134
Anal. Chem. 1987, 59, 134-139
(12) Randin, J. P.; Yeager, E. J. Elecfroanal. Chem. 1875, 58,313. (13) BjeHca, L.; Parsons, R.; Reevers, R. M. Croat. Chem. Acta 1880, 53, 211. (14) Hu, I . F. Ph.D. Thesis, Ohio State Unlverslty, Columbus, OH, 1985. (15) Christie, J. H.; Ostetyoung, R. A.; Anson, F. J . Nectroanal. Chem. 1967, 13,236. (16) h r d , A. J.; Faulkner, L. R. Nectrochemical Methods: Wiley-Interscience: New York, 1980. (17) Kazee, 6.; Welsshaar, D. E.; Kuwana, T. Anal. Chem. 1885, 57, 2736.
(18) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1882, 704, 2735. (19) Barton, S. S.; Harrison, B. H.; Dolllmore, J. J. Chem. Soc., Faraday Trans. 7 1873, 69, 1039.
RECEIVED for review April 14,1986.Accepted August 27,1986. We thank the National Science Foundation for their financial support* The work was begun at The Ohio State University, Columbus, OH.
Electrodes with Polymer Network Films Formed by y-Irradiation Cross-Linking Emory S. De Castro, Edward W. Huber, Denis Villarroel, Christos Galiatsatos, James E. Mark, and William R. Heineman*
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-01 72 P. Terrence Murray Research Institute, University of Dayton, Dayton, Ohio 45469-0001
A general method for immoMllrlng polymer f h on electrode surfaces usos y radlatkn to form a cross-Unked network. The water-soluble pdyelectrdyte, pdy(dlmethyWlaUylammonium chkrlde) (DMDAAC), Is knmobllized on graphhe and platkMl electrodes. Cycllc voltammograms of ferricyanide at DMDAAC modwled electrodes show lon exchange of the anbn into the fllm. The voltammograms are affected by supporting electrolyte Ionic strength and anion type. Variation of radlatlon dose changes the cross-link dendty of poly(acryknItrl1e) films on gold and thus can be used to control permeability of the fllm to electroacthre speck In sdutlon. An irradiated flkn of poly(ethylenlmhe) on platinum prevents electrode fouling in blood plasma.
y irradiation has been demonstrated to be an effective means of simultaneously immobilizing a water-soluble polymer such as poly(dimethyldially1ammoniumchloride) (DMDAAC) and covalently attaching to it an organic redox agent such as (2,6-dichlorophenol)indophenol(DCIP) (1). y radiation generates free radicals on the polymer chains (2)which react to both covalently cross-link the chains and attach the organic redox agent to the polymer. Cross-linking combines the polymer chains into a continuous network extending over the entire electrode. The introduction of cross-links into the polymer makes it intractable in that it is no longer soluble in any solvent (3). If, however, the un-cross-linked polymer was soluble in the medium to be used for the electrochemical measurement, the cross-linked network will now swell when placed in contact with that medium (4). Such a swollen network has a porous structure, the permeability of which can be controlled by changes in the cross-link density. y irradiation has several advantageous features as a technique for cross-linking. It is a technique that cross-links many different polymers and consequently should be a general method for forming polymer-coated electrodes. Different radiation doses can be used to control the degree of crosslinking and thus the permeability of the electrode to various solution species. An increase in radiation dose is expected to generate a greater population of free radicals and hence
Table I. Polymers name
abbreviation
poly(dimethyldiallyl- DMDAAC ammonium chloride)
structure
$TJ (1-3) X IO6
(NW
poly(acrylonitri1e)
PAN
poly(ethy1enimine)
PI
mol wt
CI-
YJ C
1.5 X lo6
W
L-fJ
1.8x 103 ~~
~
~
more cross-links, up to the point of chain scission where the net number of covalent bonds would then decrease with dose. y radiation is a good source for generating free radicals due to its ability to penetrate the polymer layer and produce a homogeneous distribution of radical sites. Another free radical source, W radiation, can exhibit limited film penetration and can require the addition of free-radical initiators. Chemical means of free-radical generation relies on the mass transfer of the radical generator into the polymer matrix and requires the ability to completely remove the chemical additive and byproducts once the reaction is finished. Of these three methods for generating free radicals, y radiation is best suited for creating a homogeneous distribution of cross-links in a polymer film. The objective of this research is to demonstrate the effectiveness of y irradiation as a technique for preparing polymer-network-coated electrodes and to describe some of the properties of these electrodes. The formation of polymer networks on electrodes by y irradiation is demonstrated with the water-soluble polyelectrolyte poly(dimethyldially1ammonium chloride). The ability to vary the dosage of y radiation to control the cross-link density, and hence film permeability, is shown with poly(acry1onitrile) (PAN). Finally, a network of poly(ethy1enimine) (PI) is demonstrated to protect a platinum electrode from fouling by proteins in a blood plasma solution. The structures of these polymers are shown in Table I. This paper focuses primarily on DMDAAC, which is a polyquaternary ammonium salt produced by the
0003-2700/87/0359-0134$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
free-radical polymerization of diallyldimethylammonium chloride (5, 6 ) . The following notation is used to designate electrodes: yx-polyrner/electrode substrate, where x is the radiation dose in megarads to which the eledrode was exposed.
EXPERIMENTAL SECTION Electrode and Polymer Preparation. Platinum wire 0.081 cm in diameter (Fisher Scientific Co., Cincinnati, OH) was cut into 4.4-cm segments and then pretreated by boiling in a 50/50 volume mixture of water and nitric acid for 1h, boiling in pure distilled deionized water for 1h, oven drying overnight at 80-110 "C, and then exposing to an air plasma (Harrick Plasma Cleaner) for 30 min. In the event previously immobilized polymer had to be removed, the above sequence was preceded by an additional 30-45 min of boiling in a solution of potassium permanganate and concentrated sulfuric acid. Spectroscopicgrade graphite rods (type AXF 5QBG1) 0.46 cm in diameter were obtained from Poco Co., Dallas, TX, and were pretreated according to one of two procedures. Early electrodes were pretreated by use of the procedure outlined above for platinum, except the oven time was extended to 24 h and the plasma cleaning was omitted. Later electrodes were pretreated by simply sonicating in distilled deionized water for 1h followed by overnight oven drying. A rough polishing step preceded the above if polymer had been previously immobilized on the electrode. The electrodes were prepared by adding the appropriate amount of polymer solution to the electrode surface and then drying. Amounts used and procedures are as follows: y2DMDAAC/graphite (Figures 2 and 4), add 10 ILLof a 15% DMDAAC solution to the end of a graphite electrode and spin dry; y15&5.5-DMDAAC/platinum(Figures 3 and 8), dip a platinum wire into a 15% solution of DMDAAC, shake off any excess solution, air-dry, repeat; y5.'-DMDAAC/graphite (Figures 5,6, and 7 ) ,add 20 ILLof a 15% DMDAAC solution to the end of a graphite electrode and spin dry; y-PANIAu, all dosages (Figures 10 and ll),add 0.1 g of PAN to 10 mL of chlorobenzene and heat in a water bath at 60 "C, apply 6 ILLof this PAN solution to a gold button electrode and dry in air with no spinning; y20-PI/ platinum (Figure 12), mix a 1% by weight solution of PI in chlorobenzene,dip platinum wire for 30 min, dry in air for 40 min, and dip again for 30 min. After application of the polymer, electrodes were purged with either nitrogen or argon in a glovebox and then sealed in 2-dram vials. The electrodes were then sent to either the University of Michigan, Phoenix Memorial Laboratory, Ford Nuclear Reactor, or the University of Cincinnati, Department of Nuclear Engineering, where they were exposed to y radiation fromoC a@ ' source. Doses varied from 0.01 to 20 Mrd. Thii dosage range was achieved by the appropriate combination of radiation flux (ca. 5-500 krd/h) and exposure time (ca. 10 min to 100 h). Electrodes were at ambient temperature during irradiation. Polymer strips were prepared by pouring undiluted polymer solution into an aluminum mold lined with Teflon fluorocarbon resin. The mold was placed in an oven at 50 "C until the polymer was dry. The polymer was removed from the mold and cut into rectangular strips weighing approximately 0.05-0.15 g. Each strip was placed in a glass vial where it was subjected to three cycles of a vacuum evacuation-argon purge and packed in a glass jar under argon. The jars were then sent to Phoenix Memorial Laboratory for irradiation. Reagents. All polymers in Table I were obtained from Polysciences, Inc., Warrington, PA, and used without further purification. Polysciences reports the molecular weight of DMDAAC to be (1-3) X lo6 as determined by light scattering, with a polydispersity index (the ratio of weight-average to number-average molecular weight) of 4. All other reagents and supporting electrolytes (MCB, Cincinnati, OH, or Sigma Chemical Co., St. Louis, MO) were used as received. Water was first distilled and then further purified through a Barnstead/Sybron system (Fisher Scientific Co., Cincinnati, OH). Cytochrome c (Horse Heart, type VI) was used as received from Sigma Chemical Co. Apparatus and Procedures. Coulometry and cyclic voltammetry experiments were performed with a Model CV-27 voltammograph and with a BAS-100 electrochemical analyzer (both from Bioanalytical Systems, West Lafayette, IN). The
w
BINDING ENERGY (eV)
135
N(ld
n'"
BINDING ENERGY (eV)
Figure 1. ESCA spectra (solid line) and results of the curve fitting routine (broken line) of DMDAAC polymer strips.
reference electrode was either a SCE or Ag/AgCl, and a platinum wire served as the auxiliary electrode. The sides of the graphite electrodes were covered with heat-shrink tubing or Teflon tape immediately before use. The degree of swelling of polymer strips was determined in 0.5 M NaCl via a 10-mL pycnometer (specific gravity flask) by measuring the amount of solvent absorbed and the swollen volume, and calculating the dry volume from the weight of solvent displaced (7). ESCA measurements on the polymer strips were made on a highly modified AEI ES-100 (Kratos) spectrometer at the Surface Science Laboratory of the Research Institute at the University of Dayton. Nitrogen determinations were performed at Galbraith Labs, Inc., Knoxville, TN.
RESULTS AND DISCUSSION Effects of y Irradiation on DMDAAC. Properties of DMDAAC cross-linked by y radiation were determined on samples of DMDAAC in the form of polymer strips that were not attached to an electrode. ESCA spectra of a 2.0-Mrd DMDAAC strip and a nonirradiated control are shown in Figure 1. The C(ls) and N(1s) peaks are shown, and a standard peak fitting routine has been used to decompose the spectra. Two observations can be made. The C(ls) peak a t 285.0 eV broadens with irradiation. The peak fitting results indicate that this is brought about by an increase in the 286.5-eV component. The observed shift is typical of the formation of a carbon-oxygen bond and is consistent with the reaction of polymer with residual O2 or water in the sample. It should be noted that ESCA is insensitive to cross-linking (Le.) substituting a C-H bond with a C-C bond) since binding energies differing by only 0.1 or 0.2 eV from the main peak would be expected. Utilizing the N(1s) peak, however, does allow one to detect degradation. The N(ls) low-energy component increases in intensity (21-32%) upon irradiation. This
136
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Table 11. Swelling of DMDAAC in 0.5 M NaCl dose. Mrd
swell ratio
std dev
re1 std dev, YC
n
0.01 0.10 0.25 0.75 2.0 8.0
1.7 1.5 1.6 1.6 1.5 1.8
0.3
18
5
0.2 0.2 0.2 0.2
12 15 13 11
5 4 5 4
1
Table 111. Specific Volume of DMDAAC in 0.5 M NaCl
dose, Mrd
specific volume, mL/g
std dev
re1 std dev, %
n
0.01 0.10 0.25 0.75 2.0 8.0
1.3 1.0 1.3 1.4 1.4 1.6
0.2 0.1 0.2 0.3 0.2 0.4
15 10 15 21 14 25
5 3
pLT*Alc c
i0.m
4 4 5 5
A
_ _ _ - - ---------M.5 ___---
/------------
€(VOLT) Figure 2. Cyclic voltammograms of (-) y*-DMDAAC/graphiteand (- - -) nonmodified graphite in 0.2 M KNO, vs. AgIAgCI. Scan rate was 40 mV/s. Cathodic current is up in this and all subsequent cyclic
voltammograms. behavior is consistent with degradation yielding tertiary, secondary, and/or primary amines indicating that chain scission is occurring at 2 Mrd. A method to test if a maximum cross-link density has been achieved with a given radiation dose is the measurement of the degree of swelling of the polymer in an appropriate solvent. A knowledge of the degree of polymer swelling is also important for calculating the thickness of a modified electrode coating when immersed in solution. Table I1 lists the swelling (calculated wet to dry volume ratios) for DMDAAC immersed in 0.5 M NaCl. No significant change in swelling occurs as radiation dosage is increased. Thus, the dosages employed here produce a constant cross-link density in the DMDAAC polymer samples. It is also apparent that cross-linking of DMDAAC can be achieved with very low dosages. The specific volume, which is the ratio of the swollen volume to the mass of dry polymer sample in mL/g, can also be calculated. This enables determination of the volume of swollen polymer retained on an electrode from the amount of dry polymer applied to the substrate. Table I11 lists the specific volumes for DMDAAC in 0.5 M NaC1. Electrochemistry of DMDAAC Networks on Platinum and Graphite. A cyclic voltammogram of a y2-DMDAAC/ graphite electrode in supporting electrolyte (Figure 2) exhibits a residualament that is not significantly different from the corresponding unmodified electrode. The presence of the polymer is visually confirmed by the presence of a swollen film 0.5-1 mm thick on the irradiated electrode. As expected for a film composed primarily of a quaternary ammonium ion based polymer, no voltammetric waves due to faradaic processes are observed. The amount of polymer remaining on an electrode after soaking in solution was measured by determination of the nitrogen content of the polymer applied to the electrode and the solution in which the electrode was soaked. The quantity
E(UOLT) Figure 3. Cyctic voltammograms of (-) y5~5-DMDAAC/phtinum and (---) nonmcdified platinum in 2 mM 1,4-benzoquinone,0.5 M KNO, vs. AgIAgCI. Scan rate was 40 mVJs.
of nitrogen applied less the amount found in the soaking solution gives the amount of polymer remaining attached to the electrode. Fifty-four percent of the DMDAAC applied to a y0,25-DMDAAC/graphiteelectrode was retained on the electrode surface after soaking in stirred 0.5 M NaCl for 48 h. The polymer lost from the film presumably consists of DMDAAC chains that were not cross-linked to the network by irradiation. Under these conditions, approximately half of the polymer is incorporated into the network that remains attached to the graphite substrate. This polymer network film was found to adhere tightly to the graphite substrate. In the case of platinum, however, attachment of the DMDAAC film hinged on the preparation of the electrode substrate prior to application of the polymer. Not all DMDAAC-coated platinum electrodes retained the DMDAAC film when immersed in solution. This indicates that immobilization is not just the insolubilization of a cross-linked network but also the anchoring of that network on the electrode surface. The greater porosity of the graphite surface aids in the retention of the polymer network. In fact, even without the irradiation process a small amount of polymer or various redox species can be retained on the graphite electrode surface (8). Cross-linked DMDAAC films on platinum and graphite are permeable to solution redox species such as ferricyanide, hexamineruthenium(II1) chloride, and 1,4-benzoquinone as evidenced by the cyclic voltammograms for 1,4-benzoquinone in Figure 3. The peak heights of voltammograms of bare and coated electrodes show almost identical current levels for cationic or neutral redox species. An increase in current with the DMDAAC-modified electrodes is observed with anionic redox species due to ion exchange of the anionic species into the cationic film (see below). At the dosage levels studied, films such as these are quite permeable when swollen, allowing even the cationic ruthenium complex to diffuse through the positively charged film to the electrode surface. Cyclic voltammograms on electrodes exposed to varying doses of radiation were essentially identical. Thus, the permeability of DMDAAC cannot be controlled by the cross-linking dosage within the range employed here. This behavior is consistent with the insensitivity of polymer swelling to radiation dose as shown in Table 11. Ferri-/ferrocyanide partitions into the DMDAAC film with repetitive cycling. Cycling gave increasing peak current (Figure 4) until a steady-state voltammogram was obtained. The polymer film was observed to shrink (from ca. 1mm to barely visible) as the ferricyanide partitioned into the film, indicative of reduction in charge repulsion in the cationic film. Figure 5 shows steady-state voltammograms for a y5.5DMDAAC/graphite electrode and an unmodified control graphite electrode in 0.4 mM ferricyanide, 0.2 M KNO,. The large increase in peak current (26X) and the shift in Eo’from
ANALYTICAL CHEMISTRY. VOL. 59. NO, 1. JANUARY 1987
137
A
I
I
-0.m
p.0
\em
Cyclic voltammograms demonstrating charge trapping of ferricyanae in y2-DMDAAClgraphne. EleCtrcde cycled continuously between +0.6 and -0.2 V vs. AgIAgCi at 40 mVls in 0.4 mM K,FeFigure 4.
EIUOLTI y5.SDhQAAClgaphiteelectrode after Flgure 5. The ektrode was removed from solution, rlnssd. and transferred into a solution containing only supporting electrolyte. Flgue 7. Cydk voltammogam of a
(CN),, 0.2 M KNO,.
h
/
r0.60
t0.60
/
0.5
at
0'
1
2 Kfl
(u)
3
4'
Figure 8. plot of m k e d peak c u m 1 vs. concentratkm suppmUrg electrciyte l a (+) y'S-DMDAAClp$tinumand (0)nonmOdRBd graphite.
EiUOLTl
Steady-state cyclic voltammograms of (top) nonmcdified qapMe and (bottm)yssDMoAAClgaphRe In 0.4 mM &Fe(CNh, 0.2 M KNO, vs. AgIAgCl. Scan rate was 40 rnV1s. Note the 20-fold difference in current scales. Figure 5.
"?
-I
+
+
++++
$"I+ "B
,
,
,
15
20
25
+ 5
10
W/s)
KCH,COO KHSO, KNOJ
0.5 0.8
1.0 1.1 1.3
+
r -%
" ?
normalized peak current
KCI
++
++;'
supporting electrolyte
KBr
++
++
Table IV. Peak Currents Obtained in Various Supporting Electrolytes
1/2
Plot of i, vs. square rwt of scan rate fa ys.+ DMDAAClgraphne In 0.4 mM K3Fe(CN),, 0.2 M KNOFigure 6.
+0.22 V (vs. Ag/AgCl) with the control electrode to +0.08 V with the modiiied electrode indicate the ferricyanide has been trapped and the oxidized form stabilized by the polymer film. A plot of peak current vs. square root of scan rate is shown in Figure 6 and indicates diffusion control of the electrolysis of the trapped ferricyanide in this relatively thick DMDAAC film. The trapping of ferri-/ferrocyanide was further demonstrated hy rinsingthe electrode, transferring it to supporting electrolyte, and recording the voltammogram as shown in Figure 7. A slow decrease in peak current was then observed
for ca. 24 h of continuous cycling a8 the ferri-/ferrocyanide slowly partitioned out of the DMDAAC film. Similar experiments with hexamineruthenium(1II)chloride showed no trapping of the cationic ruthenium complex or shift in Eo'. The charged sites of the DMDAAC film are apparently sufficiently separated so that the ruthenium complex is free to diffuse through the film to the electrode surface without any charge exclusion occurring. The nature of the supporting electrolyte influences the electrochemistry of ferricyanide as shown by cyclic voltammograms of ferricyanide at a ~~,~-DMDAAC/graphite electrode in the supporting electrolyte anions Cl-, Br-, NO,, HSO;, and OAc: A supporting electrolyte anion that is more strongly bound to the network would he expected to compete with ferri-/ferrocyanide for charged sites in the network. Binding strengths of anions are qualitatively given as "selectivities" when considering cross-linked resins for ion exchange chromatography. For the quaternary ammonium resin Durrum DA-XAF, which is somewhat similar to crosslinked DMDAAC, the selectivity is as follows: HSO; > NO; > Br- > Cl- > OAc- > OH- (9). The cathodic peak currents from cyclic voltammograms obtained in the various anions
138
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
3.
Flgure 9. Cyclic voltammograms of a y0.25-DMDAAClgraphite electrode containing cherge-trapped ferrlcyanide in solutbns of cytochrome c in 0.1 M NaCI, 0.1 M phosphate buffer, pH 7.0, vs. SCE: (A) 0.3, (B) 0.5, (C) 0.7, and (D) 1.5 mM cytochrome c . Scan rate was 40 mV/s.
show a correlation with these selectivities as shown in Table IV. Peak height increases in the order OAc- < HS04- C NO3< Br- < C1- as ferri-/ferrocyanide compete more effectively with the supporting electrolyte anion. Since the anion of the electrolyte is competing with the anionic complex for charged sites in the network, the concentration of supporting electrolyte affects the electrochemistry. Figure 8 depicts the effect of ionic strength on the cathodic peak current of the cyclic voltammograms for a y'5-DMDAAC/Pt electrode in 1mM ferricyanide. The peak heights of the ferri-/ferrocyanide voltammograms increase with increasing concentration of KCl up to 1.0 M, above which a decrease is observed. No corresponding ionic strength dependence is observed with nonmodified electrodes. This behavior is interpretable in terms of two opposing effects for the polymer modified electrode. The tendency of polyelectrolytes to shrink with increasing ionic strength would cause a less swollen, more compact film with a greater density of active groups. This would enhance the effective concentration of ferri-/ferrocyanide electrostatically bound in the film at the electrode surface and thereby cause the peaks to increase. The opposing phenomenon is the competition between C1- and ferri-/ferrocyanide for sites within the film. Thus, at sufficiently high concentrations of C1-, exclusion of ferri-/ferrocyanide from the film becomes the dominant effect and a decrease in peak height is observed. These observations are consistent with the properties of other ionic films that are bound to electrodes (10). Charge-trapped ferricyanide exhibits an apparent diffusion coefficient, Dapp,which can be measured by cyclic voltammetry or chronocoulometry when the concentration of ferricyanide in the film is known. From cyclic voltammograms and the Randles-Sevcik equation, DaPpwas found to be 1.8 x cmz/s (obtained from a 16-point plot of peak height vs. square root of scan rate with a correlation coefficient of 0.997 for scan rates varying from 10 to 500 mV/s). Chronocoulometry, suggested to give more realistic apparent diffusion coefficients in a polymer network if used over a short time period ( I I ) , gives Dapp= 3.9 X cm2/s. By comparison ferricyanide charge trapped in protonated poly(4-vinylpyridine)is reported cm2/s (12). These values are to have a Dapp= 1.5 X substantially smaller than those of the diffusion coefficient observed at a bare electrode, 0.739 X cm2/s in 1.0 M KCl (13). Electron transfer to the complex is impeded by either counterion flow within the polymer, motions by the trapped complex, or cross electron exchange reactions (14). The availability of ferri-/ferrocyanide in the DMDAAC network to mediate electron exchange with redox species in solution was tested with cytochrome c. Immersion of a ferri-/ferrocyanide-trappedyo.2s-DMDAAC/graphiteelectrode in solutions of cytochrome c gives cyclic voltammograms with
v v
Figure 10. Cyclic voltammograms for 20 mM o-hydroquinone in 0.1 M phosphate buffer, pH 6.3: (A) unmodified gold button; (B) 7'PANlAu; (C) yS-PANIAu. Scan rate was 10 mV/s. Reference electrode was SCE. enhanced peak currents. An increase in current with cytochrome c concentration shows the mediation of electron transfer from the electrode to the cytochrome via the ferri/ferrocyanide (Figure 9). The ferri-/ferrocyanide-trapped electrode was insensitive to 0.3 mM (or less) cytochrome c. Electron transfer with a large biological molecule such as cytochrome c is apparently possible with redox mediators electrostatically trapped in cross-linked networks of this type.
Poly(acrylonitri1e) Networks on Gold Size Exclusion. An interesting and potentially important property of crosslinking by y irradiation is the ability to control, to a certain extent, the degree of cross-linking in the polymer film. A change in permeability should accompany a change in cross-linking as controlled by radiation dosage for certain polymers. The network film could then be used as a means of imparting size selectivity toward species in solution. This idea of size exclusion has been explored for other polymer systems (15-19). Poly(acrylonitri1e) (PAN) on gold is a good system for demonstrating controlled access to the electrode surface (substrate metal) by varying radiation dosage. Figure 10 shows cyclic voltammograms for o-hydroquinone on an uncoated gold electrode (A) and PAN-coated electrodes with 1.0 (B) and 5.0 Mrd (C) dosages. Although the well-defined oxidation wave on the bare gold electrode is substantially attenuated by the 1.O-Mrd-irradiatedfilm, a definable oxidation wave does exist. By comparison, o-hydroquinone is completely blocked by the 5.0-Mrd-irradiated film as a result of the greater cross-link density. Excessive exposure to y radiation causes film degradation as shown by the voltammograms in Figure 11for potassium ferricyanide at a PAN-coated gold electrode. A 1.0-Mrd-irradiated film on gold severely attenuates the peak current compared to a bare gold electrode (voltammograms A and B). Further irradiation initially causes more attenuation (C, in which the electroactive species is essentially completely blocked from the electrode) until polymer degradation at very high dosages allows electroactive species access to the electrode
(D). Poly(ethy1enimine) Networks on Platinum: Prevention of Electrode Fouling. A polymer film with characteristics of size selectivity is potentially useful for the purpose of preventing the fouling of electrode surfaces by strongly adsorbing species. This is particularly important in the analysis of blood plasma, which contains strongly adsorbing proteins. Since these proteins are comparatively large, an electrode coating that excludes large molecules but allows small molecules of analytical interest to pass would be useful.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
139
shows the effect of the immediate fouling of the electrode surface. Voltammogram C for the PI-modified electrode in plasma shows considerable attenuation of ferricyanide reduction and oxidation by comparison to A. On the other hand, this electrode resisted fouling by proteins as evidenced by essentially no change in this voltammogram during 1 h of exposure to the plasma. Thus, although the network of P I reduces the sensitivity to ferricyanide, the resulting electrode is resistant to fouling and exhibits a constant analytical signal.
.5
. 2 ,1 0 -.l -.2 E,V vs, SCE
.3
.L
Flgure 11. Cyclic voltammograms of unmodified (A) and irradiated PAN/Au electrodes at various dosages in 4 mM K,Fe(CN),, 0.5 M KNO,: (6)1, (C) 5, and (0) 25 Mrd. Scan rate was 10 mV/s.
A
CONCLUSIONS The formation of polymer networks by y irradiation is a viable means for preparing polymer-modified electrodes. This procedure is especially useful for immobilizing ionic, watersoluble polymers for subsequent use in aqueous solution. The greatest advantage of immobilization through y irradiation is its generality; almost any polymer can be potentially attached to an electrode as a network through free radical cross-linking. No detailed synthetic scheme is needed. A drawback to using free radical chemistry is the potential multiplicity of products. Variation in radiation dose has little effect on “permeability” of ionic polymer networks such as DDAC but has a large effect on nonionic polymers such as PAN and PI. Registry No. DMDAAC, 26062-79-3; PAN, 25014-41-9; PI, 9002-98-6; Pt, 7440-06-4;Au, 7440-57-5; K,Fe(CN),, 13746-66-2; KNOB, 7757-79-1; KOAc, 127-08-2; KHS04, 7646-93-7; KBr, 7758-02-3; KC1,1447-40-7; NaC1,7647-14-5; graphite, 1782-42-5; o-hydroquinone, 120-80-9; 1,4-benzoquinone, 106-51-4; ferricyanide, 13408-62-3; cytochrome c, 9007-43-6.
LITERATURE CITED
.8
.5
.4
.2
0
-.2
- .4
E, V vs. SCE Flgure 12. Cyclic voltammograms of a platinum wire in 4 mM K,Fe(CN), in (A) OS M KNO, and (B)blood p l a m and IC) yZo-PIIPt electrode in 4 mM K,Fe(CN), in blood plasma. Scan rate was 15 mV/s.
This idea is demonstrated by a PI network on platinum with ferricyanide in undiluted blood plasma. Voltammogram A in Figure 12 is for ferricyanide in KNO, at a bare platinum electrode. Voltammogram B is for the same electrode and analyte in undiluted blood plasma. Comparison of A and B
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RECENEDfor review August 13,1985. Resubmitted August 11, 1986. Accepted September 10, 1986. This work was supported by the Army Research Office, Grant No. DAAG29-82-K-0161.
