Polypyrrole electrode as a detector of anionic substances

Sensors and Actuators,

POLYPYRROLE SUBSTANCES* YOSHIHITO R

12 (1987) 455 - 461

455

ELECTRODE AS A DETECTOR OF ANIONIC

IKARIYAMA

**, CHRISTOS GALIATSATOS

and WILLIAM

HEINEMAN

Department (USA) SHIGERU

of Chemzstry, Unwersity of Czncmnatl, Cmcmnatz, OH 45221 - 0172 YAMAUCHI

Research Instrtute. Nakonal Rehabrlrtatzon Tokorozawa, Saatama 359 (Japan) (Received

Center for the Dzsabkd,

4 - 1, Namzkr,

March 19, 1987, accepted June 5, 1987)

Abstract A polypyrrole film coated by electrochemical polymenzatlon on a platmum surface has been demonstrated as an electrochemrcal detector for the flow-through analysis of Ionic substances Amon detection 1s based on the dopmg and conductlvlty properties of the conductive polymer Anions are detectable at concentrations as low as 1 PM unth the use of a glycme solution as a moblle phase solution The potential of the electrode 1s controlled at +0 6 V (versus Ag/AgCl reference electrode) durmg the analysis With the use of a polymer electrolyte solution as a mobile buffer solution, amens are detected at concentratrons as low as 100 PM.

Introduction Functronal membranes that act as ion-sensltwe matenals have found use m many fields such as blome&cal technology, mdustial processes, and enwonmental monltormg Among these materials,, an Ion-selective membrane has been widely used m many fields such as chemical analysu~of ions (ion-selective electrodes) Ion-selectlve matermls are especially nnportant as transducers of blosensors, which can easily determme low molecular weight substances of clmlcal Importance, In recent years organic redox polymers such as polypyrrole and polythlophene have been shown to be conductive electrode matenals [l, 21 Conductive polymers have been the subJect of conslderable interest, smce *Paper presented at the 2nd International Meeting on Chemical Sensors, Bordeaux, France, July 7 - 10, 1986 **Present address Research Institute, National Rehablhtatlon Center for the Disabled, 4 - 1, Namrkl, Tokorozawa, Sa&ama 359, Japan

0250-6874/87/$3

50

@ Elsevler Sequo&Prmted

m The Netherlands

456

the polymer matenals can be used as a charge-storage mated m rechargeable battenes [3, 41, an orgamc electrode matinal [ 5, 6 1, a film for the prevention of photocorrosron of semiconductor electrodes [ 7 1, an ron gate membrane [8], and a presynaptlc model of neurotransmltter release w&h an electrochemical dopmg-undopmg of neurochemlcal substances [ 91 We have found that a polypyrrolecoated electrode can be used as an electrochemical detector for amens [lo] The response of the polymer-coated electrode depends on the dopmg and conductwlty properties of the polypyrrole film deposited on the flat platmum We descnbe here the electrochemical detectlon of anions by flow mJectlon analysis (FIA) with a polypyrrolecoated electrode

Expemnental section Reagents Pyrrole and tetraethylammomum perchlorate were the products of Aldnch (Milwaukee, WI) and Southwestern Analytical Chemicals, Inc (Austm, TX), respectively Sodmm acetate, acetomtie and sodium blcarbonate were purchased from Fisher Scientific Co (Fu Lawn, NJ) Sodmm dlhydrogen phosphate and glycme were the products of Matheson Coleman & Bell Manufacturmg Chemists (Norwood, OH) N-2-Hydroxyethylplperazme-N’-2-ethanesulfonlc acid [HEPES] was purchased from Sigma (St Louis, MO) Polyacryhc acid and poly(vmylsulfonlc acid) were the products of Polysclences, Inc (Warnngton, PA) Among these chemicals, tetraethylammomum perchlorate was recrystallized from water Pyrrole was chromatographed on a dry alumina column for punflcation Glycme was dissolved m redlstllled water and then recrystalhzed by the addition of ethanol Preparatwn of the polypyrrole electrode The polypyrrole-coated electrode was prepared on the flat platinum surface (diameter = 3 mm) by the electrochemical polymenzatlon of pyrrole monomers from an 80 mM pyrrole solution The polymenzatlon was performed at 0.95 V (versus Ag/AgCl) Pyrrole was dissolved In either an acetomtie solution contammg 0.1 M tetraethylammonmm perchlorate or an aqueous solution contammg 0 1 M sodium chlonde The electrochemical polymenzation was performed for 3 mm at a Pt workmg electrode m stied solution deoxygenated by bubbhng with Ar gas As Fig. 1 shows, polypyrrole electrodes prepared by this procedure were then assembled to a thm layer transducer cell for ion detection based on FIA Measurement of anwns The measurement of anions by the Ion-sensltlve detector was performed by potentiostattmg the detector with an LC-3A amperometnc detector from Bloanalytlcal Systems Inc (BAS, West Lafayette, IN) at 0 6 V uersus

457

q WE

c*--‘a In-‘----&t

I

WE

Fig 1 Assembly of the electrochemical detector system WE IS the polypyrrole-coated platinum working electrode, CE 1s a stamless steel tube counter electrode, and RE IS an Ag/AgCl reference electrode The bottom half cell IS assembled with the upper half cell through a gasket A planunetrlc drawing of the bottom half cell IS also shown

Ag/AgCl electrode All electrode potentials are reported uersus the silversliver chlonde (3 M NaCl) reference electrode FIA was done with an LCEC analyzer (BAS) with the chromatographlc column removed A 20 ~1 sample loop was used for all expenments The length of the connectmg tube (mner diameter = 0 013 cm) from the mJectlon valve to the detector was 55 cm The counter electrode was positioned at the outlet of the transducer system The distance between the working electrode and the counter electrode was approxnnately 20 cm The electrode potential was set to 0 9 V and the anodlc current was recorded, and then the electrode was undoped by applymg -0 3 V after each andytlcal response. The mobile phase solut;lons used were 0.1 M glycme, 0 1 M HEPES (pH 6 8) and polyacrylate (pH 6 8, 0.1 M as an acryhc acid). The mobile solutions were deoxygenated by bubbling with Ar gas Results and dlscusslon Anions were determined by the mJectlon of 20 ~1 samples mto the mobile buffers and the peak heights of the electrochemical detector responses were measured The polypyrrole electrode was held at 0 9 V, at which potential sample anions are doped mto the polypyrrole film Figure 2 illustrates a cahbration curve for the determmation of chlonde at a polypyrrole electrode usmg 0 1 M glycme as a mobile phase solution Lmeanty was obtmed m the concentration range 1 JIM to 1 mM A snnllar range of lmeanty was obtamed for other halides The coefflclents of vanatlon for these halides are listed m Table 1 To elucidate the contnbutlon of the dopmg current m the response of the polypyrrole electrode, cyclic voltammograms of chlonde doping were studied m the bulky electrolyte solutions where the dopmg of chlonde 1s expected to occur m preference to that of bulky amens The dopmg current was remarkably decreased to less than 20 PA m solutions such as 0 1 M HEPES (pH 6 8), poly(acrylate) (0 1 M as a monomenc acrylate, pH 6 8) and poly(vmylsulfonate) (0 1 M as a monomenc vmylsulfonate) On the other hand, the doping current of chlonde was about 500 - 400 #A m 0 1 M sodium chlonde solution The

458

l-

Ou. c

=

-1

_

CD

3 -2

0

t I

tit

I

a

I

-6 Lo-i

t&f

f

-3 M

F&g 2 Cahbratlon curve for chloride Peak height ls the maxnnum detector after a sample uqectlon The flow rate IS 13 7 ml/h TABLE

output

current of the

1

Coefflclents

of varlatlon

for hahdes

Halide

Concentration

Fluoride Chloride Bromide Iodide

1 1 1 1

*Coefficient

00 00 00 00

(mM)

cv* 12 06 18 15

(sla)

N 20 12 10 10

of vanatlon

residual current uersus potential relations for the polypyrrole electrode m flowmg solutions of glycme and HEPES buffer are shown 111Fig 3 The residual current 1s substantially greater m HEPES than m glycme In both cases the essentially lmear relation 1s suggestive of an ohmic response, with apparent resistances of 1 X log a for glycme and 3 - 4 X lo6 22for HEPES The peak current uersus potential relations for the nqectlon of chlonde sample mto glycme and HEPES moblle solutions are shown m Fig 4 In both cases apparent lmear relations were observed, however, a close look at these lmear curves mdlcates that two linear curves were linked by a less Ohmic curve at around the doping potential of chlonde ion Closer observation of the chlonde dopmg m the poly(acrylate) moblle buffer gwes a snmlar r-E relation to that obtamed w&h the HEPES buffer The relation of peak current to chlonde solution at an electrode potential of 0 15 V IS shown m Fig 5 The applied potential determines the film oxldatlon state and the resulting change m conductlvlty The dopmg potential of polypyrrole was generally observed at around 0 1 V uersus the reference electrode Higher sensltlvlty wfi be attamed at a higher applied potential The calibration curve obtamed for chlomde m the poly(acrylate) mobtie buffer was very sumlar to that obtamed m the HEPES buffer

459

200

/

100

0

.lOO

,200 -10

-05 Applied

Wg 3 HEPES

0

05

potential

10

(VI

Relation of residual current to applied buffer (0) The flow rate IS 20 ml/h

-

potential

m 0 1 M glycme

(0) and 0 1 M

50

f _/

1

a 0

t

:

0

01 0 J

?ii

d

-1 -5 -10

-05 Applmd

0 potential

05

10

50

-5

-4 Log

tV)

-3 NaCl

-2

, M

Fig 4 Relation of peak height to applied potent& for the nqectlon of 1 mM sodium chloride (20 ~1) into 0 1 M glycme (0) and 0 1 M HEPES buffer (0) The flow rate 1s 20 ml/h The z-E relation becomes less ohmic at the dopmg potential of polypmole Fig 5 Relation of peak height to chloride 0 15 V with a poly(acrylate) mobile buffer

concentration

at the electrode

potential

of

All the procedures were performed m a deoxygenated atmosphere to stabrhze the polymer electrode, which IS susceptible to degradation m the oxygen atmosphere However, the detector response gradually decreased after several sample inJections At this early stage of mvestlgation, we cannot get satisfactory and quantitative data on the dopant concentration and the correspondmg conductlvlty of the solvent- and electrdytecontammg polypyrrole electrode The conductlvlty of polypyrrole film at its oxldatlon state IS

460

reported to be =10-2 s cm-l [ll], and the reason for the low conduct~~ty of the solvent- and electrolyte-wetted polypyrrole 1snot clear There may be some reslstlve elements at the mterface between the polymer and bulk solutron or m the polymer itself, where electrolyte solution co-exists Of mterest m this regard IS the report of Nylander et al [ 121 They employed a polypyrrole film as an ammonia sensor The polypyrrole conductlvlty decreased upon exposure to gaseous ammonia Then mterpretatlon for the decrease m conductlvlty 1s that ammonia, which 1sa strong reducing agent, compensates charges m the p-type semlconductmg polymer Smce ammoma 1snot doped mto polypyrrole, the detarIed mechanism of the ammonia detection should be studled further Exposure to moisture 1s reported to result m quite complicated behavlour of the detector response m ammonia gas sensing It 1s difficult to find an effective stategy to extract only the dopmg current from the response m the anion detection, because not only the solutlon conductlvlty between the working and counter electrodes, but also the membrane conductlvlty of the polypyrrole are affected by the qectlon of lonlc substances One promlsmg approach would be the clanficatlon of the relation between the undopmg current and the correspondmg amount of released (undoped) amon This 1snow under intensive mvestlgatlon

References 1 K K Kanazawa, A F Dlaz, R H Gelss, W D G111,J F Kwak, J A Logan, J F Rabott and G B Street, Orgamc metals Polypyrrole, a stable synthetic ‘metalbc polymer, J Chem Sot Chem Commun, (1979) 854 - 855 2 T Yamamoto, K Sanechlka and A Yarnamoto, Preparation of thermostable and electric-conductmg poly(2,5_thlenylene), J Polym Scz , Polym Lett Ed , 18 (1980) 9 - 12 3 H G Heeger and A G McDlarmld, m L Alacer (ed ), The Physws and Chemwtry of Low Damensronal Sol&. Reldel, Dordrecht, 1980, pp 353 - 397 4 L W Shacklette, J E Toth, N S Murthyl and R H Baughman, Polyacetylene and polyphenylene as anode materials for nonaqueous secondary batteries, J EZectrothem Sot, 132 (1985) 1529 - 1535 5 R A Bull, R F Fan and A J Bard, Polymer films on electrodes Electrochemical behavior at polypyrrole-coated platinum and tantalum electrodes, J Electrochem Sot, 129 (1982) 1009 - 1015 6 A F Dlaz, J I Castlllo, J A Logan and W Y Lee, Electrochemistry of conductmg polypyrrole films, J EZectroanoZ Chem Interfacaal Electrochem , 129 (1981) 115 132 7 G Horowitz and F Garmer, Long-term stablllzatlon of polythlophene-protected n-GaAs photoanodes m aqueous solution, J Electrochem Sac , 132 (1985) 634 -

637 8 P Burgmayer

and R W Murray, Ion gate electrodes Polypyrrole as a switchable ion conductor membrane, J Phys Chem , 88 (1984) 2515 - 2521 9 H Shmohara, M Alzawa and H Shlrakawa, Electrochemlcally stnnulated release of neurotransmltter from a conductmg polymer thm film on the model of a synapse Chem Lett, (1985) 179 - 182 10 Y Ikarlyama and W R Hememan, Polypyrrole electrode as a detector for electrornactlve anions by flow mJectlon analysis, Anal Chem , 58 (1986) 1803 - 1806

461

11 B J Foldman, P Burgmayer and R W Murray, The potent& trlcal conductlwty and chemical charge storage of poly(pyrrole)

dependence of elecfilms on electrodes,

J Am Chem Sot, 107 (1985) 872 - 878 12 C Nylander, M Armgarth and I Lundstrom, An ammonia detector based on a conductmg polymex, Proc Int Meet on Chem Sensors, Fukuoka, Japan, Sept 1983, pp 203 -20‘7