ELSEVIER sl iv 4-10 ( I 'WV) 7.1 S :

mm.

Amperometric ion sensors based on laser-patterned composite polymer membranes

Hye Jin Lcc, Paul D. Bcallic, Brian .1. Scddon, Murray D. Osborne, Hubert H. Girault iiil'iiwKiiir it'l'.hrlidfliimic. I'.ailf I'dlvurhii'uiiir I'rdnnlc ilc l.iiiisdiiiii: ('ll-li)l^ IAIIIMIIIIU: Swilrt'ilwul

Kfi-eiwil IS NovcnilKT l')Mf,; loccivcil in an iscd liuni I4.laiuuirv l')')7

Abstiiicl

A polymer mcnibninc for llic solcclivc iimpciomcliif iranslLT :IIKI scnsinu of imilcctiliir inns liiis IICLMI ilcsiyiiL'tl iiiiil chiiiiiclcrisL'tl. '\'\K niLMiihriinc vviis r(iiim;il IVoni twn polyniiT liiyL-is. w siippuriini; I'iliii ol'piilvLMliylcMiL' icivplilhiikitt' on wliicli ;ui ulucirolytc filiii containing polyviiiylchloritic is c;isl. 'i'lic poiycslfi' l:iyL'r iuis ;i I;ISLT-CICIII-II p;illurn of ciivukir inJLTo-holcs in onL- region. 'I'IICSL- iiolc Miiicliircs ;irc arranged in a rectangular geoniclry anil iDea.sure 22 |j,ni in ilianieier wiili srp.iration tlistances ol' lO.'iixni anil I2()nni. The polyvinylchlo-ride undcrhiyer i.s a composite system comprised of a 2-nilroplienyl oclyl cllier plaslieising solution containing an electrolytic salt of tetrabulylanuMo/iiuni telrakis-(4-cliloropiienyl)horate. In this way. an array ol micro-interfaces hetvveen an imalyie solution and a FVC gel electrolyte is formed and used as a lii|iiiil|lii|uid interface for the ani|ieromelric muniloring of ion transfer reactions. 'I'he memhrane was characterised in terms of the voltanmielric response to choline transfer. The study includes an examination of the fahricalion melhodology. materials composition and memlirane slruclinv. "•) l')')7 Hlsevier .Science S.A.

Mclinex riili.': Cholii rnLMiihi Arii|)i

1. Introduction

Ion tran.sfcr rcuclions across ;i polariscci lic|uitl|liiniici interface can. in principle, be used as a base lor (he ampeiomelric delerininalion of ionic species in .solution [!]. Thi.s method has hitherto never been used in coniniei-cial cicctroanalyticai instrunienialion. The main reason for the absence of this electrochemical methodology in elec-troanalytical chemistry, has been the dilTicullies associated with the handling of two immiscible liquid phases and forming mechanically stable interlaces.

On the other hand, ion selective electrodes have been fabricated with polymer gel membranes: the working prin­ciple is based on tiie ec|iiilibrium properties of the target ion distribution between two aqueous phases on either sitie of the polymer membrane. The resulting poientiometric methodology has been a successful electrochemical method widely used in the determination of ion concentrations in many fields of analytical chemistry. The use of ion ex­changer polymer gels in l.SE is possible bectiuse the high impedance of the polymer membranes does not hinder the

sponiliiig aulhii

()()22-()72S/y7/.SI7.()() (i; I'J'W r.lscvicr Si

/ ' / / S O O 2 2 - ( ) 7 2 S ( ' ) 7 ) 0 O ( ) . S 2 - l

ll('''icp.il(.M:pri.cli.

S.A. All riiihls res

measmemenl of the Galvani potential difference between the analyte and the reference electrolyte solutions.

Using a polymeric organic electrolyte phase for the amperometric monitoring of ion concentrations has been attempted first by .Senda and coworkers [2-4] who solidi­fied a nitrobenzene electrolyte solution by adjunction of polyvinylchloride. The same approach wtis followed by Mmecek and coworkers [>-l] and by Wting and .li [8]. Unfortunately, if solidifying the organic phase allows a belter handling of lic|uid|liquid .systems, the usually high resistivity of the oig;i.nic phase makes any amperometric determinalion rather irore difficult due to the high level of uncompensaled resistance.

Instead of attempting to .solidify the organic phase, it has been proposed to support the organic phase in an inert polymer matrix. This approach has been used by Albery and Choudhery [9.10] for the construction of a rotating diffusion cell used for the measurement of solvent extrac­tion kinetics and more recently by Hundhammer and coworkers [I l-l.'i] and Wilke and coworkers [14-16] in a wall-jet configuration for the amperometric determination of ions in flow injection analysis sy.stems.

In our group, we have attempted to alleviate the diffi-

7.1 /A./. I.tr I'l (il./.liniiiiiil (if i:h;-livaiiiihliriil ClirmiMiy 4I() (IW7I 7.1 H:

cully iissociiiicil wilh IR drop liy dLneloping amiys of miL-m-lic|iiid|lic|iiid inlorfiiccs iiiul used this iipproML-h (o dcvckip a urea and a creatinine sensor [17.IS]. In this laller work, tlie organic piiase was supporleil in a glass-libie membrane.

In tile present work, we combine the use of supported membranes and organic phase gelil'ication to labricale novel composite membranes. The characterisation and de-velopmenl of tjiis ion Iransler membrane is described. The membrane structure comprises an organic polymer phase of polyvinylchloride and 2-nitrophenyl oclyl ether charged wilh a c|uaterniny ammonium electrolyte and is supported by an inert film of polyethylene lerephthalate (PliT), The polyester nim hiid been previously patterned with an array of micro-holes using the technique of IJ.V. laser photoab-ialion. By using an array ot'micro-inlerraces. as oppo.sed to a single centimetre-scale inleiTace. ihe problems normally associated with high ohmic drop in the organic phase are partially alleviated and high precision is attained.

ion are sensitise to the membrane composition. Reduced ion mobility within the membrane will also reduce ion conductivity anil these phenomena will influence the na­ture of the electrochemical response obtained.

The dilTusion-coiilrolled rate of ion flux into the mem­brane is related to the geometric structin-e of the micro-in­terface. Fi)r a membrane composed ol a system of circular channels permeable to ion movemeiit, and wilh such mi­cro-interfaces arranged at discreie and large distances apart, the diffusion-limited current for an ion transfer is given by the steady-state relationship formerly derived for an inlaid .solid micro-electrode, i,e.

4iii:.,rir('''i (3)

where /•" is the Faraday constant, r" the bulk acjueous concentration of the ion and r the radius of the micro-in-lerfaces consisting of in number of interfaces. Owing to the geometry of the micro-hole as shown in Fig, I, we may have to consider that the interface may be recessed, and in this case the sleady slate current is given by [19]

2. Elfctrochcniical re.spon.se of the membrane

Discrimination between ion transfer processes at po­larised lic|uid|lic|uid interfaces is dependent on the standard Gibbs energy of transfer AG,";" '" for the specified ions in question. This quantity is effectively the work required to transfer an ionic species / from the bulk of the aqueous phase to the bulk of the organic plia^ iid represents the difference between the standard Gib energy of hydration and solvation. For an ion sensing system based on po­larised liquidlliquid interfaces, it is of fundamental impor­tance that ions of interesl can be separated on the basis ol' their standard potentials of transfer A'"//;". This potential is related to the standard Gibbs energy of transfer as given in Fq, (I),

/ " •" = 4///.-;/••/>"f

x,<i>: IG" " / - , / • • (1)

where ,-, is the charge carried by the transferring ion. In the case of an aqueous electrolyte solutionlpolymeric mem­brane interface, the half wa e potential of a transferable ion is given by Fq. (2) in the case of a micro-disc interface. The X<^l'!''' differs substantially from the stan­dard transfer potential as both the activity and the diffusion characteristics of the ion are differeni in the membrane phase than in the ai|ueous solution.

A>y' RT / yl'D'^

r-) where /J" and D" are the diffusion coefficients of the ion in the aqueous and organic electrolyte phases respectively. -Since we can expect that the ion diffusion coefficient in the inembrane will be much less than in the aqueous electrolyte, and that the former is dependent on membrane composition, the half-wave potentials of the transferring

TTr + L, (4)

where L is the distance between the recessed micro-inter­face and the surface of the support PFT film.

If the distances between the micro-interfaces forming the array are such that the respective diffusion fields may overlap, we have to consider what has been named the shielding effect, where the diffusion regime varies from spherical to linear diffusion [20-22], In the case of,square arrays considered in this work. ScharilTxcr [21] classifies the current-time response into three categories:

/;, < (1/2: no overlap of the diffusion zones d/2 < /;, < (l/\i2: partial overlap of the diffusion zones /;, > J/v'2: total coverage by diffusion zones

where /;, is the radius of the equivalent diffusion zone given by

rj = ,•- + ,-\JTrDi (5)

Aqueous pha:

PVC-NPOE electrolyte

l-'iy. 1. Scliematic diajirani of the inicri)-iiiacliincil composite polymer tuenibraiics.

//.,/. /.(•(• (•/ iil./.l<Hinml (ifl-:iiriiviiii(ilyliriil Clwmislry -l-IDIIW?) :

iiiul (I llic centre lo ccnlrc distance between two miero-in-

leiiaces.

I'or the time interval corresponding to (he I'irsl case, tiie

cnirent density to a sc|uare array is

1 + - = = = • I (ft) 77-D| /

3. Kxperimcnti i l

3.1. Clicinircils

The aqueous and organic pli'ise solvents were de-ionised

water (Mi l l i -Q. Mill ipore, CH) and 2-nitrophenyl oetyl

ether (NPOE) (Fluka. CM) respectively. Lithium chloride

(LiCi) and choline chloride (ChCI) were supplied by Fluka

(CH). High molar mass polyvinylchloride was supplied by

Sigma (CH). Tetrahutylammoniuin letrakis(4-chioro-

phcnyDboratc (TBATPBCi) was prepared by metathesis of

tctrabutylammonium chloride and sodium tctrakis(4-chlo-

rophcnyOboratc, the precipitate being rccrystalliscd in a

methanol + ace-one mixture. Al l reagents used were ana­

lytical grade or better, with the exception of ChCl which

was > 97% purity.

3.2. Micro-iitacliincd composite polymer membrane

The .schematic of Fig. I defines the main dimensional

features of the polymer membrane and illustrates the ion

permeable regions of the structure. The membrane consists

of two fi lm layers consisting of an upper layer of F ET

which is nominally 12(jLm and acts as a supporting fi lm

for a plasticised ion conducting polyvinylchloride fi lm

< 100 fxm. The latter forms the underlying layer of the

laminate and functions as the ion-sensing component of

the device.

The PET f i lm was laser micro-machined prior to the

deposition of the PVC layer in well-defined regions, im­

parting a microscopic interfacial structure to the ion-trans­

fer membrane. Laser micro-machining of thin polyester

f i lm has been cited in a number of recent articles [23,24].

in this instance, polyester f i lm of thickness between

r-i.am and 100(xm (Mclinex type " S " from ICI Films,

UK), was etched by UV cxcinier laser photoablation.

Briefly, this consisted of the following: an argon fluoride

laser beam (ArF la.scr-LPX 200, Lambda Physik, Ger­

many) with a wavelength of 193 nm, and an energy of

approximately 200 mJ per pulse. A schematic of the laser

arrangement used is shown in Fig. 2. Pulses were directed

upon the polyester f i lm at a pulse frequency of 10 Hz. The

attenuator in the assembly controlled the laser beam inten­

sity where the beam was reflected through a field lens, and

passed through a mask. The mask consisted of a nearly

square array of 66 micro-apertures of diameter approxi­

mately 200 (im (centre separation, a = 1061 |i,m and /; =

I269ixm) in a metal surround. The mask is acting as the

object to be imaguu. The beam then passed through two

adjustable circular diaphragms (with an aperture of about

1 mm), to minimise effects due to stray cxcimcr light, and

was projected onto the target fi lm situated on an .v-v

moveable platform, via a scries of telescopic lenses which

reduced the mask dimensions by an approximate factor of

ten. Processing of the polyester fi lm involved the drilling

of a system of micro-holes in a 0.59 mnr region of the

fi lm. .Scanning electron micrographs of both sides of a

finished micro-hole array were taken using a Jeol JSM-

63()()F scanning electron microscope (SEM), Fig. 3(a)

shows SEM i)hotomicrographs of the patterned polyester

f i lm. The micro-hole array of 66 holes ( I I X 6) is etched

into the polyester in a nearly square pattern arrangement

where the diameter is 22 j i m and the hole separation,

CI ~ 105|JLm and /; = I20|xm. This distance should ideally

be a value such that no overlap of the diffusion fields of

the individual micro-interfaces wil l occur, a factor which is

time dependent [20-22].

Depending on the position of the object to be ablated

with respect to the focal plane of the optical .system, the

drill ing process leaves the holes with a sloping wall char­

acteristic, the diameter of the exit holes usually several

micrometres smaller than the entrance side. This sloping

effect is termed anisotropic etching [23]. The entrance hole

diameter was estimated to be around 22 | im and the exit

I3|xm. A polyvinylchloride film was placed over the PET

covering the region of micro-holes by a method of hot

casting of the plasticised polymer. The polyvinylchloride

layer was composed of plasticising solvent, 2-nitrophenyl

octyl ether and an organic electrolyte. The solvent is

known to po.ssess moderate boiling point (197 to 198°C). a

high relative permittivity (24.2) as well as a high viscosity

(13.8 cP = 13.8 mPas). Pla.stici.sed PVC (2.0' to 20.0%

m / m ) was prepared by dissolving the polymer into a

^

Jj_ nriiq

Aticnuiilor

f"ig. 2. SchL'iiialic diagram of the

hole I'atiricalion.

• assembly .system used I'or micro-

//.,/, /,<v ,7 til./.liHiiiHit ('fl':iirlivwiah'linil ('licnilui \ I III IIW/I ! I .SJ

y^fffs^'n' « (b)

l-'ii!, 3. (.j) ( i) Sciiniiiiit; ck'clriMi

Scjiiinin}; clcclnin pliolDinicroiini

III arniy. and ' i l ) llic conical siriic

-I? ). w ' i l l l | •V ( • l l p | l l i c^ la l ( i )55 ( •

= l()5(Lin ami h-- 12()|i.iii.

.• ol llic microliolc ii

l ( i i ) 7 ( r c . Al l appri.

solution of THATPHCI ( lOn iM) in 2-nilroplicny| oclyl

elhcr, at a tcinpcralure of approximately I2( )T (in Ihc case

of high pcR'cntage m / m of PVC the lempcralure was

increased to \MY'C). Dual layer membranes were proilucecl

by casting a hot plaslicisecl PVC mixture {\()± 2fjLl). at a

controlled temperature of 70 to 8()"C (for temperature

variation experiments the plaslieised PVC temperature was

varied from 50 to OT'C). onto the laser-machined PliT

dim. The PVC mixture was applied over the micro-hole

array region on the exit side of the hole. The drop of hot

PVC electrolyte was allowed to spread over the entire

surface of a disc of PIET fi lni (10 mm diameter). The

membrane was then cured at rooin temperature (23"C) for

3h before use. By this approach the plasticised PVC

settled into the hole structure without excessive Hov; on to

the surface of the polyester Him. One important point to

note about the present procedure is that the PVC compos­

ite nils the micro-hole structure rendering an interfacial

geometry which is inlaid to the surface of the polyester

l l lm. 'i'he thickness of the poiyvinylchloride electrolytic

layer was estimated to he of the order of 100 |xm. Fig. 3(b)

shows the polyester surface of the membrane revealing the

structure of the PVC composite in the hole regions. Il can

be seen thai the gel fills the hole so as to form inlaid

micro-interfaces. The annular pattern aroiuul each hole

filled by the PVC gel is due to an optical effect leading to

a surface ablation of the PliT. This outer ring represents a

/one of hydrophilised PliT. thereby ensuring that the hy­

drophobic polymer gel does not spread out of the hole,

Sl iM pictures at higher resolution show that the surface-

ablated I' l 'T has a higher surface roughness than the

non-ablated region, and recent work in our laboratory has

shown that this surface-ablated plastic is charged in aque­

ous solution [25].

J.J. Elcclnnlu'iiiiail iiwufiuirnwiils

HIectrochemical experiments were performed in a two

electrode mode using the cell shown in f^'ig. 4. in the

experiments canied out with pure NPOF{ solutions, (c) in

I'ig. 4 represents the organic phase, and the organic refer­

ence solution together with the respective reference elec­

trode are then located above (c).

For cyclic voltammctry. a voltage ramp was applied at

the organic reference electrode using a waveform genera­

tor, and the resultant current was then measured through

the aqueous reference electrode with a current follower

based on a high-input impedance FHT operational ampli-

//,./, lArrl,il./.liHim,l«ti:Urlri

•\. Illuslni

Iniiisrcr L'

iiiic ivl'i'iv

iiiiiiiii; };ol

llMI 111 IIK' C'k'

\|VlillH.MIIS. (l

IC sollllidll, (

11 i i i iniilu.k's.

•IniclK'iii

) A U I A L

1) I've

Clll (.•ell

CI w i a

NI'OI':

llM'll 1

(h)

,!;<.'l, (c

Ilk' lllc'l

l|llL'(lUS

) (Irillal

iliniiK' 1

ihiiM'. (

I'l'T III

l icr (I')iiiT Hiovvii O A I ' 104). 'riio volKiiiiMKignims wciv

rccorclt'cl on iin iiiiiiloj: A'K-IVL-OICUT (Aclvimccil Hiynns.

UK). 'I'hc .swivp lalc in ihc cyclic volliiiiimctiy work vva.s

2()mVs ' (unless otherwise specified). Chronoiimperonie-

l iy wiis pcirorniecl using ;i conipuler-cnnliolleil polenliosuil

(AutoslMt) ohUiinecl from Sycopel Scienliric I.Ul. UK. A.c.

inipeciiince cxperimenls for ilie ciclermiiialion ol' mcmhrane

resistance were perrornied using a Solai in"! 1286 eieclro-

cheniicai inlerlace, in conjunction with a Sohirlron \15{)

rrec|uency response analyser. A l l liic above experiments

were carrietl out at a room temperature of 23 ± 2"C".

4. Results iiiul discussion

4.1. Gcoiiiiiric clHircirlcnsciiioii

Membrane characterisation cxperimenls were concerned

with the response ol' tlie system to the transl'cr ol' choline

from an ai|ueous electrolyte piiase to the micro-inter;"'.;ce ol'

the membrane. The electrochemical system is illustrated

below, where the choline ion species is initially present in

the aqueous electrolyte phase as shown in Cell I. Here, a

polarised uciueous|membrane interlace gave rise to a mea­

surable amperometric current for the specific ion untler

study.

'V|'^"Ci|{^'^^ij"'^^'i| I'vr-Npon DiiiM ruAri'bn

IIOni.Ml.in

L.vniMChf: |AgC;i|Aj;'

uilvlical ( liciniMiy -I/O I l'/<'7l 7.1 ,S'J 77

voltammeiry and chroiioainperometry. I'ig. 5 compares the

membrane res|)oiise (2,H'/( m / n i l 'V( ') with the et|iiivalenl

responsi' of the a(Hieous|NI'()l' electrolyte interface. In

both ca.es the potential window extends beyond 700mV,

but I'or the licjiiid organic electrolyte backgroiuul currents

are slightly higher. Uotli systems show iliffusion-limiled

ciirrenis for choline liansfer. One facet of the I'VC'-based

membrane is the oceiu'ieiice of more pronounced peaketl

reliirn waves corresponding to the transfer of choline ion

back from the organic phase to the at|ueous electrolyte, a

charactenslic which is tlependent on ion mobility in the

organic phase, b'tirthermore, A" (/»,'''-' for choline transfer is

shifted from ."i.lOmV for the lii|uid organic electrolyte to

-t'J.SmV for the iilasticised I'VC system, using the same

reference electrotles and a concenlration of 0. I m M of

choline. This difference can be maiidy attributed to a

difference of IH drop due to the use of different cell

geometry (vitle supra).

I'ig. 6 shows the cyclic voltanuiietry concentration ile-

pendence for the choline ion transfer from an aqueous

lithium chloride eleclrolyle to a 2.S'/f m / m I'VC mem­

brane. The observiid shift in the half-wave potential with

ion concentration in i'ig. (i is a consequeticc of the IR droj)

across the composite polymer membra''-, resulting from

the operation of the cell in a tsvo electrode mode without

IR compensation, 'i'hirs. this apparent shift and the sub.se-

quent flattening of the voltammetric curve are mainly due

(a)

720

The electrochemical respon.se of the micro-interfacial

membrane to choline transfer was investigated by cyclic

I'ii;. .^. Cyclii; v()ll;iiiinioi;i;i

fVC'-t NI'()l-;i;cl(^uoi-piii l '--

aiicl ( i i) in Ihe pivsciiLV (if I),

AfmV (ililiiiiietl lor (;i) Nt 'Ol i

(il./.hmiiiiil(ill':l,rlir

A(t)/mV 780

I'lf,. 6. Slc;nly-sl:ilc cyclic voliaiimioynmis lor viiryiiij; ciiiiccnlriilioiiN of

ChCI (v in Cell I cc|iwls 0.1. 0.2. ().}. 0.4. (L.'S, (U), 0.7. O.K unci ().'».

Cyclic v()ll:iinm()(!r:mis recorded at 20niVs '.

lo the rcsislancc of ihe gel in ihe micro-lube niiiciiined in ihe PBT film (vide infra), the ion transfer reaction it.self being considered as reversible.

The phiteaii current plotted against concentration is shown in Fig. 7 and is found to be linear over the range of 0.1 mM to 0.9mM with a slope ( / , , / (" ) of 255nAmM '. Comparison with the liquid NPOE interface (/ , , /( ' ' = 181.3 nAn iM' ) shows that ion tluxes are depressed. Ac­cording lo the c(|uation for an inlaid micro-disc, the steady-state current given by Ec|. (3) can be used to calculate the theoretical slope expected from an array of 66 holes of 22 (jim diameter. Considering that the aqueous diffusion coefficient value of choline D" is 9.4 X I O " ' m - s ' [26], a theoretical / , , / ( " value of 263nAmM ' is found. One explanation for the above observations is the formation t)f a recessed aqueouslNPOE interface relative to the upper surface of Ihe polyester niin. From the electrochemical data and Fq. (4). which is valid for ideally cylindrical micro-holes, we can estimate that

200

150

100

50

0

--

dL L

W,** tC

> ^.t (i)

^ I I I !

I'ij;. 7. I'll

(DNI'Or-

0.0 0.2 0.4 0.6 0.8

Choline ion concentration / mM .'.iciidy-.'ilaie current vs. ion conccniraiion lor Cli *

li) l.Wi PVC-gel togcllier with the best 111 linci In

the a(|iieoiis|NI'()i; interface lies approximately lOiiin be­low Ihe polyester siuface, This also Itiads us lo conclude Ihtu the polyviiiylchloride material has com|)letely filled the holes to such a degree where the iiiterfacial structure is an inlaiti disc geometry as shown by .SliM in Fig. 3(b). In liie case of piue NFOIi. it would appear that the surface tension is such that the micro-tube cannot be fully filled upon immersion of the micro-machined I'li'l' membrane.

However. U) characterise further the composite polymer membrane, we carried out some chronoamperometric mea­surements. A shielding of the hemispherical diffusion fields is expected for membranes designed with ion penneable regions which are positioned closer than some critical distance .set by the si/.e of the micro-interface and the diffusion coefficient of the transferring ion. The diffusion-limited current for the choline ion transfer depends both on the time domain and on the geometric arrangement of /he micro-interface. Chronoamperometry for the ion transfer is shfiwn in Fig. 8. At extremely short times, the response should be similar to the Cottrellian respon.se with respect to the surface of the micro-inlerfaces. However, the corre-

( t / s )

I-ig. S, Chronoamperonietry ol' C h ' ion transfer into the momtirane

(.V'= O.Oy in Cell I), (a) I'lot of current v.s. lime and (b) plot <>r current vs.

r ' •' -. Temiiefature of casting 2.S'/f PVC. I = HfC. represents the

theoretical / „ calculated Ironi Eq. (.•?).

iil./Jiiiimil iif Hlcniviiniih'linil ('haiiistiv •/•III (IW/l 7.1 S.:

spoiitliii|i lime iloiiiiiiii is liillicr short |27| ami not iicci'ssi-i)lL- with tiic pivseiit expcriniLMital set-up.

I'or the imeiniechiiti' limes the eiiiTenI is delcfiiiined hy the tiai'isition IVom Hiiear to hemisplieiieal (hITiisioii and no real steady-state ciineiit eaii he ohseived (see I'ig. H(a)), tiie iheoietieal steady-stale cuireiil ealeiilated IVom lic|. (3) being 23.8 nA i'or the eoneentration ol'O.iWinM used. At longer limes, i.e. alter about I s. a elear change of slope ciin be observed in b'ig. K(b). intlieating the merger of the tlilTiision fields. This tiirie corresponding to the onset of liie coalescence of the dilTusion I'ields is less than that predicted by lv.\. (5). 'ibis elTecl is discussed in another publication [2K|.

It should be noted dial 30 holes out of Mi are located on llie periphery of the array, and thai only parlial shiekling occurs for these micro-inlerluces. All in all, it may be concluded thai ihe apparent sleady-slale current observed in cyclic vollamnielry at a sweep rale of 20mVs ' ap­pears as the response of an array of perl'eclly inlaid vviepi.v'.'lc:'! micro-disc inierl'aces, although the piiysical

polymer has a ;Iirec: ^iiici •'!! v. o m' .-'na;;: ^h-ruci'.-isiics: U) the ionic resiMaacc ,ii;; (ii; i\]c !!iii:;i::L::;i5 iy.-'-:;\-try. Both contribute to changes observed in ion transfer eleclrochemislry. Table I shows the variation in membrane resistance as Ihc PVC composilion is increased. The mem­brane resistances are niegaohm values which increase by approximately 800Mil from 2.8% to 15% of polymer. Fig. y shows ihe changes which occur in the cyclic vollam-mclry of choline ion transfer at membranes prepared using different percentages of polyvinyichloride in the piasli-ci.sed layer. The main vollammetric features observed arc increased background currents of Ibe polcntial window with low PVC contenl membranes as well as elevated

Tabic 1

lilcclrouhcniical cliiiraclcristics or iiiicro-iiitLTraco nicnibrane with varinu I'VC CDiiiposilidn including llic nioinhranc resistance W, return peak current /,, and iiall'-wave potential A"f/i'''-

I'VC/'/r

0 2

2.«

4

5

6

iO

15

20

•' Estimated

X.'l'' 5M)

A^)5

49.5

495

500

502

510

515

-value.

- / i i ' V / p / n A

21

26

2'>.8

30

31.0

32.5

34

39

-

A s / " A

20 22.5

26.3

26.5

26.5

27 26.5

27 15.5''

«/iV112

_ 0.069

1.07K

I. I IK

1.146

l . l«9

1.746

1.885

-

ilr. '. i';'!;..! 01 ;'VC. •• >iiii:;'.-i;:.(Mi Jy ni/ni) on llic elcctrochcinlcal a Tons-j of Cir io'i irai-.Jcr 'nio iS;e iiienilirane. (a) 2'/., (b) 5%, (c) \il-/i. (i) I'oteiuiai window Tor Coll I and (ii) in the presence of 0.1 niM Cli' ion in the aqueous phase. Sweep rale i;= 20inVs" ' . Temperature oteastiiii; 7'= 70"C and l()()"C, 15'/raiid lO'/r PVC respectively.

Steady-state currents for the aqueouslmcmbranc transfer of choline; a Altj)-/- siiifi and increased return peak current.s. Again, comparison is made with the NPOE electrolyte system (i.e. representing 0% PVC). The vollammetric plaleau current for low percentages of polymer is signifi­cantly lower than that observed at higher PVC contents. On the basis of the SEM observations and the electrochem­ical data, it would appear that the micro-interface may be formed in several positions within the hole structure giving rise to a number of geometrical forms. Calculations based on Eq. (4) for a recessed micro-interface indicate that, for membranes with 2.8% or above of PVC, the interface is one of an inlaid geometry. At levels less than 2.8%, diffusion-limited currents arc reduced over that of inlaid .systems. For a PVC concentration of 2%, the interface can be calculated to be located 5p.m below the surface of the polyester film.

It is clear from Table I that a near constant ion transfer current (mean value of 26.7 nA) is obtained for values of polymer from 2.8% m/m to 15% m/m. Moreover, this current is equivalent to the theoretical inlaid micro-inter­face value of 25.8 nA. This finding supports the view that polymer compositions over this range have little effect on

II..I. I., III./.hi ihil I'Jirin il\liiiilCliniii\liyll()ll'iii7l 7.1 ,S'..'

lliL- ion |)einii.'ahk' slriicunv of llio iiiLMnbiiiiiL- iiiid Ilial llu;

jihislicisi'd |K)l_viiiL'r phasi' iii's in Ihc pliiiH; ol' llii- polyi-'slcr

siiirafc.

I'ur hitihiT |icici'iila,UL-s ol' I'VC ( > \y/<) ihc tlilTiision

cuiTciils arc tliiiiiiiishcil; lliis may wi'll lu" a ivsiill ol'

iiicivasi-'il viscosilifs liiiulcrini; lliu flow ol'the polymer j^ol

iiUo (he iiiicro-holc anil ucnuratinj; aiiain a rccossud IMILM-

IncrcasL'il polynicr conlcnt also increases ilie niemhrane

viscosity and cleclrical resislance ol' the layer anil (his is

rel'lecleil in (he sliil'ls ohseiveil in A>A' '. JToni 2.N lo

\y.i I'VC. a gniiliial shil'l is seen in half wave polcniial:

ilie X,<I>1'' value increases hy 20niV over liie series.

Aiklilionally. the ion transfer from Ilie menihrane phase to

tiie ai|ueoiis elecirolyle is sensitive lo PVC content. In i'ig.

y, the reverse wave peak current is aiignienleii wil l i in­

creased I'VC memhiane content over the range ol' 2.S to

\5'/i. This elTect is due to the hindered mohilily of the ion

in the membrane phase leading to a greater accumulation

of the ion in the surface layers of the piasticised I'VC in

the hole region of the menihrane.

4.J. Ciisiinii i>r(i(C(liirt'

The hoi casting procedure employed in this work may

also produce significanlly dil'feienl micro-iiilerface geome­

tries depending on the casting temperature of the piasti­

cised pnlymer. The cyclic voliammelry relaling lo lithium

chloride electrolyte and of choline transfer is shown in I'ig.

10 for a ^eries of membranes produced at different casting

lemperatures f.'S.'i to 9{fC). The potential windows show

that 110 significant differences exist between the membrane

cast ul the temperatures used here. 'Very slight changes

were observed in limiting current value for choline trans­

fer, rising with the casting temperature. No changes are

found for the X/l''/' '""J reverse peak current. Also, for

membranes prepared ai lemperatures between 55 and 8()°C

the ion iraiisfer current remained independent of the tem­

perature. This is an indication that the piesenl membrane

fabrication method is reproducible, Moreover, the experi­

mental currents measured here (mean value 26.5 i iA) arc in

good agreemeiil with the theoretical inlaid interface curreiil

from Kc|. (3) (25.8 iiA). further suggesting thai the present

hoi casting approach produces a system of inlaid micro-in­

terfaces.

At higher |)oiymer temperalures ( > 8()°C). the mem­

brane current iiecomes slightly elevated as a consequence

of polyvinylchloride outflow from ihe hole during casting.

This viscosity effect leaves a slightly larger lens-shaped

interface which gives rise lo slightly higher steady slate

currents.

120'''^ ^ 700

A(|)/mV

A(t)/mV

I'ly, ID, Cyclic vcilliiniinogniiii.s oC Ch ion Iraiislcr inio Ihc iiicniliniiic

Willi vMiyiim iciiipcrauirc ol'casliny 2,,S'/( I'VC-ycl nicnibiimc, (a) 55 C

(I)) V.S'C. anil (c) y r c . (i) t'olciilial window lor Cell I ami ( i i) in ihc

presence ol' 0,1 niM Ch ' ion in Ihc a(|ueon.s pliasc.

micro-machined PHT fi lm was prepared by the excimer

process mentioned above: f i lm thickness was varied from

? to l(){)|xni. Fig. 11 shows the membrane response to the

aqueous lithium elecirolyle and choline as the PBT thick­

ness is increased from 12 to 36fxm. Most noticeable is the

shift in the choline iransfer wave to a more positive

I'.lcclrocheniical chaiaelenslic.'. ol niicro-inlciracc nicniliraiie wilh vanni;

!'!• r Ihiekiies.s incliuliii!: Ihc luenihianc ie.-.isUincc A', leuiin peak cnneiil

/|, and JKiU'-wave polenlial X'.d'' '. appioxinuilc (a) cnlrancc and (h) c i i l

.sideN of a luicro-liolc radius

I'l'T/ixni \l,l,''-/mV / /n,<\ l,,/iu\ l</Mil t/iuu'

48S.4

4')2,.'i

4y.s

4.4. Iiu'ii film ihickncss

The effect of the PET fi lm thickness on the ion transfer

voltammetry of choline was also studied (Table 2). Laser Dala are lakcn Ironi ph()loniicroi;raph.

III. / JiiiiriKil iij I'Icclivdmilyliciil Clifiiiisliyhlll I l<)"7l ".( .S',::

polciiliiil niiiinly us a icsiill of incrcustHl iiiunihi;iiii.: icsis-liiiicc. TliL' iiiL'mhiiiiic icsisiiiiiCL- is SL-nsilivc (o Imle sliuc-lurc since ion iiiohilily tiiroiij ii llie mL'miiiiinc is govcinccl liy llic riux in those regions. Fig. 12 sliovvs ihut it Id-lbkl ineieiise in lesislanee is intnuliiced for the membrane series consiciereil in Ihis slndy. 'I'iie memlirane resislanee lo polyester tiiicivness is Ibmul to he appioximately linear, possibly rel'lecting the linear ion flux through liie hole. iToni I'ig. 12, we can ealculate a value for the I'VC composite resistivity. Mere we estimate I.I X 10 11 cm, compared to 0.7 X 10 Jlcm determined for the gel com­posite by impedance measurement with a standard conduc­tivity probe.

,i(|)/mV

(C) 40

A(i)/mV

t-ig. I I. lilTccl of PliT nin\ Ihickricss on Uic clcclrocliciiiical responses of Clr"" ion transfer into organic 2M'/r PVC + NFOF- gel. (a) 12 (xm. (Ii) 23jiin. and (c) 3fi(jLin. (i) Potential window lor Cell I and (ii) in the presence ol'O. I niM Ch ' ion in the aqueous phase. Sweep rate 2()niVs~ '. Temperature of casting 7' = 7()"C.

20 40 60 80 100

PET thickness/nm

Variation of nienilirane resislanee with the thickness (

Furthermore, although the magnitude of the limiting current corresponding to aqueous lo membrane ion Irunsfer remains approximately constant for the various polyester thicknesses of 3 to 36 |xm. it is apparent thai the size of the reverse peak current is significantly increased with hole depth. Under these conditions the linear diffusion flux within longer tube-like structures means that few ionic species dilTuse into the bulk of the PVC film.

it can be noticed that in the case of the thin films, the micro-hole area is higher than for the other thicknesses due to the difficulty in handling such thin films during the laser machining procedure. However, the current density is un­changed within experimental error.

5, ConclusioiLS

Micro-interface polymer membranes sensitive to ions and structurally dellned by excimer laser patterning have been developed in this present work. A method has been developed based on the hot casting of PVC composites which renders the reproducible fabrication of membranes with an inlaid micro-interface structure. The study has demonstrated the versatility of the micro-machined com­posite membranes as an effective transducer for sensing. The membranes in the present study were characterised in terms of the ion transfer behaviour of choline in lithium chloride electrolyte. Cyclic voltammctry of choline ion transfer indicated several important factors necessary to control excessive membrane resistances.

Acknowledgements

The authors wish lo acknowledge the financial support given by the Ponds National pour la Recherche Scien-

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