Can monoclonal antibodies work only if the target of it is located in the cell's surface? or it can be used to targeting a protein located inside the cell?
Monoclonal antibodies (or antibodies in general) are large proteins (around 150 kiloDaltons according to Wikipedia), so cannot cross the plasma membrane. So monoclonal antibodies target extracellular molecules or cell surface molecules. However, there are techniques to target proteins inside a cell, like lysing the cell, or using gene modifications, or otherwise a technique covered in this paper: https://www.tandfonline.com/doi/abs/10.1517/147125184.108.40.206