12. Given that $n^{4}-3 n^{2}+9$ is a prime number, find all possible positive integer values of $n$.

 untranslated portion:
 已知 $n^{4}-3 n^{2}+9$ 是素数, 求正整数 $n$ 的所有可能值。

For a more accurate translation, the full text would be:

12. Given that $n^{4}-3 n^{2}+9$ is a prime number, find all possible positive integer values of $n$.

However, if you want to keep the original format and structure, here is the translation:

12. Given that $n^{4}-3 n^{2}+9$ is a prime number, find all possible positive integer values of $n$.

 untranslated portion:
 Given that $n^{4}-3 n^{2}+9$ is a prime number, find all possible positive integer values of $n$.