Nanostructured surfaces for sensing heavy metals and radionuclides in aqueous systems

An interdisciplinary approach was used to engineer a range of nanostructured surfaces with specificities for a number of radioactive waste contaminants that are typical of low level legacy contamination at sites across the UK. Specifically, analytes 238/234UO22+, 90Sr2+ and 137Cs+.

Because traditional mercury based electrochemical methods lacked the specificity to differentiate individual analytes from complex solutions, combining electrochemical impedance spectroscopy with receptor molecules of desired specificity was used to create sensors with specificity and sensitivity to these radionuclides and which could give responses in real time.

For UO22+, combining the evolved uranyl sequestering ability of the surface layer protein (SLP) from the bacterial strain Bacillus sphaericus JG-A12 allowed sub nanomolar levels of uranyl to be monitored in real time using the capacitative component of impedance at low frequency, whilst being largely selective against contaminant divalent cations. Whilst this approach, based on two tethering mechanisms for the SLP, has potential for fabrication of bespoke biosensors for other metal analytes, because of the lack of available metal binding proteins with appropriate cation specificity, alternative synthetic hosts were investigated as receptors.

A number of macrocycles, including crown ethers and lariat ethers with specificity to either 90Sr2+ or 137Cs+ were obtained commercially and chemically modified, or chemically synthesised, to permit surface tethering to sensor electrodes. Combining these tethered synthetic hosts with real time cyclic voltammetry, electrochemical impedance and microgravimetric interrogation methods revealed some interesting interfacial phenomena and gave insight into host interactions with the electrode surface. However, this approach did not yield devices that could be used to empirically quantify binding 90Sr2+ or 137Cs+.

Accordingly, an alternative approach of combining engineered surfaces that had high surface areas and metal ion specificity, with direct quantification of analyte by  or  counting of the captured radionuclide was investigated using  counting for 90Sr2+ and counting for 137Cs+.
This allowed differentiation and quantification of 90Sr2+ or 137Cs+ ions from their non-radioactive isotopes which exist naturally and abundantly in the environment. Functionalised silanes allowed metal chelator functionality to be deposited onto bulk silica surfaces, polymeric nanofibres and silica nano – and micro- particles. Whilst the bulk surfaces and nanofibres were able to bind significant amounts of isotopes, at very low level analyte concentrations typical of contaminated ground water, reproducibility between batches was poor.

However, the functionalised nanoparticles performed well, binding significant amounts of radionculide and exhibiting high saturation limits. They were also were able to bind low levels of radionculide in complex analyte solutions such as synthetic groundwater. Bifunctional chelators particles were also made which allowed simultaneous deposition of a solid scintillant. This allowed simultaneous binding and quantification of radioisotopes without the need of scintillation fluid for counting of -particles from 90Sr2+.