The films were found to be smooth, homogeneous and adherent. .... rated in the polymer was manifested via the weak bands at. 2192 cm- ' (-CN group) and 2966 ..... there being no adequate explanation yet for the indepen- dently measured ...
ELSEVIER
Journal of Electroanalytical Chemistry 416 (1996) 67-74
New quaternized aminoquinoline polymer films: electropolymerization and characterization H.A. Abd El-Rahman ‘, J.W. Schultze Imtitut
fir
Physikalixhr
Chrmir
und
Elektrochemir,
Heinrich-Hrine-Uniorrsitiit,
Diisseldorf,
Germuny
Received 15 August 1995; revised 12 April 1996
Abstract Poly(S-aminoquinoline) films (10 to 4OOnm) were prepared on gold electrodes by anodic polymerization in acetonitrile using cyclic voltammetry and potential step methods. The films were found to be smooth, homogeneous and adherent. The electrochemical quartz crystal microbalance and ellipsometric measurements proved that the polymer film thickness increases linearly with increasing anodic charge of formation. The electroactivity of the polymer films was found to be very small in acetonitrile and aqueous solutions. The impedance measurements indicated that the polymer films are very resistive, with a resistivity of approximately 0.4MS1 cm. In sulphuric acid solutions, capacity- and photocurrent-potential curves of the polymer films show semiconducting behaviour. XPS and IT-JR indicated the presence of perchlorate anions in the polymer films as well as in the isolated product of the constant potential electrolysis. It is proposed that the polymerization proceeds mainly via coupling at position 1 (leading to quatemary ammonium cation formation) and at position 8. The exchange of the counter anions with [Fe(CN),14- anions from neutral solutions was found to be about 10% to 18% of the total inserted anions. With a loading efficiency of about 0.15 mol I- ‘, poly(5aminoquinoline) films can find applications in the field of fixed-mediators. Keywordsr
Polymer
films;
Poly(S-aminoquinoline);
Electropolymerization;
1. Introduction Redox polymer films have received a great deal of attention during the last two decades,from both the theoretical and technical point of view, in order to develop new electrode materials which could offer better electrocatalysis than conventional electrodes [l-14]. A redox polymer film which would allow both diffusion of the reactants and charge propagation through the film with reasonablerates is expected to be a much better mediation catalyst than the soluble mediator itself on bare electrodes. Several models, mostly based on steady state measurements,have been developed for treatment of the kinetics of mediation of electrochemical reactions by redox polymers [13,15-211. The redox polymer films can be prepared by incorporation of the redox species into chemically formed polymers [7,14,22-281, during electropolymerization as counter ions [29-321 or by electropolymerization of the redox species-
’ Permanent address: Chemistry University, Giza, Cairo, Egypt.
CO22-0728/96/$15.00 PII
SOO22-0728(96)047
Department,
Faculty
Copyright 0 1996 Elsevier 18-3
of Science,
Science
Cairo
Characterization
containing monomers[33-371. Another strategy for redox polymer film formation is the electropolymerization of bifunctional molecules, where the first functional group is involved in the polymerization processand the other group is devoted to attachment of the redox species.With the proper choice of monomers, homogeneousand adherent films can be formed and then loaded with the redox species. In the present work, the electropolymerization of 5aminoquinoline in acetonitrile, the characterization of the polymer films and their ability to incorporate [Fe(CN),14anions from aqueousneutral solutions are studied.
2. Experimental The monomer S-aminoquinoline (Aldrich, 97%) and tetrabutylammonium perchlorate (Fluka, Germany, analytical grade) were used without further purification. Acetonitrile (Janssen,Germany, analytical grade) was twice distilled. The aqueoussolutions were prepared from analytical grade chemicals and de-ionized “Millipore” water. The
S.A. All rights reserved.
68
H.A.
Abd
El-Rahman,
J.W.
Schultze
/Jourd
working electrodes were either gold wires with a diameter of 400 p.m (area 0.226cm2) or gold films evaporated on glass; 200nm AullOnm Tilglass (area 0.90 cm*). The counter electrode was a gold sheet (area 2.0 cm*>. The reference electrodes were Hg IHg ,Cl 2IO.1 M TB AP(MeCN); U = -0.023 V vs. SHE [38] in acetonitrile, a saturated calomel electrode (SCE) in aqueous KC1 solutions and a HglHg2S0,10.5M H,SO,; U = 0.68OV vs. SHE in sulphuric acid solutions. All potentials are given versus the SHE. An integrated home-built potentiostat with a fast autoranging amplifier and a positive feedback ZR-compensation was used for cyclic voltammetry and potentialstep measurements. Impedance spectra were measured with a Solartron/Schlumberger 1255 frequency response analyzer using an amplitude of 5 mV. FT-IR spectra were recorded for polymer and monomer in the form of powders in KBr pellets. XPS was recorded for the polymer films deposited on 200nm Au110 nm Tilglass by cyclic voltammetry from -0.30 to 1.5OV vs. SHE at 50mVs-‘; qanodic= 0.09 C cmm2 using Mg Ka (1253.6eV) radiation. Ellipsometric measurements were carried out on polymer samples prepared on 200nm AullOnm Tilglass electrodes by cyclic voltammetry using an automatic ellipsometer @FE 401, Sentech, Germany) with a He-Ne laser; A = 632.8 nm and P = 5 mW. For the quartz crystal microbalante measurements, 5 MHz AT-cut 13 mm diameter quartz crystals (with sputtered 150nm thick gold electrodes on both sides of the crystals) were positioned in special holders to allow contact of both electrodes with the resonance circuit and the frequency counter and only one electrode surface with the electrolyte. The mass sensitivity was determined by copper deposition and found to be 18ngHz-’ cm- 2. The photocurrent was measured by a two-phase lock-in (5206, EG&G) at a chopper frequency of 30 Hz on polymer films deposited on 200 nm Au 110nm Tilglass using the direct polychromatic light of a 450 W Xe lamp to increase the signal/noise ratio. All electrochemical experiments were carried out at a temperature of 20°C under N,.
3. Results
and discussion
3.1. Polymer jilm formation Fig. 1 shows typical cyclic voltammograms for the oxidative polymerization of 10 mM 5aminoquinoline in acetonitrile (MeCN) containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The first anodic peak at 1.08 V vs. SHE is attributed to the oxidation of the primary aromatic amine, i.e. the -NH, group, into the radical cation [39,4O]. Under the same conditions, the oxidation peak of aniline was found at 1.26V. The oxidation peak of 5-aminoquinoline is followed by a shoulder (absent for aniline). The further oxidation peak at 1.96V is attributed to the oxidation of the quinoline ring
oj’Electrounulyticu1
Chemistry
2.5
416
(1996)
67-74
,
2.0 -’
1.5 ‘E i
l.Of
.0.5 -
0.0
-0.5
i
/
-0.5
I 0.0
/ 0.5
/ 1 .o U(SHE)
I 1.5
I 2.0
2.5
/V
Fig. 1. Oxidation of I OmM S-aminoquinoline + 0. I M TBAP in MeCN gold electrodes. Scan rate 0.1 Vs- ‘. The arrows refer to the direction increasing number of scans.
on of
[41]. The oxidation of 5aminoquinoline is completely irreversible since no cathodic peaks could be detected down to - 0.3 V. This indicates that the oxidation products are involved in a following chemical reaction. In the next cycles the anodic current decreases substantially and the oxidation peaks shift towards higher potentials, especially that of the quinoline ring. Insoluble reddish-brown threads (oligomers) resulting from the oxidation were seen to fall from the electrode surface. Depending on the amount of charge passed, yellow to greenish-brown, almost homogeneous, smooth and adherent polymer films were deposited. The small cathodic charge passed during the formation, around 1 mC cm- *, indicates a very low electroactivity for the polymer films, as will be seen later. Microscopic investigation of the polymer films indicated that the previously mentioned properties deteriorate as the film thickness increases. These films are insoluble in MeCN and aqueous solutions, but can easily be removed by dipping the electrode into concentrated chromic or nitric acids. The polymer films are highly resistive and an accurate constant-potential electrolysis (at potentials before the quinoline ring oxidation) could not be performed owing to the very fast and sharp decrease of the current. Using a rotating disk electrode with rotation speeds up to 4OOOrevmin’, a voltammogram similar to the first positive scan shown in Fig. 1 was recorded, and no limiting current plateaus could be detected. This indicates that the electrode surface is highly and very rapidly blocked by the depositing polymer film. Potentiostatic current-time curves for the oxidation of 5-aminoquinoline showed that the current density drops rapidly to a minimum after a few seconds. The current density then increases linearly with the square of time (e.g. up to about 20 s at U = 1.56 V>,
H.A.
Table I Assignments by oxidative Vibration
of the Ff-IR polymerization mode a
N-H -CH, -CN =C=N+ c10;
c=c
(C-H),
Abd
J.W.
Schultze/Journal
data of poly(5amincquinoline) in acetonitrile
Wavenumber/cm-
films
5-Aminoquinoline
(M,B) 3234 (VW) 2966 (W)
3330
-
2192 (W) 1636 (Ml
-
1128 (9 1096 (Sl 628 (Ml 1586 (Ml 1530 (Ml 1480 (Ml 1460(M) 984 (Ml
formed
’
Poly(S-aminoquinoline) 3378
=
El-Ruhmun,
3202
1660 1612 1588 1462 980-944
(Ml (Ml
w (Sl (Sl (9 (S)
a All vibration modes are of stretching type except for the last one (bending type) [53]. B = broad, S = strong, M = medium, W = weak, VW = very weak.
which is a characteristic relation for nucleation and growth when controlled by an electrochemical step rather than diffusion of species to the electrode [42,43]. After a broader maximum, the current density decreases again. The behaviour is common to two-dimensional nucleation and growth and the maximum can be caused by a decrease of the effective nucleation centres [43,44]. The ohmic drop under potentiostatic control due to the low electronic properties of the films was roughly estimated to be a few millivolts. Based on the experimental results, the polymerization process seems to proceed via diffusion of the monomer through the porous growing polymer film to the electrode surface according to an oxidation-deposition sequence in a similar way to the familiar dissolution-precipitation mechanism. As will be seen later, the faradaic efficiency of polymerization is low because a substantial amount of the oxidation products go into solution.
ofElectroanalytica1
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2192 cm- ’ (-CN group) and 2966 cm- ’ (-CH, group). Unfortunately, the N-N vibrational wavenumber for the system Ar’-N+-NH-Ar is not documented in the literature and is expected (if available) to be different for polymers because the N-N linkage is a part of the polymer skeleton. However, IR showed clearly the quatemization of quinoline and propagation of the polymerization via transformation of the primary to secondary amine. Analysis of the XPS data for the polymer films (Fig. 2(a)) revealed the following elemental ratios within approximately 2% error: C/N 4.5 (4.51, N/Cl 4.0 (4.01, N/O 1.0 (l.O), C/Cl 18.1 (18.0) and C/O 4.4 (4.5). The values in parentheses refer to the theoretical values of the polymer C,sH,sN4C104 (see below). In choosing 5aminoquinoline for the electropolymerization, an electroactive conducting polymer was thought to be obtained (in analogy to polyanilines) via coupling of the reactive radical cation at position 8 (the most reactive site towards electrophiles) [45]. However, the resulting polymer films showed a very small electroactivity and no faradaic pro-
5rd s E 4000. .-E % 6000.
0
0
C’ VP)
200
400
Binding
I 800
600
energy
l( DO
I eV
3.2. Structure of the polymer The most interesting El’-IR spectral assignments of the vibration modes for the polymer formed by constant potential electrolysis at 1.80 V vs. SHE and also for the monomer are given in Table 1. The two monomer bands at 3330 and 3202cm-’ (characteristic of primary amines) were replaced by one broader band at 3378 cm-’ for the polymer (the second band having almost disappeared), which signifies the transformation of the primary to a secondary amine. The band at 1636 cm-’ (absent in the monomer case) is attributed to the =C=N+ = group (aromatic C=N+ in conjugation with C=C). The strong bands at 1128, 1096 and 628 cm-’ (absent for the monomer) are attributed to the incorporated ClO; anions in the polymer. The fact that traces of the solvent, MeCN, were incorporated in the polymer was manifested via the weak bands at
0 394
396
398
400
Binding Fig 2. (al XPS for polfiS-aminoquinoline) XPS for N (IS) of polyiS-aminoquinolinel: fitting.
402
energy
404
406
4 18
/ eV films. (b) High + , measured;
resolution
70
HA.
Abd
El-Rahmun,
J.W.
of Electroanulytical
Schultze/Journol
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416
(1996)
nl = 1.26
67-74
kl = 0.04
100
50 N+ NH
0 40
20
0
60
100
80
q anod,cI mC cm.’ Fig. 3. Ellipsometric thickness qanodiC for polyi5-aminoquinoline) on 200nm AullOnm Tilglass.
(W
Scheme 1. Mechanism acetonitrile.
of anodic polymerization
of 5-aminoquinoline
in
cesses,while they contained substantial amounts of ClO; anions. A reasonableexplanation is that the coupling occurs to some extent at position 1, i.e. at the quinoline nitrogen, which leads to quatemary ammonium cation formation. The quatemary ammonium salts and complexes for quinolines with inorganic and organic reagents are numerous [45]. Based on the experimental results, it is assumedthat the polymerization processpropagatesmainly through the mutual coupling at positions I and 8 (Scheme 1). The chemical formula of the assumed polymer is (C,,H,,N,CQ), , i.e. C/N 4.5, N/Cl 4.0, N/O 1.0, C/Cl 18.0, C/O 4.5, C/H 1.38 and H/N 3.25. The fitting of the high resolution XPS for N (Is) included three peaks at 399.5 (peak 1 in Fig. 2(b)), 402 (peak 2) and 405 (peak 3). The first (main) and the secondpeaks are typical for nitrogen in aromatic aminesand quaternary ammonium saltsrespectively [46], and the latter (weak) one is usually reported for N-containing polymers, e.g. polyvinylpyridine, polyaniline and polyvinylcarbazole [47,48].
versus anodic charge of formation films deposited by cyclic voltammetry
charge passedduring the formation (Figs. 3 and 4). The optical parameters, namely the refractive index of the polymer film (nl = 1.26) and its extinction coefficient (kl = 0.04) were determined for two films of different thicknessesby measuring the ellipsometric parameters A and tj at five incident angles(44” to 75”) at a resolution of 0.01”. For each angle of incidence about 20 readings were automatically recorded and the average value was used in calculations. Before that, the values of refractive index and extinction coefficient of the substrate (n2 and k2) were
-450
3.3. Characterization of the polymer JTLrns
d
1
,
I
I
I
,
I
0
10
20
30
40
50
60
8 70
q anMIEl mC cm-’
In situ QCM and ex situ ellipsometric measurements proved that the amount of polymer deposited onto the electrode surface is directly proportional to the anodic
Fig. 4. Changes in frequency Af and mass gain m versus anodic charge of poly(5-aminoquinoline). of formation qanodic during the deposition Scan rate 20mV s- ’ . The numbers refer to the number of scans.
H.A.
Abd
El-Rohman,
J.W.
of Electroanalytic&
Schultze/Journal
logff / H7.1 Fig. 5. Bode plots for pol#i-aminoquinoline) wire electrodes in 0.1 M TBAP in MeCN thickness 173nm. Electrode area 0.226cm’.
films deposited on gold at different potentials. Film
determined experimentally using the software provided with the ellipsometer and assuming that the refractive index of air equals 1.00. The values of n2 and k2 were found to be identical at different angles of incidence. A home-written program was usedto fit the measuredparameters into a one-layer model by searchingfor the minimum value of the error function G [49,50]. The optical parameters of the films and their thicknesseswere accepted for G < 0.05. The two samplestested gave the same optical parametersand, hence, the thicknessesof the rest of the sampleswere determinedby direct measurementusing the ellipsometer software and the previously determined optical parameters. Since the film thickness was found to increaselinearly with formation charge, the assumptionof the independenceof the optical parameterson thickness up to around 300nm seemsreasonable.Scanning the polymer film surfaces with the laser beam of the ellipsometer indicated that the polymer film thickness is homogeneous; the data plotted in Fig. 3 are the average of four to six surface locations. The film thickness d is given by d =
Kl(
~anodic
-
90)
(1)
where K, = 4.2nmmC’ cm2, qanodic is the anodic charge passedduring the formation and qO = 12mC cmP2. Fig. 4 shows the results of measurementswith the quartz crystal microbalance during potentiodynamic film formation. The massdensity increaseslinearly with anodic charge according to the relation m
=
K2(
qanodlc
-
46)
(2)
with K, = 14.0g(mol electrons)-’ and qb = 8 mC cme2 (seelater). From K, the charge efficiency for the polymer film deposition was estimated to be around 8%, assuming that the chemical formula of the polymer is C ,sH ,3N,C10,. Using the slopes(1) and (2), the density of the polymer films was found to be 0.38gcmm3. As can be seen in Fig. 5, the linearity between the massgain and charge becomes visible after the first three scans.where the deviation from
Chemistry
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71
linearity at the beginning may be attributed to the non-rigid nature of the loosely attached polymer (oligomer) layer. The extrapolation of m (or Af) to zero gives a charge of about 8mCcmm2, which is close to the charge qO obtained from ellipsometry (relation (1)). The difference (approximately 4mC cmm2) may be due to the fact that different substrateswere used and the ex situ ellipsometry involves a probable mass loss during the electrode transfer and drying (i.e. higher qO observed experimentally). This amount of charge is lost in other electrochemical processes rather than polymer growth, e.g. soluble oxidation products and oligomer formation. As the polymerization proceeds, the oligomer formation is suppressed,a more compact, adherent and rigid layer is formed and the linear response of the QCM holds. The small decrease in frequency, observed during the reverse scan and in the next positive scan at potentials before polymer formation, is connected to the insertion/removal of electrolyte ions and solvent molecules into/from the growing polymer film (Fig. 4). Such behaviour is also recorded for the polymer-covered electrodes in the absenceof the monomer. The experimental mass equivalent was found to be about 164g(mol electrons)- ’ , which exceeds that of one perchlorate ion (99.5 g (mol electrons)- ’ ), since some solvent molecules are involved also. The total exchange of ions is, however, only a few 100 pC cm*, owing to the very low electroactivity of this polymer. 3.4. Electronic
properties
of the film
Cyclic voltammograms of the polymer-covered electrodes in MeCN and aqueoussolutions revealed no apparent redox peaks and a much smaller electroactivity (in terms of charge per inverse cubic centimetre) compared with conducting polymers. The impedance spectra (Bode plots) for poly(5aminoquinoline) films in 0.1 M TBAP in MeCN at different potentials (Fig. 5) are similar to those of the insulating polymer films [51]. The data were fitted using the program CNLS1521,with an equivalent circuit involving the solution and film resistances(R, = Rsolution + I?,,,,,) in serieswith a parallel element of a capacitor Cd and the charge transfer resistanceR,, (Table 2). The solution resistanceRsolut was determined using the bare electrode as 6.3 a. The film resistivity, p = 5 X lo5 fl cm, is two orders higher than that of the reduced form of polyaniline films [5 11.The very Table 2 Parameters obtained by fitting the impedance spectra in Fig. 6 using an equivalent circuit of a resistor (Rfi = R,,, + RBO,Ut,On) in series with a parallel connection of a resistor R,, and a capacitor C,
u/
R,,/
cd/
R,,
V vs. SHE
fi
pFcmm2
R
R
Rem
- 0.50
0.00
52.5 40.9
5.09 6.46
7.24X lo4 2.25 x IO4
46.2 34.6
6.05 x lo5 4.52 x 10’
0.50
39.5
7.08
6.27X
lo4
33.2
4.35 x 105
Film
/
/
P/
HA. Abd El-Rahmm,
72
J.W. Schulrze/Journal
50
0 Dark &rent I
I
I
I
I
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416 (1996)
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ters, E = (nl - Ikl)2 = 1.69. Meanwhile the decrease in l/C2 at U < 0.6 V may be attributed to shunting effects, there being no adequate explanation yet for the independently measured photocurrent. Cyclic voltammetry showed that the electron transfer reactions of the redox [Fe(CN),]4-/3on the polymercovered electrodes are highly inhibited. The electrochemistry of the redox processes is assumed to proceed via mass transfer (diffusion and, to a lesser extent, migration through the porous film). The diffusion coefficient was estimated relative to that at the bare electrode, from potential-step chronocoulometry in the presence of 0.2 M KC1 as supporting electrolyte, to be DPlym/Dsolur = 0.05.
100
3_ NC 8-P
of Electrmmalytical
I
3
3.5. Loading of the polymer films with redox species
p? LL “6
2
N 0 N z
1
0
0.0
I
I
0.2
0.4
I
I
0.6
0.8
U(SHE)
/V
/
I
1.0
1.2
1.4
Fig. 6. Dependence of the square of the photocurrent If on potential (top) and Mott-Schottky plots (bottom) for poly(5-aminoquinoline) films in aqueous 0.5M H,SO,. Scan rate 20mVs-‘. Film thickness 240nm.
high value of R,, and the fact that it is almost independent of potential indicate the negligible faradaic activity of the polymer film and are consistent with the cyclic voltammetry of the films. The increase in C, with potential was confirmed by measuring the capacitance at 1OOOHz during the slow scanning of potential in the positive direction in MeCN. The film behaves similarly to a p-type semiconductor, but the potential influence is not pronounced enough for quantitative evaluations. In aqueous solutions, the capacity has a minimum at about 0.6V (see Fig. 6). In the same potential region, a small photocurrent could be detected with the lock-in amplifier. IPh shows a maximum at the same potential, U = 0.6 V. Tentatively, a plot of Izh vs. I/ is shown in Fig. 6 too. While the right part could be interpreted by a p-type semiconductor model, the behaviour at lower potentials cannot yet be explained adequately. Similar surprising effects were observed recently with other polymers [48]. It is interesting that the flat-band potential of the polymer film at U > 0.6 V, when behaving as a p-type semiconductor, is almost the same for the two independent measurements (0.88 V from the Mott-Schottky plot and 0.86V from the photocurrent). The donor concentration, approximately 6 X 10’7cm-3, was calculated from the slope of the linear part of the Mott-Schottky plot, and the dielectric constant of the polymer was calculated (roughly owing to the different frequency domains) from the optical parame-
The polymer-covered electrodes were immersed in 20mM K,[Fe(CN),] for 15 min, washed thoroughly with de-ionized water and then transferred into 0.2M KC1 solutions where the amount of redox species loaded into the films was determined from the cyclic voltammograms recorded at 50mVs-’ (Fig. 7). From the redox charge c of the redox species in density %edox 1 the concentration the polymer film was calculated as follows: c/M
= 1000 X qredox/Fd
(3) The dependence of c on the polymer film thickness is shown in Fig. 8. The fact that c is almost independent of d indicates that the incorporated redox species are more or less homogeneously distributed through the film. However, the c values are high enough to be comparable with the conventional redox polymers [26-291. The efficiency of
60’ 40
1
-20
1 -40
I -0.2
I 0.0
I 0.2
I
1
/
0.4
0.6
0.8
U(SHE)
1 .o
/ V
Fig. 7. Typical cyclic voltammogram for polfi5-aminoquinoline) unloaded and loaded with [Fe(CN),Y anions in aqueous 0.2M solutions. Film thickness 173nm. Scan rate SOmVs-‘.
films KC1
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Acknowledgements
0.3
2 0.2
Chemistry
\ 0
One of the authors (H.A.A) is obliged to the Alexander von Humboldt Foundation for award of a fellowship. The support of the Fonds der Chemischen Industrie is gratefully acknowledged. Thanks are also due to Mr. 0. Genz, A. Michaelis, M. Hirschfeld and M. Schweinsberg for technical assistance.
0.1
04 0
50
100
150 d / nm
200
250
0"
Fig. 8. Dependence of the concentration c of the incorporated species and the percentage loading on the polymer film thickness.
References redox [l] [2]
loading of the redox speciesinto poly(5aminoquinoline) films was calculated as follows:
[3] [4] [5]
loading/% = (4 X &,,JNpu)
X 100
where rKdox is the surface concentration of the incorporated redox species and NpU is the number of polymer units (i.e. the number of quatemary ammonium cations) per squarecentimetre. /VP” was estimatedfrom the relation
Npu= qanodic(df/dqanoctic )QCM F,F2/Mpu where F, is the QCM sensitivity in grams per hertz per square centimetre, F2 is the charge efficiency and M,, is the formula weight of the polymer unit (C ,sH ,3N,C10,). As can be seen in Fig. 8, the percentage loading is only around 10% to 18% of the theoretically available capacity of the polymer. The low values of the percentage loading may be due to the following: (i) exchange of someof the incorporated redox specieswith the supporting electrolyte anion, chloride in our case;(ii) steric effects, leading to the apparentinactivity of someof the incorporated speciesdue to the low conductivity of and the ionic diffusion through the film and/or retardation of charge transfer by a hopping mechanism. However the loading efficiency of poly(5aminoquinoline) is comparable with the crosslinked poly-Cvinylpyridine when loaded with IrCli- anions [28]. In conclusion, the oxidative polymerization of 5aminoquinoline in MeCN yielded thin and homogeneous polymer films which include quaternized quinoline sites having the ability to exchange a small amount of the incorporated counter anions with redox anions from aqueous solutions. In spite of the low electronic conductivity of the polymer films, they are not too tight or rigid to allow slow but not negligible diffusion processes,responsiblefor both growth of the films and allowing electron transfer reactions.
[6] [7] [8] [Y] [lo] [ll] (121 [ 131 [14] [15] [16]
[17] [18] [lY] 1201 [21] [22] [23] [24] 1251 [26] [27] [28] [29] [30]
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