We demonstrate development of electrochemical nanosensors for planetary health applications using nanocuprous oxide synthesized from recycled materials. Laser-scribed graphene electrodes were enhanced with copper liberated from waste cables, and cuprous oxide nanospheres were synthesized via precipitation at low temperature using lactose as a reducing agent and four different surfactants as capping agents. These laser-scribed electrodes are a low-cost, lithography-free approach to direct synthesis of flexible carbon circuits. Sensors were fabricated by anchoring nanoparticles to flexible graphene electrodes, and then material properties and sensor performance were compared for each surfactant. Surfactant molecular weight and terminal group played an important role in nanoparticle size, band gap, ferromagnetic response, and electron transport. As proof of principle, we show development of catecholamine and mercury sensors for planetary health applications using the best material. Dopamine sensors were linear from 300 nM to 5 μM, with a detection limit of 200 nM, response time of 2.4 ± 0.7 s, and sensitivity of 30 nA μM cm2. Mercury sensors were linear from 0.02 to 2.5 ppm, with a detection limit of 25 ppb, response time of <3 min, and sensitivity of 10 nA ppm–1. The methods shown here are facile, environmentally friendly, and economical. Green synthesis of flexible sensors and electronic devices with recovered waste represents a sustainable approach for next-generation flexible carbon sensors for planetary health applications.