I did DPN experiments with the DNA solution last Friday. Before my fountain pen device was tested, I checked DPN writing with commercially available AFM tips to see if the DNA ink and Au substrates are fine. Unfortunately, the writing tests with commercially available tips were not successful. First, I suspected that the Au substrate was not good, which sometimes happens. I tried writing with thiol ink, and it turned out to be that the substrate was fine because I could made patterns on it. Secondly, I thought the DNA ink might be the reason. But the ink had been kept in a freezer and stayed at room temperature less than 10 minutes right before the writing experiment. While I kept trying to recoat the AFM tip with the thawed solution of DNA, I found that the solution is very hydrophobic on the surface of the chip (made of silicon nitride) with the AFM tip so that the tip is never coated with the solution. I concluded that molecules never remained on the tip after the dipping, thus no writing at all. I also just remembered that AFM tips for DNA applications need some treatment with oxygen plasma or even chemical coating to change the surface property of the tip to be hydrophilic.