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• W187924420 abstract "A series of fundamental studies are presented of the components of a prototypicalelectrochemical quartz crystal microbalance (EQCM)-based sensor for metal ions.Designed for use in the nuclear industry, the sensor consists of a gold (Au)piezoelectrode modified by a Nafion® polymer layer impregnated with a cationichydroxainic acid (HA), a ligand showing oxidation state specific selectivity towardsactinide 4+ ions and a non-radioactive demonstrator ion, Fe3+ In its final configuration,the polymer layer will also contain a redox mediator, methylene blue (MB), to facilitateelectrochemical conditioning of the analyte. The following results have been obtainedfrom EQCM and microelectrode voltammetry studies of each of the component parts.The Gold Piezoelectrode - The dissolution and redeposition processes of Au inHNO3 solution at pH I & 0 were investigated as a function of sweep rate (v). At pH Iand v +1.00 V (vs. SCE), aprocess that, at v ≤ 10 mV s-1, is kinetically hindered with respect to under potentialdeposition (UPD) of OH surface sublattices and subsequent 2-D-phase oxide formation.The inhibition arises from a closed packed layer of nitrate ions at the gold surface,although the layer is disrupted and the inhibition removed by the gold dissolution andreprecipitation processes that occur during potential cycling at low v.The Hydroxamic Acid Ligand - Study of the oxidation of acetohydroxamic acid(AHA) in nitric acid revealed that three processes occur at Au electrodes at potentials inthe vicinity of AHA oxidation at pH 0 & I: (a) oxidation of the Au surface; (b) oxidationof the AHA; (c) complexation of the AHA with electrogenerated Au3+ . The last processinhibits oxide formation and associated electrode passivation, so promoting theoccurrence of the first two. The two current peaks associated with AHA oxidation showno associated mass change, indicating that AHA oxidation is controlled by semi-infinitediffusion. The waves correspond to I and 2 electron processes with current onsets of+0.55 V and +0.80 V (vs. SCE) respectively. Oxidation of AHA appears to beirreversible and blocked at electrodes with minimal microscopic roughness by closepacking of nitrate ions at the surface.The Polymer Layer and the Demonstrator Ion, Fe 3+ - Microelectrogravimetric studiesof the Fe2+/3+ couple were conducted at bare Au, Nafion® modified Au and Nafion®modified Au impregnated with the hydrophobic cationic HA ligand, desferrioximine(DFA+) electrode. In background nitric acid (pH 1 & 0), the electrochemistry of Nafion ®layer partitioned Fe 2+/3+ ions exhibits thin layer cell behaviour with the polymer layerincreasing in mass during reduction of Fe(III) due to an influx of highly hydrated H+ ionsnecessary to maintain electroneutrality. Studies on Nafion® layers impregnated withDFA+ and Fe3+ show that while Fe3+ complexes with DFA+, Fe2+ does not and that, uponreduction of the complexed ferric ion, the resultant Fe(II)-DFA+ complex dissociatessuggesting that electrochemical expulsion of the analyte is possible.The Redox Mediator - At pH 7.9, the reduction of methylene blue (MB) toleucomethylene blue (LMB), occurs in two steps: (i) a reversible net 1.5 e transfer toform a solid charge transfer complex (LMB/LMB+) (ii) an irreversible reduction ofLMB/LMB+ to LMB, the onset of which can be directly observed as an inflection in thevoltamassogram, the first time that this has been reported. The reversibility of the MB toLMB reduction within the context of charge transfer complex electrogeneration suggeststhat MB may be a suitable mediator for actinide cation reduction within the polymerlayer of the proposed sensor. It is known that the pendular amino-groups of LMB areprotonated below pH 1, so rendering the resultant LMBH22+ soluble in aqueous solution.This means (a) difficulties associated with the irreversibile reoxidation of the solid LMBgenerated at pH 7.9 will be obviated at pH I; and (b) being a large cation, we couldexpect LMBH22+ to be retained within the Nafion® layer." @default.
• W187924420 created "2016-06-24" @default.
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• W187924420 date "2002-01-01" @default.
• W187924420 modified "2023-10-04" @default.
• W187924420 title "Development of an EQCM-based sensor for metal ions" @default.
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