The ZP depends on the pH. For nominally positively charged material in prepared form, then there is the NIST SRM 1980. These can be ordered online. It’s technically an electrophoretic mobility standard.

thanks a lot! I have forgot to mention that i would like positive zeta at pH 6-7

@ BTW They are not ‘nano’ at 300 - 700 nm... Definition of nano is typically < 100 nm...