Development of chemical sensors based on redox-dependent receptors. Preparation and characterization of phenanthrenequinone-modified electrodes

Article abstract of DOI:10.1021/ac991120w

The study of phenanthrenequinone(PQ)-modified electrodes, prepared by electropolymerization of a phenanthrenequinone-pyrrole derivative, is described. The surface-confined PQ is shown to behave similarly to PQ in solution, acting as a redox-dependent rece

Full text of DOI:10.1021/ac991120w

Anal. Chem. 2000, 72, 1860-1865
Development of Chemical Sensors Based on
Redox-Dependent Receptors. Preparation and
Characterization of Phenanthrenequinone-Modified
Electrodes
Yu Ge and Diane K. Smith*
Department of Chemistry, San Diego State University, San Diego, California 92182-1030
and transduction is not unique to optical recognition. It is also
possible to design molecules whose electrochemical properties
change in a detectable fashion upon interaction with an analyte.
Our group has focused its research efforts on one way to do this,
through the use of redox-dependent receptors.2,3
A receptor can be defined as a compound that selectively binds
another compound (substrate) through a variety of noncovalent
interactions. A redox-dependent receptor is one in which the
binding strength to a particular substrate changes upon reduction
or oxidation of the receptor. If this occurs and the redox process
is reversible, then the redox potential of the receptor will change
in the presence of the substrate. That change in redox potential
can then be used to detect and measure the concentration of
substrate by electrochemical means, even though the substrate
itself may not be electroactive.
The study of phenanthrenequinone(P Q)-modified elec-
trodes, prepared by electropolymerization of a phenan-
threnequinone-pyrrole derivative, is described. The surface-
confined P Q is shown to behave similarly to P Q in
solution, acting as a redox-dependent receptor toward
aromatic ureas in aprotic solvents. Large, positive shifts
in the E1 / 2 of the P Q0 / - redox couple are observed in the
presence of these ureas, due to a strong hydrogen bonding
interaction between the P Q radical anion and urea. The
effect is fully reversible. Of the substrates examined, only
aromatic ureas produce a significant shift. Nonaromatic
ureas or other HN and HO containing compounds have
little effect on the E1 / 2 . The magnitude of the shift is also
independent of electrode coverage, allowing reproducible
measurements to be made despite significant loss in
material from the surface.
The equilibria involved in redox-dependent receptor-substrate
binding is given in eq 1, where R stands for the receptor and S
the substrate that binds to the receptor. Kox and Kred are the
binding constants between the substrate and the oxidized and
reduced forms of the receptor, respectively. Assuming excess
substrate concentration, application of the Nernst equation and
The design and characterization of new electrochemical sen-
sors continues to be an extremely active area of research in
analytical chemistry.1 As with all types of sensors, a key compo-
nent is the recognition element, which must show some degree
of selective interaction with the analyte. However, with electro-
chemical sensors, this interaction usually does not directly
produce the analytical signal. Instead, recognition is coupled to
separate transduction components and these create an electro-
chemically detectable signal. For example, the recognition element
in a polymer membrane ISE is the selective ion-exchange agent
or neutral ionopher, but the signal is a change in potential across
the membrane. With amperometric enzyme electrodes, the
enzyme is the recognition element, but, in many cases, the enzyme
redox chemistry is not directly electrode-accessible and the signal
is actually reduction or oxidation of the enzyme cofactor through
a mediator.
(1)
the equilibrium constant expressions to this scheme leads to eq
2 at 25 °C
1 + Kred[S]
0.0592V
∆E′ )
log
(2)
(
)
n
1 + Kox[S]
where ∆E′ is the change in the formal redox potential of the
receptor after adding substrate. This equation predicts an ISE-
like response, that is, a 60/ n mV change in observed E′ per decade
change in substrate concentration over a concentration range
between approximately 1/ Kox and 1/ Kred. For example, if the weak
The fact that multiple chemical components are required to
couple recognition to signal production in most electrochemical
sensors means that the chemical side of the sensor is, by
necessity, complex. In contrast, with optical sensors, the signal
is often a change in the optical properties of the recognition
element itself. This means that recognition and transduction are
combined in a single molecule, greatly simplifying the chemical
side of the sensor. However, this ability to combine recognition
(2) (a) Smith, E. A.; Lilienthal, R. R.; Fonseca, R. J.; Smith, D. K. Anal. Chem.
1 9 9 4 , 66, 3013. (b) Lilienthal, R. R.; Smith, D. K. Anal. Chem. 1 9 9 5 , 67,
3733. (c) Lilienthal, N. A.; Enlow, M. A.; Othman, L.; Smith, E. A.; Smith,
D. K. J. Electroanal. Chem. 1 9 9 6 , 414, 107.
(1) Janata, J.; Josowicz, M.; Vanysek, P.; DeVaney, D. M. Anal. Chem. 1 9 9 8 ,
70, 179R.
(3) (a) Ge, Y.; Lilienthal, R. R.; Smith D. K. J. Am. Chem. Soc. 1 9 9 6 , 118, 3976.
(b) Ge, Y.; Smith, D. K., submitted to J. Org. Chem.
1860 Analytical Chemistry, Vol. 72, No. 8, April 15, 2000
10.1021/ac991120w CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/18/2000

binding constant was 1 M-1 and the strong binding constant was
1000 M-1 then ∆E is expected to be linear with respect to log[S]
between approximately 1 mM and 1 M. At lower concentrations,
no significant binding occurs in either oxidation state, and at
higher concentrations the receptors are saturated in both oxidation
states.
anions through hydrogen bonding, making it easier to reduce the
quinone or diimide in their presence.
The initial studies with redox-dependent hydrogen-bonding
receptors were conducted with both receptor and substrate in
solution. However, to use redox-dependent receptors as sensors,
as well as in most other applications, they will need to be attached
to surfaces. In working toward the goal of creating practical
electrochemical sensors based on redox-dependent receptors, we
have chosen to utilize phenanthrenequinone as a prototypical
example of a redox-dependent hydrogen-bonding receptor and
explore its electrochemical and redox-dependent binding behavior
when attached to electrode surfaces.
Recently, one of the most popular methods for attaching
molecular species to electrode surfaces is to take advantage of
the strong chemisorption between a thiol or disulfide and a Au
surface.7 In initial studies we explored this method by preparing
the phenanthrenequinone thiol derivative, 2 . Although we were
successful in attaching 2 to gold electrodes, the electrochemistry
Most of the work with redox-dependent receptors has focused
on the development of such receptors for ionic compounds.4 In
contrast, we have focused our efforts on the design of redox-
dependent receptors for neutral organic compounds.2,3 Most
recently, we have been exploring the concept of redox-dependent
hydrogen bonding as a particularly potent means to design such
receptors.3 The concept, which is also being actively explored by
Rotello5 and Tucker,6 is straightforward. Hydrogen bonds are
known to have substantial electrostatic character. Therefore, a
reduction or oxidation process that leads to a change in partial
charge on one of the components in a hydrogen bond will have
a significant effect on the strength of that hydrogen bond. In
particular, if the negative charge on the hydrogen acceptor or the
positive charge on the hydrogen donor is increased, the strength
of the hydrogen bond will be increased. Alternatively, if the
negative charge on the hydrogen acceptor or the positive charge
on the hydrogen donor is decreased, the strength of the hydrogen
bond will be decreased.
In our earlier work,3 we showed that substantial effects can
be produced even in very simple systems. Specifically we looked
at the behavior of 9,10-phenanthrenequinone, P Q, and 1,8-
naphthalimide, NI. Both P Q and NI undergo reversible one-
electron reductions in aprotic media to form radical anions, eq 3
and 4. Although the radical is delocalized, most of the negative
of the phenanthrenequinone was considerably different than in
solution and, even more problematic, the electrodes were not very
stable in nonaqueous solution.8
In this paper we employ electropolymerization of a phenan-
threnequinone pyrrole derivative, P QP , as an alternate method
to attach phenanthrenequinone to electrode surfaces. Like thiols,
electropolymerization of functionalized pyrroles has been widely
used to prepare modified electrodes,9 including those containing
anthraquinone10 and benzoquinone11 groups. Another type of
redox-dependent receptor, a ferrocene-containing crown ether, has
also been attached to electrodes using this method.12
(7) Finklea, H. O. In Electroanalytical Chemistry. A Series of Advances; Bard, A.
J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109.
(8) Smith, D. K.; Ge, Y.; Lilienthal, R. R. In New Directions in Electroanalytical
Chemistry; Leddy, J., Wightman, R. M., Eds.; Electrochemical Society:
Philadelphia, 1996; p 305.
(9) (a) Deronzier, A.; Moutet, J.-C. Coord. Chem. Rev. 1 9 9 6 , 147, 339. (b)
Curran, D.; Grimshaw, J.; Perera, S. D. Chem. Soc. Rev. 1 9 9 1 , 20, 391. (c)
Deronzier, A.; Moutet, J.-C. Acc. Chem. Res. 1 9 8 9 , 22, 249.
(10) (a) Moteki, S.; Sykes, A. G. J. Electroanal. Chem. 1 9 9 8 , 447, 91. (b) Allietta,
N.; Pansu, R.; Bied-Charreton, C.; Albin, V.; Bedioui, F.; Devynck, J. Synth.
Met. 1 9 9 6 , 81, 205. (c) Andrieux, C. P.; Audebert, P. J. Electroanal. Chem.
1 9 9 0 , 285, 163. (d) Audebert, P.; Bidan, G.; Lapkowski, M. J. Electroanal.
Chem. 1 9 8 7 , 219, 165.
charge resides on the oxygens due to their greater electronega-
tivity. For this reason, substrates with appropriately positioned
N-H groups hydrogen bond with the carbonyl oxygens much
more strongly in the reduced states. Examples of good substrates
are urea derivatives with P Q and the diamidopyridine derivative
1 with NI. The increase in binding strength is detected by large
positive shifts in the redox potential of P Q and NI in the presence
of these substrates. In effect, the substrates stabilize the radical
(11) (a) Aquino-Binag, C. N.; Kumar, N.; Lamb, R. N. Chem. Mater. 1 9 9 6 , 8,
2579. (b) Schuhmann, W.; Huber, J.; Wohlschlaeger, H.; Strehlitz, B.;
Gruendig, B. J. Biotechnol. 1 9 9 3 , 27, 129.
(12) (a) Ion, A. C.; Popescu, A.; Ungureanu, M.; Moutet, J.-C.; Saint-Aman, E.
Adv. Mater. (Weinheim, Ger.) 1 9 9 7 , 9, 711. (b) Ion, A. C.; Moutet, J.-C.;
Pailleret, A.; Popescu, A.; Saint-Aman, E.; Siebert, E.; Ungureanu, M. J.
Electroanal. Chem. 1 9 9 9 , 464, 24.
(4) Beer, P. D.; Gale, P. A.; Chen, Z. Adv. Phys. Org. Chem. 1 9 9 8 , 31, 1.
(5) (a) Deans, R.; Niemz, E.; Breinlinger, E.; Rotello, V. M. J. Am. Chem. Soc.
1 9 9 7 , 119, 10863-10864. (b) Niemz, A.; Rotello, V. M. Acc. Chem. Res.
1 9 9 9 , 32, 44.
(6) Carr, J. D.; Lambert, L.; Hursthouse, M. B.; Malik, K. M. A.; Tucker, J. H.
R. Chem. Commun. 1 9 9 7 , 1649.
Analytical Chemistry, Vol. 72, No. 8, April 15, 2000 1861

With functionalized pyrroles, the electroactive group is typically
attached to the pyrrole nitrogen via an alkyl chain. Oxidation of
the pyrrole creates a radical cation which couples with another
radical cation or with a neutral pyrrole followed by further charge
transfer. A neutral dimer is formed after two protons are
eliminated. The dimer is then oxidized immediately to the radical
cation at the applied potential. The oligomers and/ or polymers
of pyrrole formed in this manner contain pyrrole units connected
mainly by 2,5-linkages. Since they are considerably less soluble
than the monomer, they remain on the electrode surface. Elec-
trode derivatization is usually accomplished by cycling the elec-
trode potential through the polypyrrole and pyrrole oxidation waves.
These increase in size with each cycle as more material is de-
posited on the electrode. The coverage can be controlled by moni-
toring the current and stopping when it reaches a desired value.
As detailed in the Results and Discussion section, preparation
of phenanthrenequinone-modified electrodes via electropolymer-
ization of the pyrrole derivative, P QP , proved much more
successful than the thiol method. At low coverages the electro-
chemistry of the quinone is well-behaved and shows sensitivity
to aromatic ureas similar to that seen in solution. The electrodes
are also much more stable in aprotic solvent than those prepared
from the phenanthrenequinone thiol derivative, 2 .
(400 mL). The aqueous solution was extracted with ethyl ether
(4 × 100 mL), and the combined organic extracts were washed
with water (3 × 100 mL) and brine (1 × 100 mL) and then dried
over MgSO4. A yellow oil (1.3 g) was obtained after the solvent
was evaporated in vacuo. The crude oil was chromatographed
(silica gel, 10% CH2Cl2/ hexane) to yield a colorless oil (1.1 g, 7.47
1
mmol). HNMR (CD3OD) 1.46 (m, 2H), 1.87 (m, 2H), 2.18 (m,
3H), 3.92 (t, J ) 7.2 Hz, 2H), 6.03 (t, J ) 2.0 Hz, 2H), 6.66 (d, J )
2.0 Hz, 2H); MS m/ z 148(M+ + 1).
2-Iodophenanthrenequinone. A mixture of nitric acid (2 mL)
and sulfuric acid (5 mL) was added to a solution of phenanthrene-
quinone (2.08 g, 10.0 mmol) and I2 (2.56 g, 5.0 mmol) in acetic
acid (100 mL). The solution was then heated to reflux for 4 h,
and a small amount of orange solid formed. The mixture was
allowed to cool to room temperature and sit for 10 h without
stirring. It was then cooled to 10 °C and filtered. The solid was
washed with CH2Cl2 (3 × 5 mL) and dried under vacuum for 2 h
to afford a bright orange crystalline solid (2.43 g, 7.23 mmol, 72%
yield). 1H NMR (CDCl3) δ 7.55 (t, J ) 7.6 Hz, 1H), 7.73 (d, J ) 15
Hz, 1H), 7.75 (d, J ) 8.8 Hz, 1H), 7.99 (d, J ) 8.0 Hz, 1H), 8.04
(dd, J ) 8.4, 2.0 Hz, 1H), 8.20 (d, J ) 8.0 Hz, 2H), 8.50 (d, J ) 2.0
Hz, 1H); MS m/ z 336 (M+).
2-(6-(P yrrol-1-yl-5-hexynyl))phenanthrenequinone, P QP .
2-Iodophenanthrenequinone (159 mg, 0.476 mmol), 10% Pd/ C (101
mg, 0.0949 mmol), CuI (36.3 mg, 0.191 mmol), Ph3P (99.9 mg,
0.381 mmol), and K2CO3 (164 mg, 1.19 mmol) were mixed in a
1:1 solution of water and dimethoxyethane (10 mL) at room
temperature. The mixture was degassed with N2 for 5 min and
stirred for 30 min under N2. 1-(Pyrrol-1-yl)-5-hexyne (4 , 100 mg,
0.679 mmol) was then added, and the mixture was heated to 80
°C for 2 h. After cooling, the reaction mixture was extracted with
CH2Cl2 (5 × 20 mL). The extracts were combined, dried over
MgSO4 and filtered through Celite. The solvent was evaporated
in vacuo, and the residue was purified by flash chromatography
(silica gel, 10-20% CH2Cl2/ hexane) to produce an orange solid
(121 mg, 0.342 mmol). 1H NMR (CDCl3) 1.58-1.68 (m, 2H), 1.92-
2.02 (m, 2H), 2.47 (t, J ) 7.0 Hz, 2H), 3.97 (t, J ) 7.0 Hz, 2H),
6.04 (t, J ) 2.9 Hz, 2H), 6.70 (t, J ) 2.9 Hz, 2H), 7.49 (t, J ) 7.6
Hz, 1H), 7.69 (dd, J ) 8.4, 1.8 Hz, 1H), 7.73 (t, J ) 7.3 Hz, 1H),
7.95 (d, J ) 8.2 Hz, 1H), 8.99 (d, J ) 8.0 Hz, 1H), 8.19 (s, 1H),
8.20 (t, J ) 7.0 Hz, 1H); MS m/ z 354 (M + H+).
Electrochemical P rocedures. Voltammetry experiments
were performed with a PAR model 263 digital potentiostat with
the model 270 electrochemistry software package. The acquisition
mode was set to “ramp” in order to simulate an analogue
experiment. The CH3CN used as solvent was freshly distilled from
CaH2 and filtered through an activated alumina column right
before use. A glassy carbon disk (5-mm diameter) was used as
the working electrode in all experiments. It was first polished with
0.25 µM diamond polishing paste, rinsed thoroughly with water,
then polished with 0.05 µM alumina paste and rinsed thoroughly
with water and acetone. All measurements were conducted under
N2 in a one-compartment cell with a Pt-wire counter electrode. A
Ag wire was used as a pseudoreference electrode in the solution-
phase experiments with ferrocene as an internal reference. A Ag
wire in a solution of 0.1 M Bu4NPF6/ CH3CN was placed in a
separate compartment and used as a reference for the derivatized
electrode testing.
EXPERIMENTAL SECTION
1
Synthetic P rocedures. H NMR spectra were recorded on a
Varian Unityplus 400 MHz spectrometer. Unless specified, CDCl3
was used as the solvent for 1H NMR. All chemical shifts are
reported in ppm and are relative to TMS. J values refer to H-H
coupling constants. Mass spectra were recorded on a MS-9 AEI
spectrometer. Reaction solvents were either freshly distilled from
a drying reagent or obtained as the anhydrous grade from the
manufacturer and stored under N2. Other chemicals were of
reagent grade and used as supplied from the manufacturer.
Toluene-4-sulfonic Acid Hex-5-ynyl Ester, 3. Triethylamine
(7.0 mL, 50.0 mmol) was added dropwise to a solution of 5-hexyn-
1-ol (1.96 g, 20.0 mmol) and p-toluenesulfonyl chloride (4.56 g,
24.0 mmol) in CH2Cl2 (15 mL) at room temperature. The solution
was then stirred until the starting material disappeared (∼1 h).
Afterward, the solvent was removed in vacuo and the residue taken
into CH2Cl2 (100 mL). The solution was washed with water (1 ×
25 mL), 0.5 N HCl (3 × 25 mL), saturated sodium bicarbonate (3
× 25 mL), and brine (1 × 25 mL) and dried over MgSO4. A yellow
oil was obtained (5.44 g) after removing the solvent. The crude
product was chromatographed (silica gel column, 0-10% EtOAc/
1
Hexane) to afford a colorless oil (4.75 g, 18.8 mmol). H NMR
(CDCl3) δ 1.57 (m, 2H), 1.79 (m, 2H), 1.94 (t, J ) 2.4 Hz, 1H),
2.18 (td, J ) 11, 2.8 Hz, 2H), 2.47 (s, 3H), 4.08 (t, J ) 6.4 Hz, 2H),
7.37 (d, J ) 8.3 Hz, 2H), 7.80 (d, J ) 8.3 Hz, 2H); MS m/ z 252
(M+).
1 -(P yrrol-1 -yl)-5 -hexyne, 4 . To a solution of pyrrole (1.34
g, 20.0 mmol) in DMF (30 mL) cooled to 0 °C was added NaH
(60% dispersion in mineral oil, 0.88 g, 22.0 mol) under N2. The
mixture was allowed to warm to room temperature and stirred
for 30 min. The solution was then cooled to 0 °C again and a white
solid precipitated. Next, a solution of toluene-4-sulfonic acid hex-
5-ynyl ester (3 , 2.52 g, 10.0 mmol) in DMF (10 mL) was added,
and the resulting mixture was stirred for 2 h at room temperature.
The reaction was quenched with MeOH (10 mL) followed by water
1862 Analytical Chemistry, Vol. 72, No. 8, April 15, 2000

Scheme 1
Electrode Derivatization. A freshly polished glassy carbon
electrode was placed in a cell containing a solution of 1.0 mM
P QP in 0.1 M NBu4PF6/ CH2Cl2. A Ag wire was used as the ref-
erence electrode and Pt wire as the counter electrode. The glassy
carbon electrode was cycled between -0.9 and 1.7 V three times
at 100 mV/ s starting in the negative direction. The electrode was
then rinsed with CH2Cl2 and then acetone and dried under N2.
Once the acetone evaporated, the electrode was used immediately.
Voltammetry P rocedures. Solvents and electrolytes were
added to a cell containing working, reference, and counter
electrodes and protected under N2. The solution was degassed
with N2 for 2 min. After taking the background scan, the quinone
and internal reference, if needed, were added for solution-phase
experiments and the CV was taken. Then the guest was added
and the CV taken again. With the derivatized electrodes, two cells,
one with guest and the other without, were used. The difference
in half-wave potentials before and after adding the guest or
between the cells was then calculated.
Figure 1. Derivatization of a C electrode with PQP. Scans (a), (b),
and (c) are the first, second, and third CV scans of a glassy C
electrode in a 1 mM solution of PQP in 0.1 M Bu4NPF6/CH2Cl2. Scan
rate ) 100 mV/s.
the latter. The process was monitored by checking the redox
current of phenanthrenequinone.
Figure 1 shows cyclic voltammograms (CVs) of the derivati-
zation process. Voltammograms a, b, and c are the first, second
and third scans, respectively. The anodic peak at about 1.5 V is
the oxidation peak of pyrrole, the broad waves between 0 and 0.5
V are due to polypyrrole, and the redox couple at about -0.5 V is
phenanthrenequinone. The current of all waves increases as more
scans are conducted, a result of more material present on the
electrode surface after polymerization. The redox potential of the
phenanthrenequinone appears to shift toward more negative
potentials as the film grows, but this may be an artifact of the
Ag-wire reference electrode. The peak-to-peak separation does
increase, probably because of slower kinetics resulting from the
multiple layers. Also note that an extra reduction wave grows in
at potentials negative of the original quinone reduction. This is
probably due to the existence of a number of different environ-
ments within the film, which will change the redox potential of
quinone. Thicker films result if more cycles are conducted, but
this leads to very complicated quinone electrochemistry with
multiple overlapping voltammetric waves. To simplify interpreta-
tion, the cycles were limited so as to give very low coverage of
phenanthrenequinone on the electrode.
After the derivatization process was over, the electrodes were
rinsed with solvent, dried under N2, then placed in a CH3CN
solution containing no pyrrole. Even when the derivatization cycles
were kept to a minimum the initial CVs of the derivatized
electrodes often exhibited multiple overlapping peaks in the
quinone region as shown in Figure 2a. However, upon cycling in
fresh electrolyte, the quinone wave decreased in size and simpli-
fied, eventually reaching a stable voltammogram with the very
small peak-to-peak separation characteristic of a very thin layer
of electroactive material, Figure 2b. In other cases the shoulder
on the negative side of the reduction peak persisted even after a
stable voltammogram was obtained as shown in Figure 3a.
Behavior of C/ P QP Electrodes. Figure 3 shows the CVs of
a C/ PQP electrode in the presence of different amounts of
1-phenyl-3-propylurea. Scan a is the electrode by itself. Scans b,
RESULTS AND DISCUSSION
Synthesis. The synthesis of the pyrrole-terminated phenan-
threnequinone, P QP , is outlined in Scheme 1. Tosylate 3 was
prepared from tosyl chloride and the commercially available
alcohol using triethylamine as a base. Deprotonation of pyrrole
with NaH followed by reaction with 3 gave the alkyne-terminated
pyrrole, 4 . This was then coupled with 2-iodophenanthrene-
quinone, which was prepared from phenanthrenequinone using
a modified version of a literature procedure.13
A number of different reaction conditions, including the
standard Heck coupling procedure, have been tried in our lab to
couple 2-iodophenanthrenequinone with alkynes. Most of them
give the desired product but the yields are poor. The best results
have been obtained using the conditions reported here, which
are similar to those described by Beicher and Cosford.14
Electrode Derivatization. Glassy carbon disk electrodes were
chosen for derivatization with P QP because preliminary studies
indicated that the pyrrole polymer adheres to the carbon surface
longer than Pt or Au, giving a more stable electrode. The
electrodes were derivatized with P QP by scanning from 0 V to
-0.9 V then up to +1.6 V and back to 0 V vs Ag wire several
times in a cell containing a 1 mM solution of P QP in 0.1 M Bu4-
NPF6/ CH2Cl2. CH2Cl2 was used instead of the more commonly
used CH3CN because of the limited solubility of the monomer in
(13) Slyusarchuk, V. T.; Novikov, A. N. J. Org. Chem. USSR 1 9 6 7 , 3, 1283.
(14) Beicher, L.; Cosford, N. D. P. Synlett 1 9 9 5 , 1115.
Analytical Chemistry, Vol. 72, No. 8, April 15, 2000 1863

Figure 2. CVs of a C/PQP electrode in 0.1 M Bu4NPF6/CH3CN:
(a) first scan after derivatization, (b) 16th scan after derivatization.
Scan rate ) 100 mV/s.
Figure 4. CVs of a C/PQP electrode in the presence of 1,1-dibutyl-
3-phenylurea in 0.1 M Bu4NPF6/CH3CN: (a) 0 mM urea, (b) 5 mM
urea, (c) polished electrode. Scan rate ) 100 mV/s.
Table 1. Shifts in Half-Wave Potential, ∆E1/2, of
Phenanthrenequinone, PQ, and Derived Electrode,
C/PQP, in CH3CN upon Addition of Substratesa
∆E1/ 2PQ0/ 1- (mV)
substrate (10.0 mM)
PQ in soln
C/ PQP
1,3-di-(4-trifluoromethyl)phenyl urea
1,3-diphenyl urea
1-phenyl-3-propyl urea
butyl urea
1,3-dipropyl urea
1,1-dibutyl-3-phenyl urea
ethylene glycol
water
173 ( 10
119 ( 10
66 ( 12
17 ( 6
14 ( 9
2 ( 2
65 ( 5
107 ( 17
63 ( 8
1 ( 7
2 ( 6
3 ( 13
0 ( 15
7 ( 15
1 ( 2
2 ( 3
1 ( 2
ethanol
propylamine
3 ( 8
0 ( 11
0 ( 7
a
The reported ∆E1/ 2 values are the average of at least 3 measure-
ments. The error values correspond to the 95% confidence limits.
Figure 3. CVs of a C/PQP electrode in the presence of different
amounts of 1-phenyl-3-propylurea in 0.1 M Bu4NPF6/CH3CN: (a) 0
mM urea, (b) 5 mM urea, (c) 10 mM urea, (d) 20 mM urea, (e) fresh
cell, 0 mM urea. Scan rate ) 100 mV/s.
significant shift in the redox potential of phenanthrenequinone in
solution. Figure 4 shows the CV of (a) the derivitized electrode
by itself, (b) the electrode upon addition of 1,1-dibutyl-3-phenyl-
urea, and (c) the electrode after polishing. Just as in solution, 1,1-
dibutyl-3-phenylurea causes no significant shift in the potential of
the surface-confined quinone wave. This supports the conclusion
that the shift seen with 1-phenyl-3-propyl urea is due to the specific
interaction between this urea and the phenanthrenequinone
radical anion on the electrode surface.
Table 1 lists the shifts in half-wave potential, ∆E1/ 2, observed
for a C/ PQP electrode in the presence of 10 mM of various guests
in CH3CN and ∆E1/ 2s observed for phenanthrenequinone in
solution under the same conditions. The results show that the
surface-confined phenanthrenequinone is even more selective than
that in solution. Only the aromatic ureas, the guests which cause
a large shift in E1/ 2 of phenanthrenequinone in solution, cause a
significant shift with the derivitized electrodes. With 1,3-diphenyl-
urea and 1-phenyl-3-propylurea, the magnitude of the shift is
similar to that observed in solution. Surprisingly,1,3-di-(4-trifluoro-
methylphenyl)urea, the guest which causes the largest shift in
solution, gives a smaller shift than the other aromatic ureas with
the surface-confined phenanthrenequinone. The reason for this
is not obvious. It may be a steric effect.
c, and d are the CVs of the derivitized electrode taken in the
presence of 5, 10 and 20 mM 1-phenyl-3-propylurea, respectively.
As observed in the solution phase, the redox potential of the
phenanthrenequinone shifts toward more positive values in the
presence of urea, indicating a strong interaction between the
surface-confined quinone radical anion and the urea. Most
importantly, the process is reversible. Scan e is a CV taken with
the same electrode in a blank cell after taking the CVs in the urea
solutions. The redox potential of the primary quinone wave has
returned to close to its original value. (Interestingly, the shoulder
that was originally on the negative side of the primary reduction
peak has disappeared and a shoulder now appears on the positive
side. On electrodes with no shoulder to begin with there is still
no shoulder after returning the electrode to the blank solution.)
To confirm that the shift in redox potential of the derivitized
electrode is the result of binding between the urea and phenan-
threnequinone and not a change in a property of the solution or
the polymer film due to nonspecific interactions, the same
electrode was also examined in the presence of 1,1-dibutyl-3-
phenylurea. This urea has only one N-H and does not give a
1864 Analytical Chemistry, Vol. 72, No. 8, April 15, 2000

this electrode indicates it is possible to produce electrodes with
the stability required for chemical sensors using this methodology.
CONCLUSIONS
In this project, we successfully demonstrated that redox-
dependent receptors such as phenanthrenequinone can be con-
fined on the electrode surface by polymerizing pyrrole derivatives
of the receptors. The electrodes derivitized with phenanthrene-
quinone-pyrrole polymer are reasonably stable in organic media
such as methylene chloride and acetonitrile. Although loss in
coverage occurs, the redox potential of the surface-confined
phenanthrenequinone is stable under different conditions over a
several-hour-long testing period. Significantly, the behavior of the
phenanthrenequinone on the electrode surface is very similar to
that in solution. The redox potential of the phenanthrenequinone
in the derivatized electrodes shifts positive in the presence of
aromatic ureas, and the relative sizes of the shifts are largely in
accordance with those observed in solution. Such electrodes can
be considered as aromatic urea sensors and could be used to
detect and measure the concentration of such ureas in the
millimolar range in organic media.
Figure 5. E1/2 values for a C/PQP electrode transferred back and
forth between a blank cell (b) and a cell containing 10 mM diphenyl-
urea (2) in 0.1 M Bu4NPF6/CH3CN.
The lower detection limit for diphenylurea with the C/ PQP
electrodes is approximately 1 mM. As discussed in the Introduc-
tion, this is consistent with a binding constant of approximately
103 between the PQP radical anion and the urea. This seems
reasonable, given that the binding constant between phenan-
threnequinone and diphenylurea is 900 M-1 in DMF solution.3b
The binding constant to neutral phenanthrenequinone cannot be
determined with a great deal of accuracy, but appears to be
between 0.1 and 0.01 M-1, which would correspond to an upper
detection limit of >10 M. However, we are unable to verify this
with the C/ PQP electrodes because rapid loss of coverage occurs
above 50 mM of urea, presumably due to solubilization of the PQP
oligomers after bonding to urea.
Since loss of coverage is always an issue with modified
electrodes, this project also points out an inherent advantage that
modified-electrode sensors, in which the signal is a change in
potential, have over those in which the signal is a change in
current. With the latter, the magnitude of the signal will be
proportional to both the concentration of analyte and the amount
of electroactive material on the electrode, so a decrease in
coverage will decrease the sensitivity and alter the calibration of
the sensor. However, with sensors such as those described here,
the magnitude of the signal will depend only on the concentration
of analyte. As long as enough material remains on the electrode
that ∆E can be reliably measured, the calibration will stay the
same.
Figure 6. CVs of a C/PQP electrode in 0.1 M Bu4NPF6/CH3CN:
(a) first scan after conditioning, (b) last scan after 93 CVs in 31
different cells, about 9 h after first scan, (c) after polishing the
electrode. Scan rate ) 100 mV/s.
The reproducibility in potential of the derivatized electrodes
is also good. Figure 5 shows the variation in E1/ 2 for the surface-
confined quinone as the electrode is taken back and forth between
a blank solution and a solution containing 10 mM phenylpropyl-
urea. The E1/ 2 values in the blank solution are within 11 mV of
each other, as are those in the urea solution.
A gradual loss in coverage does occur as the C/ PQP electrodes
are used. The loss is more rapid in the presence of aromatic ureas,
probably because binding helps solubilize the pyrrole oligomers.
However, despite the loss in coverage, the potential of the pyrrole
remains constant making it possible to use some of the electrodes
for a remarkable length of time. Figure 6 shows CV’s of one
electrode (a) immediately after equilibration and (b) after 9 h of
experiments, including 93 CVs in 31 cells. Despite the significant
loss in coverage that occurred, the redox potential of the electrode
only changed from -656 ( 0 to -650 ( 4 mV between CV a and
CV b, and the electrode was still usable at the end. Although not
all C/ PQP electrodes lasted as long as this one, the behavior of
ACKNOWLEDGMENT
The authors wish to thank the donors of the Petroleum
Research Fund administered by the American Chemical Society
and the National Science Foundation for partial support of this
work.
Received for review September 29, 1999. Accepted
January 24, 2000.
AC991120W
Analytical Chemistry, Vol. 72, No. 8, April 15, 2000 1865