Development of an EQCM-based sensor for metal ions

Abstract

A series of fundamental studies are presented of the components of a prototypical
electrochemical 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 cationic
hydroxainic acid (HA), a ligand showing oxidation state specific selectivity towards
actinide 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 facilitate
electrochemical conditioning of the analyte. The following results have been obtained
from EQCM and microelectrode voltammetry studies of each of the component parts.

The Gold Piezoelectrode - The dissolution and redeposition processes of Au in
HNO3 solution at pH I & 0 were investigated as a function of sweep rate (v). At pH I
and v < 10 mV s-1 , Au dissolves through a 3e’ oxidation at E > +1.00 V (vs. SCE), a
process that, at v ≤ 10 mV s-1, is kinetically hindered with respect to under potential
deposition (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 and
reprecipitation 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 in
the vicinity of AHA oxidation at pH 0 & I: (a) oxidation of the Au surface; (b) oxidation
of the AHA; (c) complexation of the AHA with electrogenerated Au3+ . The last process
inhibits oxide formation and associated electrode passivation, so promoting the
occurrence of the first two. The two current peaks associated with AHA oxidation show
no associated mass change, indicating that AHA oxidation is controlled by semi-infinite
diffusion. 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 be
irreversible and blocked at electrodes with minimal microscopic roughness by close
packing of nitrate ions at the surface.

The Polymer Layer and the Demonstrator Ion, Fe 3+ - Microelectrogravimetric studies
of 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 layer
increasing in mass during reduction of Fe(III) due to an influx of highly hydrated H+ ions
necessary to maintain electroneutrality. Studies on Nafion® layers impregnated with
DFA+ and Fe3+ show that while Fe3+ complexes with DFA+, Fe2+ does not and that, upon
reduction of the complexed ferric ion, the resultant Fe(II)-DFA+ complex dissociates
suggesting that electrochemical expulsion of the analyte is possible.

The Redox Mediator - At pH 7.9, the reduction of methylene blue (MB) to
leucomethylene blue (LMB), occurs in two steps: (i) a reversible net 1.5 e transfer to
form a solid charge transfer complex (LMB/LMB+) (ii) an irreversible reduction of
LMB/LMB+ to LMB, the onset of which can be directly observed as an inflection in the
voltamassogram, the first time that this has been reported. The reversibility of the MB to
LMB reduction within the context of charge transfer complex electrogeneration suggests
that MB may be a suitable mediator for actinide cation reduction within the polymer
layer of the proposed sensor. It is known that the pendular amino-groups of LMB are
protonated below pH 1, so rendering the resultant LMBH22+ soluble in aqueous solution.
This means (a) difficulties associated with the irreversibile reoxidation of the solid LMB
generated at pH 7.9 will be obviated at pH I; and (b) being a large cation, we could
expect LMBH22+ to be retained within the Nafion® layer.