Master of Science
As the global population increases, the need for drinking water is an ever-present concern. Water contamination from cadmium needs to be avoided due to the damaging effects of Cd dications to the human health. The Environmental Protection Agency (EPA) has set an extremely low permissible limit for Cd2+ in water, at 5 parts-per-billion (PPB), which poses extreme challenges towards Cd2+ detection and retention in specific water purification devices. This has led to a rush in the development of low-cost and scalable devices with in-line Cd2+sensing capabilities, with the ability to simultaneously purify water and monitor its quality, as opposed to less effective off-line monitoring systems that cannot be integrated in water purification apparatuses. The goal of my thesis is to fabricate, and explore for sensing purposes, porous membranes assembled from novel two-dimensional (2D) materials. Our goal is to integrate these membranes into nanofiltration devices previously developed by Fanchini’s group, with the objective to selectively detect Cd2+ in concentrations at or below the EPA’s limit. In this thesis, membranes assembled from two distinct 2D materials have been incorporated into a computer-controlled chemiresistive sensing platform I have specially designed for this work: (1) multilayer graphene platelets exfoliated from low-defect hydrothermal graphite using block copolymers, and (2) L-cysteine functionalized molybdenum disulfide (MoS2) platelets. Platelets of 2D materials have large surface areas. Adsorption of metal ions onto their surfaces during water permeation alters the electronic properties of 2D electron gases confined in these materials, thus affecting their electrical conductivity. More critical is to attain these effects only when Cd2+, not other ions, are absorbed. Selective electrical conductivity changes due to Cd2+ absorption has been observed in this thesis, both in block-copolymer exfoliated multilayer graphene (down to about 1 PPB) and L-cysteine functionalized MoS2 (down to 0.1 PPB). In the first case, Cd selectivity has been obtained by adjusting the distance between graphene layers at values matching the hydrodynamic radius of Cd2+. In the second case, selectivity to Cd is assigned to the preferential attachment of Cd2+ to L-cysteine functional groups, which alter the density of electronic gap states in MoS2. Both types of chemiresistive sensing membranes perform at concentrations below the EPA’s permissible limit for cadmium and can be easily integrated into water nanofiltration devices.
Summary for Lay Audience
Due to the enormous use of cadmium in industry, as well as natural sources of cadmium contamination such as volcanoes, cadmium is often found in water supplies and ecological systems around the world. Cadmium has negative health effects on human populations by causing substantial lung, and renal damage, bone demineralization, and even brain damage in children. Current methods of detecting and capturing cadmium are expensive, off-line and require trained personnel. In order to address these issues, we looked at two different 2D materials, (1) graphene and (2) molybdenum disulfide. In chapter 3, multi layer graphene was exfoliated allowing the graphene flakes to be help in suspension for long periods of time. The exfoliation process was found to change the spacing between layers of multi layer graphene. This was found to be ideal for capturing and sensing heavy metal ions, specifically cadmium. The apparatus has a chemically resistive (chemiresistor) design and is simple and easy to manufacture without the need for on site trained personnel. In chapter 4, we looked at utilizing exfoliated MoS2, which has been functionalized with L-Cysteine, an amino acid commonly found in biological systems. Due to the high affinity of heavy metal ions towards L-Cysteine, functionalizing MoS2 allows for the selective capture of cadmium dications.
Van Middelkoop, Sheldon, "Porous membranes from 2D materials and their integration into chemiresistive sensors for in-line detection of cadmium in water filtration devices" (2019). Electronic Thesis and Dissertation Repository. 6359.
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