Electroanalytical studies

This thesis focuses on electroanalysis using underpotential deposition (UPD) with anodic stripping voltammetry (ASV). This method is developed for arsenic, which shows visually clear signals for sub 10 ppb concentrations in water. In this method arsenic is deposited as As(0) ad-atoms onto the surface of metal electrode, and the ad-atoms are stripped off with a linear potential scan and the Faradaic stripping signal using for quantitative analysis. The novelty and strength of this method is in its use for sub-monolayer UPD rather than bulk deposits as in previous ASV work both generally and specifically for As.
The fundamentals of electrochemistry and the principles of electrochemical methods are introduced in Chapter 1. The need for arsenic detection and quantification arising from environmental contamination by arsenic is discussed in Chapter 2. Chapter 3 summarises the generic chemicals, reagents and instrumentation used in this thesis.
Chapter 4 reports the reliability of drop cast techniques since electrodes modified with small amounts of nanoparticles are subsequently considered for As detection. The investigation is made with reference to Pt nanoparticles and reviews methods to reduce the ‘coffee ring effect’ generated during the drop cast, where most of the Pt nanoparticles were seen to be accumulated at the edge of the surface of electrode and only small amounts of the nanoparticles were distributed at the centre of the electrode. The key conclusion is that the formation of a ring-like pattern did not affect the overall linear diffusion to the electrode since both types of surface give plausible voltammetric responses unless only tiny amounts of nanoparticles are used.
Chapter 5 reports electroanalysis of arsenite, As(III), in water on Pt macroelectrodes and Pt nanoparticle modified glassy carbon electrodes via the underpotential deposition with anodic stripping voltammetry. No interference was seen from Cu(II) at realistic concentrations but high concentrations of chloride inhibited the deposition of the As ad-atoms. A visually clear detection limit was recorded at 4 ppb, suggesting that this method has practical value noting the WHO limit of 10 ppb for safe drinking water. However, many samples will contain either unknown or high levels of chloride so attention was turned to the use of gold as a substrate for UPD deposition to overcome this problem whilst retaining the benefits seen for the UPD-ASV method, notably in respect of being free from interference by Cu.
Chapter 6 investigates the electroanalysis of As(III) on Au macroelectrodes and Au nanoparticle modified glassy carbon electrodes via underpotenital deposition with anodic stripping voltammetry. The deposition was optimized for the gold surfaces and visually clear detection limits were found to be 0.8 ppb and 0.4 ppb on the Au macroelectrode or Au nanoparticles, respectively. A particular merit is that detection on Au substrate can avoid the interference from both Cu(II) and Cl(-I) in contrast to what is seen for bulk deposition or on a Pt substrate.
Chapter 7 evolves further the work reported in Chapter 6 which was exclusively focused on As(III). However, arsenate, As(V) is also common in groundwater with the contamination of arsenic. In this chapter, the total arsenic was measured without the need for an extra chemical reduction step of As(V) to As(III) as in previous work. The As(III)/As(V) speciation was found by tuning the deposition potential during the pre-concentration step. This method was used for total arsenic detection in water with a deposition potential at – 1.3 V, and As(III) can be selectively deposited at – 0.9 V, which means that the As(III)/As(V) speciation is possible. A visually clear signal of 0.8 ppb was obtained for the detection of arsenic with this method.
Overall the UPD-ASV approach using gold macroelectrodes is found to be suitable for the laboratory based analysis of As contaminated water samples, giving a LOD and visually clear signals at concentrations substantially below the WHO limit and free from interference either by chloride or copper ions.

Advisor: Compton, R