We discuss the development of aptamer functionalised, back-gated, graphene field effect transistor (GFET) biosensors for the targeted detection of Lead (Pb²⁺) ions. The widespread existence of the heavy metal Pb²⁺ in the environment is a severe threat to the health of humans. It is a neurotoxin that accumulates over time in the body restricting the cognitive, behavioural and psychological development of children along with causing irreversible harm to the human foetus. New biosensors which allow for the rapid, sensitive and low-cost detection of Pb²⁺ are required to monitor this toxicant in water sources worldwide. The GFET devices were fabricated using a scalable photolithographic patterning process with evaporated Cr and sputtered Au contacts over monolayer graphene on Si/SiO₂. The single stranded Thrombin Binding Aptamer (TBA) was immobilised onto the graphene channel either directly with a pyrene terminated 5’ end or indirectly using the 1-pyrenebutanoic acid succinimidyl ester (PBASE) molecule to facilitate DNA crosslinking with an amine modified 5’ end. Herein provides an evaluation of these two immobilisation strategies for the detection of Pb²⁺. Functionalised states were verified using Raman spectroscopy and electrically characterised using 4-probe electrical measurements to determine transfer curves allowing the calculation of field effect mobility and Dirac Point characteristics.