Electroanalytical investigations of bismuth electrodes and silver nanoparticles

Abstract

Bismuth bulk electrodes (BiBEs) have been suggested as possible replacements for mercury electrodes in electroanalytical studies in part because they are simply prepared by melting bismuth powder in a glass capillary and also because of the relative lack of toxicity of Bi. The double layer of BiBEs in aqueous media was studied using electrochemical impedance spectroscopy (EIS). The differential capacitance of BiBEs was determined in three aqueous electrolytes: sodium nitrate (NaNO3), sodium bromide (NaBr) and sodium chloride (NaCl) as well as the nonaqueous electrolyte LiClO4/acetonitrile (AN). Comparative measurements were made with a polycrystalline platinum electrode. Up to 43 µF cm-2 were recorded for the double layer capacitance at the BiBEs in the aqueous electrolytes, while more typical capacitance values of <20 µF cm-2 were obtained for the Bi|AN/LiClO4 interface. Combined investigations by EIS and x-ray photoelectron spectroscopy (XPS) suggest the high values of capacitance in the aqueous electrolytes are due to pseudocapacitance effects, owing to adsorptions of bromide and chloride ions as well as the formation/reduction of a bismuth(III) oxide film at the electrode surface. The capacitance values of the Bi|AN/LiCO4 interface are consistent with the standard Gouy-Chapman-Stern model; ClO4- anions are thought to be weakly adsorbing in non-aqueous media. The EIS measurements also enabled the determination of the potential of zero charge PZC of -0.49 V versus Ag/AgCl at BiBEs in the aqueous electrolyte mixture of NaNO3/NaCl. The differential capacitance studies provided an understanding of the nature of the BiBE interfaces required to interpret electron transfer measurements. Several redox couples were investigated by slow scan cyclic voltammetry (CV): ruthenium hexaammine, methyl viologen, sodium anthraquinone-2-sulfonate monohydrate, methylene blue, toluidine blue, hexaamminecobalt(III) chloride and cobaltocenium hexafluorophosphate. Many of these couples showed complex behaviour at Bi; either due to Bi oxidation or a lack of chemical reversibility and possible complications due to adsorption. However, ruthenium hexaamine showed reversible, uncomplicated CVs at the BiBE/KCl(aq) interface and therefore was selected for detailed study of the electron transfer kinetics by EIS.The standard rate constant (ko) and electron transfer coefficient (α) were determined for the outer-sphere one-electron transfer process for ruthenium hexaammine trichloride Ru(NH3)6Cl3 at BiBEs in KCl(aq) as supporting electrolyte. In contrast to previous work on viologen derivatives in AN in the literature, there was a marked difference between the ko values at BiBEs and Pt electrodes- the voltammetry and impedance spectra were found to be reversible at Pt. We ascribe this difference to the presence of a thin oxide layer on the BiBE at potentials near the standard potential for ruthenium hexaammine. Despite the presence of such an oxide layer, repeatable impedance spectra could be obtained for the system and the expected linear dependence of the charge transfer resistance on [Ru(III)] was observed. Further, the (dc) potential dependence of the EIS data allowed the determination of the potential dependence of the transfer coefficient. This data provided direct evidence of the importance of double layer corrections to ko because of the rapid variation of α observed near the potential of zero charge. In summary, BiBEs show somewhat complex behaviour in aqueous media; many redox couples cannot be easily studied because of the susceptibility of Bi to oxidation and complex adsorption effects, not well understood. However in the case of ruthenium hexaammine, precise voltammetric data can be obtained even though photoemission spectra and differential capacitance data indicates the presence of a thin oxide film. ii

The second part of the thesis concerns the detection of Ag(I) ions released by corrosion of silver nanoparticles (AgNPs) in aqueous media. Neither BiBEs nor Pt electrodes were found to be suitable for the detection of Ag(I), however straightforward anodic stripping voltammetry (ASV) at glassy carbon electrodes was successful. AgNPs were synthesized by the citrate reduction method and dialysed either in pure water, or different concentrations of chloride and sulphate to examine the effect of the medium on the release of Ag(I) ions. The importance of these studies relate to the fate of AgNPs in the environment; AgNPs are now widely employed for their antimicrobial activity, however it is not clear what their eventual fate is nor how much Ag(I), the putative agent is released. The experimental technique involved dialysis of the initial AgNP preparation against a particular aqueous medium and (i) analysis of the [Ag(I)] released from the dialysis membrane into the external medium and (ii) characterization of the aliquots of AgNPs remaining inside the dialysis membrane. Optical absorption spectra showed a redshift of the AgNP plasmon band throughout the dialysis, consistent with aggregation of the NPs. This is unexpected based on simple DLVO stabilization as the reduction in ionic strength against water should disfavour aggregation. However it was confirmed by dynamic light scattering (DLS) and atomic force microscopy (AFM) of drop-cast aliquots and probably arises from loss of citrate ligands. Release of Ag(I) ions was monitored by anodic stripping voltammetry at glassy carbon electrodes. The ASV data was calibrated by standard addition and indicated the presence of about 90 µM of Ag(I) ions in the initial preparation. Over time, the [Ag(I)] decreased until it reached a steady-state value of the order of a few µM. Unexpectedly, a similar steady-state concentration was observed in chloride or sulphate containing media. The presence of chloride does indeed reduce the concentration of Ag(I) in the initial preparation (to a value controlled by the solubility product of AgCl), however in that case a strong decrease in [Ag(I)] throughout the dialysis was not observed. A concentration of about 4 µM was still detected after 73 h of dialysis. This effect is interpreted in terms of the decrease in the electrode potential for the Ag/Ag(I) couple in the presence of Cl-; we suggest that the steadystate concentration of Ag(I) is determined mainly by the corrosion of the AgNPs.