Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device: 14. Enhancing sensitivity of a metal complex ion by ligand exchange

Article abstract of DOI:10.1016/j.electacta.2004.11.048

Enhancement of the sensitivity of a spectroelectrochemical sensor by ligand exchange within the sensing film during spectroelectrochemical modulation is demonstrated in this paper. This concept is illustrated with a sensor for Cu(en)₂²⁺, where en = ethylenediamine, in aqueous solution. A ligand exchange reaction increases the difference in the molar absorptivities of the two complex ions involved in spectroelectrochemical modulation, thus giving a larger optical response. The spectroelectrochemical sensor consists of a cation-selective Nafion-SiO₂ composite film spin-coated onto an indium tin oxide (ITO) glass optically transparent electrode (OTE). The film was loaded with 2,9-dimethyl-1,10-phenanthroline (neocuproine, or nc) for the ligand exchange reaction. Reduction of Cu(en)₂ ²⁺ at the OTE is accompanied by ligand exchange with nc to form Cu(nc)₂¹⁺, which has a molar absorptivity of 7950 M ^-1 cm^-1 at λ_max = 454 nm. Detection within the sensing film using attenuated total reflectance (ATR) at 454 nm during modulation was accomplished by potential cycling between 0.8 and -0.9 V versus Ag/AgCl. The effects of nc concentration in the film and potential scan rate on the absorbance-time profile for spectroelectrochemical modulation were studied. The calibration curve for Cu(en)₂²⁺ was linear within the range of 5 × 10^-6 M to 1 × 10^-3 M.

Full text of DOI:10.1016/j.electacta.2004.11.048

Electrochimica Acta 50 (2005) 3191–3199
Spectroelectrochemical sensing based on multimode selectivity
simultaneously achievable in a single device
14. Enhancing sensitivity of a metal complex ion by ligand exchange
a
b
a
a,∗
Tanya Shtoyko , John N. Richardson , Carl J. Seliskar , William R. Heineman
a
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA
Department of Chemistry, Shippensburg University, 1871 Old Main Drive, Shippensburg, PA 17257, USA
b
Received 16 July 2004; received in revised form 16 November 2004; accepted 20 November 2004
Available online 5 January 2005
Abstract
Enhancement of the sensitivity of a spectroelectrochemical sensor by ligand exchange within the sensing film during spectroelectrochemical
2
+
modulation is demonstrated in this paper. This concept is illustrated with a sensor for Cu(en)
tion. A ligand exchange reaction increases the difference in the molar absorptivities of the two complex ions involved in spectroelectrochemical
modulation, thus giving a larger optical response. The spectroelectrochemical sensor consists of a cation-selective Nafion-SiO composite
film spin-coated onto an indium tin oxide (ITO) glass optically transparent electrode (OTE). The film was loaded with 2,9-dimethyl-1,10-
2
, where en = ethylenediamine, in aqueous solu-
2
2
+
phenanthroline (neocuproine, or nc) for the ligand exchange reaction. Reduction of Cu(en)
2
at the OTE is accompanied by ligand exchange
1
+
−1
−1
with nc to form Cu(nc)
2
, which has a molar absorptivity of 7950 M cm at λmax = 454 nm. Detection within the sensing film using atten-
uated total reflectance (ATR) at 454 nm during modulation was accomplished by potential cycling between 0.8 and −0.9 V versus Ag/AgCl.
The effects of nc concentration in the film and potential scan rate on the absorbance–time profile for spectroelectrochemical modulation were
2
+
−6
−3
studied. The calibration curve for Cu(en)
2004 Elsevier Ltd. All rights reserved.
2
was linear within the range of 5 × 10 M to 1 × 10 M.
©
Keywords: Nafion-SiO2 composite film; Spectroelectrochemical modulation; Ethylenediamine; Optically transparent electrode; Neocuproine
1
. Introduction
viologen dication [12]), all of which exhibited both electro-
chemical reversibility and a substantial difference in molar
absorptivity during spectroelectrochemical modulation. Both
of these features are advantageous to development of a sen-
sitive spectroelectrochemical sensor.
New materials for films that possess properties suited
to construction of spectroelectrochemical sensors, such as
ion-exchangeability, nanoscale porosity, high optical trans-
parency, variable thickness, and physicochemical stability
have been developed [13,15,16]. These materials are essen-
tial to the functioning of this spectroelectrochemical sen-
sor. One series of materials is based on polymer blending
in a host of glutaraldehyde cross-linked poly(vinyl alcohol)
A new type of sensor exhibiting three modes of selec-
tivity (chemical partitioning into a selective thin film, elec-
trochemistry and spectroscopy) has been under development
in our laboratory for some time [1–14]. Detection of ana-
lyte occurs by observing changes in an optical signal asso-
ciated with spectroelectrochemical modulation of the ana-
lyte after it partitions into the selective film, which is coated
onto an optically transparent electrode (OTE). The sensor has
been previously demonstrated with a series of metal ion com-
3−/4−
+
plexes such as Fe(CN)
[2,10,14], [Re(dmpe)3] [7,8],
6
Ru(bipy)32 [9], and a cationic organic compound (methyl
+
[13,16]. Chemically-selective dopants in this host demon-
∗
strate property-selective separations of chemicals from
mixtures [13,16]. A second series consists of polyelectrolyte-
Corresponding author. Tel.: +1 513 556 9210; fax: +1 513 556 9239.
E-mail address: heinemwr@email.uc.edu (W.R. Heineman).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.11.048

3
192
T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
containing silica composites prepared by sol–gel processing
15].
More recently, we have begun exploring strategies
[
whereby analytes that do not exhibit ideal properties may still
be detected by the spectroelectrochemical sensor. The possi-
bility of using the spectroelectrochemical concept to detect
an analyte that itself does not exhibit a measurable optical
change associated with its electrolysis was recently demon-
strated [11]. This sensor was based on a mediated electrode
reaction in which a non-absorbing analyte (ascorbate) was
detected indirectly by its effect on the optical signal resulting
from the spectroelectrochemical modulation of the mediator
Ru(bipy)32 ), which had been trapped in a cation-selective
+
(
Nafion-SiO2 composite film [11].
2
+
A second example is the determination of Fe in aque-
ous solution [17]. Both Fe2+ and Fe ions are only weakly
absorbing in aqueous solution and thus are not directly
amenable to detection by spectroelectrochemical modula-
3+
2
+
ꢀ
Fig. 1. Diagram of Cu(en)2 sensing using nc.
tion. To overcome this problem, the ligand 4,4 -bipyridyl
(
bipy) was loaded into a Nafion film on the OTE. Fe2+
partitioned from the aqueous sample into the Nafion film
where it reacted with bipy to form the intense chro-
mophore Fe(bipy)32+ [17]. Spectroelectrochemical modula-
lower aliphatic alcohols and water), and neocuproine (98%)
by Aldrich; ethylendiamine from Aldrich; hydrochloric acid
2+
(Reagent A.C.S.), ethanol and potassium nitrite from Fisher
tion between the intensely colored Fe(bipy)3 and colorless
Fe(bipy)33 gaveasubstantiallyincreased sensitivitybecause
of the large difference in molar absorptivity (ꢀε) between the
two species [17].
+
Scientific; and cupric sulfate from Mallinckrodt.
Copper (II) sulfate solutions were made by dissolving the
appropriate amount in 0.1 M KNO3. The appropriate amount
of ethylenediamine was added to copper sulfate solutions to
make the solution 0.1 % en. A 0.2 M neocuproine solution
was made by dissolving the appropriate amount in ethanol.
The resulting neocuproine solution was dissolved in water to
make a 1:3 (v/v) nc solution. Indium tin oxide (ITO)-coated
glass(11–50 ꢁ/sq, 150 nmthickITOlayerovertinfloatglass)
was supplied by Thin Film Devices. A 1-PM101DT-R485
Photo-Resist-Spinner, manufactured by Headway Research
Inc., was used for spin-coating films onto the ITO electrode
surfaces.
The objective of this research was to demonstrate another
mechanism for sensing that enhances detection by increas-
ing ꢀε between the species involved in spectroelectrochem-
ical modulation. In this mechanism a metal complex ana-
lyte undergoes a reversible ligand exchange reaction with
another ligand in the sensing film as part of the spectro-
electrochemical modulation process. The exchange ligand
is chosen to increase ꢀε and is immobilized in the sens-
ing film prior to exposure to the analyte. We used Cu(en)22+
where en = ethylenediamine, as a representative analyte and
,9-dimethyl-1,10-phenanthroline (neocuproine, or nc) as the
,
2
exchange ligand trapped in a selective Nafion-SiO2 film. The
sequenceofeventsthatoccursinthesensorisshownschemat-
ically in Fig. 1. Cu(en)22+ first partitions into the sensing film
where it can be controlled electrochemically and monitored
optically by the evanescent wave component at the reflection
2.2. Nafion-SiO2 selective film preparation
The preparation of the Nafion-SiO2 composite film was
performed as previously described [15]. Briefly, 4 mL of
deionized water, 2 mL of TEOS, and 0.1 mL of 0.1 M HCl
were mixed for 3 h until a single-phase sol resulted. The
sol was blended with a solution of Nafion in a ratio of 1:3
(v/v). 300 ␮L of the mixture were immediately spin-coated
onto ITO-glass substrates at a spin rate of 3000 rpm for
30 s. The slides were cured in air at room temperature for
at least a day. The slides were then hydrated by soaking
in supporting electrolyte solution for at least 12 h. Unless
otherwise noted, the slides were immersed (∼1 h) in 0.04 M
neocuproine ethanol–water solution (1:3 v/v) for ligand up-
take.
2+
points of the optical beam. Weakly absorbing Cu(en)2 is re-
duced by one electron; this process is accompanied by ligand
+
exchange with nc to form the intensely absorbing Cu(nc)2 .
The large ꢀε in these two species gives rise to a large change
in absorbance, ꢀA, which is used to quantify the analyte,
Cu(en)22
+
.
2
. Experimental
2
.1. Chemicals and materials
Film thickness was estimated by the optical interference
fringe method [18] and ellipsometry (Woollam) and ranged
from 300 to 380 nm.
All chemicals were used as received without further pu-
rification: tetraethoxysilane (TEOS), Nafion (5% solution in

T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
3193
2
.3. Thin layer spectroelectrochemistry
on the coordinating ligand and in aqueous solution are pH-
−
dependent since OH and H2O are available ligands. A spec-
2+
Thin layer spectroelectrochemistry employing an opti-
trum of Cu measured in copper sulfate solution gave ε of
− −1
1
cally transparent thin layer electrode (OTTLE) [19] was used
to initially characterize the spectroelectrochemical properties
of the different complex ions. An ITO-glass slide was used
as an optically transparent electrode, and the OTTLE was
constructed using standard literature procedures [19,20]. The
reference electrode used in all electrochemical experiments
was Ag/AgCl/3M NaCl.
12 M cm at λmax = 810 nm. If en is added to an aqueous
2+
2+
solution of Cu , the very stable complex Cu(en)2 forms
(log β2 = 19.6) [21] in the pH range from 11.2 to 6.5. Visible
spectra of Cu(en)22+ at pH values between 6 and 11.2 show an
absorption peak at 547 nm and indicate that Cu(en)2 is sta-
ble over that range of pH. At pH < 6, Cu(en)22+ decomposes
due to en dissociation via protonation (spectra not shown).
Spectra of Cu(en)22+ in 0.1 M KNO3, which is the medium
2
+
−1
−1
2
.4. Attenuated total internal reflection (ATR)
usedforallofthesensingexperiments, gaveε = 75 M cm
at λmax = 547 nm. If the pH of the solution is made basic
measurements
(
∼11.2), cupric hydroxide does not precipitate, and hydrox-
2+
The instrument for spectroelectrochemical modulation
has been previously described [2]. ATR spectroelectrochemi-
cal measurements were performed using a simple Delrin plas-
tic cell, also described previously [2]. The potential and corre-
sponding ATR signal were digitized and stored on a personal
computer. Data records were manipulated and analyzed using
spreadsheet programs. The angle of incidence of light into the
coupling prism and the alignment of the cell were adjusted to
maximize the ATR throughput, as determined by measuring
the intensity of decoupled light. Sensor absorbance values
were obtained by recording the light intensity through the
prism-coupled slide when exposed to pure supporting elec-
trolyte (Io) and the intensity after being exposed to an analyte
solution (I). Sensor absorbance in the multiple reflection ar-
rangement was defined as A = log(Io/I).
ide complexes of copper do not form because Cu(en)2 is
much more stable than these species [21].
2+
Cu in aqueous solution slowly forms a square planar
2+
complex with neocuproine Cu(nc)2 [24] (log β2 = 11.4)
[21], which is not a preferred configuration. Cu(nc)2
2
+
−1
−1
is a stable orange complex with ε = 3550 M cm at
λmax = 454 nm. However, if Cu2+ is reduced to Cu in the
presence of nc, the more stable orange tetrahedral complex
Cu(nc)21+ (log β2 = 19.1) [21] shown below forms [22]. This
complex ion also has a λmax at 454 nm [22], but a much
+
−
1
−1
larger ε of 7950 M cm because the electronic transition
in question involves significant ligand molecular orbital char-
acter. Cu(nc)21+ is stable for long periods of time over a wide
pH range [22,23]. However, the absorbance of a solution of
Cu(nc)21+ decreasesdramaticallyinthepresenceoffreeCu
due to formation of a mixed ligand complex when there is not
an excess of nc in the solution [21].
2+
3
. Results and discussion
The formation constants and spectral properties of the two
complexes of copper in their different oxidation state sup-
ports the following scheme upon which the sensor is based.
Reduction of Cu(en)22+ to Cu(en)2 (log β2 = 11.2) [21] in
excess nc results in ligand substitution to form the more sta-
ble Cu(nc)21+ (log β2 = 19.1) [21]; this reaction is accompa-
nied by a large increase in absorbance at 454 nm. Oxidation
The main goal of this research was to demonstrate lig-
1+
and exchange in a metal complex analyte during spectro-
electrochemical sensing to enhance its detection. Cu(en)22+
served as the analyte and nc as the exchange ligand. Since
the analyte is cationic, a Nafion-SiO2 [15] film was used as
a cation-exchange selective layer into which Cu(en)22+ was
pre-concentrated for sensing. Nafion also served to trap the
hydrophobic exchange ligand, nc, because of its hydrophobic
domains.
1+
2+
of Cu(nc)2 to Cu(nc)2 (log β2 = 11.4) [21] in the pres-
ence of excess en should result in ligand exchange to reform
the more stable Cu(en)22+ (log β2 = 19.6) [21], and the ab-
+
sorbance decreases to its original value. Since ε for Cu(nc)2
2+
is about 800 times larger than ε for Cu(en)2 (Table 1), the
spectroelectrochemical modulation of Cu(en)22+ in excess nc
should be quite sensitive.
3
.1. Spectral characterization
Spectral characteristics of the various copper species in-
This chemistry as it applies to the spectroelectrochemi-
cal sensor is depicted in Fig. 1. Cu(en)22+ partitions from
the sample solution into the film of Nafion-SiO2, which also
volved are shown in Table 1. The near infrared absorbance
of Cu2 results from a d–d transition. For Cu complexes the
wavelength of maximum absorbance (λmax) and ε depend
+
2
+
contains the exchange ligand nc. Reduction of Cu(en)2
Table 1
Formation constants and molar extinction coefficients for copper complexes
Ligand
Cu+
Cu2+
−
1
−1
−1 −1
ε = 12 M cm (810 nm)
H2O
nc
en
ε = 9 M cm (790 nm)
−1
−1
−1 −1
log β2 = 11.4, ε = 3550 M cm (454 nm) square planer
log β2 = 19.1, ε = 7950 M cm (454 nm), tetrahedral
log β2 = 11.2, ε = 39 M cm (547 nm)
−1
−1
−1 −1
log β2 = 19.6, ε = 75 M cm (547 nm)

3
194
T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
0
+
+
results in formation of the more stable, strongly absorbing
Cu(nc)21 , which gives rise to a large increase in absorbance.
Subsequent oxidation of this complex ion forms the original
Cu(en)22 , resulting in a concomitant decrease in absorbance,
thus leading to optical modulation in response to electro-
chemical cycling of the potential.
Cu , which deposits on the electrode. Any intermediate Cu
+
2
ion formed disproportionates to form the more stable Cu
0
+
and Cu species [28]. However, Cu can be trapped by allow-
ing it to react with a strong complex forming ligand such as
neocuproine [21]. This process is evidenced by the shift in the
standard potential of the Cu /Cu couple to more positive
potentials after preferential complexation of Cu .
+
2+
+
+
2
+
3
.2. Cyclic voltammetry and spectroelectrochemistry
TheelectrochemistryandspectroelectrochemistryofCu
at a bare ITO OTE were investigated using cyclic voltamme-
try. Fig. 2A shows cyclic voltammograms of Cu2+ in 0.1 M
KNO3 at pH 7.3 (curve 2) and of supporting electrolyte (curve
1) within the potential window of 1.2 to −0.5 V. The pH of 7.3
is the natural value for the solution, and no attempt was made
Previous voltammetric studies indicate that the electro-
chemical behavior of Cu2+ in water strongly depends on the
presence (or absence) of certain ligands [21,24–27]. In the
absence of a strongly complexing ligand, Cu2+ reduces to
Fig. 2. (A) Cyclic voltammograms recorded at a bare ITO OTE in (1) 0.1 M KNO3 and (2) 1 × 10− M CuSO4 in 0.1 M KNO3. Each voltammogram was
recorded by cycling the potential in the range from 1.2 V to −0.5 V (vs. Ag/AgCl ref) at a potential scan rate of 5 mV/s. (B) ATR sensor absorbance measured
at 400 nm taken concurrently with the cyclic voltammogram shown in (A). (C) Cyclic voltammograms recorded at a bare ITO OTE of solutions containing (1)
3
−
3
−3
−3
4
× 10 M en and 0.1 M KNO3 and (2) 1 × 10 M CuSO4 with 4 × 10 M en in 0.1 M KNO3. Each voltammogram was recorded by cycling the potential
between 0.9 V and −1.2 V (ν = 5 mV/s, E vs. Ag/AgCl ref). (D) Sensor absorbance measured at 400 nm taken concurrently with the cyclic voltammogram
−
3
−3
shown in (C). (E) Cyclic voltammograms recorded at a bare ITO OTE in (1) 4 × 10 M nc, 4 × 10 M en and 0.1 M KNO3 in a 1:3 (v/v) solution of ethanol in
−3
−3
−3
water; (2) 1 × 10 M CuSO4, 4 × 10 M en, 4 × 10 M nc, and 0.1 M KNO3 in a 1:3 (v/v) solution of ethanol in water. The voltammograms were recorded
by cycling the potential in the range from 0.8 V to −0.9 V at 5 mV/s, (E vs. Ag/AgCl ref).

T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
3195
to adjust or control it. The associated optical response (trans-
mission) for the Cu2+ voltammogram is shown in Fig. 2B.
The voltammogram has a single distinctive reduction peak
for Cu2 with a small shoulder on the forward scan and mul-
tiple oxidation peaks on the reverse scan. The reduction wave
is associated with a sharp increase in absorbance. We attribute
this to deposition of Cu on the OTE because it is essentially
wavelength independent (i.e., behaves like a neutral density
filter) and is considerably larger than can be attributed to the
evidenced by the large separation in cathodic and anodic
peak potentials, this is not an electrochemically reversible
process.
+
Thin layer spectroelectrochemistry at an OTTLE was used
2+
to confirm that Cu(en)2 can be electrochemically cycled be-
tween Cu(en)22+ and Cu(nc)21+ without formation of Cu in
the presence of excess nc. The experiment consisted of load-
ing the cell with a solution of Cu(en)22+ containing excess nc,
applying an initial positive potential, and scanning the poten-
tial negatively and then positively again while recording spec-
tra simultaneously to characterize the formed species. Fig. 3
0
+
formation of Cu based on values of ε in Table 1. This be-
2
+
havior is consistent with previous work showing that Cu
+
2+
reduction in water is not by one electron to Cu , but two
electrons to Cu . As reported previously [29], the first an-
shows transmission spectra of the 7.5 mM Cu(en)2 solu-
0
tion with excess nc as taken during the reduction/oxidation
process. Spectrum (a) was recorded throughout the slow po-
tential scan from 0.8 V to −0.65 V. Although not evident on
this absorbance scale, spectrum (a) is the same as the original
spectrum of Cu(en)22+. Starting at −0.65 V and continuing to
odic peak (ca. 0.1 V in this case) was attributed to formation
of a surface Cu2O layer. The second peak (at 0.3 V in our
case) was attributed to oxidation of bulk copper to Cu2O
[29]. The concurrent decrease in absorbance during copper
1+
oxidation to Cu2O was reported before [29] and is due to
electrochromism of copper oxide thin films. The dominant
stripping peak occurring at 1.35 V (versus Ag/AgCl) corre-
sponds to the stripping of Cu2O from the ITO surface. All
of the oxidation waves are accompanied by a decrease in ab-
sorbance as expected for electrochromism and the removal
of Cu from the OTE. It is apparent that spectroelectrochem-
istry is very sensitive to the formation of solid Cu and its
oxides on the electrode surface. In fact, we have recently
reported a sensor for Cu based on the optical changes that ac-
company its deposition and stripping on a bare ITO surface
−0.9 V the spectrum of Cu(nc)2 with λmax = 454 nm grows
1+
in (spectra 3b–3c), confirming the formation of Cu(nc)2 as
the reduction product of Cu(en)22+ under these conditions.
Scanning to more negative potentials leads to hydrogen evo-
lution. Holding the potential at −0.9 V for 15 min gives an
additional large increase in absorbance at 454 nm (3c–3d),
which then stops. The baseline did not change, thus indi-
cating that no copper deposits on ITO OTE. If the potential
is scanned back from −0.9 V to 0.8 V (3d–3e), considerable
1+
time is required for Cu(nc)2 to be oxidized and exchange nc
forentoreformCu(en)22+. Thepotentialmustbeheldat0.8 V
for 30 min before the original spectrum is finally obtained
(3e–3a). Thus, the thin-layer spectroelectrochemistry shows
[30].
The addition of excess en into solution converts aqueous
Cu2+ to Cu(en)2 and raises the pH to 11.2 by hydroly-
sis of excess ligand. Cyclic voltammetry (Fig. 2C) and the
associated optical response (Fig. 2D) show the reduction
2+
quantitative cycling between Cu(en)2 and Cu(nc)2 , albeit
with a large potential separation and a very slow rate.
Repeating this experiment in the absence of nc gave no
change in absorbance when the potential was scanned from
0.8 V to −0.8 V. However, there was an increase in ab-
sorbance at all wavelengths when the potential was scanned
2+
1+
2+
of Cu(en)2 to be energetically more difficult than that of
2
+
Cu as evidenced by the negative shift in the reduction wave
(
Epc < −1.0 V, not shown in Fig. 2C due to onset of hydro-
gen reduction), as expected for the formation of the stable
complex. Reduction of Cu(en)22+ is accompanied by a sharp
0
increase in absorbance, which is also due to reduction to Cu
as evidenced by the magnitude of the change. Cu oxidizes
0
more easily (sharp stripping peak at Epa = −0.3 V) in the pres-
ence of en because stable Cu(en)22+ forms. Stripping of Cu0
returns the absorbance to its original baseline. Thus, reduc-
2+
0
tion of Cu(en)2 is to Cu , with no apparent formation of
Cu(en)21
+
.
The electrochemistry is changed significantly by adding
nc to the solution containing Cu(en)22+. During Cu(en)2
reduction with nc excess in the solution, preferential com-
2+
+
plexation of Cu with nc shifts the reduction potential to
1+
a less negative potential (Fig. 2E). Cu(nc)2 complex for-
0
mation prevents further copper reduction to Cu . No evi-
Fig. 3. Absorbance spectra of a 7.5 mM CuSO4 solution made with
0
−2
−2
2
× 10 M en, 2 × 10 M nc, and 0.1 M KNO3 in 1:3 (v/v) ethanol in
dence of deposition or stripping of Cu on the ITO OTE
was seen in the voltammogram or optically at 400 nm
not shown in Fig. 2). The anodic peak in Fig. 2E corre-
water taken during the reduction process in an ITO OTTLE in the potential
range from 0.8 V to −0.9 V (vs. Ag/AgCl ref) at 5 mV/s. (a) Cycling the po-
tential from 0.8 V to −0.65 V at 5 mV/s, (b–c) scanning the potential from
(
1+
sponds to oxidation of Cu(nc)2 accompanied by ligand
−
0.65 V to −0.9 V, (c–d) scanning the potential from −0.9 V to 0.8 V, (e–a)
exchange with en to reform the more stable Cu(en)22+. As
The potential was held at 1.0 V for 30 min.

3
196
T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
Nafion-SiO2 film. Noteworthy is the absence of a Cu2O strip-
ping peak in either case.
Fig. 4B shows the corresponding ATR optical signals
recorded concurrently with the voltammograms in Fig. 4A.
At the Nafion-SiO2 ITO OTE the absorbance at 454 nm in-
creases substantially at potentials negative of −0.5 V, which
is consistent with formation of Cu(nc)21+. The absorbance
decreases when the potential exceeds 0.5 V on the reverse
scan. At the end of the positive scan (0.2 V), the absorbance
is about 0.18 AU; however, it decreases back to its original
value of 0 AU if the potential is held at this value for about
10 min.
Comparison of the response at the modified electrode ver-
sus the bare electrode (barely visible in Fig. 4b) shows a
change in absorbance that is 250 times larger at the modi-
fied electrode. Large sensitivity enhancement was provided
by pre-concentration into the Nafion-SiO2 coated ITO OTE.
3.3. Effect of nc concentration in the Nafion film
The effect of the concentration of nc in the film was
studied by varying the concentration of nc in the contact
solution during the loading process and then by exposing
2
+
the nc-loaded film to a solution of 1.0 mM Cu(en)2
.
Fig. 4. Enhancement of the electrochemical and optical signals by the pres-
ence of Nafion-SiO2. (A) Cyclic voltammograms of a bare OTE (curve 1)
and an OTE modified with Nafion-SiO2 (curve 2). (B) Sensor absorbance vs.
time responses corresponding to voltammograms in A. The measurements
The film was exposed to each nc solution for 3 h during
loading. The concentration of nc in the contact solution
−
4
−2
was varied from 4.0 × 10 M to 4.0 × 10 M, yielding a
−3
were begun at 20 min post injection of 1 × 10 M CuSO4 solution with
−3
concentration of nc in the film varying from 5 × 10
M
−
3
4
× 10 M en in 0.1 M KNO3. The potential was scanned from 0.8 V to
0.9 V (5 mV/s, vs. Ag/AgCl ref).
to 0.84 M. The concentration of nc in the film was es-
−
timated by measuring an absorbance difference (ꢀA =
A(nc in solution before uptake)−A(nc in solution after uptake))
from
0
from −0.8 V to −0.9 V due to deposition of Cu at the elec-
4.1 mL of nc solution in a cuvette and substituting this into
−1
−1
trode surface (not shown).
Beer’s Law (ꢀA = εbc) using ε(nc at 300 nm) = 8695 M cm .
Cyclic voltammograms of Cu(en)22+ at a Nafion-SiO2
ITO OTE with trapped nc and at a bare ITO OTE with an
excess of nc in solution are shown in Fig. 4A. The voltam-
mograms are very similar except that the peak current is
three times larger at the modified electrode than at the bare
ITO electrode due to pre-concentration of Cu(en)22+ into the
A summary of these experiments is provided in Table 2.
For a constant Cu(en)22+ concentration in sample solution,
ꢀA increases with increasing nc in the film until nc in
the film exceeds Cu(en)22+ by about a factor of two.
This is the ratio that would stoichiometrically convert all
2+
1+
Cu(en)2 to Cu(nc)2 . Higher concentrations of nc gave
Table 2
Effect of ligand concentration in the film
nc concentration in
contact solution, (M)
nc concentration in
the film, (M)
ꢀAc
Concentration of
[nc]/[Cu] ratio in the
filme
a
b
Cu(en)2 in the filmd
2+
−
−
−
−
−
−
4
3
2
2
2
2
4
4
1
2
3
4
.0 × 10
.0 × 10
.0 × 10
.0 × 10
.0 × 10
.0 × 10
a
0.005
0.18
0.39
0.64
0.84
0.84
0.006
0.30
0.60
0.83
0.60
0.53
0.002
0.10
0.20
0.29
0.20
0.18
2.5:1
1.8:1
2.0:1
2.2:1
4.2:1
4.7:1
Concentration of nc used for pre-concentration (t = 4 h).
b
Estimate of concentration of nc in the film (as estimated by ꢀA = A(nc solution before uptake)−A(nc solution after uptake), and volume of the cell 4.1 mL, and ε(nc at
−1
−1
M , where A = εbc).
3
00 nm) = 8695 cm
c
2+
ꢀ
A is the change in absorbance of Cu(en)2 pre-concentrated from 1 mM solution and nc in the film.
Estimate of concentration of Cu(en)2 in the film (as estimated by penetration depth and 10 reflections, and ε(Cu(nc)2 ) = 7950 cm M , where ꢀA = εbc).
Ratio of estimated nc concentration to estimated Cu(en)2 concentration in the film.
d
e
2+
+
−1 −1
2+

T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
3197
no further increase in ꢀA, but rather caused a decrease
in signal. The reason for this behavior is not known, but
perhaps a high concentration of nc in the film makes the
film more hydrophobic, causing a decrease in partitioning
of hydrophilic Cu(en)22+. The optimal performance of the
sensor was achieved with the concentration of nc in the film
equal to 0.64 M. Here, the sensor shows the highest change
based on comparison to the data obtained using the chemical
reducing agent. Since conversion decreases as the potential
scan rate increases, very slow electromodulation is required
if maximum ꢀA is desired.
3.5. Calibration curve
2+
in absorbance, ꢀA = 0.83, for 1 mM Cu(en)2 solution
A representative calibration curve for standard solutions
2+
(
Table 2). Thus, the detection limit of Cu(en)2 should be
2+
of Cu(en)2 is shown in Fig. 6. Fig. 6A illustrates the op-
tical response driven by the electrochemical reduction and
optimized under these conditions. However, we note that the
range of the calibration curve is limited on the upper end by
the concentration of nc in the film.
2+
1+
ligand exchange converting Cu(en)2 to Cu(nc)2 as the
potential was scanned between 0.8 V and −0.9 V at 5 mV/s.
3
.4. Effect of scan rate on modulated absorbance
The rate of potential scanning during spectroelectrochem-
ical modulation has a large effect on the absorbance versus
time profile as illustrated in Fig. 5 for a Cu(en)22+ loaded
nc-Nafion-SiO2 film. At the slowest scan rate investigated
2+
1+
(
1 mV/s), complete conversion of Cu(en)2 to Cu(nc)2
during the reduction cycle is evidenced by plateauing of
the absorbance at ca. 0.75 AU. However, conversion back
to Cu(en)22 is incomplete as evidenced by plateauing at
+
0
.15 AU on the oxidation cycle, rather than returning to 0 AU.
We attribute this behavior primarily to the slow kinetics of
2+
the ligand exchange reaction to regenerate Cu(en)2 . Even
though conversion is incomplete, the signal levels off be-
cause the potential begins its second cycle in which it be-
comes too negative to oxidize Cu(nc)21+. This situation is
made worse by faster scan rates for which ꢀA systematically
decreases with increasing scan rate. Increasing the poten-
tial scan rate also causes the electrochemical diffusion layer
thickness to become thinner than the sensing film and the pen-
etration depth of the evanescent beam (i.e., the optical path).
This in turn leads to incomplete electrolysis in the film for
conversion of Cu(en)22+ to Cu(nc)2 in Nafion-SiO2. Full
1+
2+
conversion was estimated by reducing Cu(en)2 chemically
by adding excess hydroxylamine to the solution and wait-
ing for 30 min for ligand exchange to occur with the excess
of nc in the film. At 1 mV/s 100% conversion was achieved
Fig. 6. Calibration data for Cu(en)22+ taken up into a Nafion-SiO2-nc film.
(
A) The ATR optical responses driven by the electrochemical and chemi-
2
+
+
−6
−5
cal changes of Cu(en)2 to Cu(nc)2 for (1) 5 × 10 M, (2) 1 × 10 M,
−
5
−4
−3
−4
−4
(
3) 5 × 10 M, (4) 1 × 10 M, (5) 3 × 10 M, (6) 5 × 10 M, (7)
−
3
−3
Fig. 5. Modulation at 450 nm for CuSO4 taken up into a Nafion-SiO2
thin film coated on ITO-glass at steady-state film loading in nc and in
1 × 10 M, and (8) 2 × 10 M CuSO4 with 4 × 10 en. (B) Calibration
plot of ꢀA vs. concentration. (C) The linear range of the calibration plot.
All measurements were taken at 450 nm after the film was equilibrated with
CuSO4 solutions containing 4 × 10 M en and 0.1 M KNO3 supporting
electrolyte. The potential was scanned between 0.8 V and −0.9 V at 5 mV/s.
−
4
−3
5
.0 × 10 M CuSO4 and 4 × 10 M en. (0.1 M KNO3 supporting elec-
−
3
trolyte). Thepotentialwasscannedbetween0.8and−0.9 V(triangularwave)
at 1, 5, 10, 20, 30 mV/s scan rates.

3
198
T. Shtoyko et al. / Electrochimica Acta 50 (2005) 3191–3199
2
+
2+
2+
2+
These optical responses (ꢀA) were used to generate the cali-
bration curves in Fig. 6B and C. Fig. 6B shows the curve over
a broad range of concentrations, whereas Fig. 6C is restricted
to the linear region. The curve has a limited linear range span-
such as Co , Ni , Zn , and Cd . The selectivity of the
sensor for Cu(en)22+ in the presence of such interferences
will depend on stability constants, electrochemical charac-
teristics, and spectral properties of the interfering species.
For this particular list of metal ions, the stability constants
ning Cu(en)22 concentrations of 5 × 10 M to 1 × 10
+
−6
−3
M
+
and deviates negatively at higher concentrations. The non-
linearity at higher concentrations is due to one or more factors
that could include: (1) non-linear partitioning of Cu(en)22+
into the film as it becomes saturated; (2) insufficient excess
of nc in the film for stoichiometric ligand exchange; and (3)
slow ligand exchange if the potential scan rate is too fast. The
error bars in Fig. 6B represent the reproducibility achieved
at each concentration with three different electrodes. These
deviations are due to differences in the ITO electrodes, dif-
ferences in film thickness and composition, and distribution
of nc and ion-exchange sites within the film. The optimal nc
concentration in the contact solution as determined above was
used for the ligand uptake. Though the conversion at 5 mV/s
is only 48% complete, this scan rate was used to generate the
calibration curve in order to speed up the analysis time; we
note that scanning at 1 mV/s should lead to lower limits of
detection.
of their complexes with nc are much smaller than for Cu
7.0, 8.5, 7.7, 10.4, respectively) [21] and the spectroelec-
(
trochemical properties of these complexes are also very dif-
ferent. Therefore, nc is an excellent choice of ligand for the
selective determination of copper complexes. However, fu-
ture work on this sensor should include a careful evaluation
of possible interferences by these and other metal ions such as
antimony, which interferes with the determination of copper
by stripping voltammetry.
The ligand exchange mechanism for enhancing ꢀε as
demonstrated herein is a general strategy that should be ex-
tendable to other metal ions by judicious choice of appropri-
ate ligands and charge-selective films.
Acknowledgments
The sensing film can be used multiple times provided it is
washed thoroughly with 0.5 M potassium nitrate after each
use in order to remove partitioned analyte.
This work was supported by a grant awarded by the En-
vironmental Management Science Program of the U.S. De-
partment of Energy, Office of Environmental Management
(DE-FG07-99ER62311-70010 and a University of Cincin-
nati Doctoral Investment Award).
4
. Conclusions
The concept of ligand exchange within the sensing film
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