Quartz crystal microbalance determination of trace metal ions in solution
This thesis describes development of a new practical acoustic wave and electrochemical sensor for detection of trace metal ions from aqueous solutions using self-assembled monolayer modified electrodes. The gold electrodes of 10 MHz AT-cut piezoelectric quartz crystal resonators were modified with different self-assembled monolayers (SAMs) with --COOH, -SH and --NH free terminal functional groups. Frequency measurements were carried out on emersed and immersed crystals to determine ligand immobilisation. Cyclic voltammetry investigation of [Fe(CN)6] 3-/4-redox chemistry was carried out on the SAM/Au electrodes to investigate the quality and integrity of the SAM. Analysis of this data has lead to a qualitative explanation of electron transfer process involved in the cycling. These SAMs were employed for the detection of target metal ions such as Cd(II), Pb(II), Hg(II) and Cu(II) from single metal ion solutions, based on frequency measurements associated with the binding of different concentrations of metal ions from solution to each surface-bound monolayer. Data for emersed and immersed crystals were used to estimate the amount of metal ions bound to the SAM. Results for the mole ratio of metal ion:SAM were used to select the SAMs having the highest degree of sequestration. The uptake of metal ions from aqueous solution was followed as a function of concentration and attempts made to fit the data to the Langmuir, Temkin, Freudlich, Frumkin, El-Awady and Flory-Huggins isotherms. The relative merits of these isotherms are discussed and heterogeneous binding constants extracted. Selectivity for different metal ions was assessed by estimating the binding constant for target metal ions in single and mixed metal ion solutions.