Since, Cas9 separates the strands anyway before cleaving, and the activity of Cas9 is dependant on gRNA -target binding, i guess it is possible for Cas9 to cleave ssDNA.

It has already been demonstrated that Cas9 can cleave ssRNA as well! So it is reasonable to assume that it works just as well for ssDNA.

See attachment for ssRNA activity.

Article Programmable RNA recognition and cleavage by CRISPR/Cas9

And it is not that all Cas9 bind based NGG PAM specificity, the S. pyogenes Cas9 recognizes NGG. There are other Cas9 enzymes with different PAM specificity.

Found a paper on ssDNA cleavage activity of Cas9!

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0068708#pone-0068708-g005

Thanks very much, subin~

SpCas9 could cleave complementary ssDNA in vitro. You could found the result in Fig.1B of this paper, which is named "A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity".