Q-Yangians are deformations of the Yangian and also subalgebras of quantum affine algebras. We will study twisted Q-Yangians as coideal subalgebras of quantum affine algebras, and determine their centers by constructing Slyaninian dterminants. We also find various q-analogs of classical identities for q-Slyaninian determinants, including Jacobi, Schur, Sylvester, MacMahon identities etc. This is joint work with Jian Zhang.