to guarantee that the slides were properly PEGylated. It's possible that the author of your procedure is also using a procedure that he or she found in the literature, and the the rationale for the specific incubation procedure will require some digging.
The longer answer is, the reaction probably needs some incubation to run to completion. The reaction between the PEG and the ITO surface is not instantaneous. If you want the coating to be good, you need to give it time to cure, and mild heating can accelerate the process. While 18 hours seems like a long time, it would likely take interminably longer at room temperature. Could you heat at a higher temperature for less time? Probably, but I would wait until you are certain you can make the existing procedure work before messing with it. That way you have some successfully coated slides to compare to.
The 18 hour period has one of two origins. The second is more likely.
- The developer of the procedure maximized the efficiency of the process: He/she carefully experimented with the conditions until he/she discovered the exact time/temperature combination that maximized the efficiency of the reaction - getting the best coating in the shortest time.
- The developer of the procedure maximized the convenience of the process: He/she determined 1) that the coated slide needed to cure for several hours (3 or 4) at 60 oC, 2) that higher temperatures were detrimental to the quality and integrity of the coating, and 3) prolonged heating at 60 oC is not detrimental. In this case, the researcher puts the slides into the oven in the afternoon to cure overnight and picks up the cured slides the next morning. 4 PM on Monday afternoon to 10 AM on Tuesday morning is 18 hours. As a synthetic chemist, I often wrote down '18 hours', when I really meant 'overnight'. The reaction may have been done at 6 hours or 8 hours, but 18 hours is more convenient. I set up the procedure one afternoon a few hours before I left for the day so that I could make sure it was stable. I worked it up the next morning.