In-line chemiresistors based on multilayer graphene for cadmium dication sensing in water

We report that multilayer graphene exfoliated from graphite at sufficiently low point-defect density, and intercalated by poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (PEG-PPG-PEG) block copolymers of appropriate chain length, functions as a sorbent and detector for cadmium dications in water, which remain trapped within the graphene interlayers, thus changing the device's electrical resistivity. Graphitic samples at different defectiveness - synthetic graphite and high-quality natural graphite - are exfoliated by PEG-PPG-PEG at different chain lengths (M-n approximate to 1100-14,600 amu). The resulting multilayer graphene cakes are incorporated into gap-cell chemiresistors assembled onto microporous supports. The resulting devices are nanoporous and can be inserted in-line in water filtration systems. We have monitored their resistivity as a function of the concentration of specific metal dications (Cd2+, Hg2+ and Mn2+) in the water passing through the devices. Although dication concentrations (of the order of a few ppb) are too low to directly affect the solution ionic conductivity, the solid-state resistivity of our graphene-based chemiresistors decreased at increasing Cd2+ concentration, while the effect was less significant for the other types of ions. We attribute this to Cd2+ intercalation within graphene stacks. It appears that Cd2+ possesses the optimal hydrodynamic radius for being trapped within graphene layers intercalated by PEG-PPG-PEG of appropriate chain length, leading to enhanced Cd2+ sensing. Our chemiresistors are of low cost and can be easily computer controlled.