Use of different electropolymerization conditions for controlling the size-exclusion selectivity at polyaniline, polypyrrole and polyphenol films

231

J. Elecfroanal. Chem., 213 (1989) 231-242

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

Use of different electropolymerization conditions for controlling the size-exclusion selectivity at polyaniline, polypyrrole and polyphenol films Joseph Wang, Shi-Ping Chen and Meng Shari Lin Department

of Chemistry, New Mexico State University, Las Cruces, NM 88003 (U.S.A.)

(Received 2 June 1989; in revised form 17 July 1989)

ABSTRACT Controlled anodic growth of polyaniline, polyphenol and polypyrrole films is exploited for changing their permeability to solute species. In particular, fine molecular weight cutoffs are obtained by varying the electropolymerization time or monomer concentration. The exclusion of large electroactive species offers substantial improvements in the selectivity of amperometric detection in flowing streams. For example, a judicious choice of the polymerization time allows selective flow injection measurements of catechol, hydrogen peroxide, acetaminophen or hydrazine in the presence of excess of uric acid, ascorbic acid, chlorpromazine, or potassium ferrocyanide, respectively. Complex chromatograms are greatly improved. Prevention of electrode deactivation due to protein adsorption is observed in the case of polyaniline films. Scanning electron micrographs show the microstructures of films following different anodization times. The electrochemical approach for making permselective coatings is shown to be very versatile as it provides an elegant way of varying the transport properties.

INTRODUCTION

Chemically modified electrodes (CMEs) have found new and interesting applications as effective chemical sensors. A very important feature of CMEs based on permselective coatings is the ability to control access at the sensor surface. Exclusion of undesired species has been shown to enhance greatly the selectivity and stability of electrochemical sensors and detectors [l-7]. For example, various films can protect the substrate electrode from foulants present in the contacting solution phase. The resulting electrodes can thus exhibit significantly longer lifetimes than the corresponding bare surfaces [1,8,9]. Furthermore, the discriminative properties of size-exclusion films offer high specificity towards small solutes of analytical interest [1,2,7]. Similarly, polyionic films enable selective detection of counter-ionic analytes via the rejection of co-ionic electroactive interferences [3,4]. An even higher degree of specificity can be achieved by coupling two permselective films in a 0022-0728/89/$03.50

0 1989 Elsevier Sequoia S.A.

232

bilayer configuration [lo]. Coupling of permselective films with other (catalytic, preconcentration) functions can result in powerful sensing devices [11,12]. Particularly attractive for meeting the needs of numerous sensing applications is the ability to control and vary the permeability of permselective films. Approaches for manipulating the transport of electroactive species through permselective films include base hydrolysis [2] and RF-plasma [6] treatments that change the pore size of cellulose acetate and Nafion coatings, respectively, and variation of the dosage of y-radiation during the preparation of poly(acrylonitrile) films [SJ. In the present paper we describe the control of the permeability of polyaniline (PA@, polyphenol (PPh), and polypyrrole (PPy) films. The use of organic films generated on electrode surfaces by electropolymerization has gained popularity in recent years [13,14]. The electropolymerization of monomers such as aniline and pyrrole is particularly attractive because the resulting films are generally homogeneous, strongly adherent to the surface and chemically stable. As a result, electropolymerized films have been extensively evaluated for several technological applications such as storage batteries, electrochromic displays or corrosion suppression. Sensing applications can also benefit from the attractive properties of PAn and PPy films. For example, the doping-undoping features of these polymers have been exploited for the detection of electroinactive anions in flowing streams [l&16]. Molecular analogs of transistors or diodes, based on electroactive/conducting polymers, form the basis for new sensing devices [17,18]. Analytical advantages accrue also from the incorporation of redox mediators [19,20] or biocatalysts [21] into PPy and PAn matrices. Polymeric films, formed during the oxidation of phenolic compounds, have been traditionally regarded as a problem that limits the utility of finitecurrent measurements [22]. The passivating layer usually hinders the redox process of interest, resulting in poor stability. Recent work, however, has illustrated that a deliberate oxidation of phenolic compounds can be advantageous. In particular, the high permselectivity of polyphenol and poly(2,6-dimethyl-1,4phenylene oxide) films on platinum surfaces toward hydrogen ions has resulted in effective pH-response electrodes [7,23,24]. No attempt was made to control and manipulate the permeability of polyphenolic films, and hence to expand their utility towards larger analytes. The ability to vary the permeability of electropolymerized films, via their controlled anodic growth, is exploited in the following sections to obtain effective size-exclusion selectivity for use with electrochemical detectors. It is well known that the morphology of PAn and PPy coatings can be controlled by varying the electroplating conditions, particularly the amount of charge [25-271. The present work demonstrates that the use of different anodization conditions provides a very versatile way to vary the transport properties of electropolymerized films. Such precise control of surface microstructures cannot be achieved by analogous “chemical” preparation modes (based on spreading aliquots of the polymer or dip-coating). The electropolymerization approach would be advantageous also for preparing sire-selective micro sensors and for the incorporation of other (chemical or biological) functions to further enhance the selectivity.

233

Apparatus The “home-made” flow injection system and the Bioanalytical Systems Model LC-303 liquid chromatograph were described previously [2]. A platinum thin-layer electrochemical detector (Model Tl-10, Bioanalytical Systems) was used in the PAn and PPy experiments. A glassy carbon flow cell (Model Tl-5, Bioanalytical Systems) was employed in the PPh studies. The Ag/AgCl reference electrode and stainless steel auxiliary electrode were located in a downstream compartment. Sample loops of 20 ~1 (PAn, PPy films) and 100 ~1 (PPh films) were used. A Biophase C,, cohunn (25 cm x 4.6 mm) was employed. An EG& G PAR Model 264A voltammetric analyzer was used in conjunction with a Houston Omniscribe strip chart and X-Y recorders (flow and batch measurements, respectively). Scanning electron microscopy was done with a Phillips SOlB-SEM. Reagents Deionized water was used to prepare all solutions. Stock solutions of potassium ferrocyanide, ascorbic acid, sodium hydroxide and hydrazine (Baker), hydrogen peroxide (Aldrich), bovine albumin, dopamine, catechol, 4-methylcatechol, glutathione, uric acid, dihydronicotinamide adenine dinucleotide (NADH) and acetaminophen (Sigma) were prepared daily. Pyrrole, phenol, and aniline were obtained from Aldrich, Fisher and Allied Chemical, respectively. A 0.05 M phosphate buffer was used as carrier and mobile phase in the flow injection and liquid c~omato~aphy experiments. Procedure Prior to its coating, the working electrode was hand-polished with ahunina slurries of 3, 1 and 0.05 pm for 1, 1 and 5 mm, respectively. Residual polishing material was removed from the surface by sonication in a water bath for 3 mm after each polishing process. The polyaniline films were prepared in 50 ml of a quiescent 2 M sulfuric acid solution containing 0.1 M aniline, by scanning the potential between - 0.1 V and + 0.90 V at 50 mV/s. Different durations of the electroplating process were used, based on the desired permeability. Polypyrrole fihns were deposited in a 0.2 M potassium chloride solution containing 10 mM pyrrole. For this purpose the plating potential (+ 0.90 V) was applied for different periods. Polyphenol coatings were prepared in deaerated methanol solutions containing 0.3 M NaOH and different levels of phenol. For this purpose the potential was cycled three times between 0.0 and 1.0 V (vs. Ag/AgCl) at a rate of 50 mV/s; the polymerization was terminated during the third cycle by holding the potential at + 1.0 V for 1 mm. Following the electropolymerization, the electrode was rinsed with the supporting electrolyte solution and the flow cell was assembled. Amperometric measurements were made after the background current had decayed to a steady-state value. Flow injection experiments were performed with phosphate buffer (pH 5.5 (PA@, pH 6.8 (PPh), pH 7.4 (PPy)) as the carrier/electrolyte

234

solution (flowing at 1.0 ~/~) and using applied potentials of +0.8 V (PAn), +0.90 V (PPh) and +0.95 V (PPy). RE9JLl-S

AND DISCUSSION

Transport characteristics The ability to vary the permeability of PAn and PPy films via control of the electropolymerization time is illustrated in Fig. 1. Various reactants representing vastly different molecular sizes are employed. The ratio between the flow injection current at the film-coated electrode over that at the bare electrode is used as a measure of the permeability. Distinct changes in pe~~bi~ty, based on solute size, are observed. A facile transport of the small hydra&e and hydrogen peroxide through both films is observed following different polymerization times. A sharp decrease in the transport of larger species (e.g., catechol, methylcatechol, acetaminophen, ascorbic acid; mol mass 100-200 a.m.u.) is also observed upon extending the duration of the polymerization. The large NADH and ferrocyanide species are completely (or nearly completely) excluded from all films. Overall, the trends in permeability at both films are in good agreement with the variation in molecular mass (PPy: hydrazine(32) > cathechol(ll0) > acetaminophen(l51) > glutathione(307) # ADH(709 a.m.u.) and PAn: hydrogen peroxide(34) > 4-

10 lo

!!sx,o( i,b

~100

I %

I

0

0

5 Time/min

111

”

*

I”

..d

Tima/min

Fig. 1. Dependence of the permeability of the PPy (A) and PAn (B) films on the polymerization time for various solutes reptwenting different molecular sizes: (A) hydrazine (a), catechol (b), acetaminophen (c), glutathione (d), and NADH (e); (B) hydrogen peroxide (a), ascorbic acid (b) methylcatechol (c), and potassium ferrocyanide (d). Row injection experiment, as described in the Experimental section. Solute concentration. 1 X 1O-4 M.

235

methycatechol(l26) > ascorbic acid(176) > potassium ferrocyanide(422 a.m.u.)). To examine whether other factors, and particularly the net electrical charge, contribute to the resistance to mass transport we evaluated the flow injection response of reactants with similar molecular mass but different charge. Similar permeability was obtained for 1 x 10v4 M ascorbic acid, uric acid, dopamine, serotonin and dopac at the 7 min-polymerized PAn coated electrode (solution pH, 7.4; applied potential, -i-O.8 V). These data indicate that while PAn is positively charged in its oxidized state, solute charge does not play a role in the permselectivity. As the oxidation of solution species involves permeation, it appears that under the experimental (preparation) conditions used the polymers are non-conductive. It is well known that the anodization time has a promounced effect on the growth mechanism of PAn films [28]. Tbree oxidation and reduction peaks, similar to those reported [29] for preparations with high voltages, were observed throughout this study. Larger peaks were observed upon continuous scanning, reflecting the continuous growth of the film. The growth rate increased non-linearly with cycle number, with faster growth at later stages. According to Stillwell and Park [28], such an observation indicates changes in the growth mechanism of PAn films (with autocatalytic growth and side reactions at later stages). Similar voltammograms were obtained throughout this work, reflecting the reproducibility of the polymerization process. ovation of the el~~opol~e~ PAn film by scanning electron microscopy is shown in Fig. 2. Both 8(A) and 14(B) min polymerized coatings exhibit a three dimensional reticular structure of interlocking pores. However, the diameter and density of the pores is strongly affected by the anodization time. Incomplete surface coverage is observed following 8 min anodization. While the appearance of these films is not homogeneous, the micrographs of Fig. 2 illustrate clearly the structural differences between coatings prepared at different anodization times. The transport properties (described in this study) are closely related to the film microstructure and thickness. The use of different monomer concentrations can also result in different film permeabilities. Figure 3 shows the dependence of the film permeability on the phenol concentration for different analytes of vastly different molecular sizes. The film formed from the 0.5 mM phenol solution effectively excludes the large NADH and ferrocyanide species, while allowing a facile transport of the small hydrazine and methyl&echo1 molecules. A 68% attenuation of the acetaminophen peak is also observed. The film permeability decreases gradually upon increasing the monomer concentration to 1 and 5 mM; these electrodes remained insensitive to NADH and ferrocyanide. All solutes are blocked from the surface by the “50-mM phenol” film. (Such monomer concentrations were used for selective sensing of hydrogen ions [7].) Typical preparation voltammograms for polyphenol films in the presence of different concentrations of the monomer (0.5-50 mM) are shown in Fig. 4 (a-d). The first polymerization cycle (curves 1) shows distinct phenol oxidation peaks at about +0.50 V (except for the 0.5 mM phenol solution for which the peak is poorly defined). The peak heights and areas increase with the monom~ ~ncentration. No corresponding reduction peaks appear on the return cycle. Oxidation peaks are not

Fig. 2. Scanning electron micrographs of Am&rating voltage, 7.2 I&‘.

PAn coatings F&ming 8 (A) and 14 @) poiymerization. xnia

observed upon continued scanning, reflecting the complete inhibition of the redox process. (With the 0.5 mM solution, a small peak is still observed during the second scan.) As indicated in the expc~e~t~ section, the pol~e~tion is stopped after holding the potential at + 1.0 V during the third cycle. Voltammogmms similar to those of Fig. 4 were obtained throughout this study, reflecting the reproducibility of the polymerization process. Changes in the charge consumed for the oxidation of phenol at different ~n~trations, and the ~~~po~~g surface coverages* are summarized in Table I. Values of 56,110, HO, and 230 mnoljcm2 are obtained for

237

I

1

0

50 103x Conc.,/M

Fig. 3. Dependence of the film permeability on the phenol concentration for detection of hydra&e (a), methyl~t~hol (b), ~t~ophen (c), podium ferrocyanide (d), and NADH (e). Fiow injection conditions: flow rate, 0.7 ml/min; applied potential, +0.90 V, solute ~n~ntration, 1 X low4 M.

I

I

1.0

05 POTENTIAL

I 0.0 /V

Fig. 4. Cyclic volt-ark for the electrodeposition of phenol. Monomer concen~ation, 0.5 (a), 5 (b), 10 (c), and 50 (d) mM. Scan rate, 50 mV/s; methanol solution containing 0.3 M NaOH. See Experimental Section for details.

238 TABLE 1 Electrical charge and surface coverage associated with electropolymerization solutions ’

from different

Monomer concentration/mM

Q/me

lo* r/m01 cmm2

Number of monolayers

0.5 5 10 50

0.38 0.78 1.23 1.58

5.6 11 18 23

540 1100 1740 2240

phenol

a Conditions as in Fig. 4. An n value of 1 was used to calculate the surface coverage.

0.5, 5, 10 and 50 mM phenol solutions, respectively. Assuming a smooth glassy carbon surface, such coverages correspond to 540-2240 monolayers of the phenoxy radical. According to Bejerano et al. [30], a complete passivation of platinum surfaces occurs after adsorption of several hundreds of monolayers of the phenoxy product. It should be pointed out that films exhibiting complete passivation using quiescent solutions (e.g., Fig. 4) yield different extents of surface fouling under forced-convection conditions. The extent of inhibiting the redox process in hydrodyn~c-volt~et~c experiments depends primarily on the reactant size.

The fine control of the permeability, via a judicious choice of the polymerization conditions, provides the basis for the analytical utility of electropolymerized films. Electrochemical detection to flow injection and liquid chromatography systems is used to illustrate the significance of the controlled permeability in regard to sensor development. In particular, the advantages accrue from the exclusion of potentially large interferences (both electroactive, with similar redox potentials, or nonelectroactive surfactants) greatly enhance the selectivity and stability of electrochemical measurements. For example, the permeability data (profiles) of Figs. 1 and 3 indicate that a highly selective response can be obtained for small analytes in the presence of otherwise ~terfe~g substances. Figure 5 illustrates the potential of the permselective PPh and PAn fihns for selective amperomet~c detection in fIow injection systems. With the bare electrode (A), it is not feasible to detect acetaminophen or catechol selectively in the presence of NADH or uric acid, respectively. In contrast, the coated electrodes (B) effectively exclude the NADH or uric acid from reaching the surface (a); as a result, the acetaminophen or catechol response is not affected by the presence of these compounds (b-d). The facile detection of catechol (110 a.m.u.) while rejecting the uric acid (168 a.m.u.) demonstrates the fine molar mass cutoff that can be achieved by a judicious choice of the polymerization time. A selective flow-injection amperometric response was obtained also for 5 X 10e5 M hydrogen peroxide in the presence of 2 X 10S4 M ascorbic acid or 5 X lob5 M a~t~ophen in the presence of 1 X 10F4 M c~o~rorn~e (&ruin polymerized PAn film, detection at +0.8 V), for 5 X lo-’ M hydrogen

239

Time

A

B i

-.I 0.5JJ"(A) l.O)dA (81

6 mini

:

d

t I

b b

!1i ,u a

a

-

i

:

Time

Fig. 5. Flow-injection determination of acetaminophen (bottom) and catechol (top), at bare (A) and coated (B) electrodes. (a) 1 X lop4 M NADH (bottom) and uric acid (top); (b-d) same as (a) but after three additions of 5 X 10m5 M acetaminophen (bottom) and catechol (top). Anodization time (PAn, top) 7 min; phenol concentration (bottom), 0.5 mM.

in the presence of 2 x 10e4 M glutathione (10-min polymerized PPy film, detection at 4-0.95 V), for 5 X IO-* M hydrazine in the presence of 1 X 1V4 M potassi~ ferrocyanide (PPh film, as in Fig. 3 with 0.5 n&i phenol), and for 5 x lo-’ M catechol in the presence of 2 X 10V4 M NADH (3-n-& polymerized PPy film, detection at +0.95 V). In the latter case, a glassy carbon detector was employed, illustrating the applicability of PPy films to common carbon surfaces. The above mixture yielded the expected additive response when using the unmodified surface. The size-exclusion selectivity, shown in Fig. 5, is based on a judicious choice of the anodization time (in accordance with the profiles of Fig. 1). Hence, any change in the el~~oplat~g conditions (monomer concentration, scan rate, peroxide

240

potential, solvent, electrolyte, electrode material, etc.) will require re-examination of the response vs. anodization-time profiles. The data of Fig. 5 indicate also the suitability of permselective electropolymerized films for detection in dynamic flow systems. All films exhibited a rapid increase and decrease of the flow injection currents, with a slight peak broadening due to the additional diffusional resistance. For example, the peak width (at 0.6 c,,) for methylcatechol at the PAncoated electrode (7-r& anodization) was 2.7 s as compared to 2.1 s for analogous measurements at the bare electrode (not shown). Detection limits of 1.5 X 10e6 M (0.96 ng) hydrazine and 3.5 X lop6 M (7.7 ng) catechol were estimated on the basis of the signal-to-noise characteristics (S/N = 3) of the flow injection response at PPy and PAn electrodes (7 min polymerization). Hence, no major loss in the inherent (ng) detectability of amperometric detection accompanies the selectivity gains of the electropolymerized films. Obviously, some attenuation of the sensitivity is observed (vs. the bare electrode). The extent of this attenuation depends primarily upon the solute size and anodization time. For example, while similar signals are observed for the small hydrazine molecules at the bare and coated electrodes, the response for the larger catechol species is greatly attenuated. The three peaks (b-d) in Fig. 5 are a part of six concentration increments up to 3 x 10e4 M. Linearity between the peak current and concentration was observed. Liquid chromatography with amperometric detection can also benefit from the size exclusion selectivity of electropolymerized films. The rejection of large species provides simplification of chromatograms for complex samples (Fig. 6). For example, more than 30 peaks of variable sizes were observed at the bare electrode for a diluted (1: 25) urine sample, compared to 18 at the coated electrode. Similarly, only 5 peaks were observed for the diluted urine sample at the PAn or PPy coated electrodes (5-min anodization, not shown). Hence, the quantitation of small eluting compounds (e.g., ascorbic acid, uric acid; peaks 1 and 2) is facilitated by the rejection of large co-eluting components, that exhibit severe interference at the bare electrode. We investigated also the utility of electropolymerized films to serve as protective layers, aimed at minimizin g electrode poisoning effects in the presence of surfaceactive organic materials. For example, the PAn layer effectively blocks the large albumin molecules from reaching the platinum surface. As a result, a highly stable response was obtained for repetitive flow injections of a 1 X 10m4 M hydrogen peroxide solution containing 2000 ppm albumin (RSD = 0.26%, n = 20). In contrast, a rapid loss in electrode activity (up to 35%) was observed at the uncoated electrode (RSD = 11%). Unlike PAn films, PPy and PPh coatings did not offer the desired protection, probably due to the interaction of organic surfactants with these films that gradually blocked access of analytes. In conclusion, we have shown that the use of different polymerization conditions provides a versatile way to vary the transport properties of PAn, PPh, and PPy films. Although different modes of electropolymerization are described for the three polymers, it is the control of the amount of charge that is crucial for the resulting

241

I

0

I

10

.

.

20

Time /min

30

Fig. 6. Chromatograms for diluted (1:25) urine sample obtained at the bare (a) and PPh coated (b) electrodes. Monomer concentration (b), 5 mM; applied potential, +0.90 V; flow rate, 1.0 ml/min-‘; mobile phase, 0.05 M phosphate buffer (pH 6.8).

changes in permeability. Such an electrochemical approach to making permselective coatings offers an elegant alternative to “chemical” preparation schemes. The incorporation of (chemically or biologically) “active” moieties during the controlled anodic growth of the polymer may yield additional improvements in the selectivity. While molecular size has been the focus of the present work, it may also be possible to control (electrochemically) the ionic permeability of PPy and PAn films, in a manner analogous to the ion-gate experiments of Burgmayer and Murray [31]. The use of the different anodization conditions may be useful to control the size-exclusion selectivity of other conducting/electroactive polymers (e.g., polythiophene, polyacetylene). It seems likely that the improved selectivity afforded by such films might be of general utility in electroanalysis. Work along these lines is in progress in our laboratory at present. ACKNOWLEDGEMENT

This work was supported by the National Institutes of Health (Grant No. GM 30913-06). Acknowledgement is made also to the Donors of The Petroleum Re-

242

search Fund, administrated by the American Chemical Society for partial support of this research. REFERENCES 1 2 3 4 5

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