chemosensors
Article
Nitrate Ion Selective Electrode Based on Ion
Imprinted Poly(N-methylpyrrole)
Ellen M. Bomar, George S. Owens and George M. Murray *
Department of Mechanical, Aerospace, and Biomedical Engineering, Center for Laser Applications,
University of Tennessee Space Institute, 411 B. H. Goethert Parkway MS 24, Tullahoma, TN 37388, USA;
[email protected] (E.M.B.); [email protected] (G.S.O.)
* Correspondence: [email protected]; Tel.: +93-1-393-7487
Academic Editor: Peter Lieberzeit
Received: 7 October 2016; Accepted: 12 December 2016; Published: 4 January 2017
Abstract: A poly(N-methylpyrrole) based ion selective electrode (ISE) has been prepared by
electro-polymerization of N-methylpyrrole using potassium nitrate as the supporting electrolyte.
Electrochemical and chemical variables were used to optimize the potentiometric response of the
electrodes and to maximize the selectivity for nitrate over potential interferences. The selectivity,
longevity and stability of the ion-imprinted polymer give this electrode advantages over traditional
nitrate ISEs. The best prototype electrode exhibits a linear potential response to nitrate ion within the
concentration range of 5.0 × 10−6 to 0.1 M nitrate with a near Nernstian slope of −56.3 mV per decade
(R2 = 0.9998) and a strong preference for the nitrate ion over other anions. The selectivity coefficients
of the electrode were evaluated by the fixed interference method. The use of N-methylpyrrole has
advantages over pyrrole in terms of selectivity and pH insensitivity.
Keywords: ion selective electrode; conductive polymer; N-methylpyrrole
1. Introduction
Nitrate is an important ingredient in fertilizers. However, many species are susceptible to nitrate
poisoning. The Environmental Protection Agency (EPA) has set the Maximum Contaminant Level
(MCL) of nitrate as nitrogen at 10 mg/L and for nitrite 1 mg/L for the safety of drinking water [1].
Nitrate can be reduced to nitrite and ammonia, both more toxic than nitrate. The flora in the digestive
tract of ruminants produces a significant amount of nitrite. This may result in anoxia since nitrite can
oxidize the ferrous ion in hemoglobin to produce stable methemoglobin that is unable to transport
oxygen. Methemoglobinemia is also known as “blue baby syndrome.” Nitrate contamination in
ground water can result in such a large decrease in oxygen carrying capacity of hemoglobin and
in babies it may lead to death. Ingested nitrate gets into saliva. The oral flora reduces some of the
nitrate to nitrite. The nitrite that is swallowed has the potential of creating nitrosamines in the stomach
that could lead to stomach cancer [2]. The bioavailability of ingested nitrate in vegetables has been
determined to be 100% [3].
Since nitrate ion is an important analyte, a significant amount of effort has been expended
to develop ionophores for nitrate. Nitrate is well known for being a weak coordinator requiring
a complexation site of a specific size, shape and charge. Many approaches have been tried to prepare
nitrate ionophores [4–6]. Most ionophores for nitrate are employed in conventional membrane ion
selective electrodes. Conductive polymer-based solid-state ion selective electrodes are supplanting
traditional membrane ion selective electrodes because of the advantage of not requiring a fill solution
and because of their more rugged, easily miniaturized construction [7,8].
Kim et al. [9], Sun and Fitch [10] and Hutchins and Bachas [11] prepared ion selective electrodes
for nitrate ion by electro-polymerization of pyrrole using a sodium nitrate supporting electrolyte.
Chemosensors 2017, 5, 2; doi:10.3390/chemosensors5010002
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Kim’s electrode prepared by cyclic voltammetry exhibited a limited linear range, some selectivity
and pronounced pH interference. Sun and Fitch prepared electrodes with a larger linear range,
better selectivity and no pH dependence between 3 and 7. Hutchins and Bachas presented a very
comprehensive study of the properties of the electrodes prepared by varying many of the parameters
for preparation. Their optimized electrode was found to have better selectivity and sensitivity
than conventional ion selective electrodes (ISEs) for nitrate or either of the two studies above.
They did discover that the electrode also responded to changes in pH, requiring an ionic strength
adjustment buffer.
N-methyl pyrrole was chosen as an alternative to pyrrole for two reasons: first, the methyl
group on nitrogen should impede protonation in acidic media. This should impart less sensitivity to
changes in solution pH. Second, N-methylpyrrole has been shown to electro-polymerize with a higher
degree of branching and crosslinking [12]. Crosslinking is an important aspect of ionic imprinting.
Higher levels of crosslinking are expected to improve polymer selectivity. It should be possible
to prepare improved nitrate ISEs using N-methylpyrrole. Larraz, et al. [13] studied the influence
of pH on the electro-synthesis of nitrate doped poly(N-methylpyrrole) films. The paper presented
spectroscopic evidence that the polymer is protonated at low pH at the carbon alpha to the nitrogen.
The rate of polymerization was faster at low pH and at pH of 1.0 the adhesion of the film was lessened.
Later papers by some of the same authors decided that the protons were not bound but free [14].
Still, it was thought that the properties of ion-imprinted polymers prepared at differing pH values
might show differing sensitivities and selectivities, since the spectral differences could be ascribed to
structural differences in the polymer films.
2. Materials and Methods
Reagents. The N-methylpyrrole monomer, potassium salts of acetate, formate, nitrate, perchlorate,
phosphate and sulfate were obtained from Sigma-Aldrich (Milwaukee, WI, USA). The remainder of
the potassium salts was obtained from the following sources: chloride, bromide and iodide from
Fisher Scientific (Cincinnati, OH, USA) and thiocyanate from Mallinckrodt Chemical Works (St. Louis,
MO, USA). Aluminum oxide for chromatography was obtained from Fluka (Steiheim, Switzerland).
The aqueous solutions were prepared using deionized (Milli-Q water purification system; Millipore,
Bedford, MA, USA) reverse osmosis water.
Electrode preparations. The electrodes were purchased from BASi (West Lafayette, IN, USA).
A conventional one compartment three-electrode electrochemical cell was used with a glassy carbon
(GC) disk working electrode (area 0.07 cm2 ), a platinum wire auxiliary electrode, and a silver chloride
(3 M KCl) reference electrode. Prior to polymerization, the GC working electrode was polished with
0.3 µm alumina, rinsed with water, and cleaned ultrasonically. When the electrodes were exposed
to organic solvent modifiers, the electrodes were heated to 50◦ C in vacuum for thirty minutes.
N-methylpyrrole was deposited on the GC working electrode by galvanostatic electrochemical
polymerization from a de-aerated aqueous solution containing 0.10 M N-methylpyrrole and 0.1M
KNO3 as the supporting electrolyte. N-methylpyrrole was purified immediately prior to use, by using
an alumina column.
Potentiometric Measurements. All measurements were made with an eDAQ e-corder equipped
with the eDAQ potentiostat and quad pH/millivolt amplifier (Denistone East, Sydney, NSW, Australia).
3. Results
Previous work with pyrrole established that optimal films were prepared using a constant current
as opposed to a constant voltage [11]. Our tests agreed with this assertion, as we too found that
a constant current electrode performed better than ones prepared with a constant voltage. Thus,
a series of electrodes were prepared using a constant current for thirty minutes. The films were grown
on 3.0 mm diameter GC electrodes. It was originally thought that an organic modifier was needed
to obtain dissolution of the N-methylpyrrole, so the solutions were made using a 50:50 mixture of
Chemosensors 2017, 5, 2
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acetonitrile and water. (It was later discovered that N-methylpyrrole would dissolve to the extent
required in deionized water.) The current values ranged from 10 µA to 120 µA. This resulted in films
of increasing thickness. The electrodes were evaluated using serial dilutions of potassium nitrate in
deionized water. Several trends were observed: the electrodes were slow to stabilize. As the films grew
thicker, the slope of the calibration curve improved to a near Nernstian response. However, as the films
grew thicker the response at low concentration decreased. At higher amounts of current, approaching
100 µA, the films buckled or lost adhesion with an abrupt and extensive change in morphology. It was
observed that the films were damaged when higher currents resulted in a final potential in excess of 1.0 V.
Electrodes that exhibited good response to nitrate were tested with interferences. Polymer film
anion electrodes have a natural propensity to bind lipophilic anions. The binding sequence is called
the Hofmeister series [15], which gives rise to the following selectivity order: H2 PO4 − > C1− > Br−
> NO3 − > I− > C1O4 − > SCN− . Initially, two anions were selected to determine selectivity, I− and
Cl− . It was found that selectivity for these two anions was improved slightly with increasing film
thickness. However, it was observed that the best selectivity, measured for an electrode made using
80 microamps of current, degraded over the period of a week. In general, the selectivity, in terms of
selectivity coefficients as measured using separate solutions were on the order of 10−2 for I− and 10−3
for Cl− . The tests were repeated using a solution that was a 50:50 mixture of acetonitrile and water.
The responses were more rapid and the lower concentration responses were restored.
The initial study revealed several issues: the electrodes respond rapidly as long as they are
tested in a solution that has the same solvent composition as the growth solution. Dilutions of the
ions in de-ionized water responded slowly and erratically, while dilutions in the 50:50 mixture of
acetonitrile and water responded rapidly. Other organic modifiers that were less expensive and more
environmentally benign, such as ethanol and surfactants were also tested. Ethanol made films that
appeared more hydrophobic and inhibited the nitrate response versus the iodide response. There was
the expectation that a surfactant could also enhance response, since it would plasticize the membrane
and act as an auxiliary ionophore. The nonionic surfactant used, Polysorbate 80, made films that
behaved in a manner similar to ethanol solutions. Also, reusing glassy carbon electrodes that have been
exposed to organic solvents exhibited a diminishment of function, likely due to absorbance of organic
solvents. This was remedied by heating the electrodes in vacuum. When it was determined that
N-methylpyrrole would dissolve in de-ionized water given time and sonication, the use of additional
miscibility agents was eliminated.
All further work employed aqueous solutions. The response of a typical electrode is seen in
Figure 1. The electrodes responded rapidly, within seconds, and then maintained a constant potential.
The use of the eDaq recorder allowed observation of the potential for any time interval needed.
Previous reports have suggested that electro-active polymer films may be photo unstable. Thus, the
electrodes were stored in a 10−3 M KNO3 solution residing in brown glass vials. The electrodes were
observed to improve in terms of sensitivity with conditioning, Figure 2. Most of the improvement
was in a few days but small improvements could be observed for up to several weeks. This may be
due to the greater amount of crosslinking with N-methylpyrrole. It is expected that conditioning will
allow for the release of unreacted monomers or short oligomers trapped in the film during preparation.
This removal would reduce nonspecific binding sites and thus improve the selectivity and sensitivity of
the electrodes. It was noticed that using fresh conditioning solutions daily shortened the time needed
to condition the electrodes. There was no loss in function observed after storage for six months.
Another way to approach the removal of oligomer and unreacted monomers is by undoping
the polymer with an applied voltage. A fresh electrode was prepared using 70 µA of current for
30 min. The electrode was undoped by applying -300 mV for 30 min following the current graphically.
The current had essentially fallen to zero leaving the film undoped. The electrode was conditioned
overnight and tested the next day. The electrode exhibited a near-Nernstian slope of 54.7 mV/decade
and a rapid response. This is similar to the response that required two weeks for an electrode that was
not undoped. The difference from a long-term conditioned electrode was that the low concentration
de-ionized water responded slowly and erratically, while dilutions in the 50:50 mixture of acetonitrile
and water responded rapidly. Other organic modifiers that were less expensive and more
environmentally benign, such as ethanol and surfactants were also tested. Ethanol made films that
appeared more hydrophobic and inhibited the nitrate response versus the iodide response. There
was the expectation that a surfactant could also enhance response, since it would plasticize the
Chemosensors 2017, 5, 2
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membrane and act as an auxiliary ionophore. The nonionic surfactant used, Polysorbate 80, made
films that behaved in a manner similar to ethanol solutions. Also, reusing glassy carbon electrodes
that havehad
been
to The
organic
solvents
exhibited
a diminishment
of additional
function, likely
due to
response
notexposed
developed.
low-end
response
did improve
slightly with
conditioning.
absorbancethe
of undoped
organic solvents.
This
was
heating
the electrodes
in vacuum.
When
was
However,
electrode
did
notremedied
have theby
degree
of selectivity
of the
electrodes
that itwere
determined
that
N-methylpyrrole
would
dissolve
in
de-ionized
water
given
time
and
sonication,
the
not undoped,
Figure
possibly
Chemosensors
2017, 5, 3.
2 This suggests that the undoping process changed the films structure,
4 of
8
use of additional
was eliminated.
collapsing
some ofmiscibility
the pores agents
and distorting
the imprinted sites.
This may be due to the greater amount of crosslinking with N-methylpyrrole. It is expected that
conditioning will allow for the release of unreacted monomers or short oligomers trapped in the film
during preparation. This removal would reduce nonspecific binding sites and thus improve the
selectivity and sensitivity of the electrodes. It was noticed that using fresh conditioning solutions
daily shortened the time needed to condition the electrodes. There was no loss in function observed
after storage for six months.
Chemosensors 2017, 5, 2
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This may be due to the greater amount of crosslinking with N-methylpyrrole. It is expected that
conditioning will allow for the release of unreacted monomers or short oligomers trapped in the film
during preparation. This removal would reduce nonspecific binding sites and thus improve the
Figure 1.
1. Potentiometric
Potentiometric response
response of
of an
an optimized
optimized poly(N-methylpyrrole)-based
poly(N-methylpyrrole)-based film
film ion
ion selective
selective
Figure
selectivity
and sensitivity of the electrodes.
It was noticed that using fresh conditioning
solutions
−] + 121.4,
−
electrode
(ISE).
Data
are
fitted
to
a
logarithmic
curve
of
equation
E
=
−56.33
log[NO
3
electrode
(ISE). Data
are fitted
to to
a condition
logarithmic
of equation
E =no−loss
56.33
log[NO3observed
] + 121.4,
daily shortened
the time
needed
the curve
electrodes.
There was
in function
−6 −
6 M.
R22 after
= 0.9974,
for
data
from
M.
R
=
0.9974,
forfor
data
points
from [NO
[NO33−−] ]= =0.5
0.5toto1010
storage
sixpoints
months.
All further work employed aqueous solutions. The response of a typical electrode is seen in
2. Effect of conditioning
of poly(N-methylpyrrole)-based
ISE’s on
response
nitrate anion ina constant
Figure 1. Figure
The electrodes
responded
rapidly, within seconds,
and
then tomaintained
−3 M KNO3 solution immediately after film growth on
aqueous
solution.
Electrodes
were
soaked
in
10
potential. The use of the eDaq recorder allowed observation of the potential for any time interval
day 1 then tested on day indicated. Fit is to a logarithmic curve, with equation E = m log[NO3−] + b,
needed. Previous
reports have suggested that electro-active polymer films may be photo unstable.
using data points from [NO3−] = 10−1 to 10−4 M. For day 5 results, E = −51.5 log[NO3−] + 217.1,
Thus, the Relectrodes
were stored in a 10−3 M KNO3 solution residing in brown glass vials. The
2 = 0.9999. For day 21 results, E = −55.2 log[NO3−] + 202.8, R2 = 0.9998.
electrodes were observed to improve in terms of sensitivity with conditioning, Figure 2. Most of the
Another
approach
the removal
of oligomer andcould
unreacted
monomersfor
is by
theweeks.
improvement
wasway
in atofew
days but
small improvements
be observed
upundoping
to several
polymer with an applied voltage. A fresh electrode was prepared using 70 µ A of current for 30 min.
The electrode was undoped by applying -300 mV for 30 min following the current graphically. The
current had essentially fallen to zero leaving the film undoped. The electrode was conditioned
overnight and tested the next day. The electrode exhibited a near-Nernstian slope of 54.7 mV/decade
and a Figure
rapid 2.
response.
This is similar
to the response that required on
two
weekstofor
an electrode
that
of conditioning
of poly(N-methylpyrrole)-based ISE’s
response
nitrate
anion in
Figure 2.
Effect Effect
of conditioning
of poly(N-methylpyrrole)-based
ISE’s on
response
to nitrate
anion in
was not
undoped.
The
difference
from
a
long-term
conditioned
electrode
was
that
the
−3
aqueous solution. Electrodes were soaked in 10
KNO3 solution immediately after film growth on low
3M
aqueous
solution.
Electrodes
were
soaked
in 10−The
M KNO
immediately
after film
growth
on
3 solution
concentration
response
had
not
developed.
low-end
did Eimprove
slightly
with
day 1 then tested on day indicated. Fit is to a logarithmic
curve,response
with equation
= m log[NO
3−] + b,
− ] + b,
day
1
then
tested
on
day
indicated.
Fit
is
to
a
logarithmic
curve,
with
equation
E
=
m
log[NO
additional
However,
not have
the degree
of
using conditioning.
data points from
[NO3−] =the
10−1undoped
to 10−4 M.electrode
For day 5did
results,
E = −51.5
log[NOof
3−] selectivity
+ 217.1,3
− ] = 10−1 to 10−4 M. For day 5 results, E = −51.5 log[NO − ] + 217.1,
using
data
points
from
3undoped,
3
the electrodes
that
were
not
Figure
3. This
suggests
that the undoping process changed
the
R2 = 0.9999.
For
day[NO
21
results,
E = −55.2
log[NO
3−] + 202.8,
R2 = 0.9998.
−
R2films
= 0.9999.
For day
21 results,
E = −some
55.2 log[NO
202.8,
R2 = 0.9998.
structure,
possibly
collapsing
of the pores
distorting
the imprinted sites.
3 ] +and
Another way to approach the removal of oligomer and unreacted monomers is by undoping the
polymer with an applied voltage. A fresh electrode was prepared using 70 µ A of current for 30 min.
The electrode was undoped by applying -300 mV for 30 min following the current graphically. The
current had essentially fallen to zero leaving the film undoped. The electrode was conditioned
overnight and tested the next day. The electrode exhibited a near-Nernstian slope of 54.7 mV/decade
and a rapid response. This is similar to the response that required two weeks for an electrode that
was not undoped. The difference from a long-term conditioned electrode was that the low
concentration response had not developed. The low-end response did improve slightly with
additional conditioning. However, the undoped electrode did not have the degree of selectivity of
the electrodes that were not undoped, Figure 3. This suggests that the undoping process changed the
films structure, possibly collapsing some of the pores and distorting the imprinted sites.
3. Response
a conductive
polymerfilm
film ISE
ISE for
that
was
undoped
afterafter
film growth.
FigureFigure
3. Response
of aof
conductive
polymer
fornitrate
nitrate
that
was
undoped
film growth.
Response to nitrate and interferent anions iodide and perchlorate shows little selectivity for nitrate
Response to nitrate and interferent anions iodide and perchlorate shows little selectivity for nitrate
against the interferents.
against the interferents.
Chemosensors 2017, 5, 2
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The response
response of
of poly(N-methylpyrrole)
poly(N-methylpyrrole) imprinted
imprinted with
with nitrate
nitrate ions
ions was
was evaluated
evaluated with
with changes
The
changes
in pH.
pH. The
The results
results are
are shown
shown in
in Figure
Figure 4.
4. The
The results
results of
of this
this study
study is
is compared
compared to
to those
those employing
employing
in
poly(pyrrole) imprinted
imprintedwith
withnitrate
nitrateions
ions[11].
[11]. As
As is
is seen
seen in
in Figure
poly(pyrrole)
Figure 44 there
there is
is very
very little
little pH
pH dependence
dependence
for
this
electrode.
The
immunity
to
pH
changes
imparts
additional
flexibility
to
this
ISE.
for this electrode. The immunity to pH changes imparts additional flexibility to this ISE.
Figure 4. Effect
Effect of
of pH
pH on
on response
response of
ofnitrate
nitrateISE’s
ISE’susing
usingpolypyrrole
polypyrroleand
andpoly(N-methylpyrrole)
poly(N-methylpyrrole) films.
films.
Poly(N-methylpyrrole) is relatively insensitive
insensitive to
to pH.
pH. Polypyrrole
Polypyrrole data
data are
are taken
taken from
from [11].
[11].
There are
are reports
reports relating
relating differences
differences in
in poly(N-methylpyrrole)
poly(N-methylpyrrole) films
prepared at
at differing
differing pH
pH
There
films prepared
values [13,14].
[13,14]. One
α carbon
carbon when
when
values
One report
report states
states that
that some
some of
of the
the polymer
polymer was
was protonated
protonated at
at the
the α
prepared
or
conditioned
at
low
pH.
This
implies
that
the
complexing
ability
of
films
prepared
at
prepared or conditioned at low pH. This implies that the complexing ability of films prepared at
different pH
pHvalues
values
may
vary.
Electrodes
prepared
atdifferent
three different
pHand
values
and their
different
may
vary.
Electrodes
werewere
prepared
at three
pH values
their responses
responses
tested.
This
was
done
to
see
how
poly(N-methylpyrrole)
protonated
at
the
α
carbon
would
tested. This was done to see how poly(N-methylpyrrole) protonated at the α carbon would affect
the
affect
the
response
of
the
electrodes.
While
there
seems
to
be
some
effect,
the
effect
is
not
dramatic.
response of the electrodes. While there seems to be some effect, the effect is not dramatic. Table 1
Table 1the
shows
the selectivity
coefficients
for our electrodes
and those
fromselectivity
[11]. Thecoefficients
selectivity
shows
selectivity
coefficients
for our electrodes
and those from
[11]. The
coefficients
were
obtained
using
the
fixed
interference
method
for
the
purpose
of
direct
comparison.
were obtained using the fixed interference method for the purpose of direct comparison. It is apparent
It is the
apparent
that the N-methylpyrrole-based
electrodes
arefor
more
selective
for nitrate than
pyrrolethat
N-methylpyrrole-based
electrodes are more
selective
nitrate
than pyrrole-based
electrodes.
basedsupports
electrodes.
This that
supports
the claim that
N-methylpyrrole
undergoes
a greater degree
of
This
the claim
N-methylpyrrole
undergoes
a greater degree
of crosslinking.
However,
crosslinking.
However,
while
there
are
some
small
differences
in
selectivity
at
differing
pH,
there
while there are some small differences in selectivity at differing pH, there doesn’t appear to be a usable
doesn’tItappear
to be that
a usable
trend.
It does the
appear
that
raising orimproves
loweringselectivity
the pH ofagainst
fabrication
trend.
does appear
raising
or lowering
pH of
fabrication
hard
improves
selectivity
against
hard
divalent
anions.
divalent anions.
Table 1. Selectivity coefficients for nitrate ion selective
selective electrodes.
electrodes.
Interfering
IonIon
Interfering
Acetate
Acetate
Bromide
Bromide
Phosphate
Sulfate
Phosphate
Iodide
Sulfate
Formate
Thiocyanate
Iodide
Perchlorate
Formate
4. Discussion
Thiocyanate
Selectivity
Coefficients
at Specified
pH pH
Selectivity
Coefficients
at Specified
Comparison
to Literature
Comparison
to Literature
Values,
Values,
from
pH 6.14
pH 6.14
pH 3 pH 3
pH 9 pH 9
from [11] [11]
−4
−4
−4
−4 10
−4 × 10
2.00
5.015.01
× 10×
3.16 ×3.16
10−4× 10 2.00 × 10
7.94 × 10−4
7.94 × 10−4
6.31 × 10−4
−4
−4
4
7.942.82
× 10×
10−4× 10−6.31
× 10
10−3 7.94 ×3.16
3.16 × 10−4
−
4
−
5
−5
2.0
−3 10
−4 × 10
2.823.16
× 10×
3.16 ×1.99
10−4× 10−3.16
× 10
−
3
3
3.98 × 10
2.51 × 10
1.26 × 10−3
−4
−5
−5
−
4
−
4
3.167.94
× 10× 10 1.99 ×3.98
10 × 10 2.0 × 10
3.98
× 10−4
−
3
−
2
6.31 × 10
1.00 × 10
5.01 × 10−3
3.98 × 10−3 −3 2.51 × 10−3
1.26 × 10−3
5.01 × 10
5.62 × 10−3
5.01 × 10−3
7.94 × 10−4
3.98 × 10−4
3.98 × 10−4
NANA
7.6 × 10−2
−2
7.69.0
× 10
× 10−4
×−410−4
9.06.0
× 10
5.1 × 10−2
−4
6.0 × 10
NA
3.6 × 10−1
5.1 × 10−2 −2
5.7 × 10
NA
6.31 × 10−3
3.6 × 10−1
1.00 × 10−2
5.01 × 10−3
−2
Perchlorate
× 10−3 Spectroscopy
5.62 × 10−3 (FT-IR)
5.01 ×was
10−3 used to examine
5.7the
× 10films
Fourier
Transform5.01
Infrared
prepared at
differing pH values. The spectra were obtained using a diamond anvil ATR. The report cited above
Chemosensors 2017, 5, 2
Chemosensors 2017, 5, 2
6 of 8
6 of 8
4. Discussion
4. Discussion
Fourier
Transform
Infrared Spectroscopy (FT-IR) was used to examine the films prepared
Chemosensors
2017,
5, 2
6 ofat
8
Fourier
Spectroscopy
used anvil
to examine
the report
films prepared
at
differing
pH Transform
values. TheInfrared
spectra were
obtained (FT-IR)
using a was
diamond
ATR. The
cited above
differing
values.
The spectra
wereatobtained
a diamond
anvil ATR.
The report
citedatabove
also usedpH
FT-IR
to compare
the films
differingusing
pH values
[13]. Those
films were
prepared
fixed
used
FT-IR
Those
films
were
prepared
fixed
also
to
compare
the
films
at
differing
pH
values
[13].
potential of 0.8 V versus a saturated calomel electrode (SCE) resulting in considerably higher at
current
saturated
calomel
electrode
resulting
considerably
current
potential
0.8were
V versus
densities of
than
useda in
the current
study.
The IR(SCE)
spectra
of filmsingrown
at pH 9higher
and pH
3 are
than
were
used
in
the
current
study.
The
IR
spectra
of
films
grown
at
pH
9
and
pH
3
are
densities
spectra
−1
−1
given in Figure 5. There are differences in the spectra for bands around 1300 cm and 650 cm . These
−1650
−1
−1cm
−1. These
in
Figure
5.
There
are
differences
in
the
spectra
for
bands
around
1300
cm
and
cm
given
in
Figure
5.
There
are
differences
in
the
spectra
for
bands
around
1300
and
650
cm
bands have been assigned to the polymer back bone, [13] suggesting small changes in the polymer.
bands
have been
assigned
to the polymer
back
bone,
[13]
suggesting
thethis
polymer
These
bands
have
been
to
the polymer
back
bone,
[13] suggesting
small in
changes
in
the
conformations.
There
is aassigned
marked
difference
in the
spectra
obtained
onsmall
filmschanges
prepared
in
study
conformations.
is aThere
marked
in the
spectra
obtained
on
films
prepared
study
polymer
conformations.
is adifference
marked
difference
in
the
spectra
obtained
onshowed
filmsin
prepared
in
as compared
to There
this previous
report.
All of the
spectra
from
the
previous
study
athis
carbonyl
as
compared
to
this
previous
report.
All
of
the
spectra
from
the
previous
study
showed
a
carbonyl
this
study
as
compared
to
this
previous
report.
All
of
the
spectra
from
the
previous
study
showed
band at about 1700 cm−1. Of the spectra obtained in this study only two had a peak near this position.
−1 . Ofobtained
band
at about
cm−1. electrode
Of
thecm
spectra
in this
study more
only
two
hadonly
atopeak
near
position.
a
carbonyl
band
at about
1700
the spectra
obtained
in
this
study
two
hadthis
a that
peak
near
One
was
for
a1700
benzoate
that required
significantly
voltage
polymerize
had
a
One
was
for
a
benzoate
electrode
that
required
significantly
more
voltage
to
polymerize
that
had
a
this
position.
One
was
for
a
benzoate
electrode
that
required
significantly
more
voltage
to
polymerize
−1
weak band at 1736 cm . The other spectrum was obtained for a film prepared using sulfate as the
−1 . The
weak
band
at 1736
cm
. The
other
spectrum
was
for a filmfor
prepared
using
sulfate
as the
that
had
a weak
band
at−1
1736
other
spectrum
was obtained
a film
prepared
using
sulfate
supporting
electrolyte
with
acm
band
was
at 1684
cm−1obtained
. It was
observed
that
the level
of over
oxidation
−
1
−1
supporting
electrolyte
with a band
at 1684
cm1684
. It
was
observed
that the level
of
over
oxidation
as
the supporting
electrolyte
with
band
was
at
cm in
. films
It wasunsuitable
observed
that
the
level
of over
caused
by the
higher
potentials
andawas
currents
usually
result
as
sensors.
caused bycaused
the higher
potentials
and currents
result
in films
as sensors.as sensors.
oxidation
by the
higher potentials
andusually
currents
usually
resultunsuitable
in films unsuitable
Figure 5. IR spectra of films produced under conditions of varying pH and dopant anion.
spectra
of films
produced
varying
pH
and
anion.
Figure
5.5.IRIRspectra
films
produced
under
conditions
of varying
pH
and
dopant
anion.
(A) Benzoate.
(A) Benzoate.
Inset of
shows
the region
fromunder
1850 toconditions
1620
cm−1,ofand
arrow
points
to dopant
feature
in the
−1 , and
−1, and arrow points to feature in the
(A)
Benzoate.
Inset
shows
the
region
from
1850
to
1620
cm
Inset
shows
the
region
from
1850
to
1620
cm
arrow
points
to
feature
in
the
benzoate-doped
electrode
−1
benzoate-doped electrode spectrum at ca. 1736 cm , assigned to a carbonyl stretch mode; (B) Nitrate,
−1, assigned to a carbonyl stretch mode; (B) Nitrate,
benzoate-doped
electrode
ataca.
1736 cmstretch
spectrum
ca. 1736
cm9.−1 , spectrum
assigned to
carbonyl
mode; (B) Nitrate, pH 3; (C) Nitrate, pH 9.
pH 3; (C) at
Nitrate,
pH
pH 3; (C) Nitrate, pH 9.
Scanning electron
electronmicrographs
micrographs
show
the morphology
the isfilms
is to
similar
others
Scanning
show
thatthat
the morphology
of theoffilms
similar
othersto
reported
Scanning
electron
micrographs
show
that
the
morphology
of
the
films
is
similar
to
others
reported
in the literature.
An is
example
given as
in
the literature.
An example
given asisFigure
6. Figure 6.
reported in the literature. An example is given as Figure 6.
Figure 6. Scanning electron micrograph of a typical poly(N-methylpyrrole) film.
Figure 6. Scanning electron micrograph of a typical poly(N-methylpyrrole) film.
Chemosensors 2017, 5, 2
7 of 8
In order to verify that imprinting induces selectivity, control experiments were performed.
Electrodes were prepared using the optimal conditions previously established for nitrate while
substituting chloride and perchlorate as supporting electrolytes. The electrodes were conditioned
and then tested for response to the imprinting ion, nitrate and other interferences. The chloride
and perchlorate electrodes responded to changes in concentration but did not show a preference
for the imprint ion over nitrate. This is contrary to reports for pyrrole electrodes prepared for these
anions [16,17].
Additional Analytes. Due to the success with nitrate it was considered reasonable to try to prepare
electrodes for other anions. Electrodes were prepared using the optimal growth conditions found for
nitrate ions for additional anions, both univalent and divalent: benzoate, chloride, dicyanoargentate,
perchlorate, picrate, carbonate, sulfate and oxalate. The results were that dicyanoargentate and picrate
electrodes functioned polymerized with voltages similar to nitrate and performed with good sensitivity
and selectivity. The chloride and perchlorate electrodes polymerized with voltages similar to nitrate
and performed with adequate sensitivity but poor selectivity. The electrodes prepared for carbonate
and oxalate did not form films. The electrodes prepared for sulfate formed films at reasonable voltages
but the electrodes did not respond in a consistent manner. The electrode prepared for benzoate was
both selective and sensitive, but formed at significantly higher voltage. From these experiments it is
possible to make some generalizations. It appears that univalent planar anions are good candidates
for imprinting with poly(N-methylpyrrole). Other univalent non-planar anions can be used to make
electrodes that respond to anion concentration but these dopants do not form films that are selective
for the imprint. Divalent anions are unsuitable for preparing electrodes. This problem may have
an explanation. A paper examining the effect of sulfate supporting electrolyte on the polymerization
of N-methyl poly(pyrrole) found that the films grown were heavily over oxidized [18]. The paper
suggests that a divalent anion could stabilize a divalent cation that would more easily hydrolyze
and then oxidize to two carbonyl groups. This is consistent with the IR spectra that show a strong
carbonyl band for films grown using sulfate as the supporting electrolyte. Additional experiments will
be performed to obtain detailed data on the responses of the other selective electrodes.
5. Conclusions
Poly(N-methylpyrrole) can be ion imprinted by electro-polymerization of N-methylpyrrole using
potassium nitrate as the supporting electrolyte. The selectivity of the ISE produced by this process
exceeds that of other solid-state ISEs for nitrate. Changes in growth parameters such as current,
voltage and pH were investigated. The selectivity, longevity and stability of the ion-imprinted polymer
give this electrode advantages over traditional nitrate ISEs. The best prototype electrode exhibited
a linear potential response to nitrate ion within the concentration range of 5.0 × 10−6 to 0.1 M nitrate
with a near Nernstian slope of −56.3 mV per decade (R2 = 0.9998) and a strong preference for the
nitrate ion over other anions. The use of N-methylpyrrole has advantages over pyrrole in terms of
selectivity and pH insensitivity. The pH insensitivity of the poly(N-methylpyrrole) matrix and the
more extensive crosslinking suggests that this polymer is a useful matrix for the preparation of other
ion imprinted electrodes.
Acknowledgments: This work was supported by a grant from Babcock & Wilcox Technical Services Y-12, L.L.C.
Solicitation Number 6300007472. We acknowledge the assistance of Emily F. Hancock, Catherine Claire Murphy,
Matthew Dede and Kathleen S. Lansford.
Author Contributions: George M. Murray conceived and designed the experiments; Ellen M. Bomar and
George S. Owens performed the experiments; George S. Owens analyzed the data; George S. Owens and
George M. Murray wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
Chemosensors 2017, 5, 2
8 of 8
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