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Chemical sensing with zeolite materials.

Work at Guelph is concentrating on the use of zeolites as electrode modifiers

Fifteen years ago it would have been surprising that the modification of the surfaces of electrodes with nonconductive materials could effect electron transfer events in a useful way.

Today, the fabrication of such systems is playing an increasingly important role in the tailoring of electrodes for specific tasks. The electrochemistry that has evolved from chemically modified electrodes has produced much in the way of fundamental research, but beyond this there is the added bonus of practical dividends stemming from these studies. Potential applications could include; corrosion protection, electroanalysis, electrochromic devices, electrosynthesis and electrocatalysis.

At Guelph we are primarily concerned with the use of zeolites as electrode modifiers. In the electrochemical and materials oriented disciplines these materials have evoked considerable interest and several recent reviews exist |1-4~. In this article we describe one aspect of our research program at Guelph, namely the application of these electrodes as solution phase chemical sensors.

Zeolites are well known as cation-exchange materials, providing a convenient way of loading the zeolite with electroactive species. The electrochemical reduction of a charge balancing extraframework ion (|E.sup.m+~) can logically proceed via two mechanisms:

1. |E.sup.m+~(z) + |ne.sup.-~(s) + |nC.sup.+~(s) ---- > |E.sup.(m-n)+~(z) + |nC.sup.+~(z)

2. |E.sup.m+~(z) + |mC.sup.+~(s) ---- > |E.sup.m+~(s) + |mC.sup.+~(z) |E.sup.m+~(s) + |ne.sup.-~ ---- > |E.sup.(m-n)+~(s)

where (z) indicates the zeolite matrix, (s) indicates the solution phase and |C.sup.+~ is the electrolyte cation. The major distinction between these mechanisms is the site of the electron transfer to the intrazeolite electroactive moiety, |E.sup.m+~.

Mechanism 1 involves intrazeolite electron transfer, occurring for example via an electron-hopping mechanism or mediated electron transfer. In mechanism 2, the electroactive ion diffuses and/or ion-exchanges out of the zeolite and is then reduced at the electrically conductive portion of the electrode.

In both cases, if |E.sup.m+~ is a charge balancing extra-framework-ion, then a counter ion (|C.sup.+~) from the electrolyte solution must gain ingress into the zeolite interior in order to maintain electroneutrality. Thus, in the special case where |C.sup.+~ is excluded from the zeolite pore system, the electroactivity of |E.sup.m+~ is suppressed. Electrochemical systems of this type can be of potential use in the design of a chemical sensor, if the intrusion of an analyte into the zeolite pore system lifts the suppression condition.

In the mechanisms described, the rate of counter diffusion of |E.sup.m+~ and |C.sup.+~ is crucial. For slow diffusion or ion-exchange such as that occurring in non-aqueous electrolytes, the electroactivity of |E.sup.m+~ is indeed suppressed unless trace quantities of water are present in the non-aqueous solvent to facilitate rapid ion-exchange |5,6~. This is illustrated in Fig. 1 which shows the cyclic voltammograms of silver A modified electrodes in (a) DMF and (b) water.

Substantial suppression of electroactivity is displayed in DMF. This phenomenon can be harnessed for the quantitative determination of ultra-trace concentrations of water in organic solvents as is outlined below.

In Fig. 2, anodic stripping voltammograms of silver zeolite A recorded in DMF are presented. In part 2(b) of Fig. 2, 100 ppb of water were deliberately added to the electrolyte solution. This causes some lifting of the suppressed state of the intrazeolite silver ions and thus an increase in current. Note that the currents observed here, where the background water impurities are about 500 ppb, are two to three orders of magnitude smaller than those observed in pure water. The apparent detection limit of this sensor judging by the noise levels in the voltammograms is of the order of 20 ppb.

This is close to two orders of magnitude superior to Karl Fischer methods or to recently described water sensors. |7,8~ An added advantage of this sensor is specificity. Since only a small number of solution phase species can penetrate the zeolite A pore system, Fig. 3, the selectivity of the sensor is expected to be excellent. Note that even if a molecule can enter the pore system, this will not in general lift the suppressed state unless that molecule is associated with a cation of solvated complex. This is a significant advantage over widely used capacitive or resistive water sensors which are in general, not specific for water.

The response of this sensor electrode to trace water concentrations raises some interesting fundamental questions concerning the electrochemistry of zeolite modified electrodes in non-aqueous media. By virtue of their route through the dry-box ante-chamber, the zeolites used in non-aqueous experiments are partially dehydrated. (Note exhaustive dehydration requires vacuum thermal treatment at 450|degrees~C).

Significant changes in ionic conductivity and activation energies for intrazeolite cationic diffusion as a function of composition, pore size, framework charge and hydration have been discussed at length by Stamires |9~. In sodium zeolite A there is a close to five orders of magnitude change in ionic conductivity, which is proportional to the cation diffusion coefficient |10~ on going from the fully dehydrated to the hydrated form. Moreover the changes occur in discrete steps as ions located in distinct sites in the zeolite are solvated.

In zeolite A, the first extra-framework-cations to become mobile are the four sodium ions located in the eight ring (at five water molecules per unit cell) followed by the remaining eight sodium ions located on the six membered rings of the sodalite cages, which provide smaller contributions to the conductivity. The increase in current observed as water is added to the zeolite in our electrochemical experiments also show distinct stages, which perhaps reflect the solvation of silver ions in distinct sites. The sensor is indeed linear in water concentration from 1 ppm to beyond 10 ppm, but shows some non-linearity between 20 ppb and 1 ppm.

Further studies as a function of water concentration are in progress, aimed at understanding the importance of residual intrazeolitic water and extraframework cation site occupancies on the electrochemical response of the modified electrodes. The issues of ion-exchange/solvation associated with zeolites in non-aqueous systems are indeed of pivotal importance since many of the possible applications of zeolites in electrochemical environments will certainly require an understanding of these phenomena.

The projects described in this article are funded by NSERC and ICST grants.


1) D.R. Rolison, Chem. Rev. 90, 867 (1990).

2) D.R. Rolison, Talanta 38, 359(1911).

3) M.D. Baker and C. Senaratne, Electrochemistry of Novel Materials, Vol 3. VCH (1992).

4) G.A. Ozin, A. Kuperman and A. Stein, Angew. Chemie. Int. Ed. Engl. 28, 358 (1989).

5) C. Senaratne and M.D. Baker, J. Electroanal. Chem. 332, 357 (1992).

6) M.D. Baker and C. Senaratne, Anal. Chem. 64, 697 (1992).

7) H. Huang and P.K. Dasgupta, Anal. Chem. 63, 1935 (1990).

8) H. Huang, P.K. Dasgupta and S. Ronchinsky, Anal. Chem. 63, 1570 (1991).

9) D.N. Stamires, J. Chem. Phys. 36, 3174 (1962).

10) D.C. Freeman and D.N. Stamires, J. Chem. Phys, 35, 799 (1961).
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Author:Baker, Mark D.
Publication:Canadian Chemical News
Date:Nov 1, 1992
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