The efficient manipulation of the optoelectronic properties of layered semiconductors is essential for future applications of these unique materials. Here, we demonstrate that single-layer, large-area graphene can serve as a conductive spacer between an electrolyte solution and single-layer MoS2. In situ Raman and photoluminescence (PL) spectroscopies were employed to monitor the charge transfer from graphene to MoS2. The Raman G and 2D bands were used to quantify the carrier concentration in graphene. The high efficiency of the charge transfer via graphene in a broad carrier concentration range of ±2.1 × 1013 cm-2 was documented by the extreme sensitivity of the MoS2 Raman mode to the electron-doping (shift rate of ∼2.5 cm-1/1 × 1013 cm-2 electron concentration) and the high sensitivity of the PL yield, which drops by more than one and two orders of magnitude in the hole and electron doping regimes, respectively. The easy implementation, and the lithography-free effectiveness of the setup, in terms of the achievable carrier concentration range and the charge-transfer efficiency, could be an asset in near-future research and in the development of optoelectronic devices.