Spectroelectrochemical Sensor for Pertechnetate Applicable to Hanford and Other DOE Sites Page: 2 of 5
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absorbing Ru(bpy)32+ -doped Nafion films on SF11 glass were studied by internal reflection spectroscopy.
For this system, two modes of light interaction with the film are possible: attenuation due to evanescent
wave penetration and light propagation within the absorbing film itself. Unlike evanescent wave
spectroscopy, light propagation within the film can cause distortions in measured spectra due to leaky
waveguide propagation modes. For example, light propagation in a film doped with Ru(bpy)32+ can lead to
spectral red shifts up to 50 nm and even additional absorbance peaks can appear in the spectrum. These
film-based distortions depend on the complex refractive index, the thickness of the film and the angle of
incidence. These effects become significant for an extinction coefficient, k, above 0.01 and a film thickness
above 200 nm. The net result of our work has been that we must anticipate such spectral distortions in thick
films that have high values of k and that these distortions could lead to complex dynamics in the internal
reflection spectra upon analyte preconcentration in sensor films. From studying Ru(bpy)32+ partitioning
into Nafion films, we know that significant refractive index changes in the film in turn alter leaky
waveguide mode conditions in the film and, can even lead to a reduction of measured absorbance or
luminescence despite the increase in the extinction coefficient of the film. Our sensor designs for
pertechnetate will recognize this property of the films used and adjust for it as needed.
Spectroscopic ellipsometry is a nondestructive quantitative optical technique routinely used in our labs in
measurements of the optical constants n and k (n, refractive index and k, extinction coefficient) and
thickness of thin films. Previously we reported dynamic in situ measurements on the partitioning of the
model chromophore Ru(bpy)32+ into a Nafion thin film by using spectroscopic ellipsometry. We have also
documented the failure of ferrocyanide sensing films in time using similar optical techniques. Taken as a
whole, we have demonstrated how dynamic spectroscopic ellipsometry can be used to quantitatively track
both chemical and physical changes in thin sensing films on the spectroelectrochemical sensor. Continuing
fundamental work also demonstrated the detection of ferrous ions by spectroelectrochemical sensing where
2,2'-bipyridine (bpy) was first preconcentrated in the sensing layer (Nafion), and then ferrous ions, during
partitioning, formed stable colored complexes enabling efficient sensing. This model system mimics
another important feature of our pertechnetate sensor, namely, conversion of an analyte into a more easily
sensed chemical specie. Three important factors that strongly affected the sensing have been studied:(1)
optimization of the incorporation of the bpy ligand into the Nafion film,(2) retention of the iron-bpy
complex within the sensing layer upon exposure to aqueous ferrous ion solution, and (3) retention of the
charged iron-bpy complex over the time frame of the sensor measurement. A principal goal was to study
the details of metal complexation within a thin solid sensing film. To focus on sensing film changes we
used a simple prototype system composed of a thin Nafion film on a glass substrate. The film was
interrogated using a "back-side configuration" that enabled measurement of film properties only. The
influence of the different chemical species on the film properties was extracted by a stepwise experimental
approach that consisted of examining the component parts of the system (film soaking in sodium chloride,
bpy, and ferrous ion solutions one after another, respectively). Collected experimental data were then
modeled using appropriate optics software. In related studies, detection of ferrous, ferric iron and the
speciation of iron using a spectroelectrochemical sensor was also demonstrated. The optical response at 520
nm due to the electrochemical modulation of the Fe(bipy)32+/3+ complex formed in the film was
measured. The corresponding change in sensor absorbance (A) was proportional to the concentration of
ferrous or ferric ions in the film, which in turn was proportional to the bulk concentration of each iron
specie in the sample. Optimizing film thickness and ligand concentration with respect to the sensor
response yielded a detection limit of 5x10-6 M for each iron redox species and a sensor response time of 6
min. Sensor selectivity in the presence of potential interfering metal ions was also examined. Two
important general conclusions were drawn from our results:(1) the ligand complexation of an
electrochemically generated specie within the thin chemically-selective film can be efficiently
accomplished, (2) long term sensor deployment may require covalently-bound ligand to avoid leaching of
the ligand from the film.
New Ligand and Tc-complex Syntheses: New ligands for Tc are being developed primarily from the
pyridyl group of nitrogen ligands. We wish to incorporate three characteristics into the ligands for optimal
performance of the sensor: 1) the ligands should be bidentate or multidentate as this helps to maintain the
preorganized cavity for coordination of the metal within the polymer film; 2) the ligands should have a
polymerizable group such that they can be covalently attached into the polymer film; and 3) the ligands
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Heineman, William R. Spectroelectrochemical Sensor for Pertechnetate Applicable to Hanford and Other DOE Sites, report, December 1, 2004; United States. (digital.library.unt.edu/ark:/67531/metadc778636/m1/2/: accessed January 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.