The sensing properties and underlying sensing mechanisms of CuO-TiO2 p-n type core-shell (C-S) nano-wires (NWs) were investigated as a function of the TiO2 shell thickness. An extraordinary p/n transition in the sensing behavior was found to depend on the shell thickness: p-type CuO-core NWs dominate the sensing type up to a shell thickness of 40 nm, likely through a tunneling effect, but for thicker shells, n-type TiO2 shells dominate. In C-S NWs with thin shells that allow gas molecules to interact with the CuO-core NWs, less resistance modulation is developed due to the compensation of both n- and p-type conduction from the two materials. In contrast, in the case of shells that are too thick, partially depleted n-type shells deteriorate the resistance modulation. The results indicate that the shell thickness should be controlled in between these two edge cases to attain optimal sensing capabilities of the CuO-TiO2 p-n C-S NWs: thick enough to prevent the interaction of gas molecules with the core NWs, while simultaneously thin enough to achieve high electron depletion. (C) 2018 Elsevier B.V. All rights reserved.