Quantized W-algebra of ${\frak sl}(2,1)$ : a construction from the   quantization of screening operators
  Starting from bosonization, we study the operator that commute or commute up-to a total difference with of any quantized screen operator of a free field. We show that if there exists a operator in the form of a sum of two vertex operators which has the simplest correlation functions with the quantized screen operator, namely a function with one pole and one zero, then, the screen operator and this operator are uniquely determined, and this operator is the quantized virasoro algebra. For the case when the screen is a fermion, there are a family of this kind of operator, which give new algebraic structures. Similarly we study the case of two quantized screen operator, which uniquely gives us the quantized W-algebra corresponding to $sl(3)$ for the generic case, and a new algebra, which is a quantized W-algebra corresponding to ${\frak sl}(2,1)$, for the case that one of the two screening operators is or both are fermions. 
Jintai Ding, Boris Feigin