ISSN 0 9 5 8 - 2 1 1 8

1 9 9 2 / 9 An International Newsletter N u m b e r 29

Membrane Technology

C h e a p e r i o n o m e r m e m b r a n e s EIC Laboratories, developers of a new ol.RS of lonomer membrane, claims that they have lower ion resistivity and better stability than conventional cation ¢~change membranes . With a s t a r t i ~ point of cheap and plentiful hydrocarbons rather t han more expensive fluorinated chemlc~tq the new membranes should also be cheaper to manufacture .

The deve/opment work was underf~,ken with a grant from the US Depar tment of Energy and EIC are now looking to license manufac ture of the membranes . PotentiAl application areas include purification of waste streams, including metal Ion recovery, or use as fuel cell electrolytes.

The membranes are made by radical polymerization and hydrolyzation of monomers of ' superacids ' - - acids with equal or greater s trength than sulphurlc acid. Using superacids is said to give the membranes a higher ionic capacity, or more ionic groups per molecule than s tandard membranes . EIC has already m~de a hydrocarbon polymer and is synthesizing another which will have a pert luormated backbone for greater stability, bu t at a higher cost. Characterization work is continuing on all membranes.

Further tnformatlon from.. T ~ w t h y L. Rose, EIC Laboratories Inc, 111 Downey Street, Norwood, MA 02602, USA. Tel +I 617 769 9450. Fax +1 617 551 0283.

M e m b r a n e s for c h e m i c a l s e n s o r s

E L S E V E R A D V A N ( ~ED TECHNOLOGY

Ultrathin-fllm composite membranes are likely to find m a n y new applications as barrier layers in the next generation of chemical sensors. In general, barrier layers used in such sensors m u s t provide some degree of chemlcal selectivity, yet m u s t allow for high rates of analyte flux so tha t sensor response time is mln~n1~ed.

These membrane requirements m high chemical selectivity, high permeate flux, and good mechanical s trength - - ma tch the performance of today's

O 1 9 9 2 J, llmsvier l l e l e n e e P u b l i l ~ m Ltd., b ~ u ~ l @ 2 1 S 6 . O 0 per i tem . . . . . No _Imrt of this publication may ~ reprodu_ced, stored in a retrieval system, or. Iransmitt~l.b~. any Iorm or oy. an..y means, elecvromc, mechanical, pliotocopying, recormng or otherwise, without the prior permismon of the pummnors. (Reaaers m me u . ~ . - - ptease see special regulatlofis listed on back cover.)

N e w s a n d V i e w s

ultrathin-fllm composite membranes , and a team at Colorado State University has now produced several prototype sensors incorporating an Anopore/poly(dtmethytsflcomne) composite to demonstrate the practicality of this new sensor design.

The team Identified two very general types of sensor based on ultrathln-film composites, differentiated by the degree of selectivity required of the composite membrane. The first type would be based on a membrane which has specific molecTdAr specificity for the analyte species - - that is, molecular recognition chemistry would be built into the ultrathin film such that only the analyte molecule Is extracted and t ransported by the composite membrane.

In the second type of sensor the membrane would provide only a rudimentary selectivity, based on molecule size or charge, and act as a 'prefllter'. The membrane would t ranspor t the analyte molecules into an internal sensing solution which would contain the molecule recognition chemistry and the t ransducer for translating this chemistry into a measurable electrical signal. The second type of sensor is m u c h easier to fabricate and so work to date has concentrated on these devices.

In a paper in Analytical Chemistry (Vol 84, No 21, 1 November 1992, pp 2647-2651), the Colorado team describes the fabrication and t e s t ~ g of an electrochemlcal glucose sensor. This particular sensor was chosen to demonstra te the feaslbtllty of the design concept because the molecular recognition and signal t ransduct lon chemistries are already well established. As suppor t for the ul trathin film the team selected an Anopore microporous a lumina membrane - - 55 pm thick with linear, cylindrlcal pores (about 250 nm d_lameter) and about 65% porosity. One face of the membrane was rendered electronically conductive by sputtering a gold film (20-30 run) across the membrane surface. This gold film served as the worklng electrode and electrical contact was made by using a sliver-epoxy to a t tach a copper wlre to the gold surface (see Figure). The gold film is too thin to block the pores at the Anopore surface.

Working electrode lead

\

lntern~ to|ution containing glueom ogidue, -

mediator, buffer, etC.

~ltrlthin ~lm ¢~mpo~*.e membr~ae I . i "~

L~at .~n polymer film

Analyte SoDution

j Body

Sputtered Au fdm ~ working elec~xxte

Pore in support rnembrL,~

SchematSc dbngram of the prototype ultrath/n-.fflm composite membrane -based glucose sensor.

Using an interfaclal polymerlzation method developed at Colorado, the surface of the Anopore membrane opposite to the gold film was then coated with a 50 nm skin of poly(dlmethylslloxane). The film was subsequently cross-llnked and sulphonated to prevent leaching of the mediator from the internal solution into the analyte solution. Finally the gold/Anopore/polymer composite membrane was glued to the end of a glass tube which formed the body of the sensor.

One of the beauties of this approach for making sensors, says the team, is that a totally self-contained sensing device Is obtained - - external electrodes are not required. The design is compared with that of an Ion-selective electrode, in that both contain an Internal reference electrode. However, it is pointed out that an Ion-selective electrode still usual ly needs an external reference electrode whereas the new sensors do not.

Based on the series of tests reported in the paper the Colorado workers are confident in the concept and the practical design of these sensors. They believe that the design is generic and should be amenable to a wide variety of molecular recognition schemes, to other film chemistries, and to other signal t ransduct ion processes. Techniques developed at their laboratories suggest the possibility of forming ultrathln-fllm composite membranes based on almost any chemistry, and it should be possible to mlnlaturlze the sensor by using a mlcroporous hollow fibre as substrate. Work in all these areas Is continuing.

Further/nformatlonfrom: Dr Charles R. Mart/n, Department of Chemistry, Colorado State UnWerslty, Fort Col/Ins, CO 80523, USA. Fax +1 303 491 2254.

Pd memb ne catalys is In the field of membrane catalysis, high temperature membranes are classified as those that can be used at temperatures above 200 °C. Candidate materials Include inorganic oxides, carbon, palladium and its alloys and composites, and a l though m a n y problems still have to be overcome for successful process operation at these temperatures a recent review suggests progress to date gives grounds for optlmt~m.

J o h n Armor of Air Products & Chemicals Inc, writing in Chemtech (Vo122, No 9, 1992, pp 557--563), sees future potential for catalytic materials deposited on monomodal sub 8 A inorganic membranes, and with thin metal alloy coatings on mesoporous supports. He reports that studies have already been made of composite membranes consist ing of palladium and sliver-palladium deposited on the outer surface of porous glass tubes and porous a lumina cylinders by electroless plating techniques.

Further/nformattonfrom: John N. Armor, Aft" Products & Chemicals Inc, 7201 Hamllton Boulevard, Allentown, PA 18195-1501, USA. Tel +1 215 481 4911.

2 Membrane Technology No. 29