Although nanoscale sensors provide valuable data at finer levels, their size limitations impose difficulties in designing nanoscale systems. The limitations in the hardware part of such systems consequentially affect the software part too. Routing, being one of the main functions of the communication software of nano-sensors, it requires customised design, where constraints such as the energy efficiency have to be considered. Knowing that communication is the most energy consuming operation for the sensor networks, in general, the importance of routing protocol becomes clear. With this motivation, the existing routing protocols for the wireless nano-sensor networks are reviewed. The presentation of the classification and comparison of the existing protocols, followed by the discussion on the proposed simple energy-aware routing protocol based on the backward-learning' paradigm. Simulations are carried out in ns-3 by using Nano-Sim package. Results are listed to show the efficiency of the proposed method in terms of energy use and the packet statistics.